Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications1, 2, 3, 4, 5. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice6. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility7. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33 Å to 3.08 Å. We believe that 3.08 Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04 Å has been achieved through intramolecular bonding8, 9, 10). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from 0.8 cm2 V−1 s−1 for unstrained films to a high mobility of 4.6 cm2 V−1 s−1 for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.
At a glance
Inorganic semiconductor films can exhibit lattice strain due to lattice mismatch by heteroepitaxy, with strained films showing increased charge carrier mobility6. Recently, the bandgap of graphene has been modified using uniaxial strain, providing a way of tuning electronic properties11. Similarly, straining the crystal lattice of organic semiconductors could provide a way of changing the electronic properties of organic semiconductors. For organic semiconductors, the π–π stacking distance between cofacially stacked molecules critically affects the charge carrier mobility7. Therefore, a strained lattice that results in shorter π–π stacking distances can potentially greatly increase the charge transfer integral, which describes the electronic wavefunction overlap between adjacent molecules7. At the same time, the displacement distance along other molecular axes is also important in charge transport7.
Changing the chemical structure of the organic semiconductor, or the chemical composition of the dielectric interface, has been shown to affect organic semiconductor molecular packing. These are the most common methods used to design high-performance organic-semiconductor devices12, 13, 14, 15, 16. Non-synthetic techniques offer an alternative way of controlling the molecular packing. Some organic semiconductors exhibit temperature-induced solid-state transitions in molecular packing, while others exhibit a thin-film packing structure that is different from the bulk molecular packing motif17, 18. The small-molecule organic semiconductor TIPS-pentacene has a ‘brick-wall’ packing motif with a π–π stacking distance of 3.33 Å as both a thin film and a bulk crystal19. It is stable in air, processible through solution-based methods and possesses high hole mobility20, 21. Here, we report the formation of strained molecular packing in TIPS-pentacene thin films using our previously reported solution-shearing method22. The π–π stacking distance decreased from 3.33 Å to 3.08 Å for the solution-sheared thin film prepared at a shearing speed of 8 mm s−1. The hole mobility was increased from 0.8 cm2 V−1 s−1 for unstrained films prepared at a shearing speed of 0.4 mm s−1 up to as high as 4.6 cm2 V−1 s−1 in strained thin films prepared at a speed of 2.8 mm s−1.
During the solution-shearing process, a shearing plate drags the solution across a heated substrate while keeping the bulk of the solution between the plate and the substrate, with only the evaporation front exposed (Fig. 1a and Supplementary Fig. 1)22. Fig. 1b–f shows cross-polarized optical microscope images illustrating the texture of thin films as a function of shearing speed, ranging from 0.4 mm s−1 to 8 mm s−1. For a shearing speed of 0.4 mm s−1 (Fig. 1b and Supplementary Fig. 2), oriented, millimetre-wide TIPS-pentacene domains with lengths of up to a centimetre are observed with the long axis parallel to the shearing direction. Shearing at a speed of 1.6 mm s−1 (Fig. 1c) results in domains that are narrower and shorter in length than those resulting from shearing at 0.4 mm s−1. This trend in TIPS-pentacene morphology continues up to a shearing speed of 2.8 mm s−1 (Fig. 1d).
When the shearing speed is raised to 4 mm s−1, a comet-shaped morphology (known as ‘transcrystalline’) is observed (Fig. 1e). These features are hundreds of micrometres wide and several millimetres long. Such transcrystalline features have been observed for polymers crystallized in a temperature gradient and for small molecules crystallized in a concentration gradient23, 24. A concentration gradient between the bulk solution and the evaporation front also forms during solution shearing25. Increasing shearing speed to 8 mm s−1 results in an isotropic, spherulitic film (Fig. 1f); no preferential orientation relative to the shearing direction is observed for the spherulites.
