Absence of Mixed Phase in Organic Photovoltaic Active Layers

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C: Physical Processes in Nanomaterials and Nanostructures

Absence of Mixed Phase in Organic Photovoltaic Active Layers Facilitates Use of Green Solvent Processing Stefan D. Oosterhout, Victoria Savikhin, Mark A. Burgers, Junxiang Zhang, Yadong Zhang, Seth R. Marder, Guillermo C. Bazan, and Michael F. Toney J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01600 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

Absence of Mixed Phase in Organic Photovoltaic Active Layers Facilitates Use of Green Solvent Processing

Stefan D. Oosterhout,1 Victoria Savikhin,1,4 Mark A. Burgers,3 Junxiang Zhang,2 Yadong Zhang,2 Seth R. Marder,2 Guillermo C. Bazan,3 Michael F. Toney1* 1

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Building 137, Menlo Park, California 94025,

United States 2

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia

Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States 3

Department of Materials and Chemistry & Biochemistry, University of California, Santa Barbara,

California 93106, United States 4

Electrical Engineering Department, Stanford University, 350 Serra Mall, Stanford, California 94305,

United States *corresponding author: [email protected]

1 ACS Paragon Plus Environment

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Abstract We examine the morphology of a donor molecule:fullerene organic photovoltaic (OPV) active layer, processed from the ”green solvent” 2-methyl-THF (m-THF), as well as the conformation of the two OPV constituent components in the casting solution. We observe that the small molecule (X2) has a weak association with itself in chloroform solvent, while it does not self-associate in m-THF. Despite this difference, the morphology of the final processed films are extraordinarily similar: there is negligible molecularly mixed phase in the layer, and the domain sizes of the pure X2 and pure fullerene are 15-20 nm. We attribute this similarity between the final films to the strong aggregation behavior of X2 upon drying; changes in solvent or solvent additive have therefore only a minor effect on the final bulk heterojunction (BHJ) morphology. This contrasts with the majority of other OPV molecular donor semiconductor systems, which need careful tuning of solvent and/or solvent additive to achieve the optimal morphology and photovoltaic performance. We argue that the absence of a mixed phase is a result of the strong self-aggregation behavior of X2, and a key property of this material combination that makes it robust to a change in processing solvent.


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Introduction Currently, there is a need for an evolution of processing conditions for organic photovoltaic (OPV) devices towards those that are more sustainable. Specifically, many if not most OPV bulk heterojunctions (BHJ’s) to date are solution processed using chlorinated organic solvents, such as chloroform or chlorobenzene.1–7 Solution processing has many benefits compared to other deposition methods such as vacuum deposition: it allows for high-throughput roll-to-roll processing, which can make organic photovoltaics an inexpensive alternative for mass-produced solar cells. Furthermore, rollto-roll processed OPV are easily fabricated onto flexible substrates enabling new uses, such as building integrated solar cells. Sustainable mass production with minimal environmental impact, however, would be greatly facilitated by the use of less toxic processing agents and thus the use of non-chlorinated solvents. Recently, there have been reports of OPV BHJ systems that were successfully processed with more sustainable, “green” (non-halogenated) solvents or solvent mixtures.8 For example, anisol solvent can be used as processing solvent for BDTTT-S-TEG:fullerene active layers, where the solubilizing side chain on the fullerene is modified to enhance solubility in this solvent, resulting in photovoltaic devices with an efficiency of 4.5%.9 There are several reports of o-methylanisole used in polymer:fullerene10,11 as well as all-polymer11,12 active layers for OPV resulting in device efficiencies of 9.6% and 5.2% efficiency, respectively. Farahat et. al. reported 8.1% efficient devices based on a small molecule and fullerene, using a binary solvent (cyclopentyl methyl ether and toluene).13 Xu et. al. realized 12.8% efficiency using PBDTS-TDZ as polymer donor and ITIC as non-fullerene acceptor using o-xylene as processing solvent.14



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Figure 1. Chemical structure of X2. Here, we will focus our attention on the donor molecule X2:fullerene BHJ system,5,15,16 which can be processed from a more sustainable, green solvent: 2-methyl-tetrahydrofuran (m-THF).13,17 The chemical structure of X2 is displayed in Figure 1. The X2:fullerene OPV has a moderately high performance (power conversion efficiency or PCE of 6.5%)15 and benefits from significant previous characterization.5,17,15,18– 20,16

