Photogenerated Carrier Density Research Articles

Insight into a novel microwave-assisted W doped BiVO4 self-assembled sphere with rich oxygen vacancies oriented on rGO (W-BiVO4-x/rGO) photocatalyst for efficient contaminants removal

A novel W doped BiVO4 with rich oxygen vacancies (OVs) oriented on reduced graphene oxide (rGO) photocatalyst (W-BiVO4-x/rGO) was developed by a simple one-step microwave hydrothermal method for efficient removal of ciprofloxacin (CIP) and chlortetracycline (CTC) under visible light. The synergistic effect of W doping, BiVO4 with OVs, and rGO on the enhancement of photocatalytic activity was clarified from experimental and Density Functional Theory (DFT) calculations results. W doping significantly improved the density of photo-generated charge carriers. The introduction of rGO enlarged the specific surface area, expanded the light absorption range, and promoted the charge transfer of BiVO4. OVs both narrowed the band gap and enhanced the generation of O2−. W doping and OVs both enhanced the interactions between BiVO4 and rGO. The CIP degradation rate constant of W-BiVO4-x/rGO photocatalyst was 1.73, 5.32, 6.43, and 10.70 times higher than those of W-BiVO4/rGO, BiVO4/rGO, W-BiVO4, and BiVO4, respectively. Besides, W-BiVO4-x/rGO photocatalyst prepared by microwave hydrothermal method exhibited higher photocatalytic activity and better combination between materials. This study offers a new strategy to address the problem of antibiotic residues in wastewater.

Atomlike interaction and optically tunable giant band-gap renormalization in large-area atomically thin MoS2

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

Ultrahigh sensitive transient absorption spectrometer.

Transient absorption (TA) spectroscopy is considered as a powerful technique that reflects the ultrafast dynamics of photogenerated carriers in photoelectric and photocatalysis materials. However, limited by its sensitivity, the photogenerated carrier density in TA measurements of solar energy materials is usually much higher than that in the real working condition. Here, we present a combination of kHz macro-pulse and MHz micro-pulse technique for an ultrahigh sensitive TA spectrometer, which improves the sensitivity to the 10-7 level of ΔOD. It enables us to study ultrafast carrier dynamics pumped by very low power, which can avoid the influence of many-body interactions and the nonlinear effect associated with high carrier density. This work provides a novel TA method with ultrahigh sensitivity, which will play an important role in investigating the carrier dynamics of semiconductors in the working condition.

Influence of doping for InSb on ultrafast carrier dynamics measured by time-resolved terahertz spectroscopy.

The influence of doping on the ultrafast carrier dynamics in InSb has been studied by time-resolved terahertz spectroscopy with photogenerated carrier densities from 1.5×1018 to 9.5×1019cm-3 at 800 nm. The photoinduced absorption and carrier recovery process show doping type dependence. The carrier recovery time of intrinsic InSb is greater than that of p-doped InSb but less than that of n-doped InSb at low carrier densities. At high carrier densities, compared with intrinsic InSb, the doped InSb is more prone to transient Auger recombination, which indicates that the appearance of the fast decay process depends on the carrier densities. The photoinduced absorption of terahertz probe pulse of n-doped InSb is significantly less than that of p-doped and intrinsic InSb; however, that of p-doped InSb is close to that of intrinsic InSb, which demonstrates that the high concentration of electrons can accelerate the efficiency of transient Auger recombination. Our analysis provides assistance to the design, manufacture, and improvement of photovoltaic detectors.

Polythiophene-sensitized TiO2 nanotube arrays for photo-generated cathodic protection of 304 stainless steel

TiO2 nanotube arrays (TNAs) with highly ordered three-dimensional structure are a promising alternative in photo-generated cathodic protection. Organic semiconductor polythiophene (Pth)-sensitized TNAs nanocomposite was synthesized by a two-step anodic oxidation and electrochemical anodic polymerization methods in this work. The crystalline phase, morphology, optical property and photoelectrochemical performance of Pth/TNAs samples with different polymerization times were investigated. Light absorption of Pth/TNAs samples improves with appropriate Pth deposition that increases the density of photo-generated charge carriers. Meanwhile, Pth deposition induces improvement in the inner electric field that drives electron transfer from semiconductor to the connected metal. Increased carrier separation and heterojunction barrier effects in Pth/TNAs/Ti/304 SS multiple interfaces enhances the photoelectrochemical performance of the photoanode that leads to excellent protection effect of 304 stainless steel with irradiation. Moreover, electrons reflux from metal corrosion will be retarded for the unidirectional continuity of electrons in heterojunction barriers without irradiation, which shows application potentials in photo-generated cathodic protection in dark.

