Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials

Pattern-illumination time-resolved phase microscopy (PI-PM) is a technique used to study the microscopic charge carrier dynamics in photocatalytic and photovoltaic materials. The method involves illuminating a sample with a pump light pattern, which generates charge carriers and they decay subsequently due to trapping, recombination, and transfer processes. The distribution of photo-excited charge carriers is observed through refractive index changes using phase-contrast imaging. In the PI-PM method, the sensitivity of the refractive index change is enhanced by adjusting the focus position, the method takes advantage of photo-excited charge carriers to observe non-radiative processes, such as charge diffusion, trapping in defect/surface states, and interfacial charge transfer of photocatalytic and photovoltaic reactions. The quality of the image sequence is recovered using various informatics calculations. Categorizing and mapping different types of charge carriers based on their response profiles using clustering analysis provides spatial information on charge carrier types and the identification of local sites for efficient and inefficient photo-induced reactions, providing valuable information for the design and optimization of photocatalytic materials such as the cocatalyst effect.

Charge Carrier Trapping during Diffusion Generally Observed for Particulate Photocatalytic Films

Photo-excited charge carriers play a vital role in photocatalysts and photovoltaics, and their dynamic processes must be understood to improve their efficiencies by controlling them. The photo-excited charge carriers in photocatalytic materials are usually trapped to the defect states in the picosecond time range and are subject to recombination to the nanosecond to microsecond order. When photo-excited charge carrier dynamics are observed via refractive index changes, especially in particulate photocatalytic materials, another response between the trapping and recombination phases is often observed. This response has always provided the gradual increase of the refractive index changes in the nanosecond order, and we propose that the shallowly trapped charge carriers could still diffuse and be trapped to other states during this process. We examined various photocatalytic materials such as TiO2, SrTiO3, hematite, BiVO4, and methylammonium lead iodide for similar rising responses. Based on our assumption of surface trapping with diffusion, the responses were fit with the theoretical model with sufficient accuracy. We propose that these slow charge trapping processes must be included to fully understand the charge carrier dynamics of particulate photocatalytic materials.

Femtosecond laser driven precessing magnetic gratings

Manipulation and detection of spins at the nanoscale is of considerable contemporary interest as it may not only facilitate a description of fundamental physical processes but also plays a critical role in the development of spintronic devices. Here, we describe the application of a novel combination of transient grating excitation with Lorentz ultrafast electron microscopy to control and detect magnetization dynamics with combined nanometer and picosecond resolutions. Excitation of Ni80Fe20 thin film samples results in the formation of transient coherently precessing magnetic gratings. From the time-resolved results, we extract detailed real space information of the magnetic precession, including local magnetization, precession frequency, and relevant decay factors. The Lorentz contrast of the dynamics is sensitive to the alignment of the in-plane components of the applied field. The experimental results are rationalized by a model considering local demagnetization and the phase of the precessing magnetic moments. We envision that this technique can be extended to the study of spin waves and dynamic behavior in ferrimagnetic and antiferromagnetic systems.

Effect of Mn doping on the electron injection in CdSe/TiO2 quantum dot sensitized solar cells

Promotion in power conversion efficiency is an appealing task for quantum dot-sensitized solar cells that have emerged as promising materials for the utilization of clean and sustainable energy. Doping of Mn atoms into quantum dots (QD) has been proven to be one of the effective approaches, although the origin of such a promotion remains controversial. While several procedures are involved in the power conversion process, electron injection from the QD to the semiconductor oxide substrate is focused on in this work using first-principles calculations. Based on the Marcus theory, the electron injection rates are evaluated for the quantum dot-sensitized solar cell models in which the pure and Mn-doped core-shell CdSe clusters are deposited on a semiconductor titanium dioxide substrate. Enhanced rates are obtained for the Mn-doped structure, which is in qualitative agreement with the experiments. A large number of dominant injection channels and strong QD-substrate coupling are responsible for the Mn-induced rate enhancement, which could be achieved by manipulating the band structure mapping between the QD and the semiconductor oxide. By addressing the role of an Mn dopant in the electron injection process, strategies for the promotion of electron injection rates are proposed for the design of quantum dot-sensitized solar cells.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique

Photoexcited carrier dynamics in a hematite film with and without amorphous NiFeO _x on the surface was investigated using the heterodyne transient grating method. We found that two different electron/hole dynamics took place in the micro- and millisecond time regions and successfully assigned each component to the decay processes of electrons and holes trapped at surface states, respectively. It was also demonstrated that the amorphous NiFeO _x coating plays a crucial role in increasing the survival of the holes at the surface trap states, which was caused by the decrease in the surface recombination rate.

Effects of the large distribution of CdS quantum dot sizes on the charge transfer interactions into TiO2 nanotubes for photocatalytic hydrogen generation

Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum dots have revealed a hydrogen generation improvement when added to TiO2 materials under visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the capping agent. This sensitization technique permits high loading and uniform distribution of CdS quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration remains almost unchanged (homogeneous), while the concentration relative to the sulfate anion decreases by more than 80% with respect to the initial value after ∼100 nm in depth. The presence of sulfate anions is due to the oxidation of sulfide and occurs in greater proportion in the material surface. This protection for air oxidation inside the nanotubular matrix seemingly protected the CdS for photocorrosion in sacrificial solution leading to good stability properties proved by long duration, stable photocurrent measurements. The effect of the size and the distribution of sizes of CdS quantum dots attached to TiO2 nanotubes on the photocatalytic hydrogen generation were investigated. The experimental results showed three different behaviors when the reaction time of CdS synthesis was increased in the sensitized samples, i.e. similar, deactivation and activation effects on the hydrogen production with regard to TiO2 nanotubes. The deactivation effect was related to two populations of sizes of CdS, where the population with a shorter band gap acts as a trap for the electrons photogenerated by the population with a larger band gap. Electron transfer from CdS quantum dots to TiO2 semiconductor nanotubes was proven by the results of UPS measurements combined with optical band gap measurements. This property facilitates an improvement of the visible-light hydrogen evolution rate from zero, for TiO2 nanotubes, to approximately 0.3 μmol cm(-2) h(-1) for TiO2 nanotubes sensitized with CdS quantum dots.

Bilayer quantum dot-decorated mesoscopic inverse opals for high volumetric photoelectrochemical water splitting efficiency

We introduce a mesoscopic inverse opal structure sensitized by CdSe/CdS nanoparticle bilayer for use as water splitting electrodes.

Laser-induced transient grating setup with continuously tunable period

We present a modification of the laser-induced transient grating setup enabling continuous tuning of the transient grating period. The fine control of the period is accomplished by varying the angle of the diffraction grating used to split excitation and probe beams. The setup has been tested by measuring dispersion of bulk and surface acoustic waves in both transmission and reflection geometries. The presented modification is fully compatible with optical heterodyne detection and can be easily implemented in any transient grating setup.

The cause for the low efficiency of dye sensitized solar cells with a combination of ruthenium dyes and cobalt redox

What makes the efficiency of the solar cell lower for the Ru dye and the Co redox combination?

Fabrication and photoelectrochemical properties of TiO2/CuInS2/Bi2S3 core/shell/shell nanorods electrodes

The FESEM images (a and b), photocurrent density versus potential (c–v) curves (c) and schematic of the energy level arrangement (d).

Fabrication and photoelectrochemical characteristics of CuInS2 and PbS quantum dot co-sensitized TiO2 nanorod photoelectrodes

A cascade structured PbS/CuInS₂/TiO₂ photoelectrode with co-sensitization effect obtains the energy conversion efficiency of 4.11% under one sun illumination.

Charge transfer and recombination at the metal oxide/CH3NH3PbClI2/spiro-OMeTAD interfaces: uncovering the detailed mechanism behind high efficiency solar cells

Charge transfer and recombination dynamics at metal oxide (Y₂O₃or TiO₂)/CH₃NH₃PbClI₂/spiro-OMeTAD interfaces.