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2008, 112, 8507–8510 Published on Web 05/17/2008
Polymer Photovoltaic Devices Using Fully Regioregular Poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy))-1,4-phenylenevinylene] Keisuke Tajima,*,† Yuya Suzuki,† and Kazuhito Hashimoto*,†,‡ Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and HASHIMOTO Light Energy ConVersion Project, ERATO, Japan Science and Technology Agency (JST) ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: April 30, 2008
Fully regioregular poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy))-1,4-phenylenevinylene]s (MDMO-PPVs) synthesized by the Horner route were utilized for polymer photovoltaic devices, and their performance was compared with that of regiorandom MDMO-PPVs. The mixture films of the regioregular MDMO-PPVs and 1-[3-(methoxycarbonyl)propyl]-1-phenyl-(6,6)-C61 (PCBM) were successfully fabricated by spin coating using a hot chlorobenzene solution. Significant improvements in fill factor (FF) and short circuit current (ISC) were observed for the devices with regioregular MDMO-PPV compared with those for devices with regiorandom MDMO-PPV. A power conversion efficiency of 3.1% was achieved with regioregular MDMO-PPV under AM1.5 illumination, which is the highest efficiency reported for the PPV:PCBM system so far, whereas a power conversion efficiency of only 1.7% was achieved with regiorandom MDMO-PPV. It is concluded that both higher hole mobility resulting from the higher crystallinity of the polymer and better mixing morphology between the polymer and PCBM in bulk heterojunction films contributed to the improvement in the photovoltaic performance of devices with regioregular MDMO-PPV. Polymer photovoltaic devices have attracted much attention in recent years because of their potential advantages, such as their flexibility, light weight, and low fabrication cost, over their inorganic counterparts.1,2 The development of a “bulk heterojunction”, which is a simple mixture of donor and acceptor materials, has led to an increase in the efficiency of photovoltaic devices because of the efficient charge separation inside the films. The combination of poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy))-1,4-phenylenevinylene] (MDMO-PPV) and 1-[3(methoxycarbonyl)propyl]-1-phenyl-(6,6)-C61 (PCBM) has been extensively investigated,3,4 and efficiencies of up to 2.9% have been reported using this combination.5,6 However, the difference between the mobilities of the two compounds (hole mobility of 5 × 10-11 m2 V-1 s-1 for MDMO-PPV7 and electron mobility of 2 × 10-7 m2 V-1 s-1 for PCBM8) is reported to limit the charge transport inside the films, resulting in the recombination of the charges.9 Therefore, it has been predicted that an increase in the hole mobility of PPVs is necessary to achieve a balanced charge transport and a higher efficiency.10 Recently, we reported the synthesis of fully regioregular PPVs by the Horner reaction using a novel monomer.11 The high regioregularity of the polymers was quantitatively determined by 1H NMR analysis, which was also developed for the study of PPV model compounds. The polymers show high crystallinity in the solid state, as confirmed by X-ray diffraction (XRD) analysis and UV-vis absorption spectral measurement. In addition, they have moderate solubility in hot chlorobenzene, * To whom correspondence should be addressed. E-mail: k-tajima@ light.t.u-tokyo.ac.jp (K.T.) and
[email protected] (K.H.). † The University of Tokyo. ‡ Japan Science and Technology Agency (JST).
