Article pubs.acs.org/Langmuir
Determination of Fullerene Scattering Length Density: A Critical Parameter for Understanding the Fullerene Distribution in Bulk Heterojunction Organic Photovoltaic Devices Andrew J. Clulow,† Ardalan Armin,† Kwan H. Lee,† Ajay K. Pandey,† Chen Tao,† Marappan Velusamy,† Michael James,‡,§ Andrew Nelson,‡ Paul L. Burn,*,† Ian R. Gentle,† and Paul Meredith† †
Centre for Organic Photonics & Electronics, The University of Queensland, St. Lucia, QLD 4072, Australia The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia ‡
ABSTRACT: Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6−4.4) × 10−6 Å−2) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.
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INTRODUCTION
Neutron reflectometry has grown in popularity as a nondestructive technique to probe the buried interfaces and nanoscale film structure within layered organic films typically used in optoelectronic applications such as fluorescent sensing films1 and organic light emitting diode (OLED) multilayer stacks2. More recently, there has been a dramatic increase in the number of reports on the use of neutron scattering techniques and in particular neutron reflectometry to characterize the distribution of the components in organic photovoltaic (OPV) polymer/fullerene blend films.3−14 This increase in interest is due to a number of factors, including the need to correlate film structure with device performance, the importance of understanding the role of interfaces and diffusion between layers, and the fact that fullerenes such as [6,6]-phenyl-C61-butyric-acidmethyl-ester (60-PCBM) and [6,6]-phenyl-C71-butyric-acidmethyl-ester (70-PCBM) (Figure 1), which are common components in the active layers of OPV devices,15−19 do not require deuteration20 to provide the necessary neutron scattering contrast in the experiment as a result of their low relative level of protonation. It is not uncommon that the rapid uptake of a technique often leads to wide variations in values used to analyze the data leading to differing interpretations of results. For the case of published neutron scattering reports, this uncertainty has arisen directly from the scattering length densities (SLDs) of the fullerenes with a number of significantly different values reported (estimated or measured) for 60PCBM.3−13 In the case of 70-PCBM there has only been a © 2014 American Chemical Society
Figure 1. Chemical structures and molecular formulae of PCDTBT and fullerene electron acceptors 60-PCBM and 70-PCBM.
single report of its SLD.14 To characterize the distribution and packing of the fullerene molecules accurately within a bulk heterojunction (BHJ) layer by neutron reflectometry (or other neutron scattering methods), the SLDs of the individual Received: October 17, 2013 Revised: December 16, 2013 Published: January 27, 2014 1410
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10−6 Å−2.3−12 The SLD of 70-PCBM has been reported to be 4.42 × 10−6 Å−2 on the basis of a calculation using the NIST online database, although it is unclear how the exact density was determined.14 Our approach to determining a precise value for the SLDs of 60-PCBM and 70-PCBM was to measure the reflectivity profiles of films composed of different ratios of either 60PCBM or 70-PCBM blended with PCDTBT. Thin films were spin-coated directly onto silicon wafers with a natural oxide layer from 1,2-dichlorobenzene (DCB) solutions. Two studies were performed: first, films of lower-molecular-weight (lowM̅ w) PCDTBT blended with the fullerenes in weight ratios of 1:1, 1:2, and 1:4 were measured to examine the influence of the blend ratio on the film density; second, films of highermolecular-weight (high-M̅ w) PCDTBT blended with the fullerenes in the most commonly used ratio of 1:4 were measured to determine the influence of the polymer molecular weight. Neat PCDTBT films of each molecular weight were also prepared to determine the SLD of the polymer.
polymer and fullerene components must be known. In this article, we describe a method for determining the SLDs of the two most commonly used fullerene acceptors, 60-PCBM and 70-PCBM (Figure 1). For this study the polymer component of the BHJ films was chosen to be poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) because it is a high-efficiency donor-acceptor-type polymer. We show that PCDTBT with different molecular weights and polydispersities forms uniformly distributed blends with fullerenes. The neutron scattering length density of each species is defined by SLDi =
ρi NAv Mi
∑ njbj j
(1)
where ρi is the mass density of molecule i, NAv is Avogadro’s number, Mi is the molar mass of species i, nj is the number of scattering nuclei j per molecule i, and bj is the corresponding bound coherent neutron scattering length of nucleus j. A large difference in the SLDs between the molecules is important because it gives rise to the contrast that is necessary to distinguish the proportion and positioning of the components within a film. The relatively small number of hydrogen nuclei (bH = −3.74 × 10−5 Å)20 in 60-PCBM and 70-PCBM leads to their calculated molecular scattering lengths (given by the summation term in eq 1) of 4.38 × 10−3 and 5.05 × 10−3 Å, respectively. PCDTBT has protonated alkyl chains leading to a smaller calculated molecular scattering length of 1.47 × 10−3 Å for the repeat unit of the polymer. It is interesting that there has been one report using a partially deuterated d5-60-PCBM for neutron scattering measurements,3 but calculations of the molecular scattering length (bD = 6.67 × 10−5 Å)20 show that this is unnecessary for most materials that would be used in conjunction with the fullerenes in BHJ films. However, it is not the sum of the scattering lengths of the nuclei in the molecule that is key to understanding the distribution of materials in a multicomponent film but rather the SLD, and this depends on the composition and mass density of the compound as shown by eq 1. There have been two reported SLD values for neat 60-PCBM films of 3.6 × 10−6 and 4.34 × 10−6 Å−2.5,11 The former value was determined from phase-sensitive reflectometry measurements with the latter SLD calculated by co-refining data from X-ray and neutron reflectivity measurements on a film with a high surface roughness of 134 Å. At first sight, these SLDs seem similar, but using the calculated molecular scattering length for 60PCBM shows that these correspond to mass densities of 1.24 and 1.52 g cm−3, respectively. Such large differences in density would clearly give rise to significant differences in multiple properties of the films such as charge transport and the photocurrent generation profile under device-relevant conditions. It is not generally appreciated that it is difficult to create uniform neat films of the fullerenes by solution processing, and in fact, the density of such a film is not necessarily representative of the fullerenes blended with another material. In the case where the SLDs for 60-PCBM have been determined from blended films, the approach has been to choose values that enable the reflectometry profile to be fitted or alternatively to use values from films that have been sequentially deposited, so-called bilayer films. Both of these methods give SLDs for 60-PCBM in the range of (3.6−4.4) ×
