Surface Photovoltage Technique Research Articles

Cu-doped CdS-TiO2 combined with upconverting particles for NIR-induced photocatalytic CO2 reduction

The influence of copper doping of cadmium sulfide on the activity in composites with TiO2 (anatase) for photocatalytic CO2 reduction was examined. CdS and (Cu,Cd)S were synthesized by the hydrothermal method and characterized using XRD, XPS, SEM, EDS, and XRF methods. It was found that copper doping is responsible for the shift of band edge potentials. The mechanisms of photocatalytic performance of the created heterojunctions were elucidated with photoelectrochemical, spectroelectrochemical and surface photovoltage techniques. The S-scheme heterojunction was recognized for both examined systems. The obtained composites were examined under Xe-lamp radiation in the gas phase revealing CO2 reduction to CO. Moreover, the photocatalytic performance under NIR irradiation was demonstrated with the use of the upconverting nanoparticles (UCPs) added to the photocatalytic system. It was found that Cu doping improved the photocatalytic performance of CdS/TiO2 in CO2 to CO conversion mainly through the increase of the photostability of the sulfide system.

Stable MOF-In2S3/ZnFe layered dioxide heterostructures for improving photocatalytic degradation: Degradation pathways, kinetics, and charge migration

In this paper, MOF-In2S3 were modified by ZnFe-layered dioxides (LDO) with strong stability to obtain high catalytic performance composite. The successful preparation of ZnFe-LDO/MOF-In2S3 not only retains the advantages of large specific surface area of MOF-In2S3 and the stability of ZnFe-LDO, but also improves its charge separation efficiency and the light absorption region through the construction of heterojunctions. As expected, the experimental results show that ZnFe-LDO/MOF-In2S3 has superior photo-electrocatalytic properties. The photocatalytic degradation efficiency of ZnFe-LDO/MOF-In2S3 for TCH and OTC reach 92.92% and 80%, respectively, within 25 min. And the corresponding degradation rate constants are up to 1.41332 and 0.45451 L·mg−1·min−1, respectively, which are accord with the pseudo-second-order kinetic constants. The intermediate of 0.6LDO/M-I degradation of TCH was proposed by liquid mass spectrometry (LC-MS). Notably, a special transient surface photovoltage (TPV) technique was used for discussing the charge transfer kinetics and the lifetime of photogenerated carriers. Based on Mott-Schottky (M-S), Ultraviolet-visible absorption spectrum (UV), X-ray Photoelectron Spectroscopy (XPS) results, electron paramagnetic resonance (EPR) results, and active substance capture experiment results, a Z-scheme heterojunction system was proposed. The above research work proposed a new approach to enhance the stability of catalytic materials by using the aid of layered dioxides. In addition, the characterization of surface photovoltage provides a new way to study charge transfer.

Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI3 Perovskite Solar Cells.

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation.

Visible light-assisted efficient degradation of Rhodamine B by N-TiO2/Mn-HPMo/Ag ternary composites

In this paper, Ag nanoparticles were deposited on the surface of nitrogen-doped titanium dioxide coupled with manganese-substituted phosphomolybdic acid composites (N-TiO2/Mn-HPMo) using photoreduction method to construct ternary photocatalysis-Fenton composite catalyst (N-TiO2/Mn-HPMo/Ag), and the degradation performance of the catalyst was evaluated with Rhodamine B (RhB) as the target pollutant. The results showed that the N-TiO2/Mn-HPMo/Ag ternary composite catalyst had remarkable visible light activity, and the degradation efficiency of RhB could reach 96.3% in 20 min under visible light. Meanwhile, UV–vis diffuse reflectance spectroscopy illustrated the promotion effect of Ag deposition on the visible light response of N-TiO2/Mn-HPMo. The behavior of photogenerated charges was also characterized by Surface Photovoltage Technique, which further demonstrated that Ag deposition not only extended the light absorption range but also enhanced the separation efficiency of photogenerated charges. This work provides favorable information for the construction of efficient catalysts with high activity under visible light.

Visualization of charge carriers in photocatalysts

Surface photovoltage techniques combined with time-resolved spectroscopy methods provide an effective way to visualize the charge transfer dynamics in photocatalytic reactions.

