Phenyl-C61-butyric Acid Methyl Ester in DMF Solution - American

Article pubs.acs.org/Macromolecules

Charge-Transfer Complexation Mechanism of Poly(4-vinylpyridine)/ [6,6]-Phenyl‑C61-butyric Acid Methyl Ester in DMF Solution Guangmin Wei, Dongdong Yao, Zhiyong Li, Ye Huang, He Cheng,* and Charles C. Han* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, CAS, Beijing 100190, P. R. China ABSTRACT: The mechanism of charge-transfer complexation in electron-donor (D)/electron-acceptor (A) active layer was studied for a pseudobinary blend model system, poly(4vinylpyridine) (P4VP)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in DMF solution, by a combination of light scattering (LS), transmission electron microscopy (TEM), and UV−vis spectroscopy. The time evolution of the system can be characterized by four distinct stages, i.e., induction, complexation, aggregation, and precipitation. In the induction stage, a combined dynamic LS and static LS studies showed that the conformation of P4VP remained unchanged, while the UV−vis indicated that the charge-transfer complexation had almost accomplished with an obvious broadening at C60 characteristic absorption peak of 330 nm. In the complexation stage, each P4VP chain complexed with about three PCBM molecules at cP4VP = 4.1 × 10−2 g/mL, cPCBM = 5.8 × 10−4 g/mL, and the molar ratio [4VP]/[PCBM] = 57:1 and shrinked in size with almost no change in the UV−vis spectrum. In the subsequent aggregation stage, P4VP/PCBM complexes aggregated with each other to form spherical aggregates with again unchanged UV−vis signals. The free association model (FA model) can be used to explain this mechanism. In the final precipitation stage, huge P4VP/PCBM agglomerate began to phase out with a clear decrease of the scattered light intensity in the LS. The almost unchanged UV−vis spectrum after the induction stage proved that the electronic transition from ground to excited state is not necessarily to be influenced by any inter- or intrapolymer structural transition. Our kinetics study on the mechanism of the complexation/association/aggregation shed some light on the possible morphology control in the D/A active layer. nm length scale.8,9 Laiho et al. showed the first example that charge-transfer complexation between C60 and PS-b-P4VP copolymers can essentially modify the self-assembled structures.10 Sary et al. also used electron-transfer complexation between P4VP and C60 to design a PPV:C60 electron acceptor domain in 2008.11 In 2010, they further blended P3HT−P4VP block copolymers with PCBM to examine the optoelectronic properties of preliminary photovoltaic devices. 12 Then Palaniappan et al. synthesized P3HT-b-P4VP by RAFT polymerization to improve the interaction of inorganic CdSe nanocrystals with P3HT.13 Despite these intensive efforts, the mechanism, especially the time-dependent kinetics of charge-transfer complexation process, remains elusive. On one hand, the traditional UV− vis spectrum and NMR observations, which are the often-used characterization techniques, can only follow the local energy variation, and the detailed time-resolved structure evolution cannot be monitored. On the other hand, it is very difficult to observe the charge-transfer complexation process in the D/A active layer directly.6 The charge transfer normally takes an active role in governing the morphology of D/A active layer; it

1. INTRODUCTION For the past decade, considerable efforts have focused on the development of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices based on conjugated polymer−fullerene composites because the charge generation/separation film made from this type of composite material has provided a possible route to achieve large-area, low-cost, flexible, and efficient devices.1,2 Since the optoelectronic performance of those films are greatly influenced by the morphology in the D/ A active layer,3,4 it will be desirable if the D/A active layer can form a bicontinuous morphology with a characteristic length scale comparable to the excitation diffusion length.5 However, the resulting D/A domain size usually becomes much larger than the excitation diffusion length scale and ultimately diminishes the device performance due to the thermodynamically metastable states existed during the preparation, and further macroscopic phase separation and crystallization are happening at higher temperature in use.6,7 A proper modification of conjugated polymer with block chains which can form charge-transfer complexation with fullerene can decrease the D/A domain size and increase its thermal stability in bulk heterojunction devices, because the resultant block copolymer may self-assemble with fullerene through microphase separation into more stable and highly ordered nanostructures with spatial periodicities on the 1−10 © XXXX American Chemical Society

Received: September 28, 2012 Revised: December 30, 2012

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dx.doi.org/10.1021/ma3020504 | Macromolecules XXXX, XXX, XXX−XXX

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mean-square radius of gyration (⟨Rg⟩) from the angular dependence of the excess absolute scattering intensity, known as the Rayleigh ratio Rvv,15 i.e.

would be more desirable to investigate the charge transfer between donor and acceptor rather than to study the active layer as a whole. All these difficulties have motivated us to choose P4VP/PCBM pseudobinary blend as a model system and combined LS, TEM, and UV−vis spectrum techniques as the main tools to study the mechanism of charge-transfer complexation, so as to compare structure development with local energy transformation. LS and TEM showed that there are four distinct stages, i.e., induction, complexation, aggregation, and precipitation, corresponding the UV−vis spectrum measurement which changed rapidly in the induction period when the topology structure of P4VP did not change at all and kept unchanged after that (Scheme 1).12 Flory−Huggins type theory and the FA model can be used to model the complexation and aggregation processes, respectively.

