Unravelling the Thermomechanical Properties of Bulk Heterojunction

Article pubs.acs.org/Macromolecules

Unravelling the Thermomechanical Properties of Bulk Heterojunction Blends in Polymer Solar Cells Anirudh Sharma,†,‡ Xun Pan,‡ Jonathan A. Campbell,† Mats R. Andersson,*,†,‡ and David A. Lewis*,† †

Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road, Bedford Park, Adelaide, SA 5042, Australia Future Industries Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia

‡

S Supporting Information *

ABSTRACT: Glass transition temperature is a critical parameter for achieving favorable and thermally stable bulk heterojunction morphology as it determines the kinetics of molecular organization of polymeric semiconducting materials. This study presents a sensitive method of precisely determining the glass transition temperature (Tg) of conjugated polymers and polymer−PCBM blends using dynamic mechanical thermal analysis (DMTA). The method presented here is very versatile in which polymer or polymer−molecule films are reinforced using a woven glass fiber and utilizes only 5−10 mg of the material. This makes the method superior to differential scanning calorimetry (DSC) for determining the thermal properties of conjugated polymers. The effects of PCBM loading, solvents, and additive on the Tg of polymer−PCBM blends and on the miscibility of different phases are investigated using the novel DMTA method. For the P3HT:PC61BM system, two different thermal transitions were found corresponding to P3HT-rich and PCBM-rich phases when cast using CHCl3, while chlorobenzene was found to result in a single Tg for the blend which was between those of the pure components, indicating greater miscibility when cast from chlorobenzene. On the other hand, miscibility of PCBM in TQ1 was found to be relatively low, and two thermal transitions were found for all TQ1:PCBM blends. The total PCBM content or the solvent used was found to have little influence on the resultant PCBM miscibility in TQ1. Tg of a range of other polymers as measured using DMTA is also reported to prove the versatility of this technique.

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INTRODUCTION Polymer solar cells (PSCs) are promising candidates for solar energy conversion, as they offer low cost processing from solution, making them very attractive for large scale roll-to-roll production.1 With the reported efficiencies of up to 11.7%2 with lab scale devices, PSC technology is at its closest ever to the commercialization stage; however, operational lifetime and stability are still significant challenges that must be overcome. In particular, the thermal stability of solar cell materials and interfaces are an important prerequisite, as PSCs are often exposed to increased temperatures3 during fabrication and operation. Thus, in order to be certified as thermally stable as per the International standard ASTM E 1171 requirements,4 PSCs must be stable under repeated thermal treatments to at least 85 °C. The light harvesting active layer of PSCs is usually a bulk heterojunction (BHJ) layer composed of an at least partially © XXXX American Chemical Society

immiscible blend of conjugated electronic materials and an electron acceptor, where the nanostructure of the layer is crucial for achieving optimal device performance.5 For efficient devices, an intermediate degree of phase separation is desirable with intermixed donor−acceptor blends resulting in a high interfacial area between donor and acceptor regions, which enables efficient charge separation, and at the same time percolating domains of pure donor and acceptor materials for charge transport to the electrode with minimum charge recombination.5,6 Exposure of the BHJ to temperatures higher than the glass transition temperature of the BHJ blend components can lead to changes in the morphology, including increased phase separation and crystallization,7 thus changing Received: February 27, 2017 Revised: March 19, 2017

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DOI: 10.1021/acs.macromol.7b00430 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic depicting a mesh sample mounted between the clamps for DMTA measurements. (b) Optical image of glass fiber. (c) Pictures of (left to right) a piece of uncoated glass fiber, glass fiber coated with P3HT, TQ1, and PVK.

Table 1. Summary of the Chemical Structure, Transition Temperature, Melting Temperature, and Molecular Weight of Various Polymers

*

Measured using DSC, 1Liquid crystalline temperature.

