10732
J. Phys. Chem. C 2007, 111, 10732-10740
Synthesis of a Soluble n-Type Cyano Substituted Polythiophene Derivative: A Potential Electron Acceptor in Polymeric Solar Cells Christos L. Chochos,†,‡ Solon P. Economopoulos,†,‡ Valadoula Deimede,‡ Vasilis G. Gregoriou,‡ Matthew T. Lloyd,§ George G. Malliaras,§ and Joannis K. Kallitsis*,†,‡ Department of Chemistry, UniVersity of Patras, Patras 26500 Greece, Foundation for Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Processes (FORTH-ICEHT), P.O. Box 1414, Patras 26500, Greece, and Department of Materials Science and Engineering, Cornell UniVersity, Ithaca, New York 14853-1501 ReceiVed: March 13, 2007; In Final Form: May 8, 2007
A novel, easy processable n-type polythiophene derivative poly(3-cyano-4-hexylthiophene) (P3CN4HT) was synthesized and characterized with different spectroscopic techniques such as 1H NMR, size exclusion chromatography, Fourier transformed infrared spectroscopy (FT-IR), UV-vis, photoluminescence, and cyclic voltammetry. P3CN4HT is very soluble in common organic solvents (tetrahydrofyran, chloroform) and has high electron affinity. Systematic photoluminescence measurements were used to characterize several electron donating polymers such as poly(2-methoxy-5-[3′,7′-dimethyloctyloxy]-p-phenylene vinylene) (MDMO-PPV), regioregular poly(3-octylthiophene) (P3OT), and poly(4,4′-dihexylcyclopentadithiophene) (PCPDT). When P3CN4HT was employed in blends as the electron acceptor, we observed complete photoluminescence quenching for both MDMO-PPV:P3CN4HT and P3OT:P3CN4HT mixtures. Preliminary photovoltaic measurements demonstrated power conversion efficiency as high as 0.014% for the MDMO-PPV:P3CN4HT blend without any solvent screening, thickness optimation, or post-fabrication annealing of the devices.
1. Introduction Conjugated polymers are currently the subject of a broad research area, especially for application in electronic and electrochemical devices such as polymer light-emitting diodes (PLEDs),1 polymer solar cells,2 sensors,3 electrochromic devices, and field effect transistors (FETs).4 The majority of the conjugated polymers in use are electron donoring (hole transporting) in character, that is, p-type materials. In contrast, there are relatively few reports on electron accepting (n type) soluble conjugated polymers which host low-lying LUMO (lowest unoccupied molecular orbital) energy levels, that is, high electron affinity.5 The n-type conjugated polymers are important not only in light-emitting diodes as electron transport materials for increasing electron injection from the cathode but also in polymer solar cells as polymer acceptors.5d-e The development of all-plastic solar cells as an alternative to the more expensive crystalline-silicon photovoltaic devices has attracted a great deal of attention.6 The demand for inexpensive renewable energy sources has been the driving force for new approaches in the production of low-cost photovoltaic devices. The main limitations to their application are low power conversion efficiency and low stability compared to siliconbased solar cells. However, their cost of production is low and it may not be necessary to reach the performance of inorganic solar cells to put such materials into the market. Furthermore, it is anticipated that plastic solar cells have the advantages of mechanical flexibility, lightweight, and a significantly lower fabrication cost for larger area devices.7 * Corresponding author. Phone: (+30)2610-997121. Fax: (+30)2610997122. E-mail:
[email protected]. † University of Patras. ‡ Institute of Chemical Engineering and high Temperature Processes. § Cornell University.
