Recombination Kinetics Of Charge Carriers Research Articles

Deep eutectic solvent as an effective tool for enhanced photocatalytic activity of Cu2S@MoS2 nano-assemblies: A green approach

The scientific world has witnessed the emergence of plethora of green strategies to ameliorate the efficiency of nano-photocatalytic materials for waste water treatment applications. This work, for the first time, highlights the effect of ZnCl2 and urea-based DES on the morphology, electronic band structure and photocatalytic efficiency of Cu2S @MoS2 NPs compared to solvents like water, EDA and DETA. XRD and FT-IR analysis confirmed the formation of NPs and quite significant changes in the pattern of M-S bonds of Cu2S@MoS2 were observed upon variation of solvents. The FE-SEM analysis of NPs prepared in 1:1 composition of DES and water (Cu2S@MoS2 NPs - 1:1) showed entirely distinct nanoflower morphology compared to spherical arrangement shown by NPs prepared using other solvents. As a result, Cu2S@MoS2 NPs - 1:1 showed superior textural properties with a surface area of 89 m2/g in contrast to 7.4, 6.2 and 18 m2/g observed for NPs synthesized in water, EDA and DETA, respectively. Besides, wide band gap of 2.75 eV for Cu2S@MoS2 NPs - 1:1 induced favorable charge carrier recombination kinetics, which played an instrumental role in the degradation of tetracycline hydrochloride (TC) and rhodamine B (RB). Cu2S@MoS2 NPs - 1:1 showed best photocatalytic activity as 97% and 96% of TC and RB was removed after 90 min of visible light irradiation with no significant decline in removal efficiency till 5 cycles. Scavenger studies hinted towards the crucial role of h+ and ⋅ OH in degradation process and a mechanism was proposed accordingly. The superior photocatalytic activity of NPs prepared using DES compared to water and organic solvents showed that the inclusion of DES in synthesis schemes of NPs is an excellent strategy for improving their efficiency towards waste water treatment applications.

ZrO2-Based Photocatalysts for Wastewater Treatment: From Novel Modification Strategies to Mechanistic Insights

Zirconium dioxide (ZrO2) has garnered substantial research interest in the field of photocatalytic water treatment due to its appealing properties, such as thermal stability, considerable physical strength, and strong chemical resistance. However, the wide bandgap energy endorses less photoabsorption and rapid charge carrier recombination kinetics, thus restricting the photoactivity of ZrO2. Previously, vast research efforts have been made to improve the photoefficacy of ZrO2, and hence it is worth exploring the potential strategic modifications responsible for incremented photocatalytic efficiency. In this regard, the present review article emphasizes the optical, structural, and electronic features of ZrO2, which makes it an interesting photocatalytic material. The exceptional modification strategies that help to modulate the crystal structure, morphology, bandgap energy, and charge carrier kinetics are primarily discussed. The potential synthetic routes involving bottom-up and top-down methods are also outlined for understanding the rationale for incorporating these techniques. Moreover, the photocatalytic performance evaluation was done by investigating the photodegradation kinetics of various organic and inorganic pollutants degradation by ZrO2. Conclusively, in light of research advances involving ZrO2 photocatalyst, this review article may expedite further investigation for enhancing the large-scale photocatalytic applications for environmental and energy concerns.

Effect of post-annealing on thermally evaporated reduced-dimensional perovskite LEDs

Reduced-dimensional perovskites (RDPs) with self-assembled multi-quantum well structures have emerged as promising candidates for light-emitting diodes (LEDs) due to their high color purity, high photoluminescence quantum yield, and decent stability. Compared to the traditional RDP film preparation methods reported in the previous literature, thermal evaporation is an appealing option for RDP film fabrication with uniform crystallization, high repeatability, and precise control. Here, based on the vacuum deposition method, we adopted a combined strategy, including annealing treatment and device structure optimization. Meanwhile, we investigated the effects of post-annealing on charge carrier recombination kinetics, exciton energy transfer, and phase distribution of vacuum-deposited RDP films. As a result, we achieved an external quantum efficiency of 6.5% for the device, which is one of the best performances among prevailing research on vacuum-processed RDP-based LEDs.

Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

AbstractHere, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites

Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450–780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices.

