Fine-Tuned Carbazole-Based Polymers with Controlled Connectivity as Efficient Interfacial Layers in Perovskite Solar Cells

The strategic molecular design of interfacial layers presents a critical pathway to mitigate losses in perovskite photovoltaics. Here, we engineer two carbazole-based polymers with distinct connectivity patterns, 3,6-3,6-Cbz and 3,6-2,7-Cbz, to systematically investigate molecular connectivity-dependent interfacial phenomena in planar n-i-p PSCs. Comprehensive structure-property analyses reveal that the 3,6-2,7-Cbz architecture enables exceptional π-electron delocalization and optimal energy level alignment, simultaneously achieving (i) superior perovskite surface defect passivation and (ii) interfacial recombination suppression. When implemented as an interfacial modifier, the 3,6-2,7-Cbz polymer delivers a champion device efficiency of 24.16% (<i>V</i>_oc = 1.16 V, <i>J</i>_sc = 25.60 mA/cm², FF = 80.86%) with Spiro-OMeTAD as the HTL representing a significant improvement over both the control (22.61%) and 3,6-3,6-Cbz analogue (22.58%). Strikingly, this approach maintains high performance (23.17% PCE) even with undoped carbazole HTLs (Cz-Pyr), demonstrating remarkable versatility. Through correlated XPS and SCLC characterization, we establish clear structure-performance relationships between polymer connectivity and charge transport dynamics. This work provides fundamental insights into molecular topology engineering for advanced interfacial materials in next-generation photovoltaics.