Mobility In Organic Semiconductors Research Articles

Phenyl- versus cyclohexyl-terminated substituents: comparative study on aggregated structures and electron-transport properties in n-type organic semiconductors

This paper reports that specific attractive intermolecular interactions between side-chain substituents can be useful for enhancing charge-carrier mobility in organic semiconductors owing to the suppression of molecular motions.

Direct observation of the electronic structure and many-body interactions of low-mobility carriers in perylene diimide derivative

Despite the rapid progresses in the field of organic semiconductors, aided by the development of high-mobility organic materials, their high carrier mobilities are often unipolar, being sufficiently high only for either electrons or holes. Yet, the basic mechanisms underlying such significant mobility asymmetry largely remains elusive. We perform angle-resolved photoelectron spectroscopy to reveal the occupied band structures and the many-body interactions for low-mobility hole carriers in a typical n-type semiconductor perylene diimide derivative. The band dispersion exhibits strong renormalization to the calculated non-interacting electronic structure. The analysis including many-body interactions elucidate that the significant mass enhancement can be understood in terms of strong charge–phonon coupling, leading to an important mechanism of polaron band transport of low intrinsic carrier mobility in organic semiconductors.

Unequilibrated Charge Carrier Mobility in Organic Semiconductors Measured Using Injection Metal–Insulator–Semiconductor Charge Extraction by Linearly Increasing Voltage

The charge carrier mobility in tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA), a host and hole transport material typically used in organic light‐emitting diodes (OLEDs), is measured using charge carrier electrical injection metal–insulator–semiconductor charge extraction by linearly increasing voltage (i‐MIS‐CELIV). By employing the injection current i‐MIS‐CELIV method, charge transport at time scales shorter than the transit times typically observed in standard MIS‐CELIV is measured. The i‐MIS‐CELIV technique enables the experimental measurement of unequilibrated and pretrapped charge carriers. Through a comparison of injection and extraction current transients obtained from i‐MIS‐CELIV and MIS‐CELIV, it is concluded that hole trapping is negligible in evaporated neat films of TCTA within the time‐scales relevant to the operational conditions of optoelectronic devices, such as OLEDs. Furthermore, photocarrier generation in conjunction with i‐MIS‐CELIV (photo‐i‐MIS‐CELIV) to quantify the properties of charge injection from the electrode to the semiconductor of the MIS devices is utilized. Based on the photo‐i‐MIS‐CELIV measurements, it is observed that the contact resistance does not limit the injection current at the TCTA/molybdenum oxide/silver interface. Therefore, when TCTA is employed as the hole transport/electron‐blocking layer in OLEDs, it does not significantly reduce the injection current and remains compatible with the high injection current densities required for efficient OLED operation.

Carrier mobility in organic semiconductors with an exponential density of states under the framework of admittance spectroscopy

The carrier transport in amorphous organic semiconductors with an exponential density of states is systematically studied under the framework of admittance spectroscopy. Due to the exponential distribution, the slope of the double logarithmic curve of current density versus voltage is expressed as b+2, where b relates to the width of the density of states. And the mobility is proportional to nb, where n is the density of carrier. In this case, the peak frequencies of the negative differential susceptance and imaginary part of impedance turn out to be functions of parameter b. Therefore, the relation between transit time and mobility will be a linear function of parameter b. Applying our model to interpret experimental data of both small molecular and polymeric materials are illustrated. The contributions of electric field and of carrier density to mobility are discussed.

Predicting the miniaturization limit of vertical organic field effect transistor (VOFET) with perforated graphene as a source electrode

Vertical organic field effect transistors (VOFETs) are of paramount importance due to their fast switching speed, low power consumption, and higher density on a chip compared to lateral OFETs. The low charge carrier mobility in organic semiconductors and longer channel lengths in lateral OFETs lead to higher operating voltages. The channel length in VOFETs can be less than 100 nm which reduces the size of the channel and hence the operating voltages. Another important factor in the operation of VOFETs is the thickness and width of the source electrode. The channel length, source electrode thickness and width sets the miniaturization limit of the VOFETs. The graphene monolayer can be exploited as a source electrode due to its thinness, high carrier mobility, and metallic behaviors. However, for better gate modulation, perforations in the source material are desired. Here, we simulate the VOFET having perforated graphene monolayer as a source electrode and n-type organic semiconductor N, N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) as an active channel material, while aluminum as a drain electrode to predict the best-miniaturized device. The miniaturization limit of such a VOFET has a limit to the gate opening/perforation in which the minimum source width is 10 nm, as in the sub 10 nm range graphene starts behaving like a semiconductor. The subthreshold swing, deduced from the drain current (J D) versus gate voltage (V G) graph, advocates the limit of the organic semiconductor height/channel length to 50 nm, while 50 nm for the gate.