Since the discovery of efficient photoinduced charge separation at donor-acceptor interfaces, organic solar cells based on conjugated polymers and the fullerene molecule have been widely developed. Photovoltaic cells consisting of a double layer of these components showed a photovoltaic effect; however, efficiencies were quite low.8 A breakthrough in the realization of an efficient device was the development of the so-called bulk heterojunction in which a blend of an electron donor and an electron acceptor materials forms a quasi-three-dimensional network.9 Extensive studies of polymer-based photovoltaic cells have focused largely on blends or nanocomposites of donor polymers such as poly(phenylene vinylene) derivatives, poly(2-methoxy-5-[2-ethylhexoxy]-1,4-phenylene vinylene) (MEHPPV) or poly(2-methoxy-5-[3′,7′-dimethyloctyloxy]-p-phenylene vinylene) (MDMO-PPV) and regioregular poly(3-hexylthiophene) (P3HT). Traditional acceptor materials include soluble fullerenes,10,11 CdSe nanocrystals,12 TiO213 and ZnO14 nanoparticles, carbon nanotubes,15 electron transporting polymers,16 and small molecules.17 Photovoltaic cells of more exotic donor materials have also been investigated such as polyfluorene derivatives,18 2,1,3-benzothiadiazole derivatives,19 poly(thienylene vinylene),20 and other low band gap polymers.21 Although high power conversion efficiencies (∼5.0%) are observed in plastic solar cells utilizing [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor,11h the use of polymers as both the donor and the acceptor components offers clear advantages. On the basis of the low entropy of mixing for macromolecules, phase separation is expected for the polymer blends which, in the case of nanophase separation, increase the interfacial area and enhance exciton dissociation. Another important advantage of donor and acceptor polymers is the ability to individually optimize the π-π* energy gaps for maximal absorption of the visible spectrum by a single layer
10.1021/jp072030v CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
A Potential Electron Acceptor in Polymeric Solar Cells device. Initial results have demonstrated that using polymer blends as photoactive materials is a promising direction, and efficiencies of up to 0.9% have been outlined in the literature.16a,b A higher conversion efficiency from an all polymer solar cell with the bulk heterojunction concept (1.7%) has also been reported.16h This paper focuses on the synthesis and the optical properties of a new n-type polythiophene derivative (poly(3-cyano-4hexylthiophene); P3CN4HT) which has a high electron affinity and is very soluble in common organic solvents. One method to modulate the electronic properties of the conjugated polymers is the introduction of substituents with functional groups.22 To obtain n-type polythiophenes or oligothiophenes, monomers containing cyano or fluoro groups were connected with oligothiophene molecules23 or introduced in the polymer chain as side chain pendants.24 In this work, the cyano group was attached in the 4-position of the regioregular P3HT. Furthermore, blends consisting of P3CN4HT as the electron acceptor with MDMO-PPV, poly(3-octylthiophene) (P3OT) and poly(4,4′-dihexylcyclopentadithiophene) (PCPDT) as the electron donor were characterized in various compositions with photoluminescence and atomic force microscopy (AFM) techniques. Complete quenching of photoluminescence is found in MDMOPPV:P3CN4HT blends as well as in P3OT:P3CN4HT (1:1) mixtures. Finally, because of the complete photoluminescence quenching, we investigated the photovoltaic performance of devices based on MDMO-PPV:P3CN4HT and P3OT:P3CN4HT. 2. Experimental Section 2.1. Instrumentation and Measurements. The structures of the synthesized materials were clarified by 1H and 13C NMR spectroscopy with a Bruker Avance DPX 400 and 100 MHz spectrometer, respectively. Gel permeation chromatography (GPC) measurements were carried out using a polymer lab chromatographer with two Ultra Styragel linear columns (104, 500 A), UV detector polystyrene standards, and CHCl3 as eluent, at 25 °C with a flow rate of 1 mL/min. The UV spectra were recorded on a Hewlett-Packard 8452A diode array UV-vis spectrophotometer. Fluorescence was measured on a PerkinElmer LS45 spectrofluorometer. For the photoluminescence quenching experiments, the absorbance intensity of the prepared thin films was adjusted below 0.1 