Lifetime Of Photoexcited Carriers Research Articles

Effects of oxygen vacancies on the photoexcited carrier lifetime in rutile TiO2.

The photoexcited carrier lifetime in semiconductors plays a crucial role in solar energy conversion processes. The defects or impurities in semiconductors are usually proposed to introduce electron-hole (e-h) recombination centers and consequently reduce the photoexcited carrier lifetime. In this report, we investigate the effects of oxygen vacancies (OV) on the carrier lifetime in rutile TiO2, which has important applications in photocatalysis and photovoltaics. It is found that an OV introduces two excess electrons which form two defect states in the band gap. The lower state is localized on one Ti atom and behaves as a small polaron, and the higher one is a hybrid state contributed by three Ti atoms around the OV. Both the polaron and hybrid states exhibit strong electron-phonon (e-ph) coupling and their charge distributions become more and more delocalized when the temperature increases from 100 to 700 K. Such strong e-ph coupling and charge delocalization enhance the nonadibatic coupling between the electronic states along the hole relaxation path, where the defect states behave as intermediate states, leading to a distinct acceleration of e-h recombination. Our study provides valuable insights to understand the role of defects on photoexcited carrier lifetime in semiconductors.

Photocatalytic Activity Enhancement of Anatase/Rutile‐Mixed Phase TiO2 Nanoparticles Annealed with Low‐Temperature O2 Plasma

Photodecomposition and photobactericidal activities of anatase/rutile‐mixed phase TiO2 nanoparticles annealed with low‐temperature O2 plasma are clarified by comparing them with those annealed in ambient air. The photocatalytic activities of plasma‐assisted‐annealed sample greatly enhance as compared with the untreated sample, under not only UV irradiation but also visible‐light irradiation. The photocatalytic activities of air‐annealed samples do not enhance under UV irradiation but enhance under visible‐light irradiation. The enhanced photocatalytic activities due to the plasma‐assisted annealing (PAA) originate from the increased photoexcited carrier concentration. This enhancement is discussed from PAA‐induced characteristic factors. PAA facilitates the phase transformation to anatase, contributing directly to extending the photoexcited carrier lifetime. PAA introduces more oxygen vacancies, contributing to trapping more photogenerated electrons. PAA also introduces more bridging/terminal oxygen groups adsorbed on the surface, increasing the upward band‐bending, the depletion layer width at the surface, and the charge transfer from rutile to anatase. These two introductions contribute to facilitating the separation of photoexcited carriers. Furthermore, PAA reduces the aggregate size of TiO2 nanoparticles formed on the surface, contributing to increasing optical absorptions. More reactive oxygen species produced from the bridging/terminal oxygen groups by the photoexcited carriers will also enhance the photocatalytic activities.

Controlling Photocarrier Lifetime in Graphene for Enhanced Photocurrent Generation via Cascade Hot Electron Transfer.

Because of its broad absorption and high carrier mobility, graphene has been regarded as a promising photoactive material for optoelectronics. However, its ultrashort photoexcited carrier lifetime greatly restricts the device performance. Herein, we show that by constructing a graphene/WS2/MoS2 vertical heterostructure with a cascade electron-transfer pathway, the hot electrons in graphene under low-energy photoexcitation can efficiently transfer through WS2 to MoS2 in 180 fs, thus effectively photogating the graphene layer. Because of the spatial separation and energy barrier imposed by the WS2 intermediate layer which retards back electron transfer, the photocarrier lifetime in graphene is significantly prolonged to ∼382.7 ps, more than 2 orders of magnitude longer than in isolated graphene and graphene/WS2 binary heterostructure. The prolonged photocarrier lifetime in graphene leads to dramatically enhanced photocurrent generation and photoresponsivity. This study offers an exciting approach to control photocarrier lifetime in graphene for hot carrier devices with simultaneous broadband and high responsivity.

