NANO LETTERS
Photoinduced Charge Transfer and Efficient Solar Energy Conversion in a Blend of a Red Polyfluorene Copolymer with CdSe Nanoparticles
2006 Vol. 6, No. 8 1789-1793
Peng Wang,*,† Agnese Abrusci,† Henry M. P. Wong,† Mattias Svensson,‡ Mats R. Andersson,‡ and Neil C. Greenham*,† CaVendish Laboratory, J. J. Thomson AVenue, Cambridge, CB3 0HE, United Kingdom, and Materials and Surface Chemistry, Chalmers UniVersity of Technology, S-412 96, Go¨teborg, Sweden Received May 12, 2006; Revised Manuscript Received June 19, 2006
ABSTRACT We present measurements of charge transfer and the photovoltaic effect in a blend of the alternating polyfluorene copolymer poly(2,7-(9,9dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) with branched CdSe nanoparticles. Quasi-steady-state photoinduced absorption measurements identified a long-lived charged species that formed after photoexcitation at room temperature. Photovoltaic devices based on this blend system showed a spectral response extending to 650 nm and gave a solar power conversion efficiency of 2.4% under Air Mass 1.5 Global (AM1.5G) conditions.
In the past two decades, growing interest has been directed toward new generations of organic photovoltaics with the potential to provide low-cost alternatives to conventional inorganic solar cells. Several device concepts have been developed, including dye-sensitized, small-molecule, and polymer-based solar cells.1 In contrast to traditional inorganic semiconductors, photoexcitation of organic semiconductors generates a strongly bound electron-hole pair or exciton rather than free charge carriers. To dissociate excitons efficiently, the donor/acceptor bulk-heterojunction approach is typically used. Solution processable bulk heterojunction systems that have been used successfully include polymer/ fullerene,2-11 polymer/polymer,12-16 and polymer/nanocrystal17-22 donor/acceptor pairs. Efficient polymer/nanocrystal photovoltaic devices containing CdSe nanorods or branched nanoparticles have been fabricated in combination with poly(3-hexylthiophene) (P3HT)18 or poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-pphenylene vinylene) (OC1C10-PPV)19 as the hole acceptor. Polyfluorene materials are a class of stable conjugated polymers that have received recent attention because of their attractive properties for use as the active layers in lightemitting diodes,23 transistors,24 and polymer solar cells.6,11,25 Photoinduced charge transfer has been reported in composite * Corresponding author. E-mail:
[email protected] or
[email protected]. † Cavendish Laboratory. ‡ Chalmers University of Technology. 10.1021/nl061085q CCC: $33.50 Published on Web 07/19/2006
© 2006 American Chemical Society
films consisting of CdSe nanoparticles and the polyfluorene homopolymer poly(9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl) with a band-gap of 3 eV.26 For photovoltaic applications, novel alternating polyfuorene copolymers have been developed successfully by tuning the chemical structure of conjugated backbones to harvest more solar photons up to 1000 nm.6,11,27,28 Here we will study the photoinduced chargetransfer processes and photovoltaic performance in blends of branched CdSe nanoparticles with a red polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di2-thienyl-2′,1′,3′-benzothiadiazole)), [APFO-3]. Its chemical structure is shown in the inset of Figure 1. The APFO-3 copolymer was synthesized according to a procedure reported previously.6 The molecular weight relative to polystyrene standards was Mn ) 4900 and Mw ) 11800. CdSe nanocrystals were synthesized using the method of Peng and Peng29 under conditions reported previously.30 Transmission electron microscopy (Figure 1S in the Supporting Information) shows that the sample is mainly composed of branched nanoparticles that contain more than one arm, with arm diameters of about 5 nm and lengths in the range of 30-70 nm. The nanocrystals were treated with pyridine to remove the surface ligands and subsequently dissolved in a mixture of chloroform/pyridine (9/1, v/v) using an ultrasonic bath to obtain a 25 mg/mL nanocrystal solution. After mixing it with a 10 mg/mL APFO-3 solution in chloroform, the polymer/nanocrystal blends were spin-coated
Figure 1. Electronic absorption spectra for spin-coated films of pure APFO-3 (dashed line), pure CdSe nanoparticles (dotted line), and an APFO-3/CdSe blend (1/6, w/w) (solid line) from chloroform. The inset shows the chemical structure of APFO-3.
