s as Donor Materials in Bulk Heterojunction Solar Cells

Jul 30, 2012 - ABSTRACT: The manuscript describes the design, preparation and characterization of two structurally isomeric random...
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An Insight into the Potential of Random Poly(heteroarylene− vinylene)s as Donor Materials in Bulk Heterojunction Solar Cells Roberto Grisorio,†,‡ Giovanni Allegretta,†,§ Giuseppe Romanazzi,† Gian Paolo Suranna,†,* Piero Mastrorilli,† Marco Mazzeo,‡,⊥ Miriam Cezza,∥ Sonia Carallo,‡ and Giuseppe Gigli‡,⊥,○ †

Department of Water Engineering and of Chemistry, Polytechnic of Bari, Campus Universitario, via Orabona 4, 70125 Bari, Italy NNL-Istituto Nanoscienze, CNR c/o distretto tecnologico Lecce, via Arnesano 16, 73100 Lecce, Italy § Scuola Superiore ISUFI, University of Salento, via Monteroni, 73100 Lecce, Italy ⊥ Department of Mathematics and Physics “E. De Giorgi”, University of Salento, Campus Universitario, via Monteroni, 73100 Lecce, Italy ∥ Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States ○ Center for Biomolecular Nanotechnology (CBN), Italian Institute of Technology (IIT), Via Barsanti 1, Arnesano, 73010, Italy ‡

ABSTRACT: The manuscript describes the design, preparation and characterization of two structurally isomeric random poly(arylene−vinylene)s, the properties of which have been optimized for their use as donor materials in BHJ solar cells. The structure of the polymers was aimed at broadening as much as possible their absorption profile. Poly[9,9-dioctylfluorene− vinylene-co-4,7-dithiophen-2-yl-benzo[1,2,5]thiadiazole−vinylene] (P1) and poly[2,7-dithiophen-2-yl-9,9-dioctylfluorene−vinylene-co-4,7-benzo[1,2,5]thiadiazole−vinylene] (P2) were prepared using the Suzuki−Heck polymerization. The polymers were characterized by elemental analysis, NMR, UV−vis absorption and photoluminescence, cyclic voltammetry, and GPC. The electrochemical characterization of P1 and P2 revealed similar HOMO/LUMO energy levels, although the UV−vis absorption profile of P2 is markedly broader than the one exhibited by P1. The more panchromatic absorption of P2 was explained by DFT and TDDFT calculations showing that the model systems, contributing together to the description of the random polymeric structure, exhibited different calculated excitation energies, that cover a broader portion of the absorption spectrum. In BHJ solar cells, the broadness of the absorption strongly influences the BHJ solar cell performances of P2 compared to P1 leading to higher short circuit currents and to a 3-fold higher power conversion efficiency. The PCE value (0.6%) obtained with P2 is in line with those obtained for other poly(heteroarylene−vinylene)s donors and is amenable to improvement by optimizing the device construction (PC61BM amount in the blend or use of annealing processes). These results demonstrate how combination of a suitable choice of the sequence of aryl units together with the potentialities offered by random polymers, can be useful tools in the design of new light-harvesting polymers in BHJ.

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INTRODUCTION

polymeric organic semiconductors have been the subject of deep studies, being more suitable for ensuring uniform deposition of the photosensitive layer on large areas and on flexible substrates, using wet processing techniques. While considerable efforts are directed toward the improvement of

In the course of the last years, considerable basic research has been focused to the development of efficient organic photovoltaics, and in particular bulk heterojunction (BHJ) solar cells due to their prospected low fabrication cost and large scale production feasibility with respect to the consolidated inorganic semiconductor technology.1 In fact BHJ solar cells offer the intriguing perspective of fabricating flexible, lightweight and low cost photovoltaic devices.2 Within this context, © 2012 American Chemical Society

