Charge Mobility In Organic Semiconductors Research Articles

Machine learning based charge mobility prediction for organic semiconductors.

Transfer integral is a crucial parameter that determines the charge mobility of organic semiconductors, and it is very sensitive to molecular packing motifs. The quantum chemical calculation of transfer integrals for all the molecular pairs in organic materials is usually an unaffordable task; fortunately, it can be accelerated by the data-driven machine learning method now. In this work, we develop machine learning models based on artificial neutral networks to predict transfer integrals accurately and efficiently for four typical organic semiconductor molecules: quadruple thiophene (QT), pentacene, rubrene, and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). We test various forms of features and labels and evaluate the accuracy of different models. With the implementation of a data augmentation scheme, we have achieved a very high accuracy with the determination coefficient of 0.97 and mean absolute error of 4.5meV for QT, and similar accuracy for the other three molecules. We apply these models to studying charge transport in organic crystals with dynamic disorders at 300K and obtain the charge mobility and anisotropy in perfect agreement with the brutal force quantum chemical calculation. If more molecular packings representing the amorphous phase of organic solids are supplemented to the dataset, the current models can be refined to study charge transport in organic thin films with polymorphs and static disorders.

Side-chain engineering by thymine groups enables hydrogen bond in P-type donor-acceptor polymers with enhanced optoelectronic properties

Realizing ordered structures at the molecular level is a key approach to increase the charge mobility of organic semiconductors. However, the typical solution-based methods employed for the processing make the achievement of well-organized organic nanostructures difficult with the favored formation of disordered assemblies. To realize well-ordered thin films, one needs to design conjugated materials with the ability of self-assembling. In this work, two alternating donor-acceptor copolymers based on the carbazole and diketopyrrolopyrrole (DPP) units, namely P1 and P2, are successfully synthesized. Compared to P1, the polymer P2 contains an extra thymine group at the end of the alkyl chain, through which the hydrogen bonding (CO⋯NH) can be formed between the amide (NH) and carbonyl units (CO), as verified by the fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) study. The electrochemical analysis shows that the hydrogen bonding association influences the carbazole donor ability, resulting in a higher HOMO energy level of P2 compared to P1. In addition, the as-formed hydrogen bonds promote the assembling of the molecules into highly ordered structures by enabling strong aggregation as well as a narrow distance between adjacent molecules with long-range ordering packing and large nanocrystalline grains. As a result, the intermolecular charge transfer is enhanced. Consequently, the organic field-effect transistors (OFETs) constructed with P2 as the semiconductor layer present a p-type behavior with maximum hole mobility up to 1.32 cm2 V−1 s−1 upon annealing, which is almost 5 times higher than that of the pristine P2 (0.26 cm2 V−1 s−1), while the hole mobility of the annealed P1 is only 0.23 cm2 V−1 s−1. Our results indicate that the alkyl chain engineering, with the introduction of a thymine group into the donor units of the polymer, is responsible for the formation of hydrogen-bonded superstructures. This is a facile approach to enhance the optoelectronic properties of organic semiconductors.

Simple charge transport model for efficient search of high-mobility organic semiconductor crystals

Search for materials with high charge mobilities among the plethora of synthesizable organic semiconductors is of paramount importance for organic electronics, and virtual screening is expected to guide and streamline this process. However, a model for rapid but reliable prediction of charge mobility in organic semiconductors is still lacking. To address this challenge, a simple “diffusion-of-delocalized-polaron” model is suggested in this study. It considers the most essential factors governing the charge transport: intermolecular electronic coupling, electron-phonon interaction, charge delocalization, static and dynamic disorder. The suggested model reproduces experimental charge mobilities in various organic semiconductors remarkably well, and significantly outperforms another approach previously used for screening of organic semiconductors – the Marcus model. Moreover, charge mobility values predicted by our model are closer to the experiment than those provided by the two state-of-the-art approaches – quantum nuclear tunneling model and second-order cumulant expansion of the density matrix – and are on par with the popular transient localization theory. It is thus anticipated that the suggested model is an efficient tool for the search of high-mobility materials, revealing of structure-mobility relationships and rational design of organic semiconductors. Its possible integration with crystal structure prediction can boost the development of organic electronics.

