Nonradiative Defects Research Articles

Photoluminescence intensity change of GaP1−xNx alloys by laser irradiation

We report the influence of laser irradiation on photoluminescence (PL) intensity to study the evolution of nonradiative recombination centers in GaP1−xNx alloys. PL mapping measurements confirmed that defects to act as nonradiative recombination centers are permanently generated by laser irradiation, which results in irreversible degradation of the PL intensity. Real-time PL measurements revealed that stronger laser irradiation leads to a larger and faster decrease in the PL intensity with irradiation time. The decay of the PL intensity by laser irradiation is larger and faster for a lower nitrogen concentration, indicating that samples with a lower nitrogen concentration are abound with hidden defects to act as nonradiative recombination centers by laser irradiation. It was demonstrated that PL measurement using high-power density photoexcitation is useful to evaluate the generation or multiplication of irradiation-induced nonradiative defects, which causes the deterioration of optoelectronic devices during operation.

Gallium vacancies—common non-radiative defects in ternary GaAsP and quaternary GaNAsP nanowires

Nanowires (NWs) based on ternary GaAsP and quaternary GaNAsP alloys are considered as very promising materials for optoelectronic applications, including in multi-junction and intermediate band solar cells. The efficiency of such devices is expected to be largely controlled by grown-in defects. In this work we use the optically detected magnetic resonance (ODMR) technique combined with photoluminescence measurements to investigate the origin of point defects in Ga(N)AsP NWs grown by molecular beam epitaxy on Si substrates. We identify gallium vacancies, which act as non-radiative recombination centers, as common defects in ternary and quaternary Ga(N)AsP NWs. Furthermore, we show that the presence of N is not strictly necessary for, but promotes, the formation of gallium vacancies in these NWs.

Composition engineering to obtain efficient hybrid perovskite light-emitting diodes.

Metal halide perovskites have received considerable attention in the field of electroluminescence, and the external quantum efficiency of perovskite light-emitting diodes has exceeded 20%. CH3NH3PbBr3 has been intensely investigated as an emitting layer in perovskite light-emitting diodes. However, perovskite films comprising CH3NH3PbBr3 often exhibit low surface coverage and poor crystallinity, leading to high current leakage, severe nonradiative recombination, and limited device performance. Herein, we demonstrate a rationale for composition engineering to obtain high-quality perovskite films. We first reduce pinholes by adding excess CH3NH3Br to the actual CH3NH3PbBr3 films, and we then add CsBr to improve the crystalline quality and to passivate nonradiative defects. As a result, the (CH3NH3)1-xCsxPbBr3 based perovskite light-emitting diodes exhibit significantly improved external quantum and power efficiencies of 6.97% and 25.18 lm/W, respectively, representing an improvement in performance dozens of times greater than that of pristine CH3NH3PbBr3-based perovskite light-emitting diodes. Our study demonstrates that composition engineering is an effective strategy for enhancing the device performance of perovskite light-emitting diodes.

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3 Bi2 I6 Cl3 Perovskite.

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs3 Bi2 I6 Cl3 , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Understanding the interplay of stability and efficiency in A-site engineered lead halide perovskites

Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a “work in progress” due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs.

Current Transport Mechanisms in Zinc Oxide/Silicon Carbide Heterojunction Light‐Emitting Diodes

Herein, the properties of ZnO:N/n‐SiC heterojunctions (HJs) and light‐emitting diodes based on them are studied. The HJs are grown by molecular beam epitaxy. Active nitrogen generated by a radio frequency plasma source is used for p‐type doping. The location of the space charge area on the ZnO:N/n‐SiC interface is revealed by electron‐beam‐induced current (EBIC) scans. The diffusion lengths of holes and electrons are calculated. This article presents the characterization of ZnO:N/n‐SiC HJs and reveals the presence of tunneling‐related current transport in them as well as the contribution of exponentially distributed traps at large voltage bias. Electroluminescence (EL) is measured at ambient pressure by a standard EL system and also at 77 K in vacuum by a system utilizing EBIC in a scanning electron microscope. Analysis of the light output power at higher current level indicates the limited effect of nonradiative defects in this structure. EL results are compared with cathodoluminescence spectra. Color temperature for HJs based on the EL spectra is also calculated.

