Photovoltaic Cells Based on Sequentially Adsorbed Multilayers of

Figure 1 Repeat unit structures of polyelectrolytes and water-soluble C60 derivative used ..... and (b) ITO/(PPE−EDOT−SO3-/C60−NH3+)50/LiF:Al un...
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Langmuir 2005, 21, 10119-10126

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Photovoltaic Cells Based on Sequentially Adsorbed Multilayers of Conjugated Poly(p-phenylene ethynylene)s and a Water-Soluble Fullerene Derivative† Jeremiah K. Mwaura, Mauricio R. Pinto, David Witker, Nisha Ananthakrishnan, Kirk S. Schanze,* and John R. Reynolds* Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200 Received March 6, 2005. In Final Form: April 6, 2005 We describe the layer-by-layer (LBL) fabrication of multilayer films and photovoltaic cells using poly(phenylene ethynylene)-based anionic conjugated polyelectrolytes as electron donors and water-soluble cationic fullerene C60 derivatives as acceptors. LBL film deposition was found to be linearly related to the number of bilayers as monitored by UV-vis absorption. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) of the multilayer films revealed an aggregated but relatively uniform morphology devoid of any long-range phase separation. The maximum incident monochromatic photon to current conversion efficiency (IPCE) of the photovoltaic cells was 5.5%, the highest efficiency reported to date for cells fabricated by using the LBL fabrication technique, and since the thin film cells do not provide complete absorption of the incident light, the current generation per photon absorbed may be as much as 10%. The cells exhibited open circuit voltages of 200-250 mV with highest measured short circuit currents up to 0.5 mA/cm2 and fill factors around 30%. The power conversion efficiencies measured at AM 1.5 solar conditions (100 mW/cm2) varied between 0.01 and 0.04%, and similar to the IPCE results, the efficiency is a function of the thickness of the PV active layer.

Introduction Due to their unique photophysical and semiconducting properties, conjugated polymers have received considerable interest as materials for various optoelectronic applications including energy conversion, optical sensing, and optical signal transduction.1,2 Conjugated polyelectrolytes (CPEs) are a class of conjugated polymers that feature ionic side groups rendering them soluble in water and polar solvents. CPEs combine the unique properties of variable band gap, water solubility, processibility, the ability to adsorb to charged substrates, and to selfassemble into colloidal and solid-state nanoscale assemblies.3,4 Most of the CPEs investigated to date contain poly(p-phenylene) (PPP), poly(p-phenylene vinylene) (PPV), or poly(p-phenylene ethynylene) (PPE) backbones with cationic (tetraalkylammonium) or anionic pendant groups including phosphonate, sulfonate, and carboxylate functionality.5 In addition, the synthesis and applications of organic soluble PPEs have previously been investigated by several groups.6-9 * To whom correspondence may be addressed: kschanze@ chem.ufl.edu; [email protected]. † Part of the Bob Rowell Festschrift special issue. (1) McGehee, M. D.; Miller, E. K.; Moses, D.; J., H. A. Advances in Synthetic Metals. Twenty Years of Progress in Science and Technology; Elsevier: Amsterdam, 1999. (2) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (3) Thunemann, A. F.; Ruppelt, D. Langmuir 2000, 16, 3221-3226. (4) Schnablegger, H.; Antonietti, M.; Goltner, C.; Hartmann, J.; Colfen, H.; Samori, P.; Rabe, J. P.; Hager, H.; Heitz, W. J. Colloid Interface Sci. 1999, 212, 24-32. (5) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 1293-1309. (6) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605-1644. (7) Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt, H.-W.; Montali, A.; Feldman, K.; Smith, P.; Weder, C. Adv. Funct. Mater. 2001, 11, 41-46. (8) Kim, J.; Levitsky, I. A.; McQuade, D. T.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 7710-7718. (9) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574.

Stimulated by the initial discoveries by Whitten et al.,10 our group, and others, has reported on the amplified quenching of the fluorescence from CPEs by ionic energy and electron acceptors such as N,N′-dimethyl-4,4′-bipyridinium and cyanine dye cations at especially low concentrations.11-17 Quenching is believed to occur via photoinduced energy or electron transfer, where the photoexcited polymer serves as the donor and the quencher cation acts as the acceptor. These studies indicate that CPEs act to efficiently harvest optical energy and transfer it to the acceptor unit. This finding suggests that CPEs may be useful in optical-to-electrical energy conversion, e.g., as the active material in photovoltaic cells where charge transfer occurs at the interface between electron donating and accepting materials and leads to the production of charge carriers that are collected at electrodes. There has been recent considerable interest in the construction of photovoltaic cells in which the active matrix consists of a semiconducting conjugated polymer donor blended with an electron acceptor.18-23 Polymer-based (10) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Nat. Acd. Sci. U.S.A. 1999, 96, 12287-12292. (11) Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446-447. (12) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561-8562. (13) Wang, D.; Gong, X.; Heeger, P. S.; Rininsland, F.; Bazan, G. C.; Heeger, A. J. Proc. Nat. Acd. Sci. U.S.A. 2002, 99, 49-53. (14) Chen, L.; McBranch, D.; Wang, R.; Whitten, D. Chem. Phys. Lett. 2000, 330, 27-33. (15) Wang, J.; Wang, D.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A. J. Macromolecules 2000, 33, 5153-5158. (16) Wang, D.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001, 17, 1262-1266. (17) Tan, C.; Atas, E.; Mueller, J. G.; Pinto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 13685-13694. (18) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474-1476. (19) Brabec, J. C.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15-26.