10.1021/jp802688s CCC: $40.75
which enables the fabrication of films by a solution-based process. In this paper, we report the application of fully regioregular PPVs in photovoltaic devices and discuss the reason behind the performance difference. Although Mozer et al. have reported the regiospecific (i.e., one of the coupling manners is predominant in polymer chains) synthesis of MDMO-PPVs by a precursor route12,13 and the improvement in photovoltaic efficiency using the polymers and PCBM,12 no application of fully regioregular PPVs in electronic devices has been reported yet. Considering that a small difference in the regioregularity of poly(3-alkylthiophene) could markedly affect device performance,14 the electronic properties of fully regioregular MDMOPPV is an interesting research subject. Fully regioregular (regular-PPVs) and regiorandom (randomPPVs) MDMO-PPVs were synthesized via the synthetic route shown in Scheme 1. Detailed synthetic procedures are described in our previous paper.11 Both regular- and random-PPVs have sufficiently high number-averaged molecular weights (Mn) of 21 000 and 33 000, respectively, and similar polydispersities of 2.3 measured by high-temperature gel permeation chromatography (GPC). This enables their application in polymer photovoltaic devices and a comparative analysis of their performances. Bulk heterojunction films were fabricated by spin coating using mixtures of MDMO-PPVs and PCBM in chlorobenzene. In the case of regular-PPV, coating at a high temperature was necessary because of its limited solubility at room temperature (see the Supporting Information (SI) for experimental details). The UV-vis absorption spectra of the spin-coated films from the mixtures of MDMO-PPVs and PCBM are presented in the SI. The absorption peak corresponding to regular-PPV absorp 2008 American Chemical Society
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SCHEME 1: Synthetic Routes of Regioregular (Top) and Regiorandom (Bottom) MDMO-PPVs (DMO, 3,7-dimethyloctyl)
tion (540 nm) was significantly red-shifted compared with that of the random-PPV absorption (500 nm). A similar red shift was also observed in pristine polymer films in our previous study, as a result of the high crystallinity of regular-PPV.11 This result suggests the higher crystallinity of regular-PPV even in bulk heterojunction films with PCBM. For the device investigation, we first optimized the PPV: PCBM mixing ratio to obtain a high power conversion efficiency (PCE) under illumination with simulated solar light (AM1.5, 100 mW cm-2). The results are summarized in the SI. The highest efficiencies were obtained at a PCBM content of around 65 wt % for both regular- and random-PPVs. This optimum PCBM content is relatively low compared with that previously reported by Blom et al. (80 wt % PCBM) for regiorandom MDMO-PPV.15 In addition, the highest PCE achieved with random-PPV in our results (1.7%) was lower than those previouslyreportedfortheregiorandomMDMO-PPVs(2.5-2.9%).5,6 Note that the regiorandom MDMO-PPVs used in previous studies were synthesized via the Gilch route or the sulfenyl precursor route; thus, the different polymerization reactions could have varied the number of structural defects in the polymer main chains16,17 and the molecular weights of the polymers (Mn ) 3.3 × 104 by the Horner reaction, Mn ) 1.3 × 105 by the Gilch reaction,18 and Mn ) 1.5 × 105 by the precursor route18), possibly leading to a difference in the optimum conditions and the PCEs when the regiorandom MDMO-PPVs were compared. The best I-V characteristics of the devices prepared under optimum conditions under dark and AM1.5 100 mW cm-2 illumination are presented in the SI and Figure 1, respectively, and the performance parameters are summarized in Table 1. ISC and FF are higher, while VOC is lower in regular-PPV than in random-PPV. Among all of the parameters, a FF of 70% in the regular-PPV:PCBM device is a significant improvement. As a result, the power conversion efficiency for the regular-PPV was improved to 3.1% (average of 10 devices, 3.0 ( 0.05%), which is the highest value for the PPV:PCBM system reported so far. More importantly, the efficiency improvement by increasing regioregularity is clear and significant, showing the potential of the current approach. Although the origin of the improvements in device performance could be complex, we here discuss the improvement in performance parameters separately for the sake of simplicity. As for the ISC increase, the number of absorbed photons and/or the increase in the charge separation interface area could be important factors. Absorbances at the peak tops were comparable between both films, but the red shift in regular-PPV might have a better match of absorption with the solar spectrum (see the SI). However, from the calculation of the integration overlap
between the UV-vis spectra of pristine polymers and the AM1.5 spectrum, an absorption enhancement less than 12% is expected for regular-PPV compared with that for random-PPV. Therefore, the increased ISC value cannot be attributed only to the better spectral match but also to the increase in the charge separation efficiency inside the films. The interface between the donor and the acceptor could play a crucial role in charge separation, since exciton dissociation occurs only at the donor/acceptor interface. To determine the difference in interface area between the two devices, we observed the surface morphology of MDMO-PPV: PCBM bulk heterojunctions by atomic force microscopy (AFM) operated in the tapping mode (Figure 2). In the random-PPV: PCBM films, round-shaped aggregates of PCBM with a size of ca. 100 nm were observed, similarly to those previously reported.19 In contrast, the aggregations of PCBM were not observed in the regular-PPV:PCBM films. The same tendency was observed even at a higher mixing ratio of PCBM (80 wt %), as shown in the SI. This difference could be a result of the difference in the crystallization speeds of regular- and randomPPV during the spin coating. Because regular-PPV showed
Figure 1. Current density-voltage relationship of MDMO-PPV:PCBM bulk heterojunction devices under AM1.5 100 mW cm-2 illumination (solid line, regular-PPV; dashed line, random-PPV). Film thickness: 74 nm for regular-PPV and 78 nm for random-PPV.
