Auger Recombination Processes Research Articles

Optical Cooling of Dot-in-Crystal Halide Perovskites: Challenges of Nonlinear Exciton Recombination.

Highly efficient anti-Stokes (AS) photoluminescence (PL) is observed from halide perovskite quantum dots (QDs) due to their strong electron-phonon interactions. The AS PL is particularly intriguing, as it suggests the potential for semiconductor optical cooling if the external quantum efficiency approaches 100%. However, the PL quantum efficiency in QDs is primarily dominated by multiparticle nonradiative Auger recombination processes under intense photoexcitation, which impose limits on the optical cooling gain. Here, we investigate the Auger recombination of dot-in-crystal perovskites. We quantitatively estimate the maximum optical cooling gain and the corresponding excitation intensity. We further conducted optical cooling experiments and demonstrate a maximum photocooling of approximately 9 K from room temperature. Additionally, we confirmed that increasing the excitation intensity leads to a transition from photocooling to photoheating. These observations are consistent with our time-resolved measurements, offering insights into the potential and limitations of optical cooling in semiconductor QDs.

Universal Surface Repairing Strategy Enabling Suppressed Auger Recombination in CsPbBr3 Perovskite Quantum Dots for Low Threshold Lasing in a Superlattice Microcavity

AbstractLead halide perovskite quantum dots (LHP QDs) emerge as versatile photonic sources in electroluminescence and laser devices. Their applications in optoelectronic devices are hindered by surface distortion and ligand stripping during the purification process. In addition, the device performance roll‐off suffers from the ultrafast non‐radiative Auger recombination (AR). To address these issues, herein, CsPbBr3 QDs are prepared with sizes ranging from 6.5 to 9.2 nm and the universal surface repairing strategy is proved to efficiently increase the biexciton lifetime and quantum yield up to 1.4 and 3 times, respectively. The strategy provides a PbBrx rich surface and strong binding didodecyldimethylammonium bromide (DDAB) ligands to achieve near‐unity photoluminescence efficiency, which significantly improves their stability against anti‐solvents and aging in ambient conditions. A single‐mode laser device is realized by self‐assembling the QDs into superlattice as optical microcavity, with low threshold of 9.45 µJ cm−2, quality factor of 2187 and ultrafast lifetime of 3.22 ps. The superlattice laser is demonstrated as an optical encoder with large coding bandwidth of 0.133 THz. This work proposes new insight into the non‐radiative AR process in these single‐component CsPbBr3 QDs, and will pave the way for optoelectronic devices working in intense conditions.

Resolving the Anisotropic Dynamical Evolution of Wide Bandgap GeSe2 Under Polarized Photoexcitation

AbstractGermanium diselenide (GeSe2) has recently emerged as a promising material for high‐performance optoelectronic devices due to its high absorption coefficient, wide direct bandgap, and anisotropic electronic properties. For the future applications, it is necessary to understand the dynamic evolution of carriers with ultrafast response. In this work, the transient absorption spectroscopy measurements are performed to study the anisotropic dynamic evolution of GeSe2 under the polarized photoexcitation. It is find that the dynamics exhibit linear dichroism and show a significant Auger recombination process when the photoexcitation is perpendicular to the b‐axis of GeSe2. Using first‐principle and transition dipole moment calculations, it is confirmed that the more carrier localization at Γ point along the Γ‐X (perpendicular to the b‐axis) and increase the Auger recombination. In addition, the Auger recombination coefficient ≈ 1.1×10−31 cm6 s−1 is determined and demonstrated that the defect‐assisted indirect Auger recombination plays a key role in the carrier relaxation process. Through time‐resolved transient absorption spectra, it is further revealed that the ultrafast dynamical evolution of excited state absorption peaks under polarized photoexcitation. These results have implications for improving the quantum efficiency of high‐field polarization‐sensitive optoelectronics.