A previous publication demonstrated the ability to change the processing solvent from the fast-

drying chloroform to m-THF solvent, with only a minor penalty in photovoltaic performance (PCE of 5%).17 Currently it is unclear why the X2:PC61BM device efficiency is tolerant to a change in processing solvent, because in most OPV systems the final device performance can be sensitive to the exact processing conditions; choice of solvent, solvent additive, annealing temperature etc. For example, for the small molecule p-DTS(FBTTh2)2 the solvent additive 1,8-diiodooctane (DIO) is required to help the molecule crystallize during deposition, while film preparation without additive results into poor phase separation and low device performance.21 In contrast, the X2:fullerene system does not require the use of solvent additives, and X2 and fullerene have a good nanoscale phase separation when cast from the fast-drying chloroform solvent without an additive.18 Understanding why X2:PC61BM is tolerant to choice of solvent is instructive in understanding OPV systems, and could help pave the way to large-scale OPV solar cell processing by helping to provide design rules for new OPV molecules that are tolerant to changing solvents. When processing X2:fullerene from m-THF instead of chloroform, the electron acceptor phenyl-C61butyric acid methyl ester (PC61BM) is replaced by phenyl-C61-butyric acid octyl ester (PC61BC8) to ensure sufficient solubility of the fullerene. In a previous publication, the absorption profile of the completed 4 ACS Paragon Plus Environment

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active layer, hole mobility and surface topology as measured by atomic force microscopy were shown to be very similar for both solvents.17 Here, we provide insight into why this system is tolerant to solvent selection. We analyze the behavior of three X2:fullerene systems for a thorough comparison: X2:PC61BM from chloroform, X2:PC61BC8 from chloroform and X2:PC61BC8 from m-THF. We investigate the aggregation behavior of these systems by means of small-angle X-ray scattering, the degree of molecular mixing in the BHJ by using Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS), and the domain size of the BHJ nanophase separation using Resonant Soft-X-ray Scattering (R-SoXS). We find that the domain sizes are similar at 15-20 nm in all cases, and no system shows evidence of intimate X2/fullerene molecular mixing. We therefore speculate that the lack of molecular mixing is a result of X2-fullerene interactions, rather than a consequence of processing. We do however find differences in aggregation behavior of the X2 molecule in solution: in chloroform, small X2 aggregates are formed, while this is not the case for X2 in m-THF. Despite this difference, the domain sizes are similar for all systems. Because the evaporation rate of chloroform and m-THF is similar, we speculate that for X2 the domain size is dependent mainly on solvent evaporation rate due to the strong tendency for X2 to aggregate. Experimental Section Materials X2 was synthesized according to a literature procedure.18 PC61BM was purchased from Nano-C and used as received, PC61BC8 was purchased from Sigma-Aldrich and used as received (purity >99%). GIWAXS sample preparation: Silicon substrates were purchased from Silicon Quest Int’l (item 908-006) and cut to ~1x1 cm. Organic layers were spin-cast onto the substrate in a nitrogen-filled glovebox from a 20 mg/mL total solids concentration in chloroform, at a spin rate of 2000 rpm. The substrates were annealed for 10 minutes at 100 °C inside the glove box. The edges of the samples were removed to



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eliminate edge-effects in the GIWAXS experiment. The vol-% PC61BM in the layer was converted from the mass blend ratio using a density of 1.1 g/cm3 for X2 and 1.5 g/cm3 for PC61BM. Thickness measurements: the thickness of the layers was determined by analysis of the Kiessing fringes as measured in X-ray reflectivity measurements, measured on a PANalytical X’Pert diffractometer. The thicknesses obtained are summarized in Table S1 in the supporting information.18 GIWAXS measurements: GIWAXS was measured at the Stanford Synchrotron Radiation Lightsouce (SSRL) beamline 11-3 in a helium-filled chamber with an X-ray wavelength of 0.9752 Å, sample to detector distance of 30 cm at an incident angle of 0.20°. Spectra were recorded on a Rayonix MX225 X-ray detector, and processed using the Nika22 software package for Wavemetrics Igor, in combination with WAXStools.18 Further details can be found in our previous publication.18 R-SoXS sample preparation: BHJ layers were cast as described above onto PEDOT:PSS coated silicon substrates. The layers were then floated from the water-soluble PEDOT:PSS on the surface of demineralized water and picked up using 100 nm thick silicon nitride windows, purchased from Norcada (item #NX5150C). R-SoXS measurements: R-SoXS measurements were carried out at the Advanced Light Source beamline 11.0.1.2 in transmission mode. The samples were placed in a high vacuum, and the scattering intensity was captured on a Princeton Instrument PI-MTE detector cooled to -45 °C, at detector distances of 50 and 150 mm, for a total q range of 0.00230-40 vol%), a fraction of the X2 in the film is amorphous.