T g can improve 8−10 the device performance, and the crystallization of P3HT further leads to more thermally stable blend.11 On the other hand, annealing of BHJ layer of an amorphous donor polymer such as poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and PCBM12 at temperatures higher than the Tg results in

the nanostructures of the BHJ blend and impacting the performance of the device. For semicrystalline donor polymers such as P3HT (poly(3hexylthiophene)), it has been shown that thermal treatment of the BHJ blend composed of P3HT and PC61BM ([6,6]-phenyl C61-butyric acid methyl ester) at temperatures higher that the B


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Figure 2. (a) DMTA temperature scan of P3HT and (b) P3HT:PC61BM (1:1) blend prepared using CHCl3 and CB. Inset shows the tan δ peak corresponding to Tg of various samples. (c) DSC temperature scans depicting the drop in melting temperature of P3HT when blended with PC61BM. (d) Plot showing the Tg of P3HT, P3HT-rich, PC61BM-rich, and P3HT-PC61BM mixed phase in the blend (1:1) cast from CHCl3 and CB.

crystallization of PCBM and is detrimental for device performance.7,13 It is therefore of the utmost importance that the thermal properties of the materials in the BHJ are understood. It must also be emphasized that precise measurement of thermal properties such as Tg of organic semiconductors would not only be beneficial in improving the OPV technology but will have larger implications on all their applications in designing flexible and stretchable electronic devices.14 So far, various techniques15 such as variable temperature ellipsometry,16 DSC,17 plasmonic nanospectroscopy,18 and the combination of UV−vis with optical microscopy7 have been employed to estimate the glass transition temperature (Tg) of materials commonly used for fabricating PSC’s. Since only the amorphous fraction of the polymeric material displays a glass transition, measuring a traceable Tg signal in conjugated polymers using conventional techniques such as DSC is generally difficult. Direct measurement of thermal transitions of conjugated polymers using techniques such as dynamic mechanical thermal analysis (DMTA) has been limited by the fact that most of the materials used for solar cells are physically in power form, and making free-standing robust thin films for DMTA analysis is challenging and cost intensive due to the large amount of materials required. Recently, Hopkinson et al. demonstrated that DMTA could be used in a cantilever bending mode by placing the BHJ materials inside a steel pocket to perform direct measurement of the Tg of P3HT and P3HT:PCBM blends; however, the results had significant signal-to-noise issues.19 More details about the “material pocket” method can be found elsewhere.19,20

In this paper, we report a novel, highly sensitive DMTA method for measuring sub-Tg transitions, Tg, and modulus changes of a range of conjugated polymers and polymer− PCBM blends commonly used in fabricating solar cells. This was achieved by reinforcing the polymer or polymer−PCBM films using woven glass fiber and tested in tension, in a manner similar to that previously used for polymer pastes.21 This method not only enables precise measurement of Tg but also has an added advantage of utilizing only a small amount (∼5 mg) of material. This makes it a very versatile and low cost method to directly measure the thermal transitions of new solar cell materials, which often have limited supplies due to high cost and low scale synthesis in research laboratories. P3HT and P3HT:PC61BM, some of the most studied materials used in PSCs with a range of T g values reported,17,19,22 have been used to demonstrate the efficacy of the technique. This study is further extended to analyze thermomechanical properties of high performing material poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1)12 and its blends with PCBM. We also investigate the effect of different solvents, polymer−PCBM ratio, and additive on polymer−PCBM miscibility and, as a result, changes induced in the Tg of the blend. The Tg of some other polymers such as poly[N-9′-heptadecanyl-2,7-carbazolealt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT),23 poly(4-vinylpyridine) (P4PV), and poly(9vinylcarbazole) (PVK) measured using DMTA is also reported. C


