PCBM Mixtures Remain

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Mesoscopic Structural Length Scales in P3HT/PCBM Mixtures Remain Invariant for Various Processing Conditions Sameer Vajjala Kesava, Rijul Dhanker, Derek R Kozub, Kiarash Vakhshouri, U Hyeok Choi, Ralph H Colby, Cheng Wang, Alexander Hexemer, Noel C. Giebink, and Enrique D Gomez Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4011426 • Publication Date (Web): 24 Jun 2013 Downloaded from http://pubs.acs.org on June 24, 2013

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Chemistry of Materials

Mesoscopic Structural Length Scales in P3HT/PCBM Mixtures Remain Invariant for Various Processing Conditions Sameer Vajjala Kesava,1 Rijul Dhanker,2 Derek R. Kozub,1 Kiarash Vakhshouri,1 U Hyeok Choi,3 Ralph H. Colby,3 Cheng Wang,4 Alexander Hexemer,4 Noel C. Giebink2 and Enrique D. Gomez1,5* 1

Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 2

Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802 3

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 4

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

5

Materials Research Institute, The Pennsylvania State University, University Park, PA 16802

*email: [email protected] Keywords: Morphology, GISAXS, EFTEM, optical modeling, organic solar cells, organic photovoltaics Abstract Mesoscopic structural length scales in the photoactive layer of organic solar cells affect exciton dissociation into charges and thus device performance. It is currently hypothesized that these length scales are largely affected by processing conditions such as annealing temperature, annealing time and casting solvent. In our study, we utilized grazing incidence small angle Xray scattering and energy-filtered transmission electron microscopy to characterize the in-plane morphology of poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester mixtures and compared structural data with device performance. We found that the characteristic length scales of the mesostructures are dominated by the crystallization motif of the polymer and do not vary significantly for different processing conditions. For example, thermal annealing of films spun-cast from different solvents yielded similar in-plane morphologies; consequently, 1 ACS Paragon Plus Environment


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device performance was similar once thickness effects were accounted for through optical modeling.

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Introduction An intricate mesostructure in the photoactive layer of donor/acceptor organic solar cells is required to maximize device performance.1-6 The low dielectric constant and limited exciton diffusion length of organic semiconductors (5-10 nm7,8) dictates a domain size of this magnitude for efficient exciton dissociation and photogeneration of charge carriers. Furthermore, efficient charge extraction requires bicontinuous donor and acceptor domains. As such, characterizing and understanding the morphological evolution of the photoactive layer of bulk heterojunction solar cells has received significant attention.5, 9-19 Current state-of-the-art polymer/fullerene solar cells rely on the interplay between polymer crystallization and miscibility to spontaneously form the necessary mesostructure in the active layer for efficient device performance.5, 6, 10, 20-22 Electron micrographs of donor/acceptor mixtures demonstrate nanostructures composed of polymer crystals surrounded by a polymer/fullerene mixed phase.5

This suggests that polymer donors must be sufficiently

miscible with fullerene acceptors to prevent large scale macrophase separation and allow polymer crystallization to dictate the morphology. Furthermore, self-limiting crystallization of the polymer, at least in a metastable manner, is clearly advantageous to establish a length scale commensurate with the limited exciton diffusion length (5-10 nm).15,

23

Many polythiophene

conjugated donor molecules, such as P3HT, exhibit fibril-like crystal habits with fibril diameters near 10-20 nm.4,

5, 24-26

The reason for this phenomenon is currently unclear. A hypothesis

proposed by Snyder et al. suggests that defects in the backbone, such as stereoregularity defects in poly(3-hexylthiophene-2,5-diyl) (P3HT), can constrain the crystal dimension in the direction of the backbone.23

In contrast, polymers which lack self-limiting crystallization, such as



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poly[2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), are more sensitive to processing conditions as they lack a metastable nanostructure.15, 27 In the case of P3HT/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixtures, the materials are miscible at compositions greater than approximately 0.4 P3HT by volume in the amorphous phase and P3HT crystallization dictates the microstructure at these compositions.5, 6 The final morphology is composed of P3HT crystallites in the shape of fibrils surrounded by an amorphous mixture of P3HT and PCBM.