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EXPERIMENTAL SECTION
Low-M̅ w PCDTBT was synthesized and purified in our laboratory using a standard literature procedure.21 GPC (1,2,4-trichlorobenzene, 140 °C): M̅ n = 22.9 kDa, M̅ w = 45.4 kDa. High-M̅ w PCDTBT was purchased from SJPC and used as received. GPC (1,2,4-trichlorobenzene, 140 °C): M̅ n = 22.7 kDa, M̅ w = 122.2 kDa. 60-PCBM and 70-PCBM were purchased from American Dye Source, Inc. and used as received. In the following descriptions, PCBM is used to denote both 60-PCBM and 70-PCBM. The following film depositions were performed under a nitrogen atmosphere in a glovebox within a class 1000 clean room. Silicon wafers of 50 mm diameter were cleaned in piranha solution, which was a 2:1 v/v mixture of 98% sulfuric acid and 30% aqueous hydrogen peroxide. The substrates were then ultrasonicated in acetone (5 min) and 2-propanol (5 min) prior to film deposition. Low-M̅ w PCDTBT Films. A solution of low-M̅ w PCDTBT (8 mg mL−1) in 1,2-dichlorobenzene was prepared at ambient temperature and filtered. Thin films were spin-coated onto silicon substrates at 700 rpm for 60 s and dried at ambient temperature. High-M̅ w PCDTBT Films. A suspension of high-M̅ w PCDTBT (5 mg mL−1) in 1,2-dichlorobenzene was stirred at 170 °C on a hot plate for 15 min. The temperature of the hot plate was gradually reduced to 35 °C over 90 min before the solution was removed from the hot plate. The solution was filtered through glass wool before thin films were spin coated. The films were monitored visually and spinning was stopped when the Newton rings visible during film drying were no longer present. Two films were spun, the first at 500 rpm for ∼90 s and the second at 700 rpm for ∼60 s. The films were dried at ambient temperature. Low-M̅ w PCDTBT/PCBM Blend Films. Solutions of low-M̅ w PCDTBT (14 mg mL−1) and PCBM (14, 28, and 56 mg mL−1) were prepared in 1,2-dichorobenzene. The solutions were stirred at ambient temperature overnight and filtered before they were combined in 1:1 v/v ratios to give PCDTBT/PCBM solutions with weight ratios of 1:1, 1:2, and 1:4. The PCDTBT/PCBM blend films were spin coated onto silicon substrates at 1000 rpm for 60 s, and the films were then dried on a hot plate at 60 °C for 10 min. High-M̅ w PCDTBT/PCBM Blend Films. Suspensions of high-M̅ w PCDTBT in 1,2-dichlorobenzene (10 mg mL−1) were stirred at 170 °C on a hot plate for 10 min. The temperature of the hot plate was gradually decreased to 70 °C over 45 min. PCBM solutions in 1,2dichlorobenzene (40 mg mL−1) were prepared separately and were heated on the hot plate when it reached 70 °C. The PCDTBT and PCBM solutions were mixed in 1:1 v/v ratios while at 70 °C to give PCDTBT/PCBM solutions of weight ratio 1:4. The temperature of the hot plate was then gradually reduced to 35 °C over 45 min before the solutions were removed. The solutions were filtered through glass 1411
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wool before the blend films were spin coated. The films were monitored visually and spinning was stopped when the Newton rings visible during film drying were no longer present. Two films were spun from each PCDTBT/PCBM solution at 1300 rpm for ∼60 s each, and the films were dried at 60 °C on a hot plate for 10 min. Neutron Reflectometry. Neutron reflectometry measurements were performed using the Platypus time-of-flight neutron reflectometer and a cold neutron spectrum (2.8 Å < λ < 18.0 Å) at the OPAL 20 MW research reactor (Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, Australia).22 Neutron pulses (20 Hz) were generated using a disc chopper system (EADS Astrium GmbH) in medium resolution mode (Δλ/λ = 4%) and recorded on a 2D helium-3 neutron detector (Denex GmbH). Reflected beam spectra were collected at 0.5° for 20−30 min and at 2.0° for 68−182 min, giving a Q range extending out to 0.15 Å−1. Direct beam measurements were collected under the same collimation conditions. Experiments were conducted under a 4 L min−1 nitrogen flow. Analysis of the reflectivity profiles was performed using the Motofit reflectometry analysis program.23 All of the fits described included a 12 Å thick silicon oxide layer on the surface of the substrate. The uncertainties in the thickness and roughness values (Table 1),
SLDs of the films for each blend ratio were therefore independent of both the polymer molecular weight and whether 60-PCBM or 70-PCBM was in the blend. The next step in the analysis was to determine the volume fractions of PCDTBT in the PCDTBT/fullerene blend films, which were calculated by assuming that the films were fully dense; that is, all of the available volume was occupied by PCDTBT and fullerene (ϕPCDTBT + ϕPCBM = 1), yielding ϕPCDTBT SLDPCDTBT + ϕPCBM SLDPCBM = SLDblend
ϕPCDTBT =
low-M̅ w PCDTBT high-M̅ w PCDTBT (500 rpm) high-M̅ w PCDTBT (700 rpm) 1:1 with low-M̅ w 60-PCBM PCDTBT 70-PCBM 60-PCBM 1:2 with low-M̅ w PCDTBT 70-PCBM 60-PCBM 1:4 with low-M̅ w PCDTBT 70-PCBM 60-PCBM 1:4 with high-M̅ w PCDTBT 70-PCBM
thickness (Å)
roughness (Å)
331 365 303 236
11 4 5 11
1.45 1.43 1.41 2.85
WPCDTBT =
241 346
8 7
2.82 ± 0.14 3.31 ± 0.17
342 536
8 8
3.35 ± 0.17 3.94 ± 0.20
545 754
9 10
3.97 ± 0.20 3.79 ± 0.19
753 732 717
10 12 10
3.69 ± 0.18 3.85 ± 0.19 3.83 ± 0.19
ϕPCDTBTρPCDTBT (ϕPCDTBTρPCDTBT + ϕPCBMρPCBM )
× 100% (3)
where the mass densities ρi of the individual components were calculated from their SLDs using eq 1. In this model, we have assumed that the volume occupied by each PCDTBT and PCBM (macro)molecule in the blend is the same as in a neat film. The SLD of the neat film of low-M̅ w PCDTBT was found to be (1.45 ± 0.07) × 10−6 Å−2 (Figure 2), giving a corresponding mass density of 1.15 ± 0.06 g cm−3 (eq 1). The SLDs of the neat films of the high-M̅ w PCDTBT were within error the same, being measured to be (1.41 ± 0.07) × 10−6 and (1.43 ± 0.07) × 10−6 Å−2 for the two films spun at different rates. This indicated that for the molecular weights examined the degree of polymerization is sufficiently high that the SLD of the polymer is not strongly influenced by the phenyl end groups (Figure 1). For this reason, the molecular weight of the polymer repeat unit alone (MPCDTBT = 702 g mol−1) was used when calculating the mass density of PCDTBT using eq 1. When analyzing the high-M̅ w PCDTBT blend films, an average SLD of (1.42 ± 0.07) × 10−6 Å−2 was used, and this affords a corresponding mass density of 1.13 ± 0.06 