Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3

Subscriber access provided by Iowa State University | Library

C: Energy Conversion and Storage; Energy and Charge Transport

Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3-hexylthiophene) Aggregates Patrick C. Tapping, Ras Baizureen Roseli, and Tak W. Kee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00318 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3-hexylthiophene) Aggregates Patrick C. Tapping,† Ras Baizureen Roseli,†,‡ and Tak W. Kee∗,† †Department of Chemistry, The University of Adelaide, South Australia 5005, Australia ‡Current address: School of Chemistry & Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia E-mail: [email protected] Phone: +61-8-8313-5314

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In conjugated polymers used in photovoltaics, charges may be produced on ultrafast time scales without requiring exciton diffusion to a donor–acceptor interface. To investigate the role of high-energy, delocalized exciton states in charge generation within polymer domains, we apply a pump–push–probe transient absorption technique to pristine poly(3-hexylthiophene) nanoparticles. The near-infrared push pulse induces exciton dissociation through the S3 ←S1 electronic transition, which is predicted to show intramolecular charge-transfer character. We suggest the spatial extent of the high-energy exciton, which induces electron–hole separation, is sufficient to overcome the intrinsic Coulombic attraction of the electron–hole pair. We observe that ∼10% of the pushed excitons undergo dissociation to form free charges. The kinetics of charge recombination indicate that the electron and hole are separated by a distance of ∼3 nm across the polymer domains.

2


Page 2 of 20

Page 3 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction A polymer solar cell is constructed from a conjugated polymer and an electron acceptor material. A widely accepted mechanism for charge production first involves the polymer absorbing a photon to produce an exciton, or bound electron–hole pair. 1 Through diffusion, the exciton reaches an interface between the polymer and electron acceptor. There, a difference in energy of the lowest unoccupied molecular orbitals drives dissociation of the exciton, resulting in the electron residing on an acceptor molecule and the corresponding hole left on the polymer. Interestingly, photogeneration of free charges was also observed in neat polymer systems, despite the absence of an electron acceptor material. 2–4 Whether on a polymer chain or at a donor–acceptor interface, charge pairs are still in close proximity, Coulombic attraction remains strong and separation of the charges should be unfavorable. Recent work has offered insight into the exciton dissociation and charge generation processes. Gregg, Hood and Kassal reported the role of entropy 5,6 and energetic disorder 6 in separation of electron and hole with a binding energy exceeding kB T . Meng et al. showed that local nonuniform electric fields along a polymer chain contribute to excitation dissociation within 1 ps. 7 Using a quantum mechanical lattice model of a donor/acceptor heterojunction, Janković and Vukmirović demonstrated that the space-separated charges generated on an ultrafast time scale are in fact directly optically prepared, not through ultrafast transitions from exciton to charge-transfer states. 8 On the other hand, Polkehn et al. argued that mixing of Frenkel excitons with charge-transfer excitons plays a role in charge transfer from an oligothiophene to a fullerene acceptor. 9 In the event of charge separation, Matheson et al. reported generation and localization of the holes on a 100 fs time scale on the same polymer chain as their precursor excitons. 10 It is evident that despite several years of study of organic photovoltaics, there is still debate over the explanations for the high efficiency of charge generation and separation. Experiments by Kaake et al. showed ultrafast charge generation in organic donor–acceptor bilayers, demonstrating a significant proportion of charges were produced on time scales inconsistent with exciton migration through a purely incoherent hopping model. 11 While the authors attributed this phenomenon to a large spatial extent of the exciton, any miscibility of the polymer and fullerene phases may cause intermixing and an ill-defined interface structure. 12–14 Highly efficient exciton migration may be due to coherent energy transfer along conjugated polymer chains, which was observed by Collini and Scholes at room temperature on time scales of tens of femtoseconds. 15 In addition, coherent vibronic coupling between the initially delocalized photogenerated exciton and the charge-transfer state has been proposed as a mechanism for ultrafast exciton dissociation. 16 The initially delocalized exciton states were used by Zarrabi et al. to explain the charge generation kinetics in a series of pump–probe experiments where the exciton density and electron-acceptor concentrations were varied. 17 It has been proposed that excess exciton energy, or “hot” excitons, can provide a driving force for exciton migration, dissociation, and charge separation. 4,18–20 However, efficient charge generation has also been observed independent of excitation photon energy 21,22 and temperature, 23 implying that excess vibrational energy may be unimportant for efficient charge generation, but that higher-lying, delocalized electronic states are likely important. 1,24–26 The role of high-lying, delocalized electronic states in charge generation in conjugated polymers is supported by time-dependent density functional theory (TD-DFT) studies, demonstrating 3



