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New solar cell architectures use nanostructured materials to achieve high light-to-electricity conversion efficiencies while using low-cost materials and processes. One example is a dye sensitized solar cell where sunlight is absorbed by an organic dye sensitizer, and the photogenerated charge transports out of the device through a nanostructured percolating titanium dioxide (TiO₂) network. Organic dyes have a limited spectral absorption range and are not readily suited to capture all incident solar energy.  Inorganic semiconductor quantum dots are an alternative solar cell sensitizer because their spectral light absorption can be controlled by their size and composition. In principle, one can design a device having a spectral absorbance range well matched to the incident solar spectrum by using an array of differently-sized quantum dots thereby providing a pathway to higher efficiencies.

The first part of our research used solution-phase chemistry to synthesize cadmium sulfide (CdS) quantum dots with precise diameter control over the range of 2 to 10 nanometers, and corresponding control of peak optical absorption from 320nm to 365nm. We produced CdS particles using a reverse micelle method using a surfactant (AOT in heptane) allowing further integration into thin film devices using solution processing. We characterized the optical properties of thin films of both CdS and TiO₂ nanocrystal using ultraviolet-visible spectroscopy, and measured the nanocrystal film morphologies (size, structure, and thickness) using scanning electron microscopy and profilometry in order to understand the effects of different methods of film deposition (spin coating vs. doctor-blading). Spin coating of both CdS and TiO₂ nanocrystals yields uniform, three-dimensional nanocrystalline thin films. We fabricated nanocrystal thin film devices by sandwiching nanocrystalline films of either CdS or TiO₂ between a transparent indium-tin oxide electrical contact and an aluminum contact deposited by thermal evaporation. In both CdS and TiO₂ nanocrystal devices, the device current increases with applied voltage. Under simulated solar illumination, the conductance of both CdS and TiO₂ devices increases, consistent with excitation of photogenerated carriers in the semiconductor nanocrystal film network. CdS quantum dots are not the best candidate for a sensitizer due to their spectral characteristic; they absorb at wavelengths that correspond to very low radiation intensities in the solar spectrum. Due to their bangap size and position, they could be used as an electron transfer matrix performing functions similar to TiO₂.

The second part of our research used commercially available TiO₂ nanoparticles to form highly porous, nanocrystalline thin films on optically transparent indium tin oxide/glass substrates using both spin-coating and doctor-blading methods; we studied the effects of adding titanium monomers (e.g., titanium (IV) isopropoxide, titanium (IV) chloride) on the resulting TiO₂ film porosity and brittleness using scanning electron microscopy.  Next we sensitized highly porous TiO₂ films for photon absorption using both cadmium telluride and cadmium selenide quantum dots. We examined the efficacy of different quantum dot loading techniques, i.e., direct soaking and use of the linker molecule (3-mercaptopropionic acid) using ultraviolet-visible spectroscopy.  Direct soaking showed a higher degree of uptake compared to the linker modified TiO₂ matrix. We fabricated working solar cells from the quantum dot sensitized TiO₂ films employing an ionic liquid based electrolyte and a platinum counter electrode.  Our devices show overall power conversion efficiencies of 0.002% for linker-attached CdSe and 0.005% for direct-attached CdSe under simulated AM1.5G solar illumination.  In comparison to the control solar cell sensitized with Ruthenizer 620 organic dye, quantum dot sensitized cells are poor performers. This is most likely due to poor charge transfer and injection from the quantum dots to the TiO₂ layer caused by the insufficient quantum dot loading into the TiO₂ matrix compared to the organic dye.

After optimizing device performance, future experiments will explore measurements of the forward and reverse charge transfer rates from quantum dot sensitizers to the nanostructured TiO₂ to understand the relative merits of quantum dots vs. organic dyes for this device architecture. Efforts in this direction using fast laser spectroscopy collaborating with Dr. Thomas Tsang (BNL) have begun.

In addition to advancing fundamental understanding and technological development, a key goal of scientific research (particularly university-based) must be to enhance the quality of science education. The experiments performed during this year exposed undergraduates to advanced research in the areas of photochemistry, material science, and nanotechnology. My research team, undergraduate students Jason Lane, Hoang Long Nguyen, and Scott Gordon went through an extensive training for various instruments, spin coater, instrument for physical vapor deposition, plasma etcher, thermal deposition instrument, profilometer, electrical probe station, UV-VIS spectrophotometer with scattering resolution capability and SEM Scanning Electron Microscope. Some instruments were available at Adelphi and others at BNL. Students worked directly with the PI on all experiments every day learning new techniques and processes and to follow lab safety rules. They not only accrued knowledge in the scientific area but also learned about carefully planning and performing experiments that could lead to important answers and conclusions; they were part of the development process of the scientific project. The students developed/improved their writing and public speaking skills, as outcomes of the project were  preparation of an abstract, research paper, poster and oral presentation which were presented at the Adelphi Research Conference (April 12, 2010), the NY ACS 58^th Undergraduate Research Symposium (May 8, 2010) and the National Conference on Undergraduate Research (April 16, 2010, Missoula, MT).

Participation in this research had a tremendous effect on these students and their futures. Jason Lane, who graduated from Adelphi University with BSc. in chemistry/ philosophy, is currently pursuing a PhD in chemistry at Cornell University (materials science for energy conversion applications). Hoang Long Nguyen graduated from Adelphi University with BSc in chemistry /accounting and is currently applying to several top graduate schools (PhD in chemistry). Scott Gordon, currently a junior majoring in chemistry, plans to continue his education in the field of chemical engineering.

This grant had a tremendous effect on my career. It enhanced my research capabilities by purchasing new laboratory instrumentation, i.e., impedance spectroscopy module and Atomic Force Microscope, and it encouraged collaborative research with scientists from BNL in this critical area of national and international need.