Generation Of Free Carriers Research Articles

Regulation of Exciton Effects in Functionalized Conjugated Polymers by B‐N Lewis Pairs for Visible‐Light Photocatalysis

Strong excitonic effects are common in organic conjugated polymer semiconductors, severely hindering the generation of free charge carriers for conducting photocatalysis. Therefore, exploring new channels to modulate exciton dissociation in polymers is far‐reaching in facilitating photocatalysis. A series of B‐N Lewis pair functionalized conjugated polymers have been developed to minimize exciton effects by modulating charge transfer pathways. Theoretical studies have shown that introducing B‐N Lewis pairs can dramatically increase the distance of charge transfer (D index) and the amount of electron transfer and reduce the Coulomb attraction energy (EC), which contributes to breaking the equilibrium of the coexistence of excitons and charge carriers. Further experimental results show that the singlet excitons are efficiently dissociated into more free‐charge carriers under photoexcitation to participate in surface reactions. The optimized polymer PyPBM shows an exponential increase in photocatalytic hydrogen and hydrogen peroxide production performance by visible light illumination.

Regulation of Exciton Effects in Functionalized Conjugated Polymers by B-N Lewis Pairs for Visible-Light Photocatalysis.

Strong excitonic effects are common in organic conjugated polymer semiconductors, severely hindering the generation of free charge carriers for conducting photocatalysis. Therefore, exploring new channels to modulate exciton dissociation in polymers is far-reaching in facilitating photocatalysis. A series of B-N Lewis pair functionalized conjugated polymers have been developed to minimize exciton effects by modulating charge transfer pathways. Theoretical studies have shown that introducing B-N Lewis pairs can dramatically increase the distance of charge transfer (D index) and the amount of electron transfer and reduce the Coulomb attraction energy (EC), which contributes to breaking the equilibrium of the coexistence of excitons and charge carriers. Further experimental results show that the singlet excitons are efficiently dissociated into more free-charge carriers under photoexcitation to participate in surface reactions. The optimized polymer PyPBM shows an exponential increase in photocatalytic hydrogen and hydrogen peroxide production performance by visible light illumination.

Dynamics of Nonlinear Optical Losses in Silicon‐Rich Nitride Nano‐Waveguides

AbstractFree carrier absorption (FCA) is established to be the cause of nonlinear losses in plasma‐enhanced chemical vapor deposition (PECVD) silicon‐rich nitride (SRN) waveguides. To validate this hypothesis, a photo‐induced current is measured in SRN thin films with refractive indices varying between 2.5 and 3.15 when a C‐band laser light is illuminating the SRN films at various powers, indicating the generation of free carriers. Furthermore, nonlinear loss dynamics is, for the first time, measured and characterized in detail in SRN waveguides by utilizing high peak power C‐band complex shape optical pulses for estimation of free carrier generation (FCG) and free carrier recombination (FCR) lifetimes and their dynamics. Both FCG and FCR are found to decrease with an increase in the refractive index of SRN, and, specifically, the FCR lifetimes are found (92 ± 7) ns, (39 ± 3) ns, and (31 ± 2) ns for the SRN indices of 2.7, 3, and 3.15, respectively. Lastly, nonlinear losses in high refractive index SRN waveguides are demonstrated to be minimized and altogether avoided when the pulse duration reduced below the free carrier generation lifetime, thus providing a way of taking a full advantage of the large inherent SRN nonlinear properties.

