Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems.

During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

The syntheses and comprehensive characterization of 14 organic semiconductors based on perylene bisimide (PBI) dyes that are equipped with up to four halogen substituents in the bay area of the perylene core and five different highly fluorinated imide substituents are described. The influence of the substituents on the LUMO level and the solid state packing of PBIs was examined by cyclic voltammetry and single crystal structure analyses of seven PBI derivatives, respectively. Top-contact/bottom-gate organic thin film transistor (OTFT) devices were constructed by vacuum deposition of these PBIs on SiO2 gate dielectrics that had been pretreated with n-octadecyl triethoxysilane in vapor phase (OTS-V) or solution phase (OTS-S). The electrical characterization of all devices was accomplished in a nitrogen atmosphere as well as in air, and the structural features of thin films were explored by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Several of those PBIs that bear only hydr...

High-Performance Air-Stable n-Channel Organic Thin Film Transistors Based on Halogenated Perylene Bisimide Semiconductors

We discuss the properties of organic‐on‐inorganic (OI) semiconductor contact barrier diodes. A model for charge transport is developed which suggests that thermionic emission over the organic/inorganic contact barrier dominates at low current densities, whereas space‐charge effects dominate transport through the organic layer at high current densities. The effects of charge trapping in the organic layer are also considered. This model is applied to OI diodes using thin films of the prototypical aromatic compound; 3,4,9,10‐perylenetetracarboxylic dianhydride, (PTCDA) vapor‐deposited onto n‐ and p‐Si substrates. Several electrical and optical properties of PTCDA are investigated to provide a basis for analyzing the OI diodes. Both ohmic and space‐charge‐limited transport are observed in the PTCDA. We discuss mobility, transient response, and photoresponse of the thin‐film organic material. Also described are the general properties of organic‐on‐inorganic contact barrier diodes which employ PTCDA and related...

Organic‐on‐inorganic semiconductor contact barrier diodes. I. Theory with applications to organic thin films and prototype devices

Using 2‐MeV Ar+ ion beam irradiation, we have generated conducting patterns in otherwise high resistivity, organic thin films of 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA), 1,4,5,8‐napthalenetetracarboxylic dianhydride (NTCDA), and Ni phthalocyanine (NiPc). The room‐temperature resistivity of the films changes by 14 orders of magnitude from its as‐deposited value of ρ≳1010 Ω cm to ρ≅5×10−4 Ω cm at ion doses of 1017 cm−2. The temperature (T) dependence of the resistivity follows ρ(T)∝exp[+(T0/T)1/2] over a wide range of dose and temperature. The characteristic temperature T0 is found to be a function of dose. The exp(T0/T)1/2 behavior of ρ(T) is consistent with charge transport due to hopping between isolated, conducting islands.

Large conductivity changes in ion beam irradiated organic thin films

We discuss the effects of organic film thickness, film crystalline texture, contact metal, and temperature on the current‐voltage characteristics (I‐V) of organic‐on‐inorganic (OI) semiconductor contact barrier diodes, where the prototypical compound; 3, 4, 9, 10‐perylenetetracarboxylic dianhydride (PTCDA) serves as the organic thin film. The results presented can be fully understood in terms of the thermionic emission‐space‐charge‐limited (TE‐SCL) current model introduced in previous papers. Studies of the I‐V characteristics suggest the presence of a trap distribution centered at Δe=0.20 eV above the PTCDA valence band edge. Also, we have found a correlation between the hole mobility (μeff) and PTCDA crystalline texture, and obtain a mobility as high as μeff=1.4 cm2/V s for highly ordered crystalline thin films. Finally, the general criteria which must be fulfilled by an organic compound used to achieve high‐performance OI devices are outlined. The properties of several such devices using various organi...

Organic‐on‐inorganic semiconductor contact barrier diodes. II. Dependence on organic film and metal contact properties

The organic compound 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) has previously been observed to undergo a large increase in conductivity on irradiation with energetic particle beams. In this letter, we describe the electrical characteristics of novel rectifying junctions employing unirradiated PTCDA vapor deposited onto 10‐Ω cm p‐Si substrates. The PTCDA‐Si contact barrier has a height of φB = 0.74 eV. The resulting diodes undergo avalanche breakdown at VB = 230 V, and exhibit current densities at 1/2  VB of ⩽50  μA/cm2. In addition, the forward current‐voltage (IF−V) characteristics are strongly dependent on the contact metal used on the top PTCDA surface. The best results obtained were for diodes employing Ti contacts which gave nonhysteretic, stable IF‐V characteristics with an ideality factor of n = 1.7. Several properties of the as‐deposited PTCDA are also discussed. The rectifying characteristics reported here, coupled with the properties of irradiated PTCDA, suggest many unique device app...

Organic‐on‐inorganic semiconductor contact barrier devices

A new method for measuring the density of states at semiconductor surfaces using organic‐on‐inorganic (OI) semiconductor contact barriers is suggested. This work is an extension of previous models of OI diode behavior which includes the ac admittance characteristics, and which considers the range of validity of approximations to OI diode capacitance used in previous experiments. The theory describes the potential distribution across the device. We consider the case of ideal OI diodes, as well as diodes with significant densities of states at the inorganic semiconductor surface. This analysis leads to a technique whereby the low‐frequency conductance and capacitance characteristics can be used to obtain information about the magnitude and the energy distribution of surface states in the inorganic semiconductor bandgap. Also, the carrier concentration profiles of the substrate can be conveniently obtained. Due to the noninvasive nature of the organic/inorganic contact, information about the density of states at relatively undisturbed semiconductor surfaces obtained via this technique may prove useful in determining the processes involved in Schottky barrier formation and metal‐insulator‐semiconductor diode surface properties. This theory has been applied to the investigation of surfaces of III‐V alloy semiconductor‐based OI diodes, and the experimental results will be presented in a subsequent paper. In addition, an expression for the OI diode n value obtained from the forward current‐voltage characteristics is derived. It is found that the n value depends on surface states which are in equilibrium with the organic as well as the inorganic materials. Estimates of the surface state densities obtained from previously reported n values are consistent with expectations for the semiconductors under test.

P. H. Schmidt

Papers

Organic‐on‐inorganic semiconductor contact barrier diodes. I. Theory with applications to organic thin films and prototype devices

Large conductivity changes in ion beam irradiated organic thin films

Organic‐on‐inorganic semiconductor contact barrier diodes. II. Dependence on organic film and metal contact properties

Organic‐on‐inorganic semiconductor contact barrier devices

Semiconductor analysis using organic-on-inorganic contact barriers. I. Theory of the effects of surface states on diode potential and ac admittance