Electronic and catalytic engineering in two-dimensional vdW metal–organic frameworks through alloying

Bimetallic metal-organic framework (MOFs) alloys, in which heterogeneous metal clusters are incorporated into their backbone, are capable of highly selective separations and catalysis. Due to limitations in our fundamental understanding of their alloying, however, established methods result in phase-separated or amorphous two-dimensional (2D) MOFs or lack precise control over alloy ratios. Here, our results demonstrate 2D MOF alloys where metal cation ratios (M1 and M2) in M1xM21-xBDC (M1 or M2= Zn, Cu, Ni, Co, Fe, Mn) can be engineered on demand by controlling the metal salt dissociation constants. Resulting MOF alloys exhibit a highly 2D nature with excellent crystallinity and minute control over metal cation ratios. Our experimental and theoretical results show that their electronic bandgaps and photoexcited carrier lifetimes can be engineered by metal cation alloying. Interestingly, 2D alloyed MOFs enable high-efficiency photo-catalytic water reduction performance in Co/Ni MOF alloys owing to the spatially separated metal clusters in 2D MOF alloys.

Efficient hydrogen generation of vector Z-scheme CaTiO3/Cu/TiO2 photocatalyst assisted by cocatalyst Cu nanoparticles

Herein, the CaTiO3/Cu/TiO2 all-solid-state Z-scheme heterojunction is successfully designed via Cu nanoparticles situating at the interface between CaTiO3 and TiO2 with a new synthesis route. Interestingly, TiO2 nanosheets are generated in-situ on the surface of CaTiO3 in the second step hydrothermal reaction. The lifetimes of photoexcited carriers, photoluminescence emission spectra and transient photocurrent response tests have confirmed that the efficient Z-scheme charge transmission path of the CaTiO3/Cu/TiO2 is beneficial to facilitate the separation of photogenerated carriers and reduce their recombination efficiency. As expected, the hydrogen generation rate of CaTiO3/Cu/TiO2 is increased to 23.550 mmol g-1h−1 with the appropriate amount of copper loading, which is about 981 times and 93 times higher than that of pristine CaTiO3 (0.024 mmol g-1h−1) and CaTiO3/TiO2 (0.253 mmol g-1h−1), respectively. Furthermore, the CaTiO3/Cu/TiO2 sample shows good stability in cycle experiments. Particularly, experimental results show that the non-noble metal Cu nanoparticles can be an effective electron mediator. And these merits strongly demonstrate that the CaTiO3/Cu/TiO2 composites have potential application in photocatalytic field. This study can provide fundamental guidance for designing rationally efficient non-noble metal vector Z-scheme system photocatalysts with outstanding photocatalytic H2 generation performance.

Enhanced photocatalytic performance of heterogeneous hydrotalcite by spontaneously polarized ferroelectric

Two-dimensional photocatalytic materials have attracted great attention due to their large specific surface area and abundant active sites. Suppressing the recombination of photo-excited carriers is an effective approach to improve the performances of photocatalytic materials. Herein, we introduced ferroelectric PbTiO3 into the two-dimensional layered double hydroxides (LDHs) to improve the carrier separation efficiency and photocatalytic performances. A built-in electric field was generated in the polarized PbTiO3, resulting in the improvement of the carrier separation efficiency and the promotion of the lifetime of photo-excited carriers in the LDHs-PbTiO3 composites. As a result, the LDHs-PbTiO3 composites showed the decent photocatalytic performances towards water splitting under visible light irradiation. The oxygen production rate of the proposed LDHs-PbTiO3 composites was almost twice than that of pristine LDHs. These results have addressed the significance of photo-excited carriers in photocatalytic materials. This approach could undoubtedly provide the valuable information in design and construction of high efficiency photocatalysts.