Free carrier generation efficiency in organic photovoltaic films determined using photo-MIS-CELIV

Solution processed organic photovoltaic (OPV) devices are promising for low-embedded energy and large-scale renewable energy production. The efficiency of charge carrier generation is a critical factor influencing the performance of photovoltaic devices. However, quantifying charge carrier generation can be challenging, with the results from experimental methods not always being easily correlated with solar cell performance. In this paper, we describe how photoinduced metal-insulating-semiconductor charge-extraction-by-linearly-increasing-voltage (photo-MIS-CELIV) can be used to determine the free charge carrier generation efficiency (FCGE) in OPV films. One of the benefits of this approach is that the FCGE can be measured alongside the charge mobility to provide a holistic picture of the fate of charges, from generation to extraction. We demonstrate this method through quantifying the FCGE of bulk heterojunctions of PCE10:ITIC-4F, D18:Y6 and PPDT2FBT:PC71BM, obtaining values of 47.4 ± 1.6 %, 75.0 ± 2.5 % and 70.6 ± 4.6 %, respectively. The measured FCGEs for these blends were consistent with the device-based external quantum efficiencies (EQEs) at the excitation wavelength used. The use of photo-MIS-CELIV for quantifying the FCGE increases its utility beyond simple charge mobility measurements and provides an extra method to enable optimisation of OPV device performance.

Manipulating Photoconduction in Supramolecular Networks for Solar-Driven Nitrate Conversion to Ammonia and Oxygen.

For photoelectrodes to be used in practical catalytic applications, challenges exist in achieving the efficient production and transport of photogenerated charge-separated states. Analogous concepts in traditional inorganic photoelectrodes can be applied to their organic-polymer counterparts with improved charge-separation efficiencies. In this work, we develop photoconductive organic networks to form a high-performance photoelectrode for NO3- reduction to NH3. In the integrated network, interfaces between the organic electron-donating photoconductor and electron-accepting catalyst can generate charge carriers efficiently upon illumination, leading to enhanced charge separation for photoelectrocatalysis. The photoelectrode network is capable of converting NO3- to NH3 at an external quantum efficiency of 13%. By coupling with a BiVO4 photoanode in tandem, the system reduces NO3- to NH3 and oxidizes H2O to O2 simultaneously at Faradaic efficiencies of 95-98% with sustained photocurrents and production yields. Investigation of the photoconductive network by steady-state/time-resolved spectroscopies reveals the efficient generation and transport of free charge carriers in the photoelectrode, providing a basis for high photoelectrocatalytic performances.

Ferroelectric, flexoelectric and photothermal coupling in PVDF-based composites for flexible photoelectric sensors.

A ferroelectric polyvinylidene fluoride (PVDF) film with excellent flexibility possesses great potential for photodetection and wearable devices. However, the relatively weak photo-absorption and the consequent small photocurrent limit its photofunctional properties. Herein, we embedded a strongly visible-light active material system, 0.5Ba(Zr0.08Ti0.8Mn0.12)O3-0.5(Ba0.7Ca0.3)TiO3 (BZTM-BCT) loaded with Ag and Au nanoparticles, into a PVDF film, which demonstrates a significantly higher photovoltaic response in the whole visible light range with a β-phase content of over 90%. In a state of "bending + poling", the PVDF/BZTM-BCT:Au film presents an optimal response for photoelectric properties by exhibiting a photocurrent that is 57 times higher than that of a pure PVDF film when illuminated with 405 nm LED light at 100 mW cm-2. Photoexcitation and thermal excitation jointly contribute to the generation of free carriers, while the flexoelectric and ferroelectric coupling electric field provides a greater driving force for carrier separation and transport. More interestingly, composite film-based photoelectric sensors can simultaneously respond to light and the movement and deformation of contacted things, indicating its potential in versatile applications. Overall, this work puts forward a new route for designing new flexible multifunctional photoelectric devices.

(Invited) Probing Charge Carrier-Induced Changes in the Complex Dielectric Function in Carbon Nanotube Dispersions

