Roadmap on Energy Harvesting Materials

Abstract

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g., combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g., smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and electromagnetic power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyzes the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

Full text

ACCEPTED MANUSCRIPT • OPEN ACCESS
Roadmap on Energy Harvesting Materials
To cite this article before publication: Vincenzo Pecunia
et al
2023
J. Phys. Mater.
in press https://doi.org/10.1088/2515-7639/acc550
Manuscript version: Accepted Manuscript
Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process,
and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted
Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors”
This Accepted Manuscript is © 2023 The Author(s). Published by IOP Publishing Ltd.
As the Version of Record of this article is going to be / has been published on a gold open access basis under a CC BY 4.0 licence, this Accepted
Manuscript is available for reuse under a CC BY 4.0 licence immediately.
Everyone is permitted to use all or part of the original content in this article, provided that they adhere to all the terms of the licence
https://creativecommons.org/licences/by/4.0
Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content
within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this
article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions may be required.
All third party content is fully copyright protected and is not published on a gold open access basis under a CC BY licence, unless that is
specifically stated in the figure caption in the Version of Record.
View the article online for updates and enhancements.
This content was downloaded from IP address 60.240.126.68 on 18/03/2023 at 06:59

Roadmap on Energy Harvesting Materials
Vincenzo Pecunia1*, S. Ravi P. Silva2*, Jamie D. Phillips3, Elisa Artegiani4, Alessandro Romeo4, Hongjae
Shim5, Jongsung Park6, Jin Hyeok Kim7, Jae Sung Yun8, Gregory C. Welch9, Bryon W. Larson10, Myles
Creran11, Audrey Laventure11, Kezia Sasitharan12, Natalie Flores-Diaz12, Marina Freitag12, Jie Xu13,
Thomas M. Brown13, Benxuan Li14, Yiwen Wang15, Zhe Li16, Bo Hou17, Behrang H. Hamadani18,
Emmanuel Defay20, Veronika Kovacova20, Sebastjan Glinsek20, Sohini Kar-Narayan21*, Yang Bai22, Da Bin
Kim23, Yong Soo Cho23, Agnė Žukauskaitė24, Stephan Barth24, Feng Ru Fan25, Wenzhuo Wu26, Pedro
Costa27, 28, Javier del Campo29,30, Senentxu Lanceros-Mendez(27-30), Hamideh Khanbareh31, Zhong Lin
Wang32, Xiong Pu33, Caofeng Pan33, Renyun Zhang34, Jing Xu35, Xun Zhao35, Yihao Zhou35, Guorui
Chen35, Trinny Tat35, Il Woo Ock35, Jun Chen35, Sontyana Adonijah Graham36, Jae Su Yu36, Ling-Zhi
Huang37, Dan-Dan Li37, Ming-Guo Ma37, JiKui Luo38, Feng Jiang39, Pool See Lee39, Bhaskar Dudem2,
Venkateswaran Vivekananthan2, Mercouri G. Kanatzidis40, Hongyao Xie40, Xiao-Lei Shi41, Zhi-Gang
Chen41, Alexander Riss42, Michael Parzer42, Fabian Garmroudi42, Ernst Bauer42, Duncan Zavanelli43,
Madison K. Brod43, Muath Al Malki43, G. Jeffrey Snyder43, Kirill Kovnir44,45, Susan M. Kauzlarich46, Ctirad
Uher47, Jinle Lan48, Yuan-Hua Lin49, Luis Fonseca50, Alex Morata51, Marisol Martin-Gonzalez52, Giovanni
Pennelli53, David Berthebaud54, Takao Mori55,56, Robert J. Quinn57, Jan-Willem G. Bos57, Christophe
Candolfi58, Patrick Gougeon59, Philippe Gall59, Bertrand Lenoir58, Deepak Venkateshvaran60, Bernd
Kaestner61, Yunshan Zhao62, Gang Zhang63, Yoshiyuki Nonoguchi64, Bob C. Schroeder65, Emiliano
Bilotti66, Akanksha K. Menon67, Jeffrey J. Urban68, Oliver Fenwick66, Ceyla Asker66, A. Alec Talin69,
Thomas D. Anthopoulos70, Tommaso Losi71, Fabrizio Viola71, Mario Caironi71, Dimitra G. Georgiadou72,
Li Ding73, Lian-Mao Peng73, Zhenxing Wang74, Muh-Dey Wei75, Renato Negra75, Max C. Lemme74,76,
Mahmoud Wagih72,77, Steve Beeby72, Taofeeq Ibn-Mohammed78, K.B. Mustapha79 and A.P. Joshi78
1 School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1, BC, Canada
2 Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of
Surrey, Guildford, Surrey GU2 7XH, United Kingdom
3 Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware, USA
4 LAPS- Laboratory for Photovoltaics and Solid-State Physics, Department of Computer Science,
University of Verona, Ca’ Vignal 1, Strada Le Grazie 15, 37134, Verona, Italy
5 Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable
Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
6 Department of Energy Engineering, Future Convergence Technology Research Institute, Gyeongsang
National University, Jinju, Gyeongnam 52828, Republic of Korea
7 Optoelectronics Convergence Research Center, and Department of Materials Science and Engineering,
Chonnam National University, Gwangju 61186, Republic of Korea
8 Department of Electrical and Electronic Engineering, Advanced Technology Institute (ATI), University of
Surrey, Guildford, Surrey GU2 7XH, United Kingdom
9 Department of Chemistry, University of Calgary, Calgary, AB, Canada. T2N 4K9
10 National Renewable Energy Laboratory, Golden, CO 80401, USA
11 Département de chimie, Université de Montréal, Montréal, QC, Canada. H2V 0B3
12 School of Natural and Environmental Sciences, Bedson Building, Newcastle University, NE1 7RU
Newcastle upon Tyne, UK
13 CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University
of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
Page 1 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

14 International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of
Ministry of Education, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060,
China
15 Electrical Engineering Division, Engineering Department, University of Cambridge, 9 JJ Thomson
Avenue, Cambridge CB3 0FA, UK
16 School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK
17 Department of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK
18 National Institute of Standards and Technology, USA
20 Materials Research and Technology Department, Luxembourg Institute of Science and Technology
(LIST), 41 Rue du Brill, Belvaux, L-4422 Luxembourg
21 Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road,
Cambridge CB3 0FS, United Kingdom
22 Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering,
University of Oulu, Oulu, Finland
23 Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea
24 Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP
25 State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical
Engineering, Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province
(IKKEM), Xiamen University, Xiamen 361005, PR China
26 School of Industrial Engineering, Purdue University, West Lafayette, IN, 47907 USA
27 Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho 4710-053 Braga,
Portugal
28 Laboratory of Physics for Materials and Emergent Technologies, LapMET
29 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park,
48940 Leioa, Spain;
30 OIKERBASQUE, Basque Foundation for Science, 48009 Bilbao, Spain
31 Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK
32 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245,
USA
33 CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor,
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 101400 P. R.
China
34 Department of Natural Sciences, Mid Sweden University, Holmgatan 10, SE 851 70 Sundsvall, Sweden
35 Department of Bioengineering, University of California, Los Angeles, Los Angeles, CA 90095, USA
36 Department of Electronics and Information Convergence Engineering. Kyung Hee University. 1732
Deogyeong-daero, Giheung-gu, Yongin-Si, Gyeonggi-do 17104, Republic of Korea
37 Research Center of Biomass Clean Utilization, College of Materials Science and Technology, Beijing
Forestry University, Beijing 100083, PR China
38 College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027,
China
39 School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue,
Singapore 639798, Singapore
40 Department of Chemistry, Northwestern University, Evanston, IL, USA
41 School of Chemistry and Physics, Queensland University of Technology, Brisbane, QLD 4000, Australia
42 Institute of Solid-State Physics, TU Wien, A-1040 Wien, Austria
43 Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA
44 Department of Chemistry, Iowa State University, Ames, IA 50011, USA
45 US DOE Ames Laboratory, Ames, IA 50011, USA
Page 2 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

46 Department of Chemistry, University of California Davis, Davis, CA 95616, USA
47 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States
48 State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering,
Beijing University of Chemical Technology, North Third Ring Road 15, Chaoyang District, Beijing 100029,
P. R. China
49 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering,
Tsinghua University, Shuangqing Road 30, Haidian District, Beijing 100084, P. R. China
50 Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), C/Til·lers s/n (Campus UAB), Bellaterra,
Barcelona, Spain
51 Catalonia Institute for Energy Research (IREC), Jardins de Les Dones de Negre 1, 08930, Sant Adrià de
Besòs, Barcelona, Spain
52 Instituto de Micro y Nanotecnología (IMN-CNM-CSIC), C/ Isaac Newton 8, PTM, E-28760 Tres Cantos,
Spain
53 Dipartimento di Ingegneria della Informazione, Università di Pisa, Via G.Caruso, I-56122, Pisa, Italy
54 CNRS-Saint Gobain-NIMS, IRL 3629, LINK, National Institute for Materials Science (NIMS), 1-1 Namiki,
Tsukuba, 305-0044 Japan
55 National Institute for Materials Science (NIMS), WPI International Center for Materials
Nanoarchitectonics (WPI-MANA), 1-1 Namiki, Tsukuba, 305-0044 Japan
56 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, 305-
8671, Japan
57 Institute of Chemical Sciences and Centre for Advanced Energy Storage and Recovery, School of
Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
58 Institut Jean Lamour, UMR 7198 CNRS – Université de Lorraine, 2 allée André Guinier-Campus ARTEM,
BP 50840, 54011 Nancy Cedex, France
59 Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS – Université de Rennes 1 - INSA de Rennes,
11 allée de Beaulieu, CS 50837, F-35708 Rennes Cedex, France
60 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, United
Kingdom
61 Technische Bundesanstalt (PTB), Berlin, Germany, Abbestrasse 2-12, 10587, Berlin, Germany
62 NNU-SULI Thermal Energy Research Center (NSTER) and Center for Quantum Transport and Thermal
Energy Science (CQTES), School of Physics and Technology, Nanjing Normal University, Nanjing 210023,
China
63 Institute of High Performance Computing, A*STAR, Singapore
64 Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan
65 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
66 School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London
E1 4NS, UK
67 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA
30332, USA
68 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
69 Sandia National Laboratories, Livermore, CA 94551, USA
70 King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwal 23955-
6900, Saudi Arabia
71 Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Pascoli, 70/3, 20133
Milano
72 Electronics and Computer Science, University of Southampton, Highfield Campus, Southampton SO17
1BJ, United Kingdom
73 Key Laboratory for the Physics and Chemistry of Nanodevices, Peking University, Beijing 100871, China
Page 3 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

74 AMO GmbH, Otto-Blumenthal-Str. 25, 52074 Aachen, Germany
75 Chair of High Frequency Electronics, RWTH Aachen University, Kopernikusstr. 16, 52074 Aachen,
Germany
76 Chair of Electronic Devices, RWTH Aachen University, Otto-Blumenthal-Str. 2, 52074 Aachen, Germany
77 James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK
78 Warwick Manufacturing Group (WMG), The University of Warwick, Coventry, CV4 7AL, UK
79 Departments of Mechanical, Materials and Manufacturing Engineering, University of Nottingham
(Malaysia Campus), Semenyih 43500, Selangor, Malaysia
*Author to whom any correspondence should be addressed.
E-mail: vincenzo_pecunia@sfu.ca, sk568@cam.ac.uk, s.silva@surrey.ac.uk
Abstract
Ambient energy harvesting has great potential to contribute to sustainable development and address
growing environmental challenges. Converting waste energy from energy-intensive processes and
systems (e.g., combustion engines and furnaces) is crucial to reducing their environmental impact and
achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially
growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications
that can improve our quality of life (e.g., smart homes, smart cities, smart manufacturing, and smart
healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy
into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity,
piezoelectricity, triboelectricity, and electromagnetic power transfer. By bringing together the
perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive
insights into recent advances and present challenges in the field. Additionally, the Roadmap analyzes the
key performance metrics of these technologies in relation to their ultimate energy conversion limits.
Building on these insights, the Roadmap outlines promising directions for future research to fully harness
the potential of energy harvesting materials for green energy anytime, anywhere.
Keywords: energy harvesting materials, photovoltaics, thermoelectric energy harvesting, piezoelectric
energy harvesting, triboelectric energy harvesting, electromagnetic energy havesting, sustainability
1. Introduction - Vincenzo Pecunia and S. Ravi P. Silva
2. Materials for Indoor Photovoltaics
2.1 Introduction to indoor photovoltaics - Vincenzo Pecunia and S. Ravi P. Silva
2.2 III-V compound semiconductors for indoor photovoltaics - Jamie D. Phillips
2.3 CdTe solar cells for indoor applications - Elisa Artegiani and Alessandro Romeo
2.4 Kesterites for indoor photovoltaics - Hongjae Shim, Jongsung Park, Jin Hyeok Kim and Jae Sung Yun
2.5 Organic photovoltaics for indoor light to electricity conversion - Gregory C. Welch, Bryon W. Larson,
Myles Creran, Audrey Laventure
Page 4 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

