Article pubs.acs.org/JPCC
Electron Paramagnetic Resonance Tracing of Electronic Transfers in Push−Pull Copolymers/PCBM or Nanocrystal Composites B. Pépin-Donat,*,†,‡,§ C. Ottone,†,‡,§ Christophe Morell,∥ C. Lombard,†,‡,§ A. Lefrançois,†,‡,§ P. Reiss,†,‡,§ M. Leclerc,⊥ and S. Sadki†,‡,§ †
Univ. Grenoble Alpes, INAC-SPRAM, F-38000 Grenoble, France CNRS, INAC-SPRAM, F-38000 Grenoble, France § CEA, INAC-SPRAM, F-38000 Grenoble, France ∥ Université de Lyon, Université Lyon 1(UCBL) et UMR CNRS, 5280 Sciences Analytiques, 5 rue de la Doua, F-69622 Villeurbanne Cedex, France ⊥ Department of Chemistry, Université Laval, Quebec City, QC, G1V 0A6, Canada ‡
S Supporting Information *
ABSTRACT: Transfers of photoexcited electrons between poly(thieno[3,4-c]pyrrole-4,6-dione)-based copolymers (push−pull type) and fullerene or nanocrystals were studied by light-induced electron paramagnetic resonance (EPR). EPR tracing methodology has been used: EPR signatures of the various species taking part in the electron-transfer processes were determined and used to monitor potential charge transfers in the studied composites. The EPR tracing method combined with DFT calculations revealed that excited electrons are not only promoted from the HOMO of the polymer but also from its HOMO − 1 and even HOMO − 2, which are either exclusively centered on the push moiety or delocalized over both the push and pull units.
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(NCs)7−11 have been investigated as new acceptor species because of their tunable absorption properties. Light-induced electron paramagnetic resonance (LEPR) is a very efficient tool for studying electronic transfers between D and A units on the molecular scale because it allows in situ detection of the paramagnetic species resulting from these transfers.3,12−18 In addition, EPR and LEPR techniques are also valuable tools for investigating aging processes in photovoltaic active layers.19 We herein report EPR and LEPR studies of various push− pull thieno[3,4-c]pyrol-4,6-dione-based copolymer (donor)/ PCBM or CuInS2 NC (acceptor) systems. We develop an original EPR tracing methodology, which consists of determining EPR signatures for the various paramagnetic species taking part in the electron-transfer processes and following their fate as a function of illumination. This enables us to study eventual push−pull effects within some polymers and reveal from which substructures of the donor polymers electrons are transferred toward the acceptors. Theoretical calculations have been undertaken; they support the conclusions drawn from EPR tracing and allow the determination of the molecular orbitals from which electrons are promoted. This study provides key
INTRODUCTION
Polymer bulk heterojunction (BHJ) solar cells offer great opportunities because of their unique features such as low-cost production, light weight, and mechanical flexibility.1,2 Basically, an ultrafast transfer of photoexited electrons occurs at the bulk heterojunction between the donor polymer (D) and the acceptor species (A) (usually fullerene derivatives) leading to charge separation3−5 followed by the percolation of charges to the electrodes and the generation of a photocurrent. In order to improve the performance of BHJ solar cells, both the molecular scale (interactions between electron-donor and electronacceptor components) and macroscopic scale (morphology of the active layer and percolation pathways to the collecting electrodes) have to be optimized. On the molecular scale, in addition to ensuring close contact between D and A species, energy levels have to be aligned to promote electron transfer from D to A. On the polymer side (electron donor in the BHJ solar cell), using push−pull structures6 has become an efficient strategy to modulate the HOMO−LUMO energy levels and to proper match with the LUMO energy level of fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) (the most-studied electron acceptor used in BHJ solar cells), leading to a highly efficient system. However, PCBM has a narrow absorption bandwidth, which limits the absorption of light and therefore the photocurrent. Lately, nanocrystals © 2014 American Chemical Society
Received: June 11, 2014 Revised: August 13, 2014 Published: August 13, 2014 20647
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Scheme 1. Three Push−Pull Polymers Studieda
a
Push and pull units are varied step by step from P1 to P3.
