Exciton Structure and Dynamics in Solution Aggregates of a Low

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Exciton Structure and Dynamics in Solution Aggregates of a Low-bandgap Copolymer Zhi Guo, Doyun Lee, Haifeng Gao, and Libai Huang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp511949e • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Exciton Structure and Dynamics in Solution Aggregates of a Low-bandgap Copolymer Zhi Guo1,2*, Doyun Lee3, Haifeng Gao3, and Libai Huang1,4* 1

Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA 3 Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA 4 Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA 2

*

Corresponding authors; Emails: [email protected], [email protected]

ACS Paragon Plus Environment

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Abstract In this work, we elucidate exciton structure, dynamics, and charge generation in the solution phase aggregates of a low-bandgap donor-acceptor polymer, poly(4,8-bis-alkyloxybenzo[1,2b:4,5-b′]dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene-2carboxylate)-2,6-diyl(PBDTTT). The polymer aggregates in the solution phase serve as precursors for thin film morphologies. We have identified intrachain and interchain exciton transitions and resolved their relaxation pathways by comparing excitons in solution aggregates to those in isolated polymer chains. Hot intrachain excitons have led to the generation of stabilized interchain charge-separated states in solution aggregates, which could serve as the intermediate state to the hot exciton charge separation in bulk heterojunctions (BHJs). These results have important implications for controlling morphology dependent exciton dynamics in solution processed BHJs.

Keywords Conjugated copolymer, exciton dynamics, charge transfer, charge separation, interchain interactions, organic photovoltaics, ultrafast spectroscopy


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1. Introduction Morphology, including the packing of the molecules and phase segregation of different compositions, plays a critical role in charge separation and transport processes in solutionprocessed polymer-based bulk heterojunctions (BHJs).1-9 Aggregations of the conjugated polymers in solution phase serve as important precursors to the morphology of the thin films as revealed by recent Gazing incident X-Ray scattering (GIXS) studies10-12. Controlling solution phase aggregation precursors provides a means to utilize solvent engineering to modulate polymer aggregate size in thin films and to improve BHJ morphology10-11, 13-18. For instance, solvent polarity has a profound impact on polymer aggregation by its dispersion interaction with the polymer side chains for semiconducting conjugated polymers19-20. Very few studies have addressed the detailed electronic structure of the precursor solution aggregates, despite the recent X-Ray scattering studies shedding light on the role of precursor polymer aggregates in determining phase segregation in the solid state10-12. Understanding of the structure and dynamics of excitons in the solution aggregates is important for improving solar cell efficiency through solvent engineering. For instance, it has been suggested that polymer donor’s intrinsic charge-transfer properties are closely related to the BHJ device performance for the highly efficient donor-acceptor copolymers 21-22. Therefore, it is important to understand the charge transfer properties in the solution phase aggregates and compare them with those in the thin films. However, little is known for the exciton structure and dynamics of the solution aggregates for low-bandgap donor-acceptor copolymers. In solution aggregates and in polymer thin films, there are two types of excitons: along the chain (intrachain) or between chains (interchain). The intrachain excitons are typical Frenkel excitons, in other words, the collective excited states are linear combinations of electronic


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excitations of each chromophore within the conjugated unit.23-28 The interchain excitons are interchain excited state species where electron density is shared between chromophores on neighboring polymer chains. When different polymer chains in adjacency are packed in certain regular order, their ground state electron density overlap become strongly correlated and can not be physically treated individually. Under such circumstance, we name these structures "aggregates".24, 26, 29 In addition to neutral electron delocalization between chains, charge transfer can occur upon excitation of strongly interacting chromophores, leading to charge-separated interchain species. In this work, we elucidate the exciton structure and dynamics of solution aggregates of a low-bandgap donor-acceptor copolymer using steady state and ultrafast time-resolved spectroscopy. Poly(4,8-bis-alkyloxybenzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-(alkylthieno[3,4b]thiophene-2carboxylate)-2,6-diyl (PBDTTT, structure shown in Figure 1) is known as a family of low-bandgap conjugated copolymer that enables solar cells with a high power conversion efficiency (~8%)

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. Intrachain and interchain exciton transitions for the solution PBDTTT

aggregates are identified and their relaxation pathways are elucidated by comparing excitons in aggregates to those in isolated polymer chains. Hot intrachain excitons have been found to generate stabilized interchain charge-separated states, which could serve as the intermediate state to the hot exciton charge separation in BHJs. 2. Materials and Methods Sample preparation PBDTTT polymers are synthesized based on modified literature procedure31. The benzodithiophene and thienothiophene groups function as the built-in electron donor and acceptor, respectively. The PBDTTT polymers are decorated with two alkyl groups on the three substitution sites (Figure 1). The alkyl side chain groups on the R1 and R2 positions


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include two groups: 1) ethyhexyl groups (branched structure, denoted as E) that provides solubility and processability for the polymers and 2) dodecyl groups (linear structure, denoted as D) that can enable high crystallinity.

