Investigation of interfacial charge-carrier dynamics, degradation, and recombination mechanisms in single-junction perovskite solar cells with NiOx and SAM hole-transporting layers via steady-state drift-diffusion model simulations

Abstract

We investigate the stability and the degradation pathways in single-junction perovskite solar cells with four varying hole-transporting layers (HTLs): pure nickel oxide (NiO_x) and copper-doped (NiO_x:Cu) with or without self-assembled monolayer (SAM) surface passivation. The cells are aged in a continuous MPP-tracking set-up in a nitrogen environment at 25○C and the JV curves prior and after the aging are fitted via drift-diffusion simulations. By using a set of experimentally-measured input parameters and correlating the results from the experiments to the simulations, we are able to test the reliability of the model and then extract important information about the interfacial charge-carrier dynamics, recombination, and degradation mechanisms in the solar cells. We find that NiO_x induces severe electron trapping and poor band alignment at the NiO_x-perovskite interface, thereby leading to the highest quasi-Fermi level splitting to open-circuit voltage (QFLS-V_oc) offset among all HTLs. As the cells age, the density of bulk traps when NiO_x, NiO_x:Cu, and NiO_x:Cu + SAM are used increases by a factor of 36, 3, and 8, respectively, while for NiO_x + SAM it remains unchanged. For all of the HTLs, the non-radiative Shockley-Read-Hall (SRH) recombination via surface traps is the dominant recombination mechanism as it is around 2-3 orders of magnitude higher than the direct or bulk-SRH recombination pathway. Additionally, NiO_x exhibits an around 2 orders of magnitude higher rate of SRH interfacial recombination compared to the other three HTLs. However, as the cells age, the rate of the interface SRH recombination remains relatively stable, but the bulk SRH recombination increases by an order of magnitude in all cells, indicating that the degradation of the cells is directly proportional to the increase of the trap-assisted recombination in the perovskite bulk and its degradation. Finally, we investigate the correlation between the hysteresis factor (HF) and the ion concentration. We find that the devices with NiO_x have the highest HF and the highest negative-ion concentration, in good agreement with the finding of electron trapping and the highest trap-assisted recombination rate for the NiO_x samples. Combining all of this information, we can explain why NiO_x is the least stable HTL among all HTLs (15% loss in the initial PCE) and how its stability can be improved with Cu doping (8% loss in the initial PCE) and up to an extent, with SAM passivation (around 11% loss in the initial PCE).