In addition to altering TIPS-pentacene thin film crystallite orientation (texture) and domain sizes, solution-shearing speed also has an effect on the molecular packing in the thin film. Grazing incidence X-ray diffraction (GIXD) experiments were performed to characterize the texture and molecular packing. As the shearing speed increases, the (101) d-spacing decreases incrementally, from 7.70 Å to 7.26 Å, and concurrently the (010) d-spacing increases incrementally, from 7.83 Å to 8.13 Å (Fig. 2 and Supplementary Table 1). These d-spacing changes occur for the in-plane lattice parameters (a and b) only, because the vertical layer spacing—the (001) d-spacing—does not systematically change with shearing speed (Supplementary Fig. 3). To our knowledge, a non-synthetic methodology to induce incremental thin-film lattice strain has not been reported.
To study lattice strain evolution in detail, high-resolution GIXD measurements were performed to determine the crystal structure (Supplementary Fig. 4). Changes in the positions of GIXD peaks is due to the incrementally changing lattice strain in the TIPS-pentacene film as a function of solution-shearing speed. The low signal-to-noise ratio at higher shearing speeds is caused by the loss of anisotropy of the crystallite texture. Peak broadening is seen with increasing shearing speed, which is consistent with the much smaller crystallite sizes caused by fast drying at high shearing speeds. It is also possible that the entire film is not uniformly strained, causing a distribution of superimposed peaks. Owing to peak broadening beyond the shearing speed of 4 mm s−1, we cannot exclude the possibility that the peak shift seen from 4 mm s−1 to 8 mm s−1 is the result of the coexistence of differently strained crystallites. The changes in lattice strain occurring at various shearing speeds are not due to differences in film thickness (Supplementary Fig. 5). We eliminated the possibility of a solvent inclusion complex leading to the observed change in the unit cell dimensions, because heating the strained film up to 160 °C did not change the molecular packing, despite this temperature being much higher than the boiling point of the solvent used, toluene. Furthermore, the film sheared at 0.4 mm s−1 showed a GIXD pattern identical to the pattern of an evaporated thin film that was not exposed to solvent.
The unit cell geometry of the solution-sheared TIPS-pentacene thin films was obtained from the peak positions measured from the GIXD images using a least-square-error optimization procedure (Supplementary Table 1)19. The TIPS-pentacene in-plane unit cell geometry becomes increasingly more oblique with increasing shearing speed, while the in-plane unit cell area remains comparatively constant, which is also consistent with the absence of solvent inclusion in the unit cell (Supplementary Table 1).
To assess the stability of this strained crystal lattice, we exposed the strained films to toluene vapour. This induced a reorganization of the molecular packing without affecting crystallite texture or domain size, indicating that the strained structure is metastable. Upon exposure to toluene vapour a change in GIXD peak position occurred (Supplementary video 1). After an hour of toluene exposure, the lattice spacing of strained films became similar to that of the evaporated thin film, indicating strain relief (Supplementary Fig. 6). On the other hand, the strained films are thermally stable up to 160 °C (Supplementary Video 2).
The molecular packing of TIPS-pentacene has been previously determined for evaporated thin films, and was found to be identical to the bulk crystal structure19. Two distinct molecular pairs, labelled T1 and T2, can be seen (Fig. 3a). Here, the same crystal structure determination technique was applied to films sheared at a speed of 8 mm s−1; the method is described in Supplementary Fig. 7. The crystal structure for the film solution-sheared at 8 mm s−1 displayed a more oblique molecular packing motif than did that of the evaporated thin films (Fig. 3b). The π–π stacking distance of the T1 molecular pair (Fig. 3b) decreased from 3.33 Å in the evaporated thin film to 3.08 Å in thin films prepared at a shearing speed of 8 mm s−1. This is significant because the π–π stacking distance has an exponential impact on the transfer integral between cofacially arranged π-conjugated organic semiconductor molecules7. Density functional theory calculations show that the nearest-neighbour charge transfer integral of the T1 molecular pair increases from 11.7 meV in the evaporated thin film to −36.9 meV in the strained thin film (Supplementary Fig. 8 and Supplementary Table 2). The lattice strain induced in the TIPS-pentacene molecular packing also results in a molecular displacement along the molecular long and short axes. These displacements also affect the charge transfer integral, so the overall charge transfer integral may not necessarily increase with a shorter π–π stacking distance. Density functional theory calculations were performed to check the impact of molecular long and short axis displacements (Supplementary Fig. 9). We also note that the strained T2 molecular pair has a larger π–π stacking distance of 3.79 Å, and a low charge transfer integral of 0.429 meV (Fig. 3b and Supplementary Table 2). We hypothesize that charge transport in the strained TIPS-pentacene lattice is one-dimensional and mainly through T1 molecular pairs, primarily along the b axis, which is also the direction of elongated crystallite growth.
