Humidity-driven degradation of sputtered molybdenum oxide and molybdenum-titanium-oxide thin films

Molybdenum oxide (MoO 3 ) has become a popular material in its implementation as a hole-selective layer in organic light emitting diodes and solar cells, by virtue of its favourable optical and electronic properties. However, care must be taken concerning the stability of MoO 3 against water, especially for layers that are amorphous, with a considerable amount of oxygen vacancies. The present study investigates the degradation of sputtered molybdenum oxide-based thin films when exposed to controlled and elevated humidity. The investigation is mainly based on infrared spectroscopy analysis, supported by atomic force and scanning electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Detrimental modifications are observed in amorphous MoO 3 films due to the adsorption of water and hydrolysis. These modifications depend strongly on the humidity level and even lead to the film's crystallization under specific conditions. In the following, a stable alternative to MoO 3 is presented in the form of a mixed molybdenum-titanium-oxide (MTO), which was previously shown to maintain the favourable optical and electronic properties of MoO 3 . The spectroscopic analysis demonstrates that the water adsorption and subsequent hydrolysis is dramatically reduced in MTO, preserving a compact layer over the observed time period of 30 days at elevated humidity. IR measurements reveal the enhanced hydrolysis resistance of molybdenum titanium oxide compared to molybdenum oxide when exposed to controlled humidity conditions.