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.

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ROADMAP • OPEN ACCESS
Roadmap on energy harvesting materials
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ROADMAP
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 Yun2, Gregory C Welch8, Bryon W Larson9,
Myles Creran10, Audrey Laventure10, Kezia Sasitharan11, Natalie Flores-Diaz11, Marina Freitag11,
Jie Xu12, Thomas M Brown12, Benxuan Li13,14, Yiwen Wang15, Zhe Li15, Bo Hou16,
Behrang H Hamadani17, Emmanuel Defay18, Veronika Kovacova18, Sebastjan Glinsek18,
Sohini Kar-Narayan19,∗, Yang Bai20, Da Bin Kim21, Yong Soo Cho21, Agn˙e ˇ
Zukauskait˙e22,78,
Stephan Barth22, Feng Ru Fan23, Wenzhuo Wu24, Pedro Costa25,26, Javier del Campo27,28,
Senentxu Lanceros-Mendez25,26,27,28, Hamideh Khanbareh29, Zhong Lin Wang30, Xiong Pu31, Caofeng Pan31,
Renyun Zhang32, Jing Xu33, Xun Zhao33, Yihao Zhou33, Guorui Chen33, Trinny Tat33, Il Woo Ock33,
Jun Chen33, Sontyana Adonijah Graham34, Jae Su Yu34, Ling-Zhi Huang35, Dan-Dan Li35, Ming-Guo Ma35,
Jikui Luo36, Feng Jiang37, Pooi See Lee37, Bhaskar Dudem2, Venkateswaran Vivekananthan2,79,
Mercouri G Kanatzidis38, Hongyao Xie38, Xiao-Lei Shi39, Zhi-Gang Chen39, Alexander Riss40,
Michael Parzer40, Fabian Garmroudi40, Ernst Bauer40, Duncan Zavanelli41, Madison K Brod41,
Muath Al Malki41, G Jeffrey Snyder41, Kirill Kovnir42,43, Susan M Kauzlarich44, Ctirad Uher45, Jinle Lan46,
Yuan-Hua Lin47, Luis Fonseca48, Alex Morata49, Marisol Martin-Gonzalez50, Giovanni Pennelli51,
David Berthebaud52, Takao Mori53,54, Robert J Quinn55, Jan-Willem G Bos55, Christophe Candolfi56,
Patrick Gougeon57, Philippe Gall57, Bertrand Lenoir56, Deepak Venkateshvaran58, Bernd Kaestner59,
Yunshan Zhao60, Gang Zhang61, Yoshiyuki Nonoguchi62, Bob C Schroeder63, Emiliano Bilotti80,
Akanksha K Menon65, Jeffrey J Urban66, Oliver Fenwick64, Ceyla Asker64, A Alec Talin67,
Thomas D Anthopoulos68, Tommaso Losi69, Fabrizio Viola69, Mario Caironi69, Dimitra G Georgiadou70,
Li Ding71, Lian-Mao Peng71, Zhenxing Wang72, Muh-Dey Wei73, Renato Negra73, Max C Lemme72,74,
Mahmoud Wagih70,75, Steve Beeby70, Taofeeq Ibn-Mohammed76, K B Mustapha77 and A P Joshi76
1School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1 BC, Canada
2Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford, Surrey, GU2
7XH, United Kingdom
3Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, United States of America
4LAPS- Laboratory for Photovoltaics and Solid-State Physics, Department of Computer Science, University of Verona, Ca’ Vignal 1,
Strada Le Grazie 15, 37134 Verona, Italy
5Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of
New South Wales, Sydney, NSW 2052, Australia
6Department of Energy Engineering, Future Convergence Technology Research Institute, Gyeongsang National University, Jinju,
Gyeongnam 52828, Republic of Korea
7Optoelectronics Convergence Research Center and Department of Materials Science and Engineering, Chonnam National
University, Gwangju, 61186, Republic of Korea
8Department of Chemistry, University of Calgary, Calgary, AB, T2N 4K9, Canada
9National Renewable Energy Laboratory, Golden, CO 80401, United States of America
10 Département de chimie, Université de Montréal, Montréal, QC H2V 0B3, Canada
11 School of Natural and Environmental Sciences, Bedson Building, Newcastle University, NE1 7RU Newcastle upon Tyne, United
Kingdom
12 CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, Via
del Politecnico 1, 00133 Rome, Italy
13 International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education,
Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, People’s Republic of China
14 Electrical Engineering Division, Engineering Department, University of Cambridge, 9 J J Thomson Avenue, Cambridge CB3 0FA,
United Kingdom
15 School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, United Kingdom
16 School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, United Kingdom
17 National Institute of Standards and Technology, Gaithersburg, MD 20899, United States of America
