Cascade charge transfer enabled by incorporating edge-enriched graphene nanoribbons for mesostructured perovskite solar cells with enhanced performance

The ability to facilitate charge transfer in electron transport layer (ETL) is highly desirable for high-efficiency perovskite solar cells (PSCs). Herein, we report the judicious design of ETL composed of edge-enriched graphene nanoribbons (GNRs) and TiO₂ nanocrystals to enable markedly improved charge carrier conductivity and, more importantly, well-aligned energy levels at the perovskite/ETL/fluorine doped tin oxide (FTO) interfaces for imparting the cascade charge transfer, thereby leading to improved power conversion efficiency (PCE) of PSCs. The investigation into the influence of the GNR content within ETL on device performance demonstrated that excessive GNRs are detrimental for charge transfer as they block the contact between TiO₂ and perovskite. In contrast to devices using pure TiO₂ as ETL with a PCE of 15.87%, the PSCs employing GNRs-incorporated ETL display the improved photovoltaic performance with a highest PCE of 17.69%, a steady-state efficiency output of 17.05%, and a reduced current-voltage hysteresis. Subsequently, a systematic photo-carrier dynamics study revealed that the performance enhancement was a direct consequence of (a) the promoted fill factor and open circuit voltage due to the improved electron diffusion coefficient and the longer charge recombination time, (b) the increased charge collection efficiency owing to the cascade charge transfer from TiO₂ to GNRs to FTO electrode, resulting in a higher external quantum efficiency and thus a larger short circuit current density and (c) the suppressed current-voltage hysteresis ascribed to the increased charge transfer noted above. Finally, a markedly improved long-term stability was manifested. As such, the rational incorporation of edge-enriched GNRs represents a feasible and robust strategy to formulate promising ETLs for high-efficiency and stable PSCs.