⎛ 1 ⎞⎛ ⎞ Kc 1 =⎜ + 2A 2 c + ...⎟⎜1 + q2⟨R g 2⟩ + ...⎟ ⎠ R vv 3 ⎝ Mw ⎠⎝

(1)

where A2 is the second virial coefficients, K = 4π2n2(dn/dc)2/(NAλ04) and q = (4πn/λ0) sin(θ/2) with NA, dn/dc, n, λ0, and θ being the Avogadro number, the specific refractive index increment, the solvent refractive index, the wavelength of the light in vacuum, and scattering angle, respectively. In a typical SLS measurement of solution at angle θ, Rayleigh ratio Rvv = [(Isolution − Isolvent)/Itoluene](nsolvent2/ntoluene2) Rvv,toluene; here Isolution, Isolvent, and Itoluene are the scattered light intensities of solution, solvent, and toluene standard, respectively, when the corresponding incident light intensities are the same. nsolvent and ntoluene are the refractive indexes of solvent and toluene, and Rvv,toluene is the Rayleigh ratio of toluene.15 In dynamic LS (DLS), the intensity−intensity time correlation function, g(2)(τ), can be related to the normalized field−field autocorrelation function |g(1)(τ)| (≡ [⟨E(0)E*(τ)⟩/⟨E(0)E*(0)⟩]) via the Siegert relation as

Scheme 1. Schematic Illustration of the Charge-Transfer Complexation between P4VP and PCBM

⟨I(0)I(τ )⟩ − 1 = β |g(1)(τ )|2 ⟨I ⟩2

g(2)(τ ) − 1 =

(2)

where β is the instrument coherent factor and τ is the characteristic decay time. For monodisperse particles in solution, the normalized field−field autocorrelation function g(1)(τ) = exp(−Γτ), with a decay rate of Γ = Dq2, where D is the diffusion coefficient of the particles and q is the magnitude of the scattering wave vector.15,16 For a polydisperse sample, g(1)(τ) can no longer be represented as a single exponential and must be represented as a sum or integral over a distribution of decay rates G(Γ) by

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(4-vinylpyridine) was synthesized by reversible addition−fragmentation chain transfer polymerization (RAFT) according to the literature.14 In order to remove the unreacted initiator and monomers, P4VP was dissolved in dichloromethane and reprecipitated in a large amount of ether for three successive cycles at room temperature. It was dried under vacuum for 48 h before used. PCBM (+99%) from Beijing Sheng Witte Technology Co., Ltd., and reagent grade N,N-dimethylformamide (DMF) from Sinopharm Chemical Reagent Beijing Co., Ltd., were used as received. 2.2. Preparation of P4VP/PCBM Pseudobinary Blend in DMF. First, two DMF solutions were prepared separately at room temperature, i.e., 5.0 × 10−2 g/mL P4VP and PCBM saturated solution. Then 0.41 g of 5.0 × 10−2 g/mL P4VP and 4.65 g of PCBM saturated solutions were filtered into the same LS cell sequentially one after another with 0.20 μm Millex Millipore PTFE membranes. The concentrations of P4VP and PCBM after filtration were calibrated according to Lambert−Beer’s law by UV−vis, i.e., cP4VP = 4.1 × 10−2 g/ mL and cPCBM = 5.8 × 10−4 g/mL, respectively. The LS cell was equipped with a screwed cap, which could be kept for months without evaporation or contamination. No charge-transfer complexation can be observed at both 4 °C and room temperature even after 3 months, which means that it is a very slow process at these temperatures. In this study, all measurements were conducted at 40 °C if not further specified. 2.3. Light Scattering Measurement. Light scattering experiments were carried out on a commercial LS spectrometer equipped with a multi-τ digital time correlator (ALV5000). A cylindrical 22 mW UNIPHASE He−Ne laser (λ0 = 632.8 nm) was used as the light source. The spectrometer has a high coherence factor because of a novel single-mode fiber optical coupled with an efficient avalanche photodiode as the detector. The LS cell is held in a thermostat index matching vat filled with purified and dust (partial) free toluene, with the temperature controlled to within 0.1 °C. Static LS (SLS) experiments were carried out to determine the weight-averaged molecular weight (Mw) and the z-averaged root-

