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Manipulation of the Open-Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor– Donor–Acceptor Molecules Dora Demeter, Theodulf Rousseau, Philippe Leriche, Thomas Cauchy, Riccardo Po, and Jean Roncali*
are developed in two types of architectures, i.e., bilayer D/A planar heterojunctions (PHJ)[5–12] and bulk heterojunctions (BHJ), in which the D/A interface is distributed in the entire volume of a phase-segregated composite of the two materials.[1–4,13–19] Because of the short diffusion distance of excitons in organic materials, BHJ with optimized morphology present a much larger interfacial D/A contacting area, which thus allows the collection and dissociation of a larger number of excitons and hence improved power-conversion efficiency (PCE). During the past ten years, BHJ based on soluble π-conjugated polymers and fullerene derivatives have been intensively investigated and their PCE has gradually increased to reach values around 6.0% for cells based on poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PC61BM),[13–15] and 7.0%–8.0% for devices based on low bandgap polymers and PC71BM.[17–19] An emerging alternative approach involves the use of soluble small-molecular donors that combine the advantages of well-defined chemical structure, reproducible synthesis and purification, and more straightforward analysis of structure– property relationships.[20–22] In recent years this approach has led to the development of many classes of donor[20–40] and PCE values exceeding 4.0% were recently reported for BHJ based on molecular donors derived from diketopyrrolopyrrole,[37,38] and triphenylamine.[39] The molecular design of donors for OPV aims at the optimization of three parameters, i.e., light-harvesting, hole-mobility, and open-circuit voltage (Voc). The optimization of the lightharvesting properties implies the control of the band gap (Eg), width of the absorption spectrum, and molecular absorption coefficient (ε). On the other hand, improving hole mobility independently of other parameters can significantly contribute to an increased PCE of a molecular BHJ.[40,41] It is widely accepted that the Voc of a D–A heterojunction OPV depends on the energy difference between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor.[42,43] In this context, it has been shown that the creation of an internal
The synthesis of acceptor–donor–acceptor (A–D–A) molecules based on a septithiophene chain with terminal electron acceptor groups is reported. Using a dicyanovinyl- (DCV) substituted molecule as reference, another symmetrical A–D–A donor containing thiobarbituric (TB) groups is synthesized and these two acceptor groups are combined to produce the unsymmetrical A–D–A′ compound. The electronic properties of the donors are analyzed by cyclic voltammetry and UV-Vis absorption spectroscopy and their photovoltaic properties are characterized on bilayer planar heterojunction cells that include spun-cast donor films and vacuum-deposited C60 as acceptor. Optical and electrochemical data show that replacement of DCV by TB leads to a small increase of the HOMO level and to a larger decrease of the LUMO, which result in a reduced band-gap. The desymmetrized compound presents the lowest oxidation potential in solution but the highest oxidation onset in the solid state, which leads to a significant increase of the open-circuit voltage of the resulting solar cells.