10.1021/la050599m CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005

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Figure 1. Repeat unit structures of polyelectrolytes and water-soluble C60 derivative used in this study.

photovoltaic cells are of significant interest as they offer the possibility of lightweight, flexible shape and potentially cost-effective solutions to solar energy conversion.21 Relatively high optical-to-electrical power conversion efficiencies have been obtained in cells fabricated using blends of a conjugated polymer donor with buckminsterfullerene (C60) derivatives.18 With these polymer/ fullerene blends, power conversion efficiencies have been enhanced by the formation of bulk heterojunctions that greatly increase the interfacial donor-acceptor area.24 To improve on photovoltaic performance, the donoracceptor interface plays a key role and this has led to investigations in thin film deposition techniques that provide molecular-level control of the architecture of the active layer of the cell. Electrostatic layer-by-layer (LBL) deposition of polyelectrolytes has been shown to produce thin films with molecular level thickness control.25,26 This simple and versatile technique typically involves sequentially immersing a substrate into dilute solutions of oppositely charged polyelectrolytes with a rinsing step in between. This sequence is repeated in order to buildup films where the thickness is determined by the number of bilayer adsorption cycles performed. By adjustment of solution parameters such as pH, polyelectrolyte concentration, and ionic strength, the thickness and morphology of the multilayer films can be varied systematically.26 Applications of the LBL technique to fabricate optoelectronic devices, such as photovoltaic cells and light emitting diodes, have been reported by various groups.27-35 For example, Rubner, et al.36 utilized LBL assembly to produce photovoltaic cells with a PPV ionic precursor polymer (donor) and ionic C60 derivatives (acceptor) as the active materials. Other groups have used the same donor/acceptor combinations in LBL fabricated cells and achieved incident photon to current conversion efficiencies (IPCEs) of up to 2% at the maximum absorbance wavelength and AM 1.5 efficiencies ∼10-3%.27-32,37 These cell parameters are quite low compared to conventional conjugated polymer/fullerene solar cells prepared through spin-coating which can exhibit values up to 80% (IPCE) and AM 1.5 efficiencies up to 4% for the best cells.19,22,38 In the present investigation, we have used anionic CPEs with a PPE backbone as donors, combined with a watersoluble dicationic fullerene derivative as the acceptor to

fabricate thin film photovoltaic cells using the LBL technique. These photovoltaic cells performed with IPCE efficiencies of up to 5.5% at λmax of the CPE and power conversion efficiencies (at AM 1.5 solar conditions) of up to 0.04%. The synthesis, photophysics, and amplified quenching of one of the conjugated polyelectrolytes used in the present work (PPE-SO3-, Figure 1) has been reported previously by our group.11 In addition to PPE-SO3-, a new anionic conjugated polyelectrolyte, PPE-EDOT-SO3- which features a PPE-type backbone that contains alternating 1,4-phenylene and 3,4ethylenedioxythienylene repeats is presented for the first time. (20) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585-587. (21) Brabec, J. C. Sol. Energy Mater. Sol. Cells 2004, 83, 273292. (22) Organic Photovoltaics: Concepts and Realization; Brabec, J. C., Dyakonov, V., Parisi, J., Sariciftci, N. S., Eds. Springer: Berlin, 2003; Vol. 60. (23) Nelson, J. Curr. Opin. Solid State Mater. Sci. 2002, 6, 87-95. (24) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789-1791. (25) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (26) Decher, G. Science 1997, 277, 1232-1237. (27) Man, K. Y. K.; Wong, H. L.; Chan, W. K.; Kwong, C. Y.; Djurisic, A. B. Chem. Mater. 2004, 16, 365-367. (28) Durstock, M. F.; Spry, R. J.; Baur, J. W.; Taylor, B. E.; Chiang, L. Y. J. Appl. Phys. 2003, 94, 3253-3259. (29) Durstock, M. F.; Taylor, B.; Spry, R. J.; Chiang, L.; Reulbach, S.; Heitfeld, K.; Baur, J. W. Synth. Met. 2001, 116, 373-377. (30) 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. (31) Li, H.; Li, Y.; Zhai, J.; Cui, G.; Liu, H.; Xiao, S.; Liu, Y.; Lu, F.; Jiang, L.; Zhu, D. Chem. Eur. J. 2003, 9, 6031-6038. (32) Guldi, D. M.; Zilbermann, I.; Anderson, G. A.; Kordatos, K.; Prato, M.; Tafuro, R.; Valli, L. J. Mater. Chem. 2004, 14, 303-309. (33) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater. 1998, 10, 1452-1455. (34) Baur, J. W.; Rubner, M. F.; Reynolds, J. R.; Kim, S. Langmuir 1999, 15, 6460-6469. (35) Kim, S.; Jackiw, J.; Robinson, E.; Schanze, K. S.; Reynolds, J. R.; Baur, J.; Rubner, M. F.; Boils, D. Macromolecules 1998, 31, 964974. (36) Mattoussi, H.; Rubner, M. F.; Zhou, F.; Kumar, J.; Tripathy, S. K.; Chiang, L. Y. Appl. Phys. Lett. 2000, 77, 1540-1542. (37) Pradhan, B.; Bandyopadhyay, A.; Pal, A. J. Appl. Phys Lett. 2004, 85, 663-665. (38) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 45334542.

