Dominant Recombination Mechanism Research Articles

Impact of Ex-Situ Heating on Carrier Kinetics in GaN/InGaN Based Green LEDs

Self-heating has long been recognized as one of the major factors for performance deterioration of III-Nitride light emitting diodes (LEDs) during prolonged operation at high current. We have emulated the impact of self-heating on carrier kinetics in InGaN based green LED ( ∼ 516 nm at 1 mA forward current) using temperature dependent current vs voltage (T-I-V) characteristics in forward and reverse bias, where the temperature is varied from room temperature (25 °C) to 200 °C. Measured T-I-V characteristics are analyzed in detail so as to understand the dominant recombination mechanisms at different bias regimes. It is inferred that at high forward bias, Auger recombination plays a significant role during the ex-situ heating of LEDs. An exponential increase in reverse leakage current beyond a certain high temperature is attributed to the carriers attaining adequate thermal energy to escape from the active region.

Loss mechanism analyses of perovskite solar cells with equivalent circuit model

Perovskite solar cells have been attracting more and more attentions due to their extraordinary performances in the photovoltaic field. In view of the highest certified power conversion efficiency of 25.5% that is much lower than the corresponding Shockley-Queisser limit, understanding and quantifying the main loss factors affecting the power conversion efficiency of perovskite solar cells are urgently needed. At present, the three loss mechanisms generally recognized are optical loss, ohmic loss, and non-radiative recombination loss. Including the trap-assisted bulk recombination and surface recombination, the non-radiative recombination is proved to be the dominant recombination mechanism prohibiting the increase of efficiency. In this work, based on semiconductor physics, the expressions of bulk and surface recombination currents are analytically derived. Then taking the optical loss, series and shunt resistance losses, and bulk and surface recombination losses into considerations, an equivalent circuit model is proposed to describe the current density-voltage characteristics of practical perovskite solar cells. Furthermore, by comparing to the drift-diffusion model, the pre-defined physical parameters of the drift-diffusion model well agree with the fitting parameters retrieved by the equivalent circuit model, which verifies the reliability of the proposed model. For example, the carrier lifetimes in the drift-diffusion model are consistent with the recombination factors in the equivalent circuit model. Moreover, when the circuit model is applied to analyze experimental results, the fitting outcomes show favorable consistency to the physical investigations offered by the experiments. And the relative fitting errors of the above cases are all less than 2%. Through employing the model, the dominant recombination type is clearly identified and split current density-voltage curves characterizing different loss mechanisms are offered, which intuitively reveals the physical principles of efficiency loss. Additionally, through calculating the efficiency loss ratios under the open-circuit voltage condition, quantifying the above-mentioned loss mechanisms becomes simple and compelling. The prediction capability of the model is expected to be enhanced if a series of light intensity dependent current density-voltage curves are fitted simultaneously. Consequently, this model offers a guideline to approach the efficiency limit from a circuit-level perspective. And the model is a comprehensive simulation and analysis tool for understanding the device physics of perovskite solar cells.

Effects of annealing atmosphere on the performance of Cu(InGa)Se2 films sputtered from quaternary targets.

Quaternary sputtering without additional selenization is a low-cost alternative method for the preparation of Cu(InGa)Se2 (CIGS) thin film for photovoltaics. However, without selenization, the device efficiency is much lower than that with selenization. To comprehensively examine this problem, we compared the morphologies, depth profiles, compositions, electrical properties and recombination mechanism of the absorbers fabricated with and without additional selenization. The results revealed that the amount of surface Se on CIGS films annealed in a Se-free atmosphere is less than that on CIGS films annealed in a Se-containing atmosphere. Additionally, the lower amount of surface Se reduced the carrier concentration, enhanced the resistivity of the CIGS film and allowed CIGS/CdS interface recombination to be the dominant recombination mechanism of CIGS device. The increase of interface recombination reduced the efficiency of the device annealed in a Se-free atmosphere.

Mid-infrared type-II InAs/InAsSb quantum wells integrated on silicon

Direct integration of III–V semiconductor light sources on silicon is an essential step toward the development of portable, on-chip infrared sensor systems. Driven by the presence of characteristic molecular fingerprints in the mid-infrared (MIR) spectral region, such systems may have a wide range of applications in infrared imaging, gas sensing, and medical diagnostics. This paper reports on the integration of an InAs virtual substrate and high crystalline quality InAs/InAsSb multi-quantum wells on Si using a three-stage InAs/GaSb/Si buffer layer. It is shown that the InAs/GaSb interface demonstrates a strong dislocation filtering effect. A series of strained AlSb/InAs dislocation filter superlattices was also used, resulting in a low surface dislocation density of approximately 4 × 107 cm−2. The InAs/InAsSb wells exhibited a strong photoluminescence signal at elevated temperatures. Analysis of these results indicates that radiative recombination is the dominant recombination mechanism, making this structure promising for fabricating MIR Si-based sensor systems.

