SUPPORTING INFORMATION
Control of intra-chain charge transfer in model systems for block copolymer photovoltaic materials Kerr Johnson†, Ya-Shih Huang†, Sven Huettner†, Michael Sommer‡∆, Martin Brinkmann#, Rhiannon Mulherin†, Dorota Niedzialek§, David Beljonne§, Jenny Clark†, Wilhelm T.S. Huck‡^, Richard H. Friend†* †
Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, UK
‡
Melville Laboratory for Polymer Synthesis, Lensfield Road, Cambridge CB2 1EW, UK
§
Laboratory for Chemistry of Novel Materials, Université de Mons, Place du Parc, 20, 7000 Mons, Belgium
#
Institut Charles Sadron, 6 Rue Boussingault, 67083 Strasbourg, France Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg, 135 6525 AJ Nijmegen, The Netherlands ^
Contents 1. Solution Photoluminescence Quantum Efficiency. 2. Contour Length of F8-TBT-P3HT from X-ray Scattering and NMR. 3. Electron Diffraction Pattern of F8-TBT-P3HT Film grown by Directional Epitaxial Crystallization. 4. Thin Film Absorption and PL including P3HT + F8TBT Blend. 5. F8-TBT-P3HT Transient Absorption, 700 nm excitation. 6. F8-TBT-P3HT Transient Absorption, Steady State Absorption and PL Comparison. 7. F8-TBT-P3HT vs. P3HT Transient Absorption in Solution. 8. Experimental Details for TA Measurements with 40 fs Resolution. 9. F8-TBT-P3HT CT state vs. P3HT+F8TBT Blend PIA : - TA decay kinetics. 10. TBT-F8-P3HT Transient Absorption in Solution. 11. Electronic structure calculations of end-functionalized molecules at semi-empirical and ab-initio level of theory.
S1
1. Solution Photoluminescence Quantum Efficiency. Solutions, of concentration 5 µg/ml, were made by dissolving the relevant polymer in anhydrous chlorobenzene at 70°C for 2 hours followed by consecutive dilution steps. This was done in the nitrogen atmosphere of a glovebox. Quartz cuvettes were filled and sealed in the glovebox. Reference solutions of Rhodamine-6G and DCM (4-(Dicyanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4H-pyran), of approximate concentration 0.6 µg/ml in ethanol were also prepared. The absorption spectra of the solutions were measured in a 10 mm path length quartz cuvette using a Hewlett–Packard HP845× Visible–UV spectrometer. The photoluminescence (PL) spectra used for calculating the photoluminescence quantum efficiency (PLQE), were measured in a 1 mm path length quartz cuvette using a 500 mm spectrograph (SpectraPro2500i, Princeton Instruments) combined with a CCD camera (PIXIS 100-F, Princeton Instruments). The excitation source was a pulsed 470 nm, 80 ps full width at half maximum, 10 MHz diode laser (PicoQuant LDH400). PLQE values were calculated from the absorption and PL data according to the following formula1: ࡵ ࡾ ࡵࡾ ࡾ Q is the quantum efficiency, I is the integrated photoluminescence intensity (over the range 525 – 875 nm), A is the fraction of photons absorbed at the excitation wavelength (470 nm) and n is the refractive index of the solvent. The subscript R refers to the reference fluorescent material of known quantum efficiency. For this work the reference material used was DCM in ethanol with a quantum efficiency 2 of 0.45 (Rhodamine-6G in ethanol with a quantum efficiency 3 of 0.95 was also tested as a reference, resulting in similar PLQE values, although the error using Rhodamine-6G was higher due to the low absorption at the 470nm excitation wavelength). The error in this method is estimated to be ±10%. The absorption and emission spectra for F8-TBT-P3HT and TBT-F8P3HT can be fitted using the absorption and PL emission spectra of pure P3HT and F8TBT monomer to estimate the PL/Absorption ratio of each part of the molecule. These PL/Absorption ratios for each part of the molecule are then normalized by the PLQE of the relevant pure material (i.e. P3HT or F8TBT monomer). If the resultant values are close to 1 it signifies that the individual parts of the molecule are not interacting in any way and there is no energy or charge transfer. However, if these PL/Absorption ratios (normalized to pure material PLQE) differ significantly from 1 then we can estimate the magnitude of energy or charge transfer. A value lower than 1 indicates quenching, either by energy or charge transfer, while a value higher than 1 indicates that energy transfer to the chromophore is occurring. As can be seen in Table S1, for TBT-F8-P3HT the P3HT is significantly quenched while the F8TBT section of the molecule emits much more light than it absorbed (hence the 9.6 PL/Absorption ratio) meaning energy transfer to it occurs. (i.e. it emits 9.6 times more light than would be expected for its absorption) For F8-TBTP3HT the P3HT is quenched to a similar extent but there is no F8TBT luminescence. Of the 16% total PLQE for F8-TBT-P3HT then fitting the PL spectra shows 10% is from P3HT and the other 6% is from the CT state. ࡽ ൌ ࡽࡾ
