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Hybrid Materials from Diaminopyriminide-functionalized Poly(hexylthiophene) and Thymine-capped CdSe Nanocrystals: Part II s Hydrogen Bond Assisted Layer-by-layer Molecular Level Processing Julia De Girolamo, Peter Reiss,* and Adam Pron* INAC, UMR 5819-SPrAM, CEA-CNRS-UniVersite´ Joseph Fourier-Grenoble I, Laboratoire d’Electronique Mole´culaire Organique et Hybride, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: February 13, 2008; ReVised Manuscript ReceiVed: March 25, 2008
The hydrogen bonds directed molecular recognition phenomenon between 1-(6-mercaptohexyl)thymine-capped cadmium selenide nanocrystals and diaminopyrimidine-functionalized poly(alkylthiophene), that is, poly(3hexylthiophene-co-oxy diaminopyrimidine-hexylthiophene), is exploited in the layer-by-layer assembly of these components to yield a new organic/inorganic electroactive hybrid material. The elaborated molecular processing method allows for the fabrication of composite thin films of precisely controlled thickness and low surface roughness without the necessity of the substrate’s surface functionalization before the deposition of the first layer. Complementary scanning electron microscopy and small-angle X-ray scattering investigations of the deposited layers indicate that the polymer and the nanocrystal phases form a quasi-interpenetrating network, which is favorable for their application in photovoltaic devices relying on the bulk heterojunction concept. Introduction Hybrid materials consisting of semiconductor nanocrystals (NCs) and conjugated polymers have been actively studied in the past 15 years, mainly as promising components of organic/ inorganic electronic devices such as photovoltaic cells1 or light emitting diodes.2 Processing of these materials, in view of the preparation of thin films or layers of desired supramolecular organization and morphology, is, however, a challenge, because NCs and conjugated polymers have a strong tendency to phase separate in a noncontrolled manner because of their distinctly different chemical nature. The formation of aggregates of NCs within the polymer matrix is disadvantageous, especially in cases where an interpenetrating network of both components is required as, for example, in the fabrication of so-called “bulk heterojunction” solar cells.3 Several approaches have been explored with the goal to improve the compatibility of NCs and conjugated polymers. They can be roughly divided into two groups: (i) increasing the interactions between the NCs and the conjugated oligomer or polymer by introducing terminal anchoring functions in the latter (e.g., amine, phosphine oxide, etc.);4 (ii) using a two-step grafting approach, which includes the NC surface functionalization with bifunctional ligands (e.g., bromine substituted phosphine oxides or thiols), and the subsequent grafting of the conjugated macromolecules, which contain appropriate reactive groups (e.g., vinyl).5 In our previous paper6 we described a different strategy, which relies on the self-assembly of the organic and the inorganic components by molecular recognition. This approach is based on the introduction of lateral functional groups (diaminopyrimidine, DAP) to poly(3-hexylthiophene) (P3HT) to yield the copolymer poly(3-hexylthiophene-co-oxy diaminopyrimidine-hexylthiophene) (P3HT-co-P3(ODAP)HT). This copolymer is capable of strongly interacting with 1-(6* Corresponding author e-mail:
[email protected] (P.R.), adam.pron@ cea.fr (A.P.).
mercaptohexyl)thymine (MHT)-functionalized NCs via three hydrogen bonds. In a similar manner, termed a “brick and mortar” approach, Rotello and co-workers self-assembled side-functionalized polystyrene with thymine-capped gold nanoparticles.7 In general, the molecular recognition phenomenon can be exploited in different types of molecular level processing, involving NCs and/or polymers such as the deposition of layers of selfassembled semiconductor NCs on appropriate electrodes8 or selective immobilization of metal NCs on functionalized substrates.9 However, the molecular composite preparation method proposed in ref 6, although yielding materials with a uniform distribution of NCs within the polymer matrix, is not appropriate for their processing in the form of thin films of controlled thickness. Moreover, due to the lack of a common solvent for P3HT-co-P3(ODAP)HT and CdSe(MHT), the traditional thin film processing method via spin-coating cannot be applied, and special in situ ligand exchange techniques must be used. In this paper we present a new approach to the preparation of NCs/conjugated polymer hybrid films of precisely controlled composition and thickness. Taking advantage of the incompatible solubility parameters of P3HT-co-P3(ODAP)HT and MHTcapped CdSe NCs, we use the layer-by-layer (LbL) technique for the alternating deposition of NCs and polymer monolayers on the substrate. Initially the LbL method has been developed for polyelectrolytes and is still applied in the overwhelming majority of examples to this class of compounds.10 Nevertheless, this technique is also appropriate for the preparation of hybrid materials consisting of polymers and inorganic nanoparticles.11 Moreover, instead of coulombic attraction, several other interactions between the alternating layers can be exploited. Recently, the formation of covalent bonds was used as the driving force for the LbL assembly of functionalized poly(p-phenylenevinylene) and CdSe NCs.12 To the best of our knowledge, the only example of the extension of the LbL technique to the
10.1021/jp801299b CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
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Figure 1. UV-vis spectra of (a) a glass substrate before and after 5 min immersion in a P3HT-co-P3(Br)HT solution or in a P3HT-co-P3(ODAP)HT solution in chloroform, followed by rinsing and drying; (b) the same substrate with a P3HT-co-P3(ODAP)HT layer deposited as in (a), subsequently immersed for 5 min in a dispersion of stearate or MHT-capped CdSe NCs (4.7 nm), [CdSe(MHT)] ) 0.47 mg/mL, followed by rinsing and drying.
