Photocurrent Behavior Research Articles

Photo-thermionic emission and photocurrent dynamics in low crystallinity carbon nanotubes

Within this work, it is studied the photo-thermionic and photocurrent behavior of two kinds of carbon nanotube arrays, grown by chemical vapor deposition. One kind is fabricated by using a co-catalyst bilayer approach and other by a catalyst-free route by using a porous alumina membrane as a template. The carbon nanotubes fabricated by both approaches exhibit low crystallinity. To compute the thermal properties of the nanomaterials, a photothermal experiment was measured at 1064 nm wavelength, and numerical simulations were conducted to analyze the photo-thermionic emission dynamics. The high absorbance of the carbon nanotubes leads to reach temperatures above 1500 K, with time response in the millisecond order and emission currents in the nanoampere range. On the other hand, the photodetector behavior was confirmed by a single pulse experiment that induces fast photocurrent recognition associated to a photodiode. Our results highlighted the coexistence of stable states related to high-power and high-speed electronic conversion energy dependent on the growth mechanism of carbon nanotubes.

Electro-sonical deposition of nanostructured Sb2Se3 films for optoelectronic applications

In this study, a film of antimony selenide (Sb2Se3) was deposited on the Fluorine-doped tin oxide (FTO) as a substrate via electrodeposition at the potential of −0.55 V and the temperature of 25 °C. The effect of ultrasound waves during electrodeposition was also investigated at different times. The structural, morphological, optical, optoelectronic, and temperature dependence of the electrical resistance of the samples was investigated. The results showed formation of the polycrystalline phase of orthorhombic Sb2Se3. It was observed that when the ultrasound waves were used, the crystallite size (grain) and energy band gap decreased. It was found that in samples with a specified configuration, the intensity of the photocurrent decreased with increasing deposition time. Additionally, the anodic and cathodic photocurrent behaviors of the samples depended on the type and location of the electrode connection. Furthermore, the electrical resistance of the samples decreased with increasing temperature. In the sample with the lowest deposition time, conductivity and electrical activation energy were reduced by applying ultrasound waves, but they were increased by increasing the deposition time.

Substrate- and layer-effects on structural and photovoltaic properties of spin-coated Aurivillius-type Bim+1Fem-3Ti3O3m+3 thin films

Driven by their ferroelectric, semiconducting, photonic, etc. properties, the potential of Aurivillius-type oxides of binary Bi4Ti3O12-BiFeO3 system in photovoltaic area is increasingly being studied. In this work, Bi5FeTi3O15 (BFTO-4), Bi6Fe2Ti3O18 (BFTO-5) and Bi7Fe3Ti3O21 (BFTO-6) thin films on soda-lime glass, indium tin oxide (ITO) glass, silicon wafer and Pt/Ti/SiO2/Si substrate were prepared by the chemical solution deposition technique. The photovoltaic responses of the substrate/BFTO-4/Au devices measured by adapting “top-top” or “top-bottom” electrodes were observed. All devices present a similar linear dark J-V curve disjointed to zero point and their photovoltaic properties strongly depended on the substrates and electrode connection forms, arising from self-polarization and interface barrier potential. The optimal photovoltaic effect was observed in the BFTO-4 films on Pt/Ti/SiO2/Si substrate without any poling, with the short circuit current (Jsc) of 18.4 μA/cm2 and open circuit voltage (Voc) of 0.14 V under light illumination, which is attributed to larger polarization and c-axis oriented structure. Furthermore, layer effects on photovoltaic properties of the BFTO-4, BFTO-5 and BFTO-6 films deposited on ITO glass were also investigated. With the increase of layers, the devices exhibit an enhancement of Jsc along with the Voc reduction, due to the intrinsic layered structures with different lattice strains. The switching photocurrent behaviors of the BFTO-4 film by typical ON/OFF light measurements verified the stability and reliability of the photovoltaic effect.