Two different substituted polymers on a PBDTTT

backbone are used in this work: PBDTTT-EE and PBDTTT-DD, in which the first italic letter represents the species of the two identical substituted groups on the BDT unit and the second letter represents the substituted groups on the TT unit as shown in Figure 1. Details on the polymer synthesis can be found in our prior work.32.

For steady state absorption and

photoluminescence measurements, polymers are dissolved in chlorobenzene or chloroform to a concentration of 0.05 optical density (OD) and 0.5 OD at 670 nm absorption peak in a 1 cm path length cuvette. For transient absorption studies, the polymer concentration in the solutions are about 0.5~0.6 OD at the 670 nm absorption peak in a 2 mm path length cuvette. All the sample preparation and spectroscopy characterization were performed under a dry nitrogen atmosphere to ensure no undesired degradations caused by oxygen or moisture. Spectroscopic measurements Steady state absorption spectroscopy measurements were performed with a UV/vis spectrometer (Cary 50, Varian Inc.) and the steady state photoluminescence spectra were measured with a fluorometer (Fluorolog, HORIBA inc.). The femtosecond pump-probe spectrometer used to measure the transient absorption changes has been described before32. Briefly, the laser pulses were generated at a repetition rate of 1 kHz with 150 fs duration at 775 nm using a regenerative amplifier system based on a Ti: Sapphire laser (CPA-2010, Clark-MXR,). The tunable pump beam was generated with an optical parametric amplifier system (TOPAS, Light Conversion Ltd.) by feeding in 80% of the amplifier output power (~700 µJ/pulse). The probe beam, a white light continuum, was generated by focusing the 775 nm beam onto a sapphire plate (2.5 mm thick, used for the visible spectral


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region) or a sapphire rod (12 mm thick, used for the infrared spectral region). The probe light was collected and coupled to a spectrometer (HELIOS, Ultrafast System). The pulse duration before the sample is about 160 fs. To estimate of the pump power being used in the transient absorption measurements, the pump beam was collimated into an un-focused beam, and the size of the excitation area was controlled by an iris with pre-defined area. The pump power used for the experiments ranges from 6~20 µJ/cm2 depending on the pumping wavelengths. The actual absorbing photon density can be estimated from the ratio between the strength of ground state bleaching signal to that of the optical absorbance. In our experiments, the maxima of transient absorption bleaching signals were kept below 1/50 of the total sample absorbance, which implies that fewer than 1 out 50 chromophores was excited by a laser pulse. Considering that in the solution phase, the chromophore density in unit volume is several magnitude of order lower compared with that in the film phase, and the solution is constantly stirred by a magnetic bar that effectively avoids the re-excitation pulse, the high order many-body, exciton-exciton annihilation effect is negligible. The global analysis of transient absorption spectra was analyzed with theGlotaran package.33

Figure 1. Molecular structures of poly(4,8-bis-alkyloxybenzo[1,2-b:4,5-b′]dithiophene-2,6-diylalt-(alkylthieno[3,4-b]thiophene-2-carboxylate)-2,6-diyl (PBDTTT) polymers. The BDT unit is connected to two symmetric R1 substitution sites, and the TT unit is connected to the R2 substitution site. Two polymers, PBDTTT-EE and PBDTTT-DD, are composed of different alkyl side chain substitution combinations using either 2-ethylhexyl or n-dodecyl.


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3. Results and discussion Optical signatures of excitons in solution phase polymer aggregates In order to identify the optical signatures of intrachain and interchain excitons in solution aggregations, ground state absorption, PL and PL excitation (PLE) measurements of PBDTTT-EE polymer dissolved in chloroform and chlorobenzene were performed at the low (OD at 670 nm = 0.05) and high concentration (OD at 670 nm = 0.5) limits (Figure 2). The spectra for PBDTTT-DD are shown in the Figure S1 in the supporting information (SI).

Figure 2. Left panels: UV/vis absorption and PL spectra of PBDTTT-EE dissolved in chlorobenzene (a) and chloroform (b) solution at different concentration (0.05 OD and 0.5 OD at 670 nm in a 1 cm cuvette). Right panels: PLE spectra of chlorobenzene (c) and chloroform (d) solution for 0.5 OD at 670 nm concentration. The PLE spectra are obtained for three PL wavelengths: 650 nm, 700 nm and 770 nm. The intensity axes of the right panel are in log scales.

