Functionalized acenes and heteroacenes for organic electronics.

The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Electron and ambipolar transport in organic field-effect transistors.

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

Field-effect transistors based on solution-processible organic semiconductors have experienced impressive improvements in both performance and reliability in recent years, and printing-based manufacturing processes for integrated transistor circuits are being developed to realize low-cost, large-area electronic products on flexible substrates. This article reviews the materials, charge-transport, and device physics of solution-processed organic field-effect transistors, focusing in particular on the physics of the active semiconductor/dielectric interface. Issues such as the relationship between microstructure and charge transport, the critical role of the gate dielectric, the influence of polaronic relaxation and disorder effects on charge transport, charge-injection mechanisms, and the current understanding of mechanisms for charge trapping are reviewed. Many interesting questions on how the molecular and electronic structures and the presence of defects at organic/organic heterointerfaces influence the device performance and stability remain to be explored.

Device Physics of Solution‐Processed Organic Field‐Effect Transistors

One Volt Organic Transistor

All‐Organic Permanent Memory Transistor Using an Amorphous, Spin‐Cast Ferroelectric‐like Gate Insulator

High-Performance Organic Transistors Using Solution-Processed Nanoparticle-Filled High-k Polymer Gate Insulators

We report on our latest improvements in organic field-effect transistors (OFETs) using ultra-thin anodized gate insulators. Anodization of titanium (Ti) is an extremely cheap and simple technique to obtain high-quality, very thin (∼ 7.5 nm), pinhole-free, and robust gate insulators for OFETs. The anodized insulators have been tested in transistors using pentacene and poly(triarylamine) (PTAA) as active layers. The fabricated devices display low-threshold, normally “off” OFETs with negligible hysteresis, good carrier mobility, high gate capacitance, and exceptionally low inverse subthreshold slope. Device performance is improved via chemical modification of TiO2 with an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM). As the result of this combination of favorable properties, we have demonstrated OFETs that can be operated with voltages well below 1 V.

Low-Voltage, High-Performance Organic Field-Effect Transistors with an Ultra-Thin TiO2 Layer as Gate Insulator

Organic field-effect transistors (OFETs) with non-Ohmic contacts, e.g., pentacene with gold electrodes, exhibit a linearly growing threshold voltage with increased film thickness due to tunnel injection [R. Schroeder et al., Appl. Phys. Lett. 83, 3201 (2003)]. In this letter, we demonstrate gold/pentacene OFETs with a low threshold voltage independent of pentacene thickness. By doping the pentacene in the contact area with FeCl3 (iron-III-chloride), the metal-insulator-type tunneling barrier was changed to a metal-semiconductor Schottky barrier. Since the injection through a Schottky barrier depends on the potential and not on the electric field, the threshold voltage is no longer a function of the semiconductor thickness. Through selective doping of the area under the electrode, the channel remains undoped, and large on/off ratios are retained.

Leszek A. Majewski

Papers

One Volt Organic Transistor

All‐Organic Permanent Memory Transistor Using an Amorphous, Spin‐Cast Ferroelectric‐like Gate Insulator

High-Performance Organic Transistors Using Solution-Processed Nanoparticle-Filled High-k Polymer Gate Insulators

Low-Voltage, High-Performance Organic Field-Effect Transistors with an Ultra-Thin TiO2 Layer as Gate Insulator

Improving organic transistor performance with Schottky contacts