Charge Carrier Mobility in Fluorinated Phenoxy Boron

Article pubs.acs.org/crystal

Charge Carrier Mobility in Fluorinated Phenoxy Boron Subphthalocyanines: Role of Solid State Packing Jeffrey S. Castrucci,†,‡ Michael G. Helander,‡ Graham E. Morse,† Zheng-Hong Lu,‡,§ Christopher M. Yip,† and Timothy P. Bender*,†,‡,# †

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5, Canada ‡ Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, M5S 3E4, Canada # Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 § Department of Physics, Yunnan University, 2 Cuihu Beilu, Yunnan, Kunming 650091, People’s Republic of China S Supporting Information *

ABSTRACT: The electron mobilities in vapor-phase-deposited thin films of three fluorinated phenoxy boron subphthalocyanine (F5BsubPc, F12BsubPc, and F17BsubPc) molecules are measured using admittance spectroscopy. The mobilities are found to be field dependent, following a Poole−Frenkel type relation, and are in the range of ∼10−4 cm2 V−1 s−1 at typical operating field strengths for devices such as organic photovoltaic cells. A new F5BsubPc crystal polymorph (β-) is disclosed and its determination is critical to our ability to correlate charge carrier mobility and solid-state packing for each compound.

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INTRODUCTION The potential to manufacture low cost organic electronic devices from sublimable small molecules continues to attract research interest 20 years after the initial proof of concept.1,2 One parameter important to the function of electronic devices is the charge carrier mobility (μ), which can be measured via a number of techniques. These include direct measurement in thin film transistors (TFT),3 and in specially constructed single carrier devices using the time-of-flight (TOF) method4 or admittance spectroscopy (AS).5 The reported electron mobility of small molecule based n-type (electron-transport or electron accepting) materials ranges across many orders of magnitude and is typically field dependent. Films of the common n-type charge conductor tris(8-hydroxyquinolinato)aluminum (Alq3) have a zero field mobility (μ0) of ∼10−8 cm2 V−1 s−1 and a field sensitivity (commonly referred to as the Poole−Frenkel coefficient, β, the slope of the field dependent mobility plot) of ∼0.004 (V/cm)−1/2 as determined using TOF.6 A common p-type charge conductor 4,4′-N,N′-dicarbazole-biphenyl (CBP) has a μ0 of ∼10−4 cm2 V−1 s−1 and a β of ∼0.001 (V/cm)−1/2 also determined using TOF. 7 Other common charge conductors, such as rubrene (∼10 −2 cm 2 V −1 s −1 ), 8 perfluoropentacene (∼10−1 cm2 V−1 s−1),9 and C60 (∼1 cm2 V−1 s−1),10 are reported to have significantly higher mobilities measured directly in a TFT. The role of molecular packing and its influence on charge transport mobility have been demonstrated with targeted synthesis11 and theoretical calculations.12 Fluorination has been © 2012 American Chemical Society

demonstrated as a viable method of guiding crystal formation (solid state arrangement)13 with the goal of achieving molecular alignments which maximize the intermolecular overlap of frontier orbitals,14 by directing π-stacking15 while simultaneously inducing or enhancing n-type charge transport characteristics16−19 and exciton diffusion lengths,20 all desirable characteristics for an organic electronic material. Fluorination has the further advantages of improved thermal stability while undergoing a physical vapor deposition process (sublimation) and once present in the final device. Boron subphthalocyanines (BsubPcs) are an emerging class of functional small molecules. Their uncommon bowl-shaped molecular geometry21 and uniqueness compared to the common phthalocyanine have made BsubPcs the target of increasing interest and application including in organic light emitting diodes (OLEDs) as emitters22,23 and dopants,24 and in organic photovoltaics (OPVs) as both donors,25 acceptors,26 and in dyad structures.27 Recently, graded heterojunction solar cells utilizing chloro-BsubPc as a donor 28 and planar heterojunction cells with both donor and acceptor layers made from BsubPc compounds29 have been demonstrated. Commonly open circuit voltages greater than 1 V and efficiencies of 2.7−4.2% are achieved. The basic optical and electrochemical properties of BsubPcs have been examined,30 but the discussion of the charge carrier mobility of BsubPcs has Received: February 14, 2011 Published: January 9, 2012 1095

