Preparation of organic bulk heterojunction photovoltaic cells by evaporative spray deposition from ultradilute solution

An organic bulk heterojunction photovoltaic cell (OPC) having a thin layer consisting of a functionalized fullerene and a poly-$p$ -phenylenevinylene (PPV) derivative was prepared from a highly diluted tetrahydrofuran solution at the concentration below 1 ppm by the evaporative spray deposition from ultradilute solution (ESDUS) technique. The power conversion efficiency of the OPC prepared from 1:1 mixture solution of fullerene/PPV derivative at the concentration of 1 ppm was 0.69%, which was almost comparable to that prepared by the conventional spin coating technique from 0.5 wt % chlorobenzene solution of the mixture. Although the bulk heterojunction OPCs are known to show relatively high power conversion efficiency, they have two major problems for development of higher device performance. One is a solubility problem of used materials and the other is a strong dependence of the device performance on the preparation solvent. The former has been a stumbling block to improve the performance by examining various combinations of organic materials. The latter has compelled to use relatively toxic solvents such as a chlorobenzene to prepare the devices. Those problems could be solved by the ESDUS technique to fabricate an organic film of almost insoluble materials.