Advanced nanocrystallinity with widened optical gap realized via microstructural control in P-doped silicon oxide thin films used as window layer in nc-Si solar cells

Abstract

The prime objective of the present investigation was focused on simultaneous improvement in crystallinity of the oxygenated wide band gap Si-network, while increasing its doping efficiency via increased H2-dilution (RH) of the primarily He-diluted (SiH4+CO2+PH3)-plasma. The P-doped nc-SiOX:H films were grown at adequate radio frequency (rf) power (200 W) and low substrate temperature (~250 °C) by plasma-enhanced chemical vapour deposition (PECVD). Along with increasing crystallinity, significant contribution of atomic H from the increased RH resulted in the formation of enough OH-radicals that facilitated growing Si–OH bonds and produced the Si–O–Si network. The effective P-doping gradually advanced within the crystalline dominated network grown at higher RH, due to increased density of atomic H feasibly influencing sensible etching of the grain boundary and/or amorphous tissue region. In view of increased crystallinity, elevated grain-size, enhanced O-content and simultaneous continuous increase in P/Si ratio from ~1.07% to ~1.89% during increasing RH from 0 to 0.5, it was envisaged that the dopants preferably resided within crystalline-prone domain of the network which was consistently improved at elevated H2-dilution of the plasma. The P-doped nc-SiOx:H films grown with an advancement of the overall nanocrystallinity from ~7 to 60 vol% via increased H2-dilution to the SiH4 plasma attained the dark electrical conductivity increased from 10−4 to 2.3 S cm−1. Simultaneously, a significant widening of optical band gap from ~1.92–2.14 eV has been obtained via a limited increase in the Si–O bond density along with quantum confinement effect arising from increased volume fraction of Si nanocrystals with dimension <5 nm. Elevated number density of tiny Si-ncs played a significant role in maintaining both the increased electrical conductivity and the widened optical band gap, which had been attained via proper micro-structural control using H2 with He as a tricky diluent to the SiH4 plasma. Here lies the novelty of the work which led to the development of n-type nc-SiOX:H films suitably used as the window-layer of thin-film nc-Si solar cell in n-i-p superstrate configuration with an efficiency of ~5.74%.