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Prominent energy harvesting technologies covered in this Roadmap. Reproduced from Ref. [7] under the terms of a CC BY 4.0 open-access license.
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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...
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... 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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... light absorption, photo-induced electron transfer occurs from the sensitizer to the TiO 2 . 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]. ...
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... 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 TiO 2 films co-sensitized with the dye D35 and XY1 [55]. ...
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... earliest demonstrations of the indoor photovoltaic performance of perovskite solar cells can be traced to Chen et al for the PEDOT:PSS/PCBM p-i-n architecture in 2015 [61], and Di Giacomo et al [62] for the classical TiO 2 and Spiro-OMeTAD architecture. In both of these works, power conversion efficiencies as high as 24% at 200 lx and 27% at 1000 lx were obtained, which are substantially higher than those ever achieved at standard test conditions (see figure 14(a)). In 2017, Lucarelli et al reported the first flexible perovskite solar cells fabricated on PET substrates delivering a power conversion efficiency of 11%-12% in the 200-400 lx range under white light-emitting-diode illumination [63]. ...
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... studies have shown that i-PSCs are more sensitive to trap-state density and recombination currents [62,63], 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 14(a)) 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. ...
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... operating environment of indoor photovoltaics 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 Figure 14. (a) Left, distribution of power conversion efficiencies (PCE: power conversion efficiency) of the first n-i-p perovskite solar cells developed for indoor light harvesting with different preparation methods of compact TiO2 deposited either with high-temperature spray pyrolysis (HT-SP-PSC, black), spin coating a TiO2 sol-gel followed by high-temperature annealing (HT-SG-PSC, red), or atomic layer deposition (LT-ALD-PSC, blue) in a TCO/c-TiO2/meso-TiO2/ CH3NH3PbI3/Spiro-O-MeTAD/Au architecture under AM1.5 G, 1000 W m −2 standard test conditions (STC). ...
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... the former, reducing defect formation through grain boundary and interface passivation and replacing amounts of small molecular ions with inorganic ones can increase stability as well as synthesizing perovskite absorbers with more stable two-dimensional/three-dimensional structures. Cheng et al tailored a CH 3 NH 3 PbI 2−x BrCl x perovskite absorber with bandgap of 1.8 eV for indoor light harvesting, achieving a power conversion efficiency of 36.2% on 0.1 cm 2 and 30.6% on an appreciable active area of 2.25 cm 2 under fluorescent light at 1000 lx (see figure 15(a)). Furthermore, halide segregation suppressed by chloride introduction led to excellent long-term stability, sustaining over 95% of original efficiency for 0.1 cm 2 encapsulated cells under 2000 h of continuous light soaking [70]. ...
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... i-PSCs have recently attracted much attention due to improved thermal stability. Guo et al recently reported a CsPbI 2 Br all-inorganic i-PSC with a power conversion efficiency of 34.2% and a V oc of 1.14 V under light-emitting-diode illumination at 200 lx and superior thermal stability (see figure 15(b)) [71]. Commonly used approaches for encapsulation include curable adhesive and glass-glass laminated encapsulation using a variety of different adhesives [72]. ...
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... halide perovskites and derivatives (lead-free perovskites for short in the following) are a broad class of metal-halide-based compounds ( figure 16) [74]. They may present the ABX 3 perovskite structure (with A + being a monovalent cation, B 2+ a divalent metal/metalloid anion, and X -a halide anion) featuring a three-dimensional network of corner-sharing [BX 6 ] 4− octahedra. ...
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... may present the ABX 3 perovskite structure (with A + being a monovalent cation, B 2+ a divalent metal/metalloid anion, and X -a halide anion) featuring a three-dimensional network of corner-sharing [BX 6 ] 4− octahedra. Derivatives of this structure may involve metal-halide octahedra arranged in different corner-sharing or edge-sharing structural motifs, which can be either three-dimensional (e.g. as in compounds with a formula B(I) m B(III) n X m+3n , where B + and B 3+ are monovalent and trivalent metals, respectively, and X − is a halide anion) or lower-dimensional (e.g. as in zero-and two-dimensional A 3 B 2 X 9 compounds, with A + being a monovalent cation, B 3+ a trivalent metal, and X − a halide anion) ( figure 16). A common feature of all of these compounds is their being based on metals/metalloids alternative to lead-for instance, tin, germanium, antimony, bismuth, and silver. ...
