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A Robust, High-Temperature Organic Semiconductor Jeremy T. Kintigh, Jennifer L. Hodgson, Anup Singh, Chandrani Pramanik, Amanda M Larson, Lei Zhou, Jonathan B Briggs, Bruce C. Noll, Erfan Kheirkhahi, Karsten Pohl, Nicol E. McGruer, and Glen P. Miller J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp505011x • Publication Date (Web): 23 Oct 2014 Downloaded from http://pubs.acs.org on October 24, 2014

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The Journal of Physical Chemistry

A Robust, High-Temperature Organic Semiconductor Jeremy T. Kintigh,a* Jennifer L. Hodgson,a Anup Singh,b Chandrani Pramanik,a Amanda M. Larson,c Lei Zhou,a Jonathan B. Briggs,a Bruce C. Noll,d Erfan Kheirkhahi,b Karsten Pohl,c Nicol E. McGruer,b Glen P. Millera a

Department of Chemistry and Materials Science Program, University of New Hampshire,

Durham, NH 03824 USA b

Department of Electrical and Computer Engineering, Northeastern University, Boston, MA

02115 USA Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH 03824 USA d

Bruker AXS, Madison, WI 53711 USA

AUTHOR INFORMATION Corresponding Author *Griffith University, Eskitis Institute Nathan, QLD, Australia 4111 E-mail: [email protected] Tel: +61 0488 337 757

ACS Paragon Plus Environment


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ABSTRACT We introduce a new pentacene-based organic semiconductor, 5,6,7-trithiapentacene-13-one (TTPO). TTPO is a small molecule organic semiconductor that is simple to synthesize and purify, readily crystallizes, melts in air from 386 – 388 °C without decomposition, and is indefinitely stable against degradation in acid-free solution. TTPO has a high molar absorptivity, optical and electrochemical HOMO – LUMO gaps of 1.90 and 1.71 eV, respectively, and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and -1.5 V. The crystal structure for TTPO has been solved and its unique parallel displaced, head to tail packing arrangement has been examined and explained using high-level density functional theory. Highresolution scanning tunneling microscopy (STM) was used to image individual TTPO molecules upon assembly on a pristine Au(111) surface in ultrahigh vacuum. STM images reveal that vapor-deposited TTPO molecules nucleate in a unique stacked geometry with a small acute angle with respect to Au(111) surface . Preliminary TTPO-based bi-layer photovoltaic devices shows increases in short circuit current density upon heating from 25 to 80 °C with a concomitant 4-fold to 160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thinfilm electronic devices that require operation over a wide range of temperatures such as thin-film transistors, sensors, switches and solar cells.

KEYWORDS: Organic Electronics, Stability, STM, OFET, OPV


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INTRODUCTION Acenes are a widely studied class of organic semiconducting compounds composed of linearly annulated benzene rings. Tetracene and pentacene containing four and five rings respectively have been utilized as active layers in the construction of organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). However, their utility is fundamentally limited by inherent photo-instabilities and poor solubilities, leading researchers to prepare derivatives bearing either solubilizing or stabilizing substituents. The addition of silylethynyl,1,2,3,4,5 organothio,6,7,8 and cyano9 substituents leads to molecules that are less prone to photodegradation under ambient conditions and, at least in the first two cases, molecules that show improved solubilities. Even so, acenes and other organic semiconductor compounds generally lack thermal stability, especially in the presence of light and air. Only a few examples of acene-based devices operating at temperatures in excess of 100 °C are known. An OFET constructed using 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSpentacene) functioned well at lower temperatures but began degrading at 120 ºC.10 In the presence of a protective polymer package, a similar device began degrading at 180 ºC.11 Unmodified pentacene has been tested up to 210 ºC in a nitrogen atmosphere in absence of light, where a ~2.5-fold increase in mobility was seen from 30 ºC to 160 ºC.12 The availability of photostable acenes that can operate at elevated temperatures would offer new opportunities for thin film electronics including thin-film transistors on “hot” devices, as well as high temperature sensors, switches and solar cells. Here, we introduce an acene-based, small molecule organic semiconductor, 5,6,7trithiapentacene-13-one (TTPO), that shows excellent photooxidative and thermal stabilities. TTPO is prepared in one step from elemental sulfur and either pentacene-6,13-diol or 6(13H)-



