Minimizing Lateral Domain Collapse in Etched Poly(3-hexylthiophene

Feb 10, 2010 - Comparing this PSD to that from the sample that was dried at RT right ... CO2 drying provides a surface tensionless environment (∼0 m...
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Minimizing Lateral Domain Collapse in Etched Poly(3-hexylthiophene)block-Polylactide Thin Films for Improved Optoelectronic Performance Ioan Botiz,† Alex B. F. Martinson,‡ and Seth B. Darling*,† †

Center for Nanoscale Materials and ‡Materials Science Division, and Argonne-Northwestern Solar Energy Research (ANSER) Center, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received November 30, 2009. Revised Manuscript Received January 21, 2010 Thin films of poly(3-hexylthiophene)-block-polylactide block copolymer exhibiting ordered lamellar morphology have been selectively etched to produce structured films that could be used in fabrication of idealized bulk heterojunctions for organic or hybrid solar energy devices. Etched poly(3-hexylthiophene) films, after being rinsed in water to remove degraded polylactide fragments, were dried using various drying approaches that reduce or alleviate surface tension forces generated during liquid evaporation from the film. As emphasized by atomic force microscopy, X-ray diffraction, and emission photoluminescence, a reduction in domain collapse leads to improved molecular ordering in the plane perpendicular to the substrate and enhanced photoluminescence quenching when paired with fullerene C60 hydroxide electron acceptors.

Introduction Block copolymers (BCPs) have generated significant interest in recent years due to their highly tunable nanoscale selfassembly.1-4 Modern synthetic chemistry provides possibilities to design polymer molecules with specific length scales and geometries leading to (idealized) nanostructures that can target, in addition to fundamental studies, applications ranging from nanolithography3,5-10 to photonics10,11 to controlled drug delivery12,13 to organic photovoltaics (PV).14-19 For PV applications, diblock copolymers can be used either directly as active donor-acceptor materials or indirectly as structure directors in order to probe structure-property relationships and perhaps improve the performance of organic or hybrid organic-inorganic PV devices.14 A third potential route is to utilize BCPs where one of the blocks is conjugated and can be used as an active material (e.g., donor), while the other block is *To whom correspondence should be addressed. E-mail: [email protected]. (1) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (2) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (3) Segalman, R. A. Mater. Sci. Eng., R 2005, 48, 191. (4) Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152. (5) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (6) Ramanathan, M.; Nettleton, E.; Darling, S. B. Thin Solid Films 2009, 517, 4474. (7) Ramanathan, M.; Darling, S. B. Soft Matter 2009, 5, 4665. (8) Black, C. T. IEEE Trans. Nanotechnol. 2004, 3, 412. (9) Xiao, S.; Yang, X.; Edwards, E. W.; La, Y.-H.; Nealey, P. F. Nanotechnology 2005, 16, S324. (10) Lu, J.; Chamberlin, D.; Rider, D. A.; Liu, M.; Manners, I.; Russell, T. P. Nanotechnology 2006, 17, 5792. (11) Manners, I. J. Opt. A: Pure Appl. Opt. 2002, 4, S221. (12) Kataoka, K.; Harada, A.; Nagasaki, Y. Adv. Drug Delivery Rev. 2001, 47, 113. (13) van Nostrum, C. F. Adv. Drug Delivery Rev. 2004, 56, 9. (14) Darling, S. B. Energy Environ. Sci. 2009, 2, 1266. (15) Sun, S.-S. Sol. Energy Mater. Sol. Cells 2003, 79, 257. (16) Maria, S.; Susha, A. S.; Sommer, M.; Talapin, D. V.; Rogach, A. L.; Thelakkat, M. Macromolecules 2008, 41, 6081. (17) Barrau, S.; Heiser, T.; Richard, F.; Brochon, C.; Ngov, C.; van de Wetering, K.; Hadziioannou, G.; Anokhin, D. V.; Ivanov, D. A. Macromolecules 2008, 41, 2701. (18) Zhang, Q.; Cirpan, A.; Russell, T. P.; Emrick, T. Macromolecules 2009, 42, 1079. (19) Wu, P.-T.; Ren, G.; Li, C.; Mezzenga, R.; Jenekhe, S. A. Macromolecules 2009, 42, 2317.