18 Materials Research and Technology Department, Luxembourg Institute of Science and Technology (LIST), 41 Rue du Brill, Belvaux
L-4422, Luxembourg
19 Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United
Kingdom
© 2023 The Author(s). Published by IOP Publishing Ltd

J. Phys. Mater. 6(2023) 042501 V Pecunia et al
20 Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, FI-90570 Oulu,
Finland
21 Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea
22 Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, 01277 Dresden, Germany
23 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,
People’s Republic of China
24 School of Industrial Engineering, Purdue University, West Lafayette, IN 47907, United States of America
25 Physics Centre of Minho and Porto Universities (CF-UM-UP), University of Minho, 4710-053 Braga, Portugal
26 LaPMET-Laboratory of Physics for Materials and Emergent Technologies, University of Minho, 4710-057 Braga, Portugal
27 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain
28 OIKERBASQUE, Basque Foundation for Science, 48009 Bilbao, Spain
29 Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
30 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States of America
31 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, People’s Republic of China
32 Department of Natural Sciences, Mid Sweden University, Holmgatan 10 SE 851 70 Sundsvall, Sweden
33 Department of Bioengineering, University of California, Los Angeles, Los Angeles, CA 90095, United States of America
34 Department of Electronics and Information Convergence Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu,
Yongin-Si, Gyeonggi-do 17104, Republic of Korea
35 Research Center of Biomass Clean Utilization, College of Materials Science and Technology, Beijing Forestry University, Beijing
100083, People’s Republic of China
36 College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
37 School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
38 Department of Chemistry, Northwestern University, Evanston, IL 60208, United States of America
39 School of Chemistry and Physics, Queensland University of Technology, Brisbane, QLD 4000, Australia
40 Institute of Solid-State Physics, TU Wien, A-1040 Wien, Austria
41 Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, United States of America
42 Department of Chemistry, Iowa State University, Ames, IA 50011, United States of America
43 US DOE Ames National Laboratory, Ames, IA 50011, United States of America
44 Department of Chemistry, University of California, Davis, CA 95616, United States of America
45 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States of America
46 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, People’s Republic of China
47 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University,
Shuangqing Road 30, Haidian District, Beijing 100084, People’s Republic of China
48 Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), C/Til·lers s/n (Campus UAB), Bellaterra, Barcelona, Spain
49 Catalonia Institute for Energy Research (IREC), Jardins de Les Dones de Negre 1, 08930, Sant Adri`
a de Bes`
os, Barcelona, Spain
50 Instituto de Micro y Nanotecnología (IMN-CNM-CSIC), C/Isaac Newton 8, PTM, E-28760 Tres Cantos, Spain
51 Dipartimento di Ingegneria dell’Informazione, Universit`
a di Pisa, Via G. Caruso, I-56122 Pisa, Italy
52 CNRS-Saint Gobain-NIMS, IRL 3629, LINK, National Institute for Materials Science (NIMS), 1–1 Namiki, Tsukuba 305–0044, Japan
53 National Institute for Materials Science (NIMS), WPI International Center for Materials Nanoarchitectonics (WPI-MANA), 1-1
Namiki, Tsukuba 305-0044, Japan
54 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8671, Japan
55 Institute of Chemical Sciences and Centre for Advanced Energy Storage and Recovery, School of Engineering and Physical Sciences,
Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
56 Institut Jean Lamour, UMR 7198 CNRS—Université de Lorraine, 2 allée André Guinier-Campus ARTEM, BP 50840, 54011 Nancy
Cedex, France
57 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
58 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
