Covalent organic frameworks for modulating crystallization kinetics in perovskite photovoltaics

Covalent organic frameworks (COFs), recognized for their conjugated frameworks, adjustable porosity, and customizable functionalization, are emerging as versatile materials in perovskite photovoltaics with the potential to enhance both device performance and stability. However, a comprehensive understanding of their precise influence on the intricate phenomenon of perovskite crystallization kinetics is still lacking. In this study, we have successfully synthesized two distinctive COF materials, namely HIAM-0001 and HIAM-0004, utilizing the structural units of 5′,5″″-(benzo[c] [1,2,5]thiadiazole-4,7-diyl)bis(([1,1′:3′,1″-terphenyl]-4,4″-dicarbaldehyde)) (BT-TDA), with either the additional unit p-xylylenedicyanide (PDAN) or 2,2′-([2,2′-bipyridine]-5,5′-diyl) diacetonitrile (BPyDAN). By incorporating them into the PbI₂ layer, we have facilitated high-quality perovskite film fabrication through a two-step process. The COF-assisted PbI₂ films exhibited a distinct porous structure, facilitating organic salt solution permeation and reducing PbI₂ residues. Notably, in-situ UV–vis absorption characterization revealed slowed perovskite film crystallization kinetics with COF assistance, leading to the formation of larger crystal grains, fewer grain boundaries, reduced defect density, and suppressed non-radiative recombination, ultimately resulting in improved device performance. In comparison to HIAM-0001, the enhanced π-π interactions of HIAM-0004, with its bipyridine-based unit, exhibited more pronounced interactions with the perovskite, contributing to a remarkable PCE of 24.06% and excellent device stability. This study underscores the pivotal role of COF modification in shaping perovskite film crystallization kinetics, thereby enhancing film quality, reducing defects, and boosting device stability.