Creation of Hyper-Doped Crystalline Titania Thin Films by the Kinetic Doping Method for Solar Cell Anode Material

Abstract

This work outlines the general process of kinetic doping and how its benefits have been expanded from silica-based applications to use in titania thin films. While this has potential for many applications, the utilization focused on in this work is as a novel approach to the fabrication of anode material in dye-sensitized solar cells (DSSCs).

Chapter 1 discusses the general pathways of the sol-gel process and various methods of guest molecule loading. This includes the kinetic doping method which will be utilized in this work. Chapter 2 will go into further detail on the research field of dye-sensitized solar cells (DSSCs) and how this work may be a benefit to the field. Chapter 3 is a record of the various materials, instrumentation, and methods utilized to accomplish the experiments in this work.

The main work of creating the hyper-doped TiO2 thin films for utilization as a DSSC anode material is discussed in chapters 4-6. The general steps to accomplish the prototype DSSC based on our intended anode material are: (i) to establish kinetic doping in TiO2 thin films (ii) to broaden the wavelength range of light absorbed by loading multiple dyes at once (iii) to crystallize the amorphous dye-doped TiO2 thin films while minimizing the damage to dye molecules caused by thermal degradation (iv) to create and evaluated the performance of prototype DSSCs using the material established in the first three steps. Chapter 4 will discuss the challenges and results of steps (i) and (ii), establishing the protocols for creating hyper-doped amorphous TiO2 thin films and one-pot loading of multiple dyes. In theory, since the hydrolysis and condensation chemistry taking place in silica sol-gels and titania sol-gels is inherently very similar, kinetic doping in titania sol-gels is expected to follow the same general pathway as silica sol-gels. However, since the titanium dioxide precursor is more reactive than the silicon dioxide precursor, kinetic doping in titania sol-gels will clearly not be identical to kinetic doping in silica sol-gels. Concentrations as high as 0.7 - 1.9 M in thin films from 120-250 nm thick are achieved for various dyes from a 1 mM loading solution. Up to four dyes are loaded in a single-step loading process, resulting in an absorption range from 300 – 700 nm.

Step (iii), crystallization of the amorphous dye-doped TiO2 thin films is also important. The amorphous thin films may work in a prototype but would almost certainly be more effective in a crystalline form. As the traditional crystallization process of heating at high temperatures is expected to degrade the loaded organic dye molecules, a lower temperature crystallization process will likely be needed. Chapter 5 describes this low-temperature method of TiO2 thin film crystallization, and the challenges associated with establishing the protocol. The final protocol results in an 85% dye retention rate, which allows the advantage of kinetic doping to be manifested in the final crystallized thin film. Multilayer thin films are created and undergo crystal transformation in a single step, indicating a high degree of tunability for the final thickness of a DSSC anode made from this material.

The final step, (iv), the creation of a prototype, is discussed in Chapter 6. Creating a prototype is a fundamental part of establishing whether these hyper-doped TiO2 thin films have the potential to produce high efficiency anode materials for DSSC. The prototype power conversion efficiency achieved ranges in efficiency from 0.001-0.008%, which is on par with the literature efficiency of DSSCs utilizing analogous dyes and redox electrolyte solution. It is even within an order of magnitude of DSSCs that use the same analogous electrolyte and high-efficiency dyes. Avenues for further improvement to the prototype DSSCs are also discussed.