59 Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, Berlin, 10587, Germany
60 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, People’s Republic of China
61 Institute of High Performance Computing, A∗STAR, Singapore 138632, Singapore
62 Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan
63 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
64 School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom
65 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States of
America
66 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States of America
67 Sandia National Laboratories, Livermore, CA 94551, United States of America
68 King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwal 23955-6900, Saudi Arabia
69 Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy
70 Electronics and Computer Science, University of Southampton, Highfield Campus, Southampton SO17 1BJ, United Kingdom
71 Key Laboratory for the Physics and Chemistry of Nanodevices and Center for Carbon-based Electronics, School of Electronics,
Peking University, Beijing 100871, People’s Republic of China
72 AMO GmbH, Otto-Blumenthal-Str. 25, 52074 Aachen, Germany
73 Chair of High Frequency Electronics, RWTH Aachen University, Kopernikusstr. 16, 52074 Aachen, Germany
2

J. Phys. Mater. 6(2023) 042501 V Pecunia et al
74 Chair of Electronic Devices, RWTH Aachen University, Otto-Blumenthal-Str. 2, 52074 Aachen, Germany
75 James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, United Kingdom
76 Warwick Manufacturing Group (WMG), The University of Warwick, Coventry CV4 7AL, United Kingdom
77 Departments of Mechanical, Materials and Manufacturing Engineering, University of Nottingham (Malaysia Campus), Semenyih
43500 Selangor, Malaysia
78 Institute of Solid State Electronics, Technische Universität Dresden, 01062 Dresden, Germany
79 Department of Electronics and Communication Engineering, Koneru Lakshmaiah Education Foundation, Andhra Pradesh 522302,
India
80 Department of Aeronautics, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom
∗Authors to whom any correspondence should be addressed
E-mail: vincenzo_pecunia@sfu.ca,sk568@cam.ac.uk and s.silva@surrey.ac.uk
Keywords: energy harvesting materials, photovoltaics, thermoelectric energy harvesting, piezoelectric energy harvesting,
triboelectric energy harvesting, radiofrequency energy harvesting, sustainability
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
radiofrequency wireless 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 analyses 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.
3

J. Phys. Mater. 6(2023) 042501 V Pecunia et al
Contents
1. Introduction 6
2. Materials for indoor photovoltaics 9
2.1. Introduction to indoor photovoltaics 9
2.2. III–V compound semiconductors for indoor photovoltaics 12
2.3. CdTe solar cells for indoor applications 14
2.4. Kesterites for indoor photovoltaics 17
2.5. Organic photovoltaics for indoor-light-to-electricity conversion 20
2.6. Dye-sensitized photovoltaics for indoor applications 24
2.7. Lead-halide perovskites for indoor photovoltaics 27
2.8. Lead-free halide perovskites and derivatives for indoor photovoltaics 31
2.9. Quantum-dot absorbers for indoor photovoltaics 35
2.10. Accurate characterization of indoor photovoltaic performance 38
3. Materials for piezoelectric energy harvesting 41
3.1. Introduction to piezoelectric energy harvesting—lead-based oxide perovskites 41
3.2. Lead-free oxide perovskites for piezoelectric energy harvesting 44
3.3. Nanostructured inorganics for piezoelectric energy harvesting 47
3.4. Nitrides for piezoelectric energy harvesting 50
3.5. Two-dimensional materials for piezoelectric energy harvesting 53
3.6. Organics for piezoelectric energy harvesting 55
3.7. Bio-inspired materials for piezoelectric energy harvesting 58
4. Materials for triboelectric energy harvesting 60
4.1. Introduction to materials for triboelectric energy harvesting 60
4.2. Synthetic polymers for triboelectric energy harvesting 64
4.3. Nanocomposites for triboelectric energy harvesting 66
4.4. Surface texturing and functionalization for triboelectric energy harvesting 69
