Nanoestrutura e Propriedades Optoelétricas de Blendes de Polímeros de Hexiltiofeno Regio-R, Notas de estudo de Engenharia Elétrica

Baixe Nanoestrutura e Propriedades Optoelétricas de Blendes de Polímeros de Hexiltiofeno Regio-R e outras Notas de estudo em PDF para Engenharia Elétrica, somente na Docsity! C O www.MaterialsViews.com www.advmat.deM M U A New Supramolecular Route for Using Rod-Coil Block Copolymers in Photovoltaic ApplicationsN IC A T IO N By Nicolas Sary, Fanny Richard, Cyril Brochon, Nicolas Leclerc, Patrick Lévêque, Jean-Nicolas Audinot, Solenn Berson, Thomas Heiser,* Georges Hadziioannou,* and Raffaele Mezzenga*[*] Prof. T. Heiser, Dr. P. Lévêque Institut d’Electronique du Solide et des Systèmes Centre National de la Recherche Scientifique, Université de Strasbourg 23, Rue du Loess, 67037 Strasbourg (France) E-mail: thomas.heiser@iness.c-strasbourg.fr Prof. G. Hadziioannou,[+] Dr. F. Richard, Dr. C. Brochon[+] Dr. N. Leclerc Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies Université de Strasbourg, Ecole Européenne de Chimie Polymères et Matériaux 25, Rue Becquerel, 67087 Strasbourg (France) E-mail: hadzii@ecpm.u-strasbg.fr Prof. R. Mezzenga, Dr. N. Sary Department of Physics and FRIMAT Center for Nanomaterials, University of Fribourg Ch. Musée 3, CH-1700 Fribourg (Switzerland) E-mail: raffaele.mezzenga@unifr.ch Prof. R. Mezzenga Nestlé Research Center, Vers-Chez-Les-Blanc 1000 Lausanne 26 (Switzerland) Dr. J.-N. Audinot Science and Analysis of Materials Department Public Research Centre Gabriel Lippmann 41 rue du Brill, L-4422 Belvaux (Luxembourg) Dr. S. Berson Laboratoire des Composants Solaires, Institut de l’Energie Solaire Commissariat à l’énergie atomique BP 332 50 Avenue Du Lac Léman, 73377 Le Bourget Du Lac (France) [+] Present address: Laboratoire de Chimie des Polymères Organiques (LCPO) CNRS/UNIV. Bordeaux1/ENSCBP-16 av. Pey-Berland-33607 PESSACCedex (France); E-mail (from January 2010): hadzii@enscbp.fr DOI: 10.1002/adma.200902645 Adv. Mater. 2009, 21, 1–6
2009 WILEY-VCH Verlag Gmb Final page numbers not assignedThe growing interest for renewable energy technologies, such as photovoltaic (PV) devices, combined with the need for low-cost processing, have contributed to the quick expansion of organic PVs.[1] Since the pioneering work of Tang[2] on electron-donor (D)/electron-acceptor (A) double-layer devices, considerable efforts have focused on the development of bulk D/A hetero- junctions based on photoactive compounds of electron-donating conjugated polymers and fullerene derivatives.[3–6] In these devices the organic components form, throughout the entire active layer, nanometer-sized D and A domains at whose interfaces photogenerated excitons can dissociate into free charge carriers, which in turn are driven to the collecting electrodes by the built-in electric field of the device.[7,8] Applying this methodology to polythiophene/fullerene blends led to PV devices with power conversion efficiencies (PCEs) around 5%.[9,10]Despite this success, polymer/fullerene blends suffer from two major drawbacks: a poorly controlled D/A domain size distribu- tion and inherent morphological instability. The D and A domains generally originate from spinodal decomposition occurring during the film formation from a spin-coated solution and are therefore strongly dependent on the processing conditions and difficult to control.[11] Moreover, macrophase separation of both blend components may occur within the active layer upon extended device operation and considerably modify the as-deposited thin film morphology.