Excitation Energy Shuttling in Oligothiophene–Diketopyrrolopyrrole

Article pubs.acs.org/JPCA

Excitation Energy Shuttling in Oligothiophene− Diketopyrrolopyrrole−Fullerene Triads Bram P. Karsten,† Patrizia P. Smith,‡ Arnold B. Tamayo,‡ and René A. J. Janssen*,† †

Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, United States

‡

ABSTRACT: The photophysical properties of a thiophene− diketopyrrolopyrrole oligomer linked to two fullerene units via alkyl linkers of different lengths have been investigated in solution. The molecules exhibit excitation energy shuttling between the singlet and triplet photoexcited states. Photoexcitation of the oligomer is followed by singlet energy transfer to the fullerene, intersystem crossing to the triplet state, and then triplet energy transfer back to the oligomer. Competing electron transfer reactions, followed by recombination to the triplet state, are energetically possible and cannot be ruled out but were not observed and seem to have a small contribution in solution.

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INTRODUCTION Organic photovoltaics form an attractive alternative to silicon solar cell technology because of their easy processing and low cost. Significant recent advancements in the power conversion efficiency of organic solar cells have been reported,1−8 and record efficiencies are now approaching 10%.9 Among the many different materials used in organic solar cells, materials based on diketopyrrolopyrrole (DPP) constitute a particularly interesting class. Solar cells based on DPP-containing small molecules10−12 and polymers13−17 as electron donor with C60 or C70 fullerene derivatives as acceptor have resulted in solar cell efficiencies over 5%. These favorable efficiencies are caused by the small optical band gap, the deep HOMO level, and the good charge carrier mobility of DPP-based materials. After light absorption, the first step in a solar cell is the transfer of an electron from the donor to the acceptor. For the development of novel materials exhibiting enhanced performance, it is necessary to understand the charge generation and separation processes in detail. Covalent attachment of the donor and acceptor moieties within one molecule allows for a detailed study of the charge generation and recombination processes in solution where polarity can be changed and intramolecular processes prevail.18−25 Here we investigate the photophysical processes of 6T-DPPPCBCn (n = 3, 6, 11) triads (Figure 1) where two fullerene chromophores are covalently linked to a thiophene−diketopyrrolopyrrole oligomer (6T-DPP) via alkyl spacers of different lengths. The molecules exhibit strong absorption (ε = 1 × 104 to 1 × 105 cm−1) in a broad spectral range, and when used as thin solid films to form the active layer of an organic solar cell, these triad molecules produce power conversion efficiencies up to 0.49% in simulated solar light,26 and the efficiency further increases for longer oligothiophenes.27 The photophysics of the © 2012 American Chemical Society

6T-DPP-PCBCn triads is studied here in solvents of different polarity. In contrast to the thin solid films, no charge separation occurs in solution. Instead, an energy shuttling mechanism operates in which the singlet excitation on the 6T-DPP oligomer is first transferred to the fullerene, followed by intersystem crossing, and completed by back-transfer of the triplet excitation from the fullerene to the 6T-DPP oligomer.

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RESULTS AND DISCUSSION Normalized UV/vis absorption spectra of the 6T-DPP-PCBCn (n = 3, 6, 11) triads are depicted in Figure 2 and compared to those of the individual chromophores represented by the 6TDPP-C12 oligomer (Figure 1) and PCBM ([6,6]-phenyl-C61butyric acid methyl ester). The absorption spectra of the triads are a near superposition of the absorption spectra of 6T-DPPC12 and PCBM. No apparent new bands, such as charge transfer absorptions, appear in the spectra, although the spectra of the triads are slightly shifted to lower energies, most noticeably for the shorter alkyl linkers C3 and C6, and less for C11. The optical band gaps determined from the onsets of absorption of these molecules in toluene and o-dichlorobenzene (ODCB) are summarized in Table 1. For the 6T-DPPPCBCn (n = 3, 6, 11) triads, the onset was taken as that corresponding to absorption of the 6T-DPP chromophore. Several observations can be made in Figure 2 and Table 1. First, the optical band gaps of the 6-DPP chromophore in 6TDPP-C12 and 6T-DPP-PCBCn are only slightly larger than the band gap of PCBM. This indicates that their singlet excited states are nearly degenerate. Second, the optical band gap is Received: September 17, 2011 Revised: December 30, 2011 Published: January 3, 2012 1146

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further. The red shift in ODCB, which is more polar than toluene (εr = 10.12 vs 2.38), can be attributed to a stabilization of the excited state of the 6T-DPP, which most likely is a result of charge transfer from the interaction of electron-rich terthiophenes and the electron-deficient DPP unit. We tentatively ascribe the small red shift of the 6T-DPP absorption band in 6T-DPP-PCBCn relative to that in 6T-DPP-C12 to a similar stabilization resulting from the interaction between the 6T-DPP and C60 moieties. The observed alkyl chain length dependence can then be interpreted by considering that for shorter alkyl linkers, this interaction is stronger than for the long alkyl linker. Photoluminescence (PL) spectra of 6T-DPP-C12 and 6TDPP-PCBCn are depicted in Figure 3. The fluorescence of the

Figure 1. Molecular structure of the 6T-DPP-PCBCn (n = 3, 6, 11) triads and the 6T-DPP-C12 oligomer.