4.5. Nature-inspired materials for triboelectric energy harvesting 72
4.6. MXenes materials for triboelectric energy harvesting 76
4.7. Perovskite-based triboelectric nanogenerators 79
4.8. Towards self-powered woven wearables via triboelectric nanogenerators 82
4.9. Theoretical investigations towards the materials optimization for triboelectric nanogenerators 86
5. Materials for thermoelectric energy harvesting 90
5.1. Introduction on materials for thermoelectric energy harvesting 90
5.2. Chalcogenides for thermoelectric energy harvesting 93
5.3. Full Heuslers for thermoelectric energy harvesting 96
5.4. Half Heuslers for thermoelectric energy harvesting 99
5.5. Clathrates for thermoelectric energy harvesting 102
5.6. Skutterudites for thermoelectric energy harvesting 105
5.7. Oxides for thermoelectric energy harvesting 108
5.8. SiGe for thermoelectric energy harvesting 111
5.9. Mg2IV (IV =Si, Ge and Sn)-based systems for thermoelectric energy harvesting 114
5.10. Zintl phases for thermoelectric energy harvesting 116
5.11. Molybdenum-based cluster chalcogenides as high-temperature thermoelectric materials 119
5.12. Organic thermoelectrics 122
5.13. Two-dimensional materials for thermoelectric applications 125
5.14. Carbon nanotubes for thermoelectric energy harvesting 128
5.15. Polymer-carbon composites for thermoelectric energy harvesting 131
5.16. Hybrid organic–inorganic thermoelectrics 134
5.17. Halide perovskites for thermoelectric energy harvesting 137
5.18. Metal organic frameworks for thermoelectric energy conversion applications 140
6. Materials for radiofrequency energy harvesting 143
6.1. Introduction to materials for radiofrequency energy harvesting 143
6.2. Organic semiconductors for radiofrequency rectifying devices 145
6.3. Metal-oxide semiconductors for radiofrequency rectifying devices 149
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6.4. Carbon nanotubes for radiofrequency rectifying devices 152
6.5. Two-dimensional materials for radiofrequency energy harvesting 155
6.6. Materials for rectennas and radiofrequency energy harvesters 158
7. Sustainability considerations on energy harvesting materials research 160
Data availability statement 162
References 163
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1. Introduction
Vincenzo Pecunia1and S Ravi P Silva2
1School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1, BC, Canada
2Advanced 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.
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 radiofrequency 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−2when 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
6

J. Phys. Mater. 6(2023) 042501 V Pecunia et al
Figure 1. Prominent energy harvesting technologies covered in this Roadmap (RF: radiofrequency). Reproduced from [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’ (PV: photovoltaics; RF: radiofrequency).
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.
7

J. Phys. Mater. 6(2023) 042501 V Pecunia et al
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 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’.
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J. Phys. Mater. 6(2023) 042501 V Pecunia et al
2. Materials for indoor photovoltaics
2.1. Introduction to indoor photovoltaics
Vincenzo Pecunia1and S Ravi P Silva2
1School of Sustainable Energy Engineering, Simon Fraser University, Surrey V3T 0N1, BC, Canada
2Advanced 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 3(a)) that can be used for doing work. The ubiquity of light makes
photovoltaics a highly attractive energy harvesting technology due to their wide deployability [7]. 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 [14].
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−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 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 and white light-emitting
diode 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 (by contrast, solar and
building-integrated photovoltaics are growing with a CAGR of 7.4% and 16%, respectively) [15].