Revealing the importance of light extraction efficiency in InGaN/GaN microLEDs via chemical treatment and dielectric passivation

Chemical etching and Al2O3 dielectric passivation were used to minimize nonradiative sidewall defects in InGaN/GaN microLEDs (mesa diameter = 2–100 μm), resulting in an increase in external quantum efficiency (EQE) as the LED size was decreased. Peak EQEs increased from 8%–10% to 12%–13.5% for mesa diameters from 100 μm to 2 μm, respectively, and no measurable leakage currents were seen in current density–voltage (J–V) characteristics. The position and shape of EQE curves for all devices were essentially identical, indicating size-independent ABC model (Shockley–Read–Hall, radiative, and Auger recombination) coefficients-behavior that is not typical of microLEDs as the size decreases. These trends can be explained by enhancement in light extraction efficiency (LEE), which is only observable when sidewall defects are minimized, for the smallest LED sizes. Detailed ray-tracing simulations substantiate the LEE enhancements.

Extraordinary Mass Transport and Self‐Assembly: A Pathway to Fabricate Luminescent CsPbBr3 and Light‐Emitting Diodes by Vapor‐Phase Deposition

AbstractHalide perovskites have been shown to be promising materials in making light‐emitting diodes. At present, almost all of perovskite materials are made by solution‐based synthesis. There are very limited reports on fabricating perovskite LEDs by vapor‐phase deposition (VPD), a method that can be easily scaled up for commercial production. In this paper, dual‐source VPD is used to fabricate stable CsPbBr3 perovskite thin films with excellent luminescent properties. Scanning electron microscope and atomic force microscope studies show that CsPbBr3 films, when coated with a thin LiBr overlayer, demonstrate an extraordinary mass transport at room temperature to re‐assemble into well‐defined islands. LiBr is also shown to passivate nonradiative defects and boost photoluminescence performance of the CsPbBr3, improving the intensity by a factor of 11 for a nominal 18 nm perovskite film and leading to extremely narrow photoluminescence peaks (16 nm FWHM). This self‐assembled perovskite LED shows major improvement in the electroluminescence performance, almost tripling the brightness of reference devices. X‐ray photoelectron spectroscopy measurement shows that surface LiBr improves Cs/Pb chemical stoichiometry, reduces Br vacancies, and shift the Fermi energy level toward conduction band minimum.

Impact of defects on Auger recombination in c-plane InGaN/GaN single quantum well in the efficiency droop regime

We study the impact of non-radiative defects on Auger recombination in c-plane InGaN/GaN single quantum wells (SQWs) in the efficiency droop regime using high injection time-resolved photoluminescence. The defect density in the SQW is controlled by tuning the thickness of an InAlN underlayer. When the defect density is increased, apart from Shockley–Read–Hall (SRH) and standard Auger recombination, introducing an extra defect-assisted Auger process is required to reconcile the discrepancy observed between the usual ABC model and experimental data. We derive a linear dependence between the SRH coefficient and the bimolecular defect-assisted Auger coefficient, which suggests that the generated defects can act as scattering centers responsible for indirect Auger processes. In particular, in defective SQWs, the defect-assisted Auger recombination rate can exceed the radiative one. Our results further suggest that the defect-assisted Auger recombination is expected to be all the more critical in green to red III-nitride light-emitting diodes due to their reduced radiative rate.

Defect limitations in Cu2ZnSn(S, Se)4 solar cells utilizing an In2S3 buffer layer

Alternative n-type buffer layer such as In2S3 has been proposed as a Cd-free alternative in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this study, optical and electronic characterization techniques together with device analysis and simulation were used to assess nanoparticle-based CZTSSe absorbers and solar cells with CdS and In2S3 buffers. Photoluminescence spectroscopy indicated that CZTSSe absorbers with In2S3 buffer had a lower density of detrimental non-radiative defects and a higher concentration of copper vacancies VCu+, responsible for p-type conductivity in CZTSSe, in comparison to the absorber with CdS buffer. Capacitance–voltage (C–V) measurements revealed that the In2S3 buffer-based CZTSSe devices had a three times higher apparent doping density and a consequently narrower space charge region than devices with a CdS layer. This resulted in poorer collection of photo-generated charge carriers in the near-IR region despite a more favorable band alignment as determined by x-ray photoelectron and inverse photoelectron spectroscopy. The presence of interfacial defect states in In2S3 devices as determined by C–V and biased quantum efficiency measurements is also responsible for the loss in open-circuit voltage compared with reference devices with CdS.

