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A Novel Diruthenium Acetylide Donor Complex as an Unusual Active Material for Bulk Heterojunction Solar Cells Alessia Colombo,*,† Claudia Dragonetti,† Dominique Roberto,†,‡ Renato Ugo,†,‡ Luigi Falciola,*,§ Silvia Luzzati,*,|| and Dariusz Kotowski|| †
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Dipartimento CIMA “L. Malatesta” dell’ Universita degli Studi di Milano and UdR INSTM di Milano, via Venezian 21, 20133 Milano, Italy ‡ ISTM del CNR, via Golgi 19, 20133 Milano, Italy § Dipartimento di Chimica Fisica ed Elettrochimica dell’ Universita degli Studi di Milano, via Golgi 19, 20133 Milano, Italy ISMAC del CNR, via E. Bassini 15, Milano, Italy
bS Supporting Information ABSTRACT: A new dinuclear Ru(II) complex where two Ru atoms are separated by a bridge consisting of a 2,1,3-benzothiadiazole acceptor moiety flanked on either side by 2,5-thienyl donor units was synthesized. This rather simple complex appears to behave as a photoactive donor when blended with a fullerene as acceptor, thus being a first step toward novel bulk heterojunction solar cells, based on Ru donor systems.
T
he use of Ru acetylides is a novel interesting tool for the design of donor materials to combine with electron-withdrawing fullerides in bulk heterojunction solar cells. Organic photovoltaic devices (OPVs) based on conjugated polymers and oligomers have received a lot of attention because of their potential for lightweight, flexible, and low-cost photovoltaic energy conversion.1-4 Among them, the most common devices are bulk heterojunctions (BHJ) made upon blending an electron donor conjugated polymer to an electron acceptor methanofullerene derivative (as [6,6]-phenyl-C61-butyric acid methyl ester, PCBM). The photovoltaic effect in the BHJ active layer is built on the following sequence of events: (1) solar light absorption and exciton formation, (2) exciton diffusion to a donor/acceptor interface, (3) exciton dissociation into charge carriers, (4) charge transport and collection at the electrodes. The choice of components in the active layers and the morphology are crucial factors affecting the whole photovoltaic process. In the past decade, the photovoltaic conversion efficiency (PCE) of BHJ has significantly increased,2,3 and such improvements have been fueled by the search for new active materials to combine in the active layer.2 It is a common opinion that there is still a need to design and study different classes of materials in order to attain further improvements of BHJ solar cell performances.4 The use of conjugated polymers incorporating heavy atoms, in particular Pt, was proposed as a tool to enhance charge photogeneration in fullerene-based BHJ solar cells.5 The basic idea was that charge photogeneration could be more efficient by exploiting charge transfer from the triplet rather than from the singlet excitons.6-8 Therefore, to explore the potential of the presence of triplet states on OPVs, in the past few years increasing attention has been paid to conjugated polymers incorporating r 2011 American Chemical Society
Pt, which give rise to efficient intersystem crossing by enhancing the spin-orbit coupling.9,10 Several papers have been recently dedicated to the investigation of the use of Pt acetylide polymers as donors in solar cells using PCBM as acceptor.11-16 Thus, the blue-violet-absorbing Pt-acetylide polymer [-Pt(L2)-t-Th-t-]n (Th = 2,5thienyl; L = PBu3) with PCBM was investigated as the active material in OPVs, showing that the triplet excited state of the Pt-acetylide polymer plays a role in the photoinduced charge transfer process to PCBM. However, the overall power conversion efficiency was 0.16-0.27%.13 Better efficiencies were reached with Pt-acetylide polymers or oligomers absorbing in visible light.11 For instance, Mei et al. reported on a Ptacetylide-based polymer using a 2,1,3-benzothiadiazole (BTD) acceptor moiety flanked on either side by 2,5-thienyl (Th) donor units ([-Pt(L 2 )-t-Th-BTD-Th-t-]n , where L = PBu3), which absorbs strongly throughout the visible region.16 With this polymer and PCBM, the OPV devices improved to ca. 1% PCE. In this case, despite rather good photocurrents, there has been clear photophysical evidence that just singlet excitons, but not triplet excitons, are involved in photoinduced electron transfer from the platinapolyyne to PCBM.16 Recently, Ptacetylide oligomers containing a Th-BTD--Th core and oligothiophene alkynyl ligands also showed their great potential for solution-processed organic photovoltaics.17 Clearly, although still in its infancy, the use of metallapolyynes represents an innovative and challenging research area for the development of bulk heterojunction photovoltaic devices. The insertion of a Ru instead of a Pt in a conjugated backbone should Received: August 31, 2010 Published: March 03, 2011 1279