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bending mode utilizing the “material pocket” method.19 Moreover, sub-Tg features due to side chain relaxations, which have not been probed previously in cantilever bending mode, were clearly resolved for samples prepared using glass fiber reinforcement. Thermal transitions of PCBM have been widely reported in the literature.16,17,19,26−28 In this study, due to the nature of sample preparation on woven glass fiber and PCBM being a small molecule, we were not able to achieve good adhesion between PCBM and glass fiber; as a result, reliable measurements of pure PCBM could not be performed using DMTA. Thus, we use 155 °C as a reference value for the most prominent thermal transition that has previously been measured for PC61BM using DMTA.19 This is also consistent with thermal transitions of PC61BM previously measured using other techniques such as grazing angle X-ray scattering27 and DSC28 and attributed as its glass transition. The 1:1 blends of P3HT and PC61BM using CHCl3 showed an increase of 17 °C in the Tg, with the tan δ peak shifting to 55 °C, also accompanied by an additional peak at 110 °C. Such an observation of double Tg for a P3HT:PC61BM blend has previously been reported by Pearson et al.29 The shift in Tg of P3HT toward higher temperature (from 38 to 55 °C) and the appearance of second transition at 110 °C show the presence of partially mixed and compositionally different P3HT:PC61BM30,31 amorphous phases. We attribute the transitions at 55 and 110 °C to the P3HT- and PC61BM-rich phases, respectively. It must be noted that the reduction in the transition temperature of pristine PC61BM from 155 to 110 °C when blended with P3HT (1:1) also demonstrates at least partial miscibility in P3HT and agrees with the findings of Verpoegen et al.27 The tan δ peak at 215 °C is attributed to the melting of the P3HT:PC61BM blend. The depression of the melting temperature (Tm) of P3HT from 235 to 215 °C upon addition of 50 wt % PC61BM to P3HT is consistent with transitions in DSC (Figure 2c) and has previously been attributed to reduced crystallite size of P3HT due the presence of PC61BM.25,27 The second tan δ peak observed at 240 °C occurs because the sample begins to deform after melting, as shown by the sample length versus temperature plot (Figure S1, Supporting Information). Therefore, the second peak is due to the sample change, not specifically a thermal process, and should be ignored. It should be noted that the DMTA technique is not normally used to measure the melting temperature of polymer; however, in this case we find that the sample is able to withstand the applied stress up to the melting temperature due to the support provided by the substrate and possibly the cocontinuous morphology of the sample. Peak values of tan δ correspond accurately with melting temperature values obtained with complementary technique such as DSC (Figure 2c). Though such fragility of the sample can be avoided in other modes of measurements such as the cantilever bending mode as demonstrated by Hopkins et al.,19 the sensitivity of measurements using glass wool as a reinforcing agent is significantly higher than that in the cantilever bending mode using a “material pocket”. The P3HT:PC61BM blend was further studied using chlorobenzene (CB) as the solvent, since this has been reported to produce an optimum morphology and better performing devices.11 Although the melting temperature of a P3HT:PC61BM blend prepared in CB was also found to be 215 °C (Figure 2b), a broad, single tan δ peak at 80 °C was

EXPERIMENTAL SECTION

Sample Preparation. Pure polymers or polymer:PCBM blends in various ratios (as specified in the text) were dissolved in CHCl3 or oDCB (30 mg mL−1) for at least 2 h. Approximately 30 mm long pieces of glass fiber (E-glass supplied by Hexcel) mesh were cut such that the strands of the mesh were on a 45° bias (Figure 1a) in order to avoid any continuous fibers crossing the length of the sample and as a result contributing to the DMA response. Polymer solutions were then carefully drop-coated on the glass fiber to achieve uniform coverage and subsequently dried in ambient air environment. The process was repeated until sufficient material coverage on glass fiber was achieved. P3HT (Rieke Metals, Mw = 50K−70K) solution was prepared in CHCl3, and P3HT:PC61BM (nano-C) (1:1, w/w) solutions were prepared in both CHCl3 and chlorobenzene (CB). TQ1 (synthesized in our lab, Mw = 132K, Mn = 71K) and TQ1:PCBM solutions were prepared in CHCl3 and oDCB (as specified). 2 vol % CN in total solvent volume was also added as an additive for TQ1:PC71BM (1:2.5, w/w) solution in oDCB. PVK (Sigma-Aldrich, Mw = 25K−50K) solution was prepared in CHCl3, and P4VP (Sigma-Aldrich) solution was prepared in IPA. The respective chemical structures, molecular weight, and measured Tg and Tm values of various polymers are summarized in Table 1. Dynamical Mechanical Thermal Analysis (DMTA). DMTA measurements were performed on a TA Q800 DMA. The DMTA measurements were performed in strain-controlled mode at a frequency of 1 Hz. All samples were measured under a continuous flow of N2 at 60 mm min−1, and the rate of heating was set to 3 °C min−1. An initial drying run on all samples was performed from room temperature up to 80 °C in order to remove any remaining solvent and physiadsorbed water, followed by a second run from −110 to 300 °C, unless specified otherwise. DMTA measures the complex dynamic modulus E* as a function of temperature and frequency.

