Short Electron Lifetime Research Articles

Synergistic effect between ohmic contacts and localized surface plasmon resonance for enhancing CO2 photoreduction of BiOBr

The fast recombination rate of photogenerated carrier and short electron lifetime are the main factors affecting the CO2 reduction performance of BiOBr (BOB). The utilization of ohmic contacts and the localised surface plasmon resonance (LSPR) effect of metals can effectively solve the above problems. However, one mean alone is not ideal for improving the photocatalytic performance of BOB. Therefore, in this work, we reduced the metal Bi to the surface of BOB in an attempt to form a synergistic effect of ohmic contact and LSPR effect. DFT calculations confirm the ohmic contact between Bi and BOB. Meanwhile, electrochemical tests show that Bi/BOB-2 has a higher photocurrent density (1.1 μA/cm2), suggesting that the ohmic contact achieves ultrafast electron transfer from BOB to Bi under light. While the UV–Vis diffuse reflectance spectroscopy results show resonance absorption peaks from the LSPR effect of Bi, the TRPL results show that Bi/BOB-2 has a longer τave (3.05 ns), implying that longer-lived hot electrons are generated on the Bi surface. The synergistic effect of the two contributes to the efficient CO2 to CO transition on the catalyst surface. As a result, Bi/BOB-2 exhibites a 4.37-fold higher CO generation rate (11.45 μmolg-1 h−1) than that of pure BOB (2.62 μmolg-1 h−1) under light. This study provides a new blueprint for the design of BiOBr-based materials for efficient photocatalytic reduction of CO2.

Modeling the current-voltage characteristics of Sb2Se3 thin film solar cells through Sah-Noyce-Shockley recombination model

This study investigates the impact of various material parameters, such as film resistivity, the thickness of the absorber layer, uncompensated acceptor density, and carrier lifetime, on the carrier transport and device characteristics of emerging CdS/Sb2Se3 thin-film solar cells. The modeling approach is based on the rigorous Sah-Noyce-Shockley recombination model at the PN-junction, which is based on drift and diffusion collection efficiencies for total current-density calculations. The current-density and voltage have been optimized against the Sb2Se3 thickness and the depletion width at the CdS/Sb2Se3 junction. It has been determined that the lower efficiency of the cell is primarily due to the relatively short electron lifetime and the uncompensated acceptor density, which can widen or narrow the depletion width. The resistivity of the Sb2Se3 film can effectively control the voltage of the cell, however, the carrier lifetime can control this impact even to a higher extent. Therefore, the carrier lifetime and uncompensated acceptor densities are the two interdependent parameters that significantly control the current, voltage, and efficiency of the Sb2Se3 solar cells. This research provides valuable insights for the design and optimization of Sb2Se3 thin-film solar cells shedding light on carrier transport and charge collection mechanism in these emerging materials.

Preparation of NiMoO4/CdS and its Application as a Photocatalyst in H2 Evolution Reaction

AbstractThe construction of heterojunction is considered to be an effective strategy for achieving efficient photocatalyst. The pure CdS shows low hydrogen production activity, for the low charges separation efficiency and the short electron and hole lifetime, which greatly limit its practical application. Therefore, we constructed heterojunction photocatalysts of NiMoO4/CdS by a simple solvothermal method. The photocatalytic hydrogen production activity of CdS was obviously improved by NiMoO4 loaded. The NiMoO4/CdS‐2 sample shows the highest H2 generation rate, which could reach 5830.49 μmolg−1 h−1, which is 122.3 times higher than that of pure CdS. The construction of heterojunction between CdS and NiMoO4 inhibits the recombination of photogenerated charges, prolongs the lifetime of photogenerated charges and improves photocatalytic hydrogen production performance. This work is important for the construction of an efficient catalytic system for the photocatalytic H2 evolution from water splitting.

