Electron Injection Process Research Articles

Unified theory of plasmon-induced resonance energy transfer and hot electron injection processes for enhanced photocurrent efficiency.

Plasmons in metal nanoparticles (MNPs) promise to enhance solar energy conversion in semiconductors. Two essential mechanisms of enhancement in the near-field regime are hot electron injection (HEI) and plasmon-induced resonance energy transfer (PIRET). Individual studies of both mechanisms indicate that the PIRET efficiency is limited by the short lifetime of the plasmon, whereas the hot electrons result from the plasmon decay. The development of a unified theory of the coupled HEI and PIRET processes is fundamentally interesting and necessary for making reliable predictions but is complicated by the multiple interactions between various components that participate in the enhancement process. In this paper, we use the model-Hamiltonian approach to develop a combined theoretical framework including both PIRET and HEI. The coupled dynamics as well as the time evolution of hot electron energy distribution are studied. The theory further predicts an interference-induced asymmetry in the spectral dependence of PIRET, which can be used to distinguish it from HEI. As the relative contributions of PIRET and HEI strongly depend on the size of the MNPs, this presents itself as a simple route to control the strength of their contributions. The results presented here can further guide future applications of plasmonic solar energy harvesting.

Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells

Within this work, we emphasis on the importance of twisted intramolecular charge transfer (TICT) process in organic dyes based on triphenyl amine moiety to achieve high performance in dye-sensitized solar cells. Through the comparison between two recent made dyes, L1 and L1Fc, on different semiconductors (TiO2, and ZrO2), we could spectrally and dynamically detect for the first time the formation of TICT state for L1 on ZrO2 after localized charge transfer (LCT) state population, and an electron injection process from TICT state on TiO2. However, for the excited L1Fc dye, the ultrafast electron transfer from ferrocene (Fc) moiety to the L1 unit quenched the formation of TICT state in L1Fc on semiconductors, leading instead to an electron injection process from the LCT state. The electron injection from TICT state in L1 associated with structural rearrangements on TiO2 leads to slow recombination process and an efficiency improvement of about 325%, compared to solar cells based on L1Fc dye, in which TICT s...

Injection dynamics of direct-laser-accelerated electrons in a relativistically transparent plasma

The dynamics of electron injection in the direct laser acceleration (DLA) regime was investigated by means of three-dimensional particle-in-cell simulations and theoretical analysis. It is shown that when an ultra-intense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites an ion wave. The ion wave modulates the local electric field and acts as a set of potential wells to guide the electrons, located on the edge of the plasma channel, to the central region, where the DLA takes place later on. In addition, it is pointed out that the self-generated azimuthal magnetic fields tend to suppress the injection process of electrons by deflecting them away from the laser field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated X/gamma-ray sources.

The origin of slow electron injection rates for indoline dyes used in dye-sensitized solar cells

This work highlights the direct impact of selecting acceptor moiety for organic dyes on the electron dynamics at faster time scales, in which overlooked photo-physical properties are present on semiconductor surfaces with specific acceptor moieties. Four top-performing dyes of indoline family (D131, D102, D149, and D205) sharing the same donor moiety, but through different acceptor groups, were selected and compared with respect of electron injection process, using ultrafast transient-infrared probe. The presence of rhodanine moiety at the acceptor unit in D102, D149 and D205, shows an additional slow electron injection process, of picosecond time-scale, on the low band-gap semiconductor, TiO2. This slow process is expected to be present due to a twisted intramolecular charge transfer/isomerized state of the excited dye prior to electron injection. This isomerized state reduces as well the detrimental electron recombination process rates, and results of high performance in solar cells based on these rhodanine dyes. Replacing the rhodanine moiety by a cyano-acrylic group in D131 dye shows faster electron injection and recombination processes, due to the lower dipole moment present in the excited state, hindering the formation of an isomerized state. These findings will aid to enhance the organic dyes design used in dye sensitized solar cells, in which designed photo-physical processes on semiconductor surfaces can increase the efficiencies of the solar cells.