Table S1. Solution PLQE values Total PLQE (%)
P3HT: PL/Absorption ratio (normalized to P3HT PLQE)
F8TBT monomer: PL/Absorption ratio (normalized to F8TBT monomer PLQE)
P3HT
30
1
-
F8TBT monomer
52
-
1
TBT-F8-P3HT
42
0.33
9.6
F8-TBT-P3HT
16
0.33
0
To better understand the behavior of TBT-F8-P3HT in solution the photoluminescence quantum efficiency (PLQE) was compared with the theoretical PLQE values which could be obtained if a fraction of excitons undergo charge transfer and do not emit, a fraction of excitons undergo energy transfer (ET) to the F8TBT unit and emit with the PLQE of the lone F8TBT monomer, while all that do not experience energy or charge transfer emit with the pure P3HT PLQE. 42% total PLQE, 0.33 P3HT PL/Absorption ratio and 9.8 F8TBT monomer PL/Absorption ratio, which is very close to the experimentally observed values, can be obtained for 7% of the P3HT excitons undergoing charge transfer, 64% undergoing energy transfer to and emission from F8TBT, and 29% of P3HT excitons decaying intrinsically. The excitons undergoing charge transfer are assumed to eventually decay non-radiatively which is consistent with the absence of significant long lived emission in the TCSPC experiments. While somewhat rudimentary this method does give an estimate of the upper limit of intra-chain charge transfer occurring in the TBT-F8-P3HT system, 7%. Similarly, for F8-TBT-P3HT we can calculate the theoretical PLQE values which could be obtained if a fraction of excitons migrate to the CT state while the remainder (as no emission from the F8TBT is seen) emit with the pure P3HT PLQE. We assume all absorption into the F8TBT monomer results in the formation of CT states. The best fit to the experimental data is obtained for 85% of the P3HT excitons resulting in CT states and 15% decaying intrinsically. This then allows the intrinsic PLQE of the CT state to be estimated at 9%. Note that these theoretical PLQE values have been calculated taking into account the fraction of chains not functionalized, 5% in the case of TBT-F8-P3HT and 23% for F8-TBT-P3HT. 2. Contour Length of F8-TBT-P3HT from X-ray Scattering and NMR.
S2
The ≈9.1nm contour-length of the F8-TBT-P3HT chain was taken as the length of a single thiophene multiplied by the number of thiophene units, plus the length of an F8TBT unit. The length of a single thiophene unit was found to be 0.395 nm from the 002 peak (q = 16.3 nm-1) in the WAXS spectrum. The number of thiophene units was found to be 18 from the ratio of main chain to the endgroup proton NMR signals. 3. Electron Diffraction Pattern of F8-TBT-P3HT Film grown by Directional Epitaxial Crystallization.
Figure S1. Electron diffraction pattern of an oriented F8-TBT-P3HT film grown by Directional Epitaxial Crystallization.4
4. Thin Film Absorption and PL including P3HT + F8TBT Blend.
Figure S2. Steady state absorption (a) and photoluminescence (b) spectra in thin film for the end-functionalized P3HT molecules, their parent materials and a conventional blend of P3HT and F8TBT 50:50 wt%. Excitation wavelength for PL spectra is 470nm.
5. F8-TBT-P3HT Transient Absorption, 700 nm excitation. Figure S3 shows the transient absorption spectra of F8-TBT-P3HT with 700 nm excitation which directly populates the CT state with negligible absorption into the P3HT exciton band. The same spectral features are observed as for excitation at 500 nm (main
S3
text Figure 5 (b)) including the P3HT GSB peaks at 560 nm and 610 nm which indicates part of the P3HT chain is bleached when there is a CT state localized on the molecule.
