Nonradiative Recombination Processes Research Articles

Excitation power density and temperature dependence of photoluminescence study on electron-irradiated GaAs middle cell of GaInP/GaAs/Ge triple-junction solar cell

Excitation power density (2.603-9.196 W/cm2) and temperature (10-300 K) dependence of photoluminescence (PL) measurements have been presented and discussed in detail for the carrier recombination mechanism and quenching of the GaAs middle cell in 1.0 MeV electron-irradiated GaInP/GaAs/Ge triple-junction solar cell. When the excitation power density increases, the PL intensity increases. The dependence of PL intensity on excitation power density is observed to be linear below 40 K, and becomes increasingly superlinear with increasing temperature and eventually quadratic at 300 K. The observed phenomenon revealed a competing mechanism between radiative and non-radiative recombination process of photogenerated carriers. However, with the increase of temperature, the PL intensity decreases, and strong thermal quenching phenomenon of PL intensity can be observed above 40 K. By analyzing with multiple centre model, it is due to the presence of thermally activated non-radiative recombination centres named E4 electron trap with an activation energy of ∼0.76 eV and named E5 electron trap with an activation energy of ∼0.96 eV.

Improving photovoltaic performance of CsPbBr3 perovskite solar cells by a solvent-assisted rinsing step

CsPbBr3 perovskite solar cells (PSCs) have been extensively developing for high stability and open-circuit voltage applications. Typically, 2-propanol (IPA) is used as a rinsing solvent during the two-step dipping production process of CsPbBr3. However, we evidenced that the conventional IPA-rinsing step caused the formation of an undesirable Cs4PbBr6 phase resulting in poor surface topography and limited charge transport properties. Surface engineering using IPA-based rinsing was found to be inefficient owing to the poor solubility of CsBr in IPA. Alternative rinsing solvents such as ethanol and methanol were studied in this work. Notably, ethanol-rinsed CsPbBr3 (E-CsPbBr3) showed a pure-phase with a highly-compacted crystalline structure, based on suitable solubility. Because of the high purity of the perovskite crystal and large-size grains, charge extraction properties were improved, and the non-radiative recombination process was suppressed in E-CsPbBr3. Device fabrication demonstrated that average power conversion efficiency (PCE) increased from 4.77% to 6.56% by using ethanol as a rinsing solvent. The best-performing cell showed PCE of 6.89%, with exceptional stability with practically constant PCE after aging for 1030 h under humid ambient air. These remarkable experimental findings enable us to suggest a new standard process for the development of stable and high-performance CsPbBr3-based PSCs.

Temperature-dependent electroluminescence study on 265-nm AlGaN-based deep-ultraviolet light-emitting diodes grown on AlN substrates

Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and temperature dependences of the product (IQE × CIE) are derived. The temperature dependence of the EQE cannot be simply explained by the Auger recombination processes. This observation enables the CIE and IQE to be separately extracted by rate equation analysis. The room-temperature EQE of the AlGaN-based DUV LEDs is limited by the CIE and not the IQE. We propose that the relatively low CIE may originate from the nonradiative recombination process outside quantum-well layers.

Reduction of non-radiative recombination by inserting a GaAs strain-relaxation interlayer in InGaAs/GaAsP superlattice solar cells investigated by photo-thermal spectroscopy

The role of a GaAs strain-relaxation interlayer inserted into InGaAs/GaAsP superlattice solar cells was evaluated by measuring the piezoelectric photothermal (PPT) signals in the temperature range from 100 K to a device operation temperature of around 340 K. The PPT signals caused by the non-radiative recombination of electrons photo-excited to the first quantized level were observed. The temperature-dependent PPT signal intensities were assessed using an electron carrier relaxation model comprising four processes: radiative recombination, non-radiative recombination, thermionic emission, and tunneling of carriers through the e2-miniband after thermal excitation from the e1-level. The contribution of holes in the hh1 state was also included in this model, in which e1 and e2 are the first and second electron levels in the conduction band, respectively, and hh1 is the first heavy hole level in the valence band of the quantum wells. A similar analysis was conducted using photoluminescence (PL) spectra to elucidate the carrier transition dynamics in greater detail, because PPT and PL measurements are complementary to each other in terms of non-radiative and radiative electron transitions. Consequently, although the non-radiative recombination remained dominant around room temperature, the quantum yield of the carrier tunneling process increased and became comparable to that of non-radiative recombination. This implies that the recombination loss of the photo-excited carriers is suppressed by the insertion of the GaAs interlayer. By clarifying the role of the inserted interlayer with respect to the non-radiative recombination process, the usefulness of the PPT method is demonstrated.

