Layer-by-Layer Deposited Multilayer Films of Oligo(pyrenebutyric acid

Mar 7, 2008 - Langmuir , 2008, 24 (8), pp 4380–4387 ... current conversion efficiency (IPCE) of the composite film with 5 bilayers was measured to b...
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Langmuir 2008, 24, 4380-4387

Layer-by-Layer Deposited Multilayer Films of Oligo(pyrenebutyric acid) and a Perylene Diimide Derivative: Structure and Photovoltaic Properties Lu Zhao, Tai Ma, Hua Bai, Gewu Lu, Chun Li,* and Gaoquan Shi* Key Laboratory of Bioorganic Phosphorous Chemistry & Chemical Biology, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed December 12, 2007. In Final Form: January 25, 2008 Multilayer films of oligo(pyrenebutyric acid) (OPB) and N,N′-bis(N,N-dimethylaminopropylaminopropyl)-3,4,9,10-perylenediimide (BDMAPAP-PDI) were successfully fabricated by layer-by-layer deposition. Multilayer growth was monitored by ultraviolet-visible (UV-vis) spectroscopy, fluorescence spectroscopy, ellipsometry, and atomic force microscopy (AFM). It was found that extraction was scarcely observed although both components (OPB and BDMAPAP-PDI) have low molecular weights and both electrostatic interactions and π-π stacking contributed to the multilayer deposition. The multilayers exhibit a rapid photocurrent response, and excitations of both OPB and BDMAPAP-PDI can lead to the effective charge dissociation. The incident photon to current conversion efficiency (IPCE) of the composite film with 5 bilayers was measured to be 1.29% at the absorption peak of BDMAPAP-PDI. Fluorescence quenching and photovoltaic conversion studies indicated that strong photoinduced charge transfer interactions occurred at the area of OPB/BDMAPAP-PDI heterojunction in the films, which strongly enhanced the photoresponse of the multilayer films.

Introduction Organic photovoltaic devices have received increasing attention since the discovery of solar cells with the donor-acceptor structure.1-9 To organize the donor and the acceptor units efficiently, ultrathin films, such as Langmuir-Blodgett (LB) films, layer-by-layer films, and self-assembled monolayers, have been fabricated and studied extensively.10-14 Among these techniques, layer-by-layer (LBL) assembly established by Decher is a powerful method for fabricating ultrathin films with controlled architecture and compositions.15-18 This simple and * To whom correspondence should be addressed. Tel.: +86-10-62773743. Fax: +86-10-6277-1149. E-mail: [email protected] (G.S.); [email protected] (C.L.). (1) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183-185. (2) Shi, C. J.; Yao, Y.; Yang, Y.; Pei, Q. B. J. Am. Chem. Soc. 2006, 128, 8980-8986. (3) Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911-4916. (4) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wud, F. Science 1992, 258, 1474-1476. (5) Yu, G.; Pakbaz, K.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 3422-3424. (6) Yao, Y.; Shi, C. J.; Li, G.; Shrotriya, V.; Pei, Q. B.; Yang, Y. Appl. Phys. Lett. 2006, 89, 153507. (7) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85-88. (8) Wong, W. Y.; Wang, X. Z.; He, Z.; Chan, K. K.; Djurisic, A. B.; Cheung, K. Y.; Yip, C. T.; Ng, A. M. C.; Xi, Y. Y.; Mak, C. S. K.; Chan, W. K. J. Am. Chem. Soc. 2007, 129, 14372-14380. (9) Wong, W. Y.; Wang, X. Z.; He, Z.; Djurisic, A. B.; Yip, C. T.; Cheung, K. Y.; Wang, H.; Mak, C. S. K.; Chan, W. K. Nat. Mater. 2007, 6, 521-527. (10) Sgobba, V.; Giancane, G.; Conoci, S.; Casilli, S.; Ricciardi, G.; Guldi, D. M.; Prato, M.; Valli, L. J. Am. Chem. Soc. 2007, 129, 3148-3156. (11) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129-9139. (12) Marczak, R.; Sgobba, V.; Kutner, W.; Gadde, S.; D’Souza, F.; Guldi, D. M. Langmuir 2007, 23, 1917-1923. (13) Guldi, D. M.; Zilbermann, I.; Anderson, G. A.; Kordatos, K.; Prato, M.; Tafuro, R.; Valli, L. J. Mater. Chem. 2004, 14, 303-309. (14) Liu, L.; Ai, W. H.; Li, M. J.; Liu, S. Z.; Zhang, C. M.; Yan, H. X.; Du, Z. L.; Wong, W. Y. Chem. Mater. 2007, 19, 1704-1711. (15) Decher, G. Science 1997, 277, 1232-1237. (16) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395-1405. (17) Scho¨nhoff, M. J. Phys.: Condens. Matter 2003, 15, R1781-R1808. (18) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. ReV. 2007, 36, 707-718.

