Influence of rear surface pyramid base microstructure on industrial n-TOPCon solar cell performances

A passivating contact structure composition of a heavily doped polysilicon (poly-Si) and a tunneling silicon oxide plays an important role in high-efficiency n-TOPCon (tunneling oxide passivated contact) solar cell, which remarkably reduces recombination losses at both the crystalline silicon rear surface and metal/Si contact interface. However, it is a great challenge for the passivating contact structure to be effectively formed on a pyramidally textured surface due to the presence of sharp vertices, edges, and valleys. In this work, the rear surface microstructure of the TOPCon solar cells was optimized by adjusting the etching time of rear surface chemical polishing after standard alkali texturization in addition to the performance on industrial TOPCon solar cells. It can be observed that the pyramidal vertices were selectively etched and the square pyramid bases were left after the rear surface polishing by using an etching solution composed of 3.4% KOH and 0.93% additive. Besides, a pyramid base with a smaller edge but a higher height could be achieved by etching a shorter time, leading to a rougher rear surface. It could help to reduce the contact resistivity without passivation degeneration, and also assisted to improve the forming ability of metal fingers in the screen-printing process. The average FF corresponding to 170 s, 230 s, and 300 s etching time conditions were 83.62%, 83.44%, and 83.45%, respectively, which is due to the difference of roughness. While a rougher rear surface resulted in a slight difference in the open circuit voltage Voc and the short circuit current Isc. Finally, 210 mm × 210 mm industrial n-TOPCon solar cells achieved an average efficiency of 24.82% at 170 s rear etching time condition, which was an absolute efficiency improvement of more than 0.05% compared with the other two longer etching times. At the same time, batch validation of more than 10,000 pieces showed similar electrical performance improvements with excellent mass producibility.