Stability and degradation in triple cation and methyl ammonium lead iodide perovskite solar cells mediated via Au and Ag electrodes

Perovskite solar cells (PSCs), particularly based on the methyl ammonium lead iodide (MAPbI(3)) formulation, have been of intense interest for the past decade within the photovoltaics (PV) community, given the stupendous rise in power conversion efficiencies (PCEs) attributed to these perovskite formulations, where PCEs have exceeded 25%. However, their long-term stability under operational conditions and environmental storage are still prime challenges to be overcome towards their commercialization. Although studies on the intrinsic perovskite absorber stability have been conducted previously, there are no clear mechanisms for the interaction of electrode-induced absorber degradation pathways, which is the focus of this study. In this report, we have conducted a comprehensive analysis on the impact of the electrode collector layer, specifically Ag and Au, on the degradation mechanism associated with the MAPbI(3) and a triple cation absorber, Cs(0.05)FA(0.79)MA(0.16)PbI(2.45)Br(0.55). Notably, Au-based PSCs for both absorbers in an n-i-p architecture showed superior PCE over Ag-based PSCs, where the optimized PCE of MAPbI(3) and triple cation-based PSCs was 15.39% and 18.21%, respectively. On the other hand, optimized PCE of MAPbI(3) and triple cation-based PSCs with Ag electrodes was 3.02% and 16.44%, respectively. In addition, the Ag-based PSCs showed a rapid decrease in PCE over Au-based PSCs through operational stability measurements. We hypothesize the mechanism of degradation, arising from the Ag interaction with the absorber through the formation of AgI in the PSCs, leads to corrosion of the perovskite absorber, as opposed to the benign AuI when Au electrodes are used in the solar cell stack. Additionally, novel use of photoluminescence spectroscopy (PL) here, allowed us to access key features of the perovskite absorber in situ, while it was in contact with the various layers within the n-i-p solar cell stack. A quenching in the PL peak in the case of Ag-contacted MAPbI(3) provided direct evidence of the Ag corrupting the optical properties of the absorber through the formation of AgI which our X-ray diffraction (XRD) results confirmed. This was supported by the fact that an emission peak was still present in the triple cation Ag-device. For the Au-contacted MAPbI(3) the presence of a well-defined PL peak, though attenuated from the triple cation Au-device, suggested the AuI does not quell the emission spectrum for either the triple cation or the MAPbI(3) absorber. The findings should aid in the understanding and design of new electrode materials with PSCs, which will help accelerate their introduction into the commercial sector in the future.