Isothermal Crystallization of PC61BM in Thin Layers Far below the

Article pubs.acs.org/crystal

Isothermal Crystallization of PC61BM in Thin Layers Far below the Glass Transition Temperature Niko Van den Brande, Guy Van Assche, and Bruno Van Mele* Physical Chemistry and Polymer Science (FYSC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium ABSTRACT: An isothermal study was performed on 1.25 μm thin layers of PC61BM, a benchmark acceptor for organic photovoltaics. Using fast scanning chip calorimetry allowed for a scanning rate of 30 000 K·s−1, ensuring that all nonisothermal effects were avoided. The effect of isothermal treatments above and far below the glass transition was investigated. It was proven that PC61BM can crystallize below its glass transition, a behavior seen before for several organic glasses. Isothermal treatments as far as ca. 100 K below the glass transition lead to an initial enthalpic relaxation process around the glass transition, which over time gives rise to crystals melting around 300 °C. These observations show a remarkable similarity with the two-step nucleation mechanism documented in the literature. A standard crystallization process was observed for treatments above the glass transition. By combining chip calorimetry pretreatments with atomic force microscopy, different crystal morphologies were visualized for these two types of thermal treatments, a behavior reminiscent of other organic glasses that crystallize below the glass transition. free crystals.9 Despite its widespread use, thermal studies of the phase transitions of pure PC61BM have received relatively little attention,10 compared to that of the more complex donor/ PC61BM systems.8,11−14 For this reason, an isothermal study on thin layers of PC61BM will be performed here, utilizing a recently developed methodology for the study of isothermal structure formation in thin layers of P3HT based on fast scanning chip calorimetry.15 This fast scanning thermal analysis technique is based on thin membrane chips.16−19 Sample masses on the order of nanograms are used. This, together with the thin membrane setup allows for a significant increase in heating and cooling rates by reducing the mass which needs to be heated. Recent advances have made very high heating and cooling rates possible, leading to the development of “fast scanning power compensated differential scanning nanocalorimetry” (or shortened to “fast scanning chip calorimetry”), which was already utilized for the study of quickly crystallizing materials such as (nanostructured) poly(ε-caprolactone).20−22 The most recent generation of these devices has been able to reach rates over 106 K·s−1.23 Special attention will be paid to the behavior of PC61BM at temperatures below its glass transition or Tg, as these temperatures are of importance for OPV cells, which are often given thermal treatments to improve efficiency. For the important workhorse P3HT/PC61BM, a significant increase in efficiency is seen when a post processing thermal annealing

1. INTRODUCTION Organic photovoltaics (OPV) are based on the combination of donor and acceptor materials in order to form a heterojunction. As the interface is limited in a bilayer setup, the concept of a bulk-heterojunction (BHJ) was introduced, where a mixture of donor and acceptor material is allowed to phase separate to a certain point. The formed cocontinuous morphology maximizes the interface between the two materials. The donor material in such cells is often a conjugated polymer, such as the benchmark poly(3-hexylthiophene) or P3HT, or a more advanced low-bandgap copolymer.1,2 While there is a lot of variation in possible donor materials, the most widespread acceptors in the BHJ field are based on buckminsterfullerene (C60), for which ultrafast photoinduced electron transfer was demonstrated from conjugated polymers.3,4 Several functionalized C60 derivatives were introduced with greatly improved solubility.5 The most successful of these derivatives is [6,6]-phenyl-C61-butyric acid methyl ester or PC61BM, which is still one of the benchmark acceptor materials. It should be noted that to achieve the highest efficiencies, PC61BM is often replaced by its C71 analogue, PC71BM. This can be explained by the higher absorption in the visible region of PC71BM.6,7 It is known that PC61BM possesses both an amorphous and a crystalline phase in appropriate experimental conditions.8 The crystal structure is known to depend to a large extent on the pretreatment (film deposition, solvents used, thermal treatments), leading to an initial formation of solvate cocrystals, e.g., chlorobenzene PC61BM cocrystals with a 1:2 ratio, which may be transformed to solvent-free monoclinic crystals by removal of the residual solvent.9 A coordination number of 7 is expected for solvent© XXXX American Chemical Society

