A large number of non-adiabatic dynamical studies have been applied to reveal the nature of carrier transport in organic semiconductors with different approximations. We present here a "nearly exact" graphical process unit (GPU) based finite temperature TD-DMRG method to evaluate the carrier mobility in organic semiconductors as described by electron-phonon model, in particular to rubrene crystal, one of the prototypical organic semiconductors, with parameters derived from first-principles. We find that (i) TD-DMRG is a general and robust method that can bridge the gap between hopping and band picture covering a wide range of electronic coupling strength; and (ii) with realistic parameters, TD-DMRG is able to account for the experimentally observed "band-like" transport behavior ($\partial μ/ \partial T < 0$) in rubrene. We further study the long-standing puzzle of isotope effect for charge transport and unambiguously demonstrate that the negative isotope effect ($\partial μ/ \partial m < 0$ where $m$ is the atomic mass) should be universal.
Terahertz Raman spectroscopy was performed on high-mobility organic single-crystal semiconductors, by which the phonon energy at the Gamma point was qualified as a function of external uniaxial strain. The observation of peak shifts in Raman modes under uniaxial strain revealed that application of an external strain can effectively tune the intermolecular vibration that particularly correlates electron-phonon coupling. Terahertz Raman spectroscopy conducted in conjunction with molecular dynamics simulation provides an in-depth understanding of the recently discovered very large strain effect o…
Predicting charge transport in organic molecular crystals is notoriously challenging. Carrier mobility calculations in organic semiconductors are dominated by quantum chemistry methods based on charge hopping, which are laborious and only moderately accurate. We compute from first principles the electron-phonon scattering and the phonon-limited hole mobility of naphthalene crystal in the framework of ab initio band theory. Our calculations combine GW electronic bandstructures, ab initio electron-phonon scattering, and the Boltzmann transport equation. The calculated hole mobility is in very go…
We report a detailed comparison on the role of a self-assembled monolayer (SAM) of dipolar molecules on the threshold voltage and charge carrier mobility of organic field-effect transistor (OFET) made of both amorphous and polycrystalline organic semiconductors. We show that the same relationship between the threshold voltage and the dipole-induced charges in the SAM holds when both types of devices are fabricated on strictly identical base substrates. Charge carrier mobilities, almost constant for amorphous OFET, are not affected by the dipole in the SAMs, while for polycrystalline OFET (pent…
The power conversion efficiency of organic solar cells based on donor--acceptor blends is governed by an interplay of polaron pair dissociation and bimolecular polaron recombination. Both processes are strongly dependent on the charge carrier mobility, the dissociation increasing with faster charge transport, with raised recombination losses at the same time. Using a macroscopic effective medium simulation, we calculate the optimum charge carrier mobility for the highest power conversion efficiency, for the first time accounting for injection barriers and a reduced Langevin-type recombination.…
We show that infrared imaging of the charge density profile in organic field-effect transistors (FETs) can probe transport characteristics which are difficult to access by conventional contact-based measurements. Specifically, we carry out experiments and modeling of infrared spectromicroscopy of poly(3-hexylthiophene) (P3HT) FETs in which charge injection is affected by a relatively low resistance of the gate insulators. We conclude that the mobility of P3HT has a power-law density dependence, which is consistent with the activated transport in disorder-induced tails of the density of states.
Finite-bandwidth effects on the temperature dependence of the mobility of injected carriers in pure organic crystals are explored for a simplifed case of impurity scattering. Temperature-dependent bandwidth effects are discussed briefly through a simplified combination of band and polaronic concepts.
