10932
J. Phys. Chem. C 2010, 114, 10932–10936
Influence of Solvent on the Dispersion of Single-Walled Carbon Nanotubes in Polymer Matrix and the Photovoltaic Performance Cheng-Kai Chang,†,|,⊥ Jeong-Yuan Hwang,‡,⊥ Wei-Jung Lai,§ Chun-Wei Chen,§ Ching-I Huang,† Kuei-Hsien Chen,*,‡,| and Li-Chyong Chen*,‡ Institute of Polymer Science and Engineering, Centre for Condensed Matter Sciences, and Department of Material Science and Engineering, National Taiwan UniVersity, Taipei 10617, Taiwan, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ReceiVed: April 21, 2010; ReVised Manuscript ReceiVed: May 18, 2010
Solvent effects on the bulk heterojunction of poly(3-hexylthiophene) (P3HT) and single-walled carbon nanotubes (SWNTs) have been studied. Not only the morphology of P3HT but also the SWNT dispersion in P3HT matrix show strong dependence on the solvents. Atomic force microscopy images suggest that P3HT forms poor crystal structure in chloroform but better in solvents that have relatively high boiling points such as chlorobenzene, toluene, and o-xylene. However, the results of quench rate of photoluminescence and the exciton lifetime decay rate in SWNT/P3HT indicate that a good SWNT dispersion can only be obtained in chloroform and chlorobenzene cases. This suggests that more effective interfaces can be formed when these two solvents are used and thus leads to enhanced charge separation rate, which eventually will benefit the photovoltaic performance. Our photovoltaic demonstration further confirms the idea and suggests that using chlorobenzene for preparation of SWNT/P3HT photovoltaic devices will give more promising results. Introduction The development of bulk-heterojunction (BHJ) photovoltaic (PV) devices based on conjugated polymers, such as poly(3hexylthiophene) (P3HT), has drawn a lot of attention in recent years. By appropriate choice of donor and acceptor materials, the built-in electric field can help dissociate the photoexcited excitons at their interface. In addition, the separated charge carriers need to be transported away from the interface as soon as possible so that the recombination rate can be lowered. Therefore, increasing the interface area and forming a percolated structure in BHJ systems are determining factors for the performance of PV devices. One of the most successful BHJ systems is made with P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which has achieved energy conversion efficiency over 5%.1 PCBM is a fullerene derivative of C60, with an extra functional group on buckyball. The functional group improves the solubility of PCBM in organic solvents, which ensures sufficient interface area, whereas the band structure of PCBM makes it a suitable acceptor material to P3HT. In addition to these intrinsic advantages, it is known that every step in PV device fabrication affects the final performance greatly, for example, the solvent for solution preparation,2-5 annealing,4,6 and materials of buffer layers.7-9 All of these parameters affect the final performance of devices so greatly that their values of energy conversion efficiency change from 1 to ∼5%. Each step in processes has * Corresponding authors. Tel: 886-2-33665249. Fax: 886-2-23655404. E-mail:
[email protected] (L.-C.C.);
[email protected] (K.-H.C.). † Institute of Polymer Science and Engineering, National Taiwan University. ‡ Centre for Condensed Matter Sciences, National Taiwan University. § Department of Material Science and Engineering, National Taiwan University. | Academia Sinica. ⊥ Cheng-Kai Chang and Jeong-Yuan Hwang contribute equally to this work.
