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

• Characteristics of MoO3-graded IZO films were investigated for organic solar cells.
• The effect of MoO3 graded layer on work function of IZO films was investigated.
• Interface between PEDOT:PSS and MoO3 graded IZO films in OSCs.

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.

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