Carrier Recombination Processes Research Articles

Efficient photocatalytic degradation of Rhodamine B dye by undoped and zinc doped zirconium dioxide NPs

Undoped and Zinc-doped Zirconium Dioxide (ZrO2) nanoparticles (NPs) were synthesized via the co-precipitation method, employing Zirconium (IV) Oxynitrate hydrate (ZrO2N2·xH2O), Zinc Sulfate (ZnSO4·7H2O), and ammonia solution as precursor materials at room temperature. A comprehensive characterization of the structural, morphological, optical, and catalytic properties of the prepared samples was conducted using various techniques. Powder X-ray diffraction (PXRD) analysis revealed that the synthesized NPs exhibited a tetragonal phase structure, with crystallite sizes decreasing as doping concentration increased. Scanning electron microscopy (SEM) images confirmed the spherical shape of the NPs. Energy-dispersive X-ray spectroscopy (EDX) verified the presence of Zr, O, and Zn elements with high purity. Raman spectroscopy corroborated the tetragonal structure of the synthesized ZrO2 NPs. Ultraviolet–visible (UV–Vis) spectroscopy studies determined the cutoff wavelengths, while Tauc plot enabled the calculation of band gap energies. Fourier transform infrared spectroscopy (FTIR) confirmed the functional groups present in the synthesized samples. Photoluminescence spectroscopy provided evidence of defects and charge carrier recombination processes. The catalytic efficacy of the synthesized ZrO2 NPs was demonstrated through the degradation of Rhodamine B dye under visible and UV light.

Electrical Doping Regulation of Carrier Recombination Enhances the Perovskite Solar Cell Efficiency beyond 28.

With the power conversion efficiency (PCE) of perovskite solar cells (PSCs) exceeding 26.7%, achieving further enhancements in device performance has become a key research focus. Here, we investigate the impact of electrical doping in the perovskite layer using the drift-diffusion equation-based device physics model, coupled with a self-developed equivalent circuit model. Our results demonstrate that electrical doping can increase the PCE from 24.78% to >28%. In-depth theoretical analysis reveals that these improvements in performance are driven by the modulation of carrier recombination processes through doping, leading to significant increases in the open-circuit voltage and fill factor. Additionally, we explore the influence of physical parameters on device performance. Our study identifies an optimal doping concentration range from 1.0 × 1017 to 1.0 × 1019 cm-3 and a transport layer mobility of >0.01 cm2 V-1 s-1. This work provides a theoretical foundation for the development of ultra-high-performance PSCs through targeted electrical doping strategies.

Negative photoconductance in NiO/B-rGO nanocomposite-based UV photodetector: Structural and Photoelectric properties

The manifestation of uncommon negative photoconductance (NPC) in semiconductor materials has recently emerged as a significant breakthrough in various research applications. Despite its potential, nickel oxide (NiO), a transition metal oxide, has not been extensively explored in optoelectronic applications. However, when combined with carbon materials such as graphene oxide (GO), NiO demonstrates considerable promise. Additionally, doping GO with acceptor atoms like boron can modify its electronic behavior and enhance its functionality. In this study, we present a straightforward synthesis and preparation method for NiO/B-rGO nanocomposite, followed by their integration into UV photodetector devices. We investigate their structural and morphological characteristics using various analytical techniques. Furthermore, electrical characterization via I–V curve analysis is conducted to assess their photoresponse and reveal their photoconductance behavior. The results highlight negative photoconductance (NPC) under UV photoexcitation compared to the dark case, with this behavior is attributed to the trapping and recombination processes of carriers, which is thoroughly discussed, and supported by electrical noise evaluation. Additionally, the performance of the NiO/B-rGO device demonstrates superior photoresponse, with a responsivity of 0.92 A/W and a detectivity of 1.38 × 101⁰ Jones at low reverse bias.