1. Introduction The past decade has witnessed a considerable intensification of research into organic photovoltaic cells (OPVs), motivated by the possibility to develop large area, light, and flexible energy sources by means of simple, cost-effective, and environmentally friendly technologies.[1–4] OPVs basically involve a heterojunction (HJ) between a donor (D) and an acceptor (A) material. Excitons resulting from light absorption by the donor (or the acceptor) diffuse to the D/A interface where they are separated into electrons and holes by the interfacial electric field. OPVs Dr. D. Demeter, T. Rousseau, Prof. P. Leriche, Dr. J. Roncali Group Linear Conjugated Systems, CNRS, Moltech-Anjou University of Angers 2 Bd Lavoisier, F-49045 Angers, France E-mail:
[email protected] Dr. T. Cauchy Moltech-Anjou, University of Angers 2 Bd Lavoisier, F-49045 Angers, France Dr. R. Po Centro ricerche per le energie non convenzionali Istituto ENI Donegani ENI S.p.A., via G. Fauser 4, 28100 Novara, Italy
DOI: 10.1002/adfm.201101508
Adv. Funct. Mater. 2011, XX, 1–9
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charge transfer (ICT) by introducing electron-acceptor groups to the structure of the donor represents an efficient way to produce at the same time a reduction of Eg, a broadening of the absorption spectrum, and an increase of Voc due to the decrease of the HOMO level.[24] Furthermore, this lower HOMO level contributes to an improved donor stability in ambient conditions.[44] Illustrative examples of these effects are found in con jugated systems in which ICT is deliberately created,[10–12,24,27,36] or inherent to the structure of chromophores such as, e.g., diketopyrrolopyrroles,[28,37,38] squaraines,[30,32] bodipys,[31,40] or indigos.[33] The energetics and structure of organic–organic interfaces have been extensively studied in the general context of organic electronics,[45–54] and the key role of the interfacial dipole has been established. Besides the energy level of the frontier orbitals of the D–A pair, this interfacial dipole depends also on extrinsic causes such as doping,[47] interfacial exciplexes, π-stacking interactions,[44–48] or molecular orientation.[50–54] Thus, it has been shown that the ionization potential of pentacene or copper phthalocyanine and their perfluorinated derivatives strongly depends on molecular orientation,[52,54] while quite recently, a modification of Voc through the modulation of intermolecular interactions by steric hindrance has been reported.[55] In this context, we report here preliminary results on a new approach based on the manipulation of the Voc by desymmetrization of the electronic structure of a molecular donor. A–D–A molecules based on oligothiophenes have been widely investigated by Bäuerle and co-workers and efficient PHJ solar cells have been fabricated by vacuum deposition.[10–12] Recently, Chen and co-workers described the synthesis of a soluble septithiophene with dicyanovinyl (DCV) end-groups (1) that acted as donor for solution-processed BHJ cells (Scheme 1).[56–58] Herein, using compound 1 as reference, we have synthesized another symmetrical A–D–A donor containing thiobarbituric (TB)[59] acceptor groups (2) and combined these two groups to produce the unsymmetrical A–D–A′ compound 3. The electronic properties of the donors have been analyzed by means of cyclic voltammetry and UV-Vis absorption spectroscopy and their photovoltaic NC properties have been characterized on bilayer PHJ cells including spun-cast donor films and vacuum-deposited C60 as acceptor.
treated with thiobarbituric acid to give the asymmetrical target compound 3 in 72% yield. All target compounds were characterized by means of NMR spectroscopy and mass spectrometry. Figure 1 shows the UV-Vis absorption spectra of compounds 1-3 in methylene chloride and the corresponding data are listed in Table 1. The spectrum of compound 1 presents a first absorption band with λmax around 400 nm and a more intense band with a maximum at 513 nm. Replacing the DCV terminal groups of 1 by TB (2) produces a red shift of λmax to 565 nm, which is in agreement with previous results,[60,61] while the asymmetrical compound (3) shows a λmax at 544 nm. The three donors have high molecular absorption coefficients (ε ≈ 70 000 M−1 cm−1) which increases upon replacement of DCV with TB or by desymmetrization of the structure. Comparison of the spectra of spun-cast films to solution spectra reveals large red shifts of λmax (90–110 nm) and a broadening of the absorption bands. The band-gap Eg estimated from the long wavelength absorption edge decreases from 1.64 eV for 1 to 1.53 eV for 3; is this lowest value not expected in view of solution data, and suggests