The miscibility of PCBM in amorphous P3HT

suppresses the crystallization of PCBM itself, but PCBM does crystallize under some conditions (such as extended thermal annealing).16, correlated,30,

32

28-31

The crystallization of PCBM and P3HT can be

because crystallization of P3HT depletes the concentration of P3HT in the

PCBM-rich phase, leading to immiscible compositions ( < 0.4 P3HT by volume) for the amorphous mixture surrounding the P3HT fibrils. Indeed, an enhancement in the regioregularity of P3HT leads to enhanced P3HT crystallization and consequently to an increase in PCBM crystallization.16 Nevertheless, once the crystallization of PCBM begins, it proceeds remarkably quickly,28, 29

potentially due to the fast diffusion of PCBM10 and metastability of the PCBM amorphous

phase. Consequently, PCBM crystalline domains grow to micron-sized scales,16, 29, 32-34 such that crystalline PCBM interfaces do not contribute significantly to charge photogeneration because the amount of interfacial area is dominated by the nanoscaled P3HT crystalline fibrils. A consequence of P3HT crystallization dominating the nanostructure is that processing conditions, such as annealing or casting solvent, can affect the final morphology on various length scales by modulating the P3HT crystallinity.


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Recent work by Dang et al. on examining the effect of spin-casting solvents on P3HT/PCBM mixtures has shown that while non-annealed devices spun-cast from different solvents produced higher efficiencies with increasing boiling point of the solvents, thermal annealing resulted in similar device performance for all the solvents used; the cause of this interesting observation was not explored.35 In contrast, work by Ruderer et al. on P3HT/PCBM mixtures demonstrated that even after thermal annealing photovoltaic device performance could vary if films were cast from different solvents; they concluded that the morphology of the photoactive layer varies significantly with choice of casting solvent.14 This discrepancy in the literature exemplifies the current lack of understanding of the critical parameters responsible for the morphology of the photoactive layer and device performance. In this study, we examine the effect of processing conditions (spin-casting solvents, annealing temperatures) on the mesostructure of the photoactive layer of P3HT/PCBM mixtures using grazing-incidence small angle X-ray scattering (GISAXS) and energy-filtered transmission electron microscopy (EFTEM). GISAXS yields structural information on mesoscopic length scales of the photoactive layer while EFTEM is used as a complementary technique to obtain high contrast images of the same.36-42 We find that after thermal annealing differences in the initial structure created by different casting solvents is obviated, resulting in a similar final structure with mesoscopic length scales on the order of 15-30 nm. We hypothesize that polymer crystallization through nucleation at the bottom interface is driving the structure formation process, with the initial as-cast structures having no effect on the final microstructures formed. Finally, we find that these length scales do not vary significantly with different annealing temperatures and annealing times—including the choice of spin-casting solvents—suggesting that this system has a metastable mesostructure due to the self-limiting crystallization of P3HT. 5 ACS Paragon Plus Environment


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Through optical modeling, we further demonstrate that this final structure results in similar device performance when thickness variations of the photoactive layer are taken into account.

Experimental Section P3HT:PCBM solution preparation. Regioregular P3HT (95.9% H-T regioregular, Mn= 26 kg/mol, polydispersity = 2, Merck) and PCBM (>99.5%, Nano-C) solutions, in 1:1 mass ratio, were made with various anhydrous solvents (chlorobenzene, chloroform, toluene and o-xylene; Sigma-Aldrich) at 24 mg/mL concentration in a N2 glovebox.

The solutions were stirred

overnight at room temperature; solutions prepared with toluene and o-xylene were heated to 80°C and 100°C respectively for at least 1 h prior to spin-coating to ensure dissolution while solutions prepared with chlorobenzene and chloroform were heated at 80°C for 2 min and allowed to cool down to room temperature before spin-coating.

GISAXS. Grazing Incidence Small Angle X-Ray Scattering (GISAXS) experiments were conducted on beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory (λ = 1.24 Å). To simulate conditions pertinent to solar cells, P3HT/PCBM thin films were spun-cast on ca. 50 nm poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), PEDOT:PSS, (Clevios P, Heraeus) films deposited on silicon wafers; silicon wafers were cleaned with 10 min sonication each in acetone and isopropanol, and subsequent UV-ozone treatment for 10 min. Thermal annealing was done in a N2 glovebox on a calibrated digital hotplate with ensuing rapid cooling to room temperature by placing the samples on a metal surface. GISAXS data was taken at angles above the critical angle for 1:1 P3HT/PCBM mixture (ca. 0.14°) and below the silicon critical angle (0.21°)43. In-plane data at the specular reflection was 6 ACS Paragon Plus Environment

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analyzed assuming the Born Approximation and plotted as a function of the scattering vector, q (q = 4Sin(/2)/). The analysis method is described in the Supporting Information of reference 5.