Fabrication of Photostable Siloxane-Coated Quantum Dots for White-Light-Emitting Diodes

Quantum dots (QDs) are considered as excellent candidate materials for light-emitting diode (LED)-based lighting and display applications due to their excellent optical properties. However, the intrinsic unstable properties of QDs under external environments or the incompatibility between QDs and polymer composites hindered the QD-related applications. Herein, a universal modification method based on the optimization of QDs was developed for device applications. The highly dispersible and photostable QDs/siloxane composite was constructed by using the dual silica-encapsulated strategy of core/multishell CdSe/CdS/ZnCdS QDs to improve the performance of QDs. First, core/multishell QD’s size and components were optimized to enhance stability. Then, QDs@SiO2-OTMS was prepared by a combination of the reverse micro-emulsion method and surface modification. Such a double silica surface modification strategy is found to be very effective in improving the dispersion by matching the surface energy of QDs@SiO2-OTMS with that of siloxane resins. The as-prepared QDs@SiO2-OTMS/siloxane films had long-term (10 days) stability against heat and moisture and corrosive solution (HCl and NaOH). For the first time, the surface photovoltage (SPV) technique was used to study the in situ thermal stability of QDs@SiO2-OTMS, and it was found that surface defects were greatly suppressed at high temperatures. Taking these advantages, a white LED (WLED) with a color coordinate of (0.336, 0.325), color temperature of 5287 K, color gamut 123% National Television Systems Committee (NTSC,1931), and long-time stability at 20 mA had been fabricated by integrating green and red QDs@SiO2-OTMS/siloxane. These results make QDs@SiO2-OTMS/siloxane excellent candidates for QD lighting and display applications.

Charge reaction kinetics on TiO2 nanotubes under photoelectrochemical water oxidation condition

Artificial photosynthesis relies on the photoreactions between the chemical molecules and photogenerated charges. The oxygen evolution reactions are regarded as the most difficult procedure, because they are multiple-step reactions with multiple charges transfer. Notwithstanding the extensive study on photoelectrochemical and photochemical water oxidation, understanding how the surface charge reacts with the chemical molecules is limited, especially the reaction kinetics of the surface charges. We prepared anatase TiO2 nanotubes array and dense polycrystalline TiO2 films on Ti substrates, where the photocurrent increased about 4–4.5 times on nanotubes compared with the dense films. Moreover, the surface charge reaction rate and the accumulated charge densities were analyzed by transient surface photovoltage and transient photocurrent techniques. It was found a 2nd-order charge reaction behavior could be found when the charge density was high, differently, a 1st-order charge reaction could be observed when the charge density was low. The nanotubes structures would in situ grow from the Ti substrates, which simultaneously enhanced charge separation and transfer kinetics to improve the PEC performance.

(Invited) Metallic Impurity Control in Silicon Processing

In this paper we review the performances of a few techniques for the analysis of metal contamination in silicon. A few case studies are discussed to compare the ability of these techniques in detecting contaminants with different properties as silicon impurities. Common contaminants and elements recently introduced in the fabrication process are considered.TXRF (Total reflection X Ray Fluorescence), recombination lifetime measurement techniques, DLTS (Deep Level Transient Spectroscopy) and micro-photoluminescence analyses are compared. The results reported in this paper show that it is not possible to define a unique recipe that can be applied in all cases. Different approaches are required depending, on one hand, on the contaminant diffusivity and solubility, and, on the other hand, on the process step to be monitored.The techniques for monitoring metal contamination can schematically be divided into two main groups, one consisting of the techniques that measure the chemical concentration at the silicon surface and the other of the techniques that measure the electrical activity of the contaminant in the silicon volume.Mostly in case of metal contamination detection during wafer manufacturing, techniques which intentionally deposit thin films of polysilicon or thermal oxide and then perform chemical etching of them associated to ICP-MS analysis are also used, mostly addressed to detect fast diffusers like Cu and Ni [1], not always associated to clear electrical response. Indeed, these elements retain significant diffusivity even near room temperature, where the solid solubility is negligibly low, so they mostly segregate at the silicon surface or in getter sites, and for this reason they are hardly revealed in the silicon volume. However, these elements can be revealed by the recovery from deep depletion of MOS capacitors, because the segregated metals significantly increase the surface generation velocity.Some medium-fast diffusers like iron diffuse through several hundred microns during an ordinary thermal treatment, and mostly remain in the solid solution in silicon [1]. A comparison was carried out between TXRF and carrier recombination lifetime measurement for their ability to reveal iron contamination after a cleaning process. To prepare the samples for carrier lifetime measurements, the samples had to be thermally treated by RTP (Rapid Thermal Process). This study showed that TXRF and recombination lifetime measurements have essentially the same sensitivity to iron contamination. Vice versa, the detection of slow diffusers is found to be critical, because a very low concentration per unit area (≈107cm-2) may result in a non-negligible volume concentration in the device region (≈1010cm-3). As a consequence, the sensitivity per unit area required for these elements is difficult to reach with surface techniques such as TXRF. Molybdenum and tungsten [1] are examples of this sort. These elements do not diffuse deep enough to be efficiently revealed by recombination lifetime measurements, but they are easily revealed in the silicon volume by DLTS, which is probably the best approach for these elements, though it requires a considerable sample preparation. On the other hand, these elements are very common contaminants in ion implantation or epitaxial processes, and are very harmful for devices sensitive to carrier recombination, such as image sensors. Therefore, a technique able to detect near-surface contamination with no or limited sample preparation would be needed. Band-to-band micro-photoluminescence proved to be a good candidate, though DLTS still has the best sensitivity.Silicon contamination may also happen in the final part of the device process flow when the wafer frontside is protected by several dielectric layers and low-temperature thermal treatments only are allowed. In that case, contamination may come from the wafer backside only, but fast diffusers are still able to reach the wafer frontside and damage the device. In a study of palladium contamination from a contaminated chuck, TXRF proved not to be sensitive enough to palladium contamination. Palladium is best revealed by recombination lifetime measurements, such as the SPV (Surface PhotoVoltage) technique, but a high temperature treatment is required to activate it, so also in this case some sample preparation is required.Finally, we recall that the device itself can be a very sensitive monitor of metal contamination [2], though of course methods that prevent the impact of contamination of the device are generally preferred. The analysis of the dark current in image sensors not only reveals the presence of metal contamination (see the enclosed figure), but also provides some hints about the contaminant element.

