This award on solar energy research is co-funded by the Divisions of Chemistry, Materials Research, and Mathematical Sciences of the Directorate for Mathematical and Physical Sciences. A collaboration of chemistry, materials science, and mathematics groups at the University of California - Merced will explore a new kind of luminescent solar concentrator (LSC), based upon alignment of semiconductor nanorods in a liquid crystalline matrix. The idea behind this approach is that aligned nanorods will be better able to couple incident solar energy, collected over a very large area, into small area photovoltaic devices. The team will use semiconductor nanorods and heterojunction nanorods to act as light collector/emitter, and will align these particles in an anisotropic, liquid crystalline matrix. The hybrid materials will be characterized by a number of analytical techniques, and the resultant materials will be modeled with sophisticated radiative transport methods. The young scientists working on this project will have the opportunity to work in an excellent interdisciplinary environment at this Hispanic-Serving Institution, the newest campus of the University of California system.
Despite years of effort, breakthroughs in solar energy technology have been slow in coming. This unique effort by an interdisciplinary group of a chemist, a materials scientist and a mathematician is taking a new look at an old design for solar energy collection and conversion -- using a large-area device to collect and transmit energy to a small-area photovoltaic energy converter. The basic idea is to minimize the cost and increase the efficiency of the expensive component (the photovoltaic device) by decreasing its area, without sacrificing the amount of light that would be collected in going from a large-footprint to a small-footprint device. In addition, to the broad impact that improved solar-energy conversion would have for society, the postdoctoral fellows, graduate and undergraduate students working on this project will benefit from receiving an interdisciplinary education. This will help prepare them for scientific careers solving other important socially- and technologically-relevant problems.
The major goals of the project have been to design novel, highly efficient luminescent solar concentrators (LSCs) based on spherical semiconductor nanoparticles and/or aligned nanorods. Thus, the research has been in two broad categories: synthesis and characterization of highly luminescent, photostable semiconductor nanoparticles that also have absorption characteristics that are desirable for LSCs, and modeling of LSCs that incorporate nanocrystals having these optical properties. Much of the research has focused on developing methods to synthesize extremely photostable, highly luminescent semiconductor nanoparticles, both nanospheres and nanorods. The luminescence efficiency is characterized in terms of the quantum yield, the ratio of emitted versus absorbed photons, which depends on the competition between radiative and non-radiative processes. It is therefore important to understand how the radiative rates vary with particle size, shape and composition. We have shown that, to a significant extent, this is controlled by the details of the electronic structure of the semiconductor particle. As such, part of the research project has been to understand the roles of the thermal population in the bright and dark exciton sublevels and the size and shape dependent intrinsic absorption cross section in determining the radiative lifetime. Luminescence efficiencies are also controlled by the presence or absence of surface carrier trap states that can act as electron-hole nonradiative recombination centers. We have therefore sought to understand the nanoparticle surface chemistry and how this chemistry controls the rates of nonradiative processes. Such nonradiative processes can occur at defects at the core-shell interface of a core/shell particles. In general, the core and shell materials have different lattice constants and a coherent interface therefore necessarily results in lattice strain. Defects at these interfaces can occur because of excessive lattice strain, and we have sought to quantitatively understand this phenomenon. Another goal of the research has been to assess the extent to which surface charging can also result in efficient nonradiative pathways and we have considerable effort into understanding this decay mechanism. Some of the major findings of this research include the following: 1) We have synthesized very high quality CdSe particles and CdTe/CdSe core shell particles and been able to quantitatively understand the radiative lifetimes of these particles. The radiative kinetics were shown to depend on the electronic structure of the particles in ways that could be accurately calculated. 2) We have shown that it is possible to synthesize very high quantum yield CdSe/CdS core shell nanocrystals having a zincblende crystal structure. These particles are synthesized at low temperatures (<160C). Because of the lattice mismatch, these particles have considerable lattice strain, which would usually result in defect formation. However, because of the low synthesis temperature, they are metastable with respect to defect formation. 3) We have developed numerical algorithms to calculated the performance characteristics of LSCs. These calculations use the optical properties of the nanocrystals and the physical dimensions of the LSC as inputs. This research has been performed through a collaboration of three research groups. These research groups have focused on the synthesis and spectroscopic properties of semiconductor nanoparticles (led by Prof. D. F. Kelley), the electron microscopy characterization of nanoparticles (led by Prof. Valerie Leppert) and the numerical modeling of LSCs (led by Prof. Boaz Ilan). The research was performed by a combination of post-doctoral researchers, graduate students and undergraduate students at U. C. Merced. In all cases, the performance of this research was part of the scientific training process.