E* = E′ + iE″ where E′ is the storage modulus and E″ is the loss modulus. Conventionally, Tg is either defined as the peak temperature of (a) tan δ19 where tan δ = E″/E′ or (b) the peak temperature of E″.24 In this study, the peak temperature of the tan δ peak is used to define the Tg for all samples. Differential Scanning Calorimetry (DSC). DSC measurements were performed on a TA Instruments Discovery DSC series DSC10039. In order for DMA and DSC measurements to be comparable, all DSC samples were first dried by ramping the temperature from room temperature to 80 °C, at a rate of 10 °C min−1, followed by the second run. Data from the second run were used for all samples.

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RESULTS AND DISCUSSION P3HT:PCBM. A DMTA temperature scan of pure P3HT coated on glass fiber is shown in Figure 2a. A small peak at −80 °C was observed, which is attributed to the sub-Tg transition of P3HT caused by relaxation of the side chains of P3HT.24 In addition to the sub-Tg observed at −80 °C, over 1 order of magnitude loss in storage modulus was observed between temperatures ranging from 10 to 50 °C accompanied by a tan δ peak at 38 °C, which we assign to the Tg of P3HT. Though this result is consistent with previously measured values of the Tg of P3HT using DMTA,19,24 it must be noted that the absolute transition temperature also depends on the measurement technique used, as exemplified by the range of reported values;17,22 hence, it must not be considered a constant. The storage modulus was found to decrease greatly around 230 °C accompanied by a tan δ peak at 235 °C, corresponding to the melting of P3HT,25 as also confirmed using DSC (Figure 2c). As can be seen from the Figure 2a, this technique provides a very high signal-to-noise ratio, and even small changes are clearly resolved compared with those in single cantilever D


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Figure 3. (a) DMTA temperature scan of TQ1, (b) TQ1:PC61BM (1:1 and 1:2.5) blend in CHCl3, and (c) TQ1:PC61BM blend (1:1 and 1:2.5) in oDCB and in oDCB with 2% CN. (d) Plot showing the Tg of TQ1 and various TQ1:PCBM blends composed of (A−E) TQ1:PC61BM (1:1) in CHCl3, TQ1:PC61BM (1:2.5) in CHCl3, TQ1:PC61BM (1:2.5) in oDCB, TQ1:PC71BM (1:2.5) in oDCB, and TQ1:PC71BM (1:2.5) in oDCB with 2% CN.

Figure 4. DMTA temperature scans of (a) PVK, (b) PCDTBT, and (c) P4VP showing the change in storage modulus and tan δ relative to temperature. The tan δ peak corresponds to the Tg.

successfully used to study the thermomechanical properties of small amounts of polymer−PCBM blends with high resolution. TQ1:PCBM. Figure 3 shows DMTA temperature scans of TQ1 and TQ1:PCBM blends. For a pure TQ1 sample, a sub-Tg peak was observed at 10 °C and a prominent tan δ peak was found at ∼95 °C accompanied by a significant loss in storage modulus. Thus, we attribute the peak at ∼95 °C to the Tg of TQ1, which is in close agreement with that measured by Kroon et al. using variable temperature ellipsometry.32 It must be noted that when TQ1 sample was analyzed using DSC, no transition corresponding to the Tg was detected (Figure S2a, Supporting Information). For the TQ1:PC61BM (1:1) blend prepared using CHCl3 (Figure 4b), in addition to the TQ1

observed (Figure 2b), which is close to the midpoint between the reported Tg of P3HT and PC61BM transition. The increase in the Tg of P3HT from 38 to 80 °C and the disappearance of second tan δ peak at 110 °C (which was earlier attributed to the thermal transition corresponding to PC61BM-rich phase) show that casting from CB results in a blend morphology that is miscible on the probed scale of motion leading to a single Tg (Figure 2d). These findings not only agree and complement the existing literature that chlorobenzene results in a finer P3HT:PC61BM blend11 but also demonstrate that this novel technique utilizing samples deposited on glass fiber for DMTA analysis can be E