We then measured the impact of lattice strain on the electronic performance of TIPS-pentacene thin film transistors (TFTs). The in-plane hole mobility was measured in a bottom-gate, top-contact field effect transistor configuration. The hole mobility in these TFTs is higher parallel to the shearing direction than perpendicular. Although the in-plane unit cell geometry becomes increasingly oblique as shearing speed increases (Fig. 4a and Supplementary Table 1), the average mobility along the shearing direction as a function of shearing speed increased to a maximum of 2.1 cm2 V−1 s−1 for thin films prepared at a shearing speed of 2.6 mm s−1 (Fig. 4b and Supplementary Table 3). The average mobility decreased for faster shearing speeds, to a value of 0.47 cm2 V−1 s−1 for samples prepared at a speed of 8 mm s−1. We attribute the decrease in the average mobility observed at shearing speeds above 2.6 mm s−1 to the decrease in crystallite size and alignment (Fig. 1), causing an increase in the number of grain boundaries and contribution from the weaker electronic overlap of the T2 molecular pair respectively26. The best-performing device showed a mobility of 4.6 cm2 V−1 s−1, reached at a shearing speed of 2.8 mm s−1 (Fig. 4c). Mobility distributions for high-performance TFTs are shown in Supplementary Fig. 10. These TFTs were also stable for at least a year of storage in the dark and under vacuum, showing only a 10% decrease in mobility; additionally, no relaxation of lattice strain was observed for a film stored in air for more than three months (Supplementary Table 4). As a reference, the previously reported maximum mobility for a TIPS-pentacene TFT at room temperature was 1.8 cm2 V−1 s−1, and the observed threshold voltages here may be different from those reported previously owing to differences in device structure, material purity and interface properties21, 25, 27. We attribute our higher mobility to the improved electronic coupling between molecules in the strained films. This conclusion is further supported by the observation that decreased mobilities were observed for partially strain-relieved films while maintaining the same film textures. The strained films were annealed with toluene vapour for 5 min, which resulted in a partial relief of the strain, as confirmed via GIXD (Supplementary Fig. 11). Longer exposure times formed cracks in the thin film. A decrease in TFT mobility after toluene vapour annealing was observed for all films prepared at speeds above 0.4 mm s−1 (Supplementary Fig. 12). There is no change in the film texture as confirmed by cross-polarized optical microscopy and atomic force microscopy, so we attribute the decreased mobility to relaxed molecular packing strain (Supplementary Fig. 11).
During solution shearing, the liquid film thickness decreases as a function of increasing shearing speed24. A steeper temperature gradient forms in thinner liquid films, because the bottom of the film is in contact with a heating source, while the top surface is exposed to ambient temperature. The solvent evaporates faster in the thinner liquid film, which results in faster solution flow towards the growing crystal front (Supplementary Fig. 1). The faster solvent evaporation rate also gives the growing crystal front less time to reach an equilibrium state. We hypothesize that the fast crystallization and solvent evaporation kinetically traps metastable states. These states can then be relaxed to lower energy states using toluene vapour annealing. Faster rates of solvent evaporation coupled with velocity and concentration gradients are probably the key factors giving rise to strained crystals. We found that solution shearing can introduce lattice strain and charge transport enhancement in other organic semiconductors, demonstrating the utility of solution shearing as a general technique for tuning molecular packing (Supplementary Fig. 13).
