Nonradiative Recombination Mechanisms Research Articles

Non-Blinking Luminescence from Charged Single Graphene Quantum Dots.

Photoluminescence blinking behavior from single quantum dots under steady illumination is an important but controversial topic. Its occurrence has impeded the use of single quantum dots in bioimaging. Different mechanisms have been proposed to account for it, although controversial, the most important of which is the non-radiative Auger recombination mechanism whereby photocharging of quantum dots can lead to the blinking phenomenon. Here, the singly charged trion, which maintains photon emission, including radiative recombination and non-radiative Auger recombination, leads to fluorescence non-blinking which is observed in photocharged single graphene quantum dots (GQDs). This phenomenon can be explained in terms of different energy levels in the GQDs, caused by various oxygen-containing functional groups in the single GQDs. The suppressed blinking is due to the filling of trap sites owing to a Coulomb blockade. These results provide a profound understanding of the special optical properties of GQDs, affording a reference for further in-depth research.

Interactions of hydrogen atoms with boron and gallium in silicon crystals co-doped with phosphorus and acceptors

Reports showing that hydrogen and group-III acceptors play an important role in Light- and elevated Temperature-induced Degradation (LeTID) of Si-based solar cells highlight the need for a better understanding of interactions between these two species. In this contribution, a combination of junction spectroscopy techniques and first principles modelling has been used to study hydrogen-induced changes in electrical properties of either boron or gallium Czochralski-grown silicon co-doped with phosphorus in order to produce n-type material facilitating novel techniques to assess recombination active defects. The interactions of hydrogen with acceptor atoms have been induced via annealing of these co-doped hydrogenated samples with the application of reverse bias (RBA). These treatments have resulted in a significant increase in the net shallow donor concentration in depletion regions of both materials and in the appearance of a strong electron emission signal due to a trap with an energy level at about Ec −0.18 eV in the DLTS spectra of Si:P + B material. It is argued that this trap is related to the donor level of a BH2 complex. Calculations using density functional theory have shown that the BH2 defect has a charge-state dependent geometry, which turns out to be crucial for the proposed non-radiative recombination mechanism. The BH2 defect is therefore suggested to be the root cause of LeTID in boron-doped Si. In contrast, modelling results predict that GaH2 is a defect with shallow energy levels, without the characteristic features of a recombination centre. This is corroborated by the results of electrical measurements on hydrogenated Si:P + Ga subjected to RBA. Conventional annealing treatments were subsequently used to assess the thermal stability of acceptor-H related defects. Based on the obtained results, the peculiarities of hydrogen interactions with boron and gallium acceptors are discussed.

Origins of Photoluminescence Instabilities at Halide Perovskite/Organic Hole Transport Layer Interfaces

Metal halide perovskites are promising for optoelectronic device applications; however, their poor stability under solar illumination remains a primary concern. While the intrinsic photostability of isolated neat perovskite samples has been widely discussed, it is important to explore how charge transport layers─employed in most devices─impact photostability. Herein, we study the effect of organic hole transport layers (HTLs) on light-induced halide segregation and photoluminescence (PL) quenching at perovskite/organic HTL interfaces. By employing a series of organic HTLs, we demonstrate that the HTL's highest occupied molecular orbital energy dictates behavior; furthermore, we reveal the key role of halogen loss from the perovskite and subsequent permeation into organic HTLs, where it acts as a PL quencher at the interface and introduces additional mass transport pathways to facilitate halide phase separation. In doing so, we both reveal the microscopic mechanism of non-radiative recombination at perovskite/organic HTL interfaces and detail the chemical rationale for closely matching the perovskite/organic HTL energetics to maximize solar cell efficiency and stability.

Improved Carrier Lifetimes of CdSe Thin Film via Te Doping for Photovoltaic Application.

