Oligothiophene Derivatives Functionalized with a Diketopyrrolopyrrolo

Oct 15, 2008 - The field effect mobilities for annealed DHT6DPPC6 and DHT6DPPC12 ... Citation data is made available by participants in Crossref's Cit...
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J. Phys. Chem. C 2008, 112, 17402–17407

Oligothiophene Derivatives Functionalized with a Diketopyrrolopyrrolo Core for Solution-Processed Field Effect Transistors: Effect of Alkyl Substituents and Thermal Annealing Mananya Tantiwiwat,† Arnold Tamayo,‡ Ngoc Luu,‡ Xuan-Dung Dang,‡ and Thuc-Quyen Nguyen*,‡ Department of Physics and Center of Polymers and Organic Solids and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: July 31, 2008; ReVised Manuscript ReceiVed: August 22, 2008

Two new oligothiophene derivatives bearing a diketopyrrolopyrrole core, 2,5-di-n-hexyl-3,6-bis(5′′-nhexyl[2,2′;5′,2′′]terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC6) and 2,5-di-n-dodecyl-3,6bis(5′′-n-hexyl[2,2′;5′,2′′]terthiophen-5-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DHT6DPPC12), and their use in solution-processed organic field effect transistors are reported. Depending on the type of alkyl substituent and film annealing temperature, the crystal grain sizes and interlayer spacing vary as observed using atomic force microscopy and X-ray diffractometry, respectively. These changes in film morphology and interlayer spacing lead to an order of magnitude difference in the field effect mobilities. The field effect mobilities for annealed DHT6DPPC6 and DHT6DPPC12 films are 0.02 and 0.01 cm2/V s, respectively. Introduction Organic field effect transistors (OFETs) have attracted much attention in recent decades due to their potential application in electronic devices such as sensors,1,2 displays,3-5 electronics displays,3,4,6 and radio frequency identification tags.7-9 Organic semiconductors offer several advantages over their inorganic counterparts including low cost, light weight, flexibility, large area of device fabrication,10,11 and chemical tunability.12,13 Despite these advantages, the charge mobilities of thermally deposited films remain low compared to amorphous silicon. The low charge mobility is the result of weak intermolecular forces in the solid state, which typically produce disordered polycrystalline films.14,15 Organic materials used in OFETs may be conjugated polymers or small molecules. Conjugated polymers are attractive because they can easily be processed from solution.16-18 Solution-processed poly(3-hexylthiophene) (P3HT) FETs, for example, have been reported to have field effect mobilities as high as 0.1 cm2/V s.19-22 However, conjugated polymers suffer from batch-to-batch variation in terminal end groups, molecular weight, molecular weight distributions, and impurities, which can lead to large differences in mobilities.23 In contrast, thin films from small molecules prepared via thermal deposition exhibit higher charge mobilities than polymer films due to better molecular packing,9,16,24-26 with mobilities exceeding 1 cm2/V s.27-29 Recently, there has been an increasing interest in combining the solution processability of conjugated polymers with the high molecular order of small molecules. The solutionprocessed route enables large area and low temperature methods of production.17,18,25,30 Additionally, solution-processed small molecules offer several advantages over conjugated polymers, including the ease of purification, functionalization, and reproducibility. Recently, solution-processed small molecule FETs * Corresponding author. E-mail: [email protected]. † Department of Physics. ‡ Center of Polymers and Organic Solids and Department of Chemistry and Biochemistry.

