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Tuning the Morphology of All-polymer OPVs Through Altering Polymer-Solvent Interactions Eleni Pavlopoulou, Chang-Su Kim, Stephanie S. Lee, Zhihua Chen, Antonio Facchetti, Michael F. Toney, and Yueh-Lin Loo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502112z • Publication Date (Web): 19 Aug 2014 Downloaded from http://pubs.acs.org on August 23, 2014

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Chemistry of Materials

Tuning the Morphology of All-polymer OPVs Through Altering Polymer-Solvent Interactions Eleni Pavlopoulou,1,§ Chang Su Kim,1 Stephanie S. Lee,1 Zhihua Chen,2 Antonio Facchetti,2 Michael F. Toney,3 Yueh-Lin Loo1,* 1

Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544 (USA) 2

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Polyera Corporation, Skokie, Illinois 60077 (USA)

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025 (USA)

[email protected], [email protected] , [email protected], [email protected], [email protected], [email protected], [email protected] RECEIVED DATE CORRESPONDING AUTHOR FOOTNOTE *

Department of Chemical & Biological Engineering, Princeton University, Princeton, New Jersey

08544 (USA) Tel: +1-609-258-9091, Fax: +1-609-258-0211, email: [email protected] §

Present address: Institut Polytechnique de Bordeaux, Laboratoire de Chimie des Polymères

Organiques (LCPO), UMR 5629, 16 avenue Pey-Berland, F-33607 Pessac Cedex, France

ACS Paragon Plus Environment

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ABSTRACT

In this work, we investigated the effects of solvent(s)—polymer(s) interactions on the morphology of all-polymer bulk-heterojunction (BHJ) active layers cast from cosolutions. We demonstrate that altering the interactions between the solvent and both the donor and acceptor polymers in the cosolution prior to film-casting induces different solid-state morphological characteristics that subsequently leads to differences in the device performance of organic photovoltaics (OPV). Poly(3hexylthiophene), P3HT, was codissolved with poly[[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)], P(NDI2OD-T2), or otherwise known as ActivInkTM N2200, in dichlorobenzene, chlorobenzene and xylene. According to the qualitative interaction map we propose, all three solvents exhibit favorable interactions with P3HT. The extent of incompatibility these solvents exhibit with P(NDI2OD-T2), however, varies, with xylene as the worst solvent for P(NDI2OD-T2) among those examined. Polymer-polymer interactions in xylene are thus more favorable compared to P(NDI2OD-T2)-xylene interactions. Grazing-incidence wide angle x-ray scattering measurements on the cast films suggest that this preferential affinity between the two polymers disrupts crystallization in the blends; P(NDI2OD-T2) crystallinity decreases and, concurrently, results in shorter P3HT coherence lengths. Significant mixing of the two polymers is also evidenced. OPVs comprising P3HT and P(NDI2OD-T2) active layers cast from xylene exhibit the best device characteristics compared to OPVs whose active layers are cast from di-, or chlorobenzene. We attribute the improved OPV performance for the xylene-cast active layer to the presence of a more intermixed network of nanocrystalline domains of the two polymers, which originates from the affinity of P3HT and P(NDI2OD-T2) in the parent cosolution.


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INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Organic photovoltaics (OPVs) have been the focus of research activity for more than two decades, as they promise to be alternatives to their inorganic counterparts.1 The “soft” nature of organics offers better mechanical compatibility with flexible substrates. Additionally, the solution processability of organic semiconductors allows easy, large-scale, and low-cost device fabrication and implementation, all essential for wide-scale adoption. Spin-coating is the most commonly used technique for forming bulk-heterojunction (BHJ) active layers. Depending on processing parameters, such as solution concentration, donor-acceptor composition, evaporation rate of the solvent, pre- or post-deposition annealing temperatures of the solution and/or the cast film, etc., the morphological characteristics of the active layers can vary significantly.2,