TEM. Transmission Electron Microscopy experiments were conducted on a JEOL 2010F at Penn Regional Nanotechnology Facility, University of Pennsylvania and on a Zeiss LIBRA 200MC at the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory.

Bright-field images, elemental maps and thickness maps were acquired.

The

standard three-window method was used to obtain sulfur and carbon maps.37 The samples were prepared by spin-casting the active layer on PEDOT:PSS/silicon substrates, then floating off the films in distilled water and picking them up on copper TEM grids. The samples were vacuumdried for 24 h and then thermally annealed in a N2 glovebox.

Device fabrication and testing. Indium tin oxide (ITO) coated glass substrates (20 Ω/sq; Kintec, Hong Kong) were used to fabricate solar cells (device area: 0.162 cm2). The substrates were cleaned with Aquet detergent solution and water, followed by 10 min sonication each in acetone and isopropanol, and subsequent UV-ozone treatment for 10 min. PEDOT:PSS layers (thickness ca. 50 nm) were spun cast in a laminar flow hood at 4000 rpm for 2 min and then dried at 165 °C for 10 min. P3HT/PCBM active layers (thickness values in Table S1) were spun cast for 1 min in a N2 glovebox. Hot solutions prepared with toluene and o-xylene were spun cast at 80°C and 100°C respectively with substrates at room temperature (25°C). A 75 nm layer of aluminum was deposited via thermal evaporation at 10-6 torr. Thermal annealing took place before aluminum



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deposition such that films without the Al cathode utilized in GISAXS experiments could better simulate conditions for solar cells. Devices were tested in the dark and under AM 1.5G (100 mW/cm2) illumination from a 150 W Newport solar simulator using a Keithley 2636A Sourcemeter. All fabrication and testing was performed in a N2 glovebox without exposing the sample to air. Optical modeling. The number of photons absorbed in the active layer was simulated at each wavelength using the transfer matrix method implemented by Pettersson et al.44 and then integrated over the AM1.5G solar spectrum. The complex refractive indices n and k of glass, ITO, PEDOT:PSS, P3HT/PCBM (spin-coated from chlorobenzene and annealed at 150°C for 15 min) and aluminum were experimentally determined from variable angle spectroscopic ellipsometry. UV-Vis. Absorbance measurements of P3HT/PCBM films on glass slides were taken using a Beckman DU Series 500 spectrophotometer.

Results and Discussion GISAXS is a powerful technique to extract qualitative and quantitative information about the structure in P3HT/PCBM mixtures.5, 14, 28, 45-47 Utilizing a reflection scattering geometry with incident angles below the critical angle of the silicon substrate (0.21°) but above the critical angle of P3HT/PCBM films (0.14o) maximizes the signal-to-background ratio.36,40 Figure 1 shows the GISAXS intensities as a function of in-plane scattering vector qxy for 1:1 by mass P3HT/PCBM films cast from chlorobenzene, chloroform, toluene and o-xylene prior to annealing and after annealing at 150 oC for 15 min. The composition and thermal annealing conditions were chosen to match the optimum conditions for devices. In all cases the data show 8 ACS Paragon Plus Environment

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a shoulder or weak peak at q = 0.02 to 0.04 1/Å. This suggests that a characteristic length scale exists for all samples between 16 and 31 nm. Although the data is noisy due to the lack of strong order for as-cast films, the films show some differences as a function of casting solvent. This is likely due to differences in the boiling point of the solvent, such that slower evaporation during film casting leads to higher P3HT crystallization. Another possible explanation is that as the solvent evaporates and the solution concentration increases one of the components could be precipitating out of solution first. Nevertheless, after annealing films at 150 oC for 15 min (Figure 1b), the scattering profiles of all samples become nearly identical regardless of casting solvent. Sulfur elemental maps generated from EFTEM micrographs shown in Figure 2 corroborate the GISAXS data in Figure 1b; after annealing, the in-plane morphology is effectively identical for all films cast from various solvents. Figures 1 and 2 demonstrate that the structure is different after casting when different solvents are used but thermal annealing nullifies this initial effect resulting in a similar final structure.

