Auger Recombination Of Excitons Research Articles

Space charge region recombination in highly efficient silicon solar cells

The recombination rate in the space charge region (SCR) of a silicon-based barrier structure with a long Shockley–Reed–Hall lifetime is calculated theoretically by taking into account the concentration gradient of excess electron-hole pairs in the base region. Effects of the SCR lifetime and applied voltage on the structure ideality factor have been analyzed. The ideality factor is significantly reduced by the concentration gradient of electron-hole pairs. This mechanism provides an increase of the effective lifetime compared to the case when it is insignificant, which is realized at sufficiently low pair concentrations. The theoretical results have been shown to be in agreement with experimental data. A method of finding the experimental recombination rate in SCR in highly efficient silicon solar cells (SCs) has been proposed and implemented. It has been shown that at the high excess carrier concentration exceeding 1015 cm–3 the contribution to the SCR recombination velocity from the initial region of SCR that became neutral is significant. From a comparison of theory with experiment, the SCR lifetime and the ratio of the hole to the electron capture cross sections are determined for a number of silicon SCs. The effect of SCR recombination on the key characteristics of highly efficient silicon SCs, such as photoconversion efficiency and open-circuit voltage, has been evaluated. It has been shown that they depend not only on the charge carrier lifetime in SCR, but also on the ratio of hole to electron capture cross sections σp /σn. When σp /σn < 1, this effect is significantly strengthened, while in the opposite case σp /σn > 1 it is weakened. It has been ascertained that in a number of highly efficient silicon SCs, the distribution of the inverse lifetime in SCR is described by the Gaussian one. The effect described in the paper is also significant for silicon diodes with a thin base, p-i-n structures, and for silicon transistors with p-n junctions. In Appendix 2, the need to take into account the lifetime of non-radiative excitonic Auger recombination with participation of deep impurities in silicon is analyzed in detail. It has been shown, in particular, that considering it enables to reconcile the theoretical and experimental dependences for the effective lifetime in the silicon bulk.

Quantum Shell in a Shell: Engineering Colloidal Nanocrystals for a High-Intensity Excitation Regime.

Many optoelectronic processes in colloidal semiconductor nanocrystals (NCs) suffer an efficiency decline under high-intensity excitation. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, reducing the efficiency and life span of NC-based devices, including photodetectors, X-ray scintillators, lasers, and high-brightness light-emitting diodes (LEDs). Recently, semiconductor quantum shells (QSs) have emerged as a promising NC geometry for the suppression of Auger decay; however, their optoelectronic performance has been hindered by surface-related carrier losses. Here, we address this issue by introducing quantum shells with a CdS-CdSe-CdS-ZnS core-shell-shell-shell multilayer structure. The ZnS barrier inhibits the surface carrier decay, which increases the photoluminescence (PL) quantum yield (QY) to 90% while retaining a high biexciton emission QY of 79%. The improved QS morphology allows demonstrating one of the longest Auger lifetimes reported for colloidal NCs to date. The reduction of nonradiative losses in QSs also leads to suppressed blinking in single nanoparticles and low-threshold amplified spontaneous emission. We expect that ZnS-encapsulated quantum shells will benefit many applications exploiting high-power optical or electrical excitation regimes.

Temperature Dependence of Excitonic Auger Recombination in Excitonic-Complex-Free Monolayer WS2 by Considering Auger Broadening and Generation Efficiency.

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their optoelectronic properties and applications. However, even at moderate exciton densities, their light-emitting capability is severely limited by Auger-type exciton-exciton annihilation (EEA). Previous work on EEA used oversimplified models in the presence of excitonic complexes, resulting in seriously underestimated values for the Auger coefficient. In this work, we transferred monolayer WS2 on a gold substrate with hBN encapsulation, where excitons persist as the main species at 3-300 K via metal proximity. We numerically solved the rate equation for excitons to accurately determine the Auger coefficient as a function of temperature by considering laser pulse width and spatially inhomogeneous exciton distribution. We found that the Auger coefficient consists of temperature-dependent and independent terms, consistent with a theoretical model for direct and exchange processes, respectively. We believe that our results provide a guide for enhancing the luminescence quantum yield of TMDs.