g(1)(τ ) =

∫0

∞

G(Γ) exp(−Γτ ) dΓ

(3)

where G(Γ) is normalized so that ∫ ∞ 0 G(Γ) dΓ = 1. There are two ways of using DLS date to characterized G(Γ). One is the cumulants analysis first proposed by Koppel17 μ μ ⎛ ⎞ g(1)(τ ) = exp(−⟨Γ⟩τ )⎜1 + 2 τ 2 − 3 τ 3 + ...⎟ ⎝ ⎠ 2! 3!

(4)

i where μi = ∫ ∞ 0 G(Γ)(Γ − ⟨Γ⟩) dΓ is the ith cumulant. The relative line width can be obtained from cumulate analysis μ2/⟨Γ⟩2 = ∫ ∞ 0 G(Γ)(Γ − ⟨Γ⟩)2 dΓ/⟨Γ⟩2. The other is Laplace inversion; i.e., G(Γ) can be calculated from the Laplace inversion according to eq 3. For a pure diffusive relaxation, Γ is related to the translational diffusion coefficient D by Γ/q2 = D at q → 0 and c → 0. From the translational diffusion coefficient, the hydrodynamic radius Rh can be determined from the Stokes−Einstein relation Rh = kBT/6πηD, where kB, T, and η are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively.18 2.4. Ultraviolet−Visible (UV−vis) Spectroscopy. A TU-1901 UV−vis spectrophotometer equipped with a PTC-2 temperature control unit was used to measure the absorbance and transmittance of the solutions at 40 °C. For the absorption study, the wavelength was scanned from 200 to 800 nm, while the wavelength was set at 632.8 nm for the transmittance experiment. Standard quartz cuvettes with a path length of 10 mm were used. 2.5. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was performed using a JEOL 2200FS instrument at 160 kV accelerating voltage. The resultant P4VP/PCBM binary mixture solution was diluted to ∼0.02 mg/mL and then drop-casted on the copper grid with carbon membrane. The TEM grid had been placed under vacuum for 48 h to remove DMF before measurement.

B


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3.1.2. P4VP/PCBM Charge-Transfer Complexation. Figure 2 is the corresponding time correlation functions of P4VP

3. RESULTS AND DISCUSSION 3.1. Kinetics Observation by LS. To observe the mechanism of charge-transfer complexation, both the molecular weights and size distribution of P4VP and PCBM had to be calibrated first. 3.1.1. LS Calibration of P4VP and PCBM in DMF. Figure 1 calibrates P4VP and PCBM. It shows the typical intensity−

Figure 2. Typical normalized intensity−intensity time correlation functions, g(2)(τ) − 1, of P4VP, PCBM, and P4VP/PCBM freshly prepared DMF solution at θ = 20°.

before and after PCBM was introduced in DMF. The characteristic decay times before and after PCBM was added are very similar, but the intercepts, ⟨I(0)2⟩/⟨I⟩2 − 1, are different. According to UV−vis measurement, the transmittance of pure P4VP DMF solution (cP4VP = 3.3 × 10−3 g/mL) is 92.3%; it decreases to 55.8% after the PCBM (cPCBM = 5.3 × 10−4 g/mL) was added at the same P4VP concentration. The absorbance of PCBM to the incident light decreased the signalto-noise ratio and the instrument coherent factor,23 which might be the main reason for the decrease of the intercept. Because LS is a quasi-elastic scattering, the amplitude of the scattered vector should be the same with that of the incident vector. Although the absorption in our pseudobinary blend makes it not a purely quasi-elastic scattering process anymore, we still can measure the charge-transfer complexation kinetics qualitatively by LS because it is pure absorption without photon emission. Figure 3 traces the whole charge-transfer complexation process. According to the relationship between the scattered light intensity and corresponding hydrodynamic radius (Figure 3b,c), the mechanism of charge-transfer complexation can be qualitatively classified in four distinct stages: (1) induction stage, when both scattered light intensity and hydrodynamic radius keep almost constant, and there is only one mode in the intensity−intensity time correlation function; (2) complexation stage, when scattered light intensity begins to increase but P4VP coil started to collapse, and there is still only one mode in the intensity−intensity time correlation function; (3) aggregation stage, when P4VP/PCBM complexes begin to aggregate with each other, and there are two modes in the intensity− intensity time correlation function which belongs to the contribution from complexes and aggregates, respectively; (4) precipitation stage, when P4VP/PCBM agglomerate begins to phase out of the solution and the scattered light intensity starts to decrease, and there are three modes in the intensity− intensity time correlation function which corresponds to the contribution from complexes, aggregates, and agglomerates. We further discuss these four stages in detail in the following sections. 3.1.2.1. Induction Stage. Induction is the initial stage when the electrostatic interaction between P4VP and PCBM competes with their solvent affinity in DMF. In Figure 4, the time evolution of intensity−intensity time correlation function illustrates an unchanged characteristic decay time, τ, and a