Elucidating the functional form of the recombination losses in a planar perovskite solar cell: A scaling analysis

Through the analysis of scales on the drift–diffusion device model of a planar-structured perovskite solar cell, we have obtained accurate-analytical expressions that capture the recombination losses within the cell. The recombination losses are resolved into the radiative and Shockley–Read–Hall mechanisms, as well as interfacial recombination between the perovskite and electron/hole-transporting layers. After parameter calibration with the state of the art planar perovskite solar cell of 23.5% efficiency, the percentage contribution of various recombination loss channels within a planar-structured perovskite solar cell is analytically determined through derived scales and numerically verified at the condition of an open-circuit voltage and a short-circuit current, as well as gives a good prediction of a dominant recombination mechanism within the cell. On this basis, a comparison of loss analysis between the estimated scales and numeric results is carried out at the condition of an open-circuit voltage when a wide range of parameters influencing the recombination current is deviated simultaneously, and a good agreement is obtained.

The Measurement of Charge Carrier Lifetime in SIGaAs: Cr and EL2-GaAs by Pump-Probe Terahertz Spectroscopy

In the present work, the temporal dynamics of relaxation of a nonequilibrium concentration of charge carriers in SI-GaAs:Cr and EL2-GaAs semiconductor crystals has been studied using pump-probe terahertz spectroscopy. The obtained experimental data were analyzed taking into account the mechanisms of surface and bulk Shockley–Reed–Hall recombination, radiative recombination, interband and trap assisted Auger recombination. It was found that at injection levels arising upon excitation of samples by laser radiation with a pulse duration of 35 fs and an energy of 0.1 mJ per pulse at a central wavelength of 791 nm, Auger recombination mechanisms have a significant effect. Auger recombination mechanisms make a dominant contribution to the recombination rate of nonequilibrium charge carriers at injection levels above 2·1018 cm–3 for SI-GaAs:Cr and above 1018 cm–3 for EL2-GaAs. At lower injection levels, the bulk and surface Shockley-Reed-Hall recombination are the dominant recombination mechanisms.

Dominant recombination mechanism in perovskite solar cells: A theoretical study

Different recombination losses have been shown to play a role in the photocurrent and photovoltage loss of lead halide perovskite solar cells. While it is believed that trap density is extremely small in high-efficiency perovskite cells, trap-assisted recombination cannot be ruled out due to the existence of tail states, for instance at the grain boundaries. In this study, the optical and electrical performance of a perovskite solar cell of the structure: ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/LiF/Al was modelled using numerical drift-diffusion simulations with embedded optical modelling via the transfer matrix method. The total recombination was calculated as the sum of bimolecular recombination and recombination via tail states. The influence of charge carrier mobility on photovoltaic parameters (short-circuit current, fill factor, open-circuit voltage, and power conversion efficiency) of perovskite solar cells was studied. Next, the impact of Urbach energy on the recombination process was investigated by simulation of bimolecular and tail state recombination rates as a function of voltage for various values of Urbach energy. It was shown that by increasing the Urbach energy, the bimolecular recombination rate decreases, but the tail state recombination rate increases at all voltages. Furthermore, the slope of the open-circuit voltage-light intensity curve, the light ideality factor, demonstrates the dominant recombination at different Urbach energy levels. It was found that the slope increases from 1.04 at EU = 25 meV to1.65 at EU = 150 meV. It can be concluded that the bimolecular recombination is the dominant recombination at low EU, while the tail state recombination overcomes the bimolecular recombination at high EU.

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.

Reducing ultraviolet-induced open-circuit voltage loss in inverted organic solar cells by maintaining charge selectivity of the electron collection contact using polyethylenimine

The effect of using Polyethylenimine (PEI) instead of ZnO in the electron extraction layers (EELs) of inverted organic solar cells (OSCs) on their photo-stability behavior is investigated. Studies on inverted OSCs based on two different donor/acceptor bulk heterojunction material systems show that replacing ZnO by PEI substantially reduces the open-circuit voltage (Voc) loss in OSCs that is often observed under prolonged UV irradiation. Analysis of current density-voltage characteristics of the solar cells as well as of hole-only devices shows that the higher Voc stability attained upon using PEI instead of ZnO is mainly due to the ability of the ITO/PEI contact to maintain a higher degree of electron selectivity and hole blocking properties under UV exposure in comparsion to the ITO/ZnO contact. Using Voc vs light intensity analysis, a UV-induced transition in the dominant recombination mechanism from a trap-assisted bulk recombination mechanism to a surface recombination one at the contacts is found to occur in solar cells with ZnO EELs. Similar tests on PEI show that it suppresses that increase in surface recombination and confirms its ability to maintain the hole blocking properties of the contact upon UV exposure. This study provides a direct observation of increased surface recombination in OSCs due to UV irradiation and direct evidence of the importance of maintaining charge selectivity of contacts to suppress surface recombination and maintain high Voc photo-stability.