Figure 3. Photoluminescence spectra of solutions of 6T-DPP-C12, 6T-DPP-PCBCn (n = 3, 6, 11), and PCBM in toluene (a) and ODCB (b). The spectra are corrected for the optical density at the excitation wavelength and normalized to the maximum intensity of 6T-DPP-C12.

Figure 2. Normalized UV/vis absorption spectra of solutions of 6TDPP-C12, 6T-DPP-PCBCn (n = 3, 6, 11), and PCBM in toluene.

6T-DPP chromophore is strongly quenched in the 6T-DPPPBCn triads. Emission maxima and quenching factors for 6TDPP-C12 and 6T-DPP-PCBCn are summarized in Table 1. The fluorescence quenching is weaker when the flexible alkyl linker is longer. Considering that quenching is due to

smaller for the shorter chain lengths, C3 and C6, than for C11. Third, the absorption spectra of the 6T-DPP compounds are slightly more red-shifted in ODCB than in toluene, decreasing the small energy offset between the two singlet states even

Table 1. Optical Absorption and Emission Data, and Fluorescence Quenching (Q) for 6T-DPP-C12, 6T-DPP-PCBCn (n = 3, 6, 11), and PCBM Measured in Toluene and ODCB toluene 6T-DPP-C12 6T-DPP-PCBC3 6T-DPP- PCBC6 6T-DPP- PCBC11 PCBM

λmax (nm)

λonset (nm)

Eg (eV)

652 656 656 653

696 708 708 704 715

1.78 1.75 1.75 1.76 1.73

ODCB λmax

PL

(nm)

687 691 689 688 707 1147

Q (%)

λmax (nm)

λonset (nm)

Eg (eV)

λmaxPL (nm)

Q (%)

99 97 90

661 664 664 663

708 716 716 712 720

1.75 1.73 1.73 1.74 1.72

698 702 699 699 708

98 91 72

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Photoinduced absorption (PIA) spectra of the 6T-DPPPCBCn triads are depicted in Figure 5. The spectra exhibit an

interaction with the fullerene, this observation is consistent with the weaker interaction between the 6T-DPP and fullerene moieties when they are separated by a longer linker. The rate of the quenching process can be estimated using kq = (Q − 1)/τ, where Q is the quenching ratio, defined as the ratio of the fluorescence quantum yields of 6T-DPP-C12 and 6T-DPPPCBCn (Table 1) and τ = 0.5 ns the fluorescence lifetime of 6T-DPP-C12. This yields kq = 2 × 1011 s−1 and shows that the quenching is in the picosecond domain. In conjugated oligomer−fullerene systems both energy and electron transfer reactions are known to occur with high rate constants.28 The observed fluorescence quenching of the 6T-DPP chromophore in the triads can be caused by either energy transfer from the singlet excited state of the oligomer 1(6TDPP) to the fullerene 1(C60) or by electron transfer. The fluorescence is less quenched in ODCB. For electron transfer, the rate and, consequently, the fluorescence quenching would be higher in ODCB because the charge separated state is lower in energy in a more polar solvent. Hence, energy transfer, i.e., 1 (6T-DPP) → 1(C60), is the most likely explanation for the fluorescence quenching. The result that the fluorescence quenching in ODCB is less than in toluene can be explained by the slightly red-shifted absorption of the 6T-DPP oligomer in ODCB. This red shift implies that the driving force for energy transfer decreases from ∼20 meV in toluene to ∼10 meV in ODCB. This reduces the energy transfer rate and, therefore, the fluorescence quenching. The stronger quenching in systems with shorter linkers is consistent with the shorter distance between energy donor and acceptor. Given the very small energy differences between the first excited singlet states of 6T-DPP and C60 chromophores, it is interesting to see if there is an equilibration between the two states, i.e., if the 6T-DPP oligomer can be excited via the C60 moiety. For this reason, excitation spectra were measured, where the fluorescence of 6T-DPP-PCBCn was measured as a function of excitation wavelength. Figure 4 shows that the

Figure 5. PIA spectra of solutions of the 6T-DPP-PCBCn (n = 3, 6, 11) triads in (a) toluene and (b) ODCB, recorded with photoexcitation at 633 nm (1.96 eV).

