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Enhancing Indacenodithiophene Acceptor Crystallinity via Substituent Manipulation Increases Organic Solar Cell Efficiency Thomas J Aldrich, Steven M. Swick, Ferdinand S. Melkonyan, and Tobin J. Marks Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04616 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017
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Chemistry of Materials
Enhancing Indacenodithiophene Acceptor Crystallinity via Substituent Manipulation Increases Organic Solar Cell Efficiency Thomas J. Aldrich,† Steven M. Swick,† Ferdinand S. Melkonyan,*,†,‡ Tobin J. Marks*,†,‡ †Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois, 60208, United States ‡Department of Materials Science and Engineering and Argonne Northwestern Solar Energy Research Center (ANSER), Northwestern University, 2145 Sheridan Road, Evanston, Illinois, 60208, United States
Supporting Information Placeholder ABSTRACT: The post-fullerene indacenodithiophene acceptor ITIC is a highly effective n-type component of high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) for reasons that are not well-understood. Here, the impact of the ITIC alkyl substituent architecture on PSC active layer film morphology, charge transport, and photovoltaic (PV) performance is investigated with the donor polymers PBDB-T and PBDB-TF. On progressing from n-propyl to n-hexyl to n-nonyl ITIC substituents, PSC power conversion efficiency (PCE) increases from 13% have recently been reported for PSCs containing appropriate ptype donor polymers and n-type small molecule acceptors in bulkheterojunction (BHJ) photoactive layers.6 These impressive advances reflect intense, largely empirical iteration of donor semiconductor design coupled new with active layer processing techniques, and ultimately demonstrate the potential of PSCs for the practical generation of renewable energy.2,3 While fullerene derivatives have historically been the most common and effective n-type acceptors for PSCs, their weak optical absorption, high cost, morphological instability, and limited synthetic tunability present major challenges to their large-scale deployment.7 Recently, indacenodithiophene-based non-fullerene acceptors, such as ITIC, were shown to rival fullerenes in PSC performance (Figure 1a).8,9 PSCs utilizing various ITIC derivatives are attractive due to their high PCEs, which are enabled by the unique and attractive properties of ITICs. Among other attributes, ITICs generally exhibit high oscillator strength between 550–800 nm, high carrier mobility, rapid charge transfer to poly-
meric donors, good solubility in common solvents, and good environmental stability.8–12
a
Acceptor Molecules
Donor Polymers
R
Z NC S
S
S
R1
O
O
S
S S
n
S
CN R
R = n-C 3H7 n-C 6H13 n-C 9H19
R1 S
S
O R
S NC
CN
S
R O
R1 S
ITIC-C3 ITIC-C6 ITIC-C9
R1
Z
Z=H Z=F
PBDB-T PBDB-TF
R1 = 2-ethylhexyl
Figure 1. Chemical structures and properties of the ITIC-CX acceptors and donors reported here. (a) Chemical structures of the acceptors and donors. (b) Thin film optical absorption spectra of neat materials. (c) Frontier MO energetics estimated from cyclic voltammetry. It is well-established that the solubilizing alkyl substituents of organic semiconducting donors profoundly impact condensed phase properties such as crystallinity, charge transport, BHJ blend morphology, and donor molecular orientation at the donor/acceptor interface in such blends, which is critical for maximum PCE.2,13–20 Interestingly, despite their great potential importance for BHJ properties, organic semiconducting acceptor alkyl substituent effects have been sparsely investigated. ITIC derivatives with diverse fused aromatic central cores,21–28 electron-deficient end groups,6,28–33 and aromatic side groups connected at the quaternary carbon center have recently been synthe-
