Light-induced Charge Transport Research Articles

Tracking light-induced charge transport.

Precise charge dynamics could help to improve the operation of solar cells and sensors.

Light-Induced Structural Dynamics and Charge Transport in Layered Halide Perovskite Thin Films

The dynamic nature of the metal halide perovskite lattice upon photoexcitation plays a vital role in their properties. Here we report an observation of light-induced structure dynamics in quasi-2D Ruddlesden-Popper phase perovskite thin films and its impact on the carrier transport properties. By a time-resolved X-ray scattering technique, we observe a rapid lattice expansion upon photoexcitation, followed by a slow relaxation over the course of 100 ns in the dark. Theoretical modeling suggests that the expansion originates from the lattice's thermal fluctuations caused by photon energy deposition. Power dependent optical spectroscopy and photoconductivity indicate that high laser powers triggered a strong local structural disorder, which increased the charge dissociation activation energy that results in localized transport. Our study investigates the impact of laser energy deposition on the lattices and the subsequent carrier transport properties, that are relevant to device operations.

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces.

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of the spatiotemporal dynamics of photoinduced interfacial charge separation. Here, we demonstrate access to this transient charge redistribution for a model hybrid system of nanoporous zinc oxide (ZnO) and ruthenium bipyridyl chromophores. The site-selective probing of the molecular electron donor and semiconductor acceptor by time-resolved X-ray photoemission provides direct insight into the depth distribution of the photoinjected electrons and their interaction with the local band structure on a nanometer length scale. Our results show that these electrons remain localized within less than 6 nm from the interface, due to enhanced downward band bending by the photoinjected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems.

Hydrophobic-substrate based water-microdroplet manipulation through the long-range photovoltaic interaction from a distant LiNbO3:Fe crystal.

Development of photovoltaic water-microdroplet manipulation using LN:Fe crystals has to meet the requirement of the hybrid and heating-avoided design of biological lab-on-chips. To fulfill this, we demonstrate a successful manipulation of a water microdroplet on a hydrophobic substrate by utilizing the long-range photovoltaic interaction from a distant LN:Fe crystal (see Visualization 1). The maximal manipulation distance (MMD) is found to be dependent on the laser-illumination intensity at the LN:Fe crystal and it can be tuned up to a sub-centimeter level (∼4 mm). Basing on the two-center model of light-induced charge transport in the LN:Fe crystal, we establish an analytic model to describe the force balance during the microdroplet manipulation under a long-range photovoltaic interaction. Either shortening the manipulation distance or increasing the illumination intensity can enhance the photovoltaic interaction and increase the velocity of the microdroplet being manipulated. An abrupt shape change followed by a fast repelling movement of the water microdroplet is observed under a strong photovoltaic interaction (see Visualization 2).

One-step preparation (3D/2D/2D) BiVO4/FeVO4@rGO heterojunction composite photocatalyst for the removal of tetracycline and hexavalent chromium ions in water

In promoting the application of green and sustainable chemistry in wastewater treatment, it is urgent to explore effective and stable photocatalysts with excellent degradability on tetracycline (TC) and reduction of Cr (VI). However, photogenerated charge recombination and low visible light utilization limit the application of photocatalysts. To solve this problem, we successfully prepared all solid BiVO4/FeVO4@rGO photocatalyst with novel 3D/2D/2D heterojunction structure prepared via one-step hydrothermal method. The formation of this unique 3D/2D/2D heterojunction leads to the creation of nanochannels in their interfacial contact, improved light-induced charge transport and migration efficiency. The effect of rGO dosages, inorganic anions (SO42−, Cl−, Cr6+and CO32−) and organic material on the degradation of tetracycline by BiVO4/FeVO4@rGO photocatalyst was systematically investigated. Results showed that the splendid degradation performance of BiVO4/FeVO4@rGO photocatalyst was ascribed to novel 3D/2D/2D structure with the designated migrated directions of photogenerated charges, suppressed recombination rate of electrons-holes, the large specific surface area, and enhanced transport and separation efficiency of photogenerated carrier. The main active species in photocatalytic oxidation reactions were verified as h+ and ·O2− by free radical scavenging tests and electron spin resonance (ESR) analysis. Moreover, the possible degradation process of TC was analyzed with 3D EEMs and LC-MS techniques, then the mechanism of photocatalysis was proposed. Surprisingly, BiVO4/FeVO4@rGO material always exhibited high degradation efficiency of TC in simulation experiments with different water sources, exhibiting an excellent practical application value.

