Transient Photocurrent Measurements Research Articles

A two-dimensional MoS2/WSe2 van der Waals heterostructure for enhanced photoelectric performance

Fabricating a two-dimensional (2D) van der Waals heterostructure is an efficient strategy to improve the photoelectric performance of 2D materials, thus providing a new possibility for the photoanode material design in photoelectrochemistry (PEC) devices. Herein, transition metal dichalcogenide (TMD) MoS2/WSe2 heterojunction photoelectrodes are prepared via liquid-phase exfoliation and vacuum filtration. Linear sweep voltammetry, transient photocurrent, and open circuit potential measurements show that the heterojunction photoelectrodes have enhanced photocurrent intensity and improved photoresponse activity. Moreover, Nyquist impedance plots, Bode phase plots, and Mott-Schottky measurements demonstrate that the heterojunction samples have higher charge transfer rate and longer charge lifetime than MoS2 and WSe2. To reveal the mechanisms, the band alignment of MoS2/WSe2 heterojunction samples is established with the support of X-ray photoelectron spectroscopy and first-principles theory calculation. Accordingly, the improved PEC performance of the heterojunction photoanode is ascribed to the built-in electric field between MoS2 and WSe2 nanosheets, which promotes the photogenerated e-h pair separation and suppresses their recombination. This work verifies the PEC mechanism of MoS2/WSe2 heterojunction photoelectrodes, which is significant for the design of PEC devices based on TMD heterostructures.

Riboflavin-Immobilized CeO2–RGO Nanohybrid as a Potential Photoredox Catalyst for Enhanced Removal of Organic Pollutants under Visible Light

We report a facile immobilization of riboflavin (RF) over in situ synthesized CeO2–reduced graphene oxide (RGO) nanohybrid surface synthesized through a simple wet-chemical approach for photocatalytic degradation of organic pollutants under visible light (λ ≥ 420 nm). The hybrid containing 3% w/w RGO shows an excellent photocatalytic activity under visible light. The UV–visible diffuse reflectance spectra, photoluminescence (PL) analysis, and transient photocurrent measurements corroborate that the hybrid RF@CeO2–RGO significantly absorbs visible light and exhibits lower recombination of charge carriers in comparison to RF@CeO2. The catalyst, RF@CeO2–RGO, offers enhancement in photocatalytic rate, i.e., 2.8-fold increase for methyl orange (MO) and 3.5-fold for p-nitrotoluene (PNT) in comparison to RF@CeO2 under visible light (2 h). This enhancement could be attributed to the synergistic effect among CeO2, RF (broadens visible light absorption region), and RGO (an electron-shuttling agent), which lead to a...

Millimetre-long transport of photogenerated carriers in topological insulators

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi2-xSbxSe3 nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m2 s−1, which corresponds to a mobility of 6 × 104 m2 V−1 s−1 at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

The influence of the Cu2O deposition method on the structure, morphology and photoresponse of the ordered TiO2NTs/Cu2O heterojunction

The increased interest in highly ordered titania structures exhibiting tubular shape that could be directly formed onto the stable substrate is related with the intensive research on their modification enabling absorption of light within the wide solar spectrum and its further conversion into electric or chemical energy. Among others, formation of uniform 3D heterojunctions based on the TiO2 nanotubes attracts attention since porous structure provides highly developed surface area whereas tubular shape ensures facilitated charge transport. Following that, we focus on the comparison of different electrochemical routes leading to the formation of Cu2O onto the titania nanotubes platform to improve energy harvesting. Proposed approach includes optimized potentiostatic and pulsed polarization of titania immersed in the basic electrolyte composed of Cu2+ ions stabilized by lactic acid. In this work, we demonstrate that at strictly selected deposition parameters, namely time, pulse width and cycle number, one can obtain titania nanotubes overgrown by Cu2O species uniformly distributed across the whole TiO2 layer as proved by secondary ion mass-spectrometry. X-ray photoelectron spectroscopy studies indicated purely Cu(I) oxide formation for pulsed deposition route. The photoactivity of obtained materials was verified both under vis and UV–vis irradiation. Depending on the incident light spectrum, variation in photoactivity was observed that is directly related with the charges generation and their transport within the junction. The transient photocurrent measurements show strong photocurrent enhancement for Cu2O modified titania comparing to pristine support and satisfactory photostability under visible light without any protective coating.