diffractometer and Cu Kα radiation. Electrochemical measurements consisted of voltammetric measurements performed in a glovebox under an argon atmosphere. The electrochemical instrumentation consists of an Autolab3 potentiostat/galvanostat. Potentials were recorded versus an Ag/Ag+ pseudoreference electrode using an ionic liquid (1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl) imide) as the electrolyte and Pt as the counter electrode and work electrode (diameter: 1.9 mm). Samples were prepared by drop-casting 10 μL of a 20 mg/mL NC colloidal solution in chloroform on the work electrode. Free ligands were measured in 0.01 M solution in an ionic liquid. Polymer/PCBM and /PolymerNCs Blends. Preparation. First, solutions of (i) neat P1, P2, and P3 in o-dichlorobenzene (ODCB) (5 mg/mL), (ii) P1, P2, or P3 in ODCB (5 mg/mL) with PCBM (polymer/PCBM weight ratio: 1:3), and (iii) P1, P2, or P3 in ODCB (5 mg/mL) with CuInS2 NCs (polymer/ NC weight ratio: 1:2) were prepared. Second, films were obtained by drop-casting these solutions on a flexible substrate of poly(ethylene terephthalate) (PET)/ITO. Third, films deposited on the PET/ITO substrate were annealed at 110 °C for 3 h and finally introduced, once cut, into the EPR tube. It is worth noting that the EPR intensities of different samples cannot be compared. Photoluminescence Measurements of Polymer/NCs Blends. Polymers were dissolved in 1,2-dichlorobenzene (DCB) to give stock solutions with a concentration of 0.5 mg/mL. For the photoluminescence (PL) measurements, they were diluted to a concentration of 0.008 mg/mL; colloidal solutions of nanocrystals in DBC with a concentration of 0.5 mg/mL were prepared. Both solutions were freshly prepared before measurement from the stock solutions using DCB as a solvent in the glovebox. For each hybrid (70, 80, or 90 mass %
information for determining the relationship between structure and electron-transfer processes in hybrid systems dedicated to photovoltaic applications.
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EXPERIMENTAL SECTION Polymers. The synthesis and electrochemical measurements of TPD-based copolymers are reported in the literature.20 Commercial Products. PCBM was purchased from Solenne BV. Copper iodide, indium III acetate (99.99%), and dodecanethiol (DDT) (98%) were purchased from Aldrich and used in the glovebox as received. Anhydrous ethanol (99%) and anhydrous chloroform (99.99%) were purchased from Acros Organics and used in the glovebox as received. 2-Ethylhexanethiol (EHT) was purchased from Aldrich and used as received. Preparation of Nanocrystals (NCs). CuInS2 NCs were prepared according to the literature.21 The NCs were capped with DDT ligands, which are exchanged with EHT by mixing 3 mL of the colloidal solution in chloroform (27 mg/mL) with 54 μL of EHT. The mixture was stirred overnight at 60 °C before adding excess ethanol to precipitate the EHT-NCs. NCs were isolated by centrifugation. They were then resolubilized in chloroform. Optical and Structural Characterization. A Zeiss ultrascan 55 scanning electron microscope (SEM) was used in STEM mode to image the NCs. UV−vis absorption and photoluminescence data were acquired with a HP 8452A diode array UV spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. EDX analyses of samples drop-cast on Si/SiO2 substrates were carried out with a JEOL JSM-840A SEM equipped with an Oxford Instruments energydispersive X-ray analyzer. Powder X-ray diffraction (XRD) was performed in reflection geometry using a Philips X’PERT 20648
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3.45 1.15 1.25 1.05 3.50 2.64 0.80 1.04 2.16 1.40 1.48 3.45 5.25 3.52 1.96
Hpp (G) ± 0.05b
2.0036 2.0034 2.0025 2.0021 2.0028 2.0028 2.0001 2.0029 1.9999 2.0017 2.0001 2.0028 2.0046 2.0070 2.0022
g factor ± 0.0002c
100 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
B
Bf 0.0 0.0 0.0 100 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
L 95 ± 1 2.5 ± 0.3 1.7 ± 0.3 0.8 ± 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.0 ± 0.4 13 ± 1 4.1 ± 0.4 18.7 ± 0.6 0.0 4.5 ± 0.6 1.4 ± 0.2 48 ± 3 5.6 ± 0.4 0.0 0.0 0.0 0.0 0.0
L
%I P1/PCBMd
B 0.0 0.0 4.0 ± 0.6 0.6 ± 0.4 0.0 27 ± 2 27 ± 2 0.0 40 ± 3 0.0 0.0 0.0 0.0 0.0 0.0
L
%I P2/PCBMd
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 11.0 ± 0.5 89 ± 1 10.7 ± 0.5 0.0
B
L 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 15.0 ± 0.5 85 ± 1 10.7 ± 0.5 0.0
%I P3d
Bf 0.0 0.0 0.0 0.0 25 ± 2 0.0 0.0 0.0 55 ± 4 0.0 20 ± 2 0.0 0.0 0.0 0.0
L
%I P3/ PCBMd
Bf 0.0 11 ± 1 10 ± 1 2.2 ± 0.5 22 ± 2 0.0 0.0 2.4 ± 0.5 0.0 0.0 0.0 0.0 0.0 0.0 52 ± 4
L
%I P1/NCd
0.0
Bf 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7±1 28 ± 2 65 ± 3 0.0 0.0
L
%I P3/NCd
intrinsic defects of p1 or of lateral chain TPD [HOMO − 2 of P1] carbazole [HOMO of P1 and P2] carbazole [HOMO − 1 of P1 and P2] TPD [HOMO of P1] bis TPD [HOMO of P2] na na PCBM na na TPD [HOMO of P3] BDT [HOMO of P3] impurities of P3g carbazole or TPD contribution du to the transfer from P1 to the excited nanocrystal
structure mainly responsible for the EPR signal and excited orbitale
a EPR line. bLine width. cg factor. d Relative intensities of the lines observed in the different samples without illumination (B) or with illumination (L) at 20 K. eAssumed substructures responsible for the EPR line and excited orbitals at the origin of the electron transfer. fSignal too noisy to be simulated with accuracy. gThe intensity of line 13 is not taken into account to determine the relative intensities of lines 11 and 12.
