Charge Separation Mechanisms in Ordered Films of Self-Assembled

Apr 21, 2017 - Interfaces , 2017, 9 (39), pp 33493–33503 ... in 1b enhances the order of the films, but does not result in an increase in free charg...
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Charge Separation Mechanisms in Ordered Films of Self-Assembled Donor−Acceptor Dyad Ribbons Jenna L. Logsdon, Patrick E. Hartnett, Jordan N. Nelson, Michelle A. Harris, Tobin J. Marks,* and Michael R. Wasielewski* Department of Chemistry, Argonne-Northwestern Solar Energy Research (ANSER) Center, and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *

ABSTRACT: Orthogonal attachment of polar and nonpolar side-chains to a zinc porphyrin-perylenediimide dyad (ZnP-PDI, 1a) is shown to result in self-assembly of ordered supramolecular ribbons in which the ZnP and PDI molecules form segregated π-stacked columns. Following photoexcitation of the ordered ribbons, ZnP+•-PDI−• radical ion pairs form in 1 × 1013 s−1 rates typically observed,7,11,12 suggesting little influence from nuclear reorganization. © XXXX American Chemical Society

Onsager-Braun theory predicts that charge generation occurs in a thermal equilibrium between the CT and CS states,13,14 as shown in Mechanism 1 (Figure 1); however, this is inconsistent with the observed high FC yields and the temperature and electric field independence of some systems.15−17 Under

Figure 1. Charge separation (CS) mechanisms in OPVs: (1) through a vibronically cooled singlet CT state that can subsequently intersystem cross to a triplet CT; (2) through a “hot” singlet CT state; (3) directly to the CS state through delocalized bandlike states. Special Issue: Hupp 60th Birthday Forum Received: February 21, 2017 Accepted: April 17, 2017

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DOI: 10.1021/acsami.7b02585 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (A) Molecules synthesized for this study. (B) Self-assembly of donor−acceptor molecular “ribbons” using incompatible polar and nonpolar side chains.

acceptor because it is easily reduced, self-assembles readily into supramolecular structures through π-stacking, has a large crosssection for visible light absorption, and is easily modified.32 ZnP was chosen as the donor because it is relatively easy to oxidize, absorbs light at wavelengths complementary to PDI, and also self-assembles into π-stacked structures.24 To further promote chromophore segregation and prevent intermolecular electron transfer, ZnP and PDI were substituted with incompatible polar and nonpolar side chains, respectively (Figure 2B).33 In addition to 1a, three other molecules were synthesized to assess the effects of side chain length (1b), the initial intramolecular radical ion pair distance (1c), and the free energy of reaction (1d) on the FC yield (Figure 2A). Covalently linking the D−A molecules provides improved control over donor−acceptor electronic coupling, and thus, the initial charge separation rates, while at the same time controlling film morphology. The supramolecular structure also guarantees that every donor is directly next to an acceptor, minimizing complications due to exciton diffusion.31