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.

The detrimental effect of AlGaN barrier quality on carrier dynamics in AlGaN/GaN interface

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

CZTSe solar cells prepared by co-evaporation of multilayer Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se stacks

In this work, thin-film kesterite Cu2ZnSnSe4 (CZTSe) solar cells were prepared using a novel precursor configuration employing co-evaporated layer stacks of Mo/Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se. It is found that this sequential deposition of the constituants leads to the formation of large CZTSe grains on the surface and fine grains at the Mo interface of the absorber, respectively. Prototype CZTSe solar cells using this stacked approach achieve power conversion efficiencies of up to 7.9% at an open-circuit voltage of 430 mV and a fill-factor of 62%. The analysis of temperature-dependent current density–voltage characteristics indicates that bulk Schottky–Read–Hall recombination is the dominant recombination mechanism for the devices fabricated from the proposed stack. In addition, the influence of pre-annealing of each stacked layer on the absorber growth and device performance is examined and discussed.

Ag Alloying and KF Treatment Effects on Low Bandgap CuInSe2 Solar Cells

Silver alloying and KF post-deposition treatments are explored as approaches to increase the efficiency of low bandgap CuInSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIS) solar cells. The KF treatment prevents a loss in VOC when CIS has a thinner CdS layer, and also increases the carrier concentration of these devices. Compared with CIS devices, Ag alloyed CIS (ACIS) devices have lower efficiencies due to a lower carrier concentration which decreases V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> . When ACIS is exposed to the KF treatment, the resulting devices have reduced V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and fill factor (FF) compared with untreated devices; however, thinning the CdS layer with KF-treated ACIS improves V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> . ACIS+KF films show an unidentified compound in Raman spectra and the resulting solar cells have a dominant interface recombination mechanism and light-to-dark crossover. An electron microscopy study of the growth and coverage of CdS reveals that KF-treated samples in this study have faster CdS growth and ACIS+KF has complete CdS coverage with less than half of the normal deposition time. Improved coverage of CdS layers on KF-treated samples is also verified by X-ray photoelectron spectroscopy. Finally, using a heat treatment and an anti-reflection layer enables CIS+KF solar cells to obtain a high 16.0% efficiency for CIS with a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> = 526 mV and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> = 41.0 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Identifying Dominant Recombination Mechanisms in Perovskite Solar Cells by Measuring the Transient Ideality Factor

The ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used as a means to identify the dominant recombination mechanism in solar cells. However, applying this Suns-VOC technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), the light intensity, and the device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the temporal evolution of VOC at different light intensities. The initial values of the transient ideality were consistent with corresponding photoluminescence vs intensity as well as electroluminescence vs current density measurements. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, show that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both devices. Using transient photovoltage measurements superimposed on the evolving VOC of bifacial devices we further show that the charge collection interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This information could not be inferred from an ideality factor determined from only steady-state VOC values. The method we have developed will be useful for identifying performance bottlenecks in new variants of perovskite devices by comparison with the transient ideality signatures we have predicted for a range of possible recombination schemes.

Fingerprinting Electronic Structure in Nanomaterials: A Methodology Illustrated by ZnSe Nanowires.

Characterizing point defects that produce deep states in nanostructures is imperative when designing next-generation electronic and optoelectronic devices. Light emission and carrier transport properties are strongly influenced by the energy position and concentration of such states. The primary objective of this work is to fingerprint the electronic structure by characterizing the deep levels using a combined optical and electronic characterization, considering ZnSe nanowires as an example. Specifically, we use low temperature photoluminescence spectroscopy to identify the dominant recombination mechanisms and determine the total defect concentration. The carrier concentration and mobility are then calculated from electron transport measurements using single nanowire field effect transistors, and the measured experimental data were used to construct a model describing the types, energies, and ionized fraction of defects and calculate the deviation from stoichiometry. This metrology is hence demonstrated to provide an unambiguous means to determine a material's electronic structure.

Evolution of the light sensitive defects in high performance multicrystalline silicon wafers

Sequential degradation measurements have been performed on passivated high performance multicrystalline silicon wafers, first at room temperature under low intensity illumination followed by a higher intensity illumination at an elevated temperature. The presence of two main degradation mechanisms, affecting the lifetime under different conditions has been demonstrated, namely, the well-studied light induced degradation caused by boron-oxygen-complexes and the less understood light and elevated temperature induced degradation. Light and elevated temperature induced degradation is the main lifetime limiting the recombination path when fully activated, but the contribution from boron-oxygen complexes is not negligible. This separation of the two degradation mechanisms might, therefore, be necessary for proper evaluation of the dominant recombination mechanism. Experiments also show regeneration of the minority carrier lifetimes caused by deactivation of both the lifetime limiting defects at comparable time scales, and under similar illumination and temperature conditions. Wafers from different heights in a high performance multicrystalline silicon ingot have been evaluated to better understand the underlying causes for the different degradation mechanisms. Effects of the iron-boron-splitting on the carrier lifetime are only visible in ungettered wafers.