Fractal Charge Carrier Kinetics in TiO2

Charge carrier recombination kinetics of TiO2 powder samples were analyzed in the time domain ranging from 50 ns to 1 ms. The transient reflectance signals of the charge carriers observed by laser flash photolysis spectroscopy do not fit to simple second order kinetics as expected for the recombination of trapped electrons and holes. The deviation from second order reaction dynamics could rather be explained by the segregation of charge carriers and the fractal dimension of the semiconductor agglomerates. According to the fractal reaction kinetics, the time dependent rate coefficient kf (kf = k2,ft–h) has been employed instead of the second order rate constant k2, where the fractal parameter h describes the dimension of the system. This model could successfully be used to describe charge carrier signals in all observed time domains. Moreover, the model was compared with the concept developed by Shuttle et al., which proposes that the charge carrier signals decay following a power-law. The benefits of the ...

Understanding the Limiting Factors of Solvent-Annealed Small-Molecule Bulk-Heterojunction Organic Solar Cells from a Chemical Perspective.

A detailed account of the limiting factors of solvent-annealed bulk-heterojunction small-molecule organic solar cells is given. This account is based on the extensive characterisation of solar cell devices made from a library of five diketopyrolopyrole (DPP) donor dyes. Their chemical structure is designed in such a way as to provide insights into the energetics of solar cell active layer micro-structure formation. Numerous chemical and physical properties of the active layers are assessed and inter-related such as light absorption, molecular packing in the solid state, crystal-forming properties in thin films, charge carrier mobility and charge carrier recombination kinetics. A myriad of characterisation techniques are used such as UV/Vis absorption spectroscopy, photoluminescence spectroscopy, XRD, AFM and photo-induced transient measurements, which provide information on the optical properties of the active layers, morphology and recombination kinetics. Consequently, a mechanism for the solvent-vapour-annealing-assisted formation of crystalline domains of donor molecules in the active layer is proposed, and the micro-structural features are related to the J-V characteristics of the devices. According to this model, the crystalline phase in which the donor crystallise in the active layer is the key determinant to direct the formation of the micro-structure.

Photo‐Induced Charge Carrier Recombination Kinetics in Small Molecule Organic Solar Cells and the Influence of Film Nanomorphology

Small molecule organic solar cells (SMOSCs) are at the forefront of organic solar cell research and have power conversion efficiencies that match the leading polymer:fullerene organic solar cells (>10%). However, the operating physics of SMOSCs is less understood than that of their polymer:fullerene counterparts. A stronger emphasis on understanding the working mechanisms of SMOSCs is thus required. This feature article aims to highlight methods for understanding a significant loss process in SMOSCs ‐ charge carrier recombination ‐ by using photo‐induced transient optoelectronic techniques. These techniques make it is possible to probe the charge carrier density and lifetime in devices under working conditions. Employing these techniques alongside detailed morphological studies allows relationships between interfacial recombination processes, molecular packing and film nanomorphology to be obtained and can subsequently lead to more efficent devices being produced.

Charge carrier dynamics in PDPP-F/PCBM heterojunction solar cells

The morphology dependent of migration and recombination kinetics of charge carriers in the polymer solar cell based on poly {2,7′-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione}(PDPP-F) was investigated with photo-induced charge carrier extraction by linearly increasing voltage technique. The recombination coefficient of charge carriers decreased and the mobility of charge carriers increased, after the mass ratio of [6,6]-phenyl-C61-butyric acid methyl ester(PCBM) increased from 3:2 to 2:1. Meanwhile, both of them were sensitive to the applied electric field and could be together responsible for the improvement of voltage performance of polymer solar cell.

Morphology-driven photocurrent enhancement in PTB7/PC71BM bulk heterojunction solar cells via the use of ternary solvent processing blends

