Carrier Recombination Behavior Research Articles

Strategic Buried Defect Passivation of Perovskite Emitting Layers by Guanidinium Chloride for High-Performance Pure Blue Perovskite Light Emitting Diodes.

Perovskite light-emitting diodes (PeLEDs) show promise for high-definition displays due to their exceptional electroluminescent properties. However, the performance of pure blue PeLEDs is hindered by the unfavorable ionic behavior of halides and the presence of defective antisites in blue-emitting perovskite materials. An unstable buried interface between charge transport layers and the perovskite emitting layer is a major issue that limits carrier transport and recombination behavior in PeLEDs. In this study, effective buried defect passivation of pure blue perovskite emitting layers by introducing guanidinium chloride (GACl) as a bottom-passivating layer is demonstrated. The GACl bottom layer not only passivates the point defects present at the buried interface but also provides chloride anions to suppress ion migration and halide vacancy formation. Along with the defect passivation, GACl also enforces phase purity of 2D layered structure in the perovskite emitting layers to improve crystallinity and optoelectronic properties. As a result, the PeLEDs with high brightness (1200cd m-2) and excellent external quantum efficiency (6.61%) are achieved at a spectrally stable pure blue electroluminescence at 471nm (band width = 17.63nm). This study offers insights into the straightforward way for effective buried passivation for preparing high-performance PeLEDs.

Carrier, spin and optical propagation properties of CsPbBr3 hexagonal nanocrystals

The shape of nanocrystals can effectively adjust their photo-physical properties, which becomes a route to improve the nanocrystal-based semiconductor device. Herein, we further dig the photo-physical properties of CsPbBr3 hexagonal nanocrystals (HNCs), in which CsPbBr3 cubic nanocrystals (CNCs) serves as a reference. It is found that the activity of carrier in CsPbBr3 HNCs is higher than that of CsPbBr3 CNCs, owing to the slow energy dissipation path related to the phonon energy, which slows down the carrier temperature cooling process and accelerate the carrier recombination behavior simultaneously. Moreover, the spin flip process may occur in the CsPbBr3 HNCs, whose lifetime is longer than that of CsPbBr3 CNCs owing to the weakening of phonon scattering of Elliott-Yafet mechanism. In addition, the photoluminescence test of CsPbBr3 HNCs in optical fiber microcavity shows that its light wave-guiding performance is determined by the self-absorption effect and the conventional light wave-guiding effect, which are both super to those of CsPbBr3 CNCs, benefiting from its high PL quantum yield. Our research suggests that the CsPbBr3 HNCs exhibits a huge potential in the field of optoelectronics, spintronics and photo-communication.

Ionic liquids tailoring crystal orientation and electronic properties for stable perovskite solar cells

The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite component to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small- and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h.

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Nondestructive microstructural investigation of defects in 4H-SiC epilayers using a multiscale luminescence analysis approach

The development of metal oxide semiconductor field effect transistors (MOSFETs) utilizing epitaxially grown 4H-SiC has accelerated in recent years due to their favorable properties, including a high breakdown field, high saturated electron drift velocity, and good thermal conductivity. However, extended defects in epitaxial 4H-SiC can affect both device yields and operational lifetime. In this work, we demonstrate the importance of a multiscale luminescence characterization approach to studying nondestructively extended defects in epitaxial 4H-SiC semiconducting materials. Multiscale luminescence analysis reveals different aspects of excess charge carrier recombination behavior based on the scale of a particular measurement. Combining measurements of the same extended defect area at different scales tells us more about the essential nature of that defect and its microstructure. Here, we use photoluminescence imaging and cathodoluminescence spectrum imaging to investigate the recombination behavior of several different types of extended defects, including stacking faults, inclusions, and basal plane dislocations. A detailed understanding of the optoelectronic properties of extended defects in epitaxial SiC helps elucidate the microstructure of extended defects and can provide pathways to mitigate detrimental changes during device operation related to their evolution, such as the recombination enhanced dislocation glide effect that affects SiC-based MOSFETs.

