Conjugate cyclic cations engineering for high-performance 2D/3D heterojunction perovskite solar cells

•The crystallization kinetics of 2D perovskites on the 3D perovskite were regulated.•The ThFA-based 2D perovskite showed uniform distribution and orientational growth.•The PCE of the optimized device can be increased from 20.98% to 23.28%.•The 2D/3D perovskite device can maintain excellent humidity and thermal stability. Widely studied 2D/3D heterojunction structure built inside perovskite are applied into optoelectronic field due to the strong tunability and superior stability. In this work, conjugate formamidinium-based cations containing benzene and thiophene rings (benzammonium BeFA, thiophene-2-carboximidammonium ThFA) are employed to construct high-quality 2D/3D heterojunction perovskite films. Different configurations of spatial organic cations will affect the tilt degree of [PbI6]4- lattice octahedron in 2D perovskite layer, thus affecting the growth of 2D perovskite layer. According to the density functional theory (DFT) calculations, the ThFA cations obviously establish suitable binding energy for devices, which makes the films exhibit uniform distribution and induces the preferred growth of 2D perovskite. The results show that the nonradiative recombination in the 2D/3D heterojunction perovskite device is significantly hindered, which inhibits the generation of leakage current. Moreover, 2D/3D heterojunction perovskites achieve better energy level matching, which is conducive to efficient charge extraction. The optimized ThFA devices achieve the promoted power conversion efficiency of 23.28% with enhanced open-circuit voltage of 1.18 V. In addition, the modified layer can indeed inhibit the interlayer migration of small molecules and stabilize the perovskite lattice, thus contributing to the excellent humidity and thermal stability. Our work focuses on investigating the connection behind the configuration of the space cations and the performance of devices based on 2D/3D heterojunction perovskites. Besides, it provides a basis for effective regulation strategies for crystallization kinetics of 2D perovskite thin films.