Photoresponse and carrier transport of protocrystalline silicon multilayer films

Abstract

Alternating multilayer films of hydrogen diluted hydrogenated protocrystalline silicon (pc-Si:H) were prepared using a plasma-enhanced chemical vapor deposition technique. The microstructure of the deposited films and photoresponse characteristics of their Schottky diode structures were investigated by Raman scattering spectroscopy, Fourier transform infrared spectroscopy and photocurrent spectra. Microstructure and optical absorption analyses suggest that the prepared films were pc-Si:H multilayer films with a two-phase structure of silicon nanocrystals (NCs) and its amorphous counterpart and the band gap of the films showed a decreasing trend with increasing crystalline fraction. Photocurrent measurement revealed that silicon NCs facilitate the spatial separation of photo-generated carriers, effectively reduce the non-radiative recombination rate, and induce a photoresponse peak value shift towards the short-wavelength side with increasing crystallinity. However, the carrier traps near the surface defects of silicon NCs and their spatial carrier confinement result in a significant reduction of the diode photoresponse in the longwavelength region. An enhancement of the photoresponse from 350 to 1000 nm was observed when applying an increased bias voltage in the diode, showing a favorable carrier transport and an effective collection of photo-generated carriers was achieved. Both the spatial separation of the restricted electron-hole pairs in silicon NCs and the de-trapping of the carriers at their interface defects are responsible for the red-shift in photoresponse spectra and enhancement of external quantum efficiency. The results provide fundamental data for the carrier transport control of high-efficiency pc-Si:H solar cells.