Quantum shells versus quantum dots: suppressing Auger recombination in colloidal semiconductors.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of attention in recent decades. The quantum efficiency of many optoelectronic processes based on these nanomaterials, however, declines with increasing optical or electrical excitation intensity. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, whereby reducing the efficiency and lifespan of NC-based devices, including lasers, photodetectors, X-ray scintillators, and high-brightness LEDs. Recently, semiconductor quantum shells (QSs) have emerged as a viable nanoscale architecture for the suppression of Auger decay. The spherical-shell geometry of these nanostructures leads to a significant reduction of Auger decay rates, while exhibiting a near unity photoluminescence quantum yield. Here, we compare the optoelectronic properties of quantum shells against other low-dimensional semiconductors and discuss their emerging opportunities in solid-state lighting and energy-harvesting applications.

Spatiotemporal dynamics of free and bound excitons in CVD-grown MoS2 monolayer

We study photoluminescence (PL) spectra and exciton dynamics of the MoS2 monolayer (ML) grown by the chemical vapor deposition technique. In addition to the usual direct A-exciton line, we observe a low-energy line of bound excitons dominating the PL spectra at low temperatures. This line shows unusually strong redshift with an increase in the temperature and submicrosecond time dynamics suggesting indirect nature of the corresponding transition. By monitoring the temporal dynamics of exciton PL distribution in the ML plane, we observe diffusive transport of A-excitons and measure the diffusion coefficient up to 40 cm2/s at elevated excitation powers. The bound exciton spatial distribution spreads over tens of micrometers in ∼1 μs. However, this spread is subdiffusive, characterized by a significant slowing down with time. The experimental findings are interpreted as a result of the interplay between the diffusion and Auger recombination of excitons.

Simulation and characterization of planar high-efficiency back contact silicon solar cells

Short-circuit current, open-circuit voltage, and photoconversion efficiency of silicon high-efficiency solar cells with all back contact (BCSC) with planar surfaces have been calculated theoretically. In addition to the recombination channels usually considered in this kind of modeling, namely, radiative, Auger, Shockley–Read–Hall, and surface recombination, the model also takes into account the nonradiative trap-assisted exciton Auger recombination and recombination in the space charge region. It is ascertained that these two recombination mechanisms are essential in BCSCs in the maximum power operation regime. The model results are in good agreement with the experimental results from the literature.

Key parameters of textured silicon solar cells of 26.6% photoconversion efficiency

A new approach to modeling the parameters of high efficiency textured silicon solar cells (SCs) has been presented. Unlike conventional optimization formalisms, our approach additionally includes such important factors as the non-radiative Auger recombination of excitons via deep impurity levels as well as electron-hole pairs recombination in the space charge region. A simple phenomenological expression offered by us earlier for the external quantum efficiency of the textured silicon solar cells with account of the photocurrent in the long-wave part of the absorption spectrum has been also used. Applying this approach, the key parameters of textured silicon SCs, namely: short-circuit current, open-circuit voltage and photoconversion efficiency, have been theoretically determined. The proposed formalism allows calculating the thickness dependence of photoconversion efficiency, which is in good agreement with the experimental results obtained for the heterojunction SCs with the record photoconversion efficiency of 26.6%. The offered approach and the results of applying this phenomenological expression for the external quantum efficiency of the photocurrent in the long-wave part of the absorption spectrum can be used to optimize the characteristics of high efficiency textured SCs based on monocrystalline silicon.

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer.

As narrow optical bandgap materials, semiconducting single-walled carbon nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced charge-transfer (PCT) reactions. However, the unique band structure and unusual exciton dynamics of SWCNTs add more possibilities to the classical PCT mechanism. In this work, we demonstrate PCT from photoexcited semiconducting (6,5) SWCNTs to a wide-bandgap wrapping poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(6,6′)-(2,2′-bipyridine)] (PFO–BPy) via femtosecond transient absorption spectroscopy. By monitoring the spectral dynamics of the SWCNT polaron, we show that charge transfer from photoexcited SWCNTs to PFO–BPy can be driven not only by the energetically favorable E33 transition but also by the energetically unfavorable E22 excitation under high pump fluence. This unusual PCT from narrow-bandgap SWCNTs toward a wide-bandgap polymer originates from the up-converted high-energy excitonic state (E33 or higher) that is promoted by the Auger recombination of excitons and charge carriers in SWCNTs. These insights provide new pathways for charge separation in SWCNT-based photodetectors and photovoltaic cells.