Received: September 14, 2015 Revised: October 5, 2015

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DOI: 10.1021/acs.cgd.5b01331 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Complete XEN-39391 chip (left, active area indicated by arrow) and a detailed picture of the membrane with the 60 μm × 60 μm active area (center, active area indicated by arrow), both reproduced from the Xensor Integration product catalogue. An AFM topography image of the active area is also shown (right). The sample is placed on top of the active area.

procedure is used.24 Upon thermal annealing, aggregation of PC61BM was observed alongside an increased crystallinity of P3HT.25,26 For some (low molecular weight) organic glasses, it has been shown in the literature that crystallization occurs below the glass transition temperature, either in the bulk of the glass (GC growth) or on its surface.27−31 The phenomenon of GC crystal growth, starting at a temperature slightly above Tg, was first observed in the 1960s for o-terphenyl.32 Because this growth phenomenon is several orders of magnitude faster than can be expected for a diffusion-controlled process (i.e., classical crystal growth) it is also called diffusionless growth.30 Furthermore, a different crystal morphology can be observed when crystallization proceeds through the GC mode.27 The phenomenon was explained as the coalescence of homogeneous nuclei unto the surface of existing crystals.33 An alternative explanation stated that as a crystal has a higher density, this would lead to stresses when growing in a glass. These stresses may lead to a free volume increase, allowing molecules to gain mobility, and aiding crystallization.34 A more recent study expanded on this second explanation, formulating a more general model.35 A third possible explanation, named solid-state crystal growth by local mobility, was also proposed. According to this model, local molecular fluctuations native to the glass will allow minor molecular rearrangements, allowing certain crystal structures to grow.28 A fourth possible mechanism is based on nanocrystals forming a percolating web.36 None of these models can however explain all aspects of GC crystal growth.27 Crystallization on the surface of a glass seems to be much faster than the GC mechanism, as well as more common in molecular glasses.31 In this case, crystals grow from the surface of the glass to possibly hundreds of nanometers above that surface. A possible explanation for this observation is fast surface diffusion, where molecules from the glass surface join the forming crystal layer by layer.29 This also explains the appearance of a depletion zone around the surface crystal. The thermal properties of crystals grown below the glass transition of organic low molecular weight glasses are lacking in the literature. To the best of our knowledge only the thermal behavior of indomethacin under such conditions was studied.37

ca. 60 nm. The resulting coated Au foil was then cut and placed on the active area of the nanocalorimeter chips. Isothermal studies were performed using a fast scanning chip calorimeter developed at Rostock University19,38 in combination with Xensor Integration 39391 nanocalorimeter chips (see Figure 1). The XEN-39391 chips possess an active area of about 60 μm × 60 μm with a six-couple thermopile for temperature detection and two heaters. Thin layer samples were placed on the active area with the aid of silicone oil and heated and cooled at a rate of 30 000 K·s−1. As in the previous P3HT study, isothermal treatments were performed using two different preceding pathways: from the glass and from the molten state.15 The subsequent heatings were used for the interpretation of the resulting thermal transitions. The glassy state pathway is more sensitive to nucleation effects. As such, when similar results are obtained for both pathways, it is certain that all non-isothermal effects are avoided. Each experiment starts from the highest temperature in the molten state. The isothermal crystallization times were chosen in a random order, and treatment times were repeated to check reproducibility to avoid a systematic error as a result of memory effects of preceding treatments. It should be noted that the used sampling methodology, based on a gold foil substrate, makes the accurate determination of the sample weight, the heat capacity and the heat capacity step at the glass transition (ΔCp) of PC61BM difficult. Atomic force microscopy (AFM) measurements were performed on an Asylum Research MFP-3D AFM using tapping mode conditions with AC 160 TS silicon cantilevers (resonance frequency 300 ± 100 kHz, typical spring constant 26 N·m−1). In order to clarify the morphological features seen in the recorded topography images, the corresponding amplitude signal is also shown. The silicone oil used for the deposition of the thin PC61BM layer on the active area of the chip membrane required washing of the samples with isopropanol before imaging. In this way, on-chip AFM was used to image structures and morphologies that were formed through an isothermal chip calorimetry treatment.