dx.doi.org/10.1021/cg2015385 | Cryst. Growth Des. 2012, 12, 1095−1100

Crystal Growth & Design

Article

determination on a crystal grown under sublimation conditions, conditions that closely parallel the conditions under which organic electronic devices are fabricated, is an enabling piece of data making this study and its conclusions possible. Selected crystallographic data are summarized in Table 1 and detailed crystallographic data appear in the Supporting Information accompanying this article. Briefly, β-F5BsubPc (C30H12BF5N6O) formed a monoclinic crystal (P21/c) with a, b, c = 14.2834(6) Å, 11.3575(2) Å, and 115.1707(7) Å and β = 99.7330(15)°. β-F5BsubPc contains four molecules in each unit cell and a density of 1538 kg/m3. We have previously shown that α-F5BsubPc assembles in the solid-state in a concave−concave head-to-head arrangement similar to other phenoxy-BsubPcs34 and in contrast to the concave-to-ligand packing observed for F12BsubPc and F17BsubPc.23 However the solid state arrangement of βF5BsubPc resembles that F12BsubPc and F17BsubPc (Figure 1). We have a limited amount of data to suggest this packing motif may be common to the general class of phenoxydodecafluoro-BsubPcs (of which F12BsubPc and F17BsubPc belong).35 The crystal of β-F5BsubPc is less densely packed than α-F5BsubPc (2659 mol/m3 compared to 2775 mol/m3). Within the crystal there is a distinct π−π interaction between one of the six-membered rings of the isoindoline lobe (C18/ C19/C20/C21/C22/C23) of the BsubPc molecular fragments, with a neighboring axial pentafluorophenoxy fragment (C25/ C26/C27/C28/C29/C30) at a centroid-to-centroid distance of 3.7873(14) Å (see Supporting Information Figure S3 for atomic numbering). The result is that each β-F5BsubPc molecule within a column is spaced at 9.141 Å (BsubPc fragment centroid-to-centroid, along the c axis, Figure 1a) and each column is spaced at 8.023 Å. This arrangement is similar to what is seen in each of F12BsubPc and F17BsubPc (Figure 1b,c), although sequential molecules are spaced at 5.729 Å and 5.589 Å and the columns are spaced from one another by a distance of 11.661 Å and 11.902 Å (BsubPc fragment centroidto-centroid) respectively. F12BsubPc and F17BsubPc each have a lower density than β-F5BsubPc, with F12BsubPc (2496.41 mol/ m3) being more dense than F17BsubPc (2407.54 mol/m3). For the determination of the charge carrier mobility of F5BsubPc, F12BsubPc, and F17BsubPc we considered the use of a standard TFT configuration or the use of a specialized device for either TOF or AS methods. Although the TOF technique is a widely used method for the determination of charge carrier mobility, it requires films several micrometers thick which are not only impractical to fabricate, but also may not accurately reflect the transport characteristics of the films used in OPV and OLED devices which are an order of magnitude thinner (∼100 nm). Although using a TFT device configuration allows for the determination of the charge carrier mobility of films ∼50 nm thick, TFTs only yield the lateral field effect mobility (charge moves laterally between the electrodes), which may also not be applicable to OLEDs and OPVs wherein charge moves medial to the electrodes. We therefore focused on the use of admittance spectroscopy (AS) as it allows for the determination of medial charge carrier mobility as a function of electric field in films of a thickness of ∼100−500 nm. The theory behind the AS technique has been extensively discussed in the literature.36,37 In short, the AS technique is based on the measurement of the frequency-dependent capacitance of an organic thin film. The capacitance is found to exhibit a characteristic minimum as a function of frequency, which is indicative of the carrier relaxation time (τr) in the organic film.

