Optical spectroscopy is a powerful approach for detecting defects and impurities in ZnO, an important electronic material. However, knowledge of how common optical signals are linked with defects and impurities is still limited. The Cu-related green luminescence is among the best understood luminescence signals, but theoretical descriptions of Cu-related optical processes have not agreed with experiment. Regarding native defects, assigning observed lines to specific defects has proven very difficult. Using first-principles calculations, we calculate the properties of native defects and impurities in ZnO and their associated optical signals. Oxygen vacancies are predicted to give luminescence peaks lower than 1 eV; while related zinc dangling bonds can lead to luminescence near 2.4 eV. Zinc vacancies lead to luminescence peaks below 2 eV, as do the related oxygen dangling bonds. However, when complexed with hydrogen impurities, zinc vacancies can cause higher-energy transitions, up to 2.3 eV. We also find that the Cu-related green luminescence is related to a (+/0) deep donor transition level.
Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.
Optimizing the buffer layer in manufactured thin-film PV is essential to maximize device efficiency. Here, we describe a combined synthesis, characterization, and theory effort to design optimal buffers based on the (Cd,Zn)(O,S) alloy system for CIGS devices. Optimization of buffer composition and absorber/buffer interface properties in light of several competing requirements for maximum device efficiency were performed, along with process variations to control the film and interface quality. The most relevant buffer properties controlling performance include band gap, conduction band offset with absorber, dopability, interface quality, and film crystallinity. Control of an all-PVD deposition process enabled variation of buffer composition, crystallinity, doping, and quality of the absorber/buffer interface. Analytical electron microscopy was used to characterize the film composition and morphology, while hybrid density functional theory was used to predict optimal compositions and growth parameters based on computed material properties.
Process variations were developed to produce layers with controlled crystallinity, varying from amorphous to fully epitaxial, depending primarily on oxygen content. Elemental intermixing between buffer and absorber, particularly involving Cd and Cu, also is controlled and significantly affects device performance. Secondary phase formation at the interface is observed for some conditions and may be detrimental depending on the morphology. Theoretical calculations suggest optimal composition ranges for the buffer based on a suite of computed properties and drive process optimizations connected with observed film properties.
Prepared by LLNL under Contract DE-AC52-07NA27344.
Advances in thin-film photovoltaics have largely focused on modifying the absorber layer(s), while the choices for other layers in the solar cell stack have remained somewhat limited. In particular, cadmium sulfide (CdS) is widely used as the buffer layer in typical record devices utilizing absorbers like Cu(In,Ga)Se2 (CIGSe) or Cu2ZnSnS4 (CZTS) despite leading to a loss of solar photocurrent due to its band gap of 2.4 eV. While different buffers such as Zn(S,O,OH) are beginning to become competitive with CdS, the identification of additional wider-band gap alternatives with electrical properties comparable to or better than CdS is highly desirable.
Here we use hybrid density functional calculations to characterize CdxZn1-xOyS1-y candidate buffer layers in the quaternary phase space composed by Cd, Zn, O, and S. We focus on the band gaps and band offsets of the alloys to assess strategies for improving absorption losses from conventional CdS buffers while maintaining similar conduction band offsets known to facilitate good device performance. We also consider additional criteria such as lattice matching to identify regions in the composition space that may provide improved epitaxy to CIGSe and CZTS absorbers. Lastly, we incorporate our calculated alloy properties into device model simulations of typical CIGSe devices to identify the CdxZn1-xOyS1-y buffer compositions that lead to the best performance.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency and Renewable Energy (EERE) through the SunShot Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program.
Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO<sub>2</sub> (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO<sub>2</sub>. In addition, we present new results for the reflectivity of pure TiO<sub>2</sub>.