Phase Diagrams of Binary Low Bandgap Conjugated Polymer

May 24, 2019 - Finally, the glass transition temperature (Tg) elevation with increasing fullerene amounts was adequately described with the Gordon–T...
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Phase Diagrams of Binary Low Bandgap Conjugated Polymer Solutions and Blends Jung Yong Kim* Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea School of Chemical Engineering, Jimma Institute of Technology, Jimma University, P.O. Box 378, Jimma, Ethiopia

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S Supporting Information *

ABSTRACT: Liquid−liquid phase diagrams of binary polymer− solvent and fullerene−solvent mixtures were qualitatively predicted by using the Flory−Huggins (FH) lattice theory as a function of solvent, polymer, and chain length, in which the model system is a low bandgap polymer, poly[2,6-(4,4-bis(2ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), and the fullerene derivatives [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) and[6,6]-phenyl C71 butyric acid methyl ester (PC71BM). Herein, the FH interaction parameters (χ) for each binary system were estimated from the solubility parameter information, originating from the contact angle measurement leading to surface energy via the Newton−Raphson numerical method. Both polymer and fullerene solutions show an upper critical solution temperature (UCST) phase behavior based on the positive χ values, as known, “like dissolves like”. However, if there is a crystallizable component in solution, the solid−liquid phase equilibria (SLE) are present in addition to the liquid−liquid phase equilibria (LLE). Then, based on this solution phase behavior, the phase diagrams of the PCPDTBT:PC61BM and PCPDTBT:PC71BM blends were constructed on the basis of thermal, optical, and morphological analyses, indicating, when the polymer composition is >60 wt %, i.e., the miscibility limit, the fullerene nanocrystals are phase-separated out from the binary polymer−fullerene mixtures. Finally, the glass transition temperature (Tg) elevation with increasing fullerene amounts was adequately described with the Gordon−Taylor and Fox models.

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INTRODUCTION The Flory−Huggins (FH) lattice theory was established in the 1940s, which explains the phase behavior of polymer solutions, based on some ideal assumptions of no volume change upon mixing (ΔVM = 0), ideal combinatorial entropy of mixing, random mixing, mean-field theory, monodisperse polymer molecules, internal energy (enthalpy) of mixing, interactions between nearest pairwise neighbors, and so on.1−4 Then, because of these obvious artificial assumptions, other molecular thermodynamics models have been developed for a better description of a real system. Among them, the lattice-fluid model with voids in the lattice,5−7 the double-lattice model considering specific interactions,8 the free volume theory (ΔVM ≠ 0),9 the equation-of-state approach,10,11 the quasichemical model, 12,13 and the generalized FH models considering both polydispersity and composition-dependent interactions14−18 could be mentioned. However, in spite of the aforementioned improvement, the original FH theory has still an advantage coming from its remarkable simplicity, affording essential physical meaning without using any adjustable parameter.1−3 In general, the FH model can predict the liquid−liquid phase equilibria (LLE) of polymer solution, specifically, the upper critical solution temperature (UCST) in the absence of specific interactions, when an interaction © XXXX American Chemical Society

parameter (χ) is properly decided. Furthermore, when the theory of melting point depression is combined with the FH model, a solid−liquid phase equilibria (SLE) could be well explained.3 However, if a system shows a lower critical solution temperature (LCST) or closed loop or hourglass shapes, one must use other appropriate thermodynamics models instead of the original FH theory.5−19 For conjugated polymer science, the original and extended FH models have been employed by several research groups in organic electronics, particularly in polymer solar cell (PSC) fields.20−28 Currently, the power conversion efficiency (PCE) of PSCs has reached about ∼16% for a single-junction solar cell29−31 and is envisioned to reach >20% in the future.32 In this situation, for realizing high-efficiency PSC devices with a long-term operation,27 it is required to deeply understand the morphological stability of the active layer composed of polymer/fullerene,33 polymer/polymer,34 and polymer/nonfullerene acceptor from a thermodynamics perspective.35,36 Herein the morphology is directly affected by the solution processing condition through the “memory effect”.37 The Received: March 9, 2019 Revised: May 10, 2019

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DOI: 10.1021/acs.macromol.9b00477 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules polymer chain’s conformation,38 single coil or preformed aggregation in solution, is kinetically transferred to the morphology of the solid-state film. Hence, during the drying process of a photoactive layer, a phase transition mechanism, i.e., from one-phase to liquid−solid (L−S) or liquid−liquid (L−L) phase transition, should be very important. Previous studies39 have shown that in the case of the regioregular poly(3-hexylthiophene-2,5-diyl) (r-reg P3HT) in o-dichlorobenzene (DCB) solution an L−S phase transition occurs first, when one-phase solution is phase-separated through cooling process, because the LLE’s position is theoretically far below the SLE curve. In this work, the phase behavior of a low bandgap (LBG) poly[2,6-(4,4-bis(2-ethylhexyl)-4H -cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT)− solvent system as well as the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PC61BM)− or [6,6]-phenyl C71 butyric acid methyl ester (PC71BM)−solvent system was studied based on the theoretical prediction of the FH model. Then, for the purpose of comparison, the phase diagrams of both r-reg P3HT and regiorandom (r-ran) P3HT solutions were constructed together. Herein, the χ parameter was calculated from solubility parameter (δ), for which the contactangle data were converted into surface energy via Newton− Raphson’s method (a root-locating formula) according to Li and Neumann,40,41 and then the surface energy (γ) is converted into δ according to δ ∝ γ .20 Note that because a LBG polymer absorbs all visible light, making it difficult to measure cloud points and light scattering data, there has been no report about binary phase diagrams for low bandgap conjugated polymer−solvent systems. Hence, as far as the author’s knowledge, this report seems to be the first describing L−L coexistence curves for the LBG polymer PCPDTBT solutions, at least theoretically and qualitatively. Furthermore, the phase diagrams of crystallizable fullerene solutions are also calculated for the first time, showing both SLE and LLE, affecting the molecular packing status in a dried film in solid state. Finally, based on thermal, optical, and morphological analyses according to Kim and Frisbie’s approach in 2008,42 the miscibility and phase diagrams of PCPDTBT/PC61BM and PCPDTBT/PC71BM blends are studied, in which glass transition temperature (Tg) data are compared with two fundamental models: Fox43 and Gordon−Taylor44 equations.