Scrutinizing Synergy and Active Site of Nitrogen and Selenium Dual-Doped Porous Carbon for Efficient Triiodide Reduction

Electrocatalytic interconversion of iodide/triiodide is the key for state-of-the-art iodine-involved energy technologies, which is challenged by understanding the structure–activity relationship of the smart electrocatalysts with tuned active sites. Herein, active-site-enriched N,Se-co-doped porous carbon, denoted as NSeC, is crafted by a two-step approach of the template method and chemical doping. X-ray photoelectron spectroscopy and synchrotron X-ray absorption spectroscopy measurements substantiate the incorporation of extrinsic N and Se species into the carbon matrix. Raman spectroscopy reveals that the defect of densities within NSeC can be finely tuned by adjusting the doping temperature. The NSeC sample made at 900 °C shows a robust durability and a high electrocatalytic activity toward the triiodide reduction reaction with a relatively small charge-transfer resistance (0.75 Ω cm2) in parallel with a short electron lifetime (252.2 μs) participating in the triiodide reduction. Density functional theory calculations reveal that the high catalytic activity of the NSeC is due to the combined impacts, i.e., the synergy between N and Se species that helps to manipulate the adsorption process of iodine atoms and the active sites formed by the carbon atoms adjacent to quaternary N and Se atoms at the armchair edges. This work offers insights into both the design of efficient metal-free catalysts and the synergy of dual doping for efficient iodine-involved electrocatalysis for energy devices.

Dye Sensitized Solar Cells Based on Different Solvents: Comparative Study

Solution of Z907 dye in two different solvents, namely ethanol (EtOH) and chloroform (CHCl3) are prepared and utilized in dye-sensitized solar cells (DSSCs). The effects of dye solvents on the performance of these cells have been investigated and the results are represented. The DSSCs that used EtOH as a dye-adsorption solvent showed an improved solar cell efficiency relative to the DSSCs that used CHCl3. For dye-adsorption solvents, the highest conversion efficiency of 1.40% is achieved using CHCl3 and 0.15% for EtOH, respectively. Photo- and electrochemical tests of Z907 dye in ethanol, adsorbed on the TiO2 surface showed low dye loading and coverage on the surface, shorter electron lifetime and the greater resistance in the TiO2/dye/electrolyte interface, publicized to the dye aggregation at the TiO2 surface, which decreased the solar cell performance of the DSSCs.

Structural, Morphological, Photovoltaic and Electron Transport Properties of ZnO Based DSSC with Different Concentrations of MWCNTs

This study highlights the structural, morphological, photovoltaic and electron transport properties of zinc oxide-multi-walled carbon nanotubes (ZnO-MWCNTs) based dye-sensitized solar cell (DSSC) prepared at different concentrations of 0.0, 0.1, 0.3 and 0.5 wt.%. The ZnO-MWCNTs thin films were prepared by a chemical bath deposition method. X-ray diffraction (XRD) analysis proved the formation of hexagonal wurtzite of the samples. The crystallite sizes, D of ZnO-MWCNTs was measured varies from 21 nm to 11 nm. The oat-like ZnO nanoflakes structure and the presence of MWCNTs were captured by field-emission scanning electron microscopy (FESEM) analysis. Transmission electron microscopy (TEM) analysis measured the inner (~6.81 nm) and outer (~28.31 nm) diameter of MWCNTs. The optimum concentration of 0.1 wt.% MWCNTs produced the highest photocurrent density, Jsc of 13.5 mA/cm2, open-circuit voltage, Voc of 0.149 V, fill factor, FF of 0.406 and power conversion efficiency, PCE of 0.817 %. Optimum doping of 0.1 wt.% MWCNTs generated short electron lifetime, τeff of 0.67 ms, low effective electron chemical diffusion coefficient, Deff of 9.5 x 10-8 cm2 s-1 and higher electron recombination rate, keff of 1497.48 s-1. The addition of MWCNTs has influenced the structural, morphological, photovoltaic and electron transport properties of ZnO-MWCNTs based DSSC.

Cationic cetylpyridinium micelle as a novel electrolyte system for dye-sensitized solar cells

The charge transfer process within the electrolyte system is an active area for further improving the conversion efficiency of the dye-sensitized solar cell (DSSC). In this work, micelle formed by cationic surfactant cetylpyridinium (CP) chloride was used in the electrolyte to enhance the ion transport of the redox couple. Using a mixed solvent of ethylene glycol and acetonitrile at 1:9 volume ratio and 0.50 M CP, an 83% improvement in DSSC efficiency was observed. Because of a strong correlation between the efficiency and the current density as a function of the CP concentration, the presence of CP micelle likely caused a favorable shift in the ion transport within the electrolyte. According to the cyclic voltammetry, such improved ion transport can be attributed to a faster diffusion of the redox couple, particularly the I3- diffusion. In addition, the impedance analysis also revealed a short electron lifetime for the diffusion process in the presence of the CP micelle. From these results, it is plausible that the CP micelle in the electrolyte provides an extensive network of positively-charged interfaces, which facilitates the diffusion of the redox couple and enhances the overall performance of the DSSC.

