Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives

Organic Electronics 12 (2011) 1806–1814

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Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives Shuo Chai a,b, Shu-Hao Wen a, Ke-Li Han a,⇑ a b

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e

i n f o

Article history: Received 13 April 2011 Received in revised form 22 June 2011 Accepted 10 July 2011 Available online 4 August 2011 Keywords: Charge transport Anisotropic mobility Density functional theory Electron-withdrawing n-Type

a b s t r a c t A series of n-type perylene bisimide (PBI) derivatives with electron-withdrawing substituents at both bay and imide nitrogen positions were investigated. The effects of these substituents on internal energy relaxation, molecular orbitals, air stability, electronic properties and charge transport behaviors were systematically discussed with density functional theory (DFT) which has been demonstrated reliable for organic semiconductor study. The investigated derivatives with electron-withdrawing substituents show favorable performances in terms of these properties. The LUMO levels are greatly stabilized by at least 0.3 eV and these derivatives show the strong absorption from 400 to 700 nm which match with the solar spectra very well. The charge transport mainly happens between molecules in the same organic molecular layer and electronic couplings between layer-to-layer molecules are very weak, thus the mobility along layer-to-layer direction is less efficient. The variation of molecular packings and intermolecular interactions produce the highly anisotropic mobilities. The derivative with two fluorine atoms at bay positions and –CH2C3F7 at imide group has broad and strong absorption in the UV-Visible region and the electron mobility could get to 0.514 cm2 V1 s1 which is greatly encouraging for molecular and material design in organic solar-cell devices. These calculated results are in good agreement with the experimental data. However, not all the functionalization with halogen substituents would induce the increase of the electronic coupling and electron mobility. The derivatives with four halogen substituents at the bay positions could not show advantages in terms of electron mobility which is induced by the distorted conjugated structures. The theoretical understanding of these n-type organic semiconductors figures out the importance of tuning the molecular geometry to get high performance semiconductor materials. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic semiconductors have attracted remarkable research attentions in the past decades because of the outstanding advantages of low cost, flexibility, large-area applications [1–6]. The investigation of organic device technologies based on organic conjugated semiconductors, such as organic field-effect transistors (OFETs), organic solar cells ⇑ Corresponding author. Tel.: +86 411 84379293; fax: +86 411 84675584. E-mail address: [email protected] (K.-L. Han). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.07.008

(OSCs), organic biologic sensors (OBSs), are regarded as the most promising research fields [7–9]. The extensive investigation of p-type semiconductors has been performed [10– 13]. However, the development of n-type materials still largely lags behind p-type ones because of the deficiency of the high-performance materials. In the complementary circuits both p-type and n-type organic materials are required, performing with high operating speed but low power consumption [14,15]. It is desirable to search for the practical n-type and ambipolar materials with high mobility and air stability [16,17].

S. Chai et al. / Organic Electronics 12 (2011) 1806–1814

Perylene bisimides (PBIs) have been demonstrated n-type semiconductors which have large conjugated structures and strong electron affinities [18–21]. Because of the representative geometric, electronic and transport properties, PBIs show the promising applications in optical and electronic devices [22–24]. Attaching substituents, such as perfluoroalkyl, fluorine, chlorine, cyanide, etc. to the conjugated perylene bisimide core could stabilize molecular orbitals and facilitate electron transport successfully [25–28]. This kind of functionalization of material will improve device performance and increase lifetime of the device. The similar substituents have been investigated and demonstrated practical for device design [18,29,30]. Two kinds of substituents referred in our theoretical work: substituents at imide nitrogen positions with fluorinated groups and substituents at bay positions with different electron-withdrawing atoms. The compounds with substituents of fluoroalkyl and fluorophenyl groups, such as –CH2C3F7 and –C6F5 at imide nitrogen position of perylene bisimide have been demonstrated to perform the air-stable electron mobilities with the improved solubility [30,31]. The molecular models in the present work – a series of n-type electron-withdrawing derivatives based on perylene bisimide are shown in Scheme 1. Mobility is the intrinsic property of organic crystal and it is the most important characteristic when evaluate the material performance. Some groups have investigated this kind of functionalization of PBI, Chen et al. have measured the highest mobility 0.068 cm2 V1 s1 of fluorine substituents [30] and 0.35 cm2 V1 s1 have been reported by Schmidt et al. of the derivatives with both bay and imide nitrogen substituents of PBI [27]. Delgado et al. have investigated considerable PBI derivatives and found that the important impact of the intermolecular packing on the charge transport properties [18]. Here we provide the theoretical study of detailed influences of these electron-withdrawing substituents on geometric structures, electronic properties, especially the carrier mobilities. We developed a method to simulate the angular-resolution mobility and investigated the anisotropic behaviors of organic material [32].