Hybridizing Strong Quadrupole Gap Plasmons Using Optimized Nanoantennas with Bilayer MoS2 for Excellent Photo‐Electrochemical Hydrogen Evolution

AbstractPlasmonic nanoantennas with efficient scattering and strong electromagnetic field confinement are integrated into atomically thin transition metal dichalcogenides to gain insight into nanoscale light–matter interactions. Herein, unusual quadrupole gap plasmons (QGPs) are introduced in the tailored nanoantennas. The field enhancement of such QGPs is further maximized by optimizing the structure of the nanoantennas using a statistical method. In addition, the optimized nanoantennas are employed on a chemical vapor reaction‐grown bilayer molybdenum disulfide (MoS2) to tune and intensify the optical response of MoS2. The optimized QGP structure performance was enhanced by a factor of 27.87 and a beneficial continuous bilayer MoS2 can be applied in the hydrogen evolution reaction (HER) with superior outcomes. MoS2@QGP acts as an excellent photocatalyst for HER activity because of its accelerated electron injection process, which is faster than the electron–hole pair recombination, supported by the good light‐trapping capacity of the QGP structure and plentiful accepting sites of the continuous bilayer MoS2 film. The experimental results demonstrate that the QGP enhances the intrinsic catalytic properties of bilayer MoS2 for photocatalytic reactions.

Unravelling the Role of Surface Traps on Carrier Relaxation and Transfer Dynamics in Ultrasmall Semiconductor Nanocrystals

Charge carrier trapping by the surface defects of colloidal semiconductor nanocrystal (NC) is a ubiquitous process which limits the performance of NC-based photovoltaic and photocatalytic devices. Although several empirical approaches led to the enhancement of device efficiency via passivation of trap states, a systematic and unified description of trapping mechanism remains obscure. In this contribution, we present a detailed experimental investigation of the carrier dynamics of CdSe NCs with varying concentration of surface traps by means of time-resolved photoluminescence (PL) and absorption. Our study reveals that the rate of carrier cooling becomes faster as trap density increases because of the increased hot carrier trapping. A comparative dynamical study is also presented to demonstrate how trap states influence electron injection process. This enhanced understanding of the role of trap sates on charge carrier dynamics can provide valuable insight toward the rational development of more efficient N...

AgSbS2 thin film fabricated by in-situ gas-solid reaction and employed in solar cells as a light absorber

This work reports the fabrication of AgSbS2 semiconductor films via in-situ gas-solid reaction approach, including sequential solution-deposition of AgNO3 and SbCl3 percursor solutions on mesoporous TiO2 substrates, and subsequent sulfurization of the prepared films at H2S atmosphere. The resultant TiO2/AgSbS2 films show broad spectral absorption ranged from 350 nm to 800 nm with a band gap of 2.01 eV. X-ray photoelectron spectroscopy (XPS) spectra confirm the fabrication of phase-pure AgSbS2 without any impurity. Furthermore, the devices with a structure of fluorine-doped SnO2 (FTO)/TiO2/AgSbS2/Au have been prepared, exhibiting quick interfacial charge transfer and electron injection process with excellent photoresponse and high photostability.

Nanohybrid-sensitized photoelectrochemical cells for solar-to-hydrogen conversion

Abstract

Donor Effect on the Photoinduced Interfacial Charge Transfer Dynamics of D−π–A Diketopyrrolopyrrole Dye Sensitizers Adsorbed on Titanium Dioxide

In this report, we investigate the effects of a strong electron-donating group on the electron injection and recombination processes of diketopyrrolopyrrole (DPP)-sensitized TiO2 films by comparing the dynamics of triphenylamine donor-installed DPP_A and donor-absent DPP_R dyes. We show that the electron-donating group enhances the intramolecular charge transfer (ICT) characteristics of DPP_A, as demonstrated by a significant spectral redshift in the steady-state measurement of DPP_A compared to that of DPP_R. By comparing the ultrafast transient behaviors of the dye-coated films through femtosecond transient absorption (fs-TA) measurements, we provide insight into the high efficiency of DPP_A: most of the electrons from DPP_A are injected into the conduction band of TiO2 within the time resolution of TA (<150 fs), while DPP_R displays a biphasic electron injection process of ca. 340 ps (including slow decay). Additionally, due to the existence of lateral intermolecular interactions on TiO2, a phenomenon ...

