Undoped MoOX with oxygen-rich vacancies as hole transport material for efficient indoor/outdoor organic solar cells

Abstract

Molybdenum oxide (MoOX) films are typically prepared using a thermally-evaporated MoO3 source (ev.MoO3) as the hole transport layer for inverted organic solar cells (OSCs). However, their low conductivity and interface element diffusion severely limit their efficiency and stability. Herein, a novel and effective thermal evaporation method of the MoO2 source to prepare undoped MoOX films with oxygen-rich vacancies and inhibitory atom diffusion properties electrode using ev.MoO2 as the source, is reported. Electron spin resonance analysis indicates that ev.MoO2 has a stronger oxygen vacancy signal peak than ev.MoO3, which contributes to its excellent conductivity. The inverted organic solar cells modified by ev.MoO2 yielded a significant improvement in short-circuit current density (JSC) and fill factor (FF). The power conversion efficiency (PCE) increased from 16.65 % in ev.MoO3 to 17.08 % in ev.MoO2 under AM 1.5 G, which is one of the highest values ever reported for inverted OSCs. Furthermore, owing to the high transmittance and low trap density, the PCE increased from 20.18 % with ev.MoO3 OSC to 22.13 % with ev.MoO2 OSC under 500 lux. In addition, scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed that ev.MoO2 can prevent the diffusion of Al from the electrode to the functional layers, resulting in a T80 exceeding 800 h under continuous illumination.