Cadmium selenide (CdSe) solar cells have proven to be a remarkable potential top cell for a silicon-based tandem application. However, the defects and short carrier lifetimes of CdSe thin films greatly limit the solar cell performance. In this work, a Te-doped strategy is proposed to passivate the Se vacancy defects and increase the carrier lifetime of the CdSe thin film. The theoretical calculation helps to reveal the mechanism of nonradiative recombination of the CdSe thin film in depth. After Te-doping, the calculated capture coefficient of CdSe can be reduced from 4.61 × 10-8 cm3 s-1 to 2.32 × 10-9 cm3 s-1. Meanwhile, the carrier lifetime of CdSe thin film is increased nearly 3-fold from 0.53 to 1.43 ns. Finally, the efficiency of the Cd(Se,Te) solar cell is improved to 4.11%, about a relative 36.5% improvement compared to the pure CdSe solar cell. Both theoretical calculations and experiments prove that Te can effectively passivate bulk defects and improve the carrier lifetime of CdSe thin films, deserving further exploration to improve solar cell performance.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Zn-doped Pb/Sn hybrid perovskite solar cells: Towards high photovoltaic performance

Achieving a low-toxicity mixed Sn/Pb halide perovskite solar cell with the higher performance and lower cost is still a major challenge. Furthermore, the element Zn was incorporated as a substitute cation for the toxic element Pb into the CH3NH3Sn0.25Pb0.75I3 perovskite compound, as a result of its significant properties such as non-toxic nature, inability to oxidize, and abundance on earth. Thus, the structures of Zn-doped Pb/Sn hybrid perovskite solar cells (PSCs) were introduced and studied. The results showed that 3% Zn-doping improved the crystal quality and surface coverage, increased the grain size (up to about 1 µm), and reduced the non-radiative recombination mechanism by increasing the lifetime of carriers. In general, it significantly improved the photovoltaic performance of this type of solar cell. In particular, CH3NH3Pb0.72Sn0.25 Zn0.03I2.94Cl0.06 compound with a yield of 11.68% showed the highest photovoltaic performance with the lowest Pb compared to the sample without Zn-doping (Zn-0%) with a yield of 4.30%. The findings provide a new area for further research on lead-reduced hybrid perovskite solar cells.

Comparative investigation into key optoelectronic characteristics of semipolar InGaN blue laser diodes: A strategy to mitigate quantum-confine stark effect

In recent years, semipolar InGaN-based blue laser diodes (LDs) have raised indescribable appeal within solid-state lighting industry as promising alternatives for currently available polar c-plane LDs due to reduction in internal polarization field and related quantum-confined stark effect. However, any comparative study in terms of energy band gap, optical emission profile, power output (P) and forward bias response (I-V) etc. between semipolar-oriented InGaN blue LDs is inherently unattainable. This work aims to demonstrate the impact of internal polarization field on aforementioned key optoelectronic parameters of In0.17Ga0.83N/GaN double quantum well (DQW) blue LD along semipolar (101-2), (112-2), (101-1), (202-1), (303-1-), (202-1-) and (101-1-) crystal orientations by figuring out six-band k.p Hamiltonian at the brillouin zone center aided from Euler’s tensor transformation method. Three-level rate equations based LD analogous circuit model and state-space matrix are employed here to obtain P-I-V and frequency characteristics. Radiative as well as non-radiative recombination mechanisms for LD systems are portrayed by addressing modified ABC model. Our numerical investigation illustrates that best hole effective mass, optical gain, lasing power, threshold current and forward voltage can be attained from (112-2)-oriented InGaN blue LD due to insignificant presence of spontaneous plus piezoelectric (PZ) polarization charge. In addition with these, the inspection of bode plot and relevant optical power gain (dB) confirms the stability and superiority of this semipolar blue LD system for achieving high-speed visible-light communications at relatively low current densities.

