Atomic layer deposited MoOX for controlling O:Mo ratio to obtain passivation and hole selectivity in Si heterojunction solar cells
Suren Patwardhan1, 2, Arpan Dhara1, Akash Kumar1, Sandeep Maurya1, Balasubramaniam Kavaipatti1, Shaibal Sarkar1
1Indian Institute of Technology Bombay, Mumbai, India
/2K J Somaiya College of Engineering, Mumbai, India

Sub-stoichiometric molybdenum oxide (MoOX) is a promising candidate for hole extraction in dopant-free carrier selective contact type Si heterojunction solar cells. In our work, we did not use any separate passivation scheme. MoOX films are directly deposited on Si using atomic layer deposition to check their functionality in terms of passivation in addition to basic hole selectivity. We tried both the precursors tested so far first, (NtBu)2(NMe2)2Mo as Mo precursor and O2 plasma as oxidant and second, molybdenum hexacarbonyl [Mo(CO)6] as Mo precursor and Ozone as oxidant. The former pair yielded low VOC (< 300 mV) while the latter pair resulted in a better performance (around 430 mV). Although, cells made by both the routes could not match performance of our reference cell incorporating evaporated MoOX film on Si for which the VOC exceeded 600 mV, it highlights the role played by precursors and deposition conditions on device parameters. There were two main observations. The VOC was strongly affected by the mechanism of film deposition while JSC is not as significantly affected. The ALD grown MoOX films also showed stable lifetime as compared to evaporated MoOX films on Si. This is promising in terms of maintaining the work function of deposited films. In case of evaporated films, there was a drastic reduction in the carrier lifetime indicating complete loss of passivation over a week's exposure to ambient conditions. Our ALD-grown MoOX/Si cells showed about 6.7% efficiency without using intrinsic a-Si:H or any other kind of passivation. Although low, to our knowledge, this is a first report of its kind.