Generation Of Free Charge 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.

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.

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).

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.

(Invited) Signatures of Charge Carrier (De)Localization in Semiconducting Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are attractive materials for a variety of opto-electronic devices and applications due their strong and energetically tunable optical absorption, and high charge carrier mobilities that result from the delocalized nature of the π-electron system. To realize the full potential of s-SWCNTs in these applications, it is critical to understand the generation, transport, interfacial transfer, and recombination of free charge carriers (electrons and holes). As with many pi-conjugated semiconductors, there remain uncertainties regarding the degree of (de)localization for charge carriers generated in s-SWCNTs either through photogenerated exciton dissociation or through charge injection by adsorbed redox dopants or gating. In this presentation, I will compare the various signatures of charge carrier localization and delocalization that appear in spectroscopic and transport studies of s-SWCNTs in a variety of different environments and over several decades of carrier density. These comparisons help to elucidate the important roles of intrinsic and extrinsic factors on carrier (de)localization and the information that spectroscopic signatures in the visible, near-IR, and mid-IR can provide on such (de)localization.

The Role of Device Mobility on the Charge Generation Process in Polymerized Small-Molecule Acceptor Based Organic Solar Cells

We investigate the role of backbone tuning of contemporary polymerized small-molecule acceptors (PSMAs), namely PJ2, PJTVT, and PJTET, on the charge generation process when blended with JD40 polymers to explain the difference between their device short circuit current, JSC. We utilize transient absorption (TA) spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements to demonstrate the face-on π–π and lamellar stackings impact on the diffusion of excitons in pristine films. Comparing the charge generation process to data obtained from space-charge-limited current (SCLC) measurements, we show that the generation of free charge carriers, although limited by geminate recombination of charge transfer (CT) pairs, is improved with a higher charge-carrier device mobility. A higher mobility is found to also drive charge generation limited by the same recombination more efficiently than blend systems where the charges recombine nongeminately. We show that an increment of ∼0.1 eV corresponds to an ∼5% increase in the device EQE for the same mobility. Based on the data compiled from the literature and the study herein, a mobility of at least 10–3 cm2 V–1 s–1 is postulated for an EQE of at least 90%. Finally, we observe an apparent logarithmic relationship between the charge-carrier mobility and the generation of photocurrent, in lieu of a linear relationship introduced by the Braun–Onsager model. The same trend remains for surveyed literature data of contemporary Y-series acceptor-based OPV devices. This work highlights that to improve the JSC of current state-of-art OPV devices, the mobility of a device can be improved through material designs and device engineering for a higher charge generation efficiency.

Transfer matrix method-based efficiency enhancement of lead-free Cs3Sb2Br9 perovskite solar cell

This work highlights an efficiency enhancement of a lead-free Cs3Sb2Br9-based perovskite solar cell (PSC) by using the transfer matrix method (TMM). This method calculates the optical parameters such as the absorption profile of each layer, and the total reflection profile at the front surface by considering the coherent and incoherent effect in the layers of the Cs3Sb2Br9-based PSC. The calculation of absorption and reflection is done by using the optical electric field (E-field) intensity. The optical E-field intensity profile leads the calculation of the generation of free charge carriers in the absorbing layer and results in the ideal optical current density Jopt of 24.71 mAcm−2. Furthermore, the obtained optical parameters after the TMM-based simulation are used to design two different models; ‘SCAPS model’ and ‘TMM model’, for an electrical simulation using SCAPS-1D software. Finally, the lead-free Cs3Sb2Br9-based PSC using the ‘TMM model’ obtained an effective electrical performance such as JSC of 17.97 mAcm−2, VOC of 1.54 V, FF of 74.61%, and PCE of 20.67%, respectively, which encourage for the understanding of the operational mechanism of the PSC and will contribute further efficiency improvement.

Charge Carrier Dynamics in Planar Heterojunction Organic Solar Cells

The ionization energy (IE) offset of a donor–acceptor pair provides the driving force for hole transfer and subsequent free charge carrier generation in low‐bandgap nonfullerene organic solar cells (OSCs). However, the interfacial energetic landscape in bulk heterojunction OSCs is determined by the materials’ electronic structure and intermolecular interactions at the donor/acceptor interface, causing local energy‐level shifts and disorder. Herein, the impact of the IE offset on the charge transfer efficiency and charge carrier dynamics is systematically evaluated by characterizing PM6/ITIC, PM6/IT‐2Cl, and PM6/IT‐4Cl planar heterojunction (PHJ) solar cells. Ultrafast spectroscopy and time‐resolved charge carrier density measurements reveal that an IE offset of about ≈0.5 eV leads to efficient hole transfer and subsequent free charge generation. Furthermore, bimolecular charge recombination and consequently triplet generation are significantly reduced in systems with high IE offset. This work underlines the importance of sizeable donor–acceptor IE offsets in PHJ nonfullerene OSCs as critical for high‐efficiency donor/acceptor material and device design.