2.6 Dye-sensitized photovoltaics for indoor applications - Kezia Sasitharan, Natalie Flores-Diaz and
Marina Freitag
2.7 Lead-halide perovskites for indoor photovoltaics - Jie Xu and Thomas M. Brown
2.8 Lead-Free Halide Perovskites and Derivatives for Indoor Photovoltaics - Vincenzo Pecunia
2.9 Quantum-dot absorbers for indoor photovoltaics - Benxuan Li, Yiwen Wang, Zhe Li and Bo Hou
2.10 Accurate characterization of indoor photovoltaic performance - Behrang H. Hamadani
3. Materials for Piezoelectric Energy Harvesting
3.1 Introduction to Piezoelectric Energy Harvesting – Lead-based oxide perovskites - Emmanuel Defay,
Veronika Kovacova and Sebastjan Glinsek
3.2 Lead-free oxide perovskites for Piezoelectric Energy Harvesting - Yang Bai
3.3 Nanostructured Inorganics for Piezoelectric Energy Harvesting - Da Bin Kim and Yong Soo Cho
3.4 Nitrides for Piezoelectric Energy Harvesting - Agnė Žukauskaitė and Stephan Barth
3.5 2D Materials for Piezoelectric Energy Harvesting - Feng Ru Fan and Wenzhuo Wu
3.6 Organics for Piezoelectric Energy Harvesting - Pedro Costa, Javier del Campo and Senentxu Lanceros-
Mendez
3.7 Bio-inspired materials for Piezoelectric Energy Harvesting - Hamideh Khanbareh
4. Materials for Triboelectric Energy Harvesting
4.1 Introduction to materials for triboelectric energy harvesting - Zhong Lin Wang
4.2 Synthetic polymers for triboelectric energy harvesting - Xiong Pu and Caofeng Pan
4.3 Nanocomposites for triboelectric energy harvesting - Renyun Zhang
4.4 Nanoparticles, Surface texturing and functionalization for triboelectric energy harvesting - Jing Xu, Xun
Zhao, Yihao Zhou, Guorui Chen, Trinny Tat, Il Woo Ock and Jun Chen
4.5 Nature-inspired materials for triboelectric energy harvesting - Sontyana Adonijah Graham and Jae Su
Yu
4.6 MXenes materials for triboelectric energy harvesting - Ling-Zhi Huang, Dan-Dan Li and Ming-Guo Ma
4.7 Perovskite-based triboelectric nanogenerators - JiKui Luo
4.8 Towards self-powered woven wearables via triboelectric nanogenerators - Feng Jiang and Pooi See
Lee
4.9 Theoretical Investigations towards the Materials Optimization for Triboelectric Nanogenerators -
Bhaskar Dudem, Venkateswaran Vivekananthan and S. Ravi P. Silva
5. Materials for Thermoelectric Energy Harvesting
5.1 Introduction on Materials for Thermoelectric Energy Harvesting - Mercouri G. Kanatzidis and
Hongyao Xie
5.2 Chalcogenides for thermoelectric energy harvesting - Xiao-Lei Shi and Zhi-Gang Chen
5.3 Full-Heuslers for thermoelectric energy harvesting - Alexander Riss, Michael Parzer, Fabian
Garmroudi and Ernst Bauer
5.4 Half Heuslers for Thermoelectric Energy Harvesting - Duncan Zavanelli, Madison K. Brod, Muath Al
Malki, G. Jeffrey Snyder
5.5 Clathrates for thermoelectric energy harvesting - Kirill Kovnir and Susan M. Kauzlarich
5.6 Skutterudites for thermoelectric energy harvesting - Ctirad Uher
5.7 Oxides for thermoelectric energy harvesting - Jinle Lan and Yuan-Hua Lin
Page 5 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

5.8 SiGe for thermoelectric energy harvesting - Luis Fonseca, Alex Morata, Marisol Martin-Gonzalez and
Giovanni Pennelli
5.9 Mg2IV (IV = Si, Ge and Sn)-based systems for thermoelectric energy harvesting - David Berthebaud
and Takao Mori
5.10 Zintl phases for thermoelectric energy harvesting - Robert J. Quinn and Jan-Willem G. Bos
5.11 Molybdenum-based cluster chalcogenides as high-temperature thermoelectric materials -
Christophe Candolfi, Patrick Gougeon, Philippe Gall, Bertrand Lenoir
5.12 Organic Thermoelectrics - Deepak Venkateshvaran and Bernd Kaestner
5.13 Two-dimensional (2D) materials for thermoelectric applications - Yunshan Zhao and Gang Zhang
5.14 Carbon nanotubes for thermoelectric energy harvesting - Yoshiyuki Nonoguchi
5.15 Polymer-carbon composites for thermoelectric energy harvesting - Bob C. Schroeder and Emiliano
5.16 Hybrid organic-inorganic thermoelectrics - Akanksha K. Menon and Jeffrey J. Urban
5.17 Halide perovskites for thermoelectric energy harvesting - Oliver Fenwick and Ceyla Asker
5.18 Metal organic frameworks for thermoelectric energy conversion applications - A. Alec Talin
6. Materials for Radiofrequency Energy Harvesting
6.1 Introduction to materials for radiofrequency energy harvesting - Thomas D. Anthopoulos
6.2 Organic semiconductors for radiofrequency rectifying devices - Tommaso Losi, Fabrizio Viola and
Mario Caironi
6.3 Metal-oxide semiconductors for radiofrequency rectifying devices - Dimitra G. Georgiadou
6.4 Carbon nanotubes for radiofrequency rectifying devices - Li Ding and Lian-Mao Peng
6.5 2D Materials for radiofrequency rectifying devices - Zhenxing Wang, Muh-Dey Wei, Renato Negra,
Max C. Lemme
6.6 Materials for Rectennas and radiofrequency energy harvesters - Mahmoud Wagih and Steve Beeby
7. Sustainability Considerations in Energy Harvesting Materials Research - Ibn-Mohammed, T.
Mustapha, K.B. and Joshi, A.P
Page 6 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

1 - Introduction
Vincenzo Pecunia1 and S. Ravi P. Silva2
1 School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1, BC, Canada
2 Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of
Surrey, Guildford, Surrey GU2 7XH, United Kingdom
In the face of the rising global energy demand and the existential challenge posed by climate change, it is
more urgent than ever to generate green energy in order to preserve our planet and sustain human
development. Alongside the need for renewable energy technologies for the conversion of primary green
energy into electricity in large-scale installations (e.g., solar, wind, and wave farms), reducing our carbon
footprint also requires harnessing the vast energy reservoir all around us in the form of ambient light,
mechanical vibrations, thermal gradients, and radiofrequency electromagnetic waves [1]. Harvesting this
energy via compact harvesters paves the way not only for more efficient use of our energy sources (for
instance, consider the recycling of waste heat from an oven or industrial machinery) but also for
sustainably powering technologies with considerable potential to enhance our quality of life without
increasing our carbon footprint [2], [3]. Prominently, compact energy harvesters are key to enabling the
Internet of Things (IoT), which aims to make our everyday objects and environments ‘smart’ via its
ecosystem of interconnected smart sensors, thereby allowing for better functionality of technology and
its optimum use (for instance, leading to smart homes, smart cities, smart manufacturing, precision
agriculture, smart logistics, and smart healthcare) [4]. Importantly, the IoT device ecosystem will comprise
several trillions of sensors in the near future [5]. This would make it unfeasible and unsustainable to
exclusively rely on batteries as their power source—due to their environmental impacts as well as the
challenge and cost of replacing hundreds of millions of batteries globally every day. However, compact
energy harvesters could overcome this challenge by allowing IoT devices to operate continuously and in
an eco-friendly manner throughout their lifetime [6], [7]. The burgeoning of wearable electronics, with its
vast potential for health and wellness applications [8], is another related domain that would greatly
benefit from compact energy harvesters—given that, in addition to being surrounded by ambient energy,
the human body itself is a source of waste energy in the form of body heat and motion.
Energy harvesting is critically dependent on the availability of suitable materials (and devices thereof) to
convert ambient energy into usable electric energy. Therefore, research in materials and devices for
energy harvesting is key to providing energy harvesting technologies that can meet the needs of real-
world applications. Such research requires a broad, cross-cutting effort, ranging from the discovery of new
materials to the study of their energy harvesting properties, the engineering of their compositions,
microstructure, and processing, and their integration into devices and systems. Given the diverse forms
of ambient energy, materials are being developed to convert such energy through various physical
mechanisms, the most prominent of which are the photovoltaic effect, piezoelectricity, triboelectricity,
thermoelectricity, and radiofrequency power transfer (Figure 1). The rapid rise in the number of
publications in this field (Figure 2) demonstrates its growing importance and the breadth of the
community that has joined this research effort.
Page 7 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Figure 1. Prominent energy harvesting technologies covered in this Roadmap. Reproduced from Ref. [7]
under the terms of a CC BY 4.0 open-access license.
Figure 2. Publications per year for the various energy harvesting technologies covered in this Roadmap.
This data was obtained from the Web of Science by searching the phrases ‘indoor photovoltaics’,
‘piezoelectric harvesting’, ‘triboelectric harvesting’, ‘thermoelectric harvesting’, and ‘RF harvesting’.
2001 2006 2011 2016 2021
100
101
102
103
104
Publications / Year
Year
Indoor PV
Piezoelectric
Triboelectric
Thermoelectric
RF Energy
Total
Page 8 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

While the vision of ‘green energy anytime, anywhere’ may still be some way into the future, energy
harvesting technologies already offer numerous opportunities. For instance, photovoltaic harvesters have
already been commercialized to power various smart sensors, while triboelectric, thermoelectric,
piezoelectric, and RF energy harvesters have already been demonstrated to be capable of powering
wearable devices [9]–[12].
Although the various energy harvesting technologies rely on considerably different classes of materials
and devices, they all share the same overarching goals and challenges—which will continue to drive future
research pursuits in this area—as discussed below.
Efficiency. A major challenge faced by all energy harvesting technologies is the limited power density
available from ambient energy sources, which makes it essential to develop energy harvesting materials
and devices that can efficiently convert such energy. Current energy harvesting technologies typically
deliver electric power densities well below the mW cm-2 when harvesting ambient energy. This can be
limiting for energy-intensive applications that do not allow aggressive duty cycling (i.e., a system operation
pattern with long intervals in sleep mode, during which the harvested energy can be stored, alternating
with short intervals in active mode, during which the stored energy is consumed) [2], [7]. Additionally,
many emerging applications require compact energy harvesters with feature sizes in the millimetre-to-
centimetre range. Therefore, boosting power conversion efficiencies is a vital goal of energy harvesting
research. The success of this endeavour critically depends not only on characterizing and gaining insight
into the fundamental properties of energy harvesting materials, but also on the discovery of new materials
and the engineering of their device architectures to reduce loss mechanisms.
Manufacturability. Real-world applications critically require the development of energy harvesting
technologies that can be manufactured at scale. Therefore, a priority is to develop energy harvesting
materials that can be produced with simple methods, involving low capital cost and low material and
energy consumption.
Environmental Sustainability. While energy harvesters inherently have no carbon emissions during
operation, they will fully realize their purpose of providing green energy if they have minimal
environmental impacts throughout their lifecycle. Therefore, a priority is to develop energy harvesting
technologies that rely on Earth-abundant, non-toxic source materials and can be processed with low
energy consumption. Additionally, it is important to consider the fate of these materials and devices at
their end of life. Therefore, a key priority is to pursue energy harvesting materials and devices that lend
themselves to be easily recycled from cradle to grave [13]. Moreover, for applications that involve a short
life cycle, energy harvesters that are biodegradable would be highly desirable.
Cost. For any energy harvesting technology to have a practical impact, it is necessary that its cost is
sufficiently low to enable widespread deployment. In contrast to large-scale installations for the
conversion of primary green energy into electricity (e.g., solar and wind farms), the crucial cost-related
objective for ambient energy harvesters is not necessarily to minimize the cost per Watt. Indeed, the
paramount aim is to ensure that ambient energy harvesters have a cost that is a manageable fraction of
the system cost, while also being capable of supplying an energy output adequate for the application at
hand. Cost is obviously a challenging metric to evaluate at the early stage of a technology, given that
learning curves typically result in substantial cost reductions over time. Nonetheless, it is important to
keep cost considerations in context as energy harvesting technologies are being developed, prioritizing
solutions that rely on Earth-abundant materials and low-energy manufacturing processes.
Form Factors. For energy harvesters to be deployed ubiquitously, it is essential to develop them in flexible
form factors so as to seamlessly place them on all kinds of objects and surfaces. Therefore, a research
Page 9 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