Dark and illuminated electrical characteristics of Si-based photodiode interlayered with CuCo5S8 nanocrystals

Derived from the traditional dichalcogenide CuS structure, ternary transition metal chalcogenide nanoparticles in the form of CuCo5S8 are investigated under the aim of photodiode application. In addition to the detailed analysis on material characteristics of CuCo5S8 thin-film layer, the work is focused on the electrical characteristics of Au/CuCo5S8/Si diode to investigate its current–voltage, capacitance–voltage, and conductance–voltage characteristics under dark and illuminated conditions. CuCo5S8 nanocrystals with an average size of 5 nm are obtained using hot-injection method, and they are used to form thin-film interfacial layer between metal (Au) and semiconductor (Si). Under dark conditions, the diodes show about four orders in magnitude rectification rate and diode illumination results in efficient rectification with increase in intensity. The analysis of current–voltage curve results in non-ideal diode characteristics according to the thermionic emission model due to the existence of series resistances and interface states with interface layer. The measured current–voltage values are used to extract the main diode parameters under dark and illumination conditions. Under illumination, photogenerated carriers contribute to the current flow and linear photoconductivity behavior in photocurrent measurements with illumination shows the possible use of CuCo5S8 layer in Si-based photodiodes. This characteristic is also observed from the typical on/off illumination switching behavior for the photodiodes in transient photocurrent, photocapacitance, and photoconductance measurements with a quick response to the illumination. The deviations from ideality are also discussed by means of distribution of interface states and series resistance depending on the applied frequency and bias voltage.

Study on boron and fluorine-doped C3N4 as a solid activator for cyclohexane oxidation with H2O2 catalyzed by 8-quinolinolato ironIII complexes under visible light irradiation

This paper discloses that boron and fluorine-doped carbon nitride (C3N4BF), as a solid activator, can efficiently accelerate the relatively inert 5,7-dichloro-8-quinolinolato iron(III) complex (FeIII(Qb)3) to photo-catalyze the oxidation of cyclohexane to cyclohexanol and cyclohexanone by hydrogen peroxide (H2O2) in acetonitrile (MeCN) under visible light irradiation and achieve 25.87 % cyclohexane conversion, which is almost twice as high as without it and obviously higher than the results obtained using ordinary C3N4 as an activator or under thermo-catalysis. Furthermore, its such activation effect on FeIII(Qb)3 photocatalysis is also observed in the H2O2-involved oxidation of other substrates such as toluene, ethylbenzene, benzyl alcohol and thioanisole. UV–vis and PL spectra, photocurrent transient measurements and theoretical calculations support that C3N4 and especially C3N4BF can form strong π-π stacking interactions with FeIII(Qb)3, thus inducing a weakly structural distortion of FeIII(Qb)3 and enhancing its photo-reductive activation via capturing the photo-generated electrons of the activator. As a result, the synergistic catalytic effect of FeIII(Qb)3 with C3N4/especillly C3N4BF can significantly accelerate the above thermo-/especially photo-oxidations.

Spinel‐Type Mixed Metal Sulfide NiCo2S4 for Efficient Photocatalytic Reduction of CO2 with Visible Light

AbstractMixed metal oxides with a spinel structure have exhibited great opportunities for photocatalytic CO2 reduction; however, the abilities of their sulfide counterparts in this promising area are much less reported. Herein, we demonstrate the synthesis of a ternary metal sulfide, namely NiCo2S4, and its first application to catalyze the CO2 photoreduction reaction under visible light irradiation. The NiCo2S4 material is prepared through a coupled solvothermal‐ion exchange strategy, and is fully checked by diverse characterizations, including X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), energy‐dispersive X‐ray spectroscopy (EDX) and N2 sorption measurements. Under visible light irradiation in a classic tandem photochemical system, this non‐noble‐metal NiCo2S4 catalyst affords a considerable activity and high stability for CO2 deoxygenative reduction, delivering a high CO‐liberating rate of 43.5 μmol mg−1 h−1. The in‐situ PL and transient photocurrent measurements reveal that the spinel catalyst can hamper recombination and accelerate transfer of light‐excited charge carriers, and thus boosting the CO2 reduction reaction. At last, a possible mechanism of the NiCo2S4‐catalyzed CO2 photoreduction reaction is proposed.