to be carefully taken care of to enhance the exciton dissociation rate (or suppress the recombination rate) and to increase carrier mobility of the hybrid materials, therefore, giving better PV device performance. Single-walled carbon nanotubes (SWNTs), with extended structure of fullerene, are naturally considered as a good material in place of fullerene because of their pre-existing percolation structure and extremely high carrier conductivity unmatched by other materials. Although the demonstration of PV devices based on SWNT/P3HT or SWNT/P3OT has been reported,10-13 their performance was much poorer, usually because of the lower photocurrents than those of P3HT/PCBM systems. Several issues that may bring about these poor numbers have been discussed: (1) the incorporated metallic SWNTs, which provide efficient sites for exciton recombination;14 (2) the inappropriate energy levels in some semiconducting SWNTs and P3HT, which causes energy transfer instead of charge separation,15,16 and (3) the long length (up to several micrometers) of SWNTs, which results in serious problem of short-circuit in devices. In view of these shortcomings, either the separation of semiconducting and metallic SWNTs17 or the selectivity of SWNT chirality18 will be essential for promising PV devices based on systems of organic polymers and SWNTs. Among these efforts, we have learned previously18,19 that the influence of solvent on the preparation of conjugated polymer and SWNT solutions is so significant that the SWNT dispersion can be totally distinctive in different solvent background. Herein we adopted tip sonicator to shorten SWNT length and then tried several solvents to fabricate SWNT/P3HT PV devices. Issues are focused on the solvent effects on the P3HT crystallization, SWNT dispersion, and further the device performance. A clear connection to their basic properties has been observed. Experimental Methods The SWNTs grown by the “HiPCO” growth process20 were purchased from Unidym with quoted purities of >85%. Highly
10.1021/jp103601k 2010 American Chemical Society Published on Web 05/27/2010
Influence of Solvent on the Dispersion of SWNTs
J. Phys. Chem. C, Vol. 114, No. 24, 2010 10933
Figure 1. (a) Absorbance spectra of dispersed HiPCO-produced SWNTs in P3HT/benzene solution. (b) PLE maps where the false color scale represents the intensity of emission from SWNTs dispersed in P3HT/benzene solution, and the points represent the positions of SWNT resonances using the scheme proposed by Weisman and Bachilo.21
Figure 2. AFM topography and phase contrast images of pristine P3HT films spin-coated from different solvents: (a,b) CF, (c,d) CB, (e,f) o-xylene, and (g,h) toluene. The morphology image is on the left side, and the phase image is on the right for each solvent. The image size is 1 by 1 µm2 with resolution of 512 × 512 pixels.
regioregular (>98%) P3HT and four solvents, chlorobenzene (CB), chloroform (CF), o-xylene, and toluene, were all purchased from Sigma-Aldrich. Both SWNTs and P3HT were used without further purification. The powder of SWNTs was first dissolved in a solvent with a concentration of 10 µg/mL, and the solution was sonicated using a high power probe for 30 min for dispersing the nanotubes. Then, P3HT was added to the SWNT-dispersed solutions with a concentration of 10 mg/ mL and homogenized in a sonic bath for 10 min. All solutions were then kept under magnetic stirring at all of the times to prevent the nanotube aggregation before device fabrication. Indium tin oxide (ITO)-coated glasses, with a thin layer of concentrated PEDOT/PSS film deposited, were used as substrates. Before PV cell fabrication, the substrates were patterned using photolithography to obtain ITO trips of 2 mm width, but for atomic force microscopy (AFM) and optical measurement samples, these substrates were used directly without patterning. The composite films were then deposited by spin coating at 800 rpm for 60 s and annealed inside glovebox at 180 °C for 3 min to remove the solvent. Aluminum electrodes were thermally evaporated in vacuum with thickness of 100 nm, and the active area of each device is 4 mm2. Encapsulation using epoxy cured under UV light for 10 min was conducted to avoid device degradation, whereas no encapsulation was performed on samples for AFM and optical characterizations.
The morphologies were characterized using JPK instruments NanoWizardAFM. All images were acquired in tapping mode at room temperature in air with a NSC-16 cantilever (resonance frequency 180 kHz). Height and phase images were recorded with scan rates o-xylene > toluene. For SWNT dispersion, we have learned that P3HT is not a good surfactant; therefore, we supposed that the SWNT dispersion will mainly depend on solvents. Bahr et al. have studied the solubility (dispersion) of SWNTs in various organic solvents in which CF, o-xylene, and toluene were included.23 In our previous experience and the polymer solubility experiments, the SWNT solubility in CB and CF should be comparable, so the solubility in the four solvents chosen here might follow the sequence of CF g CB > o-xylene > toluene. The higher solubility indicates the better SWNT dispersion (less nanotube aggregation). Nonetheless, no nanotube images can be observed by using AFM or scanning electron microscope because the
incorporated SWNT concentration is so low that they were all embedded in P3HT matrix. Therefore, we tried to assess SWNT dispersion via analyses of optical properties rather than using real images. If SWNTs disperse better in a P3HT solution, then there must be more effective interfaces created in SWNT/P3HT composite films. This will then cause more interactions, which enhance the possibility of charge dissociation (or energy transfer), consequently resulting in higher PL quench rate and shorter PL lifetime in P3HT. Figure 4 shows the absorption and emission spectra of P3HT casted from CB solutions with and without SWNT incorporation. The absorption spectra show no significant difference, and the absorbance intensity slightly increased after SWNT incorporation. However, a clear PL quench can be observed after SWNTs were introduced, but the quench was not so substantial in every solvent’s samples. To estimate quantitatively the quench rate induced by SWNTs, we use the following formula I0
η)
/A0 - Ii/Ai I0
/A0
)1-
Ii · A0 Ai · I0
(1)
where I is defined as the integral of PL emission of P3HT in the range from 575 to 800 nm, A represents the absorption rate (in percentage) at the wavelength for sample excitation (500 nm), and subscripts 0 and i indicate the cases in the absence and presence of SWNTs, respectively. The absorption is considered to eliminate the thickness inconformity caused by solvent effect. Several samples were made, different regions were measured, and the data were summarized in Table 2. It is found that the PL quench rate in CB samples is on average >20%, ∼10% in CF and o-xylene samples, and almost no quench can be observed in toluene samples. PL lifetime of P3HT is another evidence of optical characteristic that suggests the interaction of composites. Figure 5 shows the PL decay spectroscopy for pristine P3HT (red) and SWNT/P3HT (black) samples prepared by CB (a) and toluene (b), and the fitted PL lifetimes of all samples are also listed in Table 2. Because the amount of SWNTs introduced is very low, only 0.1 wt % compared with P3HT, the change of PL lifetime was not very obvious. Overall, a decrease can usually be observed in CB and CF samples but barely observed in o-xylene or toluene samples. Combining the observations of PL quench rate and lifetime, it might be concluded that the SWNT dispersion in P3HT matrix would be better when solvents CB or CF are used, which implies that the devices made from CB or CF might give better PV performances.