Kinetic Modeling of Transient Electroluminescence Reveals TTA as an Efficiency-Limiting Process in Exciplex-Based TADF OLEDs

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) show increased efficiencies due to efficient upconversion of non-emissive triplet states to emissive singlets states via reverse intersystem crossing (RISC). To assess the influence of the characteristic efficiency-enhancing RISC process as well as possible efficiency-limiting effects in operational OLEDs, we performed temperature-dependent measurements of transient electroluminescence (trEL). With kinetic modeling, we quantify and separate the impact of different temperature-dependent depopulation processes and contributions to EL in the established donor:acceptor model system m-MTDATA:3TPYMB. The underlying rate equations adapted for EL measurements on TADF systems include radiative and non-radiative first- and second-order effects. In this way, we are able to evaluate the non-radiative recombination and annihilation processes with respect to their efficiency-limiting effects on these OLEDs. On the one hand, we evaluate the depopulation of intermolecular exciplex triplet states via non-radiative direct triplet decay, RISC and triplet-triplet annihilation (TTA). On the other hand, we determine the contribution to EL from the formation of singlet exciplex states via polarons, RISC and TTA. Our results show that TTA accounts for a significant part to triplet depopulation and contributes to EL while limiting the overall device quantum efficiency.

Computing the Effect of Metal Substitution in Metal–Organic Frameworks on the Recombination Times of Charge Carriers

Metal–organic frameworks (MOFs) have been attracting increasing attention in the fields of photoactive applications, for which understanding the factors governing their excited-state properties is critical. One of the major challenges includes reducing the energy losses due to the phonon-assisted nonradiative recombination and extending charge-carrier lifetimes. Metal substitution constitutes a way to modify the electronic structure of MOFs. Among different options, Ce-MOFs are often considered; however, from the theoretical perspective, little is known about the charge-carrier dynamics and recombination pathways in such systems. Therefore, in this work, the electron–hole recombination in a prototype Ce-based MOF is investigated. The limit of a radiative charge-carrier lifetime is estimated theoretically. The nonradiative recombination process is simulated using nonadiabatic molecular dynamics within the decoherence-induced surface-hopping approach and the corresponding electron–hole recombination rates are calculated, demonstrating that the charge-carrier lifetimes in Ce-MOFs are in excellent agreement with the experimental data. Importantly, the vibrational modes, which are the main contributors to the nonradiative decay, are analyzed. In particular, the role of the soft phonon modes in the charge-carrier recombination process is highlighted. Based on this data, the routes for modulating electron–hole lifetimes are suggested.

265 nm AlGaN-based deep-ultraviolet light-emitting diodes grown on AlN substrates studied by photoluminescence spectroscopy under ideal pulsed selective and non-selective excitation conditions

Photoluminescence (PL) spectroscopy under ideal pulsed selective and non-selective excitation conditions is used to study 265 nm AlGaN-based light-emitting diodes grown on AlN substrates. Excitation-power-density-dependent PL measurements under selective excitation conditions show that the internal quantum efficiency of the quantum-well layers is unity at cryogenic temperatures under weak excitation regime. Temperature-dependent and time-resolved PL measurements demonstrate the high internal quantum efficiency at room temperature. The PL thermal quenching behaviors differ under the two excitation conditions, indicating a nonradiative recombination process at the quantum-barrier layers. We propose that the nonradiative recombination process is a limiting factor of the external quantum efficiency.

Carrier Dynamic Investigations of AlGaInAs Quantum Well Revealed by Temperature-Dependent Time-Resolved Photoluminescence.

AlGaInAs quantum well (QW) lasers have great potential in the application fields of optical communications and eye-safety lidars, owing to the advantages of good gain performance. A large amount of experimental evidence indicated that carrier dynamic affects the resonant frequency and modulation response performance of QW lasers. However, the mechanism of carrier dynamic in AlGaInAs QW structure is still ambiguous for complicated artificial multilayers. In this paper, the carrier dynamic of AlGaInAs QW structure was investigated by temperature-dependent time-resolved photoluminescence (TRPL) in the range of 14 to 300 K. Two relaxation times (a fast component and a slow one) have a major impact on the PL emission spectra of the AlGaInAs QW below 200 K. The carriers prefer a fast decay channel in the low temperature regime, whereas the slow one a higher temperature. An unconventional temperature dependence of carrier relaxation is observed in both decay processes. The carriers’ lifetime decreases with the temperature increasing till 45 K and then increases with temperature up to 250 K. It is quite different from that in the bulk semiconductor. The mechanism of temperature-dependent carrier relaxation at temperatures above 45 K is a combination of dark state occupation and a nonradiative recombination process.

