Modulated Photocurrent Spectroscopy Research Articles

Enhancement in Photoelectrochemical Efficiency and Modulation of Surface States in BiVO4 through the TiO2 Outer Layer Using the Atomic Layer Deposition Technique

Owing to several important properties, BiVO4 is the promising photoanode material for the photoelectrochemical (PEC) water oxidation reaction; however, the poor charge transfer and transport and slow surface catalytic activity limit to achieve the expected high theoretical efficiency. In the present investigation, thin films of TiO2 have been formed over the BiVO4 photoanode surface using the atomic layer deposition technique (ALD). Films are formed at 5, 50, and 150 nm thicknesses, and the PEC performances have been evaluated. Enhancement in the performance has been observed upon inclusion of the ALD/TiO2 films over BiVO4. The performance has been observed to be higher in the case of 50 nm ALD/TiO2 films. The ALD/TiO2 films have several important roles in the enhancement in the PEC efficiency other than only the enhancement of stability through retardation of the photocorrosion process. The intensity modulated photocurrent spectroscopy measurements have been carried out for the selective evaluation of the different modes of charge transfer processes upon excitation of the photoanode materials on photoexcitation. The enhancement in the charge transfer rate constant (kt) over the relative retardation of the charge recombination rate constant (kr) indicated the specific roles of modulation of the surface states. The investigation revealed the significant scopes of the modifications of surfaces using ALD techniques to selectively modulate the surface electronic states of materials in enhancing catalytic efficiency.

Hydrothermal Synthesized CoS2 as Efficient Co-Catalyst to Improve the Interfacial Charge Transfer Efficiency in BiVO4

The bare surface of BiVO4 photoanode usually suffers from extremely low interfacial charge transfer efficiency which leads to a significantly suppressed photoelectrochemical water splitting performance. Various strategies, including surface modification and the loading of co-catalysts, facilitate the interface charge transfer process in BiVO4. In this study, we demonstrate that CoS2 synthesized from the hydrothermal method can be used as a high-efficient co-catalyst to sufficiently improve the interface charge transfer efficiency in BiVO4. The photoelectrochemical water splitting performance of BiVO4 was significantly improved after CoS2 surface modification. The BiVO4/CoS2 photoanode achieved an excellent photocurrent density of 5.2 mA/cm2 at 1.23 V versus RHE under AM 1.5 G illumination, corresponding to a 3.7 times enhancement in photocurrent compared with bare BiVO4. The onset potential of the BiVO4/CoS2 photoanode was also negatively shifted by 210 mV. The followed systematic combined optical and electrochemical characterization results reveal that the interfacial charge transfer efficiency of BiVO4 was largely improved from less than 20% to more than 70% due tor CoS2 surface modification. The further surface carrier dynamics study performed using an intensity modulated photocurrent spectroscopy displayed a 6–10 times suppression in surface recombination rate constants for CoS2 modified BiVO4, which suggests that the key reason for the improved interfacial charge transfer efficiency possibly originates from the passivated surface states due to the coating of CoS2.

Understanding the Carrier Dynamics of Complex Heterojunctions for Water Splitting through Wavelength-Dependent Intensity Modulated Photocurrent Spectroscopy

Metal oxide semiconductors are promising materials to be employed as photoanodes in photoelectrochemical (PEC) devices to drive the oxygen evolution reaction (OER) for the conversion of solar energy into chemical fuels. These materials need to efficiently absorb light, have good bulk transport properties to separate the photogenerated charges, and induce fast charge transfer from the electrode inside the solution. To this aim, the best selected materials are usually combined to form efficient heterojunction structures, often with the help of surface catalysts.The understanding of charge carrier dynamics in complex heterojunctions is of the utmost importance for the performance optimization of photoelectrochemical cells and “ operando” techniques are most valuable, because they provide information about the charge separation, bulk transport, charge transfer at each junction, and at the semiconductor-electrolyte interface of the system upon external stimuli, such as illumination, applied bias and chemical environment.In the presence of multiple competing processes and complex interfaces, which are often found in photoelectrodes for water splitting, the construction of an electrical model that describe the carrier dynamics under a light stimulus can be difficult, since the frequency response of the PEC system may depend on several interlaced resistive and capacitive contributions, which are sometimes difficult to separate or to interpret in an unambiguous fashion. The characterization of the relevant kinetic processes occurring at junctions and semiconductor/electrolyte interfaces can be effectively carried out by implementing wavelength-dependent Intensity Modulated Photocurrent Spectroscopy (WD-IMPS). This innovative approach allows to selectively probe different layer of the heterojunction and identify the electron transport properties in the bulk and at the interface.We employed this straightforward technique to study the carrier dynamics of a WO3/BiVO4/CoFe-Prussian blue heterojunction in a conventional three electrode cell for water splitting, and we identified interfacial recombination processes affecting the semiconductor heterojunction, as well as the positive contribution of the inorganic catalyst on the charge separation efficiency of the BiVO4 layer.[1] Herein, the proposed methodology is used to separate and address the role of each active component within complex interfaces coupled with different catalysts and in different electrolytic environments and represents a valuable tool for improving the understanding of dynamic processes relevant to PEC water splitting.

