Fermi Level and Light Driven Defect Generation in Silicon Solar Cells
Hydrogenated amorphous silicon (a-Si:H) has had a long standing role as a passivating dielectric for c-Si, often utilized in the early development of ICs and more recently for Si solar cells. Although it has been studied for more than 60 years, several questions about the material properties remain open, including light-induced degradation and the Fermi level dependence on the mobility of hydrogen. Here we study the origin of these phenomenon using electronic structure based calculations. First, we use density functional theory (DFT) and the nudged elastic band (NEB) method to examine defect generation via Si-H bond breaking in p-type, intrinsic, and n-type a-Si:H. We find that the energy barrier controlling this defect generation, shows the same asymmetric reduction of $\sim 0.3$ eV for p-type and $\sim 0.1$ eV for n-type, observed in experimental studies. We then develop a model based on the local Coulomb interactions at the transition state, which provides compelling evidence that the asymmetry results from the emergence of a high energy donor state created by the interstitial H. Finally, we repeat our Si-H bond breaking analysis, combining NEB with constrained density functional perturbation theory (c-DFPT) to simulate defect generation dynamics in illuminated a-Si:H. Here we find that e-p pair results in a combined effect that reduces the barrier by $\sim 0.4$, in close agreement with experimental observations.