ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ. Серия C, 2014, том 56, № 1, с. 22-32
УДК 541.64:532.7
SOLUTION PROCESSABLE SEMICONDUCTING ORGANIC SINGLE CRYSTALS
© 2014 г. Grigorios P. Rigas", b, Maxim Shkunov"
a Advanced Technology Institute, Electronic Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK b National Physical Laboratory (NPL), Teddington, Middlesex, TW110LW, UK e-mail: m.shkunov@surrey.ac.uk
Abstract — In this review we describe recent progress in fabrication, characterisation and measurements of solution processed organic single crystals based on small molecule semiconductors. We focus on single crystal applications using Field-Effect Transistors as building blocks for organic electronics.
DOI: 10.7868/S2308114714010099
INTRODUCTION
Semiconducting organic single crystals occupy a special place in organic electronics, as they represent an almost perfect example of molecular semiconductor materials with the highest degree of molecular order, lowest number of defects, and the best charge transport characteristics [1—6]. Some of the highest charge carrier mobilities were found in ultrapurified single crystals deposited by vacuum zone refining, such as reported by Karl [7], including 400 cm2/(V s) hole mobility in naphthalene single crystal (at 10 K) measured by Time-of-Flight.
Practical applications envisioned for organic electronics, such as flexible and printed organic devices, require efficient and cost—effective device fabrication methods [8, 9]. Slow and labour intensive vacuum-based processes for organic single crystal growth are highly disadvantageous, whereas solution processing and patterned deposition are considered to be essential for the development of organic electronics [10].
In this review we describe recent progress in fabrication, characterisation and measurements of solution processed organic single crystals based on small molecule semiconductors. We focus on single crystal applications using Organic Field-Effect Transistors (OFETs) as building blocks for organic electronics.
ORGANIC SINGLE CRTISTALS FOR HIGH MOBILITY OFETS
Electrical performance of organic semiconductor-based devices is strongly affected by the degree of molecular order [11]. Charge carrier mobility in semiconducting films can be varied by several orders of magnitude by changing the degree of molecular order from amorphous to highly crystalline structures [12—15]. Excellent example of such structure-performance re-
lationship was demonstrated with vapour deposited pentacene thin films OFETs, where hole mobility increased eight orders of magnitude from amorphous layer devices to highly ordered crystalline layers transistors with mobilities of up to 1 cm2/(V s) [16]. Same relationship between molecular order and charge transport is valid for solution-processed organic semiconductors. Single crystalline structures with excellent translation symmetry and low number of defects demonstrate the highest degree of molecular order achievable in organic materials. Charge transport in single crystals is considered to be in general band-like within LUMO (lowest occupied molecular orbital) for electrons and HOMO (highest occupied molecular orbital) for holes [17]. However, polaronic effects still play an important role in charge transport at room temperature due to strong electron-phonon coupling [18]. Additionally, some structural defects and impurities are still present in single crystals. As a result, charge transport at room temperature is often temperature activated, and can be described as hopping process [19]. Hopping is optimal when transfer integral is maximised due to the efficient electronic coupling between the hopping sites (molecules), including high degree of n-orbital overlap and short n-n stacking distances. It is also required that reorganisation energy associated with the changes in the surrounding media and internal molecular deformations during charge transfer is minimised [19].
There are four basic molecular packing schemes (Fig. 1) adopted in organic single crystals, including herringbone, co-facial herringbone (slipped n-stack), "slipped-stack" with 1D n-stacking and 2D "bricklayer" [20].
Currently there is no experimental evidence that a particular type of packing is superior for an efficient charge transport. Both herringbone and co-facial 2D
Fig. 1. Molecular stacking schemes: (a) herringbone packing with charge transport dominated by edge-to-face interactions, (b) co-facial herringbone packing with strong face-to-face interactions, (c) "slipped-stack" with 1D n-stacking with preferred transport along the n stack, (d) "bricklayer" arrangement with efficient 2D network charge transport. tj are the associated transfer integrals. The strongest transfer integrals are expected for face-to-face interactions (Reprinted with permission from [20]. Copyright 2013, John Wiley and Sons).
packing motifs have been associated with high mobility semiconducting crystals, including rubrene and TIPS-pentacene, respectively [21, 22].
