Improving the Performance and Stability of Inverted Planar Flexible Perovskite Solar Cells Employing a Novel NDI-Based Polymer as the Electron Transport Layer

A new naphthalene diimide (NDI)-based polymer with strong electron withdrawing dicyanothiophene (P(NDI2DT-TTCN)) is developed as the electron transport layer (ETL) in place of the fullerene-based ETL in inverted perovskite solar cells (Pero-SCs). A combination of characterization techniques, including atomic force microscopy, scanning electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, space-charge-limited current, electrochemical impedance spectroscopy, photoluminescence (PL), and time-resolved PL decay, is used to demonstrate the interface phenomena between perovskite and P(NDI2DTTTCN) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). It is found that P(NDI2DT-TTCN) not only improves the electron extraction ability but also prevents ambient condition interference by forming a hydrophobic ETL surface. In addition, P( NDI2DT-TTCN) has excellent mechanical stability compared to PCBM in flexible Pero-SCs. With these improved functionalities, the performance of devices based on P(NDI2DT-TTCN) significantly outperform those based on PCBM from 14.3 to 17.0%, which is the highest photovoltaic performance with negligible hysteresis in the field of polymeric ETLs.