Based on solution phase X-Ray scattering, polymer chains in chloroform solution adopt isolated amorphous forms. This is because chloroform is a non-aromatic solution and the solvent molecules tend to drive the polymer chains to collapse into coils that isolate from each other to


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avoid the conjugated backbone from contacting the chloroform molecules. In contrast, polymer chains in chlorobenzene solution prefer straight conformation that facilitates the aggregation10. The absorption spectra of the diluted PBDTTT-EE/chlorobenzene solution show two distinct peaks at 620 nm and 680 nm. The 620 nm and 680 nm absorbing peaks are assigned as the (0-1) and (0-0) vibronic peaks of the lowest S0→S1 transition, which is similar to a former report on PTB polymer22, 34. Similar features has also been widely documented in conjugated polymers34-38. Most interestingly, the PL emission at ~ 650 nm is stronger than the emission of ~ 690

nm

from

the

lowest

transition

PBDTTT/chlorobenzene solution.

in

PBDTTT/chloroform

and

highly

diluted

The 690 nm emission can either be identified using a

selective excitation at lower energy (>630 nm) or through de-convoluting emission profile of the low concentration sample in Figures 2a and 2c with two Gaussians. The PL excitation spectra (Figures 2c and 2d) indicate that the high energy PL peak (~ 650 nm) is only emitted from absorbing wavelength below 600 nm. The emission of ~ 690 nm becomes much more prominent in PBDTTT/chlorobenzene solution when aggregation occurs and is emitted mostly from 680 nm transition. The most likely scenario for the weak emission from the lowest 0-0 transition at ~ 690 nm and the appeared negative stokes shift in highly diluted PBDTTT/chlorobenzene solution and in PBDTTT/chloroform solution is that the radiative rate from the lowest transition (0-0) is slow compared to the that from the higher transition (0-1) probably due to certain selection rule enforced by the structural symmetry. The conformational changes and interchain interaction can relax the molecular symmetry and the radiative rate from (0-0) lowest transition increase leading to higher emission intensity from the lowest transition at higher polymer concentration in chlorobenzene solution.


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In the very diluted limit (0.05 OD), the optical spectra of the polymer in the chloroform and chlorobenzene solutions are identical reflecting the electronic structures of isolated polymer chain conformation. At this limit, the intrachain exciton PL emission is centered at ~ 650 nm. At the higher concentration limit, the PL spectra in chloroform and chlorobenzene solution deviate. For chloroform solution, PL spectra remain unchanged up to concentration as high as 0.5 OD (Figure 2b). The PLE spectra of polymer chloroform solution (Figure 2d) at different emission wavelengths revealed identical excitation spectral feature, indicating emission of at different wavelengths originated from the same transition, suggesting that interchain interaction is not sufficiently strong to influence the ensemble distribution of chain conformation.

On the

contrary, an increase in intensity of a second red-shifted PL peak at ~ 730 nm was observed as concentration increased when chlorobenzene was used as the solvent (Figure 2a). The red-shifted PL peak is a result of interchain interaction that either leads to a redistribution of polymer conformations or a direct formation of lower energy interchain exciton, as shown in the PLE spectra (Figure 2c, s1).

State-specific exciton dynamics and excited state absorption (ESA) We performed transient absorption spectroscopy to elucidate the excited state dynamics of the aggregates formed in the solution phase at high concentration (OD at 670 nm = 0.5, 2 mm cuvette).

At such

concentration, there are solution phase aggregations in the chlorobenzene but not the chloroform solution as discussed in the steady-state spectroscopy section.

We compared the transient

absorption spectra for the excitation of the two excitonic absorption bands at 580 nm and 670 nm as identified in the steady-state measurements for PBDTTT-EE in chloroform and chlorobenzene solutions in Figures 3 and 4, respectively. For the chloroform polymer solution, there is a prominent excited state absorption (ESA) peak at 800-900 nm, which was formerly observed and