Bi-doped graphitic carbon nitride nanotubes boost the photocatalytic degradation of Rhodamine B

Polymeric carbon nitride (PCN) is an emerging metal-free photocatalyst with high stability but is plagued by low photocatalytic efficiency due to the rapid charge carrier recombination behavior.

The piezotronic effect on carrier recombination processes in InGaN/GaN multiple quantum wells microwire

Understanding piezotronic correlated carrier recombination behavior in quantum wells is essential for their applications. In this work, we have studied the influence of piezotronics on carrier recombination processes in single InGaN/GaN multiple quantum wells microwire (MQW-MW) by using steady-state and time-resolved spectroscopies. We conclude that mechanical strain induced piezotronics promotes the charge separation of excitons in space, and slows down the recombination rate of free carriers. The proposed model is supported by three independent experiments: photoluminescence experiment of MQW-MW before and after peel off, strain dependent TRPL experiment, and excitation fluency dependent PL intensity experiment. Our study could provide a guideline for the application of piezotronic in MQW-MW-based optoelectronic devices.

Investigation on minority carrier lifetime, diffusion length and recombination mechanism of Mg-doped GaN grown by MOCVD

Minority carrier lifetime and diffusion length are important parameters for GaN based material and devices. In this work, we obtained the minority carrier lifetime and diffusion length of p-GaN through the photoluminescence lifetime of the band edge peak, and analyzed the recombination behavior of carriers. Besides, the dependence of photoluminescence lifetime of GaN: Mg grown by MOCVD on annealing temperature (TA) and Mg concentration ([Mg]) has been investigated through Time-resolved Photoluminescence (TRPL) measurements. We increased the activated Mg acceptors ratio by increasing TA, analyzed the fast and slow decay of carrier recombination, and compared the effects of annealing on the lifetime of GaN band-edge emission with different Mg concentrations. And it is found that the minority carrier lifetime in p-GaN is not only related to Mg concentration, but also related to the activated Mg acceptors concentration / hole concentration. Meanwhile, we compared heteroepitaxial p-GaN with homoepitaxial p-GaN, and further analyzed the influence of dislocation as non-radiative recombination centers (NRCs) on carrier lifetime and recombination behavior.

Effects of BTA2 as the third component on the charge carrier generation and recombination behavior of PTB7:PC71BM photovoltaic system

Effects of a benzotriazole (BTA)-based small molecule, BTA2, as the third component on the charge carrier generation and recombination behavior of poly [[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithio-phene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl]] (PTB7): [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) organic solar cells (OSCs) were investigated by optical simulation of a transfer matrix model (TMM), photo-induced charge extraction by linearly increasing voltage (photo-CELIV) technique, atomic force microscope (AFM), and the Onsager-Braun model analysis. BTA2 is an A2-A1-D-A1-A2-type non-fullerene small molecule with thiazolidine-2,4-dione, BTA, and indacenodithiophene as the terminal acceptor (A2), bridge acceptor (A1), and central donor (D), respectively. The short-circuit current density of the OSCs with BTA2 can be enhanced significantly owing to a complementary absorption spectrum. The optical simulation of TMM shows that the ternary OSCs exhibit higher internal absorption than the traditional binary OSCs without BTA2, resulting in more photogenerated excitons in the ternary OSCs. The photo-CELIV investigation indicates that the ternary OSCs suffer higher charge trap-limited bimolecular recombination than the binary OSCs. AFM images show that BTA2 aggravates the phase separation between the donor and the acceptor, which is disadvantageous to charge carrier transport. The Onsager-Braun model analysis confirms that despite the charge collection efficiency of the ternary OSCs being lower than that of the binary OSCs, the optimized photon absorption and exciton generation processes of the ternary OSCs achieve an increase in photogenerated current and thus improve power conversion efficiency.