Time-Resolved Changes in Dielectric Constant of Metal Halide Perovskites under Illumination.

Despite their impressive performance as a solar absorber, much remains unknown on the fundamental properties of metal halide perovskites (MHPs). Their polar nature in particular is an intense area of study, and the relative permittivity (εr) is a parameter widely used to quantify polarization over a range of different time scales. In this report, we have exploited frequency-dependent time-resolved microwave conductivity (TRMC) to study how εr values of a range of MHPs change as a function of time, upon optical illumination. Further characterization of charge carriers and polarizability are conducted by femtosecond transient absorption and stimulated Raman spectroscopy. We find that changes in εr are roughly proportional to photogenerated carrier density but decay with a shorter time constant than conductance, suggesting that the presence of charge carriers alone does not determine polarization.

Light-induced reversal of ion segregation in mixed-halide perovskites.

Bandgap instability due to light-induced phase segregation in mixed-halide perovskites presents a major challenge for their future commercial use. Here we demonstrate that photoinduced halide-ion segregation can be completely reversed at sufficiently high illumination intensities, enabling control of the optical bandgap of a mixed-halide perovskite single crystal by optimizing the input photogenerated carrier density. We develop a polaron-based two-dimensional lattice model that rationalizes the experimentally observed phenomena by assuming that the driving force for photoinduced halide segregation is dependent on carrier-induced strain gradients that vanish at high carrier densities. Using illumination sources with different excitation intensities, we demonstrate write-read-erase experiments showing that it is possible to store information in the form of latent images over several minutes. The ability to control the local halide-ion composition with light intensity opens opportunities for the use of mixed-halide perovskites in concentrator and tandem solar cells, as well as in high-power light-emissive devices and optical memory applications.

In situ NMR Investigation of the Photoresponse of Perovskite Crystal

Summary Hybrid perovskites are outstanding candidates for use in optoelectronic devices. Nonetheless, hybrid perovskites present numerous fundamental issues that are not yet fully understood. This study seeks to address this problem by reporting on the occurrence of ultraslow dynamic orientation changes in the methylammonium (MA) cations and the corresponding changes in the inorganic lattice of MAPbI3 perovskite crystals occurring over a time scale of 100 s under continuous illumination based on in situ single-crystal 2H nuclear magnetic resonance and in situ X-ray diffraction results. The demonstrated ultraslow dynamic changes in MAPbI3 are far slower than elementary molecular motions typically having time scales of femtoseconds and picoseconds and have never been reported previously. The ultraslow dynamic changes have been correlated with the reduction of band gap and the increase of absorption coefficient of MAPbI3, providing the molecular origins of the stabilization process in the photoresponse of the MAPbI3 device.

Carbon doped ultra-small TiO2 coated on carbon cloth for efficient photocatalytic toluene degradation under visible LED light irradiation

Photocatalytic activity triggered by visible light from light emitting diode (LED) is highly restricted by limited active sites, poor light absorption, and sluggish photo-generated carrier separation of the photocatalysts. Particularly for continuous degradation reaction of volatile organic compounds (VOCs), confined contact between flowing air pollutant gas and photocatalysts renders the degradation ineffective within a transient retention period. Considering the restriction of photocatalytic reaction occurred on semiconductor surface, ultra-small TiO2 (USTiO2) nanoparticles were fabricated by a facile acid-assisted solvent method, which possess copious active sites. A simple heating process was further applied for carbon doping on USTiO2 (C-USTiO2) surface. As a result, the obtained C-USTiO2 exhibited high visible light absorption, rich photo-generated carrier density, and its rapid carrier separation performance. In this study, optimized C-USTiO2 photocatalyst was deposited on carbon cloth and tested for continuous toluene degradation under LED light irradiation. Results showed that the toluene removal efficiency can reach higher than 80% with high concentration of CO2 generated and an excellent cycle stability of over 180 min.