A Comparison of the Structure and Properties of Opaque and Semi-Transparent NIP/PIN-Type Scalable Perovskite Solar Cells

For over a decade, single-junction perovskite solar cells (PSCs) have experienced an unprecedent increase in efficiencies and even offer opportunities to surpass the Shockley–Queisser limit in multijunction configuration. There is consequently an intense need for easily processable semi-transparent PSCs as a basis of affordable tandems. The current study reports the comparison of negative-intrinsic-positive (NIP) and positive-intrinsic-negative (PIN) architectures based on CH3NH3PbI3{Cl}-based perovskite. Both devices could be prepared with the same N-type (SnO2 nanoparticles) and P-type (poly-triarylamine (PTAA) polymer) materials. Each layer (except for electrodes) was deposited using solvent-based low temperature processes, contrasting with other literature studies, especially SnO2 for PIN-type purposes. A thorough experimental comparison of the two architectures reveals rather similar optical and structural properties for perovskites, whether deposited on an N- or P-type underlayer, with also comparable efficiencies in the final devices. A compatible deposition process for sputtered indium tin oxide (ITO) as a semi-transparent electrode was then performed for both architectures. Upon varying the illuminated devices’ side, the semi-transparent cells exhibited different photocurrent behaviors, the magnitude of which depended on the device’s architecture. In conclusion, despite slightly better efficiencies for the semi-transparent NIP-type devices, the semi-transparent PIN-type counterparts also appear to be optically attractive for (two-terminal) tandem applications.

A scalable approach for functionalization of TiO2 nanotube arrays with g-C3N4 for enhanced photo-electrochemical performance

Graphitic carbon nitride (g-C3N4) has manifested itself as an effective counterpart of titanium di-oxide (TiO2) to extend its photoactivity in the visible range. In this work, g-C3N4 functionalized TiO2 nanotubes were synthesized by electrodeposition of g-C3N4 in bulk on TiO2 nanotubes from a continuously rotating organic suspension at a moderate voltage (∼90 V). Additionally, samples with varying time durations were also synthesized to examine the effect of electrodeposition time on the performance of g-C3N4 functionalized TiO2 nanotubes. Photo-electrochemical measurements demonstrated strong light induced activity, with a photocurrent density of 0.14 mA/cm2 at 1 V vs Ag/AgCl in 1 M Na2SO4 electrolyte. Maximum photoresponse for g-C3N4 functionalized TiO2 nanotubes was upto 4 factors higher than bare TiO2 nanotubes. The crystallinity, morphology and chemical compositions were investigated by XRD, FESEM and XPS respectively. Further, the optical absorption edge, chemical bonds and defects states were evaluated by UV–visible, Raman, FTIR, PL and EPR spectroscopy. The properties of g-C3N4/TiO2 system were correlated with the photocurrent behaviour and a mechanism of the probable role of vacancies and defects in their photo induced activity has been proposed. The study has established a simple and scalable scheme for the functionalization of TiO2 nanotube with g-C3N4 to achieve enhanced photo-electrochemical properties.

Novel black bismuth oxide (Bi2O3) with enhanced photocurrent generation, produced by high-pressure torsion straining

Bismuth oxide (Bi2O3), a yellow semiconductor ceramic, is considered as an advanced photoactive energy material, but its activity still needs further improvement for practical applications. In this study, black Bi2O3 with nanocrystalline structure and large fraction of oxygen vacancies is synthesized by mechanical straining via the high-pressure torsion (HPT) method. The black oxide exhibits enhanced light absorbance under both UV and visible lights. It shows a bipolar photocurrent behavior with up to six times higher photocurrent density compared with the unprocessed yellow Bi2O3. This study shows the high potential of severely-strained black Bi2O3 for photovoltaic, photoconductivity and photocatalytic applications.

