Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Importance of Nucleation during Morphology Evolution of the Blade-Cast PffBT4T-2OD-Based Organic Solar Cells Zhaozhao Bi,† Hafiz Bilal Naveed,† Yimin Mao,‡,§ Hongping Yan,∥ and Wei Ma*,† †
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States § NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, Maryland 20899, United States ∥ SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States Macromolecules Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/21/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Different temperatures, solvents, and additives are used as influencing parameters to drive molecular packing and phase separation for the development of the controlled evolution of nanostructures for organic solar cells (OSCs). The temperature-dependent aggregation (TDA) features of polymers are explored by investigating aggregation in solution and solid thin-film states using solution small-angle neutron scattering (SANS), soft/hard X-ray scattering, and transmission electron microscopy (TEM) characterizations. In situ grazing incidence wide angle X-ray scattering (GIWAXS) reveals that the nucleation process is highly significant and responsible for the ultimate film morphology. Processing conditions such as temperatures, solvents, and additives were used to influence the nucleation and evolution of film morphology. The nucleation process may improve the polymer packing and phase separation. It may translate into optimized multilength scale domains and efficient charge percolation pathways, a strong implication to control the nucleation process for the efficient separation and transportation of charges in bulk heterojunction (BHJ) devices. of OSCs.17 Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)alt-(3,3‴-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2‴-quaterthiophen5,5-diyl)] (PffBT4T-2OD) is one example of this class, achieving efficiencies of over 10% with thick photoactive film utilizing fullerene derivatives.18,19 TDA being one of the most promising characteristics of PffBT4T-2OD enables a nearly ideal blend morphology containing highly crystalline yet reasonably small polymer domains, showing insensitivity to the presence of fullerene derivatives as electron-accepting materials. Those characteristics offer one feasible solution for the trade-off problem between achieving a small domain size and high polymer crystallinity, enabling a high fill factor (FF) value, even in thick active layers, and further accelerating the development toward improved OSCs.17,18 We have reported the use of different processing conditions and molecular weights on the morphology of PffBT4T-2OD:PC71BM blend films in correlation with the performance of the device in our previous collaborative work.19 Spin rates and temperature conditions are found to effectively tune molecular orientation and molecular packing; a slow spin rate and low temperature induce highly ordered face-on polymer packing, while the fast
1. INTRODUCTION Solution-processed organic solar cells (OSCs) have shown exceeding potential to be one sustainable energy source for their superior advantages of being lightweight, simple, and wearable with rapid energy payback times and large-scale rollto-roll processing.1,2 The persistent development of new organic materials and matured methods for device fabrication have provided power conversion efficiencies (PCEs) of over 13 and 15% for single-junction binary and tandem BHJ devices,3−6 highlighting the fierce competitive edge of organic photovoltaic (OPV) devices for future commercialization. However, high material consumption and the limited scale area of spin-coated devices remain a few of the major challenges toward the large-area manufacturing of commercial devices. Blade coating is an evolving technique with a promise of compatibility with large-area roll-to-roll processing and economical coatings of films from viscous solutions.7,8 For the promotion and smooth laboratory-to-factory transition, a number of reported OSCs have shown promising PCEs even with the use of a photoactive layer via the blade coating.9−16 Besides the processing methods, much of the growth of OSCs was empowered by the development of high-performing donor polymers. A family of polymers with a unique temperature-dependent aggregation (TDA) behavior has been proven to be an intimate resource for the development © XXXX American Chemical Society
Received: June 2, 2018 Revised: July 31, 2018
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Figure 1. (a) Chemical structure of PffBT4T-2OD and PC71BM. (b) Scattering geometry for in situ X-ray scattering system. (c) J−V curves of the PffBT4T-2OD:PC71BM blend processed from (i) DCB at 80 °C, (ii) DCB at 100 °C, (iii) CB at 100 °C, and (iv) DCB/DIO at 80 °C. (d) RSoXS scattering profiles at 284.2 eV for PffBT4T-2OD:PC71BM blend films at different temperatures and solvents as indicated. (e) GIWAXS curves of dry films under DCB 80 °C, DCB 100 °C, CB 100 °C, and DCB/DIO 80 °C. (f) Relative purity of PffBT4T-2OD:PC71BM blends as a function of (100) peak location processed from DCB 80 °C, DCB 100 °C, CB 100 °C, and DCB/DIO 80 °C. (g) Schematic diagram of phase separation and molecular packing of PffBT4T-2OD:PC71BM films processed from DCB 80 °C, DCB 100 °C, CB 100 °C, and DCB/DIO 80 °C, respectively.