intramolecular charge transfer characteristics of the S5 state of oligofluorenes 27 and S3 state of oligothiophenes. 28 Interestingly, the TD-DFT work on oligothiophenes showed that the excited-state absorption (ESA) band of a thiophene heptamer exhibits an excellent agreement with the experimental ESA spectrum of poly(3-hexylthiophene) (P3HT), 28 indicating that single polymer chains have sufficient size for exciton spatial delocalization. Despite the delocalized nature of the exciton, it is likely that the charge pair formed by a high-lying excited state are found on chain segments that are strongly electronically coupled. On isolated polymer chains, the charge pair are expected to undergo recombination efficiently. In a polymer aggregate or film, however, owing to significant chain–chain interactions the charge pair can undergo separation (as a result of intermolecular charge transfer). We have previously used a pump–push–probe transient absorption technique to show that (the relaxed) excitons can be dissociated by a secondary near-infrared excitation, or push pulse, in isolated chains of P3HT. 29 In solution, the P3HT exciton exhibits a strong ESA band around 1100 nm which, when excited with the push pulse, results in a persistent depletion of the exciton population. We have attributed this loss of exciton to (1) charge carrier generation owing to exciton dissociation and (2) recombination due to localization of charge carriers on isolated chains. More recently, a TD-DFT study of the nature of the P3HT ESA band assigned the absorption to the S3 ←S1 transition, and indicated the intramolecular charge-transfer character of this transition, 28 supporting the conclusions from the pump–push–probe experiments. 29 However, as charge recombination occurs efficiently on the isolated polymer chains as mentioned above, a definite spectral signature of the electron or hole-polaron was not observed. To further investigate the role of high-energy, delocalized exciton states on the generation of charge-carrier species, we turn to a polymer nanoparticle (NP) system. 30 Aqueous dispersions of conjugated polymer NPs provide several advantages for spectroscopic experiments, including film-like optical properties and semicrystalline morphology, coupled with simple preparation, good photostability and versatility. 31–33 Compared to the miniemulsion production method, 34 the reprecipitation production method used in this work produces more disordered NPs exhibiting a blend of amorphous and semicrystalline type domains, 30 and have been previously used to study exciton and polaron dynamics in P3HT systems. 35 By applying the pump–push–probe technique to pristine P3HT NPs, the push-induced exciton dissociation is achieved and the generated hole-polaron is observed spectroscopically. Analysis of the push-induced electron and hole-polaron geminate recombination kinetics indicates an initial electron-hole separation of ∼3 nm.

Experimental Methods Sample Preparation A 2.5-mg portion of regioregular P3HT (MW = 50 kg mol−1 , 99% regioregular, Rieke Metals) and 2.5 mg of Igepal CO-520 (Sigma Aldrich) were dissolved in 25 mL freshly distilled THF by sonication. The use of Igepal CO-520 as a surfactant for conjugated polymer NPs was reported by Shen et al. 36 The preparation of P3HT NPs involved rapidly injecting 2 mL of the stock solution into 8 mL of Milli-Q water under vigorous stirring. Five batches of the 4


Page 4 of 20

Page 5 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solution were collected to produce 50 mL of NP suspension with polymer concentration of 0.02 g L−1 . The THF was removed by rotary evaporation and the volume further reduced to ∼15 mL. The suspension was passed through a 0.45 µm cellulose filter to produce a sample with absorbance of 0.2 at the 400 nm pump wavelength, measured in a quartz cuvette with a 2 mm path-length (Starna Cells, 21-Q-2).

Transmission Electron Microscopy The NP sample was diluted to 1 × 10−5 g L−1 . The carbon-coated copper grid was dipped into the sample and it was left to evaporate for approximately an hour. The samples were imaged using a FEI Tecnai G2 Spirit transmission electron microscopy (TEM) at an operating voltage of 100 kV. The diameter of approximately 100 NPs were used to determine the size distribution, which was fit to a Gaussian function to give the average diameter and standard deviation of the P3HT NPs.

Steady-state and Transient Absorption Spectroscopy The steady-state absorption spectrum was obtained using the same sample and cuvette as for the transient absorption experiments on a Cary 100 UV-visible absorption spectrometer. A Ti:sapphire regenerative amplifier (Spectra-Physics, Spitfire Pro XP 100F) supplied 100 fs laser pulses centered at 800 nm with a 1 kHz repetition rate. The output was split to produce the pump, push and probe pulses. Pump light at 400 nm was produced by second harmonic generation in a 0.5 mm BBO crystal, having a pulse energy of 4.0 µJ and a FWHM spot size of 0.93 mm. Push pulses at 1200 nm were generated using an optical parametric amplifier (Light Conversion, TOPAS-C), having a pulse energy of 2.0 µJ and a FWHM spot size of 0.47 mm. The probe was a white light continuum spanning ∼850 to 1400 nm produced in a 12.7 mm sapphire crystal, with a spot size of 0.13 mm and an average power at least 1000 times lower than the pump or push. The probe was split into signal and reference beams and measured using a pair of InGaAs linear detectors (Ultrafast Systems, CAM-NIR). The push was set at a fixed delay of 2 ps relative to the pump, with the probe delay varied using a computer controlled delay line. Polarization of the push was parallel to the pump, with the probe set at the magic angle (54.7◦ ). Samples were stirred continuously during the experiments, and negligible photobleaching was observed. A Gaussian instrument response function of 150 fs full-width-half-maximum was used for fitting of kinetic data.

Results and Discussion Femtosecond Two-pulse Pump–probe Spectroscopy of P3HT NP The P3HT NPs have an average diameter of 30 ± 8 nm, as shown by the TEM image and size distribution presented in the Supporting Information. In previous work by Hu and Gesquiere, the average diameter of P3HT NPs was reported to be 37 ± 9 nm, 37 showing good agreement with the size of the P3HT NPs in this study. Figure 1 shows the steady-state and transient absorption spectra of the P3HT-NP suspension. The ground-state absorption (GSA, green)

5



displays a broad peak centered at 500 nm accompanied by shoulders at 540 and 590 nm. The structure and red-shifted position of the absorption relative to P3HT solution are characteristics of the formation of semicrystalline aggregates, and match closely the spectrum of solid films of regioregular P3HT. 35,38–40 Immediately following excitation with the 400 nm pump light, a broad ESA (red) band appears in the near-infrared region of 800–1400 nm, which has been assigned to the S3 ←S1 transition of the singlet exciton. 18,28,35 The ESA band evolves over ∼100 ps to reveal a second species, exhibiting an absorption over 800–1200 nm (blue, magnified by 2× for clarity). Studies of P3HT blended with a PCBM electron acceptor have shown that this long-lived absorption is attributable to the charged hole-polaron species formed through dissociation of excitons. 18,35 In neat P3HT films, relatively long-lived holepolaron species are also readily observed upon photoexcitation, 41 with an estimated yield of 0.15 per generated exciton. 2 The band observed here extends slightly further into the lowenergy region compared to the pure hole-polaron absorption of P3HT 18,35 and, as there is no electron acceptor present, can be attributed to the presence of the accompanying electronpolaron species. 42 Two selected probe wavelengths are indicated in Figure 1. The first, at 950 nm, is resonant with both the exciton and hole-polaron absorptions while the second, at 1350 nm, probes only the exciton population, with a negligible contribution from holepolarons. The 1200 nm push wavelength is also shown, chosen to preferentially excite the S1 exciton. GSA ESA 2 ps ESA 100 ps

