Enhanced crystallinity of CH ₃ NH ₃ PbI ₃ by the pre-coordination of PbI ₂ –DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells

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.