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In the present work, both hole and electron hopping motions are considered and the related electronic and transport properties of the two kinds of hoppings are discussed. Density functional theory (DFT) calculations have been performed to investigate the geometries, molecular orbitals, electron affinities and reorganization energies of PBI monomers. The absorption spectra of all the derivatives in the tetrahydrofuran solution have been simulated with time-dependent density functional theory (TDDFT) method. The intermolecular electronic couplings and charge transport mobilities at room temperature have been deduced with the available single crystal structures. The anisotropic mobility is simulated with our angularresolution expression. The functionalized derivatives of perylene bisimides have been compared systematically in terms of these charge transport properties. The theoretical analysis is helpful to understand the influence of the structure variations on the transport properties of these organic conjugated molecules and direct the high-performance material design. 2. Theoretical methodology Density functional theory (DFT) has been employed to calculate the structural, electronic and transport properties of these organic semiconductors. It has been demonstrated reliable to study the organic conjugated molecules [4,10,33]. In the solid state the reorganization energy is mostly from the internal energy relaxation when charge transport and the contribution from the environment to the reorganization energy could be neglected [34–36]. Thus the reorganization energy could be expressed as:

khole ¼ kþ0 þ kþ ¼ ðE0  E0 Þ þ ðEþ  Eþ Þ

ð1Þ

kelectron ¼ k0 þ k ¼ ðE0  E0 Þ þ ðE  E Þ

ð2Þ

where E0 and E+/E denote the energies of the neutral and cation/anion molecules in their lowest energy geometries, E0 and Eþ E are the energies of the neutral and cation/anion monomers with the cation/anion and neutral geometries, respectively. The calculations of reorganization

Scheme 1. Schematic of the structures of investigated PBI derivatives.

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energy are performed with B3LYP functional with 6-311G⁄⁄ basis set. Time-dependent density functional theory (TDDFT) is also employed to simulate the absorption spectra of investigated semiconductors. These calculations are carried out with Gaussian 09 program package [37]. The evolution of electronic coupling t is based on the molecular orbitals of the conjugated molecules. The coupling matrix element could be deduced as Eq. (3).

t¼

J12  S12 ðE1 þ E2 Þ=2 1  S212

ð3Þ

The electronic coupling t could be written with charge transfer integral J, overlap matrix S and site energy of monomer E, as described in the reference [38]. The monomer orbitals with proper orthogonalization are used as basis sets for the Hamiltonian of this dimer system. Assuming h is the Kohn–Sham Hamiltonian of a dimer pair system H=L and uH=L are the localized molecular orbitals of adja1 , u2 cent monomers. For hole transport the highest occupied molecular orbitals (HOMOs) of monomers are used as the basis function while for electron transport the lowest unoccupied molecular orbitals (LUMOs) are used correspondingly, and then S, J and E could be expressed as:

J 12 ¼ hu1H=L jhjuH=L 2 i S12 ¼ hu1H=L juH=L 2 i E2 ¼ hu

u

ð5Þ

where kB is the Boltzmann constant and T is the temperature which is 298 K in our calculation. The reorganization energy k and electronic coupling t between two adjacent molecules are key parameters for the charge hopping rate. The hopping mobility l could be evaluated from the Einstein relation:

l¼

eD kB T

ð7Þ

here ci is the angle of hopping path relative to the plane of interest, / is the orientation angle of the transistor channel relative to the specific crystallographic axis and hi is the angle of the projected hopping path relative to the specific axis. These calculations are based on the detailed analysis of the molecular packing of organic single crystals.

