Nonradiative Capture Research Articles

The role of hydrogen in nonradiative recombination in CsPbI3: a first-principles investigation

Defect-induced nonradiative recombination is the main factor hindering efficiency improvement in CsPbI3 perovskite solar cells. It has been recently claimed that the unintentionally incorporated H impurity can potentially cause nonradiative losses due to the deep levels. Using first-principles approaches, we show that, as a matter of fact, the H impurity has a negligible effect on carrier recombination in CsPbI3 due to its small nonradiative capture coefficient and low density. This insight rationalizes why the addition of hydriodic acid does not reduce carrier lifetime and could prevent acidic additives being discarded as potential candidates for assisting film growth.

ERCS24: An updated version of the ERCS08 program for calculations of the cross sections for atomic electron removal based on the ECPSSR theory and its variants

ERCS24: An updated version of the ERCS08 program for calculations of the cross sections for atomic electron removal based on the ECPSSR theory and its variants

Finite-size corrections to defect energetics along one-dimensional configuration coordinate

Recently, effective one-dimensional configuration coordinate diagrams have been utilized to calculate the line shapes of luminescence spectra and nonradiative carrier capture coefficients via point defects. Their calculations necessitate accurate total energies as a function of configuration coordinates. Although supercells under periodic boundary conditions are commonly employed, the spurious cell size effects have not been previously considered. In this study, we have proposed a correction energy formalism and verified its effectiveness by applying it to a nitrogen vacancy in GaN.

Metal Halide Perovskite Nanotubes for High-Performance Solar Cells with Ab Initio Analysis.

Compared to bulk metal halide perovskites, low-dimensional nanotubes can accommodate more intense atomic movement and octahedral distortion, leading to prompting the separation and localization of charge between the initial and final states and accelerating quantum coherence loss. Additionally, nonradiative carrier recombination is accompanied by weakened nonadiabatic coupling, which extends their lifetime by an order of magnitude. Common vacancy defects in perovskites act as nonradiative recombination centers, causing charge and energy loss. However, nanotubes and self-chlorinated systems can passivate and eliminate deep-level defects, resulting in a roughly two order of magnitude decrease in the nonradiative capture coefficient of lead vacancy defects. Simulation results demonstrate that the strategy of low-dimensional nanotubes and chlorine doping can provide helpful guidance and new insights for the design of high-performance solar cells.

Exploring the influence of target atomic number (Z2) on mean equilibrium charge state (q¯: A comprehensive study

We have investigated the effect of the target atomic number Z2 on the mean charge state (q¯ using various model predictions such as the Shima–Ishihara –Mikumo, Ziegler–Biersack–Littmark, Schiwietz, Schiwietz–Grande , Fermi-gas-models and theoretical codes with experimental data available in the literature. This investigation makes it possible to determine the best-fit model to calculate q¯. In this work, we discuss the post-collision charge state distribution in different targets used as thin films and projectile beams (Fq+, Siq+, Clq+, and Cuq+) with different charge states (available in literature). A detailed overview of such collision experiments has been explored over a wide energy range of 1.07–3.93 MeV/u. In this contribution, an overview of the mean charge state dependence on the Fermi velocity of target materials is provided. Finally, the influence of the non-radiative electron capture at the target exit surface on the projectile charge state distribution for fast projectiles in different targets is shown, and a comparison is made with experimental data.

Origin of Enhanced Nonradiative Carrier Recombination Induced by Oxygen in Hybrid Sn Perovskite

Oxygen ingression has been shown to substantially decrease the carrier lifetime of Sn-based perovskites, behind which the mechanism remains yet unknown. Our first-principles calculations reveal that in prototypical MASnI3 (MA = CH3NH3), oxygen by itself is not a recombination center. Instead, it tends to form substitutional OI through combining with native I vacancies (VI) and remarkably increases the original recombination rate of VI by 2-3 orders of magnitude. This rationalizes the experimentally observed sharp decline of carrier lifetime in perovskites exposed to air. The significantly enhanced carrier recombination is due to a smaller electron capture barrier of OI, resulting from lattice strengthening and the suppressed structural relaxation upon electron capture. These insights offer a route to further improve device performance via anion engineering in broad Sn-based perovskite optoelectronics operating in ambient air. Moreover, our results highlight the important role of lattice relaxation for nonradiative carrier capture in materials in general.

