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Point defects in CdTe and CdSeTe alloy: a first principles investigation with DFT+U
CdTe and its alloy CdSeTe are widely used in optoelectronic devices, such as radiation detectors and solar cells, due to their superior electrical properties. However, the formation of defects and defect complexes in these materials can significantly affect their performance. As a result, understanding the defect formation and recombination processes in CdTe and CdSeTe alloy is of great importance. In recent years, density functional theory (DFT) calculations have emerged as a powerful tool for investigating the properties of defects in semiconductors. In this paper, we use DFT+U calculations to comprehensively study the properties of intrinsic defects as well as extrinsic defects induced by commonly used dopants, such as Cu and group V elements, in CdTe and CdSeTe alloy. This work provides insights into the effects of these defects on the electrical and optical properties of the material.
Defect Stability in CdTe Based on Formation Energies and Migration Barriers
Native point defects are thought to play a key role in CdTe, either as compensation centers in intentionally doped material, as a source of conductivity in nominally undoped material, or as electron–hole recombination centers. Here, the discussion of their concentration and impact has often centered only on formation energies and transition levels. Using hybrid density functional calculations, including the effects of spin–orbit coupling (SOC), we discuss the stability of native point defects in CdTe based on their formation energies and migration barriers. We show that although Cd interstitials are the lowest energy donor defects, they are unstable at room temperature due to a low migration barrier. They are important for maintaining charge neutrality during growth or annealing at high temperatures, but once the material is brought to room temperature, they are not frozen in as often assumed and are expected to anneal out, leaving the other more stable defects to determine the conductivity. Taking this into account in the solution of the charge neutrality equation, we are able to predict the conductivity type and carrier concentrations that are in good agreement with experimental observations.
Spatially and Spectrally Resolved Defects in Polycrystalline CdTe Thin Films Revealed by Quantitative Cathodoluminescence
Increasing the grain size is a potential strategy to reduce grain-boundary recombination and improve performance of thin-film solar cells. Here, CdTe thin films with a range of grain sized were produced by varying the CdC1 2 post-deposition treatment temperature. We use high-resolution cathodoluminescence (CL) microscopy to study recombination and shallow defect levels in detail. Intensities from room temperature CL maps were compared across samples. We find that the CL intensity initially increases with grain size, as expected, but then plateaus as the grain size is increased further. The plateau is correlated with a decrease in the characteristic length-related to the carrier diffusion length-determined from CL intensity profiles near grain boundaries. In addition, low-temperature CL measurements demonstrate the evolution of the defect levels with CdC1 2 temperature.
The role of native defects and their diffusivity in CdTe
CdTe is a leading thin-film technology for solar-cell applications. Basic features such as the optimum band gap of 1.5 eV and simple manufacturing process have been driving the research on improving the current record efficiency of 22% for CdTe solar cells toward the theoretical limit of 32%. Low p-type doping and short carrier lifetimes in the CdTe absorber have been suggested as the main limiting factors and attributed to defect compensation and carrier recombination. Understanding the role of point defects and impurities in CdTe is crucial to solving these problems. Despite many years of research, the sources of compensation and the microscopic recombination mechanisms are still unclear. By using hybrid density functional calculations with spin-orbit coupling and large supercells, we investigate the electronic properties, the formation energies, and the diffusivities of the native defects in CdTe. As possible sources of compensation for p-type conductivity, we discuss the structural and electronic properties of Cd interstitial, Te vacancies, and TeCd antisites. The stability of each defect is discussed in terms of both the defect formation energy and the migration barrier. We also address possible mechanisms that lead to non-radiative recombination.
Defect Study of Group V Doped CdTe By Thermoelectric Effect Spectroscopy
Shallow acceptors in single crystal CdTe induced by Group V (P, As, and Sb) doping are identified by thermoelectric effect spectroscopy (TEES). Understanding defects in Group V doped CdTe is critical to improving and controlling the acceptor density and lifetime in the absorber material which is an important factor determining the open circuit voltage and efficiency in CdTe solar cells.
