Study of Optical Properties and Molecular Aggregation of Conjugated

Article pubs.acs.org/JPCC

Study of Optical Properties and Molecular Aggregation of Conjugated Low Band Gap Copolymers: PTB7 and PTB7-Th Fatima Bencheikh, David Duché, Carmen M. Ruiz, Jean-Jacques Simon,* and Ludovic Escoubas Aix-Marseille University, Institut Matériaux Microélectronique Nanosciences de Provence-IM2NP, UMR CNRS 7334, Marseille 13397 Cedex 20, France ABSTRACT: A comparative study of the optical properties of two well-known copolymers for photovoltaic applications, the PTB7 and the PTB7-Th, is presented. This study contributes to the understanding of the photophysics and permits correlation of the morphological change to the optical properties. For this purpose, Variable Angle Spectroscopic Ellipsometry (VASE) measurements are performed to determine the optical transitions as well as the band gap of PTB7 and PTB7-Th thin films from the imaginary parts of the dielectric functions. Then, the molecular aggregations in chlorobenzene solutions are investigated by means of absorbance and photoluminescence measurements as a function of concentration and temperature. These solution measurements are compared to film measurements in order to isolate interchain and intrachain optical transitions.

1. INTRODUCTION Improving the performance of organic solar cells requires the understanding and the mastering of the interfaces between organic−organic and organic−inorganic materials, the morphology of the active layer, as well as the molecular engineering of organic materials.1−5 Based on the latter approach, several donor−acceptor (D-A) copolymers with low band gap and suitable HOMO (highest occupied molecular orbital) level matching to fullerene are synthesized.6,7 The copolymers which give the highest power conversion efficiencies (PCEs), when blended with PC70BM ([6,6]-phenyl C71-butyric acid methyl ester) with chlorobenzene as solvent, are the poly[(ethylhexyloxy)-benzodithiophene-(ethylhexyl)-thienothiophene], commonly known as the PTB7, with PCE of 9.2% and the poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)thienothiophene], commonly known as PTB7-Th, with PCE of 10.3%.8,9 The PTB7 is based on alternating thienothiophene (TT) units as electron donor and benzodithiophene (BDT) units as electron acceptor (Figure 1(a)). The thienothiophene unit stabilizes the quinoid structure, which reduces the band gap to 1.6 eV for more efficient light absorption. Furthermore, the quinoid structure permits the enhancement of the planarity and the rigidity of the polymer backbone. The BDT unit is chosen because of its extended conjugated π system.10 In order to further reduce the band gap of the PTB7, and thus enhance the photocurrent, the PTB7-Th has been synthesized (Figure 1(b)). The PTB7-Th is obtained by incorporating a 2ethylhexyl-thiophenyl group side chain into the BDT unit in PTB7, which improves the planarity of the main chain, leading to a low band gap.11 Unlike homopolymers (e.g., poly 3-hexylthiophene, P3HT) which usually exhibit a spectral red shift from solution in chlorobenzene to solid state, some D−A copolymers do not © XXXX American Chemical Society

Figure 1. Chemical structures of (a) PTB7 and (b) PTB7-Th.

show any absorption shift from solution to solid state.12−14 Several studies with detailed analysis of the correlation between photophysical properties and morphology in high band gap homopolymers such as PPV (poly-phenylene-vinylene) derivatives and P3HT have been published.15,16 In contrast, the photophysical behavior of low band gap copolymers is less studied and understood. In this work, a comparative study of the optical properties of the PTB7 and the PTB7-Th is presented. First, variable angle spectroscopic ellipsometry (VASE) measurements are performed to extract the dielectric functions of the two copolymer

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DOI: 10.1021/acs.jpcc.5b07803 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Imaginary part of the dielectric function for (a) PTB7 and (b) PTB7-Th thin films.

thin films. In this study, the energies of the optical transitions as well as the band gap of the two materials are determined using a Gaussian model and are compared. Second, in order to identify the origin of the first two optical transitions, absorbance and photoluminescence measurements are performed as a function of concentration and temperature to modify the aggregation state in solutions. This study shows the impact of the interchain and intrachain interactions on the optical properties of the PTB7 and PTB7-Th.

