NANO LETTERS
Photovoltaic Devices Using Blends of Branched CdSe Nanoparticles and Conjugated Polymers
2003 Vol. 3, No. 7 961-963
Baoquan Sun, Eike Marx, and Neil C. Greenham* CaVendish Laboratory, Madingley Road, Cambridge, CB3 0HE, United Kingdom Received May 6, 2003; Revised Manuscript Received May 22, 2003
ABSTRACT We show that photovoltaic devices fabricated from blends of branched CdSe nanoparticles and a conjugated polymer give improved performance compared with devices made from nanorod/polymer blends. The improvement is consistent with improved electron transport perpendicular to the plane of the film. Solar power conversion efficiencies of 1.8% were achieved under AM1.5 illumination for a device containing 86 wt % of nanoparticles.
Photovoltaic devices based on solution-processable conjugated polymers are attractive for the production of low-cost solar cells.1 To obtain high efficiencies, it is necessary to have an interpenetrating network of electron-accepting and hole-accepting components within the device. This can be achieved using polymer blends2 or mixtures of conjugated polymers with C60 derivatives.3,4 Nanocrystals of inorganic semiconductors such as CdSe also act as good electron acceptors from conjugated polymers; however, the efficiency of photovoltaic devices made with spherical nanocrystals is limited by the problem of electron extraction through the nanocrystal network.5,6 The use of nanorods instead of spherical nanocrystals has been shown to give significantly higher efficiencies because of the smaller number of interparticle hops necessary for electrons to leave the device.7,8 However, nanorods have a tendency to lie in the plane of the film, which is not the optimum arrangement for electron extraction, and there is significant scope for improvement in efficiency by designing optimized microstructures. Manna et al. recently reported the synthesis of CdSe tetrapods comprising four limbs connected at a central core.9 These particles have obvious potential advantages in photovoltaic devices10 because their shape makes it impossible for them to lie flat within the film. Here, we fabricate photovoltaic devices based on blends of similar branched CdSe nanoparticles with the conjugated polymer poly(2methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylenevinylene) (OC1C10-PPV). This polymer was chosen because it has been extensively studied in photovoltaic blends with C60 deriva* Corresponding author. E-mail:
[email protected]. Tel: +44 1223 766301. Fax: +44 1223 353397. 10.1021/nl0342895 CCC: $25.00 Published on Web 06/10/2003
© 2003 American Chemical Society
Figure 1. Transmission electron microscopy images of (a) tetrapods and (b) nanorods as described in the text.
tives11 and it is known to provide a stable emitter in polymer light-emitting diodes.12 Branched CdSe nanoparticles were synthesized by a slight modification of the method of Peng and Peng.13 CdO (0.346 g, 2.7 mmol), octylphosphonic acid (OPA) (1.069 g, 5.4 mmol), and tri-n-octylphosphine oxide (2.585 g) were loaded into the reaction flask and heated to 300 °C for 5 min under an argon atmosphere. The mixture was cooled to room temperature and then reheated to 300 °C after 48 h. A refrigerated solution containing Se (0.426 g, 5.4 mmol), tributylphosphine (1.270 g), and toluene (0.3 mL) was then rapidly injected into the flask. The solution was then maintained at 250 °C for 50 min to allow the growth of nanoparticles. After cooling to 50 °C, 10 mL of methanol and 5 mL of toluene were added to precipitate the nanoparticles, which were recovered by centrifugation. The precipitate was washed five times with methanol to remove excess ligand. The OPA ligand on the surface of the tetrapods was then replaced by refluxing the particles (60 mg) in anhydrous pyridine (15 mL) at 118 °C under argon for 24 h. The
Figure 2. (a) Chemical structure of OC1C10-PPV. (b) Photovoltaic device structure.
particles were precipitated with hexanes at room temperature, recovered by centrifugation, dried for 3 min with argon, and then dispersed into a mixture of chloroform/pyridine (90: 10, vol/vol) at a concentration of 30 mg/mL. The solution was ultrasonicated for 90 min and filtered through a 1.0-µm poly(tetrafluoroethylene) syringe filter. The final concentration of tetrapods was approximately 25 mg/mL, as determined by evaporating known volumes to dryness and weighing. Figure 1a shows a transmission electron micrograph of branched nanoparticles deposited on a carbon film. A large fraction of branched nanoparticles is obtained without selective precipitation, and there is clear evidence of the formation of tetrapods. We therefore refer to this sample from now on as “tetrapods”. The tetrapod limbs are typically 50 nm long and 5 nm thick, corresponding to a total height (base to apex) of 78 nm. For comparison, nanorods of length 65 nm and thickness 5 nm were also synthesized, as shown in Figure 1b. The nanorods were obtained using a similar synthesis to that of the tetrapods but with a lower concentration of injected solution. Photovoltaic devices were fabricated as shown in Figure 2. A blend of poly(3,4-ethylene dioxythiophene) with poly(styrene sulfonate) (PEDOT/PSS) was used as the anode and was spin coated with a thickness of 70 nm onto an oxygenplasma-treated indium-tin oxide (ITO) substrate, followed by baking at 150 °C for 2 h under nitrogen. The tetrapod solution (300 µL) and 125 µL of 10 mg/mL OC1C10-PPV in 1,2-dichlorobenzene were thoroughly mixed and spin coated onto the surface of the PEDOT/PSS. Typical film thicknesses were 160-180 nm. The films were baked at 150 °C for 30 min under nitrogen to remove residual solvent, and aluminum cathodes were then deposited by thermal evaporation. Devices were encapsulated using epoxy resin and were measured under ambient conditions. Figure 3 shows the short-circuit external quantum efficiency (EQE) action spectra for polymer/tetrapod and polymer/nanorod devices. Both devices contained approximately 86% nanoparticles by weight and were prepared under identical conditions. With CdSe tetrapods, a short-circuit 962
Figure 3. Short-circuit external quantum efficiency (EQE) action spectra of photovoltaic devices containing CdSe nanorods (- - -) and tetrapods (s).
