Al-Catalyzed Si Nanowire Formation on Pre-Etched and Post-Polished Thin Si Substrates for Photovoltaic Application

Thin silicon (Si) solar cells have been a candidate to suppress Si material consumption and utilize them as flexible and durable devices. However, the advantage has come with a trading-off in solar cell efficiency. Light absorption loss in thin Si wafers is an important issue to be overcome in order to improve the conversion efficiency. Systems made with one-dimensional (1D) nanostructures of inorganic semiconductors with high crystallinity such as Si nanowires (NWs) are promising to provide significant enhancement by increasing the light trapping and carrier collection area in thin cells owing to the unique NW physical properties [1,2]. From our report [2], vapor-liquid-solid (VLS) growth using aluminum (Al) catalyst of chemical vapor deposition (CVD) on bulk Si(111) wafers was proposed to create single-crystalline SiNWs that resolve the catalyst contamination problem. The developments of vertical-oriented, Al-doped, and smooth surface Al-catalyzed SiNW array formation together with the realization of high-performance SiNW-based photovoltaic devices have been accomplished. In this study, thin SiNW-based solar cells were further demonstrated. Two different thin and bendable Al-catalyzed SiNW substrates were successfully prepared using simple techniques of pre-chemical etching in HF:CH₃COOH:HNO₃ solution before SiNW formation and post-mechanical polishing with diamond solution after SiNW growth. The formation of Al-catalyzed SiNWs on a pre-etched thin Si substrate was achieved as well as on a post-polished sample by applying optimized conditions from previous studies [2]. Although only one-half of SiNWs was vertically formed along [111] direction on pre-etched samples due to the Si surface roughness, low light reflectance (<10%) of both thin SiNW substrates is comparable. SiNW solar cells fabricated using an axial junction of p⁺-p-n structure [2] on thin pre-etched and post-polished Si wafers could enhance the power conversion efficiency (PCE) up to 6.1% and 6.6%, respectively. Both thin SiNW substrates contribute light trapping qualities while the NW-shaped increases the carrier collecting area compared to thin planar cells. The ease of Al catalyst removal and the capabilities of Al existence as a p-type dopant in SiNWs are also advantageous. The problems of catalyst contamination could be avoided and the process is capable of being scaled up. The photovoltaic characteristics of the best SiNW sample and planar sample with optimizations will be discussed. Further necessitating improvements in SiNW quality and interface engineering is in progress. [1] W. Jevasuwan, et al., JJAP, 56 [8] (2017) 085201. [2] W. Jevasuwan and N. Fukata, Nanoscale. 13 [14] (2021) 6798-6808.