Carrier Recombination Processes Research Articles

High-efficiency electroluminescence devices containing Si nanocrystals/SiC multilayers via improved carrier injection and recombination process

Realizing high efficiency of all Si-based light-emitting devices is currently one of interesting issues in order to develop monolithic opto-electronic integration on chips. Here, we report an electroluminescence device based on phosphorus (P)-doped silicon nanocrystals (Si NCs)/silicon carbide (SiC) multilayers by modulating carrier injection and recombination process. The p+-Si substrate is used instead of p-Si substrate for facilitating the hole injection into Si NCs. Additionally, the influences of annealing temperature on the device performance have been studied, and the optimized annealing temperature is achieved by balancing the crystallinity, defect state density, and recombination process.

Temperature-dependent electroluminescence of red high-In-content MQWs of dual-wavelength micro-LED

Temperature-dependent electroluminescence (TDEL) measurements have been employed to investigate the carrier transport and recombination processes of InGaN red micro-LED based on dual-wavelength InGaN/GaN MQWs structure. EL peak energy and carrier transport of the red micro-LED both show temperature dependence, due to temperature-induced changes in defect activation. In addition, the current density at which the blue peak of the low-In-content appears in the EL spectrum varies with temperature. As the temperature increases, the blue peak of the low In component tends to appear at higher current densities, which may be attributed to the increase in thermally activated defects hindering the injection of holes into the low-In-content MQWs further away from p-GaN. Furthermore, the IQEs of the high-In-content MQWs are estimated from the TDEL method and then reveal the temperature-dependent efficiency droop. The IQE decreases as temperature increases, particularly above 50 K, where it drops sharply due to temperature-dependent nonradiative recombination. And the two different variation trends in IQE of MQWs with high and low In content reveal a competitive mechanism in carrier distribution, implying that more escaping holes from high-In-content MQWs will further reduce red emission efficiency but enhance carrier injection and blue emission in low-In-content MQWs.

Enhancement the linear/nonlinear optical and magnetic properties of ZnCo2O4 nanostructures through Ni/Fe dual doping

Nano Zn0.9Ni0.1Co2-xFexO4 samples were prepared using the hydrothermal procedure in order to enhance the functional characteristics of ZnCo2O4 sample. The structural and microstructural parameters of the prepared samples were determined through Rietveld and FTIR analyses. The morphologies and compositions of the samples were examined by SEM and EDS techniques. Upon Ni2+ doping, the bandgap energy of nano ZnCo2O4 decreased from 2.22 eV to 3.61 eV–2.18 eV and 3.51 eV. Doping with Fe caused a slight decrease in bandgap energy for x = 0.05, then an increase for a higher content of Fe. Several empirical equations were used to determine the refractive index from the corresponding obtained optical energy gap values. As the Fe content rose to 5 %, the value of refractive index and nonlinear optical parameters increased; thereafter, it declined with additional Fe doping. The photoluminescence technique was used to analyze defects and processes of charge carrier recombination in the samples. ZnCo2O4 sample exhibits paramagnetic performance. With the addition of Ni to ZnCo2O4, the magnetic characteristics of the sample became superparamagnetic. Samples doped with Ni and various quantities of Fe exhibit a weak ferromagnetic behavior. The effect of doping on the magnetic transition, coercivity, saturation magnetization and remanent magnetization was explored.

Improved midgap recombination lifetimes in GaN crystals grown by the low-pressure acidic ammonothermal method

To investigate the carrier recombination processes in GaN crystals grown by the low-pressure acidic ammonothermal (LPAAT) method, the photoluminescence (PL) spectra and PL lifetimes of LPAAT GaN crystals grown on acidic ammonothermal (AAT) GaN seed crystals were correlated with the growth polarity and species/concentration of point defects. The PL spectra of LPAAT GaN grown toward the (0001¯) direction (−c region), which provided the highest growth rate, exhibited a predominant near-band edge (NBE) emission. Neither bandgap narrowing nor Burstein–Moss shifts due to high concentration residual impurities were observed in the NBE emissions, indicating higher purity than the previously reported AAT GaN crystals. In addition, strain-induced energy shift or energy broadening of excitonic emission peaks was not observed, indicating excellent crystal coherency. Because of the reduced concentration of midgap recombination centers, a record-long room-temperature PL lifetime for the NBE emission of ammonothermal GaN (40 ps) was obtained from the −c region. Meanwhile, the PL spectra also exhibited the yellow and blue luminescence bands originating from particular deep-state radiative recombination centers. The major vacancy-type defects acting as midgap recombination centers are identified as vacancy complexes comprising a Ga vacancy (VGa) and a few N vacancies (VN), namely, VGa(VN)n buried by H and/or O, where n is an integer. Further reduction of such defect complexes will allow less compensated stable carrier concentration in the LPAAT GaN crystals.

