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Hybrid tin halides with formamidinium cations become a promising candidate for eco‐friendly solar cells, which achieves the best photovoltaic performance among various lead‐free perovskite solar cells (PSCs). Unfortunately, the low quality of perovskite layers and the oxidation of Sn2+ ions degrade the performance of devices, leading to an obviously lower power conversion efficiency (PCE) compare to hybrid lead halides. Additive engineering is a facile strategy for improving the quality of perovskite layers. In this work, the 2‐aminopyrazine (APZ) additive is investigated for depositing FA0.8PEA0.2SnI2.8Br0.2 layers, and corresponding PSCs are examined. It is found that the melting point of APZ additives drops below the annealing temperature of perovskite layers by the intermolecular interaction with formamidinium iodide (FAI). The melt phase at the grain boundaries encourages the diffusion of ions in the annealing process, leading to an improved quality of perovskite layers. And the formation of APZ‐SnI2 complex also inhibits the oxidation of Sn2+ ions. Consequently, the defect density and the recombination are significantly reduced in PSCs, resulting in an increase of PCEs from 11.4% to 13.0% and a simultaneous improvement of device stability. This work demonstrates a strategy for improving tin‐based PSCs based on melt assisted annealing and coordination interaction. [ABSTRACT FROM AUTHOR]

For efficient perovskite solar cells (PSCs), hole transport layers (HTLs) are necessary components that can suppress recombination by collecting holes and blocking electrons. Inorganic hole transport materials (HTMs) are attractive because of their ease of preparation, low production cost and high stability. Although metal oxides and halides have been developed as the HTMs, metal hydroxides have not yet been reported as the HTMs of PSCs. In this work, copper doped lanthanum hydroxide (La x Cu y (OH) 3) nanorods were firstly synthesized by the hydrothermal method as a new kind of HTM for PSCs. Although the valence band edge of La(OH) 3 is slightly mismatch with that of perovskite layers, Cu2+ dopants improve the energy level alignment at HTL/perovskite interfaces. As an acceptor dopant for La(OH) 3 , Cu2+ ions also increase the hole transport ability of La x Cu y (OH) 3 HTLs. Additionally, La x Cu y (OH) 3 HTLs tend to encourage the crystallization of perovskite layers. As a consequence, Cu2+ dopants lead to an increase of PCE from 11.3 % to 20.4 % for La x Cu y (OH) 3 based PSCs. This promotion can be related to the reduced recombination process and the elevated charge extraction process. This work firstly applied metal hydroxide as the HTMs of PSCs, and demonstrates the regulation of these HTMs by the doping method. [Display omitted] • La(OH) 3 nanorods with copper dopants (La x Cu y (OH) 3) were synthesized. • La x Cu y (OH) 3 films were deposited by spin-coating at a low temperature of 100 °C. • Energy level and hole transport ability were improved by incorporating Cu2+ ions. • The Cu2+ dopants can also promote the crystallization of perovskite layers. • Cu2+ dopants lead to an improvement of PCEs from 11.3 % to 20.4 % for PSCs. [ABSTRACT FROM AUTHOR]