Far-IR to deep-UV adaptive supercontinuum generation using semiconductor nano-antennas via carrier injection rate modulation

Supercontinuum generating sources, which incorporate a non-linear medium that can generate a wideband intensity spectrum under high-power excitation, are ideal for many applications of photonics such as spectroscopy and imaging. Supercontinuum generation using ultra-miniaturized devices is of great interest for on-chip imaging, on-chip measurement, and for future integrated photonic devices. In this study, semiconductor nano-antennas are proposed for ultra-broadband supercontinuum generation via analytical and numerical investigation of the electric field wave equation and the Lorentz dispersion model, incorporating semiconductor electron dynamics under optical excitation. It is shown that by a rapid modulation of the carrier injection rate for a semiconductor nano-antenna, one can generate an ultra-wideband supercontinuum that extends from the far-infrared (Far-IR) range to the deep-ultraviolet (Deep-UV) range for an infrared excitation of arbitrary intensity level. The modulation of the injection rate is achieved by high-intensity pulsed-pump irradiation of the nano-antenna, which has a fast nonradiative electron recombination mechanism that is on the order of sub-picoseconds. It is shown that when the pulse period of the pump irradiation is of the same order with the electron recombination time, rapid modulation of the free electron density occurs and electric energy accumulates in the nano-antenna, allowing for the generation of a broad supercontinuum. The numerical results are compared with the semiempirical second harmonic generation efficiency results for validation and a mean accuracy of 99.7% is observed. The aim of the study is to demonstrate that semiconductor nano-antennas can be employed to achieve superior supercontinuum generation performance at the nanoscale and the process can be programmed in an adaptive manner for continuous spectral shaping via tuning the pulse period of the pump irradiation.

Localized-state ensemble model analysis of InGaN/GaN quantum well structures with different dislocation densities

The influence of dislocations on luminescence of InGaN/GaN multiple quantum wells was investigated by temperature-dependent and time-resolved room-temperature photoluminescence measurements and analyzed via localized-state ensemble model. The results show that dislocations decrease non-radiative recombination time and do not affect either radiative recombination time or non-radiative recombination mechanism. Moreover, dislocation-related broadening, increasing linearly with increased dislocation density, was found to take place. However, a significant part of spectral width (∼55 meV) is not defined by either dislocation-induced or alloy- and thermally-induced broadening, revealing the existence of other broadening mechanisms (e.g. carrier–carrier scattering-induced broadening).

PL, TL and Persistent Luminescence Characterization of Undoped and Eu Doped Nanostructured Hydroxyapatite

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) possesses the closest mineralized structure to natural bone tissue and has been widely using in as bone substitutes and for various biomedical applications. Undoped and doped with rare earth elements HAps are the basis of promising biomedical imaging, bone tissue engineering, gene and drug delivery applications. In the present work, we present the results of investigation of the photoluminescence (PL), thermoluminescence (TL) and persistent luminescence (PersL) of synthetic nanostructured HAp (n-HAp). Undoped and Eudoped (0.0, 0.1, 0.5, 1.0, 2.0 mol %) n-HAp powders were synthesized by solution-precipitation method and characterized by SEM-EDS, FTIR and Raman techniques. The 3D excitation-emission spectrum of undoped n-HAp exhibits a broad band with excitation/emission maximum at 376/454 nm, respectively. The maximum position slightly shifts to shorter excitation and emission wavelengths with increases of the Eu concentration that could be due to a possible overlapping between the Eu emission band and intrinsic emission of undoped n-HAp. It was found that (similarly to behavior of PL in Si nanocrystals, Xin et al., RSC Adv., 2019, 9, 8310) the emission peak is red-shifted as the excitation wavelength red-shifts. The typical Eu3+ emission bands were observed for high Eu doping concentrations. TL glow curves of n-HAp samples exposed to beta radiation from Sr-Y source display a broad TL peak with maxima in-between 145 and 165 oC depending on the Eu concentration. The highest TL intensity is observed for undoped n-HAp, showing quenching effect of Eu on TL emission. This TL quenching effect contrasted with the PersL decay emission measured immediately after irradiation exposure, which was highest for 0.1 mol % Eu. The intrinsic defects in n-HAp host is related to the PL, TL and PersL emissions; however, the nature of the radiative and non-radiative recombination mechanisms requires further investigation. The PL, TL and PersL of undoped HAp may have prospective application in the fields of radiation detection, ionizing radiation dosimetry and biomedical imaging. Keywords: Hydroxylapatite, thermoluminescence, persistent luminescence.