Understanding Defect Activation and Kinetics in Next Generation CdTe Absorbers
This project investigated the role of defects and dopants in cadmium telluride (CdTe) solar cells to improve efficiency and long-term stability. Advanced X-ray microscopy and spectroscopy techniques were used to map composition, identify defect structures, and measure charge collection at the nanoscale. These experimental results were combined with computational modeling to understand how dopants such as copper and arsenic interact with other elements during processing. The study found that many dopants become electrically inactive due to the formation of complexes with chlorine and oxygen, limiting carrier concentration and device performance. It also showed that selenium, used to enhance efficiency, can migrate within the material during both manufacturing and operation, altering local structure and potentially affecting stability. The project developed new diagnostic and analysis tools for studying photovoltaic materials and provided a mechanistic understanding of key performance limitations. These insights support the development of more efficient, stable, and cost-effective CdTe solar technologies for large-scale energy deployment.
Effects of post-deposition CdCl 2 annealing on electronic properties of CdTe solar cells
The effects of post-deposition CdCl 2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl 2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380 to 430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. Here, the best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.
Defect levels and self-compensation in iodine-doped CdTe single crystals
This study systematically documents defect levels in n-type iodine-doped cadmium telluride (CdTe:I) crystals as measured by thermoelectric effect spectroscopy, Hall-effect measurements, and photoluminescence and calculated by density functional theory. The primary donor, I Te , was identified with an activation energy of ∼0.05 eV (measured)/∼0.13 eV (calculated). Deep acceptor states, V Cd and I Te –V Cd complex, exhibited activation energies of ∼0.1 eV (measured)/∼0.19 eV (calculated) and ∼0.12 eV (measured)/∼0.24 eV (calculated), respectively. Self-compensation and Fermi level pinning were observed at ∼0.7 eV (measured)/∼1.08 eV (calculated), contributing to high resistivity in as-grown samples. Post-growth Cd annealing effectively removes compensation centers, releases iodine donors, and significantly reduces resistivity, resulting in an ideal n-type solar cell material.
Imaging CdCl 2 defect passivation and formation in polycrystalline CdTe films by cathodoluminescence
Polycrystalline thin-film solar cells are attractive for low-cost photovoltaics, but their efficiencies are hindered by material quality issues. State-of-the-art CdTe solar cells use CdCl 2 annealing treatments whose effects are still being discovered at a fundamental level. Here, a series of CdTe samples with different annealing temperatures is investigated with high-resolution hyperspectral cathodoluminescence mapping measured at both room temperature and low temperature on the same microscopic areas. A statistical analysis over a large number of grains is combined with a local analysis at grain boundaries. The results elucidate the dynamic interplay between grain boundary and intragrain defect passivation and formation, in the midst of grain growth. The CdCl 2 annealing initially contributes to an increase of the grain size and the passivation of both grain boundaries and grain interiors, increasing the overall luminescence and diffusion length. For higher annealing temperatures, a further increase of grain size is counterbalanced by the rise of bulk defects. The results illustrate the tradeoffs that lead to an optimal annealing temperature, as well as new methods for understanding defect passivation and creation in thin film solar cells.
