DOI: 10.1021/cg901533c
Control of Morphology and Orientation of a Thin Film Tetrabenzoporphyrin (TBP) Organic Semiconductor by Solid-State Crystallization
2010, Vol. 10 1848–1853
Naoki Noguchi,*,† Shen Junwei,† Haruki Asatani,† and Masakuni Matsuoka‡ †
Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama Japan 227-8502, and ‡Department of Chemical Engineering, Tokyo University of Agriculture and Technology 24-16, Naka-cho 2, Koganei-shi, Tokyo 184-8588, Japan Received December 8, 2009; Revised Manuscript Received February 8, 2010
ABSTRACT: The grain morphology of tetrabenzoporphyrin (TBP) after solid-state crystallization by annealing from tetrabicycloporphyrin (CP) was found to be influenced by the annealing temperature, but the grain orientation was not significantly changed by the temperature. From the thin film XRD measurements, the b axis, which is the most favorite direction to enhance the carrier mobility, was found to grow in parallel to the substrate, but at high temperatures, the growth direction was slightly inclined from the substrate surface. The FET performance showed dependency on the annealing temperatures, showing that the carrier mobility was three times higher for the film prepared at lower temperature compared to that at higher temperatures.
Introduction Organic semiconductors can be categorized into two classes of materials: small molecules and polymers. Polymeric semiconductors have an advantage, since they have higher solubility in organic solvents, so that they have better flexibility for solution process applications. The largest problem for polymer types is the difficulties of purification. Once impurities are introduced into a polymer chain, the chain is not possible to purify. On the other hand, for small molecule types, there are efficient methods of purification, such as sublimation, chromatography, and recrystallization. But most of the small molecule semiconductors with high mobility have good crystallinity, leading to poor solubility in common solvents. Organic devices, produced by utilizing solution processes, such as photovoltaic cells (OPVs) and field-effect transistors (OFETs), promise advantages such as low cost, large area, flexibility, and lower temperatures for device fabrication. In general, organic semiconductors are known to possess small carrier mobility compared to inorganic ones, because of the molecular nature of organic materials. Recently, the performances of organic devices have been improved. For example, the efficiency of OPVs has improved by more than 6%,1 and several papers reported that OFETs showed a mobility as high as that of amorphous silicon (0.1-1.0 cm2/V 3 s).2-4 The main advantage of using organic materials is their low manufacturing cost. To reduce further the costs of manufacturing, solution-based processes, such as spin coating or inkjet printing techniques are required. A likely problem for some materials is, as mentioned earlier, their low solubility in common organic solvents; however, evaporation processes can be used to improve this. Some small molecule compounds with a large π-conjugated ring structure, such as phthalocyanines, pentacene, rubrene, *Corresponding author. Phone: þ81-45-963-3269. Fax: þ45-963-3947. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 02/18/2010
and porphyrins, have been demonstrated as good semiconductors.5-8 All of these compounds show good crystallinity. For organic semiconductors, the charge transport is closely related to their crystallinity, crystal orientation, and grain size. To increase the device performance, controlling of film morphology, crystallinity, and orientation is important. For example, large grains can decrease the numbers of boundary gaps between the grains and, thus, increase OFET mobility.9 On the other hand, since for OPVs the charges are transported only a very short distance, they do not require large grains but defect-free films. In general, the charge transport in organic semiconductors is achieved by the π orbital overlapping of the conjugated molecules, and the carrier mobility of the organic semiconductor devices would be substantially enhanced if the molecules are aligned so as to increase the π orbital overlapping along the direction of carrier flow. Some trials to control a crystal orientation of OFET were reported:10-13 mechanical alignment methods such as rubbing treatment and using a photoaligned substrate. A controlling method of substrate temperature during sublimation was also reported.14 One of the ways to solve the low solubility problem is to fabricate devices which use a precursor for small molecule organic semiconductors.14-17 The semiconductor tetrabenzoporphyrin (TBP) has very unique characteristics among small molecule semiconductors. TBP can be processed from solution using a precursor. Tetrabicycloporphyrin (CP), which can dissolve in organic solvents, easily becomes TBP by an annealing process, as shown in Figure 1. TBP is expected as an ideal semiconductor. Similarly to other small molecule semiconductors, it is very important to control the film morphology and crystal orientation to achieve good device performances. Therefore, in this study, thin film X-ray diffraction and an optical microscope are used to characterize TBP film structures and to observe grain morphology, respectively. In addition, FET properties are also measured to ensure the possibility as an OFET. r 2010 American Chemical Society
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Figure 1. (Retro) Diels-Alder reaction from CP to TBP.