1 2 3 4 5 5′ 6 7 8 9 10 11 ≅ 5 12 13 14
line
a
%I P1d
Table 1. Various Pure EPR Lines Observed at 20 K in P1, P2, and P3 and Blends of P1, P2, and P3 with PCBM and the EHT-Nanocrystal (NC)
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Scheme 2. HOMO and LUMO Energy Levels (DFT Calculations) of All of the Considered Species
of NCs), two microtubes were preloaded with 1 mL of the polymer solution. To one tube the NCs solution was added, whereas in the second tube the NC solution was substituted with the same volume of DCB in order to have identical polymer concentrations in both tubes. Afterwards, tubes were shaken for 10 min at 1200 rpm. For the PL measurements, solutions were transferred to a quartz cuvette and sealed with Parafilm. EPR and LEPR Studies of Pure Polymers and Blends of Polymers with PCBM or NCs. Experiments were carried out using an X-band (3 cm, 9.7 GHz) ER 200D SRC Bruker spectrometer with 100 kHz field ac modulation for phase-lock detection. The sample was located in a Bruker ER 41040R optical transmission resonator (unloaded quality factor Q = 7000) and illuminated at 473.3 nm with a CW output power of 22.1 mW by a laser module Oxxius 473L-20-COL-PP-LAS01186. Low-temperature experiments were performed with an ESR 900 cryostat. Magnetic field intensities and frequencies were separately measured to ensure accurate g values (±0.0002). Operating conditions were chosen to avoid
significant power saturation and modulation broadening. When spectra are compared, they are recorded under the same conditions (gain, modulation, accumulation times, accumulation number, and microwave power); the magnetic field used to compare various spectra (Href) corresponds to an arbitrary frequency of 9.426060 GHz. For EPR and LEPR data analysis, the number of EPR lines present in the global signal, their g Landé parameters, line shapes (Gaussian or Lorentzian or both with relative g-centered contribution), peak-to-peak line width, and relative intensities were determined after the simulation of each experimental spectrum with home developed software (Supporting Information). Theoretical Calculations. With the aim to gaining a better understanding of the electronic structure of the polymer, density functional theory (DFT) calculations22,23 have been performed with Gaussian 09.24 For now, it is quite difficult to afford the modeling of a whole polymer, so only the constituting push and pull units have been calculated along with the push−pull monomer unit. All of the molecules have been fully optimized at the B3LYP25,26/6-31+G* level of 20650
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Figure 1. (A) EPR spectra at 20 K of P1 alone in the dark (dashed line) and under illumination at λ = 473 nm (solid line). (B) EPR spectra of P1/ PCBM at 20 K without illumination (solid line) (the very similar spectrum to that obtained at RT without illumination is not presented here) and EPR spectra obtained under illumination at 473 nm at RT (dotted line) and at 20 K (dashed line). (C) Spectra of P1/PCBM at 20 K without (solid line) and with (dashed line) illumination at 473 nm and recorded at 20 K after 6 and 27 mn of relaxation after stopping the illumination (square and triangular dots). After few hours, we do not observe any remaining signal.
LUMO levels of the three polymers lie in the ranges of −5.08/ −5.62 eV and −3.13/−3.53 eV, respectively. STEM images show that the CuInS2 NCs exhibit a roughly spherical shape with a mean size of 7 ±1 nm (Supporting Information); their powder X-ray diffractogram exhibits the characteristic peaks of the tetragonal chalcopyrite structure.29 EDX analyses exhibit a slightly indium-rich composition with a Cu/In ratio of 0.80. Replacing initial DDT molecules on the surface of the NCs by EHT results in a reduction of the insulating barrier (C6 alkyl chain length instead of C12) and favors charge transfer with conjugated polymers. The values of the electron affinity and ionization potential of the EHTcapped CuInS2 NCs used in this study have been determined by means of differential pulse voltammetry. They account for −4.09 and −5.56 eV, respectively, resulting in an electrochemical band gap of 1.47 eV. Photoluminescence Quenching of Polymer/NC Mixtures in Solution. In several reported examples of organic/ inorganic molecular hybrids, the photoluminescence (PL) is efficiently quenched via charge and/or energy transfer as a consequence of the relative energy-level alignment.30,31 We did not detect any PL signal for EHT-capped NCs alone. However, all three polymers exhibit broad emission peaks at around 530
theory,27 and their frequencies have been checked to determine whether the process has led to a genuine minimum. Timedependent (TD) computations have also been performed at the same level of theory28 on each monomers unit to assess their excitation energies and transition dipole moments and to unravel which orbital is actually excited. The latter calculations have been made using time-dependent DFT at the same level of calculations. This level of theory is known to give fairly accurate results regarding excitation processes. Finally, the radical cations of each monomer unit have been computed to extract their spin densities.
