Concerted Ion Migration and Diffusion‐Induced Degradation in Lead‐Free Ag₃BiI₆ Rudorffite Solar Cells under Ambient Conditions

Abstract

Silver bismuth iodide (SBI) materials have recently gained attention as nontoxic alternatives to lead perovskites. Although most of the studies have been focusing on photovoltaic performance, the inherent ionic nature of SBI materials, their diffusive behavior, and influence on material/device stability is underexplored. Herein, AgBi2I7, Ag2BiI5, and Ag3BiI6 thin films are developed in controlled ambient humidity conditions with a decent efficiency up to 2.32%. While exploring the device stability, it is found that Ag3BiI6 exhibits a unique ion‐migration behavior where Ag+, Bi3+, and I− ions migrate and diffuse through the dopant‐free hole transport layer (HTL) leading to degradation. Interestingly, this ion‐migration behavior is relatively fast for the case of antisolvent‐processed Ag3BiI6 thin‐film‐based devices contrasting the case of without antisolvent and is not observed for other SBI material‐based devices. Theoretical calculations suggest that low decomposition enthalpy favors the decomposition of Ag3BiI6 to AgI and BiI3 causing migration of ions to the electrode which is protected by using a thick HTL . The new mechanism reported herein underlines the importance of SBI material composition and fundamental mechanism understanding on the stability of Ag3BiI6 material for better solar cell design and also in extending the applications of unique ion‐migration behavior in various optoelectronics.

Solution Processable Direct Bandgap Copper‐Silver‐Bismuth Iodide Photovoltaics: Compositional Control of Dimensionality and Optoelectronic Properties

“…Profiles of the incident photon to current conversion efficiency (IPCE) spectra (Figure 5b) were qualitatively consistent with the optical absorption data (Figure 4a and Figure S25, Supporting Information) but quantitatively different, showing significantly higher IPCE for Cu 2 AgBiI 6 within the whole wavelength range examined. The IPCE recorded for the Cu‐rich material was lower than those reported for the Ag a Bi b I a +3 b ‐PTAA‐based solar cells, [ 6–8 ] but comparable to other bismuth‐based perovskite‐inspired materials. [ 4c,16a,35 ]…”

“…It is worth noting that no admixtures of CuI or AgI were detected in all samples examined herein, contrasting the results typically obtained for Ag‐rich or Bi‐rich silver bismuth iodides. [ 5a,6a,7 ] Furthermore, unlike the case of AgBiI 4 , which demonstrates cubic ( Fd 3 m ) and CdCl 2 ‐type R ‐3 m space grouped twinned structures, crystal twinning was not observed for the Cu x AgBiI 4+ x system herein, which either contained pure ( x = 1 and x ≥ 2) or two mixed phases (1 < x < 2) (Figure 2a). Relative intensities of XRD peaks for individual phases ( x = 1 and 2) were different from those reported by Sansom et al [ 23 ] for materials synthesized by high‐temperature fusing, reflecting different crystal orientations in thin‐film materials, which are often affected by the support.…”

“…[ 5a,b,6 ] However, Kulkarni et al have demonstrated recently that the Ag + ion migration induces device degradation in the highest‐performing silver‐rich Ag a Bi b I a +3 b ( a > b ) compositions. [ 7 ] Replacing Ag + with Cu + was suggested to reduce the ion migration and associated issues, but introduced new problems, including phase instability. As a result, the overall progress in the photovoltaic performance improvements has been significantly slower for the copper‐based compounds of this class (best PCE < 1.2 % reported so far).…”

Insights into Accelerated Degradation of Perovskite Solar Cells under Continuous Illumination Driven by Thermal Stress and Interfacial Junction

“…However, the iodine diffusion through PCBM or AZO layer and corroding the Ag electrode cannot be neglected in the solar cells with halogencontaining absorber layer. 13,41,42 In the NiO x device (Figures 3 and S17), one can see a much smaller effect of HLS in the HaPSC bulk than in the PTAA Unlike in the PTAA device, the NiO x /HaP interface seems to be more robust. Although there are no obvious voids in the absorber layer, the initial stage of void formation seems to be triggered by the ingress of an intergrain elemental gradient as a consequence of heat and light stress.…”

“…Buecheler and co-workers reported a similar observation of degraded HaPSCs. However, the iodine diffusion through PCBM or AZO layer and corroding the Ag electrode cannot be neglected in the solar cells with halogen-containing absorber layer. ,, …”