Photoluminescence Emission Efficiency Analysis Methodology by Integrating Raman Spectroscopy of the A1(LO) and E2(high) Phonons in a GaInN/GaN Heterostructure

Microscopic lattice vibration images of the E2(high) mode (E2H) and another mode of A1(LO) (A1L) or the higher energy branch of LO‐phonon−plasmon coupling mode (LOPC+) in a Ga0.95In0.05N film on a GaN template are obtained by Raman scattering spectroscopy using a 325 nm laser. The increase in temperature by increasing the laser power is obtained from the decrease in the energy of E2H and the theoretical formula comprising two terms based on the mode energy variation of the bulk material and the thermal strain effect. Using the obtained temperature and the energy shift of the LOPC+, the mapping images of the temperature and electron density in the x–y plane are simultaneously obtained. This image provides the spatial variation of photoluminescence (PL) emission efficiency, given as PL intensity per electron. This method enables the quantitative discussion on photo‐emission efficiency even in the regions of low or high carrier density affected by carrier transport. In the investigated area, a region with a lower PL efficiency is found despite a higher electron density and lower temperature increase than the surrounding region. This imaging analysis is feasible in integrating the carrier and thermal energy transports and recombination processes in carrier dynamics study.

An investigation of the impact of nafion polymer on surface passivation and analysis of degradation in HIT solar cells for improvement performance

The effectiveness of Heterojunction with an intrinsic thin layer (HIT) solar cell is greatly improved by using Nafion polymer to passivate the surface and reduce carrier recombination processes. The enhancement of high-efficiency solar cells has garnered considerable interest in surface passivation as a potential substitute due to their exceptional electrical properties. There is little research on the optical and electrical characteristics of different levels of Nafion concentration. The HIT solar cells were treated with varying Nafion-passivation concentrations (2.5 wt %, 5 wt %, 10 wt %). The most significant performance improvement is at a concentration of 2.5 wt% after degradation for 8 h. The open-circuit voltage (Voc) climbed to 735.05 mV, and the fill factor (FF) increased at + 3.49 %, followed by the increase of power conversion energy (PCE) at + 0.96 %, both showing a considerable increase compared to a cell made from an unaffected cell. Due to its ability to boost silicon solar cell performance, a low Nafion concentration is preferred.

Investigation of carrier transport and recombination at type-II band aligned p-NiO/AlGaN interface in p-NiO gate AlGaN/GaN HEMTs under forward bias

In this work, fine carrier transport and recombination processes in p-NiO gate AlGaN/GaN high electron mobility transistors were investigated by analyzing their electroluminescence under forward gate bias, with photoluminescence spectrum as a reference. Red luminescence with a peak of 1.9 eV was captured when the gate bias voltage exceeded 4 V, which was verified to originate from the tunneling enhanced interface recombination of injected holes from the gate metal and spilled electrons from the 2DEG channel at the type-II band aligned p-NiO/AlGaN heterostructure interface. Under higher gate bias voltage, holes were further injected into the GaN buffer layer, producing ultraviolet luminescence and yellow luminescence, corresponding respectively to the band edge emission and defect-assisted radiative recombination of GaN. Threshold voltage shift measurements under forward gate bias were conducted to further investigate the carrier transport and recombination processes.

Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping.