Organic field-effect transistors are "interfacial" devices where very thin, only few monolayers thick, conducting channel is formed between source and drain electrodes. The channel conductivity is modulated by the gate potential. As charge transport occurs at the semiconductor-insulator interface, surface properties play major role in OFET behaviour [23]. Organic crystal morphology, including molecular alignment and orientation, determines the efficiency of charge transport, and anisotropic properties of single crystals translate into mobility anisotropy, typically
found in the OFET devices [3, 24]. Injection from metal electrodes could limit current in an OFET, if there is a substantial Schottky barrier at metal-semiconductor interface. Schottky energy barrier is the difference between the Fermi energy of the metal contact and the energies of the charge transport levels of the organic material, namely electron and hole polaron states [25]. Organic FETs have been extensively discussed in the literature; detailed description of physical principles and device structures can be found, for example, in [26, 27].
In the next part of the review we focus on various deposition methods, used to produce organic semiconducting single crystals.
MATERIALS AND SOLUTION PROCESSING TECHNIQUES FOR SINGLE CRYSTALS
Solution processing of organic small molecule single crystals typically relies on several factors:
i) Semiconducting materials should have sufficient solubility in organic solvents, which typically limits this process to molecules with solubilising side-groups [28],
ii) Very slow evaporation is required to allow uninterrupted growth of a single crystal, instead of poly-crystalline films,
iii) Some control of nucleation point is typically needed for the crystal growth in pre-defined parts of the substrate.
A great majority of solution based single crystal growth techniques rely on random nucleation and, as a result, OFET devices are fabricated on top of such crystals in random substrate positions, or hand-picked and manipulated individually for device fabrication
[29]. Controlled deposition of crystals on pre-defined areas of the substrates is much more challenging, but it opens up possibilities for a reproducible single crystal OFET fabrication for organic electronics.
We start the review with most common small molecule semiconductor family based on aromatic hydrocarbons. Acenes are planar organic molecules consisting of linearly fused benzene rings, and have been extensively studied in the past as promising candidates for organic semiconductors in electronic applications
[30]. Among them pentacene, now considered a model organic semiconductor, was the topic of intense studies as the building block of OFETs due to its high field effect mobility ranging up to 3 cm2/(V s) [31]. One of the drawbacks of this molecule is its very poor solubility in common organic solvents thus making it unsuitable for solution-processed techniques. Anthony and co-workers overcame this problem by adding trialkylsilyl substituents to the conjugated core, which resulted in a new range of compounds with increased solubility, environmental stability and electrical characteristics [32]. Among them, solution processed 6,13-bis(triisopropylsilylethynyl) functionalised pen-tacene (TIPS-pentacene) was found to produce high quality films during slow evaporation of the solvent, allowing molecules to self-assemble into large n-stacked arrays [33]. The importance of uniformity of molecular stacking is based on the fact that the electrical transport properties of organic semiconductor films are highly limited by the hopping mechanism in the disordered regions of the bulk [27]. As a result, a significant effort was put into increase the molecular order by controlling the crystallisation of the films.
SOLVENT-EXCHANGE METHOD
Mixed solvent approaches, based on the solventexchange method [34], have been used to produce high quality crystals using soluble pentacene deriva-
tives. Slow evaporation of the solvent is crucial for the crystal formation. Although most small molecules have limited solubility in high boiling point solvents, this problem can be overcome by introducing a highly saturated solution of the active compound into a slower evaporating solvent. While the lower boiling point solvent evaporates quickly, the molecules are left inside the higher boiling point one to slowly self-organise into single crystalline structures.
A representative example of this process was described by Kim et al [35] for high mobility TIPS-pen-tacene transistors (Fig. 2). Small amount of TIPS-pentacene dissolved in toluene was introduced into a container of acetonitrile. According to the mixed solvent approach, molecules were transferred from the good solvent (toluene) into the poor solvent (acetoni-trile), where the material has limited solubility and thus self-assembly occurred through the n—n stacking leading to the
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