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

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) TiO ₂ 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-TiO ₂/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%.