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tangential baselines. Optical absorption was measured by an ultraviolet−visible (UV−vis) spectrophotometer (Lambda 850, PerkinElmer). X-ray diffraction (XRD, X’pert Pro) was performed to examine the molecular packing of drop-cast films out of plane direction. Transmission electron microscopy (TEM) images were obtained at 80 kV with a JSM 5410LV (JEOL). For TEM samples, a spin-coated PCPDTBT/PCBM film on the PEDOT:PSS-coated glass substrate was immersed into deionized water, and the spin-coated film was then floated onto the water and collected by a 300 mesh copper grid. Tapping-mode AFM images were taken with a Seiko instrument (SPA-400).

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RESULTS AND DISCUSSION Optical and Thermal Properties of Materials. Figure 1a shows chemical structures of PCPDTBT, PC61BM, and

Figure 1. (a) Chemical structures of PCPDTBT, PC61BM, and PC71BM. (b) UV−vis absorption spectroscopy of PCPDTBT solution and film and PC61BM and PC71BM films.

PC71BM. In Figure 1b, a LBG copolymer, PCPDTBT film (Mn = 3.2 kg/mol and PDI = 2.2), shows a maximum absorption (λmax) at 715 nm and an onset (λonset) at 850 nm; the optical bandgap Eg = 1.46 eV in the UV−vis spectra. The broad shape of the spectra indicates that the PCPDTBT material exhibits an amorphous nature, but the red-shift of the absorption implies that there are more π−π intermolecular/ intramolecular interactions through aggregation in the solid state. This amorphous nature of PCPDTBT is confirmed again through XRD (Figure S1 in the Supporting Information) and DSC data (Figure S2a, second heating curve), displaying Tg at 112 °C without any melting (Tm) of the crystalline regions. However, note that in the case of the first heating scan a very weak melting signal was observed at 190 °C (see Figure S3), indicating this PCPDTBT is a marginally crystallizable polymer. Figure S2b,c shows the thermal behavior of pure fullerene materials. PC61BM shows Tg ∼ 128 °C, Tc ∼ 179 °C, and Tm ∼ 289 °C, but PC71BM displays Tg ∼ 153 °C, Tc ∼ 250 °C, and Tm ∼ 323 °C. The thermal properties of these materials are summarized in Table 1. Through TGA experiments, the thermal stability of each material could be

EXPERIMENTAL METHODS

Materials. PCPDTBT [Mn = 3.2 kg/mol, Mw = 6.8 kg/mol, polydispersity index (PDI) = 2.2, and molecular formula = (C31H38N2S3)n] was purchased from Lumtec Co. and used as received. The r-reg P3HT [Mn = 22.0 kg/mol, Mw = 46.2 kg/mol, PDI = 2.1, and molecular formula = (C10H14S)n] and r-ran P3HT (Mn = 26.6 kg/mol, Mw = 74.5 kg/mol, and PDI = 2.8) were provided from Rieke Metals and Sigma-Aldrich, respectively, and used as received. PC61BM and PC71BM were obtained from Nano-C. Characterization. The molecular weight of polymers and its distribution were measured by a gel permeation chromatograph (GPC) (PL-GPC50) equipped with a refractive index detector with chloroform as eluent. The columns were calibrated by using standard polystyrene samples. Thermal behavior was measured using DSC (TA Instruments, DSC-Q1000) and thermogravitational analysis (TGA) (TA Instruments, TGA-Q50). To make DSC samples, 1 wt % PCPDTBT/PCBM solutions were drop-cast on glass slides in a fume hood, dried in a general convection overnight, further dried under vacuum for several hours, and finally collected into an aluminum hermetic pan. Melting temperatures were taken as peak maxima, and glass transition temperatures were taken as the midpoint between two B

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Macromolecules evaluated by examining decomposition temperature, which is around 350−450 °C (Figure S4).

xn + 1 = xn −

materials

Tg (°C)

Tc (°C)

Tm (°C)

112 128 153

179 250

190 289 323

V1̂ (δi − δj)2 + 0.34 RT

(1)

where V̂ 1, δi, R, and T are the molar volume of solvent [= (112.56 g/mol)/(1.11 g/cm3) = 101.41 cm3/mol for CB], the solubility parameter (subscript i or j = 1, 2, 3, and 4 for solvent, polymer, PC61BM, and PC71BM), the gas constant, and temperature, respectively. Note that 0.34 is an entropic correction to the original FH interaction parameter defined by enthalpy per thermal energy, which comes from the presence of free volume (ΔVM ≠ 0).1−4,9 The δ value could be estimated from surface tension calculated from contact angle (θ). According to Li and Neumann,40,41 the θ value is a function of surface free energy or surface tension based on Young’s equation (γlv cos θ = γsv − γsl where γlv, γsv, and γsl are surface energies for liquid−vapor, solid−vapor, and solid− liquid respectively, as shown in Figure 2) cos θ = −1 + 2