Figure 5. Time-resolved PL spectroscopy for the pristine P3HT (red) and SWNT/P3HT composites (black). Part a shows the samples spin-coated from solvent CB, and part b is prepared from toluene.
10936
J. Phys. Chem. C, Vol. 114, No. 24, 2010
Chang et al. References and Notes
Figure 6. I-V curves of PV devices made using solvent CB (red 9) and CF (3).
Photovoltaic Characterization. The PV devices made with SWNT/P3HT in these solvents are demonstrated, where the current-voltage (I-V) curves of samples prepared from CF and CB are shown in Figure 6. Alhough the energy conversion efficiency was not promising, probably because of the mixture of SWNTs, the results reveal crucial information of solvent effects. The open-circuit voltage in CF case (0.512 V) is slightly higher than that in CB case (0.44 V). In addition, the shortcircuit current (Isc) in CB case (0.115 mA) is 70% greater than that in CF case (0.067 mA). Although the morphological change induced by solvents24 is one of the reasons for these differences, it is believed that the significant increase in Isc can be due to better SWNT dispersion in P3HT matrix and therefore introduces more effective sites for charge separation. The devices made from toluene and o-xylene were also demonstrated (data not shown here), whereas they performed much worse, even not working at all. This is not only because of the low concentration of incorporated SWNT but also the poor SWNT dispersion in P3HT matrix. Conclusions In summary, the solvent effects on the properties of SWNT/ P3HT films have been investigated. The solvents with high bp help the P3HT crystallization, which can be well distinguished by morphology studies and optical characterization. However, the analyses of PL quench rate and PL lifetime suggest that the better SWNT dispersion was obtained when CF and CB were used (especially CB) but a bad dispersion in toluene and o-xylene cases. These indicate more effective interfaces formed in composites made from CB and CF and hence enhanced possibility of charge separation. The I-V characteristic further confirms the idea where the SWNT/P3HT PV device made from CB gave the highest Isc. Although the overall performance is not promising at the moment, improvement can be possibly reached by separating metallic and semiconducting SWNTs and using semiconducting SWNTs with smaller diameter. Acknowledgment. This research was financially supported by the Ministry of Education, National Science Council, Academia Sinica (Taiwan), and Asian Office of Aerospace Research and Development under AFOSR. Technical support was provided by the Core Facilities for Nano Science and Technology, Academia Sinica, and National Taiwan University. J.-Y.H. would like to acknowledge Dr. Y. Y. Lin for his advice in making PV devices.
