Enhancing Stability and Performance in Perovskite Solar Modules—Addressing Au/Perovskite Interface

In the perovskite module, the upper and lower electrodes are interconnected via P2 scribes. This configuration results in direct contact between Au and the perovskite layer without the intervention of a hole transport layer (HTL), potentially promoting ion migration or chemical interactions between Au and the perovskite. Therefore, it is crucial to characterize and analyze this aspect to control degradation, particularly in regions with geometric freatures such as wrinkles where Au and perovskite contact may occur.<br/>According to simulations conducted by W. Ming et al. the diffusion activation energy of Au interstitial ions within MAPbI3 is approximately 0.42 eV, which is relatively low. Interstitial defects do not inherently form deep levels but can readily become trapped within MA or Pb vacancies, thereby creating deep-level substitutional defects. While the occurrence of MA vacancies can vary, studies indicate the generation of volatile MA gas when the perovskite is irradiated with visible light under ambient conditions.<br/>Considering these factors, it was anticipated that exposing perovskite cells and modules to light soaking under ambient conditions would lead to perovskite degradation, internal diffusion of Au, and potential trapping within MA vacancies, subsequently causing non-radiative recombination and impairing long-term stability.<br/>To elucidate the degradation mechanism at the Au/perovskite interface, structural and chemical changes in cells and modules were measured pre- and post-light soaking. X-ray diffraction (XRD) was utilized to assess structural changes. Comparison of XRD peaks of perovskite before and after light soaking revealed the emergence of an intermediate phase (11.7°) and a PbI2 peak (26.3°) post-soaking. X-ray photoelectron spectroscopy (XPS) analysis of perovskite films without Au and Spiro-OMeTAD layers, post-removal, showed increased binding energy for Pb4f and I3d5 peaks after light soaking, indicating the formation of strongly bound PbI2. Additionally, fine Au peaks were detected on the perovskite surface, suggesting that light soaking induced perovskite decomposition, the creation of MA vacancies, and Au trapping at these sites, potentially acting as recombination centers and significantly affecting long-term stability.<br/>In this study, not only were the pathways for Au diffusion into the perovskite matrix identified, but it was also confirmed that Au acts as a non-radiative recombination center upon diffusing into the perovskite. Electroluminescence (EL) was employed as a technique to observe recombination patterns within devices by applying bias, where radiative recombination emits light, resulting in brightness, while non-radiative recombination appears darker. By comparing EL images of modules before and after light soaking, it was observed that pre-soaking, there was a difference in EL intensity between wrinkled and non-wrinkled areas. However, post-soaking, a significant decrease in EL intensity was noted at locations where wrinkles were present, as well as areas where Au and perovskite directly contacted near the P2 scribe. Furthermore, analysis of secondary ion mass spectrometry (SIMS) data revealed that post-light soaking, Au diffusion near the P2 region occurred significantly deeper in the depth direction compared to the reference point, indicating substantial Au diffusion and confirming the mobility of other ions.<br/>In our follow-up research, we propose inserting a diffusion barrier between the top electrode and the perovskite to prevent ion migration. Candidate materials include metal oxides that prevent diffusion without degrading hole transport. Modules with the diffusion barrier are expected to maintain stability by preventing ion migration while retaining high efficiency