γsv γlv

Δμ1α = Δμ1β

(4)

Δμ2α = Δμ2β

(5)

where Δμ1 = ∂ΔG/ ∂n1 and Δμ2 = ∂ΔG/ ∂n2 are the chemical potentials of components 1 and 2, respectively, and α and β indicate two different phases at equilibrium. According to the FH theory, the molar Gibbs energy of mixing is given by1−3,17,39

2

e−β(γlv − γsv)

(2)

where the constant β is 0.000115 (m /mJ) . Equation 2 can be algebraically re-expressed as eq 3 to find a root solution by using the Newton−Raphson method. 2

(3)

where f(xn) = xn− aeb(1−xn) , a = (cos θ + 1)/2, b = βγlv2, x2 = γsv/γlv, and γlv = 72.8 mJ/m2 for water. Then, by obtaining γsv via calculation, we can estimate δ through the relationship δ [(cal/cm3)1/2] = 1.829058√γsv, in which the proportionality (conversion) factor is obtained from Moons et al.’s reported data20 for poly(9,9-dioctylfluorenyl2,7-diyl) (F8) with δ = 9.2 (cal/cm3)1/2 and γsv = 25.3 mJ/m2. The results are summarized in Table 2. Note that for the estimation of δ the group contribution45 or intrinsic viscosity measurement46 could be used. However, the former only considers monomer structures while this chemical contains various conformations corresponding to a molecular weight distribution, and the latter requires an exponentially increasing number of experiments for determining a matching solvent with a roughly similar δ; moreover, all of these are indirect methods. Hence, the contact angle measurement remains as one of the most effective ways for finding out δ for long-chain molecules because the molar heat of evaporation, a direct method, is only available for small molecules. Importantly, rreg and r-ran P3HTs are characterized by different solubility parameters in spite of having the same monomer structure. The reason is that the former is stereoregular but the latter is not. Hence, only r-reg P3HT can undergo kinetically driven phase separation in solution over crystallization, generating a more nonpolar (hydrophobic) surface compared to r-ran P3HT. Figure 3 shows the phase diagram of binary polymer− solvent solutions, for which eqs 4 and 5 are solved simultaneously.3,39

Phase Diagrams of Binary Polymer−Solvent and Fullerene−Solvent Solutions. The aim of this section is to construct binary phase diagrams that explain the mechanism of a phase-separated morphology development in active layers for organic photovoltaics. The phase behavior of the polymer solution is largely determined by the χ parameter and the relative molar volume of polymer (r2).1−4 Herein, χ could be defined as written in eq 1. χ=

(n = 1, 2, ...) 2 2

Table 1. Glass Transition (Tg), Crystallization (Tc), and Melting Temperatures (Tm) of PCPDTBT (Mn = 3.2 kg/ mol, PDI = 2.2), PC61BM, and PC71BM PCPDTBT PC61BM PC71BM

f (xn) f ′(xn)

2

ϕ ϕ ΔG = 1 ln ϕ1 + 2 ln ϕ2 + χϕ1ϕ2 RT r1 r2

(6)

Figure 2. Contact angle measurement by using water for spin-coated films of (a) PCPDTBT, (b) PC61BM, (c) PC71BM, (d) r-reg P3HT, and (e) r-ran P3HT. Herein, γlv cos θ = γsv − γsl is Young’s equation. C

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Table 2. Contact Angle (θ), Surface Tension (γsv), Solubility Parameter (δ), and Flory−Huggins Interaction Parameter (χ) for PCPDTBT, PC61BM, PC71BM, r-reg P3HT, and r-ran P3HT When CB (δ = 9.5) Is Used as a Solvent PCPDTBT PC61BM PC71BM r-reg P3HT r-ran P3HT

θ (deg)

γsv (mJ/m2)

δ [(cal/cm3)1/2]

ria

χ

85.6 74.8 75.7 99.3 83.4

34.1 38.1 37.6 22.7 32.7

10.7 11.3 11.2 8.7 10.5

32 6 7 197 239

73.5/T + 0.34 165.4/T + 0.34 147.5/T + 0.34 33.0/T + 0.34 51.5/T + 0.34

a

Subscript: i = 2 for polymer, i = 3 for PC61BM, and i = 4 for PC71BM.

Figure 3. Phase diagrams of binary polymer solutions: solvent effect. (a) Theoretical phase diagram of PCPDTBT (Mn = 3.2 kg/mol) solutions and (b) the corresponding χ−φ2 plots. The theoretical data are displayed as symbols representing calculated data points along with a fitted line, serving to distinguish the overlapped data in (b).

Figure 4. Phase diagrams of binary polymer solutions: chain length effect. (a) Theoretical phase diagram of PCPDTBT (r2 = 40, 200, and 400) solutions and (b) the corresponding χ−φ2 plots.