High Performance Perovskite Solar Cells by Perovskites Co-Crystallized with Polymers

Extensive interest has been raised by perovskite materials as emerging alternatives for low-cost and efficient perovskite solar cells (PSCs) in the past decade. However, achieving high-performance and stable PSCs with negligible photocurrent hysteresis behavior is still a challenging task due to the intrinsic foibles of perovskites. In this work, we report efficient and stable PSCs with dramatically suppressed photocurrent hysteresis by CH3NH3PbI3 co-crystallized with poly(ethylene oxide) (PEO). It is found that PEO-CH3NH3PbI3 thin film exhibit superior film morphology and significantly enlarged crystal grains as compared with CH3NH3PbI3 thin film. Systematical investigations reveal that the introduction of PEO in CH3NH3PbI3 could improve the charge transport and reduce the charge carrier recombination and promote the charge carrier extraction time within PSCs. As a result, PSCs by PEO-CH3NH3PbI3 thin film exhibit a power convention efficiency of 20.73%, which is over 20% enhancement as compared with that (17.42%) from PSCs by CH3NH3PbI3 thin film. Moreover, photocurrent hysteresis is dramatically suppressed in PSCs by PEO-CH3NH3PbI3 thin film. In addition, PSCs by PEO-CH3NH3PbI3 thin film exhibit boosted stability. All these results demonstrate that perovskites co-crystallized with polymers is an efficient approach towards high performance PSCs.

Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination

We investigated anomalous photocurrent behavior in organic field-effect transistors(OFETs) based on C60 thin film. With increasing incident optical power, it was interesting to observe the photocurrent decreases. The inverse photocurrent was mainly dependent on intensity of the light, not the wavelength. Meanwhile, this paper revealed that the main sources of the negative photocurrent were not the electrodes, channel and the interface effect but a distinct optoelectronic property of the C60 material itself. Such mechanism of photocurrent may find potential applications in the future.

Versatile {Cp2Ti} Grafted Hetero-Polyoxotungstate Clusters: Synthesis, Crystal Structure, and Photocurrent Properties.

Polyoxotungstate supported titanocene {Cp2Ti}2+ clusters H6{K8(Cp2Ti)2P4W24O88(PO4)2}·14H2O (1), H6[Na2P4W14O58(Cp2Ti)2]·12H2O (2), and H2[K6{Cp2Ti}{PW9O33(WO2)}2{NC5H3(COOK)2}(NC5H3(CH3)COOK)·22H2O] (3) have been synthesized, and their single crystal X-ray structures have revealed unprecedented and intriguing structural features. The synthesized compounds have been characterized by various spectroscopic techniques including UV-vis, cyclic voltammogram, NMR, ESI-MS, and inductive coupled plasma spectroscopy (ICP) in solution and also by IR, TGA, and diffuse reflectance in the solid state. Clusters 1 and 2 are rare examples of lacunary POM supported titanocene clusters obtained by incorporating various phosphorus heteroatoms to form elusive phosphotungstate assemblies, whereas 3 is an unprecedented organometallic as well as heteroleptic pyridyl functionalized POM. Clusters 1-3 show transient photocurrent ON/OFF behavior upon UV-light irradiation and also exhibit characteristic TiIV/III intravalence electron transfer. This behavior is also established by their cyclic voltammograms in mixed phosphate buffers (Na2HPO4/NaH2PO4) which show the evidence of POM supported {Cp2Ti}2+/+ species in their redox solution. Furthermore, ESR line broadening is also observed in these clusters at room temperature, a fact which also confirms the formation of partially reduced/oxidized {Cp2Ti}2+/+ species leading to TiIV/III intravalence electron transfers within all three clusters. The {Cp2Ti}2+ decorated polyoxometalate cluster 3 shows improved transient photocurrent behavior which may be due to the presence of pyridyl carboxyl ions which provide better surface contact for the cluster molecule through the carboxylate moiety to the ITO electrode.

Highly sensitive in situ-synthesized cadmium sulfide (CdS) nanowire photosensor for chemiluminescent immunoassays

