Carrier Dynamics in Quantum Dot Solar Cells and Infrared Detectors

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Abstract: Abstract Title: "Understanding the enhancement mechanisms of optical energy conversion efficiency in solar cells based on confinement by quantum dots" Nontechnical: Photovoltaic is a technology that permits the conversion of abundant radiative energy from the sun to electrical energy which can ultimately be widely distributed using a smart electrical grid infrastructure. This can be done with a minimum negative impact on the environment. In spite of being a well-established clean source of energy, there is a need for improving the present conversion efficiency of Si solar cells. New materials and new approaches to achieve this goal have emerged. One appealing approach is based on using different materials with different bandgap to enhance the conversion efficiency of present solar cells. Another approach is based on using confined nanostructures, called quantum dots, to convert solar energy with high efficiency. This latter approach has the advantage of being simpler and does not require complex growth processes. This might be a very inexpensive approach to making solar cells. On the other hand, this approach has not yet demonstrated its potential. Different experimental studies are proposed with the goal of understanding the potential and limitations of this approach. The proposed approach involves an effort toward a better understanding of light absorption in these structures in order to design very efficient solar cells based on quantum dots. If successful, this project will have a major impact on clean and efficient energy conversion in this country. Technical: Photovoltaic devices based on p-n junction are the most mature technology for solar-energy harvesting. This proposal seeks to develop new highly efficient GaAs quantum dot solar cells based on the intermediate band concept. These cells have the potential of being as efficient as 3-junction solar cells but at a much lower level of complexity. The concept of intermediate band solar cells has been with us for more than 15 years. Unfortunately, these solar cells have so far demonstrated a limited conversion efficiency of 18 % or less. This is in large part related to the fact that researchers have not been able to demonstrate an intermediate band solar cell with a well-defined intermediate band Fermi energy, detached from the conduction band Fermi energy. Some key measurements on intermediate band solar cells that have not been attempted before are proposed in order to shed light on this topic. In particular, two-photon measurements using two optical sources of different wavelengths are proposed to understand if a 2-photon absorption can be measured and if the process is based on sequential photons absorption or due to simultaneous two-photon absorption. This will answer the question if intermediate band solar cells can be realized with well-defined intermediate band Fermi energy, detached from the conduction band Fermi energy. If substantially higher efficiencies cannot be reached using the concept of intermediate band solar cells, implying that the concept is basically flawed, this needs to be investigated and a final answer has to be provided with a clear explanation. Additionally, since the physics of these solar cells is similar to the physics describing photon absorption in IR-detectors, this new understanding will also lead to a better understanding of the operation of quantum dot infrared photodetectors (QDIPs). Two quantum dot solar cells will be studied with different bandgaps to optimize the predicted conversion efficiency of intermediate band solar cells.