The Role of Disorder in the Extent of Interchain Delocalization and

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Spectroscopy and Photochemistry; General Theory

The Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains Kyu Hyung Park, Sung Y. Son, Jun Oh Kim, Gyeongho Kang, Taiho Park, and Dongho Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01050 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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The Journal of Physical Chemistry Letters

The Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains Kyu Hyung Park,† Sung Yun Son,‡ Jun Oh Kim,† Gyeongho Kang,‡ Taiho Park,*,‡ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 03722, Korea

‡

Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Nam-gu, Pohang, Gyeongbuk 790-780, Korea

ACS Paragon Plus Environment

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ABSTRACT

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % of substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close packing structure in the absence of hexyl substituents.

TABLE OF CONTENTS GRAPHIC SRP33 POLARON

SP3HT

RP33 Barrierless CS

x:y = 67:33

P3HT Thermally activated CS

x:y = 100:0


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In the history of organic optoelectronic devices, polythiophenes, in particular, poly(3hexylthiophene) (P3HT), has served as an archetype of -conjugated polymers (CPs) in achieving high charge carrier mobility and charge photogeneration efficiency,1-5 while maintaining the general advantages of CPs, such as chemical tunability, facile fabrication process, and flexibility of a device.6-10 In the solid state, P3HT chains are self-assembled by - interactions between conjugated backbones and dispersion interaction between side-chains to form crystalline phase, which is surrounded by nonaggregated amorphous phase.1,3,11 Due to increased interchain coupling, the energetics of charge carriers and their precursor states are controlled sensitively by film processing conditions, such as chemical doping,12 solvent and postthermal treatment,13 and variation of donor-acceptor composition,14 which modulates the interchain distance, size of the aggregates, overall content of aggregated domains, and their relative orientations to the substrate. While a wealth of studies have focused on the effects of such extrinsic factors, control of intrinsic chemical properties and subsequent changes in the chain structures and film morphologies have also been conducted in parallel. For example, it was shown that the charge mobility and power conversion efficiency (PCE) have strong correlation with the molecular weight and long-range ordering of aggregated domains.15 The device performance is also found to be enhanced by increasing regioregularity of hexyl substituents, but is reduced upon incorporation of bulky substituents, which deteriorates long-range ordering of aggregated domains.5,16 As the mainstream of research emphasizes the importance of high crystallinity for efficient charge photogeneration and transport, polythiophenes are modified to reduce structural heterogeneity and develop larger aggregated domains. However, recent reports have demonstrated that disordered CP films showing only short-range ordering or amorphous phase


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can work as efficiently as or even outperform those with high crystallinity.17-24 In this perspective, there have been a few attempts to investigate the effects of disorder on the polythiophene-based devices. Ohkita et al. tested a number of polythiophene derivatives composed of various repeating units with different linkers and substituent positions to show that even partially crystalline polythiophenes can generate a significant amount of long-lived dissociated charges due to large free energy difference for charge separation.17 Ko et al. systematically varied the backbone twisting of polythiophenes by controlling the number and positions of alkyl substituents and found that a moderate degree of torsional disorder along the polymer chain can enhance the open-circuit voltage by 19% relative to that of P3HT.18 Gasperini et al. showed that polythiophenes including long, flexible alkyl chains as linker are also capable of forming crystalline domains which have charge carrier mobility comparable to that without flexible linkers.21 In line with this trend, we also have recently reported the effect of partial elimination of side chains in P3HT on its morphology and charge carrier mobility.24 Copolymers incorporating 3hexylthiophene and thiophene units at three different molar compositions (17, 25, and 33 mol % of thiophenes) displayed a decrease in overall crystallinity in the order of decreasing density of hexyl substituents. Interestingly, however, the hole mobilities of these copolymers were found to increase with decreasing density of side chains, reaching the value of 1.37 cm2 V-1 s-1, which is an order-of-magnitude higher than that of archetypical P3HT. While our previous study investigated the effects of disorder on the morphology and longrange charge transport, no work has been done to understand how conformational disorder within aggregate domains influences ultrafast exciton and polaron pair dynamics, which precede the formation of polarons, free charge carriers interacting with polymer matrix. In this regard, we


