Evaluating Materials Design Parameters of Hole-Selective Contacts for Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cell efficiencies are limited by parasitic absorption from the hydrogenated amorphous silicon (a-Si:H) front contact, but this may be mitigated by selecting an alternative carrier selective contact material with a wider band gap. When choosing such a material as the hole-selective contact (p-layer), the alignment of the material's valence band edge energy (E-VB) with that of crystalline silicon (c-Si) is an important criterion, but several other material parameters can also influence the band bending at the contact interface. In this article, we simulate an (n)c-Si/(i)a-Si:H/p-layer interface to explore the influence of six materials parameters in a variable p-layer on the SHJ performance. We find a strong influence on the fill factor (FF) from thickness, doping, and E-VB, and on V-OC from the interfacial defect density; notably, optimal E-VB is similar to 0.1 eV higher than the valence band edge energy of a-Si:H. Multiparameter sensitivity analyses demonstrate how performance is simultaneously influenced by E-VB and doping; thus, both parameters should be optimized alongside one another. To assess the influence of these parameters experimentally, we grow p-type NiOx as a test-case p-layer, which shows that FFs decrease with the oxygen content likely from the increased misalignment of E-VB. Although modest efficiencies are achieved experimentally (>7%), what is important is that our model simulates performance trends. With these results, we apply a materials discovery pipeline to suggest new materials (e.g., ZnTe and BeTe) to try as p-layers in the SHJ. This combination of simulations, experiments, and materials discovery informs a better understanding of contact selection in SHJ cells.

Automated summary

In this study, simulation of the (n)c-Si/(i)a-Si:H/p-layer interface explored the influence of six material parameters on SHJ performance. The findings show significant impact on fill factor (FF) from thickness, doping, and E-VB, with optimal E-VB aligning about 0.1 eV higher than a-Si:H's valence band edge energy. Sensitivity analyses highlight the importance of optimizing both E-VB and doping simultaneously for improved performance.