Received: June 7, 2012 Revised: July 20, 2012 Published: July 30, 2012 6396

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both processing techniques and device architectures,3 an everincreasing interest is also focusing on the synthesis of novel macromolecular semiconducting materials designed to enhance the figures of merit of the corresponding devices.4 To serve as an effective donor material in BHJ solar cells, a conjugated polymer should possess the following requisites: (i) a broad absorption, that ensures a better light harvesting of the solar spectrum; (ii) an efficient charge transfer to the acceptor materials (usually fullerene derivatives); (iii) an efficient hole transport; (iv) a relatively deep HOMO energy level. All these properties influence the figures of merit of the photovoltaic devices, namely the short circuit current (Jsc), the fill factor (FF) and the open circuit voltage (Voc). In particular, the band gap engineering of the π-conjugated polymer is extremely important to enhance the solar cell photocurrent, since the amount of absorbed light depends both on the absorption wavelength and on its extinction coefficient. Poly(arylene−vinylene)s (PAVs) are among the prototypical polymeric architectures for use in BHJ, though their efficiencies5 have never reached those exhibited by polythiophenes. The interest devoted to PAV is due to the presence of vinylene units, that lead to semiconductors with lower bandgaps compared to the corresponding polyarylenes. To fulfill the necessity of a panchromatic absorption spectrum,6 the polymeric materials are usually endowed with a donor− acceptor architecture, exploiting an internal charge transfer from the electron-rich to the electron-poor moiety, leading to a band gap contraction.7 For the above-mentioned purpose, heteroaromatic building blocks such as benzothiadiazole (BTZ) and its derivative dithienylbenzothiadiazole (DTBTZ) have been pinpointed as useful electron poor moieties and employed as comonomers in polyarylene-type organic semiconductors. Alkyl functionalization of the thiophene units is usually required to increase the solubility of DTBTZ-containing macromolecules, at the unfortunate price of a band gap increase, due to backbone distortion. Surprisingly, the incorporation of BTZ and DTBTZ for the design of poly(heteroarylene−vinylene)s for application in BHJ solar cells has only recently been proposed.8 Paradoxically, the risk occurring when narrow band gap poly(heteroarylene−vinylene)s are designed for photovoltaic applications is a lack of absorption at shorter wavelengths, reducing the light harvesting. This problem can be circumvented by synthesizing suitably structured random copolymers, in which the formation of several π-conjugated segments of different conjugation extension leads to broader absorption profiles and better figures of merit in BHJ solar cells with respect to those of the corresponding alternating copolymers.9 In the course of the last years we have devoted increasing attention to a copolymerization method for the obtainment of PAV by a Suzuki−Heck reaction between suitable aryl dibromides and potassium vinyl-trifluoroborate.10 On the basis of our recent experience in the application of this method to the synthesis of novel poly(heteroarylene−vinylenes) for BHJ solar cells11 we prepared two novel materials: a random PAV embodying equimolar amounts of DTBTZ and 9,9dioctylfluorene (P1) as well as its structural isomer embodying 2,7-dithienyl-9,9-dioctylfluorene and BTZ (P2). The two polymers were fully characterized and tested in blend with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as light harvesting materials in BHJ solar cells obtaining, in the case of P2, a 3-fold power conversion efficiency (PCE) with respect to P1.

Article

EXPERIMENTAL PART

All manipulations were carried out under inert nitrogen atmosphere using Schlenk techniques. All solvents were carefully dried and freshly distilled. All reactants were purchased by commercial sources and used without further purifications. 1H and 13C{1H}-NMR spectra were recorded at 295 K on a Bruker Avance 400 MHz spectrometer; chemical shifts are reported in ppm referenced to SiMe4. UV−vis spectra were recorded on a Jasco V-670 instrument and fluorescence spectra were obtained on a Varian Cary Eclipse spectrofluorimeter. FT-IR measurements were recorded on a JASCO FT/IR 4200 instrument. Gel permeation chromatography (GPC) analyses were carried out on an Agilent Series 1100 instrument equipped with a Plgel 5 μm mixed-C column. THF solutions for GPC analysis were eluted at 25 °C at a flow rate of 1.0 mL/min and analyzed using a multiple wave detector. Molecular weights and molecular weight distributions are relative to polystyrene. Thermogravimetric analyses (TGA) were carried out with a Perkin-Elmer Pyris TGA 6 thermobalance under a nitrogen flow. Cyclic voltammetry was carried out on a Metrohm Autolab PGSTAT 302-N potentiostat. The materials were drop cast on a platinum working electrode. Measurements were carried at 25 °C in acetonitrile solution containing tetrabutylammonium tetrafluoroborate (0.025 M) as supporting electrolyte. The potentials were measured versus Ag/Ag+ as the quasi-reference electrode. Subsequently to each experiment, the potential of the Ag/Ag+ electrode was calibrated against the ferrocene/ferrocenium redox couple.12 Elemental analyses were obtained on a EuroVector CHNS EA3000 elemental analyzer. Analyses of the ground-state structures for the oligomers of P1 and P2 (n = m = 2) were carried out using density functional theory (DFT). The B3LYP function13 was used in conjunction with the 631G(d,p) basis set. In order to ease the computational cost, all alkyl chains were replaced by methyl groups. Time-dependent DFT (TDDFT) calculations performed to assess the excited-state transition energies. Absorption spectra were simulated through convolution of the transition energies and their oscillator strengths with the Gaussian function using a full width at half-maximum (fwhm) of 0.2 eV. All calculations were carried out with the Gaussian 09 program package and performed in vacuo. The materials were tested in photovoltaic devices with structure indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/polymer:PC61BM (∼100 nm)/Al (150 nm). The glass/ITO substrates were cleaned by ultrasonication in acetone, water and isopropanol. On top of the ITO coating, a conductive PEDOT−PSS thin film was spun coated, and subsequently baked at 140 °C under nitrogen atmosphere for 15 min. The active layer, constituted by a blend of either P1 or P2 with PC61BM (50%wt) was deposited by spin-coating at 700 rpm from a 18 mg/mL chloroform solution. All the operations concerning the deposition of the active layer were performed in a dinitrogen-filled glovebox. The device fabrication was completed by thermal evaporation of the aluminum cathode in a Kurt J. Lesker UHV Cluster Tool with a base pressure of ∼10−8 mbar. The solar cells, with an active area of 25 mm2, were encapsulated before testing in air. The current−voltage (I−V) characteristics were recorded using a computer-controlled Keithley 2400 source meter. The solar cells where illuminated by a Spectra Physics Oriel 150W Solar Simulator at 100 mW/cm2 (AM 1.5D) white light, or by a monochromatic light using a Spectral Products DK240 monochromator. Potassium vinyltrifluoroborate14 was synthesized according to literature procedures. 2,7-Dibromo-9,9-dioctylfluorene,15 2,7-bis(5bromothiophen-2-yl)-9,9-dioctylfluorene,16 4,7-bis(5-bromothiophen2-yl)benzo[1,2,5]thiadiazole 17 and 4,7-dibromobenzo[1,2,5]thiadiazole18 were prepared by slightly adapting the literature procedures. Poly[9,9-dioctylfluorene−vinylene-co-4,7-dithiophen-2-ylbenzo[1,2,5]thiadiazole−vinylene] (P1). A solution of 4,7-bis(5bromothiophen-2-yl)benzo[1,2,5]thiadiazole (0.229 g, 0.50 mmol), 2,7-dibromo-9,9-dioctylfluorene (0.274 g, 0.50 mmol), potassium vinyltrifluoroborate (0.147 g, 1.10 mmol), Pd(AcO)2 (11.2 mg, 5 × 6397

