ARTICLE pubs.acs.org/JPCC
Spatially Resolved Spectroscopic Mapping of Photocurrent and Photoluminescence in Polymer Blend Photovoltaic Devices Thomas J. K. Brenner and Christopher R. McNeill* Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
bS Supporting Information ABSTRACT: Combined confocal photoluminescence and photocurrent microscopy is used to study the interplay between blend morphology, polymer conformation, and photocurrent generation in polymer solar cells based on blends of the polymers PFB (poly(9,90 -dioctylfluorene-co-bis-N,N0 -(4-butylphenyl)-bis-N,N0 phenyl-1,4-phenylenediamine)) and F8BT (poly(9,90 -dioctylfluorene-co-benzodiathiazole)). Two-dimensional photoluminescence spectral maps of the blend are acquired that allow for the composition of the domains to be identified, whereas the use of different laser sources allows for photocurrent imaging with selective excitation. Photoluminescence mapping additionally facilitates correlation between exciplex emission and photocurrent generation for each domain. We find that photocurrent is higher in F8BT-rich domains regardless of excitation source (375 nm, preferential absorption by PFB; 405 nm, equal absorption; 445 nm, preferential absorption by F8BT) though the discrepancy is less when PFB is preferentially excited. Prominent exciplex emission is observed in PFB-rich domains that suggests that, although PFBrich domains have a higher degree of intermixing than the F8BT-rich domains (beneficial for exciton dissociation), this intermixing may be too fine, hindering charge separation and promoting exciplex formation. We also discuss the influence of surface and capping layers on device performance finding evidence for reduced charge collection efficiency for regions in the F8BT-rich phase covered by a PFB capping layer.
’ INTRODUCTION Blends of conjugated polymers have already proven to be excellent candidates for light emitting diodes (LEDs).1 They also present a very promising approach to low-cost photovoltaics with power conversion efficiencies around 2% for polymerpolymer blends,2,3 and up to 7.5% for polymerfullerene systems.4 To reach these efficiencies, the bulk heterojunction approach5,6 has proven to be most successful. Two materials are mixed to form type-II-heterojunctions, which are necessary to provide a sufficient energy offset for exciton dissociation in polymer blends. As the typical exciton diffusion length is on the order of 10 nm, intimate mixing of the two components is mandatory to create a large interfacial area. However, there is a trade-off between efficient charge generation and charge transport, because in blends with phase separation less than the exciton diffusion length7 there can be a lack of interconnected polymer domains, which are required to transport charge to the electrodes. Furthermore, an intimately mixed blend can also hinder charge separation as it limits the number of possible routes for charges to move away from the donoracceptor interface.8 Photoluminescence studies9 have been used to clarify the role of charge generation in these systems. Furthermore, spatially resolved photocurrent mapping,10 X-ray microscopy,11 conductive atomic force microscopy (cAFM),12 electrostatic force microscopy (EFM),13 combined photocurrent and fluorescence imaging,14 and combined photocurrent and Raman spectroscopy15 have been employed to study the interplay between morphology and device operation. r 2011 American Chemical Society
In this study, we combine high-resolution photoluminescence spectroscopy with submicrometer photocurrent mapping to correlate local photophysics and charge collection in a working device. We investigate a blend of two polyfluorene copolymers, (poly(9,90 -dioctylfluorene-co-benzodiathiazole) (F8BT) and (poly(9,90 -dioctylfluorene-co-bis-N,N 0 -(4-butylphenyl)-bis-N,N 0 -phenyl1,4-phenylenediamine) (PFB), which is a good model system for investigating the photophysics of polymer blends and for which the photophysics is well-known.1622 Charge generation in polymer blends can be described as a two-step process with the formation of a neutral exciton in either polymer followed by exciton diffusion to, and efficient dissociation at, a donoracceptor interface. Exciton dissociation produces interfacial electronhole pairs (or geminate polaron pairs) initially localized at the donoracceptor interface. In the PFBF8BT system the majority of these electronhole pairs relax to form charge-transfer excitons. These charge-transfer excitons (or exciplexes) can recombine radiatively (in competition with relaxation to triplets) with characteristic delayed and red-shifted emission.16 The separation of interfacial electron hole pairs can be described by a field-dependent Onsager-like dissociation process, with photocurrent yield and quenching of exciplex emission exhibiting identical electric-field dependences Received: June 22, 2011 Revised: July 29, 2011 Published: August 23, 2011 19364