Compact Model Analysis for Low Voltage OFETs with Electrolytic Gate Dielectrics: Toward a Universal Model for Poly(3-Hexylthiophene) P3HT OFETs

The lightweight with flexibility and low-cost processing engineered the rapid growth of organic field-effect transistors (OFET) in the past three decades. Suitable compact models and parameter extraction methods are being developed to further the use of OFETs in integrated circuits, where stimulations are required to optimize the device performance. To simplify the parameter extraction, metaheuristic approaches are usually made, which otherwise is a cumbersome process. Following these, here investigations are made with the help of such a compact model to extract the operational parameters of P3HT (poly (3-hexylthiophene) based OFETs with electrolytic gate dielectrics using the genetic algorithm (GA) method. The result show that the compact model that was essentially developed in line with the successful models for inorganic material based FETs, can be used as an excellent framework for simulating low voltage OFETs made with both low and high mobility organic semiconductors. Mobility and threshold voltage calculated from the extracted parameters using GA for the two devices having mobility value differences of more than four orders are found to be nicely fitting with the experimental values. These results assume significance to the organic electronic industry as this facilitates the real-time circuit application of OFETs. KEYWORDS: Modeling, Low voltage OFET, Genetic algorithm, Ionic liquid, P3HT

Modelling the effect of dipole ordering on charge-carrier mobility in organic semiconductors

The hopping mobility in organic semiconductors is strongly influenced by the correlated on-site noise — i.e. the dipole–dipole interaction between neighbouring molecules. In this paper, we use Kinetic Monte Carlo (KMC) to study the effect of dipole moment and structural order on the hole mobility of organic molecular semiconductors. While the effect on the charge mobility of the dipole–dipole interactions can be approximately reproduced by a global Gaussian disorder model, our present results show that there are non-trivial local effects that are not incorporated into such an approach. The dipole–dipole interactions give rise to large energy barriers that restrict charged particles to smaller and smaller regions as the dipole magnitude increases. While this effect can only reduce the mobility in any given system, we note that if the dipoles self-organize, potentially driven by the dipole moment itself or by an increase in packing density, the negative effect on the mobility can be reduced.

Toward Accurate Prediction of Ion Mobility in Organic Semiconductors by Atomistic Simulation.

A multiscale scheme (MLMS: Multi-Level Multi-Scale) to predict the ion mobility (μ) of amorphous organic semiconductors is proposed, which was successfully applied to the hole mobility predictions of 14 organic systems. An inverse relationship between μ and reorganization energy is observed due to local polaronic distortions. Another moderate inverse correlation between μ and distribution of site energy change exists, representing the effects of geometric flexibility. The former and the latter represent the intramolecular and intermolecular geometric effects, respectively. In addition, a linear correlation between transfer coupling and μ is observed, showing the importance of orbital overlaps between monomers. Especially, the highest hole mobility of C6-2TTN is due to its large transfer coupling. On the other hand, another high hole mobility of CBP turned out to come from the high first neighbor density (ρFND) of its first self-solvation, emphasizing the proper description of amorphous structural configurations with a sufficiently large number of monomers. In general, systems with either unusually high transfer coupling or high first neighbor density can potentially have high μ regardless of geometric effects. Especially, the newly suggested design parameter, ρFND, is pointing to a new direction as opposed to the traditional π-conjugation strategy.

Organic single‐crystal light‐emitting transistors with external quantum efficiency over 20%

AbstractOrganic light‐emitting transistors (OLETs) have attracted increasing attention because of their potential applications in next‐generation displays and high‐energy operating devices. However, the simultaneous achievement of high luminescent efficiency and carrier mobility in organic semiconductors remains challenging because the localized excitons are advantageous for light emission, whereas the delocalized carriers are beneficial for efficient charge migration. Herein, we report an organic single crystal of a cyano‐substituted styrene derivative with balanced mobility yielding a record‐high external quantum efficiency of 20.5% in OLETs. Temperature‐dependent I–V curves and electronic structure analyses based on first‐principles calculations were performed to disclose the underlying mechanism as a band transport, which provides an efficient way to achieve high quantum efficiency in OLETs.

Effects of Molecular Encapsulation on the Photophysical and Charge Transport Properties of a Naphthalene Diimide Bithiophene Copolymer.