16 to 32.0 kg/mol. Note that because χ may increase with increasing Mn,25 the two-phase region may further increase above the calculation result. Figure 5a shows the effect of different polymers on phase behavior of binary polymer solutions. For comparison

where φ1, φ2, r1 (= 1), and r2 are the volume fraction and relative molar volumes of components 1 and 2, respectively. Note that a single lattice site is decided by a solvent’s molecular volume. For the calculation of χ according to eq 1, each solution’s δ1 is needed: toluene (TOL) has δ1 = 8.9, tetrahydrofuran (THF) δ1 = 9.1, and chlorobenzene (CB) δ1 = 9.5.47 From this, the calculated χ values for each PCPDTBT− TOL, PCPDTBT−THF, and PCPDTBT−CB system are 173.3/T + 0.34, 104.5/T + 0.34, and 73.5/T + 0.34, respectively. The corresponding critical points (φ2,c, Tc) for each system, calculated by the relationship of 1/(r1φ12) + 1/ (r2φ22) = 0, are (0.15, 482.41), (0.14, 316.08), and (0.15, 208.57) in Figure 3a. This position is governed by r2 (= 30, 40, and 32 for PCPDTBT−TOL, PCPDTBT−THF, and PCPDTBT−CB, respectively). For example, r 2 for PCPDTBT−THF is calculated from the equation r2 = (Mn/ ρ2)/(MW1/ρ1) = (3200/1)/(72.11/0.8892), in which MW and ρ are the molecular weight and density, respectively. The larger r2, the more asymmetric a binodal (coexistence) curve becomes, i.e., smaller φc. Figure 3b shows the corresponding plot of χ vs φ2, in which the critical points (φ2,c, χc) are (0.15, 0.70) (0.14, 0.67), and (0.15, 0.69) for PCPDTBT−TOL, PCPDTBT−THF, and PCPDTBT−CB systems, respectively. Here χc is ca. 0.7 due to relatively small r2 value (∼30−40) and will be 0.5 as r2 → ∞, indicating a high molecular weight polymer may show easy phase separation as usual. Figure 4 shows the phase diagrams of binary PCPDTBT− THF solutions as a function of relative molar volume (r2). A larger r2 results in a longer chain in the lattice, i.e., chain length effect (physically proportional to Mn or degree of polymerization). Herein χ (= 104.5/T + 0.34) is assumed to be the same, i.e., independent of r2. When r2 increased from 40 to 200 to 400, the critical points (φ2,c, Tc) are shifted from (0.14, 316.08) to (0.07, 448.09) to (0.05, 494.08), indicating the phase separation regions increase, as Mn increases from 3.2 to

Figure 5. Phase diagrams of binary polymer solutions: polymer effect. (a) Theoretical phase diagram of PCPDTBT (r2 = 32 and 239), r-ran P3HT (r2 = 239), and r-reg P3HT (r2 = 197) solutions. (b) Solubility parameters for polymer, fullerene, and solvent.

purposes, r-reg P3HT (with Mn = 22.0 kg/mol and PDI = 2.1) and r-ran P3HT (with Mn = 26.6 kg/mol and PDI = 2.8) are included. Specifically, for describing the L−S phase transition in r-reg P3HT-CB solution, the melting point depression theory combined with the FH model is employed.1−3,39 1 1 R Vu − 0 = (ϕ − χϕ12) Tm ΔHu V1̂ 1 Tm

(7)

T0m

where Tm and (= 490.2 K) are the melting temperature of r-reg P3HT with the solvent CB and the melting temperature of pure r-reg P3HT without CB, respectively. ΔHu (47.5 J/g)39 and Vu (151.18 = cm3/mol) are the unit enthalpy and the unit volume of r-reg P3HT, respectively. Herein χ = 33.0/T + 0.34 D

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Figure 6 shows the phase diagrams of binary fullerene solutions: (a) PC61BM−CB and (b) PC71BM−CB mixtures.

is obtained from eq 1 (solubility parameter approach) for comparison with others with consistency in this study. Figure 5b displays the solubility parameters calculated from the contact angle measurements. On the basis of the relative position in this list, we can understand the morphology development mechanism of the active layer composed of two different materials for solar cells. Three representative cases for a binary polymer−polymer (or polymer−oligomer) blend film through the phase separation process in polymer−polymer solution are as follows: Case 1: Semicrystalline Polymer−Amorphous Polymer Mixture. When r-reg P3HT (δ = 8.7) and r-ran P3HT (δ = 10.5) are mixed together in CB (δ = 9.5) at high temperature (e.g., ∼340 K) and spin-coated at room temperature (i.e., in this cooling process, the solution may pass through phase boundary), by focusing on the L−L phase transition, one may expect that r-ran P3HT is phase separated out first. However, a semicrystalline polymer has an L−S phase transition, where rreg P3HT may crystallize first, and then r-ran P3HT chain molecules are arranged in the presence of the preformed r-reg P3HT chains’ nanowire network, which should contribute to forming a nanoscale morphology via microphase separation. Note that if there is no restriction of a preformed network, there is a possibility of macrophase separation,48,49 whose morphology, if the mixtures are donor/acceptor couples, is useless for solar cell applications due to limited exciton diffusion length (∼10−20 nm). Case 2: Amorphous Polymer−Amorphous Polymer Mixture. When r-ran P3HT (with Mn = 26.6 kg/mol and δ = 10.5) and PCPDTBT (with Mn = 3.2 kg/mol and δ = 10.7) (herein, we model this polymer as amorphous material) are mixed together in CB (δ = 9.5), because Δδ is 1.2 for PCPDTBT−CB and 1 for r-ran P3HT-CB, we may expect that PCPDTBT is phase-separated out first, but as shown in Figure 5a, r-ran P3HT is phase-separated first due to the chain length effect (r2 = 239 for r-ran P3HT vs r2 = 32 for PCPDTBT). However, instead of low molecular weight PCPDTBT, if we use hypothetically high molecular weight PCPDTBT with r2 = 239 (here, χ is assumed to be independent of r2 or Mn), as shown in Figure 5a, the first phase-separated material should be PCPDTBT, not r-ran P3HT, according to the prediction based on Δδij ∝ χij (recall eq 1).

