A Quantitative Correlation between the Mobility and Crystallinity of

Article pubs.acs.org/Macromolecules

A Quantitative Correlation between the Mobility and Crystallinity of Photo-Cross-Linkable P3HT Claire H. Woo,†,‡,& Claudia Piliego,‡,& Thomas W. Holcombe,§ Michael F. Toney,∥ and Jean M. J. Fréchet*,†,‡,§,⊥ †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States ∥ Stanford Synchrotron Radiation Lightsource, Menlo Park, California 94205, United States ⊥ King Abdullah University of Science and Technology, Thuwal, Saudi Arabia 23955-6900 ‡

S Supporting Information *

ABSTRACT: The performance of polymer field effect transistors (FETs) can vary by orders of magnitude by applying different processing conditions. Although it is generally believed that a higher degree of crystallinity results in a higher mobility, the correlation is not straightforward. In addition, the effect of cross-linking on polymer thin film microstructural order is relatively unknown. This study investigates the effect of thermal annealing and UV-initiated photo-cross-linking on the FET performance and microstructural order of a photo-cross-linkable P3HT derivative. Our results demonstrate that while cross-linking did not disrupt the overall crystallinity of the polymer thin film, the photo-cross-linking process likely induced doping in the semiconductor layer, leading to the absence of saturation behavior in the FET. Annealing after cross-linking slightly improved the FET performance but only minimally affected the microstructural order of the polymer film since the 3D morphology had been “locked in” during the first cross-linking step. Importantly, annealing and cross-linking simultaneously was a successful method to preserve polymer crystallinity while also achieving effective cross-linking. Using newly developed quantitative X-ray analysis techniques, our study established a quantitative correlation between FET charge mobility and thin film crystallinity.

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INTRODUCTION The performance and reliability of solution-processed organic electronics are rapidly advancing, allowing organic field-effect transistors (OFETs) to become a reality in emerging commercial applications such as printable electronics and flexible optical displays.1 In the past decade, research efforts have focused on developing new semiconducting polymers for better charge carrier mobility. Despite the progress in synthesizing new p-type2 and n-type3 polymers, our understanding on the design criteria that should be followed to obtain high performance remain incomplete. One of the characteristics that has been identified as critical is the ability of the material to self-organize into regular structures.4 While it is possible to design organic materials that inherently possess a high tendency to self-organize, for example by using a rigid backbone or by choosing appropriate side chains, it is still difficult to control the dependence of self-organized structure and charge transport properties on processing conditions. For instance, the charge carrier mobility of polymer-based fieldeffect transistors can vary by orders of magnitude by applying different processing conditions such as casting solvent, substrate engineering, or post deposition treatments.5 However, the mechanism of how each processing step modifies the © 2012 American Chemical Society

semiconductor microstructure as well as the properties of the semiconductor/dielectric interface is relatively unknown. Field effect transistors are not only of great technological interest, but they also can be used as a material characterization tool since the charge carrier transport in thin film transistors is strongly dependent on the microstructure of the semiconducting layer.6 Transistor measurements therefore allow an indirect and qualitative estimation of the film microstructure at the buried semiconductor/dielectric interface, where charge transport takes place. Up to now, an accurate and quantitative assessment of the degree of crystallinity at this interface has been limited by the lack of appropriate and high-resolution characterization methods. Recent advances in molecular and Xray characterization techniques,7 however, have made it possible to quantitatively compare the crystallinity of polymer thin films, thus allowing us to draw a direct correlation between the microstructural order and charge transport properties of a semiconducting polymer film. In order to explore the potential of these emerging techniques, we investigated a polymer with “tunable” microReceived: September 30, 2011 Revised: January 21, 2012 Published: March 20, 2012 3057

dx.doi.org/10.1021/ma202203z | Macromolecules 2012, 45, 3057−3062

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standard procedures (see Supporting Information).13 The results are summarized in Table 1.

