Ashutosh Tiwari, PhD , University of Utah
The overall goal of this project is to design and characterize a new family of Dye-Sensitized Solar Cells (SS-DSSC) that utilizes solid hole-collectors in place of liquid electrolytes. These solid-state Dye-Sensitized Solar Cells (SS-DSSCs) are expected to be very stable, efficient and scalable for large area applications. The fund has provided 100% support to a graduate student (MS) who completed his degree in summer 2011. Presently this project is providing partial support to another graduate student (Ph.D.) who is getting excellent training in renewable energy materials and photovoltaic research. Project also supported 4% of PI’s salary (0.5 month summer).
In this project we are utilizing CuBO2, a newly reported p-type oxide, with n-type TiO2 and Ru-based dyes to fabricate SS-DSSCs. Main emphasis is on increasing the conversion efficiency of these SS-DSSCs by optimizing their nanoarchitecture, sensitizing dye and charge injection and recombination dynamics. New material configurations and processing methods are being developed to achieve a better connection between dye molecules and hole-collectors via better pore-filling and contact with the entire TiO2 film. This in conjunction with the high electrical conductivity and hole-mobility of CuBO2 is expected to result in significant improvements in the performance of the solar cell. Further efforts are being made to enhance the conversion efficiency by using coupled dye mixtures.
So far in this project, we have successfully developed: (i) a novel technique of synthesizing nano-well like electrodes for SS-DSSCs (ii) a novel method of filling solid electrolyte in the nao-well electrode. Because of the unique nanoarchitectures, nano-well electrodes possess very large surface area while maintaining sufficient spacing/opening for loading solid electrolyte in the structure. Using these TiO2 nano-well electrodes we have fabricated
The invented nano-well fabrication process includes applying a constant DC voltage bias to anode that is cleaned, degreased Titanium coated glass held against Platinum foil as a cathode. Current that flows through the system is measured by an ammeter as shown in the schematic diagram of the electrolytic bath. Monitoring and controlling the current through the system enables us to decide the completion of the process of formation of nanowells. A constant bias voltage in the range of 10-25V is applied across the electrodes in a 0.5 wt % aqueous solution of HF acid. Initial stage is oxidation of Ti metal that forms a thin TiO2 barrier layer. The bias voltage causes localized breakdown of this weak dielectric oxide layer forming pits that turn into forming the nanowell center. The Flouride ions break this barrier layer by selectively dissolving it and forming soluble Titanium salt. The dissolution rate at the base of the pits is faster than the edges of well due to localized higher pH. This structure is called anodized structure. Once anodization is set in it the formation of new oxide layer and its subsequent dissolution occurs preferentially in the base of the wells extending their lengths. Anodizing for longer times leads to formation of deeper well if bath composition and bias voltage is selected properly. Other bath parameters like temperature, viscosity, conductivity and mobility of acid cations also affect the rate of formation and morphology of the wells. All experimental variables finally control the rate of Titanium oxide growth or its dissolution and diffusion of the ionic species.
Filling the sensitized TiO2 nano-wells with solid electrolyte: We have also developed a method of filling TiO2 nanowells with solid electrolyte nanoparticles In this method, TiO2 nano-well electrodes were impregnated with the nanopowders of solid electrolyte by the capillary effect. For this, a suspension of dispersed electrolyte nanoparticles in pure ethanol is added drop wise to the sensitized electrodes, which are ultrasonicated to disperse the suspension evenly over the electrode. This process is repeated several times to uniformly fill the nano-wells.