Aqueous PEIE Soaking on ZnO for Ultraviolet Light Activation-Free Organic Photovoltaic Modules

Ultraviolet (UV) light is typically needed to activate inverted organic photovoltaics (OPVs) with zinc oxide (ZnO) as electron transporting layer (ETL) for higher efficiency. However, UV light is a major cause for the degradation of organic active layers in OPVs. This is a contradiction that UV light activation enhances the efficiency but UV illumination deteriorates the stability. It is important to solve this contradiction to develop UV light activation-free OPV devices. Herein, a method of aqueous polyethylenimine ethoxylated (PEIE) soaking on ZnO is reported to realize UV light activation-free OPV devices. The S-shape in current density-voltage (J-V) characteristics of devices tested without UV light activation is eliminated through the treatment of aqueous PEIE soaking on ZnO. The treatment reduces the oxygen adsorbates, which is confirmed by Kelvin probe and X-ray photoelectron spectroscopy. A 10.08 cm² organic photovoltaic module with the treated ZnO as ETL showed high photovoltaic performance: V_OC = 5.68 V, J_SC = 2.7 mA cm^-2, FF = 75.1%, and P_Output = 11.5 mW cm^-2 tested with the UV filter (light intensity of 0.788 sun). UV light activation is not needed for the modules to obtain high efficiency.