Figure 1. (a) Chemical structures of DHT6DPPC6 and DHT6DPPC12 and (b) schematic drawing of the top electrode FET.

have been reported to have charge mobilities ranging as high as 0.01-1.5 cm2/V s.31-36 Thiophene derivatives containing diketopyrrolopyrrole (DPP) units have been reported to self-assemble into organized and persistent domains from the solution process.37-39 Furthermore, the solubility and electronic properties of these DPP derivatives can be tuned by alkyl side chains and number of the thiophene units. Incorporating thiophene oligomers with the DPP chromophore can yield materials suitable for the fabrication of solution-processed organic solar cells.38 In this work, thin film morphology and FET characteristics of two oligothiophene derivatives containing a DPP core (Figure 1), which differ by the number of methylene units in the alkyl substitutents, are reported. OFETs fabricated using these two materials as obtained directly from solution exhibit hole mobilities of ∼10-3 cm2/V s. Higher hole mobility can be achieved by thermal annealing. Thin film morphologies as a function of molecular structure and thermal history were investigated using atomic force microscopy (AFM), and solidstate interlayer distances were obtained using the X-ray diffraction (XRD) technique. Correlations between thin film morphologies, molecular packing, and field effect mobilities were observed.

10.1021/jp8068305 CCC: $40.75  2008 American Chemical Society Published on Web 10/15/2008

DHT6DPPC6 and DHT6DPPC12 for OFETs

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Experimental Section DHT6DPPC6 and DHT6DPPC12 were prepared following a previously reported synthetic route.37 All materials were purchased from Sigma-Aldrich and used as received. Thermal Analysis. Thermal analysis was performed using a differential scanning calorimeter (DSC) (TA Instrument, Model Q20). Samples between 3 and 6 mg were weighed into aluminum calorimetry pans and hermetically sealed. The samples were then analyzed at a scan rate of 10 °C/min. Thin Film Characterization. AFM images were obtained in tapping mode using a Multimode microscope with a Nanoscope IIIa controller (Veeco) operated in a nitrogen glovebox with silicon probes having a resonant frequency of ∼75 kHz and a spring constant of 1-5 N/m (Budget Sensors). Interlayer spacing was studied using the XRD technique. Thin film XRD spectra were recorded using an X’Pert Phillips Material Research Diffractometer (MRD) at 45 kV and 40 mA with a scanning rate of 0.004 deg/s, and Cu KR radiation (with wavelength λ ) 1.5405 Å) with a 2θ-ω configuration. Device Fabrication and Characterization. OFETs with a device structure of Si/SiO2/DHT6DPPC6 (DHT6DPPC12)/Au were prepared. First, silicon substrates with 150 nm of silicon dioxide were cleaved into 1.0 × 1.0 cm2 pieces and cleaned with piranha solution, acetone, and isopropyl alcohol. Prior to use, the substrates were treated in a UV-ozone cleaner for 45 min. Solutions of DHT6DPPC6 and DHT6DPPC12 in chloroform (0.5%, w/v) were prepared under nitrogen. The solutions were stirred and heated at 60 °C overnight to ensure that the solutes were thoroughly dissolved. The DHT6DPPC6 (or DHT6DPPC12) solution, passed through a Whatman 0.45 µm PTFE membrane filter, was spun cast onto the cleaned SiO2 surface at 3000 rpm. Film thickness of ∼35 nm was confirmed using a CPII AFM microscope (Veeco Instruments). Prior to the electrode deposition, the films were annealed on a hot plate at various temperatures for 30 min; then they were removed from the hot plate and allowed to cool down inside a glovebox. FETs were fabricated using the top contact geometry (channel length L ) 20 µm, width W ) 2 mm). Source and drain electrodes were made of Au (85 nm) thermally deposited at a pressure of 9 × 10-7 Torr using a shadow mask. SiO2 (150 nm) and doped Si were used as dielectric layer and gate electrode, respectively (Figure 1b). The electrical characteristics of the FETs were measured under ambient conditions using a Signatone 1160 series probe station with Aligent Tech ICS Lite software. Results and Discussion Photophysical and Thermal Properties. The absorption spectra of DHT6DPPC6 and DHT6DPPC12 films on quartz substrates spun cast from chloroform are shown in Figure 2. The bands are characterized by a peak at ∼400 nm, attributed to the oligothiophene component, and another at ∼600 nm, which is due to the charge transfer associated with the DPP core. A shoulder at ∼720 nm is due to strong molecular interactions in the solid state, and is absent in the solution absorption spectrum (Supporting Information). The shoulder is more intense in DHT6DPPC6. Annealing the film does not result in any significant change in the film absorption spectra. DSC was used to study the thermal properties of DHT6DPPC6 and DHT6DPPC12 (Figure 3). Large melting peaks at 252 and 231 °C are observed for DHT6DPPC6 and DHT6DPPC12, respectively. Additionally, weaker transitions occur for the two compounds at ∼207 and ∼116 °C, which are attributed to the melting of the alkyl side chains. The transition temperature decreases by

Figure 2. UV-vis absorption spectra of DHT6DPPC6 and DHT6DPPC12 spun-cast films.