multilayer assembly via hydrogen bonding concerns the fabrication of thin films containing mercaptobenzoic acid-capped CdSe NCs and the conventional polymer poly(vinylpyridine).13 In addition to the description of the composite film preparation using the LbL technique, we investigate in detail the supramolecular organization and morphology of the obtained molecularly processed material as well as its spectroscopic properties. Experimental Preparation of Composite Components. The detailed description of the preparation and characterization of the two functionalized components of the molecular composite, that is (P3HT-co-P3(ODAP)HT and CdSe(MHT), can be found in the Supporting Information of our previous paper.6 Briefly, P3HTco-P3(ODAP)HT was synthesized using a postfunctionalization strategy14 comprised of (i) the preparation of regioregular poly(3hexylthiophene-co-3-(6-bromohexyl)thiophene) (P3HT-co-P3(Br)HT) by nickel-catalyzed Grignard metathesis15 and (ii) its postfunctionalization with 2,4-diamino-6-hydroxypyrimidine, leading to the copolymer P3HT-co-P3(ODAP)HT with a ratio of HT/(ODAP)HT units of 9:1. The (ODAP)HT units are randomly distributed along the polymer conjugated backbone. P3HT-co-P3(ODAP)HT 9:1 is soluble in the same solvents as P3HT, for example, chloroform and tetrahydrofuran (THF). Spherical CdSe NCs of 4.7 and 5.8 nm, covered with stearate ligands after synthesis, were functionalized with 1-(6-mercaptohexyl)thymine by ligand exchange to give CdSe(MHT). The functionalized NCs can only be dispersed in polar solvents such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF). Layer-by-layer Assembly. Two different substrates have been used for this study, a silicon substrate (untreated, p-type) for the X-ray diffraction (XRD) measurements and ITO-coated glass (sheet resistance: 20 Ω sq-1) for spectroscopic and microscopic investigations. ITO-coated glasses were first cleaned by sonication in ethanol (15 min) and acetone (15 min) and then dried in an oven. The multilayer structure was obtained by the alternated dipping of the cleaned substrate into the polymer solution and into the NCs’ dispersion. In a typical procedure, two separate solutions were first prepared, namely 2.31 g/L of P3HT-co-P3(ODAP)HT in chloroform and 0.47 g/L of CdSe(MHT) in a MeOH/DMF 1:10 mixed solvent. The ITO substrate was first immersed in the polymer solution for 2 min, then rinsed with chloroform and dried. In the next step, the substrate covered with the first
polymer layer was immersed in the NCs solution for 5 min, then washed with chloroform to remove DMF and MeOH and dried. This procedure of the bilayer deposition was repeated until the desired film thickness was achieved. After each deposition, the UV-vis spectrum was registered to monitor the film growth. The final film thickness was determined using a profilometer. To give an example, the thickness of a film consisting of 10 bilayers was ca. 50 nm. Characterization Techniques. UV-vis spectra of the deposited layers were recorded on a HP 8452A spectrometer. Small-angle X-ray diffraction (SAXS) was performed with a Philips X’Pert MPD. Scanning electron microscopy (SEM) images were obtained on a Zeiss Ultra-55 microscope. Results and Discussion The copolymer P3HT-co-P3(ODAP)HT provides a very appealing feature that facilitates the application of the LbL technique; no anchoring layer is needed for assuring its adherence on a large variety of substrates containing oxide or hydroxide groups. P3HT-co-P3(ODAP)HT can be directly deposited on several untreated substrates such as glass, ITOcoated glass, and silicon wafers with a surface silicon oxide layer. The adherence of this copolymer originates from its capability of hydrogen bond formation between the DAP groups and the oxidized surfaces. In contrast, P3HT as well as its bromo-functionalized homologue, P3HT-co-P3(Br)HT, do not show any sign of adherence on these substrates. This is clearly illustrated in Figure 1a, which shows the absorption spectra of a glass substrate after immersion into a chloroform solution of each of these polymers, followed by rinsing with the same solvent. The deposition of P3HT-co-P3(ODAP)HT is clearly visible by the π-π* transition band at 510 nm. To the contrary, no spectroscopic signs of P3HT-co-P3(Br)HT adherence can be seen. As the two compared polymers exhibit similar macromolecular parameters, the observed difference in adherence must unambiguously be ascribed to the presence of the DAP groups in P3HT-co-P3(ODAP)HT. The same behavior is observed for ITO-coated glass and Si/SiO2 substrates. After the successful deposition of the polymer layer on the different substrates, the next step is to verify whether the molecular recognition process, between the DAP groups of the copolymer and thymine functions on the surface of the NCs, takes place. This constitutes the condition sine qua non of LbL assembly of both components. Figure 1b shows the absorption spectra of
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SCHEME 1: Schematic representation of the molecular recognition between diaminopyrimidine-functionalized poly(3-hexylthiophene) (P3HT-co-P3(ODAP)HT) and 1-(6-mercaptohexyl)thymine-capped CdSe NCs (CdSe(MHT)).
