Photogeneration Of Free Carriers Research Articles

Development of an ultrafast photo-switch device using surface passivated nanocrystalline CdS thin film

The development of an ultrafast optical switch is demonstrated in this work using a surface passivated nanocrystalline cadmium sulfide (CdS) thin film. The optical switching ratio of the Ag/CdS/Ag device, after surface passivation with nitrocellulose, is 2.1 × 104 in just 0.4 ms under a constant bias of 10 V and blue light (380 nm) of intensity ∼7.4 mW/cm2, which is 100 times higher than the native device. This is because the surface sulphur vacancy defects are suppressed, increasing the concentration of photo-generated free carriers.

Piezoelectric Modulation of Excitonic Properties in Monolayer WSe2 under Strong Dielectric Screening.

We investigate the interaction of excitons in monolayer WSe2 with the piezoelectric field of surface acoustic wave (SAW) at room temperature using photoluminescence (PL) spectroscopy and report a large in-plane exciton polarizability of 8.43 ± 0.18 × 10-6 Dm/V. Such large polarizability arises due to the strong dielectric screening from the piezoelectric substrate. In addition, we show that the exciton-piezoelectric field interaction and population distribution between neutral excitons and trions can be optically manipulated by controlling the field screening using photogenerated free carriers. Finally, we model the broadening of the exciton PL line width and report that the interaction is dominated by type-II band edge modulation, because of the in-plane electric field in the system. The results help understand the interaction of excitons in monolayer transition-metal dichalcogenides that will aid in controlled manipulation of excitonic properties for applications in sensing, detection, and on-chip communication.

Computational analysis of optically controlled reconfigurable terahertz mesh filters using mesa arrays

AbstractWe report a novel approach for realizing tunable/reconfigurable terahertz (THz) mesh filters on the basis of micromachined mesa‐array structures. In this approach, different filter patterns are generated virtually using photogenerated free carriers in a semiconducting mesa‐array structure to achieve superior tunability and reconfigurability. Micromachined mesa‐array structures enable the formation of high fidelity, optically generated mesh filter structures for THz frequencies. To evaluate the proposed filter designs, the optically patterned spatial modulation properties of mesa‐array structures were first evaluated. Reconfigurable mesh filter prototypes were then designed and simulated using silicon mesa arrays with 50 × 50 μm2 square mesa unit cells. Simulations show that reconfigurable bandpass filters (BPFs) operating in the frequency range of 108–489 GHz with insertion losses of 0.82–1.13 dB can be achieved. By employing smaller unit cells, the frequency tuning range and filtering performance can be further improved. In addition to BPFs, other filter functionalities can also be realized utilizing the proposed approach. The wide tuning range and reconfigurability of the mesh filters demonstrate that the proposed approach is promising for developing tunable/reconfigurable circuits and components for advanced THz sensing, imaging, and communications.

Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells.

Metal-halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements. Their inimitable features such as strong absorption ability, direct photogeneration of free carriers, long carrier diffusion lengths, ease of fabrication, and low production cost triggered the development of perovskite solar cells (PSCs) at an incredible rate, which soon reached power conversion efficiencies up to the commercialized level. During their evolution process, it has been witnessed that alkali metal cations play a pivotal role in the crystal structure as well as intrinsic properties of hybrid perovskites, thus enabling the unique positioning of the correlated doping strategy in the development history of PSCs in the past decade. Herein, we summarize the growth and progress of the state-of-the-art alkali metal cation (Cs+, Rb+, K+, Na+, Li+) doping in the field of hybrid perovskite-based photovoltaics. To start with, the accurate identification of different alkali metal-occupied locations in the perovskite crystal lattice are discussed in detail with highlighted advanced characterization methods. Beyond that, the location-dependent functions induced by alkali metal doping are intensely focused upon and comprehensively assessed, indicating their versatile and special effects on perovskites in terms of bottleneck issues such as crystallinity modulation, crystal structure stabilization, defect passivation, and ion-migration inhibition. Thereafter, we are committed to analyze their responsible working mechanisms so as to unveil the relationship between occupied locations and crucial roles for each doped cation. The systematical overview and in-depth understanding of the superiorities of such strategies together with their future challenges and prospects would further boost the advancement of perovskite-related fields.