A plausible reason is that nucleation of P3HT crystals readily occurs at the

PEDOT:PSS or air interface such that nucleation and growth from the bottom or top of the film dominates, making the initial structure within the film irrelevant.

Indeed, crystallites are

nucleated from the buried interface in neat P3HT films.48 Alternatively, if the nucleation density at the annealing temperatures used in this study is significantly larger than the number of crystal nuclei formed from the casting process, then the final morphology will be dominated by the microstructure created during the annealing process. Nevertheless, the invariance in the structure after annealing despite the different casting solvents is a direct consequence of polymer crystallization driving the structure formation in miscible P3HT/PCBM mixtures, as demonstrated in our earlier work.5



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We note that the morphological evolution in P3HT/PCBM mixtures is likely to depend on the molecular weight of P3HT. In particular, if the molecular weight is below 7 kg/mol chains within crystals will be fully extended.49-51 The microstructure of polymers with chainextended crystals is known to be less sensitive to processing conditions,52 and as such, is likely to depend less on the choice of casting solvent. In our study, the number-averaged molecular weight of P3HT is 26 kg/mol while the weight-averaged molecular weight is 51 kg/mol. Thus, fully extended chains would be significantly longer than individual crystals and the crystallites are interconnected by tie chains, as evidenced by the high charge mobilities (5 x 10-2 cm2/Vs) obtained from thin film transistor measurements53 and the frequency-independent storage modulus determined from rheometry (see Figure S1 and S2 of the Supporting Information). As a consequence, the entanglement density and nucleation density for crystal formation in the P3HT utilized in this study will depend on the processing conditions. Regardless, the microstructure of P3HT/PCBM films does not depend on the choice of casting solvent, as demonstrated in Figures 1 and 2. GISAXS data from P3HT/PCBM mixtures at compositions near 1:1 are dominated by the structure factor (interdomain spatial correlations) such that the length scales correspond to the distance between domains.5, 54 We can quantify the structure with the use of the Teubner-Strey scattering function (TS). Originally developed for oil-water microemulsions and later extended to polymer blends,55-57 TS has been shown to be useful for modeling the scattering data from polythiophene/fullerene mixtures.5,

15

We extract average domain spacings, d, or average

distances between P3HT fibrils, from TS fits to the GISAXS data shown in Figure 1b and compare d between different solvents in Figure 3. Figure 3 also shows d obtained from TS fits to the azimuthally integrated Fourier transforms of the elemental maps shown in Figure 2 (see


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Figure S3 of the Supporting Information for further details). Domain spacings are largely invariant between different casting solvents (22-27 nm) and consistent between EFTEM results and GISAXS data. As shown in Figure S4 of the Supporting Information, annealing at 150 oC or 180 oC lead to similar results, although d increases from about 24 to 29 nm, respectively. By estimating the domain size from the domain spacing in our samples we can examine some of the potential effects on solar cell performance. For example, we can estimate the impact of reducing the interfacial area on the exciton dissociation and photogeneration of charges. The composition of the PCBM-rich phase from our elemental maps is 0.3 by volume P3HT (see Table S2 of the Supporting Information).58 Because the fibrils are essentially pure, the volume fraction of P3HT fibrils, fP3HT, must be 0.4 if the average composition of the film is 0.57 by volume. Assuming a cylindrical fibril morphology and that the domain spacing describes the distance between fibrils, the domain size of P3HT domains is given by

⁄ . Thus, the

domain size of P3HT ranges from 8 to 11 nm when d varies from 22 to 32 nm. We note that this estimate of P3HT domain size corresponds to the width of P3HT fibrils given the assumption of a cylindrical geometry. The consequence of increasing the domain size from 8 to 11 nm on the photogeneration of charges is at most 20% if exciton harvesting only takes place in the 5 nm near the PCBM interface and less than 1% if excitons can diffuse to the interface from the 10 nm of P3HT closest to a PCBM interface (exciton diffusion lengths of 5 nm and 10 nm, respectively).15 Thus, even assuming that the small changes in the in-plane morphology are due to domain coarsening, the changes in the morphology cannot account for more than a 20% difference in the device performance. Devices were fabricated with active layers spun-cast at 1000 rpm from the four solvents and annealed at 150°C for 15 min. Maintaining the spin speed constant leads to different film 11 ACS Paragon Plus Environment


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thicknesses due to the different viscosities of the solutions, as shown in Table S1 of the Supporting Information.