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, Pel,max, and the optical power reaching the cell from the indoor light source, Popt,Sn(λ), where Sn(λ)is
the spectral irradiance at the surface of the photovoltaic cell:
PCE(i)Sn(λ)=Pel,max/Popt,Sn(λ).(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. fluorescent and white-light-emitting-diode lighting) are characterized by a
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J. Phys. Mater. 6(2023) 042501 V Pecunia et al
Figure 3. (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 a
light-emitting-diode bulb is re-used with permission from Flaticon.com. (b) Normalized emission spectra of the Sun and
fluorescent and white-light-emitting-diode lamps. FL: fluorescent lighting. WLED: white-light-emitting-diode lighting. AM 1.5G:
reference Air Mass 1.5 Gobal spectrum of sunlight.
Figure 4. (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
white-light-emitting-diode spectrum shown in figure 3(b), while the data points are derived from [17–24]. (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.
rather distinct spectral content, as their emission spectra exclusively cover the visible range (by contrast,
approximately half of the terrestrial solar light falls in the near-infrared spectral region) (figure 3(b)). This
aspect is particularly consequential because it impacts the conditions that an absorber should meet to deliver
optimum photovoltaic efficiency. As shown in figure 4(a), in the radiative limit and for typical indoor light
sources (i.e. fluorescent and white-light-emitting-diode lamps), the optimum bandgap for an absorber to
deliver high-efficiency indoor photovoltaics amounts to 1.8–2.0 eV (by contrast, outdoor terrestrial solar
photovoltaics requires a bandgap of ∼
=1.1–1.5 eV for optimum performance) [7]. Moreover, the narrower
spectral range of indoor light sources compared to the solar spectrum (figure 3(b)) enables maximum
theoretical efficiencies (in the radiative limit) of up to ∼
=60% for single-junction devices (by contrast, the
corresponding limit for outdoor terrestrial solar photovoltaics amounts to 33.7%) [7]. 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).
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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) [16]. In fact, hydrogenated
amorphous silicon (a-Si:H) cells have long been the commercially dominant technology for indoor
photovoltaics [12]: not only do they deliver PCE(i) values of 4%–9% [16] 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 straightforward 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 endeavour 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.
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 4(a)). 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. 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 4(a)). Concurrently, CdTe has also shown promising indoor photovoltaic performance
(section 2.3) (figure 4(a)).
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 4(b), 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 4(a) 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 3(b)) 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 [7].
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 catalyse further advances in the field toward the
realization of the full potential of indoor photovoltaics as a green energy harvesting technology.
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2.2. III–V compound semiconductors for indoor photovoltaics
Jamie D Phillips
Department of Electrical and Computer Engineering, University of Delaware, Newark, DE 19716, United
States of America
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 (see figure 5),
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
around 50% and higher [16] at typical indoor irradiance conditions (500 lx). Experimentally, power
conversion efficiencies of >20% have been reported for AlGaAs [25] and >30% for InGaP [20], 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 (see figure 5)
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 IoT through continued advances to
improve power conversion efficiency and to develop architectures for direct system integration.
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 [26]. 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 [27]. Further reductions in dark current will require approaches to
passivate recombination centres 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 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 [28].
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 [29], hydride vapour phase epitaxy (HVPE)
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Figure 5. Calculated maximum power conversion efficiency versus bandgap energy for photovoltaic cells under AM1.5 and white
light-emitting-diode illumination. Select semiconductor materials are shown, illustrating the high potential efficiency for III–V
compound semiconductors.
[30], and roll-to-roll printing [31]. 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 [29] may be used to
accommodate a wide range of system form factors. Smaller scale systems at the millimetre 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.
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2.3. CdTe solar cells for indoor applications
Elisa Artegiani and Alessandro Romeo
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
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 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 [32].
In year 2021 almost 80% of the thin film solar cell market was CdTe based with an overall production of
6.1 GWp [33]. CdTe photovoltaic devices have achieved an efficiency of 22.1% on a small area for outdoor
solar photovoltaics [34] 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 [34].