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RESULTS AND DISCUSSION
Characteristics of the Polymer Donors and NCs Acceptors. Polymers P1, P2, and P3 consist of a push and a pull unit (Scheme 1). In P1 and P2, the push units are the same (carbazole), while the pull units are different (thienopyrroledione (TPD) and bis-TPD, respectively). Likewise, in P1 and P3 the pull units are identical (TPD) while the push units differ (carbazole- and dialkoxy-substituted benzodithiophene (BTD), respectively). The electrochemically determined HOMO and 20651
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Figure 2. EPR spectrum at 20 K, under illumination (λ = 473 nm) of (A) P1/PCBM, (B) P2/PCBM, and (C) P3/PCBM (solid lines) together with their simulation (dashed lines). Pure lines used for the simulation are presented (dotted lines) in panels A (offset) and C.
continuous-wave photoluminescence measurements do not allow us to distinguish between energy- and charge-transfer processes. Usually, time-resolved femtosecond and picosecond absorption and fluorescence spectroscopies are used to study the dynamics of these processes.32,33 Here we report a detailed LEPR study, which allows in situ detection, on the molecular scale for long-lived paramagnetic species resulting from electronic transfers between donor and acceptor units in the investigated systems. EPR and LEPR Studies. EPR Tracing Methodology. Generally, the donor moieties (D) in donor−acceptor (D− A) blends used for photovoltaic applications consist of a “homogeneous” conjugated polymer. Consequently, EPR spectra resulting from electron transfers in such materials are considered as the sum of two “pure” lines ascribed to free radicals D+° and A−°, respectively. The use of more complex polymers of the push−pull type leads us to analyze EPR spectra in more detail. Actually, it is of interest to determine which substructure of the polymer is mainly responsible for the EPR signal. For that reason, we have developed the so-called EPR tracing method simply based upon the fact that the spectrum of free radicals trapped on complex organic systems generally displays a complex signal, which can be simulated as a sum of
Table 2. Excitation Parameters for (A) P1, (B) P2, and (C) P3 Monomers, along with the Concerned Orbitals excitation number 1 2 3 4 1 2 3 4 1 2 3
ψ(i) → ψ(f) P1 (A) HOMO HOMO HOMO − 2 HOMO − 1 P2 (B) HOMO HOMO HOMO HOMO − 1 P3 (C) HOMO HOMO − 1 HOMO
λ (nm)
LUMO LUMO + 1 LUMO LUMO
448 420 445 412
LUMO LUMO + 1 LUMO + 2 LUMO + 1
526 447 425 409
LUMO LUMO LUMO + 1
481 421 410
nm (P1) and 570−580 nm (P2, P3), with a shoulder in the lower-energy/longer-wavelength spectral region for P1 and P3 (Supporting Information). For polymer/NC solutions, we observed partial PL quenching (42% quenching for P1, 39% for P2, and 38% for P3 using 90 wt % NCs). However, such 20652
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Table 3. Percentages of Total Spin Density on Each of the Substructures of the Cation Radicals of (A) P1, (B) P2, and (C) P3 as Determined from the EPR Study and DFT Calculation (Mulliken Population41) P1 (A)
P2 (B)
P3 (C)
substructure
carbazole
TPD
carbazole
bis-TPD
BDT
TPD
EPR DFT calculations
13 27
18 36
4 17
27 44
30 44
70 55
Lorentzian, only two EPR parameters are reported for each pure line, namely, the Landé g factor (g) and the line width (ΔHpp). The experimental procedure is the following: First, EPR spectra of the pure polymers are recorded in the dark and under illumination at λ = 473 nm in order to monitor potential charge transfers between its push and pull units. Second, spectra of their blends with either PCBM or EHT-CuInS2 nanocrystals (NCs) are recorded in the dark and under illumination (λ = 473 nm) to observe eventual electronic transfers between the polymer and PCBM or NCs. After each LEPR experiment, the light is turned off and the spectra are recorded as a function of time in order to observe relaxation processes and the potential irreversible residual EPR fingerprint due to laser illumination. All experimental spectra are simulated as the sum of pure lines, and a step-by-step analysis of these simulations enables us to determine the EPR signature of most of the structures and substructures under study: [PCBM]−° and push and pull units. We stress that [NC]−° is EPR-silent. Results and Discussion of EPR and LEPR Tracing Studies and DFT Calculations. As discussed earlier, P1, P2, and P3 and P1/PCBM, P2/PCBM, P3/PCBM, P1/NCs, P2/NCs, and P3/ NCs blends were observed by EPR at room temperature and at 20 K without and with illumination at 473 nm. Experimental spectra were simulated as a sum or as pure lines, which are reported in Table 1. DFT calculations were performed to obtain energy levels of HOMO and LUMO of all of the considered species. They are displayed in Scheme 2, which clearly shows that LUMO energies of PCBM and NCs are dramatically lower than those of all of the polymeric species, which confirms the accepting character of these species. EPR spectra of P1 alone were recorded at RT and at 20 K in the dark and under illumination (λ = 473 nm). Slight changes in the spectrum shape and intensity are observed under illumination at RT, and this effect is more evident at 20 K (Figure 1A). The simulation of the spectra of P1 alone obtained at RT and 20 K without and with illumination (λ = 473 nm) confirms a very small but clear effect of illumination: in the case
Figure 3. Electron densities of HOMO − 2, HOMO − 1, and HOMO of (A) P1 and (B) P2.