Influence of the donor size in panchromatic D–π-A–π-A dyes bearing 5-phenyl-5H-dibenzo-[b,f]azepine units for dye-sensitized solar cells

Two D–π-A–π-A organic dyes (YC-1 and YC-2) with 5-phenyl-5H-dibenzo[b,f]azepine derivatives as donor, thiophene as π bridge, and isoindigo and cyanoacrylic acid as acceptors were prepared. YC-1 and YC-2 show a panchromatic absorption between 300 nm and 800 nm both in solution and neat film. The photovoltaic performances of both dyes were evaluated in dye-sensitized solar cells based on iodide/triiodide electrolyte without any co-sensitizer. The YC-1 based device displays better device performance with open-circuit photocurrent density of 12.12 mA cm−2, open-circuit voltage of 0.53 V, and fill factor of 68.9%, corresponding to overall conversion efficiency (η) of 4.38%. The inferior performance of device based on YC-2 (η = 1.46%) is ascribed to short electron lifetime as evidenced from electrochemical impedance spectroscopy measurement. This research provided a potential promising donor unit for organic dyes and revealed the influence of donor size in organic dyes for photovoltaic performances.

Exploring the Heterogeneous Interfaces in Organic or Ruthenium Dye-Sensitized Liquid- and Solid-State Solar Cells

The interfacial properties were systematically investigated using an organic sensitizer (3-(5'-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2']bithiophenyl-5-yl)-2-cyano-acrylic acid (D)) and inorganic sensitizer (bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bipyridine-4,4'-dicarboxylato) ruthenium(II) (N719)) in a liquid-state and a solid-state dye-sensitized solar cell (DSC). For liquid-DSCs, the faster charge recombination for the surface of D-sensitized TiO2 resulted in shorter diffusion length (LD) of ∼3.9 μm than that of N719 (∼7.5 μm), limiting the solar cell performance at thicker films used in liquid-DSCs. On the other hand, for solid-DSCs using thin TiO2 films (∼ 2 μm), D-sensitized device outperforms the N719-sensitized device in an identical fabrication condition, mainly due to less perfect wetting ability of solid hole conductor into the porous TiO2 network, inducing the dye monolayer act as an insulation layer, while liquid electrolyte is able to fully wet the surface of TiO2. Such insulation effect was attributed to the fact that the significant increase in recombination resistance (from 865 to 4,400 Ω/cm(2)) but shorter electron lifetime (from 10.8 to 0.8 ms) when compared to liquid-DSCs. Higher recombination resistance for solid-DSCs induced the electron transport-limited situation, showing poor performance of N719-sensitized device which has shorter electron transport time and similar LD (2.9 μm) with D-sensitized device (3.0 μm).

Tetrathiafulvalene as a one-electron iodine-free organic redox mediator in electrolytes for dye-sensitized solar cells

Tetrathiafulvalene (TTF) was investigated as an organic iodine-free redox mediator in electrolytes for dye-sensitized, nanocrystalline solar cells (DSCs) and was compared to the commonly used iodide/triiodide system. The TTF system studied was determined to be a one-electron transfer system, although potentially exhibiting three well-defined oxidation states. Despite the slightly positive redox potential of TTF, electrolytes with TTF displayed around 200 mV lower open-circuit voltage than the iodide/triiodide system. This can mainly be ascribed to a much shorter electron lifetime in the TiO2 film. Mass transport limitations for redox species in TTF-based electrolytes were found to be serious. Electrochemical impedance measurements (EIS) show that the charge-transfer resistance at the counter electrode in the electrolyte with TTF is considerably larger than for the iodide/triiodide system. In addition, the light absorption of the TTF-based electrolyte is stronger than that for the iodide/triiodide system. Thus, DSCs with TTF-based electrolytes show worse photovoltaic performance than those with iodide/triiodide-based electrolytes. The differences in I–V characteristics and charge-recombination behavior have also been elucidated.

