Generation Of Free Charge Carriers Research Articles

Dielectric Constant Engineering of Organic Semiconductors: Effect of Planarity and Conjugation Length

AbstractBulk heterojunction organic solar cells continue to show steady photoconversion efficiency improvements. However, single component organic solar cells are a particularly attractive alternative due to the relative simplicity of device manufacture. It has been proposed that organic semiconductors with a high dielectric constant (≈10) could give rise to spontaneous free charge carrier generation upon photoexcitation. In this manuscript, factors are explored that affect the dielectric constant of organic semiconductors, particularly the optical‐frequency dielectric constant. The properties of monomers, dimers and trimers of two isoelectronic families of materials that have acceptor units composed of one or two dicyanovinylbenzothiadiazole moieties and one to three donor units are compared. The donor components are composed of either fluorenyl or cyclopentadithiophene moieties with the same glycol‐based solubilizing groups. It is found that chromophore planarity and orientation with respect to the substrate, and film density affect the optical and electronic properties of the materials, especially the high‐frequency dielectric constant. The results also indicate that delocalization of the highest occupied and lowest unoccupied molecular orbitals is a critical factor. The dimer with two dicyanovinylbenzothiadiazole moieties and two dithienocyclopentadiene units is found to have the highest optical frequency dielectric constant and overall performance.

Ultrafast charge carrier dynamics in potassium-doped endohedral metallofullerene Sc3N@C80 thin films

Molecular materials have emerged as highly flexible platform for photovoltaic and light-harvesting applications. One of the most important challenges for this class of materials is the trapping of charge carriers in bound electron–hole pairs, which severely limits the free charge carrier generation. Here, we demonstrate a significant modification of the exciton dynamics in thin films of endohedral metallofullerene complexes upon alkali metal doping. For the exemplary case of Sc3N@C80 thin films, we show that potassium doping results in an additional relaxation channel for the optically excited charge-transfer excitons that prevents the trapping of excitons in a long-lived Frenkel exciton-like state. Instead, potassium doping leads to an ultrafast exciton dissociation and most likely to the generation of free charge carriers. In this way, we propose alkali metal doping of molecular films as a novel approach to enhance the light-to-charge carrier conversion efficiency in photovoltaic molecular materials.

The Role of Grain Boundaries in Charge Carrier Dynamics in Polycrystalline Metal Halide Perovskites

AbstractBoundaries of perovskite grains forming polycrystalline films are the most sensitive place for the formation of the defect states and accumulation of impurities. The intra‐bandgap states located at the boundaries forming traps for charge carriers are commonly discussed as causing nonradiative carrier recombination and reducing carrier mobility. Grain boundaries (GB) may also disturb carrier motion by creating scattering centres and barriers, however, their role was less frequently considered. On the other hand, grain boundaries may also facilitate the splitting of exciton states and the generation of free charge carriers. Consequently, the grain boundaries are responsible or affect a wide range of perovskite properties and processes strongly influencing the performance of perovskite‐based optoelectronic devices. Nevertheless, abundant literature data related to the properties of grain boundaries and their influence are highly controversial. In this minireview, we discuss the most importnat literature data related to the structure and properties of grain boundaries and their role in device performance.

Molecular Chromophore-DNA Architectures With Fullerenes: Optical Properties and Solar Cells.

Supramolecular chemistry allows the construction of complex molecular architectures and the design of collective photophysical properties. DNA is an attractive template to build such supramolecular architectures due to its helical structure, the defined distances between the bases and the canonical base pairing that results in precise control of the chromophore position. The tailored properties of DNA-templated supramolecules eventually allow their implementation into optoelectronic applications. For the generation of free charge carriers from photo-generated excitons, fullerenes can be utilized. We synthesized two fullerene derivates, one of which binds by electrostatic interactions to single-stranded DNA, while the other contains two 2′-deoxyuridine moieties and assembles specifically along oligo-2′-deoxyadenosines (dA20) as DNA template. The DNA-directed assembly of both fullerenes in aqueous solution was investigated by UV/Vis absorbance and circular dichroism (CD) spectroscopy. The specific interactions with DNA make fullerenes with the 2′-deoxyuridine moieties a significantly better component for supramolecular DNA architectures. We studied the fluorescence quenching of both fullerenes with a DNA chromophore assembly. To investigate one of the key properties for optoelectronic applications, that is the supramolecular structure of the DNA-based assemblies in the solid phase, we characterized the CD of supramolecular chromophore-DNA architectures in thin films. Remarkably, the helical chirality of the chromophore assemblies that is induced by the DNA template is conserved even in the solid state. Upon implementation into organic solar cells, the external quantum efficiency measurements showed charge carrier generation on all three chromophore components of the DNA assemblies. The fullerenes with the 2′-deoxyuridine moieties enhance the quantum efficiency of the conversion process significantly, demonstrating the potential of DNA as structural element for ordering chromophores into functional π-systems, which may be employed in future organic solar cells.