low-cost method has been used for preparing sensors, capsules, and electrical devices.18-24 Photovoltaic devices with LBL multilayered films as the active layers have also been widely investigated.25-40 However, several issues remain to be dealt with. The first is the relative low-energy-transfer efficiencies of (19) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 23192340. (20) Hammond, P. T. AdV. Mater. 2004, 16, 1271-1293. (21) Jaber, J. A.; Schlenoff, J. B. Curr. Opin. Colloid Interface Sci. 2006, 11, 324-329. (22) Johnston, A. P. R.; Cortez, C.; Angelatos, A. S.; Caruso, F. Curr. Opin. Colloid Interface Sci. 2006, 11, 203-209. (23) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 37-44. (24) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203-3224. (25) Man, K. Y. K.; Wong, H. L.; Chan, W. K.; Djurisˇic´, A. B.; Beach, E.; Rozeveld, S. Langmuir 2006, 22, 3368-3375. (26) Spanggaard, H.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2004, 83, 125-146. (27) Liang, Z. Q.; Dzienis, K. L.; Xu, J.; Wang, Q. AdV. Funct. Mater. 2006, 16, 542-548. (28) Li, C.; Mitamura, K.; Imae, T. Macromolecules 2003, 36, 9957-9965. (29) Wang, S. B.; Li, C.; Chen, F.; Shi, G. Q. Nanotechnology 2007, 18, 185707. (30) Tse, C. W.; Man, K. Y. K.; Cheng, K. W.; Mak, C. S. K.; Chan, W. K.; Yip, C. T.; Liu, Z. T.; Djurisic, A. B. Chem.sEur. J. 2007, 13, 328-335. (31) Saab, M. A.; Abdel-Malak, R.; Wishart, J. F.; Ghaddar, T. H. Langmuir 2007, 23, 10807-10815. (32) Mwaura, J. K.; Pinto, M. R.; Witker, D.; Ananthakrishnan, N.; Schanze, K. S.; Reynolds, J. R. Langmuir 2005, 21, 10119-10126. (33) Baur, J. W.; Durstock, M. F.; Taylor, B. E.; Spry, R. J.; Reulbach, S.; Chiang, L. Y. Synth. Met. 2001, 121, 1547-1548. (34) Durstock, M. F.; Spry, R. J.; Baur, J. W.; Taylor, B. E.; Chiang, L. Y. J. Appl. Phys. 2003, 94, 3253-3259. (35) Durstock, M. F.; Taylor, B.; Spry, R. J.; Chiang, L.; Reulbach, S.; Heitfeld, K.; Baur, J. W. Synth. Met. 2001, 116, 373-377. (36) Man, K. Y. K.; Wong, H. L.; Chan, W. K.; Kwong, C. Y.; Djurisˇic´, A. B. Chem. Mater. 2004, 16, 365-367. (37) Li, H. M.; Li, Y. L.; Zhai, J.; Cui, G. L.; Liu, H. B.; Xiao, S. Q.; Liu, Y.; Lu, F. S.; Jiang, L.; Zhu, D. B. Chem.sEur. J. 2003, 9, 6031-6038. (38) Mattoussi, H.; Rubner, M. F.; Zhou, F.; Kumar, J.; Tripathy, S. K.; Chiang, L. Y. Appl. Phys. Lett. 2000, 77, 1540-1542. (39) Piok, T.; Brands, C.; Neyman, P. J.; Erlacher, A.; Soman, C.; Murray, M. A.; Schroeder, R.; Graupner, W.; Heflin, J. R.; Marciu, D.; Drake, A.; Miller, M. B.; Wang, H.; Gibson, H.; Dorn, H. C.; Leising, G.; Guzy, M.; Davis, R. M. Synth. Met. 2001, 116, 343-347. (40) Pradhan, B.; Bandyopadhyay, A.; Pal, A. J. Appl. Phys. Lett. 2004, 85, 663-665.

10.1021/la703884d CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

Layer-by-Layer Deposited Multilayer Films Chart 1. Chemical Structures of OPB and BDMAPAP-PDI

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These two small charged molecules can form uniform multilayers through electrostatic and π-π interactions. And the multilayer films exhibited rapid and strong photoresponse. Experimental Section

