Dopant-free hole transporting polymeric materials based on pyrroloindacenodithiophene donor unit for efficient perovskite solar cells

The hole transporting materials (HTMs) have been extensively studied since the HTMs play a critical role in promoting charge extraction/transport and reducing charge recombination between the perovskite layer and the top metal electrode, thus improving the device power conversion efficiency (PCE). In this study, novel polymeric HTMs with pentacyclic pyrroloindacenodithiophene (NIDT) as an electron-rich unit, and electron-deficient benzotriazole (BTA) as an acceptor, labelled as SGT-512 and SGT-513, were designed and synthesized. To regulate the electron withdrawing ability of the acceptor part, two thiophene units were introduced to the BTA moiety as an acceptor, yielding SGT-513 with a slightly raised HOMO energy level. The alkyl side-chain was introduced to the fused BTA unit to improve the solubility and regulate the morphology and stacking behavior of HTM. Meanwhile, for the donor part, the replacement of the bridging heteroatom in indacenodithiophene with the more electron donating nitrogen atom, helped to promote molecular orbital hybridization leading to a narrower bandgap polymer. The combination of the features of the NIDT donor and BTA acceptor enabled the production of the D-A type polymeric HTM SGT-512 with a high intrinsic hole mobility of 3.8 x 10-4 cm2V- 1s- 1, a suitable HOMO energy level (-5.25 eV) corresponding to the valence band of the perovskite layer, and excellent film-formation ability. Therefore, the PrSCs based on dopant-free SGT-512 showed a high PCE of 15.57%, which is much higher than that of the control device based on dopant free-P3HT, with a PCE of 13.52%.