On the evolution of the intensity modulated photocurrent spectroscopy (IMPS) transfer function in quadrants (IV)-(I) or quadrants (II)-(III)

The use of ac light modulated techniques to unravel the interplay between charge transfer and recombination at the interface of illuminated semiconducting photoelectrodes is becoming steadily a standard approach. In particular, the classical theory of Intensity Modulated Photocurrent Spectroscopy considers several kinetic schemes to account (or not) for the presence of surface states, and their influence on the IMPS shape. However, it is just recently that the scientific literature has put in evidence the observation of negative values of the IMPS transfer function in photoelectrochemical cells and the evolution of IMPS not only in quadrants (IV)-(I), but also in quadrants (III)-(II). This article provides a short introduction to the main published work dealing with this intriguing feature of IMPS along with the different manners these have been addressed so far.

Intensity modulated photocurrent spectroscopy to investigate hidden kinetics at hybrid perovskite–electrolyte interface

The numerous assorted accounts of the fundamental questions of ion migration in hybrid perovskites are making the picture further intricate. The review of photo-induced ion migration using small perturbation frequency domain techniques other than impedance spectroscopy is more crucial now. Herein, we probe into this by investigating perovskite–electrolyte (Pe–E) and polymer-aqueous electrolyte (Po–aqE) interface using intensity modulated photocurrent spectroscopy (IMPS) in addition to photoelectrochemical impedance spectroscopy (PEIS). We reported that the electronic-ionic interaction in hybrid perovskites including the low-frequency ion/charge transfer and recombination kinetics at the interface leads to the spiral feature in IMPS Nyquist plot of perovskite-based devices. This spiral trajectory for the perovskite-electrolyte interface depicts three distinct ion kinetics going on at the different time scales which can be more easily unveiled by IMPS rather than PEIS. Hence, IMPS is a promising alternative to PEIS. We used Peter’s method of interpretation of IMPS plot in photoelectrochemistry to estimate charge transfer efficiency ({Q}_{ste}) from the Rate Constant Model. The {Q}_{ste} at low-frequency for Pe–E interface exceeds unity due to ion migration induced modified potential across the perovskite active layer. Hence, ion migration and mixed electronic-ionic conductivity of hybrid perovskites are responsible for the extraordinary properties of this material.

Intensity Modulated Photocurrent Microspectrosopy for Next Generation Photovoltaics.

In this report, a large-area laser beam induced current microscope that has been adapted to perform intensity modulated photocurrent spectroscopy (IMPS) in an imaging mode is described. Microscopy-based IMPS method provides a spatial resolution of the frequency domain response of the solar cell, allowing correlation of the optoelectronic response with a particular interface, bulk material, specific transport layer, or transport parameter. The system is applied to study degradation effects in back-contact perovskite cells where it is found to readily differentiate areas based on their markedly different frequency response. Using the diffusion-recombination model, the IMPS response is modeled for a sandwich structure and extended for the special case of lateral diffusion in a back-contact cell. In the low-frequency limit, the model is used to calculate spatial maps of the carrier ambipolar diffusion length. The observed frequency response of IMPS images is then discussed.