the devices. This is mainly due to that, in most devices, nonelectroactive polyelectrolytes were used to interact with electroactive materials to form multilayers, which greatly reduced the conductance of the devices. The second one is the limitation of materials used for LBL deposition. Usually, electrostatic interaction is the predominant driving force for the LBL deposition in aqueous media, which are not favorable for using small molecules as the assembling components.41,42 This is due to the fact that the adsorbed small molecules on the surface are easily stripped off by their oppositely charged partners, inhibiting the deposition of functional small molecules into the multilayer films. More recently, it has been reported that the intermolecular π-π stacking between the small molecules can be used as a secondary driving force for the LBL deposition.43 The molecular aggregation resulted from π-π stacking is helpful to reduce the extraction of small molecules from the surface layers.44 Oligo(pyrenebutyric acid) (OPB) is a conjugated oligomer with a large π system, high fluorescence efficiency, and high stability under ambient conditions45 that has not been used in photovoltaic devices. Perylene diimide (PDI) derivatives, a unique class of n-type semiconductors, have attracted increasing attention in recent years due to their outstanding thermal and photochemical stabilities. They have been widely used in solar cells for their high electron affinity and high electron mobility.1,46-49 PDI can be easily positively charged by simply chemical modification. Therefore, we thought that if OPB, an electron donor with negative charges, and PDI, an electron acceptor with positive charges, can be organized into donor-acceptor alternative structured multilayers, the obtained multilayer materials would be useful for the preparation of photovoltaic devices. Herein, we desire to report the fabrication of the multilayer composite films of OPB and N,N′-bis(N,N-dimethylaminopropylaminopropyl)-3,4,9,10perylenediimide (BDMAPAP-PDI, Chart 1) by LBL assembly. (41) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 2491-2495. (42) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (43) Tang, T. J.; Qu, J. Q.; Mullen, K.; Webber, S. E. Langmuir 2006, 22, 26-28. (44) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 22242231. (45) Wu, X. F.; Shi, G. Q. J. Mater. Chem. 2005, 15, 1833-1837. (46) Shin, W. S.; Jeong, H. H.; Kim, M. K.; Jin, S. H.; Kim, M. R.; Lee, J. K.; Lee, J. W.; Gal, Y. S. J. Mater. Chem. 2006, 16, 384-390. (47) Cremer, J.; Ba¨uerle, P. J. Mater. Chem. 2006, 16, 874-884. (48) Breeze, A. J.; Salomon, A.; Ginley, D. S.; Gregg, B. A.; Tillmann, H.; Ho¨rhold, H. H. Appl. Phys. Lett. 2002, 81, 3085-3087. (49) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122.

1. Materials. 3,4,9,10-Perylenetetracarboxylic acid dianhydride (PTCDA) was purchased from Anshan HIFI Chemicals Co., Ltd. (Anshan, China). Poly(sodium 4-styrene sulfonic) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) with molar masses of 70 000 and 300 000 g, respectively, were bought from Aldrich. N,N-dimethyldipropylenetriamine, 1-pyrenebutyric acid, and (3aminopropyl)ethoxysilane (APS) were bought from Acros. All the reagents described above were used as received. Boron trifluoride diethyl etherate (BFEE) was a fresh product of Changyang Chem. Plant (Beijing, China) and purified by distillation before use. 2. Synthesis of OPB. OPB was synthesized through the reported electrochemical procedures.45 The obtained rough product was dedoped by saturated aqueous ammonia solution for 10 h. Then, OPB was precipitated from the solution by addition of 12 mol L-1 hydrochloric acid. The dedoped OPB was successively purified by extraction with n-hexane and chloroform to remove monomers and part of the oligomers with short chains. Finally, it was extracted with tetrahydrofuran to obtain soluble OPB. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry results demonstrated that the obtained OPB was composed of oligomers with chain lengths of 3-8 pyrenebutyric acid repeating units (Figure S1, Supporting Information). 3. Synthesis of BDMAPAP-PDI. BDMAPAP-PDI was synthesized by following the published procedures.50,51 Briefly, PTCDA (2.40 g, 5.81 mmol) and N,N-dimethyldipropylenetriamine (11.1 mL, 60.9 mmol) were added to 100 mL of n-butanol. The reaction mixture was heated to 90 °C and kept for 24 h under stirring and a nitrogen atmosphere. After the reaction mixture was cooled to room temperature, the crude product was separated by filtration and washed with acetone and ether. Pure BDMAPAP-PDI (3.73 g, 95%) was obtained after drying at 80 °C under vacuum. 1H NMR (CDCl3, 300 MHz) (δ): 8.50 (d, J ) 7.9, 4H), 8.33 (d, J ) 7.9, 4H), 4.27 (t, J ) 6.9, 4H), 2.75 (t, J ) 6.9, 4H), 2.71 (t, J ) 7.2, 4H), 2.35 (t, J ) 7.2, 4H), 2.22 (s, 12H), 2.0 (m, 4H), 1.70 (s, 2H), 1.67 (m, 4H). UV-vis (CHCl3) [λmax ()]: 525 (58 400), 488 (35 300 M-1 cm-1). Anal. Calcd for C40H46N6O4: C, 71.19; H, 6.87; N, 12.45. Found: C, 71.03; H, 6.74; N, 12.15. 4. LBL Deposition. Glass, quartz slides, and Si(100) wafers were cleaned by immersing in a fresh piranha solution (H2SO4:30% H2O2 ) 3:1, by volume) (warning: piranha solution is Very corrosiVe and must be treated with extreme care) and then rinsed with deionized water and dried. ITO glass slides were carefully washed successively by ethanol/acetone mixture (1:1), NaOH solution (5%, by weight), detergent, and deionized water in sequence, and subsequently, they were treated in piranha solution for 10 s to enhance the hydrophilic nature of the surface to ensure the proper anchoring of the siloxane groups without destroying the conductive oxide layer. The glass, quartz, ITO glass slides, and silicon wafers were silanized with APS in dry toluene solution (5%, by volume) for 1 h at room temperature. After several cycles of sonication and rinsing with fresh toluene and methanol, the substrates were treated at 105 °C for 1 h to proceed with the condensation of siloxane. Finally, the APS layer was positively charged by immersion in dilute HCl solution and the substrates were stored in pure water prior to use. The 0.5 mg mL-1 aqueous solutions of OPB and BDMAPAPPDI were prepared for LBL deposition. The ionic strength of each solution was adjusted to 0.5 mol L-1 by NaCl, and the pH values were adjusted to 8.0 for OPB solution and to 6.0 for BDMAPAPPDI solution with NaOH and HCl, respectively. As a result, OPB was negatively charged and BDMAPAP-PDI was positively charged. The treated positively charged substrate was alternately dipped into aqueous solution of OPB and BDMAPAP-PDI. The substrates were (50) Kern, J. T.; Thomas, P. W.; Kerwin, S. M. Biochemistry 2002, 41, 1256812568. (51) Ma, T.; Li, C.; Shi, G. Langmuir 2008, 24, 43-48.