Figure 1. Typical normalized intensity−intensity time correlation functions g(2)(τ) − 1 of 3.3 × 10−3 g/mL P4VP (△) and 5.3 × 10−4 g/ mL PCBM (○) DMF solution. The corresponding hydrodynamic radius distribution ( f(Rh)) of P4VP (□) obtained from Laplace inversion (CONTIN analysis) at room temperature at θ = 20° (data are offset vertically for clarity by factor −0.1). The inset shows the plots of Rvv/Kc vs q2 in c1 = 3.3 × 10−3 g/mL (◇) and c2 = 4.1 × 10−3 g/mL (◆) P4VP−DMF solution.

intensity time correlation function and its corresponding hydrodynamic radius distribution obtained from the CONTIN process in P4VP−DMF solution. The relative line width of the P4VP sample from cumulate analysis (μ2/⟨Γ⟩2 = ∫ ∞ 0 G(Γ)(Γ − ⟨Γ⟩)2 dΓ/⟨Γ⟩2) is ∼0.1. Therefore, its polydispersity can be calculated, Mw/Mn∼1 + 4 μ2/⟨Γ⟩2 = 1.4, according to the literature.18,19 It proves that P4VP is relatively narrowly distributed and suitable for kinetics observation. Equation 1 shows that Kc 1 = R vv Mw (5) when q⟨Rg⟩ ≪ 1 and c → 0 g/mL. P4VP is random coil in its moderate good solvent DMF (A2 ∼ 0), and its radius of gyration, ⟨Rg⟩P4VP, can be estimated, i.e., ⟨Rg⟩P4VP ∼ 1.5⟨Rh⟩P4VP = 11 nm.20,21 Then its q⟨Rg⟩P4VP at 20° scattering angle is 3.8 × 10−2, which is far smaller than 1. Besides both P4VP solutions are dilute, i.e., c → 0 g/mL. Therefore, the absolute weightaveraged molar mass of P4VP, Mw,P4VP, can be obtained from the angular independent scattered light intensity (solid line in the inset), i.e., Mw,P4VP = 4.5 × 104 g/mol. Figure 1 also shows typical normalized intensity−intensity time correlation function g(2)(τ) − 1 in 5.3 × 10−4 g/mL PCBM−DMF solution at θ = 20°. The saturated PCBM−DMF solution is very stable; both the scattered light intensity and corresponding intensity−intensity time correlation function keep unchanged after 12 months. PCBM is a small organic molecule with MPCBM = 911 g/mol. The light scattered by a small molecule is too weak to be detected in the visible range of light, even if there are many of these particles in the scattering volume.22 The absorbance of PCBM is strong, so no time correlation function can be observed in DLS in PCBM DMF solution. C


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almost unchanged, which indicates that the P4VP still behaves like random coil in DMF and its interaction with DMF solvent is still comparable to that with PCBM molecules. The dynamic balance between P4VP electrostatic interaction with PCBM and solvophilic interaction with DMF plays an important role in this stage, and the competition between them is the key to determine its structure in the following complexation stage. Finally, the electrostatic interaction becomes dominating after 7 days. 3.1.2.2. Complexation Stage. In complexation stage, the single P4VP chain (we will prove it later) begins to complex with PCBM molecules via electrostatic force. Figure 5 is the

Figure 5. Time evolution of intensity−intensity time correlation function g(2)(τ) − 1 in complexation stage. The insets show the corresponding hydrodynamic radius obtained from Laplace inversion (CONTIN analysis) and scattered light intensity variation. The arrows are used to guide the eye.

