Dissociation Of Electron-hole Pairs Research Articles

Microwave-Assisted Synthesis of SnO2@ZnIn2S4 Composites for Highly Efficient Photocatalytic Hydrogen Evolution.

This research successfully synthesized SnO2@ZnIn2S4 composites for photocatalytic tap water splitting using a rapid two-step microwave-assisted synthesis method. This study investigated the impact of incorporating a fixed quantity of SnO2 nanoparticles and combining them with various materials to form composites, aiming to enhance photocatalytic hydrogen production. Additionally, different weights of SnO2 nanoparticles were added to the ZnIn2S4 reaction precursor to prepare SnO2@ZnIn2S4 composites for photocatalytic hydrogen production. Notably, the photocatalytic efficiency of SnO2@ZnIn2S4 composites is substantially higher than that of pure SnO2 nanoparticles and ZnIn2S4 nanosheets: 17.9-fold and 6.3-fold, respectively. The enhancement is credited to the successful use of visible light and the facilitation of electron transfer across the heterojunction, leading to the efficient dissociation of electron-hole pairs. Additionally, evaluations of recyclability demonstrated the remarkable longevity of SnO2@ZnIn2S4 composites, maintaining high levels of photocatalytic hydrogen production over eight cycles without significant efficiency loss, indicating their impressive durability. This investigation presents a promising strategy for crafting and producing environmentally sustainable SnO2@ZnIn2S4 composites with prospective implementations in photocatalytic hydrogen generation.

Multi-heteroatom-doping promotes molecular oxygen activation on polymeric carbon nitride for simultaneous generation of H2O2 and degradation of oxcarbazepine.

Simultaneously realizing the efficient generation of H2O2 and degradation of pollutants is of great significance for environmental remediation. However, most polymeric semiconductors only show moderate performance in molecular oxygen (O2) activation due to the sluggish electron-hole pair dissociation and charge transfer dynamics. Herein, we develop a simple thermal shrinkage strategy to construct multi-heteroatom-doped polymeric carbon nitride (K, P, O-CNx). The resultant K, P, O-CNx not only improves the separation efficiency of charge carriers, but also improves the adsorption/activation capacity of O2. K, P, O-CNx significantly increases the production of H2O2 and the degradation activity of oxcarbazepine (OXC) under visible light. K, P, O-CN5 shows a high H2O2 production rate (1858 μM h-1 g-1) in water under visible light, far surpassing that of pure PCN. The apparent rate constant for OXC degradation by K, P, O-CN5 increases to 0.0491 min-1, which is 8.47 times that of PCN. Density functional theory (DFT) calculations show that the adsorption energy of O2 near phosphorus atoms in K, P, O-CNx is the highest. This work provides a new idea for the efficient degradation of pollutants and generation of H2O2 at the same time.

Exciton Nature of Plasma Phase Transition in Warm Dense Fluid Hydrogen: ROKS Simulation.

The transition of warm dense fluid hydrogen from an insulator to a conducting state at pressures of about 20-400 GPa and temperatures of 500-5000 K has been the subject of active scientific research over the past few decades. However, various experimental and theoretical methods do not provide consistent results. In this work, we have applied the restricted open-shell Kohn-Sham (ROKS) method for first principles molecular dynamics of dense hydrogen after thermal excitation to the first singlet excited state. The Wannier localization method has allowed us to analyze the exciton dynamics in this system. The model shows that a key mechanism of the transition is associated with the dissociation of electron-hole pairs, which allows explaining several stages of the transition of fluid H2 from molecular state to plasma. This mechanism is able to give a quantitative description of several experimental results as well as to resolve the discrepancies between experimental studies.

Boron carbonitride (BxCyNz) nanosheets based single electrode triboelectric nanogenerator for wearable UV photodetectors

