Diffusion Of Charge Carriers Research Articles

Guanidinium‐Assisted Surface Matrix Engineering for Highly Efficient Perovskite Quantum Dot Photovoltaics

Metal halide perovskite quantum dots (Pe-QDs) are of great interest in new-generation photovoltaics (PVs). However, it remains challenging in the construction of conductive and intact Pe-QD films to maximize their functionality. Herein, a ligand-assisted surface matrix strategy to engineer the surface and packing states of Pe-QD solids is demonstrated by a mild thermal annealing treatment after ligand exchange processing (referred to as "LE-TA") triggered by guanidinium thiocyanate. The "LE-TA" method induces the formation of surface matrix on CsPbI3 QDs, which is dominated by the cationic guanidinium (GA+ ) rather than the SCN- , maintaining the intact cubic structure and facilitating interparticle electrical interaction of QD solids. Consequently, the GA-matrix-confined CsPbI3 QDs exhibit remarkably enhanced charge mobility and carrier diffusion length compared to control ones, leading to a champion power conversion efficiency of 15.21% when assembled in PVs, which is one of the highest among all Pe-QD solar cells. Additionally, the "LE-TA" method shows similar effects when applied to other Pe-QD PV systems like CsPbBr3 and FAPbI3 (FA = formamidinium), indicating its versatility in regulating the surfaces of various Pe-QDs. This work may afford new guidelines to construct electrically conductive and structurally intact Pe-QD solids for efficient optoelectronic devices.

Optimization of the Epitaxial Growth of Undoped GaN Waveguides in GaN-Based Laser Diodes Evaluated by Photoluminescence

Non-intentionally doped c-plane GaN layers are generally employed as p-side waveguide layers in violet/blue-emitting laser diodes. The recombination and diffusion of charge carriers in the p-side GaN waveguide influence the injection efficiency of holes into the InGaN quantum wells of these devices. In this study, the non-radiative recombination and the diffusivity in the [000-1] direction for charge carriers in such GaN layers are investigated by the photoluminescence of buried InGaN quantum wells, in addition to the GaN photoluminescence. The vertical charge carrier diffusion length and the diffusion constant in GaN were determined by evaluating the intensity from InGaN quantum wells in different depths below a top GaN layer. Additionally, the intensity from the buried InGaN quantum wells was found to be more sensitive to variations in the non-radiative recombination rate in the GaN layer than the intensity from the GaN itself. The study enables conclusions to be drawn on how the growth of a p-side GaN waveguide layer has to be optimized: (1) The charge carrier diffusivity in the [000-1] direction at device operation temperature is limited by phonon scattering and can be only slightly improved by material quality. (2) The use of TMGa (trimethylgallium) instead of TEGa (triethylgallium) as a precursor for the growth of GaN lowers the background silicon doping level and is advantageous for a large hole diffusion length. (3) Small growth rates below 0.5 μm/h when using TMGa or below 0.12 μm/h when using TEGa enhance non-radiative recombination. (4) A V/III gas ratio of 2200 or more is needed for low non-radiative recombination rates in GaN.

Possibility of Applying the Matrix Method to Modeling the Cathodoluminscescence Caused by a Wide Electron Beam in a Planar Multilayer Semiconductor Structure

Results are presented from investigating the possibility of applying the analytical matrix method to modeling the diffusion of nonequilibrium minority charge carriers and the subsequent cathodoluminescence caused by a wide electron beam in a planar structure of semiconductor optoelectronics with any number of layers. The proposed technique allows calculations to be made with an accuracy sufficient for practical use in electron probe technologies.

Determination of charge carrier transport parameters in a polymer electrolyte intended for Li-ion batteries using electrochemical impedance analysis

Solid polymer electrolytes are key components in many electrochemical devices. For an in-depth study of the basic parameters of such electrolytes, we developed a previously proposed method in order to determine the charge carrier density (n), mobility (μ), and diffusion coefficient (D) of ionic conductors using electrochemical impedance analysis. This reinforced method was tested with a composite solid electrolyte based on polyethylene oxide, ethylene carbonate, LiCF3SO3, and alumina filler by analyzing DC conductivity, frequency dependence of AC conductivity, and the complex dielectric function. The results show a clear picture of the temperature dependence of the parameters n, μ, and D; for example, at 20 °C, more than 15% of the total number of ions available in the electrolyte are mobile, and this value decreases with increasing temperature, most probably due to increased ion association in agreement with measurements using other techniques. The increase in ionic conductivity with increasing temperature is thus due to an increased mobility of the ionic species.