Green electrodeposition synthesis of NiFe-LDH/MoOx/BiVO4 for efficient photoelectrochemical water splitting

Herein, a facile green synthetic protocol for nanoporous NiFe-LDH/MoOx/BiVO4 had been established via an electrochemical deposition method for enhanced photoelectrochemical cell (PEC) performance. The rational design of nanoporous NiFe-LDH/MoOx/BiVO4 played a vital role in improving the photocurrent density and achieving 2.7 mA /cm2 at 1.23 VRHE (3.9 - fold higher than BiVO4) with a negative onset potential of 267 mV offset. Moreover, the holes were efficiently consumed for water splitting through the cyclic reaction of NiFe-LDH layer. Thus, the nanoporous NiFe-LDH/MoOx/BiVO4 photoanode dramatically improved bulk charge transfer efficiency and surface charge injection efficiency reaching nearly 50% and 95% at 1.23 VRHE, respectively. In addition, the accumulated charge test proved that Mo oxide had the function of transferring holes. And the highest photovoltage and lowest charge recombination kinetics of composite photoanode also presented that the oxide species of Mo and NiFe-LDH had the properties of a passivation layer which were characterized by OCP (Open Circuit Potential) and IMPS (Intensity Modulated Photocurrent Spectroscopy) test. The excellent photocurrent density and facile layer-by-layer synthesis of NiFe-LDH/MoOx/BiVO4 nanocomposite made it a promising photocatalytic material for practical applications. This newly designed strategy was anticipated to be applied in future promising photoanodes for PEC water splitting.

Charge dynamics in CuInS2 photovoltaic devices with In2S3 as buffer layer

The charge carrier dynamics of photovoltaic devices composed of FTO/TiO2/In2S3/CuInS2/graphite (FTO=Fluorine doped Tin Oxide) is investigated by Intensity Modulated Photocurrent Spectroscopy (IMPS) and Intensity Modulated Photovoltage Spectroscopy (IMVS). Characteristic lifetimes and transport times are obtained. The dependence of the lifetime with light intensity can be explained by multitrapping processes, which justify the rather long lifetimes. Moreover, typical spectral features in the Nyquist plots are obtained by a model based on the continuity equation where diffusion is the major transport process, as confirmed by the dependence of the transport times on the externally applied potential. Changes from a kinetically limited process to a diffusion limited process are observed by modifying the excitation wavelength. Moreover, the existence of a potential barrier or a similar limitation process at the In2S3/CuInS2 interface could be inferred by comparing of the experimental IMPS measurements with data derived from. The existence of defect states at the In2S3 can be deduced from the IMVS dependence with wavelength. Also, different capacitive effects in the samples can be discriminated due to the spectroscopic nature of the techniques here employed.

Insight into the Roles of Metal Loading on CO2 Photocatalytic Reduction Behaviors of TiO2.

The photocatalytic reduction of carbon dioxide (CO2) into value-added chemicals is considered to be a green and sustainable technology, and has recently gained considerable research interest. In this work, titanium dioxide (TiO2) supported Pt, Pd, Ni, and Cu catalysts were synthesized by photodeposition. The formation of various metal species on an anatase TiO2 surface, after ultraviolet (UV) light irradiation, was investigated insightfully by the X-ray absorption near edge structure (XANES) technique. CO2 reduction under UV-light irradiation at an ambient pressure was demonstrated. To gain an insight into the charge recombination rate during reduction, the catalysts were carefully investigated by the intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence spectroscopy (PL). The catalytic behaviors of the catalysts were investigated by density functional theory using the self-consistent Hubbard U-correction (DFT+U) approach. In addition, Mott–Schottky measurement was employed to study the effect of energy band alignment of metal-semiconductor on CO2 photoreduction. Heterojunction formed at Pt-, Pd-, Ni-, and Cu-TiO2 interface has crucial roles on the charge recombination and the catalytic behaviors. Furthermore, it was found that Pt-TiO2 provides the highest methanol yield of 17.85 µmol/gcat/h, and CO as a minor product. According to the IMPS data, Pt-TiO2 has the best charge transfer ability, with the mean electron transit time of 4.513 µs. We believe that this extensive study on the junction between TiO2 could provide a profound understanding of catalytic behaviors, which will pave the way for rational designs of novel catalysts with improved photocatalytic performance for CO2 reduction.

