Beyond the Shockley-Queisser Limit: Understanding and Controlling Carrier Multiplication in Carbon Nanotube PN Junctions

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Non-technical Description: Solar cells built from traditional materials convert sunlight into a mixture of electrical energy and unwanted thermal energy. The fraction of thermal energy is significant, because sunlight-excited electrons transfer some of their excess energy into vibrations in the material's crystal. To improve the performance of solar cells, researchers seek a new class of materials in which sunlight-excited electrons transfer excess energy to other electrons, rather than to vibrations in the crystal lattice. Semiconducting carbon nanotubes are a promising material for this new generation of solar cell design. The electrons in carbon nanotubes interact very strongly with each other, suggesting that energy can be efficiently transferred from one electron to another. This research project addresses the challenge of optimizing a photovoltaic device to understand the physics of strongly interacting electrons in these materials. The project has a public-engagement component, with the PI working as a Science Communication Fellow associated with the Oregon Museum of Science and Industry and leading the physics outreach team at Oregon State University. The project includes summer research experience for young scientists from under-represented minorities.Technical Description: In low-dimensional materials, Coulomb interactions between charge carriers are enhanced, thereby enhancing the phenomena of carrier multiplication. Carrier multiplication can increase the energy conversion efficiency of solar cell devices because multiple electron-hole pairs can be excited by a single photon when photon energy exceeds twice the band gap. Previous experiments on zero-dimensional quantum dots have shown that carrier extraction is a major obstacle to harnessing carrier multiplication. This project focuses on carrier multiplication in a semiconducting carbon nanotube, a one-dimensional system in which carriers can be efficiently extracted. The experiments utilize individual carbon nanotube pn junctions to reveal the effect of temperature, electric field, and dielectric screening on the carrier multiplication process. The research addresses the exciton dissociation mechanism, carrier multiplication efficiency, and the fundamental limits of photocurrent quantum yield in semiconducting carbon nanotubes. By furthering our understanding of carrier multiplication in a strongly-interacting electronic system, the project supports the development of solar cell devices that beat the Schockley-Queisser limit.