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Figure 3. Mixed phase in X2:PC61BC8. (a) Observed aggregated X2 volume-% as function of PC61BC8 content. (b) Observed aggregated PC61BC8 volume-% as function of PC61BC8 content. (c) Non-scattering (amorphous) volume fraction of the BHJ as function of PC61BC8 content.



GIWAXS – influence of fullerene on X2 packing

X2:PCBM (CF) X2:PCBC8 (CF) X2:PCBC8 (m-THF)

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X2:PCBM (CF) X2:PCBC8 (CF) X2:PCBC8 (m-THF)

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Figure 4. Summary of X2 GIWAXS data. Lamellar stacking distances and peak widths as function of fullerene content. The evolution of the peak position is displayed in (a) and the peak width in (b).

In addition to the variations in the scattering intensities, there are subtle changes in peak positions of the lamellar stacking peaks of X2 upon blending with fullerene. This has been observed before in X2:PC61BM cast from chloroform,15,18 as well as in PBTTT.25 Such differences indicate subtle changes in X2 packing as a result of blending with fullerene. We investigated this behavior for the X2:PC61BC8 system for both solvents and the X2:PC61BM system in CF as shown in Figure 4. We find that the lamellar stacking position of pure X2 is slightly different for the two fullerenes, although the origin of this remains poorly understood. More importantly, although not the exact same fullerene contents are used for each material combination for a 1:1 comparison, the observed trend in lamellar peak position versus fullerene content (Figure 4a) is the same: the peak position shifts to slightly smaller q (larger lamella spacing) as fullerene content is increased, quickly for low fullerene concentrations but then more slowly. 12 ACS Paragon Plus Environment

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The trend in lamellar peak width (Figure 4b) is slightly different for different systems; the peak width for X2:PC61BC8 cast from CF and m-THF remains constant up to a fullerene content of 60 vol-%, whereas the peak width for X2:PC61BM starts to increase at around 30 vol-%. A larger peak width indicates more molecular packing disorder in crystallites. While the origin of this behavior is unclear, we speculate that the smaller side chain on PC61BM disrupts the X2 lamellar stacking near the interface with the fullerene, while the longer side chain on PC61BC8 allows X2 aggregation to a higher degree near the interface. This is consistent with the observation that there is slightly more aggregated X2 in X2:PC61BC8 layers (Figure 3a). The location and width of the π-stacking peaks were also examined, and because all values found were within experimental error (Supporting Information Figure S5), no trends were found.



Domain size (R-SoXS)

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q (A ) Figure 5. Radially integrated R-SoXS profiles of X2:fullerene with different fullerene loading. (a) X2:PC61BM from chloroform, (b) X2:PC61BC8 from chloroform and (c) X2:PC61BC8 from m-THF.

It is important to quantify the size of the donor and acceptor domains to gain insight into the solar cell performance, since the nanoscale morphology has a pronounced effect on the device performance. This is especially true for systems that do not exhibit a mixed phase, because all generated excitons in the donor will need to diffuse to an interface with the acceptor to split into electron and hole.26 For excitons


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generated in a mixed phase, this charge separation readily takes place since many photogenerated excitons are close to a donor-acceptor interface.23,27 Resonant Soft X-ray Scattering (R-SoXS) has been used in order to determine the size of the domains. The scattering intensity originates from differences in refractive indices of the two components in the layer, which makes this technique suitable to determine the domain size.28 The photon energy is tuned to maximize the contrast (C) between the two materials where C = |nX2-nfullerene|2 and nX2 and nfullerene are the complex refractive indices of the respective materials. For X2:fullerene, the maximum contrast is at a photon energy of 284.2 eV, which is the energy used here.15 The radially integrated scattering intensity vs q obtained is displayed in Figure 5. The pure materials, X2 and PC61BC8, show some scattering intensity, especially at low q, which is most likely due to surface roughness and is visible due to non-negligible contrast between the neat materials and vacuum. For layers with a low fullerene content (

Absence of Mixed Phase in Organic Photovoltaic Active Layers

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