A Furan-Substituted Polymeric Hole-Transporting Material for Energy Level Regulation and Less Planarity in Colloidal Quantum Dot Solar Cells

For efficient colloidal quantum dot (CQD) solar cells (CQD-SCs), thiol-passivated p-type CQDs are generally used as a hole-transporting material (HTM); however, there are issues with the control of optoelectrical properties, low thiol passivation rate, and poor morphology with a power conversion efficiency (PCE) of approximately 11%. Although polymeric HTMs have been introduced to address these issues, maximizing efficiency and achieving green-solvent processability and thermal stability for commercialization is necessary. Here, we synthesize a novel benzodifuran (BDF)-based HTM (asy-ranPBTBDF) showing an electron-deficient state, low steric hindrance, and low planarity compared to those of a typical benzodithiophene (BDT)-based HTM (asy-ranPBTBDT). BDF properties lead to deep high occupied molecular orbital (HOMO) levels, close π–π stacking, excellent solubility, and amorphous properties related to efficiency, green-solvent processability, and thermal stability. With these benefits, the asy-ranPBTBDF-based CQD-SC showed enhanced open-circuit voltage (V_OC) (0.65 V) and PCE (13.29%) compared to those of the asy-ranPBTBDT-based device (0.63 V and 12.22%) in toxic processes with chlorobenzene. The asy-ranPBTBDF-based CQD-SC showed a PCE of 12.51% in a green-solvent process with 2-methylanisole and improved thermal stability at 80 °C (83.8% retaining after 24 h) owing to less lateral crystallization than the asy-ranPBTBDT-based device (60.8% retaining after 24 h).