Over the past 20 years, organic transistors have developed from a laboratory curiosity to a commercially viable technology. This critical review provides a short summary of several important aspects of organic transistors, including materials, microstructure, carrier transport, manufacturing, electrical properties, and performance limitations (200 references).

Organic thin-film transistors.

The authors would like to thank M. Chabinyc, H. Ade, B. Collins, R. Noriega, K. Vandewal, and D. Duong for fruitful discussions in the preparation of this review. Stanford Synchrotron Radiation Lightsource (SSRL) is a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. This publication was partially supported by the Center for Advanced Molecular Photovoltaics (Award No. KUS-C1-015-21), made by King Abdullah University of Science and Technology (KAUST).

Quantitative determination of organic semiconductor microstructure from the molecular to device scale.

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited molecule shares its energy with a ground-state neighbor to produce two triplet excited molecules. It has been observed to occur in single-crystal, polycrystalline, and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivatives, and it is argued that a wider structural variety would be desirable. The current state of the development of molecular structure design rules, based on first-principles theoretical considerations, is described along with initial examples of implementation.

Recent Advances in Singlet Fission

Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers.

http://science.sciencemag.org/content/sci/340/6130/334.full.pdf

External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell

It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d(001). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 A polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. Here we extend the crystal truncation rod X-ray scattering technique to fiber structured thin films. We determined the complete crystal structure of this polymorph grown on technologically relevant substrates. We found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuni...

http://hasyweb.desy.de/science/annual_reports/2007_report/part1/contrib/43/20896.pdf

Determination of the crystal structure of substrate-induced pentacene polymorphs in fiber structured thin films.

The exciton dynamics in microcrystalline pentacene films is investigated by transient absorption measurements with 30 fs time resolution. It is found that the emission from photoexcited Frenkel excitons decays within 70 fs due to the ultrafast formation of an excitonic species with a strongly reduced transition dipole to the ground state and an absorption dipole in the plane of the film. We propose that an excimer exciton is formed and stabilized by changes of the local crystal structure. The subsequent dynamics is dominated by diffusion controlled annihilation and trapping.

Ultrafast exciton relaxation in microcrystalline pentacene films.

In this feature article, we will discuss to which extend the peculiar growth properties of pentacene on metallic contacts and on gate dielectrics contribute to the device performance of organic thin film transistors. The early growth state of pentacene monolayers is reviewed, as well as the molecular structure of the so called thin film phase. Then, the relation of structural defect densities to trap densities is discussed. The spatially resolved photo response of a pentacene transistor will be presented in the context of injection barriers and contact homogeneity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Pentacene devices: Molecular structure, charge transport and photo response

We report the fabrication of pentacene thin film transistors with a mobility of 2.17 cm2/Vs which is challenging amorphous silicon. Next to device mobility, large hysteresis in the IV characteristics has been an obstacle for the design of large organic circuits in the past. It is a key success factor for optimization and widespread application of organic devices to understand the underlying principles. For the first time, we demonstrate that the observed hysteresis can fully be described by transient trap charging effects. Using the commercial finite element environment ISE TCAD, transient device behavior is simulated and compared to our experimental results. Values for density and energy of acceptor-type traps at the pentacene/SiO 2 interface are extracted. Furthermore, we are able to determine the associated hole capture cross-section which has never been quantified before. To additionally emphasize the influence of trapping effects, it is outlined that transient measurement sweeps can systematically lead to misinterpreted mobilities if traps are not taken into account.

Trapping Effects in Organic Thin Film Transistors

It is demonstrated that drift-diffusion simulation is a powerful tool in design, optimization and verification of organic circuits. Starting from the simulation of single transistor structures, we treat inverter circuits with active load under both static and transient conditions while analyzing the effects of different transistor geometries. Never shown before, a dynamic finite element simulation of a full three-stage organic ring oscillator operating at 105 kHz is presented.

S. Schiefer

Papers

Determination of the crystal structure of substrate-induced pentacene polymorphs in fiber structured thin films.

Ultrafast exciton relaxation in microcrystalline pentacene films.

Pentacene devices: Molecular structure, charge transport and photo response

Trapping Effects in Organic Thin Film Transistors

Transient TCAD simulation of three-stage organic ring oscillator