Morphology Development in Solution-Processed Functional Organic

Review pubs.acs.org/CR

Morphology Development in Solution-Processed Functional Organic Blend Films: An In Situ Viewpoint Lee J. Richter,*,† Dean M. DeLongchamp,*,† and Aram Amassian*,‡ †

Material Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ‡ KAUST Solar Center (KSC) and Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ABSTRACT: Solution-processed organic films are a facile route to high-speed, low cost, large-area deposition of electrically functional components (transistors, solar cells, emitters, etc.) that can enable a diversity of emerging technologies, from Industry 4.0, to the Internet of things, to point-of-use heath care and elder care. The extreme sensitivity of the functional performance of organic films to structure and the general nonequilibrium nature of solution drying result in extreme processing−performance correlations. In this Review, we highlight insights into the fundamentals of solutionbased film deposition afforded by recent state-of-the-art in situ measurements of functional film drying. Emphasis is placed on multimodal studies that combine surfacesensitive X-ray scattering (GIWAXS or GISAXS) with optical characterization to clearly define the evolution of solute structure (aggregation, crystallinity, and morphology) with film thickness.

CONTENTS 1. 2. 3. 4.

Introduction Film Drying and Solidification Fundamentals Film Deposition Methods Measurement Fundamentals 4.1. Optical Techniques 4.2. X-ray Scattering 4.2.1. GIWAXS 4.2.2. GISAXS 5. Examples 5.1. Organic Photovoltaics 5.1.1. Summary of Structure−Function Keys 5.1.2. P3HT:PCBM 5.1.3. Other Polymer:Fullerene BHJs 5.1.4. Small-Molecule:Fullerene BHJ 5.1.5. Nonfullerene, Ternary BHJ 5.2. Organic Transistor Blends 6. Lab-to-Fab 6.1. In Situ Guided Transitions 6.1.1. Matching Kinetics of Deposition 6.1.2. When Kinetics Cannot Be Matched 6.2. Inline R2R 7. Summary and Outlook Chemical Structures for Selected Abbreviations Associated Content Special Issue Paper Author Information Corresponding Authors ORCID Notes Biographies © 2017 American Chemical Society

Acknowledgments Abbreviations References

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1. INTRODUCTION The worthy goals of sustainable and low-cost manufacturing have driven considerable interest in additive manufacturing of functional materials and devices. Additive manufacturing enables benefits of low material waste, ease of customization (bespoke manufacturing), and often a low thermal budget.1 In the additive manufacturing of thin films in electronic applications such as batteries, displays, electronics, biosensors, and photovoltaics (PV), the printing of solution-based, functional inks is a favored approach due to the ability to leverage knowledge and equipment from the traditional printing industry.2 Printed functional materials are poised to revolutionize future technologies as diverse as the Internet of things (wearables), renewable energy (PV and storage), and personal health care. Organic functional materials are commonly used in these applications because their chemical functionalization facilitates solubility and their weak molecular packing enables mechanical flexibility. The controlled deposition of films from solution inks is an ancient technology underlying writing, printing, paint (passivation and aesthetics), and more recently photographic film. It has been studied extensively from both an engineering and fundamental standpoint, with emphasis on the development of ink formulations that support the best compromises of wetting, Received: September 15, 2016 Published: April 17, 2017 6332

DOI: 10.1021/acs.chemrev.6b00618 Chem. Rev. 2017, 117, 6332−6366

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adhesion, finish (lack of craze, etc.), and drying time. The recent interest in printing functional films has challenged established knowledge in many ways. The functional material is often of limited solubility, and the target films are sufficiently thin (∼100 nm) that historical studies of high solid content, viscous inks are of limited relevance.3 Formulation options are more limited due to the recent emphasis on green solvents and a lack of knowledge about how traditional formulation additives affect functional materials. The functional performance of the dry film (energetic landscape, mobility, conductivity, quantum yield, etc.) is extremely sensitive to the film structure, both the microstructure, in terms of local and long-range order, and the mesoscale morphology. This has led to a renaissance in studies of solution ink drying. In this Review we will summarize in situ, real-time studies of functional ink drying. We will focus on the morphology (and ultimate performance) of organic (polymer and smallmolecule) based semiconductor blends targeted at both photovoltaic applications and thin-film transistors. Organic photovoltaic (OPV) and organic thin-film transistor (OTFT) devices have different geometries, functions, and requirements. The OPV device4,5 is a vertically stacked twoterminal device requiring light absorption, an energetic landscape at the donor−acceptor (D−A) interface capable of splitting excitons into free charges, and a nanoscale phase-separated blend capable of transporting the holes and electrons rapidly and without recombination to the anode and cathode, respectively, over a distance no more than the thickness of the photoactive layer, 100−400 nm. The OTFT6,7 is a three-terminal device, with source and drain electrodes laterally spaced several microns to tens of microns apart. The gate electrode creates mobile charges at the semiconductor−dielectric interface through the application of an electric field, allowing for the fast in-plane transport of charges between the source and drain electrodes. Hence, while the OPV device requires a bulk heterojunction (BHJ) with nanoscale phase separation and a distributed interfacial region, OTFTs require a continuous semiconductor film connecting the source and drain electrodes and forming a high-quality interface with few interfacial defects and lateral grain boundaries, resulting in low trap state density and high carrier mobility. The guiding principles for preparing formulations and coating materials for these two applications can differ vastly. It is the aim of this Review to demonstrate the commonalities and differences in tackling these two applications through a narrative of in situ studiesoften complemented with other experimental and computational methodsto demonstrate how solution processing of inks can provide tremendous control over the functional microstructure and morphology of semiconductors.

Figure 1. (a) Calculated reduced ΔG for a symmetric binary solution, as a function of the interaction parameter χ. (b) Calculated reduced ΔG for an asymmetric binary solution, as a function of the interaction parameter χ.

extended crystalline order, as in the case of small molecule OTFTs. A complete, microscopic picture of solid structure evolution from an initially homogeneous solution is complex and only now yielding to detailed computer modeling in simple cases (crystallization from the melt, vapor deposition, l−l separation); however, significant insights are provided by simple, analytical mean-field theories. Beginning with a homogeneous solution, the most natural model for a binary system is that developed independently by Flory11 and Huggins.12 The Gibbs free energy of mixing of the solution is given by ΔG = n1ln(ϕ1) + n2 ln(ϕ2) + χn1ϕ2 (1) RT where ni is the moles of solvent (1) and solute (2), ϕi is the volume fraction, and χ is an interaction parameter. The interaction parameter χ has a typical temperature dependence of a + b/T. Shown in Figure 1a is the Gibbs free energy for a simple solution with symmetric (matched molar volumes) solvent and solute as might be appropriate for an aromatic smallmolecule semiconductor. For χ greater than the critical value of 2, the free energy exhibits two minima and the system will thus phase separate. Shown in Figure 1b is the Gibbs free energy for a mildly asymmetric case (molar volume ratio of solute to solvent is 4). Note that the critical value of χ as defined in eq 1 has shifted, and the position of the minima (material purity of the phases) has changed. The condition for phase equilibrium is that the chemical potential ( ∂ΔG ) of each species be equal in each phase.

2. FILM DRYING AND SOLIDIFICATION FUNDAMENTALS Solution ink drying is, at its most basic, an evolution from a liquid to a solid on a substrate. The number and nature of phases present at points throughout the drying process can dramatically affect the outcome. An understanding of phase-separation processes arising from either immiscibility (liquid−liquid, hereafter l−l) or ordering (crystallization) is required to develop formulation-processing-structure-function relationships.8−10 Crystallization is of special importance in organic semiconductor films. The functional performance of organic semiconducting active layers generally requires films with high degrees of local order that typically, although not always, are associated with regions of crystallinity. These can be confined inside a nanoscale blend, as in the case of OPV BHJs, or continuous layers with

∂ni

Satisfying this constraint via the common tangent construction will give the compositions of the two equilibrium phases. Each composition pair are endpoints of the tie lines, and the locus of solutions represent the binodal line that separates the region of homogeneous phase from the two-phase region. From the shape of the free energy surface, one can see that for starting compositions above the inflection points

∂ 2ΔG ∂ni 2

= 0 the system

will spontaneously phase separate and exhibit spinodal 6333


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Figure 2. (a) Simulated morphology for spinodal phase separation. (b) Representative phase diagram for an UCST system. (c) Simulated morphology for binodal phase separation. Panels a and c are adapted with permission from ref 13. Copyright 1998 IOP Publishing.

Figure 5. Schematic of phase-separation cascades during solvent removal from an initially homogeneous solution. Images for simulated binodal and spinodal patterns are adapted with permission from ref 13. Copyright 1998 IOP Publishing.

decomposition. However, for starting compositions below the inflection points, the system is metastable and will exhibit nucleation and growth. Thus, the common binary phase diagram (for an upper critical solution temperature, UCST) is that shown in Figure 2b. As can be seen in the simulations in Figure 2,13 the characteristic long-range morphology of systems that exhibit spinodal (unstable to infinitesimal fluctuations) decomposition are quite distinct from the compact droplets of l−l separation in the metastable region between the spinodal and binodal lines. It should be noted that the elegant theory of Cahn and Hilliard, describing the evolution of density fluctuations in the spinodal region, is one of the few analytic treatments of phase-separation dynamics.14 Equation 1 can be readily generalized to ternary or greater complexity systems, simply by introduction of the relevant binary interaction parameters. The conditions for the spinodal lines at higher dimensions involve the determinate of the free energy Jacobian.15 For both l−l phase separation in the metastable regime and crystallization of one component, classic nucleation theory as originally put forth by Gibbs16 is applicable. The issue is the adverse surface tension contribution to the Gibbs free energy of small nuclei (∝ radius2) versus the volume reduction in total free energy (from eq 1, ∝ radius3). At a given temperature, these competing terms (see Figure 3a) result in a critical nucleus size (rc) and a critical free energy barrier ΔG*. The presence of ΔG* means that the system must be supercooled or supersaturated in order to phase separate, i.e., the crystallization temperature Tc

Figure 3. (a) Contributions to the free energy change upon crystallization of a finite sized nucleus of radius r. (b) Nucleation rate, I*, as a function of temperature for a typical polymer.

Figure 4. Representative temperature (T)−volume fraction (ϕ1) phase diagram including liquid−liquid separation in the nonequilibrium region between Tm and Tc.

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Figure 6. Schematic of (a) spin coating, (b) blade coating, and (c) slot-die coating.

will be less than the melting temperature Tm. Phenomenologically, the nucleation rate I* then becomes

OTFTs, where rapid lateral transport of charges is required and aided by the formation of stratified films, as will be discussed below.

