Solution Processable Semiconducting TiOx Thin Films for Photovoltaic Applications

Li, Jiguang, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Gupta, Mool, Department of Electrical and Computer Engineering, University of Virginia

Polymer based organic solar cells are promising due to advantages including light weight, mechanical flexibility and low cost roll-to-roll processing. Currently, organic solar cells have two major limitations, namely lower conversion efficiency and poor long-term environmental stability compared with inorganic solar cells. One method to address these limitations involves applying few tens of nanometer thin films of solution processable TiOx in organic solar cell applications. The TiOx thin film is used for enhancement of optical absorption and for protective coatings. The work described in this dissertation studied the properties and applications of this TiOx material for organic photovoltaic devices.
The optical, structural, and morphological properties are important for optical applications. Optical constants were calculated by ellipsometry and modeling. X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) were used to study the physical, structural and morphological properties. The results indicated that the sol-gel TiOx may be used in optical thin film applications as an antireflection layer.
Electrical properties such as electrical resistivity and photoconductivity were studied. The resistivity of TiOx was calculated as 1.5 ×107 Ω∙cm for the as-deposited films using vertical transmission line measurement model. The thermal annealing effect and temperature dependence of the resistivity of TiOx thin films were also investigated.
The degradation of the PCBM/P3HT organic solar cell was studied under different UV light intensities and gas environments. Transmission infrared (IR) and Electron Spin Resonance (ESR) spectroscopies were used to study the chemical changes of the TiOx thin films under different conditions. The results suggested that the photochemical removal of O2 by TiOx films passivated the organic solar cells from oxygen induced degradation. The protection capacity by TiOx films was calculated. Distribution of the local photocurrent in the cells with and without TiOx protection layers were obtained by laser beam induced current measurements. The results demonstrated that the introduction of the TiOx layer could effectively absorb water/oxygen from ambient air. TiOx thin films protected the cells at the edge of the devices.
Further improvement of the TiOx material properties was achieved by doping. Fe doped TiOx material improves the organic solar cell efficiency while Cs doping does not benefit efficiency. The photovoltaic characteristic parameters such as efficiency, open circuit voltage, short circuit current and fill factor were reported.
The application of the TiOx material on inorganic Si solar cells was also explored. Silicon solar cells using TiOx thin films for surface passivation and anti-reflection coatings were successfully fabricated. The obtained efficiency of 15.7 % is comparable with devices using SiO2 and Si3N4 as passivation and anti-reflection layers, respectively. The passivation effects were evaluated by near surface lifetime measurements.
This work covers the measurements of the fundamental optical and electrical properties, protection mechanism, modification and application of the TiOx material.

PHD (Doctor of Philosophy)
TiOx,, Photovoltaic, Thin Films
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