The injection of charge carriers into pi-conjugated semiconductors, whether by electrochemical injection or chemical charge transfer, is a powerful approach to tune their electrical conductivity. Recently, the concept of dielectric catastrophe, where the charge carrier density is sufficiently large to significantly alter the intrinsic dielectric properties, has been demonstrated at high doping levels in organic semiconductor thin films. The observed increase in the dielectric constant leads to reduced coulombic interactions between charge carriers and the associated counterions, promoting enhanced free carrier generation and transport.Here, we employ a combination of steady state optical spectroscopy, quantitative 19F NMR studies, and microwave dielectric loss spectroscopy to probe charge carrier density and transport in doped single-walled carbon nanotube dispersions. We demonstrate that a similar dielectric catastrophe phenomenon can be observed even in isolated single-walled carbon nanotubes dispersed in a low dielectric constant solvent AND, perhaps more surprisingly, at carrier densities less than one carrier per nanotube. The precise details of the observed changes in complex dielectric function are dependent on the chemical structure of the charge-transfer dopant, with a near-instantaneous increase in the dielectric constant and electrical conductivity observed for low carrier densities using bulky benzyloxy-substituted icosahedral dodecaborane cluster dopants, DDBs. We attribute the observations to weak coulombic interactions between the injected charge carrier and the dopant counterion, along with the large polarizability afforded by the unique physicochemical properties of single-walled carbon nanotubes. If time permits, we will show results from recent studies where we extend this approach to the doping of other nanocarbons.

High Ambipolar Mobility and Long-Range Carrier Transport in Violet Phosphorus Nanosheet.

Carrier transport capacity with high mobility and long-range diffusion length holds particular significance for the advancement of modern optoelectronic devices. Herein, we have unveiled the carrier dynamics and transport properties of a pristine violet phosphorus (VP) nanosheet by a transient absorption microscopy. Under the excitation (2.41 eV) above the exciton band, two photoinduced absorption peaks with the energy difference of approximately 520 meV emerge within a broadband transient absorption background which originates from the prompt generation of free carriers and the concomitant formation of excitons (lifetime of 467.21 ps). This observation is consistent with the established band-edge model of VP. Intriguingly, we have determined the ambipolar diffusion coefficient and mobility of VP to be approximately 47.32 cm2·s-1 and 1798 cm2·V-1·s-1, respectively, which further indicate a long-range carrier transport of approximately 2.10 μm. This work unveils the significant carrier transport capacity of VP, highlighting its potential for future optoelectronic and excitonic applications.

Organic-Inorganic Rubrene/WS2 Heterostructure for Broadband Detection and Polarization Imaging.

Organic single crystals exhibit improved carrier mobility, longer exciton diffusion length, anisotropic charge transport, and unique linear dichroism, while its high exciton binding energy seriously limits the free-carrier generation and photoelectric conversion efficiency. Layered van der Waals heterostructures, which integrate organic crystals with high mobility two-dimensional (2D) inorganic semiconductors, are promising for promoting exciton dissociation and boosting sensitivity by utilizing the interfacial potential and photogating effect. In this work, organic single-crystal rubrene is integrated with a few-layer WS2 to design the high-performance photodetector. The device exhibits an excellent responsivity of 1000 A W-1, and a fast speed of 180 μs, which is far superior to the individual WS2 device. Equally importantly, this device provides excellent polarization detection performance by virtue of the anisotropic properties of rubrene, and the dichroic ratios are 1.56, 1.5, and 1.7 for 375, 405, and 658 nm irradiation, respectively. Finally, several high-resolution single-pixel broadband polarization imaging was demonstrated. Our work shows that organic-inorganic heterostructure is an essential candidate for improving optoelectronics performance and has potential for polarization imaging.

Unraveling the nature of local field distortion-induced dual electron-trapping centers for efficient photocatalytic fuel evolution

Active electron-trapping centers play significant roles in photocatalysis. Herein, we synergize local field distortion and defects in nanocrystals to produce dual electron-trapping centers and unravel the nature of aluminum doping in porous ZnO with tunable amounts of heteroatoms and oxygen vacancies. The introduced Al atoms in the structural domain induce local field distortion and trigger the movement of Zn2+ from a singlet low spin state to a stable triplet state, resulting in the generation of free carriers trapping centers and optimized electronic energy band structure. Dual active electron-trapping centers of heteroatoms and oxygen vacancies promote the Separation of photo-generated charge carriers and reduce the adsorption energies of H* species. Furthermore, the d-band center of Al-doped ZnO with oxygen vacancies shifts to a higher energy position, thus promoting the adsorption and desorption of hydrogen intermediates and exhibiting 4-fold the photocatalytic H2 efficiency of pristine ZnO. This study provides new insights into the rational design of highly efficient oxide-based photocatalysts toward sustainable solar fuel evolution.