I * ∝ e−1/ RT(ΔΦ(T ) +ΔG * (T ))

where ΔΦ is an activation energy for transport of the critical next element to the nucleus. For dilute solutions of small molecules, this term can be taken as a constant. Lowering of the nucleation barrier with lowering temperature (due to the increased free energy change on segregation) results in an increase in nucleation rate. For polymers, the hindered dynamics of large-chain reorganization generally leads to significantly greater supercooling required for crystallization (on the order of 25 °C). In addition, the presence of a glass transition temperature (Tg) below Tc can greatly slow the dynamics. This effect is captured in the Williams−Landel−Ferry approximation, where ΔΦ(T) is of the following form: AT/(B + T − Tg). This results in a nucleation rate versus time of the form of Figure 3b, where nucleation freezes out near Tg. The rich complexity of phase evolution during solvent removal can be seen in Figure 4, which presents a potential phase diagram for a crystallizable polymer in solution as a function of solvent volume fraction and temperature. Shown are the phase transformation lines for melting (Tm) and crystallization (Tc), the glass transition temperature (Tg), and a possible l−l binodal and spinodal. Because of the supersaturation required for crystallization, the region of l−l separation can be reached in the “nonequilibrium” region between Tm and Tc, even with slow cooling. Depending on quench rate and solvent fraction, many phase-separation processes are possible: crystallization, spinodal l−l separation, binodal l−l separation, binodal l−l separation followed by crystallization, l−l separation arrested by glass formation prior to crystallization, etc. The complex interplay of l−l separation, crystallization, and glass formation is summarized in Figure 5. For polymer−solvent systems, this complexity has been leveraged in the fabrication of selectively porous membranes.17 It is also worth noting that, in the case of thin films, phase separation can occur in a vertically stratified manner, which may be a desirable outcome in applications such as

3. FILM DEPOSITION METHODS As briefly reviewed above, the material transformations that occur in a drying solution are rich and complex. The complexity of real drying films is multiplicatively larger, due to the hydrodynamics and coupled mass transfer of the evolving film. Even for apparently simple cases, such as drying at a receding meniscus, the interplay between the stick−slip receding contact angle18 and Marangoni effects19 can result in complex behavior beyond the simple “coffee stain” effect.20,21,155 For most of the examples given in this Review, care was taken to create nominally planar, simple wet films that dry only by uniform evaporation from the free interface and thus are amenable to both measurement and modeling. The three most commonly adopted deposition techniques that produce these simple films are spin coating, blade coating, and slot-die coating, as depicted in Figure 6. We will briefly describe the relevant behavior of each. For commercial-scale deposition of large-area, homogeneous films, slot-die coating is generally the most precise. However, to date, spin coating (Figure 6a) is the most commonly employed thin-film deposition technique in the laboratory. Spin coating nominally proceeds in two stages22 and is self-metered, i.e., the final film thickness is independent of the amount dispensed for solution volumes that fully cover the substrate in the as-cast state. In the first stage, the dispensed liquid is ejected from the edge of the substrate due to centrifugal forces at a rate proportional to the film thickness, h3. This leads to formation of a uniformly thick, self-leveled liquid film that can undergo radial outflow of the solution, depending on its viscosity and the centrifugal forces, before drying to form the desired film. Shown in Figure 7 are high-speed images of the early stages of spin coating, demonstrating the ejection and leveling. The ejection and outflow cause the technique to be extremely wasteful of material. In the case of solutions that can undergo l−l separation with components migrating toward the surface or the bottom of the solution, the solution outflow can be preferential to some of its components, thus dynamically changing the composition and phase-transformation behavior of the solution. When the film is sufficiently thin that the outflow is less than the evaporation rate, the wet film thickness is established and the film then thins mainly by convective evaporation due to the spinning substrate.23 The centrifugal flow of ambient or controlled atmosphere over the sample face results in extremely rapid drying and can lead to instabilities.24,25 The film meniscus never passes over the substrate, so spin coating tends to work with a wide range of ink-wetting characteristics, provided that initial wetting can be established. Typically one finds that the final film thickness (hf) will vary as a power law in spin speed, ω: hf ∝

Figure 7. High-speed images taken during the initial 1 s of spin coating of a chlorobenzene (CB) solution at 105 rad/s (1000 rpm) showing significant liquid loss during the spin-up stage of the process. Reproduced with permission from ref 28. Copyright 2013 John Wiley and Sons. 6335


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Figure 8. Side views of phase-field simulated morphologies of blends solution-processed in different conditions, including Biot number and blend composition.

[concentration]ω−n, where n varies from 0.5 to 1.0.26 A detailed review of in situ studies of nonfunctional film deposition via spin coating has recently appeared.27 Blade coating (Figure 6b) or knife-edge coating is a prototyping tool for slot-die coating and used for piece or sheet coating. Like spin coating, it is a self-metered technique. The wet film thickness is determined by the blade velocity and the film viscosity. In the horizontal dip-coating or Landau− Levich regime,29 the wet film thickness varies by ∼v2/3, where v is the substrate velocity. At low velocities, in the evaporative regime, the deposited film thickness empirically varies as v−1.30 Unlike spin coating, the technique conserves material as it is simple to dispense volumes nominally matching the final desired film area. Coating stability is determined by the wetting characteristics of the solution on the substrate, and the final film structure is determined by the drying conditions (choice of solvent and substrate temperature) after wet film deposition. Blade coating can be easily done on small (10 mm by 10 mm) substrates, resulting in extremely efficient use of small amounts of material for material screening and process optimization. Additionally, blade coating is easily adapted to high-throughput screening via programmed velocity profiles.31 Slot-die coating (Figure 6c) is an example of a premetered technique: the wet film thickness is set by the ratio of the solution delivery rate to the relative speed between the substrate (web or sheet) and slot-die head. In this regard it is very easy to control thickness, and the system is economical in terms of material consumption. However, the technique is prone to instabilities (for example, chatter) that are set by the wetting characteristics of the ink on the leading edge of the web and the underside of the die. There will be a range of web-speed and solution-delivery rates in a stability window.32 The final film characteristics are set by the evolution of the wet film in the downstream dryer, often under moderate heating ( 0, where the pattern is a convolution of the whole film off-specular reflectivity and vertical and horizontal composition fluctuations. Furthermore, such scattering is typically weak and noisy, particularly for rapidly 6339


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Figure 12. Thickness from ellipsometry versus time for a P3HT:PCBM BHJ drying at 30 °C from (a) CB and (b) TCB solutions. Note the initial linear thickness decrease, with a small region of slowing drying rate near complete drying. (c) Time evolution of the imaginary part of the dielectric function of the film. The transition from solvated and torsionally disorder P3HT to the aggregated and planarized film is clearly apparent. (d) k at selected wavelengths, showing the time dependence of the aggregation. Adapted with permission from ref 58. Copyright 2010 Royal Society of Chemistry.

Figure 13. (a) Film composition (from thickness derived by laser interferometry) of a P3HT:PCBM BHJ drying at 25 °C from an oDCB solution. (b) Intensity of the P3HT diffraction features versus time. (c) Proposed time evolution of the film structure. Adapted with permission from ref 91. Copyright 2011 American Chemical Society.

parallel to qxy, e.g., structural features and length scales parallel to the film plane. Because significant anisotropies are common in organic semiconductor films (optical, crystal orientation distributions, etc.), it is unlikely that the in-plane nanoscale

collected in situ patterns, compared to the scattering near qz = 0, where scattering is enhanced by internal reflection. Thus, most analyses of GISAXS data use only the portion near qz = 0. Importantly, these data represent only correlations that occur 6340


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5. EXAMPLES

structure would necessarily resemble the out-of-plane nanoscale structure. Attempts are underway to approach the analysis of the whole GISAXS pattern and extract both in-plane and out-ofplane information.74 It is now common to analyze the qz = 0 portion of the GISAXS pattern by searching for notable peaks and shoulders in a “Lorentz-corrected” I·q2 vs q representation, identifying the corresponding length scales of these features, and asserting that structures in the sample must match those length scales. Caution should accompany these practices because it is not clear that they are appropriate for aperiodic structures,75 although some success has been achieved with this approach.72 Length scales that might be extracted could represent either feature sizes or long periods. Such analyses are best when validated by real-space measurements of structure, such as energy-filtered TEM (EF-TEM).76 For example, a study that validates an extraction of length scales from GISAXS data of dried films by showing that length scales extracted from EF-TEM are similar might then use the same length-scale extraction method on in situ data assuming that the approach remains valid as the film evolves. Aside from characteristic length scales, GISAXS also provides a measure of the extent of phase separation in a film. For an isotropic, two-phase system, the total scattering invariant (TSI = ∫ Iq2 dq) describes the amount of phase separation and the purity of the separated phases.77 Any TSI calculated from GISAXS data at qz = 0 implicitly assumes that the out-of-plane contribution to the TSI scales linearly with the in-plane portion, which, as we discuss above, may not be a valid assumption. Nevertheless, in the absence of other measurement methods, a “pseudo-TSI” calculated in this way may be a useful means to assess phase separation. Future work to determine what extent of out-of-plane structural difference is required to produce a significant error will be important. It is also possible that a proper TSI might be extracted from analysis of the whole GISAXS pattern. If the number of phases is known, and some information about the amount of a phase and/or the phase composition is also known (for example, an integrated pole figure from GIWAXS reports the amount of a nominally pure phase), then it can be possible to model a measured pseudo-TSI with mass balance constraints (solvent contributions known from measured thickness) to extract unknowns such as the amount or composition of phases that are not directly measured (amorphous, mixed, etc.).

