A critical study on different hydrogen plasma treatment methods of a-Si: H/c-Si interface for enhanced defect passivation

Abstract

Defect engineering of amorphous-crystalline silicon interface is a critical aspect for high efficiency silicon heterojunction solar cells. In this work, we have done a comparative study of different hydrogen plasma treatment (HPT) methods namely Pre, Post and Intermediate (Inter) HPT using radio frequency plasma-enhanced chemical vapor deposition (rf-PECVD) to understand the passivation mechanism to obtain state of the art passivation. It was observed that Inter and Post HPT can give high minority carrier lifetime on both p and n type wafers and has lesser interface defect states. The analysis from Fourier Transform Infrared Spectroscopy and high resolution Transmission Electron Microscopy shows that these HPT methods results in higher bulk disorder yet atomically sharp and superior interface due to hydrogen diffusion which saturates the interface dangling bonds. While a Pre-HPT results in epitaxial growth leading to poorer performance inspite of lowest dihydrides in the bulk which further highlights the importance of the interface. We have demonstrated Inter and Post HPT of ultrathin (5 nm) a-Si:H films which could give surface recombination velocity as low as 13 cm/s. The degradation due to hydrogen effusion in these HPT a-Si:H films has also been studied.