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

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 H-2-dilution (R-H) 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 (similar to 250 degrees C) by plasma-enhanced chemical vapour deposition (PECVD). Along with increasing crystallinity, significant contribution of atomic H from the increased R-H 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 R-H, 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 similar to 1.07% to similar to 1.89% during increasing R-H 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 H-2-dilution of the plasma. The P-doped ne-SiOx:H films grown with an advancement of the overall nanocrystallinity from similar to 7 to 60 vol% via increased H-2-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 similar to 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 microstructural control using H-2 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 similar to 5.74%.