The role of front-surface charges in interdigitated back contact silicon heterojunction solar cells

Silicon (Si) heterojunction (HJ) solar cells (SCs) with interdigitated back contacts (IBCs) have drawn increasing attention due to their high efficiencies. Top surface passivation on this type of SCs is extremely important because of the need for the photogenerated carriers to be collected by the rear electrodes with minimal losses. Here, we propose that the built-in fixed charges in the front side dielectric passivation thin film (DPTF) can ideally provide excellent field-effect passivation suitable for high efficiency IBC SCs. The photoelectric mechanisms due to the polarity and density (Qf up to ±1013 cm−2) of the front side fixed charges, and interface defect density (Dit up to 1012 eV−1cm−2), as well as their overall effects on cell efficiency, were investigated via numerical simulations. It was revealed that the front-surface charges effectively promote carrier transport inside the bulk Si and suppress recombination losses at the hole transport layer/c-Si, electron transport layer/c-Si and rear gap/c-Si interfaces. Once the condition |Qf| > 5 × 1012 cm−2 is met, irrespective of the surface charge polarity, an efficiency greater than 22% can be achieved with high tolerance to interface defects. On the other hand, the negative polarity of Qf will yield superior performance only in cases of moderate or poor quality passivation of the rear interfaces, and for thinner c-Si SCs. We believe that our results provide significant guidance for the screening of dielectric passivation materials in the development of high efficiency IBC-HJ SCs.