been limited to a determination in a TFT configuration of a single BsubPc derivative (Cl-BsubPc).31 The configuration and functioning of a TFT relies on the application of an electrical field something not possible within an OPV device32,33 wherein zero field mobility is of importance. In this study, we have examined the electron mobility of three fluorinated phenoxy boron subphthalocyanine derivates (FnBsubPc, where n = 5, 12, or 17, Figure 1) using AS. Using

Figure 1. Fluorinated boron subphthalocyanines (a) β-F5BsubPc, (b) F12BsubPc, and (c) F17BsubPc and their crystal packing. Within columns the intermolecular distances are 9.141 Å, 5.729 Å, and 5.589 Å, respectively (BsubPc fragment centroid-to-centroid distance).

this technique we are able to extrapolate the zero field mobility of each derivative. We discuss correlations among the measured mobility of each FnBsubPc, the field dependence of the mobility, and the crystal structure (solid-state arrangement) of each of these compounds. The crystal structures of F12BsubPc and F17BsubPc have been previously disclosed.23 This study however was enabled by the determination of a new crystal structure for F5BsubPc (denoted as β-F5BsuPc) obtained by diffraction of crystals obtained by train sublimation.

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RESULTS AND DISCUSSION The synthesis of F5BsubPc, F12BsubPc, and F17BsubPc has been previously described.23 In our original report the single crystals from which the solid state arrangement was determined were grown from a mixture of vapor diffusion (F5BsubPc) and sublimation (F12BsubPc and F17BsubPc). We have now successfully diffracted a single crystal of F5BsubPc grown by sublimation. We denote this new polymorph as β-F5BsubPc (original polymorph now denoted as α-F5BsubPc). Its 1096



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Table 1. Selected Crystallographic Data for FnBsubPcs chemical formula formula mass method of growth crystal system density (kg/m3) density (ρ, mol/m3) (Å, calculated) a/Å b/Å c/Å α/° β/° γ/° unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z absorption coefficient, μ/mm−1 no. of reflections measured no. of independent reflections Rint final R1 values (I > 2σ(I)) final wR(F2) values (I > 2σ(I)) final R1 values (all data) final wR(F2) values (all data) goodness of fit on F2

α-F5-BsubPc23

β-F5-BsubPc

F12-BsubPc23

F17-BsubPc23

C30H12BF5N6O 578.27 solvent diffusion monoclinic 1605 2775.52

C30H12BF5N6O 578.27 sublimation monoclinic 1538 2659.65 8.547 14.2834(6) 11.3575(2) 15.1707(7) 90.00 99.7330(15) 90.00 2425.62(16) 150(1) P21/c 4 0.126 19341 5515 0.0702 0.0540 0.1164 0.1134 0.1450 1.037

C30H5BF12N6O 704.21 sublimation monoclinic 1758 2496.41 8.729 11.5399(7) 10.6340(4) 21.9972(13) 90.00 99.799(2) 90.00 2660.0(2) 150(1) P21/c 4 0.169 17217 6033 0.0756 0.0652 0.1350 0.1473 0.1709 1.037

C30BF17N6O 794.17 sublimation orthorhombic 1912 2407.54 8.835 10.8303(3) 15.0749(6) 16.9006(6) 90.00 90.00 90.00 2759.29(17) 150(1) P212121 4 0.200 20857 3539 0.0541 0.0474 0.1098 0.0808 0.1274 1.045

19.7225(4) 10.3637(3) 23.4591(6) 90.00 93.3870(15) 90.00 4786.6(2) 150(1) C2/c 8 0.128 15196 5439 0.0388 0.0432 0.1048 0.0674 0.1205 1.052

