Surface Photovoltage Measurements Research Articles

Determination of band tail widths in MOCVD grown InGaN single layer within GaN based p-i-n LED structure through photo-induced measurements

Within a p-i-n LED structure, energy band gaps of MOCVD grown %10 In content InGaN and GaN layers were obtained through traditional UV–Visible transmission-absorption measurements and utilizing surface photovoltage (SPV) and spectral photoconductivity (SPC) measurements. Moreover, all applied techniques determined that the total band offset between InGaN and GaN layers were around 0.3–0.4 eV. Furthermore, the temperature dependence of band gaps of each InGaN and GaN layer was analyzed through a modified Varshni relation where both Varshni coefficients and band tail energy width were explored. Band tails in a modified Varshni relation, SPV and SPC measurements were in agreement with each other and originated Franz-Keldysh Effect attributed to the presence of photon assisted tunneling.

Bulk Photovoltage Effect in Ferroelectric BaTiO3.

Due to the unusual charge separation mechanism and anomalous photovoltaic effects, the bulk photovoltage effect in ferroelectric semiconductors has attracted a great deal of attention in solar energy conversion, especially in attempts to utilize nonthermalized carriers. Among the various mechanisms that have been proposed for interpreting the photovoltaic effect, a shift mechanism was derived from quantum phenomena, which have been modeled and studied for many years. However, the concurrent shift and ballistic mechanism make investigating the ferroelectric bulk photovoltage effect complex and challenging. Here, taking a tetragonal ferroelectric BaTiO3 single crystal as a prototype, we report an approach for distinguishing the shift and ballistic mechanism-induced surface photovoltage. The results indicate different effects on the charge separation of the ballistic mechanism and shift mechanisms, as evidenced by surface photovoltage measurement. Interestingly, the shift and ballistic mechanisms afford charge separation in two opposite directions but on the same order of magnitude under monochromatic superband illumination. Our results provide facile and efficient methods for clarifying the shift and ballistic mechanisms in ferroelectrics.

Spatiotemporal imaging of charge transfer in photocatalyst particles.

The water-splitting reaction using photocatalyst particles is a promising route for solar fuel production1-4. Photo-induced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency5; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometres to micrometres and from femtoseconds to seconds6-8. Although the steady-state charge distribution on single photocatalyst particles has been mapped by microscopic techniques9-11, and the charge transfer dynamics in photocatalyst aggregations have been revealed by time-resolved spectroscopy12,13, spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and their exact mechanism is unknown. Here we perform spatiotemporally resolved surface photovoltage measurements on cuprous oxide photocatalyst particles to map holistic charge transfer processes on the femtosecond to second timescale at the single-particle level. We find that photogenerated electrons are transferred to the catalytic surface quasi-ballistically through inter-facet hot electron transfer on a subpicosecond timescale, whereas photogenerated holes are transferred to a spatially separated surface and stabilized through selective trapping on a microsecond timescale. We demonstrate that these ultrafast-hot-electron-transfer and anisotropic-trapping regimes, which challenge the classical perception of a drift-diffusion model, contribute to the efficient charge separation in photocatalysis and improve photocatalytic performance. We anticipate that our findings will be used to illustrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts.

Modelling of Kelvin probe surface voltage and photovoltage in dielectric-semiconductor interfaces

The characterisation of dielectric-semiconductor interfaces via Kelvin probe surface voltage and photovoltage has become a widespread method of extracting the electrical properties influencing optoelectronic devices. Kelvin probe offers a versatile, contactless and vacuum-less technique able to provide useful insights into the electronic structure of semiconductor surfaces. Semiconductor theory has long been used to explain the observations from surface voltage measurements, often by making large assumptions about the characteristics of the system. In this work I report an updated theoretical treatment to model the results of Kelvin probe surface voltage and photovoltage measurements including four critical mechanisms: the concentration of charge stored in interface surface states, the charge stored in different locations of a surface dielectric thin film, the changes to effective lifetime and excess carrier density as a result of charge redistribution, and the non-uniformity of charge observed on most large scale thin film coatings used for passivation and optical improvement in optoelectronic devices. A full model is drawn and solved analytically to exemplify the role that these mechanisms have in surface voltage characterisation. The treatment in this work provides crucial understanding of the mechanisms that give rise to surface potential in semiconductors. As such this work will help the design and development of better optoelectronic devices.