Effects of Dye‐Adsorption Solvent on the Performances of the Dye‐Sensitized Solar Cells Based on Black Dye

The effects of the dye-adsorption solvent on the performances of the dye-sensitized solar cells (DSSCs) based on black dye have been investigated. The highest conversion efficiency (10.6 %) was obtained in the cases for which 1-PrOH and the mixed solvent of EtOH and tBuOH (3:1 v/v) were employed as dye-adsorption solvents. The optimized value for the dielectric constant of the dye-adsorption solvent was found to be around 20. The DSSCs that used MeOH as a dye-adsorption solvent showed inferior solar-cell performance relative to the DSSCs that used EtOH, 1-PrOH, 2-PrOH, and 1-BuOH. Photo- and electrochemical measurements of black dye both in solution and adsorbed onto the TiO(2) surface revealed that black dye aggregates at the TiO(2) surface during the adsorption process in the case for MeOH. Both the shorter electron lifetime in the TiO(2) photoelectrode and the greater resistance in the TiO(2)-dye-elecrolyte interface, attributed to the dye aggregation at the TiO(2) surface, cause the decrease in the solar-cell performance of the DSSC that used MeOH as a dye adsorption solvent.

Linear and nonlinear transport in a small charge-tunable open quantum ring

We experimentally study the Aharonov-Bohm-conductance oscillations under external gate voltage in a semiconductor quantum ring with a radius of 80 nm. We find that, in the linear regime, the resistance-oscillation plot in the voltage-magnetic-field plane corresponds to the quantum ring energy spectra. The chessboard pattern assembled by resistance diamonds, while loading the ring, is attributed to a short electron lifetime in the open configuration, which agrees with calculations within the single-particle model. Remarkably, the application of a small dc current allows observing strong deviations in the oscillation plot from this pattern accompanied by a magnetic-field symmetry break. We relate such behavior to the higher-order-conductance coefficients determined by electron-electron interactions in the nonlinear regime.

FABRICATION OF ZINC OXIDE-BASED DYE-SENSITIZED SOLAR CELL BY CHEMICAL BATH DEPOSITION

Zinc oxide film has been fabricated by converting flower-like structure of zinc carbonate hydroxide made by chemical bath deposition technique. Flower-like structure is employed as charge transport network of dye-sensitized photoanode. Analysis of current density–voltage characteristic shows deposition temperature and deposition time of chemical bath deposition influence photovoltaic performance. Analysis of electrochemical impedance spectroscopy reveals short electron lifetime and high effective electron diffusion coefficient of zinc oxide-based dye-sensitized solar cell.

Molecular Design of Organic Dye toward Retardation of Charge Recombination at Semiconductor/Dye/Electrolyte Interface: Introduction of Twisted π-Linker

A triphenylamine (TPA) dye with two hexyl groups at the opposable 3,3′-positions of a bithiophene linker (1a) was prepared and compared to a dye with a regio regular 4,3′-dihexylbithiophene unit (1b). The absorption spectrum of 1a showed a blue shift in comparison to 1b, suggesting the structure of 1a was twisted in comparison to 1b. The twisted structure agreed with the structure optimized by DFT calculations. By replacing one thiophene unit of 1a and 1b with a pyridine ring (2a and 2b, respectively), a further blue shift was observed. Dye-sensitized solar cells (DSSCs) were prepared from these dyes and a conventional Ru dye (N719). Under one sun conditions, DSSCs/2a showed comparable or higher open-circuit voltage (Voc) than did DSSCs/N719. The high Voc was attributed solely to long electron lifetime in the DSSCs/2a. A previous study has suggested that TPA dyes with long π-conjugation unit suffer from larger dispersion forces between the dyes and acceptors, I3− and/or I2, causing short electron lifetime...

Optoelectronic Mixer Based on Composite Transparent Gate InAlAs–InGaAs Metamorphic HEMTs

In this study, sputtered indium-tin-oxide (ITO) formed ITO/Au/ITO was used to form composite transparent gate InAlAs-InGaAs metamorphic HEMTs (CTG-MHEMT), with an optoelectronic mixer significantly markedly improved front-side optical coupling efficiency. The proposed CTG-MHEMT exhibits a high responsivity (λ = 1310 nm) of 1.71 A/W under optimal bias conditions. A -3 dB electrical bandwidth of 400 MHz is produced by the photovoltaic effect and dominated by the long lifetime of the excess holes. The -3 dB electrical bandwidth associated with the photoconductive effect is 2.3 GHz, and is determined mainly by the short electron life time. A power gain cut-off frequency (fmax) of CTG-MHEMT of 18.2 GHz was achieved. This value, is much larger than that of TG-MHEMT (14.6 GHz) because Au nano particles improved the gate resistance. The optoelectronic mixing efficiency was enhanced by tuning the gate bias conditions. The CTG-MHEMT optoelectronic mixer is a cost-effective device, and based on the optical and electrical characteristics, is a promising candidate for simplifying the system architecture in fiber-optic microwave transmission applications.

