Synthesis of pyrazine-based hole transport layer and its application in p-i-n planar perovskite solar cells

Abstract

Planar p-i-n perovskite solar cells (pero-SCs) with solution-processed fabrication, low cost, flexible device fabrication, and negligible hysteresis have attracted significant interest. Hole transport material (HTM) plays a crucial role in improving the performance of p-i-n planar pero-SCs by facilitating hole extraction and then reducing surface recombination. Two types of HTMs have been used in p-i-n pero-SCs. p-Type inorganic semiconductors, such as NiO, CuI, Cu2O, CuSCN, and graphene oxide, have shown good efficiency and stability; however, organic semiconductor-based HTMs (e.g., poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA), polyarylamine (poly-TPD), poly(N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di(thien-2-yl)-2,1,3-benzothiadiazole)) (PCDTBT), and triphenylamine or thiophene derivative) have outstanding processability for simple one-step solution process at low temperatures; therefore, they should be investigated further. However, their electrical properties are usually inferior than those of inorganic semiconductors, and additives are required to improve their mobility and conductivity, which complicates device processing and results in hysteresis and poor device stability. Therefore, for the organic semiconductor-based HTMs, new hole transport layer (HTL) materials should be developed with easy synthesis process, highly reproducible photovoltaic performance, and better understanding of the structure–property relationship between the HTL materials and the device performance. Herein, an X-type HTM 4,4,4’’,4’’’-(pyrazine-2,3,5,6-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) (PT-TPA) containing pyrazine as the molecular core and triphenylamine (TPA) derivative as branches was designed and synthesized. We introduce an electron-deficient para-diazine as the core and electron-donating methoxytriphenylamine as the peripheral unit to enhance the dipole moment of PT-TPA, which could induce an intermolecular charge transfer. Compared with 4,4’’,4’’,4’’’-silanetetrayltetrakis(N,N-bis(4-methoxyphenyl)aniline) (Si-OMeTPA), the pyrazine core not only endows PT-TPA with good crystallinity but also improves the charge transfer property and plane conjugation of the molecular center, which significantly enhances the hole mobility of PT-TPA. The p-i-n planar pero-SCs based on dopant-free PT-TPA HTL showed a power conversion efficiency of 17.52%, which is approximately 15% higher than that of the device with a Si-OMeTPA HTL.