Exposure to intense UV radiation is a life threatening issue for the Military personals posted at high altitude. We report on the first ever fabrication of boron carbonitride (BxCyNz) nanosheets based single electrode triboelectric nanogenerator (STENG) on wearable textile platform for ultraviolet (UV) photodetection. The 2D nature of the BxCyNz samples has been revealed from the microscopy-based characterization tools, while the UV absorption ability of the material has been analyzed using UV-visible absorption spectroscopy. During biomechanical action, the UV detection using the STENG can be realized by the event of lowering of output voltage under UV illumination. The observation of excellent photoresponsivity ∼ 1.6 × 103 V/W can be assigned to the nitrogen deficient microstructures of BxCyNz, offering trap states to facilitate electron-hole pair dissociation and their subsequent separation. However, the basic mechanism of photodetection relies on the increment of the inherent capacitance as a consequence photoexcitation in BxCyNz. As a significant outcome of this process, the output voltage of a load capacitor is found to be elevated under illumination and thus turns out to be an alternative method of photodetection. Finally, the photodetection characteristics and the signal transmitting ability of the STENG have been demonstrated upon attaching it to the human body. Thus, BxCyNz based STENG demonstrating photodetection through capacitive voltage growth can form as the basis of next generation flexible, smart and self-powered photodetectors.

Boosting photocatalytic activity for porphyrin-based D-A conjugated polymers via dual metallic sites regulation

The incorporation of metallic sites into polymer photocatalysts has been a successful strategy to realize high photocatalytic activities. Herein, a series of bimetallic porphyrin-based D-A conjugated polymers M1TAPP-M2Bpy (M1, M2 =H2, Zn, Cu) were prepared through dual metallic sites regulation for enhanced photocatalytic degradation of bisphenol A (BPA) and benzene series pollutants. The π-conjugated polymer skeleton facilitates the adsorption of aromatic pollutants. Superior to monometallic sites, the intrinsic charge transfer characteristics of both porphyrin and bipyridine moieties can be modulated by dual metallic sites, promoting the dissociation of charge carriers. The highly-dispersed metallic reactive sites further boost the photocatalytic activities. Owing to optimized dissociation of electron-hole pairs, CuTAPP-CuBpy exhibits an outstanding photocatalytic oxidation activity for degradation of 50 ppm BPA completely in only 20 min with a relatively fast rate constant of k = 0.183 min−1. Dual metallic sites regulation highlighted in this work paves a way toward efficient polymer-based photocatalyst.

Rational design of donor-acceptor conjugated polymers with high performance on peroxydisulfate activation for pollutants degradation

Polymeric carbon nitride (PCN) has great potential for peroxydisulfate (PDS) activation but still challenge due to the sluggish electron-hole pair dissociation and tardy charge transfer dynamics. Herein, a novel PCN-based donor-acceptor conjugated polymer (PCN/4-MI D-A) is synthesized by copolymerization of urea and 4-methoxyphenyl isothiocyanate. The obtained PCN/4-MI D-A exhibited significantly improved activity on PDS activation for nitenpyram (NTP) degradation under visible light. As a result, PDS+PCN/4-MI75 shows the highest kinetic constants (0.115 min−1), which is ~6.8 times as that of the pure PCN. Density functional theory (DFT) calculations indicated that 4-methoxyphenyl as an electron-donating group promotes the charge separation/transfer of PCN/4-MI D-A. In addition, the PCN/4-MI D-A is more conducive to the PDS adsorption at the position where the amino group linked with 4-MI. The PCN/4-MI D-A generated abundant electrons which were subsequently transferred to the PDS by tunneling effect. This study provides a new design concept for the highly efficient PDS activation by constructed PCN-based donor-acceptor conjugated polymers.

Revealing generation, migration, and dissociation of electron-hole pairs and current emergence in an organic photovoltaic cell.

Using an innovative quantum mechanical method for an open quantum system, we observe in real time and space the generation, migration, and dissociation of electron-hole pairs, transport of electrons and holes, and current emergence in an organic photovoltaic cell. Ehrenfest dynamics is used to study photoexcitation of thiophene:fullerene stacks coupled with a time-dependent density functional tight-binding method. Our results display the generation of an electron-hole pair in the donor and its subsequent migration to the donor-acceptor interface. At the interface, electrons transfer from the lowest unoccupied molecular orbitals (LUMOs) of thiophenes to the second LUMOs of fullerene. Further migration of electrons and holes leads to the emergence of current. These findings support previous experimental evidence of coherent couplings between electronic and vibrational degrees of freedom and are expected to stimulate further work toward exploring the interplay between electron-hole pair (exciton) binding and vibronic coupling for charge separation and transport.

