Highly transparent and conductive MoO3/Ag/MoO3 multilayer films via air annealing of the MoO3 layer for ITO-free organic solar cells

Abstract

Air annealing of MoO3 bottom layer is a simple and highly efficient surface treatment method to prepare ultrathin, continuous, and smooth Ag layers in MoO3/Ag/MoO3 (MAM) transparent conductive films (TCFs) for indium tin oxide (ITO)-free organic solar cells (OSCs). Air annealing can readily oxidize MoO3 surface and increase surface energy of the MoO3 layer, which significantly enhances the adhesion between Ag and MoO3. When the MoO3 bottom layer is annealed at 200 °C, moreover, the treatment yields a flatter MoO3 surface, which positively influences the growth of Ag nanostructures. Eventually, an almost homogeneous, smooth, and continuous surface morphology of the ultrathin Ag films with a thickness of 6.5 nm was obtained. By employing air-annealed MoO3 bottom layer, the MoO3 (200 °C, 10 min)/Ag (6.5 nm)/MoO3 TCF had higher optical transmittance (Tav = 83.6%) and lower sheet resistance (Rs = 6.5 Ω/sq) than unannealed MAM or commercial ITO TCFs, thereby obtaining the higher value of figure of merit (FTC = 2.6 ×10−2 Ω−1). When used as the transparent electrode in organic solar cells (OSCs), the improved optoelectrical properties of the MoO3 (200 °C, 10 min)/Ag (6.5 nm)/MoO3 electrode result in increased short-circuit current density (Jsc = 2.27 mA cm−2) and fill factor (FF = 54.8%) of OSCs in comparison to devices based on unannealed MAM and ITO electrodes, leading to an improvement in power conversion efficiency (PCE = 0.62%). From the results of photocurrent measurements, air annealing MoO3 increases exciton generation in active layers and reduces the carrier back-transfer induced recombination at the anode contact, yielding the enhanced photovoltaic performance.