Electron transfer tuned by pressure-dependent aggregation-induced emission in InP/ZnS quantum dot–anthraquinone complexes

Electron transfer (ET) process is considered a substantial factor in influencing the photoelectric conversion efficiency of optoelectronic devices. While pressure has demonstrated effective tune ET, a comprehensive investigation into the mechanisms for both restraining and promoting ET remains elusive. Herein, we have performed measurements using in situ high-pressure steady-state photoluminescence (PL), Raman scattering spectra, and femtosecond transient absorption (fs-TA) spectroscopy on InP/ZnS quantum dot–anthraquinone (InP/ZnS QD-AQ) complexes. The experimental results have demonstrated that the pressure-suppressed ET process in the InP/ZnS QD-AQ complexes arises from both the aggregation-induced emission (AIE) effect of AQ in toluene and the quantum confinement effect of the InP/ZnS QDs. The reduction in the distance between InP/ZnS QD and AQ under pressure emerges as a key factor that promotes the ET process in the InP/ZnS QD-AQ complexes. Furthermore, we observed that the pressure not only enhances the ET process but also suppresses the auger recombination process in liquid phase I of toluene, consequently leading to an enhancement in the photoelectric conversion efficiency. This study contributes to understanding the mechanism of the ultrafast dynamic processes in the pressure-induced QD-receptor complexes, and it has great potential for preparing efficient and stable optoelectronic devices.

Carrier density dependent Auger recombination in c-plane (In,Ga)N/GaN quantum wells: insights from atomistic calculations

Understanding Auger recombination in (In,Ga)N-based quantum wells is of central importance to unravelling the experimentally observed efficiency ‘droop’ in modern (In,Ga)N light emitting diodes (LEDs). While there have been conflicting results in the literature about the importance of non-radiative Auger recombination processes for the droop phenomenon, it has been discussed that alloy fluctuations strongly enhance the Auger rate. However, these studies were often focused on bulk systems, not quantum wells, which lie at the heart of (In,Ga)N-based LEDs. In this study, we present an atomistic analysis of the carrier density dependence of the Auger recombination coefficients in (In,Ga)N/GaN quantum wells. The model accounts for random alloy fluctuations, the connected carrier localisation effects, and carrier density dependent screening of the built-in polarisation fields. Our studies reveal that at low temperatures and low carrier densities the calculated Auger coefficients are strongly dependent on the alloy microstructure. However, at elevated temperatures and carrier densities, where the localised states are starting to be saturated, the different alloy configurations studied give (very) similar Auger coefficients. We find that over the range of carrier densities investigated, the contribution of the electron-electron–hole related Auger process is of secondary importance compared to the hole-hole-electron process. Overall, for higher temperatures and carrier densities, our calculated total Auger coefficients are in excess of 10−31 cm6 s−1 and may reach 10−30 cm6 s−1, which, based on current understanding in the literature, is sufficient to result in a significant efficiency droop. Thus, our results are indicative of Auger recombination being an important contributor to the efficiency droop in (In,Ga)N-based light emitters even without defect-assisted processes.

Quantifying non-threshold Auger-recombination processes in mid-wavelength infrared range HgCdTe quantum wells

We study photoluminescence temperature quenching in HgTe/CdHgTe quantum wells (QWs) emitting at 3–4 μm wavelengths and recover temperature-dependent interband recombination rates. Recombination coefficients are determined for the process that we identify as a non-threshold ehh Auger process involving valence band continuum states in barriers. With the effective Auger coefficient CA reaching ∼10−13 cm4/s under resonant conditions, such a process is shown to determine stimulated emission thresholds in a wide temperature interval, while the contribution of conventional, activated Auger processes is presumably rather limited. Thus, threshold energy considerations should be used with caution for the optimization of HgCdTe QW lasers operating around 3 μm, and relatively low-barrier QWs may provide better performance than the high-barrier ones despite lower energy thresholds for thermally activated eeh-Auger recombination. It holds as long as the conduction band offset is detuned from the bandgap energy to avoid additional non-threshold eeh-processes and sufficient hole localization at elevated temperatures is maintained.