S S

15

n C6H13 Probe

Push

Probe

10

5

ESA (ΔmOD)

C6H13

Normalized GSA

×2 Pump

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 20

400

0 600

800 1000 Wavelength (nm)

1200

1400

Figure 1: Ground state absorption spectrum of P3HT NP, and ESA spectra at pump–probe delays of 2 (red) and 100 ps (blue). Inset is the structure of P3HT. The ESA at 100 ps is scaled by a factor of 2 for clarity. The results of the two-pulse pump–probe experiment are given in Figure 2a, showing the change in optical density (∆OD) at 950 nm or 1350 nm as a function of the pump–probe delay time. At early times, the two kinetic traces track closely as the exciton absorption dominates the near-infrared spectrum. The traces begin to diverge after several picoseconds, with the 1350 nm probe showing the decay of the exciton over ∼100 ps, while the 950 nm probe, which monitors the exciton and hole-polaron populations, displays an additional longlived component. The traces were fitted using a multiexponential decay function of the form P −t/τn f (t) = n An e , with the fit parameters given in Table 1. The fit of the ∆OD data at 1350 nm has two components with time constants of ∼1.5 ps and ∼25 ps, which are also present in the ∆OD fit at 950 nm. These time constants are consistent with those reported 6


Page 7 of 20

20

ΔmOD

(a)

950 nm 1350 nm

15 10 5

ΔΔmOD at 1350 nm

0 0 −1 −2 (b) −3 ΔΔmOD at 950 nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0 −0.2

(c) 0

20

40 60 Time (ps)

80

100

Figure 2: (a) Dynamics of the pump–probe experiments with a probe wavelength of 950 (blue circles) or 1350 nm (red triangles). (b, c) Dynamics of the pump–push–probe experiment showing the change in ∆OD at 1350 or 950 nm due to the 1200 nm push pulse. Solid lines are fits to a multiexponential decay model, with parameters given in Table 1.

Table 1: Fitting parameters for the pump–probe (∆OD) and pump–push–probe (∆∆OD) data with λpush = 1200 nm.a Expt. λprobe (nm) ∆OD 1350 ∆OD 950 ∆∆OD 1350 ∆∆OD 950

A1 b τ1 (ps) A2 0.67 ± 0.02 1.7 ± 0.1 0.33 ± 0.02 0.60 ± 0.02 1.2 ± 0.1 0.29 ± 0.01 −0.65 ± 0.31 1.4 ± 1.4 −0.35 ± 0.17 -0.29 1.4c -0.35

τ2 (ps) 26 ± 2 23 ± 2 24 ± 21 24c

A3 τ3 (ps) – – 0.10 ± 0.01 2100 ± 200 – – 0.36 2145c

P The ∆OD and ∆∆OD data were fit to a multi-exponential function f (t) = n An e−t/τn usingP a Gaussian instrument response function of 150 fs fwhm. b Amplitudes normalized so that n |An | = 1. c Time constants fixed. Errors in fitting parameters are 90% confidence intervals.

a

7



in our previous study and can be attributed to the exciton relaxation processes in P3HT NPs. 35 An additional component with τ3 ≈ 2 ns is present in the fit at 950 nm, corresponding to the long-lived charge-separated polaron species. The excellent agreement between the amplitudes and time constants of the two probe wavelengths supports the assignments that the absorption at 1350 nm is proportional to the exciton population only, but the probe at 950 nm contains both exciton and polaron contributions, which is consistent with the assignments in previous studies. 18,35

Femtosecond Three-pulse Pump–push–probe Spectroscopy of P3HT NP We now turn to the results of the three-pulse pump–push–probe experiment. The conditions are identical to those of the pump–probe experiment in Figure 2a, but with an additional 1200 nm push pulse arriving 2 ps after the initial pump. It is important to stress that the three-pulse signal is only observed if the pump pulse is present. As shown in the Supporting Information, attempting to excite the NPs using only the 1200 nm push gives a signal identical to that from neat water, indicating the observed data are purely due to the effect of the push pulse on the exciton. The linear dependence of the three-pulse signal on the average power of the push pulse, which is also shown in the Supporting Information, shows the one-photon characteristic of the transition induced by the push pulse. Provided that there is adequate exciton population, the three-pulse signal shows a negligible dependence on the arrival time of the push pulse (Supporting Information), which was also observed in our previous study on P3HT in solution. 29 A push-pulse arrival time of 2 ps is used in this study to ensure a substantial exciton population is available for the push pulse. The data sets are displayed in Figures 2b and 2c as the change in ∆OD due to the influence of the push, or ∆∆OD, at 1350 nm or 950 nm, respectively. In this way, the data in Figure 2b are a measure of the change in exciton population due to the push, while Figure 2c shows the combined effect of both changes in the exciton and hole-polaron populations. The solid lines indicate the multiexponential fits to the ∆∆OD data with parameters given in Table 1. The push pulse, tuned to the exciton S3 ←S1 transition, depletes the S1 exciton population, seen by the instrument response limited drop in signal at t = 2 ps at both 950 and 1350 nm (Figures 2b and 2c). A majority of the S1 exciton signal recovers rapidly with a time constant of τ1 = 1.4 ps, which can be attributed to S3 →S1 relaxation. An additional recovery component with τ2 = 24 ps is effectively identical to the exciton decay dynamics observed in the pump–probe experiment. We assign this to a permanent depletion of the S1 exciton population, since any instantaneous removal of excitons by the push will be exhibited in the ∆∆OD data simply as a proportion of the original ∆OD signal. This phenomenon was previously observed in our pump–push– probe study of solvated P3HT chains. 29 Accordingly, beyond t ≈ 5 ps the recovery in the ∆∆OD signal in Figure 2b is due to the natural decay of the entire S1 exciton population measured by the dynamics at 1350 nm in Figure 2a. Interpretation of the ∆∆OD dynamics at 950 nm in Figure 2c is complicated by a small signal level in addition to overlap of the exciton and hole-polaron absorptions. Similar to the behavior observed in Figure 2b, the characteristics of the exciton depletion are present in the results in Figure 2c, which include