in arbitrary units (in our case, all of the thin films were at 0.07 au) in order to ensure the same amount of the chromophore units in all polymers and all polymer blends were spin coated from chloroform solutions at a concentration of 12 mg/mL. Cyclic Voltammetry (CV). CV studies were performed using a standard three-electrode cell. Platinum wires were used as counter and working electrodes. Silver/silver nitrate (0.1 M AgNO3 in acetonitrile) was used as a reference electrode. Tetrabutylammonium hexafluorophosphate (TBAPF6; 98%) from Aldrich was used as electrolyte, was recrystallized three times from acetone, and was dried in a vacuum at 100 °C before each experiment. Ferrocene was provided from Aldrich and was purified by sublimation before the experiments. Acetonitrile anhydrous 99.8% CH3CN was also supplied from Aldrich and was used without further purification. All experiments were carried out in an air-sealed electrochemical cell. Before each experiment, the cell was purged with high-purity inert gas for 15 min. Before the start of the measurement, the inert gas was turned to “blanket mode”. Measurements were recorded using an EG&G Princeton Applied Research potensiostat/galvanostat model 263A connected to a personal computer running PowerSuite software. The scan rate was kept constant for all CV
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10733 SCHEME 1: Structures of PBCN4HT and P3CN4HT
runs at 20 mV/s. The working electrode was cleaned before each experiment through sonication in 65% HNO3, followed by subsequent sonication in absolute EtOH. The Ag/AgNO3 electrode was connected to the electrochemical cell through a salt bridge and was calibrated before each experiment by running cyclic voltammetry on ferrocene. The cyclic voltammogram of P3CN4HT was carried out in a methylene chloride solution, while that of PCPDT was carried out in thin film drop-casting from a chloroform solution. In both cases, the concentration of the polymers used for the CV measurements were 10 mg/mL. DeVice Fabrication. Glass substrates with 150 nm thick prepatterned indium tin oxide (ITO; Thin Film Devices, Anaheim, California) were cleaned in a sonic bath of nonionic detergent solution, followed by a deionized water rinse. Immediately following a 10 min UV/ozone treatment, a thin layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT: PSS; Baytron, P grade) was spin coated to form ∼80 nm thick films. The PEDOT:PSS films were baked for 1 h at 120 °C and cooled to room temperature under vacuum. All subsequent steps were carried out in a glovebox under a nitrogen environment with less than 1 ppm oxygen and moisture. MDMO-PPV: P3CN4HT were spin coated from a tetrahydrofuran (THF) solution, and P3OT:P3CN4HT blends were spin coated from a solution of chloroform at a concentration of 12 mg/mL. All of the polymer blends formed continuous, uniform films. The cathode was subsequently deposited under vacuum through a shadow mask at a pressure of 10-6 mbar. It consisted of a thin (∼1 nm) layer of CsF, capped with a 400 nm Al electrode to define a device area of 3 mm2. Six devices per substrate were fabricated and showed characteristics that were identical within 10%. For photovoltaic characterization, illumination was provided by a 50 W tungsten-halogen bulb. The illumination intensity was measured with a UV enhanced Si photodiode, calibrated with a UDT S370 optometer coupled to an integrating sphere. Current-voltage measurements were taken with a Keithley 237 source-measure unit. AFM Characterization. Imaging of the surface morphology of spin-coated samples was accomplished via AFM. A Topometrix Explorer SPM microscope (thermomicroscope) equipped with a scanner of maximum ranges of 100 and 10 µm in xy and z directions, respectively, was used for the AFM measurements. The concentration of the prepared mixtures was the same as for the device fabrication. 2.2. Polymer Synthesis. The regioregular P3HT and MDMOPPV were received from Aldrich and were used without further purification. Regioregular P3OT (Scheme S1; see Supporting Information) was prepared by the Grignard metathesis procedure developed by McCullough et al.25 Poly(3-bromo-4-hexylthiophene) (P3B4HT) was synthesized on the basis of known reactions reported in the literature.26 PCPDT and P3CN4HT
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Figure 1. GPC chromatograms of P3OT(I)-(II), P3B4HT, and P3CN4HT.