We present a multiscale modeling of the infrared optical properties of the rubrene crystal. The results are in very good agreement with the experimental data that point to nonmonotonic features in the optical conductivity spectrum and small optical effective masses. We find that, in the static-disorder approximation, the nonlocal electron-phonon interactions stemming from low-frequency lattice vibrations can decrease the optical effective masses and lead to lighter quasiparticles. On the other hand, the charge-transport and infrared optical properties of the rubrene crystal at room temperature…
High-performance $n$-type organic field-effect transistors were developed with ionic-liquid gates and N,N$^"$-bis(n-alkyl)-(1,7 and 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)s single-crystals. Transport measurements show that these devices reproducibly operate in ambient atmosphere with negligible gate threshold voltage and mobility values as high as 5.0 cm$^2$/Vs. These mobility values are essentially identical to those measured in the same devices without the ionic liquid, using vacuum or air as the gate dielectric. Our results indicate that the ionic-liquid and $n$-type organic semico…
We find that the hole mobility of the crystal smectic phases of the liquid crystal 1,4-di-(5-n-tridecylthien-2-yl)-benzene increases exponentially with decreasing temperature. While qualitatively consistent with transport via polaron bands, we find that it is quantitatively difficult to explain the behaviour with physically realistic parameters. In particular, the data demand either quite large typical optical phonon frequencies and/or phonon bandwidths. We also find evidence that an unusually highly ordered smectic-F phase templates the formation of lower temperature highly crystalline smecti…
We have used an electro-optic technique to measure the position-dependent infrared absorption of holes injected into a thin crystal of the organic semiconductor, 6,13-bis(triisopropylsilylethynyl)-pentacene incorporated in a field-effect transistor. By applying square-wave voltages of variable frequency to the gate or drain, one can measure the time it takes for charges to accumulate on the surface, and therefore determine their mobility.
The tunable properties of molecular materials place them among the favorites for a variety of future generation devices. In addition, to maintain the current trend of miniaturization of those devices, a departure from the present top-down production methods may soon be required and self-assembly appears among the most promising alternatives. On-surface synthesis unites the promises of molecular materials and of self-assembly, with the sturdiness of covalently bonded structures: an ideal scenario for future applications. Following this idea, we report the synthesis of functional extended nanowi…
The charge carrier mobility is a key parameter for the organic bulk heterojunction solar cell efficiency. It was recently shown that the interplay charge carrier transport and recombination, both depending on electron and hole mobilities, leads to a point of maximum power conversion efficiency at a finite mobility. Changes of bulk and surface recombination rate, however, can strongly influence this behavior. These processes were previously not considered adequately, as surface recombination velocities of infinity were implicitly assumed or bulk recombination parameters not discussed in detail.…
We report on the fabrication and characterization of single crystal field-effect transistors (FETs) based on diphenylbenzo diselenophene (DPh-BDSe). These organic field-effect transistors (OFETs) function as p-channel accumulation-mode devices. At room temperature, for the best devices, the threshold voltage is less than -7V and charge carrier mobility is nearly gate bias independent, ranging from 1cm2/Vs to 1.5 cm2/Vs depending on the source-drain bias. Mobility is increased slightly by cooling below room temperature and decreases below 280 K.