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Scheme 1. Synthesis of P1 and P2

10−2 mmol), P(o-Tol)3 (76.0 mg, 0.25 mmol), and triethylamine (0.346 g, 2.50 mmol) in DMF (3 mL) and toluene (3 mL) was refluxed overnight. After cooling to room temperature, the solvent was removed. The obtained crude product was treated with chloroform (20 mL) and the resulting solution was washed with water (3 × 20 mL), concentrated and added dropwise to methanol (200 mL). The precipitated polymer was collected by filtration and the procedure repeated twice using methanol and eventually using ethanol as precipitating solvent yielding P1 (0.189 g, 42%) as a dark-red solid. 1H NMR (298 K, CDCl3): δ (ppm) 8.15−8.05 (br, 2H), 7.92−7.84 (br, 2H), 7.71−7.62 (br, 2H), 7.56−7.44 (br, 4H), 7.39−7.11 (br, 6H), 2.07−1.94 (br, 4H), 1.25−0.98 (br, 20H), 0.86−0.59 (br, 10H). FT-IR (ATR): ν (cm−1) 2922, 2849, 1665, 1603, 1529, 1487, 1437, 1214, 1088, 944, 811. Anal. Calcd for (C47H50N2S3)n: C, 76.38; H, 6.82; N, 3.79. Found: C, 70.57; H, 6.68; N, 3.41. Poly[2,7-dithiophen-2-yl-9,9-dioctylfluorene−vinylene-co4,7-benzo[1,2,5]thiadiazole−vinylene] (P2). Following the procedure reported for P1, the polymer P2 was obtained reacting 2,7bis(5-bromothiophen-2-yl)-9,9-dioctylfluorene (0.333 g, 0.46 mmol), 4,7-dibromobenzo[1,2,5]thiadiazole (0.139 g, 0.46 mmol), potassium vinyltrifluoroborate (0.145 g, 1.23 mmol), Pd(AcO)2 (11.0 mg, 4.65 × 10−2 mmol), P(o-Tol)3 (71.0 mg, 0.23 mmol), and triethylamine (0.234 g, 2.32 mmol) in DMF (3 mL) and toluene (3 mL) in 57% yield as a dark-red solid. 1H NMR (298 K, CDCl3): δ (ppm) 8.64− 8.52 (br, 2H), 8.35−8.19 (br, 2H), 7.92−7.76 (br, 2H), 7.73−7.52 (br, 4H), 7.41−7.04 (br, 6H), 2.08−1.98 (br, 4H), 1.21−1.03 (br, 20H), 0.83−0.66 (br, 10H). FT-IR (ATR): ν (cm−1) 2921, 2853, 1603, 1447, 1412, 1264, 1231, 1087, 948, 811, 790. Anal. Calcd for (C47H50N2S3)n: C, 76.38; H, 6.82; N, 3.79. Found: C, 73.76; H, 6.87; N, 3.90.

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carried out on P1 and P2 and the results were compatible with the proposed structure. The structure of the polymers was confirmed by 1H NMR and FT-IR spectroscopy. 1H NMR analysis proved to be useful in the assessment of the copolymer composition by comparing the integral of the fluorene C9−CH2− signal (ascribable to the fluorene-based comonomers) falling at ∼2.00 ppm with the integral of the deshielded aromatic protons of the BTZ units (ascribable to the BTZ-containing comonomers) falling at 8.15−8.05 ppm in the case of P1 and at 8.64−8.52 ppm in the case of P2. The comparison revealed that, within the experimental error of the 1H NMR spectroscopy, it is reasonable to assume that the aryl comonomers of P1 and P2 are incorporated in a 1:1 ratio in the polymer backbone. Remarkably, the absence of 1,1-diarylenevinylene defects19 in the polymers, which can form after the Heck reaction step, could be confirmed by the absence of their 1H NMR fingerprint signals at 5.5−5.6 ppm, as evident from Figure 1. The

RESULTS AND DISCUSSION

Polymers Preparation and Characterization. The chosen macromolecules P1 and P2 were obtained by the Suzuki−Heck copolymerization protocol,10,11 combining a 1:1 feed ratio of the relevant dibromoaryl comonomers with an equimolar amount of potassium vinyltrifluoroborate in the presence of Pd(AcO)2/P(o-Tol)3 as catalyst and triethylamine as base in DMF/toluene at reflux (Scheme 1). The desired materials were obtained as random polymers in 45% and 57% yield for P1 and P2, respectively. The numeral-average molecular weights of the polymers, estimated by GPC, are 6700 and 8600 Da with polydispersities (PDI) of 1.4 and 2.0 for P1 and P2, respectively. Elemental analysis (C, H, N) was