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studied the energetics and interconversion of the photogenerated states in P3HT and congeneric random copolymer incorporating 33 mol % of thiophene units, RP33, which displays lower crystallinity but higher hole mobility (Figure 1). We revealed that the rate of polaron generation in RP33 is independent of temperature, implying a barrierless transition to charge separated states, in contrast to the activation energy of 0.05 eV in P3HT, which is attributed to the temperature-dependent acceleration and deceleration of polaron generation. We suggest that the two factors contribute to the efficient charge separation in RP33: (1) enhanced interchain delocalization promoted by a proximity between conjugated polymer chains absent of steric hindrance of hexyl chains and (2) torsional disorder of polymer chains within an aggregate, which locates precursor states at higher energy providing additional reduction of effective activation energy barrier.

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Figure 1. (a) Chemical structures of P3HT and RP33, where x and y denotes the molar ratio of 3-hexylthiophene and thiophene units, respectively, within a polymer chain. Schematic representation of chain conformations of (b) P3HT and (c) RP33 within aggregates. Tapping mode AFM images of (d) P3HT and (e) RP33. Details on intrachain torsional angle distribution and interchain distance is presented in the discussion section.


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Figure 2 shows the steady-state absorption and fluorescence spectra of P3HT and RP33 films measured at 297 and 77 K. In the absorption spectra, both polymers exhibit vibronic progression by a dominant symmetric C=C stretching mode (0.18 eV).25,26 As proposed by Spano et al., a suppression of 0-0 absorption vibronic band is a consequence of intermolecular exciton coupling in aggregated polythiophene domains.25-28 By separating the absorption spectrum of aggregates from that of amorphous phase, it is possible to estimate the aggregation content of the film, 26 which is 67 and 62% in P3HT and RP33, respectively (see Figure S1 in the Supporting Information for details). A small difference in aggregation contents of the two polymer films indicate that a partial removal of hexyl substituents does not hinder the formation of aggregates, but rather promotes the formation of smaller aggregates, which reduces the overall topographical roughness measured by AFM in RP33 film. Free exciton bandwidths W of P3HT and RP33 are 53 and 59 meV, respectively. These values are relatively smaller than those of other P3HT films of similar molecular weights and are similar to the J-aggregates.29 On the other hand, as detailed in the following, they also exhibit strong intermolecular delocalization, indicating that they possess mixed HJ-characater.30 Notice that the lowest energy vibronic band (E0-0) of P3HT is positioned at a slightly lower energy than that of RP33. Although RP33 film contains aggregate domains smaller than those in P3HT film, such energetic offset does not originate from the difference in the size of aggregates. According to the formulation by Spano et al., changes in the number of polythiophene chains composing aggregates little influence the positions and inhomogeneous broadenings of the vibronic states.28 The 0-n vibronic band, E0-n, to the first order is expressed as 1 𝑔 𝐸0−𝑛 = 𝐸0−0 + 𝐷 + 𝑛𝐸𝑝 + 𝑊𝑆 𝑛 𝑒 −𝑆 2 𝑔

where 𝐸0−0 is the lowest electronic transition energy of a single polymer chain in the gas phase,


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D is the gas-to-crystal shift of transition energy due to nonresonant intermolecular interactions, Ep is an energy of a local vibrational mode coupled to the electronic transition, W is exciton bandwidth, and S is Huang-Rhys parameter.28 Since the last term, the first-order correction term, is small31 and yields similar values for both P3HT and RP33, E0-0 is determined exclusively by 𝑔

𝐸0−0 and D. D of RP33 is estimated to be similar to or larger than that of P3HT, as d-spacing and - stacking distance decrease with the increasing content of thiophene units without alkyl groups.24 Thus, an apparent blue-shift of E0-0 of RP33 relative to that of P3HT is a result of higher intrachain transition energy, which originates from the torsional disorder reducing the extent of -conjugation.