Enhancing the cooling potential of photoluminescent materials through evaluation of thermal and transmission loss mechanisms

Photoluminescent materials are advanced cutting-edge heat-rejecting materials capable of reemitting a part of the absorbed light through radiative/non-thermal recombination of excited electrons to their ground energy state. Photoluminescent materials have recently been developed and tested as advanced non-white heat-rejecting materials for urban heat mitigation application. Photoluminescent materials has shown promising cooling potential for urban heat mitigation application, but further developments should be made to achieve optimal photoluminescence cooling potential. In this paper, an advanced mathematical model is developed to explore the most efficient methods to enhance the photoluminescence cooling potential through estimation of contribution of non-radiative mechanisms. The non-radiative recombination mechanisms include: (1) Transmission loss and (2) Thermal losses including thermalization, quenching, and Stokes shift. The results on transmission and thermal loss mechanisms could be used for systems solely relying on photoluminescence cooling, while the thermal loss estimations can be helpful to minimize the non-radiative losses of both integrated photoluminescent-near infrared (NIR) reflective and stand-alone photoluminescent systems. As per our results, the transmission loss is higher than thermal loss in photoluminescent materials with an absorption edge wavelength (λAE) shorter than 794 nm and quantum yield (QY) of 50%. Our predictions show that thermalization loss overtakes quenching in photoluminescent materials with λAE longer than 834 nm and QY of 50%. The results also show that thermalization, quenching, and Stokes shift constitute around 56.8%, 35%, and 8.2% of the overall thermal loss. Results of this research can be used as a guide for the future research to enhance the photoluminescence cooling potential for urban heat mitigation application.

Recombination of Polaronic Electron-Hole Pairs in Hematite Determined by Nuclear Quantum Tunneling.

Here, we investigate the intrinsic nonradiative recombination mechanism in hematite single crystals that decides the photocarrier lifetime under solar illumination. On the basis of the small polaron theory, we propose that the photogenerated electron-hole pair along with its induced lattice deformation in hematite could be treated as a pseudocoordination-complex (PCC) dispersed in a solid medium. We demonstrate that the nonradiative recombination rate at different temperatures determined from the transient absorption spectroscopy can be excellently described by the nonradiative transition theory developed previously for parallels of the PCC model. Our finding suggests that at room temperature the nonradiative recombination in hematite substantially depends on the probability of quantum tunneling of the nuclear configuration.

1.5-μm Indium Phosphide-Based Quantum Dot Lasers and Optical Amplifiers: The Impact of Atom-Like Optical Gain Material for Optoelectronics Devices

Using quantum dot (QD) structures as active material for optoelectronics was already in focus before the development of quantum well (QW) lasers and before semiconductor lasers occupied an important place in the market. The big step in reducing laser threshold conditions by substituting bulk gain materials with QWs should find a logical continuation by further reducing the dimensionality toward full 3D carrier confinement by QDs. This was predicted by theoretical considerations at the beginning of the 1980s [1], [2]. However, no practical technologies were available at that time. Top-down approaches, as originally addressed, to produce nanoscale species on very large surfaces and interface areas resulted in the accumulation of a large defect density within the active region and the domination of nonradiative carrier recombination mechanisms. Only during the early 1990s was a major breakthrough achieved with the introduction of the bottom-up approach of strain-driven, self-organized epitaxial QD growth. The first lasers were obtained in the gallium arsenide (GaAs)-based material system [3].