The Electron-Vibrational Interaction in a Thiophene—Phenylene Cooligomer and Its Relationship to the Raman Spectrum

Electron-vibrational interactions play a key role in limiting charge mobility in organic semi-conductors. This paper reports a theoretical study of the electron—phonon interaction in the 5,5′-diphenyl-2,2′-bitiophene (PTTP) molecule, which belongs to the class of thiophene-phenylene cooligomers, which are of great interest for organic optoelectronics due to their electron-transport and luminescent properties; these results are compared with anthracene, which is a model organic semiconductor. The contributions of various vibrational modes to the reorganization energy of PTTP and the anthracene molecules are revealed and it is shown that these contributions correlate with the intensities of the corresponding bands in the Raman spectrum. In particular, it is found that for the PTTP molecule the so-called I-mode with a frequency of ∼1460 cm−1, which corresponds to collective vibration of atoms of all oligomer units, has the highest intensity in both spectra. These results indicate the promise of Raman spectroscopy for studying electron-vibrational interactions in organic semiconductors. Finally, the mobility of holes in PTTP and anthracene is estimated in the framework of the jump model and the reasons for their difference are analyzed. Based on these results, we propose some ways to reduce the electron-vibrational interaction in thiophene—phenylene cooligomers, which is important for the directional molecular design of organic semiconductors.

Charge transport in an organic light emitting diode material measured using metal-insulator-semiconductor charge extraction by linearly increasing voltage with parameter variation

Charge transport measurement using the Metal-Insulator-Semiconductor Charge Extraction by Linearly Increasing Voltage (MIS-CELIV) technique is a promising method for determining charge mobility in organic semiconductors because of its ability to study electron and hole mobilities independently. However, MIS-CELIV measurements have a number of parameters that can potentially affect the calculated mobility. There are only a few reports on MIS-CELIV being used to determine the charge mobility for materials typically used in organic light-emitting diodes (OLEDs), and the impact of each of the MIS-CELIV experimental parameters on the mobility is presently unknown. We find that the pulse duration, injection time, maximum voltage, offset voltage, and external load resistance have different levels of influence on the calculated mobility. Using the hole transporting OLED host material, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), we show that having an injection time sufficient to fully charge the insulator layer, a pulse duration comparable to the transit time, and an external circuit time constant much smaller than the transit time is required to give a mobility relevant to an OLED. The optimized MIS-CELIV parameters led to the measurement having a similar current density and electric field to that of an operational OLED. Under these conditions, the hole mobility of TCTA was determined to be 2.90 ± 0.07 × 10−4 cm2 V−1 s−1, which is similar to that measured using time-of-flight techniques. Using inappropriate experimental parameters could lead to an underestimation of the mobility by an order of magnitude. Simulations of the MIS-CELIV measurements verified the effect the different parameters played in determining the charge mobility.

Direct probe of the nuclear modes limiting charge mobility in molecular semiconductors

The charge mobility of organic semiconductors are accurately predicted using first principles simulations validated by inelastic neutron scattering experiments.

Predicting Charge Mobility of Organic Semiconductors with Complex Morphology

We introduce a mesoscopic model to predict the charge mobility of organic semiconductors characterized by a coexistence of crystalline and amorphous phases. First, we validate our scheme by reproducing the trends in charge mobility observed in thin films of poly(3-hexylthiophene) (P3HT) polymers. Next, we address the problem of predicting the morphologies that lead to the highest mobility. Our main finding is the identification of a region of the model’s multidimensional parameter space, in which the charge mobility effectively depends on a single morphological feature: the average intercrystallite distance. This scaling behavior provides insight into the main physical mechanism limiting charge mobility in organic semiconductors. Our proposed framework can be adapted to study a wide class of polymeric systems and used to guide the manufacturing of new, high-performing organic semiconductor materials.