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Herein, the effect of solvent-solute(s)

affinities on the solid-state morphology of polymer-polymer BHJ is explored and correlated with OPVs device characteristics. The need to control and tailor the BHJ morphology in order to meet the prerequisites for maximum exciton dissociation at donor/acceptor interfaces and enhanced charge transport to the electrodes have been highlighted in many reports.1, 2, 4, 5 Although the majority of scientific efforts is focused in polymer-small molecules blends, polymer-polymer OPVs were introduced by Halls et al.6 as well as Yu and Heeger7 as attractive alternatives because the BHJ morphology that results can provide improved percolation pathways for the transport of charge carriers. This percolation network can be achieved through exploiting the bicontinuous, interpenetrating network formed by phase separation of the donor and acceptor polymers, which can be tuned according to established polymer physics that govern polymer blends. Polyfluorene-based polymers8-11 as well as poly(paraphenylenevinylene), PPV, derivatives have been utilized, for example, in all-polymer BHJs.6, 7, 12, 13 The efficiencies reported for all-polymer OPVs, however, have remain low compared to those of devices comprising the more commonly studied polymer-fullerene blends. The introduction of high electron mobility electron-acceptor polymers was speculated to result in higher performance.14 In 2009, the synthesis of an electron transporting polymer, poly[[N,N’-bis(2-octyldodecyl)napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)],

P(NDI2OD-T2)

or


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ActivInkTM N2200, was reported.15 Its stable electron-transport properties (0.8 cm2V-1s-1 in field1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect transistors)15,

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along with its red-shifted optical absorption relative to that of fullerene

derivatives suggest that it might be a promising electron acceptor polymer in OPVs. Devices comprising poly(3-hexylthiophene), P3HT, as donor and P(NDI2OD-T2) as acceptor, however, have exhibited disappointingly low power conversion efficiencies17-19, with the highest reported efficiency being 1.4%.20, 21 In order to achieve this performance, the authors had to cast the active layer from a mixture of xylene and chloronaphthalene, which the authors claim to suppress aggregation of P(NDI2OD-T2) at the early stage of film formation.20 They proposed that the enhanced performance originates from better intermixing of the donor and acceptor; no robust structural characterization, however, was provided to support their claim. In another report, Moore et al.18 performed a detailed study of the photophysics of P3HT:P(NDI2OD-T2) blends; geminate recombination was identified as the most probable reason for the poor performance of P(NDI2OD-T2)-containing solar cells as charges are locally confined on isolated chains of the acceptor polymer in a matrix of the donor polymer. Morphological characterization on this polymer pair, however, was restricted to the micrometer scale. The researchers concluded that understanding the thermodynamics and kinetics that govern the structure development of complex blends of these two semi-crystalline polymers is imperative for the successful implementation of new n-type polymers in OPVs. In this context, our study examines P(NDI2OD-T2) as a model electron-acceptor polymer in polymer-polymer blends designed for BHJs. P3HT is used as the hole-transport component since it is a well-studied system that exhibits hole mobilities as high as 10-2 cm2V-1s-1, depending on its regioregularity and molecular weight.22 Moreover, its absorption spectrum complements that of P(NDI2OD-T2).18 Our aim is to treat P3HT:P(NDI2OD-T2) as a model all-conjugated polymer blend to elucidate the processingstructure-function relationships in BHJ polymer solar cells. Starting with theories and practices that are commonly applied in polymer physics, we probed the different morphologies – in both solution and in the solid-state – and correlate these structural changes to differences in device performance. In this work, we opt to use polymer solution properties as a tool for inducing differences in the morphology of the active layer. The importance of solvent quality has already been recognized in


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several reports on polymer-small molecule BHJs.2, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Most reports explain the different

morphologies that result based on the boiling points of the solvents from which the blends were cast, suggesting that casting the blends from higher boiling point solvents result in higher crystallinity in the solid state since the cast films have more time to crystallize and approach equilibrium prior to vitrification upon complete solvent removal.24, 25 In polymer-fullerene systems, it has been suggested that solvent-induced structural changes result from enhanced solvent-fullerene interactions, ultimately leading to more uniform and smoother films26 with a decreased extent of phase separation between the organic semiconductor constituents.23,

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In another report it has been concluded that

when a good solvent for both the fullerene and the polymer is used, optimal phase separation can be achieved.28 Although sometimes contradictory, all reports point to the direct implication of solvent quality on the compatibility of the two constituent organic semiconductors. Despite recent efforts, how polymer affinities in solution ultimately affect the BHJ’s morphology has only been partially explored.28, 29 In this work, we focus on the interactions between the solvent, P3HT, and P(NDI2OD-T2) and explore how they induce morphological changes in P3HT:P(NDI2OD-T2) BHJs in the solid state. The polymer-polymer and polymer-solvent affinities in the cosolution are governed by interaction parameters, χ’s . We demonstrate that the interplay between the three χ’s, i.e., χP3HT-P(NDI2OD-T2), χP3HT-solvent and χP(NDI2OD-T2)-solvent, can induce different polymer-polymer affinities in solution prior to spin coating, which can subsequently affect the final film morphology. In order to quantify the solvent-polymer and polymer-polymer interactions in question, we constructed a map that is reminiscent of a qualitative phase diagram in Figure 1. We plot the square of the variance of the Hildebrand solubility parameters for P3HT–solvent and P(NDI2OD-T2)– solvent, for the three solvents under study, i.e., xylene, chlorobenzene and ortho-dichlorobenzene, as a function of the solvent’s Hildebrand solubility parameter, δsolv. The square of the variance of the solubility parameters is proportional to the enthalpic component of the polymer-solvent interaction parameter, χH, that is typically added to the entropic term, χS, to yield the interaction parameter, χ. ௏