Interfacial charge transfer between CsPbBr3 quantum dots and ITO nanoparticles revealed by single-dot photoluminescence spectroscopy

The interfacial charge transfer between single CsPbBr3 perovskite quantum dots (QDs) and indium tin oxide (ITO) is investigated by single-dot photoluminescence spectroscopy. It is found that when the Fermi level of single perovskite QDs aligns with that of ITO nanoparticles, the QD surface cannot be charged by the ITO through interfacial electron transfer. Therefore, the QD/ITO system with Fermi level alignments can exclude exciton nonradiative recombination processes involving the additional surface electrons, such as the exciton Auger recombination and the valence band hole transfer processes. Hence the photovoltaic devices based on perovskite QD/ITO system with the Fermi level alignments have the improved photoelectric conversion efficiency.

Theoretical Study of Auger Recombination of Excitons in Monolayer Transition-metal Dichalcogenides

Excitons are the most prominent features of the optical properties of monolayer transition-metal dichalcogenides(TMDC). In view of optoelectronics it is very important to understand the decay mechanisms of the excitons of these materials. Auger recombination of excitons are regarded as one of the dominant decay processes. In this paper the Auger constant of recombination is computed based on the approach proposed by Kavoulakis and Baym. We obtain both temperature dependent (from type A, A’ processes) and temperature independent (from type B, B’ processes) contributions, and a numerical estimate of theoretical result yields the value of constant in the order of 10−2 cm2s−1, being consistent with existing experimental data. This implies that Auger decay processes severely limit the photoluminescence yield of TMDC-based optoelectronic devices.

Emission of Cu2O Paraexcitons Confined by a Strain Trap: Hints of a Bose–Einstein Condensate?

We monitor the phonon sideband emission from paraexcitons confined in a strain trap in cuprous oxide at T = 1.25 K. On the low energy ank of the optical phonon replicas, both of Γ 5 − and Γ 3 − symmetry (the latter activated by application of a magnetic field), we detect sharp peaks that might represent indications for a paraexciton Bose–Einstein condensate. In contrast, such peaks are absent in the phonon-mediated emission of the orthoexcitons, and they also disappear at elevated temperatures. The results challenge our understanding of the involved physics, e.g., of the Auger recombination of excitons, which has so far been believed to prevent crossing the border to a condensate.

Emission of Cu-=SUB=-2-=/SUB=-O paraexcitons confined by a strain trap: hints of a Bose-Einstein condensate?

AbstractWe monitor the phonon sideband emission from paraexcitons confined in a strain trap in cuprous oxide at T = 1.25 K. On the low energy ank of the optical phonon replicas, both of Γ _5 ^− and Γ _3 ^− symmetry (the latter activated by application of a magnetic field), we detect sharp peaks that might represent indications for a paraexciton Bose–Einstein condensate. In contrast, such peaks are absent in the phonon-mediated emission of the orthoexcitons, and they also disappear at elevated temperatures. The results challenge our understanding of the involved physics, e.g., of the Auger recombination of excitons, which has so far been believed to prevent crossing the border to a condensate.

Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals.

We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr3, FA = HC(NH2)2) nanocrystals and the impact of postsynthetic surface treatments on the PL intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on-off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross sections, those of flickering nanocrystals show no correlation with the absorption cross sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no significant effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons.

Nearly Blinking-Free, High-Purity Single-Photon Emission by Colloidal InP/ZnSe Quantum Dots.

Colloidal core/shell InP/ZnSe quantum dots (QDs), recently produced using an improved synthesis method, have a great potential in life-science applications as well as in integrated quantum photonics and quantum information processing as single-photon emitters. Single-particle spectroscopy of 10 nm QDs with 3.2 nm cores reveals strong photon antibunching attributed to fast (70 ps) Auger recombination of multiple excitons. The QDs exhibit very good photostability under strong optical excitation. We demonstrate that the antibunching is preserved when the QDs are excited above the saturation intensity of the fundamental-exciton transition. This result paves the way toward their usage as high-purity on-demand single-photon emitters at room temperature. Unconventionally, despite the strong Auger blockade mechanism, InP/ZnSe QDs also display very little luminescence intermittency ("blinking"), with a simple on/off blinking pattern. The analysis of single-particle luminescence statistics places these InP/ZnSe QDs in the class of nearly blinking-free QDs, with emission stability comparable to state-of-the-art thick-shell and alloyed-interface CdSe/CdS, but with improved single-photon purity.