3.1. Geometry and molecular orbital

H=L 2 i

    t2 p 1=2 k exp  4kB T h kkB T

e X 2 ki ri Pi cos2 ci cos2 ðhi  UÞ 2kB T i

3. Results and discussion

In the calculation of electronic coupling, the fragment orbital approach is applied with Amsterdam density functional (ADF) software [39]. The local density functional VWN combined with PW91 gradient corrections and TZ2P basis set are selected as the calculation method. There are two widely used models to describe the charge transport in organic semiconductors: the band-like model and the hopping model [40]. The former is usually applied at very low temperature. At room temperature the charge carriers in organic single crystals are localized and the charge transport in a sequential hopping mode between neighboring molecules which could be described by Marcus theory. Hutchison and Bromley et al. have demonstrated that for organic oligomers the thermally assisted polaron hopping mechanism is reasonable [41–43]. Here we mainly investigate the electron and hole transport behaviors of PBI derivatives at room temperature and employ the hopping mechanism. According to Marcus–Hush theory the hopping rate k could expressed as [44–46]:

k¼

lU ¼

ð4Þ

E1 ¼ hu1H=L jhjuH=L 1 i H=L 2 jhj

where D is the diffusion efficient. Assuming the hopping motion is an isolated random walk and there is no correlation between charge hopping motions, the diffusion coefficient D could be evaluated from the hopping P 2 1 rate:D ¼ 2n i r i ki P i ; n is the spatial dimensionality, ki denotes the hopping rate along the specific hopping path i, ri is the intermolecular center-of-mass distance between two neighboring molecules and Pi is the hopping probability for the specific path i which can be calculated by: P Pi ¼ ki = i ki . The mobility orientation function is deduced to simulate the angular-resolution anisotropic mobility, see details in reference [32].

ð6Þ

Some selected optimized geometric parameters, including bond lengths, bond angles and dihedral angles of A1, A2, B1, C1 and C3 molecules both in neutral and anion states are listed in Table A. 1 in the Supporting Information. From the calculated results, we could tell that most bond lengths have visible change between neutral and anion states. The modification of C–C bond lengths in anion state are very big except for the ones located in two conjugated cores, e.g. the bond length difference of C1–C2 is 0.023 Å while the one of C2–C3 is only 0.002 Å for compound A1. The C–N bonds lengthen 0.006 Å in the anion state. The strengths of C–O bonds are weaker in the anion states and the bond lengths are all increased by about 0.012 Å. The electron transport also induces the bond angle change, e.g. the bond angle C2–C3–C7 of investigated molecules decrease by about 0.2° in anion state. For different PBI derivatives we could detect the geometric conformation change induced by attaching electron-withdrawing groups. When substituents are attached to nitrogen atom of perylene bisimide core, the C–N bond is lengthened by 0.01 Å and other bonds are almost changeless. However, substituents at the bay positions of perylene bisimide core bring the obvious geometric differences. The bonds have stretching vibration and the angles C5–C6–C8 increases by more than 2°. With the calculated geometries we could find that the conformation change of these compounds when electrons transport are considerable which directly induce the large reorganization energies. Compound A1 and A2 keep a relatively planar conjugated core with the dihedral angles less than 1°. The compounds with substituents at the bay positions have the visible twist, and also the planarity of the conjugated structure for this kind of derivatives is influenced by electron transport. For B1, it

S. Chai et al. / Organic Electronics 12 (2011) 1806–1814

is more planar in anion state than in neutral state. However, for compound C1, the dihedral angles of conjugated structure are increased significantly in the radial-anion state and the torsion is enhanced. The geometric structure of the isolated molecule plays an important role in determining the molecular arrangement of the single crystal. Fig. 1 shows the calculated molecular orbital levels of all the investigated PBI compounds. General n-type organic semiconductor materials are air sensitive and the electrons are easily trapped by the ambient oxidants. Because of instability of these materials, the device lifetime and performance have been limited. Attaching electron-withdrawing substituents to conjugated semiconductor could lower the molecular orbitals and improve the stability of materials. In the figure we could find both HOMOs and LUMOs have been stabilized by at least 0.3 eV for these PBI derivatives. The energy gaps, which mean the energy difference between HOMO and LUMO, are very close for these compounds. The HOMO and LUMO plots of all the investigated monomers at B3LYP/6-311G⁄⁄ calculation level are shown in Fig. A.1–A.10 in the Supporting Information. The charges are strongly located in the conjugated perylene bisimide center for all the compounds. All these derivatives are practical to produce semiconductors with improved stability. It is an effective strategy to decrease the electrons trapping by the ambient oxygen and moisture and obtain the air-stable materials. When realizing the application of organic material in electronic devices, solution process is an important operation. However, most organic conjugated semiconductors have poor solubility, thus increasing the solubility is also an object in molecular design. The added electron-withdrawing substituents in our investigated system would improve the solubility efficiently because the polarity has been increased with the introduced polar atoms.