In summary, we have demonstrated that solution shearing can be used to incrementally change the molecular packing of the TIPS-pentacene molecule. The π–π stacking distance of the T1 molecular pair decreased from 3.33 Å to 3.08 Å with increasing shearing speed, and density functional theory calculations show an increase in the charge transfer integral beyond that calculated for the evaporated thin film. The direction of elongated crystallite texture growth is along the b axis, which is also the direction for faster charge transport. We were able to achieve a maximum mobility as high as 4.6 cm2 V−1 s−1, which is more than twice the highest maximum value (1.8 cm2 V−1 s−1) previously reported27. However, we did not observe a monotonic increase in mobility as a function of increasing shearing speed owing to the formation of smaller and less oriented crystallites at high shearing speeds, which introduce grain boundary regions and domains with weaker electronic overlap28, 29. Additionally, molecular displacements along the molecular long and short axes affect the charge transfer integral, so the overall charge transfer integral for all organic semiconductors may not necessarily increase with deceasing π–π stacking distance. The advantage of our solution-shearing approach is that it allows us to access many possible new packing structures that are not possible through other commonly used methods. Therefore, in a given molecular system it provides more opportunities to enhance the charge transport. Modifying the molecular packing by tuning the extent of lattice strain instead of changing the chemical structure provides an alternative method of studying charge transport as a function of packing. We expect this work to provide a new way to improve the charge carrier mobility of organic semiconductors for practical applications.
The TIPS-pentacene (received from 3M) was used without further purification. PTS and OTS were purchased from Sigma-Aldrich and used as received (storage under an argon atmosphere to prevent hydrolysis). Highly doped n-type silicon wafers (resistivity <0.005 Ω cm) with a 300-nm thermally grown silicon oxide gate dielectric film (capacitance of the gate dielectric per unit area, Cox = 10 nF cm−2) were used as the substrates for TFT fabrication and GIXD.
Substrate preparation and characterization
The silicon wafers were cleaned in a Piranha solution (70/30 vol./vol. H2SO4/H2O2; a highly oxidative solution) for 25 min. The bottom device substrate (2 × 2 cm2) was treated with PTS to ensure proper wetting for the TIPS-pentacene solution and to reduce surface charge traps. PTS treatment was accomplished by immersing the Piranha-cleaned wafer into a toluene solution of PTS (3 wt%) and heated at 90 °C for 15 h. The substrate was subsequently removed from PTS solution and was sonicated for 2 min in toluene. It was then gently wiped with a sponge tip and rinsed sequentially with toluene, acetone and isopropanol. The water contact angle of the PTS treated surface ranged from 72° to 74°. The typical roughness (root mean square) of the surface was 0.3–0.5 nm.
For the top shearing plate, a silicon wafer with 300-nm thermally grown oxide was treated with a monolayer of OTS according to a method we published previously for forming a highly hydrophobic crystalline OTS monolayer22, 30. The OTS treatment is important to ensure deposition of TIPS-pentacene only on the bottom PTS-treated substrate. The water contact angle of the OTS modified surface ranged between 102° and 104°, and the roughness of the surface ranged between the root-mean-square value of 0.2 nm and 0.5 nm.
Solution shearing of TIPS-pentacene films
The TIPS-pentacene solution was prepared at a concentration of 8 mg ml−1 in toluene. Both the device substrate and the shearing plate were held in place by vacuum while the device substrate was mounted on a resistively heated stage (a thermoelectric module from Custom Thermoelectric) held at 90 °C. After placing about 40 μl cm−2 of TIPS-pentacene solution on the device substrate, the shearing plate was lowered with a micromanipulator to make contact with the solution. The device substrate was horizontal while the shearing plate was placed at a tilt angle of 8° from the horizontal. The gap distance (d, shown in Supplementary Fig. 1) between the device substrate and the shearing plate was fixed at 100 μm. The shearing plate was translated at different velocities by a stepper motor. The resulting sheared film was left on the heating stage for 2–3 min at 90 °C to remove residual solvent.
