In the past few years, the power conversion efficiencies (PCEs) of PSCs have been rapidly boosted from the initial 3.8% in 2009 to over 23%. Such a large increase in PCEs is inseparable from the excellent hole transport property and the effective suppression of charge recombination of hole transport material (HTM) in PSCs. At present, the most commonly used HTMs are 2,2',7,7'-Tetrakis-(N,N-di-4-methoxy- phenylamino)-9,9'-spiro bifluorene (Sprio-OMeTAD) and poly (triarylamine) (PTAA). However, because of the complicated synthetic routes of the Sprio-OMeTAD and PTAA, their synthetic costs are prohibitively high, limiting their large-scale application in future commercialization. Therefore, it is highly appreciated to develop or search for low-cost and highly efficient alternatives to Spiro-OMeTAD and PTAA, aiming at reaching state-of-the-art photovoltaic performance. Phenothiazine 5,5-dioxide (PDO) is a heterocyclic conjugated unit, which can be easily obtained by one step oxidation from phenothiazine (PTZ) (6$/kg). The 3-, 7- and N- position of PDO monomer can be easily tailored, providing easy access to a wide library of HTMs. Oxidizing PTZ to PDO would also have significant impacts on molecular conformation, which will in turn affect HTM stacking behavior in film state and charge carrier transport properties. Furthermore, the conversion of electron-donating sulfur atom to electron-withdrawing sulfone group would change the charge affinity of the core unit. To the best of our knowledge, the HTM incorporating PDO core unit has not been reported. The possible impacts of sulfone group on charge carrier transport property and photovoltaic performance are very intriguing to us and thus motivated our studies.

Based on these concerns, herein, we report two facilely synthesized PDO core building block based HTMs . PDO has a symmetrical nonplanar structure. By substituting its 3- and 7-positions with 4,4'-dimethoxydiphenylamine, and substituting the N-position with anisole or 4,4'-dimethoxytriphenylamine as the peripheral groups afforded HTMs PDO1 and PDO2, respectively. The introduction of two 4,4'-dimethoxydiphenylamine groups on PDO unit are supposed to adjust the energy level and charge carrier mobility, while the N-substitutions are to tune the spatial configuration of HTMs. The specific impact of sulfone group and different N-substituent groups on the material properties and photovoltaic performance were systematically studied. These two PDO-based HTMs show high hole mobility, high conductivity and suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Through optimization, the PSCs employing PDO2 as HTM show PCEs up to 20.2%, and good stability when aged in ambient condition in the dark for 500 h.