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Larger torsional disorder in RP33 aggregates is a consequence of reduced dispersion interaction enforcing planarization between thiophene units. In the ground state, interchain interactions in aggregated domains consist of - interaction between conjugated thiophene units and van der Waals interaction between alkyl chains.32 Through temperature-dependent phase change of polyfluorene in solution phase, Bright et al. suggested that the energy required to planarize the polymer backbone is 15.6 kJ/mol.33 This energetic barrier, given by interdigitation of alkyl chains, is surmounted by an energy barrier given by - interaction energy, which is ca. 7.2 kJ/mol as estimated by using an aromatic intermolecular interaction (AIMI) model.34 In P3HT, interaction between neighboring hexyl chains as well as -conjugated backbones forces thiophene units to retain coplanarity. In the absence of hexyl chains, as in RP33, a major energetic penalty in rotation is removed and extra torsional degree of freedom is granted. We can expect that randomly incorporated thiophene units without hexyl substituents in RP33, not only increase lowest transition energy, but also induce broader conformational and energetic heterogeneity, which can be analyzed by temperature-dependent changes in the fluorescence spectra. The steady-state fluorescence spectra are dominated by interchain excitons in H-aggregate species, which can be characterized by the reduced 0-0 vibronic band relative to the 0-1 vibronic band, while interchain coupling strengths obtained from the steady-state absorption spectra indicate significant J-character.30,35 This feature indicates that the steady-state fluorescence originates largely from the relaxed excitons at lower energy states, where H-type coupling is dominant. A small energetic offset of 0.06 eV between the 0-0 fluorescence vibronic bands of P3HT and RP33 suggests that excitons are positioned at energetically higher states in RP33 than in P3HT, due to the torsional disorder within a chain.36 Upon lowering the temperature, as shown


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in Figure 2c, the 0-0 fluorescence vibronic band of RP33 is red-shifted by ca. 0.13 eV, while the 0-0 vibronic band of P3HT is red-shifted by only ca. 0.06 eV. Below 160 K, the 0-0 fluorescence vibronic bands of the two polymers coincide. Energetic disorder in -conjugated polymer films is often described as a Gaussian density of states (DOS) with a standard deviation of energy distribution, . Excitons generated by photoexcitation initially undergo fast downhill migration towards the lower energy sites. Relaxation stops by reaching the thermal quasi-equilibrium energy level, 𝐸𝑡ℎ = −𝜎 2 ⁄𝑘𝑇, which is a function of temperature.37,38 At room temperature, downhill migration places excitons near the most populated states and excitons thereafter diffuse through nearby isoenergetic states via thermally activated hopping process. Upon lowering the temperature, Eth is shifted to the lowest part of DOS, where energetically equivalent states are sparsely distributed. At this situation, excitons relax and reside in the lower-energy tail of DOS, as thermally activated hopping process becomes unviable.36 The peak positions of 0-0 vibronic bands in the steady-state fluorescence spectra reflect the energy difference between the thermally equilibrated excitonic states and the ground state. In this regard, a temperature-dependent shift in the fluorescence spectra, E0-0, can be a measure of energetic disorder of DOS, Δ𝐸0−0 = −

𝜎2 1 1 ( − ) 𝑘 𝑇1 𝑇2

𝐏𝟑𝐇𝐓 where T1 and T2 denote the two different temperatures. Using eq 2, Δ𝐸0−0 = 0.06 eV and 𝐑𝐏𝟑𝟑 Δ𝐸0−0 = 0.13 eV yield 𝜎𝐏𝟑𝐇𝐓 = 0.023 eV and 𝜎𝐑𝐏𝟑𝟑 = 0.034 eV , respectively. As the

suppressed 0-0 vibronic bands relative to 0-1 bands in the fluorescence spectra indicate intermolecular delocalization, higher  value of RP33 suggests that the energy of intermolecular