Nearly isotropic Lyman-α 1 radiation following nonradiative electron capture in Xe54+ + Kr collisions at 197 MeV u−1

We measured the angular distribution of the subsequently emitted Lyman-α 1 transition of Xe53+* ions following nonradiative electron capture in collisions of 197 MeV u−1 Xe54+ projectiles with gaseous krypton target. The alignment of the projectile 2p 3/2 state and the relative population of its magnetic substates were further deduced. In contrast to the available lower energy results (Yang et al 2020 Phys. Rev. A 102 042803), it is found that at 197 MeV u−1 the Lyman-α 1 radiation becomes much less anisotropic and the magnetic substates are nearly statistically populated. Moreover, the experimental findings are compared with the results of the corresponding radiative electron capture mechanism, obviously different energy dependence of the magnetic-substate population is obtained and discussed using semiclassical arguments, which help reveal the population mechanism of excited states in collisions of highly charged high-Z ions with atoms.

A quantum model of charge capture and release onto/from deep traps.

The rapid development of optical technologies and applications revealed the critical role of point defects affecting device performance. One of the powerful tools to study the influence of defects on charge capture and recombination processes is thermoluminescence. The popular models behind thermoluminescence and carrier capture processes are semi-classic though. They offer a good qualitative description, but implicitly exclude the quantum nature of the accompanying parameters, such as frequency factors and capture cross sections. As a consequence, results obtained for a specific host material cannot be successfully extrapolated to other materials. Thus, the main purpose of our work is to introduce a reliable analytical model that describes non-radiative capture and release of electrons from/to the conduction band (CB). The proposed model is governed by Bose-Einstein statistics (for phonon occupation) and Fermi's golden rule (for resonant charge transfer between the trap and the CB). The constructed model offers a physical interpretation of the capture coefficients and frequency factors, and seamlessly includes the Coulomb neutral/attractive nature of traps. It connects the frequency factor to the overlap of wavefunctions of the delocalized CB and trap states, and suggests a strong dependence on the density of charge distribution, i.e. the ionicity/covalency of the chemical bonds within the host. Separation of the resonance conditions from the accumulation/dissipation of phonons on the site leads to the conclusion that the capture cross-section does not necessarily depend on the trap depth. The model is verified by comparison to the reported experimental data, showing good agreement. As such, the model generates reliable information about trap states whose exact nature is not completely understood and allows performing materials research in a more systematic way.

First-principles study of non-radiative carrier capture by defects at amorphous-SiO2/Si(100) interface

Defects have a significant impact on the performance of semiconductor devices. Using the first-principles combined with one-dimensional static coupling theory approach, we have calculated the variation of carrier capture coefficients with temperature for the interfacial defects P b0 and P b1 in amorphous-SiO2/Si(100) interface. It is found that the geometrical shapes of P b0 and P b1 defects undergo large deformations after capturing carriers to form charged defects, especially for the Si atoms containing a dangling bond. The hole capture coefficients of neutral P b0 and P b1 defects are largest than the other capture coefficients, indicating that these defects have a higher probability of forming positively charged centres. Meanwhile, the calculated results of non-radiative recombination coefficient of these defects show that both P b0 and P b1 defects are the dominant non-radiative recombination centers in the interface of a-SiO2/Si(100).