normal OPV operating conditions, Onsager-Braun theory predicts an equilibrium favoring the Coulombically trapped CT state.13,14 The initially generated singlet CT state can then recombine to ground state, equilibrate with the CS state, or intersystem cross to the triplet CT state, which can also equilibrate with the CS state or recombine to the neutral triplet state of either the donor or the acceptor. One explanation for the fast charge separation rates observed in OPVs is that a vibronically hot CT state is formed initially, followed by CS state formation coupled to vibrational relaxation, eventually leading to FCs, Mechanism 2 (Figure 1). In this case, the excess free energy for charge separation is provided by excitation above the bandgap or by a large LUMO−LUMO offset of D and A, which have been observed to increase the FC yield in some systems,12,18 while having little or no effect in others.19,20 For systems in which ultrafast charge separation occurs within 40 fs to produce an electron−hole pair separated by ∼4 nm,11 direct population of short-lived, high energy delocalized bands has been proposed to occur (Mechanism 3, Figure 1), which allows the charges to overcome their Coulombic attraction and bypass the CT state.7,8,11,16,19,21 Conflicting reports on the role of the CT state may result from the wide variety of bulk heterojunction morphologies and their inherent disorder.4,5 The majority of research on this topic has focused on the photophysics of polymer blends with fullerenes,16,19 and more recently, on high performance nonfullerene acceptors.21,22 Although these studies have led to improved device performance, they generally focus on disordered systems, where the exact morphology at the interface is unknown. Self-assembly of more ordered organic D−A materials has focused on nanostructures in solution,23,24 covalent organic frameworks,25−28 and thin films.29 For example, previous work from this laboratory on thin films of a self-assembled, covalent A−D−A molecules demonstrated the need for increased order in covalent systems to generate a high FC yield,30 a result that has been supported by other studies.21,31 Here, we investigate the photophysics of a simple D−A system in a highly ordered thin film morphology to provide deeper insights into the FC formation mechanism in the context of the mechanisms outlined in Figure 1. To simplify the morphology and photophysics, we have synthesized a zinc porphyrin (ZnP) covalently attached to a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor (1a, Figure 2A) that self-assembles into an organized supramolecular ribbon structure having segregated π-stacked hole and electron charge conduits. PDI was chosen as the

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EXPERIMENTAL SECTION

Materials Synthesis. The synthesis and characterization of compounds 1a−d are described in detail in the Supporting Information. Sample Preparation. Solution samples were prepared in toluene. Disordered films were prepared by spin-coating from a 30 mg mL−1 solution in CH2Cl2 at 1000 rpm on glass coverslips. Ordered films were prepared by doctor-blading a 30 mg mL−1 solution in N-methyl2-pyrrolidone (NMP) onto a glass coverslip. Because of the poor solubility of 1a−d in NMP, CH2Cl2 was added to fully dissolve the molecule and then the CH2Cl2 was removed, resulting in a viscous suspension, which was then doctor-bladed onto the glass coverslip. These films were pumped under vacuum for 2 days to remove any excess solvent. Electrochemistry. Electrochemical measurements were performed on a CH Instruments 660A electrochemical workstation. Samples were measured in a 0.1 M solution of tetra-n-butylammonium hexafluorophosphate (TBAPF6) in dichloromethane purged with Ar to remove O2. A 1.0 mm diameter platinum disk electrode, platinum wire counter electrode, and silver wire reference electrode were used. The ferrocene/ferrocenium couple was used as an internal standard. Steady-State Spectroscopy. Ground-state solution absorption spectra were measured using a Shimadzu UV-1800 spectrometer. Steady-state UV−vis spectroscopy of the films were measured on their glass substrates using the white light continuum generated by our previously reported fsTA apparatus.34 Fourier Transform Infrared Spectroscopy. All spectra were measured with a Bruker Tensor 37 FTIR, equipped with a mid-IR detector and KBr beamsplitter for use between 400 and 7000 cm−1 B

DOI: 10.1021/acsami.7b02585 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Transient Absorption Spectroscopy Data Analysis. The kinetic analysis was performed using home written programs in MATLAB38 and was based on a global fit to kinetic vectors following singular value decomposition. The time-resolution is given as w = 200 fs (full width at half-maximum, fwhm); the assumption of a uniform instrument response across the frequency domain and a fixed timezero (t0) are implicit in global analysis. Factoring of the two-dimensional (signal vs time and frequency) data set by Singular Value Decomposition (SVD) is performed as implemented in the MATLAB software package.38 This factoring produces an orthonormal set of basis spectra that describe the wavelength dependence of the species and a corresponding set of orthogonal vectors that describe the time-dependent amplitudes of the basis spectra.39 These kinetic vectors are then fit using the global analysis method described below. We globally fit the data set to a specified kinetic model and use the resultant populations to deconvolute the data set and reconstruct species-associated spectra. We use a first-order kinetic model with rate matrix K