absorption band centered at 1.6−1.7 eV and a bleaching band centered at 2.0 eV, characteristic for the triplet absorption of the 6T-DPP chromophore.29 Photoexcitation of 6T-DPP-C12 gives negligible intensity in the PIA spectrum (not shown) and reveals that the triplet state of 6T-DPP chromophore in 6TDPP-PCBCn is formed indirectly and not via intersystem crossing (isc) from the 6T-DPP singlet excited state. Two different processes may be responsible for triplet formation (Figure 6). The first possibility is a singlet-energy transfer from the excited 1(6T-DPP) oligomer to the fullerene, producing 1(C60), followed by intersystem crossing to yield the triplet fullerene 3(C60), and then triplet-energy transfer back to the oligomer to form 3(6T-DPP). This process can be efficient, because intersystem crossing proceeds with a quantum yield of almost unity in fullerenes.30 This energy shuttling mechanism, where the excitation energy is transferred back and forth between two different units, has previously been observed in donor−acceptor dyads and blends relevant for photovoltaics.22,31,32 The second possibility is electron transfer from the 6T-DPP oligomer to the fullerene, followed by charge recombination into the triplet state. This last pathway has been observed, when the C60 groups are directly connected to a 4TDPP oligomer backbone.29 In the first mechanism, it is essential that the triplet energy of the 6T-DPP oligomer is below that of the fullerene. For similar 2T-DPP and 4T-DPP oligomers, the triplet energies have been determined to be ∼1.1 and 1 eV.29 Assuming a similar exchange energy of 1 eV for 6T-DPP, the

Figure 4. Fluorescence excitation spectra of solutions of 6T-DPP-C12 and 6T-DPP-PCBCn (n = 3, 6, 11) in toluene. The absorption spectrum of PCBM is included for comparison.

excitation spectra of the 6T-DPP-PCBCn triads are virtually identical to the excitation spectrum of the 6T-DPP-C12. Moreover, the characteristic features that could be expected from a fullerene contribution, i.e., the small peak at 433 nm and a strong absorption at low wavelengths, are absent in the excitation spectra of the triads. From this, we conclude that energy transfer between the chromophores is a true one-way process, only occurring from the oligomer to the fullerene and not vice versa. 1148

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the 1(6T-DPP) state. However, because the 1(C60) state has virtually the same energy as the 1(6T-DPP) state (Table 1), intramolecular electron transfer in the triads remains feasible in both solvents when the two units come close. According to eq 1, formation of the charge separated state is much more exergonic in ODCB (ΔG = −0.46 eV at Rcc = 10 Å) than in toluene (ΔG = −0.09 eV at Rcc = 10 Å). Figure 5, however, shows very similar intensities for the 6T-DPP-PCPCn triplet absorptions in toluene and ODCB. The absence of a dependence on solvent polarity strongly suggests that intramolecular electron transfer from the 1(C60) state is not involved in the triplet formation. Apparently, intersystem crossing of 1 (C60) to 3(C60) and subsequent triplet energy transfer to 3(6TDPP) is faster. The 3(C60) → 3(6T-DPP) triplet energy transfer cannot occur via a long-range interaction Förster type transfer because of the negligible oscillator strengths of the 3(C60) → 1 (C60) and 1(6T-DPP) → 3(6T-DPP) transitions, but only via a short-range electron exchange Dexter-type transfer. The electron exchange triplet energy transfer could compete with intramolecular electron transfer from the 3(C60) state. The energy of 3(C60) (1.50 eV) is, however, lower than ECSS at Rcc = 10 Å in toluene (1.67 eV), meaning that this process is unlikely. In ODCB, ECSS = 1.30 eV at Rcc = 10 Å and electron transfer may occur from the 3(C60) state. This would result in a strong solvent dependence of triplet formation, which is not seen experimentally. Apparently, the larger driving force to make the low energy triplet of 6T-DPP at ∼0.75 eV enhances the rate of the triplet energy transfer compared to the electron transfer. On the basis of these considerations, we conclude that after excitation of the 6T-DPP chromophore in 6T-DPP-PCBCn producing 1(6T-DPP), singlet-energy transfer occurs, forming 1 (C60), which relaxes via intersystem crossing to 3(C60). The process ends with triplet-energy transfer from 3(C60) to the 3 (6T-DPP) (Figure 6). This conclusion seems at variance with the fact that these molecules have been employed in photovoltaic devices.26,27 We note, however, that photophysical processes in solution and solid state in oligomer-fullerene conjugates are known to differ considerably.35 The increased tendency for charge separation in the solid state is attributed to a closer orientation of the donor and acceptor that results in intermolecular electron transfer.36,36 Additionally, photogenerated electrons and holes have longer lifetimes as a result of migration of these charges to energetically more favorable sites in the film.

