Improved Power Conversion Efficiency of P3HT:PCBM Organic Solar Cells by Strong Spin-Orbit Coupling-Induced Delayed Fluorescence

Solution-processed organic bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl-C$_{60}$-butyric acid methyl ester are doped with different concentrations of iron (II,III) oxide nanoparticles (Fe$_{3}$O$_{4}$). The power conversion efficiency of the devices doped at low concentrations is improved up to 11%. The improvement finds its origin in a lower recombination current, which is a consequence of an increased effective exciton lifetime according to the J–V characteristics and the optoelectronical analysis of the films. The increase in performance cannot be attributed to changes in morphology or crystallinity according to grazing-incidence X-ray scattering experiments. The evolution of the solar cell short-circuit current at low doping concentrations is related to variations in the arrangement of the crystalline regions of P3HT. For high doping concentrations (above 1.0 wt%) the performance of the solar cell decays rapidly, ascribed to the increased leakage currents in the device caused by the presence of nanoparticles