Figure 3. DSC of DHT6DPPC6 and DHT6DPPC12.

91 °C when the hexyl group is replaced with a dodecyl group. Similar transitions have been observed for oligothiophenes (or small molecule semiconductors) that have alkyl side chains to increase solubility,40 and in poly(3-alkylthiophenes).41-44 The phase transitions have significant effects on the film morphology, as will be discussed in the next section. Thin Film Characteristics. AFM and XRD were used to characterize the film morphology and the molecular packing of DHT6DPPC6 and DHT6DPPC12 films. Thin films of both materials were prepared from 0.5% (w/v) solutions spun cast onto SiO2/Si substrates. DHT6DPPC6 films were then annealed at 150, 180, 200, and 240 °C. DHT6DPPC12 films were annealed at 100, 150, and 180 °C. Thermal annealing was done for 30 min under nitrogen. The topographic images of as-cast and annealed DHT6DPPC6 films are shown in Figure 4. The as-cast film comprises short fiberlike structures, which are approximately 70 nm wide and 80-300 nm long. Upon thermal annealing at 150 °C (Figure 4b), the fiberlike structures are replaced with flat crystalline domains, which grow larger and eventually flatten at 200 °C (slightly above the phase transition temperature of DHT6DPPC6). The crystalline domains of the film annealed at 200 °C have an average width of 700 nm (Figure 4c). This film also has an approximate average roughness of 1.5 nm, which is smaller when compared to the roughness of ∼2.2 nm of the as-cast film. Annealing the sample at 240 °C increases the crystalline domains to larger than 1 µm; however, the grooves between the domains are formed with a depth of ∼25 nm, approximately the thickness of the film (Figure 4d). This result implies that annealing at temperatures higher than 200 °C leads to a noncontinuous DHT6DPPC6 layer, which might disrupt the charge transport in FETs. The shapes of the domains do not correspond to the features of the underlying substrate because the surface topography of the Si/SiO2 substrate is very flat and featureless (Supporting Information). The surface topographic images of the as-cast and annealed DHT6DPPC12 films are shown in Figure 5. The morphology of the as-cast DHT6DPPC12 film comprises isolated fiberlike structures with a width of ∼160 nm (Figure 5a) that become

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Figure 4. 2 µm × 2 µm AFM images of DHT6DPPC6 films (a) as-cast and annealed at (b) 150, (c) 200, and (d) 240 °C.

Figure 5. 2 µm × 2 µm AFM images of DHT6DPPC12 films (a) as-cast and annealed at (b) 100, (c) 150, and (d) 180 °C.

more defined and well packed upon annealing at 100 °C (Figure 5b). Gaps of ∼2.5 nm are observed throughout the film. Annealing at 100 °C also reduces the surface roughness from ∼1.0 to 0.4 nm. The films annealed at 150 and 180 °C have approximate roughnesses of 0.9 and 0.8 nm, respectively. Unlike the DHT6DPPC6 samples, increasing the annealing temperature to 180 °C does not change the surface features significantly and the samples do not exhibit deep grooves between domains. Holes of ∼3.0 nm are observed throughout the film annealed at 150 °C (Figure 5c). Similarly, holes of ∼3.3 nm are dispersed throughout the film annealed at 180 °C (Figure 5d). However, taller features of 1.9-4.5 nm are also observed. The length of the DHT6DPPC12 molecule along the axis of the thiophene