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degradable and can be removed after the microphase separation and nanostructure formation process is complete. The newly empty domains created in the resulting porous morphology can be used as vessels for a complementary active (e.g., acceptor) material to form an ordered bulk heterojunction. One block that is frequently used to create ordered porous morphologies in thin films is poly(methylmethacrylate) (PMMA). This polymer can be easily degraded and removed by exposure to UV light.20-22 In the context of PV applications, however, UV light applied during fabrication can also induce photochemical changes in the conjugated block, which may adversely affect its photovoltaic performance. A more suitable block that can be used to create ordered morphologies and then be readily removed is polylactide. This polymer can be degraded by using an alkaline solution treatment and subsequent rinsing in water.23 This concept was recently successfully applied to a poly(3-alkylthiophene)-polylactide BCP.24 With the aim of fabricating idealized morphologies for structure-function studies in organic PV systems, we have further explored the above concept using a poly(3-hexylthiophene)-blockpoly(L-lactide) (P3HT-b-PLLA) diblock copolymer. Here, the P3HT block is the conjugated donor block. We have observed formation of lamellar domains (schematically depicted in Figure 1a) immediately after spin-casting of thin films on indium tin oxide (ITO)-covered glass and poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)-covered ITO/glass.25 Complete degradation of the PLLA block should lead to a nanoporous P3HT film morphology as schematically represented in Figure 1b. Voids (20) Thurn-Albrecht, T.; Schotter, J.; K€astle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (21) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Sibener, S. J. Adv. Mater. 2005, 17, 2446. (22) Darling, S. B. Surf. Sci. 2007, 601, 2555. (23) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer, M. A. J. Am. Chem. Soc. 2002, 124, 12761. (24) Boudouris, B. W.; Frisbie, C. D.; Hillmyer, M. A. Macromolecules 2008, 41, 67. (25) Botiz, I.; Darling, S. B. Rational design of nanostructured hybrid materials for photovoltaics. In Active Polymers (Mater. Res. Soc. Symp. Proc.); Lendlein, A., Prasad Shastri, V., Gall, K., Eds.; Materials Research Society: Warrendale, PA, 2009; Vol. 1190; pp 1190-NN03-20.

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Figure 1. Idealized, schematic representations of (a) ordered nanoscale morphology consisting of lamellae oriented perpendicular to the substrate as observed in thin solid films of P3HT-bPLLA (domain width to height is scaled as in the experiment); (b) ordered P3HT donor domains of molecular dimension ideally obtained after selective removal of biodegradable PLLA block; and (c) collapse of P3HT domains predicted after solvent removal as a result of large surface tension forces generated between the domains upon drying.

were then filled with an acceptor material by exposure to an aqueous solution of fullerene (C60) hydroxide. Photoluminescence data confirmed that charge transfer does occur from the p-type polymer to this n-type fullerene species,26 and this process can be generalized to a variety of other electron acceptors, indicating promise for fabrication of highly ordered organic or hybrid PV active layers. The molecular weight of P3HT-b-PLLA is selected to produce domain periodicity comparable to the exciton diffusion distance in P3HT, thereby optimizing the separation of excitons at the donor-acceptor interfaces within the resulting bulk heterojunction. However, a significant challenge associated with realizing these idealized structures consists of lateral collapse of the resulting (26) Botiz, I.; Darling, S. B. Macromolecules 2009, 42, 8211.