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Macromolecules glass transition at 95 °C, an additional transition at 130 °C was observed (Figure 3b), which we attribute to the thermal transition of PC61BM-rich phase in the TQ1:PC61BM (1:1) blend. It must be noted that the two transitions observed in the TQ1:PC61BM blend were (a) at 95 °C, which is the same as that measured for pristine TQ1, and (b) at 130 °C, only slightly reduced from that of pristine PC61BM at 155 °C, which shows a relatively low miscibility of PC61BM in TQ133 with the presence of separated TQ1-rich and PC61BM-rich phases. On varying the TQ1:PC61BM ratio from 1:1 to 1:2.5 (increased PC61BM), the PC61BM transition at 130 °C was found to shift to 140 °C (Figure 3b) due to an increased PC61BM content in the blend and further confirms low miscibility of PC61BM in TQ1. We further study TQ1:PC61BM and TQ1:PC71BM blend samples prepared from oDCB, as it is known to result in a more optimum BHJ morphology12 which produces better performing devices than those from CHCl3.12 In the case of TQ1:PC61BM sample prepared using oDCB, again two transitions in the blend were observed: a tan δ peak corresponding to the Tg of TQ1 at 95 °C and another peak at 145 °C, attributed to the thermal transition of PC61BM (Figure 3c). On replacing PC61BM with PC71BM, while the Tg of TQ1 was again observed at 95 °C, the tan δ peak corresponding to the thermal transition of PC71BM was found at 180 °C. The increased transition temperature of PC71BM compared to that of PC61BM in the blends corresponds to the higher melting temperature (315 °C) of PC71BM as compared to that of PC61BM (285 °C) (Figure S2b,c, Supporting Information). Both these blends display a low level of miscibility. On addition of 2% CN to the blend, both transitions corresponding to the TQ1-rich and PC71BM-rich phases further increased to 110 and 186 °C, respectively. The shift in the Tg of the TQ1-rich phase toward higher temperature shows increased miscibility of PC71BM in the TQ1-rich phase upon addition of CN. Enhanced miscibility of PC71BM in TQ1 upon addition of 2% CN as observed in this DMTA study agrees with reports on the optimization of BHJ morphology, where CN has been shown to result in finely intermixed donor−acceptor blends, leading to the enhancement of device performance.5,34,35 It must be noted that solvent additives have also been shown to enhance the purity of PCBM domains36which could explain the simultaneous increase in the transition temperature of PC71BM on addition of CN. Therefore, based on the observation of two Tg values of the TQ1:PCBM blend, it can be concluded that unlike P3HT:PCBM, miscibility of PCBM is relatively low in TQ1 and is largely independent of the solvent used. PVK, PCDTBT, and P4VP. Having successfully demonstrated the application of DMTA based on the woven glass fiber sample template for precise measurement of the Tg of both semicrystalline (P3HT) and amorphous (TQ1) polymers, we extended this study to measurement of the Tg of a range of polymers relevant for organic electronic devices. DMTA temperature scans of PVK, PCDTBT, and P4VP are briefly discussed below and are shown in Figure 4. Transition temperatures, molecular weight, and melting temperature of polymers studied in this work are summarized in Table 1. For a PVK sample, a tan δ peak at 234 °C was observed with a simultaneous decrease in the sample stiffness (Figure 4a), as indicated by the significant drop in the storage modulus. Since there are no other transitions observed in DMTA, we label the transition at 234 °C as the Tg of PVK, which is also in close