time evolution of intensity−intensity time correlation function. There is a decrease of characteristic decay time and a increase of the intercept, ⟨I(0)2⟩/⟨I⟩2 − 1. The former demonstrates the collapse of P4VP chain, while the origin of the latter is still not clear, and there are at least two possibilities. First, it may indicate that some PCBM is trapped inside P4VP/PCBM complexes and interacts with the P4VP molecule and no longer has the resonance structure to absorb 632.8 nm wavelength light, and second, it may also originate from the better signalto-noise ratio of the increased scattered light intensity contributed to the larger complexes. The insets demonstrate the corresponding hydrodynamic radius obtained from Laplace inversion and scattered light intensity. The hydrodynamic radius of P4VP decreases from 7.2 to 3.9 nm, while its scattered light intensity has a 20% increase, which means that some PCBM penetrate into P4VP random coil and behave like physical cross-linking point and intramolecularly cross-link P4VP chains. The PCBM number, NPCBM, with each P4VP chain at [4VP]/[PCBM] = 57:1 can be calculated. Because it is a dilute solution with q⟨Rg⟩ ≪ 1, eq 5 is still correct here, and the weight-averaged molar mass of the complexes can be obtained from its angular independent normalized scattered light intensity. NPCBM can be deduced

Figure 3. (a) Time-resolved intensity−intensity time correlation function g(2)(τ) − 1 of P4VP/PCBM pseudobinary blend at θ = 20°. (b) The corresponding scattered light intensity. (c) The corresponding hydrodynamic radius distribution. The solid lines and the arrows are used to guide the eye.

Figure 4. Time evolution of intensity−intensity time correlation function g(2)(τ) − 1 in induction stage. The insets show the corresponding hydrodynamic radius obtained from the Laplace inversion (CONTIN analysis) and scattered light intensity variation without transmittance correction. The arrows are used to guide the eye.

NPCBM =

decrease of the intercept, ⟨I(0) ⟩/⟨I⟩ − 1. The slight decrease of the intercept from 0.29 to 0.22 may be mainly originated from the decrease of solution transmittance from 56% to 37% at the incident light wavelength. The insets are the corresponding hydrodynamic radius obtained from Laplace inversion and scattered light intensity. Both of them keep 2

2

Mcomplexes − MP4VP MPCBM

(6)

where Mcomplexes, MP4VP, and MPCBM denote the molecular weight of complexes, P4VP single chain, and PCBM molecule, respectively. Note that Mcomplexes is just 6.7% larger than MP4VP, so only one P4VP can exist in each complexes. From this expression, we can obtain NPCBM ∼ 3 at [4VP]/[PCBM] = 57:1, which means that there are three PCBM molecules D


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Figure 6. (a) Time-resolved intensity−intensity time correlation function g(2)(τ) − 1 at θ = 20°. The inset shows the corresponding hydrodynamic radius distribution obtained from the Laplace inversion (CONTIN analysis). (b) Total, slow mode, and fast mode scattered light intensity. (c) ⟨Γ⟩agg vs q2. (d) ⟨Rg⟩agg, ⟨Rh⟩agg, and ⟨Rg⟩agg/⟨Rh⟩agg in aggregation stage.

weight fractions of the fast and slow modes, respectively. The time-resolved scattered intensities from total, single complexes and aggregates are illustrated in Figure 6b. P4VP/PCBM complexes aggregate with each other with PCBM as physical cross-linker, which leads to the decrease of intensity contribution from complexes and the increase of intensity contribution from aggregates. Because scattered light intensity is proportional to the square of molecular weight, the scattered light intensity from aggregates become overwhelming and the total scattered light intensity increases also. The structure and size evolution of the aggregates can be obtained from Figure 6c,d. A time dependence of DLS line width ⟨Γ⟩agg vs q2 is presented in Figure 6c. For purely diffusive relaxation in dilute solution, ⟨Γ⟩agg is linearly dependent on the square of the scattering vector q, passing through the origin. A straight line passing through the origin reveals that the aggregates of P4VP/PCBM are purely diffusive in aggregation stage.24 Figure 6d shows the time evolution of ⟨Rg⟩agg, ⟨Rh⟩agg, and ⟨Rg⟩agg/⟨Rh⟩agg of the aggregates in aggregation stage. The aggregates grow in size, and its basic shape can be determined from the ratio between its radius of gyration and hydrodynamic radius, ⟨Rg⟩agg/⟨Rh⟩agg. This is because they are defined from different ways: ⟨Rg⟩ is related to the square average of the segmental distribution ⟨rij2⟩ and reflects the density distribution in real space, while ⟨Rh⟩ is related to the inverse segmental distribution ⟨|rij|−1Rg⟩ and is equivalent to the radius of a hard sphere with the same translational diffusion coefficient.25 Theoretically, for a coiled chain in a good solvent, ⟨Rg⟩/⟨Rh⟩ = 1.5 and for a uniform none draining sphere ⟨Rg⟩/⟨Rh⟩ = 0.78.21 The results indicate that the ⟨Rg⟩agg/⟨Rh⟩agg of the aggregates keeps at almost constant ∼0.78, during the whole aggregation stage. Combined with the results in Figure 6c that the motion of the particle is purely diffusive; it is reasonable to conclude that the shape of the aggregates in aggregation stage is

