Type-II hot carrier solar cells: control and manipulation of non-equilibrium carriers using band engineering

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Abstract: Control and manipulation of thermal losses in quantum-engineered structures: a practical route to high efficiency hot carrier solar cells.Non-technical: Improved solar cells are vital not only to the U.S. economy and energy independence; they are essential to the reduction in global warming and for economic growth throughout the developing world. Solar cells offer a free and abundant source of clean power, but currently operate at efficiency levels that limit their economic viability. The central issue is that solar cells convert considerably less of the sun's energy to useable power than fundamentally possible. One major loss mechanism is the rapid loss of energy through heat generation when high energy photons of light are absorbed. This program addresses mitigation of these hot carrier losses via a fundamental investigation of quantum-engineered solar cell architectures that have been shown to inhibit heat generation in proof-of-principle studies. Improved versions of the solar cell structures will facilitate the control and manipulation of hot carriers to increase the fraction of solar energy that is converted into electricity. The development and operation of such structures offer a real potential for practical hot-carrier solar cells, which have the potential to impact utility-scale power generation supporting existing utility infrastructure during peak operating periods, increasing global capacity, and reducing dependence on traditional fossil fuels. Through involvement in this program, graduate and undergraduate students develop expertise in a multidisciplinary range of technical skills, and gain a unique perspective of fundamental research while acquiring an appreciation of the subtleties of novel technology development. Technical: The project focuses on quantum-engineered structures based on Antimonide and Arsenide heterostructures. Despite several potentially important advantages, these semiconductors are relatively unexplored for next generation solar cells. The research builds on recent advances at the University of Oklahoma showing stable and robust hot carrier populations at elevated temperatures in Indium Arsenide (InAs)/Aluminium Arsenide Antimonide (AlAsSb) superlattices. The generation of "hot" high energy carriers has been observed to be more stable in quantum wells, in general, particularly at low temperatures where thermal losses are inhibited. These effects, however, are rapidly quenched at higher temperatures, conditions in which real solar cells must operate. InAs/AlAsSb superlattices are ideal for studying the physics of hot carriers in semiconductor structures. The large confinement potential in these structures facilities hot carrier generation directly in the quantum wells. The degenerate valence band improves the extraction of the positive charge carriers (holes), which serves to slow hot carrier relaxation through increased electron lifetimes in the conduction band of the superlattice. In solar cells designed to harness hot carriers, direct absorption will create hot carriers in quantum wells in the upper emitter region of the cell, which are then rapidly extracted via superlattice and resonant tunnelling architectures. Confinement can be used to tune the energy-gap to optimize the operating voltage, while effectively harnessing the hot carriers generated by the UV-visible photons of the solar spectrum. The semiconductor heterostructures will be grown by molecular beam epitaxy and their hot carrier properties will be characterized by optical spectroscopy and optoelectronic measurements. Solar cell devices will be fabricated from optimized structures with the goal of improved efficiency. Investigation of these structures and devices will enable a better understanding of hot carrier dynamics in practical systems, which are of great interest to the photovoltaics community and important for the cost-effective implementation of solar cells in the domestic energy market.