agreement with the reported value.37 The Tg of PVK measured using DMTA is also supported by DSC measurements which showed a step transition at 220 °C (Figure S2e, Supporting Information). It must be noted that the Tg is not a physical constant and is instead dependent on the temperaturedependent relaxation time for α-relaxation of the polymer chain. This is reflected as the difference in the absolute Tg values measured using DMTA and DSC, as reported in this study. For PCDTBT, a tan δ peak was observed at 126 °C (Figure 4b) accompanied by a simultaneous loss in the storage modulus. DSC measurements (Figure S2d, Supporting Information) also showed a step transition at 125 °C. Thus, we attribute the tan δ peak at 126 °C to the Tg of PCDTBT, and this is close to the earlier reported Tg value of 130 °C.23,38 For the P4VP sample, two tan δ peaks at 90 and 160 °C (Figure 4c) were observed during the first run. TGA also showed 6% weight loss for P4VP (not shown here) between 120 and 150 °C, possibly due to some unknown material evaporating. Since the Tg of P4VP is known to be around 141 °C,39 a second run was performed in order to probe the existence of the transition observed at 90 °C and verify the actual Tg. For the second run, the storage modulus was found to be almost unchanged until around 140 °C and shows significant reduction between 140 and 170 °C accompanied by a tan δ peak at a slightly higher temperature of 175 °C as a result of thermal history. Therefore, we attribute the tan δ peak at 160 °C measured during the first run (without thermal history) as the Tg of P4VP, and it is also in close agreement with that measured using DSC (Figure S2f).

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CONCLUSIONS

A novel method based on a woven glass fiber template, for measuring the Tg of conjugated polymers using DMTA, is presented. This method is shown to have superior sensitivity to the glass transition as compared to conventional techniques such as DSC, and it resembles the deposition methods used for large area printing of organic electronics. We study a range of polymers and polymer−PCBM blends, commonly used in PSCs. For P3HT:PCBM blends (1:1), the miscibility of PCBM in P3HT was found to be dependent on the casting solvent, where CB was found to result in a more well-blended phase as compared to CHCl3, where both P3HT- and PCBM-rich phases were found. On the other hand, miscibility of PCBM in TQ1 was found to be limited and independent of PCBM loading and the solvent used. Solvent additive CN was found to enhance the PCBM miscibility in TQ1. These findings of phase separation and limited miscibility of PCBM in the BHJ blend have implications for establishing relevant fabrication protocols for thermally stable PSCs. The findings of this study will not only enable better understanding and control over polymer−PCBM blend morphology and its thermal stability, but this study also presents a useful and versatile method of directly measuring thermal transitions of polymers. Applications of the measurement technique presented here are not only limited to PSCs but will find relevance for studying thermomechanical properties of polymer and polymer blends for a variety of organic electronic applications. F


