Thermal Stability Analysis of Molybdenum-Oxide-Based Carrier Selective Contact Silicon Solar Cells

The thermal stability of molybdenum oxide ( ${\text{MoO}}_{x)}$ -based carrier selective contact silicon solar cells (Ag/ITO/MoO _x /n-Si/LiF _x /Al) is investigated under ambient annealing conditions for different time durations. The devices remain stable up to 150 °C for 15 and 30 min of annealing. However, at 200 °C, the cells degrade with a decrease in the fill factor and open-circuit voltage ( $V_{{\text {oc}}}$ ); the degradation rate is significant at 30 min. The illumination-dependent $V_{{\text {oc}}}$ study has indicated the charge transport barrier at the MoO _x /c-Si interface in a degraded cell. The dark current density-voltage analysis of the degraded cells has also shown charge carrier recombination and series resistance at low- and high-forward bias conditions, respectively. To identify the device degradation with the temperature, ITO/MoO _x and MoO _x /c-Si interfaces are analyzed in terms of contact resistivity ( $\rho _{c}$ ) using extended transfer length method (ETLM). After annealing at 200 °C, a significant increase in $\rho _{c}$ of the MoO _x /c-Si interface (from ~17 to $\sim 385~\text{m}\Omega $ -cm ² ) is observed in comparison to the ITO/MoO _x interface (from ~16.5 to $\sim 39~\text{m}\Omega $ -cm ² ), reflecting the crucial role of MoO _x /c-Si junction in cell degradation. The thermal degradation of the MoO _x /c-Si interface is due to a decrease in the MoO _x film's work function (WF) upon annealing, which creates a transport barrier for charge carriers. The reduction in the MoO _x WF results in misalignment of c-Si's valence band and MoO _x 's conduction band, leading to inefficient carrier transport through band-to-band tunneling, which is also confirmed from the numerical simulations.