P-type self-doping is known to hamper tin-based perovskites for developing high-performance solar cells by increasing the background current density and carrier recombination processes. In this work, we propose a gradient homojunction structure with germanium doping that generates an internal electric field across the perovskite film to deplete the charge carriers. This structure reduces the dark current density of perovskite by over 2 orders of magnitude and trap density by an order of magnitude. The resultant tin-based perovskite solar cells exhibit a higher power conversion efficiency of 13.3% and excellent stability, maintaining 95% and 85% of their initial efficiencies after 250 min of continuous illumination and 3800 h of storage, respectively. We reveal the homojunction formation mechanism using density functional theory calculations and molecular level characterizations. Our work provides a reliable strategy for controlling the spatial energy levels in tin perovskite films and offers insights into designing intriguing lead-free perovskite optoelectronics.

Effect of mild mechanical stresses on device physics of slot-die coated flexible perovskite solar cells

Halide perovskites are a promising class of novel photoactive materials for their application in flexible photovoltaic devices. Tolerance and stability of devices to various mechanical stresses are important aspects of this technology for its effective and long-term operation. Currently, this aspect has not been fully researched and addressed by the research community. Hence, this work is aimed at studying the effect of mild mechanical stresses (MMSs) induced by convex bending on the behaviour of ITO/ZnO/MAPbI3/Spiro-OMeTAD/Au slot-die-coated flexible perovskite solar cells (FPSCs) and investigating the underlying mechanisms leading to performance degradation in devices. The carrier generation, recombination, and extraction processes within FPSCs are investigated before and after the application of MMSs. The obtained results suggest that the photovoltaic performance of the FPSCs deteriorates by up to 50% after convex bending, due to increased non-radiative recombination losses. The results obtained in this work provide some valuable insights into the dynamics of recombination processes in FPSCs exposed to MMSs and can be used for developing new strategies for improving the mechanical stability of devices.

Improved Carrier Separation and Recombination by Ferroelectric Polarization in the CuBiP2Se6/C2N Heterostructure: A Nonadiabatic Molecular Dynamics Study.

The rapid recombination of photogenerated carriers heavily restricts the photocatalytic efficiency. Here, we propose a new strategy to improve catalytic efficiency based on the ferroelectric van der Waals heterostructure (CuBiP2Se6/C2N). Combining density functional theory and the nonadiabatic molecular dynamics (NAMD) method, we have systematically analyzed the ground-state properties and carrier dynamics images in the CuBiP2Se6/C2N heterostructure. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the photogenerated carriers separation. NAMD results demonstrate that the excited-state carrier transfer and recombination processes in the CuBiP2Se6/C2N are consistent with a type II mechanism. Meanwhile, constructing the ferroelectric heterostructure can effectively prolong the carrier lifetime, from ∼65.98 to ∼124.54 ps. Moreover, the high quantum efficiency and tunable band edge positions mean that the CuBiP2Se6/C2N heterostructure is an excellent potential candidate material for photocatalytic water splitting.

Self-absorption and investigation of excited carrier dynamics in two-dimensional perovskite scintillator

The PEA2PbBr4 scintillator exhibits varied performance in photoluminescence (PL) and radioluminescence (RL) profiles. PL in the reflectivity mode shows emission peaks at 414 and 434 nm, whereas PL and RL in the transmission mode exhibit only one peak at 434 nm. Temperature-dependent PL spectra measurements confirm that self-absorption contributes to this difference. PEA2PbBr4 displays decay times of 1.3 ns (90%) and 4.4 ns (10%) for PL and 11.5 ns (89%) and 47 ns (11%) for RL. To explain the distinct decay times, we construct two models for simulating laser and radiation excitation and investigate carrier dynamics using non-adiabatic molecular dynamics. Our findings suggest that the relatively long radiative recombination process of carriers contributes to the slow RL decay time.