5.1. Organic Photovoltaics

5.1.1. Summary of Structure−Function Keys. The first organic planar heterojunction solar cell was fabricated by Tang in 1986 by vapor deposition of copper phthalocyanine and a perylene tetracarboxylic derivative, achieving a power conversion efficiency (PCE) of ∼1%.78 Organic photovoltaics began to attract significant interest after the demonstration of PCE > 2% in solution processed polymer−fullerene BHJs with polyphenylenevinylene electron donors: MEH-PPV79 or MDMO-PPV80 and the polythiophene donor, regioregular-P3HT. In most cases the fullerene electron acceptor was (6,6)-phenyl C61-butyric acid methyl ester (PCBM). Although nonfullerene acceptors enabling high PCE are increasingly common, the C60 and C70 variants of PCBM are still the most common electron acceptors used today. The strongly bound exciton-polaron (hereafter called an exciton for consistency with the majority of the literature) formed upon optical excitation generally requires an interface between two materials with a potential offset for efficient charge separation. In planar heterojunctions, device efficiency is typically limited by the very short effective distance over which interfaces can effectively promote charge separation, due to the short exciton diffusion length and short distances for prompt quantum processes. Crucial to the high performance in blended films is the development of the BHJ motif: self-organization of the polymer donor and the fullerene acceptor into interpenetrating networks with nanometer characteristic dimensions, facilitating high charge separation efficiency. While the BHJ provides a route to high PCE, it requires a complex balance in often contradictory morphology characteristics. A very fine phase separation will facilitate charge separation but will make charge extraction difficult (by similarly facilitating recombination). Too coarse a phase separation can facilitate charge extraction at the expense of separation. Within the phases one needs to optimize the mobility, facilitated by the development of relevant order (pioverlap and conjugation length). Unlike the case of the planar TFT, transport must be optimized in the vertical direction in OPVs. The sensitivity of BHJs to morphology was immediately apparent in the sensitivity of device performance to solvent choice and postdeposition processing (thermal or solvent annealing) in the initial demonstration systems.81 Extreme dependence of device performance on processing details has been exhibited by nearly all high-performing systems to date.82,83 5.1.2. P3HT:PCBM. The BHJ formed by P3HT and PCBM was the first system to achieve PCE near 4% via multiple processing routes, including postdeposition thermal annealing,84 slow drying from optimal solvents,85 and the use of solvent additives.86 Due to its relative ubiquity, it became the benchmark system for the early development and demonstration of realtime, in situ measurements. 5.1.2.1. P3HT:PCBM in a Single Solvent. Although virtually all initial demonstrations of OPV systems have been done by spin coating, the ease with which blade-coating systems can support in situ measurements led to the first in situ studies of OPV systems being done on blade-coated films. Between 2009 and 2012, research groups at Sheffield58,87,88 and Karlsruhe Institute for Technology (KIT)34,89−91 contemporaneously studied the deposition and subsequent thermal annealing of P3HT:PCBM films from various solvents. The KIT effort developed a closed cell with regulated air flow, allowing independent control of substrate temperature and drying rate.34 The cell used normal

Figure 14. Power conversion efficiency of P3HT:PCBM devices as a function of film drying rate, controlled by substrate temperature and ambient gas flow rate. PCE data from ref 91 and drying rates from ref 34. 6341


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Figure 15. In situ study of spin coating of P3HT:PCBM from CB. (a) Schematic diagram of experiment. (b) Raw GIWAXS and GISAXS images. (c) Evolution of (100) diffraction intensity, GISAXS scattering intensity, and film thickness. (d) Variation of the onset of crystallization with PCBM mass fraction. Adapted with permission from ref 93. Copyright 2013 John Wiley and Sons.

Figure 16. Summary of morphology evolution behavior of P3HT:PCBM during spin coating from CB. Adapted with permission from ref 93. Copyright 2013 John Wiley and Sons.

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Figure 17. In situ UV−vis absorbance measurements performed during spin coating of P3HT in toluene in conditions of high solubility (warm solution, top left panel) and marginal solubility (room temperature, top center panel). Solution thinning and absorbance at 605 nm presented in the lower panels indicate that the preaggregated solution (due to marginal solvent) results in more aggregation as indicated by the stronger absorbance at 605 nm despite the films achieving identical thickness. The panels on the right summarize the thinning and absorbances at 460 and 605 nm for a P3HT:PCBM blend spin-cast from oDCB. The analysis shows that interrupting the spin-coating experiment prematurely but after the ejection and outflow processes are complete greatly extends the evaporation of the remaining solvent, leading to slower film formation with improved aggregation of P3HT for a film with identical thickness. Adapted with permission from ref 98. Copyright 2014 Royal Society of Chemistry.

in liquid film thickness over time, is sufficiently fast to quench the film into a state of extreme supersaturation. Independent GIWAXS studies confirmed the late development of crystalline order in the rapidly dried film.58 The solidification at extreme supersaturation described above can be contrasted with the structure evolution in the slow (no flow in the cell) drying of o-dichlorobenzene (oDCB) films shown in Figure 13.91 A slow drying rate was necessitated in this experiment due to the use of a slow imaging detector for GIWAXS. At an estimated drying rate of 28 nm/s, the initial P3HT crystallization, observed from GIWAXS, is found to occur at nominally the equilibrium solubility limit. The GIWAXS data (and use of oDCB) allowed separation of the PCBM amorphous halo from the oDCB scattering (see Figure 13c). It was possible to establish that P3HT crystallization occurs before PCBM aggregation, indicating that the phase separation is driven by polymer crystallization and is not spinodal in nature, consistent with the fine length scales observed for P3HT:PCBM BHJs. In studies of device efficiency versus drying rate, summarized in Figure 14, it was found that drying rates greater than ∼100 nm/s resulted in a significant decrease in device performance. These fast drying rates are similar to those used in the preceding SE experiments. Thus, for P3HT:PCBM in a single solvent, it seems clear that slow, nearer-equilibrium drying (oDCB, low temperature) provides better PCE than the rapid drying of CB solutions.

incidence laser interferometry for film-thickness determination and supported GIWAXS measurements at the European Synchrotron Radiation Facility. The Sheffield effort focused on SE studies, supplemented by GIWAXS measurements at the Diamond light source. These two initial studies demonstrate some common themes relevant to understanding the structure evolution of BHJ blends. Shown in Figure 12 is the film thickness (from SE) as a function of time for the chlorobenzene (CB) and trichlorobenzene (TCB) solutions studied in ref 58. Note the nominal linear drying rate at early times, consistent with Raoult’s law, the very low volume fraction of the solids, and a constant mass transfer coefficient (phase I). At late times, with film solid fractions ≈ 0.5−0.9, the drying slows, exhibiting a “knee” (phase II), followed by a period of extremely slow drying (phase III). This three-stage drying is a commonly observed feature in BHJ structure evolution. Also shown in Figure 12 is the evolution of the film dielectric function, extracted from in situ SE, in the region of the P3HT absorption. As discussed earlier, P3HT exhibits clear correlations between local order (conjugation length) and absorption spectrum. One can see a clear onset in the ordering of the P3HT, as the film entered stage II drying by the appearance of the aggregate 0−0 transition at 605 nm. It must be noted that this onset occurs at a P3HT concentration that significantly exceeds the solubility limit of P3HT in CB. The drying rate of ∼133 nm/s, when described as the rate of decrease 6343


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Figure 18. (a) (Upper row) Film thickness and composition (from SE) and false color image of (100) pole for P3HT:PCBM BHJ drying as a function of additive. The onset and end of the evolution of the features of optical aggregation are indicated by the yellow and red stars. (Lower row) Film thickness and integrated (100) pole figure. (b) (Upper row) False color image of q2 weighted GISXAS along qz ≈ 0. (Lower row) Film thickness from WLI, pseudo-TSI, and long period. Adapted with permission from refs 35 and 54. Copyright 2013 and 2015 John Wiley and Sons.

Shown in Figure 16 is a summary of the kinetics, phase morphology formation time, structural characteristics, (100) mosaicity, crystallinity, and phase contrast for the system as a function of PCBM loading, along with the as-cast and thermally annealed PCE. The study reveals that P3HT crystallization is not slowed by loading the P3HT with up to 0.125 mass fraction PCBM. Between 0.125 and 0.50 mass fraction PCBM, the crystallization of P3HT induces simultaneous phase separation during structure evolution, but for >0.50 mass fraction PCBM, the crystallization and phase separation are hindered significantly. This suggests that the presence of the higher Tg PCBM in a mixed amorphous state with P3HT is slowing the crystallization kinetics of P3HT, a phenomenon known as vitrification.94 Chlorinated solvents, such as CB and oDCB, commonly used in organic photovoltaics are denser than nonhalogenated solvents and therefore easily absorb X-ray beams. For instance, a high-energy (10 keV) X-ray beam incident at a grazing angle of 0.1−0.2° can reach the substrate and exit the solution only for a solution thickness of 1 μm or less. GIWAXS and GISAXS measurements in thicker liquid solutions or swollen and solvated films thicker than 1 μm are unlikely to capture the bulk behavior of the wet film and may reflect the near-surface structure. For these reasons, the authors of this review have developed optical absorbance measurements performed through the entire depth of the thinning solution and wet film, as this is more likely to capture aggregation and related structural changes occurring

The authors of this review extended studies of P3HT:PCBM structure evolution by performing in situ GIWAXS and GISAXS for spin-coated films28,92 and blade-coated films.35,54 In ref 93, the first comprehensive study of a spin-coated BHJ was performed, which combined fast video imaging, white light interferometry (WLI), GIWAXS, and GISAXS. The solvent was CB, and the drying rate was significantly faster than in the CB solution blade-coating experiments of ref 58 due to the rapid convection from spin coating. Shown in Figure 15 is the time evolution of the film thickness during spin coating of a P3HT:PCBM BHJ solution in both CB and pure CB solvent. The nominal two-stage drying is clearly evident, with liquid ejection and outflow at early times producing a ∼10 μm thick film by ∼1 s after starting, followed by evaporation to a nominally dry (constant thickness) film at ∼5.5 s. The drying rate in the evaporation regime is ∼1400 nm/s, nominally 10 times faster than that observed for blade coating from CB at slightly higher (30 °C) temperatures.35 Also shown in Figure 15 are representative intensities (background + peak) for both the (100) lamella peak in GIWAXS and the scattering peak due to phase separation in GISAXS. Crystallization and phase separation occur very late in the drying process, nominally simultaneously, and very rapidly, and the film structure only stops changing with the removal of the last of the solvent. The total duration of rapid film structure evolution was found to be of the order of 1 s. 6344