using spectroscopic ellipsometry.20 A thorough comprehension from the in situ view about the influence of blade-coating parameters on TDA features of PffBT4T-2OD is still absent. Morphology control and the characterization of active layers are essential for the further development of OSCs.21−24 Nucleation and subsequent growth are assumed to be the main part of morphological evolution in most organic blend films. A great number of ex situ morphology studies, however, were performed after the solidification of the film, which reveals a limited knowledge of nucleation and the growth process toward obtaining optimized nanostructures. Fortunately, a number of in situ studies have focused on OSCs, suggesting that the evolution pathways from solution to film are critical for solution processed self-organized nanostruc-
and high temperature cause poorly ordered edge-on packing. Average domain purity can also be changed by the processing conditions, while the middle domain size stays nearly identical. Moreover, the TDA properties are also highly molecularweight dependent as low molecular-weight polymers yield lessordered molecular packing and large, impure domains.19 Hyun et al. explored the TDA characteristics of PffBT4T-2OD to report a PCE of ∼10% and highlighted the thermal effects during a transformation from spin coating to blade coating (or slot-die casting) by utilizing the in situ characterization of photoactive films.14 This work promotes the transition from the laboratory to the factory of TDA-based materials and enlarges the potential of PffBT4T-2OD. Moreover, the effects of DIO on PffBT4T-2OD-based devices were investigated B
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Macromolecules tures.14,25−34 Recently, Chen’s group systematically illustrated the solvent and additive effects on PTB7 spin-coated films and pointed out that the boiling point of the solvent affects the solvent evaporation rate, which further governs the crystallization kinetics. In addition, additives can prolong the film formation time, and the additive polymer interaction plays an important role in the additive evaporation stage and the structure-forming stage of a polymer.28 Delongchamp and coworkers discussed the additive effects on the formation of a polymer and small molecular-based fullerene OSCs from an in situ view in detail.31−33 For polymers, for example P3HT, additives accelerate the bulk nucleation via controlling the solvent quality and extending the crystal growth window by delaying the onset of PCBM-induced vitrifications. Moreover, the good or poor solubility of different additives to P3HT leads to the phase’s evolution pathway into the two-phase model or three-phase model.31,32 This valuable real-time research about phase evolution deepens the comprehension about film morphology and opens the possibility to control phase evolution and morphology purposefully. As for small molecular fullerene systems, the high boiling point additive controls the nucleation density and crystal growth of the active layer by dissolution and plasticization effects during the film formation process. An additive with a proper ratio enhances nucleation density via its solubility and high boiling point. However, high additive amounts lead to less nuclei due to their good solubility with donor molecules and further contribute to large domains, highlighting the nucleation effects on domain morphology.33 It should be noted that the significant role of nucleation during the transformation from solution to dried film on film morphology and devices performance is indisputable. For TDA featured polymers, however, methods about controlling the nucleation and subsequent growth behavior for an improved crystalline morphology remains largely uninvestigated, much less a systematic study to identify the roles of individual processing parameters such as the temperature, solvent, and additive in the film drying period. In this work, we employed solution small-angle neutron scattering (SANS) characterization and in situ grazing incidence wide angle X-ray scattering (GIWAXS) to investigate the influence of the temperature, solvent, and additive on PffBT4T-2OD nucleation and subsequent growth in crystalline morphology evolution during the transformation from solution to dried films. The nucleation and growth processes during the film-forming period were distinctly affected by different processing conditions during the blade coating of PffBT4T2OD films. Integrated studies show that the nucleation process of the polymer has a profound impact on the final dry-film morphology with intermixed donor and acceptor phases, which could be carefully controlled by tuning the processing conditions. This pioneering comprehension about the role of nucleation in organic semiconducting materials has valuable implications to the optimization of TDA-based BHJ morphology toward conceiving highly efficient OSCs.