Direct measurement of surface photovoltage by AC bias Kelvin probe force microscopy.

Surface photovoltage (SPV) measurements are a crucial way of investigating optoelectronic and photocatalytic semiconductors. The local SPV is generally measured consecutively by Kelvin probe force microscopy (KPFM) in darkness and under illumination, in which thermal drift degrades spatial and energy resolutions. In this study, we propose the method of AC bias Kelvin probe force microscopy (AC-KPFM), which controls the AC bias to nullify the modulated signal. We succeeded in directly measuring the local SPV by AC-KPFM with higher resolution, thanks to the exclusion of the thermal drift. We found that AC-KPFM can achieve a SPV response faster by about one to eight orders of magnitude than classical KPFM. Moreover, AC-KPFM is applicable in both amplitude modulation and frequency modulation mode. Thus, it contributes to advancing SPV measurements in various environments, such as vacuum, air, and liquids. This method can be utilized for direct measurements of changes in surface potential induced by modulated external disturbances.

Coupling of SiC and CeO2 nanosheets to enhance solar energy utilization and optimize catalytic ozonation

The coupling of solar energy conversion and catalytic ozonation is an effective way to eliminate volatile organic compounds with high-efficiency and low-energy-consumption. In this work, we presented a non-noble-metal SiC/CeO2 system for solar-light-assisted catalytic ozonation of toluene. Temperature and light-controlled experiments and atmosphere surface photovoltage measurement were conducted to evaluate photothermal and photoelectric processes. Results show that toluene removal and mineralization efficiencies of 25%SiC-CeO2 were 69.7% and 67.1%, respectively, under full-spectrum illumination with an hourly space velocity of 6000 /h. The SiC enhanced the conversion of incident visible and infrared (IR) light to heat, thereby increasing the reaction temperature over 25%SiC-CeO2. Moreover, SiC transferred electrons to CeO2 in 25%SiC-CeO2, generating more surface Ce3+ sites and promoting charge carriers separation efficiency. Given these characteristics, 25%SiC-CeO2 exhibited outstanding performance in toluene mineralization, even better than Pd-loaded CeO2.

Exploration of sub-bandgap states in 2D halide perovskite single-crystal photodetector

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA2PbBr4) single crystals with optical bandgap of ~413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA2PbBr4. As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA2PbBr4 single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA2PbBr4 single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

Constructing Anatase-Brookite TiO2 Phase Junction by Thermal Topotactic Transition to Promote Charge Separation for Superior Photocatalytic H2 Generation.

Phase junctions of photocatalysts can promote the separation of photogenerated charge carriers for efficient utilization of the carriers. Construction of phase junctions and establishing their structure-performance relationship are still required. Herein, polycrystalline TiO2 decahedral plates with different phases were synthesized by thermal treatment-induced topotactic transition of titanium oxalate crystals. The phase of TiO2 evolved from pure anatase to anatase-brookite, anatase-brookite-rutile, and then to anatase-rutile, while the morphology of the decahedral plates was well maintained. The biphase anatase-brookite was found to be most efficient in photocatalytic hydrogen generation. Specifically, the hydrogen generation rate of the biphase anatase-brookite TiO2 was nearly 2.4 times greater than that of the biphase anatase-rutile TiO2. The spatially resolved surface photovoltage measurements indicate the more efficient separation of photogenerated charge carriers and thus greater photocatalytic activity of the former. This work provides a strategy for developing efficient phase-junction photocatalysts.