DEFECT-RELATED CARRIER TRANSPORT IN CDTE-BASEDCOMPOUNDS: COMPARISON WITH HYBRID PEROVSKITES
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First principles investigation of dopants and defect complexes in CdSe$_x$Te$_{1-x}$
Se alloying is a common approach to improve the performance of CdTe solar cells by tuning the bandgap, defect levels, and carrier density. A fundamental understanding of these improvements, specifically the effect of Se alloying on the behavior of defects and dopants in CdTe, remains unclear. Here, in this work, we present a density functional theory (DFT) study of point defect energetics in CdTe and CdSe x Te 1-x with x = 0.25, leading to a comparison of how native defects, dopants (As and Cu), impurities (Cl and O), and related defect complexes behave in CdTe vs CdSe x Te 1-x . Our calculations, performed by combining semi-local and nonlocal hybrid functionals, show a general lowering of the formation energies of native defects as well as substitutional defects formed by As and Cl upon Se addition. For successful p-type doping with As, destabilizing Cl-based defects in the CdSeTe lattice would be essential. We find evidence for some low-energy defect complexes of As, Cl, and O in CdSe 0.25 Te 0.75 . The computed defect formation energies further enable estimates of temperature-dependent defect concentrations and self-consistent Fermi levels. A comparison of defect energetics with the energies of impurity phases reveals that As, Cu, Cl, and O overwhelmingly prefer being segregated to unwanted As 2 O 5 , AsCl 3 , Cd 2 AsCl 2 , and CuO x phases rather than remain at defect sites, but such segregation is less likely to happen in CdSe 0.25 Te 0.75 than in CdTe. Overall, our work presents a list of likely defects and complexes in CdTe and Se-incorporated CdTe, paving the way to explain and mitigate limited dopant activation in experimental observations.
Distribution of Copper States, Phases, and Defects across the Depth of a Cu-Doped CdTe Solar Cell
Copper has been used as a p-type dopant in cadmium telluride (CdTe) for decades. However, the density of Cu atoms in the finished device is much higher than that of holes, which means that most Cu atoms are not activated as acceptors during incorporation. Furthermore, studies have demonstrated that the distribution of copper (Cu) atoms across the device is highly inhomogeneous, with reports citing Cu substitution on Cd sites and segregation to grain boundaries. Fast diffusion along these boundaries and Cu accumulation at the CdTe/CdS interface have also been observed and validated computationally. These levels of inhomogeneity make it difficult to accurately characterize and correlate the performance with the nature of the Cu atomic species present. To address this challenge, we utilize X-ray microscopy and, specifically, nanoscale fluorescence-mode X-ray absorption near-edge structure to resolve the atomic Cu environment throughout the depth of the CdTe layer. Our results suggest that the majority of Cu atoms are in the form of Cu x Te phases (or similar local environments) near the ZnTe|CdTe interface, Cu x O phases in the CdTe absorber, and present in various oxidation states, including Cu 1+ and Cu 2+ , near the CdS/CdTe junction. Here this work also provides experimental evidence for the first time of the presence of CuS around the ZnTe|CdTe interface and the hypothesized Cu Cd -Cl i complex in the CdTe absorber.
Numerical simulation of high-efficiency, scalable, all-back-contact Cd(Se,Te) solar cells
In this work, all-back-contact thin film photovoltaic devices in lattice back contact (LBC) configuration are studied by device simulation to set the requirements for achieving efficiency > 20% with lateral contact spacing > 50 µm. The numerical device model is first validated against published data for an LBC perovskite device and extended to predict the performance of Cd(Se,Te)-based polycrystalline devices. Recent advances in the electronic properties of Cd(Se,Te) films have made high efficiency LBC devices feasible with industrially-scalable contact dimensions. Constraints on bulk, interface, and contact properties are quantified and discussed
Self-compensation of group-V acceptors in CdTe
Cadmium Telluride is at the core of an important thin-film technology for photovoltaics that is already commercially available, yet the CdTe-based solar cell efficiency remains limited at 22%, well below the theoretical limit of ~30%. Increasing the hole concentration is crucial for higher efficiency, and group-V elements such as As, P, and Sb are potential dopants as they are shallow acceptors. Nevertheless, group-V doped p-type CdTe often exhibits low doping activation, and the compensation source remains debated. Here, we performed hybrid density functional calculations with spin-orbit coupling to investigate possible sources of hole compensation in group-V doped CdTe. First, regarding possible self-compensation of the group-V dopants, we find that the formation of AX centers is unlikely since they are found to be unstable relative to the shallow acceptor forms. However, if the group-V dopants come in during growth (such as dimer molecules As2, P2, and Sb2), we find that the impurity atoms, which would occupy nearest neighbor sites, maintain the V-V bonds, limiting the hole density. For the native defects, our study reveals that Cd interstitial is the lowest energy donor defect in p-type CdTe. Still, it has a small migration barrier of 0.5 eV, making it highly mobile and unstable at room temperature. The Te vacancy is the next lowest formation energy donor. The migration barrier of 1.4 eV indicates that the Te vacancy is stable at room temperature. The antisite CdTe is also a donor, with low formation energy and stable at room temperature, potentially limiting the hole concentration. Our results, therefore, shed light on possible compensation centers and some guidance on how to avoid them.