Unraveling Charge‐Separation Mechanisms in Photocatalyst Particles by Spatially Resolved Surface Photovoltage Techniques

AbstractThe photocatalytic conversion of solar energy offers a potential route to renewable energy, and its efficiency relies on effective charge separation in nanostructured photocatalysts. Understanding the charge‐separation mechanism is key to improving the photocatalytic performance and this has now been enabled by advances in the spatially resolved surface photovoltage (SRSPV) method. In this Review we highlight progress made by SRSPV in mapping charge distributions at the nanoscale and determining the driving forces of charge separation in heterogeneous photocatalyst particles. We discuss how charge separation arising from a built‐in electric field, diffusion, and trapping can be exploited and optimized through photocatalyst design. We also highlight the importance of asymmetric engineering of photocatalysts for effective charge separation. Finally, we provide an outlook on further opportunities that arise from leveraging these insights to guide the rational design of photocatalysts and advance the imaging technique to expand the knowledge of charge separation.

Unraveling Charge-Separation Mechanisms in Photocatalyst Particles by Spatially Resolved Surface Photovoltage Techniques.

The photocatalytic conversion of solar energy offers a potential route to renewable energy, and its efficiency relies on effective charge separation in nanostructured photocatalysts. Understanding the charge-separation mechanism is key to improving the photocatalytic performance and this has now been enabled by advances in the spatially resolved surface photovoltage (SRSPV) method. In this Review we highlight progress made by SRSPV in mapping charge distributions at the nanoscale and determining the driving forces of charge separation in heterogeneous photocatalyst particles. We discuss how charge separation arising from a built-in electric field, diffusion, and trapping can be exploited and optimized through photocatalyst design. We also highlight the importance of asymmetric engineering of photocatalysts for effective charge separation. Finally, we provide an outlook on further opportunities that arise from leveraging these insights to guide the rational design of photocatalysts and advance the imaging technique to expand the knowledge of charge separation.

Surface Photo-Voltage (SPV) and Photo electrochemistry (PEC) on TiO2

Surface charge transfer processes at semiconductors have been of great importance as the quality of the material is vital in various applications such as electronics, photovoltaics, catalysis, electrocatalysis, photocatalysis, battery, supercapacitors to name but a few. Surface Photo-voltage (SPV) is a unique and simple technique based on the spectral dependence of the photovoltage as a function of photon energy applied as non-destructive and contactless method. It has been applied for the characterization of semiconductors such as band gap, surface potential, doping concentration, especially the minority carrier lifetime and diffusion length correlating to size, shape, thickness etc. Its simplicity including sample preparation, contacts, junction formation, etc. is an advantage over several other spectroscopic methods since it can offer information on submerged heterostructure layers.We will present characterization of several size and shapes of TiO2 (thin films, nanorods, etc.) towards its hole diffusion length using SPV (solid/solid) and compared with photoelectrochemical (PEC) technique (solid/liquid). We will point out the application of SPV and PEC on TiO2 coated LiCoO2, for possible insight on the stability and increased capacity in LIB (lithium-ion battery) from a thermodynamic and kinetic point of view. We will discuss the application of SPV technique in understanding the mechanism related to decreased surface oxygen activity and change in the activation energy of Li ion transfer.