pristine or pure lines. Each pure line is ascribed to one of the polymer moieties and characterized by several features: the g Landé factor (g), the line shape (either a Lorentzian (100% L) or a Gaussian (0% L) or even a linear combination of both with the same g factor), and the peak-to-peak line width (ΔHpp(expressed in Gauss)). The Landé g factor (g) is quite sensitive to the chemical structure on which the unpaired electron is trapped. The line shape and line width are also affected by the unpaired electron environment but in a rather more complex way. Nevertheless, this set of three parameters the Landé g factor (g), line shape (%L) and line width (ΔHpp)can be considered to be the signature of the dynamics and the structure of the environment, where the unpaired electrons are trapped. In the present study, we observe that all of the pure lines are 100% Lorentzian-shaped lines, which may be ascribed, at least in part, to the free -radical dilution in the studied systems.34,35 Since all of the pristine lines determined by simulation of the experimental spectra are purely
Figure 4. DFT theoretical calculations of the spin density (blue spots) of the radical cation of (A) P2 showing its localization on the 2-TPD unit and (B) P1 showing its delocalization over all units (carbazole and TPD), in agreement with the EPR results. 20653
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Figure 5. EPR spectra of P3: (A) recorded at 20 K in the dark and (B) recorded at 20 K under illumination at 473 nm together with their simulations.
PCBM (1:3) blend. In this step, we can make two assumptions: either line 4 is only the fingerprint of charges formed on the polymer skeleton by the illumination of P1 alone or it is also due to electronic transfer between P1 and PCBM. In both cases, a glimpse of sunlight exposure may be the origin of this signal. In the second case (transfer between P1 and PCBM), a signal corresponding to anion radical PCBM−° should be observed at 20 K (PCBM−° is EPR silent at RT because of its relaxation time). Nevertheless, the absence of signal corresponding to PCBM−° does not allow us to conclude in favor of the first assumption. Actually, the very low intensity of the EPR signal of P1/PCBM in the dark and the difference in width between line 4 (ΔHpp = 1.01 G), ascribed to P1+°, and line 8 (ΔHpp = 2.2 G), ascribed to PCBM−° (see just after), may explain why PCBM−° is not detected. Indeed, with the intensity of the EPR signal being (for signals with the same line shapes) proportional to Hpp2 × Ipp, the height (Ipp) of the PCBM−° signal is then expected to be approximately 5 times lower than that of P1+°, so it might be hidden in the noise. Of course the simulation of the experimental spectrum of P1/PCBM recorded in the dark is not very precise due to high noise in the signal ratio, and the possible contribution of lines 2 and 3 observed in P1 alone under illumination cannot be excluded. Upon illumination of P1/PCBM at 473 nm at RT (Figure 1B), a dramatic increase in the signal is observed. Since PCBM−° cannot be observed at RT, the increase in the signal is ascribed to paramagnetic species of P1 resulting from electronic transfers in the P1/PCBM blend. Upon illumination of P1/PCBM at 20 K at 473 nm (Figure 1B), as expected, in addition to the intensive signal located at the g factor corresponding to P1 paramagnetic species, an additional line appears at a lower g factor (located at a higher magnetic field). The simulation of the spectrum (Figure 2A) reveals that the EPR parameters of this additional line (line 8) (g factor of 1.99992 and ΔHpp of 2.2 G) are in full agreement with those reported in the literature for the PCBM−°.36,37 Line 8 can be ascribed unambiguously to PCBM−°. After turning the light off, we observed a very fast partial decrease (much lower than a minute) of the spectrum (Figure 1C), followed by a slower decrease to finally recover the initial spectrum obtained before illumination after a relaxation that lasts approximately 1 day at 20 K. These fast and long relaxation processes have been reported in the literature and are
Figure 6. Electron densities of HOMO − 1, HOMO, LUMO, and LUMO + 1 HOMO of P3.
of P1 alone without illumination, the spectrum is satisfactorily simulated with only one line 1, which is attributed to intrinsic paramagnetic defects of P1 (Table1), while in the case of P1 alone at RT and 20 K under illumination, the simulation reveals the presence of three supplementary lines: lines 2−4 in addition to line 1 (Table 1). In this step of the study, it is not possible to ascribe lines 2−4 either to the push (carbazole) or to the pull (TDP) substructures of P1. Nevertheless, these three lines are the EPR traces of the effect of illumination on P1. The EPR trace of the illumination of P1 might be due to free radical defects trapped after exciton formation associated or not with the push−pull process. In this step, it is not possible to conclude if the push−pull effect occurs. At RT and at 20 K, in the dark, P1/PCBM exhibits the same small EPR signal (Figure 1B). We stress that although this signal is very small and noisy, it is clearly different from the one observed for P1 in the dark. The dark P1/PCBM signal can be simulated with only one line (line 4), which is also present in P1 alone under illumination. Surprisingly, line 1 observed for P1 in the dark is not detected in the P1/PCBM signal recorded in the dark; this is likely due to the “dilution” of P1 in the P1/ 20654
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Figure 7. (A) EPR spectra of the P1/NCs blend, registered at 20 K and under illumination (λ = 473 nm, six scans) and after 1 h of relaxation in the dark at 20 K (six scans). (B) EPR spectrum of the P1/NCs blend, recorded at 20 K under illumination (λ = 473 nm, 22 scans) together with its simulation. (C) EPR spectrum of very low intensity (22 scans) of the P3/NCs blend, recorded at 20 K under illumination (λ = 473 nm). (D) EPR spectra of P3/PCBM and P3/NCs (signals of very low intensity) recorded at 20 K under illumination at 473 nm under the same conditions of acquisition.