Metal oxide semiconductor nanowires enabled air-stable ultraviolet-driven synaptic transistors for artificial vision

The multi-terminal transistor can provide reliable platform to construct artificial synapses with various benefits. In particular, synaptic transistors based on wide-bandgap metal oxide semiconductors (MOSs) offer opportunities to realize ultraviolet (UV) artificial vision. In this work, via a low-cost electrospinning technique, we have fabricated bio-inspired synaptic transistors based on zinc tin oxide (ZnSnO) nanowires, which can be effectively tuned by UV laser to achieve air-stable synaptic characteristics, such as excitatory post-synaptic current, paired-pulse facilitation, and learning-forgetting process based on long-term potentiation. The adsorption/desorption of oxygen molecules governs the generation of free charge carriers in the ZnSnO nanowires. Via the combination of transmission electron microscopy and X-ray diffraction measurements, we found that the revealed SnO2/ZnO heterostructures can effectively mitigate the recombination of UV-induced free carriers in the ZnSnO nanowires. Furthermore, based on the synaptic characteristics obtained from ZnSnO nanowires, we present several applications relevant to the UV artificial vision. We believe this study can provide insights into importance of heterostructures in synaptic transistors based on low-dimension MOSs with superior environmental stability and various applications.

Luminescence Temperature Quenching in Mn2+ Phosphors

AbstractNarrower band red and green emission in phosphor‐converted white light‐emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn2+ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn2+ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn2+ luminescence is lacking. Here systematic research is reported for a variety of Mn2+‐doped phosphors; a huge variation in the luminescence quenching temperature T50, ranging from 50 K for Mn2+ in ZnTe to 1200 K in MgAl2O4, is revealed. The value T50 shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T50 providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature‐stable Mn2+ luminescence in (high power) LEDs a wide‐bandgap host material is required.

Bismuth-Based Halide Perovskites for Photocatalytic H2 Evolution Application.

Metal halide perovskites (MHPs), in particular lead-based perovskites, have earned recognized fame in several fields for their outstanding optoelectronic properties, including direct generation of free charge carriers, optimal ambipolar charge carrier transport properties, high absorption coefficient, point-defect tolerance, and compositional versatility. Nowadays, this class of materials represents a real and promising alternative to silicon for photovoltaic technologies. This worthy success led to a growing interest in the exploration of MHPs in other hot research fields, such as solar-driven photocatalytic water splitting towards hydrogen production. Nevertheless, many of these perovskites show air and moisture instability problems that considerably hinder their practical application for photocatalytic water splitting. Moreover, if chemical instability is a problem that can be in part mitigated by the optimization of the chemical composition and crystal structure, the presence of lead represents a real problem for the practical application of MHPs in commercial devices due to environmental and healthcare issues. To successfully overcome these problems, lead-free metal halide perovskites (LFMHPs) have gained increasing interest thanks to their optoelectronic properties, comparable to lead-based materials, and their more eco-friendly nature. Among all the lead-free perovskite alternatives, this mini-review considers bismuth-based perovskites and perovskite derivatives with a specific focus on solar-driven photocatalysis application for H2 evolution. Special attention is dedicated to the structure and composition of the different materials and to the advantage of heterojunction engineering and the relative impact on the photocatalytic process.

The impact of film deposition and annealing on the nanostructure and dielectric constant of organic semiconductor thin films.

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2'-[(4,4,4',4'-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4'H-{2,2'-bi[cyclo-penta[2,1-b:3,4-b']dithiophene]}-6,6'-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing.

Polariton enhanced free charge carrier generation in donor-acceptor cavity systems by a second-hybridization mechanism.