An in situ inward etching strategy for constructing a p-p heterojunction Cu2S/Cu2-xSe material based on brass as an effective counter electrode for quantum dot sensitized solar cells

Composite counter electrode Cu2S/Cu2-xSe nanosheets based on brass are fabricated by an in situ inward etching method using Cu(OH)2 nanotubes as a template. Due to superior electrocatalytic activity towards the polysulfide electrolyte, the quantum dot-sensitized solar cells constructed with Cu2S/Cu2-xSe counter electrodes deliver a considerable power conversion efficiency of 6.10% under 1 sun illumination, which is a 22% enhancement compared to that of Cu2S/brass (5.00%). According to the results of BET testing, solid-state J-V measurements and transient photocurrent measurements, the enhanced electrocatalytic activity is attributed to two aspects: one is the larger surface active area, which provides more active sites for the electrolyte, and the other is the formation of a p-p heterojunction for more effective electron transfer in the counter electrode. In addition, the band energy structure of the Cu2S/Cu2-xSe heterojunction is further investigated to understand the behavior of interfacial electron transfer. Finally, the outstanding electrochemical catalytic activity and stability of the Cu2S/Cu2-xSe counter electrode are proved by electrochemical impedance spectroscopy, Tafel polarization and cyclic voltammetry results for symmetric dummy cells, demonstrating that the Cu2S/Cu2-xSe composite is a desirable material as a counter electrode for quantum dot-sensitized solar cells.

The effect of coumarin addition on the electrical characteristics of Al/Coumarin:CdO/p-Si/Al photodiode prepared by drop casting technique

In this study, cadmium oxide thin films deposited on p-Si with different weight ratio of coumarin were prepared by drop casting technique. The effect of coumarin addition in different weight ratios was investigated on the electrical and photosensivity properties of Al/Coumarin:CdO/p-Si organic-inorganic hybrid photodiodes. The current-voltage (I–V) and capacity-conductivity-voltage (C-G–V) characteristics of the diodes were determined in the dark and under various illumination intensities. It is concluded that all samples have excellent rectification properties due to good rectifying ratios found from I–V characteristics of the devices. All photodiodes produced with different coumarin weight ratios were found to have higher photo-current under illuminated conditions than dark conditions. In addition, the highest Iphoto/Idark photosensitivity value was found as 448.78 for the device with a 5% coumarin doping rate under 5 V and 100 mW/cm2 light intensity. Transient photo-current measurements showed that all devices are sensitive to light intensity. Furthermore, capacitance/conductivity-voltage measurements were performed at frequencies ranging from 10 kHz to 1 MHz. The fact that the capacity decreases and conductivity increases with the increasing frequency values was explained with the presence of interface states. The ideality factors of Al /Coumarin:CdO/p-Si/Al photodiodes, prepared with various coumarin doping ratios, are between 4.39 and 7.83. Additionally, the interfacial density (Dit) and serial resistance (Rs) values of the devices were determined to be dependent on the frequency and the coumarin concentration. As a result, produced Al/Coumarin:CdO/p-Si/Al organic-inorganic hybrid devices can be used as photocapacitors and/or photodiodes in electro-optic applications.