3. RESULTS AND DISCUSSION 3.1. Glass Transition of PC61BM Thin Film by Chip Calorimetry. A first observation that can be made when a thin film of PC61BM on Au foil is measured using the chip calorimetry setup is that the glass transition temperature is hard to observe. Despite this, it seems the glass transition step occurs at about 200 °C. To confirm this value, different cooling rates were utilized, followed by a heating at 30 000 K·s−1. When the

2. MATERIALS AND TECHNIQUES PC61BM with a purity of 99% was purchased from Solenne BV. Thin layer samples with an average thickness of 1.25 μm were prepared by dissolving 2 wt % PC61BM in chlorobenzene and then dropcasting the solutions on gold foil with a thickness of B



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Figure 2. PC61BM endothermic relaxation peak while heating at 30 000 K·s−1, following different lower cooling rates as indicated. Reference at cooling rate of 30 000 K·s−1 indicated in red.

Figure 4. Evolution of the PC61BM endothermic peak(s) during heating at 30 000 K·s−1, formed by treatments at 117 °C (390 K) for longer isothermal times using pathway from the molten state (top) and from the glassy state (bottom).

Figure 3. Evolution of the PC61BM endothermic peak during heating at 30 000 K·s−1, formed by treatments at 117 °C (390 K) for shorter isothermal times using the pathway from the molten state (top) and from the glassy state (bottom).

preceding cooling rate is lowered, this is expected to lead to a more stable glassy phase, leading to an endothermic relaxation peak upon heating. The results of this experiment can be seen in Figure 2. An endothermic peak is indeed found, confirming that the Tg can be found around 200 °C. This is significantly higher than the 131 °C value documented.8 This can be partially explained by an increase in the scanning rate from 2.5 K·min−1 to 30 000 K·s−1, which is ca. 7 × 105 times faster. Alone, this can however not explain an increase of ca. 70 K, indicating that substrate effects due to interaction with the gold foil may reduce mobility. It should be noted that at none of these cooling rates any crystallization or melting is observed, indicating that the crystallization kinetics of PC61BM are slower than those of P3HT, where a melting peak is observed for cooling rates up to 2000 K·s−1.15 In light of the slower kinetics, a 30 000 K·s−1 heating and cooling rate, used to avoid non-isothermal effect when studying P3HT,15 can be considered sufficient for the current study. As such, similar PC61BM results for the molten

Figure 5. Evolution of the PC61BM endothermic peak during heating at 30 000 K·s−1, formed by shorter (top) and longer (bottom) isothermal treatments at 97 °C (370 K) using the pathway from the molten state.

state and glassy state pathways are expected (see also paragraphs 2, 3.2, and 3.4). 3.2. Isothermal Treatments below the Glass Transition. When isothermal treatments are performed at 117 °C (390 K), a temperature well below the Tg and the lower limit of C



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Table 1. Fitting Parameters Obtained by Fitting Equation 1 to the Enthalpies Obtained for Isothermal Treatments of PC61BM Treated at 97 °C (370 K), 117 °C (390 K), and 147 °C (420 K) ΔH∞ r (μJ) τ (s) β

97 °C (370 K)

117 °C (390 K)

147 °C (420 K)

0.126 2888 0.38

0.066 50 0.25

0.057 0.26 0.23

the glass transition region as determined by chip calorimetry (200 ± 20 °C), striking behavior is seen. Isothermal times ranging from 0.01 s to over 125 min (7556 s) were used, employing both the molten state and glassy state pathway. Only a selection of the treatment times is shown for a clearer graphical representation in all following figures. For times shorter than 1153 s, the expected behavior for any material treated below its Tg is seen, where an endothermic peak is formed around or slightly above the position of the Tg of PC61BM as determined by chip calorimetry (200 °C). This evolution can be seen in Figure 3. At times of 1153 s or above, however, this low-temperature peak at about 200 °C can be observed to decrease, while simultaneously a high-temperature peak is formed at about 300 °C. This evolution can be seen in Figure 4. From Figure 3 and Figure 4 it can be concluded that for a treatment below Tg

Figure 6. Evolution of the PC61BM endothermic peak during heating at 30 000 K·s−1, formed by shorter (top) and longer (bottom) isothermal treatments at 147 °C (420 K) using the pathway from the molten state.