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 µm [34]. Furthermore, CdTe can be successfully
deposited with a large variety of different deposition methods, the most important are: thermal evaporation
[35], close space sublimation [36], electrodeposition [37], vapour transport deposition [38].
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 indoor photovoltaic efficiency obtained from CdTe devices to date is 17.1% under white
light-emitting-diode illumination.
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−xmixed 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−xfilm 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 6.
CdTe solar cells are most successfully fabricated by physical vapour deposition processes, such as vapour
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 ◦C–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 [21]. CdSe and CdTe
are deposited by thermal evaporation, the typical standard CdCl2treatment for CdTe recrystallization,
CdSe–CdTe mixing and CdSexTe1−x/CdTe junction promotion is provided by drop casting of methanol
solution.
With a similar process, CdS was used as buffer layer, we have previously demonstrated flexible CdTe
devices on either polyimide or ultra-thin glass [32]. 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
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Figure 6. External quantum efficiency: CdS/CdTe vs. CdSexTe1−x/CdTe.
Figure 7. Efficiency of CdTe solar cells in respect to irradiation power and spectrum. LED: light-emitting diode.
towards one suggesting a change in transport mechanism corresponding to a more ideal junction [39]. This
effect has been registered also in our samples.
Advances in science and technology to meet challenges
In figure 7we compare the efficiencies of the cells measured under halogen and light-emitting-diode
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 a light-emitting diode. In both cases, this is due to a modest reduction in
open circuit voltage along with an improvement in fill factor.
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The increase of efficiency for the light-emitting-diode case is due to the irradiation spectrum of this light
which is exactly concentrated in the same range of the CdTe response (see figure 6). This, considering the
transition to light-emitting-diode 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 (decomposition occurs with temperatures above 1100 ◦C and
dissolution in water does not occur), it is subjected to RoHS (Restriction of Hazardous Substances Directive)
in the European Union (similar directives are extended to other countries). According to RoHS , 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 cm−3) and of soda lime glass (2.8 g cm−3) 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% [40].
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 light-emitting-diode 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 photovoltaic 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.
Acknowledgments
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.
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2.4. Kesterites for indoor photovoltaics
Hongjae Shim1, Jongsung Park2, Jin Hyeok Kim3and 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
4Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey,
Guildford, Surrey GU2 7XH, United Kingdom
Status
Kesterite Cu2ZnSn(S,Se)4(CZTSSe) absorbers have incontrovertible advantages for indoor photovoltaics due
to their non-toxicity, high stability, high absorption coefficient (104cm−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 photovoltaics (1.9–2.0 eV) [41]. 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
photovoltaic performance requires further improvement, given that their highest reported efficiency to date
is 8.8% under visible light-emitting-diode illumination with an irradiance of 18.5 mW cm−2[42–44].
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 commercialization
(15%). In this regard, enormous works, including optimization of their elemental compositions, interface
engineering, heterojunction optimization 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 [44]. 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 photovoltaic 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 range for indoor photovoltaics (1.8 eV to 2.0 eV); hence, it is
not preferable for the performance of indoor kesterites. Park et al [43] compared the indoor device
performance of CZTS-, CZTSe-, and CZTSSe-based solar cells (figures 8(b) and (c)), whose outdoor
performances were compatible (figure 8(a)). This study demonstrated reduced efficiency in the CZTS devices
due to a severe VOC drop under weak light (figure 8(c)). 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 as responsible for the
lower device performance (figure 8(e)). A higher sulphur ratio is needed to widen the bandgap of CZTS
absorbers for indoor photovoltaics. Hence, investigating the role of defects in kesterite films and developing
proper defect engineering methods seem to be the key priorities for kesterite indoor photovoltaics.
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Figure 8. 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 lx and 400 lx (halogen lamp). Arrhenius plots of CZTSSe (d) and CZTS (e) were
derived from temperature-dependent capacitance spectra measured by admittance spectroscopy [43].