Specifically, χ is given by ߯ = ோ் ሺߜଵ − ߜଶ ሻଶ + 0.34, where δi is the Hildebrand parameter for each ACS Paragon Plus Environment

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species and V the molar volume of the solvent.30-32 According to Flory-Huggins theory, complete 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solvent-polymer miscibility is achieved for χ < 0.5; miscibility will thus take place when the solubility parameters of the pair are similar. We estimated the solubility parameters of P3HT and P(NDI2OD-T2) by assessing the polymers’ solubility in numerous solvents with known solubility parameters. Given our empirical observation that P3HT dissolves very well in chlorobenzene, CB, while P(NDI2OD-T2) dissolves best in ortho-dichlorobenzene, DCB, we approximated δP3HT ≈ δCB = 19.4 MPa0.5 and δPNDI ≈ δDCB = 20.5 MPa0.5.30 The interaction parameter between P3HT and P(NDI2OD-T2) can thus be estimated by the variance between δP3HT and δPNDI; this quantity is depicted in Figure 1 by the solid line. This line serves as reference for estimating the relative interactions of the components of the ternary system of P3HT:P(NDI2OD-T2):solvent. Our map in Figure 1 suggests that favorable interactions exist between the polymers and CB as well as with DCB; these are thus good solvents for both P3HT and P(NDI2OD-T2). Xylene, XY, on the other hand, appears to dissolve P3HT well, but not P(NDI2ODT2). Importantly, when P3HT and P(NDI2OD-T2) are codissolved in XY, P(NDI2OD-T2) appears to be more miscible with P3HT than with xylene. It follows that P(NDI2OD-T2) should preferentially interact with P3HT rather than remain uniformly dissolved in the XY. It should be noted here, that despite the fact that we examined the polymer-polymer affinity in three solvents, we are actually only investigating two different cases: the “good solvent” case, in which polymer-solvent interactions are more favorable than polymer-polymer interactions when the polymers are dissolved in CB and DCB; and the “bad solvent” case, in which polymer-polymer interactions are favored over P(NDI2OD-T2)-XY interactions. We hypothesize that the reduced miscibility between P(NDI2OD-T2) and XY, or, accordingly, the increased miscibility between the two polymers when XY is used as the casting solvent, can drive changes in the solid-state morphology of the active layer. Grazing-incidence wide-angle x-ray scattering on the resulting thin films of the polymer blends confirms our hypothesis. Finally, the performance of OPVs is also correlated with the solvent-induced morphological characteristics.


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EXPERIMENTAL P3HT (Mw = 46 kg/mol, PDI = 2, regioregularity 96%) was purchased from Merck Chemicals Ltd. P(NDI2OD-T2) (Mw = 250kg/mol, PDI ~ 4.5) was provided by Polyera Corporation. The two polymers were codissolved at equimolar ratio to form 2 wt% solutions in o-dichlorobenzene (DCB), chlorobenzene (CB) and xylene (XY). 200-nm thick films were spun cast from the three solutions onto titania-covered ITO substrates, to emulate the bulk-heterojunction active layers of the corresponding inverted OPVs. The 30-nm thick titania (TiOx) layer was prepared by spin coating a 1 wt% solution of titanium isopropoxide (Ti[OCH](CH3)2]4, Aldrich 99.999%) in isopropyl alcohol onto pre-patterned ITO substrates, followed by hydrolysis of the precursor at room temperature for 1 h and annealing at 170 oC for 10 min.33,