g cm−3 for the higher-molecular-weight polymer. In the final step of the analysis, we used eqs 2 and 3 to determine the SLDs for 60-PCBM and 70-PCBM within the blends. This was achieved by comparing the calculated WPCDTBT values to those expected from the initial blend ratios (50% for 1:1 blends, 33.3% for 1:2 blends, and 20% for 1:4 blends). The WPCDTBT for each blend was calculated using the experimentally determined values of SLDPCDTBT and SLDblend, with the SLDPCBM for the fullerenes being varied from 4.20 × 10−6 to 5.20 × 10−6 Å−2 in 0.01 × 10−6 Å−2 increments. The resulting plots of WPCDTBT versus fullerene SLD are given in Figure 3A,B for 60-PCBM and 70-PCBM, respectively. The SLDs at which the calculated WPCDTBT crosses those expected from the blend ratio of the spin-coating solution (horizontal dashed lines) are 4.47 × 10−6 to 4.91 × 10−6 Å−2. It is important to note at this stage that the lowest SLD determined for the fullerenes is already as high as the highest previously reported value. The differences between the calculated and expected WPCDTBT values (ΔWPCDTBT) were squared (to remove differences in the sign of the errors), and these squared errors are shown in Figure 3C,D plotted against the fullerene
SLD (× 10−6 Å−2) ± ± ± ±
(2)
where ϕi is the volume fraction of species i and SLDi is the corresponding scattering length density. The weight percentage of PCDTBT (WPCDTBT) for each of the blend films was then found after calculating the volume-fraction-weighted densities of the two components in each blend (ϕiρi) using the equation
Table 1. Modeled Layer Thicknesses, Roughnesses, and SLDs for Neat PCDTBT and Blend Films with 60-PCBM and 70-PCBM film
( SLDblend − SLDPCBM ) ( SLDPCDTBT − SLDPCBM )
0.07 0.07 0.07 0.14
obtained from the parameter covariance matrix of the fitting process, were all ≤2 Å. The dQ/Q resolution for the optimized fits was in the range of 6−8%, which arises from the inherent dQ/Q resolution of the instrument (5%) and an extra contribution used to account for small variations in sample thickness across the illuminated area. In practice this extra term means that the uncertainty in layer thickness is of the order 10 Å. Modeled and calculated SLD values were ascribed a relative error of ±5%, and the corresponding errors in ρi were determined by propagating these errors throughout eq 1 using the chain rule.
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RESULTS AND DISCUSSION It was found in each case that the neutron reflectivity profiles (Figure 2A,C) could be modeled as a single layer of uniform scattering length density on top of the silicon substrates (Figure 2B,D). The fact that the reflectivity profiles could be fitted to a single uniform layer (independent of blend ratio) means that the fullerenes are evenly distributed throughout the depth of the films. The films with the same polymer/fullerene blend weight ratio had essentially the same values of thickness and bulk SLD and similar interfacial roughness (Table 1). The 1412
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Figure 2. (A, C) Neutron reflectivity profiles for neat PCDTBT and blend films. The 60-PCBM blend profile is given above the 70-PCBM blend profile for each blend ratio in plot A. Individual points indicate experimental data and solid lines indicate model fits, traces are offset for clarity. (B, D) Modeled SLD vs thickness plots, solid lines indicate 60-PCBM blends and dashed lines indicate 70-PCBM blends. Distance = 0 Å represents the silicon substrate.
SLD. For each fullerene, the squared errors for the five films were added together, and the minimum of the total squared error was taken to be the precise SLD value for the fullerene. The precise SLDs were found to be (4.66 ± 0.23) × 10−6 −2 Å for 60-PCBM and (4.74 ± 0.24) × 10−6 Å−2 for 70-PCBM. Both SLDs correspond to a mass density of 1.61 ± 0.08 g cm−3 for neat 60-PCBM and 70-PCBM (eq 1). The mass density of 60-PCBM has been reported from single-crystal structure analysis to be 1.62 g cm−3 in the absence of co-crystallized solvent24 and 1.67 g cm−3 when cocrystallized with 1,2dichlorobenzene25. These latter mass densities correspond to SLDs of 4.69 × 10−6 and 4.58 × 10−6 Å−2, respectively, for the crystals. The similarity of the SLDs from the neutron measurements and the crystal structures suggests that the blend films are composed of a uniform distribution of fullerene clusters. These results are also consistent with microwave conductivity measurements that show that 60-PCBM clusters can form at polymer/fullerene blend ratios that are as low as 99:1.26 It is therefore not surprising that we should observe such clusters in our films that have much higher fullerene loadings. Finally, we now show how the precise SLD values for the fullerenes can be used to quantify the blend composition with other materials. It has been previously reported that the deposition of 60-PCBM onto poly(3-n-hexylthiophene) (P3HT) can lead to a fullerene-rich layer at the air interface.7,13 In ref 7, XPS data showed that P3HT had migrated into the
upper 60-PCBM layer during the deposition process, and by taking the SLD (4.34 × 10−6 Å−2) of the fullerene-rich layer and using the SLD of 60-PCBM in this work along with eqs 2 and 3, we calculate that the WP3HT for the fullerene-rich layer was around 4%. In the second report, 60-PCBM was deposited onto cross-linked P3HT with the aim of reducing the intermixing of the active materials. In this latter report the SLD of the fullerene-rich layer was found to be 4.58 × 10−6 Å−2 and XPS also showed that there was P3HT near the air interface. Using the precise SLD of this work, we can determine that the concept of cross-linking had been partially successful with the WP3HT being only around 1% for the fullerene-rich layer in this latter case.13
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CONCLUSIONS
We have provided a method for determining the precise SLDs of materials that do not form neat films of sufficient smoothness for neutron reflectometry experiments. The technique also provides a means for determining the density of materials when blended. In this work, we have shown that the SLDs of 60-PCBM and 70-PCBM are similar and fall within the range of (4.47−4.91) × 10−6 Å−2, which is higher than those previously reported for these fullerenes, namely, (3.6− 4.4) × 10−6 and 4.4 × 10−6 Å−2, respectively. The determined SLD of 60-PCBM was also found to be consistent with the reported mass densities from crystallographic data. It should be noted that previous reports on the SLDs of the fullerenes do 1413
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Figure 3. (A, B) Variation of WPCDTBT with fullerene SLD for the 60-PCBM and 70-PCBM blends, respectively. (C, D) Corresponding plots of the squared difference between the calculated and expected WPCDTBT values (ΔWPCDTBT) vs the fullerene SLD.
