Improvement of MoO3/Ag/MoO3 multilayer transparent electrodes for organic solar cells by using UV-ozone treated MoO3 layer

Ultraviolet ozone (UVO) treatment of molybdenum trioxide (MoO3) appears to be a simple and efficient method for obtaining highly continuous and smooth silver (Ag) thin films in the thermally evaporated MoO3/Ag/MoO3 (MAM) multilayered structure as transparent electrodes for small-molecule organic solar cells (OSCs). It is observed that UVO treatment can oxidatively modify the non-stoichiometric MoO3 (or MoO3-x)surfaces, further increasing the Mo6+/Mo5+ composition ratio and work function of MoO3-x. Importantly, the use of UVO treatment for the MoO3 bottom layer effectively improves the wettability of Ag on MoO3 and enhances the lateral growth of Ag thin film, resulting in a reduction of the percolation threshold thickness of the continuous Ag layer. Due to the formation of an ultrathin Ag interlayer with a continuous and smooth surface morphology, the MAM multilayered electrode after UVO treatment of MoO3 for 3 min has excellent optical and electrical properties, including a high maximum transmittance of 89.1% and a low sheet resistance of 8.0 Omega/sq. When the optimal UVO-treated MoO3/Ag (7.5 nm)/MoO3 films are used as the anode in OSCs with the copper phthalocyanine (CuPc)/fullerene (C-60) planar heterojunction structure, the OSCs have a power conversion efficiency of 0.55%, which is 2.0 and 1.2 times higher than that of devices with untreated MoO3/Ag (7.5 nm)/MoO3 (0.27%) and MoO3/Ag (10 nm)/MoO3 electrodes (0.46%), respectively, and competitive with that of indium-tin-oxide based devices. Because of almost full surface coverage of the Ag interlayer, UVO treatment of MoO3 in MAM multilayered electrodes can improve charge carrier injection/extraction at the anode contact and hence improve the photovoltaic performance of OSCs.