complexed with each P4VP chain. This calculation is almost the same with that after transmittance correction because the transmittance of PCBM DMF solution with and without P4VP are the same at λ0 = 632.8 nm. Considering the degree of polymerization of P4VP is 420, we can obtain, as a statistical average, that there are 140 pyridines side groups available to form charge-transfer complexes with every one PCBM molecules. Compared with the original PCBM concentration, there must be a large number of PCBM molecules left free. The remaining “partially free” PCBM molecules in solution can still act as physical cross-linkers among complexation and lead to further intercomplexation interaction due to electrostatic force and further aggregation. 3.1.2.3. Aggregation Stage. Since aggregation is very slow, it provides us adequate time window to carry out LS measurements. In this stage, individual complexes begin to aggregate with each other. Figure 6 reveals the structure variation of P4VP/PCBM complexes in the aggregation stage. In Figure 6a, the intercept, ⟨I(0)2⟩/⟨I⟩2 − 1, increase from 0.2 to 0.85, which is caused by the better signal-to-noise ratio from the strong scattering of the huge aggregates while the transmittance did not change at all. On the other hand, two modes in the time-resolved intensity−intensity time correlation function, i.e., the fast mode which is the Brownian motion of individual P4VP/PCBM complexes and the slow mode which belongs to that of P4VP/PCBM aggregates. Their hydrodynamic radius distribution can be obtained from a Laplace inversion of corresponding intensity−intensity time correlation function. The total average scattered light intensity (Iscatt,total) comes from the summation of contributions from P4VP/ PCBM complexes (Iscatt,fast) and P4VP/PCBM aggregates (Iscatt,slow), where the solvent contribution has been substrated, namely, with Iscatt,total = Iscatt,fast + Iscatt,slow and Iscatt,fast = Iscatt,totalAscatt,fast, Iscatt,slow = Iscatt,totalAscatt,slow, and Ascatt,fast + Ascatt,slow = 1, where Ascatt,fast and Ascatt,slow are the intensity E


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close to a uniform sphere. This can be further proved by transmission electron microscopy (TEM) observations, and we will discuss it later. When t = 25 days, Iscatt,total gets to its maximum, and precipitation begins to happen. 3.1.2.4. Precipitation Stage. The aggregates grow larger and larger and finally start to phase out of solution. Then the precipitation stage begins. LS demonstrate a clear decrease of scattered light intensity due to the agglomerate concentration decrease when it grows to be micrometers in size and phase out of solution. Figure 7 illustrates the precipitation process. There

Figure 8. TEM micrographs depicting morphology of drop-casting P4VP/PCBM pseudobinary blend films in aggregation and precipitation stages with two different resolutions. Note the samples were not stained at all.

not shown here. In the following aggregation and precipitation stages, the viscoelastic effect plays its role.26−28 And the memory of aggregates structure in solvent can be carried at least in part into drop-casting film without further coalescence. Figure 8 proves that the aggregates are almost uniform spheres with a diameter of ∼700 nm, which is consistent with DLS measurement. Because the sample here is not stained at all, the contrast in TEM is mainly comes from the electron density difference between PCBM and P4VP. Therefore, it clearly indicates that PCBM acts as physical cross-linker via electrostatic force and uniformly distributes in the spherical aggregates. In precipitation stage, the aggregates grow larger and larger and finally phase out of solution. Figure 8 shows one of the large agglomerate with irregular shape and micrometers size in the precipitation stage. The visualized physical pictures obtained from TEM are consistent with our LS results. 3.3. Kinetics Observation by UV−vis Spectroscopy. Representative time-resolved UV−vis spectra in P4VP, PCBM, and P4VP/PCBM pseudobinary blend in each stage are shown in Figure 9. Note that the concentrations of P4VP, PCBM, and P4VP/PCBM pseudobinary blend in UV−vis are 100 times lower than that in LS study to avoid signal saturation in the full spectra range. Therefore, the reaction rate of charge-transfer complexation decreases according to the collision theory of chemical reaction. The kinetics of charge-transfer complexation can be assumed to be slower where the concentrations of P4VP/PCBM are decreased correspondingly. Figure 9 demonstrates that the charge-transfer complexation has almost been accomplished in the induction stage with unchanged P4VP conformation. The absorption spectra in Figure 9 are obtained by the subtraction of a normalized DMF spectrum from the original spectra of P4VP, PCBM, and P4VP/PCBM pseudobinary blend. Pure P4VP−DMF solution is colorless, with only one narrow pyridyl group adsorption peak at 265 nm.29 While the PCBM−DMF solution is purple in color with two strong absorption peaks at 256 and 330 nm, which correspond to the strong allowed transitions of the fullerene core.30 It is well-known that the C60 absorption peak at 330 nm

