TOP-HTM: New Hole Transporting Materials for Perovskite Solar Cells with High Device Stability

　Perovskite solar cells (PSCs), in which lead halide perovskite is used as a light absorber, have received increasing attention since Miyasaka’s first report in 2009.¹⁾ In recent years, the practical development of these PSCs has been conducted around the world because of their high power conversion efficiency (PCE) and the capability to fabricate them on a large scale by solution processes.
　Carrier transport materials are key for achieving high PCE and high device stability in PSCs. Spiro-OMeTAD is most widely used as a standard hole transporting material (HTM) in conventional structure PSCs. However, Spiro-OMeTAD is too expensive for the mass production of PSCs. In addition, some additives such as LiTFSI, TBP, and Co(III) complexes, which are necessary to increase PCEs in Spiro-OMeTAD-based PSCs, may promote deterioration of the resulting devices. Therefore, cost-effective and additive-free HTMs are required for the practical development of PSCs. Given these issues, TOP-HTM-α1 (1) and TOP-HTM-α2 (2) have been developed as new HTMs.²⁾ They are more cost-effective than conventional HTMs and work without any additives. The CH₃NH₃PbI₃ PSCs based on 1 and 2 gave high PCEs of 15.0% and 16.6%, respectively. Furthermore, these devices exhibited higher stability than devices based on Spiro-OMeTAD under continuous 1 sun illumination (Figure 1).