3. RESULTS AND DISCUSSION 3.1. Spectroscopic ellipsometry data analysis. The VASE technique has been used to determine the dielectric functions of the PTB7 and PTB7-Th thin films. We measure the polarization change of light upon reflection on a sample. This polarization change depends on the amplitude and phase variations of the electric fields for p- and s-polarizations. Thus, the Ψ and Δ angles measured from ellipsometry are defined from the ratio of the Fresnel reflection coefficients rp and rs as follows: rp = tan ΨeiΔ rs

2. EXPERIMENTAL SECTION 2.1. Sample preparation. Glass and silicon substrates (25 mm2 and 10 mm2, respectively) were cleaned in ultrasonic baths by using acetone, methanol, and isopropanol and then dried with nitrogen gas. The PTB7 and PTB7-Th copolymers were purchased from 1-Material. These copolymers were dissolved in chlorobenzene with a concentration of 20 mg/ mL. The solutions were then stirred for 2 h at 50 °C. The films were prepared in a glovebox by spin coating the solution with a speed of 1000 rpm and an acceleration of 150 rpm/s for 50 s. The film thicknesses were determined using a mechanical profilometer, giving 110 nm (on silicon) and 90 nm (on glass) for PTB7 films and 130 nm (on silicon) and 140 nm (on glass) for the PTB7-Th films. For the solution absorbance and photoluminescence characterization, three diluted solutions with concentrations of 0.01, 0.003, and 0.001 mg/mL were prepared and then introduced on 1 cm path length quartz cells. 2.2. Ellipsometric, spectrophotometric, and photoluminescence measurements. For thin films characterization, variable angle spectroscopic ellipsometry (VASE) measurements are performed with a Semilab rotating compensator ellipsometer using a microspot which focuses the beam on a very small diameter of the sample (∼100 μm). Data are measured from 1.3 to 2.5 eV at incident angles of 55°, 60°, 65°, 70°, and 75°. For data analysis, the films are considered as homogeneous and isotropic. The optical modeling is developed using SEA (WinElli3) software. This software uses the Levenberg−Marquardt algorithm to minimize the mean squared error (MSE) between the measured and calculated ellipsometric data Ψ and Δ. Absorbance spectra are carried out using a PerkinElmer spectrophotometer Lambda 950. The photoluminescence (PL) spectra are performed using a JASCO FP8600 spectrofluorometer. The thin films are excited at 2 eV and the solutions at 2.13 eV.

where tan(Ψ) = |rp|/|rs| represents the relative amplitude attenuation and Δ = arg(rp) − arg(rs) is the phase shift between the p- and s-polarizations. In order to determine the dielectric function of the films, a dispersion model composed of a sum of Gaussian functions is used. It is widely reported that strong electron−phonon coupling in π-conjugated molecules is better described by a Gaussian line shape function.17,18 The model parameters were varied to obtain the best fit between experimental and calculated data. The imaginary parts of the dielectric functions obtained for the PTB7 and the PTB7-Th by SE are presented in Figure 2. All the Gaussian functions used to build the dispersion models are also shown in Figure 2. The imaginary part of the dielectric function, for both copolymers, shows fine vibronic structure. For comparison, we focus only on the first two Gaussian oscillators, whose energies correspond to the transitions between the ground state and the first two vibronic levels of the excited state denoted E0−0 and E0−1. The intensities of these transitions are denoted A0−0 and A0−1. For the PTB7 thin film, the first peak is located at 1.82 eV and the second at 1.98 eV in accordance with literature data extracted from absorbance measurements.19 Concerning the PTB7-Th thin film, from our knowledge, the two vibronic levels are identified for the first time at 1.75 eV for the first peak and at 1.90 eV for the second one. Beyond the energy peak position, the relative intensities of the A0−0 and A0−1 transitions are different for the two copolymers. The ratio A0−0/A0−1 is 1.33 for the PTB7 compared to 1.27 for the PTB7-Th. These results will be used to interpret the absorbance measurements of section 3.2.1. We can also observe that the absorption bands of PTB7-Th are red-shifted compared to that of PTB7, resulting in an optical band gap of the PTB7-Th (Eg = 1.58 eV), lower by 0.06 eV than that of the PTB7 (Eg = 1.64 eV). This red shift indicates B


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Figure 3. UV−vis absorbance spectra of (a) PTB7 and (b) PTB7-Th in chlorobenzene solution with different solution concentrations.