conditions, and the detailed parameters are listed in Table S1. The blade-coated blend film processed using DCB solvent with 3% DIO at 80 °C yields the highest power conversion efficiency (PCE). Device efficiency deteriorates either by increasing the processing temperature or by replacing DCB with CB as the host solvent. In order to establish the relationship between device performance and active layer morphology, we performed transmission electron microscopy (TEM) and resonant soft Xray scattering (RSoXS) to observe the phase morphology of blends. As evidenced by TEM images (Figure S1), blend films processed from DCB or CB without DIO all exhibit large and isolated fullerene-rich domains mixed in a polymer-rich matrix; such morphology is unfavorable for charge dissociation and transport.35,36 The domain size decrease for the DCB/DIO mixture and the morphology appears to be finely blended, resulting in improved dissociation and transport efficiency and current density. RSoXS was carried out at 284.2 eV to deeply probe the phase separation of blends, as shown in Figure 1d, meanwhile avoiding K-shell absorption, which can induce background fluorescence. Blend films processed from DCB or CB without DIO show multiple peaks with higher-order structure, and those scattering peaks originate from the form factor and the structure factor, indicating isolated phases in these films, which matches up with TEM results. The scattering of DCB/DIO processed film exhibits bicontinuous phase separation and corresponds to a domain spacing of ∼30−80 nm that agrees well with previously reported values.18 Here, the degree of mixing for the donor and acceptor can be inferred by evaluating the relative domain purity through comparing the total scattering intensity (TSI) calculated from the scattering profiles,37 with a higher purity hinting at a poor mixing of donor and acceptor materials. Films processed from DCB at 80 °C and CB at 100 °C exhibit poor mixing with toohigh domain purity, as supported by TEM results. In addition, films tend to show improved mixing when blade-coated from DCB/DIO mixtures, resulting in appropriate domain purity and improved phase separation. The molecular packing of dry PffBT4T-2OD films was also investigated for a more comprehensive understanding about the structure−performance relationship. Shown in Figure 1e are 1D GIWAXS profiles of dried polymer films produced by circularly integrating 2D scattering patterns. The characteristics of (100) peaks reflecting the lamellar packing information can be extracted through peak fitting. The peaks were fitted with a Gaussian function to extract the peak location, peak width, and peak area. It is interesting to note that the relative domain purity changes as a function of (100) peak location; i.e., the degree of donor/acceptor mixing is related to the lamella packing distance as indicated in Figure 1f. There is a significant influence of the polymer-packing distance on the degree of donor/acceptor mixing as the fullerene molecules are expected to penetrate into donor polymer networks more easily. In fact, the polymer-packing distance, to some extent, is proportional to its degree of crystallinity and has been reported to play an influential role in driving the relative purity of domains and controlling the device performance.38−40 By further analyzing the d-spacing formation process with the GIWAXS measurement, we find that the lamella packing distance is influenced by the quantity of crystals of the polymer. We estimate the relative quantity of crystals (N) in PffBT4T-2OD films using the quotient of intensity and coherence length (C.L.) value (intensity value/C.L. value), considering that the integrated
2. RESULTS AND DISCUSSION 2.1. Device Characterization. We have prepared PffBT4T-2OD-based (the chemical structure shown in Figure 1a) blade-coated films choosing PC71BM as the acceptor under different temperatures, solvents, and additives to evaluate the photovoltaic performance of TDA-based BHJ devices. Figure 1c shows the current density−voltage (J−V) characteristics for champion devices under each batch of certain processing C
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Figure 2. SANS profiles of PffBT4T-2OD at 50 and 110 °C in d-DCB solvent with or without DIO and corresponding schematics exhibiting the polymer’s aggregation in solvent.
