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Dora Demeter , Theodulf Rousseau , Philippe Leriche , Thomas Cauchy , Riccardo Po , and Jean Roncali *
1. Introduction
The past decade has witnessed a considerable intensifi cation of research into organic photovoltaic cells (OPVs), motivated by the possibility to develop large area, light, and fl exible energy sources by means of simple, cost-effective, and environmentally friendly technologies. [ 1–4 ] OPVs basically involve a heterojunc- tion (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 fi eld. OPVs
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 dis- sociation of a larger number of excitons and hence improved power-conversion effi ciency ( 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-C 61 -butyric acid methyl ester (PC 61 BM), [ 13–15 ] and 7.0%–8.0% for devices based on low band- gap polymers and PC 71 BM. [ 17–19 ]
An emerging alternative approach involves the use of sol- uble small-molecular donors that combine the advantages of well-defi ned chemical structure, reproducible synthesis and purifi cation, 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 optimi- zation of three parameters, i.e., light-harvesting, hole-mobility, and open-circuit voltage ( V oc ). The optimization of the light- harvesting properties implies the control of the band gap ( E g ), width of the absorption spectrum, and molecular absorption coeffi cient ( ε ). On the other hand, improving hole mobility independently of other parameters can signifi cantly contribute to an increased PCE of a molecular BHJ. [ 40,41 ]
It is widely accepted that the V oc 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
Manipulation of the Open-Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor–
Donor–Acceptor Molecules
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 synthe- sized and these two acceptor groups are combined to produce the unsym- metrical 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 heterojunc- tion cells that include spun-cast donor fi lms and vacuum-deposited C
60as 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 com- pound presents the lowest oxidation potential in solution but the highest oxidation onset in the solid state, which leads to a signifi cant increase of the open-circuit voltage of the resulting solar cells.
DOI: 10.1002/adfm.201101508
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
FULL P APER
charge transfer (ICT) by introducing electron-acceptor groups to the structure of the donor represents an effi cient way to pro- duce at the same time a reduction of E g , a broadening of the absorption spectrum, and an increase of V oc due to the decrease of the HOMO level. [ 24 ] Furthermore, this lower HOMO level contributes to an improved donor stability in ambient condi- tions. [ 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 perfl uorinated derivatives strongly depends on molecular orientation, [ 52 , 54 ] while quite recently, a modifi cation of V oc 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 V o c by desymmetri- zation of the electronic structure of a molecular donor. A–D–A molecules based on oligothiophenes have been widely investi- gated by Bäuerle and co-workers and effi cient PHJ solar cells have been fabricated by vacuum deposition. [ 10–12 ] Recently, Chen and co-workers described the synthesis of a soluble sep- tithiophene 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′ com- pound 3 . The electronic properties of the donors have been analyzed by means of cyclic voltammetry and UV-Vis absorption spectroscopy and their photovoltaic properties have been characterized on bilayer PHJ cells including spun-cast donor fi lms and vacuum-deposited C 60 as acceptor.
2. Results and Discussion
The synthesis of the target compounds 1-3 is depicted in Scheme 2 . Hexa-octyl septithiophene 4a , the corre- sponding 2-5’’’’’’bis-carboxaldehyde 4c , and 2-5’’’’’’bis- dicyanovinyl septithiophene 1 were synthesized using the procedure reported by Liu et al. [ 56 ] Knoevenagel con- densation of dialdehyde 4c with thiobarbituric acid gave compound 2 in 99% yield. Carboxaldehyde 4b was syn- thesized 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
treated with thiobarbituric acid to give the asymmetrical target compound 3 in 72% yield. All target compounds were charac- terized by means of NMR spectroscopy and mass spectrometry.
Figure 1 shows the UV-Vis absorption spectra of com- pounds 1-3 in methylene chloride and the corresponding data are listed in Table 1 . The spectrum of compound 1 presents a fi rst absorption band with λ max 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 λ max to 565 nm, which is in agreement with previous results, [ 60,61 ] while the asymmetrical compound ( 3 ) shows a λ max at 544 nm.
The three donors have high molecular absorption coeffi cients ( 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 fi lms to solution spectra reveals large red shifts of λ max (90–110 nm) and a broad- ening of the absorption bands. The band-gap E g 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 specifi c effect of the desym- metrization 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 fi rst oxidation potential E 0 1 but produces a larger positive shift in the cathodic peak potential E pc 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-functional- theory calculations (PBE0 hybrid functional [ 62 ] and 6-31G(d) basis set). Furthermore a simple linear-response method
Scheme 1 . Chemical structure of the donor molecules.
S
S S
S S
S S
NC CN NC CN
S
S S
S S
S S
N N N N
O
O O O
S S
S
S S
S S
S S
N N
NC CN
O O
S
1
2
3 R
R R
R R R
R
R
R R
R R
R
R
R R
R R
R =n-Octyl
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(time-dependent DFT with the same functional and basis set) confi rms that the fi rst excited state corresponds to a pure HOMO to LUMO transition ( > 97%), is really intense (oscil- lator 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 com- pound 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 modifi cations 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 fl exibility of oligothiophenes, it is possible that dipolarity van- ishes in solution but reappears in the solid state. In fact, based
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 ] desymmetriza- tion can be expected to affect the packing arrangement of the molecules in the solid and and thus indirectly impact the elec- tronic properties of the material.