Highly sensitive in situ-synthesized cadmium sulfide (CdS) nanowires (NWs) for the detection of chemiluminescence in immunoassays with a photoresist (PR) layer to stabilize the CdS NWs before and after coating with a parylene film were developed. The thickness of the PR layer was controlled by adjusting the viscosity of the PR solution used for spin-coating. PR2005 was the optimal PR for passivation of the NW surface. After the addition of a parylene coating on the CdS NWs, the photocurrent increased by as much as 50% over a broad range of light intensities, and the additional PR layer increased the photoresponse over the whole range of light intensities. When the photoresponses of the CdS NWs with and without the parylene film were compared after the addition of a PR layer, significant differences were observed in the photocurrent behavior after the incident light was turned off. For the CdS NWs with a parylene film and PR layer, the photocurrent reached the baseline within milliseconds of the incident light being turned off. However, the CdS NWs without a parylene film but with a PR layer required >60 s to reach the baseline level. This difference was due to the capacitance arising from the contact between the NWs. The in situ-synthesized CdS NW photosensor passivated by the parylene film and a PR layer was used in a chemiluminescence-based immunoassay. Finally, the detection of human immunodeficiency virus antibodies was demonstrated via a chemiluminescent enzyme-linked immunosorbent assay based on the CdS NW photosensor in comparison with the optical-density measurement for the chromogenic reaction of TMB(3,3′,5,5′-Tetramethylbenzidine).

Study of the relative growth rate, the anisotropy of electrical resistivity and anomalous photocurrent behavior of the unidirectional grown Rochelle salt crystals

This research article compares the growth rate along the directions [100], [010], and [001] of Rochelle salt crystals grown using the Sankaranarayanan–Ramasamy method, reports its electrical resistivity near its two Curie temperatures along the directions [100], [010], [001], [110], [011], and [101], presents the evaluated activation energy in the paraelectric and ferroelectric phases of the crystal, points out the change in the value of activation energy at Curie temperatures of the crystal, displays the anomalous behavior of the photocurrent along the polarization direction [100] of the crystal around the two Curie temperatures, and discusses the photo-stimulated shifts of the Curie temperatures.

The Effect of Surface on Conductivity of PbSnTe:In/BaF2 Topological Crystalline Insulator in Space Charge Limited Current Regimes

The effect of surface chemical treatment on current-voltage (IV) characteristics of high-resistance MBE-grown PbSnTe:In films in space charge limited current (SCLC) regimes has been studied. At T=4.2 K depending on surface chemical treatment the current in films in SCLC regimes under no illumination could rise up to 104 times and even more. The surface chemical treatment also affected both the value and behavior of the photocurrent. The qualitative model for the observed phenomena has been discussed.

Non‐Stoichiometry Induced Switching Behavior of Ferroelectric Photovoltaic Effect in BaTiO3 Ceramics

Ferroelectric photovoltaic (FPV) effect has been studied among a series of non‐stoichiometric BaTiO3 (Ba/Ti = 0.92–1.05) ceramic chips prepared by tape casting method. The FPV performance increases abruptly when the Ba/Ti molar ratio deviates from the stoichiometry within 1%. Meanwhile, a photocurrent direction switching behavior is observed between Ti‐excess and Ba‐excess samples. The TEM analysis shows their significant difference in grain‐boundary (GB), where abnormal GB with a width of 10–15 nm is observed in Ba‐excess sample. The photocurrent switching phenomenon is described to the competition between the asymmetrical Schottky barriers induced PV effect and intrinsic FPV effect. Widen GB in Ba‐excess BaTiO3 ceramics restrains the intrinsic FPV effect and results in the switching behavior. This study offers direct evidence of the vital role of GB in FPV effect and may promote the development of photovoltaic devices.

Electrical Control of Circular Photogalvanic Spin-Valley Photocurrent in a Monolayer Semiconductor.

In a monolayer transition metal dichalcogenide (TMDC) that lacks structural inversion symmetry, spin degeneracy is lifted by strong spin-orbit coupling, and a distinctive spin-valley locking allows for the creation of valley-locked spin-polarized carriers with a circularly polarized optical excitation. When excited carriers also have net in-plane momentum, spin-polarized photocurrents can be generated at ambient temperature without magnetic fields or materials. The behavior of these spin-polarized photocurrents in monolayer TMDC remains largely unexplored. In this work, we demonstrate the tuning of spin-valley photocurrent generated from the circularly polarized photogalvanic effect in monolayer MoS2, including magnitude and polarization degree, by purely electric means at room temperature. The magnitude of spin-polarized photocurrent can be modulated up to 45 times larger, and the polarization degree of the total photocurrent can be tuned significantly (here from 0.5 to 16.6%) by gate control. Combined with the atomic thickness and wafer-scale growth capabilities of monolayer TMDC, the efficient electrical tuning of spin-valley photocurrent suggests a pathway to achieve spin-logic processing by local gate architectures in monolayer opto-spintronic devices.