a specific effect of the desymmetrization on the electronic structure of the solid. The cyclic voltammograms (CVs) of all compounds show two reversible oxidation waves which correspond to the formation of the cation radical and dication and an irreversible reduction process (Figure 2 and Table 1). Replacement of DCV with TB groups leads to an approximately 50 mV negative shift in the first oxidation potential E 01 but produces a larger positive shift in the cathodic peak potential Epc 1 for 2 and 3. These results indicate that replacing DCV groups by TB has little impact on the HOMO but essentially affects the LUMO level. The geometry and electronic structure of models A–D–A′ compounds have been investigated by using density-functionaltheory calculations (PBE0 hybrid functional[62] and 6-31G(d) basis set). Furthermore a simple linear-response method R
R S
R
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NC
R S S
R
S R
N
N 2
S R
R S
O
S R
R
S
S N
O
O
N
O
CN
R
S S
O
S R
1
S
The synthesis of the target compounds 1-3 is depicted in Scheme 2. Hexa-octyl septithiophene 4a, the corresponding 2-5’’’’’’bis-carboxaldehyde 4c, and 2-5’’’’’’bisdicyanovinyl septithiophene 1 were synthesized using the procedure reported by Liu et al.[56] Knoevenagel condensation of dialdehyde 4c with thiobarbituric acid gave compound 2 in 99% yield. Carboxaldehyde 4b was synthesized in 30% yield by Vilsmeier–Hack formylation of septithiophene 4a. Knoevenagel condensation of 4b with malonodinitrile in the presence of triethylamine gave the bis-dicyanovinyl septithiophene 5a in 97% yield. Vilsmeier–Haack formylation of compound 5a led to mono-aldehyde 5b (yield 53%) which was subsequently 2
S
R
S
O
S
S
CN
R
S
S
R
2. Results and Discussion
R
S
S S
R
S R
NC
CN
N S
3
R = n-Octyl
Scheme 1. Chemical structure of the donor molecules.
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S
X
R
S
S
S
S
S
S
R
Y
R =n-Octyl
R 4a X = Y = H
POCl3 DMF
4b X = H, Y = CHO 4c X = Y = CHO
CN + 4c
1
CN
Et3N/CHCl3
O
O
+
N
N
2
Et3 N/CHCl3
S
on the well-known effects of dipolar electrostatic interactions on the organization and properties of materials containing push– pull NLO-phores (NLO = nonlinear optics),[63] desymmetrization can be expected to affect the packing arrangement of the molecules in the solid and and thus indirectly impact the electronic properties of the material. While attempts to characterize films by means of X-ray diffraction remain unsuccessful due to the amorphous structure of the materials, a first indication as to the effect of desymmetrization on the structure and properties of the solid state is given by the atomic force microscopy (AFM) images of films (Figure 5) that clearly show that the unsymmetrical compound 3 presents a markedly different morphology to that of the symmetric compounds 1 and 2. We note also a net decrease of the surface rugosity from 2.17 nm for 1 to 0.81 and 1.16 nm for compounds 2 and 3, respectively. Further support for this conclusion is provided by the CV data of solution-cast thin films. In order to obtain accurate data, the films were cast using the same donor concentration and the
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R
R
4b CN
Et3 N/CHCl3
0.4
R
R Z
S
R
R
S
S
S
S
S
S R
NC
O
Et3N/CHCl3
R POCl3 DMF
CN
5a Z= H 5b Z = CHO O N
0.3
0.2
0.1
N S 3
0.0 200
300
400
500
600
700
800
0.5
Scheme 2. Synthesis of the donor molecules.
(time-dependent DFT with the same functional and basis set) confirms that the first excited state corresponds to a pure HOMO to LUMO transition (>97%), is really intense (oscillator strength >2.4), and is responsible for the main absorption band. For all compounds discussed the HOMO is essentially located on the oligothiophene backbone (Figure 3). For compound 1 the contribution to the LUMO is slightly higher for the terminal DCV groups of the molecule. For the desymmetrized compound 3 the HOMO to LUMO transition implies a loss of contribution of the donor (from 93% to 64%) almost entirely on the DCV side (from 3% to 27%). As expected, these structural modifications affect the dipole moment of the system which decreases from 11.5 D for 1 to 5.8 D for 2, and reincreases to 9.1 D for 3 with a change of direction due to desymmetrization (Figure 4). Based on the geometrical flexibility of oligothiophenes, it is possible that dipolarity vanishes in solution but reappears in the solid state. In fact, based
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Absorbance [a.u.]
CN
0.4 0.3 0.2 0.1 0.0 300
400
500 600 700 800 Wavelength [nm]
900
Figure 1. UV-Vis absorption spectra of the A–D–A donors in CH2Cl2 (top) and as thin films spun-cast from CH2Cl2 solutions (bottom). Dotted line (1), dashed line (2), solid line (3).
