Technical Merit: Intermediate-band (IB) solar cells have the potential to convert sunlight to electricity with record efficiency. Perhaps the most promising approach to IB semiconductors is quantum dot (QD) arrays. In principle, these QD arrays would replace the intrinsic region in state-of-the-art p-i-n solar cells. Although this concept was originally proposed in the 1990s, the anticipated efficiency records have yet to be achieved. The performance to date has been limited by a lack of fundamental design rules for energy conversion processes in the context of QD confined states and minibands. In particular, an understanding of the influence of QD shapes, sizes, strain states, and QD/matrix interface uniformity on the formation of and interaction between QD confined states and minibands is needed. Furthermore, novel miniband architectures which enable optimum sunlight to electricity conversion efficiencies must be developed. The proposed approach involves a unique combination of state-of-the-art nanofabrication methods, measurements with unprecedented spatial resolution, and multi-scale simulations. The long-term objective of the work is to provide a set of design rules to optimize sunlight-to-electricity conversion in a wide variety of nanostructured materials. Several fundamental questions critical to the development of optimum sunlight-to-electricity conversion design rules will be addressed, including: what is the critical length scale at which the electronic states associated with a collection of point defects appear similar to those of a small QD? How do QD shapes and sizes influence the QD electronic states? How do interfacial disorder and strain influence the coupling of QD electronic states, as well as the formation of electronic mini-bands in two-dimensional and three-dimensional QD arrays? Which miniband architectures are needed to optimize the sunlight to electricity conversion efficiency? Using the unique combined expertise of the co-PIs in state-of-the-art nanofabrication, measurements with unprecedented spatial resolution, and multi-scale simulations, a set of design rules to optimize sunlight-to-electricity conversion in nanostructured intermediate band solar cells will be developed.
Broader Impact: This research is expected to have a substantial impact on energy and sustainability. The PIs will also develop an outreach program in energy and sustainability. The outreach program will build partnerships in the local public high schools. This approach will lay the groundwork for a more ambitious outreach program developed by the co-PI in Ann Arbor, which has brought more than 30 local high school students to the UM campus for summer research internships. The PIs will develop a module, 'Materials for Solar Cells", to be incorporated into existing high school physics and chemistry courses. They will present this interactive and hands-on module to students from various backgrounds within several school systems. The module will draw on the excitement surrounding major new initiatives at UIUC and UM in sustainable energy research, and will serve to generate interest in engineering research.
This award supported research on the use of semiconductor quantum dots, or nanoparticles, in a novel solar cell design known as an intermediate band solar cell. The work, which involved both theoretical and experimental research, investigated the potential benefits of quantum dots in improving the efficiency of these devices. The key technical outcome of the work was the finding that, for the class of materials investigated here, the presence of quantum dots can increase the efficiency with which light is absorbed by the solar cell devices, but also generally reduces the efficiency with which electrical current can be extracted from the devices. Thus, the net effect of the quantum dots in this type of device is neutral, or even detrimental. During the performance of the research, methods for fabricating and characterizing quantum dots in these materials were improved, as were models for understanding the light absorption and electrical transport in these kinds of devices. Thus, the intellectual merit of the work was through contributions to the science and engineering of semiconductor materials for use in photovoltaic applications. The award had broad impact through the teaching and training of high school students, undergraduate researchers, graduate students, and a postdoctoral reseacher. It also enabled the publication of several referreed journal publications presenting the details of the research, and the dissemination of the findings in conference presentations.
