Spin-orbit coupling effects on predicting defect properties with hybrid functionals: A case study in CdTe

Abstract

Defect formation energies and transition levels are critical in determining doping behavior and recombination in semiconductor applications. Hybrid functionals are often used to overcome the bandgap and delocalization errors of standard density functional theory, and it is tempting to presume that the defect properties are correctly predicted once the hybrid functional mixing parameter reproduces the experimental band gap. However, pronounced spin-orbit coupling (SOC) effects can have an additional important role, this is clearly shown in this work by analyzing SOC effects originating from the Te-p orbitals in CdTe. In this work, we therefore use a hybrid functional that reproduces the experimental band gap when SOC is included, requiring a larger mixing parameter a = 0.33 compared to the conventional choice of a = 0.25. This hybrid functional was then used to predict defect properties, e.g., formation energy, transition level, and defect equilibrium. For defect states that do not directly involve the Te-p orbitals, such as the Cd interstitial, we find that the effect of SOC on the defect levels can be captured by simply considering the SOC induced band edge shift. This is not the case for the A center (ClTe+VCd defect pair), where the localized acceptor state formed by Te-p orbitals is more directly affected by the SOC. For this defect, a mixing parameter as large as a = 0.40 is required to reproduce the experimental acceptor level. Regarding the implications for photovoltaics, we suggest that the Cd interstitial, which is the dominant compensating donor, could play an important role as a recombination center. While Cdi is usually thought of as a benign shallow donor, our predicted defect levels in the fully band gap corrected calculations are deep enough to raise a concern, and we propose a new recombination mechanism for electron capture by Cdi.

Rapid Recombination by Cadmium Vacancies in CdTe

“… 1 , 2 , 6 Indeed, under the idealized detailed balance model, CdTe has an upper limit of 32% single-junction PV efficiency (based on its electronic bandgap), 7 indicating that there is still room for improvement. 6 , 8 − 11 …”

Interplay between Composition, Electronic Structure, Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN2:O

“…[11,18] However, the prediction of electron densities and doping via detailedbalance defect-equilibria is essentially limited to the dilute-defect and -impurity situation, so far. [19] Therefore, a more complete computational model, aiming at bridging these two limits, is necessary to predict the semiconducting properties for non-dilute solid solutions. As a further step, it is also desirable to extend the process parameters towards non-equilibrium synthesis.…”

Radiative Efficiency and Charge‐Carrier Lifetimes and Diffusion Length in Polycrystalline CdSeTe Heterostructures

“…Carrier lifetimes in polycrystalline CdSeTe DHs (Figure , as shown in previous studies) and in single‐crystal CdTe are comparable, which suggests similar SRH recombination center density. For CdTe, first‐principles analysis identified Te Cd antisite and Cd i interstitial point defects as possible SRH recombination centers. Using a calculated electron capture rate R e = 2.5 × 10 −7 cm 3 s −1 for Te Cd , lifetimes in Figure a can be used to estimate SRH defect density, N SRH = 5 × 10 12 cm −3 (from 1/ τ B = R e N SRH ), which is lower than that calculated and measured for CdTe.…”