Nonradiative Recombination Processes Research Articles

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.

Influence of the excitation density and temperature on the optical properties of type I InAs/GaAsSb quantum dots

In this study, the optical properties of InAs quantum dots (QDs) were characterized using photoluminescence (PL) measurements. The QDs, capped with GaAs and GaAs1−xSbx (x = 6%) strain-reducing layer (SRL), were grown by Molecular Beam Epitaxy. Temperature-dependent photoluminescence (TDPL) of both ground state (GS) and first excited state (ES) was carried out through the analysis of the PL peak position as well as the integrated PL intensity. The temperature dependence of the integrated PL intensity shows the carrier trapping in the potential barrier at the interface between the capping layer and QDs in both samples at low temperatures for the GS and ES. The Excitation density dependent photoluminescence (EDPL) showed a redshift of the GS and ES PL peak energies with increasing excitation density. We attribute this variation to the bandgap renormalization (BGR) effect. The potential barrier reduction for the GaAsSb-capped QDs increases carrier injection efficiency inside the QDs, giving rise to a larger BGR effect compared to the QDs capped with GaAs. With increasing temperature, BGR redshift varies considerably for the GaAs- capped QDs but less for the Q with GaAsSb SRL. This effect was explained using the population rate of carriers inside the QDs while taking into account the nonradiative recombination process for the two samples. Furthermore, the variation of the integrated intensity with the excitation power density grows superlinearly for the two samples for a temperature range from 10 K to 220 K. This behavior was explained by the random capture of carriers in the dots. The sample with GaAsSb SRL having a smaller potential barrier has reduced superlinear dependence at low temperatures. Losses mechanism has a significant impact on increasing the superlinear dependence at high temperatures. The ES showed a stronger superlinearity compared to the GS. This study helps to understand the optical mechanisms in some devices, such as QD lasers.

Temperature dependence of nonradiative recombination processes in UV-B AlGaN quantum well revealed by below-gap excitation light

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

Ethanedithiol treatment on zinc oxide films for highly efficient quantum dot light-emitting diodes by reducing exciton quenching

Surface modification of ZnO nanoparticles with ethanedithiol (EDT) has been studied to minimize exciton loss at the ZnO/quantum dots (QDs) interface and to improve the efficiency of QD light-emitting diodes (QLEDs). The EDT treatment has been demonstrated to fill oxygen vacancies and reduce carboxylate and hydroxyl ligands on the surface of the ZnO nanoparticles from the x-ray photoelectron spectroscopy analysis. The reduction of oxygen vacancies makes the ZnO films more hydrophobic and maintains the optical bandgap under ambient conditions. EDT treatment shifts the energy level up by 0.47 eV by modifying the surface of ZnO. Non-radiative recombination processes at the ZnO/QDs interface such as interfacial charge transfer and energy transfer are reduced through the upshifted energy level and reduced surface defects. The brightness of the QLED with EDT treatment on ZnO was improved by more than 330% having a maximum luminance of 59 , 500 c d / m 2 compared to the QLED with pristine ZnO. The maximum current efficiency of the QLEDs has been improved from 54.3 cd/A to 73.4 cd/A, which is 35% higher than ZnO-based QLEDs.

Statistical Analysis of the Photoelectric Characteristics for InGaN/GaN MQWs Solar Cells Following Proton Irradiation

The statistical photoelectric properties of InGaN/GaN MQWs solar cells under proton irradiation were studied. Three groups of devices were irradiated with 3 MeV proton beams with fluences of 1.0 × 1011 cm−2, 5.0 × 1011 cm−2, and 1.0 × 1012 cm−2. Averaged experiment results showed that the open circuit voltage (VOC) and fill factor (FF) always decreased. In contrast, the short circuit current density (JSC) and photoelectric conversion efficiency (η) increased first and then decreased with the increase of fluence. Firstly, the donor-like defects of InGaN layer in active region played a major role in increasing JSC and η when the fluence was relatively low. Due to the introduction of donor-like defects, extra free electrons were produced, which lead to an increase in the number of effective carriers. With the further increasing of proton fluence, the number of free electrons caused by donor-like defects in the used InGaN material (low indium component) increased slowly due to pinning effect. However, additional point defects (Ga vacancies) increased significantly. Ga vacancies can act as effective non-radiative recombination centers, thereby reducing the number of effective carriers. The combined effect of pinning effect and non-radiative recombination process is responsible for the first increase and subsequent decline of JSC and η.