4382 Langmuir, Vol. 24, No. 8, 2008 dipped in each solution for 5 min and subsequently rinsed three times by dipping in deionized water (1 min each). After each deposition and rinsing cycle, the sample was dried with a gentle flow of nitrogen. To study the growth mechanism and photovoltaic properties of the OPB/BDMAPAP-PDI multilayers, OPB/PDDA and PSS/BDMAPAP-PDI multilayers were prepared by following the same procedures described above. 5. Characterization. Ultraviolet-visible (UV-vis) spectra were recorded on a Hitachi 3010 UV-vis spectrometer. Fluorescence spectra were measured on a LS 55 fluorescence spectrometer (PerkinElmer). Atomic force microscopic (AFM) images were taken out by using a tapping mode AFM SPA 400 (Seiko Instruments Inc., Chiba, Japan). The thicknesses of the films were measured by using a model GES5 ellipsometer (Sopra Corp., Bois-Colombes, France). Photovoltaic properties were measured in a one-compartment twoelectrode cell using a CHI440 potentiostat-galvanostat (CH Instruments Inc.). The electrolyte was an aqueous solution with 0.2 mol L-1 KI and 0.01 mol L-1 I2 unless specially stated. All solutions were degassed with nitrogen before measurements. An OPB/ BDMAPAP-PDI multilayer-coated ITO electrode was used as the working electrode, and the counter electrode was a Pt sheet. The incident photon to current conversion efficiency (IPCE) was measured using a Keithley 2400 source meter coupled with a monochromator and a Xenon lamp as the light source. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. In the photocurrent and IPCE measurement the applied voltage was 0 V versus the Pt electrode, and all the photocurrent values were calculated on the basis of at least three individual experiment results. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO) of OPB and BDMAPAP-PDI were measured by electrochemical cyclic voltammetry in a one-compartment three-electrode cell using AgCl as the reference electrode and calculated using the onset of the oxidation and reduction peaks (Eonset) by the following equation:52

Zhao et al.

Figure 1. (A) UV-vis and fluorescence (λex ) 357 nm) spectra of OPB in water (pH ) 8.0, 10 mg L-1). (B) Plots of extinction coefficients (357 nm) and fluorescence intensities (486 nm) of OPB versus its concentrations.

EHOMO/LUMO ) [-(Eonset(vs Ag/AgCl) - E1/2(Fc/Fc+ vs Ag/AgCl)] - 4.8 eV (1)

All potentials were calculated in CH3CN and corrected by referencing the redox potential of ferrocene (Fc), E1/2(Fc/Fc+, vs Ag/AgCl) ) 0.4 V.

Results and Discussion 1. UV-Vis and Fluorescence Spectra of OPB and BDMAPAP-PDI in Solution. Figure 1A shows the UV-vis and fluorescence spectra of 10 mg L-1 OPB in water (pH ) 8.0). It can be seen from this figure that OPB exhibits a broad absorption band with a maximum at 357 nm and an emission located at 486 nm. To address the aggregated states of OPB in water (pH ) 8.0), concentration-dependent absorption and fluorescence spectra of OPB were recorded. Figure 1B plots the extinction coefficients (357 nm) and fluorescence intensities (486 nm) of OPB against its concentrations in water. It was found that the extinction coefficient is kept as a constant value with increasing OPB concentrations, indicating that OPB does not aggregate distinctly in the concentrations examined here, whereas, with increasing concentration, the emission intensity of the 486 nm band increases initially and then decreases as the concentration of OPB goes beyond 20 mg L-1. These observations suggested that the inner filter effect due to the solution absorbance is mainly responsible for the reduction in fluorescence intensity.53 It is known that the imide substituents have a negligible influence on the absorption properties of PDI derivatives due to the nodes of the HOMO and LUMO orbitals at the imide nitrogen position.54,55 Therefore, it is reliable to address the aggregated

state of BDMAPAP-PDI in water through absorption spectra by combining the fact that solvatochromism is scarcely observed for these pigments.54 Figure 2 illustrates UV-vis and fluorescence spectra of 10 mg L-1 BDMAPAP-PDI in water (pH ) 6.0) and chloroform. It can be seen that the spectrum in chloroform shows three absorption bands (525, 488, and 457 nm) and a weak broad shoulder around 429 nm, being characteristic of the S0 f S1 transition with well-resolved vibronic structures corresponding to ν ) 0 f ν′ ) 0, 1, 2, and 3 transitions, respectively.56 These spectral features are very similar to those reported for nonaggregated PDI derivatives dissolved in “good” solvents, indicating that BDMAPAP-PDI is in the monomeric form in chloroform. However, in aqueous solution, the absorption peaks are redshifted and the transitions from the ground state to the higher levels of electronic states (0-1, 0-2, and 0-3) are enhanced with respect to the 0-0 transition, in which the most intense