We analyze a model that accounts for the inherently large thermal lattice fluctuations associated to the weak van der Waals inter-molecular bonding in crystalline organic semiconductors. In these materials the charge mobility generally exhibits a "metallic-like" power-law behavior, with no sign of thermally activated hopping characteristic of carrier self-localization, despite apparent mean-free-paths comparable or lower than the inter-molecular spacing. Our results show that such puzzling transport regime can be understood from the simultaneous presence of band carriers and incoherent states …
Mobility-lifetime relation links photodegradation in spin-coated and gravure-printed organic solar cells
Technische Universität Chemnitz（开姆尼茨工业大学）；Friedrich-Alexander University Erlangen-Nürnberg（埃尔朗根-纽伦堡弗里德里希·亚历山大大学）；University of Bayreuth（拜罗伊特大学）；University of Potsdam（波茨坦大学）；Paul Drude Institute for Solid State Electronics（保罗·德鲁德固态电子研究所）
We have investigated the charge carrier transport in organic molecular semiconductors. It has been found that mobility is a function of electric field and temperature due to hopping conduction. Several theoretical models for charge transport in disordered solids have been debated over the role of spatial and energetic correlation in these systems and such correlations have been recently shown to explain the universal electric field dependence of mobility. We have compared and evaluated the applicability of different theoretically proposed models using very simple experimental results and based…
We show that the temperature dependence of conductivity of high mobility organic crystals Pentacene and Rubrene can be quantitatively described in the framework of the model where carriers are scattered by quenched local impurities and interact with phonons by Su-Schrieffer-Hegger (SSH) coupling. Within this model, we present approximation free results for mobility and optical conductivity obtained by world line Monte Carlo, which we generalize to the case of coupling both to phonons and impurities. We find fingerprints of carrier dynamics in these compounds which differ from conventional meta…
High throughput screening, crystal structure prediction, and carrier mobility calculations of organic molecular semiconductors as hole transport layer materials in perovskite solar cells
We develop a broadly applicable transport-based technique, GAte Modulated activation Energy Spectroscopy (GAMEaS), for determining the density of states (DOS) in the energy gap. GAMEaS is applied to field effect transistors made from different single crystal oligomer semiconductors to extract the free-carrier mobility, u_0, from the field effect mobility, u_eff. Samples with a lower DOS exhibit higher u_eff. Values of u_0 up to 100 +/- 40 cm2/Vs at 300K are observed, showing that performance can be greatly enhanced by improving sample purity and crystal quality.
In this paper we present an improved process for producing elastomer transistor stamps and high-mobility organic field-effect transistors (FETs) based on semiconducting acene molecular crystals. In particular, we have removed the need to use a silanized Si wafer for curing the stamps and to handle a fragile micron-thickness polydimethylsiloxane (PDMS) insulating film and laminate it, bubble free, against the PDMS transistor stamp. We find that despite the altered design, rougher PDMS surface, and lamination and measurement of the device in air, we still achieve electrical mobilities of order 1…
We explore polaronic quantum transport in three-dimensional models of disordered organic crystals with strong coupling between electronic and vibrational degrees of freedom. By studying the polaron dynamics in a static disorder environment, temperature dependent mobilities are extracted and found to exhibit different fingerprints depending on the strength of the disorder potential. At low temperatures and for strong enough disorder, coherence effects induce weak localization of polarons. These effects are reduced with increasing temperature (thermal disorder) resulting in mobility increase. Ho…
We have developed and applied a mobility edge model that takes into account drift and diffusion currents to characterize the space charge limited current in organic semiconductors. The numerical solution of the drift-diffusion equation allows the utilization of asymmetric contacts to describe the built-in potential within the device. The model has been applied to extract information of the distribution of traps from experimental current-voltage measurements of a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115] showing excellent agreement across several orders of magni…
There is compelling evidence that charge carriers in organic semiconductors (OSs) self-localize in nano-scale space because of dynamic disorder. Yet, some OSs, in particular recently emerged high-mobility organic molecular crystals, feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility OSs: DNTT (dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene), and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with fully atomistic non-adiabatic molecu…