Detailed description of the transient effect of radiative carrier transition in the bulk of semiconductor: The case of gallium antimonide

Upon solving the differential equation related to the rate of accumulation of photo-generated carriers in the respective bands of GaSb that involves radiative generation-recombination mechanisms under constant illumination in the entire bulk of the material, the transient response (time dependence) of the photo-generated carrier density and the excess carrier lifetimes during illumination and after switching-off illumination are described for a system under different injection levels. The rates of variations of the photo-generated carrier density and the radiative excess carrier lifetimes at the initial stages of the illumination and after switching-off the illumination are evaluated at different injection levels. The injection level dependences of the effective values of the photo-generated carrier density and the radiative excess carrier lifetimes are described. The competition of the thermal and the illumination effects in the band to band radiative recombination mechanism is also described thoroughly.

Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their 'working dynamic range' from which the recombination kinetics in thin-film solar cells can be determined.

Transient carrier recombination dynamics in potential‐induced degradation p‐type single‐crystalline Si photovoltaic modules

AbstractIn this study, we investigated the recombination dynamics of minority carriers through the decay profile analysis of transient diffuse reflectance spectroscopy (TDRS) for fresh and potential‐induced degradation (PID) modules. The PID‐affected region in terms of the degradation degree on the modules was firstly localized using conventional methods such as electroluminescence (EL) and lock‐in thermal images. The photogenerated carrier density and carrier lifetime were different in photovoltaic (PV) modules in fresh and PID modes. It was found that the dominant recombination involved the carrier transition via shallow trap states. The distribution of the trap states, however, was extended from the surface to the bulk of the solar cell due to Na ions–decorated defects. The behaviors of the carrier dynamics near the surface and bulk were very different, as inferred from the two different pump wavelengths of 532 and 1064 nm, respectively.

The polarization field in Al-rich AlGaN multiple quantum wells

This paper investigates the quantum confined Stark effect in AlGaN multiple quantum well structures with a high Al content grown on single-crystalline AlN substrates. The quantitative relationship between the quantum well structure parameters, photogenerated carrier density, built-in electric field and ground-level emission is discussed. It is found that the electric field strength increases from 0.5 MV cm−1 to almost 3 MV cm−1 when the Al content in the quantum well barriers is increased from 65% to 100%, which is consistent with the theory of spontaneous and piezoelectric polarization in III-nitrides. In addition, the built-in electric field increases significantly with increasing barrier thickness. Based on these results, the electric field in an Al0.55Ga0.45N single quantum well with AlN cladding is predicted to be around 5 MV cm−1.

Ultrahigh-Frequency Heterodyne Lock-In Carrierography for Large-Scale Quantitative Multi-Parameter Imaging of Colloidal Quantum Dot Solar Cells

Ultrahigh-frequency heterodyne lock-in carrierography (UHF-HeLIC) is a new near-IR dynamic photoluminescence (PL) camera-based large-area imaging technique. It is specifically developed for investigating dynamic optoelectronic events that occur at very fast diffusion-wave rates (> = 10 kHz), above which conventional camera imaging cannot be achieved. The nonlinear combination of two-frequency-modulated photogenerated carrier density waves (CDWs) with a 10 Hz beat frequency was used to generate HeLIC images in the UHF range (up to 270 kHz) above the effective CDW recombination rate. Using sequences of these high-frequency images and UHF-HeLIC theory, carrier lifetime, diffusivity, and diffusion and drift length images were reconstructed from colloidal quantum dot solar cells and used for studying the influence of surface and interface trap states on photocarrier transport processes. The new non-destructive UHF-HeLIC imaging methodology shows excellent potential for industrial in-line photovoltaic device characterization, fundamental optoelectronic physical process studies, and various other photoelectronic applications.