Improved photocatalytic efficacy of TiO2 open nanotube arrays: A view by XAS

Highly ordered TiO2 nanotube (TiNT) arrays were prepared on Ti foil. The interfacial caps between nanotube bottom and Ti foil were gradually removed by oxalic acid. The removal of caps in TiNT is known to enhance its light-harvesting capability, increase the carrier transfer, and cause effective dissociation of electron-hole pairs. The electronic and local atomic structures of TiNT at different oxalicacid immersion time were investigated by XAS. Notably, the reported enhanced light harvesting and electron collecting efficiencies in both-end open TiNT, could be strongly correlated with charge redistribution in Ti 3d orbital and structural defect. An increase in Ti 3d unoccupied states due to increase in Ti4+/Ti3+ ratio followed by a decrease in oxygen vacancy at the closed-end bottom caps after 16 h oxalicacid immersion might be one of the reasons for enhanced efficiency of both end open TiNT against one end open tube. EXAFS analysis revealed an increase in CN number and Ti–O bond length, which results in the formation of disordered TiO6 Oh symmetry that would possibly enhance the lattice phonons and eventually the efficiency of TiNT. This is one of the fundamental reasons for the improved photocatalytic properties for both-end open TiNT against closed bottom-end nanotubes.

The Quantum Efficiency Roll-Off Effect in Near-Infrared Organic Electroluminescent Devices with Iridium Complexes Emitters.

The electroluminescence quantum efficiency roll-off in iridium(III)-based complexes, namely Ir(iqbt)2(dpm) and Ir(iqbt)3 (iqbt = 1 (benzo[b]thiophen-2-yl)-isoquinolinate, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) utilized as near-infrared emitters in organic light emitting diodes with remarkable external quantum efficiencies, up to circa 3%, 1.5% and 1%, are measured and analyzed. With a 5–6 weight% of emitters embedded in a host matrix, the double-layer solution-processed structure as well as analogous three-layer one extended by a hole-conducting film are investigated. The triplet-polaron, the Onsager electron-hole pair dissociation and the triplet-triplet annihilation approaches were used to reproduce the experimental data. The mutual annihilation of triplets in iridium emitters was identified as prevailingly controlling the moderate roll-off, with the interaction between those of iridium emitters and host matrixes found as being less probable. Following the fitting procedure, the relevant rate constant was estimated to be cm3/s, values considered to be rather too high for disordered organic systems, which was assigned to the simplicity of the applied model. A coexistence of some other mechanisms is therefore inferred, ones, however, with a less significant contribution to the overall emission quenching.

Microwave-assisted synthesis of ZnO nanostructures for organic solar cell applications

In this work, a simple and efficient microwave-assisted hydrothermal method was adopted to synthesis zinc oxide nanostructures. Zinc acetate and hexamethylenetetramine were used as precursors. Various parameters involving synthesis conditions were attuned in order to achieve well-defined morphology. XRD peaks indicated the high crystallinity and hexagonal wurtzite structure of the prepared material. Field emission scanning electron microscopy revealed different morphology of the synthesized material. The prepared material can be used in organic solar cells, for effective dissociation of electron-hole pairs and thus enhancing photovoltaic performance.

Heavy Mediator at Quantum Dot/Graphene Heterojunction for Efficient Charge Carrier Transfer: Alternative Approach for High-Performance Optoelectronic Devices.

Two-dimensional (2D) material nanocomposites have emerged as a material system for discovering new physical phenomena and developing novel devices. However, because of the low density of states of most two-dimensional materials such as graphene, the heterostructure of nanocomposites suffers from an enhanced depletion region, which can greatly reduce the efficiency of the charge carrier transfer and deteriorate the device performance. To circumvent this difficulty, here we propose an alternative approach by inserting a second 2D mediator with a heavy effective mass having a large density of states in-between the heterojunction of 2D nanocomposites. The mediator can effectively reduce the depletion region and form a type-II band alignment, which can speed up the dissociation of electron-hole pairs and enhance charge carrier transfer. To illustrate the principle, we demonstrate a novel stretchable photodetector based on the combination of graphene/ReS2/perovskite quantum dots. Two-dimensional ReS2 acts as a mediator in-between highly absorbing perovskite quantum dots and a high-mobility graphene channel and a thiol-based linker between the ReS2 and the perovskite. It is found that the optical sensitivity can be enhanced by 22 times. This enhancement was ascribed to the improvement of the charge transfer efficiency as evidenced by optical spectroscopy measurements. The produced photosensors are capable of reaching the highest reported value of photoresponsivity (>107 A W-1) and detectivity compared to previously studied stretchable devices. Mechanical robustness with tolerable strain up to 100% and excellent stability make our device ideal for future wearable electronics.