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pentacenone. TTPO melts in air from 386 – 388 °C without decomposition, has an optical HOMO – LUMO gap of 1.90 eV and a high molar absorptivity (ε = 12408 M-1·cm-1 at 606 nm in dichloromethane). TTPO shows solubility in numerous solvents including chloroform, dichloromethane, THF, DMF, o-dichlorobenzene, and 1,2,4-trichlorobenzene, potentially enabling processing via methods like spin coating, blade coating and ink-jet printing, all of which require solutions or “ink” formulations. Its acid-free solutions are indefinitely stable against degradation. TTPO also shows excellent solid-state thermal stability (> 400 °C by TGA) and can be thermally evaporated to produce high-quality thin films. This compound is simple to purify and readily crystallizes. Preliminary experiments show variable temperature photovoltaic behavior with short circuit currents and efficiencies increasing by three orders of magnitude from 25 °C to 80 °C.

EXPERIMENTAL SECTION 6(13H)-Pentacenone (2). Compound 1 (0.500 g, 1.621 mmol) was dissolved in 50 mL of hydriodic acid (55-58 %, unstabilized) and 50 mL of glacial acetic acid. The mixture was heated to boiling under nitrogen for 5 hours. The reaction was cooled and the crude product was precipitated from solution using copious water, and then washed with 10 mL of saturated aqueous NaHSO3. The material was then filtered through a Büchner funnel, water washed and dried with MgSO4 to yield a brown/yellow powder (0.360 g, 75% yield, 90-95% pure). 1H NMR (500 MHz, CDCl3) δ (ppm): 9.00 (s, 1H), 8.11 (d, J = 8.2 Hz, 1H), 7.99 (s, 1H), 7.93 (d, J = 8.3 Hz, 1H), 7.65 (m, 1H), 7.61 – 7.54 (m, 1H), 4.74 (s, 1H). 13C NMR (126 MHz, CDCl3) δ (ppm): 185.41, 135.62, 135.60, 131.94, 130.38, 129.94, 129.40, 128.65, 127.13, 126.76, 126.15,32.58.


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6,13-Pentacenediol (3). Compound 1 (0.100 g, 0.324 mmol) was added to 5 mL of tetrahydrofuran and 1 mL of water. Sodium borohydride (0.100 g, 2.643 mmol) was added, and the mixture was stirred and brought to reflux under nitrogen for 3 hours. The solvent was removed by rotary evaporation and the product was precipitated from solution using water and then filtered through a Büchner funnel and dried to produce a white powder (0.095 g, 95% yield) as a mixture of cis and trans isomers. 1H NMR (300 MHz, CDCl3): δ (ppm): 8.1 (s, 1H), 7.9 (m , 1H) , 7.5 (m 1H), 6.6 (d, J = 6.2Hz, 1H), 6.1 (d, J = 6.2Hz, 1H) 6.0 (d, J = 6.2Hz, 1H), 5.8(d, J = 6.2Hz, 1H);

13

C NMR (75 MHz, CDCl3) δ (ppm): 136.9, 133.0, 127.9, 126.4, 125.0, 60.4

(consistent with a reported 13C spectrum13).

5,6,7-trithiapentacene-13-one (4) Method 1. Compound 2 (0.338 g, 1.148 mmol) and elemental sulfur (0.166 g, 5.177 mmol) were dissolved in 5 mL of N,N-dimethylformamide and heated to 153 ºC under nitrogen for 24 hours. The reaction was then cooled to room temperature and the crude product was precipitated from solution with water and filtered through a Büchner funnel. The solids were then washed with copious acetone or carbon disulfide to remove unreacted elemental sulfur and then with copious dichloromethane or chloroform to remove unreacted 6,13-pentacenequinone. Product 4 was isolated as a deep blue solid or a purple crystalline material with a metallic luster (0.192 g, 44% yield).

Method 2. Compound 2 (0.061 g, 0.195 mmol) and elemental sulfur (0.021g, 0.655 mmol) were dissolved in 5 mL 1,2,4-trichlorobenzene and heated to 214 ºC under nitrogen for 4 hours. The reaction was then cooled to room temperature and the crude product was precipitated from solution with methanol and then filtered through a Büchner funnel. The solids were then washed



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with copious acetone or carbon disulfide to remove unreacted elemental sulfur followed by copious dichloromethane or chloroform to remove unreacted 6,13-pentacenequinone. Product 4 was isolated as a deep blue solid or a purple crystalline material with a metallic luster (0.020 g, 26% yield).