Optoelectronics: High‐Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure (Small 9/2018)

In article number 1703293, Tie Lin, Weida Hu, Jianlu Wang, and co-workers present a high performance photovoltaic photodetector based on MoTe2/MoS2 Van der Waals heterojunction. The light-induced charge transport under different biases is discussed. As self-powered photodetectors, the vertical 2D p-n junctions achieve a sensitive response and broad detection wavelength range. These properties indicate that 2D van der Waals junctions are promising for application in photo-sensing, on-chip logic circuits and so on.

Investigation of light induced charge transport properties in Dy2NiMnO6 perovskite based Schottky diode

ABSTRACTHere, we have discussed the charge transport phenomena through the interface formed by silver and double perovskite oxide Dy2NiMnO6 (DNMO). The charge carrier transport mechanism in Ag/DNMO junction under dark and illuminated conditions was analyzed on the basis of the thermionic emission theory of metal-semiconductor junction. The Schottky diode parameters like ideality factor, barrier potential height and series resistance of the freshly fabricated device were evaluated under dark and illuminated conditions. The results show significant improvements in the rectification ratio, barrier potential height and ideality factor of the device upon light illumination.

Light induced charge transport in La2NiMnO6 based Schottky diode

La2NiMnO6 (LNMO) thin film is prepared on fluorine doped tin oxide (FTO) coated glass by chemical solution deposition technique. The room temperature crystal structure of LNMO is determined using X-ray diffraction and Raman spectra. The band gap (∼1.33 eV) obtained using UV–visible absorption spectrum of LNMO film is found to be very close to the Shockley-Queisser band gap (∼1.34 eV) for a single junction solar cell. A FTO/LNMO/Au type Schottky device is fabricated to explore the photo physical properties of LNMO thin film. A significant change in the resistance, barrier potential height, effective mobility, conductivity, charge carrier density and carrier diffusion length is observed when the experiment is performed under dark and white light illuminations. The thermionic emission theory is used to analyze the charge transport mechanism in the material.

Light induced charge transport property analysis of nanostructured ZnS based Schottky diode

In this report we have investigated the light sensing behavior and also discussed the induced charge transport phenomena through the junction made by aluminium and hydrothermally derived zinc-sulfide (ZnS). In this regards the structural, optical and electrical characterization of ZnS was performed well. The optical band gap energy (=3.68 eV) estimated from optical spectra and room temperature conductivity (0.49 × 10−6 S cm−1) measured from current–voltage characteristic explores the inorganic semiconductor behavior of the synthesized material. Depending upon the work function, aluminium (=4.4 eV) was preferred as metal contact to develop a potential barrier within the junction to study the underneath mechanism of charge transport through metal/inorganic-semiconductor interface. Moreover we have fabricated the sandwich structure ITO/ZnS/Al junction to study the effect of incident light on charge transport phenomena and demonstrated the potential applicability of the synthesized material to over crown the research on inorganic nano-semiconductor. The everbound charge transport phenomena within the device was analyzed by thermoionic emission theory. In searching its performance within light sensing electronic device we have deliberated the mobility-lifetime product, diffusion length and density of states (DOS) near Fermi level very aptly.

Enhanced Visible Light-Induced Charge Separation and Charge Transport in Cu2O-Based Photocathodes by Urea Treatment.

Carrier density, photocharge transfer kinetics, and charge transfer resistance of the anodized Cu-Cu2O-CuO photocathode were greatly improved using thermal treatment with urea. Time-correlated single-photon counting (TCSPC) results revealed the faster electron transfer kinetics from Cu2O to CuO in the urea-treated Cu-Cu2O-CuO composite photoelectrodes. Preservation of the metallic copper component via the intermediate Cu3N during the treatment facilitated higher bulk conductance of the Cu-Cu2O-CuO photocathode for improved charge transport. Higher carrier density was also observed in the urea-treated photoelectrode, which was possibly attributed to the presence of nitrogen as a dopant. Furthermore, the compact outer layer of CuO protected the underlayer Cu2O from being in direct contact with the aqueous solution. This suppressed the photocorrosion of Cu2O and resulted in the higher photostability of the Cu-Cu2O-CuO film. When these advantages were combined, the urea-treated Cu-Cu2O-CuO film showed a higher photocurrent of 2.2 mA/cm2 and improved stability versus that of the conventional Cu-Cu2O-CuO film (1.2 mA/cm2). To improve the charge transfer kinetics and carrier density, this paper provides a new strategy for synthesizing effective and stable Cu2O-based photoelectrodes by using urea treatment.