Remarkably Boosting Piezocatalytic Performance of Sr0.5Ba0.5Nb2O6 Piezoceramics via Size Optimization and Oxygen Vacancy Engineering

AbstractPiezocatalysis is capable of harnessing mechanical energy for environmental remediation, which is regarded as a green and promising technology to be exploited. Piezoceramics are struggling to be used as highly efficient piezocatalysts due to their grain size reaching tens of micrometers usually. Herein, a feasible and straightforward method is proposed to turn piezoceramic powders into highly efficient piezocatalysts by integrating size optimization and oxygen vacancy modulation. This strategy is validated by treating lead‐free Sr0.5Ba0.5Nb2O6 (SBN) piezoceramic powders with high‐energy ball‐milling (hBM). The rate constant k value of 46.95 × 10−3 min−1 for rhodamine B (RhB) piezocatalytic degradation of SBN‐hBM‐12h is almost 18 times higher than that of pristine SBN. Besides, the SBN‐hBM‐12 h catalyst performed superior antibacterial properties against Escherichia coli. The enhanced piezocatalytic efficiency is attributed to the introduced abundant oxygen vacancies absorbing and activating O2 into reactive oxygen species. Well‐modulated oxygen vacancy concentration can effectively accelerate the generation and separation of free carriers. However, the excess oxygen vacancies in SBN render the weakened piezoresponse thus suppressing the piezocatalytic activity. This study elucidates the critical role of oxygen vacancies in piezocatalysis and provides insights into the development of efficient piezocatalysts.

Ultrathin Niobium‐Doped Indium Oxide Active Layer Enables High‐Performance Phototransistors for Driving Quantum‐Dot Light‐Emitting Diodes

AbstractActive materials play a crucial role in the performance of phototransistors. However, the discovery of a novel and versatile active material is a big challenge. For the first time, phototransistors with ultrathin niobium‐doped indium oxide (InNbO) active layer are fabricated. The InNbO phototransistors without additional light‐absorbing layers exhibit the performance with a high average mobility of 22.86 cm2 V−1s−1, a turn‐on voltage of −0.75 V, a low sub threshold swing of 0.18 V/decade, and a high on/off current ratio of 5.74 × 108. Detailed studies show that Nb is the key to suppress the free carrier generation due to the strong bonding strength of Nb─O. In addition, the InNbO phototransistors exhibit a very broad spectral responsivity with a photocurrent of 4.72 × 10−4 A, a photosensitivity of 1.69 × 108, and a high detectivity of 3.33 × 1013 Jones under violet (405 nm) light illumination, which is significantly higher than that of the IGZO phototransistors. Furthermore, an active‐matrix quantum‐dot light‐emitting diode pixel circuit based on InNbO phototransistors is demonstrated. The findings not only indicate that InNbO is a new active material for phototransistors, but also suggest that InNbO‐based phototransistors have a great potential for the next‐generation interactive display technology.

Adiabatic weak coherent MHz linewidth O-band single-photon carrier for low erroneous phase decoding