challenging to use X-ray scattering for molecular solution study. The major concern is that the X-ray scattering signal relies on the density difference between objective molecules and its surrounding solvents, which is usually low for noncrystalline samples. Moreover, X-ray beam intensity can be severely attenuated by solvents. The SANS signal, on the other hand, stems from the scattering length density difference between different atomic nuclei, which allows us to visualize the component of interest in a mixture by appropriate isotope labeling. We used deuterated solvent (d-DCB) to dissolve the hydrogenated polymer and additive (DIO), and the 1D scattering profiles are shown in Figure 2. At 110 °C, PffBT4T2OD is well dissolved in the solvent. Scattering intensity is weak. The slight excess intensity in the q range between ∼0.01 and ∼0.1 Å−1 is mainly due to the scattering from individual PffBT4T-2OD chains. The SANS profiles, however, cannot be fitted using a simple Debye function for a Gaussian chain. The reason, besides the low intensity level, might be due to the nonideality of the solution, as well as the polydispersity in the polymer chains. Once the temperature dropped to 50 °C, a significant increase of scattering intensity was observed, indicating that the solution inhomogenity is greatly enhanced. 1D SANS profiles at the lower temperature were fitted using the Debye−Bueche model (eq 1), which is used to treat random density fluctuations.44,45
intensity is proportional to the polymer crystal quantity and C.L. is proportional to the crystallite size. Here, we do not intend to obtain the exact value of the N but to use the value to estimate the crystal features. The full width at half maximun (FWHM) values of (100) peaks are 0.058 Å−1, 0.045 Å−1, and 0.046 Å−1, respectively, through the fitting of (100) peaks of polymer films at DCB 80 °C, DCB 100 °C, and CB 100 °C, meaning that the corresponding C.L. values are ∼98 Å, 125 Å, and 123 Å, respectively. Hence, the hypothetical values of N for DCB 80 °C, DCB 100 °C, and CB 100 °C are estimated to be 4.78, 3.34, and 6.42, respectively, which means that there are a higher number of polymer crystals in films with a more pure phase purity. The extensive crystals of polymer inevitably result in limited growth space and squeeze polymer chains into more tight packing, which hinders the fullerene acceptor from getting involved in polymer networks, and this further heavily enhances the phase purity. Combining the TEM, RSoXS, and GIWAXS results, the schematic diagrams of phase separation and molecular packing can be represented in Figure 1g, which establishes the structure−property relationship and successfully explains the change of device performance. However, to figure out how these blade parameters affect the polymer packing and domain purity of a TDA-based polymer from an in situ view, further investigations before and during the morphology evolution from solution to thin film are needed. As reported previously,18 the presence of fullerene is insensitive for the crystallization of PffBT4T-2OD, and thus we have simply adopted SANS and in situ GIWAXS characterizations to study the evolution of nanostructures of PffBT4T-2OD pure films instead of the PffBT4T-2OD:PCBM blends. 2.2. Polymer Aggregation in Solution. We employed SANS to study polymer aggregation in the solution state at a length scale from a few nanometers to tens of nanometers; particular attention has been given to the influence of the temperature and additive. In principle, small-angle scattering (both X-ray and neutron) techniques are well suited for probing nanoscale structures.41−43 However, it is often
I(q) = k1
1 + k 2 exp ( −q2ξ22/4) + b (1 + q2ξ12)2
(1)
In the Debye−Bueche model, the inhomogeneity in the system is isotropic; an exponential decay is assumed for the 1D correlation function, yielding a Lorentzian-type scattering function (the first term in eq 1). The correlation length, ξ1, is the length characterizing the mesh size due to interpenetrating polymer chains. The second Gaussian term involving another correlation length, ξ2, is used to treat large-scale aggretates, producing excess intensity at low q. In eq 1, b is a constant background; k1 and k2 are scaling factors. For D
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Figure 3. (a) Circularly integrated line curves of the crystalline domain evolution for pure DCB processed blade-coated PffBT4T-2OD films at 80, 100, and 110 °C with each line curve representing an exposure time of 200 ms. (b) 2D representation images of the crystalline domain evolution for pure DCB processed blade-coated PffBT4T-2OD films at 80 °C (top row), 100 °C (middle row), and 110 °C (bottom row). (c) Evolution of the fitted (100) diffraction peak locations (left panel) and the FWHMs and intensities (right panel).