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0 6 4 2 0 -2 -4 -6

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Figure 3. Transient absorption spectra of P3HT (left blocks; a, b) and RP33 (right blocks; c, d) at 77 K upon photoexcitation at 540 nm. See Figure S5 in the Supporting Information for the transient absorption spectra measured at 297 K using higher pump fluence. exciton is modulated by the intrachain torsional disorder, which supports the arguments based on Spano’s formulation. It is interesting to note that the fluorescence lifetimes exhibit an exactly opposite trend as shown in Figure 2d. At room temperature, both P3HT and RP33 have essentially the same lifetimes, which are ca. 300 ps, whereas they diverge upon lowering the temperature. The fluorescence lifetime of P3HT at 77 K is increased to 560 ps, which is about twice that at 297 K, while that of RP33 is increased to 1.1 ns, showing four-fold increase. Disorder in the polymer aggregates often involves a number of exciton traps, which deter exciton diffusion and lower the charge generation yield in the bulk heterojunction materials. Liang et al. and Mikhnenko et al. reported that, in P3HT, the trap density is on the order of 1018 cm-3, which is similar to those of other various conjugated polymers.39,40 This suggests that exciton traps are involved in the processes of quenching of excitons and the process becomes


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significantly different at low temperatures, as will be detailed in transient absorption spectra measurements in the following sections. Figure 3 shows the transient absorption (TA) spectra of P3HT and RP33 measured at 77 K upon photoexcitation at 540 nm. The TA spectra of regioregular P3HT measured at room temperature have been extensively studied and the signatures of singlet exciton, polaron, and polaron pair have been unambiguously assigned, which are reproduced in our TA spectra of P3HT. Following the previous assignments by Guo et al., negative signals below 650 nm are attributed to ground state bleaching (GSB) bands, and a broad photoinduced absorption (PIA) band centered at around 1225 nm and another PIA band centered at around 670 nm, which appear in the initial spectrum at 250 fs delay, are assigned to the PIA of singlet excitons and polaron pairs, respectively.41 Small negative band at around 700 nm is attributed to stimulated emission (SE). SE becomes negligibly small within 30 ps as initial exciton-exciton annihilation process rapidly deactivates singlet excitons, which is also evidenced in the decay of singlet exciton PIA (Figure S6). The same spectral features also appear in the case of RP33, but their positions are different at low temperature. The PIA of singlet excitons in RP33 is noticeably redshifted to 1325 nm at 77 K, as shown in Figure 4d. Previous TA studies on the regiorandom and regioregular P3HTs demonstrated that the red-shift of PIA band of singlet excitons is correlated with interchain exciton delocalization. Despite the J-character manifested in the ratios of GSB vibronic bands, the positions of PIA of singlet excitons in both polythiophenes indicate that excitons are delocalized between chains. Furthermore, significantly red-shifted PIA of singlet excitons suggests that initial singlet excitons in RP33 at low temperature are more delocalized between chains than those generated in P3HT.


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Banerji et al., demonstrated that, in P3HT films, singlet excitons exhibit ultrafast energy relaxation in the aggregated domains, as can be inferred from the major Stokes shift of the fluorescence spectrum which takes place in the initial 200 fs.42 It has been suggested that coherent exciton transport assisted by interchain delocalization rapidly places excitons at lower energy states, where incoherent hopping process follows. This suggests that, in RP33, upon reduction of phonon-induced decoherence, excitons are transported further to lower energy sites, where extent of interchain delocalization is greater due to the tighter chain packing. However, as shown in Figure 4b, this does not enhance interchain delocalization in P3HT, since - stacking distance is limited by the presence of bulky hexyl substituents and strictly planar thiophene units.24,43 The PIA of singlet excitons rapidly decays in 100-ps time window, leaving small PIA residuals, which survives up to 4 ns. At room temperature, the residual PIA bands in P3HT and RP33 are 1.3 1.2 1.1 1.0

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Figure 4. Transient absorption spectra of P3HT (a, b, e, f) and RP33 (c, d, g, h) at 250 fs (a-d) and 3 ns (e-h) time delay. The two temperature extremes, 297 (a, c, e, g) and 77 K (b, d, f, h) were measured.