Australian Institute for Nuclear Science and Engineering (AINSE) in providing the neutron research facilities used in this work.
not always provide sufficient information to enable a direct comparison, and hence the published results on their distribution when blended with polymers might in some cases need to be reanalyzed. With the growing popularity of neutron scattering techniques for probing the structural properties of polymer/fullerene blends, it is important that the same SLD values are used to allow meaningful comparisons of observed film structures.
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REFERENCES
(1) Cavaye, H.; Smith, A. R. G.; James, M.; Nelson, A.; Burn, P. L.; Gentle, I. R.; Lo, S.-C.; Meredith, P. Solid-State Dendrimer Sensors: Probing the Diffusion of an Explosive Analogue Using Neutron Reflectometry. Langmuir 2009, 25, 12800−12805. (2) Smith, A. R. G.; Lee, K. H.; Nelson, A.; James, M.; Burn, P. L.; Gentle, I. R. Diffusion−the Hidden Menace in Organic Optoelectronic Devices. Adv. Mater. 2012, 24, 822−826. (3) Parnell, A. J.; Dunbar, A. D. F.; Pearson, A. J.; Staniec, P. A.; Dennison, A. J. C.; Hamamatsu, H.; Skoda, M. W. A.; Lidzey, D. G.; Jones, R. A. L. Depletion of PCBM at the Cathode Interface in P3HT/ PCBM Thin Films as Quantified via Neutron Reflectivity Measurements. Adv. Mater. 2010, 22, 2444−2447. (4) Kiel, J. W.; Mackay, M. E.; Kirby, B. J.; Maranville, B. B.; Majkrzak, C. F. Phase-Sensitive Neutron Reflectometry Measurements Applied in the Study of Photovoltaic Films. J. Chem. Phys. 2010, 133, 074902−074907. (5) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Nanoparticle Concentration Profile in Polymer-Based Solar Cells. Soft Matter 2010, 6, 641−646. (6) Huang, D. M.; Mauger, S. A.; Friedrich, S.; George, S. J.; Dumitriu-LaGrange, D.; Yoon, S.; Moulé, A. J. The Consequences of Interface Mixing on Organic Photovoltaic Device Characteristics. Adv. Funct. Mater. 2011, 21, 1657−1665. (7) Lee, K. H.; Schwenn, P. E.; Smith, A. R. G.; Cavaye, H.; Shaw, P. E.; James, M.; Krueger, K. B.; Gentle, I. R.; Meredith, P.; Burn, P. L. Morphology of All-Solution-Processed “Bilayer” Organic Solar Cells. Adv. Mater. 2011, 23, 766−770.
AUTHOR INFORMATION
Corresponding Author
*Phone: +61 7334 67614. E-mail: [email protected]. Fax: +61 7334 69273. Present Address
M.J.’s current address is Australian Synchrotron, 800 Blackburn Road, Clayton VIC 3168, Australia. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Australian Research Council (DP1211572). P.L.B. and P.M. are both supported by University of Queensland Vice Chancellor’s Senior Research Fellowships. We acknowledge funding from the University of Queensland (Strategic Initiative, Centre for Organic Photonics & Electronics) and the Queensland government (National and International Research Alliances Program). We acknowledge the support of the Bragg Institute, the Australian Nuclear Science and Technology Organisation (ANSTO), and the 1414
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(8) Liu, H.-J.; Jeng, U.-S.; Yamada, N. L.; Su, A.-C.; Wu, W.-R.; Su, C.-J.; Lin, S.-J.; Wei, K.-H.; Chiu, M.-Y. Surface and Interface Porosity of Polymer/Fullerene-Derivative Thin Films Revealed by Contrast Variation of Neutron and X-ray Reflectivity. Soft Matter 2011, 7, 9276−9282. (9) Staniec, P. A.; Parnell, A. J.; Dunbar, A. D. F.; Yi, H.; Pearson, A. J.; Wang, T.; Hopkinson, P. E.; Kinane, C.; Dalgliesh, R. M.; Donald, A. M.; Ryan, A. J.; Iraqi, A.; Jones, R. A. L.; Lidzey, D. G. The Nanoscale Morphology of a PCDTBT:PCBM Photovoltaic Blend. Adv. Energy Mater. 2011, 1, 499−504. (10) Chen, H.; Hegde, R.; Browning, J.; Dadmun, M. D. The Miscibility and Depth Profile of PCBM in P3HT: Thermodynamic Information to Improve Organic Photovoltaics. Phys. Chem. Chem. Phys. 2012, 14, 5635−5641. (11) Kirschner, S. B.; Smith, N. P.; Wepasnick, K. A.; Katz, H. E.; Kirby, B. J.; Borchers, J. A.; Reich, D. H. X-ray and Neutron Reflectivity and Electronic Properties of PCBM-poly(bromo)styrene Blends and Bilayers with Poly(3-hexylthiophene). J. Mater. Chem. 2012, 22, 4364−4370. (12) Pavlopoulou, E.; Fleury, G.; Deribew, D.; Cousin, F.; Geoghegan, M.; Hadziioannou, G. Phase Separation-Driven Stratification in Conventional and Inverted P3HT:PCBM Organic Solar Cells. Org. Electron. 2013, 14, 1249−1254. (13) Tao, C.; Aljada, M.; Shaw, P. E.; Lee, K. H.; Cavaye, H.; Balfour, M. N.; Borthwick, R. J.; James, M.; Burn, P. L.; Gentle, I. R.; Meredith, P. Controlling Hierarchy in Solution-processed Polymer Solar Cells Based on Crosslinked P3HT. Adv. Energy Mater. 2013, 3, 105−112. (14) Wang, T.; Scarratt, N. W.; Yi, H.; Dunbar, A. D. F.; Pearson, A. J.; Watters, D. C.; Glen, T. S.; Brook, A. C.; Kingsley, J.; Buckley, A. R.; Skoda, M. W. A.; Donald, A. M.; Jones, R. A. L.; Iraqi, A.; Lidzey, D. G. Fabricating High Performance, Donor−Acceptor Copolymer Solar Cells by Spray-Coating in Air. Adv. Energy Mater. 