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Table 1. UV-Vis absorption and cyclic voltammetric data of compounds 1-3 (conditions of Figures 1 and 2). λmax sol [nm]
εmax [M−1cm−1]
λmax film [nm]
Eg [eV]
E01, E02 [V/SCE]
Epc1 [V/SCE]
HOMO [eV]a)
LUMO [eV]b)
1
513
67600
620
1.64
0.72, 0.88
–1.20
–5.71
–3.78
2
565
72400
626
1.55
0.68, 0.87
–0.99
–5.67
–4.00
3
544
74100
633
1.53
0.67, 0.86
–1.02
–5.66
–3.97
Compound
a)Estimated
from E01 values using an offset of –4.75 eV for (NHE);[61] b)estimated using the reduction onset.
potential of oxidation onset was determined for the same value of the anodic current (2 µA). Under these conditions, the oxidation onset is found at 0.79, 0.74, and 0.83 V for compounds 1, 2, and 3 respectively (Figure 6). While for compounds 1 and 2 the results agree well with solution data, the highest potential found for compound 3 is in striking contrast with the lowest E01 measured in solution. This result suggests that the specific molecular packing resulting from the broken symmetry of the electronic structure of the A–D–A system in 3 presents a higher ionization energy than for materials derived from symmetrical donors. The photovoltaic properties of the donors have been analyzed in three series of PHJ cells consisting of spun-cast films of donors 1-3 and vacuum-deposited fullerene C60 as acceptor. Although BHJ cells can lead to higher PCE, the PHJ architecture was preferred in order to limit the number of experimental variables, thus providing more accurate structure–property relationships.
Figure 7 shows the external quantum efficiency (EQE) spectra of the three types of cells under monochromatic irradiation. The photocurrent onset observed around 750 nm for donor 1 is red-shifted to ca. 800 nm for donors 2 and 3, which is in agreement with the smaller band-gap of these materials. In each case, the spectrum shows a first maximum in the 450 nm region and a second one around 620 nm that correspond to the two transitions in the optical spectra. However, irradiation in the 620 nm band produces less photocurrent than irradiation at 420 nm. Thus, the maximum EQE around 620 nm decreases from 16% for compound 1 to 12 and 7% for compounds 2 and 3, respectively. In contrast, donor 3 gives the highest EQE value of ca. 27% at 420 nm. Although we have no definitive explanation for these different behaviors,
10 µA
-1.5
-1.0
-0.5
0.0
0.5
1.0
E [V vs SCE] Figure 2. CVs of the A–D–A donors (10−3 M substrate) in 0.10 M Bu4NPF6/ CH2Cl2, scan rate 100 mV s−1. From top to bottom; 1, 2, 3.
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Figure 3. HOMO and LUMO orbitals of compounds 1–3. Octyl chains have been replaced by methyl groups for ease of calculation.
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difference between the LUMO of the donor and that of C60, or to problems of charge collection at the interface. Furthermore, the lower surface rugosity of films of compounds 2 and 3 may also contribute to the decrease PCE by reducing the contacting area between the D and A materials. Further work is needed to clarify this point. As shown in Table 2, replacing the DCV end-groups of donor 1 by TB (2) induces a decrease of Voc from 0.72 to 0.51 V, in agreement with the CV data. However, while for compound 3 a further decrease of Voc could be anticipated in view of the CV data obtained in solution, the reverse effect is observed and the cells containing this compound have a Voc of 0.81 V, more than 0.30 V larger than for compound 2; this result is consistent with the CV data of compound 3 in the solid state. It should be underlined that both the shape of the J vs. V curves and the corresponding data are highly reproducible.
