A Discussion about Defects Passivation and Etching Mechanisms in Polycrystalline Silicon Hydrogenated by MW-ECR Plasma

Abstract- In this work we have investigated the microwave plasma discharge assisted by electron cyclotron resonance for hydrogenation of thin film polycrystalline n pp silicon solar cells in terms of defects passivation and surface etching. The polycrystalline silicon films were formed by high temperature chemical vapor deposition. Influence of various process parameters such as microwave plasma power (PMW), hydrogenation time (tH) and substrate temperature (TH) on the sheet resistance of the n emitter region and on the open-circuit voltage (Voc) of the n pp structure were investigated. The n emitter region was obtained by phosphorus diffusion using a spin-on dopant P507 solution from filmtronics. After 75 min of hydrogenation, Voc is greatly improved by a factor of 2.5 and reaches values up to 440 mV at 500°C and microwave power of 650W. This enhancement in Voc after plasma hydrogenation is originating from a reducing recombination velocity at the grain boundaries due to the passivation of dangling bonds by hydrogen atoms, leading to an important increasing of the diffusion length. Above our hydrogenation parameters values, degradation is observed, especially an etching of the emitter region. The origin of such behaviour were thoroughly explained in detail. Index Terms- Defects passivation, grain boundary, plasma hydrogenation, polycrystalline silicon, solar cells. I