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preaggregated in a theta solvent or well-dissolved in a warmer solution (Figure 17)92 clearly show differences in the aggregation state of the solution and its subsequent drying behavior, with more aggregated films formed when processing from a theta solvent resulting in improved carrier transport in OTFTs, in agreement with previous observations.95−97 Similar experiments performed on P3HT:PCBM in oDCB clearly reveal the two distinct regimes of solution ejection/outflow and structure evolution/aggregation. Having understood the process and its kinetics makes it possible, for instance, to interrupt the spin coating during outflow to impact the film thickness and drying rate by restricting the extent of outflow as compared to uninterrupted spin-coating experiments and slowing down the evaporation. Alternatively, the process can be interrupted at the onset of structure evolution (Figure 17) so as not to change film thickness but instead only extend the film drying time in the absence of substrate rotation. This was shown to promote significant aggregation of P3HT in the blend, which would otherwise have been vitrified by the combination of PCBM loading and rapid drying rate. Consequently, the solar cells yielded higher external quantum efficiency (EQE), photocurrent, and fill factor. 5.1.2.2. P3HT:PCBM with Solvent Additives. As we have discussed above, P3HT:PCBM films that are cast from relatively volatile solvents such as CB by either spin coating or blade coating typically have less developed P3HT crystallization due to the short crystallization window and potential vitrification by PCBM. Films made in this manner are typically improved by thermal annealing, which requires a moderately high thermal budget. Due to the high Tg of PCBM, the optimal annealing temperatures84 for P3HT:PCBM BHJs are ∼140 °C. An alternative to thermal annealing has been the use of solvent additives to enhance device performance.99−101

Figure 19. (a) Comparison of (100) pole figure and pseudo-TSI and two-phase model prediction for TSI for a P3HT:PCBM BHJ processed from CB solvent with a CN additive. (b) Similar comparison for a BHJ processed with an ODT additive. Adapted with permission from ref 54. Copyright 2014 John Wiley and Sons.

during solution processing with the added bonus of not having to resort to optical modeling to obtain the absorption coefficient. In situ absorbance studies carried out on neat P3HT films

Figure 20. (a) Raw signal, (b) film thickness, and (c) solvent volume fraction from laser interferometry during the drying of a PSBTBT:PCBM-71 BHJ from oDCB at 40 °C. (d) (100) pole figure and (e) integrated pole figure. Adapted with permission from ref 114. Copyright 2012 American Chemical Society. 6345


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Figure 21. (a) TEM images of PDPP5T:PCBM-71 films processed from CF:oDCB mixed-solvent solutions as a function of increasing oDCB content (moving down the column). Scale bar, 600 nm. (b) Laser reflection, (c) scattering, and (d) derived film thickness during drying of films with and without oDCB. (e) Film absorbance as a function of time, indicating the onset of aggregation. Adapted with permission from ref 116. Copyright 2015 Macmillan Publishers Limited.

systems such as PTB7, the presence of the additive decreases the length scale of phase separation.106 In refs 35 and 54, in situ measurements including SE, WLI, GIWAXS, and GISAXS were applied to the blade coating of P3HT:PCBM from CB with either the selective solvent additive ODT or the nonselective additive CN. Shown in Figure 18 is a summary of the thickness evolution (from either SE or WLI), the onset and end of optically detected aggregation, and the crystallinity (from the integrated (100) pole figure). The timeevolution of the film thickness could be quantitatively modeled with Raoult’s law.35 From the thickness, one can derive the film composition in terms of primary solvent (CB), additive, and solids. The composition is also indicated in the figures. In the absence of the additive, the film evolution was found to be consistent with the earlier blade-coated58 and spin-coated28 films. Aggregation (evidenced by the growth of the 605 nm feature in the P3HT spectrum) and crystallization (evidenced by the appearance of the (100) diffraction feature) occur very late in the drying process, at a film thickness corresponding to a P3HT concentration of 400 mg/mL, well above the solubility limit of ∼15 mg/mL. The resulting pole figure is characteristic of highly edge-on crystallites, consistent with ref 107 and suggestive of interface nucleation.108 The volume-fraction-corrected crystallinity was found to be ∼1/2 that of the P3HT film cast in the absence of PCBM, indicating that PCBM does inhibit polymer ordering, consistent with ref 91. As with the spin-coated films, the GISAXS evolution is nominally simultaneous with the GIWAXS evolution, implying that P3HT crystallization dominates the phase separation, consistent with the observation that PCBM

The use of additives came to prominence with their highly effective use in the optimization of the poly[2,6-(4,4-bis(2ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT):PCBM BHJ.102 Optimization of solvent choice and annealing schedule led to device efficiencies of only 2.8%, significantly lower than expected based on the solar spectrum overlap of the absorber. It was discovered that a high PCE of 5.5% could be achieved by the addition of 0.01−0.03 volume fraction of the low vapor pressure 1,8octanedithiol (ODT) additive. It was proposed that the additive functioned by selective solvation of the PCBM, reducing the inhibition of polymer ordering by PCBM aggregation.103 In a wide variety of more recent studies, ODT was replaced by 1,8diiodooctane (DIO). When DIO is used, the drying rate is extremely slow, such that, at room temperature, the end of detectable DIO evaporation occurs hours after initial film deposition. This allowed simple structure-evolution studies of PCPDTBT:PCBM by spin coating off the primary solvent and then transferring the additive-swollen film to a measurement stage, either for UV−vis transmission104 or GIWAXS.105 Additional studies suggested that the possible mechanisms of action for OPV BHJ formulation additives must not be confined to selective solvation alone. Additives such as 1-chloronaphthalene (CN), which dissolve both the polymer and the fullerene, can also lead to improved device performance.100 Additionally, it was observed that the morphological origins of improved performance due to additives were system-dependent. In PCPDTBT, the additive increases the length scale of phase separation and improves the level of local order. However, in 6346


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Figure 22. Onset of scattering (l−l separation) and aggregation (absorbance shift) for PDPP5T as a function of (a) oDCB or (b) DIO volume fraction. (c) Summary schematic of phase evolution versus additive volume fraction, indicating transition from l−l to nucleation and growth (aggregation) as a function of solvent quality. Adapted with permission from ref 116. Copyright 2015 Macmillan Publishers Limited.

aggregation occurs after P3HT crystallization in slowly dried systems.91 In the presence of the additive 1-chloronaphthalene (CN), clear effects are observed. The initial drying rate is rapid, as in the additive-free solution. But the drying rate slows radically after the CB is gone, due to the low vapor pressure of the remaining CN. The film at that point is ∼0.50 volume fraction additive. On the basis of SE,35 the onset of aggregation occurs slightly earlier in time (at greater wet film thickness), implying a lowering of the nucleation barrier or reduction of the formulation solvent quality, consistent with CN being a slightly poorer solvent for P3HT than CB. However, the evolution of the optical spectrum and GIWAXS persists over the entire time (200 s) that the CN evaporates. Thus, the additive both promotes earlier ordering and provides an extremely extended period for structure evolution. Close inspection of the X-ray scattering details reveals that, at the point of CB removal, a small amount of highly edgeon crystals are rapidly formed, in a manner very similar to that of the additive-free film. However, the subsequent slow increase in crystallinity is associated with a significantly greater mosaicity, implying that the CN-enhanced ordering is more bulk-like than interface-dominated. Also shown in Figure 18 is a summary of the influence of the CN on the evolution of the GISAXS. Like the evolution of the optical spectrum and the (100) pole integral, the pseudo-TSI varies over the entire time duration of the CN drying. Notably, the characteristic length scale of the phase

separation does not significantly evolve over the same time scale, suggesting the length scale is limited by the polymer characteristics, and the TSI and crystallinity evolution is due to continued nucleation of new ordered regions. Shown in Figure 19 is a detailed comparison of the evolution of the relative crystallinity (from GIWAXS) and phase purity (from the pseudo-TSI of the GISAXS feature at ∼0.2 nm−1) for the P3HT:PCBM:CN system detailed in Figure 18. If the phase separation is dominated by the crystallization of the P3HT, the scattering intensity should be given by the simple, two-phase scattering relationship, TSI =

∫ I(q)q2 dq ≈ (ρ1 − ρ2 )2 φ(1 − φ)

(2)

where ρ is the scattering length density of the two phases and ϕ is the volume fraction of one phase. Defining the two phases to be crystalline P3HT and a solvent-swollen mixed amorphous phase of P3HT, PCBM, and CN, the expected time evolution of the TSI can be calculated from the GIWAXS with only two adjustable parameters: one is the absolute crystallinity of the P3HT at the end of drying, and the other is an overall scale factor for the TSI. Also shown in Figure 19 is the best fit of this model to the experimentally measured pseudo-TSI. The two-phase model describes the data well, with a P3HT fractional crystallinity of ∼40% consistent with other estimates.109 The presence of the additive ODT results in significantly different film structure evolution, yet comparable device 6347


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Figure 23. Characterization of PDPP4T:PCBM-71 BHJ films deposited from mixed CF:oDCB solutions. (a) TEM and RSoXS results. (b) GIWAXS, (c) GISAXS, (d) summary of crystallization behavior from GIWAXS, and (e) summary of phase evolution from GISAXS. Adapted with permission from ref 118. Copyright 2014 John Wiley and Sons.

performance. The film thickness evolution with ODT (see Figure 18) is similar to that with CN, rapid evaporation of CB, followed by extremely slow evaporation of the ODT. Again, like the CN, the ODT (a nonsolvent for P3HT) promotes slightly earlier ordering as observed by the onset of the 605 nm feature in the absorption. Unlike CN, the evolution of the optical order ends with the evaporation of the CB. However, the crystallinity as observed by the (100) pole figure, and phase structure as observed by GISAXS, continuously evolve during the evaporation of the ODT. This is clear evidence that the order/ aggregation that alters the absorption spectrum of P3HT is not the same as crystallization detected by diffraction. For CN, the characteristic length scale for phase separation was constant, suggesting the feature size is limited by intrinsic chain folding of the polymer and the increase in pseudo-TSI is due to creation of new crystallites. For ODT, the characteristic length scale decreases with time, suggesting the collapse of a gel network.

