Titanium nitride thin film as a novel charge collector in TCO-less dye-sensitized solar cell

Abstract

A titanium nitride (TiN)-based dye-sensitized solar cell is developed where TiN is used as a charge collector and TCO-less glass as a substrate. A nanocrystalline TiO₂ film was deposited onto a TCO-less glass substrate using the radio frequency (r.f.) magnetron sputtering method and capped with a TiN film with a thickness of ∼66 to ∼167 nm, which was controlled by varying sputtering time. The crystal structure of TiN layers is analyzed using XRD, chemical bonding nature and composition (TiN_0.95) were confirmed by XPS and RBS, respectively. Cross-sectional scanning electron microscopic images confirmed the columnar structure of TiN films. Electrical resistance is exponentially decayed and approaches 4.4 Ω as the TiN film thickness increases up to 167 nm. The photovoltaic property is significantly influenced by the TiN film thickness. The energy conversion efficiency increases from 3.3% to 6.8% with increasing the TiN film thickness from 66 nm to 86 nm, where an increase in fill factor from 0.33 to 0.64 is mainly responsible for the efficiency improvement. The highest efficiency of 7.4% is obtained with a 136 nm-thick TiN film and declines to 5.8% at 167 nm, resulting in a one order of magnitude retarded diffusion rate of I₃⁻. A long-term stability test was performed for 1000 h and compared with a cell with pure Ti metal. The TiN-based cell maintains an efficiency of 84% after 1000 h, while the efficiency of the Ti-based cell is degraded by ∼34%, indicating that TiN is more stable than Ti in the TCO-less dye-sensitized solar cell.