Molecular stacking effect on photoluminescence quantum yield and charge mobility of organic semiconductors

The photoluminescence quantum yield (PLQY) and charge transfer property of the high mobility emissive organic semiconductors, 2,6-diphenylanthracene (DPA) and 2,6-diphenyl-9,10-bis(phenylethynyl)anthracene (DP-BPEA), are theoretically investigated. A quantum mechanics/molecular mechanics (QM/MM) method is adopted to investigate the photophysical properties, the Marcus equation is used to describe hole and electron transfer rates, and kinetic Monte Carlo simulation is performed to obtain charge mobility. In both solution and solid phase, the PLQY and the charge mobility of DPA and DP-BPEA are calculated, and the molecular stacking effect is analyzed. For DPA, the rotation motions of the terminal phenyl ring are obviously restricted in the solid phase, which results in the decrease of the Huang-Rhys factor and reorganization energy. This restricted intramolecular rotation (RIR) effect suppresses dissipation pathways of the excited state energy. As a result, the aggregation induced enhancement emission (AIEE) is thus revealed for this emitter from the tetrahydrofuran (THF) solution to the solid phase. However for DP-BPEA, the geometrical variations between the ground and excited states are almost unchanged from the THF solution to the solid phase, and only a restricted intramolecular vibration (RIV) of bond length (e.g. -C[triple bond, length as m-dash]C-) is displayed. As RIV is not obvious as RIR, a similar fluorescence efficiency is obtained for DP-BPEA in THF solution and the solid phase. In addition, DPA and DP-BPEA crystals are proven to be p-type semiconductors. The calculated mobility of the hole for DPA (3.39 cm2 V-1 s-1) is larger than that of DP-BPEA (1.62 cm2 V-1 s-1) because the number of effective transition pathways of DPA is more than that of DP-BPEA. Our study demonstrates that the different molecular stacking of organic semiconductors has an important effect on their photophysical and charge transfer properties.

Single-Component Conductors: A Sturdy Electronic Structure Generated by Bulky Substituents.

While the introduction of large, bulky substituents such as tert-butyl, -SiMe3, or -Si(isopropyl)3 has been used recently to control the solid state structures and charge mobility of organic semiconductors, this crystal engineering strategy is usually avoided in molecular metals where a maximized overlap is sought. In order to investigate such steric effects in single component conductors, the ethyl group of the known [Au(Et-thiazdt)2] radical complex has been replaced by an isopropyl one to give a novel single component molecular conductor denoted [Au(iPr-thiazdt)2] (iPr-thiazdt: N-isopropyl-1,3-thiazoline-2-thione-4,5-dithiolate). It exhibits a very original stacked structure of crisscross molecules interacting laterally to give a truly three-dimensional network. This system is weakly conducting at ambient pressure (5 S·cm(-1)), and both transport and optical measurements evidence a slowly decreasing energy gap under applied pressure with a regime change around 1.5 GPa. In contrast with other conducting systems amenable to a metallic state under physical or chemical pressure, the Mott insulating state is stable here up to 4 GPa, a consequence of its peculiar electronic structure.

Theoretical investigation on electron mobility properties of anthracenedicarboximide derivatives based n-type organic semiconductors

AbstrctThe anthracenedicarboximide (ADI) based n-type organic semiconductor materials, ADIF-Br2 and ADIF-CN2, and eight designed ADI derivatives are systematically studied using density functional theory. The eight derivatives are designed by substituting atoms and functional groups in the middle positions of the molecules and by adding functional groups at the end of the molecules. The structures, optical properties, ionization potential, electronic affinity, reorganization energy, charge coupling and charge mobility of the compounds are found to be largely influenced by the substitutions. According to the main characters of organic semiconductors, ADIF-Br2 is suggested as a good n-type organic semiconductor, and ADIF-CN2 as an ambipolar organic semiconductor. Compared to ADIF-Br2 and ADIF-CN2, all the eight designed derivations are suggested as good n-type organic semiconductors with improved performance. The main absorption spectra of the compounds are in the range 400–500 nm, which match well with the solar spectrum. The hydrogen bonds formed in ADIF-Br2 and ADIF-CN2 make a close π–π stacking, which accounst for their large charge mobility (0.208 and 0.057 cm2 V−1 s−1, respectively). The hydrogen bond is suggested as a new mechanism for charge mobility in organic semiconductors.