Figure 1. 1H NMR spectra of P1 and P2. 6398

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Photoluminescence (PL) measurements were carried out on P1 and P2 in chloroform solution exciting the samples at 400 nm. As shown in Figure 3, the PL spectra of the two polymers

formation of defect-free polyconjugated materials is extremely important to preserve their optoelectronic properties. Probably, the bulkiness of the comonomers employed in the polymerization reaction, i.e., the dibromo derivatives of dithienylfluorene or dithienylbenzothiadiazole, plays a major role in the determination of the preferential configuration of the vinylene units in the corresponding polymers. Since the 1H NMR signals of the vinylene hydrogen atoms, falling in the region of aromatic protons, could not be assigned, the configuration of the vinylene moiety was ascertained by the presence of the IR band attributed to the out-of-plane C−H bending of the trans-vinylene moiety at 944 and 948 cm−1 for P1 and P2, respectively. The thermal stability of the synthesized polymers was evaluated by thermogravimetric analyses (TGA) that revealed decomposition temperatures at 5% weight loss of 381 °C (P1) and 397 °C (P2), suggesting a satisfactory thermal stability, adequate for bearing the thermal stress that the organic materials undergo during either the fabrication processes and the operation regime of BHJ solar cells. Optical Properties. The optical characterization of P1 and P2 was carried out both in CHCl3 solution and in the solid state, as thin film on quartz (Figure 2). In solution, the

Figure 3. PL spectra of P1 and P2 in chloroform and in toluene (λex = 400 nm).

in chloroform solution were characterized by two bands: an emission with maxima at 665 nm (P1) or 637 nm (P2) and an emission at shorter wavelengths (with maxima at 457 nm for P1 and 501 nm for P2), the latter ones being of considerably lower intensity, due to an incomplete energy transfer to the low-energy excited states. In the solid state, PL could not be observed, probably due to the strong interchain interactions deriving from the high amount of BTZ units, which favor nonradiative decay pathways for the excited states. Since P1 and P2 are endowed with a donor−acceptor architecture, a charge transfer nature of the excited states can be hypothesized for both. In order to shed light on this aspect, their absorption and emission spectra were recorded in toluene, a solvent with a dielectric constant (ε = 2.38) lower than that of chloroform (ε = 4.81). While the absorption profiles of the polymers were not influenced by the polarity of the solvent, in the PL spectra a blue shift (∼20 nm) of the low energy emission maxima was observed for both polymers (Figure 3) indicating the formation of polarized excitons. In fact, an intramolecular charge transfer state can be stabilized by the solvent polarity20 due to favorable dipole− dipole interactions between the polymer chains and the solvent molecules. The excited state of both P1 and P2 is thereby lowered in chloroform respect to toluene. Differently from what observed at the ground state, an intramolecular charge transfer is favored by a plausible planarization of the π-conjugated backbone of the polymers P1 and P2 at the excited state. Conversely, the emission at shorter wavelengths of the polymers is not influenced by the solvent polarity due to the noncharge transfer character of the corresponding transitions. Theoretical Calculations. We deemed it worthwhile to provide some insight into the optical behavior of the two materials and in particular into the marked difference exhibited in their absorption profiles. To this purpose the structures of P1 and P2 were investigated by theoretical calculations carried out on appropriate model structures of the random materials. In order to obtain a reliable description of the electronic structure of the random copolymers, as well as to reduce the calculation load, the theoretical investigation was focused on oligomeric (n = m = 2) structures. Moreover, since a random

Figure 2. UV−vis absorption spectra of P1 and P2 in solution and in the solid state.

absorption spectra of P1 consists of two bands: a higher energy band with a maximum located at 411 nm and a lower energy band with maximum located at 540 nm. Also in the case of P2, the absorption spectrum showed two maxima at 400 and 546 nm. Concerning the spectral profile, however, P1 showed a double-band absorption typical of the similarly structured materials,9 characterized by a marked loss of absorption in the ∼130 nm wide region between the two maxima; conversely in the spectrum of P2 the lack of absorption between the two bands is considerably reduced, leading to a considerably more panchromatic profile. The onset of the absorption spectra at lower energy (λonset) in solution allowed the evaluation of the optical energy gaps (Egopt), calculated as Egopt = 1240/λonset, which was very similar: 1.93 eV for P1 and 1.95 eV for P2. In the solid state, while the absorption profiles resemble those recorded in solution, the Egopt of the polymers are lowered to 1.83 eV (P1) and to 1.84 eV (P2) owing to the intermolecular interactions favoring a more planar conformation of the πconjugated backbone. 6399

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Figure 4. Alternating (ABAB) and block (AABB) structures chosen as models of P1 and P2 for theoretical calculations.