Simulations of charge transport in amorphous semiconductors are often performed in microscopically sized systems. As a result, charge carrier mobilities become system-size dependent. We propose a simple method for extrapolating a macroscopic, nondispersive mobility from the system-size dependence of a microscopic one. The method is validated against a temperature-based extrapolation [Phys. Rev. B 82, 193202 (2010)]. In addition, we provide an analytic estimate of system sizes required to perform nondispersive charge transport simulations in systems with finite charge carrier density, derived f…
The pioneering experiments of Karl H. Drexhage explained the classical interaction of light with matter and the modification of the decay rates of an emitter.1 Here, we tried to mimic these experiments in a slightly different configuration and measured the electron mobility of a thin film semiconductor from weak to strong coupling regime. Perylene diimide (organic semiconductor dye) molecules are deposited on a MOSFET device. The refractive index mismatch between the silicon/silicon dioxide layer and the dye molecules forms an interference pattern. The frequency of the interference lines is tu…
We studied spin transport in four organic semiconductors (OSCs) with different electronic properties, with Fe and Co as the top and bottom ferromagnetic (FM) contacts, respectively. Magnetoresistance (MR) effects were observed up to room temperature in junctions based on an electron-carrying OSC, tris(8-hyroxyquinoline) aluminum (Alq$_3$) and a hole-carrying OSC, copper phthalocyanine (CuPc). The MR shows similar temperature dependence for these two OSCs, which suggests that the FM leads rather than the OSCs play a dominant role on the spin-transport degradation with increasing temperature. We…
We report an in-situ microbeam grazing incidence X-ray scattering study of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C$_8$-BTBT) organic semiconductor thin film deposition by hollow pen writing. Multiple transient phases are observed during the crystallization for substrate temperatures up to $\approx$93$^\circ$C. The layered smectic liquid-crystalline phase of C$_8$-BTBT initially forms and preceedes inter-layer ordering, followed by a transient crystalline phase for temperature $>$60$^\circ$C, and ultimately the stable phase. Based on these results, we demonstrate a method to produc…
Amorphous organic semiconductors based on small molecules and polymers are used in many applications, most prominently organic light emitting diodes (OLEDs) and organic solar cells. Impurities and charge traps are omnipresent in most currently available organic semiconductors and limit charge transport and thus device efficiency. The microscopic cause as well as the chemical nature of these traps are presently not well understood. Using a multiscale model we characterize the influence of impurities on the density of states and charge transport in small-molecule amorphous organic semiconductors…
The nonlocal electron-phonon couplings in organic semiconductors responsible for the fluctuation of intermolecular transfer integrals has been the center of interest recently. Several irreconcilable scenarios coexist for the description of the nonlocal electron-phonon coupling, such as phonon-assisted transport, transient localization, and band-like transport. Through a nearly exact numerical study for the carrier mobility of the Holstein-Peierls model using the matrix product states approach, we locate the phonon-assisted transport, transient localization and band-like regimes as a function o…
Southern University of Science and Technology（南方科技大学）；Shanghai University（上海大学）；Zhejiang Laboratory（之江实验室）
In the process of finding high-performance organic semiconductors (OSCs), it is of paramount importance in material development to identify important functional units that play key roles in material performance and subsequently establish substructure-property relationships. Herein, we describe a polymer-unit fingerprint (PUFp) generation framework. Machine learning (ML) models can be used to determine structure-mobility relationships by using PUFp information as structural input with 678 pieces of collected OSC data. A polymer-unit library consisting of 445 units is constructed, and the key po…
This study leverages density function theory (DFT) accompanied with Boltzmann transport equation approaches to investigate the electronic mobility as a function of inorganic substitution and functionalization in a thermally stable UiO-66 metal organic framework (MOF). The MOFs investigated are based on Zr-UiO-66 MOF with three functionalization groups of benzene dicarboxylate (BDC), BDC functionalized with an amino group (BDC + NH$_2$) and a nitro group (BDC + NO$_2$). The design space of this study is bound by UiO-66(M)-R, [M=Zr, Ti, Hf; R=BDC, BDC+NO$_2$, BDC+NH$_2$]. The elastic modulus was…
Computer simulation of the hopping charge transport in disordered organic materials has been carried out explicitly taking into account charge-charge interactions. This approach provides a possibility to take into account dynamic correlations that are neglected by more traditional approaches like mean field theory. It was found that the effect of interaction is no less significant than the usually considered effect of filling of deep states by non-interacting carriers. It was found too that carrier mobility generally increases with the increase of carrier density, but the effect of interaction…
