Materials, Interfaces, and Photon Confinement in Dye-Sensitized Solar Cells † Byunghong Lee, ‡ Dae-Kue Hwang, ‡ Peijun Guo, ‡ Shu-Te Ho, ‡,§ D. B. Buchholtz, ‡ Chiu-Yen Wang, § and R. P. H. Chang* ,‡ Materials Research Institute, Department of Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208, and Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: June 5, 2010 A series of experiments have been carried out to study the effects of materials quality, surface and interfacial modification, and photon confinement on standard dye-sensitized solar cells. For these studies, both physical and optical characterization of the materials has been performed in detail. In addition, DC and AC impedance measurements along with kinetic charge-transport modeling of experimental results have yielded information on how to systematically optimize the cell efficiency. The same kinetic model has been used to interpret the results of a series of experiments on interfacial modification studies using fluorine etching in combination with TiCl 4 surface treatment. By using specially designed photonic crystals to confine the photons in the cells, it is shown that the best cell efficiency can be further increased by about 13%. 1. Introduction Dye-sensitized solar cells (DSSCs) are good examples of where the quality of the nanomaterials and their interfacial properties are important to device performance. Numerous publications and review articles on DSSC have appeared in the literature during the past two decades. 1-4 Prototype cells for manufacturing are actively under development. 5-9 Most papers have dealt with certain aspects of the DSSC fabrication or performance. In this work, we report a systematic optimization of key parameters and processing steps in a typical DSSC to produce high cell efficiency. Our effort is guided by DC electrical measurements and AC impedance measurements and modeling. 10-12 From our studies, we note that the quality and purity of our starting materials, their surface properties, and interfacial materials compatibility play significant roles in determining the efficiency of the cell. In addition, we demon- strate the importance of improving the photon confinement of the cell. In this article, we start with a discussion of the principle of cell operation, followed by materials preparation, processing, device fabrication and assembly, measurements, and device modeling. A discussion of the experimental results will include detailed studies of interfacial properties via controlled experi- ments and the optimization of photon confinement of a typical cell. The paper will conclude with a summary of our findings and suggestions for further research and development. 2. DSSC Operational Model The principle of operation of a DSSC is well documented in the literature. 13-15 Photons, absorbed by the dye molecules which are coated around the interconnecting TiO 2 nanoparticles, generate excitons. Electrons and holes are formed upon the separation of the excitons. The electrons are conducted through a interconnecting semiconducting TiO 2 nanoparticle thin film to the anode electrode, while the holes are transported away through the redox electrolyte of iodide/tri-iodide in contact with the platinum catalyst at the cathode electrode of the cell (see illustration in Figure 1a). Under steady illumination, the DC cell response has been modeled by a simple circuit, as illustrated in Figure 1b. Device parameters of special interest to cell performance in this model include open-circuit voltage (V oc ), short-circuit current density (J sc ), and the fill factor (FF) (see Figure 1c for an ideal J-V curve). Using these basic parameters, we can optimize the DSSC efficiency, η, which is defined as η ) J sc V oc FF/P s , where P s is incident light power density. To maximize cell efficiency, the circuit in Figure 1b tells us that we need to have the series resistance, R s , be as low as possible while maintaining the shunt resistance, R sh, as high as possible. However, the dominant effect comes from lowering the value of R s . This simple and ideal circuit guides us to quickly study and optimize the cell operation in our experiments, as will be discussed below. In order to have a more in-depth understanding of the cell operation, models that include charge-transport kinetics have † Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. ‡ Northwestern University. § National Tsing Hua University. Figure 1. (a) Simplified schematic diagram of the principle of operation of a dye-sensitized solar cell (DSSC). (b) A simple equivalent circuit of a DSSC. (c) Typical current-voltage curve for a solar cell based on N719 under AM 1.5 simulated sunlight (100 mW cm -2 ). J. Phys. Chem. B 2010, 114, 14582–14591 14582 10.1021/jp102359r  2010 American Chemical Society Published on Web 07/15/2010