Method 3. Compound 3 (0.100 g, 0.342 mmol) and elemental sulfur (0.020 g, 0.623 mmol) were dissolved in 5 mL of N,N-dimethylformamide and heated to 153 ºC under nitrogen for 24 hours. After cooling the mixture to room temperature, crude product was precipitated from solution using water and then filtered through a Büchner funnel. The solids were then washed with copious acetone or carbon disulfide to remove unreacted elemental sulfur and then with copious dichloromethane or chloroform to remove unreacted 6,13-pentacenequinone. Product 4 was isolated as a deep blue solid or a purple crystalline material with a metallic luster (0.070 g, 57% yield).

Method 4. Compound 3 (2.505 g, 8.019 mmol) and elemental sulfur (3.175 g, 99.017 mmol) were dissolved in 5 mL 1,2,4-trichlorobenzene and heated to 214 ºC under nitrogen for 4 hours. The mixture was then cooled to room temperature and the crude product was precipitated from solution with methanol and filtered through a Büchner funnel. The solids were then washed with copious acetone or carbon disulfide to remove unreacted elemental sulfur and then with copious dichloromethane or chloroform to remove unreacted 6,13-pentacenequinone. Product 4 was isolated as a deep blue solid or a purple crystalline material with a metallic luster (1.492 g, 48% yield) 1H NMR (500 MHz, CDCl3) δ (ppm): 8.95 (s, 1H), 8.79 – 8.73 (m, 1H), 8.26 – 8.20 (m, 1H), 7.91 – 7.83 (m, 2H). MS (LDI) calcd. m/z = 386.0, found m/z= 385.3. X-ray quality crystals were produced via slow evaporation of a saturated solution of 4 in chloroform.


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Theoretical methods. Calculations in this work were carried out using Gaussian 03. 14 For all species, frequency calculations were performed to ensure that convergence to a local minimum was achieved; full conformational searches were carried out to ensure that global minima were located, and stabilities of the closed shell restricted wavefunctions were also assessed. Geometries of species were optimized at the M05-2X/6-31G(d) level of theory.15 The similar method M06-2X is reported to show improved handling of dispersion interactions and has been used to study the mechanism and products of the thermal dimerization of pentacene species.16 During this study, the M05-2X method was able to locate a relatively strong complex of two pentacene molecules, not shown as a local minimum using the B3LYP method. Improved energies for all reaction species were calculated at the M05-2X/6-311+G(d,p) and B3LYP/6311+G(d,p) levels with the effects of solvent included by performing the calculations in the presence of dichloromethane. Solvation effects were calculated using the polarizable continuum model (PCM)17,18,19 as implemented in the Gaussian 03 software package. This was employed at the relevant levels of theory using the recommended radii for these calculations (i.e., the united atom topological model applied to radii optimized for the PBE0/6-31G(d) levels of theory).20 HOMO and LUMO energies were determined from the Kohn-Sham eigenvalues. However, since these methods are known to underestimate the magnitude of the LUMO energies (by 0.3 – 0.5 eV for substituted acenes21), HOMO-LUMO gaps were determined from the first excitation energy in a time-dependent DFT (TD-DFT) calculation rather than from the difference in the Kohn-Sham eigenvalues. A recent study has shown that these are suitable, low cost methods for the study of the electrochemical properties of substituted acenes. 22 Charge distributions were calculated using an NBO population analysis performed in GAUSSIAN at the M05-2X/631+G(d,p) level of theory.



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Crystallographic Methods: Diffraction data for an X-ray quality single crystal of TTPO were collected on a Bruker SMART Duo diffractometer using multilayer mirror monochromated Cu K radiation (1.54178 Å). X-ray data for TTPO; monoclinic space group P21/c; a=9.0844(6) Å, b=7.3730(5) Å, c=23.5319(15) Å,



3

; additionally, a

simulated powder XRD spectrum was obtained and is available in the supporting information.

