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

  • Author(s): Woods-Robinson, R
  • Fioretti, AN
  • Haschke, J
  • Boccard, M
  • Persson, KA
  • Ballif, C
  • et al.
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.

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