Efficiency degradation induced by surface defects-assisted tunneling recombination in GaN/InGaN micro-light-emitting diodes

Two GaN/InGaN micro-light-emitting diodes (micro-LEDs) grown on the same substrate with mesa sizes of 20 μm and 100 μm are measured from 100 K to 300 K to investigate the role that surface defects play in device efficiency. The experimental results show that the surface defect-assisted tunneling process dominates the nonradiative recombination of GaN/InGaN micro-LEDs at 100 K, while the surface defect-assisted Shockley–Read–Hall recombination becomes dominant at room temperature. The temperature- and voltage-dependent tunneling current for both devices is calculated, which shows that the surface defect-assisted tunneling process is one of the major nonradiative recombination mechanisms in GaN/InGaN micro-LEDs with smaller mesa sizes at room temperature. A few potential approaches are proposed to suppress this surface defect-assisted tunneling recombination. The revised external quantum efficiency model is proposed to include the tunneling recombination effect and study the efficiency performance of GaN/InGaN micro-LEDs.

Carrier dynamics and recombination mechanisms in InP twinning superlattice nanowires.

Nominal dopant-free zinc blende twinning superlattice InP nanowires have been grown with high crystal-quality and taper-free morphology. Here, we demonstrate its superior optical performance and clarify the different carrier recombination mechanisms at different temperatures using a time resolved photoluminescence study. The existence of regular twin planes and lateral overgrowth do not significantly increase the defect density. At room temperature, the as-grown InP nanowires have a strong emission at 1.348 eV and long minority carrier lifetime (∼3 ns). The carrier recombination dynamics is mainly dominated by nonradiative recombination due to surface trapping states; a wet chemical etch to reduce the surface trapping density thus boosts the emission intensity and increases the carrier lifetime to 7.1 ns. This nonradiative recombination mechanism dominates for temperatures above 155 K, and the carrier lifetime decreases with increasing temperature. However, radiative recombination dominates the carrier dynamics at temperature below ∼75 K, and a strong donor-bound exciton emission with a narrow emission linewidth of 4.5 meV is observed. Consequently, carrier lifetime increases with temperature. By revealing carrier recombination mechanisms over the temperature range 10-300 K, we demonstrate the attraction of using InP nanostructure for photonics and optoelectronic applications.

Analysis of dominant non-radiative recombination mechanisms in InGaN green LEDs grown on silicon substrates

Relationship between the external quantum efficiency (EQE) curves and the dominant non-radiative recombination mechanisms of InGaN green LEDs grown on silicon substrates were investigated. Through the analysis of the ABC+f(n) model, the significant drop in EQE at low current levels is due to an increasingly defect-related Shockley-Read-Hall (SRH) recombination. Under extremely low current densities, the defect traps can even become the dominant channel for the leakage current through the tunneling process, thereby reducing the efficiency of carrier injection into the active region. These observations were further supported by the carrier lifetime measurement. However, this fails to explain the droop in EQE at high current densities, especially when SRH recombination has been saturated. Our results show that carrier leakage has becomes dominant at high current density when Auger recombination has been less impossible. Reduced carrier leakage may lead to increased carrier injection efficiency, which in turn alleviates EQE droop.

Photo-excited carrier relaxation dynamics in two-dimensional InSe flakes

Carrier relaxation dynamics of InSe flakes is investigated by using time-resolved pump-probe reflectivity measurement. The photocarriers associated with the Pxy orbital band-edge transition at 2.40 eV, which is coupled to the in-plane polarized light, is observed to possess a lifetime of ∼19 ps at room temperature and ∼99 ps at 10 K. The temperature and power dependent carrier lifetime suggests that Shockley–Read–Hall process is the dominant nonradiative recombination mechanism responsible for the carrier relaxation. In addition, the electron scattering with a 14.5 meV optical phonon plays an active role in the carrier relaxation with increasing temperatures. A broad absorption around 1.65–1.90 eV is observed. The photocarriers associated with this broad transition show a long lifetime of ∼200 ps that is nearly independent of temperature and photon energy. This is indicative of bound carriers by defects. Our experimental results provide essential information for the characteristics of carrier dynamics and defects in InSe flakes. The experimental findings are fundamentally important for further development of microelectronics and optoelectronics based on InSe.