Figure 4. UV−vis temperature dependent absorbance spectra of (a) PTB7 and (b) PTB7-Th in chlorobenzene solution.

and are normalized to the A0−1 transition peak intensity in order to highlight the relative change with respect to the A0−0 transition. For better visual comparison, the absorption onset for the thin film has been subtracted prior to normalizing to the A0−1 peak. Interestingly, the absorbance spectra of these two copolymers in chlorobenzene solution show fine vibronic structures with resolved peaks A0−0 and A0−1, similar to the absorbance spectrum of the film and without spectral shift between solution and solid state absorbance spectra. These results show that, already at RT, the copolymer chains of PTB7 and of PTB7-th aggregate strongly in solutions as well as in films.22 This behavior was already observed for other D−A copolymers, but it is not observed for the P3HT solutions where the absorbance spectrum of the P3HT solution is blue-shifted compared to the film absorbance spectrum.12−14,23 This is due to the high planarity and rigidity of the PTB7 and PTB7-Th backbones compared to the P3HT properties, and that means that even in solution the chains do not lose their conjugation length and their π−π stacking. By decreasing the solution concentrations from 0.01 to 0.001 mg/mL, the ratio A0−0/A0−1 decreases. We assume that this behavior is due to the size reduction of aggregates in the solutions, and both copolymers behave in the same way as a function of concentration. Then, the solutions were heated from 25 to 80 °C in order to promote the separation of the copolymer chains and hence decrease the size of the aggregates. We can observe in Figure 4 that the nominal A0−0 peak has a different temperature dependence from the A0−1 peak. In the case of PTB7 (Figure 4(a)), the intensity of the A0−0 peak decreases continuously

that interchain and intrachain interactions are relatively weak in PTB7 as compared to PTB7-Th. This may be due to the strong steric hindrance induced by the bulky side chains in the PTB7. By replacing the oxygen atom by a thiophene unit in the BDT side chains, the coplanarity is improved, and the strong steric hindrance induced by the ethylhexyl side chains is reduced and allows an enhancement of the intermolecular π−π interaction.11 3.2. Absorbance and photoluminescence spectra of the PTB7 and PTB7-Th solutions as a function of concentration and temperature. In order to isolate interchain interactions from intrachain interactions, we performed absorbance and photoluminescence measurements. As reported by Spano et al., the physical origins of absorption and of photoluminescence spectra are different: absorption is more related to the interchain interactions while the photoluminescence is sensitive to the intrachain interactions. They showed that the interchain coupling can be estimated using the absorption ration while the photoluminescence ratio is sensitive to intrachain coupling.20,21 In our study, solutions of PTB7 and PTB7-Th in chlorobenzene have been investigated because polymers in solution have more degrees of freedom to change their molecular conformation and their state of aggregation. In the next section, we report the absorbance and photoluminescence spectra of the two copolymers in solution as a function of concentration and temperature and we compare them to film measurements. 3.2.1. Solution absorbance spectra. Absorbance spectra of PTB7 and PTB7-Th in the solid and liquid phase are shown in Figure 3(a and b) at room temperature (RT). The spectra are recorded at different concentrations in chlorobenzene solution C


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Figure 5. Concentration dependent photoluminescence spectra of (a) PTB7 and (b) PTB7-Th in chlorobenzene solution.