TABLE 1: Photovoltaic Performances of MDMO-PPV:PCBM Bulk Heterojunction Devices under AM1.5 100 mW cm-2 Illumination (Shown in Figure 1) and Hole Mobilities of the MDMO-PPV Films Measured Using Space-Charge-Limited Currents
regular-PPV random-PPV
ISC (mA cm-2)
VOC (V)
FF (%)
η (%)
hole mobility (m2 V-1 s-1)
6.2 3.9
0.71 0.86
70 51
3.1 1.7
3.1 ( 0.4 × 10-10 2.2 ( 0.5 × 10-11
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Figure 2. Atomic force microscopy height images of the spin-coated films from the mixtures of 67 wt % PCBM and (a) random-PPV (rms roughness, 1.2 nm) and (b) regular-PPV (rms roughness, 1.6 nm) (image size, 3 µm × 3 µm).
higher crystallinity and lower solubility in chlorobenzene, the polymer could first form a uniform network and prevent PCBM from aggregating during the evaporation of the solvent in spin coating, resulting in the more uniform dispersion of PCBM in the polymer matrix, as shown in Figure 2b and the SI. A decrease in VOC could be caused by several factors. Yang et al. reported that the mixing morphology of the donor and acceptor can affect VOC significantly.20 The difference in mixing morphology observed above by AFM could be one of the causes of the difference in VOC. It has also been reported that the maximum VOC obtained in bulk heterojunction devices can be determined from the energy level difference between the highest occupied molecular orbital (HOMO) of the donor materials and the lowest unoccupied molecular orbital (LUMO) of the acceptor materials.21 The higher crystallinity of regular-PPV could cause a shift in the energy level of the polymer. Cyclic voltammetry (CV) on the polymer films showed onset potentials of the oxidation peaks at -5.56 eV for regular-PPV and at -5.34 eV for random-PPV versus vacuum level (see the SI). Ionization potentials (IPs) of the polymer films on ITO substrate measured by photoelectron yield spectroscopy (PYS) showed an IP of 5.28 eV for regular-PPV and 5.48 eV for random-PPV (see the SI). The CV and PYS results agreed well to indicate that the HOMO level of regular-PPV is shifted +0.2 eV compared to random-PPV. This shift could contribute to the difference in VOC of the photovoltaic devices between regular- and randomPPV (0.15 V). The increase in FF could be attributed to the balanced charge transportation, which is expected from the higher crystallinity of regioregular-PPV. We measured the hole mobility of the pristine polymer films using the space-charge-limited current (SCLC) region in current-voltage measurement. Devices for SCLC measurement were fabricated similarly to photovoltaic devices, except that no TiOx layer was used and Au instead of Al was used as the electrode to prevent electron injection. Both ohmic and SCLC regions were observed in the I-V curves for each device, and the SCLC regions were fitted to extract the hole mobility (see the SI for the details). To check the validity of the measurements, the thickness dependence of current at a voltage of 1.0 V was investigated. The log-log plot of film thickness and current fitted the line reasonably well with a slope of -3 (see the SI), which is indicative of an SCLC region. As a result, the hole mobility of MDMO-PPVs was calculated using Child’s law.7 The obtained hole mobility of 2.2 ( 0.4 × 10-11 m2 V-1 s-1 for random-PPV is reasonably close to the previously reported value (5.0 × 10-11 m2 V-1 s-1).7 Regular-