Figure 7. Change in enthalpy of the low temperature (solid symbols) and high temperature (open symbols) endothermic peaks seen for PC61BM when treated at 97 °C (370 K) (top), 117 °C (390 K) (middle), and 147 °C (420 K) (bottom) for the full range of isothermal treatment times (left) and for the shorter treatment times in detail (right). The low endothermic peak was fitted using eq 1 for enthalpic relaxation (solid line). D



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where the low-temperature peak increases initially, but starts to decrease at the same point as the high-temperature peak is formed. The low-temperature peak again appears around or slightly above the position of the Tg of PC61BM, and the hightemperature peak at about 290 °C (for the treatment at 97 °C) and slightly above 300 °C (for the treatment at 147 °C). Most likely the low-temperature peak after isothermal treatment at 97 °C, 117 °C, and 147 °C is caused by a pure relaxation phenomenon of the amorphous glass phase, but it is not entirely clear from these thermal data whether the formation of (imperfect) crystals might be involved too. The high-temperature peak is always observed around 300 °C, close to the melting temperature of PC61BM found in the literature using other techniques.8,39 It is therefore fair to assume that this endothermic high-temperature peak is caused by the melting of PC61BM crystals (see also melting of PC61BM in paragraph 3.4). This indicates that PC61BM can form crystals in isothermal conditions well below its Tg, a property that has so far only been demonstrated for a few organic molecules and never at a temperature that far below Tg.28−31,37 It is striking to see that an isothermal treatment below the Tg can give rise to crystals that melt 150 to 200 K higher than the respective treatment temperature without the interference of non-isothermal effects, which is exceptional. It is not clear how this crystallization process occurs. Two tentative explanations might be given: (i) Possibly it is a type of crystal reordering, where lower melting, less stable crystals are isothermally transformed into a more stable type. This would mean that the low-temperature endothermic peak formed in the early stages of the process is also a melting peak. (ii) Alternatively, the crystals associated with the hightemperature endotherm may form directly from the glassy state, reducing the amount of amorphous phase and thereby reducing the low-temperature endothermic relaxation peak (see also further discussion in paragraph 3.6). 3.3. Enthalpy Relaxation below the Glass Transition. In the case of explanation (ii), the low-temperature peak appearing first after a thermal treatment below Tg at 97 °C, 117 °C, and 147 °C should be interpreted as an enthalpic relaxation phenomenon. Several mathematical expressions exist in the literature to describe enthalpic relaxation, such as the stretched exponential equation:40−42

Figure 8. Evolution of PC61BM melting peak during heating at 30 000 K·s−1, formed by isothermal treatments at 217 °C (490 K) using pathway from molten state (top). For treatment times lower than 10.5 s, no change is observed in the thermograms (bottom).

Figure 9. Evolution of PC61BM melting enthalpies by isothermal crystallization at 217 °C (490 K) for short times up to 200 s, using a linear time scale. Obtained through molten state pathway, at a scanning rate of 30 000 K·s−1.

⎛ ⎛ t ⎞β⎞ ΔHr(t) = ΔHr∞⎜1 − exp⎜ ⎟ ⎟ ⎝τ⎠ ⎠ ⎝

(1)

where ΔHr(t) is the enthalpy of relaxation after a treatment time t, ΔH∞ r is the equilibrium enthalpy of relaxation, τ is the characteristic time of relaxation, and β is an exponent expressing the extension (“stretching”) of the exponential function if 0 < β < 1. Note that τ should be considered as an average value or characteristic relaxation time, as it is not staying constant over the full time scale of the relaxation process. The free volume progressively decreases with time, causing a deceleration of the relaxation process, as expressed by the exponent β < 1 in eq 1. A plot illustrating the surface under both endothermic peaks (change in enthalpy ΔH) as a function of the treatment time for the three temperatures studied can be seen in Figure 7. When the low-temperature peak is fitted using eq 1, a good fit is achieved for treatments up to a time which approximately corresponds to the maximum ΔH of the low-temperature peak

similar results for both endothermic peaks are seen when pathways from the molten and glassy state are employed if heating and cooling is performed at 30 000 K·s−1. It indicates that all non-isothermal effects are avoided, and the observed thermal transitions of PC61BM are the result of the isothermal treatments only. Therefore, the discussion on isothermal structure formation below Tg will further be based on the results obtained from the molten state pathway. Similar behavior is seen when the isothermal treatment temperature is decreased to 97 °C (370 K) or increased to 147 °C (420 K), as shown in Figure 5 and Figure 6, respectively. Both these temperatures are far below the Tg of ca. 200 °C observed by chip calorimetry. While it seems that rates of formation for the two peaks differ when the isothermal treatment temperature is modified, the same evolution is seen as for the treatment at 117 °C (390 K), E



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Figure 10. On-chip AFM topography (left) and amplitude (right) images of PC61BM dropcast on gold foil and given (a) no isothermal treatment (quenched), and (b) 7556 s at 217 °C (490 K). Dotted lines indicate line scans. Note the scale difference.