Determination of photon-driven charge transfer efficiency: Drawbacks, accuracy and precision of different methods using Hematite as case of study

The electrochemical properties of photoelectrodes must be measured accurately and precisely to enable better comparisons between different materials. Along with the flat band potential, the interfacial charge transfer efficiency, which is the ratio between charge transfer rate at the photoelectrode surface and rate of charge carrier generation in the photoelectrode, can be used to predict the current density response at a given photon flux and electrode potential. The most widely used techniques for measuring charge transfer efficiencies are Photo-Electrochemical Impedance Spectroscopy (PEIS), current density ratios in the presence and absence of hole/electron scavengers, chrono-amperometry and Intensity Modulated Photocurrent Spectroscopy (IMPS). Charge transfer efficiencies can be estimated from PEIS and IMPS spectra either by using raw data (graphically), by fitting equivalent electrical circuits or by computing the Distribution of Relaxation Times (DRT). However, these techniques have their own drawbacks and impracticalities, that require researchers to make a choice between measuring accurately or pragmatically. Hitherto, the theoretical and experimental details of these techniques have not been summarised collectively and comprehensively. Here, we report the benefits and drawbacks, the accuracy, precision and best experimental recommendations when employing different techniques for photon-driven charge transfer efficiency determination.

Creation of oxygen vacancies to activate 2D BiVO4 photoanode by photoassisted self‐reduction for enhanced solar‐driven water splitting

BiVO4 is regarded as a promising photoanode material because of its narrow band gap (∼ 2.4 eV). However, the poor bulk carrier transport capacity severely limits the actual performance of BiVO4 in photoelectrochemistry (PEC) water splitting. Here, we tried to synthesize two-dimensional (2D) morphology by adding structure-directing agents to hydrothermal solution and introduce oxygen vacancies by photoassisted self-reduction. In the scheme, the 2D morphology can shorten the carrier diffusion length and the construction of oxygen vacancies can increase the bulk carrier concentration. Through the combined effect of both, the inherent poor carrier transport capacity of BiVO4 has been improved, which has been proven by Electrochemical Impedance Spectroscopy (EIS), Intensity Modulated Photocurrent Spectroscopy (IMPS) and Mott-Schottky (M-S) plots. At 1.23 V vs. RHE, the photocurrent density increases from ∼1.60 mA/cm2 (2D BiVO4) to ∼2.30 mA/cm2 [2D BiVO4(Vox)], which is 8 times larger than that of bare BiVO4(0.3 mA/cm2). The incident photon-to-current efficiencies (IPCE) of 2D BiVO4(Vox) have reached the value of 46% (at 350 nm). The carrier separation efficiency (ƞsep) and carrier injection efficiency (ƞinj) have been improved up to 49% and 68%, respectively.

An intensity modulated photocurrent spectroscopy study of the role of titanium in thick hematite photoanodes

In this Letter, the role of Ti addition in thick hematite mesoporous photoanodes was elucidated by performing intensity modulated photocurrent spectroscopy (IMPS) monitoring its charge carrier dynamics during water oxidation. Interface engineering associated with doping of hematite is crucial to develop highly efficient thick photoanodes. Photoelectrochemical data recorded under front- and back-side illumination show that Ti insertion mitigates the collection deficit faced by hematite due to an energy barrier decrease between the grains and a change in the surface chemistry. IMPS reveals that Ti clearly influences the hematite film performance by increasing the charge separation efficiency due to its segregation at the hematite interface. However, Ti insertion does not enhance the kinetics of water oxidation at the solid–liquid interface. These facts indicate that Ti mainly affects the hematite electronic properties instead of accelerating the surface processes. This comprehensive understanding of the electronic transport and charge carrier dynamics in Ti:hematite photoanodes enables the control and design of better interfaces for more efficient photoelectrochemical water splitting.