Correlation of Defect Luminescence and Recombination in Multicrystalline Silicon

Correlations between defect-related luminescence (DRL) and recombination mechanisms of multicrystalline silicon wafers are investigated by hyperspectral photoluminescence (PL) imaging at cryogenic temperatures (∼80 K) and by PL-based techniques for charge carrier lifetime at room temperature. This unique combination of measurement techniques is used to spectrally compare the DRL in n-type and p-type wafers and to investigate the DRL as a function of block height in a p-type block. Further, the dependence of DRL on interstitial and precipitated metallic impurities has been investigated by comparison of simulated concentration profiles of interstitial and precipitated iron with the spatial distribution of DRL. Our results indicate that the origins of the dislocation-related emission lines (D-lines) are independent of the doping type and suggest that the spectral shape, rather, is determined by the dominating recombination mechanism in the material. In regions with high structural defect density, we observe increased intensities of the D-lines D1–D4 in the DRL spectrum. In regions with a high concentration of either iron or other metallic precipitates, we observe reduced emission intensities of D3 and D4. It is, thus, likely that precipitates of either iron or other impurities partly supress the D3 and D4 emission intensities.

Investigation of recombination mechanisms of CdTe solar cells with different buffer layers

It is critical to understand the dominant recombination mechanisms of thin film solar cells for further improving their performance. The dominant recombination paths of CdTe solar cells with MgxZn1-xO (MZO), MZO/CdS and MZO/CdS/CdSe buffer layers were investigated by temperature-dependent current-voltage (JVT) characterization both in light and dark, with traditional CdS/CdTe devices as a reference. The devices with MZO or MZO/CdS buffer layers are confirmed to be dominated by interface recombination, which leads to poor performance of devices. Compared to devices with traditional CdS buffer layers which are dominated by bulk recombination, the devices with MZO/CdS/CdSe buffer layer exhibit the same dominant recombination path but superior performance. It demonstrates that MZO/CdS/CdSe can be a promising composite buffer layer.

Cu(In,Ga)Se2 solar cell with Zn(S,O) as the buffer layer fabricated by a chemical bath deposition method

Zn(S,O) polycrystalline thin films were prepared using chemical bath deposition. The Zn(S,O) thin film with high-crystalline quality fully covered the substrate by controlling concentrations of starting materials, temperature of the substrate, and reaction time. The obtained Zn(S,O) thin films were characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and ultraviolet–visible spectroscopy. The optical transmittance of the Zn(S,O) thin film with a thickness of 106.84 nm exceeded 90% in the wavelength range of 350–1400 nm. The obtained Zn(S,O) films were applied as buffer layers in the CuIn1−xGaxSe2 (CIGS) solar cells. By optimizing the thickness and quality of the buffer layer and using light soaking, a 11.65%-efficiency CIGS solar cell was obtained without an anti-reflective coating layer. The short-circuit current density increased compared with that of CIGS solar cell with cadmium sulfide buffer layer due to the higher band gap of Zn(S,O) than that of CdS. The increase of the efficiency with the light-soaking time was mainly due to the fill factor increase. The CIGS solar cell showed good weak light effect and high stability. The dominant recombination mechanisms in the CIGS solar cell with Zn(S,O) buffer layer were the tunneling-enhanced recombination in the space charge region and the interface recombination at the Zn(S,O)/CIGS interface.

Deep level emission in polycrystalline CuGaSe2 thin-films observed by micro-photoluminescence

Polycrystalline CuGaSe2 thin-films with various Cu-contents were deposited by three-stage co-evaporation method. Radiative transitions in the CuGaSe2 were studied by micro-photoluminescence technique in relation to the Cu/Ga ratio in the films. In addition to a peak DAP1 around 1.60 eV, a peak DAP2 at relatively lower energy of 1.35 eV and a broadened deep-level emission DL roughly centered at 1.05 eV were observed in the photoluminescence (PL) spectra of the CuGaSe2 films. Excitation-power dependence along with temperature dependence of the PL suggests that dominant recombination mechanism due to donor–acceptor pair recombination at lower temperature becomes impurity-to-band transition at higher temperature for all the three peaks. A deep donor-level, roughly 630 meV below the conduction band, which corresponds to DL-emission has been confirmed, and was investigated further in relation to the compositional variation in the CuGaSe2 films.