The dc mobility (μdc) for nondispersive transport is then given by5

μdc =

t2 0.56τrV

(1)

where t is the device thickness and V is the applied voltage. Single carrier electron transporting only devices of the structure Al (50 nm)/FnBsubPc (500 nm)/TPBi (3 nm)/Al (100 nm) were constructed by vacuum deposition on a 50 × 50 mm2 glass substrate cleaned with a standard procedure of (sequentially) ALCONOX, acetone, methanol, and UV ozone. Individual devices were 2 × 1 mm2 in area. A TPBi layer was included for use as an electron injection layer in an attempt to form an Ohmic contact.23 Using the same method, single carrier hole only devices of the structure ITO/MoOx (0.7 nm)/ FnBsubPc (500 nm)/Ag (100 nm) were constructed, but no consistent current voltage characteristics could be captured from the devices. This may be indicative of instability toward oxidative processes related to hole transport.23 Devices were fabricated in a Kurt J. Lesker LUMINOS cluster tool with a base pressure of ∼10−8 Torr. Organic layers were deposited at a rate of 0.5 Å/s, metal oxide was deposited at a rate of 0.3 Å/s, and the Al layers were deposited at a rate of 1.0 Å/s. Samples were mounted in a vacuum cryostat,38 and impedance measurements were collected using an Agilent 4294A as a function of temperature from 120 K up to 330 K. Electron mobilities were calculated from the impedance data following the method of Tsang et al.5 Film thickness was determined by spectroscopic ellipsometry39 using a Sopra GES 5E. Figure 2a shows the frequency-dependent capacitance (C) of F5BsubPc single carrier devices. At zero applied bias the capacitance is nearly constant with frequency and is representative of the geometric capacitance (Cgeo). The increase

Figure 2. Capacitance and negative differential susceptance as a function of frequency for F5BsubPc at 300 K. The arrows denote the peak in negative differential susceptance used to identify the carrier transit time.

in capacitance at high frequency is due to the parasitic capacitance of the test fixture and does not affect the present analysis. With increasing applied bias the characteristic minimum in the capacitance curve shifts to higher frequency, indicative of the field dependent mobility of the molecule. Figure 2b shows the negative differential susceptance −ΔB = −ω(C−Cgeo) as a function of voltage for the same device. The characteristic frequency f r = τr−1, indicated by the arrows, shifts to higher frequency with increasing applied bias. This trend 1097



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BsubPc molecular fragments. In the case of β-F5BsubPc, electron transport might take place by hopping along one of two pathways each involving the BsubPc molecular fragment. The first, is an intercolumn pathway whereby the electron hopping would be roughly aligned with the plane defined by the crystallographic a- and b-axis at a distance of 8.023 Å (Figure 1a). The second where electron hopping could occur along the columns of BsubPc fragments aligned with the c axis at a distance of 9.141 Å (Figure 1a). Given the small difference it is difficult to say with certainty which pathway electrons may take as they progress through the solid. By comparison, within the crystals of F12BsubPc and F17BsubPc the distance between BsubPc molecular fragments in the columns is 5.729 Å and 5.589 Å, respectively, whereas the intercolumn distances are 11.661 Å and 11.902 Å respectively (Figure 1b,c). Because of this relatively large difference, one can infer that electron hopping would occur over the shorter distance of 5.729 Å and 5.589 Å in the direction of the b- and a-crystallographic axis for F12BsubPc and F17BsubPc, respectively. These centroid-tocentroid distances of 5.729 Å and 5.589 Å are shorter than the 8.023 Å seen within the columns of β-F5BsubPc. We must therefore conclude that it is a combination of the distances between the LUMOs of neighboring BsubPc fragments (as exemplified by the centroid to centroid distances) and their associated symmetry along a crystallographic axis leading to a distinctive pathway through the solid which is most important to achieve higher zero field charge carrier mobility, exemplified by F12BsubPc and F17BsubPc which have higher zero field mobilities. By extension, the small difference in zero field mobility between F12BsubPc and F17BsubPc may also be attributable to the small differences in centroid-to-centroid distances: 5.729 Å (μ0 = 2 × 10−5 cm2 V−1 s−1) and 5.589 Å (μ0 = 1 × 10−5 cm2 V−1 s−1) for F12BsubPc and F17BsubPc, respectively. The charge carrier mobility at 300 K of F5BsubPc is more sensitive to changes in the electric field than F12BsubPc and F17BsubPc (see Figure 3). This difference can be explained with theory.41 The slope (β) is an inverse measure of the required activation energy for the electron transport process (i.e., a lower activation energy means greater sensitivity to electric field strength, and thus a steeper slope) and is known to relate to molecular density by correlation to molecular spacing (d),41 such that β ∝ d−1/2. The average molecular spacing () can be used to explain the difference in transport activation energy. The distance is not related to any crystallographic distances but can be estimated based on the measured molar density (ρ) of the single crystals by = (ρNA)−1/3, where NA is Avogadro’s number. We have estimated for F5BsubPc, F12BsubPc, and F17BsubPc to be 8.547, 8.729, and 8.835 Å, respectively (Table 1). Given the closeness of approximation for F12BsubPc and F17BsubPc similar activation energies for charge transport can be expected and are reflected in their slopes (β). Indeed this is the case as we have measured β for F 12 BsubPc and F 17 BsubPc to be 0.0033 and 0.0031, respectively. In comparison, F5BsubPc with a significantly