Characterization of carrier transport behavior of specific type dislocations in GaN by light assisted KPFM

The direct characterization of carrier transport behavior at different types of dislocations in GaN is still questionable due to the current lack of feasible strategy. Herein, we developed a method by combining ultraviolet light-assisted Kelvin probe force microscope with defect selective etching technology. The dislocation types could be confirmed by the shape of the etching pit, and the photogenerated carrier recombination behavior at specific dislocation. Thus, it can be characterized by ultraviolet light-assisted Kelvin probe force microscope, which paves the way for analysis of carrier transport behavior at different types of dislocations in GaN. The screw dislocations are found to be the main non-radiative recombination centers, and mainly responsible for leakage current in GaN based device. This shows higher surface potential and higher electron concentration due to the donor defects introduced during dislocation growth. Conversely, a barrier is generated around the edge and mixed dislocations, which could suppress the non-radiative recombination at the dislocation. The present work provides a feasible way to direct characterization of specific type dislocations in GaN. Moreover, this method can be used to characterize other III-nitride materials.

Damp Heat Induced Degradation of Silicon Heterojunction Solar Cells With Cu-Plated Contacts

Damp heat exposure is one of the most stringent environments for testing the durability of solar cells in packaged modules. Damp heat stresses and induces a variety of degradation modes in solar cells and modules: for example, moisture-induced corrosion of electrodes and interconnections, deterioration of polymeric materials, and/or thermally activated diffusion processes. To screen for these and other potential degradation modes, we subject one-cell modules containing silicon heterojunction (SHJ) solar cells with Cu-plated contacts to extended damp heat tests at 85 °C/85% relative humidity. SHJ cells were laminated with two common encapsulants: ethylene vinyl acetate (EVA) and polyolefin elastomer (POE), and two constructions: glass–backsheet and glass–glass. We observe degradation in all components of solar cell maximum power $(P_{{\rm{MP}}})$ : current, voltage, and fill factor, and find evidence of increased carrier recombination and nonideal diode behavior with increasing stress. For glass–backsheet constructions, EVA samples generally degrade more than POE by a factor of approximately 1.5x $P_{{\rm{MP}}}$ , and the different encapsulants produce different degradation patterns. Similar trends are observed in glass–glass modules, but to a lesser degree. In a different experiment, we observe a decrease in effective minority carrier lifetime of nonmetallized SHJ precursors measured after damp heat. This implies that some degradation unrelated to the contacts is to be expected and confirms the observation of increasing recombination.

Annealing effects on the electrical and photoelectric performance of SnS2 field-effect transistor

SnS2 field effect transistor (FET) has been fabricated based on an exfoliated SnS2 flake. The effect of vacuum and sulfur-vapor annealing on the electric and optoelectronic properties of SnS2 FET have been investigated in detail. The experimental results indicate post-annealing, especially the sulfur-vapor annealing, has a strong impact on the electrical and photoelectric properties of SnS2 FET. The decreasing of source-drain current with annealing, the shift of threshold voltage and the exponential function of photocurrent varying with light intensity all exhibit a close relevance with surface states in SnS2. The intriguing characteristics can be well explained by sulfur-vacancies-related trapping mechanism. These results not only advance the current understanding of the generation, trapping and recombination behavior of photoexcited carriers in two-dimensional materials, but also stimulate the potential applications in optoelectronic devices based on SnS2.

Lattice-Mismatch Strain and Confinement in Nanoscale Si/SiO2 Structures Fabricated Using Thermal Oxidation

The behavior of electron and hole transport in semiconductor materials is influenced by lattice-mismatch at the interface. It is well known that carrier scattering in a confined region is dramatically reduced. In this work, we studied the effects of coupling both the strain and confinement simultaneously. We report on the fabrication and characterization of nanoscale planar, wall-like, and wire-like Si/SiO2 structures. As the Si nanostructure dimensions were scaled down to the quantum regime by thermal oxidation of the Si, changes to the band structure and carrier effective mass were observed by both optical and electrical techniques. Transient-time response measurements were performed to examine the carrier generation and recombination behavior as a function of scaling. Signal rise times decreased for both carrier types by an order of magnitude as Si dimensions were reduced from 200 to 10 nm, meaning that the carrier velocity is increasing with smaller scale structures. This result is indicative of decreased Si bandgap energy and carrier effective mass. Photoluminescence measurements taken at 50K showed changes in the PL response peak energies, which illustrates changes in the band structure, as the Si/SiO2 dimensions are scaled.