A correlation between electron-hole pair radii and magnetomodulation of exciplex fluorescence in electron donor-electron acceptor organic systems

Electric field dependencies of electromodulated photoluminescence and magnitudes of the magnetic-field effect on photoluminescence have been measured in vacuum-evaporated films of m-MTDATA [4,4′,4″-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine]:bathophenanthroline, m-MTDATA:BCP (bathocuproine), as well as 4,4′,4″-tris[2-naphthyl(phenyl)amino]triphenylamine:BCP. The Sano-Tachiya-Noolandi-Hong extension of standard Onsager formalism was used to investigate the electron-hole pair dissociation process which allowed us to estimate the pair intercarrier distances and the final speed of carrier recombination. A distinct correlation between the pair radii and the magnitude of the magnetic-field effect has been found.

Dynamic behavior of CH3NH3PbI3 perovskite twin domains

The recent discovery of twin domains in MAPbI3 perovskites has initiated contentious discussion on the ferroic nature of hybrid perovskites. Ferroelectric polarization is thought to facilitate the dissociation of photoinduced electron-hole pairs, helping to explain the extraordinary photovoltaic performance exhibited by this class of materials. Alternate to ferroelectricity, which has yet to be unambiguously established despite considerable efforts to do so, ferroelasticity was also proposed in these materials. Meanwhile, given the coupling of ionic states and ferroelectricity and the interconnected nature of defect chemistry and ferroelasticity, the electrochemical reactivity can no longer be ignored. In this work, using band excitation piezoresponse force microscopy, we reveal the variation in elasticity between adjacent domains, indicating the ferroelasticity and the difference in the crystallographic states of the twin domain. Moreover, using band excitation contact Kelvin probe force microscopy, we dynamically map the evolution of the twinning structure under electric bias. These results help decipher the effect of the twin domains on ionic mobility and ion diffusion pathways. Combining these results, we reveal the interaction of twin domains and ionic activity in this material. Overall, this work provides insights into the twinning structure in MAPbI3 and its potential effects on the hybrid perovskite optoelectronics.

Reduced recombination and low-resistive transport of electrons for photo-redox reactions in metal-free hybrid photocatalyst

Photoinduced charge separation against their faster recombination is a rate determinant for photocatalytic proton reduction to hydrogen. Dissociation of electron-hole pairs into free electrons and holes in carbon nitrides greatly suffered from the inherent high recombination rate. This study has shown that coupling two energetically optimized, but with different phases carbon nitrides in the form of hybrid could significantly inhibit the charge carrier recombination and facilitate the overall charge transfer processes. It is also found that the potential gradient in this homojunction delocalizes electrons and holes, and increases the spatial charge separation. Therefore, this leads to a record high apparent quantum efficiency of 5% for photocatalytic H2 production from water under visible light irradiation in the absence of a precious metal (e.g., Pt) cocatalyst.

Influence of Shell Thickness on the Performance of NiO-Based All-Inorganic Quantum Dot Light-Emitting Diodes.

The effect of shell thickness on the performance of all-inorganic quantum dot light-emitting diodes (QLEDs) is explored by employing a series of green quantum dots (QDs) (Zn xCd1- xSe/ZnS core/shell QDs with different ZnS shell thicknesses) as the emitters. ZnO nanoparticles and sol-gel NiO are employed as the electron and hole transport materials, respectively. Time-resolved and steady-state photoluminescence results indicate that positive charging processes might occur for the QDs deposited on NiO, which results in emission quenching of QDs and poor device performance. The thick shell outside the core in QDs not only largely suppresses the QD emission quenching but also effectively preserves the excitons in QDs from dissociation of electron-hole pairs when they are subjected to an electric field. The peak efficiency of 4.2 cd/A and maximum luminance of 4205 cd/m2 are achieved for the device based on QDs with the thickest shells (∼4.2 nm). We anticipate that these results will spur progress toward the design and realization of efficient all-inorganic QLEDs as a platform for the QD-based full-colored displays.

Interconversion between Free Charges and Bound Excitons in 2D Hybrid Lead Halide Perovskites.