Layer-by-Layer Fabrication of Photovoltaic Cells

Experimental Section Materials. The conjugated polyelectrolytes were synthesized according to procedures reported elsewhere.5 For the best photovoltaic performance, the conjugated polyelectrolytes were used to fabricate cells within 1 week after synthesis. Fullerene, C60, was purchased from MTR Technologies, while the watersoluble fullerene derivative was synthesized following published procedures and the analytical data were in accordance with literature values.39 Poly(allyamine hydrochloride) (PAH) with Mw ca. 70 000 g mol-1, was purchased from Aldrich and dialyzed before use. Sulfonated polystyrene (SPS) was obtained from Scientific Polymer Products, Inc., with Mw ∼500 000 g mol-1 and was used without further purification. Indium tin oxide (ITO) coated glass, Rs ) 8-12 Ω square-1 was purchased from Delta Technologies. The ITO was patterned into 5 mm wide lines by etching using aqua-regia vapor. The ITO and other glass substrates used were cleaned sequentially using an utrasonicator bath with sodium dodecyl sulfate (SDS), ultrapure water (Milli-Q system, Millipore, Inc.), acetone, and 2-propanol and then plasma (O2) treated before use. Film and Cell Fabrication. The sequential adsorption processes were carried out using a Nanostrata StratoSequence IV programmable slide stainer to both dip and rinse the substrates in polyelectrolyte solutions and rinse water, respectively. The computer-controlled procedure involved dipping the substrates in the CPE solutions for 10 min, followed by rinsing in three freshwater baths for 3, 1, and 1 min, respectively. The rinse baths were refreshed after every dip. Concentrations of the CPE solutions were adjusted to 0.001 M while the PAH and SPS solutions were 0.005 M (pH adjusted to 3.5 with HCl). The C60 derivative was not completely water soluble and therefore was dissolved in a 50:50 water/dimethyl sulfoxide solvent system at a concentration of 5 × 10-4 M. Film and Cell Characterization. The multilayer polymeric films, once assembled on glass substrates, were dried in a vacuum (10-3 Torr) and characterized by UV-vis spectrophotometry (Varian Cary UV-vis-NIR spectrophotometer) to monitor film growth. Photoluminescence (PL) was carried out on a JobinYvon Fluoromax-3 spectrometer. Film thicknesses were measured using a Dektak 3030 (Veeco Instruments Inc.) profilometer. Atomic force microscopy (AFM) images of the films on glass substrates were taken using a Nanoscope III instrument while scanning electron microscopy (SEM) images were acquired using a Hitachi S-4000 FE-SEM. For the photovoltaic cells, the films were dried under dynamic vacuum (10-3 Torr) for 6 h and then dried further in a vacuum oven at 100 °C for 2 h. Lithium fluoride (LiF) and aluminum (Al) layers were sequentially deposited through a shadow mask by thermal evaporation at 4 × 10-7 Torr without breaking the vacuum between depositions. The thicknesses of the LiF and Al layers were determined to be 5 and 2000 Å, respectively, using a calibrated oscillating quartz crystal thickness monitor. The active area for the cells was 0.25 cm2. After metal deposition, the cells were encapsulated with epoxy (Loctite quick set epoxy) in order to minimize exposure to oxygen and moisture. The completed cells were then characterized under ambient conditions. The photocurrent response spectra (IPCE) were performed by focusing monochromatic light from a xenon lamp equipped with an ISA instruments monochromator on the cells while recording the photocurrent values (at 0.0 V) using a Keithley 2400 source meter. The incident light intensity was measured using a silicon detector (UDT instruments, model 350), calibrated for each wavelength. The J-V curves both in darkness and under illumination, were taken using a Keithley 2400 source meter while the AM 1.5 solar simulation was provided by an Oriel xenon lamp assembly.