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P3HT domains, as has been reported in other nanostructured systems in the literature.27-30 Collapse (schematically shown in Figure 1c) has been observed subsequent to PLLA removal and solvent evaporation from the film surface. The result is an increase in the length scale of the structural periodicity beyond the exciton diffusion distance. Here, we propose that collapse and subsequent aggregation of the P3HT domains is the result of capillary forces present between the domains during drying. One pathway, therefore, that may alleviate capillary forces is to exchange the waterbased etchant for solvents with low surface tension prior to liquid evaporation from the film surface. The surface tension of water (∼72 mN/m) is high compared, for example, to that of ethanol (∼22 mN/m).31 The logical extension of this method to minimize domain collapse is the avoidance of a liquid-solid interface during drying, for example, by performing supercritical drying.28,29 Here, solvent (CO2) surface tension may be completely eliminated at temperatures and pressures above the critical point. The collapse of microchannels and related structures is a wellknown phenomenon and has been treated from both theoretical and experimental points of view.32 Supercritical drying in CO2, in particular, is a well established method with a broad variety of applications.33 Supercritical fluids (CO2) can be used for materials synthesis and processing,34-36 including control of the phase behavior in polymer mixtures and block copolymers.37 Our emphasis here is application of surface tension control to a nanostructured polymer system of direct relevance to photovoltaics and, importantly, the demonstration of improved optoelectronic performance that results from such control when the polymer (P3HT) is coupled with C60 acceptor material. We explore the influence of various film treatments on the resulting film morphology, that is, on the degree of film ordering and implicitly on the degree of domain collapse. A series of thin films of P3HT-b-PLLA of similar thickness that exhibited ordered lamellar morphology were prepared. After being etched in NaOH solution to degrade PLLA, these films were rinsed in water to remove degraded PLLA polymeric fragments and then dried using various approaches. Rinse water was allowed to evaporate from some etched polymer films at room temperature (RT) and pressure, while other films were transferred to containers with dry, absolute ethanol. After the ethanol exchange was executed, some films were removed from ethanol and dried at RT. The remaining films in ethanol were transferred to an ethanol-filled supercritical drier for further exchange with liquid CO2 (with which ethanol is miscible). Finally, the liquid CO2 was ramped above its critical point and slowly removed to effect surface tensionless solvent removal from the etched polymer films. Tapping-mode (TM) atomic force microscopy (AFM) and X-ray diffraction (XRD) data recorded for films that have been dried using these different recipes are presented along with photoluminescence (PL) spectra obtained after the corresponding films were filled with C60. Moreover, a set of films was prepared in (27) Haberkorn, N.; Gutmann, J. S.; Theato, P. ACS Nano 2009, 3, 1415. (28) Zhang, Y.; Lo, C.-W.; Taylor, J. A.; Yang, S. Langmuir 2006, 22, 8595. (29) Liang, Y.; Zhen, C.; Zou, D.; Xu, D. J. Am. Chem. Soc. 2004, 126, 16338. (30) Namatsu, H.; Kurihara, K.; Nagase, M.; Iwadate, K.; Murase, K. Appl. Phys. Lett. 1995, 66, 2655. (31) Won, Y. S.; Chung, D. K.; Mills, A. F. J. Chem. Eng. Data 1981, 26, 141. (32) Shih, W.-P.; Hui, C.-Y.; Tien, N. C. J. Appl. Phys. 2004, 95, 2800. (33) Tomasko, D. L.; Li, H.; Liu, D.; Han, X.; Wingert, M. J.; Lee, L. J.; Koelling, K. W. Ind. Eng. Chem. Res. 2003, 42, 6431. (34) DeSimone, J. M. Science 2002, 297, 799. (35) Pai, R. A.; Humayun, R.; Schulberg, M. T.; Sengupta, A.; Sun, J.-N.; Watkins, J. J. Science 2004, 303, 507. (36) O’Neil, A.; Watkins, J. J. MRS Bull. 2005, 30, 967. (37) Goldbach, J. T.; Lavery, K. A.; Penelle, J.; Russell, T. P. Macromolecules 2004, 37, 9639.