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

Cocrystal Engineering: Toward Solution‐Processed Near‐Infrared 2D Organic Cocrystals for Broadband Photodetection

Large-area 2D cocrystals with strong near-infrared (NIR) absorption have been designed and prepared. Driven by the intermolecular charge-transfer (CT) interactions, zinc tetraphenylporphyrin (donor) and C60 (acceptor) self-assemble into a NIR cocrystal with absorption wavelength up to 1080 nm. By tailoring the growth solvents and processes, the cocrystal morphologies can be tuned from 1D nanowires, 2D nanosheets to large-area 2D cocrystal films with length reaching several millimeters. Owing to the highly ordered donor-acceptor arrangement, the CT absorption in the 2D cocrystals is enhanced and is comparable to singlet absorption. The uniform 2D cocrystals, with enhanced CT absorption in the NIR region, displays a high responsivity of 2424 mA W-1 to NIR light and a fast response time of 0.6 s. The excellent device performance is attributed to the generation of long-lived free charge carriers as revealed by transient absorption spectroscopy and optimization of device configuration.

Effect of Palladium‐Tetrakis(Triphenylphosphine) Catalyst Traces on Charge Recombination and Extraction in Non‐Fullerene‐based Organic Solar Cells

AbstractThe effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th) donor and 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts.

Regulation of excitons dissociation in AgI/Bi3O4Br for advanced reactive oxygen species generation towards photodegradation

The electron-hole-mediated exciton effect in two-dimensional materials plays a vital role in the process of photocatalytic reactive oxygen species (ROS) generation. In photocatalytic degradation, the existence of competitive excitons is an important factor limiting the generation of free charge carriers. Herein, a binary nanohybrid of single-unit-cell Bi3O4Br and AgI nanoparticle was designed and used as a photocatalyst for ROS generation. We explored the role of excitons and hot carriers in the photo-induced process through the AgI/Bi3O4Br model, and proved that the exciton dissociation mediated by heterojunction is beneficial to the path of charge transfer in photocatalysis. The extremely high generation efficiency of •OH and O2− is attributed to the reduction of spin-orbit coupling (SOC) in AgI/Bi3O4Br heterojunction and the existence of oxygen vacancies to promote exciton dissociation. Owing to the interfacial interaction between AgI and Bi3O4Br, the generation of 1O2 during the exciton energy transfer process is reduced and further increases the generation of O2− and •OH radicals in the charge transfer path. Benefiting from the enhanced ROS generation, the AgI/Bi3O4Br shows high effective degradation of rhodamine B (RhB), phenol, acetaminophen (APAP) and ciprofloxacin (CIP). This work offers a new strategy to optimize photocatalytic ROS generation for removal of organic pollutants in wastewater.

