A Copper Coordination Polymer with Matching Energy Level for Modifying Hole Transport Layers to Improve the Performance of Perovskite Solar Cells.

Spiro-OMeTAD is currently the most widely used hole transport material for the preparation of high-performance perovskite solar cells (PSCs), usually requiring the addition of additives to achieve the desired electronic conductivity. However, the quality of the film is degraded owing to the addition of additives. Holes and defects can be observed, and the dispersion of the additives are uneven inside. Here, a copper coordination polymer, Cu-bix, with matching energy level and fluorescent properties was screened for use as an additional additive to dope Spiro-OMeTAD. The doping of Cu-bix effectively improved the dispersion state of the additives in the hole transport layers and alleviated the aggregation of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) or/and lithium salt complexes in the film. Owing to better dispersion of the additives, Spiro-OMeTAD was more fully and uniformly oxidized whereas the possibility of charge recombination was reduced in the devices. Furthermore, the flat and tightly bonded film layer obtained by optimization of the doping amount can efficiently transfer holes from the perovskite layers to the hole transport layers. Possible interaction mechanisms between additives and the copper coordination polymer are proposed and discussed. The resulting power conversion efficiency (PCE) for Cu-bix-doped PSCs was improved from 16.52 % to 18.47 % compared to the pristine devices, and this type of PSCs also showed a long stability in air owing to the increased hydrophobicity of the Cu-bix-based hole transport layers.