Effects of oxidation state and crystallinity of tungsten oxide interlayer on photovoltaic property in bulk hetero-junction solar cell

Tungsten oxide thin films with thickness of ∼30 nm are prepared from ammonium tungstate solution to be used as a hole-selective interlayer in the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)-based bulk heterojunction solar cell. The prepared tungsten oxide films are confirmed to be n-type semiconductor, as observed by Hall measurement. Photovoltaic performance is investigated in terms of tungsten oxide film annealing condition. The annealing temperatures at 150 and 300°C produce amorphous phase, whereas crystalline phase is formed at 400°C. At annealing temperature of 150°C, the annealing in vacuum shows the conversion efficiency of 0.71%, whereas the annealing in air exhibits two times higher efficiency of 1.42%. X-ray photoelectron spectroscopic (XPS) analysis confirms that only W ⁶⁺ is presented under air annealing condition, whereas W ⁵⁺ appears under vacuum-annealing condition. Shunt resistance is decreased, and series resistance is increased upon annealing in vacuum. Under air annealing condition, efficiency is further improved from 1.42 to 2.01% as temperature changes from 150 to 300°C due to a removal of the chemisorbed water and in part a complete conversion of ammonium tungstate to tungsten trioxide. Crystalline phase is formed at 400°C, where photovoltaic performance is declined to 1.27% due to the lowered shunt resistance. Hole injection efficiency is evaluated based on the ultraviolet photoelectron spectroscopy (UPS) studies. Except for the 150°C-vacuum-annealed WO _3-x, work functions (I) for the air-annealed tungsten oxide films lie in between the work function of ITO (4.66 eV) and the HOMO level (5.1 eV) of P3HT, which leads to better hole transport through the air-annealed WO ₃ interlayer than the vacuum-annealed one. Among the air-annealed samples, work function and conduction band minimum for the 300°C-annealed one are most properly positioned for hole transportation, associated with highest photovoltaic performance.