Titanium dioxide electron-selective interlayers created by chemical vapor deposition for inverted configuration organic solar cells

Abstract

We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO₂ interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P₃HT/PC₆₁BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large areas, and may be a viable non-contact alternative to electron-selective interlayer formation. Oxide-based electron-selective interlayers, such as TiO₂, need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N₂ gas stream, the CVD process provides nanometer control of film thickness to produce 12–36 nm thickness device-quality films. The best performing CVD films, processed at substrate temperatures of ca. 210 °C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO₂. Solution electrochemistries (voltammetry) of probe molecules were shown to be easily accessible indicators of film porosity and are predictive for electron harvesting selectivity (and hole-blocking) in an inverted configuration OPV platform. Small molecules whose redox potentials placed them energetically above the conduction band edge energy (E_CB) were reduced/oxidized at nearly the same rates as for bare ITO. Probe molecules whose redox potentials place them energetically within the band gap region, below E_CB, show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO₂ films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (R_P) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO₂/P₃HT:PC₆₁BM/MoO₃/Ag, using TiO₂ films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (J_SC), open-circuit photopotential (V_OC), and fill-factor (FF), versus bare ITO. OPVs using amorphous, conformal 24 nm TiO₂ interlayers showed the highest fill factors, lowest R_S, highest R_P and power conversion efficiencies of ca. 3.7%.