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.