Auger and spin dynamics in a self-assembled quantum dot

The Zeeman-split spin states of a single quantum dot can be used together with its optical trion transitions to form a spin–photon interface between a stationary (the spin) and a flying (the photon) quantum bit. In addition to long coherence times of the spin state itself, the limiting decoherence mechanisms of the trion states are of central importance. Here, we investigate in time-resolved resonance fluorescence the electron spin and trion dynamics in a single self-assembled quantum dot in an applied magnetic field of up to B=10 T. The quantum dot is only weakly coupled to an electron reservoir with tunneling rates of about 1ms−1. Using this sample structure, we can measure, in addition to the spin-flip rate of the electron and the spin-flip Raman rate of the trion transition, the Auger recombination process that scatters an Auger electron into the conduction band. The Auger effect destroys the radiative trion transition and leaves the quantum dot empty until an electron tunnels from the reservoir into the dot. The combination of an Auger recombination event with subsequent electron tunneling from the reservoir can flip the electron spin and thus constitutes another mechanism that limits the spin lifetime.

Unveiling the Impact of Organic Spacer Cations on Auger Recombination in Layered Halide Perovskites

AbstractA library of large organic cation spacers is available for engineering the performance of layered two‐dimensional (2D) halide perovskite devices. Despite extensive photophysics studies, there remains a research gap over the structure‐function relations in 2D perovskites, especially the underlying factors influencing the Auger recombination (AR) process. Herein, the contributions of exciton binding energy, exciton‐phonon coupling, and defects/film morphology to the AR process in 2D perovskites are examined. Phenyl‐alkyl‐ammonium cations with different lengths of attached alkyl groups, commonly used in blue light‐emitting diodes, are investigated. The findings reveal an order of magnitude higher threshold carrier density for the AR onset as well as a reduced AR in cations with longer alkyl chain length. Although possessing similar exciton binding energies, the exciton‐phonon coupling strength is found to play a major role in reducing the AR rate, with a smaller contribution from the defect states/film morphology. The findings can help provide further guidance on organic spacer cation engineering for highly efficient 2D perovskite light emitters.

Resolving Nonlinear Recombination Dynamics in Semiconductors via Ultrafast Excitation Correlation Spectroscopy: Photoluminescence versus Photocurrent Detection.

We explore the application of excitation correlation spectroscopy to detect nonlinear photophysical dynamics in two distinct semiconductor classes through time-integrated photoluminescence and photocurrent measurements. In this experiment, two variably delayed femtosecond pulses excite the semiconductor, and the time-integrated photoluminescence or photocurrent component arising from the nonlinear dynamics of the populations induced by each pulse is measured as a function of inter-pulse delay by phase-sensitive detection with a lock-in amplifier. We focus on two limiting materials systems with contrasting optical properties: a prototypical lead-halide perovskite (LHP) solar cell, in which primary photoexcitations are charge photocarriers, and a single-component organic-semiconductor diode, which features Frenkel excitons as primary photoexcitations. The photoexcitation dynamics perceived by the two detection schemes in these contrasting systems are distinct. Nonlinear-dynamic contributions in the photoluminescence detection scheme arise from contributions to radiative recombination in both materials systems, while photocurrent arises directly in the LHP but indirectly following exciton dissociation in the organic system. Consequently, the basic photophysics of the two systems are reflected differently when comparing measurements with the two detection schemes. Our results indicate that photoluminescence detection in the LHP system provides valuable information about trap-assisted and Auger recombination processes, but that these processes are convoluted in a nontrivial way in the photocurrent response and are therefore difficult to differentiate. In contrast, the organic-semiconductor system exhibits more directly correlated responses in the nonlinear photoluminescence and photocurrent measurements, as charge carriers are secondary excitations only generated through exciton dissociation processes. We propose that bimolecular annihilation pathways mainly contribute to the generation of charge carriers in single-component organic semiconductor devices. Overall, our work highlights the utility of excitation correlation spectroscopy in modern semiconductor materials research, particularly in the analysis of nonlinear photophysical processes, which are deterministic for their electronic and optical properties.