Figure 4a shows current densities against applied bias under

illumination for devices where the active layer was cast from different solvents. Regardless of casting solvent, the open-circuit voltage (VOC) and fill factor (FF) remain similar (Table S3 of the Supporting Information). Clearly, the short-circuit current densities (JSC) vary even though the in-plane morphology is similar for all samples (Figures 1b and 2), and devices where active layers were cast from chloroform have the highest JSC. To account for thickness variations, the device performance was normalized by dividing the JSC by the number of photons absorbed in the active layer. We model the absorption for a range of active layer thicknesses using the complex refractive indices and electric field intensity distributions in devices (Figures S5 and S6 of the Supporting Information).44,

59

Figure S6

demonstrates the need for optical modeling; the absorption is not monotonic with film thickness.60-66

Thus, we compare the overall internal quantum efficiency (IQE) between

different devices calculated from the JSC and optical absorption, as shown in Figure 5. Once changes in the absorption are accounted for due to thickness effects, it is clear that the photoconversion efficiency in terms of the overall IQE is invariant despite the use of different casting solvents and despite the thickness varying significantly (100-200 nm). To confirm this conclusion, we varied the spin speed of films cast from different solvents such that the thicknesses of the resulting films were similar (Figure S7 and Table S1 of the Supporting Information). The JSCs (Figure 4, Table S4 of the Supporting Information) and IQEs (Figure 5) of devices annealed at 150°C for 15 min where the thicknesses of the active layer were constant for various solvents were found to be similar for the different films as shown in Figure 5. Furthermore, the IQEs of devices were similar between all samples, regardless of the casting 12 ACS Paragon Plus Environment

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conditions. Examination of other performance metrics, such as the fill factor, VOC and effective series and shunt resistances (Figure 4b and Tables S3 and S4 of the Supporting Information) also demonstrate no significant differences between samples. Devices were annealed prior to deposition of the cathode in order to maximize consistency between X-ray scattering, TEM and device experiments. As a consequence, the open-circuit voltages of all devices are low, near 0.5 V. We have also fabricated devices where annealing took place after the cathode deposition, resulting in open-circuit voltages near 0.6 V, as previously reported.67, 68 Results for these devices are shown in Figure S8, Figure S9 and Table S5 of the Supporting Information, which demonstrate the same trends as the data in Figure 4b and Figure 5 for devices annealed prior to cathode deposition. The role of the processing conditions on mesoscopic structure and device performance was further probed by annealing 1:1 P3HT/PCBM mixtures and devices, spun-cast from chlorobenzene, at different temperatures for different times. Figure 6 shows the JSC vs domain spacing for different processing conditions and the detailed device characteristics are shown in Tables S6, S7, S8 and S9 of the Supporting Information. Figure 6 shows no obvious correlation. Given the small variation in the morphology of about 10 nm, a large change in the device performance is not expected.15 Nevertheless, Figure 6 demonstrates not only that the length scales of the morphology are not strongly dependent on the processing conditions of P3HT/PCBM mixtures but also shows that factors other than the in-plane morphology must strongly affect device performance. Figures 1 and 2, in conjunction with previous work which shows little coarsening of the microstructure with thermal annealing temperature and time,5, 32 demonstrate an invariance of the 13 ACS Paragon Plus Environment


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morphology and suggests that polymer crystallization governs the structural evolution and final structure of P3HT/PCBM mixtures. Furthermore, the consistency in the structure highlighted in Figure 2 suggests that self-limiting crystallization is an advantageous property of P3HT for the application to organic photovoltaics. In contrast, domains in poly[2,5-bis(3-hexadecylthiophen2-yl)thieno[3,2-b]thiophene] (PBTTT)/PC71BM mixtures can coarsen significantly depending on the processing conditions since PBTTT adopts a plate-like crystal habit without a constrained lateral dimension.15 Because the donor and acceptor materials must be compatible enough to prevent wide-scale macrophase separation, the crystallization habit or motif of the donor polymer dominates the morphological evolution and mesostructure length scales of the active layer and consequently device performance in organic solar cells.

Thus, designing novel

polymer donors with controlled crystallization may be critical for next-generation polymer/fullerene solar cells.

