I. Introduction
The passivated emitter and rear contact (PERC) has emerged as the mainstream commercial Si solar cell in the past decade with global production capacity of ~ 120 GW in 2022. The success of this technology comes from the relentless improvements in cell and module efficiency by reducing the back surface recombination loss with improved rear optics. The current record solar cell efficiency, in laboratory scale, is 24.5% for a p-type PERC solar cell demonstrated by Trina Solar [1]. The most important feature of PERC cells is the introduction of an aluminum oxide (Al2O3) rear surface contact passivation structure, which provides excellent surface passivation of p-type Si surfaces, less shunt parasitic current and enhanced light absorption by improved rear reflection. The localized laser removal of Al2O3 provides low resistance contacts [2]. The front emitter junction surface is passivated by old but established methodology and materials, i.e., by hydrogenated amorphous Si nitride and/or a stack of SiO2/a- . In this work, we have explored an alternative front emitter passivation method using a hydrogen sulfide (H2S) gas reaction. Over the years, many materials have been developed for surface passivation to improve the cell performance. Most commonly used passivating materials include , SiO2 and hydrogenated amorphous silicon (a-Si:H). All these materials can effectively reduce Si surface recombination (surface recombination velocity < 5 cm/s) on specific wafer surfaces and in different device structures. But they have their drawbacks as well, such as the fact that amorphous silicon (a-Si:H) passivation degrades at high temperatures, limiting the downstream cell processing temperature to <300°C, and suffers from parasitic light absorption loss [3]. Silicon dioxide (SiO2) is another commonly used passivation layer that can be grown either by dry oxidation or wet steam oxidation [4], [5] at temperatures >850°C, which introduces a challenge to maintain the Si bulk quality. Having a substantial negative fixed charge density makes aluminum oxide (Al2O3) passivation more suitable for p-type doped surfaces [6]. Therefore, the search for alternative passivation layers has been a subject of extensive research and a detail review of them can be found in the literature [7].