structural order: a recently developed photo-cross-linkable derivative of poly(3-hexylthiophene) (P3HT).8 Cross-linking is a valuable technique that has been utilized for the fabrication of multilayered solution processed organic light-emitting diodes,9 for increasing the thermal stability of organic solar cells,8,10 for photo patterning,11 and for the realization of polymeric dielectrics in OFETs.12 Here, we use photo-cross-linkable P3HT as a model system to illustrate the effect of cross-linking and thermal annealing on FET performance and to correlate charge mobility with thin film crystallinity. By inserting a bromine as the cross-linkable moiety tethered to the end of the alkyl chains, the functionalized polymer can be easily cross-linked by UV irradiation.8,10 We chose the P3HT-Br10 derivative, which contains 10% of the bromine functionalized monomer unit in its backbone, to ensure sufficient bromine content for complete cross-linking while minimizing the disruption of the semicrystalline nature of the P3HT polymer.8 We tested the OFET performance of the P3HT-Br10 polymer in five different conditions (1: as cast; 2: annealed (AN); 3: cross-linked (XL); 4: cross-linked and then annealed; 5: cross-linked and annealed at the same time). We used quantitative X-ray analysis techniques to compare the degree of crystallinity of all the P3HT-Br10 films. The results of our study show that the different processing conditions affect the device performance and that the mobility trend can be correlated with the evolution of the microstructure. This allowed us to draw an unambiguous correspondence between the degree of crystallinity (DoC) and the FET mobility of a polymer. This is one of the first studies exploring the effects of cross-linking on the microstructural order of a conjugated polymer.

Table 1. Summary of OFET Performance of P3HT-Br10 Films Processed under Different Conditions processing condition as cast annealed (1 h) cross-linked (1 h) cross-linked (1 h) and then annealed (1 h) cross-linked and annealed simultaneously (1 h)

μ (cm2 V−1 s−1)

ION/IOFF

VTH (V)

1.2 × 10−3 2.7 × 10−2 − 2.5 × 10−4

102 103 − 7 × 103

47 −10 − −5

8.5 × 10−3

103

−10

Figure 2 shows two examples of the output and transfer characteristics of devices under two of the five different

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RESULTS AND DISCUSSION The P3HT-Br10 polymer (Figure 1) was synthesized following the same procedure previously reported.8 The polymer used in

Figure 1. Structure of P3HT-Br10 and the BG-BC FET configuration used.

Figure 2. (a) Output and (b) transfer curves of an as cast P3HT-Br10 FET device (L = 20 μm, W = 800 μm). (c) Output and (d) transfer curves of a P3HT-Br10 FET device that has been cross-linked and annealed simultaneously for 1 h (L = 10 μm, W = 400 μm).

this study had a Mn of 19 000 g/mol, a PDI of 1.2, and regioregularity of ∼95%. OFETs were fabricated in a bottomgate, bottom-contact configuration with highly doped p-type (100) silicon wafers as the gate electrode and 300 nm of SiO2 as the gate dielectric. Patterned Au electrodes were deposited onto the substrate by lithography. The polymer layer was spun cast from chloroform on octadecyltrichlorosilane (OTS)treated Si/SiO2 substrates. Devices were fabricated with typical channel lengths of 10 or 20 μm and channel widths ranging from 200 to 800 μm. Transfer and output curves of the device were measured under vacuum. For each condition, the reported data are averaged over 10−20 devices fabricated in parallel to ensure consistency. The experiments were repeated 4−5 times to ensure reproducibility. Key device performance parameters, including field-effect mobility (μ), threshold voltage (VTH), and on/off current ratio (ION/IOFF) were extracted following

processing conditions we examined. The remaining I−V plots are shown in the Supporting Information. Looking at the summary in Table 1, it is clear that the OFET performance is highly dependent on processing conditions. By using five different fabrication procedures with two combinations of cross-linking and annealing sequences, we were able to control the mobility of P3HT-Br10 FET devices over 2 orders of magnitude. For as cast P3HT-Br10 films (see Figure 2a,b for I−V plots), the p-channel FET mobility was measured to be 1.2 × 10−3 cm2 V−1 s−1, which is comparable to as cast devices of P3HT from chloroform.14 The transfer characteristics also indicate that the threshold voltage (VTH) is in the positive VG region. This implies that the active layer has a sufficient hole concentration to conduct current even if we do not 3058


