An amorphous precursor route to the conformable oriented crystallization of CH3NH3PbBr3 in mesoporous scaffolds: toward efficient and thermally stable carbon-based perovskite solar cells

Abstract

CH₃NH₃PbBr₃ (MAPbBr₃)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr₃ deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr₃ in the TiO₂ porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 °C, affording full pore filling and smooth surface capping. The conversion to MAPbBr₃ in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr₃ crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction. Using MAPbBr₃ films in carbon-based PSCs has boosted their efficiency to 8.09% (V_oc = 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly, non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 °C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for 15 days under high heat stress (air, 80 °C and 50–85% humidity).