TABLE 1: Molecular Weight Characteristics and Polydispersities of the Synthesized Polymers polymers
Mn
Mw
PDI
P3OT(I) P3OT(II) P3B4HT PBCN4HT P3CN4HT PCPDT(I) PCPDT(II)
3800 12100 28300 27600 26900 2600 19100
7790 24200 65900 63700 61800 6500 53400
2.05 2.20 2.33 2.31 2.30 2.50 2.80
were synthesized according to the synthetic routes depicted in Scheme S2 (Supporting Information) and Scheme 1, respectively. Synthesis of P3CN4HT. P3B4HT (0.253 g, 1.0326 mmol) was placed in a flask and hexamethylphosphortriamide (HMPA, 25 mL) was added. The flask was heated at 190 °C under argon atmosphere. When P3B4HT was dissolved, the copper cyanide (CuCN; 0.185 g, 2.0652 mmol) was placed in one portion. The reaction mixture was refluxed for 48 h. The resulting polymer was precipitated in methanol, filtered, and purified by Soxhlet extraction with methanol, acetone, and THF. The desired polymer (P3CN4HT; 0.152 g) was collected from the THF solution. 1H NMR (CDCl3): δ ) 2.93 (broad, 2H), 1.68 (broad, 2H), 1.32 (broad, 6H), 0.88 (s, 3H). 13C NMR (CDCl3): δ ) 145.26, 127.78, 126.05, 119.73, 113.48, 33.04, 30.81, 30.16, 29.74, 24.01, 15.23. 3. Results and Discussion 3.1. Synthesis. The regioregular P3OT was synthesized by the Grignard metathesis procedure developed by McCullough et al.,25 which involves the Ni-mediated polycondensation of 2-bromo-3-octylthiophene-5-magnesium bromide, according to Scheme S1. The crude polymer was fractionated by Soxhlet extraction with solvents of an increased solubility for P3OT (methanol, acetone, hexane, and chloroform) which yielded two polymer fractions with different molecular weights as measured with respect to narrowly distributed polystyrene standards using GPC of the polymers dissolved in chloroform (Figure 1). The fraction from hexane is the low molecular weight P3OT (P3OT(I)) while the fraction from chloroform is the high molecular weight P3OT (P3OT(II)) (Table 1). The synthesis of PCPDT is presented in Scheme S2 (see Supporting Information). The compound 1 was prepared from
Figure 2. 1H NMR spectra in 3.2-2.3 ppm region of (a) P3B4HT, (b) PBCN4HT, and (c) P3CN4HT in CDCl3 at room temperature.
the 3-thienyllithium (obtained via metal halogen exchange between 3-bromothiophene and n-butyllithium) and 3-thiophene carboxaldehyde. Reduction of this compound with an equimolar mixture of LiAlH4 and aluminum trichloride (AlCl3) furnished the monomer 2. Dibromination of 2 with N-bromosuccinimide (NBS) at room-temperature gave compound 3 which was converted by metal-halogen interchange followed by intramolecular oxidative coupling into cyclopentadithiophene 4. The cyclopentadithiophene was dialkylated using procedures previously reported for fluorenes27 giving the 4,4′-dihexylcyclopentadithiophene 5 which thereafter was dibrominated, providing the 2,6-dibromo-4,4′-dihexylcyclopentadithiophene 6. The PCPDT was synthesized by a nickel-catalyzed Kumadatype cross-coupling reaction, according to the literature procedure.28 The resulting purple polymer was purified by precipitation into methanol and Soxhlet extraction with acetone and methanol. The molecular weights of PCPDT were measured with respect to narrowly distributed polystyrene standards using GPC (Table 1). The P3CN4HT was synthesized according to the synthetic route depicted in Scheme 1. When regioregular P3HT was treated with NBS on the basis of the known procedures,26 a yellow solid (P3B4HT) was obtained. The 1H NMR spectrum of this product shows the disappearance of the aromatic hydrogens of P3HT and a slight shift of the R-methylene hydrogen peak of the hexyl group from 2.80 to 2.71 ppm (Figure 2a). P3CN4HT was obtained from P3B4HT, as a brownish solid, by substitution of the bromine atoms from the cyano groups in the presence of copper cyanide (CuCN) with HMPA as solvent at high-temperature (∼190 °C). The reaction conditions are slightly modified from the known reaction reported in the literature.29 HMPA was selected as solvent instead of N,Ndemethylformamide (DMF) because P3B4HT is partially soluble in DMF even at high temperatures. The incorporation of the cyano groups in the polythiophene backbone of P3CN4HT was confirmed by a variety of spectroscopic techniques such as 1H and 13C NMR, size exclusion chromatography, and Fourier transform infrared spectroscopy (FT-IR). The GPC-determined
A Potential Electron Acceptor in Polymeric Solar Cells
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Figure 3. FT-IR spectra of P3B4HT and P3CN4HT casted from a chloroform solution.