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... aiming to replicate the promising performance of lead-halide perovskites in solar photovoltaics, the development of lead-free perovskites exclusively targeted their use in solar cells [74]. Departing from this narrow view, the Author and his team began investigating in early 2019 the indoor photovoltaic performance of two-dimensional lead-free perovskites comprising planes of corner-sharing metal-halide octahedra [75], which deliver superior optoelectronic properties compared to their zero-dimensional counterparts [76] ( figure 16(a)). Alongside the understanding that the typical placement of indoor photovoltaics in proximity to the end-users makes it beneficial to resort to lead-free, eco-friendly materials [7], this investigation was motivated by the realization that the wide bandgaps of many such compounds would lead to a favourable spectral match with indoor light sources ( figure 17(a)). ...
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... from this narrow view, the Author and his team began investigating in early 2019 the indoor photovoltaic performance of two-dimensional lead-free perovskites comprising planes of corner-sharing metal-halide octahedra [75], which deliver superior optoelectronic properties compared to their zero-dimensional counterparts [76] ( figure 16(a)). Alongside the understanding that the typical placement of indoor photovoltaics in proximity to the end-users makes it beneficial to resort to lead-free, eco-friendly materials [7], this investigation was motivated by the realization that the wide bandgaps of many such compounds would lead to a favourable spectral match with indoor light sources ( figure 17(a)). This effort resulted in the first-ever demonstration of the capabilities and potential of lead-free-perovskite indoor photovoltaics [77]. ...
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... have revealed that many lead-free perovskites could ultimately achieve PCE(i) values of up to ∼ =60% ( figure 17(b)) [77]. Therefore, a major challenge is to bridge the gap between current device efficiencies and this theoretical limit by developing suitable materials-, processing-, and device-based strategies. ...
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... Based on the detailed balance limit applied to artificial indoor lighting spectra, the optimal bandgap for indoor photovoltaics is 1.9 eV [12,16]. (Figure 18 presents indoor lighting sources and AM 1.5 solar spectra). Therefore, new technologies based on absorbers with tunable bandgap, such as perovskites and quantum dots, have been developed for indoor photovoltaics [17,58,89]. ...
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... date, lead-chalcogenide and lead-halide-perovskite quantum dots are the mainstream absorbers in quantum-dot-based solar cells, whereas quantum-dot-based indoor photovoltaics are still in an emerging state. As shown in figure 18, several types of quantum dots cover desirable absorption ranges for photovoltaics, and some have shown satisfying efficiency in quantum-dot-based indoor photovoltaics [17]. Meanwhile, the manufacturing cost of quantum-dot-based photovoltaic technologies is becoming competitive due to the mass production of quantum-dot-based high-definition televisions, in which solution-processed quantum dots are deposited via spin-coating, spray-coating, and blade-coating. ...
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... size-dependent photophysical properties, lead-halide-perovskite quantum dots feature more defect tolerance than PbX quantum dots, resulting in less energy loss such as photovoltage deficiency in devices. Although heavy metal Pb is dominant in both PbX and lead-halide-perovskite quantum dots due to the essential role of its 6s 2 orbitals, enabling direct bandgap transition and stable crystal dimension, the technology is now moving towards eco-friendly quantum-dot materials such as AgBiS 2 , CuInZnSSe, Sn-based-perovskite and InP nanocrystals ( figure 18). Very recently, CuInZnSSe quantum dots employed as organic replacements in dye-sensitised solar cells have shown very promising performance (solar power conversion efficiency > 15%) [92]. ...