The theoretical effects of phase separation on encounter-limited charge carrier recombination in organic semiconductor blends are investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, the dependence of the recombination rate on domain size and charge carrier mobility are quantified. Unifying competing models and simulation results, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, …
We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula, we develop a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity. The electron mobility is then calculated through a relaxation time approximation that accounts for quantum localization corrections beyond Boltzmann theory, and …
The tunability of high-mobility organic semi-conductors (OSCs) holds great promise for molecular spintronics. In this study, we show this extreme variability - and therefore potential tunability - of the molecular gyromagnetic coupling ("g-") tensor with respect to the geometric and electronic structure in a much studied class of OSCs. Composed of a structural theme of phenyl- and chalcogenophene (group XVI element containing, five-membered) rings and alkyl functional groups, this class forms the basis of several intensely studied high-mobility polymers and molecular OSCs. We show how in this …
Effect of energetic disorder on charge carrier transport in organic materials has been reexamined. A reliable method for mobility calculation and subsequent evaluation of relevant disorder parameters has been discussed. This method is well suited for a direct calculation of the magnitude of dipolar disorder $σ_\textrm{dip}$ in polar organic materials from the current transients. Calculation of $σ_\textrm{dip}$ for several transport materials with varying concentration of polar dopants gives concentration dependences that are in reasonable agreement with theoretical predictions. A possible solu…
The influence of ordered regions (micro crystallites and aggregates) in the other wise disordered polymer host matrix on field and temperature dependence of mobility (μ) has been simulated. Increase in concentration of ordered regions leads to increase in magnitude of mobility and in high field regime the saturation of the mobility occurs at lower electric field strength. The influence of different mean and standard deviation of Gaussian density of states (DOS) of ordered regions on the field dependence of mobility was studied and found to be significant only at higher concentrations. Weak inf…
In highest quality organic single-crystal field-effect transistors, electron transport occurs in the band-like regime, with the carrier mobility increasing upon lowering temperature. Neither the microscopic nature of this regime, nor why it occurs only in a small number of materials is currently understood. Here, comparative studies of closely related materials, exhibiting high-quality reproducible transport properties are needed. We performed a study of electron transport in single-crystals of different TCNQ (tetracyanoquinodimethane) molecules, combined with band structure calculations. We s…
In this work, we introduce models for deriving lower limits for the key parameters doping efficiency, charge carrier concentration, and charge carrier mobility from conductivity data of doped organic semiconductors. The models are applied to data of thin layers of Fullerene C60 n-doped by four different n-dopants. Combining these findings with thermoelectric Seebeck data, the energetic position of the transport level can be narrowed down and trends for the absolute values are derived.
We studied the influence of oxygen on the electronic trap states in a pentacene thin film. This was done by carrying out gated four-terminal measurements on thin-film transistors as a function of temperature and without ever exposing the samples to ambient air. Photooxidation of pentacene is shown to lead to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of 10(18) cm(-3). These trap states need to be occupied at first and cause a reduction in the number of free carriers, i.e. a consistent shift of the density of free holes as a function of ga…
Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic…
Predicting the electrical properties of organic molecular crystals (OMCs) is challenging due to their complex crystal structures and electron-phonon (e-ph) interactions. Charge transport in OMCs is conventionally categorized into two limiting regimes $-$ band transport, characterized by weak e-ph interactions, and charge hopping due to localized polarons formed by strong e-ph interactions. However, between these two limiting cases there is a less well understood intermediate regime where polarons are present but transport does not occur via hopping. Here we show a many-body first-principles ap…
Air-Water Interface-Assisted Synthesis and Charge Transport Characterization of Quasi-2D Polyacetylene Films with Enhanced Electron Mobility via Ring-Opening Polymerization of Pyrrole
We develop a model describing the electrochemical conversion of an organic semiconductor (specifically, the active material in a light-emitting electrochemical cell) from the undoped non-conducting state to the doped conducting state. The model takes into account both strongly concentration-dependent mobility and diffusion for the electronic charge carriers and the Nernst equation in the doped conducting regions. It is demonstrated that the experimentally observed doping front progression in light-emitting electrochemical cells can be accurately described with this model.