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holes and electrons coming across the energy barrier and injecting the metal electrodes or empty orbitals. The introduction of electron-withdrawing substituents increases the ionization potential and electron affinity obviously. According to Newman, a practical requirement for n-type material is that organic conjugated semiconductor have electron affinity between 3 eV and 4 eV [47]. These substituents are favorable for n-type semiconductors in respect of the electron affinity. An increase of electron affinity (cal. 0.3–0.9 eV) has been observed from our calculation. The enhanced electron affinity facilitates the electrons injecting into LUMOs of semiconductors effectively. Reorganization energy is the relaxation of site energy by vibrations [38]. The calculations of reorganization energies of hole and electron transport are based on the idea of adiabatic potential energy surface at the level of B3LYP/6311G⁄⁄ as well, shown in Table 1. When the substituented groups are attached to compound A1, the obvious increases of reorganization energies are observed. These increases are not helpful for charge carriers transport and the hopping rate will be decreased according to Marcus theory. The electron transport will induce larger structural relaxation than hole transport. A main reason why the mobility of n-type semiconductor is usually lower than

3.2. Ionization potential, electron affinity and reorganization energy Adiabatic ionization potentials and electron affinities of all considered compounds are calculated with DFT method at the level of B3LYP/6-311G⁄⁄, as shown in Fig. 2. The ionization potential and electron affinity relate to the ability of

Fig. 1. Energy levels (in eV) of HOMO and LUMO of PBI derivatives (the upper ones are LUMOs and the lower ones are HOMOs) with the method B3LYP/6-311G⁄⁄.

Fig. 2. Calculated adiabatic ionization potentials and electron affinities (in eV) of all the investigated PBI compounds for (a) adiabatic ionization potentials, (b) adiabatic electron affinities.

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Table 1 Calculated reorganization energies (in eV) of the hole and electron transport of PBI derivatives with the B3LYP/6-311G⁄⁄ method. Hole

A1 A2 A3 B1 B2 B3 B4 C1 C2 C3

Electron

kþ 0

k+

khole

k 0

k

kelectron

0.084 0.098 0.089 0.127 0.122 0.120 0.115 0.109 0.095 0.103

0.082 0.093 0.087 0.127 0.121 0.113 0.106 0.110 0.096 0.101

0.166 0.191 0.176 0.254 0.243 0.233 0.221 0.219 0.191 0.204

0.140 0.152 0.152 0.166 0.164 0.163 0.162 0.159 0.142 0.156

0.140 0.156 0.149 0.167 0.163 0.158 0.159 0.162 0.145 0.156

0.280 0.308 0.301 0.333 0.327 0.321 0.321 0.321 0.287 0.312

p-type one is the difference of the reorganization energy when electron and hole transport. The variation of the substituents at the bay positions has obvious influence on the charge transport reorganization energy. Fig. 3 shows the evolution of electron transport reorganization energy with electronegativity of the substituented atoms at the bay positions. A2, B1, B2 and B3 have the similar geometries only the different subsituented atoms at the bay positions. According to Pauling definition the electronegativity of H, F, Cl and Br are 2.10, 4.00, 3.16 and 2.96, respectively. The evolution of reorganization energy has been analyzed which follows a straight line expression. As the electronegativity increase the electron transport reorganization energy increase, correspondingly. When PBI molecule is substituted with F which has the largest electronegativity, the electron transport reorganization energy could come to 0.333 eV. With the fitting line we could deduce the reorganization energy substituted with other atoms. These electron-withdrawing groups could improve the stability and electron injection effectively though they would introduce the obvious increase of reorganization energy which is not expected by device design.

Fig. 4. UV-Vis absorption spectra of A2, B1, B2, B3 and C1 in tetrahydrofuran (THF) solution.

3.3. Absorption spectra Time-dependent DFT calculations at B3LYP/6-311G⁄⁄ level are performed to simulate the absorption spectra of these PBI derivatives. The theoretical simulations of absorption spectra are performed in tetrahydrofuran solution with SCM model. From the simulated spectra in Fig. 4, we find that the PBI derivatives have broad and strong absorption in the UV-Visible region especially from 400 to 700 nm. The perylene cores are the chromophore centers for these compounds and all the derivatives have the maximum absorption peak almost in the same area near 530 nm which assigns to the p–p⁄ transition. It reflects that these derivatives need almost the same optical transition energy. Attaching the strong electron-withdrawing fluorine atoms on the core positions will induce the blue shift of the absorption spectra which means the energy increase needed for the optical transition. Relative to the absorption maximum of B1, the red shift of 9 nm for B2 and the red shift of 18 nm for B3 have been observed. Though the structures of B1 and C1 have evident differences from Table A.1, the absorption spectra of the two compounds show almost the same shape and distribution only a tiny difference in the absorption strength. The calculated results are in good agreement with the corresponding experimental ones by Chen et al. [30] and Jones et al. [48]. These results indicate that the organic conjugated PBI compounds have advantages in terms of absorption spectra and they have potential to realize the application in organic solar cell. 3.4. Electronic coupling and anisotropic mobility