8


Page 8 of 20

Page 9 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a negative ∆∆OD signal with a recovery time constant of several picoseconds. A persistent positive offset dominates at later times, which is absent in the pure exciton signal, thus can be assigned to an increase in the population of the long-lived hole-polaron species induced by the push pulse. The ∆∆OD data at 950 nm were well fitted with three exponential functions. Representing the exciton recovery, τ1 = 1.4 ps and τ2 = 24 ps were fixed to those of the exciton decay probed at 1350 nm. The additional time constant τ3 = 2145 ps, which represents the persistent polaron absorption, is fixed to the value found in the pump–probe experiment. The proportion of the excitons absorbing the 1200 nm push pulse is ∼22%, determined from the magnitude of the exciton absorption bleach by ∆∆OD/∆OD at 1350 nm and t = 2 ps. From Table 1, the fitted A3 amplitude of the ∆∆OD data at 1350 nm indicates approximately one third of the pushed excitons do not return back to the S1 state, instead dissociating to form charge carriers. Given a low relative dielectric constant (r ≈ 3) of P3HT, a significant proportion the charge carriers are expected to undergo rapid (static) geminate recombination. For the portion of charge carriers that undergo separation, the push-induced increase in the hole-polaron population may be estimated at ∼10% from ∆∆OD/∆OD at 950 nm and at t > 100 ps, where the exciton absorption is negligible, as shown in Figures 2a and 2c.

Modeling of Electron–hole Geminate Recombination To gain insight into the separated charge carriers, the ∆∆OD data at 950 nm can be analyzed using a diffusive geminate recombination model. 43,44 In this model, for an electron and hole pair initially separated by a distance r0 , the survival probability at time t is given by ! coth(−rc /2a2 ) − coth(−rc /2r0 ) 1 − exp(−rc /r0 ) erfc rc , (1) Ω(r0 , t) = 1 − 1 − exp(−rc /a2 ) (16 (De + Dh ) t)1/2 where the Bjerrum length, rc , is given by rc = e2 /4π0 kB T , a2 is the reaction radius, is the relative dielectric constant of the medium, and 0 , e, kB and T = 298 K take their conventional representations as the vacuum permittivity, electron charge, Boltzmann constant, and temperature, respectively. The translational diffusion coefficients of the electron De and hole Dh are related to their respective mobilities µ in the polymer by D = µkB T /e, the Einstein relation. 45 The model given in eq 1 is derived from the Debye–Smoluchowski equation 46,47 and has been used for studying the kinetics of reaction between charges in dielectric media including solvents 43,44 and organic semiconductors. 48 Using this model, the r0 of P3HT NP in the pump-probe and pump-push-probe experiments can be determined. In reality, a range of initial R ∞ electron–hole separation distances will occur, which may be mod0 eled by Ω (hr0 i, t) = 0 Ω(r0 , t)f (hr0 i, r0 ) dr0 , where f is a modified exponential distribution determined by the average separation, hr0 i (Supporting Information). Extracting a measure of the polaron population directly from the absorption at 950 nm is not straightforward, given that the S1 exciton also absorbs in the same spectral region. However, as shown in Figure 2, the exciton population (probed at 1350 nm) is negligible at times t > 50 ps, while the polaron decay occurs over several nanoseconds. Using the absorption at 950 nm at long delay times is therefore a suitable measure of the polaron 9



population. Furthermore, because eq 1 is dominated by the dynamics of long-lived charges by diffusion, this method is suitable for obtaining a good fit to the survival probability, and thus the average initial separation distance of the charges, hr0 i. The polaron recombination in the pump–probe experiment was modeled by best fitting the value of hr0 i so that Ω0 (hr0 i, t) ∝ ∆OD950 (t)

t > 100 ps.

(2)

The inferred hole-polaron absorption kinetics in the pump–probe experiment and associated fit are presented in Figure 3a. The fit to eq 2 yielded an average initial electron-hole separation of 2.66 ± 0.02 nm. Additional input parameters to eq 1 are given in Table 2. 6

(a)

ΔmODpolaron

5

= 2.66 ± 0.02 nm

4 3 2 1 0 (b)

ΔΔmODpolaron

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 20

= 2.5 ± 0.1 nm

0.8 0.6 0.4 0.2 0 0

500

1000

1500

Time (ps)

Figure 3: Hole-polaron decay dynamics (circles) isolated by the absorption at t > 100 ps when exciton population is negligible. Pump–probe data are shown in (a), and pump–push– probe data are shown in (b). Fits to eq 2 are shown as solid curves.