Effects of Solvent Ratio on the Morphology, Growth, and Photoluminescence of Hexagonal Pyramidal β‐NaYF4:Eu3+ Microrods

Hexagonal pyramidal β‐NaYF4:Eu3+ are synthesized with a facile solvothermal method using the mixture of ethylene glycol and deionized water (DIW) as the reaction solvents. The solvent ratio has a strong effect on the morphology, growth, and photoluminescence (PL) properties of β‐NaYF4:Eu3+. Both the particle size and the aspect ratio of length/diameter of the β‐NaYF4:Eu3+ increase with the proportion of DIW in the reaction solvents, and their crystallinity is also enhanced. The growth behavior of the β‐NaYF4:Eu3+ has been analyzed and explained based on the intrinsic crystal structures, the surface free energy, and the interactions between the reacting ions. The PL of the β‐NaYF4:Eu3+ greatly increases with the proportion of DIW in the reaction solvents, and this is attributed to their enhanced crystallinity with lower density of nonradiative defects. Herein, a useful basis for preparation of luminescent materials with adjustable morphology and strong PL is provided.

Improved third harmonic nonlinear optical process upon e-beam irradiation in Cl: ZnO thin films

We report the tuning of third-harmonic generation and nonlinear absorption process in Cl doped ZnO (Cl: ZnO) thin films by energetic electron beam irradiation. The structural studies outcomes the stability of the films based on the nominal variations observed in structural parameters on irradiation. The absorption edge shows a redshift at higher irradiation dosage and bandgap of the film shows a drop from 3.28 eV to 3.22 eV at 20 kGy dosages. The morphology of the films has undergone a drastic change on irradiation and grains are found to be shattered and islands are formed at different regions of the scan area. The photoluminescence (PL) spectra show substantial change after the irradiation and number of emissions centres decreased along with peak broadening effect. The studies confirm that radiative defects in the films decreased which in turn give rise to enhancement of non-radiative defects. The Raman spectra depicts three phonon modes E2H-E2L, A1 (TO) and E2H similar to unirradiated films. The E2H mode peak has dominated in all the irradiated films confirming the hexagonal wurtzite structure of the films. On irradiation, the asymmetry observed in the core level O 1s XPS spectra has changed and a single peak positioned around 529.8 eV was observed indicating the quenching of oxygen-related defects. Third harmonic generation studies exhibit an enhancement in the THG intensity upon electron beam incorporation attributed to optical absorption and excitonic effects. Open aperture Z-scan measurement shows a high βeff of 31 × 10-2 cm/W at 20 kGy irradiated films due to enhanced scattering induced nonlinear absorption phenomena. The enhanced nonlinear optical properties exhibited by Cl: ZnO thin films show promising applications of grown material in frequency conversion and optical filtering devices.

Improved Performance for Thermally Evaporated Perovskite Light-Emitting Devices via Defect Passivation and Carrier Regulation.

Efficient inorganic perovskite light-emitting devices (PeLEDs) with a vacuum-deposited CsPbBr3 emission layer were realized by introducing an ultrathin 2-phenylethanamine bromide interlayer. The PEA+ cations not only passivated the nonradiative defects by terminating on the CsPbBr3 surface but also regulated the charge transport to balance the hole and electron transport. Consequently, the PeLEDs exhibit significantly promoted performance with a turn-on voltage of 3 V, a maximum current efficiency of 14.64 cd A-1, and an external quantum efficiency of 4.10%. Our work would provide instructive guidance for realizing efficient PeLEDs based on a vacuum processing method via focusing on the interface modification between the perovskite layer and the carrier transport layer.

Near-Unity Red Mn2+ Photoluminescence Quantum Yield of Doped CsPbCl3 Nanocrystals with Cd Incorporation.