The optoelectronic properties of hybrid perovskites can be easily tailored by varying their components. Specifically, mixing the common short organic cation (methylammonium (MA)) with a larger one (e.g., butyl ammonium (BA)) results in 2-dimensional perovskites with varying thicknesses of inorganic layers separated by the large organic cation. In both of these applications, a detailed understanding of the dissociation and recombination of electron–hole pairs is of prime importance. In this work, we give a clear experimental demonstration of the interconversion between bound excitons and free charges as a function of temperature by combining microwave conductivity techniques with photoluminescence measurements. We demonstrate that the exciton binding energy varies strongly (between 80 and 370 meV) with the thickness of the inorganic layers. Additionally, we show that the mobility of charges increases with the layer thickness, in agreement with calculated effective masses from electronic structure calculations.

Self-assembled, highly crystalline porous ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) interlayer for Si/organic hybrid solar cells

Ferroelectric polymers can effectively improve the photovoltaic performance of solar cells, inducing an electric field to promote the dissociation of electron-hole pairs, with the thus generated charges collected from open pores. Since such performance enhancement requires materials with a unique porous crystalline structure, we herein present a novel route to highly crystalline and porous poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films utilizing a modified breath figure method based on spin coating. The key feature of the above method is the addition of small amounts of water to the acetone/P(VDF-TrFE) solution to produce porous ferroelectric thin films which have significantly higher crystallinity values than nanostructures or films prepared by other methods. Furthermore, n-Si / poly(3,4-ethylene dioxy thiophene):poly(styrene sulfonate) hybrid solar cells with porous P(VDF-TrFE) interlayers are demonstrated to exhibit spontaneous polarization sufficient for increasing their open circuit voltages and fill factors. Finite-difference time-domain simulation reveals that the electric field due to the above spontaneous polarization increases the built-in electric field of the Schottky junction between n-Si and poly(3,4-ethylene dioxy thiophene):poly(styrenesulfonate) and reduces the reverse leakage current of the Schottky diode. Thus, the organic ferroelectric thin films with controlled porosity proposed in this study are well suited for a broad range of optoelectronic applications.

Improving the Performance of Graphene Phototransistors Using a Heterostructure as the Light-Absorbing Layer.

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400-700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron-hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics.

Field-induced dissociation of electron-hole pairs in organic light emitting diodes monitored directly from bias-dependent magnetic resonance techniques

The conversion processes of electron-hole (e-h) pairs into carriers and excitons in organic light emitting diodes (OLEDs) have been explored by simultaneous photoluminescence- and photocurrent-detected magnetic resonance (PLDMR and PCDMR) techniques as a function of bias. Both PLDMR and PCDMR signals are shown to disappear or extremely reduce with increasing the field strength, whereas photoluminescence intensity is unchanged and photocurrent rather increases. This is clear evidence that e-h pairs are dissociated by the field. A simple model for pair dissociation is proposed and explains the field dependence of PLDMR intensity.

Simultaneously enhancing dissociation and suppressing recombination in perovskite solar cells

This article reports the experimental studies on simultaneously enhancing the dissociation and suppressing the recombination in perovskite solar cells by using high-dielectric Nickel Oxide (NiOx) as hole transport layer. Specifically, the magneto-photocurrent, generated by the electron-hole pairs, surprisingly becomes negligible at short-circuit condition when the NiOx is used to replace the poly (3,4-ethylenedioxythiophene) poly (styrene-sulfonate) (PEDOT:PSS). This indicates that the NiOx transport layer leads to a complete dissociation of electron-hole pairs in perovskite layer. On the other hand, the negligible magneto-photocurrent can be recovered to become appreciable when a forward bias is applied towards open-circuit condition to weaken the built-in field. This magneto-photocurrent result suggests that the NiOx transport layer enhances the built-in field, completely dissociating the electron-hole pairs. Furthermore, the photoinduced capacitance studies confirm that the built-in field is enhanced essentially through static and dynamic parameters, by removing the interfacial traps and decreasing the accumulation of photogenerated carriers. The time-resolved photoluminescence shows that the NiOx/CH3NH3PbI3 interface leads to a reduction on non-radiative recombination, increasing the fraction of useful excitons available for photovoltaic actions. Moreover, the field-dependent photoluminescence measured alternatively at short-circuit and open-circuit conditions shows that the NiOx layer can also suppress the radiative recombination within available excitons, boosting the photovoltaic actions. Therefore, our studies reveal that the high-dielectric NiOx transport layer can simultaneously enhance the dissociation of electron-hole pairs and suppress both non-radiative/radiative recombination, leading to the more efficient generation of Jsc and Voc in perovskite solar cells.