Fig. 3. Correlation of reorganization energy of electron transport with electronegativity of substituented atom at the bay positions.

Fig. 5 shows the molecular packings of compound A2, B3 and C1 as well as the torsion angles of perylene bisimide plane. The dimers of molecule D with adjacent molecules in single crystal are marked as D1, D2, D3, Dp (Dp denotes the dimer in two neighboring packing layers) . . ., and the center-of-mass distance of the two closest conjugated molecules is presented. The theoretical analysis are based on the single crystals from Schmidt et al. [49]. If

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Fig. 5. Molecular packings of compounds (a) A2, (b) B3 and (c) C1 [49] with the center-of-mass distance between the conjugated centers and the torsion angles of the perylene bisimide plane by DFT calculation (cal.) as well as the experimental results (exp.). Fluoroalkyl groups have been removed for clarity.

there is no substituent at the bay position, the conjugated perylene bisimide part is almost planar, like A1, A2 and A3. The torsion angle from calculation by DFT method (0.9°) and experiment measurement (1.5°) [49] of compound A2 indicate the good planarity of this kind of molecule. The electron-withdrawing substituents at 1, 6, 7, and 12 positions can make a twisting angle as large as 35° and the conjugated PBI skeletons have been largely distorted. The calculated torsion angle of B3 molecule is 37.3 ° which is in good agreement with 37.2 ° from experiment [49]. The molecular arrangement in crystal structure is mostly determined by molecular conformation and intermolecular interactions. From the comparison of B1 and C1, we find that the addition of fluorine atoms at the bay positions enlarge the torsion of perylene backbone out of planarity which would not facilitate the dense packing and hence the intermolecular electronic coupling. The packing modes are mainly affected by the molecular conformation. The face-to-face stacking with a short distance with the large coupling area could contribute to the high mobility. The molecular packing of A2 crystal keeps a slipped stack while C1 crystal changes to a herringbone one with an increased intermolecular center-to-center distance. The charge mobility is partly dependent on the molecular distance of the conjugated compounds. The cen-

troid distance is 4.91 Å of the closest conjugated molecules for A2 while it is 7.19 Å for B3 and 5.28 Å for C1. The planar torsion hinders the approach of the molecules and obstructs the molecule packing densely. With the known crystal structures, the electronic couplings of neighboring molecules of compounds A2, B2, B3, B4 and C1 are calculated, listed in Table A.2 in the Supporting Information. For the most organic single crystals the electronic couplings between the molecules in the same organic packing layer are much stronger than that between the molecules in two neighboring layers. For A2 crystal, the electronic couplings for dimer Dp is far less than the ones for dimer D1 and D2. The weak electronic coupling along layer-to-layer molecules contributes negligibly to mobility. The large variation of electronic couplings along directions will give rise to a 2D mobility distribution in the packing layer. For our studied PBI derivatives the electronic couplings of LUMOs could come to the same order of the ones of HOMOs. Table 2 list the maximum mobilities of hole and electron of some PBI derivatives. In these PBI derivatives A2 has the preferred transport properties for both hole and electron, with the hole and electron mobilities as large as 0.552 and 0.123 cm2 V1 s1. The close packing of the molecules in the solid state allows the strong electronic couplings of the p-conjugated orbitals which determines the high mobility. When attaching the two fluorine atoms at the bay position of the perylene core, electronic coupling between LUMOs has been enhanced significantly, about two orders of magnitude larger than that for HOMOs. The compound C1 shows favorable electron transport properties. The LUMOs of conjugated molecules show strong coupling interaction and the mobility for electrons can come to 0.514 cm2 V1 s1. For compound C1 the experimentally measured electron mobility is 0.35 cm2 V1 s1 by Schmidt [49] and our theoretical simulation provides the reliable results. For the PBI derivatives with four electron-withdrawing atoms, B2, B3 and B4, the charge transport mobilities are not satisfying. Because of the repulsion of the introduced electron-withdrawing atoms, the regular molecular packing of the PBI compounds has been distorted and the distance of the conjugated center has been lengthened. The mobilities measured in experiments are very sensitive to the experimental conditions. Schmidt et al. reported that the organic thin film transistor measurements of mobility are varied a lot when the substrate is modified or the temperature is changed [49]. Our theoretical charge transport investigation is under the ideal conditions at room temperature which could provide the reasonable results and assist to understand the detailed transport behaviors of the charge carriers. The highly anisotropic character of molecular packing of organic single crystals gives rise to the highly anisotropic