Experimental Method The precursor CP used in this study was synthesized in a manner similar to that of a previous experiment.18 It was used after purification with column chromatography and recrystallization. Then, it was dissolved in chloroform at room temperature (25 °C) to saturation. The solution was then spin-coated on the surface of a substrate glass plate of 1.0 mm thickness in a nitrogen atmosphere. The size of the plate was 25.4 25.4 mm2. The spin-coating was operated at 1000 rpm, over 30 s, and most of the solvents evaporated during the coating to leave an amorphous CP layer. Then the plate was transferred onto a hot plate (HCT basic, IKA Werke GmbH & Co. KG), which was located in a glovebox filled with nitrogen, to heat the CP layer at constant rates, and the annealing time was defined as the period the layer spent on the hot plate. The thickness of the TBP layer was reduced approximately by 20% from that of CP. In the experiments, the thickness of annealed TBP layers was kept constant at around 100 nm. A hot-stage (Japan HIGH TECH CO., LTD: Type 10039 L) and a polarized light microscope were used to decide the annealing time. For this case, nitrogen was blown into the hot-stage at a rate of 0.4 L/min. The thin film X-ray diffraction (XRD) was measured with an XRD (RINT2000: X-ray; Cu KR, Rigaku) in air at room temperature. A polarized light microscope, a differential interference contrast microscope, and a scanning electron microscope (SEM) were used to observe the film morphology. The FET device was fabricated in the same way that Aramaki reported,14 except for the heating time. On top of a 300 nm thick layer of SiO2 thermally grown over a heavily doped n-type Si substrate, source and drain electrodes were formed by photolithography (lift-off) of Au (95 nm)/Cr (5 nm). The channel lengths were 10 or 100 μm with a width of 500 μm. The FET properties were measured with a semiconductor parameter analyzer (Agilent 4155C) in a glovebox filled with nitrogen.
Results and Discussion Morphology Observation with Optical (Polarized Light) and Scanning Electron Microscopes. TBP has been investigated by many authors to develop as thin film transistors.2,14-17,19,20 In the present study, the surface morphology of thin film crystals of TBP was observed with a polarized light microscope, a differential interference contrast microscope, and a scanning electron microscope (SEM) after the annealing process. The annealing times were decided from an optical microscope observation with the hot-stage to be 20 min for the case of annealing at 210 °C, 20 min at 180 °C, and 240 min at 150 °C, respectively. There were remarkable differences in the morphology observed with a polarized light microscope. The grain size when annealed at 150 °C was larger than those at the other two annealing temperatures. There were also differences in the morphology observed with the differential interference contrast microscope and a SEM. The grain surface prepared at 210 °C was apparently rougher compared to those of the
Figure 2. TBP grain morphology after annealing at several temperatures.
Figure 3. Unit lattice structure of TBP.
other two temperatures. The grain morphologies at the three temperatures are shown in Figure 2. The grain size prepared at 150 °C was on average about 5-8 μm, and each grain was homogeneous in color, indicating that the molecules were regularly oriented inside of the grain. On the other hand, the grains prepared at 180 and 210 °C were too small to determine the size with an optical microscope, but grain dots were observed clearly. Hence, the grain size is estimated to be around a few micrometers. The SEM pictures also supported the result. In general, the grain size is determined by the balance between the nucleation and growth rates. Higher annealing temperatures tend to increase both of them, and high growth rates leave rough surfaces. For the case of 210 °C, the nucleation rate was very high; thus, a large number of grains were born, and the grains did not have enough time to grow up, because they were bounded by adjacent grains quickly. For the case of 150 °C, even though the nucleation rate would be low, the grains had enough time to grow up and smooth surfaces resulted. For annealing at 180 °C, an intermediate behavior between those at 150 and 210 °C was observed. Chemical Structure. The crystal structure of TBP has been reported,18 and its unit lattice structure is shown in Figure 3. Since the charge transport in organic semiconductors is dependent on the degree of π orbital overlapping of the conjugated molecules, the carrier mobility of the organic devices would be substantially enhanced if the molecules are aligned so as to increase the π orbital overlapping along the current flow direction.21 Illustrations of conjugated molecules along each axis are shown in Figure 4, in which the gray
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Figure 4. Arrangement of TBP molecules in a crystal, as viewed from along the a to c axis.