Acquisition of Scanning Tunneling Microscopy Images. Powdered TTPO was deposited via physical vapor deposition from a tantalum crucible onto a Au(788) single crystal substrate, i.e. a vicinal surface of regularly spaced 3.9 nm wide close-packed Au(111) terraces. Experiments were performed at room temperature in an ultrahigh vacuum chamber with pressures less than 5 x 10-10 mbar. The gold substrate was atomically clean after repeated cycles of ion sputtering and annealing. Molecularly resolved images were acquired on a SPECS-Aarhus scanning tunneling microscope with a tunneling current and voltage of approximately 0.1 nA and +0.9 V sample bias, respectively.

Construction of OPV Devices. TTPO-based bilayer solar cells were constructed with TTPO as donor and C60 as acceptor, as illustrated in Figure 10. A 2.2 – 2.6 % aqueous solution of PEDOT:PSS was obtained from Sigma-Aldrich (high-conductivity grade) and filtered through a 0.45

-coated glass (ITO-140 nm, Glass-1.1 mm, resistivity-

)

was obtained from Delta Technologies and was diced into 15 mm by 15 mm chips. The chips were cleaned by ultrasonication in soap water for 10 min followed by a DI water rinse, then separate ultrasonications in acetone and IPA for 10 min each followed by a final DI water rinse for 5 min. The substrates were dried in an oven for 1 hour at 150 °C. An ICP oxygen plasma


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treatment (300/50W, plasma/bias power in Plasma-Therm 7900 etcher) was performed as a final cleaning step rendering the ITO surface hydrophilic. The filtered PEDOT: PSS solution was spin coated onto substrate at 4000 rpm and then baked for 20 min at 120 °C. Solid TTPO (70 mg) and C60 (90 mg) were loaded into separate tungsten boats and thermally evaporated at a starting base pressure of 1.2 x10-6 torr. The measured thicknesses were 75 nm for TTPO and 80 nm for C60. A 100 nm aluminum cathode was subsequently deposited in similar fashion. Cell areas varied from 0.045 to 0.06 cm2. An HP 4155 Semiconductor Parameter Analyzer was used for I-V measurements using a Dolan Jenner Fiber-lite incandescent halogen light calibrated to match the short circuit current under AM 1.5 sunlight. A 250W infrared lamp was utilized in order to heat the cells to approximately 80 °C.

Construction of TTPO-based FETs. Test transistors were fabricated on a 1-10 Ω-cm phosphorus-doped Si wafer with a 100 nm thermally grown SiO2 layer. The source/drain electrodes, 150 nm Au with a 30 nm Cr adhesion layer, were deposited by magnetron sputtering and patterned by lift-off. To achieve good electrical contact to the silicon wafer that is used as the gate electrode, the oxide on the back side of the wafer was removed using buffered oxide etch (BOE). Prior to TTPO coating, the substrates and electrodes were cleaned for 10 minutes using a piranha etch solution (2:1 H2SO4:H2O2) at 115 °C. After cleaning, the wafer was loaded into a thermal evaporator. Solid TTPO (40 mg) was loaded in a tungsten boat, and the thermal evaporator was pumped down to a pressure of 1.4 x10-6 torr. To start the evaporation, 600 W power was applied to the tungsten boat. TTPO evaporated cleanly to produce uniform 50±2 nm films as measured by a Dektak profilometer. After construction, a variable

temperature

experiment was carried out to assess the effect of temperature on carrier mobility. During these



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tests, the temperature of the chuck holding the wafer was swept from room temperature to 200 °C with the chuck itself acting as the gate electrode.

RESULTS AND DISCUSSION Synthesis and Purification of TTPO TTPO was synthesized by two different routes, both starting from the common precursor 6,13-pentacenequinone (1).

Quinone 1 was either reduced using HI/HOAc in a synthesis

modified from Athans, et al.23 to produce 6(13H)-pentacenone (2) in high yield, or was reduced to 6,13-dihydro-6,13-pentacenediol (3) using NaBH4 as detailed by Pramanik, et al24. Using conditions adapted from Goodings and co-workers in their synthesis of hexathiapentacene,25 either 2 or 3 can be dissolved in N,N-dimethylfomamide (DMF, b.p. 153 ºC) or 1,2,4-trichlorobenzene (TCB, b.p. 214 ºC) and then reacted with elemental sulfur to produce TTPO. The reaction mixtures were heated to boiling under nitrogen for either 4 hours in the case of TCB solvent or 24 hours in the case of the DMF solvent (Scheme 1). TTPO, 4, was isolated by precipitation using either water (for DMF reaction) or methanol (for TCB reaction). Crude product was purified by washing with copious quantities of either acetone or carbon disulfide to remove unreacted sulfur. This was followed by washing with either chloroform or dichloromethane to remove 1, a by-product of the reaction. TTPO was isolated as a metallic purple solid that produced a vibrant blue solution (Figure 1) when dissolved in a range of solvents including tetrahydrofuran, o-dichlorobenzene, DMF, chloroform and dichloromethane.