Solution-sheared film characterizations
The solution-sheared films were characterized using a cross-polarized optical microscope (Leica DM4000M). Thickness measurements were performed with a Dektak 150 profilometer, purchased from the Veeco Metrology Group. Tapping-mode atomic force microscopy was performed using a Multimode Nanoscope III (Digital Instruments/Veeco Metrology Group).
GIXD was performed at the Stanford Synchrotron Radiation Lightsource on beamlines 2-1, 7-2 and 11-3. On the 11-3 beamline, a photon energy of 12.73 keV was used, and the scattering intensity was recorded on a two-dimensional image plate (MAR-345) with a pixel size of 150 μm (2,300 × 2,300 pixels), located at a distance of 400 mm from the sample centre. The distance between the sample and the detector was calibrated using a LaB6 polycrystalline standard. The incidence angle was chosen in the range 0.10°–0.12°. The beam size was 50 μm × 150 μm, which resulted in a beam exposure on the sample 150 μm wide over the entire length of the 10-mm sample. The data was distortion-corrected (theta-dependent image distortion introduced by planar detector surface) before performing quantitative analysis on the images. Numerical integration of the diffraction peak areas was performed with the software WxDiff19. The overall resolution in the GIXD experiments, dominated by the sample size, was about 0.01 Å−1. The photon energy on beamlines 2-1 and 7-2 was 8 keV. Beamline 2-1 was used for high-resolution specular scanning with a point detector. Analysis software included Origin (from Microcal) and GSAS (Rietveld analysis). Beamline 7-2 was used for high-resolution grazing incidence scans, and uses a Huber high-resolution diffractometer.
The electrical characterization was performed on bottom-gate, top-contact field effect transistors created by thermally evaporating 40-nm gold source and drain electrodes on the solution-sheared samples through a shadow mask. All field effect transistors were fabricated and tested in ambient conditions, with exposure to light and air. The devices were stored in the dark under vacuum. The electrical characteristics of the TIPS-pentacene films were measured using a Keithley 4200-SCS semiconductor parameter analyser. Transfer characteristics (IDS as a function of VG, where IDS is the source–drain current and VG is the gate voltage) were tested for each transistor, and output characteristics (IDS as a function of VD at various gate voltages, where VD is the drain voltage) were measured for representative transistors. The IDS–VD curves were collected with VG increasing from −100 V to 0 V and with VD sweeping from 0 V to −100 V. The IDS–VG curves were collected with VG decreasing from 50 V to −100 V at a constant VD of −100 V. The saturation mobility μ was extracted from the slope of the transfer curve VG–√IDS, where IDS = μWCox(VG – VT)2/2L, where W ( = 1,000 μm) and L ( = 50 μm) are the channel width and length, respectively, Cox is the capacitance of the gate dielectric per unit area, and VT is the threshold voltage.
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We thank A. Salleo, J. E. Anthony, H. Li, A. Sokolov and J. Rivnay for discussions. We thank J. E. Anthony and M. M. Nelson of 3M Corp. for providing high-purity TIPS-pentacene. This publication was partially supported by the National Science Foundation DMR-Solid State Chemistry (DMR-0705687-002), the Samsung Advanced Institute of Technology, the Global Climate and Energy Project at Stanford University (SPO 25591130-45282-A) and the Air Force Office of Scientific Research (award number FA9550-09--0256). E.V. thanks the Eastman Kodak Corporation Kodak Fellows Program for support. Z.B. acknowledges support from the David Filo and Jerry Yang Faculty Fellowship from Stanford University. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of Stanford University, the Sponsors of the Global Climate and Energy Project, or others involved with the Global Climate and Energy Project.
- Supplementary Information (1.2M)
This file contains Supplementary Methods and a Discussion, Supplementary Figures 1-13 with legends, Supplementary Tables 1-4, legends for Supplementary Movies 1-2 and additional references.
- Supplementary Movie 1 (3.3M)
This movie shows in-situ strain relief of GIXD peaks during toluene vapor annealing.
- Supplementary Movie 2 (3M)
This movie shows in-situ heating TIPS-pentacene thin films, showing no strain relief of the (010) GIXD peak.