Antisite Defect SiC as a Source of the DI Center in 4H‐SiC

DI center, as a widely existed defect in 4H‐SiC, has attracted much attention in recent years, while the origin of it is still unclear. Herein, by comparing first‐principles calculated potential point defects‐related zero‐phonon lines (ZPLs) and nonradiative capture cross‐sections with the DI center‐related values in the experiment, it is proposed that the transition from the bound exciton states about 57 meV below the CBM to the “+1/+2” level of antisite defect SiC is responsible for the DI center in 4H‐SiC. This work describes the temperature and transition energy level dependence of the hole capture cross‐section of antisite defect in 4H‐SiC, which provides an idea for the optimal design of point defects in semiconductor materials.

To define nonradiative defects in semiconductors: An accurate DLTS simulation based on first-principle

Deep level transient spectroscopy (DLTS) is a crucial technique to characterize the defects in semiconductor devices. Within the framework of the state-of-the-art density functional theory (DFT), the simulation for experimental DLTS spectra is realized successfully with high accuracy and intrinsic physics in this work, of which the reliability is confirmed by nonlocal hybrid functional. The electron–phonon coupling is included to describe the thermal-activated behavior of the hot carrier nonradiative capture reflected by DLTS signal. In the case study, the divacancy in silicon is employed for the flow of DLTS calculation as a benchmark, of which the results are verified by our 40 MeV Si irradiation experiments on the NPN and PNP bipolar junction transistors and previous reports. The problem we can address is the identification of defect characterized in DLTS experiments. The realization of DLTS simulation would have a profound significance for understanding the underlying physics in experimental observations and be a powerful leverage to define defect nature in semiconductors.

Origin of Efficiency Enhancement by Lattice Expansion in Hybrid-Perovskite Solar Cells.

It has been experimentally observed that light-induced lattice expansion could enhance the solar conversion efficiency in hybrid perovskites, but the origin remains elusive. By performing rigorous first-principles calculations for a prototypical hybrid-perovskite FAPbI_{3} (FA: formamidinium), we show that 1% lattice expansion could already reduce the nonradiative capture coefficient by one order of magnitude. Unexpectedly, the suppressed nonradiative capture is not caused by changes in the band gap or defect transition level due to lattice expansion, but originates from enhanced defect relaxations associated with charge-state transitions in the expanded lattice. These insights not only provide a rationale for the efficiency enhancement by lattice expansion in hybrid perovskites, but also offer a general approach to the manipulation of nonradiative capture via strain engineering in a wide spectrum of optoelectronic materials.

Importance of surface oxygen vacancies for ultrafast hot carrier relaxation and transport in Cu2 O

${\mathrm{Cu}}_{2}\mathrm{O}$ has appealing properties as an electrode for photoelectrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (${V}_{\mathrm{O}}$) in the experimentally observed ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ reconstruction of the dominant (111) surface. Our results show that these ${V}_{\mathrm{O}}$ are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged ${V}_{\mathrm{O}}$ plays a crucial role in the nonradiative electron capture process with a capture coefficient of about ${10}^{\ensuremath{-}9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{3}$/s and a lifetime of 0.04 ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface ${V}_{\mathrm{O}}$ chemistry will be a crucial step in optimizing ${\mathrm{Cu}}_{2}\mathrm{O}$ for photoelectrode applications.

All-inorganic halide perovskites as candidates for efficient solar cells

Hybrid perovskites currently exhibit higher efficiencies than their all-inorganic counterparts in photovoltaic applications, which has led to a common belief that the organic cation somehow suppresses defect-assisted nonradiative recombination. Using first-principles calculations, here we show that the dominant nonradiative recombination center in CsPbI 3 is the iodine interstitial, which causes similar nonradiative capture rates as in MAPbI 3 (MA = CH 3 NH 3 ). However, the MA cation can give rise to additional strong nonradiative recombination centers such as hydrogen vacancies, which are absent from CsPbI 3 . One major advantage of MA + over Cs + is a better stability of the perovskite phase. Our study suggests that all-inorganic halide perovskites, if they can be stabilized, hold great promise for high-efficiency optoelectronic applications. These critical insights may prevent all-inorganic halide perovskites from being disregarded as potentially strong candidates for solar cell materials. • CsPbI 3 suffers less from nonradiative recombination than its hybrid counterpart MAPbI 3 • The inferior performance of CsPbI 3 very likely stems from poor stability • All-inorganic halide perovskites hold great promise for efficient solar cells It is commonly believed that hybrid perovskites are superior to their all-inorganic counterparts because of suppressed nonradiative losses. Using rigorous quantum mechanical calculations, Zhang et al. show that, in fact, all-inorganic halide perovskites are also very competitive potential candidates for efficient solar cells.