the solution without DIO, the fitting results of ξ1 and ξ2 are 9.2 and 42.9 nm, respectively. These lengths are 9.8 and 44.9 nm for the solution with DIO. Seen from the SANS data, the addition of DIO does not have a significant effect in the structure formation but slightly increases the incoherent background at high q for different temperatures. This is because of the incoherent scattering from hydrogen atoms in the additives. We need to point out that the fitting results of ξ2 are qualitive in nature, as our SANS data at low q were truncated at ∼0.003 Å−1. More structural details of these largescale aggregates might need ultrasmall angle-scattering probes. Nevertheless, the SANS data clearly indicated that a strong aggregation process occurred at lower temperatures. A schematic illustration of the solution structure of the PffBT4T-2OD/DCB system at high and low temperatures is shown in Figure 2. 2.3. Polymer Aggregation during Film Formation. In situ GIWAXS (scheme as shown in Figure 1b) was performed for constructing a clearer picture of the evolution of polymer molecular packing behavior. We symmetrically investigated three kinds of process factors (temperatures, solvents, and additives), attempting to construct connections between the device performance, BHJ morphology, and crystallization pathway. In situ GIWAXS data were collected and fitted in the same way as that reported in the literature.46 The location and FWHM of peak (100) are inversely proportional to the distance between the alkyl chains and coherence length of polymer crystallites, respectively; also, the integrated intensity of peak (100) is proportional to the number of polymer crystallites.47
2.3.1. Influence of Processing Temperature on Morphology Evolution. To expand our discussion, the diffraction peaks for DCB processed PffBT4T-2OD films under three different temperatures (80, 100, and 110 °C) were integrated circularly from in situ GIWAXS profiles (Figure 3a). The scattering profiles were plotted into 2D representative false-color images (Figure 3b) for a clear view of the peaks evolving as a function of time (vertical axis). There are two pronounced scattering peaks at q = 1.2 Å−1 and q = 1.8 Å−1 assigned to the DCB solvent as reported in the literature.28 The characteristics of the peaks from fitted data, plotted in Figure 3c, show a detailed evolution of polymer aggregation for different temperatures in four stages. For example, at 80 °C, the polymer solution bears no visible scattering until after t = 7.8 s (defined as the onset of blade-coating time as zero with a precision of ±0.2 s), which we mark as stage I for well-dissolved PffBT4T-2OD in relatively warm solvent. Later, the majority of solvent gradually escapes, resulting in decreasing solvent peaks until 12 s; we mark this period as stage II. At this stage, the location, FWHM, and intensity of the (100) peak keep growing, suggesting the appearance of a large amount of ordered polymer packing. With the continuous evaporation of residual solvent comes the time we mark as stage III, showing the increased location and intensity of the (100) peak attributing to the reduction of lamella spacing and the enlargement of crystalline domains. At last, morphology evolution ultimately ends, resulting in a solid polymer film at stage IV. We summarize the above stages as the (I) dissolved stage, (II) nucleation stage, (III) growth stage, and (IV) film stage for a better explanation. Similar evolution E
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Figure 4. (a) Circularly integrated line curves of the crystalline domain evolution for pure CB processed blade-coated PffBT4T-2OD films at 100 and 110 °C with each line curve representing an exposure time of 50 ms. (b) 2D representation images of the crystalline domain evolution for pure CB processed blade-coated PffBT4T-2OD films at 100 °C (top row) and 110 °C (bottom row). (c) Evolution of the fitted (100) diffraction peak locations (left panel) and the FWHMs and intensities (right panel). Different film-drying stages are distinguished by different colors.