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as an electron donor, and a component that can be utilized as an electron acceptor. In the present study, the conjugated polymers, PPE-SO3- and PPE-EDOT-SO3(structures shown in Figure 1) serve as visible light absorbing and electron-donating components. These polymers are water soluble, containing sulfonate pendant groups that give them polyanionic character. In previous investigations of PPE-SO3-, we have shown that, due to the polyanionic character, this polymer could be deposited as thin films on surfaces using the LBL approach.40 PPE-SO3- and PPE-EDOT-SO3- have been investigated in solution, where they are highly fluorescent and their fluorescence is efficiently quenched by electron acceptors, suggesting that their excited states are good electron donors.11 In addition, fluorescence quenching studies suggest that exciton transport in the solid materials is highly efficient.41,42 To investigate the spectral properties of films that contain PPE-SO3- and PPE-EDOT-SO3- alone, a series of multilayer films were prepared using PAH as a buffer polycation. The film deposition process involved first dipping the substrates in the polycation solution for 10 min followed by three rinses in freshwater for 3, 1, and 1 min, respectively. The substrates were then dipped in the CPE solutions for 10 min and then rinsed again before the process was repeated for the desired number of bilayers. Parts a and b of Figure 2 show the UV-vis spectra of the CPE/PAH multilayer films with increasing number of deposition cycles. The absorption spectra for films of various numbers of bilayers indicate that PPE-SO3(Figure 2a) has a higher band gap (Eg ) 1240/λonset) of ∼2.6 eV when compared to PPE-EDOT-SO3- (Figure 2b), which exhibits a band gap of ∼2.1 eV. This difference in the band gaps is due to the stronger donor properties of the EDOT-containing polymer relative to the phenylene moiety contained in the PPE-SO3-, which raises the HOMO energy level. In both cases, the absorbance of the multilayer films increases linearly with the number of bilayers, suggesting a linear relationship of the film thickness to the number of bilayers. It should be noted that in all of these polymer multilayers the absorbance around 330 nm is low compared to the absorption of films containing the C60 component. As the electron-acceptor component for the LBL photovoltaic cells, we selected a dicationic fullerene, C60 derivative (C60-NH3+).39 The C60 derivative, apart from being water soluble, was selected for several reasons. First, C60 is a strong electron acceptor and absorbs weakly in the visible region.18,43,44 Second, C60 and its derivatives are known to be efficient electron-transporting materials,24 and finally, C60 and its derivatives have been successfully used in organic photovoltaics (OPVs), where cells are generally processed via spin coating.18,19,22,38 To investigate the ability to prepare CPE/C60 LBL multilayer assemblies using PPE-SO3- or PPE-EDOT-SO3- as the polyanion and C60-NH3+ as the cation, films were deposited on glass substrates and their UV-vis absorption spectra as a function of the number of deposition cycles acquired are shown in parts c and d of Figure 2, respectively. There is a monotonic increase in the absorbance due to both of the components with increasing number of bilayers. The

Results and Discussion Materials and Layer-by-Layer Films. The active material used to fabricate photovoltaic cells requires several features. These include light absorption (preferably throughout the visible region), a component that can serve (39) Richardson, C. F.; Schuster, D. I.; Wilson, S. R. Org. Lett. 2000, 2, 1011-1014.

(40) Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003, 19, 6523-6533. (41) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466-1472. (42) Kim, J.; McQuade, D. T.; Rose, A.; Zhu, Z.; Swager, T. M. J. Am. Chem. Soc. 2001, 123, 11488-11489. (43) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695-703. (44) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593601.

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Figure 2. UV-vis absorption of multilayer films consisting of (a) PPE-SO3-/PAH, (b) PPE-EDOT-SO3-/PAH, (c) PPE-SO3-/ C60-NH3+, and (d) PPE-EDOT-SO3-/C60-NH3+.

