Effect of NiO x thin layer fabricated by oxygen-plasma treatment on polymer photovoltaic cell

Abstract

The performance of polymer photovoltaic (PV) cells based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61 butyric-acid methyl-ester (PCBM) bulk heterojunctions with an NiO x anode buffer layer was investigated. NiO x layers with various thicknesses (i.e., without a layer, 0.5, 1, 2, and 5 nm), were fabricated by oxygen plasma treatment on an evaporated metallic Ni layer under 200 W and 100 sccm for 200 s. The power conversion efficiency (PCE) of the polymer PV cell exhibited a similar trend to the short-circuit current density ( J sc) and fill factor (FF) but not to open circuit voltage ( V oc) for various NiO x thicknesses. This tendency is associated with the fact that the light absorption and series resistance of the polymer PV cell depends on the thickness of the NiO x layer. The PCE of the cell with the optimized 1-nm-thick NiO x layer was improved by 68.5% compared to that of the cell without an NiO x layer. With a combination of different techniques of analysis, such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), the fabricated NiO x layer was studied in detail. The NiO x layer between the ITO anode and active layer strongly influenced the PCE of polymer PV cells, which was accounted for by hole-injection and electron-blocking properties.