Area- and Thickness-Dependent Biexciton Auger Recombination in Colloidal CdSe Nanoplatelets: Breaking the “Universal Volume Scaling Law”

Colloidal nanoplatelets (NPLs) have shown great potentials for lasing applications due to their sharp absorption and emission peaks, large absorption cross sections, large radiative decay rates, and long multiexciton lifetimes. How multiexciton lifetimes depend on material dimensions remains unknown in two-dimensional (2D) materials, despite being a key parameter affecting optical gain threshold and many other properties. Herein, we report a study of room-temperature biexciton Auger recombination time of CdSe NPLs as a function of thickness and lateral area. Comparison of all NPLs shows that the biexciton lifetime does not increase linearly with volume, unlike previously reported "universal volume scaling law" for quantum dots. For NPLs of the same thickness (∼1.8 nm), the biexciton lifetime increase linearly with their lateral area (from 143.7 ± 12.6 to 320.1 ± 17.1 ps when the area increases from 90.5 ± 21.4 to 234.2 ± 41.9 nm2). The biexciton lifetime depends linearly on (1/Ek(e))7/2 (Ek(e) is the electron confinement energy) or nearly linearly on d7 (d is NPL thickness). The observed dependence is consistent with a model in which biexciton Auger recombination rate scales with the product of exciton binary collision frequency and Auger recombination probability in biexciton complexes. The linear increase of Auger lifetimes with NPL lateral areas reflects a 1/area dependence of the binary collision frequency for 2D excitons and the thickness-dependent biexciton Auger recombination time is attributed to its strong dependence on the degree of quantum confinement. This model may be generally applicable to exciton Auger recombination in quantum confined 1D and 2D nanomaterials.

Many-body effects in nonlinear optical responses of 2D layered semiconductors

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton–exciton interaction and free carrier induced band gap renormalization. The exciton–exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials.

Slow Auger Recombination of Charged Excitons in Nonblinking Perovskite Nanocrystals without Spectral Diffusion.

Over the last two decades, intensive research efforts have been devoted to the suppressions of photoluminescence (PL) blinking and Auger recombination in metal-chalcogenide nanocrystals (NCs), with significant progresses being made only very recently in few specific NC structures. Here we show that nonblinking PL is readily available in the newly synthesized perovskite CsPbI3 NCs and that their Auger recombination of charged excitons is greatly slowed down, as signified by a PL lifetime about twice shorter than that of neutral excitons. Moreover, spectral diffusion is completely absent in single CsPbI3 NCs at the cryogenic temperature, leading to a resolution-limited PL line width of ∼200 μeV.

Auger recombination of dark excitons in WS2 and WSe2 monolayers

We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in tungsten based TMDCs. Using experimental and density functional theory computed values for the exciton energies, spin–orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as , thus providing a plausible explanation for the low quantum efficiencies reported for these materials.

Fast Dissociation and Reduced Auger Recombination of Multiple Excitons in Closely Packed PbS Nanocrystal Thin Films.

Exciton decay dynamics in chemically treated PbS quantum-dot (QD) films have been studied using femtosecond transient-absorption (TA) spectroscopy. In photoconductive QD films, a decay component with a lifetime of a few nanoseconds appeared in the TA signals because of exciton dissociation under weak excitation. Increasing excitation fluence resulted in additional fast-decay components corresponding to the lifetimes of multiple excitons, which decreased with increasing photoconductivity of the closely packed QD films. Auger recombination in photoexcited QDs was suppressed in highly photoconductive films. Our findings clearly show that the carrier transfer between the QDs dominates the lifetimes of single and multiple excitons.

Quantized exciton–exciton recombination in undoped and hole-doped single-walled carbon nanotubes

We studied the quantized exciton Auger recombination in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) by means of transient absorption spectroscopy and theoretical calculations. In undoped SWCNTs, a fast decay component appears under strong photoexcitation owing to two-exciton Auger recombination. The exciton decay dynamics is well explained by the quantized exciton Auger recombination model that takes into consideration the dark-exciton state. In hole-doped SWCNTs, the fast decay component is drastically reduced even under strong photoexcitation. We calculated the temporal evolution of the exciton population in hole-doped samples by considering exciton–hole interactions and the hole-number distribution in SWCNTs, and found it to be in good agreement with the experimental results.