Structural defects in MBE-grown CdTe-basing heterojunctions designed for photovoltaic applications

Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline heterojunctions designed for photovoltaic applications have been investigated by transmission electron microscopy (TEM) and deep-level transient spectroscopy (DLTS). Lattice parameters and misfit strain in the undoped cadmium telluride (CdTe) absorber layers of the heterojunctions, grown by the molecular-beam epitaxy technique on two different substrates, GaAs and CdTe, have been determined with high-resolution x-ray diffractometry. A dense network of misfit dislocations at the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous threading dislocations and stacking faults have been shown by the cross-sectional TEM imaging of the heterojunctions. The DLTS measurements revealed five deep-level traps in the heterojunctions grown on GaAs and only three of them in the heterojunctions grown on CdTe. Four of the traps have been attributed to the electronic states of extended defects, presumably dislocations, on the grounds of their logarithmic capture kinetics of charge carriers. Two of these traps, displaying the largest values of their capture cross-section and the properties characteristic of bandlike electronic states, have been ascribed to the core states of dislocations. It is argued that they are most likely responsible for decreased lifetime of photo-excited carriers resulting in a low energy conversion efficiency of solar cells based on similarly grown heterojunctions.

Graphene/SrTiO3 interface-based UV photodetectors with high responsivity* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFF0104801 and 2018YFB0406601) and the National Natural Science Foundation of China (Grant Nos. 61804012 and 11721404).

Strontium titanate (SrTiO3), which is a crucial perovskite oxide with a direct energy band gap of 3.2 eV, holds great promise for ultraviolet (UV) photodetection. However, the response performance of the conventional SrTiO3-based photodetectors is limited by the large relative dielectric constant of the material, which reduces the internal electric field for electron-hole pair separation to form a current collected by electrodes. Recently, graphene/semiconductor hybrid photodetectors by van-der-Waals heteroepitaxy method demonstrate ultrahigh sensitivity, which is benefit from the interface junction architecture and then prolonged lifetime of photoexcited carriers. Here, a graphene/SrTiO3 interface-based photodetector is demonstrated with an ultrahigh responsivity of 1.2 × 106 A/W at the wavelength of 325 nm and ∼ 2.4 × 104 A/W at 261 nm. The corresponding response time is in the order of ∼ ms. Compared with graphene/GaN interface junction-based hybrid photodetectors, ∼ 2 orders of magnitude improvement of the ultrahigh responsivity originates from a gain mechanism which correlates with the large work function difference induced long photo-carrier lifetime as well as the low background carrier density. The performance of high responsivity and fast response speed facilitates SrTiO3 material for further efforts seeking practical applications.

Atom-by-Atom Evolution of the Same Ligand-Protected Au21, Au22, Au22Cd1, and Au24 Nanocluster Series.

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au22(SAdm)16 and Au22Cd1(SAdm)16 (SAdm = adamantanethiolate) which link up with Au21(SAdm)15 and Au24(SAdm)16 NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au22(SAdm)16 has an Au3(SAdm)4 surface motif which is longer than the Au2(SAdm)3 on Au21(SAdm)15, whereas Au22Cd1(SAdm)16 lacks one staple Au atom compared to Au24(SAdm)16 and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au22 with a 10-atom bioctahedral kernel to Au22Cd1 with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties.

Synthesis of Zn doped g-C3N4 in KCl-ZnCl2 molten salts: The temperature window for promoting the photocatalytic activity

The Zn doped g-C3N4 is successfully synthesized in eutectic KCl-ZnCl2 salt mixture with low melting temperature for the first time. More importantly, the significant improvement in the photocatalytic activity can only be achieved by locating the melting point of the salt mixture within the temperature window between dicyandiamide and melamine oligomer formation steps in the polycondensation process. The prepared g-C3N4 using dicyandiamide as the precursor (C3N4-D) shows ~9.3 times larger specific surface area (SSA), ~9.8 times larger photocurrent density and ~1.56 times longer photo-excited carrier lifetime than the bulk g-C3N4 synthesized via the conventional thermal polymerization method in air (C3N4-M-Air). In photocatalytic methyl orange (MO) degradation, the pseudo-first reaction rate constant of C3N4-D is ~4.15 times higher than the C3N4-M-Air control group. The significantly promoted MO photodegradation performance is attributed to the remarkably enlarged SSA, enhanced MO adsorption, the positively shifted valence band maximum (VBM) and suppressed electron-hole recombination due to Zn dopants.