Shown in Figure 19b is a detailed comparison of the GIWAXS and GISAXS for the ODT additive case. Also shown, as a dashed line, is the predicted time evolution of the TSI based on the twophase model that fit the CN additive case. The two-phase model clearly fails to describe the evolution of the system, and we must consider additional phases. Equation 2 can be generalized in a multiphase system to the following: TSI =

∫ (q)q2 dq ≈ ∑ (ρi − ρj )2 φφi j i>j

(3)

On the basis of the immiscibility of P3HT in ODT, a threephase model was proposed, with the three phases being (1) crystalline P3HT (given by the (100) pole figure), (2) a mixed phase of amorphous P3HT and PCBM, and (3) a mixed phase of PCBM and ODT. This model can again be fit to the measured pseudo-TSI with the introduction of only one additional 6348


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Figure 24. (Upper panels) In situ GIWAXS measurements performed immediately after spin coating, showing that the as-cast microstructure has already formed and stabilized and highlighting the need for such measurements to be performed during the coating process. (Lower panel) In situ GIWAXS measurement performed during spin coating (a) without and (b) with DIO additive, showing evidence of complex structural changes within the first minute of spin coating. Adapted with permission from ref 120. Copyright 2013 John Wiley and Sons.

the evolution of the system, which will be seen to be prototypical for systems having strongly crystallizing donors. Rapid drying, as with CB solutions, either spin coated or blade coated, exhibits extremely late polymer crystallization and a poor crystallization extent, consistent with a high nucleation barrier. Slow drying, as with oDCB with minimal ambient flow or with low vapor pressure additives such as CN, allows nucleation at earlier times and a more extended window for crystal growth, resulting in better ordered films and higher performance. In all cases, phase separation is clearly driven by P3HT crystallization and not PCBM aggregation, nor by any spinodal liquid−liquid separation process. The presence of PCBM does inhibit polymer ordering, which can be attributed to vitrification of P3HT by PCBM. It has been established that fullerenes exhibit moderate miscibility in the amorphous regions of P3HT.43,110,111 The glass transition temperature of P3HT is nominally 19 °C, near most deposition temperatures. However, the Tg’s of the common fullerenes are all in excess of 100 °C.112 Therefore, the mixed amorphous phase containing the polymer and fullerene will exhibit a T g significantly higher than room temperature. Reported Tg’s for the 1:1 blends typically used for P3HT BHJs are ∼40 °C.113 This explains the rapid cessation of crystal growth upon removal of the solvent in single-solvent systems, as the dry film is below Tg and phase ripening is kinetically frozen.

Figure 25. In situ GIWAXS measurements performed on an as-cast blend of p-DTS(FBTTh2)2 and PCBM prepared without additive and therefore quenched in the LC phase. Daily GIWAXS measurements performed over a week illustrate the slow LC to crystal transition of pDTS(FBTTh2)2. Adapted with permission from ref 122. Copyright 2015 John Wiley and Sons.

parameter: a partition coefficient of the PCBM between the mixed P3HT and mixed ODT phases. Figure 19 shows that this model describes the measured data well. The partition coefficient of the PCBM was found to be ∼1.9 between the polymer and additive phases. 5.1.2.3. P3HT:PCBM Summary. The accumulated in situ results for the P3HT:PCBM system provide a detailed model for 6349


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Figure 26. Comparison of the ability to crystallize and processing routes of p-SIDT(FBTTh2)2, p-DTS(FBTTh2)2, and X2. Adapted with permission from ref 122. Copyright 2015 John Wiley and Sons.

The recognition of vitrification in P3HT:PCBM reveals a nominally universal mechanism by which liquid additives can enhance crystallization, namely, plasticization. Plasticization can occur through multiple routes: either directly, by solvation of the mixed amorphous phase, or indirectly, by selective removal of the vitrifying agent, PCBM. Optimal additive content not only influences the extent of aggregation but increases the quality of the photophysical aggregates, which may have important benefits with respect to charge generation, transport, and recombination. 5.1.3. Other Polymer:Fullerene BHJs. Since the detailed studies of P3HT, in situ techniques have been used to study the structural evolution of BHJs formed by mixing PCBM with other polymers, including the dithienosilole-based PSBTBT and the diketopyrrolopyrrole-based PDPP5T and PDPP4T. PSBTBT is a highly crystalline polymer, like P3HT, with no reported glass transition temperature. Unlike the carbon-bridged analogue PCPDTBT, which requires the use of a solvent additive for highPCE devices, PSBTBT has been reported to enable near-optimal performance (PCE > 5%) from multiple solvents, with no additive. 5.1.3.1. PSBTBT. In ref 114 the blade-coated deposition of PSBTBT:PCBM-71 BHJs from oDCB at various substrate temperatures was studied with both laser interferometry and GIWAXS. Shown in Figure 20 is the film thickness (and solution concentration) versus time for drying at 40 °C with a drying ambient flow of 0.15 m/s. Like the drying of P3HT, the film exhibits nominally linear evaporation rate behavior over the majority of the drying, with a possible (unresolved by laser interferometry) final slow drying stage. Also like P3HT, the polymer crystal orientation distribution consists of both an edgeon component, attributed to interface nucleation, and a high mosaicity component, attributed to bulk nucleation. Again, like P3HT in oDCB (but unlike P3HT in the more rapidly drying CB), the initial appearance of crystallization is relatively early in the drying, appearing at 0.95 mass fraction solvent. The solubility limit of PSBTBT was not reported, but it was noted that the polymer was marginally soluble, requiring elevated temperature (40 °C) to avoid gelation of the 0.03 mass fraction solution. Thus, crystallization is beginning near the equilibrium solubility point. Unlike P3HT processed with additives, there is no change in the crystal orientation distribution during drying, indicating that bulk and interface nucleation are equally likely. Over the substrate temperature range of 40−85 °C, there was little variation in device performance nor final GIWAXS data.

Figure 27. (a) In situ GIWAXS data showing the time evolution of the LC (yellow band) and crystalline (green bands) phases of pDTS(FBTTh2)2 for different formulations including both DIO and polystyrene as additives. (b) In situ thickness evolutions of the different formulations along with the final dry thicknesses. Adapted with permission from ref 123. Copyright 2015 John Wiley and Sons. 6350


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Figure 28. In situ GIWAXS and GISAXS results for p-DTS(FBTTh2)2:PCBM-71, as a function of DIO additive volume fraction. (a) False color GIWAXS section versus time indicating evolution of the LC polymorph and crystal population. (b) Summary of film thickness from WLI, crystallinity, and GIWAXS. (c) False color GISAXS section versus time indicating evolution of the phase separation. (d) Summary of the pseudo-TSI (black) and characteristic length scale (blue). Adapted with permission from ref 125. Copyright 2015 Royal Society of Chemistry.

The structure evolution of PSBTBT:PCBM-71, in general, appears to be analogous to that of P3HT:PCBM. From the slow drying solvent of oDCB, the evolution is close to equilibrium, with nucleation and growth successfully competing with solvent removal and vitrification. The marginal solubility of PSBTBT, promoting early crystallization, may contribute to the relatively robust processing behavior. No in situ studies have been reported for volatile solvents. However, devices processed from chloroform115 are known to require a moderate (140−150 °C) thermal anneal to achieve optimal performance, suggesting that, also like P3HT:PCBM, sufficiently rapid drying can inhibit optimal morphology development. 5.1.3.2. PDPP5T. The polymer PDPP5T represents a significant contrast to the preceding polymers in terms of processing. Like PCPDTBT, optimal devices appear to require the use of either a solvent additive (DIO) or a mixed solvent (CF:oDCB). Thermal annealing is not sufficient to optimize the

device performance. However, the role of the additive is very different. For P3HT:PCBM and PCPDTBT:PCBM, additives allow the development of polymer order and a coarsening of the phase-separated morphology. As shown in Figure 21, for PDPP5T:PCBM-71, the additive (in this case oDCB in a CF solution) does the opposite. It promotes the refinement, or reduction in domain size, of an additive-free morphology that is too coarse. In ref 116 detailed in situ optical studies were performed on PDPP5T:PCBM-71 drying films. Laser interferometry was used to monitor film thickness. Off-specular laser scattering was used to monitor large-scale phase separation (liquid−liquid droplets), and UV−vis absorbance was used to monitor the development of polymer aggregates. Shown in Figure 21 is a comparison of the evolution of the film thickness and light scattering for solutions spin coated at 209 rad/s (2000 rpm) with and without the additive oDCB. The pronounced shift in drying rate as the film transitions from mass ejection to 6351


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Figure 29. (a) 2D GIWAXS plot of integrated scattering intensity versus time during spin coating of a P3HT:IDTBR:IDFBR ternary solution. Integration was performed for azimuthal angles ranging from 0° to 90° to consider all crystallite orientations. The scattering around q ≈ 5 nm−1 corresponds to the scattering from a Kapton foil used as a shield against ink splashes. (b) Time evolution of integrated scattering intensity of P3HT lamellar stacking peak (100) (solid lines) and solvent scattering (dash lines) during spin coating of the ternary blends. (c, d) Integrated scattering intensity of the as-cast and annealed films, respectively. The thickness of the three coated films was nearly identical as confirmed by spectroscopic ellipsometry for P3HT:IDTBR:IDFBR (85 ± 2 nm), P3HT:IDTBR:PCBM (84.7 ± 2 nm), and P3HT:IDTBR:FBR (83.5 ± 2 nm). (e) UV−vis absorption spectra obtained in situ during spin coating of P3HT:IDTBR:IDFBR ternary sample. (f) Close-up view of UV−vis absorption spectra shown in (a) to highlight that P3HT aggregation occurs earlier than that of the acceptor. (g) Time evolution of absorption intensities at selected wavelengths corresponding to P3HT aggregation (605 nm) and IDTBR aggregation (675 nm), confirming earlier aggregation of P3HT. Adapted with permission from ref 135. Copyright 2016 Macmillan Publishers Limited.

evaporation at ∼0.3 s is apparent. Similarly, the prompt cessation of drying of the CF film at ∼0.7 s, and the prolonged drying of the oDCB swollen film, is visible. The effect of the oDCB on the light scattering is quite striking. Consistent with the coarse morphology observed in TEM in the absence of additive, there is considerable off-specular scattering observed shortly before complete solvent removal in the absence of oDCB. This offspecular scattering is strongly suppressed by the addition of the additive. The suppression of the light scattering by the oDCB was accompanied by the promotion of polymer aggregation, as followed by a red shift in the absorbance spectrum, also demonstrated in Figure 21. The influence of the additive is summarized in Figure 22. It was proposed that the major role of the additive in this system is to degrade the solvent quality, consistent with negligible solubility of PDPP5T in either oDCB or DIO. In the absence of an additive, initial liquid−liquid phase separation was assumed, resulting in rapid growth of the large, fullerene-rich droplets at early time, followed by aggregation of the polymer in the polymer-rich phase. The introduction of the additive promoted polymer aggregation at an earlier time, eventually resulting in an aggregate gel, which inhibits fullerene droplet growth, that dries to the optimal finely mixed film. 5.1.3.3. PDPP4T. The BHJ of a related DPP-based polymer, PDPP4T, exhibits similar behavior to that of DPP5T when spin coated. Films coated directly from CF exhibit very long-range,

liquid−liquid-like droplet phase separation and very poor PCE. As for PDPP5T:PCBM-71, the introduction of additives (oDCB, DIO, TCB, etc.) to the PDPP4T:PCBM-71 formulation dramatically improves the PCE from 1% to ∼5% and markedly decreases the characteristic length scale of the phase separation.117 In ref 118, PDPP4T:PCBM-71 was studied by GIWAXS and GISAXS after deposition from CF:oDCB solutions using a model, small-piece slot-die coater. The residual oDCB content was estimated from the 1.9 Å−1 amorphous halo of the oDCB in the GIWAXS. The nonadditive case was not presented. Shown in Figure 23 is the variation in the final film morphology as a function of oDCB volume fraction, as observed in both TEM and resonant soft X-ray scattering (RSoXS). For the three oDCB concentrations studied of 0.05, 0.20, and 0.50 volume fraction, the characteristic length scale of phase separation is nominally invariant, but the phase contrast (pseudo-TSI) decreases with increasing additive. The PCE is nominally constant across the series at (4.9 ± 0.3)%. As seen in Figure 23, for all additive levels, the GIWAXS and GISAXS either coevolve (0.05 volume fraction oDCB) or the GIWAXS significantly leads the GISAXS (0.20 and 0.50 volume fraction oDCB), indicating that, for all additive levels studied, phase evolution is driven by the crystallization of the polymer from the unfavorable oDCB solution. This is similar to the preceding case of PDPP5T:PCBM-71, where the transition from liquid−liquid phase separation to aggregation occurred at nominally 0.03 volume fraction oDCB. Unlike in the 6352