Quadrupole Gap Surface Plasmon Integrated Bilayer MoS2 for Excellent Photoelectrochemical Hydrogen Evolution

Plasmonic nanoantennas with efficient scattering and strong electromagnetic field confinement were integrated to atomically thin transition metal dichalcogenides (TMDC) to shed light on the nanoscale light-matter interactions. Herein, we optimized the quadrupole gap surface plasmonic (QGSP) structure by Taguchi design to maximize field enhancement and integrated to CVD grown bilayer molybdenum disulfide (MoS2), beneficially tuning the optical properties of bilayer MoS2. The optimized QGSP structure with 27.87 factor enhanced performance and advantageous 1cm2 continuous bilayer MoS2 finds potential application for hydrogen evolution reaction (HER) with superior outcome. MoS2@QGSP act as an excellent photocatalyst for HER activity on account of accelerated electron injection process in comparison to electron-hole pair recombination supported from skilled light trapping capacity of QGSP structure and plentiful accepting sites of large surface continuous bilayer MoS2 film. This report demonstrates the gap surface plasmon enhanced intrinsic catalytic properties of bilayer MoS2 for photocatalytic reactions.

Organic Light-Emitting Transistors with Simultaneous Enhancement of Optical Power and External Quantum Efficiency via Conjugated Polar Polymer Interlayers.

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, an efficient injection of charges into the emissive layer is required. The introduction of solution-processed conjugated polyelectrolyte (CPE) films at the emissive layer/electrode interface represents a promising strategy to improve the electron injection process by dipole formation. However, their use in optoelectronic devices also involves some limitations because of the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs because the conjugated backbones of CPPs are functionalized with polar nonionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene-containing phosphonate groups underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET-emissive layer and, accordingly, a more than 2-fold increased light power and a 5 times higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices, allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs.

Identifying an Optimum Perovskite Solar Cell Structure by Kinetic Analysis: Planar, Mesoporous Based, or Extremely Thin Absorber Structure

Perovskite solar cells have rapidly been developed over the past several years. Choice of the most suitable solar cell structure is crucial to improve the performance further. Here, we attempt to determine an optimum cell structure for methylammonium lead iodide (MAPbI3) perovskite sandwiched by TiO2 and spiro-OMeTAD layers, among planar heterojunction, mesoporous structure, and extremely thin absorber structure, by identifying and comparing charge carrier diffusion coefficients of the perovskite layer, interfacial charge transfer, and recombination rates using transient emission and absorption spectroscopies. An interfacial electron transfer from MAPbI3 to compact TiO2 occurs with a time constant of 160 ns, slower than the perovskite photoluminescence (PL) lifetime (34 ns). In contrast, fast non-exponential electron injection to mesoporous TiO2 was observed with at least two different electron injection processes over different time scales; one (60–70%) occurs within an instrument response time of 1.2 ns...

Combining Fullerenes and Zwitterions in Non‐Conjugated Polymer Interlayers to Raise Solar Cell Efficiency

AbstractPolymer zwitterions were synthesized by nucleophilic ring‐opening of 3,3′‐(but‐2‐ene‐1,4‐diyl)bis(1,2‐oxathiolane 2,2‐dioxide) (a bis‐sultone) with functional perylene diimide (PDI) or fullerene monomers. Integration of these polymers into solar cell devices as cathode interlayers boosted efficiencies of fullerene‐based organic photovoltaics (OPVs) from 2.75 % to 10.74 %, and of non‐fullerene‐based OPVs from 4.25 % to 10.10 %, demonstrating the versatility of these interlayer materials in OPVs. The fullerene‐containing polymer zwitterion (C60‐PZ) showed a higher interfacial dipole (Δ) value and electron mobility than its PDI counterpart (PDI‐PZ), affording solar cells with high efficiency. The power of PDI‐PZ and C60‐PZ to improve electron injection and extraction processes when positioned between metal electrodes and organic semiconductors highlights their promise to overcome energy barriers at the hard‐soft materials interface of organic electronics.