Figure 7. (a) Time evolution of intensity−intensity time correlation function g(2)(τ) − 1 at θ = 20° (b) Scattered light intensity variation. (c) The corresponding hydrodynamic radius obtained from tripleexponential function analysis in precipitation stage. The arrow and dashed lines are used to guide the eye.

are three obvious decays at different time scales in its intensity− intensity time correlation function in Figure 7a, which indicated the existence of multiscale structures in the solution, namely, single complexes, aggregates, and even agglomerate. Note that the ill-defined Laplace transformation cannot be used to calculate the corresponding hydrodynamic radius distributions because imprecise boundary condition. So, we use a tripleexponential functions analysis g1(τ ) = β[a1 exp( −Γcomplexesτ ) + a 2 exp( −Γaggregatesτ ) + (1 − a1 − a 2) exp(−Γagglomerateτ )]

(7)

where a1 and a2 are the amplitudes at decay rates Γcomplexes and Γaggregates in ms−1, respectively, to fit the intensity−intensity time correlation function to obtain the corresponding hydrodynamic radius, and the fitting results are shown in Figure 7c. 3.2. Kinetics Observation by TEM. TEM is used to further explore the topological structure of the particles in aggregation and precipitation stages. Samples are taken after corresponding LS measurements and then diluted to ∼1000 times for TEM study. The characteristic images of the particles formed at aggregation and precipitation stages of P4VP/PCBM pseudobinary blend are displayed in Figure 8. Because of the effect of dilution and possible phase separation in the solvent evaporation process, the film morphologies prepared from the solutions during the induction and complexation stages are different from their corresponding solution structures and are F


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Figure 9. Time evolution of UV−vis spectra of P4VP, PCBM, and P4VP/PCBM pseudobinary blend in each stage. Arrows are used to guide the eye.

complexation stage. It can be described by the general equation37

is very sensitive to its environment, and its intensity weakens upon the chemical modification of C60 molecules.31,32 So, it can be used to identify the charge-transfer complexation between PCBM and P4VP. The freshly prepared binary blend solution is purple, and its absorption spectrum is qualitatively similar to that of PCBM. Upon aging at 40 °C, the electrostatic force became dominated, and the binary blend solution gradually changed to be dark purple in color, which made the weak adsorption peak at 330 nm broader.29 Such an observation agrees with the formation of charge-transfer complexes between the electron-donating pyridine group and the electron-accepting fullerenes.33−36 Because UV−vis spectrum only measures the energy transition from the ground to the excited state, it is no wonder that UV−vis adsorption spectra in the following complexation, aggregation, and precipitation stages keep almost unchanged because the charge transfer between PCBM and pyridyl groups has already occurred during induction stage. In LS measurement, the P4VP/PCBM concentrations are much higher, which leads to a faster charge-transfer complexation rate. Therefore, the corresponding UV−vis spectrum in complexation, aggregation, and precipitation stages should also be unchanged at LS concentration. Thus, we can conclude that UV−vis cannot be used to show further inter- or intrapolymer structure transition after the initial induction stage.

j A + k B ⇌ A jBk

(8)

which describes j molecules of species A react with k molecules of species B, forming a complexes AjBk in equilibrium. In the P4VP/PCBM pseudobinary blend, it is suitable to explain complexation stage if A denotes P4VP polymer chain and B is PCBM molecule. According to the previous calculation in section 3.1.2.2, it is simple to prove that j = 1 and k = 3; i.e., three PCBM molecules complex with each P4VP chain to form a P4VP(PCBM)3 complexes when cP4VP = 4.1 × 10−2 g/mL, cPCBM = 5.8 × 10−4 g/mL, and the molar ratio [4VP]/[PCBM] = 57:1. It should be noted that, in P4VP/PCBM pseudobinary blend, charge-transfer complexation is a reversible reaction. So j and k are depending on the ratio between [4VP] and [PCBM] in solution. According to the literature, solvent dielectric constant plays an important role in the rate constant of chargetransfer complex reaction.40 The larger the solvent dielectric constant, the weaker the electron transfer between donor and acceptor molecules.41 DMF has a large dielectric constant (ε = 36.7), so the kinetics of P4VP/PCBM charge-transfer complexation process is so slow as to occur over a month in solution at 40 °C. What is more, based on collision theory of chemical reaction, the speed of charge-transfer complexation at room temperature must be much slower than that at 40 °C when it is an endothermic reaction. In the subsequent aggregation stage, the FA model can be introduced to describe particle growth either by the addition of a single one or by the linkage of two or even more complexes once the free energy of the process is negative. The resulting aggregate forms and disintegrates in dynamic equilibrium. This model can be represented by the single kinetics equation39