Heavy Water Additive in Formamidinium: A Novel Approach to Enhance Perovskite Solar Cell Efficiency.

Heavy water or deuterium oxide (D2 O) comprises deuterium, a hydrogen isotope twice the mass of hydrogen. Contrary to the disadvantages of deuterated perovskites, such as shorter recombination lifetimes and lower/invariant efficiencies, the serendipitous effect of D2 O as a beneficial solvent additive for enhancing the power conversion efficiency (PCE) of triple-A cation (cesium (Cs)/methylammonium (MA)/formaminidium (FA)) perovskite solar cells from ≈19.2% (reference) to 20.8% (using 1 vol% D2 O) with higher stability is reported. Ultrafast optical spectroscopy confirms passivation of trap states, increased carrier recombination lifetimes, and enhanced charge carrier diffusion lengths in the deuterated samples. Fourier transform infrared spectroscopy and solid-state NMR spectroscopy validate the N-H2 group as the preferential isotope exchange site. Furthermore, the NMR results reveal the induced alteration of the FA to MA ratio due to deuteration causes a widespread alteration to several dynamic processes that influence the photophysical properties. First-principles density functional theory calculations reveal a decrease in PbI6 phonon frequencies in the deuterated perovskite lattice. This stabilizes the PbI6 structures and weakens the electron-LO phonon (Fröhlich) coupling, yielding higher electron mobility. Importantly, these findings demonstrate that selective isotope exchange potentially opens new opportunities for tuning perovskite optoelectronic properties.

Temperature‐Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells

Temperature‐dependent transport of photoexcited charge carriers through a nominally undoped, c‐plane GaN layer toward buried InGaN quantum wells is investigated by continuous‐wave and time‐resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low‐temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN‐based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.

A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) are of interest in light of their solution-processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic-acid-crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).

CsPbBr3 Single Crystal X-ray Detector with Schottky Barrier for X-ray Imaging Application

A high Z CsPbBr3 halide perovskite with large charge carrier diffusion length was used for radiation detection applications. The high sensitivity X-ray detector is expected to be used for imaging a...

Dynamics of surface charge and electric field distributions on basin‐type insulator in GIS/GIL due to voltage polarity reversal

In this study, a simulation model of surface charge accumulation has been established. The model considers three accumulation ways, i.e. electrical conduction within the gas, through insulator volume and along the insulator surface. The generation, diffusion, drift and recombination of charge carriers are also taken into account. Based on it, the influence of polarity reversal, reversal time on surface charge and electric field distribution on a basin-type insulator are studied. The polarity of the surface charges and the direction of the electric field change after the voltage polarity reversal. When the preload voltage is equal to reversal voltage, the surface charge and the electric field distributions at steady state before and after voltage polarity reversal are all the same with opposite sign, and not affected by the reversal time. However, the time to reach the steady state varies with different reversal time. The steady-state surface charge and electric field increased with the rise of reversal voltage. The transient normal and tangential electric field would not exceed the value of the steady state, which means voltage polarity reversal has no additional influence on insulation performance. This research can provide guidance to the design and manufacture of DC GIS/GIL.

Excitation wavelength dependence of photoluminescence flickering in degraded MAPbI3 perovskite and its connection to lead iodide formation

Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI3 and PbI2 which is formed in the course of degradation.

Correlation between Charge Transport Length Scales and Dielectric Relaxation Time Constant in Hybrid Halide Perovskite Semiconductors

The charge carrier diffusion length and dielectric relaxations are important parameters which decide the performance of various optoelectronic devices, in particular for photovoltaic devices. A comparative study is carried out on the charge transport length scale (L) for passivated and pristine CH3NH3PbI3 (MAPI) thin-film-based perovskite solar cells (PSCs) through scanning photocurrent microscopy (SPM). The SPM study suggested an improved L and degree of ambipolarity of photogenerated charge carriers (electron and hole) in passivated as compared to pristine MAPI-based PSCs. These results were found to be correlated with frequency-dependent photocurrent measurement, which shows that the relaxation time of the charge carrier is relatively lower in passivated MAPI-based PSCs. This mechanism could be explained by trap-assisted recombination, where trap states are induced by ion migration in halide perovskite films. Furthermore, passivation of traps showed an increased degree of ambipolarity in the perovskite semiconductor thin film.