indicates that the carrier relaxation time is shorter at higher applied bias and hence the mobility increases with electric field. Additionally, the dielectric constant (εr) can be calculated from the geometric capacitance (Cgeo = εrε0A/d). Figure 3 shows the measured electron mobility as a function of the square root of electric field strength (√F) at 300 K for

Figure 3. A plot of field dependent mobility at 300 K. Solid lines are best fits for the data and show the extrapolation back to obtain zero field mobility. Note the similarity in slope for the F12BsubPc and the F17BsubPc.

F5BsubPc, F12BsubPc, and F17BsubPc. The mobilities as a function of temperature for the three compounds are given in the Supporting Information for reference (Figure S1). The charge carrier mobilities of this series of compounds follow a Poole−Frenkel type field dependence,5 μ = μ0exp(β F )

(2)

where μ is the charge carrier mobility, μ0 is the zero field mobility, β is the Poole−Frenkel coefficient, and F is the electric field. F12BsubPc was found to have the highest zero field electron mobility of the derivatives studied, though the mobilities are comparable at higher field strengths with values in the range of ∼10−4 cm2 V−1 s−1. Table 2 summarizes the key electrical characteristics of all FnBsubPcs at 300 K. It has been shown that molecules which pack in extended πstacked or π-aligned arrays, and thus form a continuous pathway for electrical conduction, have high charge carrier mobilities in OTFTs.11 This generally results in a zero field mobility 2−3 orders of magnitude greater than systems which rely upon hopping transport.40 With this in mind, the difference in zero field mobility of the FnBsubPcs (Figure 2) may be explained by their differences in solid state arrangements and, more specifically, the arrangement of their π-electron systems. Using computational methods, we can show that the distribution of the lowest occupied molecular orbital (LUMO) density within the individual molecules is solely located on the BsubPc molecular fragment for the LUMO through to the LUMO+4 regardless of structure (see Supporting Information, Table S5). Therefore electron transport in the solid arrangement should take place between

Table 2. Electrical Characteristics of the Three Fluorinated SubPcs at a Temperature of 300 K compound

zero field mobility, μ0 (cm2 V−1 s−1)

