Performance promotion of aluminum oxide capping layer through interface engineering for tunnel oxide passivating contacts

Abstract

High-efficiency silicon solar cells depend on passivating contact structure to decrease recombination losses at the crystalline silicon (c-Si) surface and the interface of metal/Si contact. Tunnel oxide passivating contact (TOPCon) technology is one of these structures, in which heavily doped poly-Si is placed over a SiO x layer grown directly on the c-Si wafer. The passivation performance can be further improved by depositing a thin aluminum oxide (AlO x ) layer on one side of the symmetric cell structure, which is attributed to the reduced interfacial defects, facilitated formation of carrier collector layer, and excellent field-effect passivation. Atomic layer deposition (ALD) may be utilized to produce high-quality and pinhole-free AlO x capping layers due to its controllability, homogeneity, and conformality. However, insufficient substrate surface activity is a challenge faced in AlO x deposition because it can impair Al(CH 3 ) 3 nucleation, resulting in an island-like growth over the first few ALD cycles, ultimately deteriorating the passivation effect of AlO x . In this study, we optimized the deposition process of ALD AlO x through a series of single-factor experiments. Subsequently, alternative substrate treatment methods were tested to compare the performance. In particular, we proposed a two-step interface engineering process (chemical mechanical polishing + RCA SC-2 treatment) to enhance the comprehensive performance of the ALD AlO x layer, achieving a high iV OC of 726 mV and low J 0 of 5.4 fA/cm 2 , compared with a reference poly-Si/c-Si passivating contact ( iV OC = 703 mV, J 0 = 19.4 fA/cm 2 ). The result was interpreted in terms of gradually improving substrate surface quality and wettability, together with the gradually decreasing thickness of the interfacial oxide layer. The circumvent of the island growth mode improved the growth quality of the subsequent film, effectively compressed the ALD cycle needed for stable growth, and enhanced the passivation property. Altogether, the present data may guide further efforts to obtain high-efficiency silicon-based solar cells by arising the interface engineering. • The optimum ALD AlO x process parameters are determined to optimize the AlO x coating process for industrial production. • A novel two-step interface engineering process (CMP + RCA SC-2 treatment) is proposed. • The overall passivation quality of the AlO x capped symmetric structure can be improved and the ρ c can be reduced simultaneously.