Linking simulation and synthesis of nickel oxide hole-selective contacts for silicon heterojunction solar cells

In silicon heterojunction (SHJ) solar cells, one factor limiting efficiency is "parasitic absorption" from amorphous silicon (a-Si) at the front surface. Thus, discovery of a transparent hole-selective contact to replace p-type a-Si is a promising area of research, but this has proven challenging due to the additional requirements of surface passivation and band alignment, among others. In this investigation, we select nickel oxide (NiOx) as a test-case hole-selective contact material, since it is p-type and has a tunable valence band (VB) minimum aligned with that of crystalline silicon. We simulate an SHJ front interface with NiOx to investigate possible performance and optimal parameters. From this analysis, we identify a trade-off between VB alignment and hole concentration in the NiOx layer: the VBM has to be very close to its optimum value for low hole concentrations, but for higher hole concentrations there is a wider range of tolerance of VB position. To test these findings, we sputter deposit amorphous NiOx hole selective contacts to fabricate a SHJ device stack, varying oxygen flow rate during growth. NiOx is likely not a great hole-selective contact material, possibly due to VB misalignment, low carrier concentration, and interfacial defects. However, NiOx has effectively served as a test-case to demonstrate the influence of varying VB position and thickness on cell performance and verify our model. By comparing simulations to experiments, we have identified ranges of tolerance for optimal material properties that can inform future materials discovery efforts for alternative transparent hole-selective contacts.