Figure 5. Photoluminescence spectra of MDMO-PPV, PCPDT, P3OT, P3B4HT, PBCN4HT, and P3CN4HT (a) in chloroform solutions and (b) as thin films.
Figure 4. Absorption spectra of the MDMO-PPV, PCPDT, P3OT, P3B4HT, PBCN4HT, and P3CN4HT (a) in chloroform solutions and (b) as thin films.
molecular weight for P3CN4HT is smaller than that for P3B4HT (Figure 1) indicating a decrease radius of gyration of the P3CN4HT. The 1H NMR spectra of P3B4HT and P3CN4HT in the 3.2-2.3 ppm region (Figure 2b,c) show the slight shift of the R-methylene hydrogen peak of the hexyl group from 2.71 (in the case of P3B4HT) to 2.93 ppm. Furthermore, comparing the 13C NMR spectra of P3OT, P3B4HT, and P3CN4HT (see
Figures S4 and S6 of Supporting Information), one can see the clear shift of the thiophene β-carbon atom of P3OT from 129 to 115 ppm when it was substituted with the bromine atom. As shown from the 13C NMR spectra of P3CN4HT, the thiophene β-carbon atom of P3B4HT also shifts from 115 to 113 ppm when it was substituted with the cyano group, and the appearance of a new peak at 119 ppm assigned to the cyano group was also detected. In addition, comparing the FT-IR spectra of the P3B4HT and P3CN4HT, we find that a new peak centered at 2220 cm-1 assigned to the vibration stretching mode of the cyano group appeared (Figure 3). Moreover, when P3B4HT was treated with two equivalent of CuCN in a lower temperature (∼130 °C), partial substitution of the bromine atoms from the cyano groups was accomplished, and an orange solid (PBCN4HT) was obtained. As shown in Figure 2b, there are R-methylene protons at 2.71 ppm, R-methylene protons at 2.93, and R-methylene protons at 2.83 ppm assigned to hydrogens next to bromine atoms and cyano groups, respectively. The percentage of the cyano groups at the polythiophene backbone in PBCN4HT was calculated from the 1H NMR spectrum at 48%. The importance of the percentage substitution of the cyano groups at the polymer chain will be discussed below when we examine the optical and the electrochemical properties of the polymers. 3.2. Optical and Electrochemical Properties. The absorption spectra of MDMO-PPV, P3OT, P3B4HT, PBCN4HT, P3CN4HT, and PCPDT are presented in Figure 4 and in Table 2. The absorption maxima of P3B4HT (338 nm) in solution is blue-
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TABLE 2: Photophysical and Electrochemical Properties of the Polymers polymers
λabs (nm) solution
λabs (nm) film
opt. band gap (eV) sol., film
MDMO-PPV P3OT(II)a P3B4HT PBCN4HT P3CN4HT PCPDT(II)
498 448 338 369 392 578
498 514, 550, 604 338 372 408 586
2.2, 2.1 2.3, 1.9 3.0, 3.0 2.7, 2.5 2.5, 2.3 1.9, 1.8
electroch band gap (eV)
2.5 (sol)
λemis (nm) solution
λemis (nm) film
EHOMO, ELUMO (eV vs vacuum)
566 572 486 525 539 655
572 650, 715 486 563 606 739, 785
6.1, 3.6 5.1, 3.3b
a Excitation at the lower wavelength of the absorption spectra (514 nm). b ELUMO was calculated from EHOMO - Eg(opt.). Eg(opt.). Optical band gaps as calculated from the onset of the absorption spectra of the polymers.