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distribution of crystallite orientations in these semicrystalline polymer thin films, we need to use pole figures to fully capture all the crystallite orientations in order to perform valid quantitative comparisons.18 A pole figure is a plot that shows the orientation distribution of a particular Bragg reflection and provides information on the texture of the thin film. The X-ray diffraction samples were prepared under identical conditions as the FET devices. Thin films of P3HT-Br10 (∼100 nm) were spin-coated from chloroform onto OTStreated Si/SiO2 substrates and subject to the same five processing conditions mentioned above: 1, as cast; 2, crosslinked (XL); 3, cross-linked and then annealed (XL and then AN); 4, cross-linked and annealed at the same time (XL and AN together); 5, annealed (AN). To obtain the complete pole figure, we collected two sets of data: one in grazing incidence geometry (where the sample is horizontal) and the other in local-specular geometry (where the sample is tilted to satisfy the specular geometry for a particular Bragg reflection).7b For quantitative comparisons among all the samples, the X-ray data have been corrected for the distortion by the area detector, polarization, X-ray absorption, incident beam intensity, scan duration, and sample thickness. For the grazing incidence measurement, the angle of incidence (∼0.12°) was carefully chosen to allow for complete penetration of the X-rays into the polymer film but not into the Si substrate. Figure 3 shows the 2D grazing incidence X-ray

intentionally accumulate holes in the conduction channel by applying a negative voltage to the gate. We suspect that the origin of the positive threshold voltage is due to the presence of the Br moiety which may be responsible for a doping effect. A common approach to improving the performance of P3HT-based FETs is by thermal annealing.15 After annealing at 150 °C for 1 h, the mobility of the P3HT-Br10 device increased by over an order of magnitude to 2.7 × 10−2 cm2 V−1 s−1. The threshold voltage became negative, and the on/off current ratio reached a reasonable value. Annealing of the P3HT-Br10 allows the polymer chains to become mobile and improves the typical lamellar ordering and the π-stacking of the polymer.5d We were interested in probing the effect of cross-linking on the polymer FET performance. The P3HT-Br10 film was photo-cross-linked by illuminating the sample with UV light (λ = 254 nm) from a low-power hand-held lamp (1.9 mW cm−2). After 1 h of photo-cross-linking of the film, the output characteristics of the device did not show any saturation; therefore, an accurate calculation of the mobility from the transfer curve was not possible. A poor saturation behavior suggests that the dominant carrier conduction path is not at the interface between the active layer and the dielectric, as generally assumed for thin film transistors.16 A possible explanation for this phenomenon is the generation of bromine radicals during cross-linking, which could increase the conductivity of the bulk P3HT layer, leading to the formation of an additional current in the bulk which is not modulated by the field effect. This behavior is usually attributed to the presence of impurities and doping of the semiconductor layer.17 Interestingly though, we were able to recover some transistor behavior by subsequently annealing the cross-linked sample. The P3HT-Br10 device that was first photo-cross-linked for 1 h and then annealed at 150 °C for 1 h showed a mobility of 2.5 × 10−4 cm2 V−1 s−1, a threshold voltage of −5 V, and an on/off current ratio of 7 × 103. However, this performance is lower than that of the annealed device, probably because the crosslinking “freezes” the polymer chains and thus making the organization of polymer chains during annealing less effective. In order to verify this hypothesis, we tested a P3HT-Br10 device that was cross-linked and annealed at the same time with the objective of allowing the polymer chains to reorganize while cross-linking. In this case, the sample was placed on a hot plate at 150 °C while under UV illumination to perform the simultaneous cross-linking and annealing treatment. Figure 2c and d shows the I−V characteristics of this device, which achieved an FET mobility of 8.5 × 10−3 cm2 V−1 s−1. It is clear that cross-linking and annealing at the same time preserved the favorable electronic properties of the polymer, leading to improved device performance. The atomic force microscopy (AFM) analysis of the surface morphology of the films treated under the different processing procedures did not show any dramatic difference (Figure S5). All the samples displayed similar features with an average rms roughness of 1 nm, except for the annealed sample that showed slightly more pronounced features with an rms roughness of 3 nm. To investigate the correlation between charge mobility and microstructural order in the polymer thin films, we performed in-depth X-ray diffraction analysis to quantitatively compare the degree of crystallinity of the P3HT-Br10 samples processed under different conditions. We followed the method developed by Baker et al.7b,d,e for quantifying crystallographic information for textured polymer thin films. Because of the anisotropic

