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.
Detection of high-energy neutrons in the presence of gamma radiation background utilizes pulse-shape
discrimination (PSD) phenomena in organics studied previously only with limited number of materials, mostly
liquid scintillators and single crystal stilbene. The current paper presents the results obtained with broader varieties
of luminescent organic single crystals. The studies involve experimental tools of crystal growth and material
characterization in combination with the advanced computer modeling, with the final goal of better understanding
the relevance between the nature of the organic materials and their PSD properties. Special consideration is given to
the factors that may diminish or even completely obscure the PSD properties in scintillating crystals. Among such
factors are molecular and crystallographic structures that determine exchange coupling and exciton mobility in
organic materials and the impurity effect discussed on the examples of trans-stilbene, bibenzyl, 9,10-
diphenylanthracene and diphenylacetylene.
The development of high resolution, room temperature semiconductor radiation detectors requires the introduction of
materials with increased carrier mobility-lifetime (μτ) product, while having a band gap in the 1.4-2.2 eV range. AlSb
is a promising material for this application. However, systematic improvements in the material quality are necessary to
achieve an adequate μτ product. We are using a combination of simulation and experiment to develop a fundamental
understanding of the factors which affect detector material quality. First principles calculations are used to study the
microscopic mechanisms of mobility degradation from point defects and to calculate the intrinsic limit of mobility from
phonon scattering. We use density functional theory (DFT) to calculate the formation energies of native and impurity point
defects, to determine their equilibrium concentrations as a function of temperature and charge state. Perturbation theory
via the Born approximation is coupled with Boltzmann transport theory to calculate the contribution toward mobility
degradation of each type of point defect, using DFT-computed carrier scattering rates. A comparison is made to measured
carrier concentrations and mobilities from AlSb crystals grown in our lab. We find our predictions in good quantitative
agreement with experiment, allowing optimized annealing conditions to be deduced. A major result is the determination
of oxygen impurity as a severe mobility killer, despite the ability of oxygen to compensation dope AlSb and reduce the net
carrier concentration. In this case, increased resistivity is not a good indicator of improved material performance, due to
the concomitant sharp reduction in μτ.
The growth, fabrication, and device characterization of the light-emitting diodes based on InP quantum-dot within a GaP matrix and on a GaP(100) substrate are described and discussed. The diode structures are grown using gas-source molecular beam epitaxy. Electroluminescence has been measured under a variety of bias conditions and temperatures. A green emission line at about 550 nm appears to result from carrier recombination in the strained InP wetting layer. Carrier recombination in the InP quantum dots results in red emission at about 720 nm.