PTB7/PC71BM bulk heterojunction solar cell devices with high photocurrents were fabricated through processing their active layers from a ternary solvent system. The active layers were deposited from solutions where chlorobenzene (CB) was used as the main solvent, DIO (3%) as the solvent additive and cyclohexanone (CHN), a solvent in which both the polymer and fullerene are poorly soluble. The morphology of the different active layers was characterized by atomic force microscopy (AFM) and grazing incidence X-ray scattering (GIXRS) while charge extraction (CE) and transient photovoltage (TPV) techniques were used to measure charge carrier recombination kinetics. It was found that this ternary solvent system affects the J-V characteristics to a significant extent, mainly affecting the JSC and FF, in a trend where the JSC is seen to increase with increasing ratio of CHN/CB, while the FF decreases concomitantly. This increase in JSC has been correlated with an increase in crystallinity of the active layer and more specifically with an increase of the crystalline volume of the fullerene domains and the increase in the polymer crystallites size, while the decrease in FF was linked to non-optimal vertical segregation. Despite the drop in FF, the addition of CHN in the blend still leads to an overall increase in power conversion efficiency (PCE) with respect to the devices processed from pristine CB (6.86% vs. 7.31%).

Exploring the mechanisms of metal co-catalysts in photocatalytic reduction reactions: Is Ag a good candidate?

Metal co-catalysts are essential for enhancing photocatalytic performance, especially in reduction reactions using semiconductor photocatalyst materials as a consequence of the reduced recombination kinetics of charge carriers by spatial charge separation. Generally Au, Pd, Pt, and their alloys are more promising candidates than Ag for photocatalytic H2 evolution experiments, although Ag can trap more electrons having more negative reduction potential than that of Au, Pd, and Pt. Here we have synthesized and examined well-defined Au, Ag, and core-shell structured Au–Ag nanoparticles as co-catalysts for TiO2 in photocatalytic H2 evolution. By varying the dissolved oxygen in the reaction suspension, we found that selective photocatalytic reduction can be achieved by fine tuning the co-catalyst materials. Whilst Au NPs are superior for proton reduction, Ag NPs exhibits excellent performance for oxygen reduction. All core-shell structured Au–Ag NPs show non-selectivity in photocatalytic reduction of proton and oxygen.

Effects of side groups on the kinetics of charge carrier recombination in dye molecule-doped multilayer organic light-emitting diodes

Effects of side groups on the kinetics of charge carrier recombination are investigated in dye molecule-doped multilayer organic light-emitting diodes by transient electroluminescence.

Theory to carrier recombination in organic disordered semiconductors

A theoretical description for recombination kinetics of charge carriers in a disordered system with a broad energy distribution of localized states (DOS) is suggested. This kinetics is governed by the exchange of carriers between transport states and traps. Concentration transients in systems with Gaussian DOS, typical for organic semiconductors, appear much steeper than those obtained for systems with exponential DOS. This difference in recombination kinetics is caused by the difference in thermalization kinetics for these two types of the DOS functions. The comparison of the recombination transients for mobile and trapped carriers in exponential and Gaussian DOS might help to distinguish between these two possible shapes of the DOS using experimental data for transient photoconductivity and photoabsorption.

Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers

Low-bandgap diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction solar cells exhibit much faster charge carrier recombination kinetics than that encountered for less-recombining poly(3-hexylthiophene). Solar cells comprising these polymers exhibit energy losses caused by carrier recombination of approximately 100 mV, expressed as reduction in open-circuit voltage, and consequently photovoltaic conversion efficiency lowers in more than 20%. The analysis presented here unravels the origin of that energy loss by connecting the limiting mechanism governing recombination dynamics to the electronic coupling occurring at the donor polymer and acceptor fullerene interfaces. Previous approaches correlate carrier transport properties and recombination kinetics by means of Langevin-like mechanisms. However, neither carrier mobility nor polymer ionization energy helps understanding the variation of the recombination coefficient among the studied polymers. In the framework of the charge transfer Marcus theory, it is proposed that recombination time scale is linked with charge transfer molecular mechanisms at the polymer/fullerene interfaces. As expected for efficient organic solar cells, small electronic coupling existing between donor polymers and acceptor fullerene (Vif < 1 meV) and large reorganization energy (λ ≈ 0.7 eV) are encountered. Differences in the electronic coupling among polymer/fullerene blends suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based solar cells. Our approach reveals how to directly connect photovoltaic parameters as open-circuit voltage to molecular properties of blended materials.

Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis.