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Scheme 1. Synthesis of [PhtRu(L2)tTh-BDT-ThtRu(L2)tPh, Where L = Ph2PCH2CH2PPh2
Table 1. Redox Potentials E1/2 (or Peak Potentials in Italics), Scanned at 100 mV s-1 and Referenced to the Ferrocene Redox Couplea E1/2/V (vs Fcþ/Fc) compd
red3
S
red2
red1
-2.26
-1.63
2 -2.01
-1.52
ox2
0.84
ΔEg/eV 2.47
-0.04
0.82
-1.73
-0.06
1.78
-1.13
1.20
1 PCBM
ox1
1.67 2.32
The electrochemical gap ΔEg is the difference between the first oxidation and reduction potentials. The optical band gap for 2 in CH2Cl2 solution is 1.66 eV. a
in principle lead to some advantages, since a red shift of the absorption spectra and thus better solar light harvesting is expected. In the literature, there are a few examples of applications of Ru complexes in bulk heterojunction solar cells: Ruphthalocyanine complexes have been found to be active donor materials18 and Ru complexes with substituted bipyridines or phenanthrolines have been applied as hole-blocking layers in BHJ devices.19 However, to our knowledge Ru-acetylide-based materials have never been investigated for bulk heterojunction applications. With this in mind, we have prepared a new dinuclear Ru complex where two Ru atoms are separated by a bridge consisting of 2,1,3-benzothiadiazole flanked on either side by 2,5-thienyl units ([Ph-t-Ru(L2)-t-Th-BTD-Th-t-Ru(L2)-t -Ph], where L = Ph2PCH2CH2PPh2; complex 2). We have studied the photoinduced electron transfer process from this complex to PCBM, combining cyclic voltammetry and photoluminescence spectroscopy; we have tested this material as a photoactive donor when blended to PCBM in BHJ solar cells. Complex 2, easily obtained by reaction of the known [ClRu(L2)tPh]20 (complex 1) with H-t-Th-BTDTh-t-H16 (S; Scheme 1) was fully characterized by IR and 1H, 13 C and 31P NMR spectroscopies and by mass spectrometry.21,22 In CH2Cl2, complex 2 shows two strong absorption bands at 393 nm (ε = 35 011 M-1 cm-1) and 633 nm (ε = 35 340 M-1 cm-1) which overlap quite well in sunlight, being red-shifted and more intense when compared to those of S (322 nm, ε = 7592 M-1 cm-1 and 464 nm, ε = 6960 M-1 cm-1) and even to those of the related Pt complex ([Ph-t-Pt(L2)-t-Th-BTD-Th-t-Pt(L2)-t-Ph] with L = PBu3; 372 nm, ε = 40 900 M-1cm-1 and 549 nm, ε = 28 400 M-1cm-1).16 Complexes 1 and 2 were also
Figure 1. Cyclic voltammograms of complexes 1, 2, S, and PCBM (thin line), scanned at 100 mV s-1.
characterized by cyclic voltammetry in CH2Cl2, using 0.1 M [NBu4][PF6] as the supporting electrolyte.23 Table 1 summarizes the electrochemical data, while Figure 1 shows the voltammetric patterns of 1 and 2, in comparison with those of PCBM and S. Complex 2 presents a two-electron chemically reversible and electrochemically quasi-reversible oxidation peak at 0.06 V and a one-electron chemically reversible and electrochemically quasireversible reduction peak at 1.73 V. By comparison with the voltammetric behavior of S, it appears that the reduction process takes place on the organic spacer. On the other hand, by comparison with the voltammetric behavior of 1, it is clear that the oxidation can be assigned to a simultaneous one-electron removal of each of the two Ru units. This evidence suggests the absence of electronic communication (through space or through bonds) between the two Ru atoms by the organic spacer. Comparison of the voltammetric behavior of 2 and [Ph-t-Pt(L2)-t-Th-BTD-Th-t-Pt(L2)-t-Ph] (L = PBu3)16 shows that substitution of Pt by Ru shifts the oxidation potential of 300 mV toward less positive values, due to the more facile oxidation of Ru. The first oxidation and reduction potentials can be used to estimate the HOMO and LUMO energy levels by means of the equations EHOMO (eV) = EOX þ 4.8 and ELUMO (eV) = ERED þ 4.8, which involve the use of the internal ferrocene standard value of 4.8 eV with respect to the vacuum level.14a,24 Therefore, the 1280
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Figure 2. Absorption and fluorescence spectra of a thin film of 2 (continuous lines) and PCBM (dotted lines). The spectra are arbitrarily scaled.