(Figure 7, right). Note that for this fitting the results for all isothermal treatments were used, whereas only a selection was shown in the preceding figures for clarity. As would be expected for an enthalpic relaxation, but unlike crystallization, the steepest increase in ΔH is found at the start of the process, without an induction period. This seems to indicate that the low-temperature peak can be attributed completely to enthalpic relaxation of the glassy phase, making explanation (ii) most likely. The empirical fitting parameters of eq 1 are summarized in Table 1. A few important conclusions can be drawn. With decreasing treatment temperature, the relaxation process is clearly slowing down, as indicated by an increasing characteristic τ, together with a decreasing fraction of crystals at a comparable treatment time, as indicated by the evolution of ΔH of the hightemperature endotherm (see Figure 7). The fast cooling rate is important for freezing in the nonequilibrium below Tg that is responsible for the combined effect of relaxation and crystallization. The lower the temperature, the more the relaxation process seems favored against the crystallization step. From the results of Figure 7 and Table 1 the equilibrium enthalpy of relaxation (ΔH∞ r of eq 1) seems not necessarily to be reached before the low-temperature peak starts to decrease, especially not at lower temperatures. 3.4. Isothermal Treatments above the Glass Transition. Isothermal treatments were also performed above the Tg as at such temperatures it is expected this will lead to a straightforward crystallization process. Similar results were obtained for pathways from the molten or from the glassy state indicating again that, as for the study below the glass transition (see paragraph 3.2), the observed thermal transitions of PC61BM are the result of the isothermal treatments only. Therefore, only the results of the molten state pathways will be shown. When an isothermal treatment study is performed at 217 °C (490 K), a temperature above the Tg and at the upper limit of the glass transition region as measured by chip

calorimetry, a classical crystallization behavior is indeed found, as can be seen in Figure 8. This is a rather slow process, with still no detectable melting peak after a treatment of 10.5 s. For treatment times of 10.5 s or lower no change is observed in the glass transition region, indicating that no enthalpic relaxation takes place, as is also illustrated in Figure 8. When the measured melting enthalpies or ΔHm values are plotted as a function of the isothermal treatment time (see Figure 9), a clear induction period is seen, again confirming that a classical crystallization process is taking place. A melting temperature increase (from ca. 320 to 330 °C) is found as the isothermal time is increased, indicating crystal perfectioning.43−45 A similar phenomenon was observed in the earlier P3HT study.15 3.5. On-Chip AFM Study. The flat design of the chip calorimeter sensors allows AFM measurements to be performed directly on the chips after a thermal treatment. On-chip AFM is necessary as this is the only way that nonisothermal effects can be excluded from a morphological study. For this reason, an on-chip study was performed on PC61BM, using pathways from the molten state. When PC61BM was quenched at 30 000 K·s−1, no distinguishable morphology was seen (see Figure 10a). A 10 μm line scan is also shown in Figure 12, further illustrating the featureless surface. PC61BM given an isothermal treatment of 7556 s at 217 °C (490 K), a temperature clearly above Tg and at the upper limit of the glass transition region, can be seen in Figure 10b. A crystalline morphology is observed, with structures reminiscent of spherulites. As the chip calorimetry results at this temperature indicate a standard crystallization process, this is an expected result. In a next step AFM was performed on PC61BM given an isothermal treatment below the Tg, at 117 °C (390 K). Treatment times of 281 and 7556 s were chosen based on the chip calorimetry results, corresponding to the maximum enthalpy value for the low-temperature (enthalpic relaxation) F



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Figure 11. On-chip AFM topography (left) and amplitude (right) images of PC61BM dropcast on gold foil and given different isothermal treatments below Tg. (a, b) 281 s at 117 °C (390 K), and (c, d) 7556 s at 117 °C (390 K). Dotted lines indicate line scans.

