Synergistic regulation of SnO2 buried interface induced by alkyl chain length in polythiourea passivator for efficient perovskite solar cell

Interface passivation is currently one of the most widely studied approaches to improve the efficiency and stability of perovskite solar cells (PSCs). However, most reported approaches overemphasize defect passivation of the upper interface but neglect the defects of the non-exposed buried interface of the perovskite/charge transfer layer, especially for those of the buried SnO2/perovskite interface, which affects charge transfer/extraction and perovskite crystallization. In this work, three polythiourea compounds with varied alkyl chain lengths [polyethylthiourea (PTU-2C), polybutylthiourea (PTU-4C), and polyoctylthiourea (PTU-8C)] were synthesized to act as buried interface passivators in SnO2 based PSCs. The structure–property relationship of polythiourea was investigated to reveal how alkyl chain length influences interface contact and passivation capability on the SnO2/perovskite interface. The relevant studies have revealed that the shorter the alkyl chain, the greater the hydrophilic SnO2 surface area and the more conducive to producing high-quality perovskite film with enlarged grain size. Meanwhile, PTU-2C, with the shortest alkyl chain, can form the strongest chelation with various types of defect sites on the buried SnO2/perovskite interface, thus accelerating charge transfer/extraction and inhibiting nonradiative recombination. Consequently, PSCs with PTU-2C modification achieve synchronous improvement in device efficiency and stability, indicating the effectiveness of the polythiourea passivation strategy.