Modelling and Simulation of Field-effect Surface Passivation of Crystalline Silicon-based Solar Cells

An effective surface passivation plays a vital role in the performance of crystalline silicon (c-Si) solar cells. Experimental research shows that fixed charge-induced field-effect passivates the c-Si efficiently. In this work, n-type and p-type c-Si wafers symmetrically passivated by the negative-charge dielectric Al2O3 were numerically modelled with SENTAURUS TCAD. Surface recombination is traditionally modelled by employing the extended Shockley-Read-Hall (SRH) model with single energy level of interface traps. However, experiments show interface traps distribute across the silicon bandgap. Thus, we implemented the extended SRH model with the ability to describe arbitrary energy distribution of interface traps within the bandgap. The extended SRH model predicts a constant effective surface recombination velocity at low injection levels for lightly doped n-type and p-type c-Si passivated by dielectrics with either a high negative or positive charge density. However, this prediction contradicts experimental results which show a significant reduction of the effective lifetime at low injection levels if the polarity of the fixed charge is attracting the minority bulk charge carriers. One explanation is assuming the presence of a thin defect-rich layer close to the c-Si surface in which the lifetimes are degraded. However, the choice of the lifetime and depth of such damaged surface region seem rather arbitrary and its physical origin is still unclear. We show that unequal electron and hole bulk lifetime parameters can also account for the phenomenon. This proposition is physically plausible and the simulation results also show a good agreement with the experimental data from both n-type and p-type c-Si wafers passivated by Al2O3. Our modelling results predict a simple but unambiguous experiment to distinguish between these two explanations.