Thermoluminescence peak temperature shift characteristics of NaCl, NaCl:Al, and NaCl:Ca

To gain a more in-depth understanding of the thermoluminescence peak temperature shift characteristics of pure NaCl itself and its Al and Ca doped variants, a combination of the first-principles calculations and thermoluminescence experiments is used to explore how doping affects the electronic structure of the crystal and further analyze the mechanism of peak temperature shift in thermoluminescence. The calculations indicate that doping NaCl with Al slightly increases its band gap to 5.20 eV, whereas doping with Ca reduces it dramatically to 0 eV. These changes can modify the band gap width but introduce distinct defect formation energy values. Such changes may cause the thermoluminescence peak temperature to occur at lower temperatures and shift with the change of experimental conditions. The theoretical predictions are validated through thermoluminescence experiments, showing that the thermoluminescence peak temperatures of all samples rise with heating rate increasing. Notably, the change is most significant for NaCl:Al, where the peak temperature rises from 276 to 340 K. Meanwhile, as the irradiation dose increases in a range of 1–25 mGy, the growth of the thermoluminescence peak temperature turns relatively small, especially for NaCl:Ca, the peak temperature rises only from 195 to 202 K. This comprehensive analysis of the electronic structures and defect formation energy provides an insight into the thermoluminescence behavior of NaCl crystal. Doping with Al and Ca introduces mid-gap states that act as traps for charge carriers. These traps play a crucial role in the thermoluminescence process, capturing electrons during irradiation and releasing them upon heating, which leads to the observed luminescence. The presence of these traps and their specific energy levels relative to the conduction and valence bands directly influences the temperature at which the peak luminescence occurs. In addition, this study explores how the changes of electronic structure, caused by doping, affects the recombination process of charge carriers, which is very important for the thermoluminescence phenomenon. It also investigates the influence of external factors, such as the rate of heating and the dose of irradiation, on the stability and shift of thermoluminescence peak temperature. These findings emphasize the complex interactions between material composition, structural defects, and experimental conditions in determining the thermoluminescence characteristics of doped NaCl crystals. The results of this research are of great significance for the application of doped materials in various fields, including radiation dosimetry and solid-state lighting. The ability to manipulate the thermoluminescence peak temperatures through doping opens up new ways for designing materials with tailored luminescence properties for specific applications. This study not only deepens our understanding of the fundamental mechanisms of thermoluminescence but also highlights the potential of first-principles calculations combined with experimental analysis in the development of new materials with desired optical and electronic characteristics.

Blue sensitive sub-band gap negative photoconductance in SnO2/TiO2 NP bilayer oxide transistor.

Large negative photoconductance (NPC) of SnO2/TiO2 nanoparticles (NPs) heterostructure has been observed with thin film transistor (TFT) geometry and has been investigated using sub-bandgap light (blue) illumination. This negative photoconduction has been detected both in accumulation and depletion mode operation, which effectively reduces the carrier mobility (μ) of the TFT. Moreover, the threshold voltage (Vth) widely shifted in the positive direction under illumination. The combined effects of the reduction of mobility and Vth shifting led to a faster reduction of On (or Off) state current under illumination. The negative photosensitivity of this system is as high as 3.2 A W-1, which has been rarely reported in the earlier literature. Moreover, the variation of On (or Off) current, μ and Vth shift is linear with low-intensity blue light. This SnO2/TiO2NPbilayer channel has been deposited on top of an ionic dielectric (Li-Al2O3) that reduces its operating voltage of this TFT within 2 V. Furthermore, the device has achieved a saturation mobility of 0.4 cm2 V-1 s-1 with an on/off ratio of 7.4 × 103 in the dark. An energy band diagram model has been proposed based on the type-II heterostructure formation between SnO2/TiO2 semiconductors to explain this NPC mechanism. According to the energy band diagram model, adsorbed H2O molecules of TiO2 NPs created a depleted layer in the heterostructure that accelerated the recombination process of photo-generated carriers rather than its transport.

Charge carrier recombination processes, intragap defect states, and photoluminescence mechanisms in stoichiometric and reduced TiO2 brookite nanorods: an interpretation scheme through in situ photoluminescence excitation spectroscopy in controlled environment.

