Communication Cite This: Chem. Mater. 2017, 29, 10294−10298
pubs.acs.org/cm
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*,†,‡ †
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 S Supporting Information *
T
rapid charge transfer to polymeric donors, good solubility in common solvents, and good environmental stability.8−13 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.2,14−22 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,23−30 electron-deficient end groups,6,30−35 and aromatic side groups connected at the quaternary carbon center have recently been synthesized;34−37 however, to the best of our knowledge, detailed 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.38 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 ITIC-CX acceptors and their blends with two selected donor polymers (vide inf ra) were 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[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2,5thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-T, Figure 1a)10,23,31,34 and the fluorinated analogue having higher crystallinity and a lower-lying HOMO, poly{[4,8-bis[5-(2ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF, Figure 1a).39−43 The results show that ITIC-C9 achieves higher PSC performance than ITIC-C6 with both PBDB-T and PBDB-
he understanding-based invention of new organic semiconducting materials is critical to the development of inexpensive, high-efficiency, and solution-processable polymer solar cells (PSCs).1−5 Photovoltaic (PV) PCEs > 13% have recently been reported for PSCs containing appropriate p-type 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 nonfullerene 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,
Figure 1. Chemical structures and properties of the ITIC-CX acceptors and polymer 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. © 2017 American Chemical Society
Received: November 2, 2017 Revised: November 22, 2017 Published: November 27, 2017 10294
DOI: 10.1021/acs.chemmater.7b04616 Chem. Mater. 2017, 29, 10294−10298
Communication
Chemistry of Materials
Table 1. PV Parameters for ITIC-CX-Based PSCsa
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 Supporting Information, Schemes S1−S3. The ITIC-CX acceptors 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 ITIC-CX 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−1 cm−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 at 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 HOMO energy 0.14 eV lower than that of PBDB-T, which is expected to afford higher PSC open-circuit voltage (Voc) values. 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 ITICC3 exhibit no obvious thermal transitions up to 300 °C (Figure S3). Interestingly, ITIC-C6 and ITIC-C9 exhibit cold crystallization transitions in the first DSC heating cycle at 195 and 162 °C, respectively.26,44 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-diiodooctane (DIO). The coated substrates were then annealed at 160 °C for 10 min, and finally, MoO3 and Ag were vacuum-deposited (see Supporting Information 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). Relevant PV parameters for the annealed devices are summarized in Table 1. Figure 2a shows the current density− voltage (J−V) characteristics of the champion cell for each blend under simulated AM 1.5G solar illumination, and Figure 2b shows the corresponding external quantum efficiency (EQE) spectra. Surprisingly, the ITIC-C3 PSCs exhibit almost no PV response (PCE < 0.2%), suggesting poor BHJ morphology.2 In contrast, PBDB-T:ITIC-C6 PSCs exhibit 8.77% PCE, and PBDB-TF:ITIC-C6 PSCs exhibit 9.05% PCE on average. The PBDB-TF PSC performance increase tentatively reflects, among other factors, the higher Voc values accompanying the lower-lying PBDB-TF HOMO. 1 Note that ITIC-C9 PSCs exhibit significantly higher average PCEs than those with ITIC-C6, 9.01% with PBDB-T and 9.53% with PBDB-TF (Figure S7). Impressively, the champion PBDB-TF:ITIC-C9 device delivers 10.24% PCE. The Voc values of the ITIC-C9 PSCs are slightly
blend
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PBDB-T:ITIC-C3 PBDB-TF:ITIC-C3 PBDB-T:ITIC-C6 PBDB-TF:ITIC-C6 PBDB-T:ITIC-C9 PBDB-TF:ITIC-C9
0.31 0.04 0.87 0.98 0.87 0.99
0.7, 0.5b 0.6, 0.5b 15.6, 15.6b 14.4, 14.3b 15.1, 15.1b 14.5, 14.4b
29.1 22.4 65.0 63.9 68.7 66.6
