Light-induced effects on Spiro-OMeTAD films and hybrid lead halide perovskite solar cells

Perovskite (PS) solar cells have emerged as a promising technology for low-cost and efficient photovoltaics. However, the main limiting factors are related to their low long-term stability. The loss of performance of these devices is commonly attributed to degradation of the PS itself. In order to further explore the origins of such limited stability, we have developed UV–vis spectroscopy studies that clearly demonstrate that PS layers remain unalterable under the working conditions. On the contrary, significant variations in the absorption spectra of the Spiro-OMeTAD layer (Spiro), a commonly used hole transporting material (HTM), indicate that the loss of performance over the irradiation time are attributed to the degradation of Spiro. UV–vis measurements evidence a photo-induced oxidation of Spiro, both under air and inert atmosphere. This photo-oxidation is accelerated by the commonly employed additives of the Spiro (LiTFSI and tBP), as well as by the interfacial contact with the electron injecting layer (TiO2). Therefore, our results point to the Spiro degradation as the main mechanism that induces the limited functioning of the PS solar cells. Consequently, focusing on the development of alternative HTMs stable under the working conditions is one of the critical issues to be overcome for a suitable progress of PS solar cells and related opto-electronic devices.