Table 2 The calculated hole and electron transport mobility (in cm2 V1 s1) of some PBI derivatives. (The corresponding experimental values from reference [49] are in parentheses).

lhole lelectron

A2

B2

B3

B4

C1

0.552 0.123 (0.67)

0.005 0.002 (0.0005)

0.137 0.024 (0.0003)

0.014 0.002 (0.025)

0.019 0.514 (0.35)

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Fig. 6. The angular-resolution anisotropic mobility of A2 crystal: (a) hole mobility and (b) electron mobility.

mobility. Understanding the anisotropic mobility can assist in controlling the directions of transistor channel relative to reference direction of molecular crystal to obtain the highest charge mobility [50–52]. Figs. 6 and 7 describe the angular-resolution anisotropic mobilities of both hole and electron transport of compounds A2 and C1. We choose the direction of strongest electronic couplings of LUMOs as the reference direction (the 0° direction in the angular-resolution figures) and all the vectors of mobility project to this direction and then deduce the corresponding angular-resolution formula. For the hole transport of compound A2, since the electronic coupling of HOMOs for dimer D1 is very strong, the mobility maximum 0.552 cm2 V1 s1 appears in this dimer direction. Differently the electronic coupling of LUMOs for dimer D2 is comparable to the one for D1, it will contribute to the mobility distribution. Thus the mobility maximum of electron transport is along about 5° in the angular-resolution figure. For compound C1 we could find the obvious difference of the angular-resolution mobility, it is because the strongest coupling of HOMOs occurs in D2 while the strongest coupling of LUMOs occurs in D1. The hole transport mobility maximum is 0.019 cm2 V1 s1 when the angle is 71° in the angular-resolution figure and the electron transport mobility maximum is 0.514 cm2 V1 s1 along dimer

Fig. 7. The angular-resolution anisotropic mobility of C1 crystal: (a) hole mobility and (b) electron mobility.

D1 vector. Understanding the angular-resolution distribution of mobility and making use of the anisotropy of mobility can help us to improve the charge transport and get highperformance materials. 4. Conclusions In summary we theoretically investigated the perylene diimde derivatives with electron-withdrawing substituents. These derivatives show the promising performance in terms of electronic, optical and charge transport properties. Attaching the electron-withdrawing substituents at the proper positions could improve the stability of the materials, enhance the electronic coupling and facilitate the hopping motions, efficiently, though these substituents would induce the increase of the reorganization energy inevitably. All the investigated derivatives have the strong absorption in the UV-Visible area which is highly desired by the organic solar cell applications. The molecular conformation of the organic semiconductor plays a significant role in determining the electronic properties, molecular stackings and transport behaviors. In all these derivatives, compound C1 which has two fluorine substituents at the bay positions shows the favorable electron transport ability with the mobility as large as 0.514 cm2 V1 s1. Although the derivatives with

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four halogen substituents at the bay positions, B1, B2, B3 and B4, stabilize the LUMO levels and increase the electron affinities, the distorted molecular geometries are adverse to the close packings, and hence leading to the low mobilities. The charge carriers mostly transport in the basal stacked organic layer which give rise to a 2D mobility distribution. The angular-resolution anisotropic mobility analysis shows the importance to control the directions of crystals in applications to improve the material performance. The theoretical investigations of the organic semiconductors are helpful to understand the intramolecular and intermolecular charge transport behaviors and beneficial to the materials design for the electronic applications. Acknowledgment We thank the supercomputer center of Virtual Laboratory of Computational Chemistry, Computer Network Information Center, Chinese Academy of Sciences for the computational resources. This work is supported by the Natural Science Foundation of China (No. 20833008) and NKBRSF (Grant 2007CB815202).

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel.2011. 07.008.

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