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Figure 2. TM-AFM phase images showing (a) ordered morphology obtained by spin-casting P3HT-b-PLLA on ITO-covered glass (polymer film thickness was ∼70 nm); (b) morphology of the film presented in (a) after it was etched in NaOH alkaline solution, consecutively rinsed in water, and then dried at 23 °C; (c) morphology of the film presented in (a) after it was etched in NaOH alkaline solution, consecutively rinsed in water and ethanol, and then dried at RT; (d) morphology of the film presented in (a) after it was etched in NaOH solution, consecutively rinsed in water and ethanol, and then supercritically dried. (e) Corresponding power spectral densities, from which it is clear that the observed periodicity more closely resembles the original film when the surface tension is decreased during drying. The AFM image in (a) was taken after annealing in chloroform vapor for 90 h. All films were spin-cast from chloroform solution. Size of all AFM images is 1 μm2.

which the C60 acceptor material was incorporated immediately after etching/rinsing but prior to any solvent removal.

Results and Discussion Figure 2a shows the resulting morphology of a thin film (∼70 nm in thickness) obtained after spin-casting P3HT-b-PLLA chloroform solution on ITO-covered glass followed by solvent annealing in chloroform vapor for 90 h. Before annealing, the parallel stripes could not be clearly visualized by TM-AFM, indicating a less ordered initial morphology. Here, domains of lighter color correspond to the P3HT block while the darker domains are the PLLA amorphous block, consistent with a lamellar structure with the alternating domains oriented perpendicular to the substrate.26 Analyzing the AFM image and its corresponding power spectral density (PSD; green dotted line in Figure 2e), we find that the peak characteristic distance occurs at 16 nm. The AFM image shown in Figure 2b presents the morphology obtained after selective removal of the PLLA block and subsequent evaporation of rinsewater. PLLA was removed by etching the spin-cast thin film in alkaline NaOH solution for 2 days. Prior to liquid evaporation from the film overnight in ambient conditions, the etched film was rinsed in water to remove degraded PLLA fragments and NaOH. The AFM image displays alternating dark features corresponding to former locations of PLLA and light features representing the P3HT nanostructured material. These films no longer exhibit clear parallel stripes. By analyzing the PSD, we find a (peak) characteristic lateral distance of 24 nm (black line in Figure 2e), which is significantly larger than that in 8758 DOI: 10.1021/la904515z

the original film. Moreover, the PSD distribution is broader, indicating increased disorder. These results suggest a lateral collapse of some P3HT domains after the removal of PLLA (as schematically depicted in Figure 1c) due to significant forces generated by the surface tension present during water evaporation from the structured film. One simple method to curtail the lateral collapse is to ease surface tension forces by performing, prior to drying, a solvent exchange with ethanol. Experimentally, this was done by consecutive transfers of an etched film rinsed in water to a series of beakers containing dry, absolute ethanol. Subsequently, the ethanol was allowed to evaporate from the film surface overnight under ambient conditions. An AFM image obtained for this film, after drying, is depicted in Figure 2c. The resulting morphology, if compared to the morphology observed in Figure 2b, shows better ordering, that is, a morphology more similar to the original morphology observed in Figure 2a. The corresponding PSD (red line in Figure 2e) quantifies this improvement as represented by a shift toward shorter periodicities. Comparing this PSD to that from the sample that was dried at RT right after extraction from water, it is clear that the solvent exchange resulted in a larger percentage of periodicities with shorter scale, that is, closer to the original lamellar spacing. This suggests that, by reducing the surface tension forces generated during liquid evaporation from the nanostructured film surface, one can better retain the initial, ideal morphology. Extending this approach, one can further optimize this process by additional reduction of the forces occurring at liquid-solid interfaces upon film drying. Experimentally, this is achieved by Langmuir 2010, 26(11), 8756–8761