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Scheme 1. Synthesis of TTPO. Reagents and Conditions: a) N2, HI/HOAc, reflux, 6 h; b) N2, NaBH4, THF:H2O (5:1), reflux, 3 h; c) N2, DMF, S8, reflux 24 h; d) N2, TCB, S8, reflux, 4 h.

Crystal Structure

Figure 1. A saturated solution of TTPO in chloroform

X-ray quality crystals of TTPO were prepared by slow evaporation of a saturated solution in chloroform. Figure 2 shows an ORTEP image with ellipsoids of 50% probability. The crystal structure shows the unusual trithia functionality connected to the C10, C12 and C14 carbons of the pentacene backbone. Although the interior C=S bond is written as a double-bond and the



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exterior C-S bonds are written as single bonds in Scheme 1, the crystal structure indicates that all three C-S bonds have similar bond lengths (1.693 and 1.700 Å for the exterior C-S bonds and 1.719 Å for the interior C-S bond). Thus, the actual structure for TTPO must involve extensive delocalization of C-S  electrons.

Figure 2. TTPO crystal structure showing the crystallographic labeling scheme and displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radii.

Figure 3 shows a space-filling representation of TTPO molecules, based on our XRD results, making up one of the columnar stacks, which run throughout the crystal as well as the alignment of three adjacent 1D columnar stacks within the crystal. The  stacking distance between nearest molecules within a 1D column is 3.364Å. The direction of the column is not quite perpendicular to the plane of the molecules in the stack resulting in a “tilted” 1D columnar stack, with the oxygen and trithia substituents arranged in an alternating head-to-tail fashion. The stability of the crystal is enhanced by the parallel displacement of individual TTPO molecules within each column, enabling a large number of intermolecular CH-π interactions between adjacent, interdigitated stacks (Figure 3b).


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(a) (b)

Figure 3. X-ray crystal packing of TTPO: (a) space filling model of a tilted 1D columnar stack and (b) stacking of adjacent 1D columns.

Computational Modeling The density functional method M05-2X was used to investigate the structure, packing and electronic properties of both a discrete molecule of TTPO as well as 1D stacks of TTPO molecules. It was found that the bond lengths and angles in the calculated geometry of TTPO were in good agreement with those measured in the crystal structure. The geometry of a onedimensional stack composed of two TTPO molecules revealed several local geometry minima and saddle points. The global minimum geometry is the same as that seen in the crystal structure, in which the two molecules are arranged in a head-to-tail fashion offset by 1.2 Å along the longer axis of the plane of the molecule, with the O atoms above the S-S bond rather than directly above the central sulfur atom. Other local minima in which TTPO molecules are further offset also exist, but with higher energies. Geometries for 1D stacks in which the TTPO molecules are



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aligned in a head-to-head fashion show one or more imaginary frequencies, and are therefore not local minimum energy structures. Stacks made up of three and four TTPO molecules were also examined. Tilted 1D stacks composed of three and four TTPO molecules were similar to the two molecule tilted stack. For these cases, energies were also examined for the more symmetrical “zig-zag” stacks, which unlike the tilted stacks maintain a columnar direction that is perpendicular to the plane of the molecules in the stack. The energy difference for these two configurations in a stack of four TTPO molecules is less than 0.2 kcal/mol in favor of the tilted stack at the M05-2X/6311+G(d,p) level of theory, which is within the error of the calculation. The crystal structure likely maintains a tilted 1D columnar stack configuration due to stabilizing intermolecular interactions between adjacent stacks, like the previously mentioned C–H··· interactions. Figure 4 demonstrates calculated orbital diagrams for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the unit cell of the TTPO crystal. The starting molecular coordinates were taken from the crystal structure and optimized at the M05-2X/6-31G(d) level. A local geometry minimum, which closely resembles the unit cell, was found. This structure emphasizes the importance of C–H··· interactions in the X-ray crystal structure. Each molecule engages in eight C–H··· interactions, four as C–H donor and four as -acceptor. Both the HOMO and LUMO orbitals are highly delocalized. The HOMO (Figure 4a) indicates significant interaction of the co-facial π-systems (i.e.,  stacking) between the stacked molecules.


