The Next Frontier of Solar Energy: Transparent Photovoltaics
Authors: Matsuki Nobuyuki 
Published: February 2024

The development of "Transparent Photovoltaics" (TPVs[1]), i.e., solar cells that are transparent to visible light, has become more and more popular in recent years due to their contribution to low CO2 emissions through "window power generation" in fully glazed buildings (building-integrated photovoltaics: BIPV) and the growing demand for development in the promotion of farm-based solar power; Agrivoltaics [2, 3]. So far, selective light-transmission photovoltaics (SLTPVs), in which conventional bulk and thin-film solar cells are fabricated into long and thin strips and arranged like a window shade at intervals or by reducing the film thickness, have already been put into practical use [3, 4], allowing part of the incident light to escape to the rear. However, the installation applications of SLTPVs are limited since blocking the line of sight outside the window and preventing a sufficient amount of light from reaching the interior or farmland surface. If TPVs that are uniformly transparent or tinted-transparent like window glasses can be commercialized, the range of applications is expected to expand significantly.
TPVs are perceived by humans as being "nearly colorless" if they have a visible light transmittance of 80% or more, including the substrate material. Depending on the application, it is possible to produce red, and dark colors by adjusting the material characteristics so that only the short wavelength side of the visible light wavelength range, the long wavelength side, or the entire wavelength range is uniformly absorbed [2, 3]. These visible light-transmissive solar cells with visible light transmittance of 80% or less are referred to as semitransparent photovoltaics (STPVs).
It is crucial to consider the theoretical limiting efficiency (TLE) for the practical application of TPVs. Recent reports have simulated TLs for single-junction and multi-junction cells with optical absorption in the UV and NIR regions, yet not in the visible region [5]. The results demonstrate that practical conversion efficiencies of 11% and 21% can be obtained for single-junction and 16-junction cells respectively, with 80% visible transmission. The optimal band gap for cells with 100% Vis-EQE (an external quantum efficiency in the visible light range of 435 to 670 nm) is around 1.5 eV, while for cells with 0% Vis-EQE and full transmission in the visible region, the optimal Eg shifts to the lower energy side of 1.1 to 1.2 eV. To use only the UV region without using the NIR region, 2.8 to 3.0 eV or higher can be easily approached with existing technology.
Transparent photovoltaic cells (TPVs) and semi-transparent photovoltaic cells (STPVs) developed to date can be broadly classified into the following four types of power generation methods:

TPVs

(1) Heterojunction and homojunction with 3.0 eV and more wide-gap semiconductors as generating layers [6-8].
(2) Glass plate (+ wavelength conversion material) with Si solar cells installed on the end face of the glass plate to form a module [9].

STPVs

(3) Increased visible light transmittance by wider-gap semiconductor materials (colored transparent type) [3].
(4) SLTPVs [4].
If TPV can be made more cost-effective, it can be applied to various applications such as low CO2 emissions in buildings, photovoltaic power generation for agriculture, no external power source required for electric bulletin boards, auxiliary power source for electric vehicles, and so on. Research and development are categorized into the above areas (1) through (4), with (1) through (3) in particular requiring basic research and development from a materials perspective. The number of groups involved in research and requests for practical applications is increasing, and accelerated progress is expected.

References

[1] Also called transparent solar cells (TSCs).
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156, 111946 (2018).
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101, 043902 (2012).
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12, 11315 (2022).
[9] Y. Zhao et al., "Near-Infrared Harvesting Transparent Luminescent Solar Concentrators," Adv. Opt. Mater.
2, 606 (2014).