Multiphonon Raman Scattering and Strong Electron-Phonon Coupling in 2D Ternary Cu2MoS4 Nanoflakes.

The interaction between photogenerated carriers with lattice vibrations plays a fundamental role in the nonradiative recombination and charge-transfer processes occurring in photocatalysis and photovoltaics. Here, we employ Raman spectroscopy to investigate the electron-phonon interaction in ternary layered Cu2MoS4 nanoflakes. Multiphonon Raman scattering with up to fourth-order longitudinal optical (LO) overtones is observed under above-band gap excitation, indicating a strong electron-phonon coupling (EPC) that could be described by the cascade model. The Huang-Rhys factor was derived to characterize the strength of EPC and was found to be increasing with decreasing temperature. First-principles calculations of lattice dynamics and electron-phonon matrix elements suggest that the strong EPC in Cu2MoS4 is dominated by Fröhlich coupling between electron and the electric fields, which is induced by the localized phonon mode originating from a flat phonon branch. Our findings facilitate the understanding of electron-phonon interaction in 2D ternary Cu2MoS4 and pave the way for developing and optimizing optoelectronic devices.

Investigation of photoluminescence mechanisms from porous polysilicon for optoelectronic devices

The room-temperature and temperature-dependent photoluminescence spectra from as-grown and annealed porous polysilicon in the pure oxygen atmosphere have been measured and analyzed. The energy of blue emissions (B band) is independent on the measurement temperature, however; the intensity of the B band is decreased with the increasing measurement temperature. With the increasing measurement temperature, the band gap emissions of as-grown and annealed porous polysilicon at 300 °C show redshift. From the evolution of intensity with the measurement temperature, there are two different non-radiative recombination processes. At the low temperature range between 11 K to 80 K, the thermal quenching behaviors are due to the influence of the surface states. At the higher temperature range (from 80 K to 300 K), however, nonradiative recombination processes can be attributed to the thermal escape. We believe that the understanding of the defects is very beneficial for the application of the porous polysilicon in the optoelectronic device field.

Correlation between excitons recombination dynamics and internal quantum efficiency of AlGaN-based UV-A multiple quantum wells

The correlation between the recombination dynamics of excitons and the internal quantum efficiency (IQE) of AlGaN-based UV-A multiple quantum wells (MQWs) was studied via photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The probability ratio of the capture of excitons by nonradiative recombination centers (NRCs) and the radiative recombination of excitons was evaluated individually via two different experimental analyses. The IQE was evaluated via temperature- and excitation power density-dependent PL measurements and its dependence on excitation density was analyzed using a rate equation model based on the radiative and nonradiative recombination of excitons. Moreover, the radiative and nonradiative recombination lifetimes were evaluated via temperature-dependent TRPL measurements; furthermore, they were analyzed as functions of temperature and excitation energy density. The probability ratios obtained from the two individual analyses were in agreement. This quantitative agreement indicated that the analysis based on the radiative and nonradiative recombination processes of excitons, which included the process of filling NRCs, was valid for AlGaN-based UV-A MQWs.

Effects of photon recycling, trapping, and reuse on thermophotovoltaic conversion efficiency and output power

Photonic management is a key issue for the optimization of thermophotovoltaic (TPV) energy conversion systems. It is realized by selective emitters, front surface filters on TPV cells, and back surface reflectors (BSRs). Photonic management modifies photon energy transfer from the emitter to the TPV cell due to photon reuse and energy conversion processes in the TPV cell due to photon recycling and trapping in the cell. Our work has developed a comprehensive thermodynamic theory of photonic management in the TPV cell and in whole TPV systems to elucidate key optimization parameters. Our approach is based on the exact Lambert function solution of the generalized Shockley–Queisser model and the corresponding fundamental formulas of endoreversible thermodynamics for maximal electric power, emitted optical power, and dissipation losses. The model includes interrelated processes of photon recycling, photon trapping, nonradiative recombination, and parasitic absorption of the BSR. Optimization of a TPV system with photon reuse should take into account that the cell thickness that provides maximal output power does not correspond to the thickness, which gives the maximal conversion efficiency. The theory predicts the important limits for TPV efficiency and output power determined by the Auger recombination in low-bandgap semiconductor materials, various parasitic losses in the cell and conductive layers, and photon escape from the TPV system. For example, we consider the TPV system based on 0.6 eV InGaAs cells with a BSR and a front surface photon scattering layer, which provides Lambertian light trapping.

