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

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 1; no. 23; pp. 6794 - 6803
• Main Authors: Ou, Kai-Lin, Tadytin, Delvin, Xerxes Steirer, K, Placencia, Diogenes, Nguyen, Mike, Lee, Paul, Armstrong, Neal R
• Format: Journal Article
• Language: English
• Published: 01.01.2013
• Subjects: Chemical vapor deposition; Formations; Interlayers; Oxidation; Photovoltaic cells; Solar cells; Titanium dioxide; X-ray photoelectron spectroscopy
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c3ta10894e

We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12-36 nm) and conformal TiO sub(2) interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P sub(3)HT/PC sub(61)BM 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 TiO sub(2), need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N sub(2) 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 degree C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO sub(2). 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 sub(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 sub(CB), show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO sub(2) films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (R sub(P)) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO sub(2)/P sub(3)HT:PC sub(61)BM/MoO sub(3)/Ag, using TiO sub(2) films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (J sub(SC)), open-circuit photopotential (V sub(OC)), and fill-factor (FF), versusbare ITO. OPVs using amorphous, conformal 24 nm TiO sub(2) interlayers showed the highest fill factors, lowest R sub(S), highest R sub(P) and power conversion efficiencies of ca.3.7%.