Optically controlled reconfigurable terahertz waveguide filters based on photo-induced electromagnetic band gap structures using mesa arrays

In this work, we present a novel approach to the implementation of optically controlled reconfigurable terahertz (THz) waveguide filters based on photo-induced electromagnetic band gap (PI-EBG) structures using semiconductor mesa arrays. In this approach, EBG structures are dynamically generated through spatially-resolved photogeneration of free carriers in semiconductor mesa structures; the use of mesa arrays improves the spatial resolution, leading to more refined PI-EBG patterns and improved circuit operation at THz frequencies. A band-stop filter prototype in the WR-5.1 band (140-220 GHz) was designed, simulated, and analyzed in detail. Simulations show that the stop-band rejection and bandwidth of the proposed device can be adjusted by changing the filling factor of the photo-patterns illuminated onto the mesa array. The center frequency of the stop-band can also be tuned from 166 to 200 GHz by changing the period of the PI-EBG patterns. The proposed reconfigurable THz waveguide filters based on PI-EBG are promising for a wide range of applications in advanced THz sensing, imaging, and communications.

Decoupling the charge collecting and screening effects in piezotronics-regulated photoelectrochemical systems by using graphene as the charge collector

Piezotronics have shown great promises in promoting the efficiency of photoelectrochemical (PEC) reactions, while their full-gear functionalities are constrained by the longstanding contradiction between the charge collection of semiconductors and the screening effect of polarization materials. In this report, we tackle this issue by decoupling the collecting and screening trajectories using graphene as the charge collector. The moderate charge density of graphene ensures minimal screen of the adjacent electrical polarizations and concurrently delivers photogenerated free carriers toward the counter electrode. Based on a PMN-PT/graphene/TiO2 piezotronic PEC system, substantial performance gains were received through tuning the interfacial electronic energy level via ferroelectric polarizations. Forward poling could lead to a 50% improvement of photocurrent density at 1 V vs. RHE and a favorably shifted onset potential. Both calculation and experimental results suggest this outcome was beneficial from the low screening effect from graphene. In contrast, gold electrode will fully screen the polarization from PMN-PT and yield similar PEC performance despite of the poling conditions. This work reveals graphene could be an ideal conductive electrode selection for freeing piezotronic PEC devices from the performance capstone imposed by the trade-off between carrier collecting and charge screening.

Influence of Two-Photon Absorption Anisotropy on Terahertz Emission Through Optical Rectification in Zinc-Blende Crystals

We report for the first time on the observation of an angular anisotropy of the THz signal generated by optical rectification in a ZnTe crystal. This cubic (zinc-blende) crystal in the orientation exhibits both transverse isotropy for optical effects involving the linear χ(1) and nonlinear χ(2) susceptibilities. Thus, the observed anisotropy can only be related to χ(3) effect, namely two-photon absorption, which leads to the photo-generation of free carriers that absorb the generated THz signal. Two-photon absorption in zinc-blende crystals is known to be due to a spin-orbit interaction between the valence and higher-conduction bands. We perform a couple of measurements that confirm our hypothesis, as well as we fit the recorded data with a simple model. This two-photon absorption effect makes difficult an efficient generation, through optical rectification in zinc-blende crystals, of THz beams of any given polarization state by only monitoring the laser pump polarization.

Study on the photoconductive properties of charge carriers in donor-acceptor copolymer P(EDOT-FL) and P(EDOT-FL):PCBM blend

The photoinduced charge transfer properties of low band gap donor-acceptor π-conjugated copolymer poly(2,5-(3,4-ethylenedioxythiophene)-alt-2,7-(9,9-dioctylfluorene)) labelled as P(EDOT-FL) has been investigated through solvatochromic and photoconductivity studies. The effect of strong acceptor, Phenyl-C61-butyric acid methyl ester (PCBM) on the photogeneration of free carriers is studied by comparing the fluorescence spectra and photoconductive properties of the polymer-PCBM blend [P(EDOT-FL):PCBM] films with the pristine polymer films. The positive solvatochromism exhibited by the P(EDOT-FL) revealed its photoinduced charge transfer nature of the absorption and emission bands. The blend films exhibited quenching of the fluorescence intensity and showed increased photoconductivity. Internal photocurrent efficiency and photoconductive sensitivity of the pristine P(EDOT-FL) films and P(EDOT-FL):PCBM blend films were calculated as a function of electric field, by measuring the photocurrent generated in the sample. Blend films exhibited higher internal photocurrent efficiency of 86% and photoconductive sensitivity of 5.44×10−7 SW−1cm at 70V/μm. The obtained photoconductive sensitivity at low electric field (<10V/μm) is sufficient for photorefractive applications.

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites.