Figure 3. 2D grazing incidence X-ray scattering patterns of (a) as cast and (b) simultaneously cross-linked and annealed P3HT-Br10 films.

scattering (GIXS) patterns of the as cast P3HT-Br10 film and the sample that was cross-linked and annealed simultaneously. The 2D image map can be divided into a component in the plane of the substrate (qx) and a component perpendicular to the substrate (qz). Similar to P3HT,7c,19 P3HT-Br10 samples displayed three strong (h00) peaks, and they exhibited anisotropic orientation with the strongest peak intensity in the out-of-plane (qz) direction. The (010) peak at q = 1.65 Å−1 (d = 3.8 Å) is present in both samples and corresponds to the 3059


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π-stacking distance between polymer chains. Qualitatively speaking, the two samples essentially showed similar GIXS patterns. In fact, this was true for all five samples, which showed almost identical GIXS patterns despite being subjected to different processing conditions (see Supporting Information). All the samples contained ordered polymer chains with mixed orientationsome domains of the semicrystalline polymer are oriented with (100) parallel to the substrate and some are oriented perpendicular to the substrate. The mixed texture in the polymer thin film leads to the arcing of the scattering peaks in the GIXS patterns. Importantly, the similarity of all the GIXS patterns indicates that neither cross-linking nor annealing changed the overall crystallite orientation and lattice spacings of the P3HT-Br10 thin film. However, qualitatively comparing the grazing incidence patterns does not allow us to draw conclusions regarding the relative degree of crystallinity (DoC) of the P3HT-Br10 samples processed under the five different conditions. To do this, it is necessary to collect additional diffraction data in the local-specular geometry. The specular diffraction measurement was performed at both the (100) and (200) Bragg reflections for each sample. The original rectangular detector coordinates (qx, qz) were converted to polar coordinates (qr, χ) so that we could easily extract the signal within an annular region for a selected Bragg diffraction ring. For the (100) reflection, the region selected was from q = 0.2 Å−1 to 0.5 Å−1. For the (200) reflection, the signal was integrated from q = 0.7 Å−1 to 0.85 Å−1. Therefore, for each Bragg peak, there were two corresponding I(χ) plots: one from the grazing incidence measurement and one from the local-specular measurement. The two I(χ) plots were scaled and merged at χ ∼10°, giving the final pole figure as shown in Figure 4. The intensity near the pole (χ < 10°, i.e., in the out-of-plane direction) came from data collected at the local-specular geometry, whereas the intensity at χ > 10° came from data in the grazing incidence measurement. Figure 4 shows the pole figures for the (100) and (200) Bragg reflections of the as cast sample and the sample that was cross-linked and annealed simultaneously. The intensity of the X-ray diffraction signal is plotted in a semilog scale versus χ, which was the polar angle of tilting of the crystallite. First, we observed that for both samples and for both Bragg reflections examined, the signal was strongest near the pole (where χ was small), which indicated that most of the crystalline polymer chains were oriented out-of-plane, perpendicular to the substrate. In the region where χ > 15°, the signal intensity was much smaller and the χ dependence was much weaker. Comparing Figures 4a and 4b, it is clear that the χ dependence of the two samples is similar. The χ dependence was also consistent across the (100) and the (200) Bragg reflections. Second, comparing Figures 4a and 4b, it is clear that the signalto-noise ratio was higher for the (100) Bragg reflection. However, we note that the pole figure of the (200) Bragg reflection is generally more accurate because of its distance from the direct beam. The intensity of the (100) Bragg peak can be distorted by the beam stop and substrate reflectivity (see Figure 3). For this reason, we chose to use the pole figures from the (200) Bragg reflections of the five samples for quantitative comparison. Nonetheless, the data from the (100) Bragg peak showed a similar trend in all the samples, and the comparison using the (100) reflection is included in the Supporting Information.

Figure 4. Comparing pole figures of the as cast P3HT-Br10 sample and the sample that was cross-linked and annealed at the same time (XL = cross-linked, AN = annealed). (a) Pole figures from the (100) peak. (b) Pole figures from the (200) peak.