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exchanging ethanol for liquid CO2 in a small, temperature controlled pressure vessel followed by the slow removal of the CO2 under supercritical conditions. Supercritical CO2 drying provides a surface tensionless environment (∼0 mN/m), thus eliminating capillary forces at the liquid-solid interfaces. The AFM image shown in Figure 2d presents the resulting nanoporous morphology obtained for a thin film that, after being etched and rinsed in water and ethanol, was dried in a supercritical CO2 chamber. Analyzing the corresponding PSD, we found a peak corresponding to a characteristic lateral distance of about 18 nm, well below the 24 nm obtained for films from which water was evaporated. This distance nearly matches the original characteristic distance of 16 nm (blue line in Figure 2e). The PSD peak following CO2 drying is not as narrow as the one representing the original morphology (green dotted line in Figure 2e), suggesting some domain collapse/rearrangement still occurs. It is possible that some domains collapse during film etching, water rinsing, and/or solvent exchange, prior to supercritical drying. Another source of the persistent disorder could be the result of swelling or a lowering of the glass transition temperature of the P3HT in the supercritical environment, facilitating some level of undesirable reorganization of polymer chains. Nonetheless, we have shown that, by reducing forces generated by the surface tension of the water in the drying film, we can reduce domain collapse. The influence of this control on processes important to photovoltaics is examined below. P3HT exhibits characteristic photoluminescence (PL) centered around 750 nm when excited by 500 nm light. Coupling to an electron acceptor provides a pathway for nonradiative quenching of this PL. All nanoporous films that were etched in NaOH alkaline solution, regardless of the drying procedure, provide a significant enhancement of interfacial area available for electron transfer to C60 moieties infiltrated into the voids (relative to a simple bilayer) and should, therefore, exhibit measurable PL quenching. Infiltration of C60 hydroxide is performed by dipcoating thin nanoporous films in an aqueous solution of the acceptor. Results are summarized in Figure 3. The first three PL spectra (top, dotted lines) were taken for three thin films: (1) P3HT homopolymer and P3HT-b-PLLA diblock copolymer (2) before and (3) after PLLA removal to serve as references with no C60 content. The rest of the spectra were recorded for nanoporous films that, after etching, were (from top to bottom, the solid lines in Figure 3) (i) rinsed in water, dried at RT, and then dip-coated in C60 solution for 72 h; (ii) rinsed in water and ethanol, dried at RT, and then dip-coated in C60 solution for 72 h; (iii) rinsed only in water and then directly dip-coated in C60 solution for 72 h; and (iv) rinsed in water and ethanol, supercritically dried, and then dip-coated in C60 solution for 72 h, respectively. The amount of C60 incorporated into each of these samples may be different, but only due to the different morphologies that result from the different drying approaches. In all samples, the total amount of C60 represents an overabundance. That is, the films are infiltrated with C60, but there is also a substantial overlayer of C60 across the entire surface. It is the degree of infiltration that differs from sample to sample based on the nanostructure. The data in Figure 3 obtained from the block copolymer films, which were initially prepared with identical thickness, are presented on the same scale so a direct comparison can be made. As one can observe in Figure 3, following infiltration with the C60, the PL spectra exhibit significant quenching. The quenching is maximally enhanced (Figure 3 bottom spectrum) for films that were dried using a supercritical CO2 drying process and thus presented a characteristic distance of nanostructured domains (18 nm) corresponding to donor/acceptor Langmuir 2010, 26(11), 8756–8761

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Figure 3. Photoluminescence spectra of thin films of P3HT homopolymer, P3HT-b-PLLA before and after NaOH etching (dashed top spectra) and etched P3HT-b-PLLA films (see the solid spectra from top to bottom) that were (i) rinsed in water, dried at RT, and then filled with C60; (ii) rinsed in water and then ethanol, dried at RT, and then filled with C60; (iii) rinsed only in water and then directly filled with C60 (no drying); and (iv) rinsed in water and ethanol, dried in a supercritical chamber in presence of CO2, and then filled with C60, respectively. The block copolymer film exhibits PL characteristic of P3HT both before and after etching. Substantial quenching is observed, however, upon filling with the electronaccepting material, suggesting that many excitons are being separated at the nanostructured interfaces. Excitation wavelength was 500 nm for all spectra.