2013, 3, 505− 512. (15) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Efficient Methano[70]fullerene/MDMO-PPV Bulk Heterojunction Photovoltaic Cells. Angew. Chem., Int. Ed. 2003, 42, 3371−3375. (16) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297−303. (17) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Inganäs, O.; Andersson, M. R. HighPerformance Polymer Solar Cells of an Alternating Polyfluorene Copolymer and a Fullerene Derivative. Adv. Mater. 2003, 15, 988− 991. (18) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.; Williams, S. P. Polymer−Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2010, 22, 3839−3856. (19) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. 2.5% Efficient Organic Plastic Solar Cells. Appl. Phys. Lett. 2001, 78, 841−843. (20) Sears, V. F. Neutron Scattering Lengths and Cross Sections. Neutron News 1992, 3, 26−37. (21) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletête, M.; Durocher, G.; Tao, Y.; Leclerc, M. Toward a Rational Design of Poly(2,7-Carbazole) Derivatives for Solar Cells. J. Am. Chem. Soc. 2008, 130, 732−742. (22) James, M.; Nelson, A.; Holt, S. A.; Saerbeck, T.; Hamilton, W. A.; Klose, F. The Multipurpose Time-of-Flight Neutron Reflectometer “Platypus” at Australia’s OPAL Reactor. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 632, 112−123. (23) Nelson, A. Co-refinement of Multiple-Contrast Neutron/X-ray Reflectivity Data Using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273− 276. (24) Casalegno, M.; Zanardi, S.; Frigerio, F.; Po, R.; Carbonera, C.; Marra, G.; Nicolini, T.; Raos, G.; Meille, S. V. Solvent-Free PhenylC61-Butyric Acid Methyl Ester (PCBM) from Clathrates: Insights for
Organic Photovoltaics from Crystal Structures and Molecular Dynamics. Chem. Commun. 2013, 49, 4525−4527. (25) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Influence of the Solvent on the Crystal Structure of PCBM and the Efficiency of MDMO-PPV:PCBM ‘Plastic’ Solar Cells. Chem. Commun. 2003, 2116−2118. (26) Ferguson, A. J.; Kopidakis, N.; Shaheen, S. E.; Rumbles, G. Dark Carriers, Trapping, and Activation Control of Carrier Recombination in Neat P3HT and P3HT:PCBM Blends. J. Phys. Chem. C 2011, 115, 23134−23148.
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Determination of Fullerene Scattering Length Density: A Critical Parameter for Understanding the Fullerene Distribution in Bulk Heterojunction Organic Photovoltaic Devices Andrew J. Clulow,† Ardalan Armin,† Kwan H. Lee,† Ajay K. Pandey,† Chen Tao,† Marappan Velusamy,† Michael James,‡,§ Andrew Nelson,‡ Paul L. Burn,*,† Ian R. Gentle,† and Paul Meredith† †
Centre for Organic Photonics & Electronics, The University of Queensland, St. Lucia, QLD 4072, Australia The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia § School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia ‡
ABSTRACT: Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6−4.4) × 10−6 Å−2) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.
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INTRODUCTION
Neutron reflectometry has grown in popularity as a nondestructive technique to probe the buried interfaces and nanoscale film structure within layered organic films typically used in optoelectronic applications such as fluorescent sensing films1 and organic light emitting diode (OLED) multilayer stacks2. More recently, there has been a dramatic increase in the number of reports on the use of neutron scattering techniques and in particular neutron reflectometry to characterize the distribution of the components in organic photovoltaic (OPV) polymer/fullerene blend films.3−14 This increase in interest is due to a number of factors, including the need to correlate film structure with device performance, the importance of understanding the role of interfaces and diffusion between layers, and the fact that fullerenes such as [6,6]-phenyl-C61-butyric-acidmethyl-ester (60-PCBM) and [6,6]-phenyl-C71-butyric-acidmethyl-ester (70-PCBM) (Figure 1), which are common components in the active layers of OPV devices,15−19 do not require deuteration20 to provide the necessary neutron scattering contrast in the experiment as a result of their low relative level of protonation. It is not uncommon that the rapid uptake of a technique often leads to wide variations in values used to analyze the data leading to differing interpretations of results. For the case of published neutron scattering reports, this uncertainty has arisen directly from the scattering length densities (SLDs) of the fullerenes with a number of significantly different values reported (estimated or measured) for 60PCBM.3−13 In the case of 70-PCBM there has only been a © 2014 American Chemical Society
Figure 1. Chemical structures and molecular formulae of PCDTBT and fullerene electron acceptors 60-PCBM and 70-PCBM.