Heterostructured Bismuth Vanadate/Cobalt Phosphate Photoelectrodes Promote TEMPO‐Mediated Oxidation of 5‐Hydroxymethylfurfural

AbstractMotivated by replacing the kinetically unfavorable oxygen evolution reaction (OER) and producing value‐added products in photoelectrochemical cells (PECs), we report that bismuth vanadate (BiVO4) photoelectrodes modified with a cobalt phosphate (CoPi) overlayer facilitate 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐mediated selective oxidation of 5‐hydroxymethylfurfural (HMF). CoPi layers with sufficient thickness were found to reduce the potential required for TEMPO oxidation by 0.5 V, as well as increase charge injection efficiency sevenfold compared to BiVO4 without CoPi. Furthermore, the undesired OER was completely suppressed when using the heterostructured photoanodes. Transient photocurrent measurements suggested that CoPi alleviates recombination losses resulting from the back reduction of oxidized TEMPO. The PECs with BiVO4/CoPi bilayer achieved 88 % yield to 2,5‐furandicarboxylic acid (FDCA) from HMF oxidation under mild conditions, whereas <1 % FDCA was generated with BiVO4. These findings suggest that suppression of the back reduction process substantially improves the efficiency of the oxidation, giving a potential route to more efficient solar fuel/chemical production.

Electrical and Photoresponse Properties of Al/p-Si/Y1-xSrxMnO3/Al Heterojunction Photodiodes

The electrical and photoresponse properties of Al/p-Si/Y1-xSrxMnO3/Al diodes were investigated by using current-voltage and transient photocurrent measurements. The average ideality factor and barrier height has been calculated as 4.2568 and 0.613 eV respectively. The calculated ideality factor for Al/p-Si/Y1-xSrxMnO3/Al diodes is higher than unity because of the interface states, native oxide layer and series resistance. Also, diodes have exhibited property of photosensitivity. These indicated that the fabrication of diodes can be used optoelectronic device applications.

Molecularly imprinted Ag/Ag3VO4/g-C3N4 Z-scheme photocatalysts for enhanced preferential removal of tetracycline

High selectivity and fast charge separation are two important factors for photocatalytic wastewater treatment. Herein, we prepared a molecular imprinted Ag/Ag3VO4/g-C3N4 photocatalyst (MIP) that exhibited great specific recognition ability along with excellent photocatalytic activity. The ultrathin g-C3N4 nanosheets with high surface are used to prepare. The Z-scheme Ag3VO4/g-C3N4 heterostructure and the surface plasmon resonance of the photoreduced Ag0 together contributed to the improvement of the separation efficiency of photogenerated electrons and holes. In addition, MIP provides the specific recognition ability to preferentially adsorb the target pollutant. The selectivity of photocatalysis was evaluated by the degradation of oxytetracycline and tetracycline solutions. Photoluminescence and transient photocurrent measurements further prove the improved charge separation efficiency of MIP. A plausible photocatalytic reaction mechanism is proposed based on electron spin resonance measurement and the active species trapping experiments, where indicates that the main active species of this photocatalytic process are O2− and h+. This research provides an effective strategy for the simultaneous enhancement of selectivity and activity via molecular imprinting.

The synthesis of new bola-amphiphile TPEs and the comparison of current transformer mechanism and structural properties for Al/Bis(HCTA)-TPE/p-Si and Al/Bis(HCOA)-TPE/p-Si heterojunctions

In this study, the new bola-amphiphile tetraphenylethylene derivatives Bis(HCTA)-TPE and Bis(HCOA)-TPE were synthesized to use in device application by a green approach. Al/organic layer/p-Si heterojunctions were fabricated by the usage of Bis(HCTA)-TPE and Bis(HCOA)-TPE organic interfacial layers. The mentioned organic layers were coated on the polished side of silicon wafer using the spin coating method. The surface and morphological analyses of materials coated on silicon substrate were investigated by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) images. Besides, the electrical characteristics of fabricated heterojunctions were examined by the use of current-voltage (I–V) and current-time (I-t) measurements and compared with each other. I–V characteristics were investigated under various illumination conditions, and also the behavior of these characteristics against changing temperature conditions has been determined. At the same time, transient photocurrent measurements were performed to determine the photoresponse behaviors. The results show that the fabricated device with Bis(HCTA)-TPE organic interfacial layer exhibits better electrical characteristics than the Bis(HCOA)-TPE organic layer. The other result of this experiment is that the fabricated devices could be used in optoelectronic and photovoltaic applications.