PPV films show a hole mobility of 3.1 ( 0.4 × 10-10 m2 V-1 s-1 under the same conditions, which is about 1 order of magnitude higher than that of random-PPV. The reason for the higher mobility could be the higher crystallinity and ordering of regular-PPV, as shown by powder XRD analysis and UV-vis absorption measurement reported in our previous paper.11 In conclusion, the photovoltaic performance of MDMO-PPV: PCBM bulk heterojunction devices can be significantly improved using fully regioregular PPVs synthesized by the Horner route. This improvement could be attributed to both the fine mixing morphology of the two components and the higher hole mobility of the films. These results give us prospects for designing new conjugated regioregular polymers applicable in high-performance polymer electronic devices. Acknowledgment. We thank Dr. Kouske Hirota (Mitsui Chemicals, Inc.) for fruitful discussion and Prof. Kubo (The University of Tokyo) for PYS measurements. This work was partly supported by a Grant-in-Aid for Exploratory Research No. 18651045 from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese government. Supporting Information Available: Experimental details, results of the device optimization, SCLC, CV, PYS, I-V curves under dark and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324–1338. (2) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. AdV. Mater. 2007, 19, 1551–1566. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789–1791. (4) Brabec, C. J.; Padinger, F.; Sariciftci, N. S.; Hummelen, J. C. J. Appl. Phys. 1999, 85, 6866–6872. (5) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841–843. (6) Munters, T.; Martens, T.; Goris, L.; Vrindts, V.; Manca, J.; Lutsen, L.; De Ceuninck, W.; Vanderzande, D.; De Schepper, L.; Gelan, J.; Sariciftci, N. S.; Brabec, C. J. Thin Solid Films 2002, 403, 247–251. (7) Blom, P. W. M.; deJong, M. J. M.; vanMunster, M. G. Phys. ReV. B 1997, 55, R656–R659. (8) Mihailetchi, V. D.; van Duren, J. K. J.; Blom, P. W. M.; Hummelen, J. C.; Janssen, R. A. J.; Kroon, J. M.; Rispens, M. T.; Verhees, W. J. H.; Wienk, M. M. AdV. Funct. Mater. 2003, 13, 43–46. (9) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88. (10) Koster, L. J. A.; Smits, E. C. P.; Mihailetchi, V. D.; Blom, P. W. M. Phys. ReV. B 2005, 72.
8510 J. Phys. Chem. C, Vol. 112, No. 23, 2008 (11) Suzuki, Y.; Hashimoto, K.; Tajima, K. Macromolecules 2007, 40, 6521–6528. (12) Mozer, A. J.; Denk, P.; Scharber, M. C.; Neugebauer, H.; Sariciftci, N. S. J. Phys. Chem. B 2004, 108, 5235–5242. (13) Mozer, A. J.; Denk, P.; Scharber, M. C.; Neugebauer, H.; Sariciftci, N. S.; Wagner, P.; Lutsen, L.; Vanderzande, D.; Kadashchuk, A.; Staneva, R.; Resel, R. Synth. Met. 2005, 153, 81–84. (14) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M. Nat. Mater. 2006, 5, 197–203. (15) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M.; Melzer, C.; de Boer, B.; van Duren, J. K. J.; Janssen, R. A. J. AdV. Funct. Mater. 2005, 15, 795–801. (16) Holzer, W.; Penzkofer, A.; Tillmann, H.; Horhold, H. H. Synth. Met. 2004, 140, 155–170.
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