AABB) models, respectively. Both configurations can be considered near-planar and their HOMO electronic distributions are fully delocalized over the whole carbon skeleton with the exclusion of the heteroatoms on the thienyl and BTZ units, while the LUMO electron density is mainly localized on the BTZ moieties. Notwithstanding the different nature of the aryl units involved in the description of the HOMO frontier orbitals, the HOMO energy values of the optimized configurations of P1-ABAB (−4.77 eV) and P1-AABB (−4.76 eV) are very similar. Conversely, although the LUMO electronic distributions of the two configurations are mainly localized on the BTZ units, the electronic coupling occurring between neighboring BTZ units leads to a stabilization of the LUMO energy value of P1-AABB (−2.89 eV) with respect to P1-ABAB (−2.76 eV). Concerning the results obtained for P2, in the case of the P2ABAB structure (Figure 7) the HOMO electronic distribution

copolymer is constituted by alternating and block segments, both contributing to the definition of the electronic and optical properties of the materials, for each polymer we studied both the alternating (ABAB) and the block (AABB) model structures reported in Figure 4. DFT and TDDFT calculation were carried out at the B3LYP/6-31G(d,p) level on all model structures. Concerning P1, the calculated electron density distributions of HOMO and LUMO levels along with the optimized geometry are shown in Figures 5 and 6 for the alternating (P1-ABAB) and block (P1-

Figure 5. HOMO (bottom) and LUMO (top) frontier orbitals of the alternating (ABAB) model systems of P1.

Figure 7. HOMO (bottom) and LUMO (top) frontier orbitals of the alternating (ABAB) model systems of P2.

is mainly localized on the thienylene−vinylene−BTZ−vinylene−thienylene system, while in the case of P2-AABB (Figure 8), it is principally localized on the thienylene-vinylenethienylene-fluorenylene-thienylene system. As described for P1, also the LUMO level of P2 is mainly localized on the BTZ units. However, the presence of two adjacent BTZ units in P2-AABB leads to a stabilization of the LUMO energy value of P2-AABB (−2.82 eV) with respect to P2-ABAB (−2.70 eV). On the other hand, the HOMO energy values of P2-ABAB (−4.79 eV) and P2-AABB (−4.80 eV) are

Figure 6. HOMO (bottom) and LUMO (top) frontier orbitals of the alternating (AABB) model systems of P1.

6400

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copolymers. Table 1 summarizes the main calculated transitions together with the corresponding excitation energies, configurations and oscillator strengths for P1-ABAB, P1-AABB, P2ABAB, and P2-AABB. TDDFT calculations suggest that the S0 → S1 transition has a main HOMO → LUMO character for all model molecules, though other close-lying valence molecular orbitals (in particular, HOMO−1 and LUMO+1) contribute to the excitation. Interestingly, for each model segment, TDDFT calculations invariably revealed that the main transition is accompanied by a second, higher-energy transition with a strong oscillator force, mainly associated with a HOMO → LUMO+2 transition, though other close-lying valence molecular orbitals contribute to the excitation. The LUMO+2 frontier orbitals of the model segments are mainly localized on the fluorene and thiophene units, i.e. on the less polar region of the structure. These theoretical findings nicely explain both the dual emission and the solvatochromic behavior observed for P1 and P2. It is reasonable in fact, on the basis of the theoretical findings, to associate the shorter wavelength PL emission of the synthesized polymers to a LUMO+2 → HOMO transition while the low energy band should originate from a LUMO → HOMO transition. Furthermore, since the excited state of the latter transition (the LUMO frontier orbital) is mainly localized on the more polar part of the polymeric chain (i.e., the BTZ units) it is justifiable that a polar solvent can stabilize its energy level by dipole−dipole interactions, thereby further red-shifting the emission wavelengths with respect to what observed in non

Figure 8. HOMO (bottom) and LUMO (top) frontier orbitals of the alternating (AABB) model systems of P2.

very similar, notwithstanding the different nature of the aryl units involved in the description of the frontier orbital. The observation of similar HOMO energy values for the alternating (ABAB) and the block structures (AABB) of the structures leads to the prediction that the oxidation potentials of P1 and P2 are independent of the microstructure of the related polymers. On the other hand, the reduction potential of the two polymers are predictably governed by the block conjugated segments (showing lower LUMO energies) rather than by the alternating ones. A TDDFT study was also carried out, aimed at providing insight into the spectroscopic behavior of the random

Table 1. Calculated (TDDFT) Excited-State Transition Energies, Electronic Configurations, and Oscillator Strengths of the Main Transitions (f > 0.15) As Determined by TDDFT at the B3LYP/6-31G(d,p) Level of Theory for the Model Systems P1ABAB, P1-AABB, P2-ABAB, and P2-AABB model molecule

excitation energy (eV; nm)

oscillator strength f (au)

P1-ABAB

1.73; 2.18; 2.56; 2.81; 2.87; 3.13; 3.29;

716 585 485 442 432 396 377

2.56 0.17 1.65 0.23 0.28 0.28 0.17

P1-AABB

1.61; 2.57; 2.58; 2.77; 2.94; 3.08;

767 482 481 447 422 403

2.28 0.45 1.28 0.35 0.59 0.22

P2-ABAB

1.84; 2.26; 2.65; 2.87; 2.02; 2.25; 2.42; 1.79; 2.01; 2.42; 2.46;