Cavity quantum electrodynamics has been studied as a potential approach to modify free charge carrier generation in donor-acceptor heterojunctions because of the delocalization and controllable energy level properties of hybridized light-matter states known as polaritons. However, in many experimental systems, cavity coupling decreases charge separation. Here, we theoretically study the quantum dynamics of a coherent and dissipative donor-acceptor cavity system, to investigate the dynamical mechanism and further discover the conditions under which polaritons may enhance free charge carrier generation. We use open quantum system methods based on single-pulse pumping to find that polaritons have the potential to connect excitonic states and charge separated states, further enhancing free charge generation on an ultrafast timescale of several hundred femtoseconds. The mechanism involves polaritons with optimal energy levels that allow the exciton to overcome the high Coulomb barrier induced by electron-hole attraction. Moreover, we propose that a second-hybridization between a polariton state and dark states with similar energy enables the formation of the hybrid charge separated states that are optically active. These two mechanisms lead to a maximum of 50% enhancement of free charge carrier generation on a short timescale. However, our simulation reveals that on the longer timescale of picoseconds, internal conversion and cavity loss dominate and suppress free charge carrier generation, reproducing the experimental results. Thus, our work shows that polaritons can affect the charge separation mechanism and promote free charge carrier generation efficiency, but predominantly on a short timescale after photoexcitation.

Regulating Excitonic Effects in Covalent Organic Frameworks to Promote Free Charge Carrier Generation

Strong excitonic effects, induced by the Coulombic interactions between photogenerated electrons and holes, seriously hinder the generation of free charge carriers in organic semiconductors for conducting photocatalysis. Herein, we report a built-in control of the donor–acceptor (D–A) interaction strategy to regulate excitonic effects within benzobisthiazole-bridged covalent organic frameworks (Tz-COFs). Theoretical calculation and ultrafast spectroscopy reveal that strengthening D–A interactions by this built-in control strategy in Tz-COFs can accelerate exciton dissociation, thus generating more long-lived photogenerated charge carriers for photoredox reactions. As a result, the optimized Tz-COF-3 exhibits high photocatalytic H2 evolution activity as high as 43.2 mmol g–1 h–1, with an apparent quantum yield of 6.9% at 420 nm. This work guides the development of COFs from excitonic aspects for photocatalysis.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems.

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

NaNbO3 Nanorods: Photopiezocatalysts for Elevated Bacterial Disinfection and Wastewater Treatment.

In the present work, ferroelectric sodium niobate (NaNbO3) nanorods are formulated to attain photopiezocatalysis for water pollutant degradation and bacterial disinfection. NaNbO3 nanorods, integrating the advantages of photocatalysis (generation of free charge carriers) and piezocatalysis (separation of these charge carriers), possess synergistic effects, which results in a higher catalytic activity than photocatalysis and piezocatalysis alone. Active species that are involved in the catalytic process are found to be •O2– < OH• < h+, indicating the significance of piezocatalysis and photocatalysis. The degradation efficiency of sodium niobate (NaNbO3) nanorods for Rhodamine B in the presence of both sunlight and ultrasonic vibration is 98.9% within 60 min (k = 7.6 × 10–2 min–1). The piezo potential generated by NaNbO3 nanorods was reported to be 16 V. The antibacterial activity of the produced sample was found to be effective against Escherichia coli. With inhibitory zones of 23 mm, sodium niobate has a greater antibacterial activity.

Regulating the exciton binding energy of covalent triazine frameworks for enhancing photocatalysis

Strong excitonic effects are the major constraints hindering free charge carrier generation in polymeric semiconductors. Minimizing the potential Coulomb interaction of Frenkel excitons is ideal for boosting the generation of charge carrier.

Dopant-dependent thermoelectric performance of indoloindole-selenophene based conjugated polymer

The development of organic conductors, which exhibit high electrical conductivity through chemical doping using non-acidic molecular dopants, has been challenging because of inefficient generation of free charge carriers. In this study, we synthesized a new conjugated polymer (CP) containing electron-donating indoloindole and biselenophene to devise an efficient doping platform. When the synthesized CPs were doped with electron acceptor (F4TCNQ) and Lewis acid (AuCl3) type dopants, both dopants realized high electrical conductivities (184 S·cm−1 for F4TCNQ and 283 S·cm−1 for AuCl3). Furthermore, the binding energies of the electron-donating atoms in the doped CPs and the degree of stabilization energy before/after doping indicated that Se and N atoms tended to interact more favorably with F4TCNQ and AuCl3, respectively. However, the Seebeck coefficient and power factor of the AuCl3-doped CP were lower than those of the F4TCNQ-doped CP, which was attributed to the larger amount of charge carriers generated by the AuCl3 dopant.