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. Park et al [41]
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 kesterite indoor
photovoltaics compared to outdoor photovoltaics, opening possibilities for high-performance indoor
photovoltaics based on kesterites with wider bandgaps. 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 Cui et al [45]. The passivation reduced the local potential fluctuation of band edges and resulted
in the widening of bandgap and enhancement of VOC (see figure 9(a)). For further efficient absorption of
indoor light entering from all directions for more homogenous intensities, dedicated device engineering
efforts are also required. For instance, Deng et al [42] designed robust bifacial CZTSSe-based photovoltaics
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 9(b)).
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Figure 9. (a) (Top) Diagram illustrating the band gap fluctuation in CZTS with and without Al2O3treatment and (bottom)
surface modification mechanism in CZTS subjected to ALD-Al2O3treatment [45]. (b) Device structure of bifacial CZTSSe
photovoltaic cells on a flexible substrate [42]. Roadmap for efficient and eco-friendly kesterite photovoltaics for indoor
applications.
Concluding remarks
Among various candidates for indoor photovoltaic absorbers, kesterites can be promising due to their
non-toxicity, high stability, high absorption coefficient, and direct and tuneable bandgap. In figure 9(c), 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 photovoltaics. 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 photovoltaics market.
Acknowledgments
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)
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2.5. Organic photovoltaics for indoor-light-to-electricity conversion
Gregory C Welch1, Bryon W Larson2, Myles Creran3and Audrey Laventure3
1Department of Chemistry, University of Calgary, Calgary, AB, T2N 4K9, Canada
2National Renewable Energy Laboratory, Golden, CO 80401, United States of America
3Département de chimie, Université de Montréal, Montréal, QC, H2V 0B3, Canada
Status
Organic photovoltaics are a widely investigated clean energy (light-to-electricity) conversion technology in
academia [46–48]. 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 module
flexibility/conformability and form factors (i.e. shapes and sizes), and tunable light harvesting properties.
Limitations include the power conversion efficiency 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 [12,49,50]. Indeed, the global
indoor light harvesting market is expected to grow from $140 M in 2017 to >$1 B (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 the
IoT. Potential application has been recently demonstrated with an organic photovoltaic device reaching 25%
efficiency under 1000 lx (i.e. a standard light-emitting-diode bulb) [51].
The organic semiconductors (p- and n-types, π-conjugated compounds) that comprise the photoactive
layers of organic photovoltaics are ideally suited for utility in harvesting light from artificial sources including
light-emitting diodes 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
[52]. 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 10(b) [53].
Current and future challenges
Materials design
Most reports on indoor organic photovoltaics have simply used known materials developed for outdoor
(1 sun, i.e. 100 mW cm−2) 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 power
conversion efficiencies, 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 favour 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 organic photovoltaics characterization is easier when the reference spectrum is always our
Sun (outdoor photovoltaics). 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 [54]. When the reference condition is not the Sun (the case for the
majority of intended uses for indoor organic photovoltaics), existing translational equations are invalid.
Since new translational methodologies do not exist yet for indoor photovoltaics standards, substantial
uncertainty exists in reported indoor power conversion efficiencies 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 do not apply to the indoor spectrum.
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Figure 10. (a) Chemical structures of reported organic semiconductors with demonstrated utility as effective light-emitting-diode
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 light-emitting-diode emission.
(c) Current–voltage curves of an organic photovoltaic device with a PTQ10:tPDI2N-EH bulk-heterojunction exhibiting a high
open-circuit voltage, a result of tailored electronic energy levels. Reproduced from [53] with permission from the Royal Society of
Chemistry.
Device engineering
Given the different environmental operating constraints of indoor vs. outdoor light harvesting, indoor
organic photovoltaics requires new design criteria (in many ways relaxed relative to outdoor organic
photovoltaics) for device stack materials that are tailored specifically for indoor conditions. Indeed,
metal-oxide ultraviolet-light 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
an indoor organic photovoltaic 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
(cf figure 11) 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.