For weak coherent single-photon secure data communication among short-reach metropolitan intra-/inter-city networks at the O-band (1250-1350 nm), the commercially available semiconductor laser sources are emerging but still suffering from high single-mode-fiber (SMF) loss, broad linewidth, and unstable wavelength. To overcome such disadvantages for enabling the efficient phase-coding link with sufficient secure key rate, a specifically designed adiabatic package with active temperature-/current-feedback control is proposed for the paired O-band MHz-linewidth master-to-slave injection-locked DFBLDs and a polarization-maintaining 1-bit-delay interferometer is stabilized with using a passively adiabatic cell to achieve accurate differential phase decoding. Even though, the phonon-induced phase fluctuation still occurs at rising and falling edges of the decoded long-pattern secure data bits delivered from the slave DFBLD, which is mainly attributed to the intra-cavity heating under excessive free-carrier generation via the master DFBLD injection. To stabilize the differential-phase-shift (DPS) keying protocol, the phase-code distortion caused by over-injection-induced Auger heating is effectively suppressed by reducing the overly biased injection with precise master-injection-level control. The rising-/falling-edge damping distortion of the phase-shift-encoded secure bit-stream envelope is suppressed by appropriately decreasing the DC bias current and adjusting the AC encoding amplitude of the master DFBLD. Such operation reduces the incorrect π phase shift in the injection-locked slave DFBLD biased at optimized below-threshold DC offset, thus allowing single-photon DPS-keying data transmission over 15-km SMF with slightly increasing the single-photon bit-error ratio from <3% (0-km) to 6.2% (15-km).

Delocalizing Excitation for Highly-Active Organic Photovoltaic Catalysts.

Localized excitation in traditional organic photocatalysts typically prevents the generation and extraction of photo-induced free charge carriers, limiting their activity enhancement under illumination. Here, we enhance delocalized photoexcitation of small molecular photovoltaic catalysts by weakening their electron-phonon coupling via rational fluoro-substitution. The optimized 2FBP-4F catalyst we develop here exhibits a minimized Huang-Rhys factor of 0.35 in solution, high dielectric constant and strong crystallization in the solid state. As a result, the energy barrier for exciton dissociation is decreased, and more importantly, polarons are unusually observed in 2FBP-4F nanoparticles (NPs). With the increased hole transfer efficiency and prolonged charge carrier lifetime highly related to enhanced exciton delocalization, the PM6 : 2FBP-4F heterojunction NPs at varied concentration exhibit much higher optimized photocatalytic activity (207.6-561.8 mmol h-1 g-1) for hydrogen evolution than the control PM6 : BP-4F and PM6 : 2FBP-6F NPs, as well as other reported photocatalysts under simulated solar light (AM 1.5G, 100 mW cm-2).

Quasi-Homogeneous Photocatalysis in Ultrastiff Microporous Polymer Aerogels.

The development of efficient and low-cost catalysts is essential for photocatalysis; however, the intrinsically low photocatalytic efficiency as well as the difficulty in using and recycling photocatalysts in powder morphology greatly limit their practical performance. Herein, we describe quasi-homogeneous photocatalysis to overcome these two limitations by constructing ultrastiff, hierarchically porous, and photoactive aerogels of conjugated microporous polymers (CMPs). The CMP aerogels exhibit low density but high stiffness beyond 105 m2 s-2, outperforming most low-density materials. Extraordinary stiffness ensures their use as robust scaffolds for scaled photocatalysis and recycling without damage at the macroscopic level. A challenging but desirable reaction for direct deaminative borylation is demonstrated using CMP aerogel-based quasi-homogeneous photocatalysis with gram-scale productivity and record-high efficiency under ambient conditions. Combined terahertz and transient absorption spectroscopic studies unveil the generation of high-mobility free carriers and long-lived excitonic species in the CMP aerogels, underlying the observed superior catalytic performance.

Low-temperature synthesis of transparent conducting La-doped BaSnO3via rejuvenation of the dried peroxo-precursor