stages can be expected in films processed at higher temperatures of 100 and 110 °C. A detailed analysis would briefly illustrate the influence of temperature on the nucleation stage and the evolution of polymer morphology. As shown in Figure 3c, the time in stages I, II, and III is negatively correlated with temperature, which means that evaporation is faster at higher temperature, leading to a shorter polymer aggregation time due to its higher concentration. Moreover, an obvious difference can be observed in stage II. The FWHM value keeps stable after stage II at about 0.064 Å−1, 0.048 Å−1, and 0.042 Å−1 corresponding to 80, 100, and 110 °C, respectively, implying that the coherence lengths were ∼88, 120, and 133 Å, respectively. We can estimate the relative nucleation quantity (n) in stage II in PffBT4T-2OD films using the quotient of intensity and the C.L. value as discussed in section 2.1. The hypothetical n values are estimated to be 3.94, 1.78, and 1.59 for 80, 100, and 110 °C, respectively, which means that there is a higher number of polymer nuclei at relatively lower temperatures after stage II. The massive nucleation of the polymer inevitably results in limited growth space, and this can greatly influence the final film crystalline morphology (such as packing distance, total crystallization amount, etc.). The stage after nucleation is the residual solvent evaporation stage (stage III), during which polymer nuclei grow gradually and remain similar for all three temperatures. Here, we are able to conclude that temperature affects the crystalline morphology by impacting the nucleation behavior at stage II. Moreover, the formed crystalline morphology could affect the mixing degree of the donor and acceptor due to the difference in the polymer packing distance and would further influence the device performance. 2.3.2. Influence of Solvent on Morphology Evolution. The influence of solvent during the morphology evolution of
PffBT4T-2OD films was investigated by in situ GIWAXS. The integrated 1D diffraction curves (Figure 4a) and the fitted data illustrating the detailed polymer crystallization process in CB solvent are plotted in Figure 4c. The initial diffraction peak at q = 1.4 Å−1 originates from CB as reported in the literature.28 Note that the length of evolution time at 100 and 110 °C is no more than 10 s, which means that the transition from solution to dried film is faster compared to that of using DCB as a solvent. Therefore, we adopted a 0.05 s exposure time per frame in order to obtain a detailed evolution process. We observed that the CB solvent scattering peak disappears at 5.6 and 3.4 s for 100 and 110 °C, respectively, according to the fitting results. Two-dimensional representative false-color images (Figure 4b) can offer a more intuitive display of the evolution of the solvent and polymer peaks as a function of time. Note that the (100) scattering peak occurs until the disappearance of the CB scattering signals (shown in Figure 4b,c). It clearly exhibits that the alkyl chains of PffBT4T-2OD dissolved in CB until the majority of solvent evaporated, and this crystallization behavior is quite different from that in DCB solvent. We divide the film-forming process for PffBT4T-2OD in CB solvent into three stages. In stage I, the majority of CB gradually evaporates, and the polymer keeps disaggregating in warm solvent. We combine the middle two stages, the evaporation of residual solvent with nucleation and the followed growth stage of polymer chains, as stage II (III) for a better comparison with the DCB case. Last, stage IV concludes the complete evaporation of CB to obtain a final dry film. Different nucleation behaviors between DCB and CB solvent processed PffBT4T-2OD films can be found through the in-depth analysis of in situ GIWAXS data in both cases, which leads to an ultimate different crystalline morphology. The main reason for these differences is the different F
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Figure 5. (a) Circularly integrated line curves of the crystalline domain evolution for DCB processed PffBT4T-2OD blade-coated films with 3% v/v DIO at 80 °C. Each line curve represents an exposure time of 200 ms. (b) 2D representative images of the crystalline domain evolution for DCB processed PffBT4T-2OD blade-coated films with 3% v/v DIO at 80 °C. (c) Evolution of the fitted (100) diffraction peak locations (left panel) and the fwhm’s and intensities (right panel).
