Passivation at the interface between liquid-phase crystallized silicon and silicon oxynitride in thin film solar cells

The passivation quality at the interface between liquid-phase crystallized silicon (LPC-Si) and a dielectric interlayer (IL) was investigated in terms of the defect state density at the IL/LPC-Si interface (D_it) as well as the effective fixed charge density in the IL (Q_IL,eff). Both parameters were obtained via high-frequency capacitance–voltage measurements on developed metal–insulator–semiconductor structures based on a molybdenum layer sandwiched between the IL and the glass substrate. D_it and Q_IL,eff were correlated to the open circuit voltage (V_oc) and the integrated external quantum efficiency (J_sc,EQE) obtained on corresponding solar cell structures as well as to V_oc and J_sc,EQE results based on two-dimensional simulations. We found that D_it was reduced by one order of magnitude using a hydrogen plasma treatment (HPT) at 400 °C. Irrespectively of the HPT, Q_IL,eff was > 10¹² cm⁻². We suggest that field-effect passivation dominates chemical passivation at the IL/n-type LPC-Si interface. We attribute the significant enhancement of V_oc and J_sc,EQE observed after HPT on n-type LPC-Si solar cells mainly to improvements of the passivation quality in the n-type LPC-Si bulk rather than at the IL/n-type LPC-Si interface. For p-type absorbers, the HPT did not improve V_oc and J_sc,EQE significantly. We propose that this is because of an insufficient passivation of bulk defects by positively charged hydrogen, which dominates in p-type silicon, in combination with an insufficient interface passivation.