primary absorption bands of both the C60 and CPE components are preserved in the films, namely, the visible absorption of the polymers in the 400-600 nm region and the UV absorption characteristics of the C60-NH3+. There is evidence for some visible absorption by the C60 derivative as indicated by the tail on the long wavelength side of the polymer absorption spectra. While part of this absorption may be attributed to scattering from C60 aggregation, it has been shown that C60 derivatives with [6,6] addition exhibit a weak absorption peak at ∼700 nm.45 Comparison of the absorption spectra of the bilayer films consisting of the CPEs and PAH with those of the CPEs with C60-NH3+ shows that the latter films exhibit considerably more absorption at wavelengths below 400 nm. This difference clearly shows that for the CPE/ C60-NH3+ films, light absorption in this wavelength region is dominated by the C60 component. This feature will become important below when we consider the photovoltaic response of the films in the near-UV region. An important feature to understand with respect to these films is their chemical composition. Both the CPE and the C60 derivative have two ionic pendant groups (per repeat unit for the polymers). If there were an ideal electrostatic pairing during the film deposition, a 1:1 stoichiometry is expected. To probe the stoichiometry in the films, X-ray photoelectron spectroscopy (XPS) studies were done to determine the nitrogen-to-sulfur ratio in multilayer films of PPE-SO3- and C60-NH3+. These studies indicate a 2:1 N/S ratio suggesting that there is more C60-NH3+ than the expected 1:1 electrostatically derived stoichiometry. This may be due to hydrophobic interactions between the donor and acceptor components as the multilayer film is constructed, or possibly due to aggregation of the C60 component. A combined atomic force microscopy (AFM) and scanning electron microscopy (SEM) study was carried out to probe the surface morphology of the CPE/C60-NH3+ multilayer films on glass substrates. The films fully cover the glass substrates with a continuous and reproducible morphology as illustrated in Figure 3. The AFM (done in (45) Zheng, L.; Zhou, Q.; Deng, X.; Yuan, M.; Yu, G.; Cao, Y. J. Phys. Chem. B 2004, 108, 11921-11926.

tapping mode) height images show relatively small nodular features with a surface roughness ranging between 1.5 and 2.5 nm (root mean square). The phase images indicate a similar morphology devoid of any longrange phase separation. For both types of the CPE/C60 films, SEM micrographs indicate a similar aggregated morphology as was observed with AFM. The PPE-SO3films exhibit a more aggregated texture compared to the PPE-EDOT-SO3- films, possibly due to the PPE-SO3based films being thicker, as this polymer was found to deposit more efficiently. In summary, the morphology studies indicate that the films are overall uniform. Previous studies of polymer blends indicated that these features of highly interacting donor and acceptor components would be favorable for photovoltaic cells, due to the fact that a close-packed structure enhances charge separation and mobility.19 Photovoltaic Cells: Energetic Considerations. Before describing the photovoltaic response of cells fabricated using PPE-SO3-, PPE-EDOT-SO3- and C60-NH3+ as the active materials, it is useful to consider the approximate band structure diagram for these materials (see Figure 4). The HOMO and LUMO levels shown for C60-NH3+ in this diagram are based on the previously reported redox values for methanofullerene compounds such as PCBM,19,46,47 which (like C60-NH3+) is a methanoC60 derivative. The HOMO level for PPE-SO3- is based on the oxidation potential of the polymer (+1.0 V vs SCE)48 whereas the LUMO energy is extrapolated from this value by using the optical band gap (2.6 eV). Using redox potentials obtained for poly(p-phenylenes) both with and without the 3,4-ethylenedioxythiophene (EDOT) from our previous work,49,50 the HOMO level for PPE-EDOT-SO3is estimated to lie approximately 0.5 eV above that of (46) Kesharvarz-K, M.; Knight, B.; Haddon, R. C.; Wudl, F. Tetrahedron 1996, 52, 5149-5159. (47) Hummelen, J. C.; Knight, B. W.; LePeq, F.; Wudl, F. J. Org. Chem. 1995, 60, 532-538. (48) Funston, A. M.; Silverman, E. E.; Miller, J. R.; Schanze, K. S. J. Phys. Chem. B 2004, 108, 1544-1555. (49) Child, A. D.; Reynolds, J. R. Macromolecules 1994, 27, 19751977. (50) Wang, F.; Wilson, M. S.; Rauh, R. D.; Schottland, P.; Thompson, B. C.; Reynolds, J. R. Macromolecules 2000, 33, 2083-2091.

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Figure 3. Tapping mode AFM (a) height and (b) phase images (1 µm × 1 µm) showing the surface morphology of 10 bilayer films of PPE-SO3-/C60-NH3+ (left) and PPE-EDOT-SO3-/C60-NH3+ (right), with corresponding (c) SEM (1 µm × 1 µm) images.

Figure 4. Band diagram generated for ITO/(PPE-SO3-/C60-NH3+)50/Al and ITO/(PPE-EDOT-SO3-/C60-NH3+)50/Al cells showing possible pathways, a and b, for photoinduced electron transfer.