675 549 467 431 411 382 362 691 617 511 504

2.47 0.29 1.74 0.15 0.26 0.17 0.17 1.34 0.69 1.13 1.34

3.11; 399

0.20

P2-AABB

configuration H → L [0.66]; H−1 → L+1 [0.17]; H → L+1 [0.15] H−1 → L+1 [0.66]; H → L [0.18]; H−1 → L [0.11] H → L+2 [0.63]; H → L+3 [0.21]; H−2 → L [0.13]; H−1 → L+4 [0.12]; H−1 → L+3 [0.11] H → L+3 [0.54]; H−3 → L [0.26]; H−1 → L+2 [0.24]; H → L+2 [0.17]; H−1 → L+3 [0.12] H−1 → L+3 [0.58]; H → L+4 [0.28]; H → L+2 [0.14]; H−3 → L [0.14] H−1 → L+3 [0.48]; H → L+4 [0.35]; H−4 → L [0.21]; H−2 → L+2 [0.15]; H−1 → L+4 [0.15] H−1 → L+4 (0.56); H → L+5 [0.27]; H−2 → L+2 [0.14]; H−2 → L+3 [0.12]; H−1 → L+3 [0.11]; H → L+2 [0.10] H → L [0.69] H−3 → L [0.57]; H → L+4 [0.32]; H−1 → L+3 [0.17] H → L+2 [0.58]; H−3 → L [0.28]; H → L+3 [0.19]; H−2 → L+1 [0.13] H → L+3 [0.56]; H−3 → L [0.29]; H → L+3 [0.22]; H−2 → L+1 [0.13] H−1 → L+2 [0.67] H−1 → L+3 [0.59]; H−4 → L [0.25]; H → L+5 [0.12]; H → L+4 [0.11]; H−2 → L+3 [0.11]; H → L+3 [0.10] H → L [0.69]; H−1 → L+1 [0.10] H−1 → L+1 [0.66]; H−1 → L [0.16]; H → L+1 [0.12]; H → L [0.11] H → L+2 [0.57]; H → L+3 [0.32]; H−2 → L+1 [0.17]; H−1 → L+3 [0.11] H → L+3 [0.48]; H−3 → L [0.38]; H → L+2 [0.25]; H−1 → L+2 [0.16] H−1 → L+2 [0.60]; H−3 → L+1 [0.28]; H → L+2 [0.14] H−1 → L+3 [0.45]; H−4 → L [0.38]; H → L+4 [0.33]; H−1 → L+4 [0.12] H−1 → L+4 [0.61]; H−2 → L+3 [0.22]; H−1 → L+3 [0.14]; H−5 → L [0.10] H → L [0.68]; H−1 → L [0.16] H−1 → L [0.67]; H → L [0.17] H−1 → L+1 [0.43]; H → L+2 [0.41]; H → L+1 [0.33]; H−2 → L [0.11] H → L+2 [0.52]; H−2 → L [0.26]; H−1 → L+1 [0.26]; H → L+1 [0.23]; H−3 → L [0.12]; H−1 → L+3 [0.10] H−2 → L+2 [0.38]; H−4 → L [0.37]; H−1 → L+3 [0.36]; H → L+4 [0.16]; H−3 → L+1 [0.15] 6401

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polar media such as toluene, as actually observed (vide supra). In the case of the shorter wavelength emission, the absence of solvatochromism experimentally observed for P1 and P2 can also be explained by the theoretical calculations. The electron density of the relevant excited states (the LUMO+2 frontier orbitals) are, in fact, localized on the less polar region of all the chosen models. Simulated absorption spectra based on the TDDFT analysis of P1-ABAB, P1-AABB, P2-ABAB, and P2AABB are shown in Figure 9 and Figure 10, respectively. In the

red-shifted with respect to the experimental data due to the well-known propensity of the TDDFT B3LYP method to favor delocalized solutions induced by the self-interaction error in DFT.22 This drawback however does not affect the core finding of the theoretical study, i.e. the justification of the “camelback” absorption profile observed for P1. A completely different scenario crops up from the TDDFT study of P2-ABAB and P2-AABB: while the simulated absorption profile of P2-ABAB evidenced two prominent absorption peaks, the simulated spectrum of P2-AABB gave three peaks, the more intense of which is comprised between those of the two simulated for P2-ABAB. Also for the model systems of P2, the low-energy excitation peaks are red-shifted with respect to the empirical data. Again, the UV−vis absorption profile of the random P2 can be thought as the convolution of the simulated spectra of P2ABAB and P2-AABB model molecules. On the basis of this assumption, our results provide a nice justification of the broadness of the UV−vis absorption profile experimentally observed for P2 that arises from the different excitation energies observed for the alternating and block segments together describing the random polymeric structure. Electrochemical Properties. Investigating the electrochemical features of a semiconducting polymer is mandatory in order to investigate how the copolymer primary structure affects its energy levels. Oxidation and reduction potentials for P1 and P2 in the solid state were obtained using cyclic voltammetry (CV) as summarized in Table 2. The electro-

Figure 9. Calculated (TDDFT) absorption spectra for the alternating and block oligomer of P1.