nucleation growth model of polymer chains. We fitted the evolution curves of intensity in both solvents at 100 and 110 °C (shown in Figure S6), representing the effective crystallization process. It is clear that the crystallization evolution pathway for the DCB case exhibited an S-like curve, representing the continuous nucleation and growth stage. However, the intensity evolution for the CB case looks like an upper half side S curve, just as the nuclei have already formed, and there was only the growth of crystals during this stage. What’s more, the solvent evaporation rate in the CB case is faster than that in the DCB case, resulting in a limited time window for polymer chain movement. We see the unique nucleation process in CB via a plausible strong polymer alkyl chain−solvent interaction, allowing CB swells in the alkyl chains network and covering up the ordered packing of polymer chains in solvent. Hence, the ordered packing might not be examined though formed early due to the inferior solubility. Once the majority of CB escaped out, polymer chains returned to a well-organized arrangement and grew up quickly. To investigate this possibility, we considered the Hansen solubility parameters and the resulting interaction radii, Ra, to approximate the polymer−solvent molecule affinity. As reported by Manley et al., one can evaluate the solvent−polymer affinity from similarities in dispersion, polarity, and hydrogen-bonding components of the solvent− polymer cohesive energy density, and a small Ra indicates a greater solvent−polymer affinity.28,48 To the best of our knowledge, the Hansen solubility parameter for PffBT4T-2OD has not been determined experimentally so far, so we constructed an approximate parameter for PffBT4T-2OD using the modified group contribution method by Stefanis et al.49,50 We consider PffBT4T-2OD as a rigid rod-like polymer and divide the interaction between the solvent and polymer backbone into two parts, (a) the interaction between the solvent and polymer backbone and (b) the interaction between the solvent and alkyl chains. The Hansen parameter for both solvents used was from the literature.51 Approximate Ra values
for the interaction between the solvent and alkyl chains were then computed to be 7.9 and 9.2 for the CB and DCB cases, projecting a greater solvent−polymer alkyl chain affinity in CB compared to in DCB. Thus, a plausible scenario for CB processed films is that CB molecules interact strongly with the PffBT4T-2OD alkyl chains and swell well in alkyl chains network in the solution state. Once a large amount of CB escaped out from the polymer network, polymer alkyl chains returned to an ordered arrangement leading to a quick growth time. This evolution of polymer crystallization behavior is consistent with the above-explained fast nucleation speed creating inadequate mixed phases with the introduction of a large amount of tight crystalline structures. 2.3.3. Influence of Additive on Morphology Evolution. We finally investigated the effect of the additive on crystalline morphology evolution and the device performance of bladecoated PffBT4T-2OD films from the DCB/DIO mixture. The corresponding integrated line curves, 2D images, and fitting results are presented in Figure 5. The film-forming process is divided into four continuous stages as shown in Figure 5c: stage I, dissolved polymer in warm solution; stage II, nucleation and growth of polymer along with the evaporation of host solvent, DCB; stage III, nucleated polymer followed by crystallite growth with the continuous evaporation of high boiling point additive, DIO; and stage IV, the final dried polymer film. Note that the location and FWHM kept unchanged along with the increase of intensity in the early period of stage III, implying that the polymer continually nucleated during the evaporation stage of DIO, and this contributes to the well bicontinuous phase separation. Moreover, we fitted the (100) peak intensity against evolution time to evaluate the role of DIO (shown in Figure S4). The slopes of intensity during DCB evaporation in pure DCB and DCB/DIO mixtures at 80 °C were 0.87 and 0.53, respectively, meaning DIO slows down the nucleation and growth rate. Meanwhile, the in situ data show that the film-forming time is G
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active layers were obtained by using a JEM-F200 transmission electron microscope operated at 200 kV. 4.4. Resonant Soft X-ray Scattering (RSoXS) Characterization. RSoXS transmission measurements were performed at beamline 11.0.1.252,53 at the Advanced Light Source (ALS). Samples for RSoXS measurements were prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication and then transferred by floating in water to a 1.5 × 1.5 mm2, 100 nm thick Si3N4 membrane supported by a 5 × 5 mm2, 200 μm thick Si frame (Norcada Inc.). Then, 2D scattering patterns were collected on an in-vacuum CCD camera (Princeton Instrument PIMTE). The sample detector distance was calibrated from the diffraction peaks of a triblock copolymer poly(isoprene-b-styrene-b2-vinylpyridine), which has a known spacing of 391 Å. The beam size at the sample is approximately 100 by 200 μm2. 