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Figure 30. (a) In situ thickness and absorbance measurements performed during the two-step spin coating of diF-TESADT:PTAA blends in tetralin solvent. The ratio of absorbance at 526 nm (diF-TESADT) and 383 nm (PTAA) reveals a faster rate of loss of the small molecule at higher spin speed. The final thickness distributions of the trilayer and bilayer structures are obtained from cross-sectional EF-TEM measurements. (b) Phase-field simulations showing the ternary phase diagram of the ink formulation and its predicted vertical stratification at low- and high-speed conditions of spin coating. Adapted with permission from ref 143. Copyright 2016 John Wiley and Sons.

case of P3HT:PCBM from rapidly evaporating CB with additive, where interfacial nucleation of crystallization occurs upon removal of the primary solvent, followed by increased bulk crystallization promoted by the additive, in the case of PDPP4T, there is no significant early time step in the diffraction. The GIWAXS seems to monotonically evolve with the removal of the oDCB. PDPP5T:PCBM-71 and PDPP4T:PCBM-71, both based on DPP-containing polymers, are distinct among polymer−fullerene BHJ systems in that pronounced liquid−liquid phase separation is readily observed and must be avoided with formulation or processing approaches to achieve a reasonably high PCE. In these systems, the morphology issues are solved by forcing crystallization to be the dominant structure-evolution mechanism. In many other systems, crystallization naturally dominates the structure evolution and liquid−liquid phase separation is not observed in commonly tried formulation and processing approaches. 5.1.4. Small-Molecule:Fullerene BHJ. While the BHJ motif was first introduced in polymer donor systems, significant interest lies in the development of similar OPV architectures

based on small-molecule (SM) electron donors. The small molecules have the benefit of simpler synthesis and purification, eliminating common polymer issues with polydispersity, regioregularity, and isomerization. BHJ solutions with SM electron donors tend to be more difficult to process due to low viscosity and a tendency to comprehensively crystallize. The small molecules p-DTS(BTTh2)2 and related p-DTS(FBTTh2)2, developed in the laboratory of Prof. Bazan,119−121 formed some of the first SM BHJs to exhibit performance (>7% PCE) comparable to optimized polymer systems. In both of these SM systems, it was empirically found that higher PCEs could be obtained by processing with an additive (DIO) versus thermal processing. Strikingly, it was observed that both systems exhibit an extreme sensitivity to the additive loading, optimizing within a very small window of additive volume fraction. This has spurred significant in situ work on the p-DTS(FBTTh2)2 system, in an attempt to identify the detailed role of the additive. 5.1.4.1. p-DTS(FBTTh2)2 spin coating. In ref 120 the deposition of p-DTS(FBTTh2)2:PCBM-71 via spin coating, with and without additive, was characterized by time-resolved GIWAXS. In an initial experiment, the as-cast films (with and 6353


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coating.125 Shown in Figure 28 is the evolution of the GIWAXS and film thicknesses for 0, 0.003, 0.006, and 0.01 volume fraction DIO. The film thickness evolution is very similar to that of P3HT:PCBM with CN or ODT additive: a rapid linear decrease in thickness due to evaporation of the primary solvent followed by significantly slower evaporation of the low vapor pressure DIO. Even at relatively low DIO loadings (0.001 volume fraction) and elevated temperature (sample stage 40 °C), evaporation of the DIO takes ∼20 min. Also shown is the integrated pole figure intensity as a proxy for relative crystallinity. As observed in spin coating, the additive-free solution dries to a very disordered, poorly crystalline film that exhibits a distinctly different diffraction pattern than the optimal film. It is likely a polymorph of the crystal present in optimal films. In the presence of the additive, one sees the marked increase in crystallinity. There is also a rapid evolution from an early time polymorph, which appears similar to the polymorph in the additive-free films, to the final crystal structure, which is nominally that of bulk pDTS(FBTTh2)2 crystals. There are no apparent changes in the degree of crystallinity or the crystal structure even when an excessive amount (0.01 volume fraction) of additive is used. Also shown in Figure 28 is the evolution of the GISAXS (qx2 weighted qx scattering extracted from evanescent field enhanced, Yoneda scattering at qz ≈ 0) and the extracted pseudo-TSI and characteristic dimension (peak in Kratky plot). The origin of the extreme sensitivity to additive loading becomes clear in the GISAXS: the additive is altering the characteristic dimension of phase separation and excess additive results in excessive coarsening (too large a characteristic dimension). It is notable that the observed pseudo-TSI does not significantly evolve with time. This suggests that the role of DIO is in determining the nucleation density, as opposed to altering the crystal growth rate. The coarsening (decrease in nucleation density) with additive level suggests a decrease in the degree of supersaturation during nucleation, consistent with the observation that DIO is a solvent for p-DTS(FBTTh2)2 (24 mg/mL solubility limit at room temperature). 5.1.4.3. p-DTS(FBTTh2)2 Summary. The collective studies of p-DTS(FBTTh2)2:PCBM-71 structure evolution reveal similarities and differences with the evolution of the polymer−fullerene films. Because of its strong propensity for crystallization, this SM system is similar in many regards to those based on the highly semicrystalline polymers P3HT and PSBTBT. Rapid drying from a volatile (CB) solvent results in a kinetically trapped mixed amorphous or liquid crystalline state that cannot phase-separate due to rapid quenching below the glass transition. For pDTS(FBTTh2)2:PCBM-71, the Tg of the donor125 was found to be ∼45 °C while that of the acceptor was ∼160 °C, consistent with the fullerene vitrifying the p-DTS(FBTTh2)2. A significant role of the DIO additive (a solvent for the donor in its LC mesophase and acceptor) is thus to plasticize the amorphous state, allowing both crystallization of the donor and phase separation. However, in contrast to the semicrystalline polymers, where the self-limiting crystal size is controlled by polymer characteristics such as relative molecular mass and polydispersity, the additive has a second role in SM systems of defining the crystal size by affecting nucleation. This is the origin of the extreme sensitivity of SM systems to the amount of additive, as this tunes the final length scale for phase separation. It is also found that, unlike polymers, the rich diversity in the crystal landscape afforded by SM (both crystal polymorphs and apparent liquid crystalline solution phases) can complicate the time evolution of the drying film. Understanding these intricate

without DIO) were placed on the beamline immediately after spin coating, as shown in the upper panel of Figure 24, only to find that the films’ microstructures had ceased to evolve. The solution cast in the presence of DIO showed evidence of significant crystallization, whereas the formulation cast without additive forms a quenched film with little evidence of order beside a scattering feature near 4.4 nm−1. To capture the evolution of microstructure, time-resolved GIWAXS was performed during the spin-coating process (lower panel, Figure 24). It revealed that, upon the drying of the process solvent (CB), the as-cast film forms an intermediate mesophase of pDTS(FBTTh2)2, which remains quenched if no additive is used or transforms into the equilibrium crystalline phase of pDTS(FBTTh2)2 in the presence of DIO. In a subsequent study,122 this mesophase was identified as a liquid crystalline intermediate and was shown to gradually convert into the crystalline phase over a period of days (Figure 25). The same authors used a combination of flash DSC (F-DSC) and in situ GIWAXS measurements on two additional donors (p-SIDT(FBTTh2)2 and X2) to evaluate the role of the additive. They found that the additive is needed in small-molecule systems where the donor tends to get easily quenched into an intermediate mesophase (e.g., liquid crystalline or amorphous) during spin coating. The study went on to link the need for a solvent additive to the ease of quenching of the donor into a mesophase (Figure 26) by identifying an alternative donor molecule (X2) that has the ability to easily crystallize (F-DSC) and to do so directly from solution (in situ GIWAXS), achieving a maximum PCE of 7.6% without any additive. It is quite clear from these in situ investigations that the additive helps to transition the donor from a mesophase toward a more stable crystalline phase. However, this is not the only pathway to crystallization. Reference 123 reported an in situ investigation of the effects of including a small amount of polystyrene (PS) into the blend as a means of increasing the viscosity of the solution and increasing the dry thickness of the BHJ. In situ GIWAXS measurements shown in Figure 27 indicate that the presence of PS promotes the crystallization of some of the LC phase of p-DTS(FBTTh2)2 in addition to increasing the dry thickness from ∼95 to ∼130 nm. In situ optical measurements of thickness reveal this to be due to the fact that PS helps the blend film retain more solvent, effectively providing more time for the residual solvent to interact with the LC phase of pDTS(FBTTh2)2, converting it into the more stable crystalline phase via solvent annealing.124 The above kinetic arguments in support of the mechanism of LC to crystal transition should a priori also be valid for explaining the role of the additive. Indeed, p-DTS(FBTTh2)2 powders have been shown to exhibit non-negligible solubility in DIO.125 One could therefore assume that DIO performs a similar solventannealing task as did the residual CB solvent. However, under normal drying circumstances, it can take an hour for the DIO to be evaporated from the film, but the LC-to-crystal transition occurs within seconds of CB leaving the film, soon after the solution inversion has taken place from CB-dominated to DIOdominated. This suggests that the influence of the additive is not so simple and requires further study. 5.1.4.2. p-DTS(FBTTh2)2 Blade Coating. In ref 125, GIWAXS, GISAXS, and in situ WLI were used to follow blade-coated pDTS(FBTTh2)2 films, allowing full correlation of the drying to the evolution of both the film crystallinity and the phase segregation. Stable blade coating required slightly different solution concentrations than were employed in the spin 6354