Multi-functional organic field effect transistor based on a dual doped P3HT

<abstract> <p>A dual doped regio-regular poly(3-hexylthiophene-2, 5-diyl) (P3HT) was investigated to develop a multi-functional organic field effect transistor (OFET). OFETs based on a pristine P3HT and a dual doped P3HT (P3HT:PCBM:I<sub>2</sub> blend) were fabricated to study the impact of doping on the electrical properties of the samples, and to examine the mechanism through which it amplified the output performance of the doped OFETs. A series of experimental techniques such as device electrical characterization, active layer surface analysis, and photon absorptivity measurements were conducted to quantitatively characterize the principal parameters that are susceptible to change as a result of doping. Topographic mapping revealed the expected doping-induced improvements in surface morphology, which could be associated with the ability of iodine to improve interdigitation between adjacent P3HT chains. Similarly, absorption spectra showed a 3 nm red-shift of the light absorbance spectrum of the doped samples compared to the undoped samples. The electrical conductivity of the samples was also examined at various conditions of temperature and light intensity, and the values obtained from the doped sample were approximately one order of magnitude higher compared to those of the undoped sample at room temperature, which explains the reason behind the higher output current drawn from the doped device compared to that of the undoped OFET. The explanation for this is two-fold, both PCBM and iodine promote the generation of free charge carriers, which increases the electrical conductivity of the active layer; and in addition to that, the improved P3HT main-chain interdigitation brought about by the introduction of iodine results in an increase in charge-carrier mobility, which also results in higher electrical conductivity. The findings of this study offers valuable information that could be instrumental in further advancing the future organic semiconductors based studies.</p> </abstract>

Charge Photogeneration in Non‐Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

AbstractIn organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, the contributions of excess energy and molecular quadrupole moments to the charge separation process are considered. The charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7‐Th and two non‐fullerene acceptors, ITIC and h‐ITIC, is investigated. CT state dissociation in these donor–acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. The electric field dependence of charge carrier generation is studied at different excitation energies by time delayed collection field (TDCF) and sensitive steady‐state photocurrent measurements. Upon excitation below the optical gap, free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. The average distance between electron–hole pairs at the donor–acceptor interface is determined from empirical fits to the TDCF data. The delocalization of CT states is larger in PTB7‐Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor–acceptor interface on the dissociation of CT complexes.

Molecular Quadrupole Moments Promote Ground‐State Charge Generation in Doped Organic Semiconductors

AbstractThe role of local environmental interactions on the generation of free charge carriers in doped organic layers is investigated. Via a classical micro‐electrostatic model, a dual effect of molecular quadrupole moments of host and dopant molecules on doping is demonstrated. Namely, electrostatic interactions ease ionization of the dopant by altering the energy level alignment between the host and the dopant and reduce the barrier for charge dissociation by flattening the energy landscape around the ionized dopants. These results indicate that tailoring molecular quadrupole moments of the host and/or dopant are an attractive strategy toward improved doping efficiency in organic semiconductors.

Charge carrier mobility dynamics in organic semiconductors and solar cells

Charge carrier mobility in organic semiconductors is not a constant value unambigously characterizing some particular material, but depends on the electric field, temperature and even on time after it was generated or injected. The time dependence is particularly important for the thin-film devices where charge carriers pass the organic layer before mobility reaching its stationary value. Here we give a review of experimental techniques with ultrafast timeresolution enabling one to address the mobility kinetics and analyse properties of the time-dependent mobility in conjugated polymers and organic solar cells. We analyse kinetics during the charge carrier generation and extraction of free charge carriers. The mobility typically decreases by several orders of magnitude on a picosecond-nanosecond time scale; however, its kinetics also depends on the investigation technique. The mobility kinetics in blends for bulk heterojunction solar cells strongly depends on the stoichiometric ratio of donor and acceptor materials.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

Impact of molecular quadrupole moments on the energy levels at organic heterojunctions

The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

Long-Lived Free Charge Carriers at Heterojunctions between Semiconducting Single-Walled Carbon Nanotubes and Perylene Diimide Electron Acceptors