Table 2. Electrochemical Potentials and Electronic Energy Levels of the Polymers as Thin Film polymer

Eoxonset (V)a

Eredonset (V)b

Egelc (eV)c

HOMO (eV)

LUMO (eV)

P1 P2

1.08 1.14

−1.07 −1.09

2.15 2.23

−5.40 −5.46

−3.25 −3.23

a

Onset of the anodic event. bOnset of the cathodic event. cEgelc = Eoxonset − Eredonset.

chemical HOMO−LUMO energy gap was determined as the difference between the onsets of the oxidation and the reduction potentials (Egelc = Eoxonset − Eredonset). The HOMO and LUMO energy values were estimated from the onset potentials of the first oxidation and reduction event, respectively. After calibration of the measurements against Fc/Fc+, the oxidation potential of which is assumed at 4.8 eV below the vacuum level, the HOMO and LUMO energy levels were calculated according to the following equations:

Figure 10. Calculated (TDDFT) absorption spectra for the alternating and block oligomer of P2.

case of P1-ABAB and P1-AABB, the simulated absorption profiles evidenced two prominent absorption peaks. Under the reasonable assumption that the random polymer structure consists of an equal amount of alternating and block segments, the convolution of the simulated spectra of the aforementioned P1-ABAB and P1-AABB molecular models provides a theoretical evaluation of the UV−vis absorption profile of the random P1. These results are in agreement with the dual peak absorption profile experimentally observed for P1 as well as with the theoretical and experimental data reported by Canestraro et al.21 for a copolymer corresponding to P1 but endowed with an alternating structure. However, it should be noted that, while a good agreement is observed for the absorption peak recorded at shorter wavelength, the excitation peaks at lower energies for both P1-ABAB and P1-AABB are

E HOMO (eV) = −[Eox onset − E1/2(Fc/Fc+) + 4.8] E LUMO (eV) = −[Ered onset − E1/2(Fc/Fc+) + 4.8]

where E1/2(Fc/Fc+) is the half-wave potential of the ferrocene/ ferrocenium couple measured relatively to Ag/Ag+. The cyclic voltammograms of the polymers, shown in Figure 11, revealed quasi-reversible p-doping (oxidation/rereduction) processes over a positive potential range (up to 1.4 V) and reversible n-doping (reduction/reoxidation) processes over a negative potential range (up to −1.6 V). The reduction potentials observed for P1 and P2 were very similar, consequently, similar LUMO values could be calculated as −3.25 eV (P1) and−3.23 eV (P2). Analogously, for the HOMO energy levels, comparable values of −5.40 eV (P1) and 6402

dx.doi.org/10.1021/ma301163p | Macromolecules 2012, 45, 6396−6404

Macromolecules

Article

cells with the following configuration: ITO/PEDOT−PSS/ polymer:PC61BM (1:1wt)/Ca/Al, in which the active layer was deposited by spin-coating without any further thermal treatment. As shown by the J−V behavior reported in Figure 13, the

Figure 11. CV curves obtained for of P1 and P2.

−5.46 eV (P2) could be obtained from the onset of oxidation potential. The found electrochemical gaps are 2.15 eV (P1) and 2.23 eV (P2) respectively (Table 2). The optical behavior of P1 and P2 was also tested under electrochemical stress, in conditions resembling those occurring during the operation of a BHJ solar cell. In particular, the polymers were studied under p-doping conditions, in order to mimic the state of the materials after the electron transfer from the polymeric donor to the fullerene acceptor. To this purpose, a film of the polymers was spin-coated on a indium−tin oxide (ITO) substrate and submitted to a voltage of +1.3 V (vs Ag/ Ag+) for 2 min. The applied potential corresponds to the peak voltage of the anodic event during the CV scans for both P1 and P2. It is noteworthy that, during the measurement, both polymer films exhibited a marked color switch from purple to colorless, as confirmed by their UV−vis spectrum quickly recorded after submitting the film to the p- doping with the formation of a new broad polaronic absorption band (λmax at ∼900 nm), as shown in Figure 12. Interestingly, if the film on ITO is subsequently submitted to a potential of 0 V (vs Ag/Ag+), the absorption spectra of the polymer films recover their original profiles. Photovoltaic Characterization. The photovoltaic properties of the synthesized polymers were investigated in BHJ solar

current densities at short circuit conditions, Jsc (V = 0 V) are 1.20 mA/cm2 and 2.91 mA/cm2 for P1 and P2, respectively, while the open circuit voltages Voc (J = 0 mA/cm2) are 0.62 and 0.67 V, respectively. The fill factor is 0.30 for both the devices and the resulting PCE are 0.2% and 0.6% for P1- and P2-based solar cells, respectively. Differently from what observed for most polymer-based BHJ solar cells, that typically require donor:acceptor weight ratios much lower than 1:1 to be significantly performing, our devices made use of a relatively low fullerene content (50% w/w). It is apparent that the different performances of the two devices originate from the 3-fold current density value of P2 with respect to that of P1. The similarity of the Voc exhibited by the two devices stems from the similar HOMO−LUMO energy levels. Moreover the structural analogies between P1 and P2 lead to exclude a dependence of the number of the photogenerated carriers from the polymer/PC61BM interactions. Therefore, we conclude that the better performances of P2 can be ascribed to its much broader absorption spectrum with respect to P1 related to its previously discussed random nature.

Figure 12. Comparison between the absorption spectra of P1 and P2 (film on ITO) as deposited and after p-doping (1.3 V vs Ag/Ag+) conditions.