Figure S3. Transient absorption spectra of F8-TBT-P3HT with 700 nm excitation.
6. F8-TBT-P3HT Transient Absorption, Steady State Absorption and PL Comparison. Figure S4 shows a comparison of the F8-TBT-P3HT transient absorption spectrum at 1ps with steady state absorption spectra of P3HT and F8-TBT-P3HT and the photoluminescence spectrum of F8-TBT-P3HT. As can be clearly seen, the feature peaking at ≈700 nm in the TA spectrum does not correspond to the absorption of P3HT, nor the emission spectrum of F8-TBT-P3HT. Rather, this is the bleaching of the CT state and the spectral position agrees with the shoulder in the F8-TBT-P3HT absorption spectrum.
Figure S4. Comparison of F8-TBT-P3HT transient absorption spectrum at 1ps with steady state absorption spectra of P3HT and F8-TBT-P3HT, as well as the photoluminescence spectrum of F8-TBT-P3HT
7. F8-TBT-P3HT vs. P3HT Transient Absorption in Solution. Figure S5 shows the transient absorption spectra of (a) P3HT and (b) F8-TBT-P3HT in chlorobenzene solution. For P3HT the GSB can be seen for wavelengths 2 ns.
10. TBT-F8-P3HT Transient Absorption in Solution. Figure S8 (a) shows the transient absorption spectra of TBT-F8-P3HT in chlorobenzene solution (excitation is 500 nm for times < 2 ns, 532 nm for > 2 ns). The positive feature in the 550 – 700 nm range is a combination of stimulated emission from the P3HT and F8TBT parts of the molecule. Disentangling the two quantitatively is complicated as there is energy transfer from the P3HT to the F8TBT and also a dynamic redshift caused by solvent relaxation. The region from 700 – 800 nm is a combination of P3HT and F8TBT exciton PIA at early times. By 10 ns these excitons have decayed either to the ground state in the case of those on the F8TBT or, in the case of those originally situated on the P3HT, to the P3HT triplet T1 state via intersystem crossing, as is seen to occur in solution but not thin film.5 Figure S8 (b), shows the normalized TA spectra at 100 ns for TBT-F8-P3HT, P3HT and a mixture of P3HT and F8TBT monomer in the same ratio as the end-functionalized molecule. By 100 ns all the singlet excitons have decayed, and for P3HT and the P3HT + F8TBT monomer mixture only the P3HT triplet T1 state is observed. However, for TBTF8-P3HT there is an additional PIA in the 500 – 650 nm region. This may be evidence of a small fraction of intra-molecular charge separated states formed in the TBT-F8-P3HT molecule. Figure S8 (c) shows the decay kinetics (averaged over the wavelength ranges indicated) of TBT-F8-P3HT, the F8TBT monomer, and a mixture of P3HT and F8TBT monomer in the same ratio as the end-functionalized molecule. The F8TBT monomer and the P3HT + F8TBT monomer mixture are scaled to the TBT-F8-P3HT kinetics at 750-800 nm and 600-620 nm respectively. This shows that the P3HT triplet (e.g. the P3HT + F8TBT monomer 750-800 nm kinetic trace) and the additional PIA for TBT-F8-P3HT in the 500-650 nm region have somewhat similar lifetimes which could be explained if the decay processes (e.g. triplet-triplet and triplet-charge annihilation) are limited by molecular diffusion mediated collisions.
Figure S8. (a) Transient absorption spectra of the end-functionalized TBT-F8-P3HT in chlorobenzene solution. (b) Comparison of normalized TA spectra at 100 ns for TBT-F8-P3HT, P3HT, and a mixture of P3HT and F8TBT monomer in the same ratio as the end-functionalized molecule. (c) The decay kinetics (averaged over the wavelength ranges indicated) of TBT-F8-P3HT, F8TBT monomer, and a mixture of P3HT and F8TBT monomer in the same ratio as the end-functionalized molecule. The F8TBT monomer and the P3HT + F8TBT monomer mixture are scaled to the TBT-F8-P3HT kinetics at 750-800 nm and 600-620 nm respectively. (λex = 500nm).