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both centered at 1000 nm as shown in Figure 4e and 4g, which has been ascribed to polarons.44-46 Unlike monomodal PIA bands of polarons observed at 297 K, the PIA of P3HT at 77 K is bimodal. The two PIA bands are centered at 1000 and 1150 nm (1.24 and 1.08 eV, respectively) and have similar intensity ratio. The one observed only at low temperatures has previously been reported by steady-state photomodulation measurements and attributed to trapped singlet excitons.44-46 Emergence of trapped excitons is correlated with thermal activation of polaron generation as will be discussed in the following. It is interesting to note that, as shown in Figure 4h, the monomodality of PIA is retained in the case of RP33. This suggests that in RP33, trapped excitons are not formed and polarons are still generated despite reduced thermal energy. This suggests that the polaron generation is not temperature-dependent in RP33. The center wavelength of the PIA band is slightly more redshifted than that measured at 297 K and those in P3HT. This is due to torsional disorder given by thiophenes without hexyl chains, which is supported by model quantum mechanical calculations detailed in the Supporting Information (Figure S7). To analyze the temperature-dependence of polaron generation dynamics, it is required to extract the pure decay dynamics of polarons by eliminating the decay dynamics of PIA of singlet excitons, which spectrally overlaps with the PIA of polarons. Fortunately, as shown in Figure 4a4d, both P3HT and RP33 exhibit monomodal PIA bands in the initial TA spectra, indicating that it is possible to selectively populate singlet excitons at any temperatures. 41 By subtracting pure singlet exciton dynamics at 1350 nm from that at 1000 nm, where the dynamics of both polarons and singlet excitons are mixed, it is possible to extract pure decay profiles of polaron as shown in Figure 5a.47 Temperature-dependent singlet exciton deactivation processes in picosecond timescale are plotted in the Supporting Information (Figure S8 and S9) for comparison.


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The extracted decay profile exhibit a clear rise dynamics which reaches the maxima at 10 ps and decays steadily in 4 ns time window (Figure S10). The rise dynamics is analyzed by single exponential function with a constant residual. The rate constants of the rise dynamics are plotted against the inverse of temperature. A linear correlation in P3HT is a clear evidence of a presence of an activation energy barrier in the formation of polaron. Fitting the plot with Arrhenius equation yields an activation energy barrier of 0.05 eV, which is smaller than that estimated in P3HT:C60 blends.48,49 On the other hand, the rate constants of polaron generation in RP33 do not show clear temperature dependence, indicating barrierless transition. Given the timescale of polaron generation, which varies within a few picoseconds, the barrierless formation of polarons can be attributed to an efficient exciton delocalization of hot excitons in an early timescale. The electron-hole binding energy of polaron pairs in pristine and bulk heterojunction conjugated polymer films is typically a few hundreds of millielectronvolts, but an order of magnitude higher and lower values were also reported.48-50 Such large energy barrier is overcome by interchain charge delocalization, which partially reduces electron-hole Coulomb attraction by distributing the charges over a number of polymer chains and/or molecules. In analogy to the coherent exciton migration process, excitons can also move more effectively to the lower energy within a timescale of a few hundreds of femtoseconds. Unlike energetically homogeneous P3HT crystalline structure, RP33 can contain sites where interchain distance is small enough to promote effective interchain delocalization and consequent reduction of activation energy barrier. Change of initial interchain delocalization of polaron pairs in RP33 is larger than those in P3HT, leading to an apparent acceleration of charge separation process. The activation energy barrier of P3HT estimated through TA spectroscopy is an order of magnitude smaller than those reported using photoelectron spectroscopy, which is 0.7 and 0.3 eV


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for excitons and polaron pairs, respectively.50 This large discrepancy originates from the inability of steady-state measurements to detect the dynamic character of excitons and polaron pairs, which can rapidly migrate through energy sites within aggregated domains to enhance interchain delocalization. Therefore, it can be suggested that an effective activation barrier for polaron generation can be greatly reduced upon dynamic delocalization process of excitons and polaron pairs in the early timescale, which can only be captured by the TA spectroscopy. Temperature-independent polaron generation dynamics has also been reported in various donor-acceptor blend films. By employing microwave photoconductance and photocurrent techniques, Grzegorczyk et al. and Lee et al., respectively, demonstrated that quantum yield of charge separation in P3HT:PCBM blend film has little temperature dependence.48,51 By using transient vibrational spectroscopy, Pensack et al. demonstrated that, in P3HT:PCBM blend film,

