Enhanced crystallinity of CH3NH3PbI3 by the pre-coordination of PbI2–DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells

Abstract

Solution processable CH₃NH₃PbI₃ has received considerable attention for highly-efficient perovskite solar cells. However, the different solubility of PbI₂ and CH₃NH₃I is problematic, initiating active solvent engineering research using dimethyl sulfoxide (DMSO). Here we investigated the pre-coordination of PbI₂–DMSO powders for planar heterojunction perovskite solar cells fabricated by a low-temperature process (≤100 °C). Pre-coordination was carried out by simple mechanical mixing using a mortar and pestle. The composition of PbI₂–DMSO_x (x = 0, 1, or 2) in the powder mixture was investigated by gradually increasing mechanical mixing time, and a dominant composition of PbI₂–DMSO₁ was obtained after a 10 min mixing process. The pre-coordinated PbI₂–DMSO powders were then blended with CH₃NH₃I in DMF to make the CH₃NH₃PbI₃ film by toluene-assisted spin-coating and heat treatment. Compared with the one-step blending of CH₃NH₃I, PbI₂, and DMSO in DMF, the pre-coordination method resulted in better dissolution of PbI₂, larger grain size, and pinhole-free morphology. Consequently, absorption, fluorescence, carrier lifetime, and charge extraction were enhanced. The average open-circuit voltage (1.046 V), short-circuit current (22.9 mA cm⁻²), fill factor (73.5%), and power conversion efficiency (17.6%) were increased by 2–12% with decreased standard deviations (13–50%), compared with the one-step blending method. The best efficiency was 18.2%. The simple mechanical pre-coordination of PbI₂–DMSO powders was very effective in enhancing the crystallinity of CH₃NH₃PbI₃ and photovoltaic performance.