structures likely corresponding to crystals, for which melting was observed in the chip calorimetry study (see Figure 11c,d as well as the corresponding line scans in Figure 12). These results give a visual confirmation of crystallization below the Tg for PC61BM. The observed structures are significantly smaller than those formed at 217 °C (490 K), as indicated by the smaller lateral size of the AFM images and the line scans in Figure 12. Furthermore, besides the scale difference, the formed crystals also seem morphologically different from the crystals seen at 217 °C (490 K). Such a difference between crystallization above and below Tg has been seen before in literature for low molecular weight organic glasses,27 and may be explained by a difference in the crystallization mechanism.

and the high-temperature (melting) peak, respectively (see Figure 7). When on-chip AFM is performed on a PC61BM layer that has been treated for 281 s at 117 °C (390 K), the surface has transformed from featureless to a more rough morphology (see Figure 11a,b as well as the corresponding line scans in Figure 12). On the basis of the relaxation study, it is unlikely that this roughness can be attributed to crystallinity but rather to a “progressive increase of heterogeneity” in the glassy state, where the kinetics of density fluctuations gradually slow down, provoking heterogeneity and eventually crystallization.46 After a treatment time of 7556 s, however, a clear morphology is observed which gives rise to larger height differences, with large G



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melting transition, without the presence of a crystallization peak, implying that crystals/nuclei were formed isothermally.37 In this case it was also noted that relaxation of the glassy phase took place before any crystals were detected. However, by using a rather modest heating rate of 10 K.min‑1−1, non-isothermal reordering effects before melting could not be excluded in this study. It is important to note that while the enthalpic effects are observed, a densification of the formed glass toward the equilibrium glass takes place at the same time. If densification is required before crystals can form, which seems to be observed in the experimental conditions applied by chip calorimetry, this process is remarkably similar to the two-step nucleation mechanism,47−51 according to which dense mesoscopic clusters need to form prior to crystal nucleation inside such clusters in a second step. While initially conceived for protein crystals in solution, it has been shown to apply to several other systems, both organic and inorganic.49−52 On the basis of the observations made here, it seems likely that crystallization below the glass transition in PC61BM follows such a two-step nucleation mechanism. The densification step may then be required to bring several PC61BM molecules together in such a way that specific interactions can give rise to crystals. As the glassy state relaxes to a more stable form, the crystallization process can occur, which transforms part of the glass to crystals. This constitutes a further decrease in enthalpy, from the relaxed glass to the still more stable crystals, as illustrated in Figure 13. A schematic representation of this mechanism can be seen in Figure 14. The effect of the decrease in enthalpy related to this entire process will then no longer be observed at the Tg, but at the melting point Tm of the formed crystals. It is expected that this would lead to a decrease in the size of the relaxation peak (drop in ΔHr) and the heat capacity step (ΔCp) at the Tg, while simultaneously a melting peak will be formed (increase in ΔHm), which is exactly what is observed. The remaining amorphous glass in case of partial crystallization might appear as a rigid amorphous phase, intimately mixed with and captured in between the crystals. The mobility of this rigid amorphous phase will only released during melting of the crystals in a concerted process. This limiting condition of simultaneous melting of crystals and devitrification of the rigid amorphous glass is schematically depicted in Figure 13. It is also noticed in some polymer systems, e.g., poly(phenyleneoxide).53 As such, it is possible for the relaxation peak to disappear entirely even though an amorphous glass fraction still remains. An amorphous glass phase with a delayed mobility might also appear as an intermediate condition along the two-step nucleation and crystallization pathway. The information on ΔCp at Tg, which is useful for a more detailed evaluation of the rigid amorphous glass fraction, is not accurate due to the sampling procedure (see paragraph 2) and is not used in this respect. Note that all previously described processes of densification followed by crystallization could also be considered as a “progressive increase of heterogeneity in the dynamics of supercooled liquids by drastically slowing down their kinetics, eventually leading to crystallization in glasses”.46 It should be stressed that other tentative pathways could be suggested for the observed enthalpy relaxation followed by crystallization of PC61BM. An alternative route might be crystal nucleation in domains of locally increased free volume adjacent to the densified clusters. This might be seen as a variant of the

Figure 12. On-chip AFM line scans of the AFM images shown in Figures 10 and 11, illustrating the significant height differences induced by different thermal treatments. Note that a common scale is used for all measurements (except for scan 2, the isothermal treatment of 7556 s at 217 °C (490 K), where the larger lateral size of the formed structures requires a larger lateral scale. Line scan 6 is included on the same lateral scale as an insert for comparison.