Figure 4. Calculated dipole moments (red arrows) for compounds 1–3.
a possible change in the interfacial dipole due to the specific molecular packing of the unsymmetrical compound 3 could play a role. Figure 8 shows the current vs. voltage curves of the three types of cells under simulated solar illumination. As suggested by EQE spectra, donor 1 produces the highest shortcircuit current density (Jsc) of 6.0 mA cm−2 and a PCE of 1.64% (Table 2). In contrast, the TB derivative 2 gives only a Jsc of 2.16 mA cm−2 and a PCE of 0.36%. For this donor, the J/V curves present an S-shape often attributed to problems of charge accumulation due to unbalanced charge transport or to limited charge-collection efficiency at the material/electrode interfaces.[64,65] As could be expected, use of the asymmetric donor 3 leads to intermediate performances with maximum Jsc and PCE values of 3.70 mA cm−2 and 1.20%, respectively. The results obtained with compounds 1 and 3 are comparable to those reported for PHJ cells based on oligothiophenes with trifluoroacetyl end groups.[12] Better PCE values (2.60%–2.80%) were recently obtained with oligothiophenes end-capped by DCV groups.[11] However it should be noticed that these cells are based on a more sophisticated architecture and present a smaller active area.[66] The hole mobility (µh) of the three compounds has been determined using the space-charge-limited current method on devices ITO/PEDOT:PSS/donor/Au to be 7.93 × 10−5, 6.9 × 10−5, and 3.28 × 10−5 cm2 V−1s−1 for compounds 1, 2, and 3 respectively (see the Supporting Information). The value obtained for 1 is in reasonable agreement with the reported result of 1.4 × 10−4 cm2 V−1s−1.[57] These results show that the incorporation of the TB group has little effect on µh, while desymmetrization seems to have a larger impact on hole transport, presumably because of a less homogeneous electron distribution than in the symmetric compound. In this context, the decrease of PCE observed for compound 2 could result from a less efficient exciton dissociation due to the decrease of the
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Figure 5. AFM images of films of A–D–A donors spun-cast on glass from dichloromethane solutions. From top to bottom: 1, 2, 3.
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10.0 5.0
10 µA
0.0 -5.0 -10.0
0.0
0.2
0.4
0.6
0.8
Current density [mA cm-2 ]
4.0 1.0
E (V vs SCE)
Figure 6. CVs of films of A–D–A donors drop-cast on platinum electrodes from 4.4 × 10−3 M solutions in dichloromethane. Electrolytic medium 0.10 M Bu4NPF6/CH3CN, scan rate 20 mV s−1. Dotted line (1), dashed line (2), solid line (3).
It is generally admitted that the Voc of D–A heterojunction OPV depends on the difference between the ionization potential of D (p-type material quasi-Fermi level) and the electron affinity of A (n-type material quasi-Fermi level).[42,43] These quantities are generally assimilated to the energy level of the HOMO of D and LUMO of A and evaluated using cyclic voltammetry in solution.[67] However, this procedure implicitly assumes that conjugated molecules or polymer chains weakly interact in the solid and that the electronic structure of the solid largely preserves that of a molecule or single conjugated chain.[45]
2.0 0.0 -2.0 -4.0 4.0 2.0 0.0 -2.0 - 4.0 -1.5 -1.0 -0.5
40
0.0
0.5
1.0
1.5
Voltage [V] Figure 8. Current density vs. voltage curves for bilayer solar cells based on donors 1–3 and C60 fullerene. Dotted lines: in the dark, solid lines: under simulated solar irradiation in AM 1.5 conditions with a light intensity of 90 mW cm−2. From top to bottom 1, 2, 3.
EQE [%]
30
20 Table 2. Photovoltaic parameters of bilayer cells based on donors 1–3 (conditions of Figure 8). The data correspond to the average of 11 cells for compounds 2 and 3 and seven cells for compound 1. Data in bold correspond to the best results of each series.
10 Donor
0 400
500
600
700
800
900
Wavelength [nm] Figure 7. EQE of PHJ cells based on the A–D–A septithiophenes and C60. Dotted line (1), dashed line (2), solid line (3).
