Mitigating parasitic optical losses in bifacial TOPCon solar Cells through localized thinning of polysilicon

Yalun Cai¹, Shuo Deng¹, Jialiang Huang¹, Zicheng Wang¹, Yuhao Cheng¹, Jialin Cong¹,

Tang Qiao², Geng Zhang², Jian Song², Feng Li², Zhuo Xu², Ning Song¹

¹ School of Photovoltaics and Renewable Energy Engineering, UNSW, Australia

² Jolywood (Taizhou) Solar Technology Co., Ltd

E-mail: n.song@unsw.edu.au

Abstract

The parasitic absorption of incident illumination in the poly-Si layer on the rear side of a bifacial tunnel oxide passivated contacts (TOPCon) cell can compromise the short-circuit current density (J_SC) and cell efficiency. In this work, we demonstrate a local thinning (LT) approach on the polysilicon (poly-Si) layer in TOPCon cells to mitigate parasitic absorption. The LT approach is achieved by combining laser and alkaline etching. The 355 nm ultraviolet picosecond (UV-ps) pulsed laser thickens the surface silicon oxide (SiO_x) to 4 nm and functions as an etch barrier in the subsequent alkaline etching. Consequently, lasered regions in the poly-Si layer can maintain a thickness of 110 nm while regions without laser treatment are thinned to 30 nm. This local thinning process can suppress parasitic absorption while preserving excellent passivation. The laser also amorphizes the first 30 nm of the poly-Si into a-Si, which recrystallizes into poly-Si in later firing step with an increased doping concentration to 7E20 cm^-3. Experimental J–V measurements show an increase in J_SC by 0.13 mA/cm², a V_OC gain of 1.9 mV, and an absolute efficiency gain of 0.12% under one-sun illumination. Device-level simulations with Quokka3 further predict the efficiency gain of TOPCon cells incorporating this LT process under optimized contact and passivation conditions. These results highlight that the LT-TOPCon approach can effectively suppress rear-side parasitic absorption while maintaining passivation quality, offering a practical route to boost the efficiency of next-generation bifacial silicon solar cells.