Elucidating the role of interfacial MoS₂ layer in Cu₂ZnSnS₄ thin film solar cells by numerical analysis

In this study, the effects of transition metal dichalcogenide, MoS₂ interfacial layer formation between the Cu₂ZnSnS₄ (CZTS) absorber layer and Mo back contact in a conventional CZTS thin film solar cell (TFSC) structure have been studied by numerical simulation using wxAMPS-1D software. The goal of this study is to elucidate the effects of both n and p-type MoS₂ on the overall CZTS solar cell's performance from the viewpoint of metal-semiconductor junction and heterojunction band alignment. Interestingly, CZTS device, regardless of p or n-type MoS₂ largely outperforms device without any MoS₂ due to lower back contact barrier value. Significant transition in efficiency is noticed when acceptor (increases efficiency) or donor (decreases efficiency) concentration has a transition from 10¹⁶ cm⁻³ to higher concentration of 10¹⁸ cm⁻³ or more. Also, effect of variable electron affinity and band gap of MoS₂ has been discussed from band alignment perspective. Generally, MoS₂ layer with lower electron affinity and band gap is preferred to induce desirable band alignment and subsequently result in higher efficiency. All-in all, the formation of p-type MoS₂ in CZTS solar cells can be tuned to improve the cell performance mainly by doping with higher acceptor doping concentration and limiting layer thickness. However, the detrimental effect of n-MoS₂ can be prevented by maintaining thinner layer in the vicinity of ∼30 nm with low to moderate donor doping (<10¹⁶ cm⁻³).