Quantitative description of carrier dynamics in GaSb/GaAs quantum-ring-with-dot structures

Self-assembled GaSb/GaAs quantum-ring-with-dot structures (QRDSs) are the nanostructures exhibiting type-II band alignment. Each QRDS consists of both quantum ring (QR) and quantum dot (QD) parts which have their own energy levels. In this work, the carrier dynamics in the GaSb/GaAs QRDSs are explored and quantitatively described by a rate equation model which is developed from experimental photoluminescence (PL) spectra in literature. The model is comprised of the carrier transition rates and activation energies involved the transfer processes of thermal-excited carriers. The difference between the QR and QD PL intensities is also taken into account in the model. Electronic structures of a single QRDS are calculated in order to determine the overlap between electron and hole wave functions that can be used for estimating the radiative transition rates. Numerical values of the radiative and non-radiative transition rates, carrier capture and escape rates, and activation energies are presented. The PL spectra obtained from the model are consistent with the reported experimental data.

State-selective nonradiative electron capture in collisions of 95–197−MeV/uXe54+ with Kr and Xe

Lyman transitions of ${\mathrm{Xe}}^{53+*}$ produced by nonradiative electron capture in collisions of 95, 146, and 197 ${\mathrm{MeV/u}\phantom{\rule{4pt}{0ex}}\mathrm{Xe}}^{54+}$ ions with Kr and Xe atoms have been studied experimentally. It shows that the captured electrons preferentially populate lower states. The intensity ratios $I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\beta})/I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\alpha})$ and $I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\gamma})/I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\alpha})$ are found to decrease with increasing projectile velocity, and the ratios for Xe decrease faster than those of Kr. The obtained results are compared with the relative cross sections calculated with the relativistic eikonal approximation. For both targets, the theoretical results overestimate the experimental ratio $I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\beta})/I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\alpha})$ but underestimate $I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\gamma})/I(\mathrm{Ly}\text{\ensuremath{-}}\ensuremath{\alpha}).$ Moreover, the theory gives parallel decreasing curves for the two targets, which does not agree with the present experiment. In order to well understand the experimental findings, a more sophisticated theoretical approach is urgently required.

Nonrad: Computing nonradiative capture coefficients from first principles

Point defects in semiconductor crystals provide a means for carriers to recombine nonradiatively. This recombination process impacts the performance of devices. We present the Nonrad code that implements the first-principles approach of Alkauskas et al. (2014) [8] for the evaluation of nonradiative capture coefficients based on a quantum-mechanical description of the capture process. An approach for evaluating electron-phonon coupling within the projector augmented wave formalism is presented. We also show that the common procedure of replacing Dirac delta functions with Gaussians can introduce errors into the resulting capture rate, and implement an alternative scheme to properly account for vibrational broadening. Lastly, we assess the accuracy of using an analytic approximation to the Sommerfeld parameter by comparing with direct numerical evaluation. Program summaryProgram Title: NonradCPC Library link to program files:https://doi.org/10.17632/xmfj4zxmn3.1Developer's repository link:https://doi.org/10.5281/zenodo.4274317Code Ocean capsule:https://codeocean.com/capsule/5582062Licensing provisions: MIT LicenseProgramming language: Python 3Nature of problem: Nonradiative carrier capture at point defects in semiconductors and insulators significantly impacts the performance of devices. A first-principles approach to calculating nonradiative capture rates is necessary to guide materials design and provide atomistic insight into the nonradiative capture process.Solution method: Our code, written in the Python language, implements the first-principles methodology developed by Alkauskas et al. (2014) [8]. Nonradiative capture rates are calculated using a one-dimensional approach, which maps the prohibitively large phonon problem onto a single, effective phonon mode. Harmonic phonon matrix elements may then be computed using an analytic technique or by direct numerical integration of the harmonic oscillator wavefunctions. The electron-phonon coupling is evaluated using the VASP code [Kresse and Furthmüller (1996) [27]] to linear order for the effective mode.