Detection of Single Charge Trapping Defects in Semiconductor Particles by Evaluating Photon Antibunching in Delayed Photoluminescence.

Time-resolved analysis of photon cross-correlation function g(2)(τ) is applied to photoluminescence (PL) of individual submicrometer size MAPbI3 perovskite crystals. Surprisingly, an antibunching effect in the long-living tail of PL is observed, while the prompt PL obeys the photon statistics typical for a classical emitter. We propose that antibunched photons from the PL decay tail originate from radiative recombination of detrapped charge carriers which were initially captured by a very limited number (down to one) of shallow defect states. The concentration of these trapping sites is estimated to be in the range 1013-1016 cm-3. In principle, photon correlations can be also caused by highly nonlinear Auger recombination processes; however, in our case it requires unrealistically large Auger recombination coefficients. The potential of the time-resolved g(2)(0) for unambiguous identification of charge rerecombination processes in semiconductors considering the actual number of charge carries and defects states per particle is demonstrated.

4H-SiC Auger recombination coefficient under the high injection condition

The on-resistance of bipolar devices depends on the carrier lifetime, which is determined by Shockley–Read–Hall, surface, radiation, and Auger recombination processes. Values for the Auger recombination coefficient have been previously reported, but the values were constant in each report. However, the Auger recombination coefficient should depend on the concentration of excited carriers and the presence of traps. In this study, we observed excited carrier recombination in 4H-SiC under the high injection condition using time-resolved free carrier absorption measurements. Consequently, we discovered that the Auger recombination coefficient is dependent on the excited carrier concentration and that the traps have negligible effects on the coefficient.

Effect of trifluoroacetic acid on InP/ZnSe/ZnS quantum dots: mimicking the surface trap and their effects on the photophysical properties.

Understanding the precise effects of defects on the photophysical properties of quantum dots (QDs) is essential to their development with near-unity luminescence. Because of the complicated nature of defects in QDs, the origins and detailed roles of the defects still remain rarely understood. In this regard, we used detailed chemical analysis to investigate the effect of surface defects on the optical properties of InP/ZnSe/ZnS QDs by introducing shell defects through controlled trifluoroacetic acid (TFA) etching. TFA treatment on the InP/ZnSe/ZnS QDs partially removed the ZnS shell as well as ligands and reduced the quantum yield by generating energetically deep surface traps. The surface defects of QDs by TFA cause charged trap sites inducing an Auger recombination process with a rate of ca. 200 ps. Based on these results, we proposed possible trap-assisted non-radiative decay pathways between the band-edge state and surface deep traps in InP/ZnSe/ZnS QDs.

Auger scattering dynamic of photo-excited hot carriers in nano-graphite film

Charge carrier scattering channels in graphite bridging its valence and conduction band offer an efficient Auger recombination dynamic to promote low energy charge carriers to higher energy states. It is of importance to answer the question whether a large number of charge carriers can be promoted to higher energy states to enhance the quantum efficiency of photodetectors. Here, we present an experimental demonstration of an effective Auger recombination process in the photo-excited nano-graphite film. The time-resolved hot carrier thermalization was analyzed based on the energy dissipation via the Auger scattering channels. We split the Auger recombination occurrence centered at 0.40 eV energy state into scattering and recombination parts, for characterizing the scattering rate in the conduction band and the recombination rate toward the valence band. The scattering time with respect to the energy state was extracted as 8 ps · eV−1, while the recombination time with respect to the energy state was extracted as 24 ps · eV−1. Our study indicates a 300 fs delay between the hot carrier recombination and generation, leading to a 105 ps−1 · cm−3 Auger scattering efficiency. The observed duration for the Auger recombination to generate hot carriers is prolonged for 1 ps, due to the hot carriers energy relaxation bottleneck with optical-phonons in the nano-graphite. The presented analytic expression gives valuable insights into the Auger recombination dynamic to estimate its most efficient energy regime for mid-infrared photodetection.