Modeling light-induced charge transfer dynamics across a metal-molecule-metal junction: bridging classical electrodynamics and quantum dynamics.

We develop a numerical approach for simulating light-induced charge transport dynamics across a metal-molecule-metal conductance junction. The finite-difference time-domain method is used to simulate the plasmonic response of the metal structures. The Huygens subgridding technique, as adapted to Lorentz media, is used to bridge the vastly disparate length scales of the plasmonic metal electrodes and the molecular system, maintaining accuracy. The charge and current densities calculated with classical electrodynamics are transformed to an electronic wavefunction, which is then propagated through the molecular linker via the Heisenberg equations of motion. We focus mainly on development of the theory and exemplify our approach by a numerical illustration of a simple system consisting of two silver cylinders bridged by a three-site molecular linker. The electronic subsystem exhibits fascinating light driven dynamics, wherein the charge density oscillates at the driving optical frequency, exhibiting also the natural system timescales, and a resonance phenomenon leads to strong conductance enhancement.

Charge Transfer in Nanocrystalline Semiconductor Electrodes

Nanocrystalline electrodes in liquid junction devices possess a number of unique properties arising from their convoluted structure and the dimensions of their building units. The light-induced charge separation and transport in photoelectrochemical systems using nanocrystalline/nanoporous semiconductor electrodes is discussed here in connection with the basic principles of the (Schottky) barrier theory. Recent models for charge transfer kinetics in normal and unipolar (dye-sensitized) cells are reviewed, and novel concepts and materials are considered.

Threshold behavior of the Einstein oscillator, electron-phonon interaction, band-edge absorption, and small hole polarons in LiNbO3:Mg crystals

Based on the Einstein oscillator model and the Urbach rule, we study the electron-phonon interaction and the band-edge structure of LiNbO${}_{3}$:Mg. We report on the concentration threshold behavior of the Einstein oscillator, the electron-phonon interaction, the band-edge absorption, and the light-induced small hole polarons O${}^{\ensuremath{-}}$ in LiNbO${}_{3}$:Mg crystals with increasing Mg-doping concentration. The result gives a fundamental microscopic mechanism of the concentration threshold behavior related to the band-edge optical properties and the light-induced charge transport of LiNbO${}_{3}$:Mg crystals, and is helpful to optimize LiNbO${}_{3}$ for applications such as nonlinear optics.

Light-induced charge-transport in undoped LiNbO3 crystals

Light-induced charge transport in nominally undoped LiNbO3 crystals essentially differs from that in Fe- and Cu-doped crystals. Furthermore, it is of relevance for numerous effects ranging from optical damage to domain reversal. Here, we review the state of the art of the studies of light-induced charge transport in this important material. This includes a summary of the facts known for doped crystals, a description of the experimental methods relevant to undoped crystals, a summary of the main experimental data for this material, especially the data on the intensity dependences, presentation of the methods of purification from unwanted photo-active electrons, and a discussion of possible mechanisms responsible for the charge-transport peculiarities.

Light-Induced Charge Transport within a Single Asymmetric Nanowire

Artificial photosynthetic systems using semiconductor materials have been explored for more than three decades in order to store solar energy in chemical fuels such as hydrogen. By mimicking biological photosynthesis with two light-absorbing centers that relay excited electrons in a nanoscopic space, a dual-band gap photoelectrochemical (PEC) system is expected to have higher theoretical energy conversion efficiency than a single band gap system. This work demonstrates the vectorial charge transport of photogenerated electrons and holes within a single asymmetric Si/TiO(2) nanowire using Kelvin probe force microscopy. Under UV illumination, higher surface potential was observed on the n-TiO(2) side, relative to the potential of the p-Si side, as a result of majority carriers' recombination at the Si/TiO(2) interface. These results demonstrate a new approach to investigate charge separation and transport in a PEC system. This asymmetric nanowire heterostructure with a dual band gap configuration and simultaneously exposed anode and cathode surfaces represents an ideal platform for the development of technologies for the generation of solar fuels, although better photoanode materials remain to be discovered.