Highly efficient ruthenium complexes with acetyl electron-acceptor unit for dye sensitized solar cells

Two ruthenium complexes sensitizers RC-61 and RC-62 with acetyl electron-acceptor ancillary ligands are reported for highly efficient dye-sensitized solar cells, of which RC-62 obtains a power conversion efficiency of 9.10% compared to 8.23% for the conventional N3 dye. Dye-sensitized solar cells based on RC dyes receive a satisfactory photocurrent density (16.82 and 17.78 mA cm−2 for RC-61 and RC-62, respectively) attributable to the acetyl units ancillary ligands. The electrochemical measurement manifest that the acetyl electron-acceptor group is beneficial to stabilize the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels, which are conducive to the processes of dye regeneration and electron injection. The kinetics of dye recombination and regeneration are investigated by transient absorption spectroscopy. The faster rate of the RC-62 dye cation decay is, the higher efficiency of regeneration process contributing a higher photocurrent. Besides, RC-62 dye exhibits a good long-term stability and maintains more than 92.6% of the initial power conversion efficiency under light soaking for 1000 h. The above conclusions strongly corroborate the superiority of acetyl ancillary ligands at enhancing photocurrent density and power conversion efficiency, propose a strategy to develop ruthenium complexes sensitizers.

Fundamental Aspects of Photoinduced Charge Flow at a Quantum-Dot-Sensitized Single-Crystal TiO2 Semiconductor Interface

The fundamental aspects of charge transfer from photoexcited CdSe quantum dots to a single crystal of TiO2, a wide band gap metal oxide semiconductor, were investigated and compared with that of a dye-sensitized system in relation to the operation of quantum-dot-sensitized solar cells (QDSCs) and dye-sensitized solar cells (DSSCs). Due to the stark differences in both physical and electronic properties of quantum dots versus molecular dyes, it was hypothesized that the fundamental behavior of the two systems could differ greatly. The large size and surface area of the quantum dots relative to molecular dyes present the possibility for the positively charged hole to move a greater distance away from the QD/oxide interface during the electron injection process. This increased distance influences the Coulombic interaction between the trapped hole and injected electron, leading to differences and increased complexity of the recombination pathways when compared to the dye system.

Molecular engineering of rhodanine dyes for highly efficient D-π-A organic sensitizer

Due to the strong electron-withdrawing capability, the rhodanine unit is widely used as the acceptor of the D-π-A organic sensitizer. However, in this unit, the carboxyl group is shut out from the whole conjugation system, which affects the injection of the excited electrons, leading to poor photovoltaic performance. Herein, with the introduction of cyanoacetic acid into the rhodanine segment, two novel indoline sensitizers, CS-22 and CS-24, based on D-π-A construction are designed and synthesized. With this modification, the carboxyl group is conjugated with the whole molecule, leading to sharp enhance of light-harvesting capability of the dye and more important, quite smooth electron injection process. Moreover, the introduced long alkyl chain can also prevent the π-aggregation on TiO2 surface and restrain the charge recombination effectively. Accordingly, using the modified acceptor, the photovoltaic performance is significantly improved: an extremly high PCE of 8.48% is obtained by CS-22 sensitized DSSC, which is the highest value among rhodanine dyes without any co-adsorbent. Our work demonstrates that the introduction of conjugated carboxyl group is greatly benefit for the improvement of rhodanine dyes in photovoltaic performance, providing a powerful strategy for the development of rhodanine based highly efficient D-π-A organic sensitizer in the future.

Improved performance of laser wakefield acceleration by tailored self-truncated ionization injection

We report on tailoring ionization-induced injection in laser wakefield acceleration so that the electron injection process is self-truncating following the evolution of the plasma bubble. Robust generation of high-quality electron beams with shot-to-shot fluctuations of the beam parameters better than 10% is presented in detail. As a novelty, the scheme was found to enable well-controlled yet simple tuning of the injected charge while preserving acceleration conditions and beam quality. Quasi-monoenergetic electron beams at several 100 MeV energy and 15% relative energy spread were routinely demonstrated with a total charge of the monoenergetic feature reaching 0.5 nC. Finally these unique beam parameters, suggesting unprecedented peak currents of several 10 kA, are systematically related to published data on alternative injection schemes.