Figure 2. Effect of the immersion time on the thickness of the deposited CdSe(MHT) or P3HT-co-P3(ODAP)HT layer, deduced from the absorbance at 504 nm for the polymer, at 390 nm for the NCs (NCs’ diameter: 5.8 nm). NCs concentration: 0.36 g/L; polymer concentration: 0.54 g/L.
a P3HT-co-P3(ODAP)HT film deposited on a glass substrate after its dipping in NCs dispersions, followed by rinsing with chloroform and drying. Two types of NCs dispersions were tested: (i) as-synthesized, stearate-capped CdSe NCs in chloroform; or (ii) MHT-functionalized NCs in a DMF/MeOH mixture (10:1). In the first case, the spectrum remained unchanged with respect to that of the polymer-covered substrate, which means that no NCs were deposited or that they were washed away during the rinsing step due to the absence of attracting interactions with the polymer film. In the second case, the deposition of CdSe(MHT) on the polymer film resulted in an increase of the absorbance in the spectral range below 450 nm and in the appearance of a signal corresponding to the first excitonic peak at 604 nm (NCs’ diameter ) 4.7 nm). One should also note that after the NCs’ layer deposition the band corresponding to the π-π* transition in the polymer is hypsochromically shifted from 510 to 490 nm. We attribute this spectral shift to a conformational modification of the poly(thiophene) chains induced by molecular recognition with the thymine groups on the NCs’ surface (see Scheme 1). As a summary of this part, the results of the UV-vis spectroscopy confirm that the molecular recognition process via hydrogen bonding between DAP and Thy groups is the driving force of the deposition process. This hydrogen bond directed interlayer assembly is not unexpected taking into account a high value of the P3HT-co-P3(ODAP)HT-MHT association constant (850 M-1), determined from the nuclear magnetic resonance (NMR) investigations. The formation of H-bonds between ODAP and Thy is also confirmed by the expected shifts of the corresponding IR bands.6 The fabrication of thicker hybrid films can be achieved by alternated dipping of the substrate into a P3HT-co-P3(ODAP)HT solution in chloroform and into a dispersion of CdSe(MHT) in a DMF/MeOH mixture (10:1) with an intermediate step of rinsing with chloroform and drying under argon flow after each dipping step. The LbL deposition process is evidenced by UV-vis spectroscopy, because both the copolymer and the NCs, as already mentioned, exhibit distinct absorption bands. Several experimental parameters can modulate the amount of matter deposited during each dipping step. We have explored the influence of the concentration of the solution and of the immersion time in a systematic study by means of UV-vis spectroscopy.