units as estimated from modeling is approximately 4.3 nm, while it is 4.0 nm along the axis of alkyl side chains. These taller features might indicate a monolayer of DHT6DPPC12; however, the orientation of the molecules with respect to the substrate is still unclear. Comparison between the AFM images of DHT6DPPC6 and DHT6DPPC12 indicates that long alkyl chains influence the film morphology. Thin film XRD was used to obtain the intermolecular spacing, d, in the film as a function of the alkyl chain length and the annealing temperature. The XRD results of DHT6DPPC6 films obtained after annealing at different temperatures are displayed in Figure 6. One observes that the diffraction angles (the peak location) and the peak intensity increase in the following order:

DHT6DPPC6 and DHT6DPPC12 for OFETs

Figure 6. XRD patterns of DHT6DPPC6 films as-cast and annealed at 150, 200, and 240 °C.

the as-cast film, film annealed at 150 °C, film annealed at 240 °C, and film annealed at 200 °C. The increase in the peak intensity implies a higher degree of crystallinity,45 while the diffraction angles correspond to intermolecular spacing. In this case, d signifies the distance from the center of a DHT6DPPC6 molecule in the solid state to the next. The intensity of the ascast film diffraction peak at 2θ ) 3.47° is much weaker than those of the annealed samples. Using the Bragg equation, λ ) 2d sin θ (λ ) 1.54 Å), the intermolecular spacing, d, is calculated to be 25.4 Å. As the annealing temperature increases to 150 °C, the peak intensity increases and the diffraction peak is observed at 2θ ) 3.62°, indicating a d-spacing of 24.4 Å. The film annealed at 200 °C shows an even stronger diffraction peak at 2θ ) 3.74°, indicating a d-spacing of 23.6 Å, the smallest among the DHT6DPPC6 films. The decrease of d-spacing suggests that the molecules pack closer in the solid state upon thermal annealing, implying that the alkyl side chains interdigitate more tightly.46 Increasing the annealing temperature to 240 °C reduces the diffraction peak to 3.68°, indicative of a d-spacing of 24.0 Å. This slight increase in d-spacing can be explained by the fact that the annealing temperature has approached the melting point of DHT6DPPC6; therefore, the film might begin to melt. This increase in the d-spacing is also accompanied by the crystalline domain separation observed in the AFM results (Figure 4d). Overall, the XRD results of the DHT6DPPC6 films are consistent with the morphological transformation observed via AFM. In particular, the annealed films are highly crystalline with large crystal domains and small d-spacing. The thin film XRD results of DHT6DPPC12 also show shifts in the diffraction angles and the change in the peak intensities upon thermal annealing (Supporting Information). For DHT6DPPC12, we find that the d-spacings are 33.8, 30.7, and 29.9 Å for the as-cast and annealed films at 100 and 180 °C, respectively. The d-spacing of DHT6DPPC12 is 8.40 Å larger than that of DHT6DPPC6, implying a distance of a little more than five carbon-carbon single bond lengths less. This difference can only be caused by the change in the length of the alkyl chains connected to the DPP core. The approximate lengths of DHT6DPPC6 and DHT6DPPC12 molecules along this axis are 2.10 and 3.96 nm, respectively. The results suggest that the d-spacing obtained from XRD corresponds to the distance between layers spaced by the akyl chains connected to the DPP core, not the alkyl chains on the thiophene rings, which are similar for DHT6DPPC6 and DHT6DPPC12 molecules. FET Characteristics. Given the considerable morphological and intermolecular spacing changes that take place upon film annealing, we anticipated significant changes in the device characteristics with annealing temperature.26,47-49 OFETs were fabricated using a top contact geometry with a channel length (L) ) 20 µm and width (W) ) 2 mm. Both DHT6DPPC6 and