Engineering the molecular structure of conjugated polymers is key to advancing the field of organic electronics. In this work, we synthesized a molecularly encapsulated version of the naphthalene diimide bithiophene copolymer PNDIT2, which is among the most popular high charge mobility organic semiconductors in n-type field-effect transistors and non-fullerene acceptors in organic photovoltaic blends. The encapsulating macrocycles shield the bithiophene units while leaving the naphthalene diimide units available for intermolecular interactions. With respect to PNDIT2, the encapsulated counterpart displays an increased backbone planarity. Molecular encapsulation prevents preaggregation of the polymer chains in common organic solvents, while it permits π-stacking in the solid state and promotes thin film crystallinity through an intermolecular-lock mechanism. Consequently, n-type semiconducting behavior is retained in field-effect transistors, although charge mobility is lower than in PNDIT2 due to the absence of the fibrillar microstructure that originates from preaggregation in solution. Hence, molecularly encapsulating conjugated polymers represent a promising chemical strategy to tune the molecular interaction in solution and the backbone conformation and to consequently control the nanomorphology of casted films without altering the electronic structure of the core polymer.

14GHz Schottky Diodes Using a p-Doped Organic Polymer.

The low carrier mobility of organic semiconductors and the high parasitic resistance and capacitance often encountered in conventional organic Schottky diodes hinder their deployment in emerging radio frequency (RF) electronics. Here, these limitations are overcome by combining self-aligned asymmetric nanogap electrodes (≈25nm) produced by adhesion lithography, with a high mobility organic semiconductor, and RF Schottky diodes able to operate in the 5G frequency spectrum are demonstrated. C16 IDT-BT is used, as the high hole mobility polymer, and the impact of p-doping on the diode performance is studied. Pristine C16 IDT-BT-based diodes exhibit maximum intrinsic and extrinsic cutoff frequencies (fC ) of >100and 6GHz, respectively. This extraordinary performance is attributed to the planar nature of the nanogap channel and the diode's small junction capacitance (<2pF). Doping of C16 IDT-BT with the molecular p-dopant C60 F48 improves the diode's performance further by reducing the series resistance resulting to intrinsic and extrinsic fC of >100and ≈14GHz respectively, while the DC output voltage of an RF rectifier circuit increases by a tenfold. Ourwork highlights the importance of the planar nanogap architecture and paves the way for the use of organic Schottky diodes in large-area RF electronics of the future.

Crystal engineering of alkylethynyl fluoroanthradithiophenes

2-Dimensional brickwork packing is desirable for high mobility organic semiconductors. We synthesised acene derivatives that adopt this packing without using heavier group 14 elements to investigate substituent effects on bulk transport properties.

Solution Processable Pentafluorophenyl End‐Capped Dithienothiophene Organic Semiconductors for Hole‐Transporting Organic Field Effect Transistors

AbstractTwo solution‐processable organic semiconductors, DFPT‐DTTR (1) and DFPbT‐DTTR (2), composed of pentafluorophenyl (FP) end‐capped 3,5‐dialkyl dithienothiophene (DTTR) core with thiophene (T) or bithiophene (bT) as π‐bridged spacers are developed and investigated for their optical, electrochemical, microstructural, and electrical properties. With more conjugated bithiophene units, compound 2 exhibits a red‐shifted UV–vis absorption band and upshifted HOMO/downshifted LUMO energy levels. According to the density functional theory, compound 2 features a more twisted molecular structure due to the intrinsic non‐coplanar blocks in the π‐backbones. Compound 1‐based organic field effect transistors exhibit efficient hole transport with mobility up to 0.48 cm2 V−1 s−1. This is one of the high mobility organic semiconductors exhibiting p‐channel characteristics based on solution‐processable small molecular FP end‐capped fused/oligothiophenes. With large and interconnected crystalline morphologies, decreased π–π stacking distance, and less steric hindrance, compound 1 exhibits two orders of magnitude higher mobility than the more distorted 2, which exhibits lower hole mobility of 1.82 × 10−3 cm2 V−1 s−1.

High Mobility Organic Lasing Semiconductor with Crystallization‐Enhanced Emission for Light‐Emitting Transistors

AbstractThe development of high mobility organic laser semiconductors with strong emission is of great scientific and technical importance, but challenging. Herein, we present a high mobility organic laser semiconductor, 2,7‐diphenyl‐9H‐fluorene (LD‐1) showing unique crystallization‐enhanced emission guided by elaborately modulating its crystal growth process. The obtained one‐dimensional nanowires of LD‐1 show outstanding integrated properties including: high absolute photoluminescence quantum yield (PLQY) approaching 80 %, high charge carrier mobility of 0.08 cm2 V−1 s−1, Fabry‐Perot lasing characters with a low threshold of 86 μJ cm−2 and a high‐quality factor of ≈2400. Furthermore, electrically induced emission was obtained from an individual LD‐1 crystal nanowire‐based light‐emitting transistor due to the recombination of holes and electrons simultaneously injected into the nanowire, which provides a good platform for the study of electrically pumped organic lasers and other related ultrasmall integrated electrical‐driven photonic devices.