In this article, we develop a low-temperature route for the synthesis of the high figure-of-merit transparent conductor, La-doped BaSnO3. Towards this, we first study the evolution of the cubic perovskite phase of the undoped BaSnO3 during solution combustion synthesis. A peroxo/superoxo precursor synthesized via H2O2 assisted co-precipitation is employed as the oxidizing agent. Typically, precursor drying is detrimental to the low-temperature conversion of the precursor to crystalline BaSnO3. The synthesis procedure developed herein is agnostic to the altered precursor composition obtained under the particular drying conditions. This is achieved by re-solvation of the precursor with ethyl alcohol prior to the annealing. The added ethanol plays the role of fuel, enabling an exothermic reaction of the components of the mixture, providing the heat of reaction for conversion and crystal growth. Complete conversion of the precursor to phase-pure BaSnO3 with a three-fold increase in the crystallite size at lower temperatures is thus achieved. We then demonstrate the facile incorporation of lanthanum as a dopant through this method. The low-temperature synthesized lanthanum substituted BaSnO3 displays optical absorption in the NIR region corresponding to free carrier generation, and n-type conductivity.

Tight-binding model predicts exciton energetics and structure for photovoltaic molecules.

Conjugated molecules and polymers are being designed as acceptor and donor materials for organic photovoltaic (OPV) cells. OPV performance depends on generation of free charge carriers through dissociation of excitons, which are electron-hole pairs created when a photon is absorbed. Here, we develop a tight-binding model to describe excitons on homo-oligomers, alternating co-oligomers, and a non-fullerene acceptor - IDTBR. We parameterize our model using density functional theory (DFT) energies of neutral, anion, cation, and excited states of constituent moieties. A symmetric molecule like IDTBR has two ends where an exciton can sit; but the product wavefunction approximation for the exciton breaks symmetry. So, we introduce a tight-binding model with full correlation between electron and hole, which allows the exciton to coherently explore both ends of the molecule. Our approach predicts optical singlet excitation energies for oligomers of varying length as well as IDTBR in good agreement with time-dependent DFT and spectroscopic results.

Dielectric Screening and Charge-Transfer in 2D Lead-Halide Perovskites for Reduced Exciton Binding Energies.

Layered lead-halide perovskites have shown tremendous success as an active material for optoelectronics. This is attributed to the electronic structure of the inorganic sublattice and large exciton binding energies due to quantum and dielectric confinement. Expanding functionalities for applications that depend on free-carrier generation requires new material design routes to decrease the binding energy. Here we use electronic structure methods with model Bethe-Salpeter equation (BSE) to examine the contributions of the dielectric screening and charge-transfer excited-states to the exciton binding energy of phenylethylammonium (PEA2PbBr4) and naphthlethylammonium (NEA2PbBr4) lead-bromide perovskites. Our model BSE calculations show that NEA introduces hole acceptor states which impose charge-transfer character on the exciton along with larger dielectric screening. This substantially decreases the exciton binding compared to PEA. This result suggests the use of organic cations with high dielectric screening and hole acceptor states as a viable strategy for reducing exciton binding energies in two-dimensional halide perovskites.

Tuning the Inter-Nanoplatelet Distance and Coupling Strength by Thermally Induced Ligand Decomposition.

CdSe nanoplatelets (NPLs) are promising 2D semiconductors for optoelectronic applications, in which efficient charge transport properties are desirable. It is reported that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. Combining optical absorption, transmission electron microscopy, and thermogravimetric analysis, it is revealed that the thermal decomposition of ligands (e.g., cadmium myristate) governs the inter-NPL distance and thus the inter-NPL electronic coupling strength. Employing ultrafast terahertz spectroscopy, it is shown that this enhanced electronic coupling increases both the free carrier generation efficiency and the short-range mobility in NPL solids. The results show a straightforward method of controlling the interfacial electronic coupling strength for developing functional optoelectronic devices through thermal treatments.

Tunable in situ near-UV pulses by transient plasmonic resonance in nanocomposites.

We propose a concept for generation of ultrashort pulses based on transient field-induced plasmonic resonance in nanoparticle composites. Photoionization and free-carrier plasma generation change the susceptibility of nanoparticles on a few-femtosecond scale under the action of the pump pulse. This opens a narrow time window when the system is in plasmonic resonance, which is accompanied by a short burst of the local field. During this process, frequency-tunable few-fs pulses can be emitted. This paves a way to ultra-compact yet efficient generation of ultrashort pulses at short wavelengths.