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assigned to intramolecular “pseudo- charge transfer” state by Rolczynski et al.21 As shown in Figure 3, no apparent excitation wavelength dependence is observed in the chloroform solution, which is consistent with the PL and PLE spectra that the intrachain exciton is the only dominate specie in the chloroform solution. Therefore we assigned the dominant 800-900 nm exciton absorption to an intrachain charge transfer state formed in disordered chain configurations. In direct contrast, the ESA of PBDTTT-EE in chlorobenzene solution strongly depends on the excitation wavelength as shown in Figure 4. Higher excitation energy at 580 nm creates a prominent ESA band at 900 nm at early pump probe delay time similar to that of chloroform solution, while the lower energy excitation at 670 nm results in an ESA peak at 1300 nm. Both of the 900 nm and 1300 nm ESA bands are created soon after excitation (within our instrument response, ~200 fs) and we assign them to the intrachainn and interchain excitons form in the chlorobenzene solution, respectively. Because 1300-1400 nm band is much more prominent with the 670 nm excitation, we assign it to the more delocalized interchain exciton formed in solution phase aggregates. The exciton decays in chloroform and chlorobenzene solutions probed at the exciton ESA bans are compared in Figure 5. The exciton decay kinetics in chloroform solution is not strongly dependent on the pump wavelength, which again is consistent with that both excitation wavelengths create the same intrachain excitonic state. The fastest excited state decay in chloroform solution is about 2 ps, which is considerably faster than that in chlorobenzene solution (~ 10 ps). The faster excited state quenching in chloroform likely reflects more rapid solvent molecule facilitated internal conversion process39-40. The slower exciton decay in chlorobenzene solution is likely due to difference in solvent molecule relaxation (vibration & rotation). In chlorobenzene solution, two exciton ESA bands (~ 900 nm and ~ 1300 nm) have


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different decay life times again reflecting complex relaxation processes. More details on the relaxation pathways from the two different exciton transitions in the aggregates in the chlorobenzene solution will be discussed in the next section.

Figure 3. Transient absorption spectra obtained in PBDTTT-EE/cholroform solution with two different excitation wavelengths: 580 nm (a) and 670 nm (b).

Figure 4. Transient absorption spectra obtained in PBDTTT-EE/chlorobenzene solution with two different excitation wavelengths: 580 nm (a) and 670 nm (b).


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Figure 5. Comparing the exciton ESA decay of PBDTTT-EE polymer dissolved in chlorobenzene (CB) and chloroform (CF) solutions by monitoring intrachain and interchain excitons at different probe wavelength. Intrachain and interchain excitons and charge-separated state in solution aggregates To better resolve the lifetimes and excited species of the solution phase aggregates we perform pump wavelength dependent measurements and global analysis. Figure 6 shows the transient absorption spectra of PBDTTT-EE in the chlorobenzene solution at high concentration (OD at 670 nm = 0.5, 2 mm cuvette) with 387 nm excitation from the visible to IR spectral range. Global analysis (Figure 6b) of the transient absorption spectra in Figure 6a to decompose the spectra into principal decay components and to resolve the time constants associated with each decay spectra (also known as decay associated spectra, DAS). More details on global analysis are presented in the SI. The DAS (Figure 6b) shows clearly that there are multiple excited state species associated with distinct lifetimes. The fastest DAS with a lifetime of 2.7 ps has bleaching of both ground state features and ESA bands at 800- 900 nm and 1300- 1400 nm. This indicates that both intrachain and interchain excitons have a 2.7 ps decay channel.

Notably, the

intermediate DAS with an 100 ps lifetime contains only the lower energy ground state bleach at


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700 nm corresponding to the interchain excitons and a blue-shifted excited state absorption centered at ~1300 nm. This observation confirms the assignment of the 1300-1400 nm absorption band to the interchain exciton absorption as discussed above.21, 32 The slowest decay DAS with a lifetime of 1.3 ns is distinctly different from those at early times (2.7 ps and 100 ps) with excited absorption spectral feature (~1150 nm) and the higher energy bleaching feature at ~ 600 nm. The 1130-1150 nm band absorption is consistent with the polymer cation absorption formerly using steady state electrochemistry method as well as the transient absorption studies in the BHJ film.21, 32, 41 Based on the spectral feature evolution in the infrared region, we assigned this long-lived ESA at ~ 1150 nm to charge-separated state (here we defined the charge separated state as electron-hole pair of large separation distance and characterized by nanosecond life time).