High Mobility Organic Lasing Semiconductor with Crystallization-Enhanced Emission for Light-Emitting Transistors.

The development of high mobility organic laser semiconductors with strong emission is of great scientific and technical importance, but challenging. Herein, we present a high mobility organic laser semiconductor, 2,7-diphenyl-9H-fluorene (LD-1) showing unique crystallization-enhanced emission guided by elaborately modulating its crystal growth process. The obtained one-dimensional nanowires of LD-1 show outstanding integrated properties including: high absolute photoluminescence quantum yield (PLQY) approaching 80 %, high charge carrier mobility of 0.08 cm2 V-1 s-1 , Fabry-Perot lasing characters with a low threshold of 86 μJ cm-2 and a high-quality factor of ≈2400. Furthermore, electrically induced emission was obtained from an individual LD-1 crystal nanowire-based light-emitting transistor due to the recombination of holes and electrons simultaneously injected into the nanowire, which provides a good platform for the study of electrically pumped organic lasers and other related ultrasmall integrated electrical-driven photonic devices.

High-performance 2,9-DPh-DNTT organic thin-film transistor by weak epitaxy growth method

2,9-DPh-DNTT, an isomeric of diphenyl-dinaphtho[2,3-b:2′,3′-f]-thieno[3,2-b] thiophene (DPh-DNTTs), is an emerging candidate of high mobility organic semiconductor material. In this work, a high performance 2,9-DPh-DNTT organic thin-film transistor (OTFT) is fabricated by the method of weak epitaxy growth. The quality of 2,9-DPh-DNTT thin film was significantly improved when its epitaxial layer grows on an inducing layer of para-sexiphenyl (p-6P). Continuous large-area, highly ordered and terraced 2,9-DPh-DNTT polycrystalline thin films are obtained. The hole mobility of as-fabricated 2,9-DPh-DNTT thin-film transistor reaches up to 6.4 cm2 V−1s−1. This simple process of preparing high mobility 2,9-DPh-DNTT thin-film transistor supplies a facile route of large-area OTFT fabrication.

Suppression of dynamic disorder by electrostatic interactions in structurally close organic semiconductors.

Dynamic disorder manifested in fluctuations of charge transfer integrals considerably hinders charge transport in high-mobility organic semiconductors. Accordingly, strategies for suppression of the dynamic disorder are highly desirable. In this study, we suggest a novel promising strategy for suppression of dynamic disorder-tuning the molecular electrostatic potential. Specifically, we show that the intensities of the low-frequency (LF) Raman spectra for crystalline organic semiconductors consisting of π-isoelectronic small molecules (i.e. bearing the same number of π electrons)-benzothieno[3,2-b][1]benzothiophene (BTBT), chrysene, tetrathienoacene (TTA) and naphtho[1,2-b:5,6-b']dithiophene (NDT)-differ significantly, indicating significant differences in the dynamic disorder. This difference is explained by suppression of the dynamic disorder in chrysene and NDT because of stronger intermolecular electrostatic interactions. As a result, guidelines for the increase of the crystal rigidity for the rational design of high-mobility organic semiconductors are suggested.

The Effect of Electric Field on Carrier Mobility in Organic Semiconductors Based on Exponential Type Density of States and Genuine Transport Energy

Carrier mobility plays a crucial role in organic semiconductors because it limits the efficiency of organic devices. Based on the exponential type density of states (EC DOS) and genuine transport energy, we introduce a model to describe the dependence of electric field and carrier mobility in organic semiconductors by using effective medium approximation. The theoretical results show that the carrier concentration is a key factor to influence the carrier mobility, which increases with the increasing carrier concentration. The Poole-Frenkel behavior is reproduced in our model. The calculated carrier mobility as functions of electric field at different temperatures are in good agreement with analytical data for 40’-(3,7-dimethyloctyloxy)-1,1’-biphenylene-2, 5-vinylene] (NRS-PPV).

Thermal Vibration Correlation Function Formalism for Molecular Excited State Decay Rates

Thermal Vibration Correlation Function Formalism for Molecular Excited State Decay Rates

Finite-Temperature TD-DMRG for the Carrier Mobility of Organic Semiconductors.

A large number of nonadiabatic 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-based finite-temperature time-dependent density matrix renormalization group (TD-DMRG) method to evaluate the carrier mobility in organic semiconductors, as described by the electron-phonon model, in particular, in 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 pictures, covering a wide range of electronic coupling strengths and (ii) with realistic parameters, TD-DMRG is able to account for the experimentally observed "band-like" transport behavior (∂μ/∂T < 0) in rubrene. We further study the long-standing puzzle of the isotope effect for charge transport and unambiguously demonstrate that the negative isotope effect (∂μ/∂m < 0 where m is the atomic mass) should be universal.