4.5. Solution Small-Angle Neutron Scattering (SANS) Characterization. Solution SANS measurements were carried out at the NGB 30 m SANS beamline in the NIST Center for Neutron Research (NCNR), National Institute of Standards and Technology. The neutron wavelength was 6 Å, with a wavelength spread of ∼12%. The beam diameter was 12.7 mm. Three sample-to-detector distances, namely, 1.3, 4.0, and 13.2 m, were used to collect scattering data in a q range between ∼0.003 and ∼0.5 Å−1. The solution sample was contained in a quartz cell, and the solution concentration was 9 mg/mL; the thickness of sample was 1 mm. The conversion of 2D scattering data to 1D profiles was carried out using the IGOR macro developed at NCNR.54 The reduced data were corrected for sample transmission, scattering from an empty cell, and environmental background. 4.6. In Situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) Characterization. In-situ GIWAXS measurements were performed in the air without any special treatment at Beamline 7-2 of the Stanford Synchrotron Radiation Light source (SSRL) with a 2D area Pilatus detector (Dectris, 300k). The blade parameters were kept consistent with those used for device fabrication. The sample-todetector distance was 336.6 mm, and the incidence angle was 0.2°. The X-ray wavelength was 0.8856 Å, corresponding to an X-ray beam energy of 14 keV. The X-ray beam flux was attenuated by a factor of 30 to reduce beam damage to the sample. Three different protocols were used to follow the morphological evolution of the sample over different time windows: 15 s (CB), 300 s (DCB), and 600 s (DCB/ DIO). GIWAXS images of the first acquisition protocols were collected every 50 ms over the 15 s, with each exposure extending for 45 ms followed by 5 ms for the readout of the pixel array detector. For the two longer acquisition times, 195 ms of exposure time was taken in 200 ms exposure intervals within 60 s, followed by 1 exposure every 3 s. The start of the timing is always defined as the point when blading begins. The conversion of 2D scattering data to 1D profiles was carried out using the IGOR macro developed at ANL55 and specific procedures developed by Dr. H. Yan. Full data processing for each film analyzed by in situ GIWAXS is provided in the Supporting Information.
prolonged by about 13 times owing to the high boiling point of DIO. In general, we are able to conclude the effects of DIO for PffBT4T-2OD-based OSCs as it transforms the phase separation into a well bicontinuous form, leading to better charge transport, and it slows down the nucleation and growth rate while enlarging the evolution time of polymer crystallization so as to form a better polymer packing. These two effects enable a reasonable mixing degree of the polymer and fullerene acceptor in dried film and account for the improvement of device performance. The morphology evolution of PffBT4T-2OD in DCB/DIO at 100 and 110 °C was also investigated, and a similar conclusion can be drawn when correlating the RSoXS results with device performance (shown in Figures S4 and S7).
3. CONCLUSIONS We explored the unique TDA features of polymers for a realtime study of the processing conditions influencing the filmforming dynamics. We have found that the ultimate morphology is highly dependent on the nucleation process stage, which can be controlled by adjusting the processing parameters such as the temperature, solvent, and additive. The mixing degree of the donor and acceptor is also influenced by the nucleation stage, which has to be adjusted for better charge transport pathways. It is shown that nucleation processing drives the degree of mixing and optimizes multilength scale morphology, which account for the overall improvement of the performance of the device. 4. EXPERIMENTAL SECTION 4.1. Materials. PffBT4T-2OD was purchased from Solarmer Materials Inc. PC71BM was purchased from Solenne BV Inc. CB, DCB, and DIO were purchased from Sigma-Aldrich Inc. All chemicals were used as received without purification. 4.2. Device Fabrication. The BHJ organic solar cells were fabricated using the inverted structure of the device (ITO/ZnO/ PffBT4T-2OD:PC71BM/MoO3/Al). The ITO substrate was sequentially cleaned by sonication in detergent water, deionized water, acetone, and isopropanol, 20 min for each step. After UVO treatment for 20 min, the ZnO precursor (zinc acetate dihydrate, dissolved in 2methoxyethanol with ethanolamine) was spin-coated on the cleaned ITO substrate to form a ∼28 nm electron-transporting layer, followed by thermal annealing at 200 °C for 1 h. Then, the solution with the donor and acceptor materials was bladed on the surface of ZnOcoated ITO with different processing conditions in the air. The solution was prepared 1:1.2 (w/w) in pure chlorobenzene (CB) or dichlorobenzene (DCB) with or without a 3% volume fraction of additive 1,8-diiodooctane (DIO), accompanied by stirring on a hot plate at 100 °C. During the blade coating, the gap between the hydrophobic-treated silicon blade and silicon substrate was fixed at 200 μm. A printing speed of 60 mm s−1 was used to obtain a relatively thick film at 80, 100, and 110 °C as solution and substrate temperatures. The blade-coated films were transferred directly to a vacuum environment for electrode deposition without any thermal treatment, and 10 nm MoO3 and 80 nm Al were sequentially deposited as the hole-transporting layer and anode at a vacuum level of