[3] The resulting domain size can ultimately become much larger than the exciton diffusion length (about 10 nm in semiconducting polymers[12,13]) and diminish the device performances. The use of rod–coil block copolymers as photoactive material in bulk heterojunction devices is a possible way to overcome these drawbacks. Rod–coil block copolymers are indeed well known to self-assemble through microphase separation into highly ordered nanostructures that are thermodynamically stable and exhibit spatial periodicities on the 1–10 nm length scale.[14–19] Block copolymers composed of an electron-donating and an electron- accepting block are therefore particularly interesting for PV applications and are presently studied worldwide by several research groups.[20–37] Particularly, rod–coil block copoly- mers using poly[(2,5-di(2-ethyl)hexyloxy)-1,4-phenylenevinylene] (DEH-PPV) as electron donor and various coil blocks (such as polystyrene or polybutylacrylate) with covalently linked fullerene moieties as electron acceptor have been investigated inten- sively.[23–28] Although these studies have given considerable insight into the physics of copolymer self-assembly, their efficient utilization as the active layer in PV devices has not yet been fully demonstrated. In the present work, we report on the thin film nanostructure of blends of regio-regular poly(3-hexylthiophene)-block-poly(4- vinylpyridine) (P3HT-b-P4VP) rod–coil block copolymers with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and on the optoelectronic properties of preliminary PVdevicesmade thereof. We anticipate that a conjugated polymer with strong p–p stacking interactions, such as regio-regular P3HT (rr-P3HT), used as the rod block should stabilize the copolymer nano- structure in the presence of fullerene derivatives and allow good hole transport. Furthermore, we show that the utilization of a P4VP coil block demonstrates a new way to form electron- acceptor domains within a block copolymer self-assembled nanostructure. Indeed, polyvinylpyridines are known to experi- ence weak supramolecular interactions with electron-deficient chemical species.[38–40] These interactions would make free C60 molecules preferentially accumulate within the coil domains and,H & Co. KGaA, Weinheim 1 C O M M U N IC A T IO N www.advmat.de www.MaterialsViews.com Figure 1. a) P3HT-P4VP block copolymer/PCBM compound used for the polymer bulk heterojunction active layer: b) standard and c) inverted PVdevice structure used in this study. (PEDOT:PSS¼ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), ITO¼ indium tin oxide, HTL¼ hole transport layer). 2 when combined with an electron-donating rod block, lead to the endeavored D/A interpenetrated networks. Also, in the case of block copolymer/fullerene blends, the copolymer microphase separation has been found to be less affected by the C60 crystallization than for fullerene-grafted block copolymers.[34,39] In particular, the pristine DEH-PPV-b-P4VP nanostructure was shown to be preserved when blended with 10% C60 and to be thermally stable up to at least 16 h at 180 8C.[39] Finally the PCBM fullerene derivative was chosen for its high solubility in common solvents. In this paper, special emphasis is put on the copolymer nanostructure, the morphology thermal stability, and the device properties, for different PCBM contents. We find that the P3HT ordering as well as the copolymer nanostructure can be maintained even at relatively large fullerene contents (36 vol%) and that the thermal stability is dramatically improved in comparison to P3HT:PCBM blends. Finally, we show that a high photon-to-current conversion efficiency (above 40%) and an overall PCE of 1.2% can be reached even with non-optimized PV devices, which positions the present solar cells among the best-performing PVdevices having block copolymers as themajor constituent in the active layers. Figure 1a sketches the blend of the P3HT-P4VP block copolymer and PCBM studied in the present work. P3HT-P4VP has been obtained by anionic polymerization of 4-vinylpyridine and quenching with an aldehyde end- functionalized P3HT. This route has been adapted from the synthesis of a PPV-based block copolymer previously described[17,18] and will be discussed in detail in a separate manuscript. The synthesized diblock copolymer (Fig. 1a) has a total molecular weight of 11.6 kg mol