EQE of 45% was obtained under 0.39 mW cm-2 illumination at 480 nm. The nanorod device had a maximum EQE of 23%, almost a factor of 2 smaller than that in the tetrapod device. The efficiency improvement in the tetrapod device is consistent with improved electron transport perpendicular to the plane of the film as a consequence of the shape of the tetrapods. The action spectra show clear evidence of response in the region where only the nanoparticles absorb, beyond the absorption region of the polymer, indicating hole transfer from nanoparticles to polymer. The surface topography of polymer/nanorod and polymer/ tetrapod films was studied using atomic force microscopy, as shown in Figure 4. The surface roughness in both cases is rather similar (about 20 nm), and in both cases, there is some evidence of the aggregation of nanoparticles to form features of size 40-100 nm. The morphology of the films was sensitive to the degree of replacement of the OPA ligand with pyridine. Incomplete replacement led to poor solubility of the nanoparticles in the chloroform/pyridine mixture, producing rough surfaces and reduced device efficiencies. The dependence of the efficiency on processing conditions has been studied in detail by Huynh et al. for polymer/ nanorod composites;14 we also find that efficiency is sensitive Nano Lett., Vol. 3, No. 7, 2003
Figure 4. Tapping-mode atomic force micrographs showing the surface topography of OC1C10-PPV/nanoparticle blend films (1:6 w/w) with a thickness of 160-180 nm. (a) CdSe tetrapods. (b) CdSe nanorods. The image size is 1 µm × 1 µm, and the vertical scale is 20 nm.
Figure 5. (a) Current density vs voltage for a tetrapod/OC1C10PPV device in the dark (- - -) and under 0.39 mW cm-2 illumination at 480 nm. Voc ) 0.53 V, Isc ) -0.069 mA cm-2, FF ) 0.49, and η ) 4.45%. (b) Current density vs voltage for the same device as in plot a illuminated with simulated AM1.5 global light at an intensity of 93 mW cm-2, Voc ) 0.65 V, Isc ) -7.30 mA cm-2, FF ) 0.35, and η ) 1.8%.
to the solvent mixture used. However, we find that the tetrapods give consistently higher efficiencies than rods under the same processing conditions. Although the quantum efficiencies of our devices under low-intensity excitation are smaller than those of poly(3-hexylthiophene)/nanorod devices reported by Huynh et al.7 (which contain higher nanoparticle concentrations and are expected to have higher hole mobilities in the polymer), our results indicate that the use of tetrapods is an attractive route to obtain high efficiencies by improving electron transport perpendicular to the plane of the film. Future improvements in efficiency are likely to involve both the further optimization of electron extraction through the tetrapod network and the improvement of hole transport in the polymer. Figure 5 shows the current density as a function of voltage for a polymer/tetrapod device under 0.39 mW cm-2 illumination at 480 nm. The short-circuit current density (Isc),
Nano Lett., Vol. 3, No. 7, 2003
open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) are -0.069 mA cm-2, 0.53 V, 0.49, and 4.5%, respectively. Under illumination at AM1.5 global conditions, Voc, Isc, and FF are 0.65 V, -7.30 mA cm-2, and 0.35 respectively, and the solar power conversion efficiency is 1.8%. Under these conditions, the power conversion efficiency is slightly higher than that of poly(3hexylthiophene)/nanorod devices.7 Although our lowintensity quantum efficiencies are smaller than those of Huhnh et al., they do not fall off so strongly at high intensities, which accounts for our high power conversion efficiencies under solar illumination. This is consistent with improved electron transport in the tetrapod devices, avoiding space-charge build-up and bimolecular recombination at high intensities. In conclusion, we have shown that 3D CdSe tetrapods give improved electron extraction in photovoltaic devices compared with 1D nanorods. The control of nanoparticle shape in 3 dimensions provides an additional tool to control morphology and optimize efficiency in polymer/nanoparticle photovoltaic devices. Acknowledgment. We thank A. S. Dhoot, H. J. Snaith, and L. J. Schmidt-Mende for helpful discussion and assistance. This work was funded by the Engineering and Physical Sciences Research Council, U.K. E.M. thanks the Deutsche Akademische Austauschdienst for support. References (1) Nelson, J. Curr. Opin. Solid State Mater. Sci. 2002, 6, 87. (2) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (4) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (5) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628. (6) Ginger, D. S.; Greenham, N. C. Phys. ReV. B 1999, 59, 10622. (7) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (8) Huynh, W. U.; Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (9) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (10) Scher, E. C.; Manna, L.; Alivisatos, A. P. Philos. Trans. R. Soc. London, Ser. A 2003, 361, 241. (11) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (12) Liedenbaum, C.; Croonen, Y.; van de Weijer, P.; Vleggaar, J.; Schoo, H. Synth. Met. 1997, 91, 109. (13) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (14) Huynh, W. U.; Dittmer, J. J.; Libby, W. C.; Whiting, G. L.; Alivisatos, A. P. AdV. Funct. Mater. 2003, 13, 73.
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