Forming a Metal-Free Oxidatively Coupled Agent, Bicarbazole, as a Defect Passivation for HTM and an Interfacial Layer in a p–i–n Perovskite Solar Cell Exhibits Nearly 20% Efficiency

In this study, we synthesized three simple and inexpensive (34–120 USD/g) 3,3′-bicarbazole-based hole transporting materials (BC-HTMs; NP-BC, NBP-BC and PNP-BC) through a metal-free oxidative coupling, in excellent yields (≥95%). These bicarbazoles contain phenylene or biphenylene substituents on the carbazole N atom, with extended π-conjugation achieved through phenylene units at the 6,6′-positions of the bicarbazole. When using NBP-BC as a dopant-free HTM in a p–i–n perovskite solar cell (PSC), we achieved a power conversion efficiency (PCEs) of 12.22 ± 0.54% under AM 1.5G conditions (100 mW cm^–2); this PCE was comparable to that obtained when using PEDOT:PSS as the HTM (11.23 ± 1.02%). BC-HTMs showed the large grain size (μm) of perovskite than PEDOT:PSS-based, due to defect passivation on indium tin oxide (ITO) substrate and good hydrophobicity. Furthermore, we realized highly efficient and stable PSCs when using the p–i–n device structure ITO/NiO_x/NP-BC/perovskite/PC₆₁BM/BCP/Ag. The bifacial defect passivation effect of the interfacial layer improved the grain size of the perovskite layer and also enhanced the performance; the best performance of the NiO_x/NP-BC device was characterized by a short-circuit current density (J_sc) of 22.38 mA cm^–2, an open-circuit voltage (V_oc) of 1.09 V, and a fill factor (FF) of 79.9%, corresponding to an overall PCE of almost 20%. This device structure has competitive potential because its performance is comparable to that of the record-high-efficiency PSCs. Under an Ar atmosphere, the PCE of the NiO_x/NP-BC PSC device decayed by only 4.55% after 168 h; it retained 90.80% of its original PCE after 1000 h. A morphological study revealed that the films of the BC-HTMs were indeed smooth and hydrophobic and that the perovskite films spin-coated upon them were uniform and featured large grains (micrometer scale). Time-resolved photoluminescence (TRPL) spectra of the perovskite films suggested that the hole extraction capabilities of the NiO_x/BC-HTMs were better than that of the bare NiO_x. The superior film morphologies of the NiO_x/BC-HTMs were responsible for the performances of their devices being comparable to those of bare NiO_x-based PSCs.