Performance Investigation of the lifetime and efficiency of solar cell using surface photovoltage (SPV) method

In this paper the optical characterisation of mono crystalline fabricated solar cells has been investigated focusing on the minority carrier flow using surface photovoltage (SPV) technique and the efficiency of the solar cells has been evaluated by means of the sun simulator analysis. A computer-controlled incidence measurement system was designed for SPV measurements based on a mini mono-chromator, which was driven by a stepper motor to various wavelengths in the spectral range of 400–1200 nm. SPV is typically measured using Standard Research 510 lock-in amplifiers. A LabVIEW interface is employed for system control and data acquisition. Every step of the measurement has been described in detail throughout the whole experimental procedure. After calculating the experimental data obtained from monocrystalline silicon solar cell measurement, the length ( L ) of the minority carrier diffusion and lifetime (?) were calculated and results derived were 92 µm and almost 3.135 µs respectively. Typically in the laboratory experiment the efficiency is found in the range of 8–12% but with our experimental data we have successfully found the efficiency as 16.05% using the Sun Simulator K3000 LAB55. The results from the experimental data has promising application to assess the quality of the solar cells that are being used.

Combination of μW-PCD and SPV techniques for bulk and surface defects densities measurements

In semiconductor applications, surface and interface defects play a key-role on the electronic properties of the device. Several non destructive tools can be combined to characterize these defects. From microwave-Photo-Conductivity Decay (μW-PCD) measurements, one can access to an effective measurement of lifetime of minority carriers that characterizes both the surface and the bulk semiconductor material. On the other hand, Surface Photo-Voltage (SPV) is a powerful technique that gives credible bulk lifetime values from minority carriers’ diffusion length measurements. In this paper, we use a nondestructive method by which we determine the surface and bulk defects densities in crystalline silicon wafers by combining μW-PCD and SPV tools using an adequate theoretical approach. We apply this method to three surface morphologies; a mirror-polished sample, a CP4 etched sample and a pyramidal textured sample.

Determining limitations of capacitance-voltage measurements of built-in voltage as an alternative to surface photovoltage for a-Si:H/c-Si heterojunctions

Strong band bending within crystalline silicon is important for improving carrier selectivity in the contact regions of silicon heterojunction solar cells. In this work, we compare the use of capacitance-voltage as an alternative to the surface photovoltage method for probing the built-in voltage associated with crystalline silicon band bending. The surface photovoltage technique is incapable of measuring structures with non-transparent contacts, while the capacitance-voltage technique shows saturation at high built-in voltages due to the formation of an inversion layer. The shift in built-in voltage and effect on open circuit voltage are demonstrated for variation across a range of amorphous Si p-doping levels. These experimental results are supported by simulation work, confirming the underlying band structure and carrier concentrations of photovoltaic Si heterojunctions and clarifying the limitations of capacitance measurements at high built-in voltages. Errors arising in capacitance-voltage measurements of these structures are explained and linked to deviations from the Mott-Schottky model approximations of a one-sided junction in full depletion.

Ultrathin nanoflake-assembled hierarchical BiOBr microflower with highly exposed {001} facets for efficient photocatalytic degradation of gaseous ortho-dichlorobenzene

A series of BiOBr catalysts with different microstructures were synthesized by tuning the solvothermal conditions. Assisted by the surface photovoltage, transient photovoltage and time-resolved photoluminescence techniques, the photoexcited charge carriers separation and transfer dynamics were comparatively investigated. The results indicated that the hierarchical BiOBr microflower (BiOBr MF) consisting of well-organized ultrathin nanoflakes were most efficient in promoting charge carrier separation and migration, resulting from the nearly 100 % {001} facets exposed with more oxygen defects. Furthermore, the photocatalytic performance was estimated through the degradation of gaseous ortho-dichlorobenzene (o-DCB) under visible light irradiation, and the BiOBr MF exhibited superior photoactivity. The enhanced mechanism underlying the charge transfer was disclosed by the energy band structures and the reactive oxygen species which were examined by room temperature electron spin resonance. In addition, the catalytic oxidation process of o-DCB over the BiOBr MF and the corresponding surface intermediates was revealed by in situ FTIR spectroscopy.

