Understanding the Degradation Mechanism of Spiro-OMeTAD during the Long-Term Operation of Perovskite Solar Cells and Mitigating it via ALD.

Metal halide perovskites (MHPs) have emerged as promising photovoltaic (PV) materials with a power conversion efficiency (PCE) of 26% on the laboratory scale. Despite achieving high performance in less than a decade, device degradation under operating conditions is outcompeted in the PV market by silicon and CdTe technologies. To overcome this challenge, interface passivation with organic molecules and composition engineering are commonly explored to improve the device stability. While most studies have focused on the intrinsic stability of the perovskite active layer, there is a lack of understanding of the degradation of other charge-extraction layers and electrodes, and its effect on the final device performance upon aging.<br/>In this work, triple-cation-based PSCs with a planar n-i-p architecture and a champion PCE of 20.1% were aged under maximum power point tracking (MPPT) for 1000 min under 1 sun illumination without any encapsulation in an ambient atmosphere. After aging, we performed soft Ar atom cluster sputtering XPS depth profiling to study the ionic motion/interactions between the perovskite active layer and the overlying charge extraction layer. In addition to iodine migration from the perovskite to the Spiro-OMeTAD/electrode interface, we observed a shift in the binding energies (BE) of the constituent species present in Spiro-OMeTAD with p-dopants lithium bistrifluoromethanesulfonimidate (LiTFSI) and 4-tertbutylpyridine (tBP).<br/>When looking specifically at the core level spectra of S 2p centered at 169.3 eV are present uniformly from the Spiro/electrode interface to the Spiro/perovskite interface in the fresh PSC. Upon aging, the S 2s exhibit a lower binding energy 168.3 eV attributing to the reduction of sulfur present in the Spiro layer. Notably, the F 1s spectrum also showed an extra peak at 685.5 eV for the aged sample from 689.0 eV for the fresh sample. Additionally, the F/S ratio changed after MPPT aging, leading to a final change in the charge extraction properties of the Spiro-OMeTAD layer, leading to the final device degradation. We observed that this chemical redistribution created pinholes in the films, leading to easier penetration of H₂O and O₂, and perovskite instability. Applying near-ambient pressure XPS (NAP-XPS) on the perovskite film with and without the spiro-OMeTAD layer at 1 mbar of H₂O at 50 ^°C, 100 ^°C, and 150 ^°C showed the formation of metallic lead (Pb⁰) and iodine due to the degradation of Spiro-OMeTAD, accelerating the decomposition of the perovskite. Interestingly, our XRD results confirm that the interaction between the degraded Spiro-OMeTAD and perovskite leads to the amorphization of perovskite, which is different from what is commonly observed for PbI₂. The cross-sectional SEM image of the aged PSCs highlights the spiro/perovskite interface as the breaking point under constant illumination.<br/>Finally, to mitigate the instability caused by the degradation of Spiro-OMeTAD, robust passivation of ALD-deposited AlOx of &lt;1 nm has proven to be an effective strategy for the long-term stability of PSCs. We examined the conformity of this sub-nanometer layer via HRTEM and observed the complete coverage of a rough perovskite layer (RMS ± 20 nm) by AlOx, which is difficult to obtain by the chemical deposition of organic passivation layers. In-situ electron diffraction also showed that the grown ALD layer was amorphous and acted not only as a surface passivation layer for perovskite but also as a barrier layer to prevent the degradation of perovskite as a result of the decomposition of Spiro-OMeTAD.<br/>Overall, our work provides chemical and mechanical insights into the degradation of the overlying layer on perovskite by combining macroscopic, microscopic, and spectroscopic studies. We also decoupled the role of ALD as a passivation layer for perovskite intrinsic stability and as a nanoencapsulant layer for mitigating the phase transition to amorphous phases upon Spiro-OMeTAD degradation.