Simulation study of the sub-terawatt laser wakefield acceleration operated in self-modulated regime

Laser wakefield acceleration (LWFA) can be accomplished by introducing a sub-terawatt (TW) laser pulse into a thin, high-density gas target. In this way, the self-focusing effect and the self-modulation that happened on the laser pulse produce a greatly enhanced laser peak intensity that can drive a nonlinear plasma wave to accelerate electrons. A particle-in-cell model is developed to study sub-TW LWFA when a 0.6-TW laser pulse interacts with a dense hydrogen plasma. Gas targets having a Gaussian density profile or a flat-top distribution are defined for investigating the properties of sub-TW LWFA when conducting with a gas jet or a gas cell. In addition to using 800-nm laser pulses, simulations are performed with 1030-nm laser pulses, as they represent a viable approach to realize the sub-TW LWFA driven by high-frequency, diode-pumped laser systems. The peak density which allows the laser peak power PL∼2Pcr of self-focusing critical power is favourable for conducting sub-TW LWFA. Otherwise, an excessively high peak density can induce an undesired filament effect which rapidly disintegrates the laser field envelope and violates the process of plasma wave excitation. The plateau region of a flat-top density distribution allows the self-focusing and the self-modulation of the laser pulse to develop, from which well-established plasma bubbles can be produced to accelerate electrons. The process of electron injection is complicated in such high-density plasma conditions; however, increasing the length of the plateau region represents a straightforward method to realize the injection and acceleration of electrons within the first bubble, such that an improved LWFA performance can be accomplished.

A rational design of high efficient and low-cost dye sensitizer with exceptional absorptions: Computational study of cyanidin based organic sensitizer

We have computationally designed and characterized a series of new organic D−π−A architected dyes that have originated from cyanidin, which is vastly available in nature, for effective sensitization of DSSCs with absorption spectra extending up to near infrared region. Cyanidin acts as the donor group while cyanoacrylic acid and thieno [3, 2-b] thiophene are employed as the acceptor and π-spacer, respectively. Sensitization performance, depending on the substituted position of the π-spacer-acceptor (π-A) combination on cyanidin molecule, is examined by the results of density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. The calculated data of free energy change driving force (ΔGinject), electron regeneration driving force (ΔGregen), open circuit potential eVOC and light harvesting efficiency (LHE) suggest two preferred substitutions of π-A combination to cyanidin molecule that leads to an efficient DSSC. At LUMO the designed sensitizers have denser electron cloud towards acceptor group that leads to an efficient electron injection process. All π-A substitutions resulted a broader absorption spectrum with a redshift up to 2500 nm which is a significant improvement compared to the vast majority of reported sensitizers.

A combined experimental and computational investigation on pyrene based D-π-A dyes.

The geometry (twist vs. planar) of a dye is one of the most pivotal factors for determining intramolecular charge transfer (ICT), light harvesting and photovoltaic properties of dye-sensitized solar cells. In order to comprehend the role of dye geometry on the above properties, we have devised the pyrene based D-π-A dyes namely 2-cyano-3-(5-pyren-1-yl-furan-2-yl)-acrylic acid (PFCC) and 2-cyano-3-(5-pyren-1-ylethynyl-furan-2-yl)-acrylic acid (PEFCC). The synthesized pyrene dyes were well characterized by NMR and EI-MS spectrometry. In both the dyes, the donor (pyrene) and acceptor (cyanoacrylic acid) segments remained the same. The varied π-spacers were furan and ethynyl furan. The influences of the ethynyl spacer on the energy levels, light absorption, dynamics of excited states and photovoltaic properties of the DSCs were systematically investigated via theoretical calculations and spectroscopic measurements. UV-visible absorption spectral measurements indicated that the introduction of the ethynyl spacer enhances the molar absorptivity of a dye (PEFCC) in the order of 2, but does not shift the absorption range, which is consistent with the results obtained from density functional theory (DFT) calculations. The theoretical analysis indicated that the charge transfer transition is mainly constituted of the HOMO to the LUMO that were found to be located on donor and acceptor segments, respectively, which is supportive for efficient charge separation and electron injection processes. TDDFT calculations highlighted that the LUMO of the PEFCC dye is more stabilized by the incorporation of the ethynyl group between the pyrene and furan moieties that aid to inject electrons efficiently into TiO2 thereby resulting in an enhanced power conversion efficiency of 2.47% when compared to the PFCC dye. Notably, the overall conversion efficiency of the PEFCC dye reached 60% with respect to that of an N719-based device (4.12%) fabricated under similar conditions. Transient absorption kinetic studies demonstrated that a slower charge recombination rate is an essential factor behind enhanced efficiencies in PEFCC based cells.