Perovskite cobaltates/p-silicon heterojunction photodiodes

Sr doped Y1-xSrxCoO3/p-Si photodiodes were fabricated by the drop-casting technique. The electrical and photoresponse properties of Al/p-Si/Y1-xSrxCoO3/Al diodes were investigated by using current-voltage, capacitance-voltage, and transient photocurrent measurements. The calculated average barrier heights and ideality factors are 0.343 eV (x = 0), 0.356 eV (x = 0.01), 0.386 eV (x = 0.05) and 0.393 eV (x = 0.15), 13.43 (x = 0), 17.27 (x = 0.01), 12.71 (x = 0.05) and 15.75 (x = 0.15) for Al/p-Si/Y1-xSrxCoO3/Al diodes at different illumination intensities, respectively. The lowest ideality factor and series resistance were observed from x = 0.05 doped diode. These results suggest that the Al/p-Si/Y1-xSrxCoO3/Al diodes can be used optoelectronic device applications.

Electrochemical nano-patterning of brass for stable and visible light-induced photoelectrochemical water splitting

A novel propitious nano-patterned brass oxide nanowires were fabricated via controlled anodization of α-brass in aqueous electrolytes at room temperature. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS) techniques were used to investigate the morphology, structure, and composition of the fabricated materials. The morphology of the resulted structures was found to depend on the concentration of the sodium bicarbonate used for anodization as well as the post treatment. The XRD analysis confirmed the existence of both ZnO and CuO. The XPS results suggest the formation of CuZnO nanowires. The fabricated nanowires showed exceptional optical activity with an absorption wavelength extending to 800 nm, corresponding to a bandgap energy of 1.7 eV. This bandgap energy was also confirmed via DFT calculations. The fabricated nanostructures were used to split water photoelectrochemically under AM 1.5 illumination. They showed very promising results towards visible light water splitting with a photocurrent of 1.88 mA/cm2 at −0.5 V versus Ag/AgCl, an incident photon-to-current efficiency (IPCE) of ∼15% at 400 nm, and a production of ∼875 μmol of H2 gas upon illumination for 5 h. The obtained photocurrent is at least five times higher than that reported for ZnO and TiO2. The transient photocurrent measurements showed the fabricated electrode to be photostable under the operating conditions.

Copper Iodide Interlayer for Improved Charge Extraction and Stability of Inverted Perovskite Solar Cells.

Nickel oxide (NiO) is one of the most promising and high-performing Hole Transporting Layer (HTL) in inverted perovskite solar cells due to ideal band alignment with perovskite absorber, wide band gap, and high mobility of charges. At the same time, however, NiO does not provide good contact and trap-free junction for hole collection. In this paper, we examine this problem by developing a double hole transport configuration with a copper iodide (CuI) interlayer for efficient surface passivation. Transient photo-current (TPC) measurements showed that Perovskite/HTL interface with CuI interlayer has an improved hole injection; CuI passivation reduces the concentration of traps and the parasitic charge accumulation that limits the flow of charges. Moreover, we found that CuI protect the HTL/perovskite interface from degradation and consequently improve the stability of the cell. The presence of CuI interlayer induces an improvement of open-circuit voltage VOC (from 1.02 V to 1.07 V), an increase of the shunt resistance RSH (100%), a reduction of the series resistance RS (−30%), and finally a +10% improvement of the solar cell efficiency.

Facile synthesis of Z-scheme Se/BiVO4 heterojunction with enhanced visible-light-driven photocatalytic performance

The Z-scheme Se/BiVO4 photocatalysts were successfully prepared by chemical reduction and hydrothermal methods. Ethanol had the effect of solubilizing a-Se and insoluble t-Se during aging, which was critical for the rapid growth of t-Se rods. Compared with pure Se rods and BiVO4, the Z-scheme Se/BiVO4 exhibits enhanced photocatalytic activity for rhodamine B (RhB) degradation under visible-light irradiation. The photoluminescence detection technology of terephthalic acid spectra shows that ·OH is the major active radical in the degradation of RhB by Se/BiVO4. Based on the transient photocurrent measurements and band structures, it can be concluded that the Z-scheme structure of Se/BiVO4 not only enhances the photogenerated electrons and holes migration rates but also to provide higher redox ability. With the enhanced photocatalytic performance, Se/BiVO4 can be potentially applied for the environmental remediation.

