High efficiency organic solar cells based on amorphous electron-donating polymer and modified fullerene acceptor

The current manuscript examines the structure and property relationship of amorphous conjugated polymer based bulk heterojunction solar cells. Solvent vapor annealing (SVA) is used to optimize the morphology. It is seen that the structure order of the blended thin film does not change but phase separation becomes prominent. The space charge limited current and admittance spectroscopy reveals that the charge transport ability of the SVA-treated device is significantly strengthened. Furthermore, both the current-voltage analysis and impedance spectroscopy illustrates that SVA can effectively promote the fill factor (FF) and short circuit current density (J(sc)) due to the enhanced charge transport properties. However, the increased mobility after the SVA treatment causes non-geminate charge carrier recombination and reduces geminate recombinations. In particular, the reduction in open circuit voltage (V-oc) of the solvent vapor annealed devices is found to originate from the charge recombination induced by the quasi-Fermi level variation between the donor and acceptor materials.