Chapter 1. Introduction
1.3 Transparent solar cells
1.3.3 Types of the transparent solar cells
The main concept to develop a transparent solar cells is to penetrate the incident visible light.
To carry out this concept, main strategies are devied into three types (Figure 1.10): (1) thin film type, (2) selective light transmission type, (3) luminescent solar concentrator type using luminescent materials. Type (1) and (2) are the solar cells that partially transmits of the visible light (400 nm – 700 nm) and absorbs remains of it to generate electricity. In order to impart transparency to solar cells, the transmission of the visible light region. Because the visible light can be recognized to the humans, is required. There are two main strategies. The first strategy is to fabricate transparent solar cells is thin film technology. This technology is partially transmits visible light by decreasing the light absorbing layer’s thickness. The second strategy is a selective light transmission technology. This strategy is penetrate visible light through selective regions.
Figure 1.10 Main strategies for development of the transparent solar cells. Reprinted from Lee et al.4 CC BY-NC-ND, 4.0. Lim et al.22 Copyright 2017, Wiley-VCH. Lee et al.14 Copyright 2020, Elsevier.
Zhao et al.23 Copyright 2014, Wiley.
１３ 1.3.3.1 Thin film type
Thin film type is a method to develop transparent solar cells by decreasing the thickness of the light absorbing layer of an opaque solar cells with inducing a transmitted light. Generally, the light absorbing layer of an opaque solar cells uses a material with bandgap of below 2.0 eV. Therefore, the light absorbing layers absorb visible light region, making the solar cells opaque to the human eye. If the thickness of the light-absorbing layer reduces, light can be penetrated according to the Beer-Lambert law (Equation 1.6 and Figure 1.11).24
𝐼𝑇 = 𝐼0× 𝑒−𝛼𝑙 (eq 1.6)
Where, 𝐼0 , 𝐼𝑇, 𝛼 , and 𝑙 indicates incident light intensity, transmitted light intensity, absorption coefficient, and the absorbing layer’s thickness, respectively. Therefore, as the thickness of the light absorbing layer decrease, the light transmittance increases. In other words, thin film type transparent solar cells use a technology that realizes light transmittance by thinning the thickness of the light absorbing layer.25 Thus, it is possible to fabricate various type of transparent solar cells by using almost types of light absorbing layers such as inorganic (CIGS, CdTe, a-Si, Quantum dot), organic (Polymer, Dye), and perovskite, which are typically used in opaque solar cells. (Figure 1.12A-H)
Figure 1.11 Schematic illustration of thin film type for realizing light transmission
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Figure 1.12 Photograph diagram of a representative three types thin film transparent solar cells (organic, inorganic, and perovskite). Figures reproduced with permission from: (A); Beiley et al.26 Copyright 2013, Wiley-VCH. (B);27 CC BY 4.0 license. (C); Shin et al.28Copyright 2019, Elsevier. (D); Alrashidi et al.29Copyright 2020, Elsevier. (E); Yang et al.30 Copyright 2018, Elsevier. (F); Zhang et al.31 Copyright 2017, Royal Society of Chemistry. (G); Xue et al.32Copyright 2017, Wiley-VCH. (H); Heo et al.33Copyright 2015, Royal Society of Chemistry.
1.3.3.2 Selective light-transmission type
The second type is a selective light transmission technology. This type is a method that transmits visible region light using selective light transmission areas. This approach is generally used to fabricate light-transmissive solar modules for windows for the building. As shown in Figure1.13A, opaque solar cells are formed with some gap on transparent substrates like transparent plastic substrates, and glass. This technology has an advantage of being easily developed, it reveals the main disadvantage which the solar cells’ regions are opaque. Therefore, there is a limit to the transmission region, and these types of the solar module could detract the aesthetics of the applications.
Some researchers have tried to fabricate transparent solar cells by forming holes on the active layer of solar cells. As a representative example, see-through amorphous silicon solar cells by forming holes on an amorphous silicon solar cell have been developed by sanyo corporation.
(Figure 1.13B). In addition, some researchers have been developed see-through solar cells by
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forming apertures.34, 35 However, when the solar cells fabricated, holes were formed without considering the invisible conditions of holes, resulting in opaque regions and holes being inevitably recognized. Therefore, these solar cells are not perfect transparent solar cells.
Figure1.13 (A) Illustration of light-transmissive solar cell modules, (B) Illustration of light transmissive solar cells. Reprinted from Takeoka et al.36 Copyright 1993, Elsevier.
1.3.3.3 Luminescent solar concentrator type
Luminescent solar concentrator type solar cells use luminescent materials which absorb and emit light in the Ultraviolet/Near-infrared wavelength region, so it has reported very high average visible transmittances over 85%. The luminescent materials embedded in transparent polymer substrates absorb ultraviolet or near-infrared photons and emit, the longer wavelength region light than those of the absorbed light. The emitted light moved to the transparent substrates’ edge. At the edge side, conventional opaque solar cells are installed (Figure 1.14A). The luminescent solar concentrator type consists of two parts. The first part is a light absorbing-regions that include the luminescent materials.
The second part is the edge region where conventional opaque solar cells are installed, which produces electricity. The luminescent solar concentrator based transparent solar cells have the advantage of able to realize perfect good transmittance which have no visible light absorption like that of glass, because the edge area is not important region for vision (Figure 1.14B). The crystalline silicon, gallium arsenide solar cells have been used as the edge solar cells and organic materials, and quantum dots are usually used materials as the luminescent materials embedded in transparent substrates. Although the idea of
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luminescent solar concentrator is promising, more researches are required to improve the power conversion efficiency of the luminescent solar concentrator type transparent solar cells. In addition, there is no stability evaluation’s standard of luminescent solar concentrator-based transparent solar cells.
Thus, it is necessary to establish the standard of stability test.
Figure 1.14 (A) Schematic of luminescent solar concentrator type solar cells. Reprinted from Lee et al.4 CC BY-NC-ND. (B) Photograph of luminescent solar concentrator Reprinted from Zhao et al.23 Copyright 2014, Wiley.
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