Group-V acceptors and compensation centers in CdTe
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Cadmium telluride (CdTe) is a leading thin-film material for photovoltaic applications. Owing to its high efficiency, cost-effectiveness, and scalability, CdTe-based solar cells are strong competitors to conventional silicon-based technology. These solar cells offer comparable efficiency with a simpler manufacturing process and a lower carbon footprint. Moreover, due to CdTe’s high absorption coefficient and direct band gap of 1.5 eV, the absorber layer can be significantly thinner than that of silicon, reducing material usages and production costs. ☐ The efficiency of CdTe-based solar cells has steadily improved, recently surpassing 23%. Further progress toward the theoretical limit of 33% remains an active area of research. A key challenge is enhancing p-type doping in the CdTe absorber layer. Group-V elements—P, As, and Sb—are commonly used to increase hole concentrations from the intrinsic level of ∼1014 to ∼1016 cm−3. However, doping efficiency is often low, with hole concentrations significantly lower than the dopant levels. The origin of this compensation remains unclear, though defects and grain boundaries are expected to play important roles in this low doping efficiency. ☐ This thesis investigates the role of defects, dopants, native defects, and their complexes, on carrier dynamics in CdTe. We employ first-principles calculations based on density functional theory (DFT) to study defect electronic structures. To accurately capture the band gap and band edge positions, hybrid DFT with spin-orbit coupling (SOC) is used. Calculations using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional with 33% Hartree-Fock mixing and SOC yield an improved description of the band structure, including a ∼0.3 eV shift in the valence band maximum due to relativistic effects from heavy Te atoms. ☐ The study first revisits group-V substitutional defects as acceptors. These shallow-level defects exhibit extended wavefunctions, leading to artificial interactions in finite-size supercells. To address this, ionization energies were extrapolated to the dilute limit using 64-, 216-, and 512-atom supercells. The extracted values of 93, 99, and 116 meV for P, As, and Sb, respectively, agree with temperature-dependent Hall measurements. Notably, AX centers, previously thought to be dominant compensation mechanisms, are shown to be unstable under typical doping conditions and thus unlikely to be major sources of compensation in group-V doped CdTe. ☐ The thesis then examines native point defects, focusing on their formation energies and migration barriers. While Cd interstitials are the lowest-energy donor defects, their low migration barriers make them unstable at room temperature. These interstitials are crucial for charge neutrality during high-temperature growth or annealing but tend to diffuse out upon cooling. Consequently, only more stable defects determine the material’s conductivity. Accounting for this in the charge neutrality equation yields carrier concentrations and conductivity types consistent with experimental data. In p-type doped samples, Cd interstitials may still be present in complex forms. ☐ One such complex involves group-V acceptors binding with Cd interstitials to form donor-type complexes that compensate for hole doping. Thermal annealing can dissociate these complexes, releasing Cd interstitials to the surface. The results suggest that annealing conditions are critical. Cd-rich environments favor complex formation, while Cd-poor conditions increase hole concentration. This complex explains experimentally observed trends in hole concentration and highlights its significant role in limiting doping efficiency in group-V doped CdTe. ☐ While group-V incorporation on Cd sites can also act as donor defects, they are not dominant under Cd-rich conditions, which is the most favorable environment for incorporating group-V atoms on Te sites and is commonly used in doping. The study also explores group-V pairing configurations, emphasizing the importance of using isolated dopant form as the doping source and controlling dopant concentrations. ☐ The analysis further highlights that the ionization energies of group-V acceptors (∼100 meV) are not-so-shallow compared to those of conventional shallow acceptors, contributing to lower doping activation at high dopant concentrations. ☐ Finally, the study extends to CdSeTe alloys. It is shown that group-V dopants can become deep acceptors in these materials. For example, the Sb acceptor level in CdSe0.25Te0.75 is found to lie 0.3–0.4 eV above the valence band maximum—significantly deeper than in CdTe—due to local structural distortions that break the ideal Td symmetry. ☐ In summary, this thesis provides new insights into doping and defect physics in CdTe. It demonstrates that group-V elements can serve as effective p-type dopants, given that doping and annealing conditions are carefully controlled. The findings underscore the importance of considering both defect formation energy and migration barrier in understanding defect stability. These results represent a critical step toward improving CdTe photovoltaic technology by enabling more effective doping strategies and a deeper understanding of defect behavior.
Description
Keywords
Defects, Dopants, First-principles calculations, Solar cells, Cadmium telluride