Top-Interface Molecular Engineering with Carbazoles Mitigates Phase Segregation in Mixed-Halide Perovskite Solar Cells

Photoinduced phase segregation remains a major roadblock to the long-term stability of photovoltaic devices employing mixed-halide perovskite absorbers. To address this issue, we use carbazole derivatives, often referred to as “self-assembled monolayers” (SAMs) with phosphonic acid as anchoring groups, typically used as hole transport materials at the buried interface, as surface treatments. Beyond their affinity for metal oxides, the phosphonic acid anchoring groups in these SAMs bind strongly to undercoordinated Pb 2+ ions. 2PACz and MeO-2PACz exhibit distinct functionalities due to the energy alignment between the perovskite valence band maximum and the carbazole HOMO levels. While 2PACz passivates surface defects as evidenced by the increase in PL emission, only MeO-2PACz suppresses phase segregation and I 2 expulsion via combined effects of defect passivation and hole extraction. By incorporating “SAMs” between the mixed-halide perovskite film and the spiro-OMeTAD hole transport layer, we fabricated photovoltaic devices with improved open-circuit voltage and stability under continuous illumination in open-circuit conditions.