(1) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. AdV. Funct. Mater. 2005, 15, 1617– 1622. (2) Moule´, A. J.; Meerholz, K. Controlling Morphology in PolymerFullerene Mixtures. AdV. Mater. 2008, 20, 240–245. (3) Cook, S.; Furube, A.; Katoh, R. Mixed Solvents for Morphology Control of Organic Solar Cell Blend films. Jpn. J. Appl. Phys. 2008, 47, 1238–1241. (4) Li, L.; Lu, G.; Yang, X. Improving Performance of Polymer Photovoltaic Devices Using an Annealing-Free Approach via Construction of Ordered Aggregates in Solution. J. Mater. Chem. 2008, 18, 1984–1990. (5) Kawano, K.; Sakai, J.; Yahiro, M.; Adachi, C. Effect of Solvent on Fabrication of Active Layers in Organic Solar Cells Based on Poly(3hexylthiophene) and Fullerene Derivatives. Sol. Energy Mater. Sol. Cells 2009, 93, 514–518. (6) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stu¨hn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells. AdV. Funct. Mater. 2005, 15, 1193–1196. (7) Tao, C.; Ruan, S.; Xie, G.; Kong, X.; Shen, L.; Meng, F.; Liu, C.; Zhang, X.; Dong, W.; Chen, W. Role of Tungsten Oxide in Inverted Polymer Solar Cells. Appl. Phys. Lett. 2009, 94, 043311. (8) Ahlswede, E.; Hanisch, J.; Powalla, M. Comparative Study of the Influence of LiF, NaF, and KF on the Performance of Polymer Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2007, 90, 163504. (9) Vogel, M.; Doka, S.; Breyer, Ch.; Lux-Steiner, M. Ch.; Fostiropoulos, K. On the Function of a Bathocuproine Buffer Layer in Organic Photovoltaic Cells. Appl. Phys. Lett. 2006, 89, 163501. (10) Mallajosyula, A. T.; Iyer, S. S. K.; Mazhari, B. A Comparative Study of Poly(3-octylthiophene) and Poly(3-hexylthiophene) Solar Cells Blended with Single Walled Carbon Nanotubes. Jpn. J. Appl. Phys. 2009, 48, 011503. (11) Kymakis, E.; Servati, P.; Tzanetakis, P.; Koudoumas, E.; Kornilios, N.; Rompogiannakis, I.; Franghiadakis, Y.; Amaratunga, G. A. J. Effective Mobility and Photocurrent in Carbon Nanotube-Polymer Composite Photovoltaic Cells. Nanotechnology 2007, 18, 435702. (12) Kymakis, E.; Koudoumas, E.; Franghiadakis, I.; Amaratunga, G. A. J. Post-fabrication Annealing Effects in Polymer-Nanotube Photovoltaic Cells. J. Phys. D 2006, 39, 1058–1062. (13) Kymakis, E.; Amaratunga, G. Single-Wall Carbon Nanotube/ Conjugated Polymer Photovoltaic Devices. Appl. Phys. Lett. 2002, 80, 112. (14) Kanai, Y.; Grossman, J. Role of Semiconducting and Metallic Tubes in P3HT/Carbon-Nanotube Photovoltaic Heterojunctions: Density Functional Theory Calculations. Nano Lett. 2008, 8, 908–912. (15) Schuettfort, T.; Nish, A.; Doig, J.; Nicholas, R. J. Observation of a Type II Heterojunction in a Highly Ordered Polymer-Carbon Nanotube Nanohybrid Structure. Nano Lett. 2009, 9, 3871–3876. (16) Collison, C. J.; Pellizzeri, S.; Ambrosio, F. Spectroscopic Evidence for Interaction of Poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] Conformers and Single-Walled Carbon Nanotubes in Solvent Dispersions. J. Phys. Chem. B 2009, 113, 5809–5815. (17) Tanaka, T.; Jin, H.; Miyata, Y.; Fujii, S.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Simple and Scalable Gel-Based Separation of Metallic and Semiconducting Carbon Nanotubes. Nano Lett. 2009, 9, 1497–1500. (18) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640–646. (19) Hwang, J.-Y.; Nish, A.; Doig, J.; Douven, S.; Chen, C.-W.; Chen, L.-C.; Nicholas, R. J. Polymer Structure and Solvent Effects on the Selective Dispersion of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2008, 130, 3543–3553. (20) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-Phase Catalytic Growth of Single-Walled Carbon Nanotubes from Carbon Monoxide. Chem. Phys. Lett. 1999, 313, 91–97. (21) Weisman, R. B.; Bachilo, S. M. Dependence of Optical Transition Energies on Structure for Single-Walled Carbon Nanotubes in Aqueous Suspension: An Empirical Kataura Plot. Nano Lett. 2003, 3, 1235–1238. (22) Chang, J.-F.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; So¨lling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Enhanced Mobility of Poly(3hexylthiophene) Transistors by Spin-Coating from High-Boiling-Point Solvents. Chem. Mater. 2004, 16, 4772–4776. (23) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Dissolution of Small Diameter Single-Wall Carbon Nanotubes in Organic Solvents. Chem. Commun. 2001, 2, 193–194. (24) Liu, J.; Shi, Y.; Yang, Y. Solvation-Induced Morphology Effects on the Performance of Polymer-Based Photovoltaic Devices. AdV. Funct. Mater. 2001, 11, 420–424.
JP103601K