Table 2: Parameters used in the diffusive recombination model. a2 µe µh

description value relative permittivity of medium 3.5 reaction radius 0.5 nm −8 electron mobility 10 m2 V−1 s−1 −8 hole mobility 10 m2 V−1 s−1

ref. 49 44 50–55 50–55

The push-induced dissociation of the exciton and subsequent recombination of the generated charge carriers was modeled in the same way as for the pump–probe experiment using eq 2, but instead using the ∆∆OD data (Figure 3b). The fit yielded a value for the average initial electron–hole separation of hr0 i = 2.5 ± 0.1 nm, similar to that found in the pump–probe experiment. The choice of model parameters listed in Table 2 will influence the 10


Page 11 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


determined value of hr0 i, with the electron and hole mobility having the greatest impact. Note that from eq 1 the charge carrier survival probability, and hence hr0 i, is dependent on the sum of the electron and hole mobilities. The values used in our analysis are towards the lower bound of those measured in neat P3HT films using field effect transistor (FET), time of flight, or space-charge limited techniques. 50–55 Due to the disordered nature of conjugated polymers, charge mobility can be dependent on the particular morphology of the aggregates. Several studies have shown significant dependence on the molecular weight (MW) of P3HT, 53,55,56 with a maximum mobility found using a MW of ∼50 kg mol−1 . Thermal annealing of films has also been shown to increase mobility, 52 though to a much lesser extent than that of MW. Electron and hole mobilities that are much greater (up to 1 or 2 orders of magnitude) than those used in this work have been observed in some FET devices. 55,56 The high mobility values have been attributed to highly crystalline morphologies in which the polythiophene backbones are arranged in-plane to the substrate, allowing efficient charge transport through the interchain π–π system. 57 Fitting our experimental data using a charge mobility that is an order of magnitude greater results in only a 60% increase in the value of hr0 i to ∼4.5 nm (Supporting Information). As the P3HT NPs appear spectroscopically similar to neat films, but do not exhibit a high degree of order, the typical values of µe = µh = 10−8 m2 V−1 s−1 are expected to be appropriate for our analysis. The magnitude of the reaction radius parameter a2 does not have a strong influence on the resulting initial separation distance. 44 We argue that a value of 0.5 nm is a reasonable estimate, which would correspond to the case when the charges are located on adjacent thiophene rings. Alternative analytical models for semiconductor charge recombination exist in the literature, 58 and have previously been applied to P3HT. 59 However, we note that these studies aimed to reproduce experimental data obtained at liquid helium temperatures on 100 ns to µs time scales. On these long time scales, charge recombination proceeds by tunnelling, as evidenced by the power-law decay with weak temperature dependence, while the initial recombination dynamics on the ps to ns time scale follow a exponential-like decay and are affected by temperature. 59 In neat P3HT, the magnitude of sub-ns recombination can be significant. For example, Piris et al. found that ∼25% of photogenerated charges in a neat P3HT film recombined on a time scale of 100 ps. 2 For the ps to ns time window of this study, a model considering short time scale dynamics and thermal effects, such as eq 1, is necessary. The P3HT NPs are stable in water and form a colloidal suspension due to the presence of a negative surface charge. 60,61 Given the diameter of the NPs is ∼30 nm (Supporting Information), it is possible that some proportion of the photogenerated holes may interact with the surface charges, rather than their geminate electron partner. For example, if a 5 nm thick surface shell is designated, then only 30% of the total NP volume can be considered as the non-interacting core. The NP samples used in this study were stabilized using a nonionic surfactant (Igepal CO-520), which would provide a shell over the photoactive P3HT core and help reduce any interactions with the charged surface or solvent. Nevertheless, the model represented by eq 1 considers only geminate recombination and if holes are able to recombine with additional intrinsic charges in a similar manner, the lifetime will appear shortened, thus the fitted value of hr0 i may be an underestimate by up to 24%. This effect is relatively minor and should not be expected to influence the conclusions from our analysis. Figure 4a summarizes our results thus far. First, an exciton is produced as a result of excitation with the 400-nm light. Second, the exciton undergoes migration to be trapped in a 11



2 1

sh

pu

3

4

5

P3HT CO-520 H 2O (a)

Push

S3

NR

(b) Exciton Dissociation

Charge Carriers

Recombination

Fluor.

NR

S1 Pump

Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 20

S0

Figure 4: (a) Illustration for events occurring during the pump–push–probe experiment: (1) Exciton produced by 400 nm pump laser. (2) Migration of exciton through polymer, possibly becoming trapped at low-energy chromophore sites. (3) Push laser induces S3 ←S1 transition, delocalizing exciton. (4) Delocalized exciton dissociates, separating electron and hole over several nanometers. (5) Electron and hole diffuse through polymer, recombining to form ground-state P3HT. (b) Energy level diagram showing processes involved in the pump–push–probe experiment. NR and Fluor. are non-radiative decay and fluorescence, respectively. The surfactant CO-520 forms the interface between the P3HT NP and H2 O.