Although Mn2+ doping in semiconductor nanocrystals (NCs) has been studied for nearly three decades, the near 100% photoluminescence (PL) quantum yield (QY) of Mn2+ emission has never been realized so far. Herein, greatly improved PL QYs of Mn2+ emissions are reported in Mn2+-doped CsPbCl3 NCs with various Mn2+ doping concentrations after CdCl2 post-treatment at room temperature. Specifically, the near-unity QY and near single-exponential decay of red Mn2+ emission peaking at 627 nm in doped CsPbCl3 NCs are obtained for the first time. The temperature dependence of steady-state and time-resolved PL spectra reveals that the CdCl2 post-treatment significantly reduces the nonradiative defect states and enhances the energy transfer from host to Mn2+ ions. Moreover, the Mn2+:CsPbCl3 NCs after CdCl2 post-treatment exhibit robust stability and high PL QYs after multipurification. The results will provide an effective route to obtain doped perovskite NCs with high performance for white lighting emitting diodes.

All-Inorganic Perovskite Solar Cells With Both High Open-Circuit Voltage and Stability

Metal halide perovskite solar cells based on all-inorganic CsPbBr3 have attracted considerable attentions recently, due to their high open-circuit voltage and good stability. However, the fabrication of CsPbBr3 film is limited by the poor solubility of cesium precursors in organic solvents by the one-step method. Here, we successfully fabricated CsPbBr3 film solar cells by employing colloid nanocrystal. The effects of technique parameters, including purification times, anneal temperatures, and spin-coating times on film morphology, optical spectra, and device performance are investigated in detail. The highest power conversion efficiency of 4.57% has been achieved based on a large open-circuit voltage of 1.45 V and a large short-circuit current of 9.41 mA cm−2. A large open-circuit voltage results from the reduced non-radiative energy loss channels and defect states while a large short-circuit current is related to the high conductivity induced by the removal of organic ligands with the increased nanocrystal electronic coupling. Furthermore, excellent stability in air is disclosed on the unencapsulated device suggesting the enormous potential for developing high open-circuit photovoltaic devices with high stability in future.

Influence of the cationic composition on the optical properties and photostability of AgInS2 and AgInS2/ZnS quantum dots

Semiconductor I-III-VI nanocrystals are highly luminescent low toxic materials with direct band gap and tunable emission in visible and near infrared region which can be synthesized in aqueous media via simple procedure making them competitive object in compare with well developed and investigated II-VI quantum dots. However the nature of radiative recombination pathways definition and effect of synthesis conditions on spectral characteristics of the nanocrystals investigation remains an important task. In the present work influence of the cation precursors ratio as well as the heat treatment duration on the composition, optical properties and photostability of AgInS2 and AgInS2/ZnS nanocrystals synthesized in aqueous media have been investigated. Due to the low reactivity of indium salts in aqueous solution coused by formation of a stable complexes with stabilizer or hydroxycomplexes the nearest stoichiometric Ag1,1InSx quantum dots are formed at the initial [In]:[Ag] ratio 7:1. Under high excess of Ag formation of Ag2S/AgInS2 core/shell quantum dots confirming by presence of large 12-15 nm nanocrystals and red shift of the photoluminescence maximum with increasing [In]:[Ag] ratio from 1 to 3 (shell thickness should increase proportionally) is possible. With a further increase of the [In]:[Ag] ratio the absorption edge and the photoluminescence maximum are green shifted indicating increase of the quantum dots band energy. That can be explained by lowering of the valence band ceiling energy and rising of the conduction band bottom energy due to decrease of density of states of Ag 4d orbitals and increase of density of states of In 5s and 5p orbitals involved in the AgInS2 band gap formation. Increase of Indium content leads to significant increase of the photoluminescence intensity of AgInS2 nanocrystals eliminating nonradiative defects such as Agi. It have been shown that the ZnS shell epitaxial grow occurs due to the cation exchange between Zn2+ and In3+ and takes place only at low concentration of Zinc precursor ([Zn]:[Ag] ≤ 4). With further enhancement of the ratio the solid solution of AgInS2-ZnS is forming. To achieve the highest possible photoluminescence intensity and energy the AgInS2-ZnS nanocrystals should be heated at 950C at least for 120 minutes. It have been shown that the nearly stoichiometric Ag1,1InSx nanocrystals posess the highest photostability under UV light irradiation.

Synergistic morphology control and non-radiative defect passivation using a crown ether for efficient perovskite light-emitting devices

Promoted CsPbBr3 PeLED electroluminescent performance has been achieved using HM12C4 via ameliorating the film morphology and passivating defects.