Photoluminescence properties of hybrid perovskites in solar cells with TiO2 and Mg0.2Zn0.8O electron transport layers

This paper presents a study of the photoluminescence properties of hybrid perovskite films deposited on titanium and magnesium zinc oxide films, as electron transport layers, using the spin-coating technique. The subject of the investigation was continuous wave photoluminescence versus temperature, excitation power and transient photoluminescence. Moreover, the paper discusses possible carrier recombination mechanisms. Complex temporal decay was approximated through the use of several models, but only the four-exponent model and the model using the sum of two hyperbolic functions provided a good agreement with the experimental data. The first attempt to replace titanium dioxide with magnesium zinc oxide in conjunction with the perovskite layer showed improved optical properties such as a weaker non-radiative recombination process and a longer decay time constant.

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

AbstractRadical‐carrying organic molecules have received significant attention to bypass the issue related to harvesting triplet excitons in current light‐emitting materials. While the computational efforts conducted so far have treated these radical emitters as isolated entities, in actual devices, they are embedded in a host matrix and subject to emitter–host interactions. Here, by combining molecular dynamics simulations and density functional theory calculations, the impact of the host matrix on the optoelectronic performance of radical emitters is evaluated, taking as a representative example the (4‐ncarbazolyl‐2,6‐dichlorophenyl)bis(2,4,6‐trichlorophenyl)‐methyl (TTM‐3NCz) radical emitter dispersed in a 4,4‐bis(carbazol‐9‐yl)biphenyl (CBP) host. A morphological analysis shows that steric effects around the radical centers, carried by the TTM electron‐poor moieties of the emitters, disfavor π–π interactions with the host molecules, which leads to random intermolecular orientations around the TTM moieties. The 3NCz electron‐rich moieties of the emitters, however, have much lesser spatial hindrance for intermolecular π–π stacking, which modulates the structural and electronic properties of the emitters in the host matrix. The influence of dynamic and static disorders on the radiative and nonradiative recombination processes is also investigated and it is found that the rates of nonradiative recombination are small, which opens the way to 100% internal quantum efficiency for the doublet‐based emission process.

Light-Induced Passivation in Triple Cation Mixed Halide Perovskites: Interplay between Transport Properties and Surface Chemistry.

Mixed halide perovskites have attracted a strong interest in the photovoltaic community as a result of their high power conversion efficiency and the solid opportunity to realize low-cost and industry-scalable technology. Light soaking represents one of the most promising approaches to reduce non-radiative recombination processes and thus to optimize device performances. Here, we investigate the effects of 1 sun illumination on state-of-the-art triple cation halide perovskite thin films Cs0.05(MA0.14, FA0.86)0.95 Pb (I0.84, Br0.16)3 by a combined optical and chemical characterization. Competitive passivation and degradation effects on perovskite transport properties have been analyzed by spectrally and time-resolved quantitative imaging luminescence analysis and by X-ray photoemission spectroscopy (XPS). We notice a clear improvement of the optoelectronic properties of the material, with a increase of the quasi fermi level splitting and a corresponding decrease of methylammonium MA+ for short (up to 1 h) light soaking time. However, after 5 h of light soaking, phase segregation and in-depth oxygen penetration lead to a decrease of the charge mobility.

Mechanism Analysis of Proton Irradiation-Induced Increase of 3-dB Bandwidth of GaN-Based Microlight-Emitting Diodes for Space Light Communication

Influences of 170-keV protons beam irradiation on the static and dynamic properties of blue light InGaN/GaN multiple quantum wells (MQWs) microlight-emitting diodes (Micro-LEDs) were investigated. It was interesting to find out that, although threshold voltage and light output power of Micro-LEDs deteriorated after proton irradiation, a 3-dB bandwidth was greatly improved. In quantitative terms, at the forward current density of 1 kA/cm2, 3-dB bandwidth increased from original 7.34 to 119.74 MHz when Micro-LED exposed to proton beam with the fluence of $5\times 10^{14}$ p/cm2. Based on the frequency response data analysis, differential carrier lifetimes of Micro-LEDs, including Shockley–Read–Hall (SRH) lifetime and differential radiation recombination lifetime, were compared. The results pointed out that both SRH and recombination lifetimes became shorter after proton irradiation, indicating that competition between the nonradiative and radiative recombination processes was enhanced by proton beam. To reveal the origination of the 3-dB bandwidth improvement, photoluminescence (PL) and time-resolved PL (TRPL) spectrums of Micro-LEDs were measured. MQWs’ PL spectrum with a peak wavelength of 450 nm was observed and its intensity decreased as proton fluence increasing. Meanwhile, a PL peak at 550 nm, which was well known as defect-related PL spectrum, was enhanced by proton irradiation, especially at a proton fluence of $5\times 10^{13}$ and $5\times 10^{14}$ p/cm2, proving the increase of defects in epitaxial thin films as proton fluence increasing. In the TRPL experimental study, the nonradiative recombination lifetime decreased with proton fluence, which was consistent with the results analyzed by frequency response. Overall, the improvement of 3-dB bandwidth could be mainly attributed to the decrease of carrier lifetime in MQWs, which were caused by the generation of defects due to the atom displacement effect of proton irradiation.