single report of its SLD.14 To characterize the distribution and packing of the fullerene molecules accurately within a bulk heterojunction (BHJ) layer by neutron reflectometry (or other neutron scattering methods), the SLDs of the individual Received: October 17, 2013 Revised: December 16, 2013 Published: January 27, 2014 1410
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10−6 Å−2.3−12 The SLD of 70-PCBM has been reported to be 4.42 × 10−6 Å−2 on the basis of a calculation using the NIST online database, although it is unclear how the exact density was determined.14 Our approach to determining a precise value for the SLDs of 60-PCBM and 70-PCBM was to measure the reflectivity profiles of films composed of different ratios of either 60PCBM or 70-PCBM blended with PCDTBT. Thin films were spin-coated directly onto silicon wafers with a natural oxide layer from 1,2-dichlorobenzene (DCB) solutions. Two studies were performed: first, films of lower-molecular-weight (lowM̅ w) PCDTBT blended with the fullerenes in weight ratios of 1:1, 1:2, and 1:4 were measured to examine the influence of the blend ratio on the film density; second, films of highermolecular-weight (high-M̅ w) PCDTBT blended with the fullerenes in the most commonly used ratio of 1:4 were measured to determine the influence of the polymer molecular weight. Neat PCDTBT films of each molecular weight were also prepared to determine the SLD of the polymer.
polymer and fullerene components must be known. In this article, we describe a method for determining the SLDs of the two most commonly used fullerene acceptors, 60-PCBM and 70-PCBM (Figure 1). For this study the polymer component of the BHJ films was chosen to be poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) because it is a high-efficiency donor-acceptor-type polymer. We show that PCDTBT with different molecular weights and polydispersities forms uniformly distributed blends with fullerenes. The neutron scattering length density of each species is defined by SLDi =
ρi NAv Mi
∑ njbj j
(1)
where ρi is the mass density of molecule i, NAv is Avogadro’s number, Mi is the molar mass of species i, nj is the number of scattering nuclei j per molecule i, and bj is the corresponding bound coherent neutron scattering length of nucleus j. A large difference in the SLDs between the molecules is important because it gives rise to the contrast that is necessary to distinguish the proportion and positioning of the components within a film. The relatively small number of hydrogen nuclei (bH = −3.74 × 10−5 Å)20 in 60-PCBM and 70-PCBM leads to their calculated molecular scattering lengths (given by the summation term in eq 1) of 4.38 × 10−3 and 5.05 × 10−3 Å, respectively. PCDTBT has protonated alkyl chains leading to a smaller calculated molecular scattering length of 1.47 × 10−3 Å for the repeat unit of the polymer. It is interesting that there has been one report using a partially deuterated d5-60-PCBM for neutron scattering measurements,3 but calculations of the molecular scattering length (bD = 6.67 × 10−5 Å)20 show that this is unnecessary for most materials that would be used in conjunction with the fullerenes in BHJ films. However, it is not the sum of the scattering lengths of the nuclei in the molecule that is key to understanding the distribution of materials in a multicomponent film but rather the SLD, and this depends on the composition and mass density of the compound as shown by eq 1. There have been two reported SLD values for neat 60-PCBM films of 3.6 × 10−6 and 4.34 × 10−6 Å−2.5,11 The former value was determined from phase-sensitive reflectometry measurements with the latter SLD calculated by co-refining data from X-ray and neutron reflectivity measurements on a film with a high surface roughness of 134 Å. At first sight, these SLDs seem similar, but using the calculated molecular scattering length for 60PCBM shows that these correspond to mass densities of 1.24 and 1.52 g cm−3, respectively. Such large differences in density would clearly give rise to significant differences in multiple properties of the films such as charge transport and the photocurrent generation profile under device-relevant conditions. It is not generally appreciated that it is difficult to create uniform neat films of the fullerenes by solution processing, and in fact, the density of such a film is not necessarily representative of the fullerenes blended with another material. In the case where the SLDs for 60-PCBM have been determined from blended films, the approach has been to choose values that enable the reflectometry profile to be fitted or alternatively to use values from films that have been sequentially deposited, so-called bilayer films. Both of these methods give SLDs for 60-PCBM in the range of (3.6−4.4) ×
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EXPERIMENTAL SECTION
Low-M̅ w PCDTBT was synthesized and purified in our laboratory using a standard literature procedure.21 GPC (1,2,4-trichlorobenzene, 140 °C): M̅ n = 22.9 kDa, M̅ w = 45.4 kDa. High-M̅ w PCDTBT was purchased from SJPC and used as received. GPC (1,2,4-trichlorobenzene, 140 °C): M̅ n = 22.7 kDa, M̅ w = 122.2 kDa. 60-PCBM and 70-PCBM were purchased from American Dye Source, Inc. and used as received. In the following descriptions, PCBM is used to denote both 60-PCBM and 70-PCBM. The following film depositions were performed under a nitrogen atmosphere in a glovebox within a class 1000 clean room. Silicon wafers of 50 mm diameter were cleaned in piranha solution, which was a 2:1 v/v mixture of 98% sulfuric acid and 30% aqueous hydrogen peroxide. The substrates were then ultrasonicated in acetone (5 min) and 2-propanol (5 min) prior to film deposition. Low-M̅ w PCDTBT Films. A solution of low-M̅ w PCDTBT (8 mg mL−1) in 1,2-dichlorobenzene was prepared at ambient temperature and filtered. Thin films were spin-coated onto silicon substrates at 700 rpm for 60 s and dried at ambient temperature. High-M̅ w PCDTBT Films. A suspension of high-M̅ w PCDTBT (5 mg mL−1) in 1,2-dichlorobenzene was stirred at 170 °C on a hot plate for 15 min. The temperature of the hot plate was gradually reduced to 35 °C over 90 min before the solution was removed from the hot plate. The solution was filtered through glass wool before thin films were spin coated. The films were monitored visually and spinning was stopped when the Newton rings visible during film drying were no longer present. Two films were spun, the first at 500 rpm for ∼90 s and the second at 700 rpm for ∼60 s. The films were dried at ambient temperature. Low-M̅ w PCDTBT/PCBM Blend Films. Solutions of low-M̅ w PCDTBT (14 mg mL−1) and PCBM (14, 28, and 56 mg mL−1) were prepared in 1,2-dichorobenzene. The solutions were stirred at ambient temperature overnight and filtered before they were combined in 1:1 v/v ratios to give PCDTBT/PCBM solutions with weight ratios of 1:1, 1:2, and 1:4. The PCDTBT/PCBM blend films were spin coated onto silicon substrates at 1000 rpm for 60 s, and the films were then dried on a hot plate at 60 °C for 10 min. High-M̅ w PCDTBT/PCBM Blend Films. Suspensions of high-M̅ w PCDTBT in 1,2-dichlorobenzene (10 mg mL−1) were stirred at 170 °C on a hot plate for 10 min. The temperature of the hot plate was gradually decreased to 70 °C over 45 min. PCBM solutions in 1,2dichlorobenzene (40 mg mL−1) were prepared separately and were heated on the hot plate when it reached 70 °C. The PCDTBT and PCBM solutions were mixed in 1:1 v/v ratios while at 70 °C to give PCDTBT/PCBM solutions of weight ratio 1:4. The temperature of the hot plate was then gradually reduced to 35 °C over 45 min before the solutions were removed. The solutions were filtered through glass 1411
