The sun represents the most abundant potential source of sustainable energy on earth. Solar cells made from thin films of nanometer sized crystals of titanium oxide are potentially much less expensive that crystalline silicon materials currently used in commercial solar cells, but suffer from low solar energy conversion efficiencies. The goal of this project is to determine how the addition of rare earth materials into the titanium oxide layer can improve solar energy conversion efficiency. The research plan will focus on neodymium oxide as model rare earth material. The fundamental science gained in this research may also lead to the discovery of new quantum mechanical processes for enhancing current production from solar cells. As part of this project, students at the University of Louisville will collaborate with a world-renowned solar energy research facility at the Swiss Federal Institute of Technology.
Dye-sensitized and perovskite solar cells containing titanium oxide nanocrystal thin films which act as the acceptor for photoexcited electrons offer low-cost fabrication relative to doped crystalline silicon materials, but solar energy conversion efficiencies are still below 20%. The overall goal of the proposed research is to understand how the incorporation of rare earth oxides into the titanium oxide layer of perovskite and dye-sensitized photovoltaic materials can be optimally realized to increase solar energy conversion efficiency. The incorporation rare earth oxides into the titanium oxide layer can potentially enhance short circuit current densities exceeding the Shockley-Queisser limit by three possible mechanisms. First, the coupling of optical transitions between the rare earth and the sensitizer provides the opportunity for carrier multiplication effects. Second, exchange interactions between the spin moment of the f-state electrons in the rare earth and the sensitizer causes mixing between the triplet and singlet excitons, leading to enhanced optical absorbance. Third, the low electron affinity and high dielectric constant of rare earth oxides enable efficient hole transport. The research plan will elucidate the efficiency enhancement mechanisms through correlation of the optical and electronic properties with the composition and morphology of the rare-earth oxide doped films using electron and scanning probe microscopy, spatial photoelectrochemical measurements, scanning energy dispersive X-ray elemental mapping, and Raman chemical mapping. This knowledge will then be used to develop, fabricate, and test rare-earth oxide / perovskite hybrid solar cells using neodymium oxide as the model rare earth oxide material. At the most fundamental level, these studies could lead to the discovery of new mechanisms for photocurrent enhancement involving electro-optic and electron spin interactions which can potentially be applied to other types of photovoltaic materials. The analytical and materials synthesis methods used in the research will be adapted for use in nanotechnology instructional modules at the undergraduate level. Outreach and broadening participation activities will be coordinated through the Technology Fellows Program and the Conn Center for Renewable Energy Research at the University of Louisville.