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Figure 31. (Upper row) (a) Efficiency of blade-coated PTB7:PCBM solar cells prepared by spin coating and blade coating in different conditions and formulations. (b) In situ thickness and absorbance of PTB7:PCBM solutions in CB and DIO additive spin-cast and blade-cast. (Bottom row) (a) PL quenching and EF-TEM data (b−f) for BHJs prepared using different methods and formulations. Adapted with permission from ref 148. Copyright 2016 Royal Society of Chemistry.

spectrum and a reduced lowest unoccupied molecular orbital (LUMO) offset with P3HT. The performance of P3HT:NFA systems falls well below the efficiency (10%) attainable with NFAs blended with low-bandgap polymers.133,134 A ternary approach has thus been explored by Baran et al.135 with P3HT as donor and two NFAs, the primary acceptor (A1, IDTBR) and secondary acceptor (A2, IDFBR), to achieve a PCE of 7.7 ± 0.1% [P3HT:IDTBR:IDFBR (1:0.7:0.3)] compared with 3.6 ± 0.2% when using A2:PCBM (1:0.7:0.3). The same approach was also shown to yield 11 ± 0.4% using PCE10:IDTBR:IDFBR (1:0.5:0.5) without resorting to any solvent additives. An interesting observation made in ternary blends is the suppression of the aggregation of IDTBR (A1) when blended with any other A2 (FBR, PCBM, or IDFBR), even after annealing, contrary to the binary IDTBR:PCBM blend. In situ GIWAXS and in situ UV−vis absorbance measurements (Figure

mechanisms has already allowed chemists to propose new oligomer systems that can form crystalline and optimally phaseseparated BHJs without the need for solvent additives, an important requirement for facile manufacturing of these films. 5.1.5. Nonfullerene, Ternary BHJ. To date, the most common BHJ has consisted of a low-bandgap polymer donor blended with a fullerene acceptor. However, such donor materials often require multistep and complex syntheses as compared with P3HT. While P3HT is readily scalable,126 is compatible with green solvents, and yields robust solar cells with PCBM, its open-circuit voltage and short-circuit current are limited, yielding PCE < 5%.127 The emergence of nonfullerene acceptors (NFAs),128−131 such as IDTBR, which can be mixed with P3HT and yield PCE up to 6.4%,132 has revived interest in this workhorse donor polymer of the OPV community. The NFA improves PCE thanks to increased absorption in the visible 6355


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29) were performed during spin coating on P3HT:IDTBR:A2 (A2 = FBR, IDFBR, or PCBM). In all cases, P3HT aggregation was observed in the very last moments of solvent evaporation and preceded the crystallization of all other components. The choice of A2 strongly influences the crystallization behavior of P3HT, with PCBM playing a vitrifying role whereas IDFBR does not vitrify P3HT as much. Additional DSC investigations and static GIWAXS studies on binary and ternary blends established that the ternary blend is composed of three partially miscible components, namely, a mostly unperturbed P3HT phase, which hosts a molecular dispersion of IDFBR molecules, and an IDTBR-rich crystalline phase, which also contains IDFBR. With increasing IDFBR content, the IDTBR is vitrified, resulting in a disordered solid solution of two acceptor molecules, whereas P3HT mostly maintains its crystalline order. 5.2. Organic Transistor Blends

Blending of organic small-molecule semiconductors with amorphous polymers, whether conjugated or insulating, has been shown to yield OTFTs with consistently superior and more reproducible device performance,136−138 both by spin coating and blade coating. Some reports show carrier mobility nearly on par with single-crystal FET devices of the same small molecules.139−141 Key to their success is the vertical stratification of the phases into a very thin and highly crystalline smallmolecule semiconductor film and a polymer layer. The stratification tends to be either in bilayer or trilayer structures, as confirmed via ex situ measurements, such as TEM, secondary ion mass spectrometry (SIMS),136 and neutron scattering.142 A recent in situ investigation of the spin coating of OTFT blend inks has shown that process conditions play an important role in dictating the vertical stratification outcome of the experiment.143 As summarized in Figure 30, it was shown that, for a starting formulation containing 45% by volume of small-molecule 2,8difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diFTESADT) and 55% semiconducting amorphous polymer (PTAA) in tetralin, the spin-coating speed determines the final volume fraction of small molecule in the final film, with no more than 30% at 105 rad/s (1000 rpm) and 5% at 523 rad/s (5000 rpm). In situ absorbance measurements demonstrated the presence of a speed-dependent preferential loss of the small molecule during spin coating through a combination of ejection and outflow. This preferential loss was attributed to the unstable nature of the tetralin-based ink, which led to growing spinodal instabilities, as predicted using phase-field simulations. It was shown that, for processing conditions yielding excessive outflow of the solution, the buried interfacial layer of the small molecule does not form, leading to formation of bilayer structures. By contrast, reducing the spin speed so as to minimize outflow leads to a film that maintains the composition of the starting formulation and yields a trilayer structure with a well-formed buried layer.

Figure 32. Current density−voltage characteristics for PBDTTTCT:PCBM and PTB7-Th:PCBM solar cells prepared by spin coating and blade coating using the same formulation and adjusted to achieve thickness and morphological parity with spin coating, thus achieving performance parity. Adapted with permission from ref 148. Copyright 2016 Royal Society of Chemistry.

(≤25 μm) makes in situ studies difficult, and the authors are not aware of any detailed studies. Recently, in situ studies have been performed on the receding edges of large drying droplets.145 For large-area applications, such as solar cells, the difficulty of adapting spin-coated materials to flexible web continuous manufacturing have been well-documented.146 While smallarea, spin-coated cells have demonstrated PCE over 10% for multiple materials, hero performance of blade-coated cells has, until recently, been limited to ∼7%. This can be attributed to many issues: fundamental differences in the morphological and microstructural evolution between the rapid drying intrinsic to spin coating and the slower drying of web-coaters, difficulties in managing coating defects upon scaling to large areas, and difficulties in producing air-stable, high-performing, low-cost electrodes (both transparent and reflective). Both blade coating and piecewise slot-die coating should model the drying dynamics of continuous slot-die web coating well and allow direct engineering of the first issue: correction for the different drying dynamics. It is generally accepted that material sets that allow optimal PCE to be achieved at dry film thicknesses of ≥250 nm are required to address the second issue (significantly decreasing the pool of candidate materials), while material engineering advances are required to address the third.147

6. LAB-TO-FAB The dominant use of spin coating to evaluate materials in research laboratories results in a significant Lab-to-Fab transition barrier in commercializing novel organic materials. This is particularly an issue with transistor and organic light-emitting diode (OLED) applications, where production entails additive patterning by either inkjet or traditional (gravure, offset, etc.) printing. The rheological requirements of patterned deposition are significantly different from spin coating, and considerable ink design is required.144 The small scale of the final design features

6.1. In Situ Guided Transitions

6.1.1. Matching Kinetics of Deposition. When empirically optimizing BHJ coating, after choice of solvent, at minimum three parameters must be addressed: donor-to-acceptor ratio, which to first order determines the percolation network and hole and electron transport balance; film thickness, which determines the balance between recombination and increased light harvesting; and additive volume fraction, which controls, as described above, the phase separation. When transitioning an empirically optimized recipe from one coating technique to another, it is 6356


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Figure 33. (a) UV−vis absorbance, as false color, versus temperature for PffBT4T-2OD:PCBM-71 coating solution. (b) PCE of blade-coated films as a function of deposition temperature and film thickness. (c) GIWAXS and (d) RSoXS characterization of PffBT4T-2OD blade-coated films. Adapted with permission from ref 152. Copyright 2016 Royal Society of Chemistry.

as an absorber.149 This polymer is known to have a relatively high diffraction strength, and its high performance was, in part, attributed to its strong aggregation, even in solution. This polymer−fullerene system is particularly well-suited to large-area coating because high PCE can be achieved in relatively thick films (>300 nm), providing a generous manufacturing tolerance. However, the strong tendency to aggregate complicates processing. In the initial demonstrations of spin coating, it was found that it was necessary to spin hot (∼100 °C) solution onto a preheated (∼100 °C) substrate. Hot spin coating has become a common deposition technique for many high-performance (≥10% PCE) polymer BHJs.149,150 As both blade coating and slot-die coating involve nominally constant solution and substrate temperatures, matching the dynamic temperature evolution of the spin-coated system is unlikely. Shown in Figure 33 is in situ UV−vis absorbance data for a PffBT4T-2OD:PCBM-71 1:1.2 coating solution at 44 mg/mL total solids in a 1:1 CB:oDCB mixed solvent. The spectra redshift due to polymer aggregation is clearly evident near 60 °C. Also shown in Figure 33 are PCE results for blade-coated films as a function of both film thickness (varied by blade velocity) and solution/substrate temperature. PCE comparable to those achieved by spin coating are obtained when blade coating near 90 °C. Additionally shown in Figure 33 are GIWAXS and RSoXS results for the blade-coated films at both the optimal (90 °C) and

critical to consider the drying rate of the primary solvent in the two processes. The Amassian lab recently demonstrated how careful matching of the kinetics of drying, based on in situ optical studies of both spin and blade coating, enables rapid and rational transition of optimized spin-coating parameters to blade coating.148 Figure 31 shows that matching the film thickness of the spin-cast BHJ by blade coating the exact same formulation does not necessarily imply performance parity. The study also showed that, in conditions where film thickness of blade coating and spin coating cannot be matched using the same formulation, the latter can be modified, for instance, by increasing the concentration. In such a scenario, it is critical to maintain the additive-to-solute ratio, which effectively means increasing the volume ratio of additive in the solution. Taking care to match the drying kinetics of spin coating by gently heating the sample holder of the blade coater helps achieve qualitative morphological parity, as indicated by EF-TEM analysis and PL quenching. In these cases, PBDTTT-CT:PC 7 1 BM, PTB7:PC71BM, and PTB7-Th:PC71BM solar cells with PCEs of 6.5%, 9.0%, and 9.7%, respectively, can be achieved by blade coating the BHJ layer, matching the performance of spin-cast devices (Figure 32). 6.1.2. When Kinetics Cannot Be Matched. Very high power conversion efficiencies (>10%) were recently achieved in spin-coated polymer−fullerene solar cells using PffBT4T-2OD 6357


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Figure 34. (a) In situ absorbance at 700 nm and film thickness during blade coating of PffBT4T-2OD at a fixed temperature of 90 °C and spin coating starting at ∼90 °C. (b) GIWAXS and film thickness data during blade coating at both 55 °C (aggregated solution) and 90 °C (disaggregated solution). Adapted with permission from ref 152. Copyright 2016 Royal Society of Chemistry.

