SOLAR: Enhanced Photovoltaic Efficiency through Heterojunction Assisted Impact Ionization

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This project combines thin film synthesis, analysis, and modeling expertise from three universities to explore the possibility of achieving quantum efficiency greater than one in unbiased semiconducting heterostructures and nanostructured thin films. The approach, called heterojunction-assisted impact ionization, combines a narrow band gap host material with wide band gap absorbers in the form of thin films, nanorods, or nanoparticles. Carriers generated by higher energy photons in the wide-gap material drift into the narrow-gap material where they decay through impact ionization to produce additional carriers. The project explores promising combinations of semiconducting materials with appropriate band alignment and growth characteristics to promote impact ionization while minimizing other recombination pathways. Success in this endeavor will lead to improved solar cell efficiency using environmentally benign materials.One of the grand scientific challenges of the 21st century is the development of a technology that can convert sunlight to electricity on a large scale at reasonable cost. The efficiency of most solar cells is limited by unit quantum efficiency of light absorption, that is, each photon of light produces at most one electron of charge. This is particularly problematic for shorter wavelength light, which corresponds to higher energy photons, since a large fraction of an absorbed photon's energy is converted into waste heat. This project aims to design and optimize nanostructured semiconducting materials that preferentially channel this excess energy into additional charge generation rather than waste heat. The approach can be readily adapted to materials used in existing photovoltaic devices and also suggests entirely new classes of environmentally benign materials for application in solar energy conversion. Through the training of students, the project will help foster the next generation of scientists and engineers, well-prepared to address problems relevant to society.