Photocatalytic properties of graphene-supported titania clusters from density-functional theory.

Density-functional theory calculations of (TiO2 )n clusters (n = 1-5) in the gas phase and adsorbed on pristine graphene as well as graphene quantum dots are presented. The cluster adsorption is found to be dominated by van der Waals forces. The electronic structure and in particular the excitation energies of the bare clusters and the TiO2 /graphene composites are found to vary largely in dependence on the size of the respective constituents. This holds in particular for the energy and the spatial localization of the highest occupied and lowest unoccupied molecular orbitals. In addition to a substantial gap narrowing, a pronounced separation of photoexcited electrons and holes is predicted in some instances. This is expected to prolong the lifetime of photoexcited carriers. Altogether, TiO2 /graphene composites are predicted to be promising photocatalysts with improved electronic and photocatalytic properties compared to bulk TiO2 .

Fabrication of carbon quantum dots/1D MoO3-x hybrid junction with enhanced LED light efficiency in photocatalytic inactivation of E. coli and S. aureus

Abstract Novel carbon quantum dots/1D MoO3-x hybrid junction (noted as CQDs/MoO3-x) was obtained by a facile hydrothermal route. The CQDs/MoO3-x photocatalyst was estimated as an effective photocatalyst for inactivating E. coli and S. aureus under LED light. Compared with pure MoO3 and MoO3-x, CQDs/MoO3-x photocatalyst displayed better performance in inactivating E. coli and S. aureus. The results of UV–visible diffuse reflectance spectrum (UV–vis DRS), transmission electron microscopy (TEM) and electrochemical impedance spectrum (EIS) indicated the enhanced photoactivity is ascribed to: (1) MoO3-x has good absorption property of visible light; (2) the 1D structure of MoO3-x prolongs the lifetime of photo-excited charge carriers; (3) CQDs modifying 1D MoO3-x is not only favorable for absorption of visible light, but also for the fast transport and surface separation of the charge carriers. The capture experiment showed that holes (h+) are main active species in the photocatalytic process. This work presents an efficient photocatalyst for water antibacterial treatment.

Suppressing photoexcited electron–hole recombination in MoSe2/WSe2 lateral heterostructures via interface-coupled state engineering: a time-domain ab initio study

Interface-coupled states play a vital role in photoexcited carrier lifetime of two-dimensional lateral heterostructure-based photovoltaic and photoelectric devices.

Charge-Carrier Recombination in Halide Perovskites.

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.

Balancing Light Absorption and Charge Transport in Vertical SnS2 Nanoflake Photoanodes with Stepped Layers and Large Intrinsic Mobility

AbstractSignificant optical absorption in the blue–green spectral range, high intralayer carrier mobility, and band alignment suitable for water splitting suggest tin disulfide (SnS2) as a candidate material for photo‐electrochemical applications. In this work, vertically aligned SnS2 nanoflakes are synthesized directly on transparent conductive substrates using a scalable close space sublimation (CSS) method. Detailed characterization by time‐resolved terahertz and time‐resolved photoluminescence spectroscopies reveals a high intrinsic carrier mobility of 330 cm2 V−1 s−1 and photoexcited carrier lifetimes of 1.3 ns in these nanoflakes, resulting in a long vertical diffusion length of ≈1 µm. The highest photo‐electrochemical performance is achieved by growing SnS2 nanoflakes with heights that are between this diffusion length and the optical absorption depth of ≈2 µm, which balances the competing requirements of charge transport and light absorption. Moreover, the unique stepped morphology of these CSS‐grown nanoflakes improves photocurrent by exposing multiple edge sites in every nanoflake. The optimized vertical SnS2 nanoflake photoanodes produce record photocurrents of 4.5 mA cm−2 for oxidation of a sulfite hole scavenger and 2.6 mA cm−2 for water oxidation without any hole scavenger, both at 1.23 VRHE in neutral electrolyte under simulated AM1.5G sunlight, and stable photocurrents for iodide oxidation in acidic electrolyte.