Both PC61BM and PC71BM are crystallizable oligomers with r3 = 6 and r4 = 7, respectively. Hence, these fullerene−solvent solutions may display SLE and LLE, just like r-reg P3HT-CB solution. Herein, the SLE curve is calculated based on eq 7: for PC61BM−CB, ΔHu = 47.4 J/g and Vu = 101.15 g/mol (= MW/density50/r2 = 910.30/1.5/6); for PC71BM−CB, ΔHu = 62.2 J/g and Vu = 98.19 g/mol (= 1031/1.5/7). Note that ΔHu is estimated from XRD and DSC results. From the XRD spectra in Figures S5 and S6, the crystallinity (xc) is defined as follows:51 Ic xc = Ic + Ia (8)

Case 3: Semicrystalline Polymer−Semicrystalline Polymer Mixture. Like r-reg P3HT, a polymer has L−S and L−L phase transitions (e.g., SLE > LLE by temperature); we need to compare the two materials’ SLE curves for predicting which material is phase-separated out first. However, in the case where both crystalline polymers have binodal curves with LLE > SLE, we need to compare LLE curve locations just like the amorphous polymer−amorphous polymer system. In this instance also, just like case 1, the preformed (phase-separated) polymer may guide the morphology formation for the second phase-separated polymer. Then, eventually, the L−L phase transition is followed by the L−S phase transition, indicating that the film has a semicrystalline nature. In the case of the rreg P3HT−PC61BM or r-reg P3HT−PC71BM system, it may be included in this case 3 and SLE > LLE. Note that in this study PC61BM (with r3 = 6) and PC71BM (with r4 = 7) are modeled as an oligomer in comparison to a solvent. Finally, in the case of a ternary (e.g., polymer−polymer−fullerene) blend film, the development of morphology could be understood by combining two cases among the aforementioned three.

where Ic and Ia are the areas of crystalline and amorphous peaks, respectively. Accordingly, xc for PC61BM is 22.6% (pristine) and 55.7% (annealed at 160 °C for 24 h), and that for PC71BM is 0% (pristine) and 23.8% (annealed). Then, through DSC, the enthalpy of samples (preannealed at the same condition) is obtained, which is 26.4 J/g for PC61BM (xc = 55.7%) and 14.8 J/g for PC71BM (xc = 23.8%). Therefore, when xc = 100%, ΔHu becomes 47.4 J/g for PC61BM and 62.2 J/g for PC71BM. Note that the smaller ΔHu, the higher melting point depression, i.e., a large ΔTm (= ΔTm − T0m). PC61BM is known to have a triclinic unit-cell structure with a = 13.8333, b = 15.288, c = 19.249 Å, α = 80.259°, β = 78.557°, and γ = 80.406°.52,53 Hence, just like the r-reg P3HT solution case, as shown in Figure 6c, the L−S phase transition (path I) may occur first in PC61BM−CB, solution, and then the L−L phase transition (path II) may follow. In the case of the PC71BM−CB solution, the overall phase separation mechanism should be similar to the PC61BM−CB solution. However, because the molecular shape of PC71BM slightly deviates from a sphere, the L−S transition leading to phase-

Figure 6. Phase diagrams of binary fullerene solutions: PC61BM vs PC71BM. (a) Theoretical phase diagram of the PC61BM−CB solution, (b) theoretical phase diagram of the PC71BM−CB solution, and (c) L−S phase transition (path I: SLE) and L−L phase transition (path II: LLE) of the PC61BM−CB system.