Thermo-diffusion of excited photothermal semiconductor under laser pulse and ramp heating with acoustic pressure

The goal of this work is to provide a novel mathematical model that explains how certain physical variables propagate (acoustic-thermal-mechanical diffusive) as waves in a photoexcited non-Gaussian laser pulse semiconductor medium. Under the impact of acoustic pressure, the isotropic and homogeneous semiconductor medium is discussed concerning the fundamental equations according to charge carrier recombination processes with optoelectronic properties. Given the impact that relaxation times have on the governing equations. Laplace transforms were utilized in a one-dimensional (1D) context to examine essential non-dimensional properties such as displacement, stress components, carrier density, temperature, and acoustic pressure in order to mathematically answer the required problem. By imposing specific initial and boundary conditions, inverse Laplace transformations were employed to generate precise solutions for the numerical modeling of different physical quantities depicted graphically. The graphical representation of wave propagation data was followed by a theoretical analysis and interpretation, emphasizing the influence of other factors (such as heat rise time, relaxation times, and laser pulse effects) on the observed occurrences.

Investigation into the carrier recombination in Sb2Se3: Photo thermal effect, trapped carrier absorption and hot carrier cooling

Understanding the carrier recombination processes in Sb2Se3 is essential for its optoelectronic applications. In this work, carrier recombination dynamics in Sb2Se3 were studied by broad band transient absorption spectroscopy. Firstly, the contribution of photothermal effect to the transient absorption spectrum was thoroughly discussed. It is confirmed that the excited state absorption (ESA) band with lifetime of several nanoseconds results from co-contribution of photo thermal effect and deep trapped carrier absorption. Secondly, the features of transient absorption spectrum on picosecond time scale were interpreted. The short-lived ESA band around 1000 nm was assigned to shallow trapped carrier absorption, while not band gap renormalization (BGR) or free carrier absorption. By globally fitting the transient absorption spectrum, the hot carrier cooling time and time constant for free carrier relax into deep trap state were determined to be 0.25∼0.45 ps and 3.1∼8.7 ps, respectively. Finally, we built up the carrier recombination model of Sb2Se3. The experimental results in this work will improve the understanding on the carrier recombination in Sb2Se3.

Effect of cap layer and post growth on-site hydride passivation on the surface and interface quality of InAsP/InP hetero and QW structures

The performance of devices made from III-V compound semiconductors relies heavily on their surface and interface properties. The InAsP/InP material system is recently gaining interest due to its suitability for the next generation of long-haul classical and quantum communication applications. Researchers are studying how surface and interface properties affect the efficiency of the device and concludes that the epitaxial growth conditions responsible for creating surface and interface states require further improvement. To address this, we have studied the effect of the top (cap) layer and post-growth on-site hydride passivation on the properties of InAsP/InP hetero-structures grown using metal-organic vapor phase epitaxy technique. The flow rate and flux ratios of the hydrides significantly affect the surface reaction rates and gas-phase diffusion coefficients, which in turn impact the As↔P exchange mechanisms. Our findings suggest that post-growth on-site arsine passivation encourages the As↔P substitutions at the vicinity of the desorption sites of phosphorus, leading to the arsenic-rich surface of InAsP with the diffused hetero-interfaces. The activation energy for such As↔P exchange reaction mechanism is evaluated as ∼ (1.4 ± 0.3) eV. On the other hand, it has been observed that the thin cap layer of InP protects the surface of the InAsP epilayer and thereby preserving the sharp interface. Further, surface photovoltage and photoluminescence spectroscopy is employed to examine the role of the diffused and sharp interface of InAsP/InP hetero-structures in charge carrier transport, their redistribution and recombination process. Our analysis of the energy transitions occurring in the surface photovoltage and photoluminescence spectrum reveal that the energy band alignment and the subsequent charge carrier redistribution processes is influenced by the surface and interface states. These changes in the surface photovoltage phase spectra of InAsP/InP system also support the role of surface and interface states in the generation and separation of the charge carriers. The study provides insights into the effects of As↔P exchange on surface and interfacial quality, highlighting the implications for optoelectronic device development using InAsP/InP material system.

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.

Luminescence efficiency and carrier dynamics for InGaAs/GaAs surface quantum dots in coupled heterostructures

InGaAs/GaAs surface quantum dot (SQD) heterostructures have long been viewed as having great potential for realizing environmental gas detection devices. The research has recently been steered into discovering ways to inject carriers into the available SQD energy states to facilitate readable devices. In the present research, carrier injection from buried QDs (BQDs) to SQDs was controlled by changing the interlayer coupling. Photoluminescence (PL) measurements show that, by increasing the stacking period of BQDs, the carrier collection efficiency and subsequently the luminescence intensity for SQDs can be effectively improved. A rate equation simulation of the carrier recombination process is used to quantitatively describe the luminescent quantum efficiency (IQE) for the SQDs, indicating an increase with additional BQD layers due to an increase in carrier injection. The PL spectra along with the rate equation simulations further indicated that for these SQD hybrid structures nonradiative recombination dominates, with Auger recombination becoming significant at high excitation intensities. These results may help to understand the carrier dynamics in coupled SQDs hybrid structures. In addition, they provide a path to manipulating the properties of InGaAs SQDs for the development of gas sensors.