Engineering Interfacial Band Bending over ZnIn2S4/SnS2 by Interface Chemical Bond for Efficient Solar‐Driven Photoelectrochemical Water Splitting

AbstractDeveloping a simple and effective strategy to modulate the energy band bending of heterojunction photoelectrodes is pivotal in terms of photoelectrochemical (PEC) water splitting. Herein, it is demonstrated that the introduction of the interfacial InOSn chemical bonds at the ZnIn2S4/SnS2 interface regulates the band bending of ZnIn2S4/SnS2 heterojunction photoanodes, reverses the charge transport direction, and reduces the oxygen evolution reaction (OER) overpotential. Detailed analysis indicates that the interfacial InOSn bond makes band adaptation to promote carrier separation and transfer through ultraviolet photoelectron spectrometry, hydroxyl radical production tests, and surface photovoltage measurements. Due to the special nanosheet morphology with exposed edges of the heterojunction interface, the InOSn bonds are partially exposed, which can reduce the OER overpotential and boost the surface injection efficiency according to the PEC impedance spectroscopy and density functional theory calculations. The synergistic modulation of InOSn bond yields a photocurrent of 4.57 mA cm−2 at 1.23 V (vs reversible hydrogen electrode, AM 1.5 G) and a low onset potential of −0.14 VRHE. This work provides a new solution for energy band regulation to improve the performance of heterojunction photoelectrodes for PEC water splitting.

Kelvin probe force microscopy for material characterization.

Kelvin probe force microscopy is a scanning probe method for imaging the surface potential by atomic force microscopy. The surface potential is one of the most important surface properties and is correlated to e.g. the work function, surface dipoles, localized surface charges and structural properties. It gives detailed information on the electrical properties and can be combined with optical and electrical excitation mechanisms providing additional properties like surface band bending and charge carrier mobilities. We will introduce the main concept and will briefly describe the major methods of operation. Based on the analysis of a Si superjunction device, structures dopant profiling and the concept of surface photovoltage measurements will be introduced. The influence of local charge accumulation on these devices will be presented and the effect on the measured contact potential values will be discussed.

Wavelength dependent photoinduced charge carrier dynamics of heterojunction materials: The case of CuO/ZnO

Heterojunction materials composed of narrow and wide band-gap semiconductors possess different photoelectric responses under different wavelength incident light, and show distinct photogenerated charge carrier dynamics. In this work, taking the hydrothermal method fabricated CuO nanosheets / ZnO nanorods heterojunction materials as example, the wavelength dependent photoinduced charge carrier dynamics in this heterojunction material have been carefully studied. Transient surface photovoltage (TSPV) measurements reveal that the photogenerated holes gather at the surface of the fabricated CuO / ZnO with 355 nm light illumination, but photogenerated electrons gather at the surface under 532 nm light. Photocatalytic degradation experiments suggest that CuO / ZnO heterojunction material has better performance under ultraviolet light than that under visible light.

Surface photovoltage measurement of PM10 atmospheric aerosols collected over SRMIST-Kattankulathur campus (12.81° N & 80.03° E): a step towards utilization of atmospheric aerosols in optoelectronic applications

Surface photovoltage measurement of PM10 atmospheric aerosols collected over SRMIST-Kattankulathur campus (12.81° N & 80.03° E): a step towards utilization of atmospheric aerosols in optoelectronic applications

Charge Transfer in Self-Assembled Fullerene-Tetraphenylporphyrin Non-Covalent Multilayer

Self-assembly of organic molecules is a promising method for generating multilayer systems for fabrication of functional devices. In particular, fullerene (C60) and porphyrin molecules offer a variety of binding modes, including π–π interactions, dipole electrostatic attraction, and hydrogen bonding, to tailor the charge separation and charge recombination limiting device performance. Here, we investigate multilayer systems obtained by the sequential physical vapor deposition of C60 and tetraphenylporphyrin (H2TPP) layers, focusing on the effect of the interfaces on the charge transfer processes. Absorbance spectra indicate noncovalent-like π-stacking, with the increment of fullerene interfaces shifting the porphyrin Soret band toward the blue. Similarly, surface photovoltage measurements in the multilayer systems show that as the number of interfaces increases, so does the photogeneration of charge. Charge separation follows carrier generation given that the recombination time, associated to trap states, decreases. This behavior indicates that the Donor-Acceptor nature of the fullerene-porphyrin bilayer system is conserved, and even enhanced in the multilayer film, and that the number of interfaces aids to the formation of selective paths for charge carrier collection, demonstrating its potential in optoelectronic devices.