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH3NH3PbI3 is blended with solution-processed n-type TiOx nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH3NH3PbI3:TiOx bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH3NH3PbI3 mixed with 5 wt% of TiOx, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm2, an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.

Electron‐Beam‐Related Studies of Halide Perovskites: Challenges and Opportunities

AbstractElectron beam microscopy and related characterization techniques play an important role in revealing the microstructural, morphological, physical, and chemical information of halide perovskites and their impact on associated optoelectronic devices. However, electron beam irradiation usually causes damage to these beam‐sensitive materials, negatively impacting their device performance, and complicating this interpretation. In this article, the electron microscopy and spectroscopy techniques are reviewed that are crucial for the understanding of the crystallization and microstructure of halide perovskites. In addition, special attention is paid to assessing and mitigating the electron beam‐induced damage caused by these techniques. Since the halide perovskites are fragile, a protocol involving delicate control of both electron beam dose and dose rate, coupled with careful data analysis, is key to enable the acquisition of reliable structural and compositional information such as atomic‐resolution images, chemical elemental mapping and electron diffraction patterns. Limiting the electron beam dose is critical parameter enabling the characterization of various halide perovskites. Novel methods to unveil the mechanisms of device operation, including charge carrier generation, diffusion, and extraction are presented in scanning electron microscopy studies combined with electron‐beam‐induced current and cathodoluminescence mapping. Future opportunities for electron‐beam‐related characterizations of halide perovskites are also discussed.

Call for papers on special issue “Perovskite photovoltaics and optoelectronic devices”

Aim & Scope: Metal halide perovskitehave been regarded as promising classes of materials for photovoltaics and optoelectronic devices, owing to the unique characteristics, such as long charge carrier diffusion lengths, precise tunable bandgaps, high light absorption coefficients, and high defect tolerance. Research on perovskite in the fields including photovoltaics, light-emitting diodes, lasers, X-ray imaging, and photodetectors has been gaining increasingly interest over the past years. Up to now, the efficiency of perovskite solar cells has grown from 3.8% in single-junction solar cells in 2009 to more than 25%, catching up the efficiency level of commercial silicon cells. Up to now, the key issues of perovskite photovoltaics and optoelectronic devices have become the stability, performance and large-scale production. This requires optimization of the film morphology, interface, device structure and the fabrication process. A lot work has been done on this issue and has made remarkable progress. We kindly invite you to submit a manuscript(s) for this Special Issue. Full papers, communications, and reviews are all welcome.

Stochastic Resonance of Charge Carriers Diffusion in a Semiconductor Layer under a Nonuniform Low Temperature

We study the dynamics of charge carriers jumping from one trap to the other of potential trap depth Φ in a one-dimensional semiconductor layer with the help of thermal noise. Applying a nonuniform temperature, colder around the center and hotter on moving away from it, favors the charge carriers to migrate toward the center and populate around the center. However, exposing the system to another additional nonuniform temperature, hotter around the center, forced the charge carriers to redistribute around two points. The trap potential together with the nonuniform temperature forms a system similar to having bistable potential. Diffusion of charge carriers and thermally transition rate, in a high barrier limit, as a function of controlling parameters has been explored. Due to a time-varying signal the system shows stochastic resonance (SR). The noise that made phase transition also favors the system to exhibit SR by applying a time varying signal. Using two-state approximation, SR of charge carriers diffusion, both analytical and numerical simulation, has been investigated. Our finding shows a strong spectral amplification η at a low temperature.