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tion. In the case of PCPDTBT:PC71BM blends, the overall morphologies in film (Figure S10e−h) are very similar to those of PCPDTBT:PC61BM blends. However, in the 60−70% films (Figure S10g,h), the phase-separated nanoscale domains look much smaller than those of the PCPDTBT:PC61BM system (Figure S10c,d), which is consistent with our expectations based on the OM images in Figures S8 and S9. These smaller domains might originate from the asymmetric structure of PC71BM, which makes it difficult to crystallize when compared with PC61BM. Note that in polymer science, a stereoregular polymer can be crystallized, but a stereoirregular cannot.4 In the same vein, the molecular shape of fullerene in a threedimensional space give direct effects on its packing nature into a unit cell when crystallized. The AFM height images corresponding to Figure S10 are reported in Figure S11. Structural and Thermal Properties of Blend Films. To investigate the phase behavior of the blends more clearly, X-ray scattering was employed. At first, unannealed blend films were examined as a function of composition (Figure 8a,c). In the PCPDTBT:PC61BM system, a relatively strong peak at 2θ ∼ 19° from PC61BM crystals was observed for the 60−100 wt % PC61BM films, indicating phase separation may start at 60 wt % PC61BM. However, in the PCPDTBT:PC71BM system, most peaks in XRD spectra are too broad to determine the phase separation point. Thus, an XRD study was repeated after annealing all the films at ∼160 °C for 24 h.42 As shown in Figure 8b,d, both PCPDTBT:PC61BM and PCPDTBT:PC71BM systems show a clear peak from 60 wt % PCBM, indicating that the phase separation really starts at 60 wt %. However, 60 wt % PC61BM in Figure 8b and 60−80 wt % PC71BM in Figure 8d show very broad peaks. Thus, to clarify phase separation, the PCBM crystallite size (t) in each film was estimated using Scherrer’s relation, t = 0.9λ/(B cos θ), where B is the full width at half-maximum (fwhm) of the peak at angle θ and λ is the wavelength of X-ray. The results of these calculations are summarized in Table 3. The PC61BM crystallite size overall decreases gradually from 82 nm (100 wt %) to 2 nm (60 wt %), whereas the PC71BM crystallite size is ∼2 nm (60−80 wt %) and ∼19 nm (90−100 wt %). Note that this crystallite size may not be necessarily equal to a phase-separated domain because there is a possibility that some fullerene crystallites may stay in aggregates with partial crystallinity. XRD spectra for two PCBMs in both pristine and annealed are found in Figures S5 and S6. As shown in the above XRD experiments, the phase separation point is more clearly determined in the annealed samples than in the pristine ones. Note that spin coating is a nonequilibrium procedure to deposit thin films on a substrate, where a polymer conformation is usually quenched before reaching equilibrium. Thus, through an annealing process providing thermal energy, chain molecules may have more chances to be organized by taking an energy-minimized conformation before further crystallization or aggregation. Thus, considering the general crystallization mechanism of materials, while thermal analysis was carried out, the DSC samples were preannealed in the same condition and then thermally scanned for data acquisition. The result is summarized in Figure 9, in which the DSC thermograms (a), (c) and (b), (d) are the first and the second heating curves, respectively. In the PCPDTBT:PC61BM system, the melting of PC61BM is observed from 100 to 60 wt % PCB61M, indicating that this system shows phase separation at 60%

separated crystallite formation may kinetically require more time. Hence, as shown in Figure S5, the spin-coated film of PC61BM shows a crystalline structure, but that of PC71BM displays amorphous structure which needs further annealing for crystallization. Morphologies of Solid-State Films. In the previous section, the phase diagrams of binary polymer−solvent and fullerene−solvent mixtures with varying solvent, chain length, polymer, and fullerene size were qualitatively described based on the FH theory. Now, let us investigate phase behavior of solid-state PCPDTBT:PC61BM and PCPDTBT:PC71BM films as a function of composition. These donor/acceptor systems, from a film-forming mechanism point of view from solutions, may be relevant to case 1 (amorphous−crystalline mixture; see Figure S7) or case 3 (crystalline−crystalline mixture), depending on the composition of polymer−fullerene mixture in solution. The microscale morphologies of film were investigated by using optical microscopy (OM) before nanoscale analysis. According to the DSC thermal results, PCPDTBT is a semicrystalline polymer with a large amorphous nature, but two fullerenes (PC61BM and PC71BM) are crystals. When we see the OM images of PCPDTBT film in Figure S8a, it appears amorphous because there is no microscale spherulite (e.g., ∼10−100 μm) grown from the lamellae. However, both fullerenes PC61BM and PC71BM show crystalline morphologies in Figure S8b,c. Specifically, the PC61BM film displays both grains and grain boundaries, indicating that there is uneven growth in the crystallization process, whereas PC71BM shows uniformly distributed granular structures without any grain boundaries. For the OM images of blend films are shown in Figure S9, indicating evidence of phase separation in blends. Then, for a high-resolution study, the nanostructural morphologies of films were investigated by using TEM and AFM. The PCPDTBT film exhibits some nanoscale domains from aggregated polymeric chains without noticeable fibril-like structure, demonstrating a largely disordered structure of the film (see Figure 7a,b). Through grain size analysis using AFM software, we found that the average grain in a solid-state PCPDTBT film has a diameter of ca. 60 nm.

Figure 7. (a) TEM and (b) AFM height images of PCPDTBT polymer, in which average grain has a diameter of ca. 60 nm.

Figure S10 shows the TEM images as a function of blend composition. In the case of PCPDTBT:PC61BM blends, the 40−50 wt % films (Figure S10a,b) show similar morphologies to the pure PCPDTBT film (Figure 7a). However, the 60−70 wt % films (Figure S10c,d) display clearly distinct morphologies from the previous two ones by showing some nanoscale flakes originating from the polymer−fullerene phase separaF

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Figure 8. XRD patterns for (a) the pristine and (b) the annealed PCPDTBT/PC61BM films (160 °C, 1 h) and XRD patterns for (c) the pristine and (d) the annealed PCPDTBT/PC71BM films (160 °C, 1 h) as a function of composition at room temperature.