Polaron Structure and Transport in Fullerene Materials: Insights from First-Principles Calculations

Organic semiconductors offer a low-cost alternative to inorganic semiconductors. However, their usefulness is limited by a relatively low mobility of polaron charge carriers. Past research indicates a positive correlation between charge density and charge mobility in organic semiconductors. This relationship is usually attributed to the phenomenon of excess charges filling traps. Here, we explore whether charge density may also affect mobility via influence on intermolecular couplings. Density functional theory (DFT) with a long-range corrected (LC-BLYP) functional is used to calculate charge densities and electronic couplings of negative charges on C70 fullerenes in the presence of nearby negative point charges, which provides an upper limit calculation of the influence of nearby polarons. We find that in C70 systems with relatively low couplings, the presence of additional charges has an effect of maximizing intermolecular couplings and hence transport. This effect drops off quickly with distance, suggesting that it is relevant only at extremely high charge densities that are an unlikely event in current C70 devices. The effect of charge density on couplings may be useful in understanding transport in very limited regions of C70 materials where the local charge density is high; however, it is unlikely to affect overall device performance.

Transient space-charge-perturbed currents of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine in diode structures

The time-of-flight (TOF) experiment is the standard method to measure the charge mobility in organic semiconductors. Typically, this measurement assumes space-charge-free (SCF) transport. However, pure SCF current cannot be achieved in experiment since carriers have charges, which will cause space-charge perturbation. For a long time, the charge transport under space-charge-perturbed (SCP) conditions was not understood for organic semiconductors. Until recently, it is learned from Monte Carlo simulations that the TOF measurement under SCP conditions may lead to lower mobility. Our previous work also suggests that the influence of space-charge perturbation might be overestimated in the past. These results, if verified in experiment, might motivate us to review the requirements of the TOF measurement. Here, the transient SCP currents in organic semiconductors are studied using two commonly used hole transporting materials—N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD). The results confirmed the prediction of earlier peak time, longer transit time, and lower charge mobility. Further investigations using Monte Carlo simulations indicate that the enhanced carrier dispersion and low overall carrier movement under SCP conditions are caused by the inhomogeneous electric field. For the systems we studied, it is found that at small SCP degrees, determining the transit time by the “half plateau value” has small precision loss in mobility compared with the SCF condition.

Strong Photocurrent Enhancements in Highly Efficient Flexible Organic Solar Cells by Adopting a Microcavity Configuration

Organic solar cells often show inefficient light harvesting due to a short absorption path length limited by the low charge mobility of organic semiconductors. We demonstrate a flexible organic solar cell in a microcavity configuration using a TeO2/Ag semitransparent electrode to confine the optical field within the device with significant performance improvements and reaching a power conversion efficiency of 8.56%.

From charge transport parameters to charge mobility in organic semiconductors through multiscale simulation

This review introduces the development and application of a multiscale approach to assess the charge mobility for organic semiconductors, which combines quantum chemistry, Kinetic Monte Carlo (KMC), and molecular dynamics (MD) simulations. This approach is especially applicable in describing a large class of organic semiconductors with intermolecular electronic coupling (V) much less than intramolecular charge reorganization energy (λ), a situation where the band description fails obviously. The charge transport is modeled as successive charge hopping from one molecule to another. We highlight the quantum nuclear tunneling effect in the charge transfer, beyond the semiclassical Marcus theory. Such an effect is essential for interpreting the "paradoxical" experimental finding that optical measurement indicated "local charge" while electrical measurement indicated "bandlike". Coupled MD and KMC simulations demonstrated that the dynamic disorder caused by intermolecular vibration has negligible effect on the carrier mobility. We further apply the approach for molecular design of n-type materials and for rationalization of experimental results. The charge reorganization energy is analyzed through decomposition into internal coordinates relaxation, so that chemical structure contributions to the intramolecular electron-phonon interaction are revealed and give helpful indication to reduce the charge reorganization energy.