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The Journal of Physical Chemistry C for PFBF8BT blends.17 Thus interfacial electronhole pairs can be thought of as an intermediate state that can either undergo separation to form free charges or relaxation and recombination through the exciplex state.18 Indeed, the red-shifted emission from the exciplex state can be used as an indicator of the efficiency of interfacial charge separation. Here we study laterally phase-separated blends of PFB and F8BT spin-coated from xylene. Although the micrometer-sized domains exhibited by these blends are much larger than desired for optimizing device efficiency, they exhibit a hierarchy of phase separation with finer-scale intermixing within these mesoscale domains. Furthermore, xylene-processed blends allow study with techniques such as confocal microscopy and have been used as a test-bed for examining the relationship between domain structure and device performance.11,13,23 Indeed, the structure and formation of such polymer blends is of interest in their own right24 and the use conjugated polymers allows for new insight into film microstructure.1 Whereas X-ray microscopy has been used previously to two-dimensionally map the (vertically averaged) composition of such blends,11 photoluminescence maps are presented here that provide information about the nature of intermixing on the molecular level. We compare these photoluminescence maps with photocurrent maps acquired using different laser sources to selectively excite each polymer component. This allows us to correlate local photocurrent generation efficiency with photoluminescence quenching efficiency and exciplex emission. We additionally examine the influence of capping layers on photocurrent collection.
’ EXPERIMENTAL DETAILS Devices were fabricated on ITO-covered glass substrates. After cleaning in methanol, acetone, and 2-propanol, the sample was oxygen plasma treated for 10 min at 250 W (Tegal). A 60 nm thick poly(3,4-ethylenedioxythiophene)polystyrenesulfonic acid (PEDOTPSS) layer was spin-coated after plasma etching and annealed under nitrogen flow for 30 min at 230 °C. The photoactive layer, a 1:1 blend of PFB (Mw = 60 kg/mol, Figure 1a) and F8BT (Mw = 100 kg/mol, Figure 1b, both from Cambridge Display Technology) was deposited on top of the PEDOTPSS layer by spin-coating (20 mg/mL in xylene). A spin-speed of 1500 rpm was used to achieve a thickness of 90100 nm (Dektak 6 M profilometer). Regular devices were completed by evaporating a 100 nm thick aluminum top electrode at a pressure of less than 106 mbar to define an active device area of 4.5 mm2. Semitransparent devices were completed by the evaporation of a 25 nm thick aluminum electrode. Spin coating and evaporation were carried out in a glovebox with water and oxygen levels of 90%) compared to those of PFB-rich domains (∼70%).11 Spectrally resolved mapping can be used to identify the two polymers simply from their photoluminenscence fingerprint.25 Also the bright spots within the isolated domains can be identified as F8BT-rich spots embedded in PFB-rich regions, which occur in PFBF8BT blends spun from xylene. The assignment is in very good agreement with previous work on photoluminenscence and Raman spectroscopy of PFB and F8BT.23,26,27 Photocurrent maps for different excitation wavelengths (λex = 375, 405, and 445 nm) are presented in Figure 4. Starting with the photocurrent map acquired at λex = 405 nm, Figure 4b,
Figure 3. Two-dimensional photoluminescence spectra of the PFBF8BT blend. Total integrated PL (410690 nm) (a), PFB PL (435475 nm) (b), F8BT PL (535565 nm) (c), excitation wavelength λex = 405 nm.
