Cu-assisted chemical etching of bulk c-Si: A rapid and novel method to obtain 45 μm ultrathin flexible c-Si solar cells with asymmetric front and back light trapping structures

Abstract

Ultrathin c-Si solar cells (≤50 μm) are believed to be applied in military, aerospace and other special circumstances in the future due to their flexibility and high specific power density, and thus has attracted a great deal of research interest. However, until now, lacking fabrication means of high-quality ultrathin c-Si materials accompanied with their inefficient absorption of near-infrared light greatly limits their further application. In this work, we present a simple and novel method to realize rapid thinning and texturing of bulk c-Si at room temperature by varying the ρ ([HF]/([HF] + [H2O2])) values during the one-step Cu-assisted chemical etching process, followed by a systematic investigation of the formation mechanism of the surface structures. It is found that the sizes of surface structures accompanied with the etching rate increase with increasing the ρ values from 40% to 95% during the double sided etching process, and a high etching rate of 29.6 µm/min is obtained under the ρ value of 95%. For rapid thinning and efficient absorption of near-infrared light, 45 µm c-Si solar cell with asymmetric front and back light trapping structures is rapidly fabricated by directly immersing as-sawn bulk c-Si substrate into the thinning (ρ = 95%) and texturing (ρ = 60%) solution successively for only a few minutes. A high short-current density (Jsc) (36.12 mA/cm2) and energy-conversion efficiency (17.3%) are achieved, which are 1.09 mA/cm2 and 0.4% higher respectively than that in 45 µm c-Si with flat back surface. Based on the absorption spectra, it is demonstrated that the 45 µm c-Si cell with our asymmetric structures yields a high theoretical Jsc of 42.47 mA/cm2, which nearly approaches the Yablonovitch limit of 42.56 mA/cm2. All the findings offer additional insight into the structure formation mechanism and pave a rapid and novel way for exploration of next-generation flexible photovoltaics.