direction of paramount importance is to develop energy harvesting materials and devices capable of high
power conversion efficiencies while being mechanically flexible or stretchable and optimized for small
areas. The ubiquity and functionality of this novel system will act as an overlay to future wearables.
This Roadmap provides extensive insights into the status and prospects of the various energy harvesting
technologies being researched to address the aforementioned challenges. It does so by covering the
various classes of materials being developed for photovoltaic (Section 2), piezoelectric (Section 3),
triboelectric (Section 4), thermoelectric (Section 5), and radiofrequency (Section 6) energy harvesting. In
particular, this Roadmap highlights the key trends in materials properties and device performance
underlying these prominent energy harvesting technologies, also discussing the open challenges and the
potential strategies to overcome them. Finally, a perspective is presented on the key sustainability
challenges that need to be tackled in energy harvesting materials research (Section 7). Based on these
insights, it is envisaged that this Roadmap will catalyse further advances in energy harvesting materials
and devices, bringing us closer to realizing the vision of ‘green energy anywhere, anytime’.
References
[1] L. B. Kong, T. Li, H. H. Hng, F. Boey, T. Zhang, and S. Li, “Introduction,” in Waste Energy Harvesting,
Heidelberg, Germany: Springer, 2014, pp. 1–18. doi: 10.1007/978-3-642-54634-1_1.
[2] L. Portilla et al., “Wirelessly powered large-area electronics for the Internet of Things,” Nat
Electron, vol. 6, no. 1, Dec. 2022, doi: 10.1038/s41928-022-00898-5.
[3] V. Pecunia, M. Fattori, S. Abdinia, H. Sirringhaus, and E. Cantatore, Organic and Amorphous-Metal-
Oxide Flexible Analogue Electronics. Cambridge University Press, 2018. doi:
10.1017/9781108559034.
[4] Q. Hassan, Ed., Internet of Things A to Z. Hoboken, NJ, USA: John Wiley & Sons, Inc., 2018. doi:
10.1002/9781119456735.
[5] J. Bryzek, “Roadmap for the Trillion Sensor Universe”, iNEMI Spring Member Meeting and Webinar,
Berkeley, CA, 2013.
[6] P. Harrop, “Battery Elimination in Electronics and Electrical Engineering 2018–2028,” Cambridge,
UK, 2018.
[7] V. Pecunia, L. G. Occhipinti, and R. L. Z. Hoye, “Emerging Indoor Photovoltaic Technologies for
Sustainable Internet of Things,” Adv Energy Mater, vol. 11, no. 29, p. 2100698, Aug. 2021, doi:
10.1002/aenm.202100698.
[8] G. Wang, C. Hou, and H. Wang, Eds., Flexible and Wearable Electronics for Smart Clothing.
Weinheim, Germany: Wiley, 2020. doi: 10.1002/9783527818556.
[9] S. Bose, B. Shen, and M. L. Johnston, “A Batteryless Motion-Adaptive Heartbeat Detection System-
on-Chip Powered by Human Body Heat,” IEEE J Solid-State Circuits, vol. 55, no. 11, pp. 2902–2913,
Nov. 2020, doi: 10.1109/JSSC.2020.3013789.
[10] Y. Song et al., “Wireless battery-free wearable sweat sensor powered by human motion,” Sci Adv,
vol. 6, no. 40, Oct. 2020, doi: 10.1126/sciadv.aay9842.
[11] A. J. Bandodkar et al., “Battery-free, skin-interfaced microfluidic/electronic systems for
simultaneous electrochemical, colorimetric, and volumetric analysis of sweat,” Sci Adv, vol. 5, no.
1, Jan. 2019, doi: 10.1126/sciadv.aav3294.
[12] I. Mathews, S. N. Kantareddy, T. Buonassisi, and I. M. Peters, “Technology and Market Perspective
for Indoor Photovoltaic Cells,” Joule, vol. 3, no. 6, pp. 1415–1426, Jun. 2019, doi:
10.1016/j.joule.2019.03.026.
[13] S. Nandy, E. Fortunato, and R. Martins, “Green economy and waste management: An inevitable
plan for materials science,” Progress in Natural Science: Materials International, vol. 32, no. 1, pp.
1–9, Feb. 2022, doi: 10.1016/j.pnsc.2022.01.001.
Page 10 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

2. Materials for Indoor Photovoltaics
2.1 Introduction to indoor photovoltaics
Vincenzo Pecunia1 and S. Ravi P. Silva2
1 School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1, BC, Canada
2 Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of
Surrey, Guildford, Surrey GU2 7XH, United Kingdom
Photovoltaic devices enable the direct conversion of light into electricity: the energy harvested from the
light impinging on the photoactive material of choice (absorber) excites the charges within the material;
the resultant mobile charges are transported to and collected at two opposite electrodes, thereby
delivering an electric current and voltage (Figure 1a) that can be used for doing work. The ubiquity of light
makes photovoltaics a highly attractive energy harvesting technology due to their wide deployability [1].
Moreover, the wave nature of light—with its very short wavelength in comparison to the thickness of the
material layers typically found in photovoltaic devices—ensures that much energy density can be
transported even in the case of non-line-of-sight situations without a physical medium being present.
Additionally, based on typical ambient illumination levels, the power density that photovoltaics can supply
is at the high end of all the energy harvesting technologies (Section 1), with the intermittency of its
underlying energy transmission being generally manageable due to the predictability of its temporal
patterns. By virtue of all these factors, photovoltaics occupies a prominent role both in indoor energy
harvesting and for large-scale outdoor deployment, with its significance and impact being expected to
grow dramatically in the future as the current materials challenges are addressed [2].
Figure 1. a) Sketch of the operation of a photovoltaic device: photons (each having an energy equal to hν) are
dissipated in the absorber and are thus converted into mobile charges, i.e., electrons (denoted by a solid black circle)
and holes (denoted by an empty circle), whose transport to and collection at the electrodes result in an electric
current and voltage. The icon representing the LED bulb is re-used with permission from Flaticon.com. b) Normalized
emission spectra of the Sun (AM1.5G) and fluorescent (FL) and white LED (WLED) lamps.
The use of photovoltaics for the conversion of terrestrial outdoor solar light into electricity—through
arrays of solar panels deployed on rooftops or in large-scale solar farms (outdoor solar photovoltaics)—is
undoubtedly dominant and justifiably so due to the high energy density (in the range of 10–100 mW cm-
Page 11 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

2) of outdoor terrestrial solar illumination. Harvesting outdoor solar light also applies to building-
integrated photovoltaics, which involves solar panels embedded into the building envelope (e.g., as part
of façades, roofs, and windows). Alongside solar photovoltaics, recent years have witnessed the rapid rise
of indoor photovoltaics, which involves the conversion of ambient indoor light into electricity. This trend
parallels the strong demand for energy technologies that could sustainably power the exponentially
growing ecosystem of IoT smart sensors—expected to grow to several trillion units in the near future—
that are an essential constituent of the Internet of Things (IoT). Indeed, most IoT applications rely on smart
devices placed indoors (e.g., for smart homes, smart buildings, and smart manufacturing). From an energy
harvesting perspective, indoor photovoltaics presents a more stringent scenario, given the considerably
lower power density found in the ambient indoor light (in the range of 70–350 μW cm-2) supplied by
fluorescent (FL) and white light-emitting diode (WLED) lighting. Therefore, developing photovoltaic
technologies that can function at such low illumination levels is critical to sustainably powering the vast
IoT smart sensor ecosystem. The significance and potential of indoor photovoltaics are also confirmed by
the growth of its market size, which is projected to reach 1 billion dollars within the next few years,
meanwhile having a compound annual growth rate (CAGR) of 70 % and hence being the fastest-growing
segment within the entire photovoltaic market (cf., solar photovoltaics and building-integrated
photovoltaics are growing with a CAGR of 7.4 % and 16 %, respectively) [3].
The performance of an indoor photovoltaic cell can be quantified in terms of its indoor power conversion
efficiency (PCE(i)), which expresses the ratio between the maximum electric power generated by the cell,
, and the light intensity reaching the cell from the indoor light source, 󰇛󰇜, where 󰇛󰇜 is
the spectral irradiance at the surface of the photovoltaic cell:
󰇛󰇜󰇛󰇜  󰇛󰇜
 (1)
Note that, due to the current lack of a standard spectrum for the assessment of PCE(i), a given indoor
photovoltaic cell may present differing PCE(i) values depending on the light source considered—with the
relative efficiency variations depending on the level of spectral match between the absorber and the
illuminant as well as the irradiance of the latter. Additionally, note that the acronym PCE(i) is adopted
herein to refer to the power conversion efficiency of indoor photovoltaic cells to prevent confusion with
the power conversion efficiency of outdoor terrestrial solar cells (commonly indicated as PCE). Indeed,
while conceptually analogous, the two metrics are not directly comparable due to the different luminous
sources relevant to the two scenarios.
In addition to their much lower irradiance levels compared to outdoor terrestrial solar light, typical
artificial indoor light sources (i.e., FL and WLED lighting) are characterized by a rather distinct spectral
content, as their emission spectra exclusively cover the visible range (cf. approximately half of the
terrestrial solar light falls in the near-infrared spectral region) (Figure 1b). This aspect is particularly
consequential because it impacts the conditions that an absorber should meet to deliver optimum
photovoltaic efficiency. As shown in Figure 2a, in the radiative limit and for typical indoor light sources
(i.e., FL and WLED lighting), the optimum bandgap for an absorber to deliver high-efficiency indoor
photovoltaics amounts to 1.8–2.0 eV (cf. outdoor terrestrial solar photovoltaics requires a bandgap of
1.2–1.4 eV for optimum performance) [1]. Moreover, the narrower spectral range of indoor light sources
compared to the solar spectrum (Figure 1b) enables maximum theoretical efficiencies (in the radiative
limit) of up to  60 % for single-junction devices (cf. the corresponding limit for outdoor terrestrial solar
photovoltaics amounts to 33.7%) [1]. This implies that indoor photovoltaics could theoretically deliver
power densities of up to  40–200 μW cm-2 (for typical indoor illumination levels of 70–350 μW cm-2).
Page 12 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