First-principles identification of deep energy levels of sulfur impurities in silicon and their carrier capture cross sections

Studies of the deep energy levels and nonradiative carrier capture induced by sulfur doping in silicon were initiated 60 years ago; however, the defect configurations, their deep energy levels, and the carrier capture cross sections are still not well understood. In this study, we focus on SSi substitution, and perform a first-principles study of its defect configurations and the deep energy levels using hybrid exchange-correlation functional. We discover a new distortive configuration for SSi + besides the previously obtained structure with higher symmetry. For both SSi + configurations, the deep transition levels (0/+) and (+/2+) are determined as 0.35 eV and 0.68 eV below the conduction band minimum, respectively. As a benchmark calculation, the hole-capture cross sections for neutral and +1 charged states are obtained based on the distortive structure. The cross section for SSi + agrees with the experiment, demonstrating the multi-phonon process for SSi + capturing a hole, whereas the cross section of SSi 0 is significantly lower than the experimental data because hole capture by SSi 0 is an Auger-type process. Our calculations provide a benchmark for the evaluation of the cross section of carrier capture in semiconductors using multi-phonon nonradiative recombination theory.

Alignment of the projectile 2p3/2 state created by nonradiative electron capture in 95- and 146-MeV/u Xe54+ with Kr and Xe collisions

Anisotropic distributions of the Lyman-${\ensuremath{\alpha}}_{1}$ transition of the down-charged $\mathrm{X}{\mathrm{e}}^{53+*}$ ions are observed in fast $\mathrm{X}{\mathrm{e}}^{54+}$ with Kr and Xe collisions, in which the electron is transferred by the nonradiative electron capture mechanism. At the energy of 95 MeV/u, the alignment parameter of the projectile $2{p}_{3/2}$ state, ${\mathcal{A}}_{20}$, is determined to be \ensuremath{-}0.35 \ifmmode\pm\else\textpm\fi{} 0.02 for both targets. While the projectile energy is 146 MeV/u, the parameter for the Kr and Xe target is \ensuremath{-}0.28 \ifmmode\pm\else\textpm\fi{} 0.02 and \ensuremath{-}0.29 \ifmmode\pm\else\textpm\fi{} 0.04, respectively. The present results demonstrate that the electron capture to the ${m}_{j}=\ifmmode\pm\else\textpm\fi{}1/2$ magnetic substates is about two times more probable than to the ${m}_{j}=\ifmmode\pm\else\textpm\fi{}3/2$ ones.

Anharmonic lattice relaxation during nonradiative carrier capture

Lattice vibrations of point defects are essential for understanding non-radiative electron and hole capture in semiconductors as they govern properties including persistent photoconductivity and Shockley-Read-Hall recombination rate. Although the harmonic approximation is sufficient to describe a defect with small lattice relaxation, for cases of large lattice relaxation it is likely to break down. We describe a first-principles procedure to account for anharmonic carrier capture and apply it to the important case of the \textit{DX} center in GaAs. This is a system where the harmonic approximation grossly fails. Our treatment of the anharmonic Morse-like potentials accurately describes the observed electron capture barrier, predicting the absence of quantum tunnelling at low temperature, and a high hole capture rate that is independent of temperature. The model also explains the origin of the composition-invariant electron emission barrier. These results highlight an important shortcoming of the standard approach for describing point defect ionization that is accompanied by large lattice relaxation, where charge transfer occurs far from the equilibrium configuration.