Hybrid photovoltaic-thermoelectric structure based on multilayer combinations of Cr and Fe oxides

Cr and Fe oxide-based multilayer structures with enhanced photovoltaic properties were prepared by a reactive pulsed laser deposition on Si(100) substrate. Their charge carriers’ transport properties were studied by surface photovoltage technique. The results reveal that optimal bilayer structure grown on a cold substrate exhibits a photovoltage enhancement that corresponds to the high separation rate of photo-induced charges. Integral responsivity in the spectral region 360–1100 nm is about 500 V/W. A synergetic effect of thermoelectric and photovoltaic phenomena in multilayer combinations of oxides is discussed.

(Invited) Surface Photovoltage Analysis of Photocatalytic Materials

A brief review of surface photovoltage (SPV) techniques and their application to photocatalytic materials is given. SPV is defined by the light-induced change of the contact potential difference (∆CPD) between two electrodes, i.e. SPV signals can be obtained whenever photogenerated charge carriers are separated in space [1]. SPV signals depend on processes of light absorption, photogeneration, directed charge transfer, charge transport and recombination. Therefore, SPV techniques can be well applied to the analysis of electronic, transport and recombination properties of photoactive materials [2].Basics of SPV are introduced including the relaxation of a disturbed space charge, the center of charge approach and mechanisms of charge separation. Principles of SPV measurements will be given and illustrated with applications. Depending whether fixed capacitors, Kelvin probes, electron beams or photoelectrons are applied (see also figure 1), SPV signals can be measured at crystals, layers or powders of photocatalysts with high sensitivity over wide ranges in time or high resolution in space and in different ambient (gases, vacuum, electrolytes). In-operando SPV measurements are realized.For measurements with fixed capacitors, SPV signals are coupled out directly with a high-impedance buffer. Due to the discharge of the measurement capacitor via a very large measurement resistance, the application of fixed capacitors is limited to transient [3] and modulated [4] SPV measurements. With fixed capacitors, the widest range in time ((sub)ns...ms...h, transient, example: nano-composite of TiO2/In2S3 [5]) and the highest sensitivity (10...100 nV, modulated, see [6] for defect analysis in (FA0.85MA0.1Cs0.05)PbBr3) are reached whereas no additional interactions are involved. Attention will be paid to the application of transient SPV spectroscopy [7] at constant photon flux for the investigation of defect states and charge transfer across interfaces [8] (c-Si(n++)/TiO2). Furthermore, principles of random walk simulations for the modeling of transient [9] and modulated [10] SPV signals in small systems will be discussed [2].With Kelvin probes, the ac current between a sample and a vibrating electrode (vibrating electrode [11], most applied technique, see, for example [12]) or the electrostatic force between the sample surface and a vibrating tip electrode (Kelvin probe force microscopy [13] [14], KPFM) are nulled by applying an external bias potential. The highest resolution in space is reached for SPV measurements with KPFM (tens of nm). Examples will be shown for SPV measurements with vibrating electrodes for TiO2 [15] and KPFM for Cu2O [16] and BiVO4 with co-catalysts [17].In SPV measurements with electron beams [18, 19], a low electron current is kept constant with a compensating bias potential. In photoelectron spectroscopy, core levels shift with regard to ∆CPD. Additional interactions of the sample with electrons and/or x-rays are possible. The application of synchrotron radiation allows for SPV measurements at extremely short times [20, 21]. Further opportunities opened for SPV measurements by ambient pressure photoelectron emission spectroscopy [22].[1] review of L. Kronik, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1.[2] Th. Dittrich, S. Fengler, Surface photovoltage analysis of photoactive materials, World Scientific (2019), ISBN: 978-1-78634-765-7.[3] E. O. Johnson, J. Appl. Phys. 28 (1957) 1349.[4] V. Duzhko, et al., Phys. Rev. B 64 (2001) 075204.[5] Th. Dittrich, et al., Rev. Sci. Instrum. 88 (2017) 053904 and 90 (2019) 026102.[6] I. Levine, et al., ACS Energy Lett. 4 (2019) 1150.[7] Th. Dittrich, et al., Appl. Phys. Lett. 110 (2017) 023901.[8] S. Fengler, et al., submitted.[9] S. Fengler, et al., J. Phys. Chem. C 117 (2013) 6462.[10] S. Fengler, Th. Dittrich, J. Phys. Chem. C 120 (2016) 17777.[11] K. Besocke, S. Berger, Rev. Sci. Instrum. 47 (1976) 840.[12] M. A. Melo et al., Nano Lett. 18 (2018) 805.[13] M. Nonnenmacher, et al., Appl. Phys. Lett. 58 (1991) 2921.[14] T. Glatzel, S. Sadewasser (edts.) Kelvin probe force microscopy – from single charge detection to device characterization, Springer, Cham. (2018).[15] M. K. Nowotny, et al., Mater. Lett. 64 (2010) 928.[16] R. Chen, et al., Nano Lett. 19 (2019) 426.[17] J. Zhu, et al., Nano Lett. 17 (2017) 6735; R. Chen, et al., Chem. Soc. Rev. 47 (2018) 8238.[18] F. Steinrisser, R. E. Hatrick, Rev. Sci. Instrum. 42 (1971) 3014.[19] J. Clabes, M. Henzler, Phys. Rev. B 21 (1980) 625.[20] W. Widdra, et al., Surf. Sci. 543 (2003) 87.[21] S. Yamamoto, I. Matsuda, J. Phys. Soc. Jpn. 82 (2013) 021003.[22] J. R. Harwell et al., Phys. Chem. Chem. Phys. 18 (2016) 19738. Figure 1