The study of titanium dioxide (TiO2) in the brookite phase is gaining popularity as evidence has shown the efficient photocatalytic performance of this less investigated polymorph. It has been recently reported that defective anisotropic brookite TiO2 nanorods display remarkable substrate-specific reactivity towards alcohol photoreforming, with rates of hydrogen production significantly (18-fold) higher than those exhibited by anatase TiO2 nanoparticles. To elucidate the basic photo-physical mechanisms and peculiarities leading to such an improvement in the photoactive efficiency, we investigated the recombination processes of photoexcited charge carriers in both stoichiometric and reduced brookite nanorods via photoluminescence excitation spectroscopy in controlled environment. Through an investigation procedure employing both supragap and subgap excitation during successive exposure to oxidizing and reducing gaseous agents, we firstly obtained an interpretation scheme describing the main photoluminescence and charge recombination pathways in stoichiometric and reduced brookite, which includes information about the spatial and energetic position of the intragap states involved in photoluminescence mechanisms, and secondly identified a specific photoluminescence enhancement process occurring in only reduced brookite nanorods, which indicates the injection of a conduction band electron during ethanol photo-oxidation. The latter finding may shed light on the empirical evidence about the exceptional reactivity of reduced brookite nanorods toward the photo-oxidation of alcohols and the concomitant efficiency of photocatalytic hydrogen generation.

Determining the Turn-On Voltage of GaN-Based Light-Emitting Diodes: From Near-Ultraviolet to Green Spectra

This work investigates the turn-on voltage (V on) estimation methods from experimental data of InGaN-based multiple-quantum-well light-emitting diodes (LEDs) with emission spectra from near-ultraviolet to green wavelengths. Three different methods are examined: fixed current intercept, linear fitting of current-voltage curve, and the minimum ideality factor (n ideal). The voltage at minimum n ideal consistently represents the on-state of LEDs since it describes the carrier recombination processes reflected in light output power (LOP) and the external quantum efficiency (EQE). Beyond the voltage at minimum n ideal, samples’ LOPs start to increase abruptly, with the EQEs reaching ≥ 54% of the peak EQE values. For V on determined by other methods, samples’LOPs are already turned on and the EQEs exceed their peak value. Therefore, using the minimum n ideal is a suitable method for determining Von of an LED.

Piezo-phototronic enhanced dual-wavelength InGaN/GaN multiple quantum wells Micro-LED arrays

As a promising technology for next-generation displays due to their high brightness and low power consumption, InGaN-based micro light-emitting diodes (Micro-LEDs) have attracted great attention in recent years. In this work, we detailedly investigate the carrier recombination and transportation process in dual-wavelength InGaN/GaN multiple quantum wells (MQWs) Micro-LED arrays under piezo-phototronic effects using electroluminescence (EL) and time-resolution photoluminescence measurements. With the increase of external strain, the violet EL intensity of dual-wavelength Micro-LED arrays first increases obtaining a maximum enhancement of ∼12% and then decreases, whereas blue EL emission almost maintains constant. Additionally, as the size of Micro-LED decreases, the enhancement obtained via piezo-phototronic effect will reduce, which is attributed to their inherently weaker piezoelectric polarization effect. Combining with dynamic analysis of carriers in the blue quantum well (QW), it is concluded that strain-induced interface polarized charges promote the wave function overlap of electron–hole pair, but reduce the injection of hole carriers in blue QW. Superposition of the above two factors enables the blue EL intensity stable under piezoelectric coupling. These results present a promising potential of piezo-phototronic effects to improve the Micro-LEDs devices.

Bulk Heterojunction Antimony Selenosulfide Thin‐Film Solar Cells with Efficient Charge Extraction and Suppressed Recombination

AbstractAs an emerging earth‐abundant light harvesting material, antimony selenosulfide (Sb2(S,Se)3) has received tremendous attention for photovoltaics. Manipulating the carrier separation and recombination processes is critical to achieve high device efficiency. Compared to the conventional planar heterojunction (PHJ), the bulk heterojunction (BHJ) configuration affords great potential to enable efficient charge extraction. In this work, BHJ Sb2(S,Se)3 solar cells are constructed based on CdS nanorod arrays (NRAs). Highly ordered CdS NRAs with appropriate nanorod lengths and diameters are obtained by regulating the growth environment and screening different substrates for CdS deposition. A low‐temperature oxygen doping strategy implemented on CdS NRAs is further developed to improve the optoelectronic and defect properties as well as form a favorable cascade band structure for CdS NRAs, so as to realize more efficient charge extraction and suppressed recombination at the heterointerface. As a result, the CdS NRAs‐based superstrated BHJ Sb2(S,Se)3 solar cell yields a considerable power conversion efficiency of 8.04%, outperforming that of the PHJ device. A careful comparative study of PHJ and BHJ based on electrostatic field simulations indicates that the BHJ allows more efficient charge extraction and transport. This work highlights the great potential of BHJ configuration for constructing high‐performance antimony chalcogenide solar cells.