condition. However, in the second heating scan, Tg is well observed, in which Tg is determined at the midpoint of two tangential lines. When a polymer has a Tg higher than 120 °C, it is called an engineering plastic. Here, although PCPDTBT has a Tg = 112 °C, lower than 120 °C, when it is blended with PC61BM (Tg = 128 °C) or PC71BM (Tg = 153 °C), the blend’s Tg could be higher than 120 °C. For example, the PCPDTBT:PC61BM blend has Tg = 122 °C at 30 wt % PC61BM and the PCPDTBT:PC71BM blend has Tg = 124 °C at 30 wt % PC71BM, indicating that the blends with having or exceeding 30 wt % fullerene could be called an engineering plastic. Hence, if a conjugated polymer is amorphous, it is recommended that the polymer/fullerene blend should have a Tg higher than 120 °C for stability purposes because when the temperature is higher than Tg, a linear polymer can creep, i.e., losing dimensional stability. Importantly, as shown in Figure 9b,d, Tg is elevated by increasing PC61BM or PC71BM wt %, which is good for enhanced device stability. Phase Diagrams of Binary Polymer−Fullerene Blends. Based on the DSC data with the combined XRD and TEM results, the binary phase diagrams were constructed for the PCPDTBT:PC61BM (Figure 10a) and PCPDTBT:PC71BM systems (Figure 10b), where S2, S3, S4, L, and G stand for PCPDTBT in the solid state (flexible thermoplastic), PC61BM in the solid state (crystal), PC71BM in the solid state (crystal), liquid state, and glassy state at lower temperatures than Tg, respectively. First of all, in both PCPDTBT:PC61BM and PCPDTBT:PC71BM systems, when fullerene contents are within 0−20 wt % fullerene, the PCPDTBT molecules act as a semicrystalline polymer with the crystalline region showing a melting point. However, when

Table 3. Fullerene’s Crystallite Size and Enthalpy of Fusion as a Function of Composition crystallite size (nm)

enthalpy of fusion (J/g)

PCPDTBT:PCBM (wt ratio)

PC61BM

PC71BM

PC61BM

PC71BM

40:60 30:70 20:80 10:90 0:100

2 26 29 34 82

2 2 2 19 19

4.3 9.0 9.4 9.6 26.4

2.0 6.4 8.2 14.8

PC61BM, i.e., the miscibility limit (recall the XRD data in Figure 8). In the case of the PCPDTBT:PC71BM system, the melting of PC71BM is also observed from 100 to 60 wt %, but it is very weak at 60 wt % PC71BM (Figure 9c). The enthalpy of fusion for each blend is summarized in Table 3. Here it should be mentioned that, as shown in Figure 9c, the PCPDTBT:PC71BM system does not show any melting point depression of PC71BM with increasing PCPDTBT amounts, although the PCPDTBT:PC61BM blends display it clearly in Figure 9a. The absence of Tm depression in the PCPDTBT:PC71BM system means that in the nanoscale size (∼2−20 nm) of PC71BM [2 nm at 60−80 wt % or 19 nm at 90−100 wt %] Tm = ΔH/ΔS maintains a similar value through the proportional change between the heat (ΔH) and the entropy of fusion (ΔS). Figure 9b,d shows the second heating curves, in which only 100 wt % PC61BM or PC71BM shows a melting transition, indicating that it is difficult for PCBM to be crystallized in the presence of PCPDTBT chain molecules in this kinetic G

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Figure 9. DSC thermogram at a scan rate of 10 °C/min: (a) first heating and (b) second heating for the PCPDTBT:PC61BM system; (c) first heating and (d) second heating for the PCPDTBT:PC71BM system.

Figure 10. Phase diagram of binary polymer−fullerene blends: (a) PCPDTBT−PC61BM, (b) PCPDTBT−PC71BM systems, and (c) phase separation at miscibility limit of 60 wt % PC61BM (or PC71BM).

it is >20 wt % fullerene, the PCPDTBT polymer becomes 100% amorphous without any crystallinity because the

presence of fullerene destroys the crystalline regions completely (Figure 10a,b). In Figure 10a, when small amounts H

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Macromolecules of PC61BM are added in PCPDTBT, they are miscible with the PCPDTBT by dissolving into the amorphous regions of PCPDTBT (S2 + L and L regions). However, once the miscibility limit (60 wt % PC61BM) was reached, the PC61BM molecules are phase-separated out and form crystalline domains (S3 + L region). When the temperature is lower than Tg, the three regions (S2 + L, L, and S3 + L) enter a glassy state (G), in which segmental movements of chain molecules are highly restricted and frozen. In the case of the PCPDTBT:PC71BM system, the overall phase behavior is very similar to that of PCPDTBT:PC61BM, except for a subtle difference coming from PC71BM’s high Tg and Tm (Figure 10b). At this moment, it would be worth mentioning the characteristics of the S2 + L region. PCPDTBT is a semicrystalline polymer composed of large amorphous and small crystalline regions, both of which are intrinsically nonseparable. Hence, when fullerene molecules are uniformly dissolved into amorphous regions of PCPDTBT before reaching miscibility limit, the resulting phase could be considered a single phase (from classical polymer science point of view), although it is composed of small PCPDTBT’s crystals and PCPDTBT/PCBM miscible regions. Therefore, both S2 + L and L regions are thought of as one phase, but the S3 + L and S4 + L regions are phase-separated, i.e., two phases. In a condition of S2 + L or L or S3 + L or S4 + L, when the temperature decreases, each phase may reach a glassy state (G) in which, according to the composition, i.e., 0−20, 20−60, and 60−100 wt % PCBM, there are three kinds of glassy states as depicted in Figure 10a,b. Note that Hillmyer et al.54 also reported the L region as a single-phase melt for the poly(thienylene vinylene) (PTV):PC61BM system.55,56 In addition, the L−L phase transition of PCPDTBT:PC61BM and PCPDTBT:P71CBM is included in Figure S12. For explaining the Tg elevation in a binary system of (a) PCPDTBT:PC61BM and (b) PCPDTBT:PC71BM, as shown in Figure 11, the Fox43 and the Gordon−Taylor (GT)44

Tg =

x PCPDTBTTg,PCPDTBT + k GTxPCBMTg,PCBM x PCPDTBT + k GTx PCBM

(10)

where kGT is an adjustable parameter determined from experimental data and represents asymmetric contributions of components to the binary system. In this work, kGT is 9.5 for PCPDTBT:PC61BM and 1.0 for PCPDTBT:PC71BM, indicating the former shows a nonlinear behavior whereas the latter displays a linear one. As shown in Figure 11, the GT model describes the Tg behavior very well. Herein, note that because the segmental size for observing Tg is known to be about 10− 50 repeat segments,57 a single Tg does not necessarily mean a system is homogeneous. The nanoscale phase separation at >60 wt % PCBM might be too weak to generate additional Tg (see Table 3). Hence, in this work, only a single Tg is observed over the entire range of compositions (Figure 11).