12


Page 13 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


low-energy site (e.g., a semicrystalline domain). Then, the trapped exciton is re-excited using a push pulse (λpush = 1200 nm), which delocalizes the exciton and in turn dissociates it to form an electron and hole pair that are separated over 3 nm. The electron and hole undergo diffusion in P3HT and eventually recombine to give a ground-state species. Figure 4b shows the energetics of the pump–push–probe experiment and results. First, the pump pulse excites P3HT to a nonequilibrated S1 state, forming an exciton. Upon relaxation in the S1 manifold, the exciton is “pushed” to the S3 excited state. While a great portion of the S3 excitons undergo rapid nonradiative decay to the S1 state, approximately 10% of the high-energy exciton population undergoes dissociation to yield charge carriers. Finally, through diffusion the electron and hole undergo recombination to yield the ground state. In our previous work, pump–push–probe spectroscopy was used to study charge carrier generation in solvated P3HT chains. 29 In that work, the presence of charge carriers was in fact inferred because they are isolated on a single chain, which facilitates a rapid recombination before they can be detected. As a result, only limited knowledge can be gained from that study. 29 In the present work, however, charge carriers can separate to a further distance across multiple chains and hence exist for an extended period of time. The extended lifetime of the charge carriers, which is limited by diffusion, enables observation of the hole-polaron produced by the push pulse, and in turn, measurement of its recombination dynamics to offer insight into hr0 i. As indicated in process 3 and 4 in Figure 4a, hr0 i can be used to estimate the spatial extent of the delocalized exciton initially prepared by the push pulse, which we show to dissociate to form charge carriers. The ∼3 nm spatial extent is similar to the size of a heptamer of 3HT, which we showed in a previous report to be sufficiently large to calculate the S3 ←S1 excited-state absorption spectrum. 28 It is interesting to compare the results reported herein with the hr0 i values determined in other studies. Barker et al. used low-temperature, tunneling-controlled recombination dynamics in polymer/PCBM blends to conclude photogenerated charge pairs are produced with 3–4 nm separation within the thermalization time. 62 However, they note that their experiment can not distinguish between fast electron and hole diffusion, versus a direct method involving dissociation of a highly-delocalized exciton. In contrast, in the current study we produce charges from excitons that have had time to thermalize, and the push pulse delocalizes the exciton to form charges that have an initial separation of ∼3 nm. In another study by Matheson et al., a mean electron-hole distance of 7 nm was reported for a P3HT/PC60 BM bulk heterojunction, which was deduced using transient absorption data over a range of excitation densities. 10 This electron-hole distance is larger than the reported distance in the current study and this difference is expected because the presence of PC60 BM in a bulk heterojunction (as opposed to neat P3HT) promotes electron and hole separation. Recent work by Kaake et al. showed that charge transfer from a conjugated polymer to a fullerene acceptor occurs before localization of excitons, 11,63 highlighting the role played by delocalized excitons. While the results in the current study appear to agree with the mechanism of charge carrier generation through exciton delocalization, Kaake et al. suggested that the extent of delocalization should be similar to the exciton diffusion length, 11 which is expected to be tens of nanometers, approximately an order of magnitude larger than what is obtained in the current study. Interestingly, a recent report summarized exciton diffusion lengths of 14–20 nm for P3HT, 64 which were estimated from experiments and calculations. Exciton diffusion lengths of this magnitude show agreement with our result because the 13



observed diffusion length is likely to have contributions from exciton delocalization and hopping.

Conclusions In conclusion, we have used pump–push–probe spectroscopy to show that excitons in a P3HT nanoparticle can be delocalized using a secondary excitation, a push pulse, by accessing the S3 ←S1 electronic transition. Approximately 10% of the “pushed” excitons undergo dissociation to yield charge carriers. By using a diffusive electron-hole recombination model, an initial electron-hole distance of ∼3 nm is obtained, which can be used to infer the spatial extent of the high-energy excitons generated as a result of the push pulse excitation.

Acknowledgement The authors acknowledge the Australian Research Council for funding support (LE0989747 and DP160103797).

Supporting Information Available TEM and size distribution, push power dependence, negligible push arrival time dependence, push-only experiment/coherent artifacts from solvent, details of recombination modeling.

References (1) Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512–516. (2) Piris, J.; Dykstra, T. E.; Bakulin, A. A.; van, P. H. M., Loosdrecht; Knulst, W.; Trinh, M. T.; Schins, J. M.; Siebbeles, L. D. A. Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends. J. Phys. Chem. C 2009, 113, 14500– 14506. (3) Marsh, R. A.; Hodgkiss, J. M.; Albert-Seifried, S.; Friend, R. H. Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy. Nano Lett. 2010, 10, 923–930. (4) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Near-IR Femtosecond Transient Absorption Spectroscopy of Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films with Different Regioregularities. J. Am. Chem. Soc. 2009, 131, 16869–16880. (5) Gregg, B. A. Entropy of Charge Separation in Organic Photovoltaic Cells: The Benefit of Higher Dimensionality. J. Phys. Chem. Lett. 2011, 2, 3013–3015.

14


Page 14 of 20

Page 15 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Hood, S. N.; Kassal, I. Entropy and Disorder Enable Charge Separation in Organic Solar Cells. J. Phys. Chem. Lett. 2016, 7, 4495–4500. (7) Meng, R.; Li, Y.; Gao, K.; Qin, W.; Wang, L. Ultrafast Exciton Migration and Dissociation in π-Conjugated Polymers Driven by Local Nonuniform Electric Fields. J. Phys. Chem. C 2017, 121, 20546–20552. (8) Janković, V.; Vukmirović, N. Origin of Space-separated Charges in Photoexcited Organic Heterojunctions on Ultrafast Time Scales. Phys. Rev. B 2017, 95, 075308. (9) Polkehn, M.; Tamura, H.; Burghardt, I. Impact of Charge-transfer Excitons in Regioregular Polythiophene on the Charge Separation at Polythiophene-fullerene Heterojunctions. J. Phys. B: At., Mol. Opt. Phys. 2018, 51, 014003. (10) Matheson, A. B.; Pearson, S. J.; Ruseckas, A.; Samuel, I. D. W. Charge Pair Dissociation and Recombination Dynamics in a P3HT-PC60 BM Bulk Heterojunction. J. Phys. Chem. Lett. 2013, 4, 4166–4171. (11) Kaake, L. G.; Zhong, C.; Love, J. A.; Nagao, I.; Bazan, G. C.; Nguyen, T.-Q.; Huang, F.; Cao, Y.; Moses, D.; Heeger, A. J. Ultrafast Charge Generation in an Organic Bilayer Film. J. Phys. Chem. Lett. 2014, 5, 2000–2006. (12) Huang, D. M. Computational Study of P3HT/C60 -Fullerene Miscibility. Aust. J. Chem. 2014, 67, 585–591. (13) Chen, H.; Peet, J.; Hu, S.; Azoulay, J.; Bazan, G.; Dadmun, M. The Role of Fullerene Mixing Behavior in the Performance of Organic Photovoltaics: PCBM in Low-bandgap Polymers. Adv. Funct. Mater. 2014, 24, 140–150. (14) Chen, H.; Hegde, R.; Browning, J.; Dadmun, M. D. The Miscibility and Depth Profile of PCBM in P3HT: Thermodynamic Information to Improve Organic Photovoltaics. Phys. Chem. Chem. Phys. 2012, 14, 5635–5641. (15) Collini, E.; Scholes, G. D. Coherent Intrachain Energy Migration in a Conjugated Polymer at Room Temperature. Science 2009, 323, 369–373. (16) Falke, S. M.; Rozzi, C. A.; Brida, D.; Maiuri, M.; Amato, M.; Sommer, E.; De Sio, A.; Rubio, A.; Cerullo, G.; Molinari, E. et al. Coherent Ultrafast Charge Transfer in an Organic Photovoltaic Blend. Science 2014, 344, 1001–1005. (17) Zarrabi, N.; Burn, P. L.; Meredith, P.; Shaw, P. E. Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor–Acceptor Organic Solar Cell Blends. J. Phys. Chem. Lett. 2016, 7, 2640–2646. (18) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154–6164.