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Figure 4. M05-2X/6-311+G(d,p)//M05-2X/6-31G(d) orbital diagrams showing (a) the highest occupied molecular orbital (HOMO) and (b) lowest unoccupied moleuclar orbital (LUMO) of the unit cell of TTPO. Figure 5 depicts the distribution of charges within TTPO molecules calculated at the M05-2X/6-311+G(d,p) level on the two molecule TTPO offset stack. Even though TTPO is net neutral, significant charge character exists within the molecule. Much of the charge is localized on the interior sulfur atom (+0.41) and the carbonyl O atom (-0.57). These localized charges in turn cause opposite charge character at the respective carbon atoms adjacent to these sites (C=S 0.11; C=O +0.56). This alternating pattern of localized charges no doubt drives the observed head-to-tail stacking pattern observed in the crystal structure, while destabilizing the head-tohead configuration.



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Figure 5. Density functional theory NBO population analysis of TTPO based on a two molecule stack at the M05-2X/6-311+G(d,p)//M05-2X/6-31G(d) level of theory.

To further prove the accuracy of the models, a comparison was made between the measured distances of the XRD crystal structure and the calculated values. The centroid-centroid distance between two mean planes, as defined by the pentacene backbone of two TTPO molecules from the crystal structure, was shown to be 3.3663(13) Å. This compared with the analogous centroid-centroid measurement from the calculated two molecule TTPO offset stack at 3.3864 Å shows a high degree of confidence in the models.

Scanning Tunneling Microscopy To better understand organic-metal interfaces, individual TTPO molecules were deposited on the close-packed terraces of the gold surface and then analyzed by scanning tunneling microscopy (STM). Understanding the structure of the organic-metal interfaces with molecular level precision potentially allows for tailoring of those interfaces in order to maximize charge carrier transport.


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STM imaging of TTPO molecules on the close-packed gold surface was challenging due to significant molecular diffusion at room temperature. The TTPO molecules were highly mobile on the gold surface until they interacted with a herringbone reconstruction elbow site, an energetically favorable location for molecular adsorption. The well-studied herringbone reconstruction of the Au(111) terraces presents itself in the STM images as a sequence of characteristic ‘V’ patterns.26 The ‘V’s are marking the transition lines between the hcp and fcc stacking of the Au(111) top layer; i.e. Shockley Partial dislocations. The highly reactive elbow sites at its bottom is an edge dislocation formed by one 5-fold coordinated Au atom, while all other Au surface atoms have 6 nearest neighbors.27,28



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Figure 6. (A) STM image of TTPO on a close-packed (111) terrace of the Au(788) surface. Scale bar = 10 nm. (B) High resolution STM image of alignment of five TTPO molecules at a elbow site of the herringbone surface reconstruction. Scale bar = 1 nm. (C) Height profile along white dotted scan line in image A, crossing a TTPO molecule at ~5 nm, and two monatomic steps at ~ 16 nm and 22 nm, respectively.


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In Figure 6, the close-packed gold (111) terrace with its herringbone reconstruction is visible. Blurred agglomerations of organized TTPO molecules are observed at numerous elbow sites. At one elbow site, there is a more stable molecular arrangement of five TTPO molecules (see inset of Figure 6 and Figure 7). Scan lines 1 and 2 of Figure 7B and C, respectively, indicate a length of approximately 2 nm for each assembled structure. A range of widths between 0.6 and 0.9 nm, and a range of heights between 0.17 and 0.21 nm are observed. The measurements are consistent with five TTPO molecules that are lying down on the close-packed gold surface, but not lying flat. The pentacene derivative 6,13-dichloropentacene is known to assemble on a close-packed gold surface in a more or less perfectly flat fashion.29,30 TTPO absorbs differently because of its asymmetrical nature, in contrast to the 4-fold symmetric DCP molecule. That is, the trithia bridge on one side of TTPO is expected to bind more strongly to gold than the ketone function on the opposite side. Although further STM and theoretical work will be required to fully elucidate the structures, we surmise that each assembled TTPO molecule has its trithia bridge angled down toward the gold surface with a small cant angle between the molecular plane and the gold surface. Cant angles are well known for self-assembled monolayers involving alkanethiols on gold, for example, a cant angle of ~ 28° is reported for octadecanethiolate on Au(111).