CONCLUSIONS Isomeric random poly(arylene−vinylene)s containing (i) dithienyl benzothiadiazole and fluorene (P1) or (ii) dithienylfluorene and benzothiadiazole (P2) as aromatic units have been prepared by the Suzuki−Heck polymerization route aiming at applying them as donor materials in BHJ solar cells. The two polymers exhibited remarkably different UV−vis absorption spectra in that the one of P2 covered a broader portion of the visible region with respect to P1. Theoretical calculations were carried out on suitable alternating and block model systems of the random copolymers, both contributing to the description of the random materials P1 and P2. The calculated excitation energies for the alternating and block models for P2 showed different values covering a broader portion of the spectrum with respect to those obtained for P1, thereby explaining the more panchromatic absorption observed in its UV−vis spectrum of the former. The broader absorption of P2 is responsible for the better PCE obtained for P2 with respect to P1 in BHJ solar cell devices, stressing how

Figure 13. J−V curves of the solar cells based on P1 and P2.

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Part A: Polym. Chem. 2009, 47, 2093. (c) Grisorio, R.; Piliego, C.; Striccoli, M.; Cosma, P.; Fini, P.; Gigli, G.; Mastrorilli, P.; Suranna, G. P.; Nobile, C. F. J. Phys. Chem. C 2008, 112, 20076. (d) Grisorio, R.; Piliego, C.; Fini, P.; Cosma, P.; Mastrorilli, P.; Gigli, G.; Suranna, G. P.; Nobile, C. F. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6051. (e) Grisorio, R.; Suranna, G. P.; Mastrorilli, P.; Nobile, C. F. Org. Lett. 2007, 9, 3149. (f) Grisorio, R.; Mastrorilli, P.; Nobile., C. F.; Romanazzi, G.; Suranna, G. P.; Gigli, G.; Piliego, C.; Ciccarella, G.; Cosma, P.; Acierno, D.; Amendola, E. Macromolecules 2007, 40, 4865. (11) Colella, S.; Melcarne, G.; Mazzeo, M.; Gigli, G.; Grisorio, R.; Suranna, G. P.; Mastrorilli, P. Polymer 2011, 52, 2740. (12) For nonaqueous electrochemistry, IUPAC recommends the use of a redox couple such as ferrocene/ferrocenium ion (Fc/Fc+) as an internal (or marker) standard: (a) Gritzner, G.; Kůta, J. Pure Appl. Chem. 1984, 56, 461. (b) Gritzner, G. Pure Appl. Chem. 1990, 62, 1839. (13) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (b) Lee, C. B.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter. Mater. Phys. 1988, 37, 785. (c) Becke, A. D. Phys. Rev. A: At. Mol. Opt. Phys. 1988, 38, 3098. (14) Molander, G. A.; Rivero, M. R. Org. Lett. 2002, 4, 107. (15) Grisorio, R.; Mastrorilli, P.; Nobile, C. F.; Romanazzi, G.; Suranna, G. P.; Acierno, D.; Amendola, E. Macrom. Chem. Phys. 2005, 206, 448. (16) Sahu, D.; Padhy, H.; Patra, D.; Kekuda, D.; Chu, C.-W.; Chiang, I.-H.; Lin, H.-C. Polymer 2010, 51, 6182. (17) Kim, J.; Park, S. H.; Cho, S.; Jin, Y.; Kim, J.; Kim, I.; Lee, J. S.; Kim, J. H.; Woo, H. Y.; Lee, K.; Suh, H. Polymer 2010, 51, 390. (18) Mancilha, F. S.; DaSilveira Neto, B. A.; Lopes, A. S.; Moreira, P. F., Jr.; Quina, F. H.; Goncalves, R. S.; Dupont, J. Eur. J. Org. Chem. 2006, 4924. (19) Klingelhöfer, S.; Schellenberg, C.; Pommerehne, J.; Bässler, H.; Greiner, A.; Heitz, W. Macromol. Chem. Phys. 1997, 198, 1511. (20) Dias, F. B.; King, S.; Monkman, A. P.; Perepichka, I. I.; Kryuchkov, M. A.; Perepichka, I. F.; Bryce, M. R. J. Phys. Chem. B 2008, 112, 6557. (21) Canestraro, C. D.; Rodrigues, P. C.; Marchiori, C. F. N.; Schneider, C. B.; Akcelrud, L.; Koehler, M.; Roman, L. S. Sol. Energy Mater. Sol. Cells 2011, 95, 2287. (22) Gierschner, J.; Cornil, J.; Egelhaaf, H. J. Adv. Mater. 2007, 19, 173.

combination of a suitable choice of the sequence of aryl units together with the potentialities offered by random polymers, can be useful tools in the design of new light-harvesting polymers in BHJ, a concept that can be easily extended from PAVs to the more performing poly(arylenes). The PCE value (0.6%) obtained with P2 are comparable with those reported for other poly(heteroarylene−vinylene)s. Moreover, these results were obtained using a donor/acceptor blend with a relatively low amount (50% w/w) of PC61BM and without annealing processes. We expect that further improvements will be achieved by a careful optimization of the Suzuki−Heck polymerization conditions aimed at increasing the molecular weights of the polymers as well as by suitable comonomer choice for the fine-tuning of absorption, redox and film-forming properties for this class of new semiconductors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Telephone: +390805963603. Fax: +390805963611. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Regione Puglia (APQ-Reti di Laboratorio, Project PHOEBUS, cod. 31) is gratefully acknowledged for funding. REFERENCES

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dx.doi.org/10.1021/ma301163p | Macromolecules 2012, 45, 6396−6404