Chapter 3. Transparent crystalline silicon solar cells
3.5 Efficiency enhancement of the transparent solar cells with surface treatment
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he silicon to the solution. With excess HNO3, the silicon oxidation is much faster than the fo rmation of Si-F bonds. Therefore, the etching process is independent of a crystal orientation o f the crystalline silicon. A more detailed mechanism of the HNA etching was reported by Ro bbins and Schwartz.64-67 Since HNA solution has isotropic etching properties, etching occurs i n all directions equally regardless of the crystal direction of the crystalline silicon. Because of this characteristic, conformal etching is possible on the damaged region of the light transmissi on regions of the transparent crystalline silicon substrates. We performed a study to remove t he surface damage of the light transmission regions of the transparent crystalline silicon subst rates using RSE-100 (a ratio of HF:HNO3:CH3COOH = 2:7:1), which is a representative HN A solution. First, to check the etching rate, a transparent crystalline silicon substrate was imm ersed in the HNA solution (Figure 3.16A). As a result, as shown in Figure 3.16B, it was co nfirmed an etching rate was around 0.16 µm/sec. It was also confirmed that the entire surfac e of the light transmission windows’ sidewall was smoothed, as shown in Figure 3.17. After that, to determine the effect of removing surface damage, the minority carrier lifetime accordi ng to the HNA solution (RSE-100) treatment time was measured. As a result, before the surf ace treatment, the minority carrier lifetime was 2.21 µs, and the minority carrier lifetime incr eased rapidly for the surface treatment time (Figure 3.18). Finally, when the surface treatment time was after 30 seconds, the lifetime became saturation and showed a minority carrier lifeti me of approximately 10 µs. In addition, after the Al2O3 passivation, in the case of samples without surface treatment, there is almost no minority carrier lifetime enhancement after passi vation. However, after surface treatment, the minority carrier lifetime is increased from 10.1 µ s (before passivation) to 191.1 µs (after passivation). This can also be confirmed through imp lied open-circuit voltage measurement, and it was confirmed that the implied open-circuit volt age was significantly improved from 537.6 mV (w/o surface treatment) to 670.7 mV (w/ surf ace treatment). This wet-chemical surface treatment not only has the advantage of removing si dewall surface damage of the microhole region but also allows the diameter control of the mi crohole (Figure 3.19). As the microhole size increased, the light transmittance can be tuned.
We fabricate a transparent crystalline silicon substrate with a hole size of 70 μm, and immers ed it in HNA solution, and adjusted the hole size to 190 μm (Figure 3.20). As a result, as s hown in figure Figure 3.21, as the microhole size increased, the light transmittance at the visi ble wavelength was systematically adjusted from 10% to up to 70% at the visible wavelength of 400-800 nm.
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Figure 3.14 (A) Photograph of transparent crystalline silicon solar cells, and schematics of a cross- sectional view of the transparent crystalline silicon solar cells. Lee et al.14 Copyright 2020, Elsevier. (B) Cross-sectional SEM images of light transmission windows’ wall. (C) Bird’s-eye view SEM image of light-transmission windows.
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Figure 3.15 Mechanism of surface treatment to microhole region of the transparent crystalline silicon via isotropic wet etching
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Figure 3.16 (A) Illustration of experimental set-up, (B) The etching rate verse etching time
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Figure 3.17 SEM image of the light-transmission window in a transparent crystalline silicon solar cells (A) before surface treatment, and (B) after surface treatment
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Figure 3.18 The minority carrier lifetime of the transparent crystalline silicon substrate according to the post wet etching treatment time.
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Figure 3.19 (A) Comparison of effective carrier lifetime of the transparent crystalline silicon solar cells before/after wet-etching treatment. (B) Comparison of implied open-circuit voltage before and after wet-etching treatment.
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Figure 3.20 Control of the hole size of the light transmission windows via HNA wet etching treatment
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Figure 3.21 Total transmittance spectra of the transparent crystalline silicon substrates (Control of the transmittance of the transparent crystalline silicon substrates via HNA wet etching treatment)
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Finally, the transparent crystalline silicon solar cells fabricated with the transparent crystalline sil icon substrate with surface treatment exhibited an open circuit voltage increase of 20 mV compared w ith that of the transparent crystalline silicon solar cell without surface treatment. Whereby, the dev ice showed a power conversion efficiency of 13.0% (short circuit current density = 28.8 mA/
cm2, open-circuit voltage = 608 mV, and fill factor =74.5%) at an average visible transmittan ce of 20% and cell size of 1 cm2. In particular, when the size of the transparent crystalline s ilicon solar cells is scaled up, as the surface area increases, so the effect of surface treatment becomes more critical. Therefore, when a solar cell with a size of 25 cm2 without surface tr eatment was fabricated, the solar cells exhibited a power conversion efficiency of 9.6% with a short circuit current density of 25.5 mA/cm2, the open-circuit voltage of 528.3 mV, and the fill factor of 71.3%. On the other hand, when the transparent crystalline silicon solar cell w as fabricated after surface treatment, the open-circuit voltage increases 100 mV than that of t he transparent crystalline silicon solar cell without surface treatment. Therefore, the best devic e exhibited a power conversion efficiency of over 13.0% (short circuit current density = 27.0 mA/cm2, open-circuit voltage = 628 mV, and fill factor =79.6%).
Table 3.4 Photovoltaic properties of the 1 cm2 sized transparent crystalline silicon solar cells Size of the solar cells Surface
treatment
Jsc
(mA/cm2) Voc
(mV)
FF (%)
PCE (%) 1 cm2
(K.Lee et al. Joule, 4, 1-12. (2020)) w/o 29.2 588 71.1 12.2
1 cm2 w 28.8 608 74.5 13.0
* The width of microgrid electrode: 4 μm
Table 3.5 Photovoltaic properties of the 25 cm2 sized transparent crystalline silicon solar cells Size of the solar cells Surface
treatment
Jsc
(mA/cm2)
Voc
(mV)
FF (%)
PCE (%)
25 cm2 w/o 25.5 528.3 71.3 9.6
25 cm2 w 27.0 628.1 79.6 13.5
* The width of microgrid electrode: 10 μm
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Figure 3.22 Current Density-Voltage curve of the 1 cm2-sized transparent crystalline silicon solar cells without surface treatment (Green line), and with surface treatment (Orange line).
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Figure 3.23 (A) Current Density-Voltage curve of the 25 cm2-sized transparent crystalline silicon solar cells without surface treatment (Red line), and with surface treatment (Green line). (B) Quantum efficiency and reflectance of the transparent crystalline silicon solar cells.
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Figure 3.24 (A) Photograph of the large-sized (25 cm2) transparent crystalline silicon solar cell. (B) Summary of the power conversion efficiency and open-circuit voltage of the transparent crystalline silicon solar cells.
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