Modulated-photocurrent spectroscopy of single-crystal organic semiconductor rubrene with pristine and trap-dominated surfaces

High-performance, benchmark organic semiconductor rubrene is investigated by modulated-photocurrent (MPC) spectroscopy. In this technique, periodically modulated and steady-state photocarrier populations are simultaneously generated in the sample by two independent light beams, with the resultant photocurrent measured by a lock-in amplifier. The technique allows identifying a bandlike carrier motion (as opposed to incoherent hopping) as the dominant charge transport mechanism in this highly crystalline molecular semiconductor, in agreement with prior studies, yet without the need for fabrication of complex transistor devices. Moreover, MPC spectroscopy is used to determine the important parameters, such as the density of states for traps, their carrier capture coefficient, and attempt to escape frequency, as well as the quantum efficiency of photocarrier generation and the photocarrier mobility. The MPC spectroscopy is also shown to be able to discriminate charge transport at the surface of the crystal from that occurring in the bulk. The wealth of the thus obtained information is essential for our better understanding of the microscopic mechanisms governing photoconductivity in this class of materials.

Light Intensity Modulated Photo-Electrochemical Methods and Distribution of Relaxation Times As Tools to Investigation Photovoltaic Materials

Electrical Impedance Spectroscopy (EIS, sometimes also Electrochemical Impedance Spectroscopy) found its place in material studies as a routine experiment. EIS proliferation became possible through advances in electronics. Current-voltage characteristics were standard features of potentiostats. Impedance techniques require additional components embedded into a potentiostat, namely signal generator and signal analyzer. Relatively simple devices are available, although dedicated instrumentation with little noise, broad frequency control and perhaps sample temperature control or automated sample handling are particularly desired and command premium in cost.EIS as an electrical technique uses as the perturbing signal alternating electrical signal, either potential of current. In general, impedance studies are intended to study a transfer function and the input and output do not have to be only voltage-current or current-voltage as it would be in EIS. In mechanical engineering are for example useful studies focused on mechanical perturbation. Electrical response by its nature makes EIS amenable to study electrochemical systems.Along with the impedance vs. frequency data collection comes also the need of meaningful data interpretation. This is typically done through data fitting to components of an equivalent electrical circuit. The acquired data -- real and imaginary impedance components gathered over a vast range of frequencies -- are evaluated by fitting to some equivalent circuit using nonlinear regression.Oftentimes both finding the equivalent circuit and the physical component assignment are tedious, if not impossible. Recently a new approach to data reduction has been a treatment via distribution of relaxation times (DRT) [1, 2]. In principle, this approach has been outlined already in 1936 [3]. Its present use was made possible by available computation power of current PCs [4, 5]. While this is not a substitute to equivalent circuits and subsequent physical data interpretation, it is possibly a method, when used appropriately and in automated way, that can provide more information that simple comparison of the Bode or Nyquist graphs. This note is presenting some examples of this approach.In the field of photovoltaic studies of light-gathering materials experimentation on systems responding to illumination makes use of impedance studies in which the perturbing signal is light of certain wavelength (or white light) with variable intensity. Here we will look at the link between EIS and electrical response spectroscopy induced by fluctuating light. This is sometimes for short dubbed as optical spectroscopy although it does make a use of the instruments for optical spectroscopy (such as UV-VIS, IR etc.) as they are known from analytical chemistry literature. Rather, an electric signal in a photoactive material is generated by intensity modulated light at different frequencies (similar range to electrical impedance spectroscopy) and a response (the magnitude and the phase shift) is analyzed. In this respect the magnitude and phase shift correspond to the magnitude and a vector of resistance on the polar plot of electrical impedance measurement. However, the data obtained serve a different purpose. For example, in a photovoltaic shorted system, the time constant associated with the response corresponds to mobility of the photogenerated charge in the material. For an open-circuit potential of such a photovoltaic system the measured time constant response corresponds to the rate of recombination of the generated hole-electron species.Such materials of general structure (ABX3) could be replacements for silicon-based photovoltaic cells. These materials, due to the possible variability of both cations and anions in their composition, offer almost endless possibilities for synthetic preparation. For the active layer of a solar cell is often used the CH3NH3PbI3-2Cl2 perovskite structure, i.e., a structure containing an organic component. We have built and then tested such photovoltaic cells by static and dynamic methods using a unique photoelectric research instrument using the method CIMPS (Controlled Intensity Modulated Photocurrent Spectroscopy).AcknowledgementThis work was supported through the support of The Czech Science Foundation (GACR), contract GA19-23718S.References BioLogic, Distribution of Relaxation Times (DRT): an introduction, BioLogic, Editor. 2017.Weese, J., A reliable and fast method for the solution of Fredhol integral equations of the first kind based on Tikhonov regularization. Computer Physics Communications, 1992. 69(1): p. 99-111.Yager, W.A., The Distribution of Relaxation Times in Typical Dielectrics. Physics, 1936. 7(12): p. 434-450.Shoar Abouzari, M.R., et al., On the physical interpretation of constant phase elements. Solid State Ionics, 2009. 180(14): p. 922-927.Wan, T.H., et al., Influence of the Discretization Methods on the Distribution of Relaxation Times Deconvolution: Implementing Radial Basis Functions with DRTtools. Electrochimica Acta, 2015. 184: p. 483-499.