Unravelling the nature of the interactions between photogenerated charge carriers in solar energy conversion devices is key to enhance performance. In this perspective, we discuss electroabsorption spectroscopy (EAS), as the spectral bandshape of the electroabsorption (EA) signal directly depends on the strength of the charge carrier interactions. For instance, the electroabsorption response in molecular or confined excitonic systems can be modelled perturbatively yielding the Stark effect. In contrast, most solids exhibit weaker interactions, and a perturbative approach cannot be taken in general. For solids with negligible charge carrier interactions, one resorts to the Franz-Keldysh theory of a continuum in a field, that, in the low-field limit, simplifies to the low-field FKA effect. Alternatively, when the continuum approximation breaks down, the problem of a Wannier exciton in a field has to be solved, and numerical methods emerged as the best solution. We illustrate our discussion with two examples involving lead-halide perovskites, a new, high-stake solar cell material. In the first example, we discuss the lineshape of the electroabsorption response for thin-films of lead-iodide perovskite, that sustains the photogeneration of free carriers. In the second example, we address a confined excitonic case with lead-bromide perovskite nanoparticles, and demonstrate the presence of so-called charge-transfer excitons.

Direct Free Carrier Photogeneration in Single Layer and Stacked Organic Photovoltaic Devices.

High performance organic photovoltaic devices typically rely on type-II P/N junctions for assisting exciton dissociation. Heremans and co-workers recently reported a high efficiency device with a third organic layer which is spatially separated from the active P/N junction; but still contributes to the carrier generation by passing its energy to the P/N junction via a long-range exciton energy transfer mechanism. In this study the authors show that there is an additional mechanism contributing to the high efficiency. Some bipolar materials (e.g., subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc)) are observed to generate free carriers much more effectively than typical organic semiconductors upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than those of reported single-layer devices. In addition, internal quantum efficiencies (IQEs) of bilayer devices with opposite stacking sequences (i.e., SubNc/SubPc vs SubPc/SubNc) are found to be the sum of IQEs of single layer devices. These results confirm that SubNc and SubPc can directly generate free carriers upon photoexcitation without assistance from a P/N junction. These allow them to be stacked onto each other with reversible sequence or simply stacking onto another P/N junction and contribute to the photocarrier generation.

Theoretical study on dynamic acoustic modulation of free carriers, excitons, and trions in 2D MoS2 flake

We present a self-consistent model calculation to study the influence of a radio-frequency surface acoustic wave (SAW) on the dynamic processes of photo-generated free carriers, excitons and trions in a 2D MoS2 flake with an odd number of layers. We found that the concentrations of excitons and trions decreased, while the lifetime of the free carriers increased when an external SAW field was applied. We attribute this phenomenon to the spatial separation of the electrons and holes, which are confined by a SAW-induced periodic type-II band-edge. We further studied the effect of the modulations on the spatiotemporal distributions of free carriers and excited quasiparticles by varying the amplitude and wavelength of the SAW field. We finally calculated the photoluminescent properties of the MoS2 flake under modulation of the SAW field and compared our results with those of recent experiments.

Non-degenerate two-photon absorption in silicon waveguides: analytical and experimental study.

We theoretically and experimentally investigate the nonlinear evolution of two optical pulses in a silicon waveguide. We provide an analytic solution for the weak probe wave undergoing non-degenerate two-photon absorption (TPA) from the strong pump. At larger pump intensities, we employ a numerical solution to study the interplay between TPA and photo-generated free carriers. We develop a simple and powerful approach to extract and separate out the distinct loss contributions of TPA and free-carrier absorption from readily available experimental data. Our analysis accounts accurately for experimental results in silicon photonic crystal waveguides.

Ultrafast all-optical modulation in a silicon nanoplasmonic resonator

Ultrafast all-optical modulation in silicon-based metal-insulator-semiconductor-insulator-metal nanoring resonators through photogeneration of free-carriers using two-photon absorption is presented 3-D through finite difference time domain simulations. In a compact device footprint of only 1.4 µm(2), a 13.1 dB modulation amplitude was obtained with a switching time of only 2 ps using a modest pump pulse energy of 16.0 pJ. The larger bandwidth associated with more compact nanorings is shown to result in increased modulation amplitude.

Biexciton, single carrier, and trion generation dynamics in single-walled carbon nanotubes

We present a study of free carrier photogeneration and multicarrier bound states, such as biexcitons and trions (charged excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.