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17405 DHT6DPPC12 devices demonstrate typical p-channel FET behavior. When a negative gate bias is applied from 0 to -40 V, the source-drain current (IDS) increases according to the change in the magnitude of the negative gate voltage. I-V characteristics were collected for as-cast and annealed DHT6DPPC6 and DHT6DPPC12 devices at different temperatures. Only a sample of the annealed DHT6DPPC6 device characteristics is shown in Figure 7. The output curves are the plots of IDS as a function of source-drain voltage (VD) at various gate biases (VG) (Figure 7a), while the transfer curves are the plots of IDS as a function of VG (Figure 7b). The output curves display expected saturation behavior as VD > VG. The field effect mobility (µFET) was calculated in the saturation regime (VD ) -80 V) using the equation24

ID ) (WCi/2L)µFET(VG - VT)2 where Ci is the capacitance per unit area of SiO2 dielectric layer and VT is the threshold voltage. DHT6DPPC6 devices exhibit a significant increase in hole mobility after thermal annealing before the electrode deposition. The mobility of the as-cast DHT6DPPC6 device is ∼4.7 × 10-4 cm2/V s; however, as the DHT6DPPC6 films were annealed at temperatures below 200 °C, the mobilities increased up to 6.2 × 10-3 cm2/V s and further to 1.6 × 10-2 cm2/V s when the annealing temperature reached 200 °C, and then decreased slightly to 1.2 × 10-2 cm2/V s when the annealing temperature reached 240 °C. This trend can be explained by change in the thin film morphology discussed in the previous section. Annealing may increase the degree of film crystallinity while reducing defects and intermolecular spacing, hence leading to improved charge mobility.26,49-51 For these samples, the annealed films have much larger grain size and smaller d-spacing; thus, these improvements facilitate better charge transport.51-53 At the phase transition temperature ∼206 °C, the DHT6DPPC6 molecules are allowed to pack with large platformlike crystal domains (Figure 4c) with the smallest d-spacing of 23.6 Å, therefore yielding the highest hole mobility (∼0.016 cm2/V s) among all DHT6DPPC6 devices. Annealing past the phase transition temperature decreases the mobility slightly to 0.012 cm2/V s. This phenomenon can be explained by the increase in spacing between the crystal domains (Figure 4d), which causes discontinuity at the interface between DHT6DPPC6 and SiO2. In OFETs, charge carriers are transported within the first few monolayers at the interface between the organic semiconductor and the dielectric layer;54,55 thus, these film interruptions could be responsible for the slight drop in the mobility of the devices. Figure 8 summarizes the relationship between the change in the field effect mobilities and the intermolecular spacing of the molecules in the solid state. It can be noted that the field effect mobilities closely correlate to the d-spacing of in DHT6DPPC6 films. Each error bar was computed from several devices. Similarly, the DHT6DPPC12 device mobility is improved by thermal annealing. The as-cast DHT6DPPC12 device shows a mobility of 3.9 × 10-3 cm2/V s; however, after thermal annealing at 100 °C, close to the DHT6DPPC12 transition temperature, the mobility increases to 9.1 × 10-3 cm2/V s. It decreases slightly to 8.0 × 10-3 cm2/V s at 180 °C and decreases drastically after thermal treatment beyond 200 °C. The plot of the change in field effect mobilities and the intermolecular spacing of the molecules in the solid state of DHT6DPPC12 shows trends similar to those observed for DHT6DPPC6 (Supporting Information). These results support the hypothesis that optimal performance of DHT6DPPC6 and DHT6DPPC12 is achieved when the films are annealed approximately at their

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Figure 7. Output I-V characteristic curves of the DHT6DPPC6 FET device annealed at 200 °C at different gate biases (left) and the transfer plot at constant VD ) -80 V (right).