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wool before the blend films were spin coated. The films were monitored visually and spinning was stopped when the Newton rings visible during film drying were no longer present. Two films were spun from each PCDTBT/PCBM solution at 1300 rpm for ∼60 s each, and the films were dried at 60 °C on a hot plate for 10 min. Neutron Reflectometry. Neutron reflectometry measurements were performed using the Platypus time-of-flight neutron reflectometer and a cold neutron spectrum (2.8 Å < λ < 18.0 Å) at the OPAL 20 MW research reactor (Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, Australia).22 Neutron pulses (20 Hz) were generated using a disc chopper system (EADS Astrium GmbH) in medium resolution mode (Δλ/λ = 4%) and recorded on a 2D helium-3 neutron detector (Denex GmbH). Reflected beam spectra were collected at 0.5° for 20−30 min and at 2.0° for 68−182 min, giving a Q range extending out to 0.15 Å−1. Direct beam measurements were collected under the same collimation conditions. Experiments were conducted under a 4 L min−1 nitrogen flow. Analysis of the reflectivity profiles was performed using the Motofit reflectometry analysis program.23 All of the fits described included a 12 Å thick silicon oxide layer on the surface of the substrate. The uncertainties in the thickness and roughness values (Table 1),
SLDs of the films for each blend ratio were therefore independent of both the polymer molecular weight and whether 60-PCBM or 70-PCBM was in the blend. The next step in the analysis was to determine the volume fractions of PCDTBT in the PCDTBT/fullerene blend films, which were calculated by assuming that the films were fully dense; that is, all of the available volume was occupied by PCDTBT and fullerene (ϕPCDTBT + ϕPCBM = 1), yielding ϕPCDTBT SLDPCDTBT + ϕPCBM SLDPCBM = SLDblend
ϕPCDTBT =
low-M̅ w PCDTBT high-M̅ w PCDTBT (500 rpm) high-M̅ w PCDTBT (700 rpm) 1:1 with low-M̅ w 60-PCBM PCDTBT 70-PCBM 60-PCBM 1:2 with low-M̅ w PCDTBT 70-PCBM 60-PCBM 1:4 with low-M̅ w PCDTBT 70-PCBM 60-PCBM 1:4 with high-M̅ w PCDTBT 70-PCBM
thickness (Å)
roughness (Å)
331 365 303 236
11 4 5 11
1.45 1.43 1.41 2.85
WPCDTBT =
241 346
8 7
2.82 ± 0.14 3.31 ± 0.17
342 536
8 8
3.35 ± 0.17 3.94 ± 0.20
545 754
9 10
3.97 ± 0.20 3.79 ± 0.19
753 732 717
10 12 10
3.69 ± 0.18 3.85 ± 0.19 3.83 ± 0.19
ϕPCDTBTρPCDTBT (ϕPCDTBTρPCDTBT + ϕPCBMρPCBM )
× 100% (3)
where the mass densities ρi of the individual components were calculated from their SLDs using eq 1. In this model, we have assumed that the volume occupied by each PCDTBT and PCBM (macro)molecule in the blend is the same as in a neat film. The SLD of the neat film of low-M̅ w PCDTBT was found to be (1.45 ± 0.07) × 10−6 Å−2 (Figure 2), giving a corresponding mass density of 1.15 ± 0.06 g cm−3 (eq 1). The SLDs of the neat films of the high-M̅ w PCDTBT were within error the same, being measured to be (1.41 ± 0.07) × 10−6 and (1.43 ± 0.07) × 10−6 Å−2 for the two films spun at different rates. This indicated that for the molecular weights examined the degree of polymerization is sufficiently high that the SLD of the polymer is not strongly influenced by the phenyl end groups (Figure 1). For this reason, the molecular weight of the polymer repeat unit alone (MPCDTBT = 702 g mol−1) was used when calculating the mass density of PCDTBT using eq 1. When analyzing the high-M̅ w PCDTBT blend films, an average SLD of (1.42 ± 0.07) × 10−6 Å−2 was used, and this affords a corresponding mass density of 1.13 ± 0.06 g cm−3 for the higher-molecular-weight polymer. In the final step of the analysis, we used eqs 2 and 3 to determine the SLDs for 60-PCBM and 70-PCBM within the blends. This was achieved by comparing the calculated WPCDTBT values to those expected from the initial blend ratios (50% for 1:1 blends, 33.3% for 1:2 blends, and 20% for 1:4 blends). The WPCDTBT for each blend was calculated using the experimentally determined values of SLDPCDTBT and SLDblend, with the SLDPCBM for the fullerenes being varied from 4.20 × 10−6 to 5.20 × 10−6 Å−2 in 0.01 × 10−6 Å−2 increments. The resulting plots of WPCDTBT versus fullerene SLD are given in Figure 3A,B for 60-PCBM and 70-PCBM, respectively. The SLDs at which the calculated WPCDTBT crosses those expected from the blend ratio of the spin-coating solution (horizontal dashed lines) are 4.47 × 10−6 to 4.91 × 10−6 Å−2. It is important to note at this stage that the lowest SLD determined for the fullerenes is already as high as the highest previously reported value. The differences between the calculated and expected WPCDTBT values (ΔWPCDTBT) were squared (to remove differences in the sign of the errors), and these squared errors are shown in Figure 3C,D plotted against the fullerene
SLD (× 10−6 Å−2) ± ± ± ±
(2)
where ϕi is the volume fraction of species i and SLDi is the corresponding scattering length density. The weight percentage of PCDTBT (WPCDTBT) for each of the blend films was then found after calculating the volume-fraction-weighted densities of the two components in each blend (ϕiρi) using the equation
Table 1. Modeled Layer Thicknesses, Roughnesses, and SLDs for Neat PCDTBT and Blend Films with 60-PCBM and 70-PCBM film
( SLDblend − SLDPCBM ) ( SLDPCDTBT − SLDPCBM )
0.07 0.07 0.07 0.14
obtained from the parameter covariance matrix of the fitting process, were all ≤2 Å. The dQ/Q resolution for the optimized fits was in the range of 6−8%, which arises from the inherent dQ/Q resolution of the instrument (5%) and an extra contribution used to account for small variations in sample thickness across the illuminated area. In practice this extra term means that the uncertainty in layer thickness is of the order 10 Å. Modeled and calculated SLD values were ascribed a relative error of ±5%, and the corresponding errors in ρi were determined by propagating these errors throughout eq 1 using the chain rule.
