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

The parasitic absorption 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 introduces a 4 nm surface silicon oxide (SiO X ) which 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 about 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 from 2E20 to 7E20 cm −3 . Experimental J–V measurements show an increase in J SC by 0.13 mA/cm 2 , 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 idealized 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. • Laser-assisted local thinning (LT) of rear poly-Si reduces parasitic optical losses in bifacial TOPCon cells. • 0.13 mA/cm 2 J SC increase, 1.9 mV V OC gain, 0.12 % absolute efficiency boost and no fill factor loss are achieved . • Laser induces a 30 nm highly doped poly-Si layer that may enhance field-effect passivation in a finished cell. • Quokka3 simulations predict up to 0.35 % efficiency gain with ideal contact and passivation parameters.