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 CH 3 NH 3 PbI 3 has received considerable attention for highly-efficient perovskite solar cells. However, the different solubility of PbI 2 and CH 3 NH 3 I is problematic, initiating active solvent engineering research using dimethyl sulfoxide (DMSO). Here we investigated the pre-coordination of PbI 2 -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 PbI 2 -DMSO x ( x = 0, 1, or 2) in the powder mixture was investigated by gradually increasing mechanical mixing time, and a dominant composition of PbI 2 -DMSO 1 was obtained after a 10 min mixing process. The pre-coordinated PbI 2 -DMSO powders were then blended with CH 3 NH 3 I in DMF to make the CH 3 NH 3 PbI 3 film by toluene-assisted spin-coating and heat treatment. Compared with the one-step blending of CH 3 NH 3 I, PbI 2 , and DMSO in DMF, the pre-coordination method resulted in better dissolution of PbI 2 , 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 PbI 2 -DMSO powders was very effective in enhancing the crystallinity of CH 3 NH 3 PbI 3 and photovoltaic performance.