Transient photoconductivity responses in amorphous In-Ga-Zn-O films

We studied the photoconductivity responses in amorphous In-Ga-Zn-O (a-IGZO) films using a time-resolved microwave photoconductivity decay (μ-PCD) technique. The a-IGZO film characteristics are correlated with three components in the photoconductivity response: the peak value and two decay constants. The peak value originated from the density of the photo-generated free carriers through carrier generation and recombination processes during laser pulse irradiation. Power law characteristics indicated that the peak values are attributed to recombination process related to the exponential distribution of the conduction band tail states. After the laser pulse was turned off, the reflectivity signal decreased rapidly, indicating fast recombination of the photo-generated carriers. This fast decay component is suggested to be related to the recombination processes through the deep level states. Following the fast decay, a slow decay with a decay constant on the order of microseconds appeared. This slow decay was attributed to the reemission of trapped carriers with an activation energy of ∼0.2 eV. In addition, both the fast and slow decays for the wet annealed a-IGZO film were longer than those of the as-deposited a-IGZO film. The decay constants are considered to reflect the density of the subgap states that act as trapping or recombination centers. The μ-PCD method provides a useful estimation of the film quality, such as the density of the defect states, and the physical properties of electronic devices using a-IGZO films.

Ultrafast all-optical switching in a silicon-based plasmonic nanoring resonator

A silicon-based plasmonic nanoring resonator is proposed for ultrafast, all-optical switching applications. Full-wave numerical simulations demonstrate that the photogeneration of free carriers enables ultrafast switching of the device by shifting the transmission minimum of the resonator with a switching time of 3 ps. The compact 1.00 μm² device footprint demonstrates the potential for high integration density plasmonic circuitry based on this device geometry.

Effect of self assembled monolayers on charge carrier photogeneration in sexithiophene based diodes

Photogeneration process in organic materials may occur via multiple mechanisms depending on experimental conditions. In this work we investigate how self-assembled monolayers of two types of benzoic acid molecules (ABA and NBA) affect photocarrier generation in a sexithiophene based photovoltaic cell. Photogeneration of free carriers in organic photovoltaic cells results from dissociation of Frenkel excitons. This dissociation is either thermally assisted process and/or results from the interaction with structural defects in the bulk or with the surface states. A theoretical model is used for quantitative description of the open circuit voltage as a function of generation rates at the electrodes. The orientation and the magnitude of dipole moment of SAM molecules are the crucial factors that affect the organic photovoltaic parameters.

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells

The improvement of the performance of organic photovoltaic cells (OPVCs) and the photogeneration process in these devices may occur via multiple mechanisms depending on their structure and/or architecture. For this purpose we investigate how self-assembled monolayers of thiol molecules (C12H25SH and 3T(CH2)6SH) and benzoic acid molecules (ABA and NBA) affect the efficiency and the photogeneration of free carriers in a sexithiophene based photovoltaic cells. Firstly, we provide the results of absorption spectra for samples with SAM of thiol that show there effect on orientation of 6T molecules on these structures and the organization degree of the thiol molecules on ITO substrate. Afterward, we describe from current vs. applied voltage after illumination, the enhancement of the performance of these cells. In the second, we study the effect of SAM of benzoic acids molecules on the photovoltaic behavior. A theoretical model is used for quantitative description of the open circuit voltage as a function of carrier’s generation rates at the electrodes. The results of I-V characterization under illumination show that open circuit voltage as well as short circuit current is dramatically affected by the dipolar layer. The orientation and the magnitude of dipole moment of benzoic acid molecules are the crucial factors that affect the organic photovoltaic parameters.

Deep level contribution to the carrier generation and recombination in high resistivity Si irradiated by neutrons

Deep level spectroscopy in neutron irradiated FZ and MCz Si was performed by extrinsic photoconductivity spectra measurements in the range of temperature of 18–120 K. The Lukovsky model was used, and the Gaussian distribution of deep level energy demonstrated the best fit of simulation and experimental data. The non-monotonous change of deep levels during annealing, and non-monotonous dependence of their concentration on the fluence were observed. The photoconductivity decay was investigated by the transient photo-Hall effect, recombination parameters of the main recombination centre were determined, and the recombination centre model was proposed. The photoconductivity and thermally stimulated current measurements were used to demonstrate the existence of photogeneration of free carriers by a cascade of optical and thermal transitions.

Photophysical properties of indolo[3,2-b]carbazoles as a promising class of optoelectronic materials

The photophysical properties of new synthesized indolocarbazoles, i.e., indolo[3,2-b]carbazole and its derivatives, have been comparatively analyzed. It is shown that their photosensitivity (total photosensitivity to (5–8) × 10−2 (lx s)−1, spectral photosensitivity ∼105 cm2 J−1, and free carrier photogeneration quantum yield of 0.1) and transport (effective mobility in 5,11-dioctyl indolo[3,2-b]carbazole is more than 10−5 cm2/(V s)) parameters significantly exceed those of pentacene, which, among molecular media (organic crystals), exhibits the highest carrier photogeneration quantum yields. The high photoluminescence intensity of synthesized indolo[3,2-b]carbazole derivatives shows promise for their applicability in electroluminescent devices.