ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ. Серия C, 2014, том 56, № 1, с. 6-21
УДК 541.64:539.2
RECENT TRENDS IN CRYSTAL ENGINEERING OF HIGH-MOBILITY MATERIALS FOR ORGANIC ELECTRONICS
© 2014 г. Abderrahim Yassar
LPICM, UMR 7647CNRS, Ecole Polytechnique, Route de Saclay, 91128Palaiseau Cedex, France e-mail: abderrahim.yassar@polytechnique.edu
Abstract — Acenes and heteroacenes are receiving great attention in fundamental and applied science due to their interesting optoelectronic and charge transport properties. Their easy synthesis and functionalization have enabled the rapid development of a large number of molecular materials with remarkable charge-transport properties. This perspective provides an overview of their fundamental properties, molecular packing/morphology and charge transport properties and summarizes the progress made in recent years in the development of new high-mobility small-molecule materials focusing in particular on crystalline materials that have been able to approach or surpass mobilities of amorphous silicon.
DOI: 10.7868/S2308114714010117
INTRODUCTION
Organic semiconductors have emerged as a subject with intensive academic and commercial interest over the past few decades. The design and synthesis of new functional n-conjugated materials is a major issue in the development of next generation of organic optoelectronic devices. Organic solar cells and organic field effect transistors (OFETs) stand out as key elements of flexible, large-area and low-cost organic devices. Continuous research efforts have contributed to the great advances in the performances of OFETs and organic solar cells devices during the last years. For example, high carrier mobility of up to 30 cm2/(V • s) has been achieved in solution-processed thin film OFETs [1] and high power conversion efficiencies (PCE) exceeding 10% have been recorded with both vapour deposition and solution-processed system1. The progresses made are result of the improvements in fabrication processes that yielded improved active layer morphology, and a better understanding of the design rules to yield efficient active materials. Oligoth-iophene, acene and heteroacene are the most representative and widely studiedorganic semiconductors that form highly ordered crystalline materials. The state-of the-art ^-type crystal materials are 17.2 cm2/(V s) in the case of vacuum-deposited OFETs [2], and 31.3 cm2/(V s) for solution-processed OFETs. Mobility as high as 15—40 cm2/(V s) has been reported for bulk single crystal devices [3—6]. Diketo-pyrrolopyrrole-based copolymers have achieved record high mobility values for ^-type (hole mobility: 10.5 cm2/(V s)), «-type (electron mobility: cm2/(V s)), and
1 Best research—cell efficiencies of various types of solar cells including OPVs are found at: http://www.nrel.gov/ncpv/.
ambipolar (hole/electron mobilities: 1.18/1.86 cm2/(V s)) [7]. To accomplish high mobility materials two distinct but related issues must be addressed. Firstly, molecule must have an extended n-conjugated core, which is a prerequisite for a molecule to act as an organic semiconductor. Secondly the molecule must crystallize in such a way that the strong interactions between neighbouring molecules, which is characterized as the transfer integral, led to efficient n—n overlap. The commonly used strategy to achieve n-overlap extend-edsolids relies on crystal engineering using non-cova-lent interactions as directional intermolecular forces. As stated by Desiraju, the crystal engineering is the design and synthesis of molecular solid-state structures with desired properties, based on an understanding and exploitation of intermolecular interactions [8]. Non-covalent interactions include, hydrogen, halogen bonds, dipole-dipole, C—H-n and n—n interactions, etc. The herringbone structure is a common molecular packing mode in organic semiconductors; nevertheless it limits the realization of high carrier mobility materials (Fig. 1). Recently it has been shown that through structural modification, the herringbone packing could be forced into a 1D or 2D n-stack motif by destroying edge-to-face interactions, C—H-n, resulting in a high mobility. A clear correlation between the mobility and molecular packing has been found recently in some family of substituted pentacene and heteroacene derivatives. In the following, we review recent high-mobility small-molecule materials with a focus on crystalline materials that have been able to approach or surpass mobilities of amorphous silicon (Fig. 2). Special emphasis will be on the impact of the chemical modification on molecular packing, charge transport and OFET performance. This review is less
(b)
Herringbone
Slipped stack
materials in which strong intermolecular electronic coupling between the packed molecules are present. We highlight also results that have aimed to address the important question of whether 1D, 2D co facial n-stacking or herringbone modes are more beneficial to efficient charge carrier transport and high mobility.