Comment on “Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities”

Comment on “Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities”

Ultrasonic synthesis of Zn-doped CdO nanostructures and their optoelectronic properties

Abstract The effect of Zn dopant on the growth of cadmium oxide (CdO) nanostructures through a sonochemical method was investigated. The X-ray diffraction (XRD) patterns of the nanoparticles show CdO cubic structures for the produced samples. Field emission scanning electron microscope (FESEM) images reveal that morphologies of the samples change, when they are doped with Zn atoms, and their sizes reduce. Room temperature photoluminescence (PL) and UV-Vis spectrometers were used to study optical properties of the samples. Evaluation of optical properties indicates that different emission bands result from different transitions and the value of CdO energy band gap increases due to doping. Studies of electrical properties of the nanostructures demonstrate that Zn dopant enhances electrical conductivity and photocurrent generation as the result of light illumination on the nanostructures due to improved density of photo-generated carriers. Considering the obtained outcomes, Zn dopant can alter the physical property of the CdO nanostructures.

Photomodulated Reflectivity Measurement of Free-Carrier Dynamics in InGaN/GaN Quantum Wells

We describe a novel technique for measuring carrier dynamics in solid-state optical materials based on photomodulated reflectivity (PMR) and, as an example, apply it to a study of an InGaN/GaN multi-quantum-well (QW) structure grown on a c-plane sapphire substrate. The technique is a form of frequency modulation spectroscopy and relies on probing changes in refractive index induced by fluctuations in free-carrier density during optical excitation. We show that it is possible to accurately determine both carrier density and lifetime, independent of any photoluminescence (PL) measurement and with no knowledge of the incident, or fraction of absorbed, laser power, quantities that can give rise to considerable uncertainties in PL studies. We demonstrate that such uncertainties can lead to an order of magnitude underestimation of the total photogenerated carrier density and compromised accuracy in determining carrier lifetime. We determine, by a comparison of the two techniques, PMR and PL, the nonradiative Shockley–Reed–Hall (SRH), radiative (excitonic), and nonradiative Auger-related coefficients (from the standard ABC model). We find marked differences in the carrier-density-dependent lifetime, determined from PMR, translate to significant differences in the SRH and excitonic coefficients, which we believe relate to the more accurate determination of carrier densities in PMR than in PL. We also find evidence from the PMR for a change in effective mass of the photoexcited carriers with excitation intensity, which points to a complex localization/delocalization mechanism, likely facilitated by random fluctuations in indium content and QW width, consistent with previous findings by independent methods.

Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide

Many time-resolved techniques to study charge carrier recombination involve pulsed high-power optical excitation and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. In this report, we demonstrate a steady-state contactless microwave conductivity technique to evaluate the photoconductivity of carriers in semiconductors at low illumination intensity, as a function of optical power density. We studied characteristics of both thin films and single crystals of a hybrid halide perovskite compound, methylammonium lead iodide (MAPbI3). The aggregate mobility-lifetime product of majority and minority carriers in thin films of MAPbI3 was determined and found to be highly-dependent on incident optical power density, even at sub-1-sun illumination intensities, and attributed to trap states within the films.

Carrier dynamics in the potential-induced degradation in single-crystalline silicon photovoltaic modules

In this study, transient diffuse reflectance spectroscopy (TDRS) and electroluminescence (EL) analysis have been employed to clarify the carrier dynamics in potential-induced degradation (PID) in single-crystalline Si (sc-Si) solar cell. We first localized the PID-affected region in terms of degradation intensity on the modules on the basis of EL. The carrier dynamics in that region are then studied and clarified in terms of carrier lifetime, defect level, and photogenerated carrier density. Our results suggest that carrier relaxation in a fresh solar cell proceeds via band to band and/or shallow and deep donor–acceptor recombination. However, the dominant recombination in solar cells with PID is intercenter charge transfer via shallow-to-deep and/or deep–deep defects for which the carrier lifetime decreased drastically. Also, it is found that the carrier dynamics near the surface and bulk do not progress similarly as confirmed using 532- and 1064-nm-wavelength pumps.

Optical absorption mechanism of GaAs photoconductive semiconductor switch

The output charges of linear and nonlinear semi-insulating GaAs photoconductive semiconductor switches (PCSSs) triggered by a 1064 nm nanosecond pulse laser with an energy of millijoule level are measured under different bias voltages. On the basis of the operation mechanism of the linear PCSS, the photogenerated carrier densities of the PCSS are measured. From the results of the comparison between the measured values and the values determined by different absorption mechanisms, the absorption mechanism of the switch is analyzed. As an important parameter of the avalanching charge domain, the effect of the photogenerated carrier density on the output performance of GaAs PCSSs is discussed.