Thin air - plasma - treated alkali fluoride layers for improved hole extraction in copper phthalocyanine/C_(70) - based solar cells

摘要

Alkali fluorides, mostly LiF and CsF, are well - known to improve electron injection/ extraction in organic light - emitting diodes (OLEDs) and organic solar cells (OSCs). They are also utilized, though to a lesser extent, for hole injection in OLEDs. Here we demonstrate a new role for such fluorides in enhancing OSCs' hole extraction. We show that an ultrathin air-plasma- treated alkali fluoride layer between the indium tin oxide (ITO) anode and the active layer in copper phthalocyanine (CuPc)/C_70-based OSCs increases the short circuit current by up to ~ 17% for cells with LiF and ~7% for cells with NaF or CsF. The effects of the fluoride layer thickness and treatment duration were evaluated, as were OSCs with oxidized and plasma-treated Li and UV-ozone treated LiF. Measurements included current voltage, absorption, external quantum efficiency (EQE), atomic force microscopy, and x-ray photoelectron spectroscopy, which showed the presence of alkali atoms F and O at the treated ITO/fluoride surface. The EQE of optimized devices with LiF increased at wavelengths >560 nm, exceeding the absorption increase. Overall, the results indicate that the improved performance is due largely to enhanced hole extraction, possibly related to improved energy-level alignment at the fluori- nated ITO/CuPc interface, reduced OSC series resistance, and in the case of LiF, improved absorption.