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... are currently under way to meet the challenges discussed above. Recently, the standard SEMI PV80-0218 [99], Specification of Indoor Lighting Simulator Requirements for Emerging Photovoltaic was Figure 19. For current vs. voltage measurements of indoor photovoltaics, a calibrated reference cell (right) is placed next to the indoor photovoltaic cell (left) and used to measure and adjust the effective irradiance of the incident light-emitting-diode illumination. ...
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... a calibrated reference solar cell can now be obtained from the National Institute of Standards and Technology in the USA. NIST uses the absolute irradiance spectral responsivity method to calibrate an appropriate reference solar cell (see figure 19) under any adopted reference spectrum, including three unique NIST-proposed reference spectra based on white light-emitting diodes of different correlated colour temperatures [97]. The chosen reference spectra are in absolute spectral irradiance (units: W m −2 nm −1 ) and they are designed such that they correspond to a total illuminance of 1000 lx. ...
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... inset in figure 21 clearly shows that the power density per unit area is proportional to the thickness of piezoelectric material. This essentially disqualifies very thin films for energy harvesting, as long as one considers 1 mW cm −2 as the power density target. ...
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... using substrates compatible with large strains, such as polymers, is a very interesting approach. Piezoelectrics are generally prepared on stiff and brittle substrates able to withstand very high temperatures, typically beyond 900 • C (see figure 21). Therefore, transferring top-quality piezoelectric layers on compliant substrates is a fundamental approach. ...
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... second term ∂Ps ∂t is the displacement current due to non-electric field but owing to external strain field. The first term is dominant at high frequency for wireless communication, while the second term is the low frequency or quasi-static term that is responsible for the energy generation ( figure 31). The term that contributes to the output current of a triboelectric nanogenerator is related to the driving force of ∂Ps ∂t , which is simply named as the Wang term in the displacement current. ...
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... to the thin film or bulk structures, fibres and textiles have more sophisticated patterns and higher surface roughness, which can further satisfy the functionality and aesthetic requirements of wearable applications with enhanced triboelectric output. Figure 41(a) shows different forms of fibres and textiles based on their various structural dimensions and fabrication methods [252,253]. Fibres are the basic building blocks, which can be coiled alone or twisted with other fibres to form yarns. Different yarns can be further processed to construct a hierarchical structure with better mechanical strength. ...
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... existing challenges are still centred on how to prepare the hybrid energy devices in the fibre/textile form and efficiently integrate them into one garment without affecting their original breathability and comfortability. An overview of the storage of energy via self-charged human-body bioenergy is shown in figure 41(b) [256]. ...
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... other classes of thermoelectric materials, half Heuslers are known for their mechanical robustness and thermal stability in the mid-high temperature range [312]. This is depicted in high resistance to elastic deformation (average Young's modulus ∼187 GPa) [312] and the best reported creep resistance in thermoelectric materials (see figure 51) [304]. However, half-Heusler alloys, among most of the thermoelectric materials, lack the required fracture resistance that is essential to minimize the impact of expanding cracks initiated during processing or operation of thermoelectric devices. ...
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... to the imperfect matching of n-and p-type legs, parasitic losses due to electrical and thermal contacting, and the reduced fill fraction needed to avoid contact between legs. An overview of calculated power outputs and efficiencies for near ambient harvesting (300-310 K) and up to the stability limit of the materials is given in figure 61. This shows that high efficiencies are possible for large temperature differences, and that Mg 3 (Sb/Bi) 2 is promising for near ambient harvesting, both in terms of efficiency and power output. ...
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... example, a flexible rectenna system has been realized for Wi-Fi band wireless energy harvesting based on MoS 2 material, as shown in figure 81(a). The core of this device is a Schottky diode based on a MoS 2 phase heterojunction, which operates up to 12 GHz, covering most of the unlicensed industrial, scientific and medical radio bands, including the bands for Wi-Fi [510]. ...