Photoacoustic study of elastic deformations in silicon membranes

In an optically excited semiconductor micromechanical structure, photogenerated carriers (plasma waves) can produce elastic deformations (local strains and stresses)–plasmaelastic (PE) deformations. On the other hand, the generation of excess carriers will produce heat due to carrier thermalization and recombination processes. The generated heat can produce other elastic deformations–thermoelastic (TE) deformations. For these two components of elastic deformation, it is possible to consider two types of elastic displacements (two mods of elastic vibrations): elastic expanding and elastic bending. A theoretical model of the photoacoustic (PA) signal for optically excited Si membranes is given, which includes thermal diffusion (TD), TE, and PE mechanisms, in order to study elastic expansion and bending. The relations for the PA amplitude and the phase of elastic expanding and bending in the excited membrane are derived. Analysis of the PA signal indicates that the TD component is dominant for all thicknesses and practically the entire range of observed frequencies. The calculated PA amplitude and phase spectra show that the PA elastic expanding component has a significant influence on the total PA signal at low frequencies. On the other side, the calculated PA spectra show that the PA elastic bending component has a significant influence on the total PA signal at high frequencies. Experimental PA signals of Si membranes were measured using the sample-gas-microphone detection technique with a transmission configuration in relation to the modulation frequency of the optical excitation for different membrane thicknesses. The experimental PA spectra were compared with the theoretical ones.

Lattice dynamics and carrier recombination in GaAs/GaAsBi nanowires

GaAsBi nanowires represent a novel and promising material platform for future nano-photonics. However, the growth of high-quality GaAsBi nanowires and GaAsBi alloy is still a challenge due to a large miscibility gap between GaAs and GaBi. In this work we investigate effects of Bi incorporation on lattice dynamics and carrier recombination processes in GaAs/GaAsBi core/shell nanowires grown by molecular-beam epitaxy. By employing photoluminescence (PL), PL excitation, and Raman scattering spectroscopies complemented by scanning electron microscopy, we show that increasing Bi-beam equivalent pressure (BEP) during the growth does not necessarily result in a higher alloy composition but largely affects the carrier localization in GaAsBi. Specifically, it is found that under high BEP, bismuth tends either to be expelled from a nanowire shell towards its surface or to form larger clusters within the GaAsBi shell. Due to these two processes the bandgap of the Bi-containing shell remains practically independent of the Bi BEP, while the emission spectra of the NWs experience a significant red shift under increased Bi supply as a result of the localization effect.

Recent advances of modification effect in Co3O4-based catalyst towards highly efficient photocatalysis

Tricobalt tetroxide (Co3O4) has been developed as a promising photocatalyst material for various applications. Several reports have been published on the self-modification of Co3O4 to achieve optimal photocatalytic performance. The pristine Co3O4 alone is inadequate for photocatalysis due to the rapid recombination process of photogenerated (PG) charge carriers. The modification of Co3O4 can be extended through the introduction of doping elements, incorporation of supporting materials, surface functionalization, metal loading, and combination with other photocatalysts. The addition of doping elements and support materials may enhance the photocatalysis process, although these modifications have a slight effect on decreasing the recombination process of PG charge carriers. On the other hand, combining Co3O4 with other semiconductors results in a different PG charge carrier mechanism, leading to a decrease in the recombination process and an increase in photocatalytic activity. Therefore, this work discusses recent modifications of Co3O4 and their effects on its photocatalytic performance. Additionally, the modification effects, such as enhanced surface area, generation of oxygen vacancies, tuning the band gap, and formation of heterojunctions, are reviewed to demonstrate the feasibility of separating PG charge carriers. Finally, the formation and mechanism of these modification effects are also reviewed based on theoretical and experimental approaches to validate their formation and the transfer process of charge carriers.