Emergence of Z-Scheme Photocatalysis for Total Water Splitting: An Improvised Route to High Efficiency.

Photocatalytic water splitting to spontaneously produce H2 and O2 is a long-standing goal in solar energy conversion, presenting a significant challenge without using sacrificial electron donors or external biases. Inspired by natural photosynthesis, the design of artificial Z-scheme photocatalytic systems is at the forefront of this field. These systems achieve higher redox potential by separating photogenerated electrons and holes through a fast interlayer recombination process between valence and conduction band edges. Z-scheme photocatalysis involves using two different semiconductors with distinct bandgap energies. Here, we explore potential systems based on two-dimensional (2D) heterostructures composed of carbon, nitrogen, or similar main group elements. The advantages and disadvantages of these systems are discussed, with a focus on enhancing their efficiency through strategic design. Special emphasis is placed on the dynamics of excited charge carrier transfer and recombination processes, which are crucial for developing efficient photocatalytic systems for overall water splitting.

Investigation of Thermo-Electrical Instabilities in a Semiconductor as 2D Dynamical Systems

A semiconducting sample placed in cryogenic media with applied electric field generates low frequency oscillations of electric current and sample temperature and known to be thermo-electrical instabilities. Although observation of current oscillations on oscilloscope is possible, change of sample temperature cannot be detected experimentally. Description of the phenomenon through mathematical equations helps to understand relationship of the two variables as well as their connection to deep trap behavior that are involved in supporting the instability. Mathematical model for thermo-electrical instabilities in an n type semiconductor based on the two deep trap level model with non-degenerate electron statistics has been introduced in order to investigate the unique relationship between the change in time of both electric current flowing through a semiconductor sample and the sample temperature. The 3D dynamical system of nonlinear inhomogeneous ordinary differential equations has been investigated as component 2D dynamical systems (n,T), (n,n<sub>t</sub>) and (n<sub>t</sub>,T) for local behavior at isolated equilibrium and at points on individual trajectories, where n, n<sub>t</sub> and T are free electron concentration at conduction band, electron concentration at deep traps and temperature of a semiconductor sample accordingly. Each of the planar systems is expressed in canonical form and investigated as a Cauchy problem with a set of appropriate initial values. This paper presents investigation results of phase trajectories of the planar systems depending on a single parameter – the temperature of cooling media T<sub>0</sub>. Based on obtained calculation results of time sequences of the three variables n, n<sub>t</sub> and T, phase differences among these variables have been determined for different values of T<sub>0</sub>. It has been established that the change in sample temperature lags behind change in current and this lag increases with T<sub>0</sub>. Clearly defined correlations among systems (n,T), (n,n<sub>t</sub>) and (n<sub>t</sub>,T) are seen, being the result of balance between field aided and thermal ionization mechanisms for charge carrier generation and recombination processes. Thermal and field assisted generation mechanisms compete with one another in achieving steady non equilibrium state in the system depending on temperature of cooling media T<sub>0</sub>.

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.

Magnetic-order-mediated carrier and phonon dynamics in MnBi2Te4

We investigate the quasiparticle dynamics in MnBi2Te4 single crystals using ultrafast optical spectroscopy. Our results show that there exist anomalous dynamical optical responses below the antiferromagnetic (AFM) ordering temperature TN. In specific, we reveal that both the initial carrier decay and recombination processes can be modulated via introducing the AFM order in subpicosecond and picosecond timescales, respectively. We also discover a long relaxation process emerging below TN with a timescale approaching the nanosecond regime, and can be attributed to the T-dependent spin-lattice interaction. There also emerges an unusual phonon energy renormalization below TN, which is found to arise from its coupling to the spin degree via the exchange interaction and magnetic anisotropy. Our findings provide key information for understanding the dynamical properties of nonequilibrium carrier, spin, and lattice in MnBi2Te4. Published by the American Physical Society 2024

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.