Machine learning defect properties in Cd-based chalcogenides
Impurity energy levels in the band gap can have serious consequences for a semiconductor's performance as a photovoltaic absorber. Data-driven approaches can help accelerate the prediction of point defect properties in common semiconductors, and thus lead to the identification of potential deep lying impurity states. In this work, we use density functional theory (DFT) to compute defect formation energies and charge transition levels of hundreds of impurities in CdX chalcogenide compounds, where X = Te, Se or S. We apply machine learning techniques on the DFT data and develop on-demand predictive models for the formation energy and relevant transition levels of any impurity atom in any site. The trained ML models are general and accurate enough to predict the properties of any possible point defects in any Cd-based chalcogenide, as we prove by testing on a few selected defects in mixed chalcogen compounds CdTe 0.5 Se 0.5 and CdSe 0.5 S 0.5 . The ML framework used in this work can be extended to any class of semiconductors.
Final Report: Preparation and Evaluation of N-Type CdSeTe and CdTe as an Absorber in Thin Film PV
The primary objective of this project was to demonstrate n-type CdTe and CdSeTe films prepared by in-situ doping methods using pre-doped source material. Our goal was to achieve electron density (>10 17 cm -3 ) and lifetime (>10 ns) in n-type CdTe and CdSeTe films. The focus was on indium as the dopant, as it is suitable for incorporation during melt growth of CdTe. It was thought that indium doped bulk crystals might exhibit self compensation and high resistivity, so post-growth annealing studies were planned to manipulate crystal stoichiometry, defects and electrical activity. Single crystals of CdTe:In and CdSe 0.4 Te 0.6 :In, with target In concentration 10 19 cm -3 , were grown by the vertical Bridgman crystal growth method at Washington State University (WSU). Incorporation of the intended In into the crystals was ~25%-30%, (2.5-3) ×10 18 cm -3 , according to glow discharge mass spectrometry (GDMS). Carrier concentration in CdTe:In crystals were very low until after a Cd anneal, at which point the concentration reached 8 ×10 17 cm -3 (~27% activation) according to Hall effect and time resolved photoluminescence (TRPL). CdSeTe:In, on the other hand, had ~100% carrier activation with no post-growth treatments, resulting in ~3×10 18 cm -3 carriers with a lifetime of 1.5 ns, near the radiative limit for this material.
Hybrid Functional Calculations for Antimony Doping in CdTe
CdTe-based solar cells are leading thin-film photovoltaic technology, with efficiencies over 22%, but still much lower than the theoretical maximum of 29%. Further improvements will rely on increasing the open-circuit voltage Voc, which, in turn, depends on carrier density and lifetime. Using hybrid density functional calculations, we investigate Sb doping of CdTe, focusing on its limitation as shallow acceptor and the formation of compensating AX center. Paying special attention to supercell size and effects of spin-orbit coupling, we predict an ionization energy of 116 meV in the dilute limit, much closer to recent experimental value of 103 meV from temperature-dependent Hall measurements in bulk single crystals, and in contrast to much larger values from previous calculations. We also find that the Sb-related AX centers are not major compensation centers in Sb-doped CdTe.