calculated HOMO and LUMO energies for 2 are 4.74 and 3.07 eV, respectively. Under the same experimental conditions, the LUMO level of PCBM is estimated at 3.67 eV. These energy levels suggest that 2 might act as a donor in a bulk heterojunction solar cell. Photoinduced charge transfer and charge separation at the donor/acceptor interface are key steps in the mechanism by which photovoltaic cells convert optical energy to electrical power. Complex 2 exhibits (Figure 2) a structureless fluorescence band which, upon blending with PCBM, is effectively quenched (20 times for 1:1 w/w), suggesting that an efficient exciton dissociation at the donor/acceptor interface is taking place, by photoinduced electron transfer from 2 to PCBM. Fluorescence quenching could, in principle, arise also from energy transfer, since PCBM has a much weaker emission than 2. However, as the singlet excited state of PCBM is slightly higher in energy with respect to the singlet of complex 2 (Figure 2), a process of energy transfer from 2 to PCBM can be reasonably ruled out. The energetics of charge separation in the 2/PCBM blend can be roughly estimated by combining the measured redox potentials and the optical spectra. The singlet excited states, taken from the fluorescence maxima, are respectively at 1.56 eV (2) and 1.73 eV (PCBM). The energy of the charge-separated state (ECS) is roughly estimated at 1.06 eV, from the expression ECS = EOX(2) ERED(PCBM).12,13 Therefore, the singlet excited state of 2 is about 0.5 eV above the charge-separated state and thus photoinduced electron transfer from the singlet manifold is quite exothermic and should be rapid. It is interesting to note that the substitution of Pt by Ru hardly varies the energetics of charge separation to PCBM.16 To test in a preliminary way this new class of Ru-based metallorganic complexes as photoactive donor materials for BHJ solar cells, we have prepared and characterized devices based on blends of 2 and PCBM, fabricated with the following architecture: glass/ITO/PEDOT-PSS/2:PCBM (1:2 w/w)/ Ca/Al. The active layer has been spin-coated from chloroform (stabilized with amylene) solutions. The external quantum efficiency (EQE) measurement shows a band peak at 640 nm that matches the lowest absorption band of 2, with an IPCE maximum of 4% in the blend but just 0.04% in the pristine film (Figure 3). This strong photocurrent enhancement in the blend demonstrates that charge photogeneration arises from photon absorption by 2 followed by electron transfer to PCBM. In comparison to the devices made with the corresponding Pt-based material,16 the photocurrent spectral
Figure 3. External quantum efficiency (EQE) spectra (solid lines) and absorption spectra of the active layer (dotted lines): (A) pristine complex 2 film device; (B) 2:PCBM (1:2 w/w) blend.
response is red-shifted, demonstrating that the use of Ru-based materials is a good strategy to improve the overlap with the solar spectrum. Preliminary tests on the photovoltaic performances, under AM1.5 solar simulation (100 mW/cm2), of BHJ made upon blending complex 2 and PCBM at 1:2 (w/w) afforded the following photovoltaic parameters: Voc = 0.4 V; FF = 0.31; Jsc = 0.66 mA/cm2; PCE = 0.1% (see the Supporting Information for details). Although the performance is relatively low, probably due to poor morphologies (see Supporting Information), it shows the potential of Ru-acetylides as new donor candidates for BHJ solar cells. We have observed a strong tendency to phase segregation between 2 and PCBM, and this should strongly reduce the photocurrent, affecting both charge photogeneration yields and charge transport to the electrodes. In fact, relevant phase segregation reduces the number of excitons that reach the donor/acceptor interface where charge separation is occurring; moreover, bad filming properties forced us to deposit relatively thick active layers (about 130 nm) to avoid electrical shorts; thus, also the probability for the charges to reach the electrodes is reduced. For the above reasons, work is now in progress to improve the morphology of the blend by choosing suitable substituents and by extending the investigation to Ru polymers. In conclusion, we have assessed that charge photogeneration by means of electron transfer from complex 2 to PCBM is occurring. The energetic driving force for this process does not differ from that recently reported for similar Pt-based materials, but there is a better solar light coverage. Although morphological issues have to be solved to improve the photovoltaic performances, it appears from this first result that the use of Ru-acetylides is a novel interesting tool for the design of donor materials to combine with electron-withdrawing fullerides in BHJ solar cells.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, figures, and a table giving details of the preparation and morphological characterization of the photoactive films. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*A.C.: tel, þ39 0250314399; fax, þ39 0250314405; e-mail,
[email protected]. L.F.: tel, þ39 0250314057; fax, þ39 0250314300; e-mail,
[email protected]. S.L.: tel, þ39 0223699372; fax, þ39 0270636400; e-mail,
[email protected].