31

However, no examples exist for rigid sulfur-based structures like

TTPO. While a tilted TTPO structure is not expected to impact S-Au bonding, it may allow for additional S- interactions between adjacent molecules that stabilize the assembled superstructure.

Structural

variations in the TTPO derivative may allow for fine-tuning of the TTPO cant angle and ultimately improved charge carrier transport.



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Figure 7. (A) STM image of five TTPO molecules adsorbed on a Au(788) surface with profile scan lines 1 (dashed) and 2 (solid). Scale bare = 1 nm. (B) Scan profile line 1. (C) Scan profile line 2.

Physical and Electronic Properties TTPO in both its amorphous powder and crystalline forms has a melting point of 386 – 388 ºC in air. No decomposition is observed during melting. Thermogravimetric analysis of both the amorphous powder and crystalline forms of TTPO indicate an onset of evaporation at approximately 420 °C. The UV/vis spectrum of TTPO recorded in dichloromethane reveals a molar absorptivity ε = 12408 M-1·cm-1 at 606 nm. Based on the UV/vis spectrum, the optical HOMO – LUMO gap for TTPO is 1.90 eV, somewhat smaller than the 2.1 eV, optical gap for pentacene.32 Cyclic voltammetry (CV) studies of TTPO were performed (Figure 8) using a BAS100B electrochemical analyzer in a three-electrode single-compartment cell with a glassy carbon


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working electrode, Ag/AgCl reference electrode, a Pt wire as auxiliary electrode, tetrabutylammonium

hexafluorophosphate,

(TBAPF6) as

a supporting electrolyte,

o-

dichlorobenzene as solvent, a scan rate of 100 mV/s, and ferrocene as an internal standard. HOMO and LUMO energies, -5.27 and -3.56 eV respectively, were determined from the onsets of the first oxidation and the first reduction waves. The corresponding electrochemical HOMO – LUMO gap of 1.71 eV is 0.19 eV smaller than the optical HOMO-LUMO gap. Although time dependent DFT methods have previously been successful in the calculation of the electronic properties of substituted pentacenes,22 it is likely that they are not appropriate methods for TTPO derivatives due to the exotic trithia functionality. The method TD-B3LYP/6-311+G(d,p)//M052X/6-31G(d) was used to calculate the HOMO and LUMO energies of TTPO and the associated gap. While the calculated HOMO – LUMO gap of 1.99 eV shows decent agreement with the measured optical gap of 1.90 eV, calculated values for the individual HOMO and LUMO energies differ from the electrochemically measured values by up to 0.45 eV.

Figure 8. Cyclic voltammogram of a saturated solution of TTPO at room temperature using a freshly cleaned glassy carbon working electrode, TBAPF6 as supporting electrolyte, odichlorobenzene as solvent, ferrocene as internal standard, and a 100 mV/s scan rate. ACS Paragon Plus Environment


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TABLE 1. Optical, electrochemical and computational data for TTPO Low Energy λmax (nm)

Eg,Optical (eV)

E1/2 [Ox] mV

E1/2 [Red] mV

EHOMO (eV)

ELUMO (eV)

Eg,EChem (eV)

606 nm

1.90

1058

-750, -1133

-5.27

-3.56

1.71

ELUMO,DFT a (eV)

Eg,DFT(eV)a

-3.73

1.99

EHOMO,DFT a (eV)

-5.72 a

DFT based on a four-molecule tilted stack calculated at the TD-B3LYP/6-311+G(d,p)//M05-2X/6-31G(d) level in

a dichloromethane solvent field implementing the PCM solvation method.