(52) Sensfuss, S.; Al-Ibrahima, M.; Konkina, A.; Nazmutdinovaa, G.; Zhokhavetsb, U.; Gobschb, G.; Egbec, D. A. M.; Klemmc, E.; Rotha, H.-K. Proc. SPIE 2004, 5215, 129-140. (53) Fanget, B.; Devos, O.; Draye, M. Anal. Chem. 2003, 75, 2790-2795.

(54) Wu¨rthner, F. Chem. Commun. 2004, 1564-1579. (55) Langhals, H. HelV. Chim. Acta 2005, 88, 1309-1343. (56) Wu¨rthner, F.; Chen, Z. J.; Dehm, V.; Stepanenko, V. Chem. Commun. 2006, 1188-1190.

Figure 2. UV-vis and fluorescence spectra of BDMAPAP-PDI (10 mg L-1) in water (pH ) 6.0, λex ) 499 nm) and chloroform (λex ) 488 nm).

Layer-by-Layer Deposited Multilayer Films

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Figure 4. Thickness of multilayer film against the numbers of bilayer.

Figure 3. (A) UV-vis spectra of OPB/BDMAPAP-PDI multilayer films with different numbers of bilayer. (B) Absorbance at 361 and 539 nm of the multilayer films against the numbers of bilayer.

absorption maximum was the 0-1 transition at 500 nm, and the lowest energy vibronic band (0-0) was reduced in intensity and red-shifted to 541 nm. These spectral results indicate that BDMAPAP-PDI molecules are mainly in the aggregated states in water. Intermolecular π-π stacking plays a decisive role in controlling the emissive properties of π-conjugated molecules, and thus, emission spectra were also used to monitor the aggregated states of BDMAPAP-PDI in different solvents.54 As shown in Figure 2, free BDMAPAP-PDI molecule in chloroform exhibits the same peak structure in a mirror image of the absorption as reported previously,57,58 whereas, in water, the emission from the perylene chromophores decreases in intensity due to the formation of larger nonemitting aggregates with the forbidden low-energy excitonic transition and the observed emission is characteristic of PDI moiety in the monomeric form. On the basis of UV-vis and fluorescence results, it is believed that BDMAPAP-PDI can form a one-dimensional assembly in water through π-π stacking and hydrophobic interactions.59-61 The BDMAPAP-PDI aggregates formed by π-π stacking are thought to be a kind of supramolecular polymer, enlarging the apparent molecular weight, and to be beneficial for the LBL deposition. 2. Deposition of LBL Films of OPB and BDMAPAP-PDI. LBL deposition of OPB and BDMAPAP-PDI was performed by alternative dipping the substrates into aqueous solutions of OPB and BDMAPAP-PDI. Figure 3 shows the UV-vis spectra of OPB/BDMAPAP-PDI multilayers on glass slides with 1-12 (57) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J. L.; Oitker, R.; Yen, M.; Zhao, J. C.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390-7398. (58) Balakrishnan, K.; Datar, A.; Oitker, R.; Chen, H.; Zuo, J. M.; Zang, L. J. Am. Chem. Soc. 2005, 127, 10496-10497. (59) Arnaud, A.; Belleney, J.; Boue, F.; Bouteiller, L.; Carrot, G.; Wintgens, W. Angew. Chem., Int. Ed. 2004, 43, 1718-1721. (60) Beckers, E. H. A.; Jonkheijm, P.; Schenning, A.; Meskers, S. C. J.; Janssen, R. A. J. ChemPhysChem 2005, 6, 2029-2031. (61) Wang, W.; Han, J. J.; Wang, L. Q.; Li, L. S.; Shaw, W. J.; Li, A. D. Q. Nano Lett. 2003, 3, 455-458.

Figure 5. Absorbance at 361 and 539 nm of multilayer films against the numbers of bilayer.