1 and a P3HT52-P4VP28 architecture. Three series of P3HT-P4VP:PCBM blends, based on either 8, 17, and 36% volume fractions of PCBM were investigated. These blends and related PV devices are hereafter referred to as C8, C17, and C36. Standard P3HT:PCBM (1:1 weight ratio) blends were used as reference material. The details about film formation and device elaboration procedures are described in the Experimental Section. The thin film nanostructure was investigated by transmission electron microscopy (TEM) and UV–vis absorption spectroscopy. PVdevices using the C8, C17, and C36 blends as photoactive layer were elaborated according to both, standard (Fig. 1b) and inverted (Fig. 1c) configurations, for reasons which will become evident in what follows. The devices current–voltage characteristics were measured under darkness and under air mass 1.5 (AM1.5) illumination. The incident photon-to-current conversion effi- ciency (or IPCE) of the device was measured with a standard experimental set-up. According to Ikkala and co-workers,[38] each PCBM molecule can form noncovalent bonds with up to six 4-vinylpyridine (4VP) monomer units. Considering the number of PCBM molecules per 4VPmonomer unit actually present in the active layers, this is only possible in the case of C8, whereas at higher PCBM content only partial binding to 4VP units can be achieved. Figure 2 compares the thin film morphology of the 1:1 P3HT:PCBM blend to that of the C36 copolymer blend, after annealing at 150 8C for either 30min (Fig. 2a,c) or 24 h (Fig. 2b,d). The morphologies of the corresponding pristine P3HT-P4VP block copolymer are shown in the insets. After 30min of
2009 WILEY-VCH Verlag Gannealing, both the reference and block copolymer active layers exhibit comparable nanostructures, with a high level of mixing of the blend components. Major differences arise however after longer annealing times. Micrometer-sized dark domains, which most likely correspond to PCBM crystallites, are present in the PCBM:P3HT blend and point out significant macrophase separation. On the other hand, the C36 nanostructure shows an increased structural order and no formation of microdomains, maintaining a morphology similar to that of the pristine block copolymer (see Fig. 2 insets). These results therefore suggest that i) the high PCBM loading does not perturb the copolymer self-assembly and ii) the block copolymer:PCBM system provides a significantly improved structural stability. Most importantly, these findings show that supramolecular bonding between the fullerene and P4VP coil block avoids the formation of macroscopic fullerene crystals without hampering the copolymer self-assembly. This behavior contrasts with that of previously reported fullerene-grafted block copolymer self-assembly.[34] Furthermore, from the pronounced thermal stability of thembH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–6 Final page numbers not assigned C O M M U N IC A T IO N www.MaterialsViews.com www.advmat.de Table 1. PV performances of solar cells with inverted device structure, before and after annealing at 150 8C. Before annealing Composition Voc [V] Jsc [mA cm