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Domains on the Ultrafast Charge Generation in Bulk Heterojunction Materials. J. Phys. Chem. C 2015, 119, 26889−26894. (7) Lindqvist, C.; Bergqvist, J.; Bäcke, O.; Gustafsson, S.; Wang, E.; Olsson, E.; Inganäs, O.; Andersson, M. R.; Müller, C. Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend. Appl. Phys. Lett. 2014, 104, 153301. (8) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nat. Mater. 2005, 4, 864− 868. (9) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends. Nat. Mater. 2008, 7, 158−164. (10) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene. Appl. Phys. Lett. 2005, 86, 063502. (11) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (12) Wang, E.; Hou, L.; Wang, Z.; Hellström, S.; Zhang, F.; Inganäs, O.; Andersson, M. R. An Easily Synthesized Blue Polymer for HighPerformance Polymer Solar Cells. Adv. Mater. 2010, 22, 5240−5244. (13) Bergqvist, J.; Lindqvist, C.; Backe, O.; Ma, Z.; Tang, Z.; Tress, W.; Gustafsson, S.; Wang, E.; Olsson, E.; Andersson, M. R.; Inganas, O.; Muller, C. Sub-glass transition annealing enhances polymer solar cell performance. J. Mater. Chem. A 2014, 2, 6146−6152. (14) Bao, Z.; Chen, X. Flexible and Stretchable Devices. Adv. Mater. 2016, 28, 4177−4179. (15) Müller, C. On the Glass Transition of Polymer Semiconductors and Its Impact on Polymer Solar Cell Stability. Chem. Mater. 2015, 27, 2740−2754. (16) Muller, C.; Bergqvist, J.; Vandewal, K.; Tvingstedt, K.; Anselmo, A. S.; Magnusson, R.; Alonso, M. I.; Moons, E.; Arwin, H.; CampoyQuiles, M.; Inganas, O. Phase behaviour of liquid-crystalline polymer/ fullerene organic photovoltaic blends: thermal stability and miscibility. J. Mater. Chem. 2011, 21, 10676−10684. (17) Zhao, J.; Swinnen, A.; Van Assche, G.; Manca, J.; Vanderzande, D.; Mele, B. V. Phase Diagram of P3HT/PCBM Blends and Its Implication for the Stability of Morphology. J. Phys. Chem. B 2009, 113, 1587−1591. (18) Nugroho, F. A. A.; Diaz de Zerio Mendaza, A.; Lindqvist, C.; Antosiewicz, T. J.; Müller, C.; Langhammer, C. Plasmonic Nanospectroscopy for Thermal Analysis of Organic Semiconductor Thin Films. Anal. Chem. 2017, 89, 2575−2582. (19) Hopkinson, P. E.; Staniec, P. A.; Pearson, A. J.; Dunbar, A. D. F.; Wang, T.; Ryan, A. J.; Jones, R. A. L.; Lidzey, D. G.; Donald, A. M. A Phase Diagram of the P3HT:PCBM Organic Photovoltaic System: Implications for Device Processing and Performance. Macromolecules 2011, 44, 2908−2917. (20) Tg and Melting Point of a Series of Polyethylene Glycols Using the Material Pocket http://www.perkinelmer.com/lab-solutions/ resources/docs/APP_TgandMeltofPolyethylene.pdf. (21) Chartoff, R. P.; Menczel, J. D.; Dillman, S. H. Dynamic Mechanical Analysis (DMA). In Thermal Analysis of Polymers; John Wiley & Sons, Inc.: 2008; pp 387−495. (22) Hugger, S.; Thomann, R.; Heinzel, T.; Thurn-Albrecht, T. Semicrystalline morphology in thin films of poly(3-hexylthiophene). Colloid Polym. Sci. 2004, 282, 932−938. (23) Blouin, N.; Michaud, A.; Leclerc, M. A Low-Bandgap Poly(2,7Carbazole) Derivative for Use in High-Performance Solar Cells. Adv. Mater. 2007, 19, 2295−2300. (24) Kuila, B. K.; Nandi, A. K. Structural Hierarchy in MeltProcessed Poly(3-hexyl thiophene)−Montmorillonite Clay Nanocomposites: Novel Physical, Mechanical, Optical, and Conductivity Properties. J. Phys. Chem. B 2006, 110, 1621−1631.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00430. DMTA scan of P3HT:PC61BM casted from CB; DSC scans of TQ1, TQ1:PC61BM blend, TQ1:PCB71BM blend, PCDTBT, PVK, P4VP, and P3HT (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(M.R.A.) E-mail mats.andersson@flinders.edu.au. *(D.A.L.) E-mail david.lewis@flinders.edu.au. ORCID

Anirudh Sharma: 0000-0003-4841-0108 Jonathan A. Campbell: 0000-0003-3439-8531 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of South Australia, Flinders University and the South Australian government (PRIF) for financial support. The facilities at Flinders University are supported by the Australian Nano Fabrication Facility (ANFF) and the Australian Microscopy and Microanalysis Research Facility (AMMRF), which are gratefully acknowledged. Authors wish to thank Helena Andersson for GPC measurements.

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ABBREVIATIONS P3HT, poly(3-hexylthiophene); TQ1, poly[2,3-bis(3octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl]; PC61BM, [6,6]-phenyl C61-butyric acid methyl ester; PC71BM, [6,6]-phenyl C71-butyric acid methyl ester; P4VP, poly(4vinylpyridine); PVK, poly(9-vinylcarbazole); PCDTBT, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole); P(NDI2OD-T2), poly([N,N′-bis(2octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-12 biothiopene)).

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REFERENCES

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Unravelling the Thermomechanical Properties of Bulk Heterojunction

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