interfaces spaced comparably to the exciton diffusion distance of e10 nm.38 Also presented in this figure are results indicating an additional way to minimize domain collapse, that is, to directly dip-coat the etched films, previously rinsed in water, in C60 solution for filling without drying beforehand (Figure 3 second spectrum from the bottom). The objective here is to get the C60 molecules into the pores before the P3HT domains have had an opportunity to collapse during drying. The PL spectra show that this recipe is slightly less efficient than drying films in a supercritical chamber, but notably better than drying films at RT. It may be the case that the C60 cannot efficiently diffuse into the voids and displace extant water molecules during this process. Though enhanced, the PL quenching is still not complete in these materials. This might be due to the nonideal P3HT morphology obtained after etching and drying of thin films of P3HT-b-PLLA, that contain not only P3HT domains of a characteristic lateral distance comparable to the mean exciton diffusion distance but also P3HT domains of a characteristic lateral distance larger than the exciton diffusion length. Though recent studies have suggested exciton separation can be achieved at larger distances in P3HT/C60 systems,39 the bulk of the literature on bulk heterojunctions points to the need for structure on the scale of 10 nm.40 Note in Figure 2e that the width of the PSD peak from the original morphology is narrower than those of the morphologies obtained after film etching and drying. Another possible contributing factor is that the hydroxylated (hydrophilic) (38) Knupfer, M. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 623. (39) Ayzner, A. L.; Tassone, C. J.; Tolbert, S. H.; Schwartz, B. J. J. Phys. Chem. C 2009, 113, 20050. (40) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297.

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domain ordering correlates with improved exciton separation and, likely, improved charge carrier mobility in the p-type domains.42

Conclusions

cast on chromium-coated SiO2 recorded before (dotted) and after (solid) NaOH etching. Etched films (see the solid spectra from bottom to top) were (i) rinsed in water and then dried at RT; (ii) rinsed in water and ethanol and then dried at RT; and (iii) rinsed in water, soaked in ethanol, and then supercritically dried. The (100) peak corresponds to crystalline vertical stacking of P3HT chains.

Thin films of P3HT-b-PLLA exhibiting ordered lamellar morphology have been etched in NaOH solution to selectively degrade PLLA in order to obtain nanoporous P3HT films that could be used in fabrication of idealized bulk heterojunctions for organic or hybrid solar energy devices. Etched films were rinsed in water to remove degraded PLLA polymeric fragments and then dried using various approaches. Drying methods that reduce or alleviate surface tension forces generated during liquid evaporation from the film led to less P3HT domain collapse, improved molecular ordering in the plane perpendicular to the substrate, and enhanced photoluminescence quenching when paired with C60 electron acceptors. The approach outlined here could be adapted to other semiconducting polymers, so long as it is synthetically feasible to bond them with PLLA; indeed, the PLLA block could be replaced as well with alternate degradable materials presuming they could still be selectively removed without adversely affecting the optoelectronic performance of the semiconducting block. Future studies will probe the influence of the nanoscale morphology on photovoltaic device performance.

C60 moieties may avoid significant direct contact with the hydrophobic conjugated polymer domains, thereby inhibiting charge transfer at the donor-acceptor interfaces. One can conclude that the amount of quenching depends on the film morphology and implicitly the degree of order. Therefore, there is likely a correlation between film drying treatment and the resulting degree of crystallinity present in thin films. In addition to characterizing the lateral domain morphology with microscopy, the crystallinity in the plane perpendicular to the substrate was also probed with XRD (Figure 4). Four films of ∼70 nm thickness were obtained by spin-casting chloroform P3HT-b-PLLA solution on chromium-coated SiO2 substrates. Chromium was selected both because of its high X-ray reflectivity and because of its favorable wetting properties for the BCP film. One of these films was used as a reference. Its XRD spectrum exhibits a strong (100) diffraction peak just above 5° (dotted scan in Figure 4). This peak corresponds to crystalline vertical stacking of the P3HT chains, mediated by the alkyl side chains.41 The other three films, after being etched, were (i) rinsed in water and then dried at RT; (ii) rinsed in water and ethanol and then dried at RT; (iii) rinsed in water and ethanol and then dried in supercritical CO2, respectively. Their corresponding XRD spectra are shown in Figure 4 (solid lines from bottom to top). The film dried in supercritical CO2 exhibited higher peak intensity than the films that were dried using water or ethanol; that is, there was higher molecular order in the plane perpendicular to the substrate. Also, the film that was rinsed in water and then exchanged in dry absolute ethanol exhibited higher molecular order than the film that, before drying, was rinsed only in water. Collapsed domains will have orientations that are shifted away from the direction probed by the X-ray beam. Using the (instrument function-corrected) peak widths to extract average crystalline domain sizes using the Scherrer equation, we find that the initial size of 18 nm is best preserved using the supercritical CO2 processing: 16, 14, and 10 nm for CO2, ethanol, and water processing, respectively. These results support the correlation between the film drying method and the resulting degree of internal order. Improved molecular and