Conclusions In summary, we have shown that the mesostructure of P3HT/PCBM is robust. We found that thermal annealing nullifies the initial effect of solvent on the P3HT/PCBM structure; this is likely a consequence of structure formation being driven by polymer crystallization rather than the initial state of the system. Taking the thicknesses of the active layers into account, the performance of devices and the active layer morphology is similar when P3HT/PCBM mixtures were spun-cast from different solvents. Further, thermal annealing at different temperatures and for different times was found not to affect the mesoscopic length scales of the active layer significantly despite significant changes in the device performance. Thus, some of the critical parameters which link processing, morphology of the active layer and device performance remain poorly understood. 14 ACS Paragon Plus Environment

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Acknowledgements Major funding for this work was provided by NSF under Grant No. DMR-1056199. The authors gratefully acknowledge the Penn Regional Nanotechnology Facility, University of Pennsylvania and the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, which is supported by the U. S. Department of Energy under Contract No. DEAC02-05CH11231 for TEM access. The authors also acknowledge support of the Advanced Light Source, Lawrence Berkeley National Laboratory, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-C02-05CH11231. The authors thank Dr. Susan Trolier-McKinstry for use of the spectroscopic ellipsometer and Dr. John Asbury at the Pennsylvania State University for use of the spectrophotometer.

Supporting Information Available: Text and figures giving the experimental details, quantitative comparisons between GISAXS and EFTEM, domain compositions from EFTEM, detailed device characteristics, and optical modeling results. This material is available free of charge via the Internet at http://pubs.acs.org.



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1

(a)

0.6

o

I/I (a. u.)

0.8

Chlorobenzene Chloroform Toluene o-xylene

0.4 0.2 0 0.01

0.1

-1

q (Å ) xy

1

(b)

0.8 0.6

o

I/I (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4


0.2 0 0.01

-1

0.1

q (Å ) xy

Figure 1. GISAXS intensity vs in-plane scattering vector, qxy, of 1:1 by mass P3HT/PCBM (a) non-annealed films cast from various solvents and (b) films cast from various solvents and annealed at 150°C 15 min.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 2. 2 Sulfur ma aps generateed from EFT TEM of 1:1 by mass P33HT/PCBM M mixtures sspuncast from m (a) chlorrobenzene, (b) chlorofform, (c) tooluene, and (d) o-xylen ne. Films were annealed d at 150°C for f 15 min. Scale bars: 100 nm.


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35

Domain spacing (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

EFTEM GISAXS

25 20 15 10 5 0 Chlorobenzene

Chloroform

Toluene

O-Xylene

Solvent Figure 3. Comparison of domain spacings calculated from the Teubner-Strey fits to GISAXS curves of Figure 1b and to azimuthally-integrated Fast Fourier Transforms of elemental maps of Figure 2. Error bars represent the standard deviation from multiple measurements.




2

Current density ( mA / cm )

5

0

(a) 1000 rpm -5

-10 -0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.5

0.6

Applied voltage (V)

5


2

Current density ( mA / cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

0

(b) variable rpm -5

-10 -0.1

0

0.1

0.2

0.3

0.4

Applied voltage (V) Figure 4. Average current densities vs applied bias for 1:1 by mass P3HT/PCBM solar cells were the photoactive layer was spun cast at (a) 1000 rpm or (b) at various rpm such that the thickness is roughly the same (~ 90 nm). All devices were annealed at 150°C for 15 min. The error bars represent the standard deviation from multiple measurements.


Page 27 of 29

100

1000 rpm

variable rpm

80

IQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 Chlorobenzene

Chloroform

Toluene

O-Xylene

Solvent

Figure 5. Overall internal quantum efficiencies for 1:1 by mass P3HT/PCBM solar cells cast from various solvents and annealed at 150°C for 15 min. The number of photons absorbed was obtained from optical modeling shown in Figure S6 of the Supporting Information



7.5 7

2

(mA/cm )

6.5 6 5.5

SC

J

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

5 4.5

o

165 C

o

o

190 C

100 C

o

140 C 4 24

26

28

30

32

34

Domain spacing (nm) Figure 6. Short-circuit current density, JSC, of devices vs. domain spacing of 1:1 P3HT/PCBM mixture annealed at different temperatures for different durations. The active layer was cast from chlorobenzene solutions. Error bars represent the standard deviation from multiple measurements.


Page 29 of 29

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Table off Contents Graphic


PCBM Mixtures Remain

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