The comparison of transient photocurrent spectroscopy measurements of Pulsed Electron Deposited ZnO thin film for air and vacuum ambient conditions

Photoconduction mechanism of ZnO thin films that produced by Pulsed Electron Deposition method is systematically investigated by taking Transient Photocurrent Spectroscopy measurements for different atmospheres including high vacuum and air environments. Response and recovery rates of photocurrent in the air are faster than the rates in high vacuum condition. The results in the presented work clearly indicate that the photoconduction of ZnO thin films with high surface-area-to-volume ratio are surface-related and mostly governed by adsorption/desorption of oxygen and water molecules in the atmosphere. Therefore, the high surface interaction tendency of ZnO surface with the atmosphere inevitably leads to charge transfer from surface to adsorbates and/or vice versa.

Stable isoporphyrin copolymer: Electrochemical mechanism and behavior and photovoltaic properties

Two types of porphyrin copolymer thin films were obtained by electropolymerization of zinc-5,15-bis(p-tolyl)porphyrin. Electrogenerated porphyrin radical cation and dication are powerful electrophiles which can rapidly react with 1,1″-(1,3-propanediyl)bis-4,4′-bipyridinium hexafluorophosphate salt (bpy+–(CH2)3–+bpy·2PF6−) as nucleophiles to form copolymers containing stable isoporphyrin radicals or porphyrins, depending of the applied upper potential limit (1.0 V versus 1.6 V). Electrochemical routes leading to these electroactive copolymers are discussed and the unusual redox properties of copolymers containing stable isoporphyrin radicals are studied. The electropolymerization of the two copolymers was monitored by electrochemical quartz crystal microbalance (EQCM). The copolymers were characterized by UV–Vis–NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), electrochemistry, electron spin resonance (ESR) and atomic force microscopy (AFM). Their electrical properties have been studied by electrochemical impedance spectroscopy and their photovoltaic performances have been investigated by photocurrent transient measurements under visible-NIR light irradiation. The isoporphyrin radical copolymer exhibited considerably better photocurrent generation when compared to the porphyrin copolymer.

Electrical and fotoconducting characterization of Al/coumarin:ZnO/Al novel organic-inorganic hybrid photodiodes

Coumarin doped ZnO/p-Si photodiodes were prepared by using the drop casting technique. The effect of coumarin contribution on the photoresponse properties of fabricated Al/Coumarin:ZnO/p-Si/Al organic-inorganic hybrid devices was investigated. Electrical characterization of Al/Coumarin:ZnO/p-Si/Al photodiodes was carried out in the dark and under various illumination intensities with current-voltage (I–V) and capacity/conductivity-voltage (C-G-V) characteristics. The photocurrent of the diodes were found to be higher under illuminated conditions than the dark situation. It was determined that the prepared photodiodes exhibit photovoltaic behavior. Also, the photosensitivity properties of photodiodes were investigated. The highest photosensitivity expressed as Iphoto/Idark ratio was found as 894.73 for the specimen having 3% coumarin weight ratio under the conditions of 100 mW/cm2 and 5 V. Transient photocurrent measurements confirmed that the photocurrent is sensitive to light intensity. In addition, capacitance/conductivity-voltage measurements were accomplished in frequencies between 10 kHz and 1 MHz. As a result of the continuous distribution of interface states, the capacity decreased with increasing frequency. It was determined that all photodiodes produced exhibit a linear photoconductivity behavior. The calculated ideality factors for Al/Coumarin:ZnO/p-Si/Al photodiodes prepared with various coumarin weight ratios exhibited that the non-ideal behaviors due to surface states and oxide layer. In addition, it was understood that the values of the barrier height and the interfacial density of the diodes could be changed with the doped of coumarin. As a result, it is concluded that the produced organic-inorganic devices can be used as optical sensors for optoelectronic applications.