Figure 8. Averaged field effect mobilities (blue squares) and d-spacings (red circles) of DHT6DPPC6 devices as a function of annealing temperatures.

transition temperatures and that the field effect mobilities of the materials depend on grain size of the crystal domain and interlayer spacing of the molecules in the solid state. Even though the field effect mobility can be optimized by thermal annealing, the problems encountered in these devices are their low Ion/Ioff ratios. Without any surface treatment, the device only achieves Ion/Ioff as high as 103. Octadecyltrichlorosilane (OTS) has been previously reported to improve field effect mobility and decrease leakage current.56,57 Treating oxide dielectric layer with OTS reduces its surface energy and changes the property of the SiO2 layer from hydrophilic to hydrophobic.58,59 Also, OTS treatment has been shown to affect the degree of film crystallinity and also increase the carrier mobility.58-60 Thus, OTS treatment is used to increase the low Ion/Ioff value by reducing the off current and increasing the on current for DHT6DPPC6 devices. The cleaned substrates were soaked in 0.001 M OTS solution in hexane for 30 min and rinsed with hexane prior to material spin casting. The OTS surface treatment further improved the mobility to 0.02 cm2/V s for the DHT6DPPC6 devices. Moreover, the off current in the OTStreated device reduced by an order of magnitude, thus the improving Ion/Ioff value to 104. Since AFM and XRD techniques do not provide information regarding the gate dielectric-organic semiconductor interface, where the charge transport takes place, it is important to examine the surface topography of an ultrathin film. The topographic images of DHT6DPPC6 deposited from a 0.05% (w/v) solution onto OTS-treated and untreated SiO2 surfaces were collected. At this concentration, the substrate is not completely covered by the organic layer (Supporting Information). The film thicknesses are approximately 3.8 and 7 nm for the untreated and the OTS-treated SiO2 surfaces, respectively. The OTS-treated film consists of larger crystalline domains, and the domains are better connected than those of the film deposited onto the untreated SiO2 substrate. Similar results have been observed for other organic semiconductors.61,62 The AFM results of ultrathin films provide the film morphology close to the dielectric

layer and the organic semiconductor interface and, hence, provide insight into the origin of the improved mobility measured in OTS-treated devices. The thin film XRD patterns of DHT6DPPC6 on OTS-treated and untreated SiO2 surfaces show the same diffraction angle (peak position), indicative of similar d-spacing; however, the peak pattern obtained from the film on OTS-treated SiO2 surface has a slightly higher intensity, which suggests a higher degree of crystallinity (Supporting Information). Thus, AFM and XRD results of thin films support that the increase in the field effect mobility upon treating the SiO2 substrate with OTS is due to an increase of the crystal domain size and overall degree of crystallinity and that the electronic changes are not due to a decrease of intermolecular spacing. Conclusions In summary, we have designed, synthesized, and characterized two new oligothiophene derivatives bearing a DPP core (DHT6DPPC6 and DHT6DPPC12) for use in solution-processed FETs. Thin film morphologies and FET characteristics were investigated as a function of alkyl chain length and thermal annealing. Annealing the films prior to electrode deposition improves the field effect mobilities of the devices by increasing the size and regularity of the crystalline domains and reducing the interlayer spacing of both DHT6DPPC6 and DHT6DPPC12. OTS treatment on Si/SiO2 substrates also increases the current on/off ratio. Increasing the alkyl chain length increases the intermolecular spacing and, hence, reduces the field effect mobility. This is the first demonstration of a DPP-based small molecule for FET application. Multiple substitution sites allow for tuning solubility, leading to ease of processing, and control of thermal properties and intermolecular spacing in the solid state. These features argue in favor of using DPP as a versatile platform for the design of novel organic small molecule semiconductors for electronic applications. Acknowledgment. We thank the Office of Naval Research Young Investigator Program and the Department of Energy for the financial support. M.T. thanks the Royal Thai Government Scholarship for financial support. Supporting Information Available: Absorption spectra of DHT6DPPC6 and DHT6DPPC12, XRD patterns and mobility and d-spacing of DHT6DPPC12 as a function of annealing temperature, AFM images of a bare Si/SiO2 surface, and AFM images and XRD patterns of DHT6DPPC6 deposited onto OTStreated and untreated Si/SiO2 surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Crone, B.; Dodabalapur, A.; Gelperin, A.; Torsi, L.; Katz, H. E.; Lovinger, J.; Bao, Z. Appl. Phys. Lett. 2001, 78, 2229.

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