with an ATR attachment. Disordered spectra were acquired by drop casting a CH2Cl2 solution sample onto the ATR surface then drying for 20 min. Samples for the ordered spectra were prepared in the same way as the film preparation and then the sample was scraped off the glass coverslip and placed onto the ATR surface. Optical Microscopy. All images were collected using a Nikon upright microscope equipped with a camera. SAXS/WAXS Measurements. Small- and wide-angle X-ray scattering (SAXS/WAXS) measurements were carried out at beamline 12ID-B at the Advanced Photon Source (APS), Argonne National Laboratory. Samples were loaded into 2 mm quartz capillaries with a wall thickness of 0.2 mm. The X-ray scattering instrument utilizes a double-crystal Si(111) monochromator and a two-dimensional mosaic CCD detector. Scattering intensity is reported as a function of the modulus of the scattering vector q, related to the scattering angle 2θ by the equation q = (4π/λ) sin θ, where λ is the X-ray wavelength. The sample-to-detector distance was adjusted to measure across two detection ranges of q, 0.006−0.3 Å−1 and 0.1−1.6 Å−1. Subtracting the solvent scattering intensity (Isolvent) from the sample scattering (Isample) gives the scattering contributed by the solute (Isolute = Isample − Isolvent). GIWAXS Measurements. Grazing incidence X-ray scattering (GIWAXS) measurements were carried out at Beamline 8-ID-E of the Advanced Photon Source at Argonne National Laboratory. The Xray beam (7.35 keV, 1.6868 Å) was incident on all samples at an angle of 0.2° in order to maximize film scatter and minimize background scatter from the glass substrate. Samples were measured under vacuum conditions with a Pilatus 1 M detector. Scattering coordinates are expressed in terms of q = 4πsin(θ)/λ, and can be related to the dspacing by q = 2π/d. GIXSGUI. A MATLAB-based program, was used to apply pixel efficiency, polarization, flat field, and solid angle corrections to the detector images, as well as to display images and take linecuts.35 Correlation lengths were calculated using a modified version of Scherrer analysis that accounts for instrument broadening and detection limits.36 The Scherrer constant was set to K = 0.866. The reported correlation lengths represent a lower limit as broadening from crystal structure defects was not accounted for. Femtosecond Transient Absorption Spectroscopy (fsTA). Solution transient absorption experiments were performed as described previously.34 The samples were excited at 525 nm with 0.5 μJ/pulse and a 200 μm diameter spot size. Solutions were prepared in toluene with an optical density of 0.3 in a 2 mm glass cuvette. Low fluence transient absorption experiments for thin films were performed as described previously.37 All samples were excited with a 525 nm, 20 nJ, 100 fs laser pulse. The spot size was enlarged to 1 mm diameter (fluence = 2.5 × 10−6 J/cm2) to avoid singlet−singlet annihilation. Low temperature transient absorption measurements were performed using a Janis VNF-100 cryostat and a Cryo-Con 32B temperature controller. The outer edges of the film samples were coated with a thermally conductive paste and then cooled from 295 to 95 K. Room temperature measurements were conducted before and after the sample was cooled to ensure no degradation occurred. Nanosecond Transient Absorption Spectroscopy (nsTA). Nanosecond transient absorption (nsTA) experiments were performed by exciting the sample with 7 ns, 1.5 mJ, 500 nm pulses using the frequency-tripled output of a Continuum Precision II 8000 Nd:YAG laser pumping a Continuum Panther OPO. The probe pulse is generated using a xenon flashlamp (EG&G Electro-Optics FX-200), and is overlapped with the pump pulse at the sample, with the pump focused to a spot size slightly larger than the probe. Samples were kept under vacuum inside a cryostat (VPF-100, Janis Research) throughout the duration of experiments to minimize photochemical degradation. Kinetic traces were acquired using a monochromator and photomultiplier tube (Hamamatsu R928) with high voltage applied to only 4 dynodes, and recorded with a LeCroy Wavesurfer 42Xs oscilloscope interfaced to a customized LabVIEW program (LabVIEW v. 8.6.1). Spectra were constructed from the average of the single wavelength kinetic traces taken from 700 to 720 nm, in 5 nm intervals. Each kinetic trace is representative of an average of 150 shots over a 5 μs time window.