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been thoroughly investigated as the components of various photovoltaic cells due to several advantages such as spectral tunability, absence of charge-transfer (CT) states, giant aspect ratio, chemical robustness, and hydrophobicity. In the previous study, it was reported that the heterojunctions between s-SWCNTs and perylene diimide (PDI)-based electron acceptors yield long-lived charge separated states whose lifetimes are more than 1.5 µs. Besides the potential of PDI-based electron acceptors to substitute fullerene-based electron acceptors, this work noted the significance of the molecular geometries of PDI-based electron acceptors which result in molecular aggregation and the associated charge delocalization in the acceptor phase. However, the true characteristics of the long-lived charge carriers for these heterojunctions have not been revealed yet. For instance, the degree to which the charge carriers generated at these heterojunctions are free or trapped was not yet clear. Moreover, it was not determined whether the charge recombination process from these heterojunctions is monomolecular-like or bimolecular-like. In this study, we explored the nature of charge carriers for the heterojunctions between (6,5) s-SWCNTs and two different PDI-based electron acceptors by combining two effective spectroscopic techniques: transient absorption (TA) and time-resolved microwave conductivity (TRMC). Two PDI-based electron acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were synthesized and coated on (6,5) s-SWCNT films to form donor-acceptor heterojunctions. TA and TRMC studies reveal that the dynamics of the charge-separated states across the (6,5) s-SWCNT/PDI-based acceptor heterojunctions remain similar over three orders of magnitude in absorbed photon flux. This fluence independence of the heterojunctions indicates that the charge carriers recombine ‘pseudo’-monomolecularly. Moreover, the charge recombination kinetics from TA and TRMC studies are well-matched, indicating that most of the generated charge carriers are free, not trapped. The unconventionally strong suppression of bimolecular charge recombination from these heterojunctions, supported by fluence independence of charge recombination dynamics, may be attributed to the high carrier mobility and good charge delocalization in both (6,5) s-SWCNTs and PDI-based acceptors. These factors can also be regarded as the origin of high free charge carrier generation in these heterojunctions. These photophysical studies provide the fundamental understandings of the charge generation process in s-SWCNT-based heterojunctions and how different electron acceptor materials can impact the nature of charge generation with respect to the heterojunction energetics and molecular orientations. The results can inform rational design strategies for s-SWCNT-based optoelectronic applications.

Ultrafast charge transfer coupled with lattice phonons in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics. For efficient generation of free charge carriers, the donor–acceptor (D-A) conjugation has been adopted for two-dimensional (2D) COFs recently. In the 2D D-A COFs, photoexcitation would generate a polaron pair, which is a precursor to free charge carriers and has lower binding energy than an exciton. Although the character of the primary excitation species is a key factor in determining optoelectronic properties of a material, excited-state dynamics leading to the creation of a polaron pair have not been investigated yet. Here, we investigate the dynamics of photogenerated charge carriers in 2D D-A COFs by combining femtosecond optical spectroscopy and non-adiabatic molecular dynamics simulation. From this investigation, we elucidate that the polaron pair is formed through ultrafast intra-layer hole transfer coupled with coherent vibrations of the 2D lattice, suggesting a mechanism of phonon-assisted charge transfer.

Photoelectrocatalytic phenol oxidation employing nitrogen doped TiO2-rGO films as photoanodes

Semiconductor photoanodes based on TiO2 have been widely studied due to their high photonic-to-electron conversion capability, high thermal and chemical stability and low cost. However, the absorption of high-energy regions and poor conductivity limits their applicability in photo(electro)chemistry and energy conversion processes. In this work, we studied how the incorporation of dopants as nitrogen and a good carrier conductor as graphene (rGO) generates a synergistic effect improving the visible-light harvesting of photoanodes and increasing charge carrier separation and mobility into the material, to produce a high oxidizing power for organic compounds oxidation in the photoanode/electrolyte interface. Nitrogen-doped rGO-modified TiO2 photoanodes (NTG) were prepared by a sol − gel method and immobilized on stainless steel (SS) by dip-coating technique to evaluate their photoelectrocatalytic activity in the phenol oxidation. The surface of the photoanodes was characterized by FESEM-EDS, GIXRD, DRS, Raman and X-ray photoelectron spectroscopy. The photoelectrochemical properties of photoanodes were studied by photovoltammetry in 0.1 M HClO4 (pH 1). FESEM images showed a cracked surface due to the evaporation of solvent from precursor solution. GIXRD patterns and Raman spectra exhibited the presence of anatase phase in the films. DRS spectra of NTG films displayed an absorption range between 450–560 nm, indicating the extension of the spectral response of the semiconductor to visible region. XPS spectra showed the presence of substitutional and interstitial nitrogen into the TiO2 lattice, indicating the N-doping of TiO2. The photocurrent generated by the NTG electrode was higher than those of rGO-modified TiO2 (TG) and N-doped TiO2 (NT), associated to the generation of freecharge carriers under visible light by the presence of N, and their subsequent transport through carbon conjugated structure of added rGO to current collector. This synergistic behaviour allowed more efficient phenol degradation around 45% by using photoelectrocatalysis in comparison with other oxidation process as photolysis (1.5%) photocatalysis (21%) and electrocatalysis (24%).