Figure 6. Temperature dependent photoluminescence spectra of (a) PTB7 and (b) PTB7-Th in chlorobenzene solution (0.001 mg/mL).

to the interchain π−π* transition due to the π−π stacking of the polymer chains.24,25 Thus, the change of the size of the aggregates is related to the variation of the peak intensity ratio RAbs = A0−0 /A0−1 and of the spectral blue shift. Increasing temperature and decreasing concentration for both PTB7 and PTB7-Th lead to a decrease of A0−0. This means that A0−0 is sensitive to the size of the aggregates. The aggregates formation is reversible when the solution is cooled down from 80 °C to RT. 3.2.2. Solution photoluminescence spectra. In addition to the previous results, photoluminescence (PL) measurements have been carried out on PTB7 and PTB7-th films and solutions. Photoluminescence is the reverse physical process of absorption and gives information on the nature of the excited states in polymers. After absorption of a photon with energy equal to or greater than the energy gap of the organic material, an electron is promoted from the ground state S0 to the vibrational levels of the first excited electronic state S1. This electron undergoes radiationless relaxations between the different vibrational level and reaches the lowest vibrational level in the first excited state S1, satisfying the law of Kasha.26 Then, this electron returns to the vibrational level of the ground state S0 by emitting a photon giving rise to photoluminescence.17 The normalized PL spectra of PTB7 and PTB7-Th thin films are shown in Figure 5 and present one dominant peak at 1.61 and 1.63 eV, respectively. In comparison with the PTB7 (Figure 5(a)), the PL spectrum of the PTB7-Th (Figure 5(b)) is red-shifted, indicating an enhancement of the conjugation length which leads to an increase of the π−π* interactions. In

with temperature, but it remains dominant over the peak A0−1, whose intensity shows no significant variation. On the other hand, for PTB7-Th spectra, the intensity of the A0−0 peak decreases also continuously with temperature, but from 70 °C it becomes less intense than the A0−1 peak, whose intensity remains the same. In order to understand such a difference, we need to have a look at the dielectric function models determined by spectroscopic ellipsometry. As shown in Figure 2(a), the intensity of the first Gaussian oscillator, A0−0 for the PTB7, is greater than that of the second one, A0−1. On the contrary, for the PTB7-Th (Figure 2(b)), the intensity of the first Gaussian oscillator, A0−0, is smaller than that of the second one, A0−1. This difference can explain why the A0−0 peak becomes smaller than the A0−1 peak for higher temperatures in the absorbance spectrum of the PTB7-Th (Figure 4(b)). For both copolymers, the intensity variations with the temperature are accompanied by a spectral blue shift, which is more pronounced in the case of the PTB7-Th than in the case of the PTB7. These results show that the PTB7 and PTB7-Th chains are strongly aggregated in the RT solution, showing a high intensity A0−0 peak. By heating the solution, interchain interactions decrease and copolymer chains are allowed to separate. During this morphology modification, the copolymer chains lose their planarity, the conjugation length decreases, and a spectral blue shift appears. As a consequence of interchain and intrachain disorder, the aggregates are broken and both the attenuation of the A0−0 peak intensity and a spectral blue shift are observed. As seen for other D−A copolymers (e.g., PSBTBT: poly dithienosilole-benzothiadiazole), the A0−0 peak should be more sensitive D


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of chains coplanarity. In order to go further in the comparison and the understanding, we have presented a detailed analysis of absorbance and PL spectra of the two copolymers in chlorobenzene solutions. We have shown that it is possible to resolve the solution morphology and its changes by following the evolution of the intensity of the 0−0 and 0−1 peaks. Concentration and temperature dependent absorbance and PL spectra show that diluting and heating the chlorobenzene solutions of the aggregates can break the aggregates. We observe that the variations of the PL peak intensity and the spectral shift are more pronounced in the PTB7-Th than in the PTB7. Moreover, the PL spectra analysis shows that the emission of the film and of the concentrated solution arises from H-aggregates and that these H-aggregates are more easily broken in the PTB7-Th solution than in the PTB7 one. This study contributes to the understanding of the photophysics of two well-known copolymers in organic photovoltaics and permits correlation of the morphological change in the optical properties.

