Technical: This research project aims to improve the power conversion efficiency of solar cells by investigating a multiband material, a semiconductor with an intermediate band. Such a material has effectively three absorption bands and a single junction could be equivalent to a more complicated, but high-efficiency, three-junction solar cell. The growth and processing of such a material is a major challenge, along with understanding and modeling of a multiband solar cell. The emphasis of this project is on materials growth and processing of a particular class of materials, dilute nitride GaNAsP, which has an intermediate band so that electrons can be absorbed by below-bandgap photons, be excited to the intermediate band, and on to the conduction band providing current. These below-bandgap photo-excitations are in addition to the absorption of above-bandgap photons to excite electrons from the valence band to the conduction band directly. The PI, his students and collaborators at the National Renewable Energy Laboratory will study fundamental aspects of the growth, processing and characterization, with the aim toward greater understanding of fundamental materials science and optimization of the multiband material for solar cell applications. The approach includes fabrication and testing, in comparison with single-band solar cells. Greater understanding of the physical properties of such multiband semiconductors is expected as an outcome, advancing the field generally, and the performance of solar cells.
This project addresses basic research issues in a topical area of materials science with high technological relevance. High-efficiency solar photovoltaic cells have broad potential impact on economy and human life through more efficient electrical energy generation. Compared to single-junction and multijunction solar cells, the multiband solar cells investigated in this project could provide significantly higher efficiency and lower cost. The research activities will promote interdisciplinary training for students in growth and characterization of novel compound semiconductors, prototype solar cell device design, and fabrication. Undergraduate students will also participate in the research project; they will be recruited from various sources, including the McNair Program, which serves low-income students and/or underrepresented minorities. The undergraduate students' activities will include advanced characterizations, such as atomic force microscopy, and participation in various undergraduate research conferences at UCSD. Additionally, the PI is the director of the COSMOS (California State Summer School for Mathematics and Science) residential summer program for 150 high school students. The results of this research project can be incorporated into the curriculum and extend to the high schools through the Teacher Fellow program of COSMOS.
Global warming, finite supply of fossil fuels, and increasing demand in energy consumption require a multitude of alternative energy sources. Solar cells are an important component of energy independence. In a conventional single-junction solar cell based on one semiconductor, like silicon solar panels on rooftops, the electrons in a low-energy band absorb a portion of sunlight energy (Figure a) and jump to a high-energy band. They then travel to a terminal of the solar cell and, thus, produce electricity. The maximum efficiency in converting sunlight to electricity has been calculated to be 31%. We can increase this efficiency to over 60% by stacking different semiconductors, for example, three, one on top of another, and they absorb different portions of the solar spectrum (Figure b). However, adding more junctions increases cost, complicates the cell design, and leads to only incremental improvements in efficiency. This funded project tries a different approach by investigating a material with an intermediate band between the low-energy band and the high-energy band. Electrons from the low-energy band can absorb a portion of sunlight energy and jump to the intermediate band. Electrons from the low-energy band and from the intermediate band can absorb different sunlight energies and jump to the high-energy band to produce electricity. Thus, the effect is similar to a three-junction solar cell, but the fabrication process is simpler because only one junction from the intermediate-band material is to be fabricated. This novel intermediate-band material is not easy to grow, however, and the main purpose of the project, which the project has accomplished, is to optimize the growth of this material and demonstrate it has an intermediate band. The science is interesting, and the impact would be great if this material can be demonstrated to be as efficient as a three-junction solar cell but less costly because it is only one junction.