photocurrent is significantly enhanced in regions with F8BT as the majority component (Figure 3) with values of up to 4.7 nA at ∼2 μW laser illumination compared to 2.22.6 nA in areas with predominantly PFB. The interface between the two phases does not show enhanced photocurrent generation. This observation is consistent with the results of Snaith et al.23 and Coffey et al.13 who investigated the dependence of performance on blend composition and found a high F8BT content to be beneficial. The simultaneous acquisition of photocurrent and photoluminescence maps allows us to unambiguously assign regions of efficient photocurrent generation to local film composition. Comparing the photocurrent maps acquired using selective excitation, in general we observe that the magnitude of photocurrent increases with excitation wavelength, supporting the finding that predominantly F8BT domains are contributing to photocurrent generation. At 445 nm (Figure 4c), the highest photocurrent (maximum 8.1 nA) is obtained. F8BT-rich spots 19366
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Figure 5. Cross-sectional images taken from the photocurrent images shown in Figures 4 and 5. Regular device architecture after background subtraction (a), and semitransparent device after background subtraction (b).
Figure 4. Two-dimensional photocurrent images (20 20 μm) of a PFBF8BT blend: λex = 375 nm (a), λex = 405 nm (b), λex = 445 nm (c).
in PFB islands can also be located and identified in the photocurrent map with a resolution of ∼700800 nm, which is comparable to the resolution of the photoluminescence maps of ∼700 nm. Interestingly, even at λex = 375 nm, where PFB predominantly absorbs, photocurrent generation in F8BT-rich domains is slightly more efficient than in PFB-rich domains (Figure 4a). However, photocurrent contrast is small and the generation profile appears quite uniform throughout the film with only 17% difference at most between F8BT-rich and PFB-rich areas. For λex = 405 nm, both absolute current and current contrast increase. As expected for a larger number of F8BT molecules excited at 405 nm, there is a difference in photocurrent of 30% between the F8BT-rich and PFB-rich domains, as shown in the cross sections in Figure 5. Domain contrast is further increased with excitation at 445 nm, where only F8BT absorbs, with current contrast
increasing to 41%. The observation of both increased total current and increased current contrast when going from PFB excitation to F8BT excitation points to photocurrent generation being more efficient in F8BT domains than in PFB domains. This observation is somewhat surprising given the higher degree of intermixing within the PFB-rich domain and more efficient exciton quenching. The larger thickness of F8BT-rich domains compared to PFB-rich domains could in principle explain the difference in observed photocurrent and photocurrent constrast. To demonstrate that film thickness alone does not explain the observed results, we have carried out calculations on the basis of a semiquantitative model. On the basis of thickness and domain purity data obtained with scanning transmission X-ray microscopy (STXM),11 which have used films prepared in identical fashions, and the absorption data of Figure 2a, we first calculated the number of absorbed photons per second in each domain for 405 nm excitation, assuming a single pass of the light through the device for a constant laser power of 2 μW. The F8BT domain thickness was taken to be 100 nm with a domain purity of 95% and PFB domain thickness was taken to be 70 nm with a purity around 70%. For the same incident photon flux, coincidentally each phase is calculated to absorb 1.6 1012 photons per second due to the slightly higher absorption coefficient of PFB at 405 nm. When the ratios of collected current to absorbed photons are compared, the F8BT-rich domain is calculated to have a quantum efficiency of 1.8% compared to only 0.9% for the PFB-rich domain. Table 1 summarizes these results. Thus even though the F8BT-rich domain is slightly thicker than the 19367
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Table 1. Absorbed Photon Flux, Photocurrent, F8BTPFB Absorbed Photon Ratio, and Quantum Efficiency for Different Domains Obtained from a Semiquantitative Analysis of the Photocurrent Data Depending on Excitation Wavelength excitation wavelength (nm)
majority component in domain
405
F8BT
405
PFB
photons/s total
charges/s total
F8BTPFB absorption ratio
QE (%)
1.6 10
2.9 10
10
13.4
1.8
1.6 1012
1.4 1010
0.3
0.9
12
Figure 6. Cross sections from a regular device containing both photocurrent and photoluminescence data taken at λex = 405 nm (a). Normalized PL spectra acquired at different regions of the film. Exciplex emission is observed in the region from ∼600 to 660 nm, increasing with PFB content (b). Photocurrent image showing where spectra 14 from (b) were taken (c). Schematic cross-section of film structure depicting PFB capping layers (d).