While crystalline silicon (c-Si) is the dominant semiconductor technology for terrestrial solar
photovoltaics, the PCE(i) values of commercial c-Si solar cells are rather low (up to 12 %) because of
their low shunt resistance and comparatively narrow bandgap of c-Si (1.12 eV) [4]. In fact, hydrogenated
amorphous silicon (a-Si:H) cells have long been the commercially dominant technology for indoor
photovoltaics [5]: not only do they deliver PCE(i) values of 4–9 % [4] in commercial devices due to the
wider bandgap of a-Si:H (1.6–1.8 eV), but they also allow simpler manufacturing and mechanical
flexibility, thereby enabling significant cost reductions and the facile integration of indoor photovoltaics
in a wide range of objects and environments. Notwithstanding the commercial dominance of a-Si:H in the
indoor photovoltaics arena, commercial a-Si:H cells are characterized by a particularly large efficiency
deficit with respect to the ultimate efficiency potential of indoor photovoltaics (i.e., the radiative-limit
efficiency PCE(i)RL), which can be traced to a significant extent to the inherent optoelectronic and materials
properties of a-Si:H. Consequently, a grand challenge in indoor photovoltaics research is to develop
absorbers and device architectures that are capable not only of reliably surpassing the commercial state
of the art (essentially, the 10 % PCE(i) threshold) but that can also approach the ultimate PCE(i) limit of 60
%. This endeavor is all the more worthwhile because its success would enhance the reach and impact of
indoor photovoltaics as a leading energy harvesting technology: it would enable indoor photovoltaics to
extend the operational lifetime of battery-less smart devices, enhance their miniaturization, and deliver
sufficient power for smart devices capable of more complex, multifunctional, and energy-intensive
computing.
Figure 2. a) PCE(i) of the champion devices of various indoor photovoltaic technologies for an illuminance of or close
to 1000 lx. We calculated the radiative limit (denoted by the green trace) for single-layer device structures based on
the WLED spectrum shown in Figure 1b, while the data points are derived from Refs. [6]–[13]. b) Relative PCE(i)
deficit with respect to the PCE(i) in the radiative limit, PCE(i)RL, for the various technologies presented in a). PVSK:
halide perovskites; Pb-Free PVSK: lead-free halide perovskites; DSSC: dye-sensitized solar cells; OPV: organic
photovoltaics; QD: quantum dots.
The ambition to realize indoor photovoltaics with efficiencies approaching the radiative limit has
prompted the exploration of new materials and new device architectures, as well as the revisitation of
conventional photovoltaic technologies in the context of indoor light harvesting. In regard to emerging
technologies, remarkable progress has been achieved with organic (Section 2.5), dye-sensitized (Section
2.6), and perovskite (Section 2.7) cells, all delivering PCE(i) values in the range of 30–40 % in champion
devices (Figure 2a). Meanwhile, alternative families of absorbers—e.g., kesterites (Section 2.4), lead-free
perovskite derivatives (Section 2.8), and non-toxic quantum dot semiconductors (Section 2.9)—have been
pursued to address the toxicity or material scarcity issues faced by some of the emerging technologies.
Page 13 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Moreover, the re-energized exploration of silicon-based absorbers and the optimization of III-V
technologies (Section 2.2) for indoor photovoltaics have delivered significant progress, with efficiencies
in the 30–40 % being achieved in some cases (Figure 2a). Concurrently, CdTe has also shown promising
indoor photovoltaic performance (Sections 2.3) (Figure 2a).
Despite these considerable research achievements, the ultimate radiative efficiency limit of 60 % is far
from being reached, as also revealed by the plot in Figure 2b, which presents the relative PCE(i) deficit of
the various photovoltaic technologies with respect to the PCE(i) in the radiative limit, PCE(i)RL. This plot
highlights a particularly large relative efficiency deficit of 40–70 % for nearly all technologies, which
therefore prompts the pursuit of new materials and device architectures to advance the state of the art—
e.g., compositional engineering and defect passivation strategies to reduce recombination losses, as well
as device engineering to enhance the shunt resistance. Additionally, even when a smaller deficit is
achieved (as in the perovskite case), Figure 2a highlights that bandgap optimization remains crucial for
the 60% efficiency limit to be approached, which points to the need to focus on materials technologies
that allow bandgap tuning toward the 1.9 eV optimum.
Furthermore, the lack of a standard indoor light spectrum (e.g., see Figure 1b) has made the
characterization of the efficiency of indoor photovoltaics prone to ambiguities (see Equation 1), thereby
giving rise to the need to establish solid characterization protocols for the field to advance further, as
discussed in Section 2.10.
Apart from performance considerations, the future progress of indoor photovoltaics will also considerably
depend on the use of technologies that are based on eco-friendly, Earth-abundant materials and
straightforward manufacturing processes. Toxicity issues are particularly significant because indoor
photovoltaics are intended to be deployed in everyday objects and environments. Hence, it is crucial to
avoid that the end-users could be exposed to toxic materials accidentally released from indoor
photovoltaic cell. Therefore, materials engineering and materials discovery toward eco-friendly, Earth-
abundant, and easy-to-make indoor photovoltaics are key to ensuring its sustainability [1].
The following contributions (Sections 2.2–2.10) discuss the specifics of these grand challenges in the
context of various indoor photovoltaic technologies, also identifying potential avenues to overcome these
challenges. By presenting a comprehensive roadmap of indoor photovoltaic materials, we envisage that
the insights provided herein and in the rest of this section could catalyze further advances in the field
toward the realization of the full potential of indoor photovoltaics as a green energy harvesting
technology.
References
[1] V. Pecunia, L. G. Occhipinti, and R. L. Z. Hoye, “Emerging Indoor Photovoltaic Technologies for
Sustainable Internet of Things,” Advanced Energy Materials, vol. 11, no. 29, p. 2100698, Aug. 2021,
doi: 10.1002/aenm.202100698.
[2] S. R. P. Silva, “EDITORIAL: Now is the Time for Energy Materials Research to Save the Planet,”
ENERGY & ENVIRONMENTAL MATERIALS, vol. 4, no. 4, pp. 497–499, Oct. 2021, doi:
10.1002/eem2.12233.
[3] “BCC Research (2018). Global Markets, Technologies and Devices for Energy Harvesting: EGY097C,”
Wellesley, MA, USA, 2018. Accessed: Jun. 19, 2022. [Online]. Available:
https://www.bccresearch.com/market-research/energy-and-resources/global-markets-
technologies-and-devices-for-energy-harvesting.html
Page 14 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

[4] M. Freunek, M. Freunek, and L. M. Reindl, “Maximum efficiencies of indoor photovoltaic devices,”
IEEE Journal of Photovoltaics, vol. 3, no. 1, pp. 59–64, Jan. 2013, doi:
10.1109/JPHOTOV.2012.2225023.
[5] I. Mathews, S. N. Kantareddy, T. Buonassisi, and I. M. Peters, “Technology and Market Perspective
for Indoor Photovoltaic Cells,” Joule, vol. 3, no. 6, pp. 1415–1426, Jun. 2019, doi:
10.1016/j.joule.2019.03.026.
[6] B. Hou et al., “Multiphoton Absorption Stimulated Metal Chalcogenide Quantum Dot Solar Cells
under Ambient and Concentrated Irradiance,” Advanced Functional Materials, vol. 30, no. 39, p.
2004563, Sep. 2020, doi: 10.1002/adfm.202004563.
[7] J.-J. Cao et al., “Multifunctional potassium thiocyanate interlayer for eco-friendly tin perovskite
indoor and outdoor photovoltaics,” Chemical Engineering Journal, vol. 433, p. 133832, Apr. 2022,
doi: 10.1016/j.cej.2021.133832.
[8] L.-K. Ma et al., “High-Efficiency Indoor Organic Photovoltaics with a Band-Aligned Interlayer,” Joule,
vol. 4, no. 7, pp. 1486–1500, Jul. 2020, doi: 10.1016/j.joule.2020.05.010.
[9] Y. Dai, H. Kum, M. A. Slocum, G. T. Nelson, and S. M. Hubbard, “High efficiency single-junction
InGaP photovoltaic devices under low intensity light illumination,” in 2017 IEEE 44th Photovoltaic
Specialist Conference (PVSC), Jun. 2017, pp. 222–225. doi: 10.1109/PVSC.2017.8366547.
[10] I. Mathews et al., “Analysis of CdTe photovoltaic cells for ambient light energy harvesting,” Journal
of Physics D: Applied Physics, vol. 53, no. 40, p. 405501, Sep. 2020, doi: 10.1088/1361-
6463/ab94e6.
[11] X. He et al., “40.1% Record Low‐Light Solar‐Cell Efficiency by Holistic Trap‐Passivation using
Micrometer‐Thick Perovskite Film,” Advanced Materials, vol. 33, no. 27, p. 2100770, Jul. 2021, doi:
10.1002/adma.202100770.
[12] H. Michaels et al., “Dye-sensitized solar cells under ambient light powering machine learning:
towards autonomous smart sensors for the internet of things,” Chemical Science, vol. 11, no. 11,
pp. 2895–2906, 2020, doi: 10.1039/C9SC06145B.
[13] G. Kim, J. W. Lim, J. Kim, S. J. Yun, and M. A. Park, “Transparent Thin-Film Silicon Solar Cells for
Indoor Light Harvesting with Conversion Efficiencies of 36% without Photodegradation,” ACS
Applied Materials & Interfaces, vol. 12, no. 24, pp. 27122–27130, Jun. 2020, doi:
10.1021/acsami.0c04517.
2.2 III-V compound semiconductors for indoor photovoltaics
Jamie D. Phillips
Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware, USA
Status
Photovoltaic energy harvesting from indoor lighting sources presents unique challenges and constraints
in comparison to conventional solar photovoltaics, where the differing cost and performance tradeoffs
lend themselves to consider III-V compound semiconductor devices. The narrowband spectrum of indoor
lighting sources, irradiance levels that are orders of magnitude lower than typical sunlight conditions, and
desire for direct integration of power systems on devices place emphasis on high cell performance and
integration capabilities. Compound semiconductors based on III-V materials provide a range of available
bandgap energies that span the desired window in the visible spectrum for indoor lighting, while also
demonstrating very high optical absorption coefficients and carrier transport properties that can achieve
high power conversion efficiency at low irradiance conditions. The optimal bandgap energy of
approximately 2 eV for photovoltaics operating under high-efficiency indoor lighting are well matched to
mature III-V materials including AlGaAs and InGaP, with a theoretical power conversion efficiency of
Page 15 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

around 50 % and higher [1] at typical indoor irradiance conditions (500 lux). Experimentally, power
conversion efficiencies of >20 % have been reported for AlGaAs[2] and >30 % for InGaP[3], dramatically
outperforming prior commercial indoor photovoltaic devices based on amorphous silicon (<10%). The
high performance of III-V devices, and opportunities for further improvements in efficiency, are linked to
the outstanding fundamental electronic and optical material properties and the ability to “bandgap
engineer” sophisticated device structures. The superior power conversion efficiency offered by III-V
photovoltaics provides unique opportunities for miniaturized self-powered systems, where unlike solar
photovoltaics, power density is more important than cost per unit area. Indoor photovoltaics based on III-
V compounds are likely to play a major role in self-powered devices for the internet of things (IoT) through
continued advances to improve power conversion efficiency and to develop architectures for direct
system integration.
Figure 1. Calculated maximum power conversion efficiency versus bandgap energy for
photovoltaic cells under AM1.5 and White LED illumination. Select semiconductor materials are
shown, illustrating the high potential efficiency for III-V compound semiconductors.
Current and Future Challenges
Primary research challenges for realizing the potential of III-V indoor photovoltaics can be categorized as
issues related to cell performance, cost feasibility, and system integration. While high power conversion
efficiency has already been realized for AlGaAs and InGaP devices, performance is still only about half of
the theoretical limit for indoor lighting conditions. Continued optimization of device structures for the
indoor lighting spectrum and irradiance levels would be expected to result in substantial gains in
efficiency. Device structure optimization parameters to pursue include epitaxial structure thickness,
bandgap, and doping concentration of emitter, base, window, and barrier layers[4]. Reduction of dark
current is a primary limitation for low irradiance conditions, which is dominated in III-V devices by non-
radiative surface/sidewall perimeter recombination[5]. Further reductions in dark current will require
approaches to passivate recombination centers or to isolate minority carriers from these interfaces. Cost
feasibility is a pervasive challenge for III-V devices due to the higher cost of substrate materials and
epitaxial growth processes in comparison to silicon microelectronics. However, the high performance
Page 16 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

offered by III-V photovoltaics make them preferable for applications where power density is at a premium
provided that the cost of the energy harvesting device is a manageable fraction of the overall system cost.
Cost-effective approaches to realize III-V photovoltaics are a current research challenge, which includes
strategies for large scale production to leverage economies of scale. The adoption of III-V photovoltaics
will depend on the ability for direct system integration to provide an effective power unit. Target
applications will likely be miniaturized self-powered systems where the overall efficiency of the power
unit require optimization and co-design of the photovoltaic device, power management circuitry, and any
energy storage devices. At the photovoltaic device level, there are research challenges to consider the
development of appropriate multi-junction devices and series-connected modules on a chip for efficient
voltage up-conversion[6].
Advances in Science and Technology to Meet Challenges
A primary thrust for improving the efficiency of III-V indoor photovoltaics should explore means of
reducing non-radiative surface/sidewall recombination. Example techniques may include continued
exploration of chemical surface treatments and surface passivation layers. High-quality epitaxial regrowth
of wide-bandgap III-V materials on exposed interfaces may offer dramatic improvements in interface
quality. There have been several techniques that have been proposed to develop cost-effective III-V
photovoltaics (primarily for solar energy conversion) including epitaxial liftoff[7], hydride vapor phase
epitaxy (HVPE)[8], and roll-to-roll printing[9]. The epitaxial liftoff approach offers a method for substrate
reuse, eliminating the major cost of substrates as a part of the overall cell cost. Techniques such as HVPE
can reduce the cost of epitaxial growth through efficient use of precursor materials. Large scale
production such as roll-to-roll printing offer a means to achieve low cost at high volume. Technology
advances will be needed to enable III-V device integration with systems ranging from the macroscale to
the microscale. Larger scale devices and systems (approximately 1 cm and larger) will require
development of suitable packaging technologies to interface with systems. An attractive advantage of III-
V devices is the ability to use thin device layers via wafer liftoff, where mechanically flexible photovoltaic
devices[7] may be used to accommodate a wide range of system form factors. Smaller scale systems at the
millimeter size and below will rely on continued advances in heterogeneous integration with silicon
microelectronics. This is a technology area that is rich for development, where there is a desire to leverage
both the high-performance of III-V materials and sophistication of silicon microelectronics for applications
including photonic integrated circuits, microelectromechanical systems, high-frequency electronics, and
energy conversion systems such as described in this work. Heterogeneous integration strategies include
epitaxial liftoff and cold welding, direct wafer bonding, micro-transfer printing, and selective epitaxy of
III-V materials on silicon.
Concluding Remarks
The outstanding optical and electronic properties of III-V compound semiconductors offer the highest
power conversion efficiency for indoor photovoltaics, with continued room for performance
improvement through optimization of device structures and material interfaces. The adoption of III-V
photovoltaics will depend on the cost practicality of these devices for a given system application. The most
likely applications to incorporate III-V indoor photovoltaics will be systems where size, weight, and power
are at a premium. Furthermore, the continued trajectory of advanced self-powered microscale systems
may be enabled by the high power density provided by III-V photovoltaics.
References
[1] M. Freunek, M. Freunek and L. M. Reindl, "Maximum efficiencies of indoor photovoltaic devices," IEEE Journal of Photovoltaics,
vol. 3, no. 1, pp. 59-64, Jan. 2013, doi: 10.1109/JPHOTOV.2012.2225023.
Page 17 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