Unveiling the Influence of the Spectral Irradiance of Indoor Light‐Emitting Diodes on the Photovoltaics of a Methylammonium Lead Iodide‐Based Device

An understanding of the spectrum–property relationship of perovskite solar cells when illuminated by light‐emitting diodes that are used for indoor applications is necessary. Herein, it is aimed to explore the influences of correlated‐color temperatures on a MAPbI3‐based device under low‐light conditions. Given an irradiance of approximately 3 W m−2 (or ≈1000 lx), a maximum free carrier generation rate of 1.0 × 1021 m−3 s−1 was found. Additionally, power conversion efficiencies (PCEs) up to 31.97%, 30.36%, and 28.98% with maximum power outputs of 13.66, 13.02, and 16.09 μW could be reached at 3000, 4000, and 6500 K, respectively. Additional increases in the PCEs were observed when high‐energy blue light (in a range of 400–550 nm) was excluded during the current–voltage sweeps. In combination with the surface photovoltage measurements, intense blue light (under 6500 K) had a minimal influence on the photoinduced charge separation signals when compared to those caused by 3000 and 4000 K light. As a solar cell, the PCE reached as high as 34.52%, which corresponded to 73.08% of the thermodynamic limit of its bandgap at 3000 K.

Tunable visible light enhanced triethylamine adsorption on pH dependent ZnO nanostructures: An investigation by scanning Kelvin probe

In this work, we reported the effect of adsorption of volatile organic compounds on surface potential of zinc oxide nanostructures synthesized at four different pH (5, 7, 9, and 11) at 80°C about 45 min. For all zinc oxide nanostructures, the contact potential difference was determined by exposing ethanol, n-hexane, and triethylamine under dark and visible light in ambient air environment using scanning Kelvin probe with surface photovoltage module. X-ray diffraction studies revealed that the crystallite size decreased with an increase in pH. Field-emission scanning electron microscopy and high-resolution transmission electron microscopy studies showed that the shift in pH resulted in multiple morphologies ranging from dumbbells to nanocones decorated nanorods. Photoluminescence, Raman spectra and surface photovoltage measurements revealed the presence of defects (oxygen and zinc vacancies) that enhanced the photosensitivity of zinc oxide nanostructures in the visible region. Out of all samples, the zinc oxide nanostructures obtained at pH 9 and 11 have demonstrated ∼ 3 times enhanced photovoltage towards triethylamine.

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

Hydrothermally grown rutile titanium dioxide (TiO2) nanorod array photoanode with various nanorod lengths were investigated for understanding the charge carrier separation including hole diffusion length using Photoelectrochemistry (PEC) and surface photo voltage (SPV) measurements for their importance in photoconversion as photoelectrode for solar water splitting. Various lengths of TiO2 nanorods with lengths 1.35, 1.85, 2.5 and 4.5 micrometers were prepared and used in the present investigation. Front and back illumination in SPV and PEC Mott-Schottky plots were investigated and found to show significant difference in charge carrier separation. The hole diffusion length calculations using PEC was up to 200 nm and using SPV was in the order of 600-700nm.