Figure 13. Idealized schematic representation of crystallization after relaxation of the glass below Tg.

3.6. Proposed Two-Step Nucleation Mechanism. Results obtained by chip calorimetry indicate that when PC61BM is given a thermal treatment below the Tg a relaxation of the glassy phase first occurs, after which crystallization can take place. Enthalpic relaxation as observed in thermal analysis techniques is linked to the relaxation of a glass phase toward equilibrium. This process is paired with a decrease in the volume, entropy and enthalpy of the glass, of which the enthalpy decrease is detected by chip calorimetry. Similar behavior was observed for indomethacin, where isothermal treatments as far as 56 K below the Tg gave rise to a detectable H



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Figure 14. Schematic representation of the possible two-step nucleation mechanism by which PC61BM crystallizes below Tg.

Interestingly, PC61BM crystallization below Tg only seems to occur after enthalpy relaxation takes place, and the enthalpy relaxation peak can be seen to decrease during the crystallization process. Both of these processes can be interpreted by using a kind of two-step nucleation mechanism of crystallization, in which a densification step (consistent with enthalpy relaxation) is required before crystal formation by specific interactions can proceed. While more experimental confirmation may be required, this seems to be a new system where the two-step theory of crystal nucleation applies. It would also be interesting to compare the thermal behavior observed for PC61BM with other small molecules known to crystallize below the Tg. Thermal data are however lacking for such materials. A thermal analysis study could therefore determine whether below Tg an enthalpic relaxation precedes structure formation as in PC61BM, in order to determine if the two-step process is generally valid.

two-step nucleation mechanism where the crystal nucleus is not growing in the bulk of a densified cluster but is surface-induced by the densified cluster in the surrounding zone of higher free volume. This approach is again fitting in the general view of the gradually slowing down kinetics of density fluctuations in the glassy state, provoking heterogeneity and eventually crystallization.46 Finally, although less probable, densification (enthalpy relaxation) and crystal nucleation and crystal growth might happen in independent parallel pathways starting from the initially formed glass, leading to the observed enthalpy relaxation and melting profiles according to different kinetics. It is clear that more detailed experimental investigation and modeling is needed to distinguish between these different mechanistic proposals. In all above-mentioned isothermal phase transformations of PC61BM and their mechanistic interpretations, the role of the gold substrate should not be neglected. Heterogeneous surfaceinduced effects will influence the vitrification process in the Tg region and the crystallization process well above Tg. The role of homogeneous crystal nucleation is probably getting more important in isothermal conditions around and below Tg.54 The effect of different substrates on the thermal transformations in thin layers will be studied in future work.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +32 2 629 32 76. Fax: +32 2 629 32 78. E-mail: bvmele@ vub.ac.be. Notes

The authors declare no competing financial interest.

4. CONCLUSIONS Isothermal chip calorimetry studies could be performed on thin films of PC61BM at several temperatures above and below the Tg. While standard crystallization seems to occur above the Tg, the behavior below Tg was surprising. For temperatures as far as 100 K below the Tg, crystallization was observed too. While crystallization below the Tg has been reported in the literature for some small organic molecules, this is the first time it was shown to exist in PC61BM. This finding is of major importance for the field of OPV, where morphology and stability as a function of temperature are crucial. The thermal methodology developed could be applied to other electron acceptors such as PC71BM, which is now more and more used for OPV instead of PC61BM. On-chip AFM was also employed on PC61BM crystallized above and below the Tg, yielding different crystal morphologies. The appearance of a different crystal morphology when crystals form below the Tg has been seen before in the literature for some small organic molecules.

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ACKNOWLEDGMENTS

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REFERENCES

Prof. Peter Vekilov is thanked for valuable discussions on the two-step nucleation mechanism. The authors acknowledge the Research Foundation Flanders (FWO) for a Ph.D. grant and the foundation of public utility Hercules for financial support.

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