Enhanced photoelectrocatalytic water oxidation using CoPi modified GaN/MWCNTs composite photoanodes

GaN is promising material for photoelectrochemical (PEC) water oxidation owing to its adjustable band gap and suitable band edge locations, which straddle the H+/H2 and O2/H2O redox conditions. However, due to its wide band gap and unavoidable surface defects, the PEC conversion efficiency of GaN is insufficient to be used for practical applications. To improve the PEC efficiency of GaN, we report fabrication of a cobalt phosphate (CoPi)-modified GaN/multiwall carbon nanotube (MWCNT) composite photoanode through combination of metal organic chemical vapor deposition, dip and dry methods, and electrodeposition. Here, GaN photoanodes with different MWCNT surface coverages were obtained by cost effective ‘dip and dry’ method. Different deposition cycles were performed to prepare a series of samples, which were evaluated for their photoelectrochemical water oxidation. The highest photocurrent density (Jph) of 2.23 mA/cm2 at (0 V vs. Pt electrode) was achieved in a sample with moderate incorporation of MWCNTs, formed by through three cycles of the dip and dry coating (i.e., GaN/MWCNT 3T). We attribute this result to the MWCNTs facilitating light sensitization. However, excessive MWCNT loading produced a shielding effect, which lowered the photocurrent density, and an undesirable two-plateau photocurrent behavior indicating sluggish reaction kinetics. The maximum photocurrent density Jph of 2.81 mA/cm2, (vs. Pt electrode) with solar to hydrogen conversion efficiency 0.91% was achieved for GaN/MWCNTs 3T/CoPi ternary composite photoanode, which was approximately 2.51 and 1.26 times as high as that of bare GaN and GaN/MWCNTs 3T photoanodes.

Synthesis and characterization of ruthenium-doped CdO nanoparticle and its n-RuCdO/p-Si junction diode application

In the present work, Ru doped CdO nanoparticles were prepared with different doping concentrations by microwave irradiation technique for P-N junction diode application. The effect of doping agent on the CdO nanoparticles were investigated via various techniques such as XRD, FT-IR, SEM, EDS, TEM, UV-DRS, and XPS techniques. The XRD results confirmed that the prepared nanoparticles have cubic structure with preferred orientation along (111) direction and this plane crystallite size is in the range 18.7 to 16.8 nm. SEM and TEM morphologies reveal the CdO nanoparticles are strongly influenced by the doping of Ru and grain size reduces with increasing Ru doping levels. UV–vis DRS results depict the optical band gap value decreases with increase of Ru content. FTIR, EDS and XPS results are good evidence for formation of pure and Ru doped CdO. The DC electrical conductivity result shows that the 5 wt% Ru doping level have a maximum conductivity compared to pure CdO. The n-RuCdO/p-Si diode parameters (ideality factor (n) and barrier height (Фb)) were studied using J-V method. The photocurrent and dark current behaviors of the fabricated n-RuCdO/p-Si diode show good photoconducting response.

Noble metal free Fe and Cr dual-doped nanocrystalline titania (Ti1−x−yMx+yO2) for high selective photocatalytic conversion of benzene to phenol at ambient temperature

Metal (Fe and Cr)-incorporated mesoporous nanocrystalline titania (Ti1−x−yMx+yO2, or TiMxO2) was synthesized using a simple solution combustion method. Structural characterization, microscopy, and spectroscopy techniques confirmed the incorporation of Fe and Cr into the TiO2 lattice. Photocatalytic benzene oxidation to phenol was investigated under ambient conditions, and under solar stimulator (AM1.5 or one sun condition) and ultraviolet (UV) irradiation. Compared to pristine TiO2, TiO2 doped with 2 at% of Fe (Ti0.98Fe0.02O2) exhibited a higher catalytic activity for benzene-to-phenol oxidation, highlighting the importance of the introduction of the Fe dopant. Introduction of Fe and Cr into the TiO2 (Ti0.98Fe0.01Cr0.01O2) lattice further enhanced the oxidation of benzene to phenol under UV light and gave a phenol yield that was ∼2 times higher than that given by Ti0.98Fe0.02O2. The high photocatalytic activity of Ti1−x−yMx+yO2 and good photocurrent behaviour were mainly ascribed to the generation of an electron trapping level composed of Fe3+ and Cr3+ in the TiO2 conduction band, disordered mesoporosity, high mobility, and a short diffusion length for charge carriers.