PPE-SO3- and the LUMO is extrapolated by using the polymer’s optical band gap (2.1 eV). Charge carrier generation in these materials occurs by photoinduced electron transfer at an interface between the donor polymer (PPE-SO3- or PPE-EDOT-SO3-) and the acceptor, C60-NH3+. Two possible pathways exist for photoinduced electron transfer: (a) transfer of an electron from the singlet excited state of the donor polymer

to C60-NH3+ (path a, Figure 4); (b) transfer of a hole from photoexcited C60-NH3+ to the donor polymers (path b, Figure 4). In order for photoinduced electron transfer via path a to be thermodynamically feasible, the band offset between the LUMO of the photoexcited donor polymer and the LUMO of C60-NH3+ must exceed the singlet exciton binding energy (ca. 0.4 eV).51 Inspection of Figure 4 shows that this condition is met for both CPEs,

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Figure 5. Schematic representation of the photovoltaic cell structure showing the alternating donor (D) and acceptor (A) layers, forming the active material.

e.g., photoinduced electron transfer from the excited-state polymers to C60-NH3+ is exothermic by approximately 0.3 eV in each system. (In both cases, the band offset exceeds the 0.4 eV exciton binding energy by ca. 0.3 eV.) On the other hand, in order for photoinduced electron transfer via path b to be thermodynamically feasible, the band offset between the HOMO of the polymer and the HOMO of C60-NH3+ must exceed the 0.4 eV singlet exciton binding energy. This condition is clearly met for the PPE-EDOT-SO3-/C60-NH3+ pair, where the polymer’s HOMO is ca. 0.9 eV higher in energy compared to the C60-NH3+ HOMO. The situation in the PPE-SO3-/ C60-NH3+ system is less clear because in this case the band offset between the polymer’s HOMO and that of C60NH3+ is only 0.4 eV. Thus, electron transfer from the PPESO3- to photoexcited C60-NH3+ is at best only weakly exothermic, and therefore the process is likely to be relatively inefficient. The difference in the energetics of photoinduced electron transfer via path b for the two donor CPEs arises because C60-NH3+ has a lower energy excited state compared to the polymers, coupled with the fact that PPE-SO3- is a poorer donor compared to PPE-EDOT-SO3-. The manifestation of this difference in thermodynamics of photoinduced electron transfer between the two pairs of materials in the experimental data will be discussed below. Photovoltaic Cells: Characterization. The layerby-layer self-assembly approach was used to fabricate the active material layer for photovoltaic cells as illustrated schematically in Figure 5. Starting with an ITO substrate, cleaned as described in the Experimental Section, multilayer films were sequentially built up beginning with a polycationic buffer polymer (PAH) in order to positively charge the ITO surface. Subsequently, deposition of either PPE-SO3-or PPE-EDOT-SO3- served as the first active layer. Exposure to a solution of the C60-NH3+ completed the bilayer. Repeating this process, 40-60 bilayers were deposited to construct the active films, which were then vacuum-dried. A layer of 5 Å lithium fluoride was deposited followed by a layer of 2000 Å of aluminum in order to cap the cell and provide a counter electrode. The cells were encapsulated in air with epoxy to protect them from the atmosphere during testing. As illustrated by the doubly expanded schematic of the layer-by-layer structure in Figure 5 there is an intermingling of the cationic and anionic layers in the LBL process. Although this schematic shows an ideal picture of discrete CPE/C60 layers, it is believed that, in LBL assemblies, there is an interpenetration of the polymer (51) Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Hwang, D.-H.; Bredas, J. L.; Friend, R. H. Phys. Rev. B 1999, 60, 5721-5727.

chains within the layers leading to a more disordered stucture.25,26 In this case, the disorder gives rise to intimate mixing of the donor and acceptor components within the film, and assuming that phase separation occurs within the layers, this leads to the possibility of the formation of a bulk heterojunction needed for efficient charge separation and transport through the cell.24 While the use of the buffer layer improves the adhesion of the first active layer, very little effect has been seen on cell performance. This is not surprising as the layers are not discrete but are molecularly mixed, and thus, electroactive material deposited on top of the first layer is in close proximity to the electrode. As measured using a profilometer, a 50 bilayer film has a thickness of approximately 50 nm. The maximum absorbance of a typical 50 bilayer film is approximately 0.5-0.7 absorbance units, and such a film captures visible light with an efficiency of approximately 50%. Cells constructed with both PPE-SO3- and PPEEDOT-SO3- as the donor material and C60-NH3+ as acceptor produce a substantial photovoltaic response when illuminated with monochromatic or broad-band (AM 1.5) light. Photocurrent action spectra for the cells were obtained by measuring the short circuit current at zero bias while illuminating with wavelength-tunable monochromatic light from a tungsten lamp (100 µW/cm2 at peak maximum). The intensity of the monochromatic source was calibrated allowing us to use the photocurrent data to calculate the incident photon to current efficiency (IPCE) spectra, and the resulting data are shown in Figure 6. Qualitative comparison of the spectral response for the cells that contain the two conjugated polyanions shows that the photocurrent action spectrum for PPE-EDOT-SO3- is broader and extends farther into the red when compared to that for PPE-SO3-. This difference in the photoaction spectra mirrors the absorption spectra for the two polymers shown in Figure 2. An interesting feature in these results is that the IPCE for the PPE-SO3- cell falls sharply at wavelengths below 400 nm, whereas that for the PPE-EDOT-SO3- cell continues to increase for λ < 400 nm. Comparison of the absorption spectra of multilayer films that contain conjugated polyelectrolytes only with those that contain the polyelectrolytes as well as C60-NH3+ shows that the absorption in the region below 400 nm is due mainly to the C60-NH3+ component (see Figure 2). Thus, the photoaction spectra imply that in the PPE-EDOT-SO3-/ C60-NH3+ cells light absorbed by the C60-NH3+ component gives rise to photocurrent. By contrast, in the PPE-SO3-/C60-NH3+ cells, light absorbed by C60-NH3+ seems to not efficiently generate photocurrent. A possible explanation for this difference comes from consideration