Photogalvanic effect induced by an electric field

Light-induced charge transport that occurs in crystals containing centres with permanent dipole moment due to their polar orientation is considered. Principal attention is paid to the situation when such dipolar centres are oriented by application of an external electric field. In this case, we may regard the current generation as caused by an electric-field-induced photogalvanic effect. In contrast to conventional photogalvanic phenomena, the effect under consideration can be observed in crystals of arbitrary symmetry and even in any condensed matter containing photoactive dipolar centres. A remarkable feature of the effect is noncollinear orientation of the current vector with respect to the electric field vector, which is a consequence of the discrete set of orientations available for the dipole centres' positions. It is shown that this feature could significantly facilitate experimental observation of the electric-field-induced photogalvanic current. Here, a phenomenology description and qualitative model of the effect are presented and its possible impact on the effect of absolute negative conductivity is discussed.

Light-induced absorption in reduced congruent and near-stoichiometric Er:LiNbO3 crystals

Light-induced absorption (LIA) in thermally reduced congruent and near-stoichiometric Er:LiNbO3 crystals, which have different cuts of crystal (X- and Y-cut) and different doping levels of Er3+ (0.2, 0.4, 0.6 and 1.0 mol %), has been studied within continuous-wave light intensity by using polarized 632.8 nm beam as probe light and polarized 488 nm beam as pump light. Effects of thermal reduction condition, pump intensity, Li/Nb ratio and Er-doping level on the LIA are summarized and discussed. Studies on spectral characteristics of both linear absorption and light-induced absorption show that photo-ionization of bipolarons into small polarons plays a predominant role in the LIA process. The role of the Er3+ ion in the LIA is investigated by studying the doping level and the thermal reduction effects on the absorption and emission properties of Er3+. The results show the possibility that the Er3+ ion directly participates in the light-induced charge transport is small. It affects the LIA only via the bypassing part of pump intensity to excite near-infrared emissions of Er3+. The thermaleffect on the LIA is demonstrated. A two-center model with inclusion of the thermal contribution can qualitatively explain two major LIA characteristics that include the appearance of a slow stage in the LIA procedure and the unsaturation behavior of the LIA coefficient in high pump intensity regime. In addition, the pump depletion effect on the LIA characteristics is discussed. The possibility of pump beam fanning arising from light-induced scattering is ruled out experimentally.

Characterization of Proton Transport across a Waveguide-Supported Lipid Bilayer

Cellular energy transduction processes are often driven by transmembrane ion gradients, and numerous artificial biomembrane systems have been developed that allow for chemically or light-induced charge transport into/out of liposomes. Liposomal architectures, however, are not readily interfaced to a solid-state transducer. Formation of an ion gradient across a planar-supported membrane, "wired" to a substrate electrode, may ultimately allow utilization of the potential energy to drive other electrochemical processes. Described here is a novel conductive polymer/planar waveguide assembly that provides for highly sensitive transduction of proton transport across a planar-supported lipid bilayer (PSLB). A quinone proton shuttle is embedded in the PSLB, which is coupled to the planar optical waveguide electrode through a pH-sensitive, self-assembled conductive polymer film. Interfacial potential and absorbance changes in the conductive polymer film provide for sensitive characterization of transmembrane proton transport. The general and flexible nature of this architecture makes it adaptable to many different types of transmembrane transport chemistries, particularly light-activated systems.

Intensity and wavelength dependence of the photoconductivity in Cr-doped Sr $\mathsf{_{0.61}}$ Ba $\mathsf{_{0.39}}$ Nb $\mathsf{_2}$ O $\mathsf{_6}$

We examine the light-induced charge transport properties of a series of chromium-doped \(\rm {Sr}_{0.61}\rm {Ba}_{0.39}\rm {Nb}_2\rm {O}_6\) single crystals by measurements of the optical absorption and the electric conductivity. By comparing the wavelength dependence of the specific photoconductivity and the optical absorption we show that both effects stem from the same center. The intensity dependence of the photoconductivity shows the applicability of a one-center charge transport model for high doping concentrations, while for low doping concentrations a more sophisticated model is needed. The validity of a one-center model is exemplarily verified for a crystal doped with 0.51 mol % Cr over a wide intensity range using a holographic method. The product of mobility and recombination time of photoexcited electrons is deduced from the specific photoconductivity.