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We heated the polymer cosolutions at 70°C to ensure

complete dissolution; the cosolutions were then cooled to room temperature and stirred overnight. Given that P(NDI2OD-T2) exhibits limited solubility in XY, cosolutions in XY were subjected to additional stirring at 50°C for at least one hour prior to spin coating in order to form homogeneous films. We chose to fabricate inverted OPVs to exploit the air stability of P(NDI2OD-T2). Gold served as anodes due to its high work function. Inverted OPVs were prepared by thermal evaporation of 100-nm thick gold on the P3HT:P(NDI2OD-T2)/TiOx/ITO stack, through a shadow mask, to define an active device area of 0.18 cm2. All active layers were annealed at 170 oC for 1 min prior to gold evaporation. The same annealing procedure was applied to the thin films that were used for optical and structural characterization. Current density–voltage (J-V) characteristics under AM 1.5G 100mW/cm2 illumination were collected 2 days after device fabrication35 using a Keithley 2400 source measurement unit. Before testing, each device was illuminated for 15 min to fill shallow electron traps in the TiOx electron transport layers.33 All the processing steps and testing of the devices took place in air. In order to characterize the structure of the P3HT:P(NDI2OD-T2) active layers, we carried out grazing-incidence wide-angle x-ray scattering (GIWAXS) on beamline 11-3 at the Stanford


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Synchrotron Radiation Lightsource. The energy of the x-rays was 12.7 eV and the angle of incidence 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

was set at 0.11o. The diffracted intensity was recorded with a 2-D detector and was normalized by the incident photon flux and the acquisition time. The diffracted intensity was first backgroundsubtracted before it was radially averaged to derive traces of one-dimensional intensities as a function of the scattering vector, q. Finally, UV-vis-NIR absorption spectra of the constituent polymer solutions, as well as of the cosolutions, and of the corresponding films were acquired using an Agilent 8453 Spectrophotometer. We utilized a very thin home-made liquid cell formed by two optical microscopy glass slides to decrease the optical path through the solutions under study. Data were corrected for solvent and cell absorption. The films were spun from the same cosolutions onto titania-covered ITO substrates, similar to the procedure followed for device fabrication and GIWAXS, while a titania-covered ITO substrate was used as reference for background subtraction.

RESULTS AND DISCUSSION In order to provide experimental evidence on the validity of the miscibility map presented in Figure 1, we performed UV-vis absorption measurements, both of P3HT:P(NDI2OD-T2) solutions and of films cast from the three solvents. These measurements were made on solutions having the same concentration, namely 2 wt%, and at the same temperature, namely room temperature, to those used to form films. The corresponding spectra are presented in Figure 2. The absorption spectra of the polymer blend dissolved in CB and DCB (Figure 2, left) comprise 3 peaks; a broad absorption associated with the π-π* absorption band of P3HT36 located at 460 nm and those characteristic of the π-π* transition and the charge transfer band of P(NDI2OD-T2) at around 400 and 700 nm, respectively.18 The position of P3HT’s absorption confirms that it is well-dissolved in CB and DCB, in agreement with the miscibility map in Figure 1. In the spectrum acquired on the XY cosolution, this peak is red-shifted and we observe the emergence of three distinct vibronic shoulders, at 525, 565, and 615 nm, respectively. The presence of these vibronic structures in the UV-vis-NIR


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spectrum is indicative of the formation of loose P3HT aggregates.36-38 We also observe a red shift of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the P(NDI2OD-T2) absorption peak from 680 nm in DCB, to 702 nm and 710 nm in CB and XY, respectively, indicating that P(NDI2OD-T2) also tends to aggregate in this cosolution as well.20, 39 It is interesting to note that the absorption of the cosolution in XY is similar to those of the three films presented in the right part of Figure 2, which suggests that P3HT:P(NDI2OD-T2) aggregates readily in the XY cosolution. These P3HT:P(NDI2OD-T2) aggregates do not originate from a reduced miscibility of P3HT in XY but from a preferential affinity of the two polymers in this specific solvent, driven by the reduced miscibility of P(NDI2OD-T2) in XY. While the affinity of P3HT and P(NDI2OD-T2) in the XY cosolution cannot be directly detected in the UV-vis data, the differences in the absorption features of the two polymers in the cosolution described above support our hypothesis that altering the polymer-solvent interactions can induce different polymer-polymer affinities in solution. We were not, however, able to further quantify these aggregates in the cosolutions via Dynamic Light Scattering because our samples absorb in the visible and are thus opaque to the incident laser beam (632 nm). In order to investigate the solid-state morphology of P3HT:P(NDI2OD-T2) films, we performed GIWAXS experiments; the two-dimensional diffraction images are presented in Figure 3a. In all x-ray scattering patterns, we observe a series of periodically spaced reflections near the meridian (qxy = 0); these reflections represent the (h00) reflections of P3HT. Referring to Rivnay et al.,40 reflections associated with P(NDI2OD-T2) can be seen along the in-plane direction (qz = 0) in the two-dimensional x-ray diffraction patterns. To quantify, we have plotted the corresponding inplane (qz = 0) and near out-of-plane (qxy ≈ 0) traces in Figures 3b and 3c, respectively. Reflections associated with P(NDI2OD-T2) are indexed according to the study of Rivnay et al.40 The (100) reflection of P(NDI2OD-T2) is evident in the in-plane traces at 0.25 Å-1, followed by higher-order (200) and (300) reflections at 0.5 Å-1 and 0.75 Å-1, respectively. The (001) reflection of P(NDI2ODT2), associated with the polymer backbone periodicity, is also apparent in the in-plane traces at 0.47 Å-1. While the (100) and (200) reflections of P(NDI2OD-T2) are observed in the out-of-plane traces, their intensities are lower than those in the in-plane direction for all three films (see insets of Figures