Charge transfer dynamics of the CdTe quantum dots fluorescence quenching induced by ferrous (II) ions

Size dependence of glutathione capped CdTe quantum dots (GSH-CdTe QDs) on the sensitivity and selectivity in the fluorometric detection of ferrous (II) ions (Fe2+) has been systematically investigated. Smaller-size QDs show higher sensitivity in the detection of Fe2+, resulting in higher quenching efficiency and red shift of the fluorescence peak of QDs. Stern–Volmer plots indicate that the charge transfer model can be employed to account for the observed fluorescence quenching effect. Fe2+ is bound to the surface of QDs by GSH and excited electrons are transferred from QDs to Fe2+, which facilitates a nonradiative recombination process and a decrease in the PL efficiency. In addition, the results from time resolved photoluminescence and a confocal scanning fluorescence microscope have shown that smaller-size QDs have a faster decrease in the fluorescence lifetime compared with that of larger-size QDs with Fe2+ addition, suggesting that the fast charge transfer in smaller-size QDs should be responsible for the observed fluorescence quenching effect. This Letter provides a comprehensive understanding of the mechanism of the fluorescence for the CdTe QDs quenched by Fe2+.

Ethylenediamine chlorides additive assisting formation of high-quality formamidinium-caesium perovskite film with low trap density for efficient solar cells

Dual-cation formamidinium-caesium (FACs) perovskite shows good thermal stability, but a great number of trap states existing in the FACs perovskite film impair its photoelectric conversion efficiency and long-term stability. In this work, ethylenediamine chlorides (EDACl2), a new type Lewis base trap deactivator, is synthesized and then applied to the perovskite precursor solution to assist the formation of FACs perovskite film with less trap states. A series of photoelectric measurements reveal that the charge transport of FACs perovskite film, mediated by EDACl2 additive, is promoted and the non-radiative recombination process is reduced as well. By optimizing the EDACl2 additive amount and Br composition, the EDACl2-engineered perovskite solar cells (PSCs) with structures of ITO/SnO2/Cs0.15FA0.85PbI2.7Br0.3/Spiro-MeOTAD/Ag and ITO/SnO2/Cs0.15FA0.85PbI2.7 Br0.3/carbon, achieve their respective champion efficiencies of 19.68% and 15.56%, along with negligible hysteresis. Particularly, the carbon-based FACs device with EDACl2 exhibits good air-, thermal- and photostability. This work provides a facile and efficient ion additive engineering for fabrication of high-quality perovskite films and high-performance PSCs.

Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

Summary Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.

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.

Quantum Yield Enhancement in Graphene Quantum Dots via Esterification with Benzyl Alcohol

The quantum yield of graphene quantum dots was enhanced by restriction of the rotation and vibration of surface functional groups on the edges of the graphene quantum dots via esterification with benzyl alcohol; this enhancement is crucial for the widespread application of graphene quantum dots in light-harvesting devices and optoelectronics. The obtained graphene quantum dots with highly graphene-stacked structures are understood to participate in π–π interactions with adjacent aromatic rings of the benzylic ester on the edges of the graphene quantum dots, thus impeding the nonradiative recombination process in graphene quantum dots. Furthermore, the crude graphene quantum dots were in a gel-like solid form and showed white luminescence under blue light illumination. Our results show the potential for improving the photophysical properties of nanomaterials, such as the quantum yield and band-gap energy for emission, by controlling the functional groups on the surface of graphene quantum dots through an organic modification approach.

Crystal Structure Ideality Impact on Bimolecular, Auger, and Diffusion Coefficients in Mixed-Cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 Perovskites

Nonlinear nonradiative recombination processes are considered as a main obstacle to efficient lead halide perovskite light emitters as light emitting diodes and lasers. In this work, crystal structure ideality, described by the Goldschmidt tolerance factor, impact on Auger and bimolecular recombination rates in spin-coated mixed-cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 perovskite layers was investigated. The excitation-dependent carrier lifetime and diffusion coefficient were determined using optical pump–probe techniques: time-resolved photoluminescence and light-induced transient grating. Layers with a Goldschmidt tolerance factor closer to unity revealed Auger and bimolecular recombination rates smaller by up to an order of magnitude. The bimolecular coefficient (0.6–5 × 10–10 cm3/s) increased simultaneously with the Auger coefficient (0.05–2 × 10–27 cm6/s) upon increase of x and simultaneous decrease of tolerance factor, due to variation in the localized carrier population and Rashba splitting. A larger...

Organic Solar Cells Based on Non-Fullerene Small-Molecule Acceptors: Impact of Substituent Position

Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end-groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end-groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end-groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton dissociation and nonradiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.