Device Physics of Solution-Processed Physics of Solution-Processed Organic ... and device physics of solution-processed organic field-effect transistors, ... small-molecule organic

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  • Device Physics of Solution-ProcessedOrganic Field-Effect Transistors**

    By Henning Sirringhaus*

    1. Introduction

    Following the initial demonstration of field-effect conduc-tion in small organic molecules[1,2] and conjugated poly-mers,[35] the community of industrial and academic researchgroups that are interested in using organic semiconductors asthe active layer in organic field-effect transistor (OFET) de-vices has been growing steadily, particularly over the last fourto five years. The Institute for Scientific Information (ISI)Web of Science counts 393 scientific publications in the fieldof organic transistors in 2004, up from 304 in 2003, and 80 in1999. The reasons for this surge of interest are manifold. Theperformance of OFETs, which is generally benchmarked

    against that of amorphous silicon (a-Si) thin-film transistors(TFTs) with field-effect mobilities of 0.51 cm2 V1 s1 andON/OFF current ratios of 106108, has improved significantly.Currently, the record mobility (l) values for thin-film OFETsare 5 cm2 V1 s1 in the case of vaccum-deposited small mole-cules[6] and 0.6 cm2 V1 s1 for solution-processed polymers.[7]

    As a result, there is now a serious level of industrial interest inusing OFETs for applications that are currently incompatiblewith the use of a-Si or other inorganic transistor technologies.OFETs are most commonly manufactured using standard top-gate (Fig. 1A) and bottom-gate TFT architectures. One oftheir main technological attractions is that all the layers of anOFET can be deposited and patterned at low/room tempera-ture by a combination of low-cost solution-processing anddirect-write printing, which makes them ideally suited for rea-lization of low-cost, large-area electronic functions on flexiblesubstrates (see the reviews by Sirringhaus et al.[8] and For-rest[9]). The first applications in which we can realisticallyexpect OFETs to be used within the next three to five yearsare flexible, active-matrix electronic-paper displays, for whichimpressive demonstrations have been developed recently,[10,11]

    and simple, low-cost, radiofrequency identification (RFID)tags[12] and sensing devices. Other applications, such as active-matrix liquid crystal or organic light-emitting diode (OLED)displays, or high-performance RFID tags compatible withexisting communication standards, are also being envisioned,

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    Adv. Mater. 2005, 17, 24112425 DOI: 10.1002/adma.200501152 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2411

    Field-effect transistors based on solution-processible organic semiconduc-tors have experienced impressive improvements in both performance andreliability in recent years, and printing-based manufacturing processesfor integrated transistor circuits are being developed to realize low-cost,large-area electronic products on flexible substrates. This article reviewsthe materials, charge-transport, and device physics of solution-processed organic field-effecttransistors, 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 ofthe gate dielectric, the influence of polaronic relaxation and disorder effects on charge trans-port, charge-injection mechanisms, and the current understanding of mechanisms for chargetrapping are reviewed. Many interesting questions on how the molecular and electronic struc-tures and the presence of defects at organic/organic heterointerfaces influence the device perfor-mance and stability remain to be explored.

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    [*] Prof. H. Sirringhaus

    Cavendish Laboratory, University of CambridgeCambridge CB3 OHE (UK)E-mail: hs220@phy.cam.ac.ukProf. H. SirringhausPlastic Logic Ltd.34/35 Cambridge Science ParkCambridge CB4 OFX (UK)

    [**] It is a pleasure to acknowledge stimulating discussions on scientificissues discussed in this review with many wonderful students, post-docs, and colleagues, in particular, Dr. Lukas Buergi, Jana Zaumseil,Shalom Goffri, Tim Richards, Jui-Fen Chang, Dr. Jerome Cornil,Dr. Janos Veres, Dr. Catherine Ramsdale, and Prof. Richard Friend.

  • but require a transistor performance with mobilities exceed-ing 1 cm2 V1 s1, which is still difficult to achieve with solu-tion-processed OFETs.