Understanding the Origin of Degradation of InP‐Quantum Dot Light‐Emitting Diodes

AbstractThe origin of degradation of InP‐QLED (quantum dot LED) is reported by comparing the stability of Cd‐QDs and organic Bebq2:Ir(mphmq)2(tmd) emissive layers (EMLs). The degradation causes of InP‐QLED are checked by measuring the stability of hole and electron only devices (HOD and EOD) against hole/electron, exciton stress, and hole/electron‐exciton stress conditions. The results show that the InP‐QDs layer is more vulnerable to exciton and electron‐exciton stress compared to the Cd‐QDs and Bebq2:Ir(mphmq)2(tmd) due to the increase of surface defects in the InP‐QDs after exciton and electron‐exciton stress, which increases non‐radiative Auger recombination process. However, InP‐QDs are relatively less stable against electron and hole stress than the Cd‐QDs and Bebq2:Ir(mphmq)2(tmd). To reduce the electron‐exciton stress on the InP‐QDs, an inverted red QLED with InP‐QDs:DBTA EML is fabricated. The QLED with InP‐QDs: hole transport layer (DBTA) shows a low driving voltage of 5.7 V, high external quantum efficiency (EQE) of 10.2%, and longer lifetime (T50:557 h at 1000 cd m−2) than the reference InP‐QDs device (T50: 29 h at 1000 cd m−2).

Photoluminescence Spectroscopy of the InAsSb-Based p-i-n Heterostructure.

Photoluminescence in a double heterostructure based on a ternary InAsSb solid solution was observed in the mid-infrared range of 2.5–4 μm. A range of compositions of the InAs1−ySby ternary solid solution has been established, where the energy resonance between the band gap and the splitting-off band in the valence band of the semiconductor can be achieved. Due to the impact of nonradiative Auger recombination processes, different temperature dependence of photoluminescence intensity was found for the barrier layer and the narrow-gap active region, respectively. It was shown that efficient high-temperature photoluminescence can be achieved by suppressing the nonradiative Auger recombination (CHHS) process. Increased temperature, for which the energy gap is lower than the split-off band energy, leads to violation of the resonance condition in narrow gap antimonide compounds, which explains the observed phenomenon. This finding might influence future application of the investigated material systems in mid-infrared emitters used for, e.g., optical gas sensing.

Tailoring Transition Dipole Moment in Colloidal Nanocrystal Thin Film on Nanocomposite Materials (Advanced Optical Materials 4/2022)

A novel underlying optical interplay between quantum dot thin films and nanocomposite materials via image dipole interaction is revealed by Kwang Jin Lee, Kwang Seob Jeong, Minhaeng Cho, and co-workers (see article number 2102050). This study demonstrates that the image dipole interaction plays a critical role in reducing the net transition dipole moment amplitude, suppressing the biexciton Auger recombination process. This work provides a pure optical mechanism for development of novel optoelectronic applications including light-emitting devices, photodetectors, and solar cells.

Tailoring Transition Dipole Moment in Colloidal Nanocrystal Thin Film on Nanocomposite Materials

AbstractControlling the transition dipole moment is extremely important for various photophysical characteristics in semiconductors. Especially, suppression of Auger recombination in quantum dots (QDs) is essential for the development of novel applications, including bioimaging, lasing, and optoelectronic devices. To date, most of the studies on the Auger process are conducted on the basis of manipulating the material property such as wavefunction of electron and hole, energy band, and confinement potential. However, a new way of tuning the Auger process using nanocomposite materials is not reported. In this work, the biexciton Auger recombination (BAR) process in CdSe/CdS(1 ML) nanocrystal thin‐film is successfully controlled by introducing nanocomposite materials. Performing pump intensity‐dependent transient absorption experiments, a significant reduction (up to 30%) of BAR rate is observed in the presence of nanocomposite structures. This notable suppression effect is attributed to the modulation of the net transition dipole moment. These findings will provide further insight into the rational design of QDs combining with a nanostructure that efficiently suppresses Auger recombination rates.

