Oxygen Plasma Treatment of Thermally Evaporated MoO3-x Films: An Approach to Tune the Work Function

The hole selectivity of molybdenum oxide (MoO3-x) in organic and inorganic heterojunction solar cells depends on its work function value. MoO3-x with a higher work function value has superior selectivity and facilitates the flow of holes through it. The oxidation state of the Mo atoms and the oxygen vacancy affect the work function of MoO3-x. Here, for the first time, thermally evaporated MoO3-x films are subjected to oxygen (O2) plasma treatment using a plasma-enhanced chemical vapor deposition method to tune the work function. The effect of O2 plasma treatment on work function is studied using Kelvin probe force microscopy. The work function of thick MoO3-x films increased from 4.91 ± 0.01 eV for as-deposited films to 5.22 ± 0.02 eV by proper tuning of rf power, oxygen flow rate, and O2 plasma treatment time. This increase in work function is accompanied with the increase in O/Mo ratio in these films as confirmed by EDX. Oxygen plasma treatment has also resulted in the enhancement of work function, transmittance, and band gap of thin (23 and 14 nm) MoO3-x films. An optimum increase in work function for thin films by ∼0.40 eV is observed for 5 min plasma treatment at 80 W rf power with a 30 SCCM oxygen flow rate. The studies suggest oxygen plasma treatment as an effective approach to recover or tune the work function of molybdenum oxide films.