The experimental spectra of P1/PCBM recorded at 20 K under illumination is satisfactorily simulated, with lines 2−9 (Figure 2A and Table 1). At this stage, lines 2−4 (observed for P1 under illumination) can be ascribed either to carbazole or to TPD units of P1. We stress that the effect of light on the EPR signal of P1 alone is very slight if compared to that observed on the P1/PCBM signal. Nevertheless, it allows us to conclude that lines 2−4 assign the P1 contribution to the electronic transfer that occurs in the P1/PCBM blend. At this step, it is impossible to assign lines 6 and 7. They quite likely do not sign the polymer because their g factor is too low. It can be noticed that the EPR parameters of line 6 are similar to those of the “spike” sometimes found in the EPR spectrum of fullerenes and fullerene derivative anion radicals. A few different hypotheses have been put forward to explain the existence of this spike,39,40 among which is the presence of an inevitable dimer impurity C120O in air-exposed samples of C60.
Figure 8. Electron density map of the P3 HOMO isovalue of 0.002 au.
respectively ascribed to bimolecular recombination processes between mobile charge carriers and to the slower recombination of less-mobile polarons trapped in deeper states.18,38 After full relaxation, a second illumination leads to the same spectrum as that observed upon the first illumination. 20655
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Scheme 3. Exciton Formation at 473 nm and EPR-Detected Electron Transfersa
a
“?” means that a process may occur but is not fully demonstrated.
In order to further confirm the results of the simulation, we recorded the EPR spectrum of P1/PCBM (1:3) at 20 K under illumination for different values of microwave power (from 0 to 40 dB). Unfortunately it was not possible to better resolve the global signal by a saturation process, likely because relaxation
times of the constituting moieties (carbazole and TPD) are not different enough. Nevertheless, a clear difference is seen between the shapes of the signals of P1 recorded at high and low microwave power, which demonstrates the presence of at least two different paramagnetic species in the polymer. 20656
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Lastly, remaining line 2 might belong to the TPD unit contribution to the HOMO − 2 orbital of P1, which can also be excited by the laser as can be seen in Table 2. This ascribing is based upon the fact that line 2, while present in every spectrum of P1, is absent from the P2 spectrum. It is to be noticed that in the P2/PCBM signal recorded under illumination, the intensity of line 5′, representing bisTPD, is much lower than that of PCBM (line 8). This leads us to assume that line 6 is due to impurities interacting with PCBM to give PCBM−° and not to the “spike” found in the EPR spectrum of fullerene and the fullerene derivative anion radical. Actually none of the hypotheses evoked to explain the presence of this spike in ref 16 and17 are in agreement with our results (all hypothesis should lead to a ratio of PCBM−°/P2+° < 1) In dealing with eventual push−pull processes in P1 (and P2), in the case of a strong push−pull process, electrons should be quickly transferred from carbazole to TPD (or bis-TPD) and then to PCBM. Then the EPR signal of TPD (or bis-TPD) should be lower than that of carbazole. But it turns out that the intensity of TPD (or bis-TPD) observed in P1/PCBM (or P2/ PCBM) is approximately equal (or much higher) that that of carbazole. Consequently, we conclude that both the push (carbazole) and pull (TPD or bis-TPD) units are involved in exciton formation on P1 and P2 and electronic transfer between P1 and PCBM processes. The push−pull effect may occur, but we can claim that it is not predominant. This is in total agreement with the DFT calculation and particularly with a HOMO delocalized over the whole push−pull unit. For P3, we observed an effect of illumination at 20 K on the shape of the spectrum. The simulation of the spectra under dark conditions and under illumination at 473 nm at 20 K was performed. The results are presented in Figure 5 and Table 1 The simulation of the spectrum of neat P3 recorded at 20 K in the dark shows the presence of three lines 11, 12, and 13. Because of its g factor (2.007002), line 13 is ascribed to impurities. Interestingly, line 11 is similar to line 5 observed in P1, which has been ascribed to the TPD group. Consequently, it is assumed that line 11 belongs to the TPD group. The g factor of line 12 ranging in expected values of polymers is therefore attributed to BDT. The relative intensities of lines 11 and 12 are about 10 and 90%, respectively. After illumination (473 nm) at 20 K, the spectrum of P3 exhibits a slight but observable change in the shape, and its simulation is made with the same lines 11, 12, and 13. Nevertheless, the relative intensities of lines 11 and 12 are slightly changed and now equal 15 and 85%, respectively. The change in the relative intensities either signals a push−pull process from BDT to PD or trapped photoformed radicals on both the push BDT and pull PD units upon illumination. As for P1 and P2, push−pull process cannot be demonstrated for P3. At RT and at 20 K, the EPR spectrum of P3/PCBM is characterized by a very low intensity dark EPR signal, which cannot be simulated because of its high noise to signal ratio. Under illumination, a significant evolution of the spectrum occurs. From the simulation of the spectra it was possible, as expected, to reveal the presence of line 8, previously assigned to the PCBM anion radical, and of line 5 already, ascribed to TPD (Figure 2C and Table 1). In this case, as for P2, we face a problem since the intensity of line 5, representing TPD, is much lower than that of PCBM (line 8), and we can wonder if line 10 of P3 is coming from species interacting with PCBM. At the moment, we are not able