Figure 6. State diagram of the 6T-DPP-PCBCn triads and photophysical processes occurring after excitation of the 6T-DPP chromophore. The solid arrows indicate the energy shuttling mechanism between the 6T-DDP and C60 chromophores to form the 3(6T-DPP) T1 state that is consistent with the experimental observations. The dashed arrows show alternative processes, i.e., direct intersystem crossing and intramolecular electron transfer, that are in competition with the energy shuttling mechanism, but for which no experimental evidence was found. The two gray lines for the charge separated state (CSS) represent the energy of that state in toluene (1.66 eV) and ODCB (1.29 eV) for Rcc = 10 Å based on eq 1.

triplet state energy will be at ∼0.75 eV, i.e., well below the triplet energy of the fullerene at 1.50 eV. The likelihood of the second mechanism depends on the energy of the charge separated state (CSS) state that would be formed. Using a continuum model, the energy of the charge separated state (ECSS) in different solvents can be calculated by33

e2 4πε0εsR cc 1 ⎞⎛ 1 1⎞ e2 ⎛ 1 ⎜ ⎟⎜ − + − ⎟ − 8πε0 ⎝ r + r ⎠⎝ εref εs ⎠

ECSS = e(Eox (D) − Ered(A)) −

(1)

In this equation, Eox(D) is the oxidation potential of the oligomer (measured to be 0.23 V vs Fc/Fc+ in ODCB) and Ered(A) is the reduction potential of PCBM (−1.2 V vs Fc/ Fc+). Rcc is the center-to-center distance of the positive and negative charges. r+ and r− are the radii of the positive and negative ions formed. For C60 the value of r− = 5.6 Å has been estimated using the density of C60.30 r+ can be estimated using a similar approach, using a density of 1.5 g/cm3, the value for terthiophene.34 This gives a value of 5.5 Å for r+. εref and εs are the relative permittivities of the reference solvent (used to measure oxidation and reduction potentials) and the solvent in which electron transfer is studied. Because of the flexible nature of the linkers, the value of Rcc is not determined. Equation 1 reveals that ECSS is below the singlet energy of 6T-DPP oligomer in toluene for Rcc ≤ 11 Å and in ODCB for any value of Rcc (i.e., also when Rcc would exceed the length of the alkyl spacer), meaning that electron transfer can occur. Because the rate for electron transfer reaction decays exponentially with Rcc, electron transfer can only occur when the two moieties are (temporarily) in close proximity. Considering that the fluorescence of 6T-DPPPCBCn is less quenched in ODCB than in toluene, we already concluded that electron transfer in 6T-DPP-PCBCn is not occurring from the 6T-DPP singlet state but that intramolecular singlet energy transfer from 1(6T-DPP) to 1(C60) deactivates

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CONCLUSION We have studied the photophysical properties a 6T-DPP-based oligomer coupled to a fullerene, via different alkyl linkers using UV/vis absorption, fluorescence, and photoinduced absorption spectroscopy. After excitation of these 6T-DPP-PCBCn triads, fast energy transfer occurs from the DPP oligomer to the fullerene, followed by intersystem crossing and back-transfer of the excitation to the triplet state of the oligomer. Competing electron transfer reactions, followed by recombination to the triplet state, are energetically possible and cannot be ruled out but were not observed and seem to have a small contribution in solution.

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EXPERIMENTAL SECTION UV/vis absorption spectra were recorded using a Perkin Elmer Lambda 900 spectrophotometer. Fluorescence spectra were recorded on an Edinburgh Instruments FS920 double1149

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monochromator spectrophotometer with a Peltier-cooled redsensitive photomultiplier. The emission spectra were corrected for the wavelength dependence of the sensitivity of the detection system. Near steady-state PIA spectra were recorded by exciting with a mechanically modulated Ne/Ne laser (λ = 633 nm, 275 Hz) pump beam and monitoring the resulting change in transmission of a tungsten-halogen probe light through the sample (ΔT) with a phase-sensitive lock-in amplifier after dispersion by a grating monochromator and detection, using Si, InGaAs, and cooled InSb detectors. The pump power incident on the sample was typically 35 mW with a beam diameter of 2 mm. The PIA (ΔT/T) was corrected for the photoluminescence, which was recorded in a separate experiment. Photoinduced absorption spectra and photoluminescence spectra were recorded with the pump beam in a direction almost parallel to the direction of the probe beam. The solutions were studied in a 1 mm near-IR grade quartz cell at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Mindaugas Kirkus for providing us with 6T-DPPC12. The research was financially supported by the European Union (the European Regional Development Fund - ERDF) in the frame of Organext project of the Operational Program INTERREG IV-A Euregio Meuse-Rhine.

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