Bias voltage-dependent photoinduced current and photoluminescence in organometal perovskite layers on silicon substrates

Thin organometal perovskite layers on crystalline silicon substrates under excitation with nanosecond laser pulses were prepared and investigated to reveal an effect of the bias voltage and parameters of the perovskite/silicon structure on photoinduced current and photoluminescence. Numerical simulations of the diffusion, separation and recombination of photoinduced charge carriers in perovskite/silicon structures were carried out to describe both the photoluminescence transient and time-dependent photocurrent at different bias voltage. While the nonradiative surface recombination does not significantly influence the photoluminescence transients for times shorter than 100 ns, the photoinduced current decreases strongly with increasing the surface recombination velocity. The simulation results were compared with experimental ones obtained for organometal perovskite deposited on substrates of optically polished crystalline silicon wafers of p-type conductivity and those covered with silicon nanowires. The obtained results are discussed in view of possible applications of the investigated structures in photovoltaics and light emitting optoelectronics.

High-resolution light-activated electrochemistry on amorphous silicon-based photoelectrodes.

Light-activated electrochemistry (LAE) consists of employing a focused light beam to illuminate a semiconducting area and make it electrochemically active. Here, we show how to reduce the electrochemical spatial resolution to submicron by exploiting the short lateral diffusion of charge carriers in amorphous silicon to improve the resolution of LAE by 60 times.

Anisotropic carrier diffusion in single MAPbI3 grains correlates to their twin domains

The anisotropic charge carrier diffusion coupled to ferroelastic twin domains in methylammonium lead iodide opens possibilities for further optimization of perovskite solar cells and optoelectronic devices via strain engineering and heat treatments.

Optimal ranges determination of morphological parameters of nanopatterned semiconductors quality for solar cells

Purpose: The paper aims to determine the values of the main morphological characteristics of nanopatterns, which can be considered as the reference for use as surfaces of solar cells. Design/methodology/approach: The article uses an approach based on the definition of reference indicators of nanopatterns for solar cells by analysing the main parameters of solar cells and comparing them with the possible values of morphological parameters. Correlations of pore radius and visible wavelength, porosity and visible range, wavelength of de Broglie, nanopatterned layer thickness and charge carriers diffusion length, etc., are analysed. Compliance verification of morphological characteristics of nanopatterns with the specified criteria was performed on the example of porous silicon layers. Findings: The conducted research allowed to define the basic values of morphological parameters of porous nanopatterns, namely porousness, pore size (effective diameter), the thickness of the porous layer, and form factor. Reference ranges of morphological parameters of nanopatterns formed on the surface of semiconductors for applications in solar cells are established. Research limitations/implications: The article is devoted to the choice of optimal morphological characteristics of porous nanopatterns on the surface of semiconductors for solar cells. However, for solar cells, other types of nanopatterns can also be applied, for which it is also necessary to develop methods for selecting optimal parameters. Moreover, the prospect of research on this topic is to check the intrusion into a certain range of values of real nanopatterns formed on the surface of semiconductors. Practical implications: In the article the methodology allowing to choose optimal values of morphological parameters of nanopatterns for its application for solar cells is considered. Such studies are of great practical importance for the production of high-quality solar cells based on nanopatterned semiconductors. Originality/value: The article for the first time considers the choice of the nanopattern type and the ranges of morphological parameters in terms of quality assurance of the final product – the solar cell. It is determined that it is necessary to take into account such factors as porousness, pore size, thickness of the porous layer and roundness. A range of optimal values is selected for each of the indicators.

Mutual contaminants relational realization and photocatalytic treatment using Cu2MgSnS4 decorated BaTiO3

Emerging contaminants and related health risks restrain aged treatment approaches. A better-premise understanding of water remediation attained by mutual contamination relational realization (MCRR). Precisely engineered Cu2MgSnS4 (CMTS) decorated BaTiO3 (BTO) composites demonstrated a remarkable removal of organic contaminants and inactivation of harmful bacterium. It showed a valuable 97 % degradation of organic dyes and strong bacterial antagonism for both Escherichia coli and Staphylococcus aureus. Implantation of P-type CMTS on the surface of n-type BTO caused formation of a space charge layer (SCL), which affected diffusion of charge carriers. Junction at the interface of constituent materials and built-in electric field increased negative and positive carrier’s separation. Bandgap of the composites decreased from 3.4 eV to 2.6 eV by adjusting the composition of CMTS, which drastically augmented their ability to harvest the visible light. Therefore this study opens a new door for potential photocatalytic applications.