Photo-Induced Charge Transfer on Pt/Bi₂WO6 Composite Photocatalysts.

Pt/Bi₂WO6 composite photocatalysts were prepared by a facile photoreduction method. Pt nanoparticles with an average size of 5-8 nm were successfully deposited on the surface of Bi₂WO6 microspheres and the photocatalytic activity of Bi₂WO6 was greatly improved by Pt nanoparticles. The photo-induced charge transfer properties of samples were studied by means of surface photovoltage (SPV) and transient photovoltage (TPV) techniques, giving an insight into the intrinsic reasons of the improvement in photocatalytic activity. The SPV and TPV results revealed that the deposited Pt nanoparticles could trap photo-induced electrons and then largely enhance the separation efficiency of photo-induced charge carriers.

ZnO nanocrystal-coated MoS2 nanosheets with enhanced ultraviolet light gas sensitive activity studied by surface photovoltage technique

In this paper, ZnO nanocrystals and MoS2/ZnO composites were fabricated by a simple sol-gel method and ultrasonic concussion. Several characterization techniques including XRD, TEM, XPS, Raman spectroscopy and surface photovoltage were employed to explore the samples. The doping of MoS2 nanosheet into the ZnO nanocrystal composites was studied for its effect on NO2 gas-sensitivity under UV irradiation at room temperature. Compared to gas sensors developed from pure ZnO, those developed from ZnO/MoS2 exhibited higher sensitivity with a reversible response. Surface photovoltage results suggest that these desirable sensing features can be attributed to MoS2/ZnO heterojunctions and an abundance of diffusion channels which reduce the energy barrier to promote ready transference and target gas adsorption/desorption. These results offer new opportunities to fabricate composite materials for gas sensors.

Numerical simulation and validity of the surface photovoltage method in amorphous silicon with a Schottky contact

The steady-state surface photovoltage technique (SPV) is widely used to evaluate minority photocarriers diffusion length for the characterization of photovoltaic device performance. This contact technique shows the difficulty to evaluate the true minority photocarriers diffusion length (Lp) of hydrogenated amorphous silicon (a-Si:H) even though the space charge region is reduced. In this work, we propose a formula to predict more accurately minority diffusion length Lp depending on apparent diffusion length (Lapp) and space charge width (W). This formula is deduced by Moore’s analytical solution in relation to other forms. All transport phenomena taken into account, a complete one-dimensional numerical simulation of the SPV on an Au/a-Si:H Schottky structure is developed. The apparent diffusion length Lapp and the space charge width W are determined using SPV simulation. The standard density of states (DOS) of the a-Si:H is used. Poisson’s equation and the two continuity equations for charge carriers are numerically solved. The SPV assumptions involving W are explored for a set of parameters of the a-Si:H DOS. This numerical simulation shows a linear dependence of Lapp up on the valence band tail energy, which agrees well with the experimental results. The Lp values obtained by the new formula are compared to Lapp and discussed for a typical DOS parameters of a-Si:H.