It appears that for both the polymer and the NCs, the immersion time, independently of the solution concentrations tested, has only a rather small impact on the thickness of the deposited layer on the substrates after rinsing (Figure 2). Therefore, rather short immersion times can be used without changing the quality of the formed layers. Consequently, in subsequent experiments we used immersion times of 2 min for the polymer solution and 5-10 min for the NCs dispersion. To the contrary, the concentration of the solution strongly influences the thickness of the deposited layer. In the case of the polymer deposition, a steady increase in the film absorbance was measured up to the polymer concentration of 1.5 g/L, and then a plateau was reached (Figure 3a). The presence of this plateau indicates that the maximum amount of the deposited matter is limited by the number of specific sites on the substrate surface capable of interacting with the DAP groups of the copolymer. In case of the NCs, deposited on a substrate containing a polymer layer, a similar behavior was observed (Figure 3b), even though the concentration value of saturation was not yet reached within the explored experimental range of 0.05-0.3 g/L. Figure 4a shows the UV-vis absorption spectra recorded during consecutive depositions of 10 bilayers of P3HT-coP3(ODAP)HT/CdSe(MHT) on an ITO-coated glass substrate. The steady increase of the absorbance at both 504 and 622 nm (wavelengths of the polymer π-π* transition peak and the excitonic peak of the used NCs, respectively) indicates that the growth of the film is a progressive and reproducible process. From the linear increase of the polymer and NCs absorbance versus the number of bilayers (Figure 4b), we conclude that the same amounts of polymer and NCs are deposited at each dipping step. The multilayer film obtained by the LbL process does not present any visible defects detectable with the naked eye. With the goal to determine the surface coverage by the NCs, we characterized both silicon and ITO-coated glass substrates after deposition of the first polymer/NCs bilayer by SEM (Figure 5). In both images, the NCs (diameter: 5.8 nm) are clearly identifiable as small white grains. The surface coverage is incomplete and could be estimated as 30-40% by numerical image treatment. The heterogeneous structure seen in the background of Figure 5 (right) is related to the irregular surface of the ITO substrate having an inherent roughness of 3 nm (root
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Figure 3. Influence of the concentration on the thickness of the deposited layer, evidenced by absorbance measurements of (a) P3HT-co-P3(ODAP)HT and (b) CdSe(MHT). (NCs diameter: 5.8 nm). The immersion times are 2 min for the polymer and 5 min for the NCs.
Figure 4. (a) UV-vis absorption spectra recorded during the successive deposition of 10 bilayers of P3HT-co-P3(ODAP)HT /CdSe(MHT) (NCs’ diameter: 5.8 nm), using the LbL method. In the order of increasing absorbance, each spectrum corresponds to the addition of one bilayer. (b) Absorbance measured at 504 and 622 nm as a function of the number of bilayers.
Figure 5. SEM images of a single bilayer of P3HT-co-P3(ODAP)HT/CdSe(MHT), deposited on an oxidized silicon substrate (left) or on an ITO-coated glass substrate (right, same scale).
mean square [rms]). Atomic force microscopy (AFM) measurements of a film consisting of 25 bilayers, deposited on an ITOcoated glass substrate, resulted in a rms of 15.6 nm, indicating a rather smooth surface of the 135 nm thick film. Further structural and morphological studies of the hybrid films produced by LbL were performed by means of SAXS analysis of a silicon substrate containing 15 bilayers of P3HTco-P3(ODAP)HT/CdSe(MHT). The diffractogram presented in Figure 6 revealed Kiessig fringes due to the interference of the
X-rays reflected from the front and the back of the film. The amplitude of the fringes decreases for higher values of 2θ, which can be attributed to the roughness of the film.16 These results show that the LbL film is relatively uniform in thickness and is flat, because no Kiessig fringes (specular reflection) would otherwise be visible. From the periodicity of the fringes we can extract a characteristic distance of 67 nm, corresponding to the total thickness of the film. However, this value is inferior to the expected thickness of 15 monolayers of hexagonally packed
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J. Phys. Chem. C, Vol. 112, No. 24, 2008 8801 consisting of inorganic semiconductor NCs and semiconducting polymers. This approach enabled us to propose a convenient layer-by-layer preparation method of a material constituted of functionalized poly(thiophene) and CdSe NCs, in which the recognition is assured by the thymine group of the NCs surface ligands and the diaminopyrimidine group of the polymer sidechains. SAXS and microscopic studies of this material show that it exhibits a quasi-interpenetrating networks morphology rather than a multilayer one, which is suitable for various applications, including photovoltaic ones. The generality of the proposed approach and the simplicity of its application to the preparation of electroactive thin films of strictly controlled thickness and morphology, unequivocally indicate that it can be successfully applied in the development of new generations of hybrid materials.
Figure 6. SAXS diffractogram recorded for a multilayer film of 15 bilayers of P3HT-co-P3(ODAP)HT/CdSe(MHT) (NCs’ diameter: 5.8 nm) on a silicon substrate.