onto quartz glass and annealed for 30 min at 150 °C under nitrogen for optical measurements. The electronic absorption spectra for solutions and thin films were acquired with a Hewlett-Packard 8453 diode array spectrometer. Steady-state photoluminescence (PL) spectra were recorded with a VARIAN Cary Elipse fluorescence spectrophotometer. Time-correlated single-photon counting (TCSPC) measurements were excited with a 407 nm diode laser (PicoQuant LDH 400) producing pulses with a 100 ps full width at half-maximum at a 10 MHz repetition rate. PL was detected with a microchannel plate photomultiplier tube (Hamamatsu Photonics) coupled to TCSPC electronics (Lifespec-ps and VTC900 PC card, Edinburgh Instruments). Our setup for quasi-steady-state photoinduced absorption (PIA) measurements has been described elsewhere.31 Excitation was provided by an argon-ion laser at 488 nm, and a 150 W halogen lamp produced a wideband probe beam. Detection was provided by a monochromator and a silicon photodiode. Changes in transmission were detected by a lockin amplifier referenced to the modulation frequency and were normalized to the unmodulated transmission at each wavelength. Figure 1 shows the electronic absorption spectra of spincoated films of pure APFO-3, branched CdSe nanoparticles, and their blend. The absorption spectrum of the pure APFO-3 film is composed of two distinct, featureless bands of almost the same optical density peaking at 388 and 560 nm. As shown in Figure 2S of the Supporting Information, the highenergy and low-energy bands of a diluted chloroform solution containing 10 µg/mL APFO-3 peak at 388 and 540 nm, respectively. The low-energy band shows a 20 nm red-shift in going from solution to film, which is not present in the high-energy band. This is consistent with a higher degree of charge-transfer character in the low-energy band,32 giving more sensitivity to the dielectric environment. The electronic absorption spectrum of the nanocomposite blend represents a superposition of the respective spectra of pure materials, indicating that there is no electronic interaction between the APFO-3 polymer and the CdSe nanoparticles in the ground state. Steady-state PL measurements were employed to study the excited-state charge-transfer event in the blend films. Charge transfer leads to quenching of the PL because the photogenerated exciton is dissociated before luminescence 1790
Figure 2. (A) PL spectra for films of pure APFO-3 (squares), pure CdSe nanoparticles (line), an APFO-3/CdSe blend (1/1, w/w) (circles), and (d) an APFO-3/CdSe blend (1/6, w/w) (triangles) spincoated from chloroform. The excitation wavelength was 550 nm, and the exitation power and collection geometry were the same in each case. (B) PL decay (a) at 650 nm for a 10 µg/mL APFO-3 solution in chloroform and at 670 nm for films of (b) pure APFO3, (c) an APFO-3/CdSe blend (1/1, w/w), and (d) an APFO-3/CdSe blend (1/6, w/w) spin-coated from chloroform. The excitation wavelength was 407 nm.
can occur. As shown in Figure 2A, the branched CdSe nanoparticles used in our experiments are nonluminescent but APFO-3 luminescence is observed in the spin-coated polymer and blend films. At a concentration of 86% nanoparticles by weight in the blend, over 90% of the PL from the APFO-3 polymer is quenched. The incomplete quenching is consistent with the morphology of this blend, which shows a phase separation on the scale of tens of nanometers, compared with the typical exciton diffusion length of 5-10 nm. Because there is a negligible overlap between the emission spectrum of APFO-3 and the electronic absorption spectrum of CdSe nanoparticles, it is likely that the PL quenching is due to charge-transfer rather than resonance energy transfer. TCSPC measurements were performed to give additional information on the charge transfer dynamics in the blends, by probing the characteristic 670 nm emission from APFO-3 films (Figure 2B). An almost monoexponential fluorescence decay is observed for the pure APFO-3 film with a lifetime of approximately 1 ns for the singlet excited state, which is shorter than the lifetime of over 3 ns measured at 650 nm for a 10 µg/mL APFO-3 solution in chloroform. As the nanocrystal concentration in the blend films is increased, the PL decay became faster, indicating rapid charge transfer from the APFO-3 polymer to CdSe nanocrystals. PIA measurements were performed to detect the presence and measure the lifetime of long-lived charged species after photoexcitation and exciton dissociation in the APFO-3/CdSe Nano Lett., Vol. 6, No. 8, 2006
Figure 4. Intensity dependence of the PIA signal at 1.4 eV for an APFO-3/CdSe blend (1/6, w/w) at 293 K. The modulation frequency was 225 Hz.
Figure 3. (A) In-phase photoinduced absorption spectra for spincoated films from chloroform of pure APFO-3 at 293 K (dotted line), pure APFO-3 at 18 K (solid line), and an APFO-3/CdSe blend (1/6, w/w) at 293 K (dashed line). (B) Temperature dependence of PIA signals at 1.4 eV for (a) pure APFO-3 and (b) an APFO-3/ CdSe blend (1/6, w/w). Both curves have been normalized so that their signals at 40 K are equal. Curve c is obtained by subtracting curve a from curve b. The laser modulation frequency was 225 Hz, and the intensity was 80 mW/cm2.