Figure 6. Calculation HOMO surfaces of the eight TBP molecules. White and red surfaces represent the phases of the orbital.
Figure 7. Schematic figure of thin film X-ray diffraction. Figure 5. Stacking model of eight TBP molecules used for the calculation.
surfaces correspond to the van der Waals radii of the carbon atoms. Along the a axis, the distance between the two molecules is short, but the π orbital overlapping area is very small. On the contrary, for the b axis, the distance is also short and the π orbital overlapping area is very large. The TBP molecules are planar, and they are orderly stacked in the b axis with an angle which is parallel to the (114) or (1,-1,4) plane. For the c axis, the π orbital overlapping area is small, but the distance between the two molecules is short. Based on this molecular arrangement, the most favorite orientation to enhance the carrier mobility for TBP can be concluded to be along the b axis. Simulation. TBP is a p-type semiconductor; therefore, the HOMO surface is also as important for the carrier transport by hopping as the π-π stacking surfaces. If the HOMO and π-π stacking surfaces orient in the same direction, the hole mobility would be large. Before measurement of the OFET characteristics, computer simulation of the TBP crystal was conducted. Computational Details. A cluster model with 8 TBP molecules according to their layer stacking in the crystal structure18 was constructed and is shown in Figure 5. The electronic structure was evaluated at the HF/6-31G level with a single point self-consistent field (SCF) calculation. All calculations for the cluster were performed with the GAUSSIAN03 program package.22 The calculated HOMO surfaces are shown in Figure 6, and the red and white surfaces correspond to the antibonding orbital between the contiguous molecules. The calculations demonstrate that the HOMO surfaces exist in parallel to the flat TBP molecules, and it corresponds that HOMO surfaces are aligning to the b axis direction.
Thus, the b axis is expected to be the most favorite direction to transport a carrier from the point of view of the π-π stacking and the HOMO surfaces. Thin Film X-ray Diffraction (XRD). Schematic illustrations of thin film X-ray diffraction are shown in Figure 7. From the diffraction patterns of “out-of plane (OP)”, surfaces parallel to a substrate are observed, while, from “in plane (IP)” patterns, information can be acquired for surfaces perpendicular to the substrate. The thin films prepared at three temperatures of 150, 180, and 210 °C were analyzed with thin XRD, and the patterns are shown in Figure 8, where quartz glass plates were used as the substrates. Different diffraction patterns between the OP and the IP were clearly observed. They also show different intensities in the diffraction patterns as a function of the annealing temperatures. The notable peaks are 2θ = 8.5, 10.3, 12.1, 14.6, 25.0, and 30.1°. They correspond to the (1,0,-1), (101), (002), (200), (113), and (114) planes, respectively. As mentioned in the section Chemical Structure, the (114) plane is set to be parallel to the b axis. However, the (113) plane is use to discuss the direction of the b axis, since the peak is clearer compared to that of the (114) plane. The angle between the (113) and (114) planes is 7.15°. Comparing the patterns between IP and OP, the most remarkable diffarence was found to be that the (113) of IP was stronger than that of OP at each temperature. This result indicates that the (113) plane is oriented perpendicularly to the substrate. It is, however, not possible to decide the orientation from the observations. Three remarkable peaks of the (1,0,-1), (101), and (002) in addition to the (113) plane were noticed. In Figure 9, the blue, green, and yellow surfaces in the morphologies correspond to the (1,0,-1), (101), and (002) planes, respectively. While the red plane coresponds to the (113) plane.
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Figure 8. Thin film X-ray diffraction patterns of TBP after annealing at several temperatures.
Figure 9. Morphology of the TBP compound orientation in a thin film after annealing at several temperatures. This morphology was calculated with the software Mercury2.2 (CCDC).
