Optical and microstructural characterization of MoO₃ thin films for organic solar cells applications

ABSTRACT

Organic solar cells are very attractive for solar energy conversion due to their low cost, flexibility, lightness, and potential for manufacturing via roll-to-roll process. One of the materials most used as hole transport layer in these cells is the PEDOT: PSS polymer. However, this polymer easily degrades under environmental conditions, limiting the device lifetime. In this work MoO₃ films were deposited by reactive radio frequency sputtering for being used as hole transport layer in organic solar cells. The effect of deposition parameters, such as working power and substrate temperature, was investigated. The x-ray diffraction data showed that the films deposited at 200 °C were amorphous or nanocrystalline, regardless the deposition power, except for those deposited at the lowest power, 125 W, which contained a small amount of the β-MoO₃ phase. The transmittance of these films, measured in a UV-VIS spectrophotometer, ranged from 60% to 75%. Unlike deposition power, the substrate temperature significantly affected the microstructural and optical properties of the films. The results showed that films deposited above 250 °C were crystalline and consisted of a mixture of β-MoO₃ and α-MoO₃ phases, the percentage of the α-MoO₃ phase increasing with temperature. The surface images of the films confirmed the important role of the substrate temperature on the crystallization of the films, showing the increase of crystallization with the temperature. The results of UV-Vis spectroscopy showed that the substrate temperature can be used as a parameter for bandgap tuning, with the values changing from 2.7 eV to 3.1 eV as temperature increases.

Keywords
molybdenum trioxide; optical properties; microstructure; sputtering