While attempts to characterize fi lms by means of X-ray dif- fraction remain unsuccessful due to the amorphous structure of the materials, a fi rst indication as to the effect of desym- metrization on the structure and properties of the solid state is given by the atomic force microscopy (AFM) images of fi lms ( Figure 5 ) that clearly show that the unsymmetrical compound 3 presents a markedly different morphology to that of the sym- metric 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 fi lms. In order to obtain accurate data, the fi lms were cast using the same donor concentration and the
Scheme 2 . Synthesis of the donor molecules.
S
S S
S S
S S
S
S S
S S
S S
Z
R
R R
R R R
R
R
R R
R R
R =n-Octyl
X Y
4aX = Y = H 4bX = H, Y = CHO 4cX = Y = CHO
4c
POCl3 DMF
N N
O
S O
Et3N/CHCl3 CN
CN
Et3N/CHCl3 1
2
4b
Et3N/CHCl3 CN
CN
NC CN 5aZ= H
5bZ = CHO POCl3
DMF
N N
O
S
O Et3N/CHCl3
3 +
+
Figure 1 . UV-Vis absorption spectra of the A–D–A donors in CH 2 Cl 2 (top) and as thin fi lms spun-cast from CH 2 Cl 2 solutions (bottom). Dotted line ( 1 ), dashed line ( 2 ), solid line ( 3 ).
Absorbance [a.u.]
200 300 400 500 600 700 800
0.0
300 400 500 600 700 800 900 0.1
0.2 0.3 0.4
0.0 0.1 0.2 0.3 0.4 0.5
Wavelength [nm]
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potential of oxidation onset was determined for the same value of the anodic current (2 µ A).
Under these conditions, the oxidation onset is found at 0.79, 0.74, and 0.83 V for compounds 1 , 2 , and 3 respec- tively ( Figure 6 ). While for compounds 1 and 2 the results agree well with solution data, the highest potential found for com- pound 3 is in striking contrast with the lowest E 0 1 measured in solution. This result suggests that the specifi c 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 ana- lyzed in three series of PHJ cells consisting of spun-cast fi lms of donors 1-3 and vacuum-deposited fullerene C 60 as acceptor.
Although BHJ cells can lead to higher PCE , the PHJ architec- ture was preferred in order to limit the number of experimental variables, thus providing more accurate structure–property relationships.
Figure 7 shows the external quantum effi ciency ( EQE ) spectra of the three types of cells under monochromatic irra- diation. 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 fi rst maximum in the 450 nm region and a second one around 620 nm that corre- spond 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 defi nitive explanation for these different behaviors,
Figure 2 . CVs of the A–D–A donors (10 − 3 M substrate) in 0.10 M Bu 4 NPF 6 / CH 2 Cl 2 , scan rate 100 mV s − 1 . From top to bottom; 1 , 2 , 3.
E [V vs SCE]
-1.5 -1.0 -0.5 0.0 0.5 1.0 10 µA
Figure 3 . HOMO and LUMO orbitals of compounds 1– 3 . Octyl chains have been replaced by methyl groups for ease of calculation.
Table 1. UV-Vis absorption and cyclic voltammetric data of compounds 1-3 (conditions of Figures 1 and 2 ).
Compound λ max sol [nm]
ε max [M − 1 cm − 1 ]
λ max fi lm [nm]
E g [eV]
E 0 1 , E 0 2 [V/SCE]
E pc 1 [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
a) Estimated from E 0 1 values using an offset of –4.75 eV for (NHE); [ 61 ] b) estimated using the reduction onset.
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a possible change in the interfacial dipole due to the specifi c 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 sug- gested by EQE spectra, donor 1 produces the highest short- circuit current density ( J sc ) of 6.0 mA cm − 2 and a PCE of 1.64% ( Table 2 ). In contrast, the TB derivative 2 gives only a J sc 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 effi ciency at the material/electrode interfaces. [ 64,65 ]
As could be expected, use of the asymmetric donor 3 leads to intermediate performances with maximum J sc 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 trifl uoroacetyl 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 ( µ h ) 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 cm 2 V − 1 s − 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 cm 2 V − 1 s − 1 . [ 57 ] These results show that the incorporation of the TB group has little effect on µ h , while desymmetrization seems to have a larger impact on hole trans- port, presumably because of a less homogeneous electron dis- tribution than in the symmetric compound. In this context, the decrease of PCE observed for compound 2 could result from a less effi cient exciton dissociation due to the decrease of the
difference between the LUMO of the donor and that of C 60 , or to problems of charge collection at the interface. Furthermore, the lower surface rugosity of fi lms 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 V oc from 0.72 to 0.51 V, in agreement with the CV data. However, while for compound 3 a further decrease of V oc 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 V oc 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 cor- responding data are highly reproducible.
Figure 4 . Calculated dipole moments (red arrows) for compounds 1– 3 .