We discuss the progress and challenges in the application of impedance spectroscopy analysis to determine key processes and parameters in organic bulk-heterojunction solar cells. When carrier transport or outer interface extraction do not severely influence the solar cell performance, a simple method to quantify the open-circuit voltage loss caused by the kinetics of charge carrier recombination is provided, based on the determination of chemical capacitance and recombination resistance. This easily allows distinguishing between energetic and kinetic effects on photovoltage, and establishes a benchmark for the performance comparison of a set of different cells. A brief discussion of impedance analysis in the much less studied case of collection-limited solar cells is introduced.

Direct Observation of Surface-Mediated Electron−Hole Pair Recombination in TiO2(110)

The kinetics of surface and bulk electron−hole pair recombination have been measured separately on TiO2(110) in ultrahigh vacuum under well-controlled surface conditions for the first time. Using 100 eV incident electrons, excitation of electron−hole pairs occurs within ∼6 A of the surface, and the rate of stimulated desorption of O2 was measured as a means of determining the charge carrier recombination kinetics, which are found to be mediated by the surface and to be first-order in charge carrier concentration. Comparison with a previous O2 photodesorption experiment, where excitation by 3.4 eV photons was used for electron−hole pair generation in a deeper region below the surface (∼100 A), shows that bulk recombination in TiO2 occurs by second-order kinetics in charge carrier concentration.

Studies of kinetics of charge carrier recombination in organic light-emitting diodes based on beryllium complexes by transient electroluminescence

The kinetics of the charge–carrier recombination in double-layer organic light-emitting diodes composed of indium–tin–oxide/N,N′-di(naphthalene-1-yl)-N,N′ -diphenyl-benzidine/beryllium complexes/LiF/Al were studied by transient electroluminescence (EL). Two blue-emitting materials of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2) and bis[2-(2-hydroxytoluenyl)-pyridine]beryllium (Betp2) were used as the emissive layer. It was found that the EL efficiency of the Bepp2-based devices was higher than that of the Betp2-based devices. The improvement of the EL efficiency was well interpreted by the results that the Bepp2 shows higher electron mobility and electron–hole recombination coefficients (γ) than Betp2, which are determined from the onset of the transient EL after a rectangular voltage pulse is applied, and from the long-time component of the temporal decay of the EL intensity after the voltage pulse is turned off. The electron mobility and the recombination coefficients γ of the Bepp2 reached 6 × 10−6 cm2 V−1 s−1 and 7.3 × 10−12 cm3 s−1, respectively, which are approximately twice the magnitude of the Betp2.

Kinetics of optically excited charge carriers and defects in hydrogenated amorphous silicon and related alloys

Electron spin resonance and optically induced electron spin resonance measurements on hydrogenated amorphous silicon and germanium are useful probes of charge carriers in localized electronic states near the edges of the conduction and valence bands (band-tail states) and of defects, such as silicon and germanium dangling bonds. We review recent results on the recombination kinetics of charge carriers trapped in band-tail states. We also review recent results on the kinetics of metastable optically induced germanium dangling bonds in hydrogenated amorphous germanium. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetics of charge carrier recombination in organic light-emitting diodes

The temporal behavior of the electroluminescence (EL) from thin-film indium-tin-oxide (ITO)/aromatic diamine (TPD)/Al(III) 8-hydroxyquinoline (Alq3)/Mg/Ag light-emitting diodes (LEDs) upon the application of a rectangular driving voltage is analyzed in terms of the electron-hole recombination on the TPD/Alq3 interface. From the long-time component of the temporal decay of the EL intensity the electron-hole recombination coefficient γ=(1.1±0.5)×10−10 cm3/s is determined in good agreement with the value expected on the basis of the Langevin theory of recombination with the recombination process controlled by the motion of holes in the hole-transporting TPD layer of the LED.

Charge carrier geminate recombination in polyacetylene crystals with defects

Charge carrier recombination kinetics in polymer crystals is investigated in the framework of a model of one-dimensional diffusion-like charge carrier motion in confined chains with absorbing or reflecting ends. The averaged survival probability for a pair of photogenerated charge carriers is calculated. A crossover from the t −1 2 behavior describing the recombination kinetics in an infinite ideal chain to an exponential-type dependence ∝ exp(−τ 1 3 ) , τ = 10 π 2c 2Wt (W is the diffusion rate, c is the defect concentration), is predicted. The results obtained are compared with the observed decay of transient photocurrent and photoinduced absorption in polyacetylene crystals.