15



(19) Chen, K.; Barker, A. J.; Reish, M. E.; Gordon, K. C.; Hodgkiss, J. M. Broadband Ultrafast Photoluminescence Spectroscopy Resolves Charge Photogeneration via Delocalized Hot Excitons in Polymer:Fullerene Photovoltaic Blends. J. Am. Chem. Soc. 2013, 135, 18502–18512. (20) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G. Hot Exciton Dissociation in Polymer Solar Cells. Nat. Mater. 2013, 12, 29–33. (21) Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F. et al. Efficient Charge Generation by Relaxed Charge-transfer States at Organic Interfaces. Nat. Mater. 2014, 13, 63–68. (22) Albrecht, S.; Vandewal, K.; Tumbleston, J. R.; Fischer, F. S. U.; Douglas, J. D.; Fréchet, J. M. J.; Ludwigs, S.; Ade, H.; Salleo, A.; Neher, D. On the Efficiency of Charge Transfer State Splitting in Polymer:Fullerene Solar Cells. Adv. Mater. 2014, 26, 2533–2539. (23) Petersen, A.; Ojala, A.; Kirchartz, T.; Wagner, T. A.; W¨ urthner, F.; Rau, U. Fielddependent Exciton Dissociation in Organic Heterojunction Solar Cells. Phys. Rev. B 2012, 85, 245208. (24) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Voorhis, T. V. Charge Transfer State Versus Hot Exciton Dissociation in PolymerFullerene Blended Solar Cells. J. Am. Chem. Soc. 2010, 132, 11878–11880. (25) Bassler, H.; Kohler, A. “Hot or Cold”: How Do Charge Transfer States at the Donoracceptor Interface of an Organic Solar Cell Dissociate? Phys. Chem. Chem. Phys. 2015, 17, 28451–28462. (26) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340–1344. (27) Ling, S.; Schumacher, S.; Galbraith, I.; Paterson, M. J. Excited-State Absorption of Conjugated Polymers in the Near-Infrared and Visible: A Computational Study of Oligofluorenes. J. Phys. Chem. C 2013, 117, 6889–6895. (28) Roseli, R. B.; Tapping, P. C.; Kee, T. W. Origin of the Excited-State Absorption Spectrum of Polythiophene. J. Phys. Chem. Lett. 2017, 8, 2806–2811. (29) Tapping, P. C.; Kee, T. W. Optical Pumping of Poly(3–hexylthiophene) Singlet Excitons Induces Charge Carrier Generation. J. Phys. Chem. Lett. 2014, 5, 1040–1047. (30) Wu, C.; Szymanski, C.; McNeill, J. Preparation and Encapsulation of Highly Fluorescent Conjugated Polymer Nanoparticles. Langmuir 2006, 22, 2956–2960. 16


Page 16 of 20

Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Pecher, J.; Mecking, S. Nanoparticles of Conjugated Polymers. Chem. Rev. 2010, 110, 6260–6279. (32) Jiang, Y.; McNeill, J. Light-Harvesting and Amplified Energy Transfer in Conjugated Polymer Nanoparticles. Chem. Rev. 2017, 117, 838–859. (33) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; G¨ untner, R.; Scherf, U. Novel Approaches to Polymer Blends Based on Polymer Nanoparticles. Nat. Mater. 2003, 2, 408–412. (34) Landfester, K.; Montenegro, R.; Scherf, U.; G¨ untner, R.; Asawapirom, U.; Patil, S.; Neher, D.; Kietzke, T. Semiconducting Polymer Nanospheres in Aqueous Dispersion Prepared by a Miniemulsion Process. Adv. Mater. 2002, 14, 651–655. (35) Clafton, S. N.; Huang, D. M.; Massey, W. R.; Kee, T. W. Femtosecond Dynamics of Excitons and Hole-Polarons in Composite P3HT/PCBM Nanoparticles. J. Phys. Chem. B 2013, 117, 4626–4633. (36) Shen, X.; Li, L.; Wu, H.; Yao, S. Q.; Xu, Q.-H. Photosensitizer-doped Conjugated Polymer Nanoparticles for Simultaneous Two-photon Imaging and Two-photon Photodynamic Therapy in Living Cells. Nanoscale 2011, 3, 5140–5146. (37) Hu, Z.; Gesquiere, A. J. PCBM Concentration Dependent Morphology of P3HT in Composite P3HT/PCBM Nanoparticles. Chem. Phys. Lett. 2009, 476, 51–55. (38) Clarke, T. M.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Free Energy Control of Charge Photogeneration in Polythiophene/Fullerene Solar Cells: The Influence of Thermal Annealing on P3HT/PCBM Blends. Adv. Funct. Mater. 2008, 18, 4029–4035. (39) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Dependence of Regioregular Poly(3-hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight. Macromolecules 2005, 38, 3312–3319. (40) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Sthn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells. Adv. Funct. Mater. 2005, 15, 1193–1196. (41) Cook, S.; Furube, A.; Katoh, R. Analysis of the Excited States of Regioregular Polythiophene P3HT. Energy Environ. Sci. 2008, 1, 294–299. (42) Takeda, N.; Asaoka, S.; Miller, J. R. Nature and Energies of Electrons and Holes in a Conjugated Polymer, Polyfluorene. J. Am. Chem. Soc. 2006, 128, 16073–16082. (43) Clifford, P.; Green, N. J. B.; Pilling, M. J. Analysis of the Debye-Smoluchowski Equation. Approximations for High-Permittivity Solvents. J. Phys. Chem. 1984, 88, 4171– 4176.