[2] A. S. Teran, J. Wong, W. Lim, G. Kim, Y. Lee, D. Blaauw, and J. D. Phillips, "AlGaAs Photovoltaics for Indoor Energy Harvesting
in mm-Scale Wireless Sensor Nodes," IEEE Transactions on Electron Devices, vol. 62, no. 7, pp. 2170-2175, July 2015, doi:
10.1109/TED.2015.2434336.
[3] Y. Dai, H. Kum, M. A. Slocum, G. T. Nelson and S. M. Hubbard, "High efficiency single-junction InGaP photovoltaic devices
under low intensity light illumination," 2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017, pp. 222-225, doi:
10.1109/PVSC.2017.8366547.
[4] J. Phillips, E. Moon, and A. Teran, “Indoor Photovoltaics Based on AlGaAs”, Indoor Photovoltaics, M. Freunek Müller (Ed.). doi:
10.1002/9781119605768.ch9
[5] A. S. Teran, E. Moon, W. Lim, G. Kim, I. Lee, D. Blaauw, J. D. Phillips., "Energy Harvesting for GaAs Photovoltaics Under Low-
Flux Indoor Lighting Conditions," IEEE Transactions on Electron Devices, vol. 63, no. 7, pp. 2820-2825, July 2016, doi:
10.1109/TED.2016.2569079.
[6] E. Moon, M. Barrow, J. Lim, D. Blaauw and J. D. Phillips, "Dual-Junction GaAs Photovoltaics for Low Irradiance Wireless Power
Transfer in Submillimeter-Scale Sensor Nodes," in IEEE Journal of Photovoltaics, vol. 10, no. 6, pp. 1721-1726, Nov. 2020, doi:
10.1109/JPHOTOV.2020.3025450.
[7] K. Lee, J. D. Zimmerman, T. W. Hughes, and S. R. Forrest, “Non-Destructive Wafer Recycling for Low-Cost Thin-Film Flexible
Optoelectronics”, Adv. Funct. Mater., 24: 4284-4291 (2014). doi: 10.1002/adfm.201400453
[8] K. A. W. Horowitz, T. Remo, B. Smith and A. Ptak, “A Techno-Economic Analysis and Cost
Reduction Roadmap for III-V Solar Cells“, National Renewable Energy Laboratory Technical Report NREL/TP-6A20-72103, (2018).
[9] D. Khatiwada, C. A. Favela, S. Sun, C. Zhang, S. Sharma, M. Rathi, P. Dutta, E. Galstyan, A. Belianinov, A. V. Ievlev, S. Pouladi,
A. Fedorenko, J. H. Ryou, S. Hubbard, and V. Selvamanickam, “High-efficiency single-junction p-i-n GaAs solar cell on roll-to-roll
epi-ready flexible metal foils for low-cost photovoltaics”, Prog Photovolt Res Appl., 28: 1107– 1119 (2020). doi: 10.1002/pip.3308
2.3 CdTe solar cells for indoor applications
Elisa Artegiani1 and Alessandro Romeo1.
1LAPS- Laboratory for Photovoltaics and Solid-State Physics, Department of Computer Science,
University of Verona, Ca’ Vignal 1, Strada Le Grazie 15, 37134, Verona, Italy
Status
CdTe thin film based solar cell technology has achieved, so far, the largest deployment on large scale
manufacturing among thin film technologies. These have a very good advantage for PIPV (product
integrated photovoltaics) that is the capability to be deposited on every type of substrate, from glass to
metal and they can become extremely lightweight and flexible when deposited on flexible substrates [1].
In year 2021 almost 80 % of the thin film solar cell market was CdTe based with an overall production of
6.1 GWp [2]. CdTe photovoltaic devices have achieved an efficiency of 22.1 % on a small area for outdoor
solar photovoltaics [3] but most important, unlike other technologies, the efficiency of the solar modules
in production is not far from the record values: 18.7 %. The main reason for this achievement is due to
the intrinsic properties of the CdTe absorber, which has a very simple phase diagram. The required
stoichiometry (tendentially 50-50 for Cd and Te) is easily obtained also at relatively low temperatures
(below 400 °C) and with no issues of unexpected and uncontrolled secondary phases [3].
CdTe is a direct band gap semiconductor with a value of 1.45 eV which is very near to the Shockley-
Queisser optimum for outdoor solar photovoltaics, it also has a high absorption coefficient that allows to
collect the photons with thicknesses down to 2 micrometers [3]. Furthermore, CdTe can be successfully
Page 18 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

deposited with a large variety of different deposition methods, the most important are : thermal
evaporation [4], close space sublimation [5], electrodeposition [6], vapour transport deposition [7].
In low light conditions, the spectrum changes with a wavelength shift towards the blue region, so the
light power is concentrated in the 300 – 500 nm range. In this case, CdTe, which is mainly operating in the
visible part of the spectrum, is much better performing, as also shown by different independent analysis.
The highest efficiency indoor obtained from CdTe devices to date is 17.1 % under white LED.
Fig. 1. External quantum efficiency: CdS/CdTe vs CdSexTe1-x/CdTe
Current and Future Challenges
CdTe thin film solar cells are typically fabricated in superstrate configuration, on a transparent conductive
oxide coated soda lime glass.
Historically the cell is based on a CdS buffer layer for the separation of the charges. CdS is a semiconductor
with a band gap of 2.4 eV and needs to be very thin in order to reduce the parasitic absorption in the 400
nm range. However, in recent times the structure of the solar cell has been radically changed with the
introduction of a graded band gap. In this case CdTe compound forms a junction with a CdSexTe1-x mixed
compound (with a band gap of 1.4 eV). In this configuration CdS is no longer necessary and a high
resistance layer (HRT) is interposed between the CdSexTe1-x film and the front contact.
This is particularly important for indoor application because it increases the spectral response in the
spectral region of indoor lights, as shown in figure 1.
CdTe solar cells are most successfully fabricated by physical vapor deposition processes, such as vapor
transport deposition/ close space sublimation that are based on the sublimation of the CdTe in an inert
gas atmosphere at a substrate temperature of about 500 °C, as well as thermal evaporation where CdTe
is deposited in vacuum at a substrate temperature in the range of 300-350 °C.
The device made in our laboratories consists of a commercial SnO2/SnO2:F (TO/FTO) coated soda lime glass
(TEC12), so in our case the HRT layer is TO, while also MgxZn1-xO can be used [8]. CdSe and CdTe are
deposited by thermal evaporation, the typical standard CdCl2 treatment for CdTe recrystallization, CdSe-
CdTe mixing and CdSexTe1-x/CdTe junction promotion is provided by drop casting of methanol solution.
Page 19 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

With a similar process, CdS was used as buffer layer, we have previously demonstrated flexible CdTe
devices on either polyimide or ultra-thin glass [1]. This is a very important feature for consumer electronics
and so for indoor applications since it allows to adapt to different products, and it does not increase the
weight of the final device.
CdTe solar cells have a demonstrated superior performance under low light irradiance where the
reduction of open circuit voltage is restrained. With a very simple 1-diode model the ideality factor
reduces towards one suggesting a change in transport mechanism corresponding to a more ideal junction
[9]. This effect has been registered also in our samples.
Advances in Science and Technology to Meet Challenges
In figure 2 we compare the efficiencies of the cells measured under halogen and LED illumination, with
different light irradiances. These values are normalized to the efficiency measured in standard conditions
(AM 1.5 - 1 sun).
Under low light irradiance, the efficiency is only slightly reduced in the case of halogen lamp, while it even
increases when illuminated by LED. In both cases, this is due to a modest reduction in open circuit voltage
along with an improvement in fill factor.
The increase of efficiency for the LED case is due to the irradiation spectrum of this light which is exactly
concentrated in the same range of the CdTe response (see fig. 1). This, considering the transition to LED
lighting, further favours CdTe technology for indoor applications.
One very important limitation for CdTe technology to indoor application is the national and international
regulations that consumer electronics are subjected to. These set a maximum concentration of chemical
elements that are considered dangerous to humans. Despite that CdTe is not registered as carcinogenic
due to its impressive stability (1-decomposition occurs with temperatures above 1100 °C; 2-solubility in
water does not occur), it is subjected to ROHS (Restriction of Hazardous Substances Directive directive)
in EU (similar directives are extended to other countries). For ROHS regulation the maximum permitted
concentrations in non-exempt products are 0.1 % or 1000 ppm (except for cadmium, which is limited to
0.01 % or 100 ppm) by weight. The restrictions are attributed to a so-called homogeneous material in
the product, this applies to each part that can be separated mechanically. In the latest design of CdTe
solar cells where CdS is removed, cadmium, which is a heavy metal, is present only in the absorber.
Considering the different densities of CdTe (5.85 g/cm3) and of soda lime glass (2.8 g/cm3) as well as
considering the thickness of the two encapsulating glasses (≥3 mm) we can roughly estimate a required
CdTe thickness of less than 0.8 µm. Our standard CdTe thickness is about 4.5 µm, about 5 times above
the limit. However, we have demonstrated that is possible to fabricate CdTe devices with 0.7 µm with
still an overall conversion efficiency of 8 % [10].
Page 20 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Fig.2. Efficiency of CdTe solar cells in respect to irradiation power and spectrum.
Concluding Remarks
CdTe has shown to be a very good candidate for product integrated photovoltaics due to its robustness,
reliability, high efficiency, and its suitability to be deposited on flexible devices.
Moreover, it has shown a remarkable behaviour for low light irradiation as demonstrated by different
reports and papers also mentioned in this section. Furthermore, we have proved that with the new
CdSexTe1-x/CdTe configuration the device is very well performing under low light, and it is particularly
suitable for LED indoor irradiation.
The main limitation to its application on devices is the ROHS directive which allows only 0.01 % of Cd
weight compared to the complete PV device. This however can be overcome if the solar cell is made with
ultra-thin CdTe, below 0.8 µm. This configuration has been already demonstrated also at our laboratory.
Efficiencies could become quite interesting if a back reflecting mirror would be applied on the back
contact.
Acknowledgements
We would like to acknowledge Matteo Bertoncello, Matteo Meneghini and Gaudenzio Meneghesso from
the Department of Information Engineering, University of Padua, for the EQE measurements. This work
was partially made in the framework of BIntegra (Building Integrated Solar Energy Generation and
Agrivoltaics) project funded by FSE-REACT-EU PON- CUP B39J21025850001.
References
[1] A. Salavei et al., “Comparison of high efficiency flexible CdTe solar cells on different substrates at
low temperature deposition,” Sol. Energy, vol. 139, pp. 13–18, 2016.
[2] Fraunhofer Institute for Solar Energy Systems and PSE Projects GmbH, “Photovoltaics Report -
2022- Fraunhofer ISE,” no. February, p. https://www.ise.fraunhofer.de/conte%0Ant/dam/ise/d,
2022.
Page 21 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

