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 cellsdepends on its work function value. MoO3-x with a higher work function value has superior selectivityand facilitates the flow of holes through it. The oxidation stateof the Mo atoms and the oxygen vacancy affect the work function ofMoO(3-x ). Here, for the first time,thermally evaporated MoO3-x filmsare subjected to oxygen (O-2) plasma treatment using a plasma-enhancedchemical vapor deposition method to tune the work function. The effectof O-2 plasma treatment on work function is studied usingKelvin probe force microscopy. The work function of thick MoO3-x films increased from 4.91 & PLUSMN;0.01 eV for as-deposited films to 5.22 & PLUSMN; 0.02 eV by proper tuningof rf power, oxygen flow rate, and O-2 plasma treatmenttime. This increase in work function is accompanied with the increasein O/Mo ratio in these films as confirmed by EDX. Oxygen plasma treatmenthas 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 filmsby & SIM;0.40 eV is observed for 5 min plasma treatment at 80 Wrf power with a 30 SCCM oxygen flow rate. The studies suggest oxygenplasma treatment as an effective approach to recover or tune the workfunction of molybdenum oxide films.