Plasmon‐Enhanced Charge Separation and Surface Reactions Based on Ag‐Loaded Transition‐Metal Hydroxide for Photoelectrochemical Water Oxidation

AbstractCoating photoanodes with transition‐metal hydroxides (TMH) is a promising approach for improving photoelectrochemical (PEC) water oxidation. However, the present system still suffers from high charge recombination and sluggish surface reactions. Herein, effective charge separation is achieved at the same time as boosting the surface catalytic reaction for PEC water splitting through decoration of plasmon metal (Ag) in a semiconductor/TMH coupling system. The kinetic behavior at the semiconductor/TMH and TMH/electrolyte interfaces is systematically evaluated by employing intensity modulated photocurrent spectroscopy, scanning photoelectrochemical microscopy, and oxygen evolution reaction model. It is found that both charge transfer and surface catalysis dynamics are enhanced through local surface plasmon resonance of Ag nanoparticles. The as‐prepared BiVO4/Co(OH)x‐Ag exhibits remarkable activity (≈4.64 times) in PEC water splitting in comparison with pure BiVO4. Notably, this smart approach can be also applied to other TMH (Ni(OH)2), reflecting its universality. This work provides a guiding design method for solar energy conversion with the semiconductor‐TMH system.

Effect of auxiliary acceptor on D-π-A based porphyrin sensitizers for dye sensitized solar cells

Among the tetrapyrrolic class of compounds, porphyrin based sensitizers have attracted considerable attention due to their excellent performance in dye-sensitized solar cells (DSSCs). Herein, two new donor-[Formula: see text]-acceptor (D-[Formula: see text]-A) concept based porphyrin sensitizers having donor 3-ethynylphenothiazine tethered at meso- position, zinc porphyrin as [Formula: see text]-spacer and acceptor either 3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (LG22), and 3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-yl)-2-cyanoacrylicacid (LG23) were designed and synthesized. Both sensitizers were characterized by various spectroscopic techniques and electrochemical methods. Optical, electrochemical and optimized studies suggest that these dyes are suitable for sensitization of nanocrystalline TiO2. The optimized device studies showed the power conversion efficiencies (PCE) of 1.86% and 1.37% for LG22 and LG23 dyes, respectively using [Formula: see text]/[Formula: see text] liquid redox shuttle. Finally, Charge Extraction (CE), Intensity Modulated Photovoltage Spectroscopy (IMVS), Intensity Modulated Photocurrent Spectroscopy (IMPS) and Open Circuit Voltage Decay (OCVD) measurements were adopted to understand in detail the electron transfer properties at various interfaces in a way to understand the performance limiting processes in these novel porphyrin sensitizers.

Preparation and photoelectrochemical properties of hierarchical heterostructure ZnO/CuO array

In this paper, a ZnO array is first prepared on the conductive glass by hydrothermal method, and then a seed layer of CuO nanoparticles is deposited on the surface of ZnO nanorods to form a core–shell ZnO/CuO nanoarray. Following, the dendritic CuO nanorods are grown on ZnO nanorods by hydrothermal method, constructing a three-dimensional ZnO/CuO nano-heterogeneous hierarchical structure array and applied in photoelectrochemical cell. Our three-dimensional ZnO/CuO nano-heterogeneous hierarchical structure shows obvious absorption of visible light from 500 to 900 nm. The current–time curves reveal that the photocurrent is greatly enhanced after dendritic CuO grows on the ZnO nanorod array. Furthermore, the charge dynamics processes in the core–shell ZnO/CuO nano-heterostructure array and the three-dimensional ZnO/CuO nano-heterostructure array are studied by comparing electrochemical impedance spectroscopy and intensity modulation photocurrent (photovoltage) spectroscopy. Although the photo-generated charge transfer rate in the ZnO/CuO core–shell structure is slightly faster than the three-dimensional ZnO/CuO hierarchical structure, the recombination life of the photo-generated charge is increased by nearly two orders of magnitude in the three-dimensional ZnO/CuO nano-heterostructure array. The three-dimensional ZnO/CuO nano-heterostructure array has a good potential application in the field of photoelectric conversion devices and photocatalysis.

