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

Abstract

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 ( $\boldsymbol{E_{\text{VB}}}$ ) 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 $\boldsymbol{E_{\text{VB}}}$ , and on $V_{\text{OC}}$ from the interfacial defect density; notably, optimal $\boldsymbol{E_{\text{VB}}}$ is $\sim$ 0.1 eV higher than the valence band edge energy of a-Si:H. Multiparameter sensitivity analyses demonstrate how performance is simultaneously influenced by $\boldsymbol{E_{\text{VB}}}$ and doping; thus, both parameters should be optimized alongside one another. To assess the influence of these parameters experimentally, we grow p-type $\text{NiO}_{x}$ as a test-case p-layer, which shows that FFs decrease with the oxygen content likely from the increased misalignment of $\boldsymbol{E_{\text{VB}}}$ . 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.