Let us recall that P2 is similar to P1 except that the TPD unit is now a bis-TPD unit (Scheme 1), which should strengthen the acceptor power of the pull moiety. In the dark, we did not observe any signal for P2/PCBM, while under illumination at 473 nm we observed two signals located in the polymer and PCBM g-factor ranges (Figure 2B). The simulation of the P2/ PCBM signal recorded under illumination (Table 1) reveals the presence of lines 3, 4, 6, 5′, and 8. Lines 3, 4, 6, and 8 were also detected in P1/PCBM under illumination. Line 8 was ascribed to PCBM, while line 6 was demonstrated not to belong to the polymer. Lines 3 and 4, already ascribde to P1 and also present in P2, can be attributed to the carbazole unit present in both polymers. P2/PCBM under illumination also exhibits line 5′ with the same g factor as line 5 present in P1/PCBM under illumination but with a narrower line width. Similar g factors can be assigned to similar chemical structures while a decrease in the line width can be ascribed to a larger delocalization of the free electron. This leads us to ascribe lines 5 and 5′ to the TPD and bis-TPD structures, respectively (Table 1) DFT calculation of the excited states of P1, P2, and P3 has been undertaken. The results show that the laser is able to promote electrons either from HOMO, HOMO − 1, or HOMO − 2 to different virtual orbitals for polymers P1, P2, and P3. Results for P1, P2, and P3 are gathered in Table 2. The electron densities of HOMO, HOMO − 1, and HOMO − 2 of P1 and P2 are displayed in Figure 3. Both HOMO − 1 values of P1 and P2 are mainly located on the carbazole unit. On the other hand, both HOMOs of P1 and P2 are delocalized over the whole molecule even though the main contribution is due to the pull unit (TPD and bis-TPD for P1 and P2, respectively). Because line 4 is the only pure line constituting the spectrum of P1/PCBM without illumination and is present in the spectra of P1, P1/PCBM, and P2/PCBM under illumination, we attribute it with fair confidence to the signature of a radical formed by the abstraction of one electron from the HOMO − 1 (fully located on the carbazole unit) of both P1 and P2. Line 3 appearing in the spectra of P1, P1/ PCBM, and P2/PCBM might be attributed to the contribution of the carbazole unit in the HOMO orbitals of P1 and P2 (delocalized over the carbazole and TPD or bis-TPD units, respectively). Line 5′ signaling the pull unit (bis-TPD) and being the principal contributor to the EPR signature of P2/ PCBM under illumination is attributed to the contribution of bis-TPD to the HOMO of P2. Consequently, line 5 is attributed to the contribution of TPD to the HOMO of P1. DFT calculations were carried out to obtain the spin densities of P1 and P2 cation radicals, i.e., the ultimate thermodynamic state of the charge transfer (Figure 4). As can be seen in Figures 3 and 4, the spin density and HOMO electronic density are quite similar, as expected, thus validating conclusions previously drawn. DFT calculations reveal that for P2 the unpaired electron density is mostly localized on the bisTPD unit (Figure 4A), while in P1 it is much more delocalized (Figure 4B), in full agreement with EPR results. To further confirm the agreement between EPR experimental data and DFT calculations, we have compared the spin densities determined by these two methods. Of course, we are aware that comparing the absolute values makes little sense, but still the same trends should be observed. Actually, as expected, the same variation of electronic density is observed: an increase from carbazole to TPD and to bis-TPD in P1 and P2 respectively (Table 3A,B) 20657
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by the laser, see Table 2C) and spin density of the P3 radical cation (not shown) are mostly localized on the BDT unit (Table 3C and Figure 8), while the EPR signal of P3/PCBM under illumination does not reveal the presence of BDT. We are not able to explain this result at the moment. We stress again that the effect of illumination observed by EPR for blends of P1 or P3 with EHT-NCs is much lower if compared to that obtained for P1 and P3/PCBM blends (Figure 7D). Although EPR intensities cannot be compared with accuracy, the difference in EPR intensities is so large that it can be interpreted either by a smaller active interface promoting charge transfer in P1 or P3/NCs than in P3/ PCBM, which can due either to larger dimensions (7 to 8 nm) of NCs if compared to PCBM (∼3 nm) or to less-efficient electron transfers from polymer to NCs than to PCBM or both. All of the conclusions drawn from this study are summed up in Scheme 3