concentrated chlorobenzene solution, the PL spectrum of the PTB7 shows one dominant peak at 1.64 eV, as in the film but with a slight blue shift. The PL spectrum of the PTB7-Th solution presents also a peak at 1.64 eV with a slight blue shift, and a second peak at 1.86 eV appears (Figure 5(b)). When the solutions are diluted, a slight blue shift of the dominant peak is observed for both copolymer solutions compared to the film and concentrated solution spectra. For the PTB7, a weak high energy band appears in dilute solution while, in the case of the PTB7-Th solution, the intensity of the high energy peak PL0−0 relative to the second peak PL0−1 increases with decreasing the concentration. This indicates that the PL at high concentration and in thin films originates from weakly interacting H-type aggregates where the copolymer chains are more planar and more elongated.16 The 0−0 transition is theoretically strictly forbidden in H-aggregates, but disorder breaks the symmetry allowing 0−0 photoluminescence in diluted solutions.27 Indeed, aggregates act as shallow traps for excitons, serving as nonradiative recombination centers, but with increasing disorder, the interactions between chains become weak and the PL spectrum approaches a broad, isolated-molecule PL spectrum.28 At the same time, the ratio RPL = PL0−0/PL0−1 increases with increasing disorder, as mentioned by Yamagata et al. and as observed in our study when solutions are more diluted.29 In order to confirm this assumption, temperature dependent PL spectra are performed on the same solutions. Figure 6 shows the photoluminescence spectra of PTB7 and PTB7-Th for the most diluted solution during the heating process (from 25 to 80 °C). For temperatures ranging between 25 and 80 °C, we observe an increase of photoluminescence for the two copolymers. The increase of the photoluminescence is accompanied by a slight increase of the PL0−0 peak, which remains weak relative to the PL0−1 peak for the PTB7 solution (Figure 6(a)). In the case of the PTB7-Th (Figure 6(b)), the increase of the PL0−0 peak with temperature is more important and the ratio RPL= PL0−0/ PL0−1 goes from 0.98 to 1.44 when heating the PTB7-Th solution from 25 to 80 °C. The aggregates are broken, showing a dominant PL0−0 peak whose intensity increases with increasing temperature. Thus, this PL temperature dependent behavior shows the transition from the aggregated phase (RPL < 1) to the coiledamorphous phase (RPL > 1) and confirms that the aggregates behave like H-type aggregates. Finally, the relative ratio of the first and second peaks in the PL spectra of PTB7-Th solutions depends not only on the concentration but also on the heating temperature whereas the PL spectra of the PTB7 show a weak PL0−0 peak which increases slightly with concentration and temperature. These two different behaviors confirm that aggregates are more easily broken in the PTB7-Th chlorobenzene solution than in the PTB7 chlorobenzene solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. 06.15.31.89.43. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the French Fond Unique Intermisteriel (FUI) under the project “SFUMATO” (Grant number: F1110019 V/201308815) for their financial support. The authors acknowledge with gratitude Dr. Silvio Balaban and Florian-Xuan Dang (ISM2, Aix Marseille University) for help with photoluminescence measurements.

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ABBREVIATIONS PCE, power conversion efficiencies; PC70BM, [6,6]-phenyl C71-butyric acid methyl ester; PTB7, poly[(ethylhexyl-oxy)benzodithiophene-(ethylhexyl)-thienothiophene]; PTB7-Th, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)thienothiophene]; TT, thienothiophene; BDT, benzodithiophene; P3HT, poly 3-hexylthiophene; PPV, poly-phenylenevinylene; VASE, variable angles spectroscopic ellipsometry; PSBTBT, poly dithieno-silole-benzothiadiazole

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REFERENCES

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4. CONCLUSION A comparative study of the optical properties and the molecular aggregation of the two copolymers PTB7 and PTB7-Th is presented. By spectroscopic ellipsometry, it has been shown that the differences of the chemical structure between these two copolymers induces a change in the morphology and the optical properties. By replacing the oxygen atom in the BDT side chains of the PTB7 by a thiophene unit, the optical transitions in the imaginary part of the dielectric function of PTB7-Th are red-shifted, resulting in a lower bad gap due to an enhancement E


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Study of Optical Properties and Molecular Aggregation of Conjugated

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