Lewis acid-base adduct-type organic hole transport material for high performance and air-stable perovskite solar cells

Highlights: • A new class of Lewis acid-base adduct-type hole transport material (HTM) is reported. • It exhibited an impressive performance in perovskite solar cells with outstanding long-term durability. • The average PCE was 34% higher than that of the state-of-the-art HTM, without any hygroscopic additives/dopants. • The MAPbI₃/HTM interface became more selective for holes due to Lewis acid-base adduct formation of pyridine in HTM and BCF. • The mobility and conductivity were enhanced and the film was uniform with smooth morphology and improved hydrophobicity. -- Abstract: Since hole transport materials (HTMs) play a significant role in enhancing the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs), which are the key factors for their commercialization, an effective design strategy is necessary for the potential HTMs in the current emerging field of PSCs. Here, we present a new class of HTM with pyridine as a central core with an extended π-conjugated molecular structure with electron-donating blocks. We have systematically investigated its photophysical, thermal, electrochemical, and charge transport properties and found that 4,4′-(5,5′-(pyridine-2,6-diylbis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(N,N-bis(4-methoxyphenyl)aniline) (PyThTPA) is a potential HTM candidate for making PSCs. The PyThTPA HTM-based PSC attained an average PCE of 16.57% with outstanding long-term durability of over 720 hrs with minimal reduction of its initial PCE and negligible hysteresis. This PSC performance was 34% higher than that of the state-of-the-art HTM, Spiro-OMeTAD with tris(pentafluorophenyl)borane (BCF). We speculate that the Lewis acid-base adduct (LABA) formation of pyridine in the HTM and BCF interacted with methylammonium lead iodide (MAPbI₃), resulting in the MAPbI₃/HTM interface becoming more selective for holes. This also enhanced the film uniformity and afforded a smoother morphology with improved hydrophobicity that further increased the long-term durability. Furthermore, the mobility and conductivity were increased for PyThTPA with BCF. To the best of our knowledge, this is the first report of pyridine being incorporated into the HTM with continuous π-conjugation and with a high performance of nearly 17%. Overall, we believe that this approach will be an effective design strategy capable of enhancing the performance of PSCs with less hysteresis and improved long-term durability.