Inverted organic photovoltaic (OPV) cells containing various concentration of Ga-doped ZnO (GZO) as an electron buffer layer (EBL) were deposited by using a solution-process to enhance their power conversion efficiency (PCE). Atomic force microscopy (AFM) images showed that the measured root mean square roughnesses of 0, 5, and 10% GZO EBL were approximately 11.2, 2.3, and 4.6 nm, respectively, and that the 5% GZO EBL have very smooth surface morphology. The optical transmittance of the 5% GZO thin films in the wavelength range between 300 and 800 nm is 88% higher than those of GZO EBLs. X-ray photoelectron spectroscopy (XPS) measurement was investigated to confirm the valence state of Zn, Ga, and O in the GZO EBL. Current density-voltage (J-V) results showed the highest PCE of the device of the fabricated inverted OPV cells utilizing 5% GZO with 2.61%. The enhancement in the PCE of inverted OPV cells were attributed to an enhancement of the surface smoothness and the modification of the work function of the GZO EBL.

Keywords: Organic photovoltaics; Inverted structure; Electron buffer layer; Ga-doped ZnO; Solution-process