The photo-induced electron and hole transfer across the semiconductor-dielectric interface in trap dominated p-type organic field-effect transistors (OFETs) has been investigated. It has been observed that the transfer of electrons into the dielectric results in a decrease of the field-effect mobility of polarons, suggesting that additional shallow traps are generated in the conduction channel. Using this effect, the dependence of the field-effect mobility on the density of shallow traps, mu(N), has been measured, which allowed to estimate the average polaron trapping time, tau_tr = 50 +- 10 p…
We have observed the Hall effect in the field-induced accumulation layer on the surface of small-molecule organic semiconductor. The Hall mobility mu_H increases with decreasing temperature in both the intrinsic (high-temperature) and trap-dominated (low-temperature) conduction regimes. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, n_H, coincides with the density n calculated using the gate-channel capacitance, and becomes smaller than n in the trap-dominated regime. The Hall data are consistent with the diffusive band-like m…
In this article, we present an alternative method for simulating charge transport in disordered organic materials by using a buffer lattice at the boundary. This method does not require careful tracking of carrier's hopping pattern across boundaries. Suitability of this method is established by reproducing the field dependence of mobility, carrier relaxation and carrier diffusion in disordered organic systems obtained by simulating the charge transport for the full length of the systems along the field direction without and boundary condition. The significance of the buffer lattice is emphasiz…
The dynamics and internal structure of doping fronts in organic semiconductors are investigated theoretically using an extended drift-diffusion model for ions, electrons and holes. The model also involves the injection barriers for electrons and holes in the partially doped regions in the form of the Nernst equation, together with a strong dependence of the electron and hole mobility on concentrations. Closed expressions for the front velocities and the ion concentrations in the doped regions are obtained. The analytical theory is employed to describe the acceleration of the p- and n-fronts to…
Hall effect measurements are important for elucidating the fundamental charge transport mechanisms and intrinsic mobility in organic semiconductors. However, Hall effect studies frequently reveal an unconventional behavior that cannot be readily explained with the simple band-semiconductor Hall effect model. Here, we develop an analytical model of Hall effect in organic field-effect transistors in a regime of coexisting band and hopping carriers. The model, which is supported by the experiments, is based on a partial Hall voltage compensation effect, occurring because hopping carriers respond …
We propose lyotropic chromonic liquid crystals (LCLCs) as a distinct class of materials for organic electronics. In water, the chromonic molecules stack on top of each other into elongated aggregates that form orientationally ordered phases. The aligned aggregated structure is preserved when the material is deposited onto a substrate and dried. The dried LCLC films show a strongly anisotropic electric conductivity of semiconductor type. The field-effect carrier mobility measured along the molecular aggregates in unoptimized films of LCLC V20 is 0.03 cm^2 V^(-1) s^(-1). Easy processibility, low…
We explore the magnetoresistance (MC) effect in an organic semiconductor device based on the magnetic field related bipolaron formation. By establishing a group of dynamic equations, we present the transition among spin-parallel, spin-antiparallel polaron pairs and bipolarons. The transition rates are adjusted by the external magnetic field as well as the hyperfine interaction of the hydrogen nuclei. The hyperfine interaction is addressed and treated in the frame work of quantum mechanics. By supposing the different mobility of polarons from that of bipolarons, we obtain the MC in an organic s…
The active layer crystallinity has long been associated with favourable organic solar cells (OSCs) properties such as high mobility and Fill Factor. In particular, this applies to acceptor materials such as fullerene-derivatives and the most recent Non-Fullerene Acceptors (NFAs), which are now surpassing 19% of Power Conversion Efficiency. Despite these advantages are being commonly attributed to their 3-dimensional crystal packing motif in the single crystal, the bridge that links the acceptor crystal packing from single crystals to solar cells has not clearly been shown yet. In this work, we…