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3b and 3c). Additionally, the characteristic reflection associated with interchain π-π stacking, (010), 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of P(NDI2OD-T2) only appears in the out-of-plane traces, at around 1.63 Å-1. Collectively, these observations indicate that P(NDI2OD-T2) is preferentially oriented face-on, with its π stacking direction normal to the substrate, consistent with Rivnay et al.40, although a small population of edge-on oriented crystals exist as well, as evinced by the presence of weak (h00) reflections in the out-of-plane traces. On the other hand, the interchain π-π stacking reflection of P3HT is only apparent in the in-plane traces, at 1.66 Å-1. Concurrently, the (h00) family of P3HT reflections are largely only observed in the out-of-plane traces, indicating that P3HT crystallites – unlike P(NDI2OD-T2) – are substantially more oriented in an edge-on fashion. We plot in Figure 4 the low-q range of the radially-averaged diffraction patterns collected on these same films. The same intensity axis has been used for all three graphs for ease of comparison. Background scattering from the TiOx/ITO substrate has been subtracted to allow for quantitative comparison across the three samples. These radially-averaged scans allow us – to first order – account for the diffracted intensities of crystallites oriented in most directions, recognizing that this analysis ignores any geometric corrections. In the low q-range, the (100) reflections of P(NDI2OD-T2) and P3HT appear at 0.26 and 0.39 Å-1, respectively. These reflections correspond to d100 spacings of 24.2 ± 0.6 Å for P(NDI2ODT2) and 16.1 ± 0.4 Å for P3HT, and the d-spacings are independent of the casting solvent used. This invariance in the lattice spacing plus the similarity in the 2D GIWAXS images indicate that the unit cells of the polymers are not affected by differences in the affinities in the cosolution. Instead, they are strong evidence that the solvents don’t swell the unit cells, neither are polymers forming cocrystals. Interestingly, we observe varying extents of diffracted intensity between the (100) reflections of the two polymers, at around 0.32 - 0.36 Å-1, depending on the solvent from which the films were cast. This extra scattering is reminiscent of the diffuse halo that is present in the diffraction patterns of amorphous polymers and possibly points to an enhanced presence of amorphous P3HT and P(NDI2OD-T2) regions within the films. The presence of the halo suggests the existence of a distribution of poorly correlated d-spacings that are close to the d100 spacings of


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the two polymers.41 We speculate that the appearance of this extra intensity results from a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

perturbation of the crystalline structure along the (100) direction and of possible intermixing of P3HT and P(NDI2OD-T2), giving rise to a broad distribution of d-spacings intermediate of the (100) reflections of P3HT and P(NDI2OD-T2). The morphological picture that arises for these polymer:polymer BHJs is similar to the three-phase model proposed for the polymer:fullerene BHJs;42-45 phase separation between the two polymers is moderate, resulting in two rather pure P3HT and P(NDI2OD-T2) phases that coexist with a third phase where the polymers are molecularly mixed. Information concerning the crystallite coherence length and the relative crystallinity of P3HT and P(NDI2OD-T2) in the three films can be acquired by fitting the low q-region of the backgroundsubtracted, radially averaged intensity patterns to three Gaussian peaks, each corresponding to the (100) reflections of P3HT and P(NDI2OD-T2), and the diffuse halo. For each film, the best fit and the corresponding Gaussians are included in Figure 4. For the accurate determination of the characteristics of the (100) reflections, it is imperative to take into account the presence of extraneous intensity from the diffuse halo, since the two peaks associated with the (100) reflections of P3HT and P(NDI2OD-T2) appear to be partially superimposed on it. The full peak widths at half the maximum intensities of the Gaussians associated with the background-subtracted, radiallyaveraged (100) reflection for both polymers, β100, were used to estimate the coherence length along the (100) planes, D100, for P3HT and P(NDI2OD-T2), following Scherrer’s equation: ‫ܦ‬௛௞௟ = ఉ