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N a– N d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10 − 10 –10 − 9 s and too thin (3–5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28–30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20–30 µm, ≥ 10 16 cm − 3 , and ≥ 10 − 6 s, respectively.

Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with High Molar Extinction Coefficient Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells

We report two new heteroleptic polypyridyl ruthenium complexes, coded C101 and C102, with high molar extinction coefficients by extending the pi-conjugation of spectator ligands, with a motivation to enhance the optical absorptivity of mesoporous titania film and charge collection yield in a dye-sensitized solar cell. On the basis of this C101 sensitizer, several DSC benchmarks measured under the air mass 1.5 global sunlight have been reached. Along with an acetonitrile-based electrolyte, the C101 sensitizer has already achieved a strikingly high efficiency of 11.0-11.3%, even under a preliminary testing. More importantly, based on a low volatility 3-methoxypropionitrile electrolyte and a solvent-free ionic liquid electrolyte, cells have corresponding >9.0% and approximately 7.4% efficiencies retained over 95% of their initial performances after 1000 h full sunlight soaking at 60 degrees C. With the aid of electrical impedance measurements, we further disclose that, compared to the cell with an acetonitrile-based electrolyte, a dye-sensitized solar cell with an ionic liquid electrolyte shows a feature of much shorter effective electron diffusion lengths due to the lower electron diffusion coefficients and shorter electron lifetimes in the mesoporous titania film, explaining the photocurrent difference between these two type devices. This highlights the next necessary efforts to further improve the efficiency of cells with ionic liquid electrolytes, facilitating the large-scale production and application of flexible thin film mesoscopic solar cells.

Semiclassical model for the relative intensity noise of intersubband quantum cascade lasers

We present a semiclassical noise model for the relative intensity noise of quantum cascade (QC) lasers based on a 3-level rate equation approach for a cascaded active region. We analyze the intensity noise properties with respect to the individual noise sources contributing to the RIN as well as the influence of the number of gain stages incorporated into the active region. We find that by increasing the number of gain stages in the active region, the overall contribution of non-radiative losses of carriers out of the upper laser level become the dominant noise source. Furthermore, we investigate the influence of the electron lifetime on the intensity noise properties. We find that the different noise properties of QC lasers compared to interband semiconductor lasers result from the joint effect of the short electron lifetime and the cascaded active region.

Electron Lifetime in Armchair Carbon Nanotubes

The excited conduction electrons in metallic armchair carbon nanotubes can decay by inelastic electron–electron scattering. The deexcitation channels include the single-particle and collective excitations of different angular momenta. The higher conduction subbands have more deexcitation channels, or shorter electron lifetimes. Each conduction subband has certain states with very long lifetimes, and a simple relationship between the inverse electron lifetime and the state energy is absent. Such results directly reflect the characteristics of the 1D excitation spectra. The inverse electron lifetime contrasts sharply with those of semiconducting carbon nanotubes, graphite, and 2D and 3D metallic systems. Dimensionalities play an important role in the many-body effects. The predicted results could be verified by femtosecond time-resolved optical spectroscopy.

Traps and Trap-Assisted Tunneling in HgCdTe Photodiodes

ABSTRACTThe performance of HgCdTe photodiodes, realized for the purpose of thermal imaging in the 8–12μm atmospheric window, is often limited by traps and by the tunneling mechanism associated with them. This mechanism dominates the leakage current and the dynamic resistance in diodes operated in reverse bias above 1OOmV and at temperatures below 77K. In usual working conditions, namely low reverse bias and a temperature of 77K, the presence of traps and the tunneling mechanism associated with them, degrades the performance of the diodes. Although the diffusion mechanism dominating the leakage current and the dynamic resistance in diodes operated under the above conditions is of major concern, the presence of the traps should not be ignored. The traps result in shorter electron lifetime in the bulk as well as in the generation of 1/f noise, which degrades the device's figures of merit such as NETD. Therefore, a thorough understanding of the trapassisted tunneling mechanism is a prerequisite to studying its impact on the device performance.In this paper we present a model which describes the connection between the leakage current associated with the traps and the trap characteristics: concentration, energy level and capture cross-section. The validity of the model is confirmed by the good fit between measured and calculated characteristics of the diodes.