Work function and interface control of amorphous IZO electrodes by MoO₃ layer grading for organic solar cells

To control the work function and the interface of transparent anodes for use in organic solar cells (OSCs), we fabricated an amorphous Zn-doped In₂O₃ (IZO) films with graded MoO₃ top layers by using a graded sputtering technique. Electrical, optical, structural, and morphological properties of the MoO₃ graded IZO films (MIZO) were investigated in detail by studying MIZO films with various thickness of the MoO₃ graded layer. Graded sputtering of the MoO₃ layer on the top region of IZO films led to the high work function of 5.23 eV for the amorphous IZO, which was higher than that of the MoO₃/IZO double layer, even though MIZO had resistivity similar to that of the IZO single layer. Due to the high work function and high transparency of the MIZO films, OSCs based on the MIZO anode exhibited a power conversion efficiency (PCE) of 3.2%, greater than that of OSCs based on IZO single layer and MoO₃/IZO double layer anodes. Based on Kelvin probe measurements and transmission electron microscope examinations, we suggested a possible hole extraction mechanism at the interfaces between the MIZO anode and the PEDOT:PSS buffer layer to explain the higher PCE of OSCs based on MIZO anodes. The successful operation of OSCs on graded MIZO indicated that the graded sputtering technique is a promising coating process allowing modification of surface properties of amorphous IZO anodes without requiring additional solution coating processes.