High efficiency Si quantum dot heterojunction solar cells using a single SiOX:B layer

Si quantum dots (QDs) have been fabricated from SiO2/SiOx multilayer structures to create a homogeneous size. However, this structure achieved much lower efficiencies than would be expected in the Si QD photovoltaic field. This is because Si QD generation and photoexcited carrier transport is restricted by the adjacent SiO2 layer. In this study, we applied a single SiOx:B layer fabrication method to the Si QD heterojunction solar cells. The number of generated Si QDs and the photo-excited carrier lifetime was maximized when the oxygen partial pressure and boron doping concentration parameters were 2.7 × 10−5 Torr and 2.27 × 1021 atoms cm−3, respectively. As a result, in excess of 17% power conversion efficiency for the Si QD heterojunction solar cell was achieved using the single layer method.

Relating Carrier Dynamics and Photovoltaic Device Performance of Single-Crystalline Cu2ZnSnSe4

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination to structure and processing of photovoltaic absorber materials is critical to designing efficient solar cells. Using three complementary techniques, the authors elucidate photoexcited carrier dynamics in a high-quality kesterite crystal. These dynamics help to explain limitations in charge-carrier collection observed in a device's $J\ensuremath{-}V$ relation and external quantum efficiency. Combining ultrafast spectroscopy and device measurements in this way can lead to a more detailed understanding of the performance-limiting photophysical processes, accelerating the improvement of solar cells.

Visible-infrared dual-mode MoS2-graphene-MoS2 phototransistor with high ratio of the Iph/Idark

2D materials such as graphene and transition metal dichalcogenides exhibit novel electrical and optoelectrical properties, which are promising for the application in sensing and imaging. However, the photoresponse ability of pure monolayer graphene in room temperature and ratio of Iph and Idark are still unsatisfactory, due to the weak light absorption itself, short lifetime of photo-excited carriers and large dark current, especially in the infrared range. Here, we successfully constructed a broadband graphene-based phototransistor consisting of lateral MoS2-graphene-MoS2 heterostructure by chemical vapor deposition technique. It shows broadband photodetection and a low dark current less than 1 pA at applied bias of 0.5 V, resulting in excellent specific detectivity more than 1012 Jones and nearly 109 Jones in visible and infrared band, respectively. Moreover, the ratio of Iph and Idark could be modulated by the back-gate voltage when the phototransistor is under the infrared illumination. And a high ratio of 105 was exhibited. This type of graphene-based phototransistor by chemical vapor deposition demonstrates an approach for the room temperature broadband photodetection of monolayer graphene by energy engineering. All the results address key challenges for broadband detection by graphene-based phototransistor, and are promising for the large scale fabrication and integration in the future electronic and optoelectronic applications.

Photoexcited Small Polaron Formation in Goethite (α-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV, whereas the polaron formation probability (85 ± 10%) remains constant. By comparison to hematite (α-Fe2O3), an oxide analogue, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.

Improved near infrared energy harvesting through heterogeneously coupled SK on SML quantum dot heterostructure

Near Infra Red (NIR) energy harvesting is one of the advanced strategies to improve the overall performance of new generation solar photovoltaics. We introduce here a new heterogeneously coupled Stranski–Krastanov (SK) on Submonolayer (SML) quantum dot (QD) heterostructure, which is capable of enhanced energy conversion efficiency in that typical spectral range. The coupling in the proposed structure has been optimized by varying the growth rate and the barrier thickness between seed SML and top SK QDs. The optimal configuration has been incorporated into a quantum dot solar cell (QDSC). The carrier transitions in this hybrid QD assembly have been investigated through luminescence spectroscopy. An existence of carrier tunnelling between SML and SK QDs has been revealed from the photoluminescence and the photoluminescence excitation spectroscopy. The unique configuration of such coupled QDs shows the presence of resonating energy states. These phenomena ameliorate the absorption efficiency in the NIR regime with increased photoexcited carrier lifetime, which has been reflected in the external quantum efficiency.