Most interestingly, the long-live charge-separate state at ~1150 nm is associated with

only the higher energy ground state bleaching feature at ~ 600 nm corresponding to the intrachain excitons. This observation suggests that generation of the long-lived charge-separated state in the solution aggregates resulted only from the intrachain exciton states. To further validate the formation of the charge-separated state from intrachain excitons, we carried out excitation wavelength dependent experiments. Based on the transitions found in the ground state absorption spectra and the PLE spectra, four different pump wavelengths, 387 nm, 580 nm, 670 nm and 730 nm, were used to monitor the generation of long-lived charge separated state. The ground state bleaching spectra at 500 ps for PBDTTT-EE/chlorobenzene induced by different pump wavelengths are shown in Figure 7a. The ground state bleaching is mainly composed of two spectral components: one is centered at 600 nm (intrachain exciton) and the other at 700 nm (interchain exciton). Lower energy pump at 730 nm results in long-lived excitation only at the 670-700 nm, while higher energy at pump at 387 nm leads to selectively


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bleaching of the transition ~ 600 nm. 580 nm and 670 nm excitations result in bleaching that consists of both exciton states. Figure 7b shows the corresponding IR spectra for 387 nm and 730 nm excitation at 1.5 ns. It demonstrates that long-lived charge separated state feature at 1150 nm can result from 387 nm excitation but not 730 nm. The excitation wavelength dependent measurement further confirms that the long-lived charge separated state can only be generated from the intrachain exciton state.

Figure 6. (a) Transient absorption spectra of PBDTTT-EE in chlorobenzene solution at different delay times. The excitation wavelength was 387 nm. (b) Decay associated spectra (DAS) obtained from global analysis on the spectra shown in part (a).


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Figure 7. (a) Ground state bleaching spectra of PBDTTT-EE/ chlorobenzene solution at 500 ps delay. Samples were pumped at four wavelengths (387 nm, 580 nm, 670 nm and 730 nm). (b) transient absorption spectra at 1.5 ns delay in the IR spectral region for 387 nm and 730 nm pump excitation.

We compare the yield of generating the long-lived charge-separated state with different pump wavelengths by comparing the ground state bleaching signal at 580 nm at 1.5 ns to the maximum bleaching signal at 0 ps and the results are shown in Figure 8. The higher pump energy leads to higher yield of charge-separated state, with 387 nm excitation resulting in a yield of ~ 30% (Figure 8a). For 730 nm excitation, this yield is dropped to ~ 1% (Figure 8a). This observation can be explained by hot exciton charge generation, where the vibrational heat bath serves as the main source of the energy that allows charge carriers to escape from the potential well formed by Coulomb attraction.42-43 Similar phenomenon has been observed in thin films as well, where above-gap excitation allow for a fast generation of charge separated state through these vibrationally hot excitons.44-46 The stabilized interchain charge-separated states in these aggregates observed here could serve as the intermediate state to the hot exciton charge separation in BHJs.


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Figure 8. Normalized and overlaid kinetic traces probed in PBDTTT-EE (a) and PBDTTT-DD (b) at 580 nm for different pump wavelengths. The absorbance units are in log scales.

Most interestingly, the long-lived charge-separated state is not observed in the chloroform solution where no solution aggregates are formed (Figure 3 and Figure S2). This suggests that the long-live charge-separated state requires electron and hole to reside on different polymer chains. This is consistent with the fact that the charge separation yield is increased with the linear side chain (PBDTTT-DD). Because linear side chains lead to better interchain packing32 and stabilized the charge-separated state. As a result, significantly higher chargeseparated state population of can be produced with pump wavelengths of 580 nm and 670 nm for PBDTTT-DD than for PBDTTT-EE (Figure 8).


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4. Conclusions In this article, we have employed steady-state and ultrafast time-resolved optical spectroscopy to elucidate intrachain and interchain exciton structure and dynamics in solution aggregates of a low-band gap donor-acceptor copolymer, PBDTTT. These polymer aggregates in solution phase are precursors for thin film morphologies. At high concentration, solution aggregations are formed in polymer chlorobenzene solution leading to mixed intrachain and interchain exciton transitions. We have elucidated the relaxation pathways of excitons in solution aggregates by comparing excitons in polymer/chlorobenzene solution to those in polymer/chloroform solution where no aggregation is formed. Hot intrachain excitons have been found to generate stabilized interchain charge-separated states, which could serve as the intermediate state to the hot exciton charge separation in BHJs.

Acknowledgement: Z. Guo and D. Lee acknowledge support from the Sustainable Energy Initiative of the University of Notre Dame. The part of work carried out at the Radiation Laboratory was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the US Department of Energy through grant DE-FC0204ER15533.

Supporting Information Available Steady-state optical spectroscopy of PBDTTT-DD, transient absorption and decay associated spectra of PBDTTT-EE/chloroform solution, and details on global analysis. This information is available free of charge via the internet at http://pubs.acs.org.


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Exciton Structure and Dynamics in Solution Aggregates of a Low

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