The P3HT-b-PLLA linear diblock copolymer was purchased as a custom synthesis from Polymer Source, Inc. prepared according to the literature procedure24 and used without further purification. The number average molecular weight was Mn = 3300 for P3HT and Mn = 4100 for PLLA. The polydispersity index (Mw/Mn, PDI) was 1.28, as measured by size exclusion chromatography performed on a Varian liquid chromatograph equipped with refractive and UV light scattering detectors. Three columns from Supelco were used with triple detectors from Viscotek. The molecular weight was calculated based on polystyrene standards. NMR spectra were recorded in deuterated chloroform to verify the functionality and the composition of copolymer. Fullerene C60 hydroxide [chemical structure: C60(OH)22-26] was purchased from Materials Technologies Research, Ltd. This acceptor material is water-soluble at pH g 8. Thin solid films with an average thickness of ∼70 nm (measured by ellipsometry and profilometry) were obtained by spin-casting chloroform polymer solution onto solid substrates including ITO-coated boro-aluminosilicate display grade glass and chromium-coated (10 nm) SiO2 [∼500 nm oxide thermally grown on Si(100)]. Spin-coating was performed at 2000 rpm for 30 s from a 15 mg/mL solution of P3HT-b-PLLA in chloroform. Solvent annealing was achieved by placing the sample in a sealed bell jar with saturated chloroform vapor. The sample was removed by venting the vapor over the course of approximately 15 s. The surface of the thin films was characterized in detail by tapping-mode atomic force microscopy [Nanoscope V, Veeco]. The etching process used to selectively remove the biodegradable PLLA block consisted of treatment of thin polymer films in 0.5 M NaOH solution (60:40 v/v water/ methanol). Thin films were submerged in NaOH solution for 48 h and then either carefully rinsed with water in order to remove degraded PLLA fragments and NaOH or transferred consecutively to a series of beakers containing dry, absolute ethanol to execute a thorough solvent exchange. After rinsing, solvent was allowed to evaporate from the thin film surface overnight at RT or removed via supercritical CO2 extraction. Supercritical drying was performed in a manual critical point drier (Tousimis, SamdriPVT-3D). The etched, rinsed, ethanol exchanged sample was rapidly transferred to the absolute-ethanol-filled pressure vessel.

Figure 4. Smoothed XRD spectra of P3HT-b-PLLA films spin-

(41) Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233.

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(42) Darling, S. B. J. Phys. Chem. B 2008, 112, 8891.

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Botiz et al. After sealing the stainless steel vessel, liquid CO2 was slowly flushed through the chamber until the exhaust gas was free from ethanol vapors. The vessel was raised above the critical point for CO2 over ∼15 min and allowed to equilibrate for an additional 15 min. Finally, supercritical CO2 was bled from the chamber as an ambient pressure gas at a rate less than 1 L/min. Fullerene filling was achieved by dip-coating etched films for 72 h in an aqueous solution of 15 mg/mL concentration. Films were then extracted from the C60 aqueous solution and dried at RT for several hours. X-ray diffraction data were obtained using a Bruker D8 Discover

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Article analytical X-ray system in a grazing incidence geometry (θi = 0.19). Emission photoluminescence spectra were recorded using a Perkin-Elmer LS-55 luminescence spectrometer with a 250 nm/ min scan rate and excitation wavelength of 500 nm.

Acknowledgment. The authors thank B. Fisher for assistance with the X-ray diffraction. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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