bilayers. The absorption peak at 361 nm originated from OPB, and the peak at 539 nm associated with BDMAPAP-PDI was used to monitor the multilayer deposition. It is clear from Figure 3B that the multilayers grew linearly with sequential deposition of the OPB and BDMAPAP-PDI. These results indicate that successful film deposition was achieved by LBL assembly and an equal amount of OPB/BDMAPAP-PDI was deposited in each bilayer. This fact was also confirmed by the linear correlation between the number of bilayer and the thickness of the multilayers measured by an ellipsometer on a silicon wafers as shown in Figure 4. The average thickness of one bilayer was estimated to be about 2.8 nm from the slope of the line. It is noted here that, in each deposition cycle, the absorption at 361 nm was magnified in intensity after an OPB layer (0.5 bilayer) was absorbed to the substrate, and the intensity is kept as a constant value during the subsequent deposition of BDMAPAP-PDI onto the substrate (Figure 5). An integer number used here denotes that BDMAPAP-PDI is the outmost layer and the multilayers are composed of intact bilayers, whereas the multilayers with additional 0.5 layer are terminated with an OPB layer. Similarly, absorption at 539 nm related to BDMAPAPPDI is also kept constant during the deposition of OPB layer. These results indicate that neither OPB nor BDMAPAP-PDI was extracted from the film surfaces during the alternative depositing processes. 3. Mechanism for the Multilayer Deposition. Usually, multilayer deposition from small charged organic molecules and polyelectrolytes did not proceed well. In this case, the polymer tends to extract some of the small charged molecules from the surface rather than simply adsorbing at the surface, decreasing the efficiency of multilayer formation. Therefore, it remains a challenge to deposit multilayer films from small charged molecules. However, in the system described above, extraction was scarcely observed although two components (OPB and

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Figure 6. (A) UV-vis spectra of OPB (red line), BDMAPAP-PDI (blue line), and mixture of OPB and BDMAPAP-PDI with the ratio of 1 to 1 (black line) in water and the sum of spectra of OPB and BDMAPAP-PDI (dashed black line). (B) Fluorescence spectra of OPB (dashed red line), BDMAPAP-PDI (dashed blue line), and 1:1 mixture of OPB and BDMAPAP-PDI aqueous solution (solid lines). [OPB] ) [BDMAPAP-PDI] ) 10 mg L-1.

BDMAPAP-PDI) with low molecular weight were used, being quite different from those reported previously.41,42,44,62,63 To elucidate the good performances for the multilayer deposition from OPB and BDMAPAP-PDI, UV-vis and fluorescence spectra of OPB, BDMAPAP-PDI, and the mixture of OPB and BDMAPAP-PDI with the molar ratio of 1 to 1 in aqueous solution were compared in Figure 6. It can be seen from Figure 6A, in the mixture of OPB and BDMAPAP-PDI, both absorption bands of OPB and BDMAPAP-PDI are red-shifted and broadened with respect to that of OPB and BDMAPAP-PDI, respectively. These results indicate that mixing these two components can induce the distinct aggregation of OPB and BDMAPAP-PDI. Moreover, the emission of OPB (λex ) 357 nm) was strongly quenched as mixing with BDMAPAP-PDI (Figure 6B), and the fluorescence intensity of BDMAPAP-PDI was also strongly decreased (λex ) 499 nm), suggesting that photoinduced electron transfer occurs between OPB and BDMAPAP-PDI.64 On the basis of UV-vis and fluorescence results, we can conclude that primary electrostatic interactions further induce the π-π interactions between each components or the donor (OPB) and acceptor (BDMAPAPPDI) molecules, which would be responsible for the distinct film growth process irrespective of molecular weight. To further address the above-mentioned possible mechanism for the film growth, fluorescence spectra of OPB/BDMAPAPPDI multilayers (λex ) 360 and 500 nm) were recorded to follow (62) Sun, J. Q.; Zou, S.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Mater. Sci. Eng., C 1999, 10, 123-126. (63) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178-182. (64) Koeppe, R.; Sariciftci, N. S. Photochem. Photobiol. Sci. 2006, 5, 11221131.

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Figure 7. (A) UV-vis and fluorescence spectra (λex ) 360 nm) of (OPB/BDMAPAP-PDI)5 and (OPB/PDDA)5. (B) UV-vis and fluorescence spectra (λex ) 500 nm) of (OPB/BDMAPAP-PDI)5 and (PSS/BDMAPAP-PDI)5.

the film deposition process (Figure S3). As one layer of OPB was deposited (n ) 0.5) on the substrate, a strong emission band at 480 nm was observed, whereas this emission was quenched after sequential depositing a BDMAPAP-PDI layer on the substrate and cannot be observed in the following deposition process. On the other hand, when BDMAPAP-PDI layer was deposited after the OPB layer, the emission band of BDMAPAPPDI (λex ) 500 nm) did not appear throughout the deposition process. These fluorescence quenching results suggest a distinct photoinduced electron transfer between OPB and BDMAPAPPDI, giving direct evidence for the interlayer π-π interaction between OPB and BDMAPAP-PDI. Control experiments were also performed to examine the interactions between OPB and BDMAPAP-PDI in the films. Figure 7 compares the absorption and fluorescence spectra between (OPB/PDDA)5, (PSS/BDMAPAP-PDI)5, and (OPB/ BDMAPAP-PDI)5 (the number, 5, denotes the number of multilayers). PDDA and PSS have no absorption in the spectral region examined here; therefore, the absorption intensity can be used to compare the amount of OPB and BDMAPAP-PDI deposited in the multilayers. It can be seen that the amount of OPB in (OPB/PDDA)5 film is around 1.5 times that in (OPB/ BDMAPAP-PDI)5 multilayers, whereas the amount of BDMAPAP-PDI in the (PSS/BDMAPAP-PDI)5 film is comparable to that in the (OPB/BDMAPAP-PDI)5 film. The fluorescence spectrum of (OPB/PDDA)5 excited at 360 nm exhibits a broad emission band with a maximum at 480 nm, which can be attributed to the emission of OPB. In contrast, in the spectrum of the (OPB/ BDMAPAP-PDI)5 film, this band is strongly quenched. Similarly, an emission band of BDMAPAP-PDI at 664 nm (λex ) 500 nm) observed in the spectrum of the (PSS/BDMAPAP-PDI)5 film has been extensively quenched in the spectrum of the (OPB/ BDMAPAP-PDI)5 film. Compared with the fluorescence quenching in aqueous solutions described above, it is reasonable to conclude that the strong quenching of OPB and BDMAPAP-