PFB-rich domain, for 405 nm illumination each phase absorbs approximately the same photon flux, indicating that the F8BTrich domain is indeed significantly more efficient. Although we acknowledge the limitations of our semiquantiative analysis, refinements to our modeling such as the use of the transfer matrix model to account for interference effects are unlikely to alter this conclusion. Furthermore, observations of devices with semitransparent aluminum electrodes where interference effects would be limited exhibit similar contrast ratios (see below). In Figure 6a, a direct comparison between photocurrent and photoluminescence signal is shown, using cross sections of the data presented in Figures 3 and 4. First, a clear correlation between F8BT PL and photocurrent is evident. Regions of strong F8BT PL also show enhanced photocurrent generation. The anticorrelation between PFB PL and photocurrent is also observed; even photocurrent increases within the F8BT-rich spots in PFB (at x = 3 μm) can be identified. That the higher efficiency of F8BT-rich domains is correlated with increased F8BT emission is counterintuitive. The photoluminescence observed from the F8BT-rich domain indicates that many excitons are not able to reach interfaces. One possible explanation for why the F8BTrich domain is more efficient given the high domain purity of this phase (∼95%) is the existence of a thin PFB-wetting layer at the bottom of the F8BT-rich phase,28,29 between the ITO/PEDOTPSS electrode and the active layer. PFBF8BT bilayers are observed to be surprisingly efficient, outperforming even blends processed from chloroform with a nanoscale intermixed morphology.21 Thus dissociation of excitons at, and the efficient separation of
electronhole pairs from, the PFBF8BT interface at the bottom of F8BT-rich phases may provide an explanation for the surprisingly higher efficiency of this phase. Further insight into the correlation between photophysics and photocurrent generation can be obtained by studying photoluminscence spectra of different regions of the blend. Figure 6b contains a plot of normalized photoluminescence intensity versus wavelength for four different regions of the blend, with Figure 6c indicating where in the blend the spectra were taken. For an F8BT-rich area (1) hardly any PFB can be detected, whereas an area where there is evidence for a PFB capping layer (a region on top of the F8BT-rich domain capped by PFB,29 Figure Figure 6d for a schematic) has a spectrum of type 2. Although a small amount of PFB PL is evident, F8BT is still the majority component and there is no significant change in the shape of the spectra. For an F8BT-rich spot inside a PFB island (3), however, the situation is different. Besides a substantial amount of PFB and F8BT being present, there is a pronounced tail present in the region from ∼600 to 660 nm, which can be attributed to exciplex emission.17 Even more pronounced exciplex emission is found in regions that are PFB-rich (4). Distinct exciplex emission is found in addition to a relatively weak F8BT signal. The observation of exciplex emission in xylene blends with coarse phase separation is slightly surprising, as it has not been previously obseved. Although there is micrometer-sized phase separation, there is still pronounced intermixing of F8BT in PFB in the PFB-rich phase. Thus without spatially resolving the PL emission this exciplex emission is swamped by F8BT emission from the almost-pure F8BT-rich phase. If we compare areas of strong 19368
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The Journal of Physical Chemistry C exciplex emission with areas of inefficient photocurrent generation, we find a very good correlation between the two. As exciplex photoluminescence is a form of radiative recombination across the heterojunction, it represents a useful local probe for interfacial recombination losses. Exciplexes that have recombined radiatively can no longer be split into free charges and contribute to the photocurrent. This explains why photocurrent is low in regions of strong exciplex emission and increased photocurrent generation is observed where less exciplex emission is found. On a molecular level the F8BT molecules are likely to be finely dispersed in PFB-rich domains, which is reflected in the efficient photoluminescence quenching of F8BT excitons in these regions. The strong exciplex emission in these regions shows that a significant number of exciplexes can be created but not converted to photocurrent, which is also consistent