⎛− kA → B 0 0 ⎜ ⎜ kA → B − kB → C 0 K=⎜ kB → C − k C → D ⎜ 0 ⎜ 0 kC→D ⎝ 0

0⎞ ⎟ 0⎟ ⎟ 0⎟ ⎟ 0⎠

(1)

The MATLAB program numerically the solves the differential equations through matrix methods,40 then convolutes the solutions with a Gaussian instrument response function with width w (fwhm), before employing a least-squares fitting using a Levenberg−Marquardt or Simplex method to find the parameters which result in matches to the kinetic data. The PDI−• feature for all film data was integrated from 620 to 750 nm to improve the signal-to-noise. Kinetic analysis for films of molecules 1a−d kinetic analysis is performed in Origin 2015 by fitting the time-dependent signals S(t) to the convolution of Gaussian instrument response with temporal width w and (i) a multiexponential decay with amplitudes ai and time constants τi, (ii) a delta function with amplitude a0, centered at the zero of pump−probe delay (t0) to account for instrument-limited coherence artifacts, and (iii) offsets for before (S0) and after (S0′) t0 to account for any signals present beyond the experimental window: 2

2

S(t ) = e−t / w ⎧ S0 ⎪ ⎪ ⎨ S ′ + a0δ(t − t0) + ⎪ ⎪ 0 ⎩

t < t0 N

∑ aiexp[−(t − t0)]/τi

t ≥ t0

i=1

(2) The time-resolution is given as 2w ln 2 = 200 fs (full width at halfmaximum, fwhm). Time-Resolved EPR (TREPR) Spectroscopy and TimeResolved Microwave Conductivity (TRMC). Film samples on glass slides (2.5 cm2) were sliced to fit in dry quartz tubes (4.0 mm o.d. × 3.0 mm i.d.), degassed, and flame-sealed under vacuum ( 0), where 2J (spin−spin exchange interaction) depicts a large S-T energy splitting, where S is the singlet state of the radical pair and T+1, T0, and T−1 are the triplet sublevels split by the Zeeman interaction in the high magnetic field limit. (C) Radical pair energy levels and transitions in the high magnetic field limit following S-T+1 mixing to yield ΦA and ΦB, which are initially populated. The arrows indicate the microwave-induced transitions responsible for the observed spin-polarized TREPR spectra.

population has a lifetime on the order of 100 μs, which is attributed to the formation of FCs, and is consistent with that observed for FCs in other systems.48 The slow decay of the 720 nm PDI−• feature is fit to a power law with α = 0.3 (Figure 5C), similar to those seen in many polymer/fullerene systems for dispersive bimolecular recombination of separated charges.49 This suggests that the electrons and holes are reaching energetic traps during recombination, whereas α = 1 would correspond to trap-free bimolecular recombination.50 Possible traps include the ends of the ribbons or defects resulting from small amounts of alternate ZnP - PDI stacking modes within the ribbons. The long-lived FCs are not present in the fsTA spectra of the disordered film, where the CT state population has a fast biexponential decay of τCR1 = 89 ps (53%) and τCR2 = 460 ps (47%), which is likely due to intermolecular charge recombination between ZnP+• and PDI−• (Figure 5B, Figure S6). Probing the CT State. The time-resolved EPR (TREPR) spectrum of ZnP+•-PDI−• in the ordered film of 1a shown in Figure 6A was measured at 350 ns following a 7 ns, 525 nm laser pulse using direct microwave detection. The TREPR spectrum is emissively spin-polarized as a result of the radical ion pair formation mechanism described below (see Discussion). The narrow width (∼1 mT) of the TREPR spectrum indicates that the primary photogenerated transient species is a radical ion pair rather than the neutral excited triplet states of either ZnP51 or PDI23 (usually ∼100 mT wide), supporting the assignments in the transient absorption experiments. The emissive TREPR spectrum indicates that the spin−spin exchange interaction (2J), which depends exponentially on the distance between the two spins is large, implying that the hole and electron comprising the CT state in the ordered film of 1a have not separated more than about 2 nm.52 If the radical ion pair distance were larger, the corresponding value of 2J would be much smaller, so that one would observe the mixed emission-absorption TREPR spectrum characteristic of S-T0 mixing.52,53 We also confirmed the presence of long-lived FCs with timeresolved microwave conductivity (TRMC) measurements. Qualitatively, the ordered films of 1a show a large increase in the photoconductivity signal over that of the disordered films (Figure 7). The signal in the ordered films persists past the 50

