Dopant free mixture of Spiro-OMeTAD and PTAA with tunable wettability as hole transport layer enhancing performance of inverted CsPbI2Br perovskite solar cells

Abstract To date, doped 2,2,7,7-Tetrakis(N,N-dip-methoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD) is still state-of-the-art hole transport material (HTM) for inverted perovskite solar cells (PeSCs). Nevertheless, hydroscopic dopants usually lead to accelerated degradation of perovskite and hence reduce the device durability especially those based on all-inorganic perovskite. In this work, we report a series of dopant-free mixtures (SpiPA) of hydrophilic Spiro-OMeTAD and hydrophobic poly(triarylamine) (PTAA), which exhibit tunable surface wettability due to the dominant lateral phase separation. When applying them as HTM for inverted CsPbI2Br PeSCs, SpiPA-II based devices yield enhanced efficiency up to 12.52% with significantly reduced hysteresis, as compared to the devices with other mixed HTLs (SpiPA-I, SpiPA-III, SpiPA-IV) and conventional HTLs including pristine Spiro-OMeTAD (10.26%), PTAA (9.05%) and NiOx (10.75%). Additionally, the corresponding mechanism has been studied in detail.