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Figure 8. AFM images of the OPB/BDMAPAP-PDI multilayer films with (A) 1, (B) 5, and (C) 11 bilayers, respectively. The imaged area is 1 µm × 1 µm.

PDI emissions in the (OPB/BDMAPAP-PDI)5 film is probably due to the photoinduced electron transfer between OPB and BDMAPAP-PDI through interlayer π-π interaction. Combining lines of evidence discussed above, we also can conclude that π-π interaction is another driving force for the formation of stable multilayers except for the electrostatic interaction. That is, the primary electrostatic interactions stimulate further secondary π-π interactions, and the cooperative interactions facilitate the final efficient deposition. Given the facts that OPB and BDMAPAP-PDI are easily organized into ultrathin films through simple procedures, and there exists efficient photoinduced electron transfer between these two components, one can expect that the multilayer of OPB/BDMAPAP-PDI is a promising photoresponsive material. 4. Photovoltaic Properties. Prior to checking the photoelectric conversion properties of OPB/BDMAPAP-PDI multilayers, morphologies of the films were examined by AFM. Figure 8 shows the AFM images of the multilayer films with 1 µm × 1 µm dimension each deposited on silicon. After deposition of a single bilayer, the silicon substrate was covered uniformly by the materials. Along with increasing the number of the bilayers, the roughness of films is increased slightly. The root-meansquare (rms) roughnesses were measured to be 2.63, 3.21, and 4.66 nm for the films with 1, 5, and 11 bilayers, respectively. Figure 9A illustrates the photovoltaic responses of a (OPB/ BDMAPAP-PDI)5 film on ITO irradiated by 60 mW cm-2 white light. I3- in aqueous solution serves as an electron acceptor, and the applied potential was 0 V vs Pt electrode. The sample was exposed to white light for 10 s and kept in dark for another 10 s. As can be seen from this figure, the current response was rapid and steady as the irradiation was switched on and off. Repeated photoexcitation of the multilayer films did not lead to decrease the photocurrent over several hours of alternating light-dark cycles. The photogenerated current flow was from the solution to ITO electrode. The photocurrent density increased with the number of multilayers (n) to a maximum at n ) 5 and then gradually decreased (Figure 9B). With increasing the number of multilayers, both light absorption and resistance of the films were increased.40,65 The former factor enhances the photocurrent densities of the multilayers, whereas the latter one could reduce electron-transfer rate to the ITO electrode. Therefore, there exists a maximum at the point of 5 bilayers. The formation of an OPB/BDMAPAP-PDI heterojunction strongly enhanced the photoresponse of the multilayered films. As shown in Figure 10, the photocurrent densities of OPB monolayer and single bilayer of PSS/BDMAPAP-PDI were (65) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 36933723.

Figure 9. (A) Photocurrent density response of 5-bilayer OPB/ BDMAPAP-PDI film irradiated with 60 mW cm-2 white light. (B) Plot of photocurrent densities of OPB/BDMAPAP-PDI multilayers versus the numbers of bilayer.

measured to be about 0.49 and 0.25 µA cm-2, respectively. However, the photocurrent density of the bilayer film of OPB/ BDMAPAP-PDI was detected to be about 5.13 µA cm-2. It has been confirmed that the amounts of OPB and BDMAPAP-PDI in OPB monolayer and PSS/BDMAPAP-PDI bilayer, respectively, are comparable to those in the OPB/BDMAPAP-PDI bilayer (Figure S4). Therefore, one can conclude that the photoinduced charge-transfer effect between OPB and BDMAPAPPDI efficiently enhanced the photocurrent response of the film. Figure 11 plots the photocurrent action spectrum of 5-bilayer OPB/BDMAPAP-PDI film. The incident photon to current conversion efficiency (IPCE) was calculated using the following equation:

IPCE (%) ) 100 × 1240 × Jsc/[λPi]

(2)

Here λ and Pi are the wavelength (nm) and intensity (mW cm-2) of incident light, respectively, and Jsc is the short circuit density

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Figure 10. Photocurrent density response of a single bilayer of OPB/BDMAPAP-PDI, OPB monolayer, and a single bilayer of PSS/ BDMAPAP-PDI irradiated with 60 mW cm-2 white light at a bias of 0 V vs Pt electrode. Figure 12. Band diagram of the ITO/OPB/BDMAPAP-PDI/I3-, I-/Pt device.