Study on two-photon induced ultrafast carrier dynamcis in Ge-doped GaN by transient absorption spectroscopy

Gallium nitride (GaN) is a key material in blue light-emitting devices and is recognized as one of the most important semiconductors after Si. Its outstanding thermal conductivity, high saturation velocity, and high breakdown electric field have enabled the use of GaN for high-power and high-frequency devices. Although lots of researches have been done on the optical and optoelectrical properties of GaN, the defect-related ultrafast dynamics of the photo-excitation and the relaxation mechanism are still completely unclear at present, especially when the photo-generated carrier concentration is close to the defect density in n-type GaN. The transient absorption spectroscopy has become a powerful spectroscopic method, and the advantages of this method are contact-free, highly sensitive to free carriers, and femtosecond time resolved. In this article, by employing optical pump and infrared probe spectroscopy, we investigate the ultrafast photo-generated carriers dynamics in representative high-purity n-type and Ge-doped GaN (GaN:Ge) crystal. The transient absorption response increased as probe wavelengths increased, and hole-related absorption was superior to electron-related absorption, especially at 1050 nm. The transient absorption kinetics in GaN:Ge appeared to be double exponential decay under two-photon excitation. By modelling the carrier population dynamics in energy levels, which contained both radiative and non-radiative defect states, the carrier dynamics and carrier capture coefficients in GaN: Ge can be interpreted and determined unambiguously. The faster component (30–60 ps) of absorption decay kinetics corresponded to the capturing process of holes by negatively charged acceptor C<sub>N</sub>. However, the capturing process was limited by the recombination of electron and trapped holes under higher excitation after the saturation of deep acceptors. As a result, the slower component decayed slower as the excitation fluence increased. Moreover, the experimental and theoretical results found that, the carrier lifetime in n-GaN can be modulated by controlling the defect density and carrier concentration under a moderate carrier injection, making GaN applicable in different fields such as LED and optical communication.

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

AbstractColloidal semiconductor nanoplatelets (NPLs) have recently emerged as highly promising optical gain medium because of their superior optical properties. Here, the shell‐thickness‐dependent optical gain properties of CdSe/CdS core/shell NPLs synthesized by high‐temperature growth are systematically investigated for the first time. The core/shell NPLs show the increased quantum yields and enhanced photostability as well as clear reduced emission blinking, thanks to the preferable passivation of nonradiative surface defects by the growth of the high‐quality CdS shells under high reaction temperature. Meanwhile, the amplified spontaneous emission (ASE) performance of CdSe/CdS NPLs indicates a nonmonotonic dependence on the shell thickness. The ASE threshold is achieved as low as ≈4.4 µJ cm−2 for thick‐shell NPLs with six monolayer CdS shells, and exhibiting ultrafast transient dynamics process (≈11 ps). Besides, an extremely long lifetime (>800 ps) and large bandwidth (>140 nm) of optical gain are observed by employing ultrafast transient absorption spectroscopy. Finally, a thick‐shell NPLs vertical cavity surface‐emitting laser is developed, which demonstrates spatially directional single‐mode operation with an ultralow lasing threshold of ≈1.1 µJ cm−2. These excellent results are attributed to the remarkable optical gain performance of core/shell NPLs and represent an important step toward practical NPL laser devices.

Influence of thin MOCVD-grown GaN layer on underlying AlN template

Abstract We have studied the direct growth of a GaN film on an AlN template/sapphire substrate by metalorganic chemical vapor deposition. It was found that the GaN layer causes marked deformation of the underlying AlN template at the initial growth. The intensity of x-ray diffraction from AlN drops by a factor of 5 and the full widths at half maximum of the rocking curves of the on- and off-axis planes are increased from 50 to 300 arcsec. With increasing GaN growth time, these values gradually recover, and the crystalline quality of the GaN film is improved. No alloy formation is observed at the interface between AlN and GaN. An AlN template on a sapphire substrate seems to act as a buffer layer, adjusting the lattice constant to improve the crystallinity of the direct grown GaN. Compared with a GaN film grown on a sapphire substrate, GaN grown directly on an AlN template forms a smoother surface with better crystalline quality in a shorter growth time, and at a lower temperature with fewer nonradiative defects in the band gap.