Internal quantum efficiency and time signals from intensity-modulated photocurrent spectra of perovskite solar cells

Intensity Modulated Photocurrent Spectroscopy (IMPS) is a small-perturbation optoelectronic technique that measures the quantum efficiency of a photoelectrochemical device as a function of optical excitation frequency. Metal Halide Perovskites (MHPs) are mixed electronic–ionic semiconductors with an extraordinary complex optoelectronic behavior and a record efficiency surpassing 25%. In this paper, we propose a simplified procedure to analyze IMPS data in MHPs based on the analysis of the internal quantum efficiency and the time signals featuring in the frequency spectra. In this procedure, we look at the change of each signal when optical excitation wavelength, photon flux, and temperature are varied for an archetypical methyl ammonium lead iodide solar cell. We use drift-diffusion modeling and comparison with relatively simpler dye-sensitized solar cells (DSC) with viscous and non-viscous electrolytes to help us to understand the origin of the three signals appearing in MHP cells and the measurement of the internal quantum efficiency.

Copper Vanadate Nanobelts as Anodes for Photoelectrochemical Water Splitting: Influence of CoOx Overlayers on Functional Performances.

The design and development of environmentally friendly and robust anodes for photoelectrochemical (PEC) water splitting plays a critical role for the efficient conversion of radiant energy into hydrogen fuel. In this regard, quasi-1D copper vanadates (CuV2O6) were grown on conductive substrates by a hydrothermal procedure and processed for use as anodes in PEC cells, with particular attention on the role exerted by cobalt oxide (CoOx) overlayers deposited by radio frequency (RF) sputtering. The target materials were characterized in detail by a multitechnique approach with the aim at elucidating the interplay between their structure, composition, morphology, and the resulting activity as photoanodes. Functional tests were performed by standard electrochemical techniques like linear sweep voltammetry, impedance spectroscopy, and by the less conventional intensity modulated photocurrent spectroscopy, yielding an important insight into the material PEC properties. The obtained results highlight that, despite the fact that the supposedly favorable band alignment between CuV2O6 and Co3O4 did not yield a net current density increase, cobalt oxide-functionalized anodes afforded a remarkable durability enhancement, an important prerequisite for their eventual real-world applications. The concurrent phenomena accounting for the observed behavior are presented and discussed in relation to material physico-chemical properties.

Modulated Photocurrent Spectroscopy Study of the Electronic Transport Properties of Working Organic Photovoltaics: Degradation Analysis.

Electronic transport measurement using modulated photocurrent (MPC) spectroscopy is demonstrated herein in working organic photovoltaics (OPVs) before and after AM1.5G irradiation. OPVs with bulk heterojunction (BHJ) using prototypical donor and acceptor materials, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1–2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] = hieno [3–4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), were fabricated. The OPVs had inverted structures (BHJs are formed on transparent conductive oxide substrates). The photovoltaic performance of PTB7:PC71BM OPVs was characterized and the best power conversion efficiency was obtained at PTB7 content of 40 wt%. Electron and hole mobility were determined with MPC spectroscopy in PTB7:PC71BM OPVs and were well balanced at PTB7 content of 40 wt%. Degradation of the photovoltaic performance of PTB7:PC71BM OPVs with PTB7 content of 40 wt% caused by AM1.5G irradiation was studied. MPC spectroscopy showed that the well-balanced mobility was not affected by AM1.5G irradiation. The degradation of OPVs was not due to changes in the electronic transport properties, but mainly to the reduced short circuit current (Jsc) and fill factor (FF). The origin of this reduction is discussed.