Impact of UV spot position on forward and reverse photocurrent symmetry in a gold-diamond-gold detector

A UV detector based on a planar quadrant electrode structure with a 100-μm isolating cross-gap deposited on chemical vapor deposition diamond was fabricated. An asymmetrical behavior was observed for the amplitude of photocurrent when in forward and reverse bias. A linear relationship between the sum of the photocurrents at biases U and −U, and the light spot position was also observed. The aim of this study is to clarify the mechanism underlying this asymmetrical behavior and exhibit its potential application based on this behavior. A mechanism involving lowering of the contact barrier height between gold and i-diamond by carrier generation upon UV illumination was adopted to account for the photocurrent behavior and confirmed when an opposite asymmetrical photocurrent behavior was observed in other samples. In addition, position-sensitive detection performance, based on this asymmetrical photocurrent behavior, was preliminarily studied. The results exhibited the potential for application as a position-sensitive detector.

Structural and Temperature-Dependent Electrical/Optical Behaviors of Hot-Wall Deposited BaGa2Se4 Layers

The characteristic behavior of hot-wall deposited BaGa2Se4 layers was investigated as a function of temperature. The structural quality of the layers was found to be affected by tensile strain due to the coincidence-lattice mismatch. The carrier mobility showed an aspect of two scattering mechanisms. As the temperature was increased to 100 K, the mobility increased as a function of T1 due to impurity scattering. Also, at temperatures higher than 100 K, its behavior was reduced to two different-temperature slopes caused by lattice scattering. One was a function of T−3/2 at T > 200 K, and the other was a function of T−1/2 for 100 200 K. From the behavior of log n(T) versus 1/T, three donor-trap levels due to native defects were observed. By tracking the spectral photocurrent (PC) behavior with decreasing temperature, we found that the three PCpeak positions shifted toward shorter wavelengths and their intensities were dramatically decreased. These spectral PC peaks were due to the band-to-band transition. These band-gap variation were well matched by Eg(T) = Eg(0)−2.06×10 −3T2/(T +230.7). By the selection rule, the crystal-field and the spin-orbit splitting were found to be 0.2031 and −0.2259 eV, respectively. In the log Jph versus 1/T plot, we found that the spectral PC-intensity behavior was related to two donor-trap levels by comparing with the carrier concentration results. In conclusion, the dramatic decrease in the spectral PC intensity was attributed to trapping centers.

36% Enhanced Efficiency of Ternary Organic Solar Cells by Doping a NT-Based Polymer as an Electron-Cascade Donor.

In recent years, ternary organic photovoltaic cells (OPVs) have been dedicated to improving power conversion efficiency (PCE) by broadening optical absorption spectra. Ternary OPVs with different poly[thieno[3,2-b]thiophene-2,5-diyl-alt-4,9-bis(4-(2-decyltetradecyl)thien-2-yl)naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole-5,5′-diyl] (PTT-DTNT-DT) doping concentrations were designed and the effect of PTT-DTNT-DT as a complementary electron donor on the performance of OPVs was investigated. The optimized PCE of OPVs was increased from 3.42% to 4.66% by doping 20 wt % PTT-DTNT-DT. The remarkable improvement in the performance of the ternary device is mainly attributed to the sharp increase in the short-circuit current density and fill-factor. The major reasons have been systematically studied from atomic force microscopy, electrochemical impedance spectroscopy, surface energy, space charge limited current and photocurrent behavior. It has been found that the separation of excitons and the transportation of charge are enhanced while light absorption is increased, and the charge recombination also decreases due to the optimization of the cascade energy level and the morphology of the ternary active layer. The results show that it is feasible to improve the performance of ternary OPVs by their complementary absorption.