[3] A. Romeo and E. Artegiani, “Cdte-based thin film solar cells: Past, present and future,” Energies,
vol. 14, no. 6, 2021.
[4] E. Artegiani et al., “Analysis of a novel CuCl2 back contact process for improved stability in CdTe
solar cells,” Prog. Photovoltaics Res. Appl., vol. 27, no. 8, pp. 706–715, 2019.
[5] A. Bosio, A. Romeo, D. Menossi, S. Mazzamuto, and N. Romeo, “Review: The second-generation
of CdTe and CuInGaSe2 thin filmPV modules,” Cryst. Res. Technol., vol. 46, pp. 857–864, 2011.
[6] A. A. Ojo and I. M. Dharmadasa, “15.3% efficient graded bandgap solar cells fabricated using
electroplated CdS and CdTe thin films,” Sol. Energy, vol. 136, pp. 10–14, 2016.
[7] B. E. McCandless, W. a Buchanan, and R. W. Birkmire, “High Throuput Processing of CdTe / CdS
Solar Cells,” in Conference Record of the 33rd IEEE Photovoltaic Specialists Conference, 2008, pp.
1–6.
[8] I. Mathews et al., “Analysis of CdTe photovoltaic cells for ambient light energy harvesting,” J.
Phys. D. Appl. Phys., vol. 53, no. 40, 2020.
[9] D. L. Bätzner, A. Romeo, H. Zogg, and A. N. Tiwari, “CdTe/CdS Solar Cell Performance under Low
Irradiance,” in Proceedings of 17th European Photovoltaic Solar Energy Conference and
Exhibition, 2002, vol. 1, no. October, pp. 1180–1183.
[10] A. Salavei, I. Rimmaudo, F. Piccinelli, and A. Romeo, “Influence of CdTe thickness on structural
and electrical properties of CdTe/CdS solar cells,” Thin Solid Films, vol. 535, pp. 257–260, May
2013.
2.4 Kesterites for indoor photovoltaics
Hongjae Shim1, Jongsung Park2, Jin Hyeok Kim3 and Jae Sung Yun4
1Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable
Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia
2Department of Energy Engineering, Future Convergence Technology Research Institute, Gyeongsang
National University, Jinju, Gyeongnam 52828, Republic of Korea
3Optoelectronics Convergence Research Center, and Department of Materials Science and Engineering,
Chonnam National University, Gwangju 61186, Republic of Korea
4Department of Electrical and Electronic Engineering, Advanced Technology Institute (ATI), University of
Surrey, Guildford, Surrey GU2 7XH, United Kingdom
Status
Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorbers have incontrovertible advantages for indoor photovoltaics
(PV) due to their non-toxicity, high stability, high absorption coefficient (104 cm-1), and direct bandgap.
Their bandgap tuneability (from 1.0 to 1.94 eV) by adjusting the [S]/[S+Se] ratio and substituting their
constituents (Cu and Sn with Ag and Ge, respectively) allows the deposition of kesterites with optimum
bandgap for indoor PV (1.9–2.0 eV) [1]. Furthermore, the possibility of depositing kesterite absorbers on
flexible substrates facilitates their integration into diverse shapes and dimensions, as needed for IoT
devices. In fact, related Cu(In,Ga)(S,Se)2 (CIGS) chalcopyrite absorbers also have the same advantages;
however, their rare constituents (In, Ga) may hinder the mass production of indoor energy harvesters.
Therefore, kesterite absorbers can be a promising solution for indoor applications. However, their indoor
PV performance requires further improvement, given that their highest reported efficiency to date is 8.8%
under visible LED light illumination with an irradiance of 18.5 mW/cm2 [2]–[4].
Page 22 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Figure 1. J-V curves of CZTS, CZTSe, and CZTSSe devices under 1 sun illumination (a). Low light intensity J-V curves of
CZTSSe (b) and CZTS (c) devices under 200 lux and 400 lux (halogen lamp). Arrhenius plots of CZTSSe (d) and CZTS (e)
were derived from temperature-dependent capacitance spectra measured by admittance spectroscopy (AS). [3]
Current and Future Challenges
The advantages of kesterites have expanded their potential for both outdoor and indoor applications.
However, these assets may only be exploited if the efficiencies reach levels attractive for
commercialisation (15%). In this regard, enormous works, including optimisation of their elemental
compositions, interface engineering, heterojunction optimisation and treatment, and defect engineering
have been conducted. Despite the diverse approaches to improving device performance, the evolution of
their outdoor efficiency has stagnated at around 13% for a decade. The considerably high density of
intrinsic defects (e.g., CuZn and ZnCu antisites) and defect clusters (e.g. [CuZn + ZnCu]), originating from
compatible ionic radii of Cu and Zn, have been widely reported [4]. The charged point defects and defect
clusters in bulk and near the interface of the p-type absorber layer and n-type buffer layer have been
suspected to reduce VOC, thus limiting the device performance. Considering the vulnerability of indoor PV
devices caused by charge carrier traps under weak light, the role of defects and defect clusters that can
induce trap levels would become more significant. Furthermore, CZTSSe-based devices with a high
composition of Se possess the highest outdoor performance whilst increasing S incorporation generally
deteriorates the device performance by forming Sn-related defects. A higher Se composition results in the
reduction of bandgap around 1.1 eV, which is far away from the optimum bandgap of indoor PV device
(1.9 eV to 2.0 eV); hence, it is not preferable for the performance of indoor kesterites. J. Park et al. [3]
compared the indoor device performance of CZTS-, CZTSe-, and CZTSSe-based solar cells (Figure 1b-c),
whose outdoor performances were compatible (Figure 1a). This study demonstrated reduced efficiency
in the CZTS devices due to a severe VOC drop under weak light (Figure 1c). The deeper defect level at 218
meV in the CZTS device compared to those (at 53 and 91 meV) in the CZTSSe counterpart was regarded
Page 23 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

as responsible for the lower device performance (Figure 1e). A higher sulphur ratio is needed to widen
the bandgap of CZTS absorbers for indoor PV. Hence, investigating the role of defects in kesterite films
and developing proper defect engineering methods seem to be the key priorities for kesterite indoor PV.
Advances in Science and Technology to Meet Challenges
One powerful approach to alleviate the dysfunctionalities of the antisite defects and defect clusters in
kesterites for indoor applications is substituting Cu ions with cations such as Ag and Ge. J. Park et al. [1]
demonstrated that the antisite defects related to Cu, Zn, and Sn were effectively suppressed by the
cations, achieving enhanced device performance at low light illuminations. Under 1 sun illumination, the
performance enhancement by incorporation of both Ag and Ge was comparable (from 7.5 to 9.04 and
9.05 %, respectively). Nevertheless, at low-intensity light conditions, the CZTSSe device with Ag
incorporation exhibited higher performance than that with Ge incorporation. While the Ag device had a
higher defect density (~2 × 1017 cm-3eV-1) than the Ge device, its lower defect energy level (86 meV) was
found to be beneficial under low light illumination. Therefore, the superior performance of the Ag device
in weak light despite high defect density can be interpreted based on its lower defect energy level. This
work gives an insight into the necessity of different approaches to defect engineering for indoor PV
devices compared to the outdoor PV counterpart, opening possibilities for high-performance indoor PV
based on kesterites with wider bandgap. Also, improved efficiency in CZTS solar cells was achieved by
inserting a passivation layer (Al2O3) between the kesterite absorber and Cd-free buffer layer was reported
by X. Cui et al. [5]. The passivation reduced the local potential fluctuation of band edges and resulted in
the widening of bandgap and enhancement of VOC (see Figure 2a). For further efficient absorption of
indoor light entering from all directions for more homogenous intensities, dedicated device engineering
efforts are also required. For instance, H. Deng et al. [2] designed robust bifacial CZTSSe-based PV with
outdoor efficiency of over 9% and indoor efficiency of 8.8%, which could harvest energy from light
absorbed via both the front and back surfaces, using flexible Mo-foil substrates (see Figure 2b).
Page 24 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Figure 2. (a) (Top) Diagram illustrating the band gap fluctuation in CZTS with and without Al2O3 treatment and
(bottom) surface modification mechanism in CZTS subjected to ALD-Al2O3 treatment [5]. (b) Device structure of
bifacial CZTSSe photovoltaic cells on a flexible substrate [2]. Roadmap for efficient and eco-friendly kesterite PV for
indoor applications.
Concluding Remarks
Among various candidates for indoor PV materials, kesterites can be promising due to their non-toxicity,
high stability, high absorption coefficient, and direct and tuneable bandgap. In Figure 2c, we illustrate a
schematic of the roadmap for efficient and eco-friendly kesterites for indoor applications. Firstly, the right
bandgap composition has to be set with appropriate defect engineering to mitigate the defect density as
well as the defect level positions. Also, the Cd buffer layer must be replaced with other materials to realize
fully eco-friendly PV. Finally, novel device engineering (for instance, a device architecture with kesterite
absorbers coated on both sides of a flexible substrate) will lead kesterites to be competitive in the future
indoor PV market.
Page 25 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Acknowledgements
This work was supported by Priority Research Centers Program through the National Research Foundation
of Korea(NRF) funded by the Ministry of Education, Science and Technology(2018R1A6A1A03024334), by
the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT)
(No.2022R1A2C2007219)., and by Basic Science Research Program through the National Research
Foundation of Korea(NRF) fund by the Ministry of Education (NRF-2022R1I1A3069502)
References
[1] J. Park et al., “Suppression of Defects Through Cation Substitution: A Strategic Approach to Improve
the Performance of Kesterite Cu2ZnSn(S,Se)4 Solar Cells Under Indoor Light Conditions,” Solar RRL,
vol. 5, no. 4, Apr. 2021, doi: 10.1002/solr.202100020.
[2] H. Deng et al., “Novel symmetrical bifacial flexible CZTSSe thin film solar cells for indoor
photovoltaic applications,” Nature Communications, vol. 12, no. 1, Dec. 2021, doi:
10.1038/s41467-021-23343-1.
[3] J. Park et al., “Investigation of low intensity light performances of kesterite CZTSe, CZTSSe, and
CZTS thin film solar cells for indoor applications,” Journal of Materials Chemistry A, vol. 8, no. 29,
pp. 14538–14544, Aug. 2020, doi: 10.1039/d0ta04863a.
[4] P. D. Antunez, D. M. Bishop, Y. Luo, and R. Haight, “Efficient kesterite solar cells with high open-
circuit voltage for applications in powering distributed devices,” Nature Energy, vol. 2, no. 11, pp.
884–890, Nov. 2017, doi: 10.1038/s41560-017-0028-5.
[5] X. Cui et al., “Cd-Free Cu2ZnSnS4 solar cell with an efficiency greater than 10% enabled by Al2O3
passivation layers,” Energy and Environmental Science, vol. 12, no. 9, pp. 2751–2764, Sep. 2019,
doi: 10.1039/c9ee01726g.
2.5 Organic photovoltaics for indoor light to electricity conversion
Gregory C. Welch1, Bryon W. Larson2, Myles Creran3, Audrey Laventure3
1Department of Chemistry, University of Calgary, Calgary, AB, T2N 4K9, Canada
2National Renewable Energy Laboratory, Golden, CO 80401, USA
3Département de chimie, Université de Montréal, Montréal, QC, H2V 0B3, Canada
Status
Organic photovoltaics (OPV) are a widely investigated clean energy (light to electricity) conversion
technology in academia.1–3 Recently, the technology has been commercialized with various products
available to the general public. Key advantages compared to traditional silicon-based photovoltaics
include solution processability of the photoactive layer and charge transport interlayers components,
which enables ultra-low-cost manufacturing via coating and printing techniques, a high degree of OPV
module flexibility/conformability and form factors (i.e. shapes and sizes), and tunable light harvesting
properties. Limitations include the power conversion efficiency (PCE) and operational lifetimes, both
inhibiting widespread utilization.
Owing to the high molar absorptivity of organic molecules in the visible region of the electromagnetic
spectrum (i.e. white light) and the exciton-based processes involved in organic photovoltaics, they have
been predicted to be capable of converting indoor light into useable electricity.4–6 Indeed, the global
indoor light harvesting market is expected to grow from $140M in 2017 to >$1B (USD) by 2023, with a
projected demand for such devices by then exceeding 60 million per year. While the output power is by
default low (microwatts per cm2) such devices are suitable for low-power, wireless electronic sensors for
Page 26 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