Poole-Frenkel coefficient, β (V/cm)−1/2

dielectric constant, εr

electron mobility μe F = 0.01 MV/cm

electron mobility, μe F = 1 MV/cm

F5BsubPc F12BsubPc F17BsubPc

4 × 10−7 2 × 10−5 1 × 10−5

0.0076 0.0033 0.0031

2.7 3.2 3.3

8 × 10−7 3 × 10−5 1 × 10−5

8 × 10−4 6 × 10−4 2 × 10−4

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copper substrate.43 Comparing the three samples, films of F5BsubPc have considerably fewer grain boundaries than the films of F12BsubPc or F17BsubPc. Since charge mobility across grain boundaries is expected to be slower than that through a crystalline domain, we can then conclude that the mobility difference between F5BsubPc and F12BsubPc/F17BsubPc is even larger in idealized thin films engineered to have minimal grain boundaries. We can further conclude that the good film forming properties of this series of FnBsubPcs does nothing to counter our conclusions made above with regard to the correlation of the electrical performance and the metrics taken from the X-ray determined structures of each compound.

smaller average separation distance showed lower activation energy for charge transport and we have measured a steeper slope (roughly twice as large at β = 0.0076). We have demonstrated that the electrical performance of FnBsubPcs can be correlated to their respective solid-state arrangements and the associated metrics as determined from Xray diffraction of single crystals grown by sublimation. However the bulk films used for the AS measurements, while fabricated under similar sublimation conditions, are not expected to be single crystals and will contain grain boundaries. To examine the presence and frequency of grain boundaries and the general film forming properties of the FnBsubPcs, we grew 150 nm thick films of each on glass substrate and examined the films using AFM (Figure 4). If we consider that topology is caused

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CONCLUSION In conclusion, the electron mobilities for a series of fluorinated phenoxy boron subphthalocyanines  F5BsubPc, F12BsubPc, and F17BsubPc  were measured in single carrier devices made from vacuum deposited thin films using AS. While F5BsubPc was found to have the lowest zero field mobility within the set, its mobility increased much more significantly in response to an increasing electric field. The dependence of mobility on applied field was correlated to the average molecular spacing (). All three compounds were found to have electron mobilities of at least ∼10−4 cm2 V−1 s−1 at high electric fields, and the differences can be attributed to differences in solid state arrangement of each compound. On the basis of these observations, we can propose desirable solid-state packing characteristics to achieve high charge carrier mobility in a BsubPc while simultaneously having a minimal field dependence: the BsubPc compound should have good alignment (close association) of its molecular orbitals in a onedimensional column (close intracolumn distance), while each column should be relatively separated from one another (larger intercolumn distance). The larger intercolumn distance leads to an increase in the average molecular spacing and a smaller Poole−Frenkel parameter (β). Additionally, the BsubPc should have good film forming properties (on the order of 100−500 nm) with minimized inter-grain boundaries. We are currently working to design and engineer additional BsubPc systems which have these characteristics such that we can further support this hypothesis.

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ASSOCIATED CONTENT

* Supporting Information S

Detailed charge carrier mobility data; Gaussian disorder model parametric fit charts; tables of molecular orbital distributions calculated using DFT methods for FnBsubPcs; X-ray crystallographic information file (CIF), and detailed crystallographic information for β-F5BsubPc. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 4. AFM images of 150 nm thick films of (a) F5BsubPc, (b) F12BsubPc, and (c) F17BsubPc on glass.

AUTHOR INFORMATION

Corresponding Author

*Telephone: 416-978-6140. E-mail: [email protected].

by grain boundaries, then less frequent changes in topology mean fewer grain boundaries. As can been see from Figure 4, 150 nm thick films of FnBsubPcs are relatively flat having no more than a 5−6 nm variation in their thickness across the ∼5 μm sampling area. For charge transport, these continuous films are better than and in contrast to the fiber-like film morphology reported for the related Cl-BsubPc on a potassium bromide substrate42 or the pyramidal morphology found by Torres on a

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ACKNOWLEDGMENTS We gratefully acknowledge funding for this research from the Natural Sciences and Engineering Research Council (NSERC) of Canada in the form of a DG Grant (T.P.B., Z.H.L.), a CGSM Scholarship (J.S.C.), a Vanier Scholarship (M.G.H.), and a CGS-D Scholarship (G.E.M.). 1099



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