Neutral-color Semi-transparent Crystalline Silicon Solar Cells

Abstract

Semi-transparent solar cells (STSCs) are an attractive energy conversion device because it can be used in various applications in our daily life such as building-integrated photovoltaics, and vehicle-integrated photovoltaics. Currently, organic photovoltaics (OPV), and dye-sensitized solar cells (DSSC) based STSCs have been developed so far. However, the developed STSCs up to now have limitations in power conversion efficiency (PCE), and stability. Furthermore, they have shown specific colors owing to the colors of polymers or dyes. To address this issue, some researchers have been developed near infrared (NIR)-absorbing STSCs to fabricate neutral-color STSCs like that of glass. However, NIR-absorbing STSCs have been shown very low PCE under 1% until now. Accordingly, a different approach is required to develop STSCs with neutral color, high PCE, and long-term stability. As a method to develop new-concept neutral-color STSCs with high PCE and stability, crystalline silicon (c-Si) based STSCs can be considered. c-Si would be one of the best candidates to develop STSCs because conventional c-Si solar cells are known to exhibit high PCE and long-term stability compared to other solar cells. However, the development of c-Si based STSCs is extremely challenging due to the opaque characteristic of c-Si wafers with thicknesses of 200 µm. To fabricate STSCs using c-Si, thinning of c-Si has been considered as a method to increase transparency. However, as the absorption of long-wavelength light is extremely limited in thin films, their application to solar cells is not suitable. In addition, a thin c-Si film has a particular color because of absorption spectrum cutoffs in the visible light wavelength range. In this presentation, we show a novel approach to develop neutral-color transparent c-Si solar cells. First of all, we have developed a neutral-color transparent c-Si substrate using a 200-µm thick c-Si wafer, which is known to be opaque. Transparent c-Si substrates were fabricated by placing hole-shaped light transmission windows on a bare c-Si wafer. These windows were designed to enable the transmission of all incident visible light through the substrate, resulting in a colorless substrate. In addition, the spacing between the holes was appropriately selected by considering the minimum angle of resolution for humans to ensure that the individual transmission windows are not visible to the human eye. A light absorption area was also designed on the substrate to efficiently absorb incident light of the spectral range between 300 nm and 1100 nm. The transmittance of the transparent c-Si substrate was systematically tuned from 20% to 50 % under the full solar spectrum. The STSCs fabricated with the substrate exhibit a PCE of up to 12.2%, with Jsc = 29.2 mA/cm2; Voc = 588 mV; and FF = 71.1% with a transmittance of 20 %. The cell performance of the transparent c-Si solar cells is higher than those of other neutral-color semi-transparent solar cells reported thus far. Hence, our novel c-Si STSCs presents a unique opportunity to develop next-generation neutral-color STSCs which would satisfy high efficiency as well as high stability. Furthermore, we believe that this study makes a significant contribution to not only photovoltaics fields but also various transparent optoelectronics.