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VOC [V]
Jsc [mA cm−2]
FF [%]
PCE [%]
1
0.71
4.81
36
1.38
1
0.72
6.00
34
1.64
2
0.48
2.13
27
0.31
2
0.51
2.16
28
0.36
3
0.81
3.10
36
1.00
3
0.81
3.70
36
1.21
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3. Conclusions A–D–A systems based on septithiophene end-capped with DCV and TB electron-acceptor groups have been synthesized. Optical and electrochemical data determined in solution show that replacement of DCV with TB leads to a small increase of the HOMO level and to a larger decrease of the LUMO resulting in a contraction of the band-gap. Desymmetrization of the electronic structure of the A–D–A system by replacement of only one DCV group by TB leads to a lower oxidation potential in solution but to the reverse effect in the solid state, thus demonstrating the intermolecular collective origin of this phenomenon. The characterization of PHJ solar cells with C60 as acceptor layer shows that replacement of DCV groups by TB leads to a decrease of conversion efficiency due possibly to less efficient charge collection at the donor/electrode interface and/or to the lower offset between the LUMO of the donor and that of C60. However, our results show that breaking the symmetry of the donor structure can lead to a significant increase of the opencircuit voltage. While this phenomenon poses many questions that will require further experimental and theoretical work, these findings can open several interesting lines of investigation for fundamental research on the physics of organic–organic interfaces with possible implications for other areas of organic electronics such as organic field-effect transistors (OFETs) or organic lightemitting diodes (OLEDs). On the other hand, they can also open new perspectives for the molecular and supramolecular engineering of active materials for OPV. The synthesis of other classes of unsymmetrical A–D–A′ compounds with different conjugated systems and acceptor groups is now underway to explore the scope and limitations of this original approach.
4. Experimental Section Bis-2,5′′′′′′bis-(thiobarbituric)-hexaoctylseptipthiophene (2): Diformylseptithiophene 4c (150 mg, 0.11 mmol) was dissolved in a solution of 1,3-diethyl-2-thiobarbituric acid (177 mg, 0.88 mmol) in dry CHCl3, (25 mL) three drops of triethylamine were added and the solution was stirred for 4 h under argon, at room temperature. The reaction mixture was then diluted with CH2Cl2, washed with water and brine. After removal of solvent the residue was chromatographed on silica gel using
Adv. Funct. Mater. 2011, XX, 1–9
dichloromethane as eluent to afford a dark violet solid (190 mg, 99%). mp 176 °C–178 °C; 1H NMR (300 MHz, CDCl3, δ): 7.69 (s, 2H), 7.52 (s, 2H), 7.21 (s, 2H), 7.13 (s, 2H), 7.06 (s, 2H), 4.63–4.58 (m, 8H), 2.87–2.80 (m, 12H), 1.72–1.70 (m, 12H), 1.46–1.40 (m, 12H), 1.37–1.27 (m, 60H), 0.89–0.85 (m, 18H); 13C NMR (300 MHz, CDCl3, δ): 178.6, 161.0, 159.9, 149.8, 149.1, 148.9, 140.6, 140.2, 135.7, 134.5, 143.4, 133.2, 132.5, 131.6, 131.3, 129.3, 126.2, 109.6, 43.9, 43.0, 31.8, 30.6, 30.5, 29.8, 29.6, 29.4, 29.3, 29.2, 22.6, 14.1, 12.5, 12.3; matrix-assisted laser desorption ionization MS (MALDI-MS): 1668.4; HRMS (MALDI): calcd for [M]+ 1668.7729, found 1668.7729. 