Investigation of carrier localization in InAs/AlSb type-II superlattice material system

We investigate carrier localization in the InAs/AlSb type-II superlattice (T2SL) material system using temperature- and excitation power (Iex)-dependent photoluminescence (PL). Evidence of carrier localization in T2SLs was observed by an S-shaped temperature dependence of the PL peak position. Analysis of the Iex-dependent PL at various temperatures also shows the existence of carrier localization in the T2SLs. The thermal activation energies in T2SLs were extracted to identify the nonradiative recombination mechanisms and the possible origins of localized states. We found that there are two thermal activation energies, E1 = 8.2–1.2 meV and E2 = ∼60 meV at various Iex. We interpret E1 as a thermal activation energy that comes from Anderson localization, associated with roughness due to As2 diffusion into the interfaces. This is because the extracted E1 values are comparable to the exciton binding energy of localization in various quantum structures. Carrier trapping at a state in the InSb interfacial layer (Tamm state) may account for the origin of E2. Based on previous reports, we believe that the 60 meV state might be a Tamm state if we consider thickness variations in the InSb interfacial layer for the T2SLs.

Role of CuAlO2 as an absorber layer for solar energy converter

A delafossite semiconductor, namely CuAlO2, with a lower band gap is a potential candidate for the absorption of solar radiation due to its excellent electronic and optical properties. In this paper, structural, electronic, vibrational and optical properties of delafossite CuAlO2 have been calculated using state-of-the-art first principles calculations based on density functional theory (DFT). The optimized structural parameters, electronic properties and Raman active vibrational modes namely the Eg and A1g are in good agreement with experimental and other theoretical data. The full phonon dispersion curves (PDC) together with the phonon density of states (PHDOS) of CuAlO2 depict its dynamical stability due to non-existence of any imaginary phonon mode in the entire Brillouin zone (BZ). To understand the mechanism of optical transition properties, we have further calculated the dielectric functions, absorption coefficient and joint density of states (JDOS) for delafossite, CuAlO2. The dielectric constant and absorption coefficient show a significant anisotropic nature in the components of polarization directions. Solar cell parameters of CuAlO2 are also calculated and the highest theoretical efficiencies of 14.8% and 12.5% have been observed with Shockley-Queisser (SQ) limit and spectroscopic limited maximum efficiency (SLME) respectively. The SLME predicted efficiency agrees better to the experimental value as it includes the thickness dependent absorbance coefficient in account of non-radiative recombination and carrier loss mechanism.

Carrier recombination mechanism at defects in wide band gap two-dimensional materials from first principles

Identification and design of defects in two-dimensional (2D) materials as promising single photon emitters (SPE) requires a deep understanding of underlying carrier recombination mechanisms. Yet, the dominant mechanism of carrier recombination at defects in 2D materials has not been well understood, and some outstanding questions remain: How do recombination processes at defects differ between 2D and 3D systems? What factors determine defects in 2D materials as excellent SPE at room temperature? In order to address these questions, we developed first-principles methods to accurately calculate the radiative and non-radiative recombination rates at defects in 2D materials, using h-BN as a prototypical example. We reveal the carrier recombination mechanism at defects in 2D materials being mostly dominated by defect-defect state recombination in contrast to defect-bulk state recombination in most 3D semiconductors. In particular, we disentangle the non-radiative recombination mechanism into key physical quantities: zero-phonon line (ZPL) and Huang-Rhys factor. At the end, we identified strain can effectively tune the electron-phonon coupling at defect centers and drastically change non-radiative recombination rates. Our theoretical development serves as a general platform for understanding carrier recombination at defects in 2D materials, while providing pathways for engineering of quantum efficiency of SPE.