2] FF [%] PCE [%] C17 0.61 3.93 40.2 0.96 C36 0.61 4.06 40.2 0.99 After 14 h at 150 8C C36 0.62 4.51 43.8 1.22electron-acceptor counterpart. This approach allows the design of the optimal composition of the system for preserving both suitable structural morphologies and hole/electron mobilities. This settles the compositional/structural control well beyond the possibilities offered by block copolymers with covalently bound electron-acceptor moieties. By following this route, active layers for PV cells have been designed with highly improved thermal stabilities, photon-to-current conversion efficiencies equivalent to those of polymer heterojunction reference cells, and overall energy conversion efficiencies beyond those reported for PV cells whose active layer contain a block copolymer as its major constituent. Thus, the present approach can offer a new viable route toward the design of active layers for bulk heterojunction block-copolymer-based PV cells with improved stability and competitive optoelectronic properties. Experimental Section Materials: The copolymer was synthesized following a similar pro- cedure to the one reported for the synthesis of PPV-P4VP block copolymers [18]. Details about the P3HT-P4VP synthesis will be reported in a separate article. In the present case, only 80% of the P3HT polymers used were end-functionalized: the polymeric material was thus constituted of 80% of block copolymers and 20% of residual P3HT homopolymer. This excess homopolymer is expected to stabilize the nanodomains interfaces [17]. The 4-vinylpyridine block size was intentionally maintained short so that swelling of the coil phase by fullerenes does not jeopardize the possibility of thermodynamically stable interfaces. The blend of the P3HT52-P4VP28 with PCBM was formed by mixing overnight adequate amounts of PCBM and P3HT-P4VP solutions in o-dichlorobenzene. The PCBM and the rr-P3HT, employed for the reference blends, were used as received. TEM Sample Preparation: P3HT-P4VP:PCBM blend films were spin- coated from a o-dichlorobenzene solution on top of a PEDOT:PSS layer previously deposited on a silicon wafer. By dissolving the PEDOT:PSS layer in water, the active layer is floated on water and recovered on a copper grid. The P4VP phase is stained with iodine for 4 h andmicrographs are taken on a SIS Morada CCD mounted on a CM100 Philips TEM operated at 80 kV. Cross-Section Preparation: The aluminum electrode of the device is covered with the four components epoxy resins used for TEM samples preparation. After 3 h curing at 70 8C, the epoxy coated device is left in cold water until the PSS-PEDOT layer is dissolved by the water diffusing from the uncoated side of the device (typically a few hours). The epoxy layer with the organic film is then carefully recovered and embedded in epoxy resin to form a block which is used to produce ultrathin cut using a Leica Ultramicrotome. Device Elaboration and Characterization Procedures: The device structure for the standard device configuration was glass/ITO/PEDOT:PSS/active layer/Ca/Al (Fig. 1b), while the inverted device structure was glass/ITO/ TiOx/active layer/HTL/Al (Fig. 1c), where HTL represents a conductive polymer used as hole transporting layer. The PEDOT:PSS, HTL and TiOx layers as well as the active layer were obtained by spin-coating. For the active layer deposition, a 25mg mL
1 blend solution was used. All theAdv. Mater. 2009, 21, 1–6
2009 WILEY-VCH Verlag Gmb Final page numbers not assignedsamples were annealed at 150 8C for 15min prior to metal deposition. The metal electrodes were deposited by thermal evaporation. The device active area was 28 mm2. The whole process was done in a nitrogen filled glovebox. The device current–voltage characteristics were measured under both, darkness and AM1.5 (100mW cm
