The Non-Innocent Role of Hole-Transporting Materials in Perovskite Solar Cells

The race to the future generation of low-cost photovoltaic devices continuously takes on added momentum with the appearance of novel practical solutions for the fabrication of perovskite solar cells (PSCs), a paradigm technology for ultracheap light-to-electricity conversion. Much has been done in the past few years toward defining standard protocols for the assessment of their efficiency and stability, aiming at achieving a worldwide consensus on the issue, that will allow reliable reporting of new data. While this is undoubtedly a step ahead toward commercialization of these devices, it also often triggers researchers to test record architectures using benchmark configurations, mainly for what regards the ancillary layers that extract electrical charges from the photoexcited perovskite. In particular, the mostly used hole-transporting material (HTM) is the small-molecule spiro-OMeTAD, which is also well known to be the origin of PSC degradation after prolonged operation. Herein, it is aimed to remark the huge impact of the HTM on PSC performance, recalling major issues associated with the conventional spiro-based one and providing an overview of state-of-the-art alternatives. Finally, possible scenarios for the future development of smart HTMs are also envisioned, as charge-extracting layers, with a real active role in ensuring PSC operational stability