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

Abstract

Molybdenum oxide (MoO₃) 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 MoO₃ 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 MoO₃ 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 MoO₃ 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 MoO₃. 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.