Figure 5 . AFM images of fi lms of A–D–A donors spun-cast on glass from dichloromethane solutions. From top to bottom: 1, 2, 3.
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It is generally admitted that the V oc of D–A heterojunction OPV depends on the difference between the ionization potential of D (p-type material quasi-Fermi level) and the electron affi nity 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 pre- serves that of a molecule or single conjugated chain. [ 45 ] Figure 6 . CVs of fi lms of A–D–A donors drop-cast on platinum electrodes from 4.4 × 10 − 3 M solutions in dichloromethane. Electrolytic medium 0.10 M Bu 4 NPF 6 /CH 3 CN, scan rate 20 mV s − 1 . Dotted line ( 1 ), dashed line ( 2 ), solid line ( 3 ).
0.0 0.2 0.4 0.6 0.8 1.0
10 µA
E (V vsSCE)
Figure 8 . Current density vs. voltage curves for bilayer solar cells based on donors 1 – 3 and C 60 fullerene. Dotted lines: in the dark, solid lines:
under simulated solar irradiation in AM 1.5 conditions with a light inten- sity of 90 mW cm − 2 . From top to bottom 1 , 2 , 3 .
-10.0 -5.0 0.0 5.0 10.0
-4.0 -2.0 0.0 2.0 4.0
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 - 4.0
-2.0 0.0 2.0 4.0
Voltage [V]
Current density [mAcm-2]
Figure 7 . EQE of PHJ cells based on the A–D–A septithiophenes and C 60 . Dotted line ( 1 ), dashed line ( 2 ), solid line ( 3 ).
Wavelength [nm]
400 500 600 700 800 900
EQE[%]
0 10 20 30 40
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.
Donor V OC
[V]
J sc [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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In fact, many examples have shown that the electronic prop- erties of material based on π -conjugated systems are strongly infl uenced by intermolecular interactions and molecular orienta- tion. These parameters can affect the band-gap and redox poten- tials, [ 68 ] or charge mobility. [ 69–70 ] Furthermore, the strong impact of these effects on the interfacial dipole of organic–organic inter- faces 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 mole- cules. [ 63 ] In this context, the above results provide a coherent pic- ture 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 indi- rectly affect the energy levels of the resulting material.
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 poten- tial 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 C 60 as acceptor layer shows that replacement of DCV groups by TB leads to a decrease of conversion effi ciency due possibly to less effi cient charge collection at the donor/electrode interface and/or to the lower offset between the LUMO of the donor and that of C 60 . However, our results show that breaking the symmetry of the donor structure can lead to a signifi cant increase of the open- circuit voltage.
While this phenomenon poses many questions that will require further experimental and theoretical work, these fi nd- ings can open several interesting lines of investigation for fun- damental research on the physics of organic–organic interfaces with possible implications for other areas of organic electronics such as organic fi eld-effect transistors (OFETs) or organic light- emitting 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 ): Diformyl- septithiophene 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 CHCl 3 , (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 CH 2 Cl 2 , washed with water and brine. After removal of solvent the residue was chromatographed on silica gel using
dichloromethane as eluent to afford a dark violet solid (190 mg, 99%).
mp 176 ° C–178 ° C; 1 H NMR (300 MHz, CDCl 3 , 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); 13 C NMR (300 MHz, CDCl 3 , 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 POCl 3 (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 CH 2 Cl 2 and stirred with a solution of NaCOOCH 3 for 2 h. The organic layer was washed with water and brine, and dried over MgSO 4 . 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%). 1 H NMR (300 MHz, CDCl 3 , 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 CHCl 3 (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 CH 2 Cl 2 , 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%). 1 H NMR (300 MHz, CDCl 3 , 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 POCl 3 (75 µ L, 1.75 mmol) in dry DMF (80 µ L, 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 CH 2 Cl 2 , and stirred with a solution of NaCOOCH 3 for 2 h. The organic layer was washed with water and brine, dried over MgSO 4 . 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. 1 H NMR (300 MHz, CDCl 3 , 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-2- thiobarbituric acid (56.78 mg, 2.83 mmol) in dry CHCl 3 (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 CH 2 Cl 2 , 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; 1 H NMR (300 MHz, CDCl 3 , 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); 13 C NMR (300 MHz, CDCl 3 , 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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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 CH 2 Cl 2 solutions. The CVs were then recorded in 0.10 M Bu 4 NPF 6 / MeCN in which the fi lms 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 modifi ed by a spun-cast layer of PEDOT:PSS (Clevios P VP. AI 4083 (HC-Starck) fi ltered through a 0.45 µ m 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 nm 2 ) were spun-cast in atmospheric conditions from chloroform solutions containing 4 mg donor mL − 1 . After fi lm deposition the devices were introduced to an argon glovebox (200B, MBraun) equipped with a vacuum chamber. A 25-nm fi lm of fullerene C 60 (99 + %) (MER Corporation) and a 100-nm thick aluminum electrode were thermally evaporated onto the top of the donor fi lm under a pressure of 2 10 − 6 mbar through a mask defi ning two cells of 6.0 mm diameter (0.28 cm 2 ) 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 effi ciency 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 amplifi er (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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