2) illumination. J–V and ICPE measurements as well as device annealing (150 8C for up to 14 h) were performed under nitrogen atmosphere. Acknowledgements The authors acknowledge the Swiss Science National Foundation, BASF Aktiengesellschaft, and the French Agence Nationale de la Recherche (HABISOL program) for financial support. This work is dedicated to the memory of Dr. Bert de Boer, who has produced a deep and lasting contribution to the field of block copolymers in organic photovoltaics. Supporting Information is available online from Wiley InterScience. Received: August 5, 2009 Revised: August 21, 2009 Published online:[1] H. Spranggaard, F. C. Krebs, Sol. Energy Mater. Sol. Cells 2004, 83, 125. [2] C. W. Tang, Appl. Phys. Lett. 1986, 48, 183. [3] B. C. Thompson, J. M. J. Frechet, Angew. Chem, Int. Ed. 2008, 47, 58. [4] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009, 21, 1323. [5] R. A. J. Janssen, J. C. Hummelen, N. S. Sariciftci, MRS Bull. 2005, 30, 33. [6] G. Hadziioannou, MRS Bull. 2002, 27, 456. [7] L. J. A. Koster, E. C. P. Smits, V. D. Mihailetchi, P. W. M. Blom, Phys. Rev. B 2005, 72, 085205. [8] M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789. [9] W. Ma, C. Yang, X. Gong, K. Lee, A. J. Heeger, Adv. Funct. Mater. 2005, 15, 1617. [10] Y. Kim, S. Cook, S. Tuladhar, S. A. Choulis, J. Nelson, J. R. Durrant, D. D. C. Bradley, M. Giles, I. Mcculloch, C.-S. Ha, M. Ree,Nat. Mater. 2006, 5, 197. [11] M. K. Riede, K. O. Sylvester-Hvid, M. Glatthaar, N. Keegan, T. Ziegler, B. Zimmermann,M. Niggemann, A. W. Liehr, G.Willeke, A. Gombert, Prog. Photovoltaics: Res. Appl. 2008, 16, 561. [12] J. J. M. Halls, K. Pichler, R. H. Friend, S. C. Moratti, A. B. Homes, Appl. Phys. Lett. 1996, 68, 3120. [13] D. E. Markov, J. C. Hummelen, P. W. M. Blom, A. B. Sieval, Phys. Rev. B 2005, 72, 045217. [14] M. Reenders, G. ten Brinke, Macromolecules 2002, 35, 3266. [15] B. D. Olsen, D. Alcazar, V. Krikorian, M. F. Toney, E. L. Thomas, R. A. Segalman, Macromolecules 2008, 41, 58. [16] B. D. Olsen, R. A. Segalman, Macromolecules 2005, 38, 10127. [17] N. Sary, R. Mezzenga, C. Brochon, G. Hadziioannou, J. Ruokolainen, Macromolecules 2007, 40, 3277. [18] N. Sary, L. Rubatat, C. Brochon, G. Hadziioannou, J. Ruokolainen, R. Mezzenga, Macromolecules 2007, 40, 6990. [19] N. Sary, C. Brochon, G. Hadziioannou, R. Mezzenga, Eur. Phys. J. E 2007, 24, 379. [20] M. Sommer, S. M. Lindner, M. Thelakkat, Adv. Funct. Mater. 2007, 17, 1493. [21] S. S. Sun, C. Zhang, A. Ledbetter, S. Choi, K. Seo, C. E. Bonner, M. Drees, N. S. Sariciftci, Appl. Phys. Lett. 2007, 90, 043117. [22] G. Sauve, R. D. McCullough, Adv. Mater. 2007, 19, 1822. [23] C. Brochon, N. Sary, R. Mezzenga, C. Ngov, F. Richard, M. May, G. Hadziioannou, J. Appl. Polym. Sci. 2008, 110, 3664. [24] B. de Boer, U. Stalmach, P. F. van Hutten, C. Melzer, V. V. Krasnikov, G. Hadziioannou, Polymer 2001, 42, 9097. [25] U. Stalmach, B. de Boer, C. Videlot, P. F. van Hutten, G. Hadziioannou, J. Am. Chem. Soc. 2000, 122, 5464.H & Co. KGaA, Weinheim 5 C O M M U N IC A T IO N www.advmat.de www.MaterialsViews.com 6 [26] U. Stalmach, B. de Boer, A. D. Post, P. F. van Hutten, G. Hadziioannou, Angew. Chem, Int. Ed. 2001, 40, 428. [27] M. H. van der Veen, B. de Boer, U. Stalmach, K. Van de Wetering, G. Hadziioannou, Macromolecules 2004, 37, 3673. [28] K. Van De Wetering, C. Brochon, C. Ngov, G. Hadziioannou, Macromol- ecules 2006, 39, 4289. [29] J. S. Liu, E. Sheina, T. Kowalewski, R. D. McCullough, Angew. Chem, Int. Ed. 2002, 41, 329. [30] C. L. Chochos, J. K. Kallitsis, V. G. Gregoriou, J. Phys. Chem. B 2005, 109, 8755. [31] P. K. Tsolakis, J. K. Kallitsis, A. Godt, Macromolecules 2002, 35, 5758. [32] S. Kim, Y. Kakuda, A. Yokoyama, T. Yokozawa, J. Polym. Sci, Part A 2007, 45, 3129. [33] F. Richard, C. Brochon, N. Leclerc, D. Eckhardt, T. Heiser, G. Hadziioannou, Macromol. Rapid Commun. 2008, 29, 885. [34] S. Barrau, T. Heiser, F. Richard, C. Brochon, C. Ngov, K. Van de Wetering, G. Hadziioannou, D. V. Anokhin, D. A. Ivanov, Macromolecules 2008, 41, 2701.
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