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Langmuir, Vol. 21, No. 22, 2005 10125

Figure 6. IPCE spectral response for CPE/C60-NH3+ multilayer film cells (a) ITO/(PPE-SO3-/C60-NH3+)50/LiF:Al and (b) ITO/(PPE-EDOT-SO3-/C60-NH3+)50/LiF:Al under 0 V bias.

of the thermodynamics for photoinduced charge transfer in the two cells. As noted in the previous section and Figure 4, photoinduced electron transfer via path b (photoexcited C60-NH3+) in the PPE-EDOT-SO3-/C60-NH3+ cell is very exothermic, whereas it is at best only weakly exothermic in the PPE-SO3-/C60-NH3+ cell. Thus, in the PPE-SO3-/C60-NH3+ cell when light is absorbed by C60-NH3+ photoinduced electron transfer is inefficient and consequently photocurrent generation is not efficient in the spectral region where the fullerene component dominates the absorption. In the cells that contain either conjugated polyelectrolyte donor, the maximum IPCE is approximately 4.5-5.5%. Because the multilayer films do not absorb all of the incident light (%T ≈ 50% at the absorption maxima of the polymers), the current generation efficiency per photon absorbed by the materials is greater than 10%. Comparison of these IPCE results with those from other cells constructed using the LBL method shows these, to the best of our knowledge, to be the most efficient reported to date. Previously reported IPCEs from cells prepared using the LBL technique had maximum single wavelength values ranging between 0.01 and 2.2%,27-32,37 with the optimum coming from cells fabricated using a bis(2,2′:6′,2′′-terpyridine)ruthenium(II) complex incorporated into a PPV-like main chain and assembled with sulfonated polyaniline.27 To characterize the power conversion efficiencies for the PPE-SO3-/C60-NH3+ and PPE-EDOT-SO3-/ C60-NH3+ cells, their current-voltage characteristics were measured under illumination with AM 1.5 solar simulated light (100 mW/cm2). Repeated experiments were carried out using several cells for each active layer composition (50-bilayer films), and example current-voltage (J-V) plots for the two cell compositions are presented in Figure 7. For either of the donor polymers, the cells give negligible dark current under forward bias, whereas there is a significant photovoltaic response when the cells are illuminated. From the data presented in Figure 7a for the PPE-SO3-/C60-NH3+ cell, we calculate an open circuit voltage (Voc) of 260 mV, a short circuit current (Jsc) of 0.50 mA/cm2, and a fill factor (FF) of 31%. This affords an overall optical to electrical power conversion efficiency of η ) 0.04 ( 0.001% (calculated by using the formula η% ) [Voc × Jsc × FF × 100%]/[total incident power], reported value is an average of data from three cells).

Figure 7. Current-voltage characteristics for CPE/C60-NH3+ multilayer film cells (a) ITO/PPE-SO3-/C60NH3+)50/LiF:Al and (b) ITO/(PPE-EDOT-SO3-/C60-NH3+)50/LiF:Al in the dark and under simulated solar light irradiation (100 mW/cm2).

The overall photovoltaic response of the PPE-EDOTSO3-/C60-NH3+ is not as good as that PPE-SO3- based cell. In particular, cells using this donor polymer exhibit Voc ≈ 200 mV, Jsc ≈ 0.2 mA/cm2, FF ≈ 25%, and an overall power conversion efficiency of η ≈ 0.01 ( 0.002%. Because the visible absorption of PPE-EDOT-SO3- is broader and red-shifted compared to that of PPE-SO3-, the former absorbs a greater fraction of the solar spectrum. On this basis one would a priori expect that the PPE-EDOTSO3- cells would afford higher optical to electrical power conversion efficiency. However, visual observation of the PPE-EDOT-SO3- cells reveals that the polymer bleaches while it is illuminated with the high intensity AM 1.5 solar light. This photobleaching may be partly responsible for the poor photoresponse of the cells that contain PPEEDOT-SO3-. In photovoltaic cells there is an inherent trade-off in film thickness effects. Thicker films are preferred to optimize the light absorption cross section of the material, whereas thin films are needed to optimize the transport of excitons to interfaces and charge carriers to the collecting electrodes. To explore the effect of film thickness on the conversion efficiency of LBL films, cells were constructed using 40, 50, and 60 bilayers of PPE-SO3-/ C60-NH3+, and the photovoltaic response of the cells under AM 1.5 illumination was compared. The results of these tests are compiled in Table 1, and the data suggest that Voc and Jsc is smaller (within experimental error) for devices that contain 40 bilayers. Although on average the performance of the 50 bilayer cells appears to be optimal, the cell-to-cell variation in Jsc is such that it is not possible to draw a definitive conclusion as to whether the performance of the 50 bilayer device is truly better compared to the 60 bilayer device (there is an approximately 10-