As is apparent in Figure 4, the film that was simultaneously cross-linked and annealed had much higher diffraction intensity across the entire range of χ, which means that this sample had a higher degree of crystallinity than the as cast sample. To obtain the relative DoC for each sample, we integrated the area under the pole figure (corrected for the geometric factor of sin(χ))7b from χ = 0° to χ = 90°. Figure 5 shows the DoC results from the five samples. Included in Figure 5 are the corresponding FET mobility values of each sample. This master graph shows the observed correlation between the FET mobility and the relative degree of crystallinity of the films, where a film with a higher degree of crystallinity shows better FET performance. This trend confirms that processing conditions have a huge effect on the microstructural order of the thin film, which in turn determines the charge carrier mobility of the sample. Our results are in agreement with the recent publication by Boudouris et al.7e in which the authors correlate the real time crystallization process of poly(3-2′-ethyl)hexylthiophene with the evolution of its optoelectronic properties. In particular, they observe a clear transition in the mobility of the polymer after the film has developed a reasonable DoC due to the formation of a percolating crystalline network. This interpretation is 3060


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and microstructural order of the photo-cross-linkable P3HTBr10 polymer. Cross-linking did not disrupt the overall crystallinity of the polymer thin film, but the photo-crosslinking process likely induced a doping of the semiconductor layer, leading to the absence of saturation behavior. Annealing after cross-linking partially recovered FET activity but only minimally affected the microstructural order of the polymer film since the 3D morphology had been “locked in” during the first cross-linking step. In addition, we discovered that annealing and cross-linking simultaneously was a successful method to preserve polymer crystallinity while also achieving effective cross-linking. Using a newly developed quantitative Xray analysis technique, our study established a quantitative correlation between FET mobility and thin film crystallinity.

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Figure 5. FET mobility (left axis) of P3HT-Br10 thin films processed under five different conditions and their relative degree of crystallinity (DOC) (right axis) obtained from the integration of the corresponding pole figures from the (200) Bragg peak.

ASSOCIATED CONTENT

S Supporting Information *

Synthetic details, FET device fabrication, additional device data, X-ray diffraction data and analysis, atomic force microscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

consistent with our results: all the samples exhibit a certain degree of crystallinity, but the mobility increases only when the processing induces a more extended crystallization and an even higher DoC. Our results show that cross-linking did not disrupt the overall degree of crystallinity of the polymer film, as the cross-linked sample had a similar DoC compared to the as cast sample. The lack of FET activity (insufficient current modulation) might have been due to the generation of Br radicals during crosslinking, which could have induced a doping effect. As mentioned above, some FET activity could be recovered by subsequent thermal annealing of the cross-linked sample. However, there was little change in crystallinity upon annealing in a cross-linked sample. This was most likely because the 3D structure of the P3HT-Br10 was “locked in” during crosslinking, and annealing the sample above the glass transition temperature of the polymer did not induce the formation of crystalline domains. We also attempted to first anneal the sample and subsequently perform photo-cross-linking. However, this procedure did not lead to effective cross-linking of polymer chains. Therefore, for effective cross-linking, UV irradiation had to be performed before the polymer chains were organized into regular crystal structures. This result has implications on the cross-linking mechanism of this material, which is currently under investigation. To circumvent the problem of “locking in” the 3D morphology too early and to achieve effective cross-linking, we simultaneously cross-linked and annealed the P3HT-Br10 film. This processing condition led to effective cross-linking of the polymer film (see Supporting Information for the test of the cross-linking) and afforded FET performance better than that of the as cast devices but not as good as the annealed devices. More importantly, this trend was correlated well with our crystallinity measurements. The sample that was crosslinked and annealed at the same time possessed a degree of crystallinity that was in between that of the as cast sample and the annealed sample. Therefore, this new procedure was able to preserve a high degree of crystallinity in the thin film and was an effective way to achieve good FET mobility.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Author Contributions &

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support by the U.S. Department of Energy, Basic Energy Sciences, under Contract DE-AC03-76SF00098. Partial support from the Frechet “other donors” fund is also acknowledged. C.H.W. and T.W.H. thank the National Science Foundation for Graduate Research Fellowships. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The authors also thank Leslie Jimison and Jonathan Rivnay for helpful discussions.

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CONCLUSIONS In this study, we investigated the effect of thermal annealing and UV-initiated photo-cross-linking on the FET performance 3061


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