Spiro-Bifluorene-Cored Dopant-Free Conjugated Polymeric Hole-Transporting Materials Containing Passivation Parts for Inverted Perovskite Solar Cells

Two spiro-bifluorene-based dopant-free HTMs (X22 and X23) have been synthesized by facilely condensing spiro-bifluorene diamine with 3,4-ethylenedioxythiophene (EDOT)-5,7-dicarbonyl dichloride and 2,3,5,6-tetrafluoro-terephthaloyl dichloride, respectively. In the X22 molecule, lone pairs of electrons on the sulfur (S) and oxygen (O) functional groups interact with the perovskite materials. The hole mobility (μ_h) of X22 (3.9 × 10^-4 cm² V^-1 S^1-) is more than twice that of X23 (1.4 × 10^-4 cm² V^-1 S^1-). The conductivity (σ₀) of X22 is 2.73 × 10^-4 S cm^-1, which is also higher than that of X23 (2.39 × 10^-4 S cm^-1). The EDOT moiety benefits the contact angle of CH₃NH₃PbI₃ precursor solutions on HTMs as low as 24°. The X22-based device with an indium-doped tin oxide/hole transport material (HTM)/CH₃NH₃PbI₃/phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)/bathocuproine/Ag structure achieves a power conversion efficiency (PCE) of 19.18%. The PCE of the device based on X23 containing fluorine is 18.70%, and the contact angle between HTM and the perovskite precursor solution is 32°. The X22- and X23-based devices at ambient temperature (≈25 °C) in N₂ retain 86% and 79% of the initial PCE after 150 days. The effect of S, O, and F heteroatoms plays an important role in the side chain modification of HTMs, improving defect passivation in HTM/CH₃NH₃PbI₃ interfaces by multiple functional groups.