MoOx work function, interface structure, and thermal stability analysis of ITO/MoOx/a-Si(i) stacks for hole-selective silicon heterojunction solar cells

Abstract

Abstract Silicon heterojunction (SHJ) solar cells are gaining prevalence owing to their high conversion efficiency and simple, low-cost, and industrially compatible fabrication process. However, the high parasitic absorption exhibited by the doped amorphous silicon a:Si(p/n) layer substantially limits their performance. This can be alleviated by substituting traditional amorphous doped p/n layers with alternative materials such as transition metal oxides. MoOx is one of the most promising candidates for replacing the a:Si(p) layer, and it has yielded impressive results in recent studies; however, the thermal instability of MoOx-based devices is a major limitation. In this work, the MoOx work function, thermal stability of the ITO/MoOx/a-Si(i) interface, and the effect of MoOx thickness variation on the performance of a MoOx SHJ solar cell were thoroughly studied. The MoOx composition and work function were analyzed using X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and ultraviolet photoelectron spectroscopy. The O2-treated MoOx film showed minimal oxygen deficiencies accompanied by a higher work function of 6.22 eV compared with those of H2-, N2-, and Ar-treated MoOx films. The ITO/MoOx/a-Si(i) stacks have a clear contact interface up to 150 °C and in-out diffusion between the layers is initiated upon further increase of the annealing temperature. Thus, 150 °C was proposed as the optimal temperature and used for Ag paste sintering without degrading the device performance. Additionally, a thin sub-stoichiometric SiOx layer was formed on a-Si(i) during MoOx deposition, irrespective of the annealing environment, temperature, and thickness of MoOx. The fabricated SHJ solar cell with an optimized thickness of MoOx (7 nm) exhibited an efficiency of 19.86% with a Voc of 716 mV, Jsc of 37.50 mA/cm2, and FF of 74.01%. The achieved Jsc is 1.40 mA/cm2 higher and the external quantum efficiency in the range of 300–600 nm is 13%–15% higher than those for the reference SHJ solar cell. Therefore, our results show that MoOx is an efficient and convenient alternative emitter material for Si solar cells, and the proposed optimized parameters for MoOx processing pave the way for more efficient SHJ solar cells.