For the case of annealing at 150 °C, a strong peak representing the (1,0,-1) planes and weak peaks for the (101) and (002) planes are present in the OP patterns. On the other hand, a weak (1,0,-1) peak and stronger (101) and (002) peaks are observed in the IP patterns. These results reflect the majority of grains orientating. The (1,0,-1) planes are oriented parallel to the substrate. In the film annealed at 150 °C, the grains would settle in a manner as shown in Figure 9a, where the b axis should be set parallel to the substrate. For the film annealed at 180 °C, three notable peaks were observed both in the OP and in the IP patterns, but the strong peaks of the (1,0,-1) and (101) planes were observed only in the OP pattern. This may suggest that the (002) planes are almost perpendicular to but slightly inclined to the substrate. Therefore, the film annealed at 180 °C would have a majority of grains settling as show in Figure 9b, where the b axis should also be set parallel to the substrate. Finally, for the film annealed at 210 °C, a unique point was that the strong peak of the (1,0,-1) plane was observed in the OP pattern. For the IP analysis, the pattern was very similar to that annealed at 180 °C, and three notable peaks were observed having similar peak intensities. These results indicate that the (1,0,-1) plane is oriented almost in parallel to but inclined by a small angle to the substrate. Therefore, the film annealed at 210 °C would have a majority of grains settling as shown in Figure 9c, where the b axis should be slightly inclined from the substrate surface. In this study, the orientation was not varied significantly depending on the annealing temperatures, but as Shea20 et al. mentioned, other factors, such as film thickness, substrate annealing speed, and solvents, could affect the TBP film orientation. Further research is needed to reveal the effects of operating conditions on TBP crystallization.
FET Property. The values of mobility (μ) and threshold 05 -Vg plot, by voltage of TBP were determined from the Isat using eq 1. I sat ¼ ðCi W =2LÞμðV g -V t Þ2
ð1Þ
where Isat, Ci, W, and L represent the saturation current, the capacitance of the insulator layer per unit area, the channel width, and the channel length, respectively. In this study, W = 500 μm and L = 10 or 100 μm. The experiments using the hot-stage confirmed that there was no difference in the crystallization rate and morphology between those on the quartz glass and annealed Si surfaces. Typical FET characteristics measured at 150, 180, and 210 °C are shown in Figure 10, and FET parameters are shown in Table 1. The carrier mobility calculated using eq 1 was found to increase as the crystallization temperature decreased, as shown in Figure 11. Xu et al.23 reported that grain boundaries should be the cause of such mobility decreases for TBP. They also showed that rough surface domains decreased the potential energy. In the present research, the orientations were found to be similar for the films prepared at the different annealing temperatures; the b axis, which is the most favorite direction to enhance the carrier mobility, grew in parallel to the substrate surface. But the grain size was different; low annealing temperatures increased the domain size and, thus, decreased the number of domain boundaries between the channels. This result is also supported from the previous research.9,23 Lower annealing temperature also created a smooth grain surface. The dependence of mobility on the annealing temperature can be related to crystal morphology, grain size, and surface roughness. A much lower annealing temperature may increase the mobility, but Noguchi24 et al. have shown that the crystallization process of CuTBP needs temperatures higher than
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Figure 10. I/V characteristics of TBP OFET properties after annealing at several temperatures. Table 1. OFET Parameters of TBP after Annealing at Several Temperatures (L = 10 μm) annealing temp mobility on/off ratio Vt
°C
210
180
150
cm2/V 3 s V
1.6 10-2 5 104 5
5.9 10-2 2 105 15
7.0 10-2 2 105 5
have provided an advantage for carrier mobility. The FET performance was also influenced by the annealing temperature; actually, the carrier mobility was three times higher, as the annealing temperature was lower. These differences were concluded to have been caused by the grain morphologies but not by the crystal orientations.
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Figure 11. Mobility after annealing at several temperatures.
about 145 °C. Thus, 150 °C was chosen as a minimum annealing temperature in the present research. Conclusions In a solid state crystallization (annealing) process of TBP from CP, the grain morphology was found to be influenced by the annealing temperature. Annealing at lower temperatures made larger grains and decreased the amount of grain boundaries existing between the channels. It also resulted in smooth surfaces. Additionally, the direction along the b axis was found to have an advantage over other directions to enhance the carrier mobility from the point of π-π stucking and HOMO surfaces. The grain orientations were not changed significantly by the annealing temperature. The b axis grew almost in parallel to the substrate surface; hence, this must
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