the Internet of Things (IoT). Potential application has been recently demonstrated with an OPV device
reaching 25% efficiency under 1000 lux (i.e. a standard LED light bulb).7
The organic semiconductors (p- and n-types, π-conjugated compounds) that comprise the photoactive
layers of OPVs are ideally suited for utility in harvesting light from artificial sources including LEDs and
incandescent bulbs. Fine control of the chemical structure of these compounds allows for tailoring of
optoelectronic properties. Design rules related to the p- and n-type organic semiconductors are now well
established and optical absorption of photoactive blends can be matched to specific light emission and
energy levels optimized to minimize energy loss and maximize operating voltages.8 In addition, such
materials can be (1) prepared via atom-economical synthetic procedures rendering them low-cost and
accessible and (2) be processed into photoactive films from halogen-free solvents using roll-to-roll
compatible coating methods facilitating a transition from laboratory-to-fabrication, as shown in Figure
1B.9
Figure 1. (A) Chemical structures of reported organic semiconductors with demonstrated utility as
effective LED light harvesters. PTQ10 is a donor polymer (p-type) and tPDI2N-EH is a non-fullerene
acceptor (n-type). Both materials can be made on scale and are processible from halogen-free solvents.
(B) Optical absorption spectrum of a PTQ10:tPDI2N-EH bulk-heterojunction film (rainbow) overlapping
with the irradiance of warm and cool LED emission. (C) Current-voltage curves of a OPV device with a
PTQ10:tPDI2N-EH bulk-heterojunction exhibiting a high open-circuit voltage, a result of tailored electronic
energy levels. Reproduced from Ref. 9 with permission from the Royal Society of Chemistry.
Page 27 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Current and Future Challenges
Materials Design. Most reports on iOPVs have simply used known materials developed for outdoor (1 sun,
i.e., 100 mW/cm2) environments. Thus, there is a great opportunity to develop new custom-made
photoactive materials with matched optical absorption to the emission from specific light sources
(approximately 400-700 nm). In the design of such organic semiconductor materials, minimizing energy
loss and maximizing operating voltages is far more important than reaching higher and higher PCEs as the
intended use is to run low-power devices. Materials should adopt a facile synthesis and be processible
from halogen-free solvents, and thus be compatible with large area roll-to-roll coating. In this case, classic
organic semiconductors that have fallen out of favor such as P3HT and PCDTBT may find new life owing
to a low-cost synthesis and strong absorption of indoor lighting.
Accurate Photovoltaic Measurements. Standardization of OPV characterization is easier when the
reference spectrum is always our Sun (outdoor PV). The task of standardizing non-solar light conversion
is a challenge that must be overcome so that reliable power output specifications to design IoT or sensors
around are known. Translational equations are used by institutions like NREL, EST-JRC, and AIST to
interpret device output under a given reference condition.10 When the reference condition is not the sun
(the case for the majority of iOPV intended uses), existing translational equations are invalid. Since new
translational methodologies don’t exist yet for indoor PV standards, substantial uncertainty exists in
reported indoor PCEs to date, especially when lux meters, as opposed to spectral radiometric equipment,
are used to establish incident power. One major current challenge is that the traceable reference cells
that are used to measure indoor light power, were calibrated against 1 sun when certified, and therefore
don’t apply to the indoor spectrum.
Device Engineering. Given the different environmental operating constraints of indoor vs. outdoor light
harvesting, iOPV requires new design criteria (in many ways relaxed relative to outdoor OPV) for device
stack materials that are tailored specifically for indoor conditions. Indeed, metal oxide UV-soaking does
not happen inside, reinforcing the need to use different charge transport layer materials.
Advances in Science and Technology to Meet Challenges
Matching the indoor light emission wavelength range to the absorbance spectrum of the photoactive layer
of the iOPV device can be achieved by a rational design and/or blending of the p- and n-type organic π-
conjugated compounds. To meet this challenge faster, the conventional experimental trial-and-error
approach would benefit from pairing up with computational simulations and predictions. Feedback loops
could also be developed where the results of molecular design and/or processing conditions act as inputs,
while output designs and conditions are suggested via artificial intelligence tools. Once the formulation of
the photoactive layer is selected, another major challenge lies in its processing, especially since processing
can dramatically affect the resulting microstructure of the film, and thus, its absorption profile. It needs
to be compatible with industrially relevant coating techniques, such as blade- and slot-die coating (c.f.
Figure 2) and be conducted in ambient conditions (no spin-coating nor glove-box processing). Photoactive
layer formulations that present a performance that is thickness independent also need to be targeted.
A way to ensure a proper comparison of device performance is to move away from PCE and instead
compare W/m2 values produced directly against appropriate reference incident total irradiance spectra.
The latter will require the most effort to produce, since many indoor reference spectra, in W/m2, will need
to be collected, but afterwards new reference cells can be certified against these spectra and then the
traditional translational equation methodologies can be applied.
Page 28 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Alternatives to charge transport layers requiring post-processing high temperature annealing and UV
activation or soaking are required for iOPV prepared on flexible (mostly polymer-based) substrates and
operating with indoor light. Such charge extracting interfaces design is a paramount to ensure iOPV
operational lifetime in a context where the absence of UV light, humidity and temperatures swings in
indoor conditions impact far less the photoactive layers than under 1 sun conditions. Formulations that
can be coated using roll-to-roll compatible techniques without requiring a high temperature annealing,
like SnO2 nanoparticles, need to be further developed. Module fabrication (different form factors) and
circuit integration would greatly benefit from electrical engineering inputs, where connection of devices
(series or parallel) can tune the module power delivery.
Figure 2. Required transition for OPV application in indoor light recycling. (A) Common lab-scale OPV
devices made via spin-coating. (B) Affordable roll-coaters for research and development. (C) OPV module
(5 cell) made via roll-coating using halogen free solvents with current photoactive and interlayer materials.
Photos original from Welch lab.
Concluding Remarks
Overall, the organic photovoltaic for indoor light conversion to electricity presents several challenges for
the scientific community that are yet to be tackled. Seizing this opportunity to develop the next generation
of iOPV calls for interdisciplinary research efforts, leading to advances within the materials chemistry,
device engineering and metrology landscapes. Moreover, three key concepts need to be kept in mind
during this endeavor towards a lab-to-fab transition for iOPV devices: scalability, sustainability, and
standardization. The scalability and the sustainability of the photoactive layer and charge transport
Page 29 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

interlayer compounds synthesis and of their thin film coating processes, and the standardization of the
device performance evaluation. These key concepts stand as sine qua non conditions to ensure a perennial
technology transfer from research and development to a widespread adoption of iOPV as power sources
for wireless, low-voltage devices.
Acknowledgements
M.C. thanks the Centre Québécois sur les Matériaux Fonctionnels (CQMF, a Fonds de recherche du
Québec – Nature et Technologies strategic network) and A.L. thanks the Canada Research Chairs program
for financial support. G.C.W thanks the University of Calgary. This work was authored in part by the
National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S.
Department of Energy (DOE) under Contract No. DE-AC36-08GO28308 with writing support for BWL by
ARPA-E DIFFERENTIATE program under grant no. DE-AR0001215. The views expressed in the article do
not necessarily represent the views of the DOE or the U.S. Government.
References
(1) Zhang, J.; Tan, H. S.; Guo, X.; Facchetti, A.; Yan, H. Material Insights and Challenges for Non-
Fullerene Organic Solar Cells Based on Small Molecular Acceptors. Nat. Energy 2018, 3, 720–
731.
(2) Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-Generation Organic Photovoltaics Based on Non-
Fullerene Acceptors. Nat. Photonics 2018, 12 (3), 131–142.
(3) Hou, J.; Inganäs, O.; Friend, R. H.; Gao, F. Organic Solar Cells Based on Non-Fullerene Acceptors.
Nat. Mater. 2018, 17 (2), 119
(4) Mathews, I.; Kantareddy, S. N.; Buonassisi, T.; Peters, I. M. Technology and Market Perspective
for Indoor Photovoltaic Cells. Joule 2019, 3 (6), 1415–1426.
(5) You, Y.-J.; Song, C. E.; Hoang, Q. V.; Kang, Y.; Goo, J. S.; Ko, D.-H.; Lee, J.-J.; Shin, W. S.; Shim, J.
W. Highly Efficient Indoor Organic Photovoltaics with Spectrally Matched Fluorinated
Phenylene-Alkoxybenzothiadiazole-Based Wide Bandgap Polymers. Adv. Funct. Mater. 2019, 29
(27), 1901171.
(6) Shin, S.-C.; Koh, C. W.; Vincent, P.; Goo, J. S.; Bae, J.-H.; Lee, J.-J.; Shin, C.; Kim, H.; Woo, H. Y.;
Shim, J. W. Ultra-Thick Semi-Crystalline Photoactive Donor Polymer for Efficient Indoor Organic
Photovoltaics. Nano Energy 2019, 58, 466–475.
(7) Cui, Y.; Wang, Y.; Bergqvist, J.; Yao, H.; Xu, Y.; Gao, B.; Yang, C.; Zhang, S.; Inganäs, O.; Gao, F.; et
al. Wide-Gap Non-Fullerene Acceptor Enabling High-Performance Organic Photovoltaic Cells for
Indoor Applications. Nat. Energy 2019, 1–8.
(8) Dayneko, S. D.; Pahlevani, M.; Welch, G. C. Indoor Photovoltaics: Photoactive Material Selection,
Greener Ink Formulations, and Slot-Die Coated Active Layers. ACS Applied Materials and
Interfaces. 2019. 11, 49, 46017-46025.
(9) Tintori, F.; Laventure, A.; Koenig, J. B. D.; Welch, G. C. High Open-Circuit Voltage Roll-to-Roll
Compatible Processed Organic Photovoltaics. Journal of Materials Chemistry C. 2020. 8, 13430-
13438.
(10) Osterwald, C.R. Translation of device performance measurements to reference conditions. Solar
Cells. 1986. 18, 269-279.
2.6 Dye-sensitized photovoltaics for indoor applications
Kezia Sasitharan1, Natalie Flores-Diaz1 and Marina Freitag1
Page 30 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

1School of Natural and Environmental Sciences, Bedson Building, Newcastle University, NE1 7RU
Newcastle upon Tyne, UK
Status
Dye-sensitized solar cells (DSCs) comprise of a mesoporous semiconducting layer (usually TiO2)
functioning as a working electrode (WE), to which sensitizer molecules are adsorbed. The counter
electrode (CE) faces the sensitizer, with a redox mediator between CE and the WE. Upon light absorption,
photo-induced electron transfer occurs from the sensitizer to the TiO2. The redox mediator enables
regeneration of the dye, facilitating the transfer of positive charges from the WE to the CE, as
demonstrated in Figure 1. DSCs primarily absorb in the visible region (from 400 to 650 nm) and
outperform GaAs solar cells under diffuse light conditions, while also being inexpensive and environment
friendly.[3]
Even under ambient light illumination, DSCs can maintain a high photovoltage. This is attributed to the
tuneable energy levels in Cu(II/I) electrolyte systems and reduced recombination along with fast charge
separation processes in organic dyes. Molecular engineering of the dyes, and their combination (co-
sensitizers) for improved matching of their absorbance with the emission of the artificial light sources (as
shown in Figure 2) has significantly pushed the efficiency of DSCs for indoor photovoltaics (IPVs). A PCE of
28.9% was observed under 1000 lux fluorescent light tube using [Cu(tmby)2]2+/1+ redox coupled with TiO2
films co-sensitized with the dye D35 and XY1. [4] The continued development of panchromatic rigid-
structure dyes, alternative hole transport materials and design flexibility has enabled improved PCEs,
currently reaching 13% under AM1.5G conditions and 34% under indoor light.[5]
In recent years, DSCs have shown remarkable progress in harvesting energy from artificial light sources,
making them a suitable option for various low power devices used indoors. In 2020, DSCs were
successfully tested to power battery-free IoT devices capable of machine learning under ambient light
conditions.[6]
Page 31 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Figure 1. Working principles of DSCs with a co-sensitized system and a redox mediator Ox/Red.
Current and Future Challenges
The conversion of ambient light into usable energy paves the way for the widespread implementation of
self-powered wireless devices.[7] To attain their full potential as IPVs, DSCs must achieve PCEs closer to
the maximum theoretical value of 52%. [8] Their integration as a sustainable power source for sensors
and wireless electronics will lead to self-powered monitoring systems, data collection, and wireless
communication, thereby saving energy in buildings, industries, and households. [9] Indoor DSCs with co-
sensitized systems can achieve high PCEs employing dyes absorbing at 550-600 nm and a co-sensitizer
absorbing in the blue region of the visible spectra, offering an excellent match to the ambient light spectra.
This approach also reduces electron recombination rates from the conduction band of the TiO2 to the
redox mediator. Interfacial engineering and redox mediators with high redox potentials are required to
further reduce electron recombination and maintain high photovoltage. The commercialization of DSCs is
hindered by the inability to develop solid-state devices on a large scale. Solid-amorphous copper-based
HTMs showed significant power densities at 1000 lux (approximately 110 mW/cm2), and scalable
deposition methods are being developed. [10]
Advances in Science and Technology to Meet Challenges
To advance the field of DSCs for ambient applications, it will be necessary to simultaneously improve DSCs
and IoT devices with innovative hardware and software, combining chemistry, engineering, and computer
science. By developing new materials, it is possible to increase the Voc above 1.0 V at 1000 lux. For
instance, novel preparation methods and surface treatments for semiconductors with higher conduction
band energies than TiO2 (such as Zn2SnO4, SrTiO3, and BaTiO3) should be investigated to increase dye
loading and decrease interfacial electron recombination.
Since recombination processes cause most DSC performance losses, research should focus on developing
a fundamental understanding of the interaction between dyes and charge transport materials and their
impact on photovoltaic processes. This will enable the development of alternative charge carrier materials
with improved charge transport, reduced recombination losses, and enhanced long-term stability.
Furthermore, liquid electrolytes should be replaced with solid-state charge transport materials to reduce
leakage, solvent evaporation, dye photodegradation, dye desorption, and counter electrode corrosion.
Novel architecture designs are needed to allow backside illumination and incorporate carbon-based
composites at the counter electrodes, enabling solid-state monolithic devices. [8]
Page 32 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Figure 2. Normalized emission spectra of warm white CFL and LED bulbs, and of the AM1.5G spectrum
with absorption overlay of prominent indoor dyes: L1, XY1 and MS5.
Future research will focus on efficient converters with low power fluctuations and energy buffers to
maintain a constant voltage throughout the operation/sleep cycles of indoor electronic devices. Due to
the scarcity of battery recycling facilities and materials, supercapacitor research must be addressed.
Coupling supercapacitors or novel energy storage devices to DSCs will enable constant power delivery
when the indoor light is unavailable.[9]
Concluding Remarks
Self-powered and ‘smart’ IoT devices and networks can now be powered by ambient light harvesters, a
previously untapped energy source. Indoor dye-sensitized photovoltaics with nontoxic materials and high
efficiency will play an important role in IoT sustainability.
Considering that indoor environments are more stable in terms of temperature, humidity, and light,
designing materials with higher performance properties is plausible. New electronic configurations
custom designed for various IoT applications must be developed to provide constant voltage and power.
With these further developments in both material design and device architecture, the PCEs of DSCs under
indoor light can be pushed closer to the theoretical limit to generate a new era of autonomous, smart and
self-powered devices.
The progress of these systems can depend on the following factors:
• Upscaling DSCs to minimodules delivering 3-5 V to fulfil IoT requirements.
• Advances in the stability and efficiency of commercially available IPVs
• Novel energy storage devices, particularly non-conventional storage approaches.
• New methods to improve the energy of CPUs/MCUs, wireless communication devices and sensors.
• Computing algorithms and topologies enabling self-powered IoT devices with environmental- and
self-awareness.
Page 33 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

