Conjugated polyelectrolytes as promising hole transport materials for inverted perovskite solar cells: effect of ionic groups

Conjugated polyelectrolytes (CPEs) have developed as promising hole transport materials for perovskite solar cells (PSCs). The conjugated backbone serves as an efficient vehicle for transporting holes, and the electric dipole layer formed through the organization of ionic groups on CPEs may improve the hole collection efficiency. In this work, three CPEs anchored with -N(CH3)(3)(+), -SO3- and -NH3+ ions, denoted as BF-NMe3, BF-SO3 and BF-NH3, respectively, were synthesized and applied as the hole transport material (HTM) of inverted planar PSCs. Replacing the benchmark material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), with the three CPEs as the HTM considerably improves the photovoltaic performance of PSCs. The results from scanning electron microscope imaging, X-ray diffraction and time-resolved photoluminescence indicate that the structure of the ionic species rather than the type of charge has a decisive impact on the perovskite morphology. Both cationic BF-NH3 and anionic BF-SO3 layers enable methylammonium lead iodide (MAPbI(3)) to grow into larger crystals and grains with fewer defects. Moreover, the electrochemical impedance spectroscopy measurements demonstrate that the BF-NH3 and BF-SO3 devices have comparable charge recombination resistance, which is apparently higher than that of the BF-NMe3 and PEDOT:PSS devices. Consequently, the cationic BF-NH3 can act as an excellent HTM as the anionic BF-SO3 and the champion cell based on BF-NH3 exhibits a superior power conversion efficiency of 17.7%.