଴.ଽఒ

೓ೖ೗ ௖௢௦ఏಳ

(1)

Herein, λ is the x-ray wavelength, θB the Bragg angle, and β100 is estimated in radians. The result of this analysis is presented in Figure 5. P(NDI2OD-T2) forms crystallites with a coherence length of about 12 nm; this length appears to be invariant with the solvent from which the blends were cast. On the other hand, we see significant variations in P3HT crystallite coherence length depending on the solvent from which the blends were cast. More specifically, D100 for P3HT is 24.9 nm for films cast from DCB, and 23.6 nm for films cast from CB. It further decreases to 18.3 nm when the film is cast from XY. Such a significant decrease in the coherence length is not observed in case of pure ACS Paragon Plus Environment

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P3HT films cast from the same solvents, under the same conditions (Supporting Information, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figures S1-S3). We can correlate this decrease in crystal coherence length to the polymers-solvent interactions described in Fig. 1, as we believe that the affinity P(NDI2OD-T2) exhibits towards P3HT relative to XY obstructs the aggregation of P3HT in solution. The absorption spectrum in Figure 2 indicates that P3HT aggregates do form in solution; the formation of crystallites with a smaller coherence length in the solid state suggests that the growth of perfectly ordered domains must be hindered by preferential interactions with P(NDI2OD-T2). Separately, we note that the boiling temperature of XY is 144oC,30 higher than that of CB (Tb = 131oC) but lower than that of DCB (Tb = 174oC).30 Contrary to what had been proposed in earlier studies,24, 25 we do not believe the smaller P3HT coherence length can be attributed to differences in solvent evaporation rates as DCB is the slowest solvent to evaporate given its high boiling point relative to the other solvents, with all other processing parameters being equal. Moreover, solvent evaporation should be similar for pure and blend films, leading to similar trends of the coherence lengths of the two polymers, which is not the case herein. Rather, these observations are consistent with our initial hypothesis that reduced miscibility of P(NDI2OD-T2) and XY in cosolution can drive structural changes in the cast films and it further stresses the importance of polymers-solvent interactions in influencing the final morphology of the active layer blends. Apart from the peak width of the (100) reflections, we can examine the integrated (in polar angle) peak area to derive qualitative information on the effect of solvent on the relative crystallinity of P3HT and P(NDI2OD-T2) in the three films. Since the texture of the films is not significantly affected by solvent choice, the relative constituent crystallinities is proportional to the integrated intensities of the corresponding (100) peaks in Figure 4.46,47 The integrated (100) peak intensity suggests that P(NDI2OD-T2) is most crystalline in the film cast from CB; it is moderately crystalline in the film cast from DCB, and least crystalline in the film case from XY. This is also evident by comparing the area beneath the fitted Gaussian peaks corresponding to the (100) reflection of P(NDI2OD-T2). Given that the P(NDI2OD-T2) crystallites have comparable coherence lengths in the three films, we conclude that less crystallites are formed in the film cast from XY with respect to


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those formed by spin-coating the cosolutions from good solvents of DCB and CB. It thus appears 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