11. Electronic structure calculations of end-functionalized molecules at semi-empirical and ab-initio level of theory.
S6
The optimal ground-state geometry of each F8, T and B repeating unit, and the torsion angles between them, were found by DFT using B3LYP functional with 6-31G** basis. With these structural parameters, we constructed a reasonable molecular structure and used instead an INDO/SCI approach that has been fully validated in the past for the description of charge-transfer like excited states.6,7 To check the robustness of our semi-empirical results, we run as a reference B3LYP and long-range corrected wB97XD TD-DFT calculations on a smaller, representative end-capped oligomer and found excellent agreement between the semi-empirical and ab initio results. As expected, B3LYP overestimates delocalisation while the long range corrected functional shows a more localised character for the excited state in agreement with the INDO/SCI calculations.8,9 It is especially pronounced in the TBT-F8P3HT molecule case where there is mainly HOMO→LUMO transition contributing to the lowest excited state calculated with B3LYP, while we do not observe this transition in the wB97XD TD-DFT calculation. Instead, lower and more delocalized (including over the TBT units) orbitals (namely HOMO-2 and HOMO-1) contribute to lowest excited state obtained with the long-range corrected functional, similar to the INDO/SCI results. A detailed comparison is presented in Figures S9 and S10 below. Finally, the extra advantage of using a semi-empirical Hamiltonian is that it allows literally dissecting the anatomy of the electronic excited states via the calculation of the electron-hole density matrix. This is done in the following manner: The electron-hole wavefunction for a hole on atomic orbital p and an electron on atomic orbital q are defined as:
ψ ( xh = p, ye = q) = ∑ Cia χip χ aq i ,a
where Cia is the configuration interaction coefficient for the single electron-hole excitation involving molecular orbitals i (occupied) and a (empty) and ߯ (߯ ) is the LCAO coefficient on atomic orbital p (q) in molecular orbital i (a). To ease the visualization, we then ‘coarse grain’ the probability density, │ψ (x ,y)│2, into a site representation where each site represents a monomer unit. This allows us to generate the two-dimensional grids shown in Figure 7 of the manuscript where each point, (x ,y), represents the probability, │ψ (x ,y)│2, of finding the hole in the π-orbital on site x and the electron in the π-orbital on site y.
S7
Figure S9. HOMO and LUMO topology of the P3HT-F8-TBT (left panel) and P3HT-TBT-F8 (right panel) molecules calculated at the ZINDO/SCI a) and b), B3LYP/6-31G(d) c) and d) and wB97XD/6-31G(d) e) and f) level. The HOMO for both molecules is delocalized along the thiophene part, while the LUMO is mainly localized on the TBT units. The size and colour of the spheres represent the amplitude and sign of LCAO (Linear Combination of Atomic Orbital) coefficient, respectively; For P3HT-F8-TBT at the ZINDO/SCI and wB97XD/6-31G(d) levels the HOMO-1 and HOMO-2 orbitals contribute to the lowest excited state (See Figure S10.) and are also displayed.
S8
ES1
Transitions
Wavelengths [nm] (OS)
ZINDO
B3LYP
ωB97XD
ZINDO
B3LYP
ωB97XD
20(P3HT)
H→L
H→L
H→L
624 (6.19)
671 (6.75)
503 (7.85)
F8-TBT-10(P3HT)
H→L
H→L
H→L
551 (1.71)
636 (0.99)
476 (1.56)
H→L
H-2→L H-1→L
519 (1.19)
597 (0.56)
455 (1.04)
TBT-F8-10(P3HT)
H-2→L
Figure S10. a) and b) ES-GS charge distribution for the lowest excited state calculated at the ZINDO/SCI level. Blue and red colors correspond to negative (electron) and positive (hole) charges, respectively. NTOs for the hole and electron for the lowest excited state calculated at the B3LYP/6-31G(d) c) and d) and wB97XD/6-31G(d) e) and f) level. Left panel displays the orbital densities for P3HT-F8-BT and the right panel for P3HT-TBT-F8. The table displays results for the lowest excited state calculated for 20P3HT polymer and both investigated end-functionalized molecules using different theoretical approaches.
AUTHOR INFORMATION
S9
Corresponding Author *
[email protected] Present Addresses ∆ Institute of Macromolecular Chemistry, University of Freiburg, Stefan-Meier-Str. 31, 79104 Freiburg, Germany
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