Facile Determination of Bulk Charge Carrier Concentration in Organic Semiconductors: Out-of-Plane Orientation Hopping Conduction Characteristics in Semicrystalline Polythiophene

In this report, we demonstrate a straightforward two-terminal-device-based electrical measurement and analysis scheme that can simultaneously determine the bulk free charge carrier concentration and out-of-plane charge mobility in organic semiconductors by understanding the transition behavior of device current–voltage characteristics from ohmic to space charge limited conduction. As a model system, we characterize the properties of a semicrystalline poly(3-hexylthiophene) (P3HT) conjugated polymer film in which free carrier concentration is systematically controlled by adjusting oxygen doping level. The observed dependence of out-of-plane charge mobility on the carrier concentration is analyzed in the context of percolative variable range hopping conduction, and we identify the rate-limiting charge hopping process in P3HT and correlate it with the role of disordered polymer regions in mediating the charge transport between neighboring crystalline polymer lamellar domains.

Toward Quantitative Prediction of Charge Mobility in Organic Semiconductors: Tunneling Enabled Hopping Model

A tunneling-enabled hopping mechanism is proposed, providing a pratical tool to quantitatively assess charge mobility in organic semiconductors. The paradoxical phenomena in TIPS-pentacene is well explained in that the optical probe indicates localized charges while transport measurements show bands of charge.

Influences of molecular packing on the charge mobility of organic semiconductors: from quantum charge transfer rate theory beyond the first-order perturbation

The electronic coupling between adjacent molecules is an important parameter for the charge transport properties of organic semiconductors. In a previous paper, a semiclassical generalized nonadiabatic transition state theory was used to investigate the nonperturbative effect of the electronic coupling on the charge transport properties, but it is not applicable at low temperatures due to the presence of high-frequency modes from the intramolecular conjugated carbon-carbon stretching vibrations [G. J. Nan et al., J. Chem. Phys., 2009, 130, 024704]. In the present paper, we apply a quantum charge transfer rate formula based on the imaginary-time flux-flux correlation function without the weak electronic coupling approximation. The imaginary-time flux-flux correlation function is then expressed in terms of the vibrational-mode path average and is evaluated by the path integral approach. All parameters are computed by quantum chemical approaches, and the mobility is obtained by kinetic Monte-Carlo simulation. We evaluate the intra-layer mobility of sexithiophene crystal structures in high- and low-temperature phases for a wide range of temperatures. In the case of strong coupling, the quantum charge transfer rates were found to be significantly smaller than those calculated using the weak electronic coupling approximation, which leads to reduced mobility especially at low temperatures. As a consequence, the mobility becomes less dependent on temperature when the molecular packing leads to strong electronic coupling in some charge transport directions. The temperature-independent charge mobility in organic thin-film transistors from experimental measurements may be explained from the present model with the grain boundaries considered. In addition, we point out that the widely used Marcus equation is invalid in calculating charge carrier transfer rates in sexithiophene crystals.

Influences of Crystal Structures and Molecular Sizes on the Charge Mobility of Organic Semiconductors: Oligothiophenes

Both crystal packing and molecular size have strong influences on the charge mobility for organic semiconductors. The crystal structures for oligothiophene (nT) can be roughly classified into two types: the Z = 2 (two molecules in one unit cell) or high temperature (HT) phase and the Z = 4 or low temperature (LT) phase. Through first-principles calculations within the Marcus electron transfer theory coupled with random walk simulation for room temperature charge diffusion constants, we found that the hole mobility of the HT phase is about 3–4 times larger than that of the LT phase because the molecular packing in the HT phase favors the hole transfer (i.e., the frontier orbital wave function phases of the dimer are constructive, which tends to maximize the overlap), while for the LT phase, the molecules are packed in a position that reduces the intermolecular orbital overlap due to phase cancellation. As the molecular size increases from 2T to 8T, the hole mobility tends to increase because the reorganiza...

Charge Mobility of Organic Semiconductors under High Pressure. Anthracene

Abstract The charge mobilities of anthracene single crystals were measured under high pressures up to 7.4 kbar. An experimental apparatus like Kepler’s was used. A sample holder was attached in a Drickamer-type high-pressure photocell, with kerosene employed as the pressure-transmitting fluid. The electron mobilities along the a and b crystal axes increased as the pressure applied increased; the values of the charge drift mobility at 7 kbar were 1.7 times as large as those at atmospheric pressure. The electron mobility along the c′ axis was nearly constant. The hole mobility, however, could not be measured, because the tail of the pulse signal, observed on the oscilloscope, was deformed.