Reports: AC1048556-AC10: Core-Shell Heterostructures for Photovoltaic Energy Conversion

Igor V. Vasiliev , New Mexico State University

Our research project was focused on accurate theoretical prediction of the structural, electronic, and optical properties of core-shell semiconductor nanocrystals composed of wide band gap semiconductors, such as ZnSe, ZnTe, CdSe, and CdTe. The study was carried out using first principles density functional and time-dependent density functional methods. The aim of this research project was to expand the knowledge of chemistry and physics of semiconductor core-shell heterostructures and facilitate the development of nanocomposite materials for efficient conversion of solar energy.

Inorganic and organic composites containing luminescent wide-gap semiconductor nanocrystals have recently emerged as a promising new class of photovoltaic materials that can be utilized in the next generation of solar cells. These materials represent an attractive alternative to traditional light harvesting assemblies made from bulk semiconductors. Semiconductor nanocrystals exhibit a size-dependent variation of the absorption and emission optical gaps due to quantum confinement effects. The ability to tune the size of the optical gap in these nanocrystals makes it possible to design photovoltaic systems that can capture a wider portion of the solar spectrum than bulk semiconductors. Furthermore, polymeric and amorphous composite materials are generally less expensive to produce than pure crystalline semiconductors. As a result, the use of nanocomposite materials in photovoltaic cells can potentially improve the efficiency and reduce the cost of photovoltaic solar panels.

Semiconductor nanocrystals composed of two or more layers with different chemical composition exhibit enhanced functionality and possess more degrees of freedom than single-component nanocrystals and quantum dots. The additional degrees of freedom can be used to fine-tune the properties of heterostructural nanocrystals for specific needs. Recent studies demonstrated that the rate of Auger recombination can be substantially suppressed in multicomponent core-shell semiconductor nanocrystals due to the spatial separation of the electron and hole wave functions in these structures. This makes core-shell semiconductor nanoparticles highly attractive for use in nanocomposite solar cells.

During the extension year of the project, we have completed the study of the structural and electronic properties of core-shell semiconductor nanocrystals composed of group II-VI semiconductors. Our calculations were performed using a massively-parallel version of the PARSEC (Pseudopotential Algorithm for Real-Space Electronic Calculations) ab initio pseudopotential electronic structure code. The optical gaps of core-shell nanocrystals were computed using a linear response algorithm implemented in the framework of time-dependent density functional theory. The accuracy of time-dependent density functional calculations was improved by using asymptotically correct exchange-correlation functionals.

The studies of wide-gap semiconductor nanocrystals had been hampered in the past by the lack a simple surface passivation scheme for group II-VI semiconductors that could remove dangling bonds from the nanocrystalline surfaces and render them chemically inert. Standard passivation techniques tends to produce localized surface states situated within the optical gap of group III-V and II-VI semiconductor nanocrystals, which makes it unsuitable for studying confinement effects in nanoparticles and quantum dots composed of wide-gap semiconductors. In our study, we implemented a recently developed passivation technique based on the use of fictitious partially charged hydrogen atoms. The value of fractional charge was selected according to the type of covalent bond. We found that this approach completely removed the electronic states associated with the surface hydrogen atoms from the gap of group II-VI semiconductor nanocrystals.

Using this technique, we calculated the structures, electronic densities of states, and optical absorption gaps of traditional CdSe/ZnSe and CdTe/ZnTe and inverted ZnSe/CdSe and ZnTe/CdTe core-shell semiconductor nanocrystals containing up to approximately 300 atoms. The structures of surface-passivated core-shell semiconductor nanocrystals were optimized using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm implemented in the PARSEC electronic structure code. The densities of states and the optical gaps were computed for the optimized geometries of core-shell nanocrystals. We found that the sizes of the optical absorption gaps in both traditional and inverted core-shell nanocrystals decreased with increasing the external diameters of the nanocrystals. The evolution of the calculated absorption gaps was consistent with the predictions of the quantum confinement model. Overall, the gaps computed using the time-dependent density functional formalism were found to be approximately 0.3-0.5 eV larger than those obtained in time-independent density functional calculations. Our calculations also demonstrated that the HOMO-LUMO gaps of core-shell nanocrystals could be tuned by more than 1 eV by adjusting the size of the core without changing the external diameters of the nanocrystals. This result indicates that the optical and electronic characteristics of core-shell nanocrystals can be tuned over a wide range by changing the ratio between the radius of the core and the thickness of the shell. The results of our study were presented at the 2011 March Meeting of the American Physical Society in Dallas, Texas. A paper based on this study is currently in preparation and will be submitted to Physical Review B before the end of this year.

Our theoretical studies were carried out in collaboration with the Theoretical Chemistry Group at Los Alamos National Laboratory and the Center for Computational Materials at the Institute for Computational Engineering and Sciences and the University of Texas at Austin. The collaborative partnerships created through this project helped advance the PI's research program at New Mexico State University (NMSU). The research conducted in connection with this project has generated several peer-reviewed publications in high-ranked U.S. and international physical and chemical journals. The results of this study will help attract additional external research funding to NMSU.

This research project has also benefited graduate students at NMSU. The major part of the project budget was used to support graduate students at the Physics Department of NMSU through stipends and benefits. The research carried out under this project provided a basis of Ph.D. dissertations for two graduate students.

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