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CONCLUSIONS AND FUTURE WORKS Phase diagrams of binary polymer−solvent and fullerene− solvent mixtures were qualitatively constructed based on the FH lattice theory, in which the effects of polymer (PCPDTBT, r-ran P3HT, r-reg P3HT), solvent (CB, THF, TOL), chain length (r2 = 40, 200, 400), and fullerene (PC61BM, PC71BM) on phase behavior were examined. These phase diagrams provide insight for understanding the mechanism of morphology development by showing potential L−S and L−L phase transitions. Then, with the understanding of solution phase behavior for crystalline−amorphous, amorphous−amorphous, and crystalline−crystalline mixtures, the phase behavior of binary PCPDTBT:PC61BM and PCPDTBT:PC71BM blends was further studied, and resultantly, their phase diagrams were constructed based on DSC, XRD, and TEM results, indicating 60 wt % is a miscibility limit. Finally, the glass transition temperature (Tg) elevation was well explained with the GT and Fox equations. Specifically, the PCPDTBT:PC61BM blend showing a nonlinear Tg behavior could be described by the GT model with kGT = 9.5. In future work, it may be a good strategy to employ a mature membrane-formation technology58,59 for fabricating small bandgap conjugated polymer−polymer (or polymer− oligomer) active layers with “stable” nanoscale morphologies, for which ternary phase behavior and phase diagram should be critically studied and constructed, respectively. Importantly, because the use of LBG copolymers (e.g., PTB7, PTB7-Th, PNTz4T, PffBT4T-2OD, and PBDB-T-SF)29−31,60−64 as electron donors and of LBG non-fullerenes (e.g., IEIC, IDTBR, m-ITIC, IDIC, and SF(DPPB)4)65−69 as electron acceptors has been a key strategy for realizing high-efficiency PSCs, constructing the phase diagrams for these materials would be essential in the near future. Note that the full names of each chemical are available in the Nomenclature of Materials.

Figure 11. Glass transition temperature of (a) PCPDTBT−PC61BM and (b) PCPDTBT−PC71BM systems, in which ● and ■ are experimental data and the red and blue solid lines are theoretical calculations derived from the Gordon−Taylor and Fox equations, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

models are employed. The Fox equation could be expressed as follows: x x 1 = PCPDTBT + PCBM Tg Tg,PCPDTBT Tg,PCBM (9)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00477. XRD, DSC, and TGA data of PCPDTBT, PC61BM, and PC71BM; comparison of phase behavior of PCPDTBT− CB with PC61BM−CB or PC71BM−CB solution; OM, TEM, and AFM images of PCPDTBT:PC61BM and

where xPCPDTBT and xPCBM are weight fractions of PCPDTBT and PCBM, respectively. However, because the Tg data are nonlinear, the GT equation might be a better model. I

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PCPDTBT:PC71BM blends; amorphous−amorphous phase separation of PCPDTBT−PC 61 BM and PCPDTBT−PC71BM based on the Flory−Huggins lattice theory (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jung Yong Kim: 0000-0002-7736-6858 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government through the Brain Korea 21 Program.

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NOMENCLATURE OF MATERIALS P3HT, poly(3-hexylselenothiophene-2,5-diyl); PBDB-T, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))]); PBDB-T-SF, poly[(2,6-(4,8-bis(5-(2-ethylhexylthio)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′c′]dithiophene-4,8-dione)]; PCPDTBT, poly[2,6-(4,4-bis(2ethylhexyl)-4H-cyclopenta[2,1-b 3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]; PffBT4T-2OD, poly[(5,6-difluoro2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3‴-di(2-octyldodecyl)2,2′,5′,2″,5″,2‴-quaterthiophen-5,5‴-diyl)]; PNTz4T, poly(naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-C24); PTB7, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]; PTB7-Th, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b,5-b′]dithiophene-2,6-diyl-alt-(4-(2ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate2−6-diyl)]; PTV, poly(thiennylene vinylene); IDTBR, indacenodithiophene−benzothiadiazole-3-ethylrhodanine; IDIC, 2,2′-[(4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis[methylidyne (3-oxo-1H-indene2,1(3H)-diylidene)]]bispropanedinitrile; IEIC, indaceno[1,2b:5,6-b′]dithiophene and 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile; m-ITIC, 3,9-bis(2-methylene-((3-(1,1dicyanomethylene)-6/7-methyl)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6b′]dithiophene; SF(DPPB)4, spirobifluorene(diketopyrrolopyrrole benzene)4; PC61BM, [6,6]-phenyl C61 butyric acid methyl ester; PC71BM, [6,6]-phenyl C71 butyric acid methyl ester.

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REFERENCES

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DOI: 10.1021/acs.macromol.9b00477 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00477 Macromolecules XXXX, XXX, XXX−XXX