Reports: UNI1049464-UNI10: Development and Investigation of Multijunction Hybrids of Nanocrystalline: TiO2/quantum dots/conducting Polymer Nanowires for Solar Cells Applications

Justyna Widera, PhD , Adelphi University

Solar cells based on inorganic quantum dots are promising candidates for the next generation of energy production because they can be made using low cost materials and processes. Quantum dots have advantages over organic molecule-based sensitizers. 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. By creating a cell with an array of differently sized quantum dots, a device with an absorbance range corresponding to the solar spectrum can be attained.

This research project consisted of two parts. The first part of this project focused on fabrication of semiconducting CdTe nano- and microstructures using two novel deposition techniques: electrochemical (one step method) and electrochemical/chemical (one step method).

In the one-step electrochemical deposition method, the influence of the applied cathodic potential, the composition of the electrochemical bath solution, bath temperature and heat treatment of the created thin films on the thickness, composition, morphology and photoelectrochemical properties of the deposits was studied. The photoresponse of CdTe thin films were studied in acetic buffer solution (pH=2) containing sulphite (IV) ions as a reducing agent. The intensity of the observed photocurrent is largely a function of the cathodic potential value applied during the electrodeposition process. The resulting photocurrent can be improved by changing the annealing temperature of the generated CdTe films.

In the two-step electrochemical/chemical method, the Cd deposition time, the electrode substrate, the composition of the synthetic bath solution and deposit annealing temperature was studied. The intensity of the observed photocurrent is mostly a function of the Cd deposition time during the first electrochemical step.

Both, the electrochemical one-step deposition and electrochemical-chemical two-steps deposition methods are novel, easy synthetic routes for preparation of CdTe thin films. Morphology of CdTe deposits depends on the synthesis method. Morphology of CdTe deposits depends on the potential applied and on the composition of synthetic bath during the electrodeposition process. Both techniques lead to the formation of photosensitive deposits, however the application of different electrode substrates and deposition methods revealed differences in measured photocurrents. These novel types of the cadmium telluride microstructures present themselves as promising material for application in high conversion-efficiency photoelectrochemical solar cells.

The second part of this project focused on understanding the relationship between the composition and geometry of the nanocrystal layer and the performance of the solar cell by varying its layer thickness, the quantum dots’ size and arrangement, and coupling between the nanocrystals.

The effects of depositing layers of cadmium selenide (CdSe) quantum dots using a controllable thermal annealing technique were studied. Initially, titanium dioxide was deposited via atomic layer deposition onto indium tin oxide coated glass whose surface was dehydrated in order to ensure an even surface coating of CdSe. The nanocrystals were capped with an organic octadecylamine ligand that solubilized them in toluene, yet the organic ligands needed to be removed during layer-by-layer deposition. Annealing the CdSe dots at 150ºC removed the ligands making it possible for another layer to be deposited without dissolving the previous one. Spin coating and high annealing temperatures between consecutive layer depositions increased the attachment of CdSe deposit and allowed the formation of multilayer CdSe films. Scanning electron microscope images were taken to characterize the surface morphology of the deposited films. Ultraviolet–visible spectra of CdSe thin films were recorded in order to monitor the deposited film thickness and any change in its optical properties during processing. We also studied the effect of the contact material on the solar cell performance. We fabricated working CdSe quantum-dot-based solar cells using different configurations of the contact material. We measured our device’s performance under simulated air mass 1.5 G solar illumination to determine its energy conversion efficiency. The presence of the top and bottom oxide layers contacts gave the best cell electrical performance. Also, the improvement of the cell performance was observed for increased annealing temperature and for mixture of different quantum dot sizes. Current efficiencies of ~0.1% have been attained. Future goals include optimizing multilayer quantum dot deposition in order to achieve higher-powered devices.

This grant enhanced also the quality of education of the students working in my research group. The experiments performed during this year exposed undergraduates to advanced research topics in the areas of photochemistry, material science, and nanotechnology. They had an opportunity to learn how to operate state-of-the-art equipment in my research laboratory at Adelphi University as well as at the Center for Functional Nanomaterials, Brookhaven National Laboratory. 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, 2011), the NY ACS 59th Undergraduate Research Symposium (May 5, 2011).  Currently, we are preparing an abstract for submission to the National Conference on Undergraduate Research (NCUR 2012, Weber State University Ogden, Utah March 29-31, 2012).

This grant had a tremendous effect on my career. It enhanced my research capabilities, which helped me to continue collaborative research with scientists from BNL and to start collaboration with the Department of Chemistry, Warsaw University in this critical area of national and international need. Recently, I was awarded tenure at Adelphi University and I am currently applying for a sabbatical leave in order to spend one semester continuing collaborative research at the Department of Chemistry, Warsaw University. Currently, I am also preparing a manuscript summarizing my research efforts and finings to publish it in the peer reviewed scientific journal.

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