Highly efficient ambient DSCs are ultimately intended to assist us in maximising the benefits of
interconnected IoTs and wireless device innovations while reducing their own environmental and energy
impacts. [10]
Acknowledgements
M.F. acknowledges the support by the Royal Society through the University Research Fellowship
(URF\R1\191286), Research Grant 2021 (RGS\R1\211321), and EPSRC New Investigator Award
(EP/V035819/1). N.F-D. acknowledges the support by the EU Horizon 2020 MSCA-IF funding, project
101028536.
References
[3] I. Mathews, S. N. Kantareddy, T. Buonassisi, and I. M. Peters, “Technology and Market Perspective for
Indoor Photovoltaic Cells,” Joule, vol. 3, no. 6, pp. 1415–1426, 2019.
[4] M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P. Liska, J. Hua, S. M. Zakeeruddin, J. E. Moser,
M. Graetzel, and A. Hagfeldt, “Dye-sensitized solar cells for efficient power generation under ambient
lighting,” Nature Photonics, vol. 11, no. 6, pp. 372-378, 2017.
[5] H. Michaels, M. Rinderle, R. Freitag, I. Benesperi, T. Edvinsson, R. Socher, A. Gagliardi, and M. Freitag,
“Dye-sensitized solar cells under ambient light powering machine learning: towards autonomous smart
sensors for the internet of things,” Chemical Science, vol. 11, no. 11, pp. 2895–2906, 2020.
[6] M. Li, F. Igbari, Z. K. Wang, and L. S. Liao, “Indoor Thin-Film Photovoltaics: Progress and Challenges,”
Advanced Energy Materials, vol. 10, no. 28, pp. 1–25, 2020.
[7] C. Zheng, Q. Wu, S. Guo, W. Huang, Q. Xiao, and W. Xiao, “The correlation between limiting efficiency
of indoor photovoltaics and spectral characteristics of multi-color white LED sources,” Journal of Physics
D: Applied Physics, vol. 54, no. 31, p. 315503, aug 2021.
[8] B. Li, B. Hou, and G. A. J. Amaratunga, “Indoor photovoltaics, The Next Big Trend in solution-processed
solar cells”, InfoMat, vol. 3, no. 5, pp. 445–459, 2021.
[9] H. Michaels, I. Benesperi, and M. Freitag, “Challenges and prospects of ambient hybrid solar cell
applications,” Chemical Science, vol. 12, no. 14, pp. 5002–5015, 2021.
[10] E. Hittinger and P. Jaramillo, “Internet of things: Energy boon or bane?” Science, vol. 364, no. 6438,
pp. 326–328, 2019.
2.7 Lead-halide perovskites for indoor photovoltaics
Jie Xu and Thomas M. Brown
CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of
Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy
ORCID: 0000-0003-2141-3587 (T. M. Brown); ORCID: 0000-0003-0416-5200 (J. Xu)
Status
Recent and ever-increasing published literature has shown that perovskite solar cells (PSCs) are clearly
one of the best candidates for indoor photovoltaics (IPV) owing to their exceptional power-conversion-
efficiency (PCE) under low light conditions in the visible range, i.e. those emitted by white light sources
such as LEDs and compact fluorescent lamps used for indoor illumination. The earliest demonstrations of
IPV performance of PSCs can be traced down to Chen et al. for the PEDOT:PSS/PCBM p-i-n architecture in
2015 [1], and Di Giacomo et al. [2] for the classical TiO2 and Spiro-OMeTAD architecture. In both these
works, PCEs as high as 24% at 200 lx and 27% at 1000 lx were obtained, which are substantially higher
Page 34 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

than those ever achieved at standard test conditions (see figure 1(a)). In 2017 Lucarelli et al. reported the
first flexible PSCs fabricated on PET substrates delivering a PCE of 11-12% in the 200 - 400 lx range under
LED white light [3]. After only a few years, efficiencies of indoor perovskite solar cells (i-PSCs) have
continued to soar, reaching current records in rigid cells of 34.8% at 200 lx, the most common range found
in homes [4], and 40.2% at the higher 1000 lx found less often, in environments like supermarkets [5], and
30.7% (212 lx), 31.9% (1062 lx) in flexible PEN cells [6], and 20.6% (200 lx) in ultrathin flexible glass [7].
Notably, these record efficiencies of i-PSCs have surpassed their competitors, both commercial such as a-
Si as well as new generation PV such as DSSCs and OPV as shown in figure 1(b) [4]. The continuing
improvement in efficiency, together with corresponding step forwards in module performance and
stability that is required in the future, as well as overcoming environmental/toxicity concerns of
perovskite semiconductors, can open up huge markets for this technology. In fact, the low-power (20-50
μW average power consumption) Internet of Things (IoT) and wireless sensor networks fields are
becoming more and more prevalent in our daily lives, with more than tens of billions of IoT devices
predicted to be deployed by 2025. IPV can become a key enabling technology to power these indoor
microelectronic devices (for example, sensors, watches, and calculators) [8].
Figure 1. a) Left, distribution of power conversion efficiencies (PCE) of the first n-i-p perovskite solar cells
developed for indoor light harvesting with different electron transport layers (ETLs) deposited either with
atomic layer deposition (ALD, black), spray pyrolysis (red), or double ETL of ALD deposited c-TiO2 together
with a meso-TiO2 ETL in a TCO/c-TiO2/meso-TiO2/CH3NH3PbI3/Spiro-O-MeTAD/Au architecture (blue)
under AM1.5 G, 1000 W m-2 standard test conditions (STC). Right, distribution of maximum power density
(MPD) and estimated PCE produced by the same devices measured under a CFL lamp at an illuminance of
200 lx; The efficiencies depend greatly on the quality of the ETL, much more than at STC, and the maximum
efficiencies reached are significantly higher than those at STC; Reproduced with permission from [2]. b)
Summary of power-conversion-efficiency reached of representative works for indoor perovskite solar cells
Page 35 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

(PSCs, red squares) and competitors such as a-Si (purple rhombus), DSSC (blue triangle) and OPV (green
circle), with PCEs reaching 34.8% at 200lx and 40.2% at 1000 lux; Adapted with permission from [4].
Current and Future Challenges
Stability
PSCs have achieved impressive efficiencies in a short period of time. Indoor stability remains an aspect of
concern, although they operate in a milder environment compared to outdoors. Many of IoT devices need
to last only a few years rather than more than 25-years under the much more taxing conditions under the
sun, outdoors. Thus, systematic studies of degradation under indoor illumination as well as developing
device and encapsulation materials that can match commercial lifetime requirements should be carried
out.
Large-area fabrication
Scaling up the manufacturing processes is key for commercialization. Previous studies have shown that i-
PSCs are more sensitive to trap-state density and recombination currents [2, 3], due to the lower incident
optical power of indoor lights compared to sunlight, resulting in a higher ratio of recombining electrons
to photo-generated electrons. Preparation of high-quality transport and perovskite films (Figure 1a) over
large areas as well as maintaining high shunt resistances in contacting series-connected cells in monolithic
modules is crucial for device performance. Ideal geometries for modules will also differ depending on
illumination conditions.
Manufacturing Costs
Reducing manufacturing costs is an important factor for successful commercialization. Achieving
comparable market price with its competitors, currently a-Si PV, as well as by other means such as
conventional batteries, is required as well as reducing as much as possible the cost of its integration with
the product.
Possible Toxicity Concerns
The operating environment of indoor PV consists in most places of spaces where there is human activity,
such as the living room, office, and shops. Therefore, the possible toxicity of lead-halide perovskite
inevitably becomes an important question to answer even if the quantity of lead is minuscule in such
devices. On the one hand this is a regulatory issue, on the other a technical one which can be tackled with
formulation of new materials (e.g. lead-free alternatives) or proper encapsulation and sealing.
Standardization protocols
As an emerging technology, IPV does not yet have a generally accepted standard measurement protocol
similar to established PV technologies for power generation outdoors. Moreover, there is only one
standard light source outdoors (i.e. the sun), while artificial light sources are continuously updated in
pursuit of higher efficiency and longer lifetimes. Establishment of measurement protocols will greatly help
the field.
Advances in Science and Technology to Meet Challenges
Continuing to improve efficiency towards its theoretical limit of 56% under LED light [9], can be achieved
by band gap (the optimal one is around 1.9 eV, higher than that at STC) and defect-passivation engineering
[4, 8, 10] both in the perovskite layer as well as the transport layers and its interfaces [2, 3, 5]. Stability
needs to be tackled by new material and device architectures (intrinsic) as well as encapsulation (extrinsic)
to avoid permeation of moisture and oxygen. For the former, reducing defect formation through grain
boundary and interface passivation and the amount of small molecular ions with inorganic ones can
Page 36 of 211AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

increase stability as well as synthesizing perovskite absorbers with more stable 2D/3D structures. Cheng
et al. tailored a CH3NH3PbI2−xBrClx perovskite absorber with bandgap of 1.8 eV for indoor light harvesting,
achieving a PCE of 36.2% on 0.1 cm2 and 30.6% on an appreciable active area of 2.25 cm2 under fluorescent
light at 1000 lux (see figure 2(a)). Furthermore, halide segregation suppressed by chloride introduction
led to excellent long-term stability, sustaining over 95% of original efficiency for 0.1 cm2 encapsulated
cells under 2000 h of continuous light soaking [11]. However, this work does not report performance in
the more common 200-500lx range found in home and offices. All-inorganic i-PSCs have recently attracted
much attention due to improved thermal stability. Guo et al. recently reported a CsPbI2Br all-inorganic i-
PSC with a PCE of 34.2% and a VOC of 1.14 V under LED at 200 lx and superior thermal stability (see figure
2(b)) [12]. Commonly used approaches for encapsulation include curable adhesive and glass–glass
laminated encapsulation using a variety of different adhesives [13]. Glass is an excellent permeation
barrier, thus should also be considered as a possible substrate together with plastic ones even for flexible
devices [7]. Encapsulation can become a solution also for the possible toxicity issue via lead sequestration
engineering. Researchers have initially explored materials including epoxy resin, hydroxyapatite, and
sulfonic acid-based resins to supress leakage of internal lead [14]. Research on lead free perovskites for i-
PSCs is described in section 2.9. To reduce manufacturing costs, it is necessary to develop new cost-
effective constituent materials and fabrication technologies that enable very uniform films over large
areas as well as low costs, such as slot-die coating.
Figure 2. (a) 30.6% for indoor perovskite solar cells (i-PSCs) with area >1 cm2 (i.e. 2.25 cm2) with a tailored
triple-anion MAPbI2−xBrClx Perovskite material under fluorescent light at 1000 lux, Reproduced with
Page 37 of 211 AUTHOR SUBMITTED MANUSCRIPT - JPMATER-100717.R1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Accepted Manuscript

permission from [11]. (b) All-inorganic CsPbI2Br i-PSCs with a record-high efficiency of 34.2% with VOC of
1.14 V under LED illumination at 200 lux, Reproduced from [12].
Concluding Remarks
The huge market for IoT devices worth tens of billions in the future brings unprecedented opportunities
for IPV. Perovskite solar cells have numerous advantages for integration with indoor IoT electronic
devices, such as being mechanical flexible, light weight, and low cost. Simultaneously, i-PSCs have
surpassed their counterparts and achieved outstanding indoor efficiency of over 34% at 200lx and 40% at
1000 lx, becoming one of the prominent leaders for potential commercialization. However, the above-
mentioned achievements were obtained under laboratory conditions. Many challenges such as stability,
maintaining performance over large sizes, possible toxicity, reducing manufacturing and integration costs
must be addressed in the future to promote the commercialization of i-PSCs. The path to
commercialization for indoor products compared to outdoor installations may be more evident not only
because of the outstanding performance but also of the more lenient environment for stability. This
review has aimed to propose some guidelines and ideas to overcome these challenges. Furthermore, the
outstanding performance under low intensity light can be appealing not only for photovoltaics but also
for imaging/vision devices of the future.
Acknowledgements
JX gratefully acknowledges financial support from the China Scholarship Council (CSC, No.202004910288).
The project has received funding from Italian Ministry of University and Research (MIUR) through the
PRIN2017 BOOSTER (project n.2017YXX8AZ) grant. TMB gratefully acknowledges funding by the Air Force
Office of Scientific Research’s Biophysics program through award number FA9550-20-1-0157.
References
[1] Chen C, Chang J, Chiang K, Lin H, Hsiao S, and Lin H 2015 Perovskite