blend. As shown in Figure 3A, a very small signal is observed for the pure APFO-3 polymer at 293 K, while a broad photoinduced absorption centered at 1.4 eV is clearly seen at 18 K. T1 f Tn transitions in this energy range are typical for conjugated polymers, and hence we assign this low-temperature absorption mainly to triplet excitons.33-36 Figure 3B (curve a) shows that the signal at 1.4 eV for the pure polymer decreases continuously with increasing temperature as the triplet exciton lifetime is reduced. Upon addition of CdSe nanoparticles to the film, even at ambient temperature a significant photoinduced absorption band peaking at 1.4 eV arises because of the allowed polaron transition located below the energy gap of the polymer, evidence of charge transfer between the APFO-3 polymer and CdSe nanoparticles. The feature around 2.1 eV is ascribed to the bleaching of conjugated polymer via groundstate depletion. A similar high-energy polaron band was also reported recently in the APFO-3/fullerene blend along with another low-energy polaron band centered at 0.35 eV.37 In the low temperature regime, the signal at 1.4 eV for the APFO-3/CdSe nanocrystal blend shows a similar temperature dependence to that of the pristine polymer. Thus, we subtracted the contribution due to triplet exciton transitions from the total signal for the blend to see the temperature dependence of polaron absorption. Above 110 K, the signal starts to grow until 190 K, in contrast to the decrease seen for the pure polymer in this temperature regime. The recombination rate for polarons is expected to increase with increasing temperature; hence, the rise in polaron absorption Nano Lett., Vol. 6, No. 8, 2006
Figure 5. Frequency dependence of the PIA signal at 1.4 eV for the APFO-3/CdSe blend (1/6, w/w) at 293 K. The laser intensity was 60 mW/cm2.
in this temperature regime is likely to be caused by an enhancement of charge generation, perhaps producing charge pairs that are more spatially separated and therefore less susceptible to early-time recombination.31 Above 220 K the polaron signal drops slightly, consistent with an increasing recombination rate for the long-lived polarons. Figure 4 shows the excitation intensity dependence of the 1.4 eV PIA peak for the APFO-3/CdSe blend at room temperature. At laser intensities lower than 30 mW/cm2, a power law dependence of -∆T/T ∝ I R with R ≈ 1 is observed as expected for a monomolecular decay process, and at higher intensities R ≈ 0.5, implying a bimolecular recombination process. As shown in Figure 5, the PIA signal decreases steadily with increasing modulation frequency, and does not fit the behavior expected for simple monomolecular or bimolecular recombination. Variation in the signal over the frequency range shown implies that the polaron lifetimes span a range at least from 330 µs up to 10 ms. Such a broad distribution of lifetimes can stem from a diffusive recombination process38 as was also observed in other semiconducting polymer/nanocrystal blend systems.31,39 For the recombination of electrons and holes to occur, both charges need to diffuse through their respective materials to meet each other. A distribution in pair separation can therefore produce a distribution in lifetimes (and trapping can further contribute to these effects). The PIA response on modulated excitation can be fitted by the equation39,40 -
(∆T/T)0 ∆T ) T 1 + (ωτ)γ 1791
where (∆T/T)0 is the PIA amplitude at ω ) 0, ω is the modulation frequency, τ is the mean lifetime, and the dispersion constant γ < 1. The parameters τ and γ for fitting the data shown in Figure 5 are 1.3 ms and 0.89, respectively. These data are measured with a laser intensity of 60 mW/cm2, and hence an element of bimolecular recombination will contribute to the observed lifetimes. In view of the efficient photoinduced charge transfer and the long lifetime of charged species in the blend of APFO-3 and CdSe nanoparticles, it is interesting to make photovoltaic devices based on this new system. Solar cells were fabricated in a typical sandwich structure using glass as the substrate coated with indium-tin oxide (ITO) and poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as an electrode for hole collection. Here we made two types of devices (abbreviated as the chloroform device or the p-xylene device) depending on the initial solvent (chloroform or p-xylene) used to dissolve the APFO-3 polymer. The polymer/nanoparticle blends were spin-coated to give an active layer with a thickness of 80-100 nm. The weight ratio between polymer and CdSe was about 1:6. After annealing at 150 °C for 30 min under a nitrogen atmosphere to remove any residual solvent and pyridine, 100 nm of aluminum was evaporated to form devices with a typical area of 4.5 mm2. The photocurrent action spectra and current density versus voltage (J-V) curves were measured under nitrogen at room temperature using a Keithley 237 source-measure unit with illumination from a tungsten lamp dispersed through a singlegrating monochromator. Power conversion efficiencies under approximately Air Mass 1.5 Global (AM1.5G) conditions were measured using a solar simulator (Oriel Instruments 81160). As shown in Figure 6A, photocurrent action spectra of hybrid photovoltaic devices consisting of APFO-3 and CdSe are consistent with the electronic absorption spectrum for this blend (see Figure 1), giving direct evidence of efficient photoinduced charge transfer between the electron-donating polymer and hole-donating nanocrystals. The incident photon to collected electron conversion efficiency (IPCE) of the p-xylene device is over 40% in the broad spectral range from 510 to 590 nm with a maximum value of 44% at 565 nm. The IPCE of the chloroform device is found to be about a factor of 2 lower over the whole wavelength range. Figure 6B presents the current density-voltage characteristics of devices under 0.1 mW/cm2 illumination at 550 nm. The short-circuit current density (Jsc), open-circuit photovoltage (Voc), and fill factor (ff) for the chloroform device are 8.90 µA/cm2, 460 mV, and 0.366, yielding a monochromatic power conversion efficiency (ηm) of 1.5%. The photovoltaic parameters (Jsc, Voc, ff, and ηm) for the p-xylene device are significantly enhanced to 19.82 µA/cm2, 660 mV, 0.550, and 7.2%, respectively. Under simulated AM 1.5G sunlight at 100 mW/cm2, the photovoltaic parameters (JSC, VOC, ff, and power conversion efficiency) for the p-xylene device are 7.23 mA/cm2, 950 mV, 0.380, and 2.6%, respectively (Figure 6C). Considering the spectral mismatch between our solar simulator and the true AM1.5G spectrum, the corrected efficiency is 2.4%. The high Voc is consistent 1792
Figure 6. (A) Photocurrent action spectra of a chloroform device (dashed line) and a p-xylene device (solid line) based on the APFO3/CdSe blend (1/6, w/w). (B) J-V characteristics for the chloroform device (dashed line) and the p-xylene device (solid line) under 0.1 mW/cm2 illumination at 550 nm. The inset shows the dark currents for the chloroform device (dotted line) and the p-xylene device (solid line). (C) J-V characteristic for the p-xylene device under simulated AM 1.5G sunlight at 100 mW/cm2.
with a lower-lying highest occupied molecular orbital (HOMO) in APFO-3 (5.8 eV)27 compared to those for P3HT (4.9 eV) and OC1C10-PPV (5.0 eV).24 The enhanced device performance for the p-xylene device is not due to better light-harvesting because it has a slightly thinner active layer than the chloroform device. In addition, the IPCE of the chloroform device did not increase even with a thinner active layer thickness comparable to that of the p-xylene device. The improvement in device performance in the p-xylene device must therefore arise from a difference in internal structure within the blend film. Because APFO-3 has a much lower solubility in p-xylene than in chloroform,6 it is possible that a pure polymer or polymer-rich layer forms on the PEDOT:PSS electrode at an early stage in the spincoating process. Figure 6B shows that the dark current is significantly higher in the chloroform device than in the p-xylene device, both in reverse bias and under small positive biases. This is consistent with the existence in the chloroform device of continuous pathways between the two electrodes in either the polymer or the nanoparticle component, which Nano Lett., Vol. 6, No. 8, 2006
are suppressed because of the polymer layer at the bottom electrode in the p-xylene device. We note that devices made using pure nanoparticles also show high dark currents in this voltage regime, consistent with the above hypothesis. The existence of vertical stratification and the suppression of parallel pathways between the electrodes are known to improve the photovoltaic performance in polymer/nanoparticle,19 polymer/polymer,13,41,42 and dye-sensitized devices.43 In conclusion, we have studied photoinduced charge transfer between an alternating polyfluorene copolymer and branched CdSe nanoparticles by PL quenching and quasisteady-state photoinduced absorption measurements. On the basis of the blend of this polymer with CdSe nanoparticles, we have fabricated a photovoltaic device with over 40% IPCE in the spectral range from 510 to 590 nm and a power conversion efficiency of 2.4% under AM1.5G conditions. This work demonstrates that the polyfluorenes are an important class of materials for use with nanoparticles in photovoltaic devices. Their stability and potential for further chemical tuning should allow rapid progress to be made in device performance. Acknowledgment. This work is supported by the EU 6FP NAIMO Integrated Project No. NMP4-CT-2004 500355 and by the Engineering and Physical Sciences Research Council, United Kingdom. Supporting Information Available: TEM image of CdSe nanocrystals and UV-vis spectra of APFO-3. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) For a special issue on organic-based photovoltaics see MRS Bull. 2005, 30, 10-53. (2) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (3) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (4) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (5) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371. (6) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Ingana¨s, O.; Andersson, M. R. AdV. Mater. 2003, 15, 988. (7) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (8) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (9) Reyes-Reyes, M.; Kim, K.; Dewald, J.; Lo´pez-Sandoval, R.; Avadhanula, A.; Curran, S.; Carroll, D. L. Org. Lett. 2005, 7, 5749. (10) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, N. D. C.; Giles, M.; McCulloch, I.; Ha, C.S.; Ree, M. Nat. Mater. 2006, 5, 197.
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