to attribute lines 6 and 10 to specific structures. We did not observe any line corresponding to the BDT unit in P3/PCBM, although this line was observed in pure P3 without and with illumination. For BDT and TPD exhibiting approximately the same line width, we interpret the absence of signal attributed to BDT by the fact that the transfer between P3 and the fullerene is essentially due to TPD. The blank signal of P3 alone under illumination containing both lines corresponding to TPD and BDT and the intrinsic defects (line 13) is too low to be detected in the P3/PCBM signal. As observed for P1/PCBM, blends of P2 and P3 with PCBM exhibit two kinds of relaxation: a very fast partial decrease of the signal upon stopping the illumination followed by a very slow decrease to recover approximately the initial signal after around 1 day. Photoluminescence Studies of P1 (or P3)/CuInS2 Nanocrystals Blends. P1 and P3 were also tested with another type of electron acceptor, namely, nanocrystals of copper indium disulfide (CuInS2 NC). Studies on the photoluminescence quenching in organic/inorganic hybrid materials demonstrate significant quenching via charge and/or energy transfer in accordance with the relative energy level alignment. In our case, the quenching of photoluminescence was efficient: 42 and 38% for P1 and P3, respectively, at the maximum weight % of NCs in the blend (Supporting Information). EPR Studies of P1 and P3 CuInS2 Nanocrystal (NC)(1:3) Blends. Like PCBM, inorganic semiconductor nanocrystals (NCs) can act as electron acceptors when blended with polymers. Figure 7A shows the EPR spectrum of P1/NCs recorded at 20 K under illumination and after relaxation (10 min) in the dark. A clear decrease in the spectrum intensity was observed during the relaxation, showing a slight but clear, at least partially reversible, illumination effect. The results of the spectra simulations are shown in Figure 7B and Table 1. The spectrum of P1/NCs is satisfactorily simulated as the sum of lines 2 (ascribed to the TPD contribution to HOMO − 2), 3 (ascribed to the carbazole contribution to the HOMO), 4 (ascribed to the carbazole contribution to HOMO − 1), and 5 (ascribed to the TPD contribution to HOMO − 1). We noticed new line (line 14) with a nonnegligible contribution (54%). This line is interpreted as the photoexcitation of the nanocrystal, followed by an electron transfer from the bestfitting occupied orbital of P1 (HOMO or HOMO − 1) (Scheme 2 and Table 2A) to the HOMO of NC (cf. mechanism described in ref 42). We stress that the signal of P1/NCs is very low and the simulation is difficult to achieved. The important result is that the lines previously ascribed to push and pull units are also found in this case. P3/NCs were studied by EPR. Figure 7C shows its spectrum registered at 20 K under illumination (λ = 473 nm). It is very weak and quite undetectable with a low signal-to-noise ratio. Nevertheless, a large accumulation of scans allows the detection of a line of the same parameters as those obtained for the radical formed in P3 upon illumination of the P3/PCBM (1:3) blend (Table 1), which can be (very carefully) interpreted as a sign of a slight transfer from P3 to the nanocrystal. The simulation reveals the presence of lines 10 (non-attributed), 11 (ascribed to the TPD unit), and 12 (ascribed to BDT). The major contribution is due to BDT. Here we face a problem: EPR results obtained for the P3/NCs blend are in full agreement with DFT calculations, which show that the electronic density of the P3 HOMO (which can be excited
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CONCLUSIONS In this article, we have applied the so-called EPR tracing method to study electronic transfers in push−pull copolymers and in their blends with either PCBM or CuInS2 nanocrytal as acceptor materials. The EPR tracing method consists of attributing EPR fingerprints to various substructures of a material and following them under various experimental conditions. In addition, the DFT calculation of the monomers of each studied polymer has been undertaken to gain better insight into their electronic structure. It appears that DFT results are consistent with the conclusion drawn for the EPR tracing method. For all of the polymers under investigation, several electrons can be excited. As expected, the excited electrons are generally promoted from the HOMO, but they also can come from other occupied orbitals such as the HOMO − 1 and even the HOMO − 2. The excited orbital is either centered on the push moiety or delocalized over both the push and pull units. We did not get any experimental evidence of the push−pull effect, even though quantum calculations indicate that such an effect might exist. Theoretical investigations are in progress regarding this issue. It is believed that such an EPR tracing study can also be applied to various other topics such as degradation studies of active photovoltaic layers upon aging in various atmospheres.
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ASSOCIATED CONTENT
S Supporting Information *
TEM observation of DDT nanocrystals. X-ray patterns of DDT-CuInS2 nanocrystals. Differential pulse voltammetry (DPV) study of EHT-NCs. Determination of HOMO and LUMO levels for DDT and EHT-NCs from DPV data. Photoluminescence study of blends of P1 and P3/EHT-NCs as a function of the EHT-NC/P1 or P3 weight ratio. EPR fingerprint determination. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank Dr. Serge Beaupré for fruitful discussions. 20658
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