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sized,32–35 however, to the best of our knowledge, systematic investigation of ITIC alkyl substituent effects on the chemical, physical, and electronic properties of the neat materials and their high-PCE BHJ blends are virtually unexplored.36 Here we compare and contrast the structural, physical, photophysical, and BHJ photovoltaic properties of an ITIC series, ITIC-CX (Figure 1a), in which the n-alkyl substituent dimensions are incremented from n-propyl to n-hexyl to n-nonyl. The ITICCX acceptors and their blends with two selected donor polymers (vide infra) are characterized by optical absorption spectroscopy, cyclic voltammetry (CV), differential scanning calorimetry (DSC), atomic force microscopy (AFM), X-ray diffraction (XRD), grazing incidence wide-angle X-ray scattering (GIWAXS), space-charge limited current (SCLC) mobility measurements, photovoltaic response, and PV response as a function of light intensity. The PSC performance of the ITIC-CX series is evaluated in blends with the large-bandgap, high-efficiency donor polymers poly{[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2b:4,5-b']dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3diyl]]} (PBDB-T, Figure 1a),10,21,29,32 and the fluorinated analog having higher crystallinity and a lower-lying HOMO, poly{[4,8bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5b']dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-1,3diyl]]} (PBDB-TF, Figure 1a).37–41 The results show that ITICC9 achieves higher PSC performance than ITIC-C6 with both PBDB-T and PBDB-TF due to increased structural order and the superior charge transport properties of the BHJ blends. The ITIC-C3 blends have poor BHJ morphologies and minimal PSC performance. The synthesis, purification, and characterization of the ITICCX acceptors and donor polymer PBDB-TF are summarized in the SI and in Schemes S1–S3. The ITIC-CX are soluble in chloroform and chlorobenzene (PhCl), although ITIC-C3 exhibits reduced solubility unless heated. The number-average molecular masses of PBDB-T and PBDB-TF are similar at 17.4 and 17.9 kg/mol, respectively, with dispersities of 2.55 and 2.73, respectively, as measured by high-temperature gel permeation chromatography (GPC). The optical absorption properties of the ITICCX and donor polymers were studied as thin films and dilute solutions (Figures 1b and S1). The ITIC-CX extinction coefficients in PhCl are nearly identical ~1.7 × 105 M–1cm–1 (Table S1), and the optical bandgaps calculated from the thin-film absorption onsets for ITIC-C6 and ITIC-C9 are identical at 1.59 eV, while that of ITIC-C3 is slightly lower, 1.57 eV. The two donor polymers exhibit similar optical bandgaps ~1.85 eV and absorbance spectra (Figure 1b). CV oxidation potentials were next employed to estimate donor polymer and ITIC-CX HOMO energies, while the ITIC-CX LUMO energies were estimated from the CV reduction potentials (Figures 1c and S2). Fluorinated donor polymer, PBDB-TF, exhibits a 0.14 eV lower HOMO energy versus that of PBDB-T, which is expected to afford higher PSC open-circuit voltages (Vocs). Interestingly, the ITIC-CX series exhibits a slight upward trend in LUMO energy with increasing X dimensions (Table S2). Finally, the thermal properties of the donor polymers and ITIC-CX acceptors were studied by DSC. The donor polymers and ITIC-C3 exhibit no obvious thermal transitions up to 300 °C (Figure S3). Interestingly, ITIC-C6 and ITICC9 exhibit cold crystallization transitions in the first DSC heating cycle, at 195 °C and 162 °C, respectively.24,42 PSC devices were next fabricated with inverted cell architectures, ITO/ZnO/active layer/MoO3/Ag with the active layers consisting of donor polymer:ITIC-CX blends. Active layers were spun-cast onto the ZnO layer from PhCl solutions containing the donor polymer and ITIC-CX (1:1 mass ratio) and 1 vol% 1,8-
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diiodooctane (DIO). The coated substrates were then annealed at 160 °C for 10 min. and finally MoO3 and Ag were vacuumdeposited (see SI for details).10 For comparison, devices annealed at 190 °C and without the thermal annealing step were also fabricated and found to exhibit similar or reduced PCEs; consequently further discussion will focus on the devices thermally annealed at 160 °C (Table S5).
Table 1. PV Parameters for ITIC-CX Based PSCsa Blend PBDB-T:ITIC-C3 PBDB-TF:ITIC-C3 PBDB-T:ITIC-C6
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.31
0.7, 0.5b
29.1