’ ACKNOWLEDGMENT We deeply thank Chiara Trabattoni for experimental help. This work was supported by the MIUR (FIRB 2003 RBNE033KMA: Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices), by the CNR (PROMO 2006: Sistemi molecolari e nanodimensionali con proprieta funzionali: Nanostrutture organiche, organometalliche, polimeriche ed ibride; ingegnerizzazione supramolecolare delle proprieta fotoniche e dispositivistica innovativa per optoelettronica), and by the Fondazione CARIPLO (2008.2205: Progettazione e utilizzo di nuovi materiali organometallici o di coordinazione per celle solari organiche di terza generazione). ’ REFERENCES (1) Kippelen, B.; Bredas, J.-L. Energy Environ. Sci 2009, 2, 251. (2) (a) G€unes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324. (b) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (c) Brunetti, F. G.; Kumar, R.; Wudl, F. J. Mater. Chem. 2010, 20, 2934. (3) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, 1. (4) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. Adv. Mater. 2006, 18, 789. (5) (a) K€ohler, A; Wittmann, H. F.; Friend, R. H.; Khan, M. S.; Lewis, J. Synth. Met. 1996, 77, 147. (b) Beljonne, D.; Wittmann, H. F.; Kohler, A.; Graham, S.; Younus, M.; Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Bredas, J. L. J. Chem. Phys. 1996, 105, 3868. (c) Chawdhury, N.; Younus, M.; Raithby, P. R.; Lewis, J.; Friend, R. H. Opt. Mater. 1998, 9, 498. (6) Shao, Y.; Yang, Y. Adv. Mater. 2005, 17, 2841. (7) Giebink, N. C.; Sun, Y.; Forrest, S. R. Org. Electron. 2006, 7, 375. (8) Holten, D.; Gouterman, M.; Parson, W. W.; Windsor, M. W.; Rockley, M. G. Photochem. Photobiol. 1976, 23, 415. (9) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press: New York, 1970. (10) Clem, T. A.; Kavulak, D. F. J.; Westling, E. J.; Frechet, J. M. J. Chem. Mater. 2010, 22, 1977 and references therein. (11) Wong, W.-Y.; Ho, C.-L. Acc. Chem. Res. 2010, 43, 1246. (12) Selected examples: (a) Wong, W.-Y. Dalton Trans. 2007, 4495. (b) Wang, X.-Z.; Wong, W.-Y.; Cheung, K. Y.; Fung, M.-K.; Djurisic, A. B.; Chan, W.-K. Dalton Trans. 2008, 5484. (c) Wong, W.-Y. Macromol. Chem. Phys. 2008, 209, 14. (d) Wu, P.-T.; Bull, T.; Kim, F. S.; Luscombe, C. K.; Jenekhe, S. A. Macromolecules 2009, 42, 671. (13) (a) Guo, F. Q.; Kim, Y. G.; Reynolds, J. R.; Schanze, K. S. Chem. Commun. 2006, 1887. (b) Guo, F.; Ogawa, K.; Kim, Y. G.; Danilov, E. O.; Castellano, F. N.; Reynolds, J. R.; Schanze, K. S. Phys. Chem. Chem. Phys. 2007, 9, 2724. (14) (a) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Djurisic, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.; Chan, W.-K. Nat. Mater. 2007, 6, 521. (b) Wong, W.-Y.; Wang, X.-Z.; He, Z.; Chan, K.-K.; Djurisic, A. B.; Cheung, K.-Y.; Yip, C.-T.; Ng, A. M.-C.; Xi, Y. Y.; Mak, C. S.K.; Chan, W.-K. J. Am. Chem. Soc. 2007, 129, 14372. (15) Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.; Jen, A. K.-Y. Chem. Mater. 2008, 20, 5734. (16) Mei, J.; Ogawa, K.; Kim, Y.-G.; Heston, N. C.; Arenas, D. J.; Nasrollahi, Z.; McCarley, T. D.; Tanner, D. B.; Reynolds, J. R.; Schanze, K. S. Appl. Mater. Interfaces 2009, 1, 150.
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