with finely dispersed F8BT molecules in PFB-rich domains. On a macroscopic scale, increased exciplex emission accompanied by reduced photocurrent has also been reported for poly(phenylenevinylene) (PPV) blends by Yin et al.30 Hence measuring and monitoring the exciplex emission of a blend where it is spectrally separated from the excitonic emission of the individual components can be a useful way of screening the potential of a particular blend morphology for efficient photocurrent generation. Finally, we comment on the influence of surface capping layers on charge collection. During spin coating, polymer blends undergo a complex behavior where wetting layers of one polymer can form at the substratefilm or the filmair interface.28,31 As briefly referred to above, there are portions on top of the F8BTrich domain that are capped by a pure PFB capping layer.29 In Figure 7 we present photocurrent images where we highlight regions corresponding to PFB-surface capping regions. These images were taken by a device with a semitransparent aluminum electrode with illumination through this top electrode. We observe similar effects with standard devices and illumination through ITO; however, the effects are more pronounced for illumination through a semitransparent electrode. Due to the smaller fraction of light transmitted to the active layer (a result of the lower transmittance of the semitransparent aluminum electrode compared to that of the ITO electrode) the photocurrent is lower compared to that of the regular device structure. In Figure 7 three two-dimensional photocurrent maps of semitransparent devices excited at λex = 375 nm (a), λex = 405 nm (b), and λex = 445 nm (c) are displayed. Reduced current in some areas of the interconnected F8BT phase is observed in the semitransparent device, which is not present in the λex = 375 nm case (indicated by green circles in Figure 7), but pronounced in the λex = 445 nm map. The presence of these regions of lower photocurrent is attributed to a capping layer of PFB at the surfaceair interface of the active layer.29 Thus, although the presence of this capping layer has been shown to produce increased surface charging with scanning Kelvin probe microscopy,32 we present direct evidence here that this buildup of charge can facilitate charge recombination in an operational device. The reason this effect is more prominent for illumination through the semitransparent aluminum electrode may be due to the different excitation profiles. For illumination through the ITO/PEDOTPSS electrode more charges will be generated at the bottom of the device that would have a better chance of navigating around the PFB capping layer. For illumination through the aluminum electrode more charges will be generated in the vicinity of the capping layer, enhancing recombination there.
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Figure 7. Two-dimensional photocurrent images (20 20 μm) of a PFBF8BT blend: λex = 375 nm (a), λex = 405 nm (b), λex = 445 nm (c). The green circles indicate the presence of a PFB capping layer.
’ SUMMARY AND CONCLUSIONS Using simultaneous photoluminescence and photocurrent mapping, we have characterized the photovoltaic model system PFBF8BT on a submicrometer scale, revealing the interplay between local composition and photocurrent generation. We find that regardless of excitation wavelength the F8BT-rich domain is more efficient at generating photocurrent despite a relatively high degree of exciton recombination. Pronounced exciplex emission is observed in the PFBrich domain, suggesting that the increased intermixing of F8BT and PFB in this phase results in efficient geminate recombination rather than promoting charge separation. Finally, we have also presented evidence that the PFB surface capping layers present in PFBF8BT blends hinder charge 19369
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’ ASSOCIATED CONTENT
bS
Supporting Information. Two-dimensional photoluminescence spectra of the PFBF8BT blend. Unnormalized spectra at different regions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Present address: Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia. E-mail: christopher.
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Engineering and Physical Sciences Research Council (T.J.K.B. and C.R.M., EP/E051804/ 1). We thank Dr. Dhritiman Gupta and Dr. Bettina Friedel for useful discussions, and Cambridge Display Technology for the provision of PFB and F8BT.
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