that the affinity between the two polymers in the XY cosolution not only promotes the formation of P3HT crystallites with a smaller coherence length, it also suppresses the crystallization of P(NDI2OD-T2). Interestingly, such a comparison of the relative integrated intensity of the P3HT (100) reflection suggests that P3HT is most crystalline when the film is cast from CB, then XY, and finally DCB (Fig. 4). We now return to the additional intensity between the (100) reflections of P3HT and P(NDI2OD-T2) in the x-ray diffraction traces in Figure 4. The area under the Gaussian fit to this extra intensity is the largest in the x-ray trace acquired on the XY-cast film. Consistent with the morphological picture that is emerging based on our discussion thus far, we speculate that this observation is due to better intermixing between the two constituent polymers when XY is used to cast the films. With enhanced mixing, we observed hindered crystallization of both P3HT and P(NDI2OD-T2),as well as a smoother surface of the XY-cast film i.e., 8.6 nm as opposed to 11.1 nm in case of the films cast from CB, both estimated from 10×10µm AFM images. In order to complete our study, we fabricated inverted solar cells having P3HT:P(NDI2ODT2) active layers cast from the three different solvents using the same casting and annealing conditions as those used to create the thin films used in our x-ray and optical absorbance studies. Representative current density–voltage (J-V) curves are presented in Fig. 6. The mean values and standard deviations were derived by testing 15 devices at each condition. Not surprisingly, the solidstate morphology, reminiscent of the polymer-polymer affinity in the cosolution, has a severe effect on device performance. When good solvents CB and DCB are used to cast the active layers, devices comprising these films perform poorly. The fill factor is very low (27±2% for devices whose active layers are derived from both solvents) and the short-circuit current density, Jsc, is only 0.23±0.09 mA/cm2 for devices with active layers spun from DCB and 0.26±0.04 mA/cm2 for those spun from CB, resulting in power conversion efficiencies of 0.012±0.006% and 0.025±0.008%, respectively. When XY is used to cast the active layer, the performance of such devices is markedly improved in comparison. The Jsc increase by an order of magnitude to 2.30±0.06 mA/cm2 compared to the other


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devices in our study, resulting in a corresponding increase in efficiency to 0.38±0.02%. The fill 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

factor and open-circuit voltage, Voc, also improve (40±3% and 0.46±0.01 V, respectively). While the characteristics of devices having XY-cast active layers remain poor compared to the best polymer-polymer devices today, our comparison between devices having active layers cast from different solvents have nonetheless shed light on the processing-structure-function relationships that govern this polymer pair. The improved device performance recorded with devices having XY-cast active layers is strongly correlated with differences in the solid-state morphology that is induced by casting the film from a XY cosolution. As revealed by GIXD, the affinity of P3HT and P(NDI2OD-T2) when codissolved in XY results in films with less coherent P3HT crystallites and enhanced intermixing between the donor and acceptor polymers. Yet, these morphological characteristics prove not to be detrimental for device performance. Contrary to common belief that well-defined donor/acceptor phase separation is necessary for enhanced photovoltaic performance, our results point to intermixing of the two phases as being favorable. This observation is in-line with recent reports on the role of intermixing in polymer:fullerene BHJs. Bartelt et al.48 suggest that, in addition to percolation between interconnected aggregates, the presence of an intermixed polymer:fullerene phase also helps enhance photovoltaic performance by increasing exciton quenching and charge carrier extraction.48 Similar observations were made by Westacott et al.49 in which transient absorption studies indicate that intermixing between the donor and acceptor phases results in higher charge generation. The enhanced performance obtained for devices containing XY-cast active layers is consistent with the picture put forth by these studies. To the best of our knowledge, this is the first time intermixing is discussed in the context of polymer:polymer BHJ OPVs. Our study shows that polymer(s)-solvent affinities can be a powerful tool for tailoring intermixing and provides experimental support on the hypothesis of Schubert et al.20 that judicious selection of the solvent used for casting active layers can dramatically improve the extent of intermixing between the donor and acceptor semiconductors.


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CONCLUSIONS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In this report, we have explored the morphological development of P3HT:P(NDI2OD-T2) blends cast from different solvents, based on polymer-polymer and polymer-solvents interactions in the corresponding cosolutions. An integrated study of the relationships between polymer-polymer affinity in solution, the resulting solid-state structure and device performance is presented. The qualitative interactions map of a P3HT–P(NDI2OD-T2)–solvent cosolution suggests the affinity of the two polymers when the polymer-solvent interactions are not favorable. As a consequence, crystallization of the two polymers is retarded in the cast film. In particular, our results point to more intermixing between P3HT and P(NDI2OD-T2) when XY is used for casting. We believe that these morphological characteristics are responsible for the improved performance of the XY-cast devices. Our results are in line with recent reports on the role of donor:acceptor intermixed phase on photovoltaic performance.