Signatures of ultrafast electronic and atomistic dynamics in bulk photoluminescence of CVD and natural diamonds excited by ultrashort laser pulses of variable pulsewidth

Inter-band photoexcitation in bulk natural and synthetic diamonds by tightly focused near-IR (1030 nm) ultrashort laser pulses of variable duration (0.3–6.3 ps) promotes characteristic UV photoluminescence A-band and phonon-mediated free-exciton recombination bands, respectively, with distinct phonon progressions. The photoluminescence yield in these diamonds is very similar and highly-nonlinear versus laser intensity with the power slopes, gradually decreasing at higher intensities for all the laser pulsewidths to indicate varying contributions of multi-photon photoexcitation, free-carrier impact ionization and Auger recombination processes in electron-hole plasma (EHP). The peak position of the main free-excitonic photoluminescence band in the synthetic diamond demonstrates universal, pulsewidth-independent “red” spectral shift as a function of laser intensity, representing the electronic bandgap renormalization at the increasing plasma density. The related phonon progression indicates the predominating interaction of free carriers with near-center-zone optical phonons, exerting distinct dynamic increase in phonon energy at the increasing plasma density. Dynamic photoluminescence micro-spectroscopy appears as a simple, but informative experimental tool, paving the way to clarification and modeling of ultrafast electronic and lattice dynamics, as well as laser energy deposition, in different types of diamonds for their micromarking and tracing.

Förster resonance energy transfer outpaces Auger recombination in CdTe/CdS quantum dots-rhodamine101 molecules system upon compression.

Förster resonance energy transfer (FRET) and Auger recombination in quantum dots (QDs)-molecules system are important mechanisms for affecting performance of their optoelectronic and photosynthesis devices. However, exploring an effective strategy to promote FRET and suppress Auger recombination simultaneously remains a daunting challenge. Here, we report that FRET process is promoted and Auger recombination process is suppressed in CdTe/CdS QDs-Rhodamine101 (Rh101) molecules system upon compression. The greatly improved FRET is attributed to the shortened donor-acceptor distance and increased the number of molecules attached to QDs induced by pressure. The reduced Auger recombination is ascribed to the formation of an alloy layer at the core/shell interface. The FRET can occur 70 times faster than Auger recombination under a high pressure of 0.9 GPa. Our findings demonstrate that high pressure is a robust tool to boost FRET and simultaneously suppress Auger recombination, and provides a new route to QDs-molecules applications.

Size‐dependent Auger recombination in CdSe quantum dots studied by transient absorption spectroscopy

AbstractAs closely related to exciton quenching, Auger recombination (AR) is very important to colloidal semiconductor quantum dots (QDs) in applications such as lasing, solar energy conversion, and light‐emitting diodes. It is beneficial to understand the kinetic mechanism of AR. In this article, colloidal CdSe QDs with a narrow range of diameters, 2.7, 2.5, 2.3, 2.2, and 2.0 nm, are synthesized by adequately modulating the reaction temperature and time during hot injection synthesis. With sizes less than the Bohr exciton radius, the resultant collision probability differs for various CdSe QDs attributed to the varying extent of the spatial confinement effect. To analyze the bi‐exciton AR in these QDs, the ultrafast time‐resolved absorption spectrum is utilized to derive the bi‐exciton lifetime (τBX) in high precision as a function of QD size. The τBX varying with volume (V) obeys the universal scaling law; that is, τBX = γV. In our experiments, the scaling factor γ is 1.649 ± 0.097 ps/nm3. This study could provide further insight for understanding the theory of the AR process and rational design of optoelectronic devices based on colloidal QDs.