4. THEORETICAL MODEL IN THE COMPLEXATION AND AGGREGATION STAGES Stimulated by the theoretical work, i.e., the free association model (FA) by Karl F. Freed,37−39 we used Flory−Huggins type theory to explain the complexation stage kinetics and structural model and to demonstrate the subsequent aggregation stage kinetics and model, respectively. Flory−Huggins type theory of mutual association, which containing single-step model, is appropriate to describe the

M i + Ml ⇌ M i + l , G

i , l = 1, 2, ...

(9)


Macromolecules

Article

Figure 10. (a) Time dependence of the molecular weight (Magg) and the hydrodynamic radius (⟨Rh⟩agg) for the aggregates formed in aggregation stage. (b) Scaling relationship between the molecular weight (Magg) and the hydrodynamic radius (⟨Rh⟩agg) for the aggregates formed in aggregation stage.

Scheme 2. Schematic Representation of the Structure Evolution of P4VP/PCBM Pseudobinary System in DMF

“Partially free” PCBM molecules have already bonded with P4VP random coil with excluded volume chain conformation in the induction stage. However, the structure is not thermodynamically stable. In the subsequent complexation process, three PCBM molecules bond strongly with one P4VP chain to form intrapolymer complexes at [4VP]/[PCBM] = 57:1. As time goes by, “partially free” PCBM molecules act as physical crosslinker and joint these complexes together to form spherical aggregates and the aggregates grow in size and finally precipitate. The mechanism of charge-transfer complexation in the P4VP/PCBM pseudobinary blend can be schematically illustrated in Scheme 2. Further experiments should be carried out to study whether the four stages should still be observed when we accelerate the charge-transfer complexation.

where M denotes P4VP(PCBM)3 complexes. Assuming that the density of the complexes is the same with that of the aggregates, and there are no “partially free” PCBM molecules inside, the molecular weight of aggregates, Magg, can be approximated as 4

Magg = Mcomplexes

ρ 3 π ⟨R h⟩agg 3 4

ρ 3 π ⟨R h⟩complexes 3

⎛ ⟨R h⟩agg ⎞3 ⎟⎟ = Mcomplexes⎜⎜ ⎝ ⟨R h⟩complexes ⎠

(10)

with ⟨Rh⟩agg and ⟨Rh⟩complexes being the hydrodynamic radius of aggregates and complexes, respectively. Because the molecular weight of complexes, Mcomplexes, has been measured according to eq 5 (see section 3.1.2.2), the molecular weight of aggregates can be estimated according to eq 10. Figure 10a clearly indicates that the mass and size of aggregates grow exponentially with time, so it is a reaction limited cluster aggregation process (RLCA),42 and compact spherical aggregates should be formed. Figure 10b calculates that the fractal dimension of the aggregates is about 2.9 ± 0.1. In RLCA, not all of collision in complexation can lead to the formation of aggregates; the mass growth of aggregates may be selfaccelerated, which agrees well with the FA model. Seriously speaking, eq 10 underestimates the mass of aggregates because the “partially free” PCBM molecules have to be involved as physical cross-linker in aggregation stage, and their contribution to Magg is not included in eq 10.

5. CONCLUSION LS and TEM were used to study the topological evolution in a model charge-transfer complexation system, P4VP/PCBM pseudobinary blend in DMF. Four distinct stages, i.e., induction, complexation, aggregation, and precipitation stages, are identified. The corresponding energy mapping by UV−vis spectrum proved charge-transfer complexation has already been accomplished in the induction stage when the mass and shape of P4VP random coil almost did not change with PCBM molecules “partially joint” to the corresponding pyridine groups. The almost unchanged UV−vis spectrum in the subsequent complexation, aggregation, and precipitation processes demonstrates UV−vis alone cannot be used to distinguish inter- or intrapolymer complexation. Flory− H


Macromolecules

Article

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Huggins type theory of mutual association together with the FA model can be used to explain the complexation and aggregation kinetics, respectively. According to our kinetics study, a proper P4VP block concentration as electron donor to form chargetransfer complexation with fullerene, a proper induction time, and the temperature control in solution before spin-coating are crucial to the formation of a desirable morphology in D/A active layer in bulk heterojunction (BHJ) devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.C.H.), [email protected] (H.C.); Tel +86-10-8261-8089; Fax +86-10-6252-1519. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Scientific Foundation of China (No. 21174152) is gratefully acknowledged.

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REFERENCES

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