    On the materials front, improving field-effect mobilities re-mains an important topic, although, compared to the situationin 2002,[13] there has been less emphasis on improving headlinemobility numbers and more on developing materials that al-low the combination of high mobilities with good materialsstability under air, moisture, and light exposure. Very signifi-cant progress has been made in this respect recently. Fig-ure 1B shows the output characteristics of a state-of-the-art,unencapsulated polymer FET, comparing measurements per-formed in ambient air and light directly after device manufac-ture and several weeks later, after the device had participatedin a customer trial and had crossed the Atlantic twice.[11] Noevidence for device degradation is observed. Improvements in

    shelf as well as operational life have been achieved as a resultof using organic semiconductors with better inherent stability,better understanding of the requirements for gate dielectrics,and by more controlled manufacturing processes. It is gener-ally well appreciated now that the choice of the right dielectricis crucial for achieving optimum field-effect mobility (lFE),device stability, and reliability. While most of this work hastraditionally focused on the p-type conduction regime, therehas been a significant effort made to understand the conduc-tion processes involving negative electrons, with the aim ofrealizing solution-processible n-type as well as ambipolar or-ganic semiconductors for use in complementary metal oxidesemiconductor (CMOS)-type circuits and light-emitting FETs.

    There is a wealth of fundamental scientific questions re-garding the charge-transport and charge-injection physics oforganic semiconductors, and their structureproperty rela-tionships, for which FET devices provide a useful scientifictool through their ability to control the charge-carrier concen-tration electrostatically rather than chemically. A significanteffort has been focused on understanding the fundamentalelectronic structure of the organic semiconductor, in particu-lar at the interface with the dielectric, and how microscopic,molecular-scale transport processes determine the electricalcharacteristics of macroscopic devices. This is a challengingtask because of the complex microstructure of solution-pro-cessed organic semiconductors, which in many cases cannotbe fully characterized by conventional diffraction and micros-copy techniques. An important related topic is the under-standing of electronic-defect states and associated device deg-radation mechanisms, which are becoming an increasinglyimportant topic as OFETs are nearing their introduction intofirst products with strict reliability and lifetime requirements.

    This article is focused on reviewing the current state ofknowledge of the materials and the device and charge-trans-port physics of solution-processed OFETs. Due to limitationsof space, no attempt is made to review the device physics ofpolycrystalline, small-molecule organic semiconductors de-posited by vacuum evaporation, nor to give an overview ofthe different approaches to manufacturing OFETs. For theseimportant subjects we refer the reader to other excellent andrecent review articles.[8,9,14,15] Section 2 discusses the materialsphysics of solution-processible p- and n-type organic semicon-

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    H. Sirringhaus/Device Physics of Solution-Processed OFETs

    2412 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Adv. Mater. 2005, 17, 24112425

    Henning Sirringhaus is the Hitachi Professor of Electron Device Physics at the Cavendish Labo-ratory. He has been working in the field of organic transistor devices since 1997. He has an un-dergraduate and Ph.D. degrees in physics from ETH Zrich (Switzerland). From 19951996 heworked as a postdoctoral research fellow at Princeton University (USA) on a-Si TFTs for active-matrix liquid crystal displays. His current research interests include the charge-transport physicsof molecular and polymeric semiconductors, the development of printing-based nanopatterningtechniques, and the use of scanning probe techniques for electrical characterization of functionalnanostructures. He is co-founder and Chief Scientist of Plastic Logic Ltd., a technology start-upcompany commercializing printed organic transistor technology. He was awarded the BalzersPrize of the Swiss Physical Society in 1995 for his Ph.D. work on ballistic-electron-emission mi-croscopy of epitaxial metal/semiconductor heterointerfaces, and the Mullard award of the RoyalSociety in 2003.

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    Figure 1. A) Schematic diagram of a top-gate OFET using a standard TFTdevice architecture. B) Output characteristics (drain voltage, Vd, vs.source current, Is) of a state-of-the-art, unencapsulated OFET measuredin air and light (closed circles: device measured after manufacture; opencircles: device measured two weeks later).

  • ductors and dielectrics. Section 3 fo-cuses at a more fundamental levelon the electronic structure of solu-tion-processed organic semiconduc-tors and the charge-transport pro-cesses at the active interface, andhow these are affected by disorderand molecular-relaxation effects.Finally, in Section 4 we review thecurrent understanding of electronic-defect states and degradation mech-anisms in OFETs, which lead to d