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... it comes to the parasitic capacitance, C, two-dimensional materials have a clear advantage over other materials. For example, in the MIG diode configuration shown in figure 81(b), the parasitic capacitance can be largely reduced due to the 1-dimensional junction. In a geometrical diode like shown in figure 81(c), the parasitic capacitance is largely reduced since there is no dielectric involved in the operation of such diodes, although the resistance is a limiting factor due to the small dimension of the junction. ...
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... example, in the MIG diode configuration shown in figure 81(b), the parasitic capacitance can be largely reduced due to the 1-dimensional junction. In a geometrical diode like shown in figure 81(c), the parasitic capacitance is largely reduced since there is no dielectric involved in the operation of such diodes, although the resistance is a limiting factor due to the small dimension of the junction. These aspects must be considered when designing a rectenna for high frequency operation. ...
Citations
We recently identified CaCuP as a potential low cost, low density thermoelectric material, achieving zT = 0.5 at 792 K. Its performance is limited by a large lattice thermal conductivity, κL, and by intrinsically large p-type doping levels. In this paper, we address the thermal and electronic tunability of CaCuP. Isovalent alloying with As is possible over the full solid solution range in the CaCuP1–xAsx series. This leads to a reduction in κL due to mass fluctuations but also to a detrimental increase in p-type doping due to increasing Cu vacancies, which prevents zT improvement. Phase boundary mapping, exploiting small deviations from 1:1:1 stoichiometry, was used to explore doping tunability, finding increasing p-type doping to be much easier than decreasing the doping level. Calculation of the Lorenz number within the single parabolic band approximation leads to an unrealistic low κL for highly doped samples consistent with the multiband behavior in these materials. Overall, CaCuP and slightly Cu-enriched CaCu1.02P yield the best performance, with zT approaching 0.6 at 873 K.
The direct conversion of thermal energy into electrical current via thermoelectric (TE) effects relies on the successful integration of efficient TE materials into thermoelectric generators (TEGs) with optimized characteristics to ensure either optimum output power density or conversion efficiency. Successfully employed for powering deep-space probes and extraterrestrial rovers since the 1960s, the development of this technology for waste-heat-harvesting applications faces several key issues related to the high temperatures and oxidizing conditions these devices are subjected to. This Perspective provides a brief overview of some prospective thermoelectric materials/technologies for use in radioisotope thermoelectric generators utilized in space missions and highlights the progress made in the field over the last years in the fabrication of TEGs. In particular, we emphasize recent developments that enable to achieve increased power densities, thereby opening up novel research directions for mid-range-temperature applications. In addition to showing how using lower quantities of TE materials may be achieved without sacrificing device performance, we provide an outlook of the challenges and open questions that remain to be addressed to make this technology economically and technologically viable in everyday-life environments.
Flexible perovskite solar cells are lightweight, bendable, and applicable to curved surfaces. Polyethylene terephthalate (PET) has become the substrate of choice compared to other plastic substrates like polyethylene naphthalate. PET is not only stable but also much cheaper to manufacture, an important factor for photovoltaics (PV). Herein, highly efficient devices on PET are demonstrated using a dual low‐temperature (≤100 °C) approach, first by anion mixing (replacing I with Br) of the lead‐containing perovskite composition, increasing bandgap (42% improvement), and then by interfacial engineering with tetrabutylammonium bromide (TBAB) (a further 26% improvement), reaching efficiencies of 28.9% at 200 lx and a record 32.5% at 1000 lx. The TBA ⁺ cation intercalates into the structure, substituting formamidinium cations at the perovskite/TBAB interface, inducing the formation of large‐sized, lower dimensional structures over the 3D perovskite matrix. The resulting PV cell has 1.4 times higher carrier lifetime, one order of magnitude lower leakage currents, and 3 times lower defect densities, suppressing recombination. Importantly, stability (ISOS‐D1 protocol) improves by more than double with treatment. Highly efficient and stable cells on PET films enable seamless integration with wearable, portable, smart building, and Internet of Things electronic devices, expanding the reach of indoor applications.