Figure 7. TRMC for ordered and disordered films of 1a.

μs window of this experiment, which is consistent with the time scale of slow charge recombination and detrapping processes.28,48,54

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DISCUSSION Effect of Order on Free Charge Carrier Yield. Increasing order in the ZnP-PDI dyad film increases the FC yield, which is most likely due to confinement of the holes and electrons in their respective ZnP and PDI segregated π stacks, allowing the FCs to move through the segregated stacks before recombining. However, polymer/fullerene BHJs have been observed with nearly 100% internal quantum efficiencies (IQE)55 suggesting quantitative FC yields compared to the present 30% yield. Although films of 1a appear to be highly ordered, alternating D−A stacking, other forms of disorder, or facile charge recombination mechanisms (Figure 1) could result in the lower yield of free carriers. As many researchers have suggested, creating a more highly ordered system could in principle reduce traps and improve the overall FC yield.21,30,31,56,57 In order to test this idea, an analog of molecule 1a was prepared with extended tails to promote further ZnP and PDI segregation (1b, Figure 2A). SAXS/ WAXS measurements on the preassembled NMP solution of 1b show increased order over 1a (Figure S3B), where the oscillations and diffraction peaks present in the data suggest a far more ordered assembly in solution than for 1a.44 Furthermore, a comparison of the GIWAXS data for films of F

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on the radical ion pair distance, rDA, reflecting the distance dependence of the electronic coupling matrix element between ZnP+• and PDI−•,52 whereas the magnetic dipolar coupling (D) between the spins depends on 1/r3DA, thus providing radical ion pair distance gauges. The isotropic spin−spin exchange interaction splits the singlet and triplet spin manifolds (Figure 6B). For CT states having large negative free energies of charge recombination, such as ZnP+•-PDI−•, where recombination occurs in the Marcus inverted region of the rate vs free energy dependence, it has been shown that 2J > 0, which places the singlet energy level above those of the triplet sublevels at zero magnetic field using the standard sign convention of TREPR spectroscopy.62 Application of a magnetic field results in Zeeman splitting of the 3(D+•−A−•) triplet sublevels, which are described by the T+1, T0, and T−1 states quantized along the field in the high field limit typical of EPR spectroscopy, whereas the 1(D+•− A−•) energy level (S) remains field invariant (Figure 6B).53 At the relatively short 1.4 nm ZnP+•-PDI−• distance within covalent dyad 1a, 2J is large enough to preclude significant spin state mixing at magnetic field strengths accessible for EPR spectroscopy.63 However, as the radical ion pair distance increases as a result of intermolecular electron and/or hole delocalization and migration in the segregated π stacks, 2J decreases to values for which either S-T+1 (2J > 0) and/or S-T0 mixing can occur, driven largely by electron−nuclear hyperfine interactions within the two radical ions. At that point, intersystem crossing can occur to produce 3(D+•−A−•) from 1 (D+•−A−•).53 The spins of the two radical ions remain correlated until spin relaxation, which usually occurs on the order of a few microseconds at room temperature, destroys it. When the radical ion pair distance is ∼1.5−2 nm, 2J is usually large enough that S-T+1 mixing occurs to produce states |ΦA⟩ and |ΦB⟩ (Figure 6C).64 When the radical ion pair distance is ≳ 2 nm, 2J decreases further and S-T0 mixing usually dominates.64 Given that rapid charge separation within covalent dyad 1a initially forms a singlet CT state, |ΦA⟩ and |ΦB⟩ are populated exclusively, so that microwave-induced transitions between these states and the unpopulated |T0⟩ state result in an all emissive spin-polarized EPR spectrum at early times.53 Additionally, S-T+1 mixing may occur during diffusional encounters of charge carriers, briefly increasing 2J.65 The decay of the spin-polarized TREPR signal resulting from the CT state is much longer (τ1 = 0.42 μs (80%); τ2 = 3.1 μs (20%)) than the 70% of ion pairs that recombine back to the ground state (τ < 1 ns). Given the radical ion pair distance implied by the magnitude of 2J, the long CT state lifetime, and the fact that ΔGCS = −0.32 eV, it is possible that some of the ZnP+•-PDI−• population responsible for the TREPR signal can overcome the Coulombic barrier to produce FCs. Note that we do not see any evidence for the CT state decay leading back either to 3*ZnP or 3*PDI. Onsager-Braun theory predicts that the FC yield resulting from the CT state should be sensitive to the so-called capture radius, rc, which is the distance below which the ions comprising the ion pair remain Coulombically bound, eq 3

