Structural and electronic properties of nanocrystalline silicon thin film solar cells fabricated by hot wire and ECR-plasma CVD techniques

Abstract

Nanocrystalline silicon has become the material of interest recently, for solar cell applications and also in the fabrication of thin film transistors. The material contains crystalline grains surrounded by amorphous tissues and when used as intrinsic layer in solar cell devices, greatly enhances the device stability against the light induced degradation which is a critical problem with amorphous silicon solar cells. The conventional PECVD techniques used for the deposition of high efficiency devices have a major drawback of very low growth rates. This project deals with a systematic study of structural and electronic properties of nanocrystalline Si:H films and devices fabricated using a relatively new technique called the Hot Wire CVD (HWCVD). In addition, we study the influence of ions on the crystalline ratio, grain size and orientation of the nanocrystalline films. Our apparatus allows us to add plasma ions separately from the primary growth process, which is growth using only radicals that are generated by the thermal dissociation of silane and hydrogen at the hot wire. In this way, we have also deposited the first ever nanocrystalline silicon solar cells by the combined HWCVD and Electron Cyclotron Resonance (ECR) PECVD technique. While hot wire deposition of nanocrsytalline Si:H has been studied in the past, virtually all previous work utilized a close-proximity hot wire deposition condition that creates a varying temperature profile during deposition because of the intense heating of the growing film due to radiation from the filament. In contrast, in this work, we use a remote filament to minimize sample heating, a conclusion verified by experimental measurements of surface temperatures during growth conditions. We have found that low energy ion bombardment, by either inert (helium) or reactive (hydrogen) ions significantly helps in