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Figure 11. Required transition for the application of organic photovoltaics in indoor light recycling. (a) Common lab-scale
organic photovoltaic devices made via spin-coating. (b) Affordable roll-coaters for research and development. (c) Organic
photovoltaic module (5 cell) made via roll-coating using halogen-free solvents with current photoactive and interlayer materials.
Photos original from Welch lab.
A way to ensure a proper comparison of device performance is to move away from power conversion
efficiency and instead compare W m−2values 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 m−2, 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.
Alternatives to charge transport layers requiring post-processing high temperature annealing and
ultraviolet-light activation or soaking are required for indoor organic photovoltaics prepared on flexible
(mostly polymer-based) substrates and operating with indoor light. Such charge extracting interfaces design
is a paramount to ensure operational lifetime in a context where the absence of ultraviolet light, humidity
and temperature 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 SnO2nanoparticles, 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.
Concluding remarks
Overall, organic photovoltaics 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
indoor organic photovoltaics calls for interdisciplinary research efforts, leading to advances within the
materials chemistry, device engineering and metrology landscapes. Moreover, three key concepts need to be
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kept in mind during this endeavour towards a lab-to-fab transition for indoor organic photovoltaics:
scalability, sustainability, and standardization. Scalability and sustainability concern the photoactive layer
and charge transport interlayer compounds synthesis and their thin-film coating processes. Standardization
concerns 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 indoor organic
photovoltaics as power sources for wireless, low-voltage devices.
Acknowledgments
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.
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2.6. Dye-sensitized photovoltaics for indoor applications
Kezia Sasitharan, Natalie Flores-Diaz and Marina Freitag
School of Natural and Environmental Sciences, Bedson Building, Newcastle University, NE1 7RU Newcastle
upon Tyne, United Kingdom
Status
Dye-sensitized solar cells comprise a mesoporous semiconducting layer (usually TiO2) functioning as a
working electrode, to which sensitizer molecules are adsorbed. The counter electrode faces the sensitizer,
with a redox mediator between the counter electrode and working electrode. 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 working electrode to the counter
electrode, as demonstrated in figure 12. Dye-sensitized solar cells 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 [12].
Even under ambient light illumination, dye-sensitized solar cells 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 13) has significantly pushed the efficiency of dye-sensitized solar cells for
indoor photovoltaics. A power conversion efficiency of 28.9% was observed under a 1000 lx fluorescent light
tube using [Cu(tmby)2]2+/1+redox coupled with TiO2films co-sensitized with the dye D35 and XY1 [55].
The continued development of panchromatic rigid-structure dyes, alternative hole transport materials and
design flexibility has enabled improved power conversion efficiencies, currently reaching 13% under
AM1.5G conditions and 34% under indoor light [23].
In recent years, dye-sensitized solar cells 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,
dye-sensitized solar cells were successfully tested to power battery-free IoT devices capable of machine
learning under ambient light conditions [56].
Current and future challenges
The conversion of ambient light into usable energy paves the way for the widespread implementation of
self-powered wireless devices [57]. To attain their full potential for indoor photovoltaics, dye-sensitized solar
cells must achieve power conversion efficiencies closer to the maximum theoretical value of 52% [58]. 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 [59]. Indoor dye-sensitized solar cells with co-sensitized systems can achieve high
power conversion efficiencies 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 TiO2to 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 dye-sensitized solar cells
is hindered by the inability to develop solid-state devices on a large scale. Solid-amorphous copper-based
hole-transport materials showed significant output power densities at 1000 lx, and scalable deposition
methods are being developed [60].
Advances in science and technology to meet challenges
To advance the field of dye-sensitized solar cells for ambient applications, it will be necessary to
simultaneously improve dye-sensitized solar cells 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 lx. 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 performance losses in dye-sensitized solar cells, 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
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