NCs of a diameter of 5.8 nm. This discrepancy can be explained by the incomplete surface coverage of the NCs’ layers. The interstices between the NCs of the first layer can be subsequently filled with polymer and/or NCs of the subsequent layers. However, this type of material’s assembly leads to a heterostructure much more resembling a 3D interpenetrated network than a multilayered one. Finally, a large hump occurring at 2θ ) 1.5° and corresponding to a distance of 6.8 nm is noticeable on the X-ray curve, which can be attributed to the elementary brick of the film, that is, a NC with the surrounding organic layer. Concluding this section, complementary studies of hybrid films containing a large number of polymer/NCs bilayers indicate that their morphology resembles an interpenetrated network of the NCs and the polymer phases rather than a multilayer assembly. The LbL technique yields films of precisely controlled thickness (by the number of bilayers), which in addition show a low surface roughness. Thus, they should, in principle, be well suited for the use as bulk heterojunction layers in organic/inorganic hybrid photovoltaic cells, especially in view of the fact that the photoluminescence (PL) of both components of the composite is efficiently quenched. In particular, no PL originating from the NCs, expected as a narrow (full width at half-maximum [fwhm] of ca. 30 nm), symmetric peak with a maximum at 640 nm (5.8 nm CdSe NCs) could be detected in the composite film. The only weak signal registered arises from broad residual emission of the functionalized polymer (570-750 nm), which is several times weaker as compared to that of the pure polymer. Preliminary results, obtained without any attempt to optimize the cell parameters, show that a composite layer, sandwiched between an ITO and an Al electrode, exhibits a measurable photovoltaic effect (device active surface ) 28 mm2, open circuit voltage Voc ) 0.82 V, short circuit current density Jsc ) 6.5.10-5 A/cm2, fill factor FF ) 0.28, and power conversion efficiency ) 0.015% when irradiated with 100 mW cm2 under AM 1.5 conditions). Further studies are in progress. Conclusion To summarize, we have applied the molecular recognition concept to the preparation of self-assembled hybrid materials
Acknowledgment. Remi De Bettignies and Severine Bailly from the National Institute of Solar Energy (INES) are thanked for the solar cell device measurements. Jean-Pierre Travers is gratefully acknowledged for stimulating discussions. This work was supported by the French Agence Nationale pour la Recherche (project NANORGYSOL). References and Notes (1) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324–1338. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354–357. (3) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585–587. (4) (a) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Frechet, J. M. J. AdV. Mater. 2003, 15, 58–61. (b) Liu, J. S.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550– 6551. (c) Locklin, J.; Patton, D.; Deng, S. X.; Baba, A.; Millan, M.; Advincula, R. C. Chem. Mater. 2004, 16, 5187–5193. (5) (a) Zhang, Q.; Russell, T. P.; Emrick, T. Chem. Mater. 2007, 19, 3712–3716. (b) Xu, J.; Wang, J.; Mitchell, M.; Mukherjee, P.; Jeffries-EL, M.; Petrich, J. W.; Lin, Z. Q. J. Am. Chem. Soc. 2007, 129, 12828–2833. (c) Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. J. Am. Chem. Soc. 2004, 126, 11574–11582. (6) De Girolamo, J.; Reiss, P.; Pron, A. J. Phys. Chem. C 2007, 111, 14681–14688. (7) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748. (b) Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem.—Eur. J. 2004, 10, 5570–5579. (8) Baron, R.; Huang, C. H.; Bassani, D. M.; Onopriyenko, A.; Zayats, M.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 4010–4015. (9) van den Brom, C. R.; Arfaoui, I.; Cren, T.; Hessen, B.; Palstra, T. T. M.; De Hosson, J. T. M.; Rudolf, P. AdV. Funct. Mater 2007, 17, 2045–2052. (10) (a) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569–594. (b) Decher, G.; Hong, J. D. Makromol. Chem—M Symp. 1991, 46, 321–327. (11) (a) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12, 1526–1528. (b) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Susha, A. S.; Caruso, F. Coll. Surf. A 2002, 202, 135–144. (c) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065– 13069. (d) Sinan, V. A.; Koktysh, D. S.; Yun, B. G.; Matts, R. L.; Pappas, T. C.; Motamedi, M.; Thomas, S. N.; Kotov, N. A. Nano Lett. 2003, 3, 1177–1182. (e) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530– 5533. (12) Liang, Z. Q.; Dzienis, K. L.; Xu, J.; Wang, Q. AdV. Funct. Mater. 2006, 16, 542–548. (13) Hao, E. C.; Lian, T. Q. Langmuir 2000, 16, 7879–7881. (14) Zhai, L.; Pilston, R. L.; Zaiger, K. L.; Stokes, K. K.; McCullough, R. D. Macromolecules 2003, 36, 61–64. (15) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. AdV. Mater. 1999, 11, 250–253. (16) Hazra, S.; Gibaud, A.; Sella, C. J. Phys. Appl. Phys. 2001, 34, 1575–1578.
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