Figure 6. Cyclic voltammograms (a) of P3CN4HT in CH2Cl2 solution, scan rate 20 mV/s, (b) the oxidation run of PCPDT in thin film cast from CHCl3, scan rate 100 mV/s.
shifted compared with that of P3OT (448 nm) indicating that P3B4HT has a lower effective conjugation length.26a Strong evidence for partial and efficient incorporation of the cyano group into the polythiophene backbone ensues from the absorption spectra of P3B4HT, PBCN4HT, and P3CN4HT. The absorption maxima of PBCN4HT and P3CN4HT in solution are 369 and 392 nm, respectively, which are red-shifted compared with P3B4HT on the basis of the fact that the cyano group is more planar and less bulky than the bromine atom, which probably allows a more efficient packing of the polymer chain. In the solid state, the absorption spectrum of P3B4HT is similar to that of the solution. Conversely, PBCN4HT exhibits an absorption maximum at 372 nm which is red-shifted by 3 nm compared with the solution maximum and 34 nm higher than that of P3B4HT. Moreover, the solid state absorption maximum of P3CN4HT is centered at 408 nm, which is 16 nm red-shifted compared to the solution maximum. This value is 36 nm higher than PBCN4HT, indicating an efficient packing of the polymer chains and an increase of the effective conjugation length. Because of the π-π* electronic transition, the absorption spectrum of MDMO-PPV displays an absorption maximum at 498 nm which does not change between the solution and the solid state. On the contrary, the absorption spectrum of P3OT is different in the solid state compared with the solution, with the appearance of three peaks at 514, 550, and 604 nm characteristic of the regioregular poly(3-alkylthiophenes).25,30 Finally, the absorption spectrum of PCPDT reveals a red shift of 8 nm with the maximum peak shifting from 578 nm in solution to 586 nm in the solid state. The photoluminescence spectra of the conjugated polymers MDMO-PPV, P3OT, P3B4HT, PBCN4HT, P3CN4HT, and PCPDT both in solution and in solid state are presented in Figure 5 and Table 2. The emission spectra of P3B4HT are nominally identical both in solution and in solid state, in contrast to the emission maxima of PBCN4HT and P3CN4HT which shift upon passing from solution to the solid state. PBCN4HT and P3CN4HT both emit red light in thin film form and more
specifically the emission maximum of P3CN4HT (606 nm) is higher than that of MDMO-PPV (572 nm). Finally, the emission spectra of P3OT and PCPDT in the solid state reveal two emission peaks centered at 650, 715 nm for P3OT and 739, 785 nm for PCPDT. CV was used to assess the ionization potentials (EHOMO) and the electron affinity (ELUMO) of P3CN4HT and PCPDT. The EHOMO and ELUMO of MDMO-PPV and P3OT were obtained on the basis of values reported in the literature.31 The potential values obtained versus Ag/Ag+ were converted versus saturated calomel electrode (SCE). The energy levels were calculated using the following empirical equations:32 HOMO ) 4.4 + (Eoxonset) and LUMO ) 4.4 + (Eredonset), where the Eoxonset and Eredonset are the onset oxidation potential and the onset reduction potential versus SCE, respectively. The CV voltammographs of the P3CN4HT and PCPDT are presented in Figure 6, and the energy levels are tabulated in Table 2. P3CN4HT exhibited irreversible characteristics (oxidation and reduction), and the electrochemical band gaps determined by the oxidation and reduction onsets of the polymers were in excellent agreement with the optical band gaps as calculated from the onset of the absorption spectra in dichloromethane solution for P3CN4HT. On the basis of these onsets, the values for the HOMO and the LUMO levels were calculated at 6.1 and 3.6 eV for P3CN4HT, and at 5.1 and 3.3 eV versus vacuum for PCPDT, respectively. 3.3. Photovoltaic Characterization. The inspiration for investigating the photovoltaic response of the polymer blends stemmed from the high degree of photoluminescence quenching for MDMO-PPV, P3OT, and PCPDT subsequent to the introduction of P3CN4HT. Polymer blends of MDMO-PPV: P3CN4HT, P3OT:P3CN4HT, and PCPDT:P3CN4HT, in various compositions (2:1, 1:1, and 1:2), were prepared, and their photoluminescence spectra are presented in Figure 7. Complete photoluminescence quenching is observed for MDMO-PPV: P3CN4HT in all compositions and P3OT:P3CN4HT in a 1:1 composition (Figure 7a,b). This behavior suggests that the LUMO of P3CN4HT is sufficiently below the LUMO of both
A Potential Electron Acceptor in Polymeric Solar Cells
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10737
Figure 8. I-V curves of an ITO/PEDOT:PSS/polymer blend/CsF:Al device under 100 mW/cm2 illumination, where the polymer blend is (a) P3OT:P3CN4HT and (b) MDMO-PPV:P3CN4HT in (2:1, 1:1, 1:2) compositions.