Fabrication and Behavior of Preliminary TTPO-Based Bi-layer Photovoltaics

Figure 9. Schematic cross-section of photovoltaic device

Figure 9 shows a schematic of preliminary TTPO-based bi-layer photovoltaic devices. Four devices were constructed via thermal evaporation of TTPO, C60 and aluminum on top of PEDOT:PSS that was spin-coated onto ITO coated glass. Under illumination from an incandescent halogen lamp at room temperature, the best performing cell (0.045 cm2) had a short circuit current of 0.35 µA, a short circuit current density (Jsc) of 7.8 µA/cm2, an open circuit voltage (Voc) of 0.45 V, a fill factor of 0.12 and a power conversion efficiency (PCE, %) of 1.26x10-3. Upon heating the devices to approximately 80 °C using an IR lamp, the short circuit


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current densities increased between 4-fold and 141-fold with concomitant 4-fold to 160-fold increases in PCE. Most organic photovoltaic devices show an increase in efficiency but then a decrease as temperature increases,33 due to a combination of increased hopping conduction followed by induced structural disorder and degradation. We postulate that TTPO thin films could show increased order at elevated temperatures due to a tightening of the intermolecular interactions observed in the crystal structure.

Fabrication and Behavior of Preliminary TTPO-Based FETs

TTPO film

S

D

SiO2 insulator n-Si Gate Figure 10. Schematic cross-section of the fabricated transistors

Preliminary field effect transistors and photovoltaics have been constructed using TTPO in order to assess the semiconductor properties of this compound. A schematic cross-section of the OFET device is shown in Figure 10. The device was fabricated using thermal evaporation of TTPO. The carrier mobility of the FETs was measured from room temperature up to 200 °C and for a range of gate voltages from 0 to - 50 V. Figure 11 shows the transistor behavior for a device tested between 25 and 200 °C with Vgs = - 50 V. As temperature increases, the drain source current increases, corresponding to increasing conductivity of the device.



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Figure 11. Variable temperature transistor behavior of a TTPO device at Vgs= -50 V

TTPO does not conduct at room temperature unless a high gate voltage is applied. Figure 12 shows the extracted carrier mobilities as a function of temperature. The mobilities increase in a more or less linear fashion up to 150 °C, which would be consistent with hopping conduction . The relatively low mobility values are consistent with unoptimized devices with relatively large interfacial surface tensions between TTPO either the SiO2 dielectric or the Au electrodes, or both and are consistent with the low observed photovoltaic conversion efficiency. Even so, these preliminary devices show a nearly ten-fold increase of carrier mobility when heated from room temperature to 150 °C, a result which has rarely been observed in organic FETs.


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Figure 12. Extracted carrier mobilities versus temperature for a TTPO thin film transistor device

CONCLUSIONS This work describes the first synthesis of a new pentacene-based organic semiconductor, 5,6,7trithiapentacene-13-one (TTPO). TTPO is soluble in numerous organic solvents, has a high molar absorptivity and is indefinitely stable against degradation in acid-free solution. In the solid state, TTPO shows excellent thermal stability. It melts in air from 386 – 388 °C without decomposition. TTPO is simple to synthesize and purify, readily crystallizes and can be thermally evaporated to produce highly uniform thin films. Its cyclic voltammogram reveals one reversible oxidation and two reversible reductions between +1.5 and -1.5V. TTPO has optical and electrochemical HOMO – LUMO gaps of 1.90 and 1.71 eV, respectively. The crystal structure for TTPO has been solved and its unique parallel displaced, head-to-tail packing arrangement has been examined and explained using high-level density functional theory. STM studies indicate that TTPO assembles on close-packed gold with a small, acute cant angle, likely smaller than those observed with SAMs composed of alkanethiols on Au. Preliminary and nonoptimized TTPO-based bi-layer photovoltaic devices are active. Upon heating from 25 to 80 °C,



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an increase in short circuit current densities is observed with a concomitant 4-fold to 160-fold increase in power conversion efficiencies. TTPO has the potential to be used in thin-film electronic devices that require operation over a wide range of temperatures such as thin-film transistors on “hot” devices as well as high temperature sensors, switches and solar cells.

ACKNOWLEDGEMENTS This work was funded by the National Science Foundation through the Nanoscale Science & Engineering Center for High- rate Nanomanufacturing (grant NSF EEC-0832785). The authors also acknowledge Dr. Weimin Lin for his assistance.

Supporting Information Available: 1H and

13

C NMR spectra, 2D gCOSY NMR spectrum,

laser desorption ionization mass spectrum, UV-vis spectrum, thermal gravimetric analysis, X-ray crystal structure data, CIF data file, simulated XRD, ATR-FTIR spectra and Gaussian archive entries. This information is available free of charge via the internet at http://pubs.acs.org.

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.


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