Figure 11. IPCE of 5-bilayer OPB/BDMAPAP-PDI film in an aqueous solution with 0.2 M KI and 0.01 M I2 at a bias of 0 V vs Pt. The red line shows the absorbance spectrum of the (OPB/ BDMAPAP-PDI)5 film.

(mA cm-2). It is clear from this figure that IPCE of 5-bilayer film is 1.29% at 520 nm. The action spectrum of IPCE agrees well with the absorption spectrum of OPB/BDMAPAP-PDI multilayers, indicating that both absorption of OPB and BDMAPAP-PDI can result in efficient photoelectric conversion. With the combination of absorption and emission spectroscopic results of the multilayers, the mechanism of photoelectric conversion is explained as follows. Upon excitation of the OPB (λex ) 357 nm), the emission from OPB is strongly quenched by energy and electron transfer to BDMAPAP-PDI. In this process, the initial energy transfer may be from the OPB excitedstate to BDMAPAP-PDI, followed by a rapid electron transfer from OPB to the BDMAPAP-PDI excited state,66 leading to the photocurrent. However, upon the excitation of BDMAPAP-PDI (λex ) 499 nm), the energy-transfer process is unfavorable. The emission quenching of BDMAPAP-PDI is mainly originated from the electron transfer by accepting an electron from OPB. Combining the facts that strong fluorescence quenching irrespective of the irradiation wavelength in the mixed solution and the multilayers, it is reasonable to conclude that photoinduced electron transfer is mainly responsible for the photoelectric conversion as irradiated the PDI moieties. The band diagram of the ITO/OPB/BDMAPAP-PDI/I3-, I-/Pt device is shown in Figure 12. The HOMO energy level of OPB was calculated to be -5.4 eV, and the LUMO energy level of BDMAPAP-PDI was calculated to be -3.6 eV through electrochemical cyclic voltammetry. The optical band gaps of OPB and BDMAPAP-PDI were calculated to be 2.5 and 2.1 eV (66) Go´mez, R.; Veldman, D.; Blanco, R.; Seoane, C.; Segura, J. L.; Janssen, R. A. J. Macromolecules 2007, 40, 2760-2772.

according to the onset wavelengths of the absorption spectra of (OPB/PDDA)5 and (PSS/BDMAPAP-PDI)5, respectively (Eg ) 1240/λonset). Figure 12 indicates that both excitations of OPB and BDMAPAP-PDI can lead to rapid charge transfer in the film. This process should have a high efficiency because the thickness of the bilayer (2.8 nm) is smaller than the exciton diffusion length.65 The generated electrons and holes then transport to I3- in the solution and ITO electrode, respectively, resulting in the cathodic photocurrent. This result is also accordance with the IPCE diagram that both excitations of OPB and BDMAPAP-PDI are efficient for photon-to-current energy conversion. To further shed light on the photoelectric conversion mechanism, photocurrent measurements with a sacrificial electron donor in the solution were performed. It was found that anodic photocurrent can be generated by irradiating a bilayer of OPB/ BDMAPAP-PDI in an aqueous solution of 0.1 M NaH2PO4 containing 0.01 M ascorbic acid as a sacrificial electron donor (Figure S5), and the photocurrent response is also enhanced by the OPB/BDMAPAP-PDI heterojunction. In contrast with the cathodic process described above, after the charge separation at OPB/BDMAPAP-PDI heterojunctions, the generated holes transport to the sacrificial electron donor, ascorbic acid, in the solution and electrons transfer to the ITO electrode, resulting in the anodic photocurrent (Figure S6).

Conclusions Multilayered composite films of OPB and BDMAPAP-PDI have been successfully prepared by LBL deposition. The large π systems of the two components lead to strong π-π interactions with each other. The π-π interaction prevents the extraction of inner small molecules in the deposition process. An equal amount of OPB/BDMAPAP-PDI was deposited in each bilayer, and a uniform multilayered film was obtained. The LBL films showed strong and steady photoresponse with an IPCE of 1.29% at the absorption peak of BDMAPAP-PDI. The formation of OPB/ BDMAPAP-PDI heterojunctions strongly enhanced the photoresponses of the multilayered films. This is the first example of fabricating multilayers heterojunctions by LBL deposition of two components with low molecular weights. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 50533030, 20604013, 20774056) and 863 project (Grant 2006AA03Z105).

Layer-by-Layer Deposited Multilayer Films

Supporting Information Available: The MALDI-TOF mass spectrum of OPB, UV-vis spectra of OPB/BDMAPAP-PDI multilayers terminated with OPB, emission spectra of OPB/BDMAPAP-PDI multilayers, UV-vis spectra of a bilayer of OPB/BDMAPAP-PDI and PSS/BDMAPAP-PDI and an OPB monolayer, photocurrent responses of a bilayer of OPB/BDMAPAP-PDI and PSS/BDMAPAP-PDI and

Langmuir, Vol. 24, No. 8, 2008 4387 an OPB monolayer in an aqueous solution containing ascorbic acid, and a band diagram of the ITO/OPB/BDMAPAP-PDI/ascorbic acid/Pt device. This material is available free of charge via the Internet at http://pubs. acs.org. LA703884D