Polyaniline based hybrid bionanocomposites with enhanced visible light photocatalytic activity and antifungal activity

Nickel doped polyaniline/cellulose bionanocomposites have been synthesized via in-situ polymerization of aniline and hydrothermally prepared nickel nanoparticles. The as prepared materials were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV–vis) spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). Cyclic Voltammetry (CV) was used to determine the electrochemical surface area (ECSA) of the materials by using the electrochemical double layer capacitance (EDLC) data. SEM images reveal that synthesized nanocomposites contain spherical Ni nanoparticles scattered uniformly within PANI-Cellulose matrix. The as prepared materials exhibited significant degradation of reactive orange (RO-16) dye under visible light. Incorporation of Ni nanoparticles in to the polymer matrix causes strong adsorption of dye in case of PANI/C/Ni, PANI/Ni as compared to that of PANI. The photodegradation of RO-16 was achieved by the electron-hole pair separation and formation of reactive species by trapping of the photo generated electron from the surface of photo catalyst. Scavengers were added to identify the primary reactive species. Fluorescence spectroscopy was to study the recombination behavior of charge carriers (electron-hole pair) during photodegradation. Moreover, the anti-fungal nature of the bionanocomposites was also examined and the materials were found to be effective in growth control of two pathogenic fungal strains- Rhizoctonia solani and Alternaria alternate.

A Study of Efficiency Droop Phenomenon in GaN-Based Laser Diodes before Lasing.

Carrier recombination behavior in c-plane GaN-based laser diodes (LDs) is numerically investigated by using the commercial software LASTIP. It is found that efficiency droop phenomenon does exist in GaN-based LDs before lasing, which is confirmed by experimental results. However, the current density corresponding to the peak efficiency of GaN-based LDs before lasing, Jmax, is nearly 40 A/cm2, which is much lower than that reported by other studies. The reported Jmax, measured from the cavity facet side is modulated by the absorption of quantum wells, which shifts the Jmax to a higher value. In addition, the currents due to various recombinations are calculated. It is found that Auger recombination affects the threshold current greatly, but it only plays a small role at high current injection levels.

Temperature-dependent charge transport in solution-processed perovskite solar cells with tunable trap concentration and charge recombination

Based on control of the perovskite film thickness, we investigate temperature-dependent charge carrier transport, recombination, traps, and solar cell behavior based on methylammonium lead triiodide films.

Modeling the Free Carrier Recombination Kinetics in PTB7:PCBM Organic Photovoltaics

Currently the exact recombination mechanism of free carriers in organic photovoltaic (OPV) devices is poorly understood. Often a reduced Langevin model is used to describe the decay behavior of electrons and holes. Here we propose a novel, simple kinetic model that accurately describes the decay behavior of free carriers in the PTB7:PCBM organic photovoltaic blend. This model needs to only take into account free and trapped holes in the polymer, and free electrons in the fullerene, to accurately describe the recombination behavior of free carriers as measured by time-resolved microwave conductivity (TRMC). The model is consistent for different PTB7:PCBM blend ratios and spans a light intensity range of over 3 orders of magnitude. The model demonstrates that dark carriers exist in the polymer and interact with photoinduced charge carriers, and that the trapping and detrapping rates of the holes are of high importance to the overall carrier lifetime.

Temperature-dependent photoluminescence spectra of ZnO nanorod arrays grown on transparent conducting substrates by hydrothermal routes

Ordered ZnO nanorod (NR) arrays have been prepared on transparent conducting oxide (TCO) substrates by a facile seed-assisted hydrothermal route. The optical properties of the samples were investigated by temperature-dependent photoluminescence (PL) spectra. Two sharp peaks were clearly identified in the ultraviolet (UV) band, located at 3.34 and 3.26 eV in 10 K spectrum, and were labeled as peaks A and B. Both the peaks exhibited different quenching behaviors, although their red-shift characteristics were almost the same with temperature rising. Something gave rise to a possible mechanism to explain such a behavior. Specifically, peaks A and B were assigned to the bound excitons and carrier transitions from shallow donor levels to the valence band, respectively. The detailed analysis will be illustrated in this study. It is hoped that this work could render guided information for understanding the carrier recombination and dynamics behavior in single-crystal ZnO nanostructures.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

Understanding how the complex intermolecular- and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.