Chapter 3. Transparent crystalline silicon solar cells
3.6 Efficiency enhancement of the transparent solar cells with light-harvesting film
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solar cell, the power conversion efficiency of the solar cells is enhanced from 16.2% to 17.9% (Figure 3.26).20
We applied the light-harvesting film (inverted-pyramid-structured Polydimethylsiloxane film) to transparent crystalline silicon solar cells to improve the power conversion efficiency. However, as a result, the solar cell was no longer transparent, as shown in Figure 3.27. This is because the inverted- pyramid-structured Polydimethylsiloxane film with a high haze ratio of over 97% covers the light transmission area of the transparent crystalline silicon solar cells. Therefore, to apply the inverted- pyramid-structured Polydimethylsiloxane film to transparent crystalline silicon solar cells, the inverted- pyramid-structured Polydimethylsiloxane film needs to be only located on the light absorption region of the transparent crystalline silicon solar cells. Therefore, to apply the film to the transparent crystalline silicon solar cells, the light-harvesting film needs to be only located on the light absorption region of the transparent crystalline silicon solar cells.
Therefore, a microhole-patterned textured Polydimethylsiloxane film is required. To fabricate microhole-patterned textured Polydimethylsiloxane films, as shown in Figure 3.28, a casting process or a punching method was considered. However, to apply fabricated microhole-patterned textured Polydimethylsiloxane film to transparent crystalline silicon solar cells, the alignment process is required.
An alignment process is not an easy process because the microhole pattern is not recognizable to the naked eyes of humans. Therefore, a new fabrication process to develop microhole-patterned textured Polydimethylsiloxane film is required, only located on the light absorption region. In this study, we develop the novel fabrication process for transparent crystalline silicon solar cells with Polydimethylsiloxane film. The schematic illustration of the fabrication process is shown in Figure 3.29.
First, pour a Polydimethylsiloxane solution to the transparent crystalline silicon solar cells. Second, stamp the textured mold on the less cured Polydimethylsiloxane. Third, remove the Polydimethylsiloxane on the microhole area using suction in the vacuum chamber. Then, finally, we can fabricate microhole patterned light-harvesting film on the transparent crystalline silicon solar cells.
As a result, as shown in Figure 3.30, the transparent crystalline silicon solar cells with film maintain transparency. This can also be confirmed by the total transmittance measurement (Figure 3.31). As a result of comparing the total transmittance spectra in the wavelength range of 300-1100 nm, the transmittance spectra of the transparent crystalline silicon solar cells with the light-harvesting film is almost the same as that of the transparent crystalline silicon solar cells without light-harvesting film. In addition, when we check the haze ratio of the transparent crystalline silicon solar cells with textured film, it is 1.2% which is almost no transmission haze ratio. In the case of the glass and transparent crystalline silicon substrate, the haze ratio is 0.89%, and 0.95% respectively (Figure 3.32).
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Figure 3.25 Total transmittance spectrum of the Polydimethylsiloxane film over the wavelengths 300–1100 nm
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Figure 3.26 (A) Current Density-Voltage curves and, (B) External quantum efficiency of the opaque crystalline silicon solar cells with and without light-harvesting film. Hwang et al.20 Copyright 2020, Elsevier.
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Figure 3.27 Photograph and schematic illustration of the transparent crystalline silicon solar cells without/with light-harvesting film
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Figure 3.28 A considerable fabrication process of the microhole-patterned textured Polydimethylsiloxane films.
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Figure 3.29 The fabrication process of the transparent crystalline silicon solar cells with textured film
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Figure 3.30 Transparent crystalline silicon solar cells with light-harvesting film (A) full contact, (B) selective contact.
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Figure 3.31 Total transmittance spectra of the transparent crystalline silicon solar cells with/without light-harvesting film
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Figure 3.32 Haze ratio of the transparent crystalline silicon solar cells with light harvesting film (Full contact (Blue line) and Selective contact (Red line))
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We checked that the power conversion efficiency of the transparent crystalline silicon solar cells is improved by attaching the light-harvesting film because of the current density enhancement (Figure 3.33-3.34). As shown in Table 3.6, the transparent crystalline silicon solar cells with a light-harvesting film show a power conversion efficiency of 14.6% with a short-circuit current of 30.9 mA/cm2, the open-circuit voltage of 618 mV, and a fill factor of 76.3%. In the case of the transparent crystalline silicon solar cells without light-harvesting film, the device shows a power conversion efficiency of 13.5 with a short-circuit current density of 28.3 mA/cm2, an open-circuit voltage of 618 mV, and fill factor of 77.0%. In addition, this film not only improves power conversion efficiency but also has additional advantages. In a real situation, the sun’s position changes continuously in season and time (Figure 3.35A). So, minimizing the efficiency changes of solar cells due to the incident angle is an important factor. Therefore, in the case of conventional opaque solar cells, some researchers are developing a sun tracking system that moves solar panels according to the location of the sun (Figure 3.35B). However, in the case of the transparent solar cells, since there is a high probability of being installed in a fixed area, such as a window in a building or vehicle, it is difficult to apply a sun tracking system. Thus, new strategies are required for transparent solar cells to minimize the difference of power conversion efficiency according to the angle of the incident light. We solve this issue using a light-harvesting film.
When we attach the film on the transparent solar cells, because of the light-harvesting effect, the difference in power conversion efficiency can be minimized according to the angle of the incident light.
To check this property, we obtained the relative change in the current density for several incident angles.
Figure 3.36 shows the relative change of the current density for incident angles. As a result, in the case of transparent crystalline silicon solar cells with light-harvesting films, the current density change according to incident angle is smaller than those of transparent crystalline silicon solar cells without films or planar crystalline silicon solar cells. In addition, because of Polydimethylsiloxane has low thermal conductivity which is 0.15 W/m∙K, Polydimethylsiloxane-based light-harvesting film has an additional advantage, which is maintaining the temperature of the solar cells (Figure 3.37).
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Table 3.6 Photovoltaic performance of the transparent crystalline silicon solar cells with light- harvesting film
Light harvesting film Voc
(mV)
Jsc
(mA/cm2)
FF (%)
PCE (%)
w/o 618 28.3 77.0 13.5
w 618 30.9 76.3 14.6
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Figure 3.33 Current Density-Voltage curves of the transparent crystalline silicon solar cells with/without light harvesting film.
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Figure 3.34 External quantum efficiency of the transparent crystalline silicon solar cells with/without light harvesting film.
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Figure 3.35 (A) Illustration of the sun path. (B) Schematic of the sun tracking system
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Figure 3.36 Relative ratio of short-circuit current, with respect to short-circuit current, for different incidence angles of the light for the planar solar cells and transparent solar cells with and without light harvesting film.
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Figure 3.37 Current Density-Voltage curves of the transparent crystalline silicon solar cells over time with heat supply, (A) without light harvesting film, (B) with light harvesting film.
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