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

The race to the future generation of low-cost photovoltaic devices continuouslytakes on added momentum with the appearance of novel practical solutions forthe fabrication of perovskite solar cells (PSCs), a paradigm technology forultracheap light-to-electricity conversion. Much has been done in the past fewyears toward defining standard protocols for the assessment of their efficiencyand stability, aiming at achieving a worldwide consensus on the issue, that willallow reliable reporting of new data. While this is undoubtedly a step aheadtoward commercialization of these devices, it also often triggers researchers totest record architectures using benchmark configurations, mainly for whatregards the ancillary layers that extract electrical charges from the photoexcitedperovskite. In particular, the mostly used hole-transporting material (HTM) is thesmall-molecule spiro-OMeTAD, which is also well known to be the origin of PSCdegradation after prolonged operation. Herein, it is aimed to remark the hugeimpact of the HTM on PSC performance, recalling major issues associated withthe conventional spiro-based one and providing an overview of state-of-the-artalternatives. Finally, possible scenarios for the future development of smartHTMs are also envisioned, as charge-extracting layers, with a real active role inensuring PSC operational stability.