2-Formyl-hexaoctyl-septithiophene (4b): Vilsmeier reagent, prepared with POCl3 (0.09 mL, 0.99 mmol) in dry DMF (0.07 mL, 0.99 mmol), was added to a cold solution of septithiophene 4a (1.04 g, 0.83 mmol) in 1,2-dichloroethane (15 mL) at 0 °C under argon. After 12 h stirring at 60 °C, the solution was cooled to room temperature, diluted with CH2Cl2 and stirred with a solution of NaCOOCH3 for 2 h. The organic layer was washed with water and brine, and dried over MgSO4. After removal of solvent the residue was chromatographed on silica gel using a mixture of dichloromethane and petroleum ether (1:1) as eluent to afford 0.30 g of a reddish solid (yield 30%). 1H NMR (300 MHz, CDCl3, δ): 9.82 (s, 1H), 7.58 (s, 1H), 7.17 (d, 1H), 7.13 (s, 1H), 7.11 (s, 2H), 7.01 (s, 1H), 6.98 (s, 1H), 6.94 (s, 1H), 6.92 (d, 1H), 2.85–2.76 (m, 12H), 1.72–1.68 (m, 12H), 1.39–1.28 (m, 60H), 0.90–0.86 (m, 18H). 2-Dicyanovinyl-hexaoctyl-septithiophene (5a): 2-Formylseptithiophene 4b (300 mg, 0.23 mmol) was dissolved in a solution of malonitrile (19 mg, 0.25 mmol) in dry CHCl3 (25 mL), after addition of three drops of triethylamine the resulting solution was stirred 3 h, under argon, at room temperature. The reaction mixture was then diluted with CH2Cl2, washed with water and brine. After removal of solvent the residue was chromatographed on silica gel using dichloromethane as eluent to afford the desired compound (300 mg, 97%). 1H NMR (300 MHz, CDCl3, δ): 7.68 (s, 1H), 7.51 (s, 1H), 7.21 (s, 1H), 7.17 (d, 1H), 7.11 (s, 2H), 7.04 (s, 1H), 6.98 (s, 1H), 6.94 (s, 1H), 6.93 (d, 1H), 2.83–2.76 (m, 12H), 1.71–1.68 (m, 12H), 1.41–1.28 (m, 60H), 0.90–0.85 (m, 18H). 2-Formyl-5’’’’’’-dicyanovinyl-hexaoctylseptithiophene (5b): Vilsmeier reagent, prepared with POCl3 (75 µL, 1.75 mmol) in dry DMF (80 µL, 1.75 mmol), was added to a cold solution of 5a (0.30 g, 0.22 mmol) in 1,2-dichloroethane (20 mL) at 0 °C under argon. After being stirred at 60 °C overnight, the solution was cooled to room temperature, diluted with CH2Cl2, and stirred with a solution of NaCOOCH3 for 2 h. The organic layer was washed with water and brine, dried over MgSO4. After removal of solvent the residue was chromatographed on silica gel using a mixture of dichloromethane and petroleum ether (1:1) as eluent to afford 0.16 g, (53%) of the target compound. 1H NMR (300 MHz, CDCl3, δ): 9.82 (s, 1H), 7.68 (s, 1H), 7.58 (s, 1H), 7.52 (s, 1H), 7.21 (s, 1H), 7.12 (s, 3H), 7.04 (s, 1H), 7.01 (s, 1H), 2.86–2.78 (m, 12H), 1.71–1.68 (m, 12H), 1.41–1.28 (m, 60H), 0.88–0.87 (m, 18H); MALDI-MS: 1353.1. 2-Dicyanovinyl-5’’’’’’-thiobarbituric-hexaoctylseptithiophene (3): Compound 5b (160 mg, 1.18 mmol) was dissolved in a solution of 1,3-diethyl-2thiobarbituric acid (56.78 mg, 2.83 mmol) in dry CHCl3 (25 mL), three drops of triethylamine were added and the resulting solution was stirred 6 h, under argon, at room temperature. The reaction mixture was then diluted with CH2Cl2, washed with water and brine. After removal of solvent the residue was chromatographed on silica gel using a mixture of dichloromethane and hexane (2:1) as eluent to afford a dark blue solid (130 mg, 72%). mp 160–162 °C; 1H NMR (300 MHz, CDCl3, δ): 8.58 (s, 1H), 7.69-7.68 (m, 2H), 7.52 (s, 1H), 7.38 (s, 1H), 7.21 (s, 1H), 7.13 (m, 2H), 7.06 (s, 1H), 7.04 (s, 1H), 4.66-4.55 (m, 4H), 2.89–2.78 (m, 12H), 1.75–1.66 (m, 12H), 1.46–1.40 (m, 12H), 1.38–1.28 (m, 60H), 0.89–0.85 (m, 18H);13C NMR (300 MHz, CDCl3, δ): 178.6, 161.0, 159.9, 149.8, 149.1, 148.9, 143.9, 141.8, 140.6, 140.2, 135.7, 135.6, 143.4, 133.2, 132.5, 131.9, 131.6, 131.5, 131.4, 131.3, 131.2, 129.4, 129.3, 126.2, 109.6, 43.9, 43.0, 31.8, 30.6, 30.5, 29.9, 29.6, 29.4, 29.3, 29.2, 22.6, 14.1, 12.5, 12.3; MALDI-MS: 1535.5; HRMS (MALDI): calcd for [M] + 1534.7328, found 1534.7318. Characterization: Electrochemical experiments were carried out with a PAR 273 potentiostat-galvanostat in a three-electrode single-compartment cell equipped with platinum working electrodes, a platinum wire counter
