Bifunctional Molecular Bridges for Enhanced Efficiency and Stability in Simplified Carbon-Based Perovskite Solar Cells: A Combined Experimental and Theoretical Study

Planar hole-transporter-free carbon-based perovskite solar cells (p-CPSCs) have recently attracted considerable interest due to their simplified architecture, energy-efficient fabrication, and use of cost-effective carbon electrodes. However, the absence of mesoporous scaffolds─such as mesoporous TiO2 and ZrO2─intensifies interfacial charge recombination, thereby limiting device performance compared to their mesoscopic counterparts. To address this challenge, we systematically engineer the interface between the compact TiO2 electron transport layer (ETL) and the perovskite absorber in p-CPSCs by introducing bifunctional organic molecules─cinnamic acid (Cin), p-coumaric acid (Cou), and caffeic acid (Caf)─as molecular bridges. The chemical anchoring of these molecules to the TiO2 surface via their carboxyl (−COOH) groups appears to be associated with improved surface crystallinity. The presence of a single hydroxyl (−OH) group in Cou facilitates uniform perovskite film formation, while the dihydroxyl configuration of Caf promotes intermolecular aggregation, resulting in Caf molecular misalignment and inferior perovskite film quality. Density functional theory (DFT) calculations indicate that Cin and Cou favor efficient electron transfer from the perovskite to TiO2, whereas aggregated Caf introduces localized trap states that impede charge injection. Consequently, devices incorporating the TiO2–Cou ETL exhibit the best performance, achieving an open-circuit voltage (Voc) exceeding 1.0 V and a power conversion efficiency (PCE) of 15.37%. Additionally, TiO2–Cou-based devices demonstrate enhanced stability, retaining over 80% of their initial PCE after 1200 h of storage under high-humidity conditions (∼70% RH at 30 °C). These improvements are attributed to a significant suppression of trap states both within the perovskite layer and at the TiO2–Cou/perovskite interface.