Additive-Free Oxidized Spiro-MeOTAD Hole Transport Layer Significantly Improves Thermal Solar Cell Stability

Achieving thermal stability of perovskite solar cells is a significant challenge. While <i>pin</i>-perovskite devices demonstrate high power conversion efficiency and thermal stability, they face issues with the C60 electron transport layer, hindering commercialization. Therefore, it is advisable not to limit perovskite solar cell research to one device architecture. The <i>nip</i>-perovskite solar cell architecture, which typically uses Spiro-MeOTAD with additives as a hole transport layer, shows promising power conversion efficiencies. However, the device's stability at elevated temperatures is a concern. It is generally believed that additives necessary for enhancing electrical conductivity and optimizing energy level alignment cause reduced stability—suggesting that Spiro-MeOTAD-based hole transporting layers are inherently unstable. This study presents a reliable noble metal-free synthesis of Spiro-MeOTAD(TFSI)₄, which is used as the oxidizing agent. Without the need for additives, the stability of the Spiro-MeOTAD-based hole transporting layers improves significantly, offering a potential solution to the stability issue in perovskite solar cells.<br/>The electrical conductivity is essentially developed with the first 1% of oxidation. Further oxidation shifts the energy levels away from the vacuum level, allowing tuning of the energy level alignment without additives—contradicting the current understanding of this system. Without additives, devices demonstrate efficiencies up to 24.2%. The thermal stability of the <i>nip</i>-perovskite solar cell strongly depends on the perovskite composition, demonstrating stable devices at elevated temperatures up to 85 °C under one sun over 1400 hours of continuous illumination with a power conversion efficiency of around 6% (with an unoptimized perovskite layer). Notably, the intrinsic thermal stability of the hole transport layer is demonstrated. At the same time, degradation may be activated from perovskite degradation and ion diffusion, which may be further suppressed by compositional engineering and adequate ion barrier layers.