Figure 35. Illustration depicting differences in solidification evolution of a PffBT4T-2OD system blade-coated at different temperatures.

cool (55 °C) temperatures. Strikingly, while the optimal conditions result in nearly identical PCE to spin-coated devices, they result in radically different morphology. This is in stark contrast to the preceding results on BDTTT polymers. The morphology of the PffBT4T-2OD:PCBM-71 spin-coated films,149,151 characterized by a face-on orientation distribution and a monotonic phase-separation distribution, is very similar to the morphology of the 55 °C suboptimal blade-coated film. The optimal blade-coated film exhibits an edge-on orientation distribution and a clearly bimodal phase separation. In situ studies of polymer aggregation (via UV−vis) and crystallization (GIWAXS) clearly identified the origin of the morphology changes upon optimized blade coating. Shown in Figure 34a is the absorbance at 700 nm, the peak of the aggregate absorption, and the film thickness during blade coating at a constant 90 °C and spin coating at an initial ∼90 °C. The evolution of the blade-coated film is very similar to that of the strongly crystallizing P3HT in the presence of ODT: aggregation occurs late during the primary solvent drying, indicative of supersaturation and a significant nucleation barrier. It does not

evolve during the additive drying stage. In contrast, aggregation occurs much earlier during spin coating, attributed to cooling of the solution. Thus, solidification in the spin-coated case occurs from an aggregated gel state, whereas during blade coating, morphology development occurs in an additive-swollen film. The differences are clearly evident in Figure 34 b, which shows GIWAXS results for blade coating from the aggregated 55 °C solution, resulting in a moderately face-on orientation distribution, and from the dissolved 90 °C solution (resulting in an edge-on crystal orientation distribution). The influence of the temperature-dependent aggregation state on film structure evolution is summarized schematically in Figure 35. The PffBT4T-2OD system provides important insight into the dependence of PCE on BHJ morphology because the final morphology of its optimal spin-coated film is decidedly different than that of its optimal blade-coated film. In the spin-coated film, the polymer crystals have a preference for face-on orientation of the conjugated planes, whereas in the blade-coated film, the crystals have an edge-on preference. RSoXS measurements of the spin-coated film show a broad, monomodal domain size 6358


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Figure 36. Schematic of lab-scale roll coater compatible with X-ray scattering measurements. Raw GIWAXS images taken (a) 120 (14 s) mm and (b) 195 mm (23 s) from the slot-die head. (c) Image taken after unrolling (1 h drying). Reproduced with permission from ref 153. Copyright 2014 American Institute of Physics.

above by both spin- and blade coating. The web speed was 0.5 m/ min, and the total web length was 30 m, allowing a total of ∼1 h of data acquisition. The concentration of the solution (60 mg/mL total solids) was such that the ∼10 μm thick wet film dried to a ∼480 nm final film. The evolution of the (100) pole figure is nominally similar to that observed in the preceding model studies: at early times (14 s, short distances = 120 mm, Figure 36 a) the P3HT crystallites exhibit a tight, edge-on orientation, attributed to interface nucleation. At later times (1 h drying, Figure 36 c), a clear isotropic ring (increased mosaicity) is present. Interestingly, it is clear that the film continued to evolve beyond 25 s (195 mm) as the final dry film exhibits the greatest isotropic contribution. However, at the elevated drying temperature (70 °C), complete evaporation of the CB would be expected in 1 V Open Circuit Voltages. Energy Environ. Sci. 2016, 9, 3783−3793. (135) Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.; Gasparini, N.; Röhr, J. A.; Holliday, S.; Wadsworth, A.; Lockett, S.; Neophytou, M.; et al. Reducing the Efficiency−stability−cost Gap of Organic Photovoltaics with Highly Efficient and Stable Small Molecule Acceptor Ternary Solar Cells. Nat. Mater. 2016, 16, 363. (136) Ohe, T.; Kuribayashi, M.; Yasuda, R.; Tsuboi, A.; Nomoto, K.; Satori, K.; Itabashi, M.; Kasahara, J. Solution-Processed Organic ThinFilm Transistors with Vertical Nanophase Separation. Appl. Phys. Lett. 2008, 93, 053303. (137) Kang, J.; Shin, N.; Jang, D. Y.; Prabhu, V. M.; Yoon, D. Y. Structure and Properties of Small Molecule−Polymer Blend Semiconductors for Organic Thin Film Transistors. J. Am. Chem. Soc. 2008, 130, 12273−12275. (138) Smith, J.; Hamilton, R.; McCulloch, I.; Stingelin-Stutzmann, N.; Heeney, M.; Bradley, D. D. C.; Anthopoulos, T. D. Solution-Processed Organic Transistors Based on Semiconducting Blends. J. Mater. Chem. 2010, 20, 2562. (139) Smith, J.; Zhang, W.; Sougrat, R.; Zhao, K.; Li, R.; Cha, D.; Amassian, A.; Heeney, M.; McCulloch, I.; Anthopoulos, T. D. SolutionProcessed Small Molecule-Polymer Blend Organic Thin-Film Transistors with Hole Mobility Greater than 5 cm2/Vs. Adv. Mater. 2012, 24, 2441−2446. (140) Niazi, M. R.; Li, R.; Qiang Li, E.; Kirmani, A. R.; Abdelsamie, M.; Wang, Q.; Pan, W.; Payne, M. M.; Anthony, J. E.; Smilgies, D.-M.; et al. Solution-Printed Organic Semiconductor Blends Exhibiting Transport Properties on Par with Single Crystals. Nat. Commun. 2015, 6, 8598. (141) Niazi, M. R.; Li, R.; Abdelsamie, M.; Zhao, K.; Anjum, D. H.; Payne, M. M.; Anthony, J.; Smilgies, D.-M.; Amassian, A. ContactInduced Nucleation in High-Performance Bottom-Contact Organic Thin Film Transistors Manufactured by Large-Area Compatible Solution Processing. Adv. Funct. Mater. 2016, 26, 2371−2378. (142) Shin, N.; Kang, J.; Richter, L. J.; Prabhu, V. M.; Kline, R. J.; Fischer, D. A.; DeLongchamp, D. M.; Toney, M. F.; Satija, S. K.; Gundlach, D. J.; et al. Vertically Segregated Structure and Properties of Small Molecule-Polymer Blend Semiconductors for Organic Thin-Film Transistors. Adv. Funct. Mater. 2013, 23, 366−376. (143) Zhao, K.; Wodo, O.; Ren, D.; Khan, H. U.; Niazi, M. R.; Hu, H.; Abdelsamie, M.; Li, R.; Li, E. Q.; Yu, L.; et al. Vertical Phase Separation in Small Molecule: Polymer Blend Organic Thin Film Transistors Can Be Dynamically Controlled. Adv. Funct. Mater. 2016, 26, 1737−1746. (144) Calvert, P. Inkjet Printing for Materials and Devices. Chem. Mater. 2001, 13, 3299−3305. (145) Roth, S. V.; Herzog, G.; Körstgens, V.; Buffet, A.; Schwartzkopf, M.; Perlich, J.; Abul Kashem, M. M.; Döhrmann, R.; Gehrke, R.; Rothkirch, A.; et al. In Situ Observation of Cluster Formation during Nanoparticle Solution Casting on a Colloidal Film. J. Phys.: Condens. Matter 2011, 23, 254208. (146) Hösel, M.; Dam, H. F.; Krebs, F. C. Development of Lab-to-Fab Production Equipment Across Several Length Scales for Printed Energy Technologies, Including Solar Cells. Energy Technol. 2015, 3, 293−304. (147) Angmo, D.; Gevorgyan, S. A.; Larsen-Olsen, T. T.; Søndergaard, R. R.; Hösel, M.; Jørgensen, M.; Gupta, R.; Kulkarni, G. U.; Krebs, F. C. Scalability and Stability of Very Thin, Roll-to-Roll Processed, Large Area, Indium-Tin-Oxide Free Polymer Solar Cell Modules. Org. Electron. 2013, 14, 984−994. (148) Zhao, K.; Hu, H.; Spada, E.; Jagadamma, L. K.; Yan, B.; Abdelsamie, M.; Yang, Y.; Yu, L.; Munir, R.; Li, R.; et al. Highly Efficient Polymer Solar Cells with Printed Photoactive Layer: Rational Process Transfer from Spin-Coating. J. Mater. Chem. A 2016, 4, 16036−16046.

(149) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293−5298. (150) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (151) Ma, W.; Yang, G.; Jiang, K.; Carpenter, J. H.; Wu, Y.; Meng, X.; McAfee, T.; Zhao, J.; Zhu, C.; Wang, C.; et al. Influence of Processing Parameters and Molecular Weight on the Morphology and Properties of High-Performance PffBT4T-2OD:PC 71 BM Organic Solar Cells. Adv. Energy Mater. 2015, 5.10.1002/aenm.201570126 (152) Ro, H. W.; Downing, J. M.; Engmann, S.; Herzing, A. A.; DeLongchamp, D. M.; Richter, L. J.; Mukherjee, S.; Ade, H.; Abdelsamie, M.; Jagadamma, L. K.; et al. Morphology Changes upon Scaling a High-Efficiency, Solution-Processed Solar Cell. Energy Environ. Sci. 2016, 9, 2835−2846. (153) Rossander, L. H.; Zawacka, N. K.; Dam, H. F.; Krebs, F. C.; Andreasen, J. W. In Situ Monitoring of Structure Formation in the Active Layer of Polymer Solar Cells during Roll-to-Roll Coating. AIP Adv. 2014, 4, 087105. (154) Andersen, T. R.; Dam, H. F.; Hösel, M.; Helgesen, M.; Carlé, J. E.; Larsen-Olsen, T. T.; Gevorgyan, S. A.; Andreasen, J. W.; Adams, J.; Li, N.; et al. Scalable, Ambient Atmosphere Roll-to-Roll Manufacture of Encapsulated Large Area, Flexible Organic Tandem Solar Cell Modules. Energy Environ. Sci. 2014, 7, 2925. (155) Yu, L.; Niazi, M. R.; Ngongang Ndjawa, G. O.; Li, R.; Kirmani, A. R.; Munir, T.; Balawi, A. H.; Laquai, F.; Amassian, A. Programmable and Coherent Crystallization of Semiconductors. Sci. Adv. 2017, 3, No. e16022462.

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