Temperature-dependent photoluminescence of H2TPP and ZnTPP thin films on Si substrates

Temperature-dependent photoluminescence (PL) properties of tetraphenyl porphyrin (H2TPP) and Zinc-tetraphenyl porphyrin (ZnTPP) thin films on the silicon substrates has been investigated. The photoluminescence (PL) properties are observed within the temperature range of 163–543 °K. The 405 nm diode laser beam is used to excite the molecule during the photoluminescence measurement. The integrated PL intensity exhibits abnormal behaviour, the intensity decreases with temperature in the temperature range of 163-273 °K, and then followed by the increase in the further temperature range of 273-543 °K. This anomaly reflects the competition between the radiative recombination process and the non-radiative recombination processes, i.e. carrier capture, thermalization, and carrier relaxation processes. The peak energy of 0-0 and 0-1 transitions show a red-shift with temperature, while their full width at half maximum increases.

Burn-In Degradation Mechanism Identified for Small Molecular Acceptor-Based High-Efficiency Nonfullerene Organic Solar Cells.

Organic solar cells (OSCs) have again become a hot research topic in recent years. The record power conversion efficiency (PCE) of OSCs has boosted to over 17% in 2020. Apart from the high PCE, the stability of OSCs is also critical for their future applications and commercialization. Recently, many studies have proposed that burn-in degradation can be considered as an ineluctable barrier to long-term stable OSCs. However, there is still lack of studies to explain the detailed mechanism of this burn-in process. In this work, we first investigated the mechanism of the burn-in process in the high-efficiency PM6:N3-based nonfullerene OSCs. The PM6:N3-based device achieved a profound average PCE of 14.10% but also showed a significant performance loss after the burn-in degradation. Following characterizations such as dark J-V, photoluminescence (PL), time-resolved PL, Urbach energy estimation, and electrochemical impedance spectroscopy reveal that the burn-in degradation observed is closely related to the current extraction, energy transfer, nonradiative recombination, and charge transport process in the PM6:N3-based device. At the same time, it has small effects on the exciton dissociation process and energetic disorder in the PM6:N3-based device. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and grazing incidence X-ray diffraction measurements gratifyingly found that the morphology of the PM6:N3 active layer is relatively stable during the burn-in degradation. Therefore, these observed degradations are suspected results from the instability of interfaces and electrodes. The atoms in carrier transport layers and electrodes may diffuse to the active layer during the degradation, which changes the energy levels of each layer and causes traps at the interface and in the active layer. Conquering the instability of interfaces and electrodes is proposed as the prior task for PM6:N3-based OSCs to achieve long-term stability. Our study provides insights into the mechanism behind the burn-in degradation of the PM6:N3-based OSCs, which takes the first step to conquer this barrier.

Bright trion emission from semiconductor nanoplatelets

The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. Here, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

Photogenerated charge behavior of BiOI/g-C3N4 photocatalyst in photoreduction of Cr (VI): A novel understanding for high-performance

Z-scheme photocatalytic system was considered as a promising strategy to tackle environmental pollution crises. However, photogenerated charges behaviors of Z-scheme photocatalyst were still ambiguous, particularly for charge-recombination process occurring at the interface of Z-scheme heterojunctions. In this work, a solid-state heterojunction photocatalyst consisting of BiOI and g-C3N4 was fabricated to reveal the Z-scheme transfer process of photogenerated charge. The morphology, physical and optical properties were characterized in detail. The as-prepared BiOI/g-C3N4 composites exhibited higher reduction efficiency of Cr(VI) than that of pure BiOI and g-C3N4, and 79.2% of Cr(VI) removal was achieved after 15 min visible light irradiation (λ > 420 nm). The remarkable degradation performance was due to efficient separation and transfer of photogenerated charge in a direct Z-scheme path. More importantly, the non-radiative recombination process of photoinduced carriers occurring at the interface between BiOI and g-C3N4 was confirmed by the photoluminescence (PL), photoacoustics (PA) spectroscopy, work function (WF) and electron spin-resonance spectroscopy (ESR) measurement. This work shows that appropriate increasing the non-radiative recombination center may help to improve the charge recombination occurring at the interface so as to enhance photocatalytic degradation activity of Z-scheme photocatalytic system.