TABLE 3: Tabulation of the Device Performance under 100 mW/cm2
blend P3OT:P3CN4HT P3OT:P3CN4HT P3OT:P3CN4HT MDMO-PPV:P3CN4HT MDMO-PPV:P3CN4HT MDMO-PPV:P3CN4HT
Figure 7. Photoluminescence spectra of (a) MDMO-PPV:P3CN4HT (b) P3OT:P3CN4HT, and (c) PCPDT:P3CN4HT blends in (2:1, 1:1, 1:2) compositions as thin films.
MDMO-PPV (2.7-2.9 eV)16g,31a-c and P3OT (2.9-3.1 eV)31d,e to provide the driving force for exciton dissociation via charge transfer. For the PCPDT:P3CN4HT blend, however, only incomplete photoluminescence quenching is observed in all compositions (Figure 7c), indicating that charge transfer is not efficiently carried out in this system. It is interesting to note that a P3OT:P3CN4HT blending ration of 1:1 results in complete photoluminescence quenching while only partial photolumines-
short open circuit circuit ratio voltage current (wt %) (mV) (mA/cm2) 2:1 1:1 1:2 2:1 1:1 1:2
590 510 500 800 530 620
0.02 0.02 0.01 0.06 0.08 0.09
FF
n (%)
0.27 0.27 0.27 0.25 0.28 0.26
0.003 0.003 0.001 0.012 0.012 0.014
cence quenching is exhibited in 2:1 and 1:2 compositions. Nevertheless, the significant photoluminescence quenching is a strong indication that the MDMO-PPV:P3CN4HT and P3OT: P3CN4HT systems may be useful for photovoltaic applications. For device preparation, the high molecular weight P3OT was used. Figure 8 shows the current-voltage (J-V) characteristics of MDMO-PPV:P3CN4HT and P3OT:P3CN4HT devices. The photovoltaic results obtained for all of the investigated blends under illumination of 100 mW cm-2 are summarized in Table 3. The devices showed shunt resistances (measured in the dark) that were reasonable, with values ranging between 0.1 and 1.6 MΩ cm2. The series resistances, on the other hand, were high, with values nearing 3000 Ω cm2. The high series resistance of these cells is likely to be responsible for reducing the shortcircuit current and fill factor to values below what the materials system can produce. Series resistance can be improved by reducing the thickness of the active polymer layer (which was kept thick during initial characterization to reduce the likelihood of short circuits). Also, an enhancement in micro-phase segrega-
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Figure 9. AFM tapping-mode micrographs of surface topography formed by the P3OT:P3CN4HT mixture from a chloroform solution in (a) 2:1, (b) 1:1, and (c) 1:2 compositions.
tion between the two polymer components should increase shortcircuit current. This has been shown to occur when the heterojunction interface is lengthened in polymer blends.33 It is interesting that the open circuit voltage changes with the polymer blending ratio. Generally, the open circuit voltage is determined by the donor/acceptor band-edge offset and should not vary significantly with device thickness or blending ratio.34 One would expect that the power conversion efficiency of the P3OT:P3CN4HT blend should be higher than that of
Chochos et al.
Figure 10. AFM tapping-mode micrographs of surface topography formed by the MDMO-PPV:P3CN4HT mixture from a tetrahydrofuran solution in (a) 2:1, (b) 1:1, and (c) 1:2 compositions.