17



(44) Goulet, T.; Jay-Gerin, J.-P. On the Reactions of Hydrated Electrons with OH· and H3 O+ . Analysis of Photoionization Experiments. J. Chem. Phys. 1992, 96, 5076–5087. ¨ (45) Einstein, A. Uber die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Fl¨ ussigkeiten suspendierten Teilchen. Ann. Phys. 1905, 322, 549–560. (46) Debye, P. Reaction Rates in Ionic Solutions. Trans. Electrochem. Soc. 1942, 82, 265. (47) Smoluohowski, M. V. Versuch einer mathematischen Theorie der Koagulationskinetik kolloider Lösungen. Z. Phys. Chem. 1917, 92, 129–168. (48) Fuentes-Hernandez, C. In Photorefractive Organic Materials and Applications; Blanche, P.-A., Ed.; Springer International Publishing, 2016; pp 65–127. (49) Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of Interchain Interactions on the Absorption and Emission of Poly(3–hexylthiophene). Phys. Rev. B 2003, 67, 64203. (50) Mills, T.; Kaake, L. G.; Zhu, X.-Y. Polaron and Ion Diffusion in a Poly(3hexylthiophene) Thin-film Transistor Gated with Polymer Electrolyte Dielectric. Appl. Phys. A 2009, 95, 291–296. (51) Yang, K.; Wang, Y.; Jain, A.; Samulson, L.; Kumar, J. Determination of Electron and Hole Mobility of Regioregular Poly(3-hexylthiophene) by the Time of Flight Method. J. Macromol. Sci., Part A 2007, 44, 1261–1264. (52) Mihailetchi, V. D.; Xie, H. X.; de Boer, B.; Koster, L. J. A.; Blom, P. W. M. Charge Transport and Photocurrent Generation in Poly(3-hexylthiophene): Methanofullerene Bulk-heterojunction Solar Cells. Adv. Funct. Mater. 2006, 16, 699–708. (53) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Fréchet, J. M. J. Molecularweight-dependent Mobilities in Regioregular Poly(3-hexyl-thiophene) Diodes. Appl. Phys. Lett. 2005, 86, 122110. (54) Kim, Y.; Cook, S.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. Organic Photovoltaic Devices Based on Blends of Regioregular Poly(3-hexylthiophene) and Poly(9,9-dioctylfluorene-co-benzothiadiazole). Chem. Mater. 2004, 16, 4812–4818. (55) Kline, R.; McGehee, M.; Kadnikova, E.; Liu, J.; Fréchet, J. Controlling the Field-Effect Mobility of Regioregular Polythiophene by Changing the Molecular Weight. Adv. Mater. 2003, 15, 1519–1522. (56) Fumagalli, L.; Binda, M.; Natali, D.; Sampietro, M.; Salmoiraghi, E.; Di Gianvincenzo, P. Dependence of the Mobility on Charge Carrier Density and Electric Field in Poly(3-hexylthiophene) Based Thin Film Transistors: Effect of the Molecular Weight. J. Appl. Phys. 2008, 104, 084513.

18


Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(57) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; LangeveldVoss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P. et al. Two-dimensional Charge Transport in Self-organized, High-mobility Conjugated Polymers. Nature 1999, 401, 685. (58) Brosseau, C.-N.; Perrin, M.; Silva, C.; Leonelli, R. Carrier Recombination Dynamics in Inx Ga1−x N/GaN Multiple Quantum Wells. Phys. Rev. B 2010, 82, 085305. (59) Paquin, F.; Latini, G.; Sakowicz, M.; Karsenti, P.-L.; Wang, L.; Beljonne, D.; Stingelin, N.; Silva, C. Charge Separation in Semicrystalline Polymeric Semiconductors by Photoexcitation: Is the Mechanism Intrinsic or Extrinsic? Phys. Rev. Lett. 2011, 106, 197401. (60) Clafton, S. N.; Beattie, D. A.; Mierczynska-Vasilev, A.; Acres, R. G.; Morgan, A. C.; Kee, T. W. Chemical Defects in the Highly Fluorescent Conjugated Polymer Dots. Langmuir 2010, 26, 17785–17789. (61) Jin, Y.; Ye, F.; Wu, C.; Chan, Y.-H.; Chiu, D. T. Generation of Functionalized and Robust Semiconducting Polymer Dots with Polyelectrolytes. Chem. Commun. 2012, 48, 3161–3163. (62) Barker, A. J.; Chen, K.; Hodgkiss, J. M. Distance Distributions of Photogenerated Charge Pairs in Organic Photovoltaic Cells. J. Am. Chem. Soc. 2014, 136, 12018– 12026. (63) Kaake, L. G.; Moses, D.; Heeger, A. J. Charge Transfer from Delocalized Excited States in a Bulk Heterojunction Material. Phys. Rev. B 2015, 91, 075436. (64) Tamai, Y.; Ohkita, H.; Benten, H.; Ito, S. Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement in Photovoltaic Conversion Efficiency. J. Phys. Chem. Lett. 2015, 6, 3417–3428.

19



Graphical TOC Entry

20


Page 20 of 20

Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3

Recommend Documents