(c)
Brikwork
Fig. 1. Different molecular packing modes of organic semiconductor crystals.
concerned with other important aspects of the organic synthetic chemistry for the preparation of new organic semiconductor materials, processing methods for OFET fabrication, the role of OFET architecture (top and bottom configuration) in device performance, and the importance of efficient charge injection from the source—drain contacts. We restrict the discussion to the main competing factors, C—H- n, n—n and non-covalent interactions that govern molecular packing and intermolecular interactions in these materials and we present recent trend in crystal engineering approaches that have been developed to synthesis
FUNCTIONALIZED PENTACENE AND TETRACENE
Acenes and heteroacenes have been the main focuses of research and their structural analysis have that they crystallize in herringbone motif in which the molecules are arranged edge-to-face manner. Although these packing modes are not ideally appropriate to achieve efficient n-orbit overlap, they exhibit high FET performance with mobility up to 5 cm2/(V s). Recent theoretical studies have shown that both the transfer integral and reorganization energy are strongly dependent on the molecular structures and their corresponding molecular packing, therefore high mobility can be achieved by maximizing face-to-face interactions within the crystal [9—11]. One straightforward approach to control the molecular packing and to improve n-orbital overlap of acene family is to func-tionalize their peri position with trialkyl-silyl-ethynyl, bulky groups. The advantage of this approach is that
R I
R-Si-R
R = CH3, TEMS R = CH3CH2, TES
-Si-I
R Si_R R = isopropyl, TIPS R
,, , , u = 2.5 cm2 V-1 s-1 ref (17)
U = 4.6 cm2 V-1 s-1 ref (16)
Cl
Cl Cl
U = 1.6 cm2 V-1 s-1 ref (24) u = 1.7 cm2 V-1 s-1 ref (27) U = 9 cm2 V 1 s 1 ref (25)
U = 0.31 cm2 V-1 s-1 ref (20)
U = 4.28 cm2 V-1 s-1 ref (18) R,
R I
R-Si-R
R I
R-Ge-R
Vf
U = 2.5 cm2 V-1 s-1 ref (17)
Rj
U = 2.5 cm2 V-1 s-1 ref (17)
U = 12 cm2 V-1 s-1 ref (39)
R
T!2 V-1 C-1 T
R-Ge-R I
R
■n2 V-1 c-1 r
S___ S
U = 10 cm2 V-1 s-1 ref (41) u = 1 cm2V-1 s-1 ref(44) u = 6 cm2 V-1 s-1 ref(46) u = 5.4 cm2V-1 s-1 ref(47)
Sv .S._
yJTYJ-
S
O-O U S S I
CnH^^ ^ 'CnH23 R1 R1
u = 9.1 cm2 V-1 s-1 ref (50) u = 17.2 cm2V-1 s-1 ref(2) u = 8.3 cm2V-1 s-1 ref(51) u = 10.2 cm2 V-1 s-1 ref (54) u = 9.5 cm2V-1 s-1 ref(55)
T!2V-1 C-1 r
U = 16 cm2 V-1 s-1 ref (55)
Fig. 2. Chemical structure of some high-mobility small-molecule materials.
— Si —
Cl
Cl Cl
Cl
Cl
Pk
Si(CH3)3 (H3C)3Si-Si-Si(CH3)3
(H3C)3Si-Si-Si(CH3)3 Si(CH3)3
Fig. 4. The evolution of molecular packing of pentacene derivatives as the length of the substituents increases. Reprinted with permission from References [12] and [13], Copyright 2013, American Chemical Society.
the solubilizing group is held away from the conjugated core by a small acetylenic bridge allowing the n-con-jugated core of adjacent molecules to achieve close contacts. Anthony et al. first interested in the impact of the functionalization, the steric effect and relative length of the substituents, in tuning n—n interactions of the pentacene [12].
The most popular synthetic route to introduce sol-ubilizing groups at the peri-positions of pentacene is shown in Fig. 3. Usually functionalized pentacene -quinone serves as starting materials. The addition ethynyllithium, or ethynyl Grignard reagent, to pen-tacenequinone followed by deoxygenation of the resulting diol using either HI or SnCl2 furnish the functionalized pentacene in moderate to high yield [12].
The molecular packing modes of silylethynepenta-cene derivatives are strongly dependent on the relative length of the substituent. Small changes in the substitution of the silicon lead to significant changes in the crystal packing (Fig. 4) [13]. Roughly when the length of the substituent is approximately half the length of the pentacene core, an ideal 2D lamellar packing motif is achieved, otherwise a 1D n-stacking packing or herringbone mode is adopted. Indeed, pentacene itself packs in a herringbone mode, and the same pattern is adopted by 6,13-trimethyl-silyl-ethynyl pentacene
because of increased edge-to-face interactions. Replacement of trimethyl-silyl-ethynyl group with trii-sopropyl-silylethynyl (TIPS) group, which is essentially spherical and has an approximate diameter equal to one-half of the pentacene length, induces a change in the packing arrangement, a 2-D parallel mode, with a 3.5 A separation between nearest-neighbour penta-cenes, whereas «-propyl substitution results in a slipped-stack.
The molecular packing of thermal evaporated TIPS-pentacene thin film has been also solved by Bao et al. and was found to be identical to the bulk crystal structure [14]. Final device performance depends strongly on the strength of n-orbital interactions between co-facially stacked molecules within the c
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