Hole-selective and impedance characteristics of an aqueous solution-processable MoO₃ layer for solution-processable organic semiconducting devices

We herein report an investigation of aqueous solution-processable molybdenum-oxide (MoO₃) hole-selective layers fabricated for solution-processable organic semiconducting devices. A homogeneous MoO₃ layer was successfully deposited via spin-coating using aqueous solutions of ammonium heptamolybdate as a MoO₃ precursor. The use of the solution-processable MoO₃ layer as a hole-injecting layer (HIL) on an indium-tin-oxide (ITO) anode in solution-processable organic light-emitting diodes (OLEDs) resulted in excellent device performance in terms of the brightness (maximum brightness of 37,000 cd m⁻²) and the efficiency (peak efficiency of 25.2 cd A⁻¹), comparable to or better than those of a reference OLED with a conventional poly(ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) HIL. Such good device performance is attributed to the water-processable MoO₃ hole-selective layers, which allowed the formation of a high-quality film and provided good matching of the energy levels between adjacent layers with improved hole-injecting properties, impedance characteristics, and stability. Furthermore, polymer solar cells (PSCs) with a MoO₃ layer used as a hole-collecting layer (HCL) showed improved power conversion efficiency (3.81%), which was higher than that obtained using the PEDOT:PSS HCL. These results clearly indicate the benefits of using a water-processable MoO₃ layer, which effectively acts as a hole-selective layer on an ITO anode and provides good hole-injection/collection, electron-blocking and energy-level-matching properties, and improved stability. They, therefore, offer considerable promise as an alternative to a conventional PEDOT:PSS layer in the production of high-performance solution-processable organic semiconducting devices.