molecules 1a and 1b shows that the longer tails in 1b result in a higher degree of order and crystallinity, evident by the increase in the intensity and sharpness of the lamellar stacking peaks, resulting in an average correlation length of 25.3 nm as compared to 14.5 nm for 1a (Figure 4, Table S2). However, the lamellar peaks of 1b are not evenly spaced as in 1a, likely due to interdigitation of the long tails. The decay of the long-lived PDI−• absorption in the nsTA data for 1b was fit to a power law with α = 0.6 (Figure S10), suggesting fewer or shallower traps during recombination than 1a.50 This confirms that there is enhanced order in films of 1b relative to those of 1a. Interestingly, despite the increased order in films of 1b, the fsTA spectra of 1b show no difference in the FC yield within experimental error relative to that of 1a (Table 1, Figure S9). Therefore, structural disorder is most likely not Table 1. FC Yield in Ordered Films of Molecules 1a−d molecule 1a 1b 1c 1d

FC yield (%) 30 27 20 27

± ± ± ±

3 3 2 1

the principal factor limiting FC yield in this system, so that a closer examination of the possible charge separation mechanisms is necessary, as discussed below. Charge Separation Mechanism 1. The free energy of reaction for charge separation in BHJs can be approximated by the LUMO−LUMO offset of the donor and acceptor materials. Many studies have focused on optimizing the energy offsets as a means to generate a higher FC yield.58 Other studies have shown that optimizing the morphology and domain size rather than the free energy of reaction has a greater effect on the FC yield.19,20 While the effect of the energy offsets at the D−A interface on the overall FC yield is currently debated, free energies of reaction greater than the exciton binding energy (∼0.3 eV) generally tend to cause fast charge separation.59 In covalent dyad 1a, the free energy of charge separation, calculated using the Weller equation, is ΔGCS = −0.32 eV, which should result in rapid charge separation because it is similar to the total nuclear reorganization energy for this process (Table S1).60 Based on our earlier work probing the reduction of PDI-containing thin films,30 we do not expect the redox potentials for ZnP and PDI to change significantly in these films relative to their values in solution. A quantitative yield of ZnP+•-PDI−• appears within 200 fs, exceeding the charge separation time constant of dyad 1a in toluene by almost 2 orders of magnitude, which is commonly seen in many BHJ systems.11,12,19,61 The additional low amplitude growth component (τ = 2.1 ± 0.2 ps) in the PDI−• transient absorption kinetics of the ordered film may result from a small CT state population that does not form within the