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Table 1. Properties of ITO/(PPE-SO3-/C60-NH3+)n/LiF:Al Photovoltaic Cells as a Function of the Number (n) of Bilayers Used in Active Material Deposition (results are average of three cells) device

thickness (Å)

Voc (V)

Jsc (mA/cm2)

FF

%η

40 BL 50 BL 60 BL

390 510 630

0.11 0.26 0.23

0.24 ((0.1) 0.50 ((0.05) 0.39 ((0.08)

0.30 0.31 0.31

0.008 0.041 0.028

20% variation in Jsc when many cells are tested, although the FF and Voc values were consistent). Although the PPE-SO3-/C60-NH3+ cells display a reasonably good J-V response, the overall power conversion efficiency is comparatively low. Several factors contribute to this relatively low efficiency. First, PPE-SO3absorbs only in the blue region of the visible spectrum, and consequently a significant fraction of the incident solar energy is not harvested by this polymer. In addition, the LBL films have a relatively high resistance, with a typical 50 bilayer CPE/C60 film exhibiting R ) 6 kΩ/cm2 (measured at 1 kHz frequency). Nevertheless, when compared to results of others investigating the LBL method for photovoltaic cells, the results obtained on the PPE-SO3-/C60-NH3+ cells are significant because in previous studies little attention has been paid to overall power conversion efficiencies at AM 1.5 solar conditions.27,28,31,52,5352-53 To the best of our knowledge, the highest AM 1.5 power conversion efficiency previously reported for an LBL deposited material prior to our work is 0.0022% from cells fabricated using a bs(2,2′:6′,2′′terpyridine)ruthenium(II) complex incorporated into a PPV-like main chain and LBL assembled with sulfonated polyaniline.27 Summary and Conclusions In this report, we have demonstrated the application of the LBL approach to fabricate the active materials for organic photovoltaic cells. Two conjugated polyelectrolytes which feature anionic sulfonate solubilizing groups were used as electron donors, and a cationic methanofullerene, (52) Guldi, D. M.; Luo, C.; Prato, M.; Maggini, M.; Menna, E.; Mondini, S.; Kotov, N. A.; Koktysh, D. AIP Conf. Proc. 2001, 591, 553-557. (53) Baur, J. W.; Durstock, M. F.; Taylor, B. E.; Spry, R. J.; Reulbach, S.; Chiang, L. Y. Synth. Met. 2001, 121, 1547-1548.

C60-NH3+ was used as the acceptor. The LBL films constructed using the CPE/C60-NH3+ materials were characterized by absorption spectroscopy, AFM, SEM, and XPS. These data reveal that the deposited layers are spatially uniform, relatively smooth, and free of long-range phase segregation between the donor CPE and acceptor C60-NH3+ components. Photovoltaic cells fabricated using 50 bilayer films exhibit moderately efficient IPCE response under low-intensity monochromatic light illumination. The photoaction spectra mirror the absorption spectra of the PPE-SO3- and PPE-EDOT-SO3- (donor) components; however, interestingly only the PPE-EDOTSO3-/C60-NH3+ cell shows a marked photovoltaic response at wavelengths where the C60-NH3+ dominates the absorption. This finding is explained on the basis of the difference in thermodynamics for photoinduced electron transfer in the PPE-SO3-/C60-NH3+ and PPE-EDOT-SO3-/C60-NH3+. Although the overall power conversion efficiency of the cells under AM 1.5 solar simulated light is relatively low, this work represents the best conversion efficiencies yet reported on photovoltaic cells constructed using the LBL approach. It is important to note that the LBL technique affords the ability to prepare donor-acceptor films with relatively precise (molecular level) control over the structure and energetics of the active layers of photovoltaic cells. Thus, this technique makes it possible to study the effects of features such as energy gradient driven exciton and/or charge transfer on photoconversion efficiency in organic photovoltaic cells. We are in the process of using this approach to explore various conjugated polyelectrolytes in different LBL architectures and better understand fundamental aspects of photoinduced charge transfer and energy and exciton transport in nanostructured films. It is hoped that the understanding gained through such fundamental studies will ultimately lead to improved performance of polymer-based photovoltaic cells. Acknowledgment. We gratefully acknowledge financial support of this work from the Department of Energy, Basic Energy Sciences Program (Grant DE-FG-2-96 ER 14617). LA050599M