A Cross‐Linkable Electron‐Transport Layer Based on a Fullerene−Benzoxazine Derivative for Inverted Polymer Solar Cells

The synthesis, optoelectronic characterization and device properties of a cross‐linkable fullerene derivative, [6,6]‐phenyl‐C61‐butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]‐phenyl‐C61‐butyric methyl ester (PCBM). Thermal cross‐linking of the benzoxazine moiety in PCBB resulted in the formation of cross‐linked, solvent resistive adhesive films (C−PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT‐PCBM polymer blend film upon incorporation of C−PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C−PCBB/P3HT‐PCBM/V2O5/Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT‐PCBM/V2O5/Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C‐PCBB on P3HT/C‐PCBB and PCBM/C‐PCBB heterojunctions. Fullerene films: Heat‐triggered ring‐opening polymerisation of a cross‐linkable fullerene derivative (PCBB) functionalized with a benzoxazine moiety resulted in crosslinked, solvent‐resistive adhesive film (C‐PCBB). Inverted bulk heterojunction solar cells with cross‐linked PCBB as electron transport layer showed 25 % enhancement in the power conversion efficiency compared to a reference device without C‐PCBB.