A spatially smoothed device model for meso-structured perovskite solar cells

Meso-structured perovskite solar cells (PSC), utilizing a mesoporous absorber layer consisting of mesoporous metal oxide and the perovskite material inside, are still delivering the highest solar cell efficiency for perovskite-based solar cells up to date. Their outstanding performance critically depends on the nanoscopic morphology formed inside the mesoporous absorber layer. This, however, is not accounted for in most of the perovskite device models, as they are based on an effective-medium formulation for the mesoporous absorber layer, and the details of its underlying morphology are ignored. The mesoporous absorber layer is treated as a two-phase model that describes intrinsic solar cell physics such as free charge carrier generation, carrier transport, and recombination within the two phases, as well as at the interface between the two phases. We derive a spatially smoothed device model for meso-structured PSCs based on volume-averaging of electric potential and electron and hole concentrations of the two-phase model, and this spatially smoothed formulation captures two essential morphological descriptors that are not found in existing effective-medium formulations for meso-structured PSCs, namely, surface-to-volume ratio and porosity inside the mesoporous layer. Furthermore, we determine the explicit functional forms of the effective parameters in the spatially smoothed model for the case of an ideal “Spaghetti” blend morphology.

Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe2/MoS2 Type-II Interface.

Atomically thin transition-metal dichalcogenides (TMDC) have become a new platform for the development of next-generation optoelectronic and light-harvesting devices. Here, we report a Kelvin probe force microscopy (KPFM) investigation carried out on a type-II photovoltaic heterojunction based on WSe2 monolayer flakes and a bilayer MoS2 film stacked in vertical configuration on a Si/SiO2 substrate. Band offset characterized by a significant interfacial dipole is pointed out at the WSe2/MoS2 vertical junction. The photocarrier generation process and phototransport are studied by applying a differential technique allowing to map directly two-dimensional images of the surface photovoltage (SPV) over the vertical heterojunctions (vHJ) and in its immediate vicinity. Differential SPV reveals the impact of chemical defects on the photocarrier generation and that negative charges diffuse in the MoS2 a few hundreds of nanometers away from the vHJ. The analysis of the SPV data confirms unambiguously that light absorption results in the generation of free charge carriers that do not remain coulomb-bound at the type-II interface. A truly quantitative determination of the electron-hole (e-h) quasi-Fermi levels splitting (i.e., the open-circuit voltage) is achieved by measuring the differential vacuum-level shift over the WSe2 flakes and the MoS2 layer. The dependence of the energy-level splitting as a function of the optical power reveals that Shockley-Read-Hall processes significantly contribute to the interlayer recombination dynamics. Finally, a newly developed time-resolved mode of the KPFM is applied to map the SPV decay time constants. The time-resolved SPV images reveal the dynamics of delayed recombination processes originating from photocarriers trapping at the SiO2/TMDC interfaces.

Mechanism of the free charge carrier generation in the dielectric breakdown

Many studies have been conducted to investigate the effect of environmental, mechanical and electrical stresses on insulator. However, studies on physical process of discharge phenomenon, leading to the breakdown of the insulator surface are lacking and difficult to comprehend. Therefore, this paper analysed charge carrier generation mechanism that can cause free charge carrier generation, leading toward surface discharge development. Besides, this paper developed a model of surface discharge based on the charge generation mechanism on the outdoor insulator. Nernst’s Planck theory was used in order to model the behaviour of the charge carriers while Poisson’s equation was used to determine the distribution of electric field on insulator surface. In the modelling of surface discharge on the outdoor insulator, electric field dependent molecular ionization was used as the charge generation mechanism. A mathematical model of the surface discharge was solved using method of line technique (MOL). The result from the mathematical model showed that the behaviour of net space charge density was correlated with the electric field distribution.