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RESULTS AND DISCUSSION It was found in each case that the neutron reflectivity profiles (Figure 2A,C) could be modeled as a single layer of uniform scattering length density on top of the silicon substrates (Figure 2B,D). The fact that the reflectivity profiles could be fitted to a single uniform layer (independent of blend ratio) means that the fullerenes are evenly distributed throughout the depth of the films. The films with the same polymer/fullerene blend weight ratio had essentially the same values of thickness and bulk SLD and similar interfacial roughness (Table 1). The 1412
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Figure 2. (A, C) Neutron reflectivity profiles for neat PCDTBT and blend films. The 60-PCBM blend profile is given above the 70-PCBM blend profile for each blend ratio in plot A. Individual points indicate experimental data and solid lines indicate model fits, traces are offset for clarity. (B, D) Modeled SLD vs thickness plots, solid lines indicate 60-PCBM blends and dashed lines indicate 70-PCBM blends. Distance = 0 Å represents the silicon substrate.
SLD. For each fullerene, the squared errors for the five films were added together, and the minimum of the total squared error was taken to be the precise SLD value for the fullerene. The precise SLDs were found to be (4.66 ± 0.23) × 10−6 −2 Å for 60-PCBM and (4.74 ± 0.24) × 10−6 Å−2 for 70-PCBM. Both SLDs correspond to a mass density of 1.61 ± 0.08 g cm−3 for neat 60-PCBM and 70-PCBM (eq 1). The mass density of 60-PCBM has been reported from single-crystal structure analysis to be 1.62 g cm−3 in the absence of co-crystallized solvent24 and 1.67 g cm−3 when cocrystallized with 1,2dichlorobenzene25. These latter mass densities correspond to SLDs of 4.69 × 10−6 and 4.58 × 10−6 Å−2, respectively, for the crystals. The similarity of the SLDs from the neutron measurements and the crystal structures suggests that the blend films are composed of a uniform distribution of fullerene clusters. These results are also consistent with microwave conductivity measurements that show that 60-PCBM clusters can form at polymer/fullerene blend ratios that are as low as 99:1.26 It is therefore not surprising that we should observe such clusters in our films that have much higher fullerene loadings. Finally, we now show how the precise SLD values for the fullerenes can be used to quantify the blend composition with other materials. It has been previously reported that the deposition of 60-PCBM onto poly(3-n-hexylthiophene) (P3HT) can lead to a fullerene-rich layer at the air interface.7,13 In ref 7, XPS data showed that P3HT had migrated into the
upper 60-PCBM layer during the deposition process, and by taking the SLD (4.34 × 10−6 Å−2) of the fullerene-rich layer and using the SLD of 60-PCBM in this work along with eqs 2 and 3, we calculate that the WP3HT for the fullerene-rich layer was around 4%. In the second report, 60-PCBM was deposited onto cross-linked P3HT with the aim of reducing the intermixing of the active materials. In this latter report the SLD of the fullerene-rich layer was found to be 4.58 × 10−6 Å−2 and XPS also showed that there was P3HT near the air interface. Using the precise SLD of this work, we can determine that the concept of cross-linking had been partially successful with the WP3HT being only around 1% for the fullerene-rich layer in this latter case.13
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CONCLUSIONS
We have provided a method for determining the precise SLDs of materials that do not form neat films of sufficient smoothness for neutron reflectometry experiments. The technique also provides a means for determining the density of materials when blended. In this work, we have shown that the SLDs of 60-PCBM and 70-PCBM are similar and fall within the range of (4.47−4.91) × 10−6 Å−2, which is higher than those previously reported for these fullerenes, namely, (3.6− 4.4) × 10−6 and 4.4 × 10−6 Å−2, respectively. The determined SLD of 60-PCBM was also found to be consistent with the reported mass densities from crystallographic data. It should be noted that previous reports on the SLDs of the fullerenes do 1413
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Figure 3. (A, B) Variation of WPCDTBT with fullerene SLD for the 60-PCBM and 70-PCBM blends, respectively. (C, D) Corresponding plots of the squared difference between the calculated and expected WPCDTBT values (ΔWPCDTBT) vs the fullerene SLD.
Australian Institute for Nuclear Science and Engineering (AINSE) in providing the neutron research facilities used in this work.
not always provide sufficient information to enable a direct comparison, and hence the published results on their distribution when blended with polymers might in some cases need to be reanalyzed. With the growing popularity of neutron scattering techniques for probing the structural properties of polymer/fullerene blends, it is important that the same SLD values are used to allow meaningful comparisons of observed film structures.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*Phone: +61 7334 67614. E-mail: [email protected]. Fax: +61 7334 69273. Present Address
M.J.’s current address is Australian Synchrotron, 800 Blackburn Road, Clayton VIC 3168, Australia. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Australian Research Council (DP1211572). P.L.B. and P.M. are both supported by University of Queensland Vice Chancellor’s Senior Research Fellowships. We acknowledge funding from the University of Queensland (Strategic Initiative, Centre for Organic Photonics & Electronics) and the Queensland government (National and International Research Alliances Program). We acknowledge the support of the Bragg Institute, the Australian Nuclear Science and Technology Organisation (ANSTO), and the 1414
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