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no highly ordered crystalline phase is present, a clear temperature dependence of charge separation dynamics is observed.52 All of the previous studies commonly attribute the temperature-independent charge generation in blend films to the efficient interchain charge delocalization, which reduces the activation energy barrier between charge transfer states and charge separated states. Polaron generation of RP33, in this regard, can be benefited from the torsional disorder resulting from partial elimination of hexyl substituents. Activation energy barrier of charge separation is sensitively modulated within a few nanometer scale of charge delocalization, as simulated by Devižis et al.53 Although both P3HT and RP33 films contain highly ordered crystalline phases, as shown in the steady-state absorption and fluorescence spectra (Figure 2a and 2b), intermittently formed close chain-chain contact points in RP33 promotes superior interchain delocalization at early timescale, as can be inferred from the positions of singlet exciton PIA bands (Figure 4). Furthermore, when the excitons and polarons are not yet relaxed to the thermal quasiequilibrium states, their energy distributions follow the DOS determined by torsional disorder within a chain. Since polaron generation is completed within a few tens of picoseconds, dissociation of excitons and polaron pairs in RP33 can be assisted by both low activation energy barrier provided by superior interchain delocalization and higher energetic position profitable for barrier crossing. This effect is clear in the nanosecond timescale at 77 K, where thermal energy cannot boost dissociation process. Due to high efficiency in palaron generation, excitons in RP33 are completely dissociated, while those in P3HT are partially trapped to leave bimodal PIA bands in the TA spectra (Figure 4f and 4h). The DOS of singlet excitons as well as temperature-dependent polaron generation dynamics in P3HT and RP33 are summarized in Figure 6.


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SRP33 P SP3HT

Exciton Trapping

RP33 Barrierless CS P3HT Thermally activated CS

Figure 6. Schematic energy level diagram illustrating the polaron generation dynamics in P3HT (red) and RP33 (blue) at 297 (thick straight arrow) and 77 K (thin curved arrows). Downward arrows indicate deactivation of singlet excitons via fluorescence (straight arrows)

and trapping (wavy arrow). In short, we have investigated the ultrafast polaron generation and associated dynamics of excitons and polaron pairs in P3HT and RP33. The results herein suggest important mechanistic prerequisites in charge generation processes in CPs: (1) To generate polarons upon photoexcitation, the formation of π-π stacked aggregates is required. As an efficiency of charge separation is determined by charge delocalization, reduction of π-π stacking distance, rather than long-range ordering of an aggregated domain, becomes more important. (2) In terms of energetics, disorder can aid charge separation process. By placing excitons and polaron pairs at higher energy states, disorder can act as a driving force for crossing activation energy barriers in pristine CP films. Although energetic imbalance is already given at donor-acceptor heterojunctions, control of torsional disorder can influence polaron generation process in the


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same manner as chemical modifications.54 However, this effect can be subtle since torsional disorder cannot be added to a degree over which disrupts polymer aggregation, as in the case of regiorandom P3HT, which shows poor charge separation yield.47 Complex energetics from blend morphology may contribute to the energetics and dynamics of the system as well.55 Overall, a manipulation of intrachain and interchain disorder parameters carries a great significance in charge generation processes in CP materials. We believe that our findings in the disordered CP film will provide valuable information for future designing of solar cell materials for higher efficiency.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work at Yonsei University was supported the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2016R1E1A1A01943379). The quantum calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with supercomputing resources including technical support. The work at Pohang University of Science and Technology was supported by grants from the Center for Advanced Soft Electronics under the Global Frontier Research Program (Code No. NRF-2012M3A6A5055225).

ASSOCIATED CONTENT Supporting Information. Experimental details, steady-state absorption spectra analyses, additional transient absorption spectra, decay profiles, fitting parameters, quantum calculation results.


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The Role of Disorder in the Extent of Interchain Delocalization and

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