Low-temperature LPE growth and characterization of GaAsSb layers for photovoltaic applications

In this paper, we present an original study on metamorphic GaAsSb layers grown by liquid phase epitaxy (LPE) on GaAs substrates for photovoltaic applications. To ensure a controlled growth of the layers with reproducible composition and properties a low-temperature (below 600 °C) variant of LPE was used combined with a precise choice of the technological conditions. This allowed obtaining layers with ∼8% Sb in the As sublattice. The crystal structure, composition, surface morphology, local arrangement and chemical bonding of Sb in the obtained layers were investigated by a variety of measurement methods including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Hall-effect measurements were performed in the temperature range of 80–300 K to clarify the influence of defects on the transport properties. Temperature-dependent photoluminescence spectra at low excitation power were measured to evaluate the optical quality and identify localized states in the grown layers. The optical absorption properties and the transport of the photogenerated carriers in the grown samples were investigated using surface photovoltage (SPV) spectroscopy at room temperature. The SPV measurements have shown that the long-wavelength photosensitivity of the GaAsSb layers is extended down to 1.2 eV.

Hollow InVO4 Nanocuboid Assemblies toward Promoting Photocatalytic N2 Conversion Performance.

The unique InVO4 mesocrystal superstructure, particularly with cubical skeleton and hollow interior, which consists of numerous nanocube building blocks, closely stacking by stacking, aligning by aligning, and sharing the same crystallographic orientations, is successfully fabricated. The synergy of a reaction-limited aggregation and an Ostwald ripening process is reasonably proposed for the growth of this unique superstructure. Both single-particle surface photovoltage and confocal fluorescence spectroscopy measurements demonstrate that the long-range ordered mesocrystal superstructures can significantly retard the recombination of electron-hole pairs through the creation of a new pathway for anisotropic electron flow along the inter-nanocubes. This promising charge mobility feature of the superstructure greatly contributes to the pronounced photocatalytic performance of the InVO4 mesocrystal toward fixation of N2 into NH3 with the quantum yield of 0.50% at wavelength of 385nm.

Surface Photovoltage Spectroscopy over Wide Time Domains for Semiconductors with Ultrawide Bandgap: Example of Gallium Oxide

A nonconventional approach is proposed for the measurement of surface photovoltage (SPV) signals over very wide ranges in photon energy and time. Regimes for AC, DC, and combined AC–DC measurements are defined and applied for the characterization of a β‐Ga2O3 crystal by transient and modulated SPV spectroscopy, spectroscopy in the mode of a Kelvin probe and single‐pulse SPV transients from 10 ns to 1000 s with the same electrode. Numerous electronic transitions are distinguished in β‐Ga2O3 depending on the measurement regime, the time response and the history of measurement. An accumulation of negative charge at the surface of β‐Ga2O3 is observed at long times independent whether the SPV signals are positive or negative before. The nonconventional approach of measuring SPV signals opens new opportunities for investigating electronic properties of semiconductors with ultrawide bandgaps and any other photoactive materials.

Fine structures of conduction and intermediate bands of NaBiO3•2H2O/NaBiO3•xH2O heterostructures investigated by surface photovoltage measurement with external bias

The fine structures of conduction and intermediate bands of semiconductors determine photoelectronic properties of devices, whose investigation is developed with surface photovoltage(SPV) measurement. With external bias, the differences of SPV intensities at 400 and 375 nm of NaBiO3•2H2O/NaBiO3•xH2O heterostructures increase with temperature and reach the maximum for the sample heated at about 110 °C. The SPV at about 470 nm NaBiO3•2H2O/NaBiO3•xH2O heterostructures (30 °C) is enhanced about 8.8 times after ethanol treatment. The results suggest that the possible excited state corrsponding to 3.1 eV (400 nm) above the conduction band bottom is better matching conduction band of NaBiO3•2H2O than that corresponding to 3.3 eV (375 nm). The intermediate band within band gap corresponding to 2.6 eV (470 nm) due to ethanol treatment inducing surface defects contributes to the enhanced SPV. The investigation suggests that the SPV measurement is a helpful tool to investigate the fine structures of conduction and intermediate bands of NaBiO3•2H2O/NaBiO3•xH2O heterostructures and related photovoltaic and catalytic materials.