SUPPORTING INFORMATION GIXD data and coherence lengths for pure P3HT and P(NDI2OD-T2) films. This information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGMENT We acknowledge funding from the Photovoltaics Program of ONR (N00014-11-10328), the MRSEC program at the NSF through Princeton Center for Complex Materials (DMR-0819860), and the Solar Initiative at the NSF (DMR-10135217). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC0276SF00515. MFT thanks the Center for Advanced Molecular Photovoltaics (CAMP) (Award No KUS-C1-015-21), made possible by King Abdullah University of Science and Technology (KAUST). ACS Paragon Plus Environment

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LIST OF FIGURES

χP3HT-solv

(δsolv−δpoly)

2

χPNDI-solv χP3HT-PNDI

XY

CB

DCB

miscibility

18.0 18.5 19.0 19.5 20.0 20.5 1/2

δsolv (MPa ) Figure 1. Miscibility map that illustrates the interactions between P3HT, P(NDI2OD-T2) and the three solvents; xylene, chlorobenzene and dichlorobenzene. The horizontal line represents the interaction between P3HT and P(NDI2OD-T2). Below this line, polymer-solvent interactions are more favorable than polymer-polymer interactions, while above it they are less favorable. Note that due to space limitations P(NDI2OD-T2) is abbreviated to PNDI in all Figures.

1.2

Films XY CB DCB

Solutions

1.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8 0.6 0.4 0.2 0.0

400

600

800

Wavelength (nm)

400

600

800

Wavelength (nm)

Figure 2. Normalized absorption spectra of solutions and films of the P3HT: P(NDI2OD-T2) blend, for the three solvents studied herein.


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(a)

6

PNDI (400)

1

10

0

0.2

0.3

0.4

PNDI (001)

10

PNDI (200)

2

P3HT (100)

10

PNDI (100)

PNDI (001)'

PNDI (300)

10

PNDI (100)

4

P3HT (100) PNDI (001) PNDI (200)

Intensity (a.u.)

10

0.5

P3HT (010)

2

10

DCB CB XY

0

10

0.5

1.0

qxy (

Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

1.5

2.0

)

(b)


17


8

10

P3HT (200)

4

10

PNDI (100)

200

PNDI (200)

P3HT (100)

6

PNDI (100)

Intensity (a.u.)

400

10

P3HT (100)

600

0 0.2

0.3

P3HT (300)

0.4

0.5

PNDI (010)

DCB

2

CB

10

XY

0.5

1.0

Å -1

qz (

1.5

2.0

)

(c)

Figure 3. (a) 2D GIXD images collected at grazing incidence (α = 0.11o) for the P3HT: P(NDI2ODT2) blends cast from dichlorobenzene (DCB), chlorobenzene (CB) and xylene (XY), and the respective in-plane (b) and out-of-plane (c) traces. These traces have been displaced along the intensity axis for clarity. In the insets we present the corresponding (in-plane or out-of-plane) traces for the low-q range, obtained after background subtraction. These traces are presented as is in order to allow for direct comparison between the samples. Note that due to space limitations P(NDI2ODT2) is abbreviated to PNDI.

Intensity (a.u.)

100 XY

CB

80

DCB

60

experimental data fit crystalline PNDI crystalline P3HT amorphous halo

40 20 0 0.2

0.3

0.4

0.5

0.2

0.3

0.4

ÅÅÅÅ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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q(

)

-1

0.5

0.2

0.3

0.4

0.5

0.6


18

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Figure 4. The low q-range of the background-subtracted, radially-averaged I vs q diffraction patterns of the P3HT:P(NDI2OD-T2) films cast from xylene, chlorobenzene and dichlorobenzene, and the corresponding fits to the sum of three Gaussian peaks. Two of them represent the (100) reflections of P3HT and P(NDI2OD-T2) respectively while the third represents an effective diffuse halo that results from the intermixing of the two polymers. Note that the intensity axis is the same in all three graphs.

Coherence Length (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

28 DCB

CB

24 XY

20 P3HT P(NDI2OD-T2)

16 12 8 18.0

18.5

19.0

19.5

20.0

20.5

1/2

δH (MPa ) Figure 5. Crystallite coherence length of P3HT (triangles) and P(NDI2OD-T2) (circles) for the P3HT:P(NDI2OD-T2) films cast from xylene, chlorobenzene and dichlorobenzene. The coherence length was estimated from the full width at half maximum of the (100) reflections of P3HT and P(NDI2OD-T2), respectively, according to Scherrer’s analysis.


19


3.0 2.5 2

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

2.0 1.5 1.0 0.5

XY CB

0.0

DCB

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1

V (V) Figure 6. J-V curves of BHJ P3HT: P(NDI2OD-T2) inverted OPVs, measured under 100 mW/cm2 illumination. The active layers were spun cast from xylene, chlorobenzene and dichlorobenzene.


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For Table of Contents use only:


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Tuning the Morphology of All-Polymer OPVs ... - ACS Publications

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