MDMO-PPV:P3CN4HT due to the better properties of P3OT. The absorption spectrum of P3OT captures longer wavelengths in the visible spectrum and has demonstrated higher charge carrier mobility than MDMO-PPV.10g,h,35 Despite its suboptimal properties, the power conversion efficiencies of the MDMOPPV:P3CN4HT devices are 1 order of magnitude higher than those of the P3OT:P3CN4HT devices. 3.4. Morphological Characterization. Tapping-mode AFM was employed to gain insight into the structure of the different MDMO-PPV:P3CN4HT and P3OT:P3CN4HT mixtures. Fig-
A Potential Electron Acceptor in Polymeric Solar Cells ures 9 and 10 show the AFM images of the P3OT:P3CN4HT and MDMO-PPV:P3CN4HT blends, respectively. The topography images reveal clear phase separation for the P3OT: P3CN4HT blend in all compositions with an average phase separation length on the order of 390 nm. These preliminary morphological results do not allow a strict correlation of the photoluminescence quenching behavior with the thin film morphology. Similar results were obtained for MDMO-PPV: P3CN4HT blends which differed primarily because of a significantly higher surface roughness. Though the MDMO: P3CN4HT 1:1 blend showed much coarser phase segregation, photoluminescence quenching behavior (Figure 7a) appears to be insensitive to this morphological parameter. 4. Conclusions In summary, by the addition of CuCN in P3B4HT at high temperatures, we have succeeded in synthesizing a new n-type semiconductor (P3CN4HT) with high electron affinity and excellent solubility in common organic solvents. The chemical structure of P3CN4HT was characterized by a variety of spectroscopic techniques. The optical and the electrochemical properties of PBCN4HT and P3CN4HT were modulated as a function of fractional cyano group composition. Complete photoluminescence quenching is observed for the MDMO-PPV: P3CN4HT blend in all compositions and the P3OT:P3CN4HT mixture in a 1:1 composition. Initial photovoltaic characterization of the MDMO-PPV:P3CN4HT and P3OT:P3CN4HT systems indicated power conversion efficiencies as high as 0.014%. Further studies, including optimization of the devices are underway for the full characterization of these systems. Acknowledgment. The authors are grateful to Konarka Technologies of Lowell, MA and the Greek Ministry of Development, under Grant EPAN E13, for the financial support of the project. In addition, we would like to thank Dr. Russell Gaudiana of Konarka Technologies for the helpful discussions. Finally, we also thank Dr. Vasilis Dracopoulos of FORTHICEHT for the help in acquiring the AFM images. Supporting Information Available: Details of the synthesis of the monomers and the conjugated polymers P3OT and PCPDT as well as the 1H and 13C NMR spectra of the synthesized compounds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (b) Cho, N. S.; Park, J.-H.; Lee, S.-K.; Lee, J.; Shim, H.-K.; Park, M.-J.; Hwang, D.-H.; Jung, B.-J. Macromolecules 2006, 39, 177. (2) (a) Hou, J. H.; Tan, Z.; Yan, Y.; He, Y.; Yang, C. H.; Li, Y. F. J. Am. Chem. Soc. 2006, 128, 4911. (b) Zhou, E. J.; Tan, Z.; Yang, C. H.; Li, Y. F. Macromol. Rapid Commun. 2006, 27, 793. (3) (a) Cheng, F.; Zhang, G. W.; Lu, X. M.; Huang, Y. Q.; Chen, Y.; Zhou, Y.; Fan, Q. L.; Huang, W. Macromol. Rapid. Commun. 2006, 27, 799. (b) Lee, K.; Cho, J. C.; Heckb, J. D.; Kim, J. Chem. Commun. 2006, 1983. (4) (a) Kim, Y. M.; Lim, E.; Kang, I.-N.; Jung, B.-J.; Lee, J.; Koo, B. W.; Do, L.-M.; Shim, H.-K. Macromolecules 2006, 39, 4081. (b) Li, Y.; Wu, Y.; Ong, B. S. Macromolecules 2006, 39, 6521. (5) (a) Tonzola, C. J.; Alam, M. M.; Kaminsky, W.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13548. (b) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556. (c) Hancock, J. M.; Gifford, A. P.; Zhu, Y.; Lou, Y.; Jenekhe, S. A. Chem. Mater. 2006, 18, 4924. (d) Veenstra, S. C.; Verhees, W. J. H.; Kroon, J. M.; Koetse, M. M.; Sweelssen, J.; Bastiaansen, J. J. A. M.; Schoo, H. F. M.; Yang, X.; Alexeev, A.; Loos, J.; Schubert, U. S.; Wienk, M. M. Chem. Mater. 2004, 16, 2503. (e) Kietzke, T.; Egbe, D. A. M.; Horhold, H. H.; Neher, D. Macromolecules 2006, 39, 4018.
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