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In fact, many examples have shown that the electronic properties of material based on π-conjugated systems are strongly influenced by intermolecular interactions and molecular orientation. These parameters can affect the band-gap and redox potentials,[68] or charge mobility.[69–70] Furthermore, the strong impact of these effects on the interfacial dipole of organic–organic interfaces has been demonstrated.[48,51,54] From a different viewpoint, the organization and electro-optical properties of materials based on push–pull conjugated chromophores are largely determined by the spontaneous head-to-tail arrangement of the dipolar molecules.[63] In this context, the above results provide a coherent picture which suggests that breaking the symmetry of the electronic structure of an A–D–A donor can represent a possible approach to modify the molecular organization in the solid and thus indirectly affect the energy levels of the resulting material.
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electrode and a saturated calomel reference electrode (SCE). Films for solid-state cyclic voltammetry were cast on platinum electrodes from 4 × 10−3 M solutions in CH2Cl2 solutions. The CVs were then recorded in 0.10 M Bu4NPF6/ MeCN in which the films are insoluble. AFM images were obtained on a Veeco Thermimicroscope CR-Research apparatus. Device preparation: Indium–tin oxide (ITO) coated glass slides of 24 × 25 × 1.1 mm dimensions with a surface resistance of 10 Ω/! were purchased from Kintec. Part of the ITO layer was etched away with 37% HCl. The ITO electrodes were then cleaned in ultrasonic bath with successively: Deconex from VWR international GmbH, distilled water (15.3 MΩ cm−1), acetone, ethanol, and distilled water again for 10 min each and dried in an oven at 100 °C. The dried electrodes were then modified by a spun-cast layer of PEDOT:PSS (Clevios P VP. AI 4083 (HC-Starck) filtered through a 0.45 µm membrane just prior to use). Spin-casting was achieved at 5000 rpm (r = 10 s, t = 60 s), and the electrode was then dried at 130 °C for 15 min. Films of donor materials (ca. 20 nm2) were spun-cast in atmospheric conditions from chloroform solutions containing 4 mg donor mL−1. After film deposition the devices were introduced to an argon glovebox (200B, MBraun) equipped with a vacuum chamber. A 25-nm film of fullerene C60 (99+%) (MER Corporation) and a 100-nm thick aluminum electrode were thermally evaporated onto the top of the donor film under a pressure of 2 10−6 mbar through a mask defining two cells of 6.0 mm diameter (0.28 cm2) on each ITO electrode. The J vs. V curves of the devices were recorded in the dark and under illumination using a Keithley 236 source-measure unit and a home-made acquisition program. The light source was an AM1.5 Solar Constant 575 PV simulator (Steuernagel Lichttecknik, equipped with a metal halogen lamp). The light intensity was measured by a broad-band power meter (13PEM001, Melles Griot). The devices were illuminated through the ITO electrode side. The efficiency values reported here are not corrected for the possible spectral mismatch of the solar simulator. EQE was measured using a halogen lamp (Osram) with an Action Spectra Pro 150 monochromator, a lock-in amplifier (Perkin-Elmer 7225), and a S2281 photodiode (Hamamatsu).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Received: July 5, 2011 Revised: July 27, 2011 Published online:
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