High-Crystallinity Ï-Conjugated Small Molecules Based on

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High-Crystallinity #-Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties Seiichi Furukawa, Hideaki Komiyama, Naoya Aizawa, and Takuma Yasuda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17056 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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ACS Applied Materials & Interfaces

High-Crystallinity -Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties Seiichi Furukawa,†,‡ Hideaki Komiyama,‡ Naoya Aizawa,‡ and Takuma Yasuda*,†,‡ †Department

of Applied Chemistry, Graduate School of Engineering and ‡INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ABSTRACT: Narrow-bandgap small molecules with -extended backbones are promising donor materials for solutionprocessed bulk-heterojunction (BHJ) organic solar cells (OSCs). Herein, a series of acceptor–donor–acceptor (A–D–A) photovoltaic small molecules incorporating thienylene-vinylene-thienylene (TVT) as a central D unit and alkylsubstituted rhodanine or 2-(1,1-dicyanomethylene)rhodanine as terminal A units are designed and synthesized. Their physical properties including photoabsorption, electronic energy levels, hole mobility, and morphological characteristics are systematically investigated. Using solvent vapor annealing (SVA), the morphologies of the BHJ photoactive layers composed of these small-molecule donors and a [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) acceptor can be properly modulated. As a result of increased crystallinity of the donors and desired phase segregation between the donors and PC71BM upon rapid SVA treatment, the photovoltaic performances of the resultant OSC devices undergo drastic enhancement. The results reported here indicate that high-efficiency small-molecule OSCs can be achieved through rational design of the TVT-based molecular framework and optimization of the nanoscale phase-segregated morphology via proper SVA treatment. KEYWORDS: organic solar cells, small molecule, thienylene-vinylene-thienylene, solvent vapor annealing, self-organization

INTRODUCTION Organic solar cells (OSCs) have attracted considerable attention as a next-generation renewable energy technology owing to their fascinating characteristics, such as low cost, light weight, mechanical flexibility, transparency, and high-throughput manufacturing processes.1–8 This OSC technology essentially relies on the use of bulk heterojunction (BHJ) architectures, which consist of binary blends of p-type (donor) and n-type (acceptor) organic semiconductors, for dissociating the photo-generated excitons into free charges and thereby generating electrical current. Recently, the certified power conversion efficiencies (PCEs) for the state-of-theart OSCs using semiconducting polymer donors and a non-fullerene acceptor,9,10 with single-junction and double-junction tandem structures have reached 14%11–13 and 17%,14 respectively. Compared to their polymer counterparts, small-molecule (SM) donor materials offer some inherent attractions, including well-defined chemical structures and molecular weights, potential higher open-circuit voltages (Voc) by easier control of energy levels, and the capability of forming a well-ordered thin-film morphology without chain entanglement. During the past few years, materials innovation benefitted from diversity of molecular design and synthetic protocols has boosted the PCEs of fullerene-based SM-

OSCs to over 10%,15–24 gradually approaching those of polymer-based devices.25–28 To attain higher photovoltaic performance of the SMOSCs, the intrinsic physical properties of SM donors, including their absorption spectra (i.e., absorption wavelengths and coefficients), frontier energy levels, charge carrier mobilities, and thin-film morphologies, must be properly tuned. As for SM donor materials, utilizing the -conjugated acceptor–donor–acceptor (A– D–A) structure, consisting of a central electron-rich core (D) and two terminal electron-deficient units (A) with strong intramolecular charge transfer (ICT), is currently regarded as one of the most promising design strategies to widen the optical absorption range and tune the energy levels and bandgaps.15–21,29–34 Accordingly, a judicious selection of D and A units is of great importance for producing high-performance SM donor materials. Moreover, designing SM donors with high hole mobilities can contribute to an efficient charge transport with reduced charge recombination within the active layer, which should be beneficial for enhancing the short-circuit current density (Jsc) and fill factor (FF) in SM-OSCs.35–37 In general, highly crystalline -conjugated molecules are expected to form well-ordered assemblies via intermolecular – stacking, allowing enhanced charge carrier mobilities in their thin films. However, this feature typically results in the self-aggregation of SM donors and

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hence the formation of largely phase-segregated domains in BHJ blends, which tends to cause a significant photocurrent loss of the OSC device. Therefore, it is still challenging to develop ideal SM-based BHJ systems that can simultaneously retain their high crystallinity and a fine nanoscale phase-segregated morphology for further facilitating exciton dissociation and charge transport in SM-OSCs. In this work, taking the foregoing insight into account, new A–D–A-structured SM donors, 1–4 (Figure 1), were designed, in which thienylene-vinylene-thienylene (TVT) was adopted as the central D core to couple with two terminal rhodanine (Ro) or dicyanorhodanine (CNRo)based A units16–21,29,30,33 through different numbers of 3hexylthiophene rings. Because of its high coplanarity and extended -conjugation, the introduction of the TVT core is expected to not only enhance the self-organization capability and carrier mobility, but also lower the bandgap energy. Solid-state molecular packing can be controlled by alkylation at the 3-position of the thiophene rings, and sufficient solution processability can be induced. Although a variety of TVT-based semiconducting polymers have been developed for applications in OSCs as well as organic field-effect transistors,38–49 there are very few reports on TVT-based photovoltaic SM materials.50–52 As expected, 1–4 exhibited photoabsorption over the entire visible range and possessed relatively low-lying HOMO energy levels. Notably, these coplanar -conjugated molecules showed superior crystallinity and crystalline coherence in the thin films upon solvent vapor annealing (SVA) treatment,53–60 leading to significantly improved hole mobilities close to 0.1 cm2 V−1 s−1, which are far higher than those of common SM and polymer donors. It was also found that by simply optimizing the phase-segregated morphology and crystallinity in the BHJ active layers via SVA, photovoltaic

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performances for the SM-OSCs using 1–4 were drastically improved and the resulting PCEs were increased by 4–6 times.

Figure 1. Molecular structures of TVT-based SM donors 1–4.

RESULTS AND DISCUSSION Materials Synthesis and Characterization. The general procedure for the synthesis of 1–4 is outlined in Scheme 1. First, McMurry reductive coupling of 2-formyl-3hexylthiophene (5) was performed using a low-valent titanium regent (TiCl4/Zn) to produce a TVT derivative, (E)-1,2-bis(3-hexylthiophen-2-yl)ethene42,49 (6). As-

Scheme 1. Synthetic Routes for 1–4


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prepared 6 was then distannylated via dilithiation and subsequent treatment with trimethyltin chloride to afford 7. Meanwhile, Vilsmeier–Haack formylation of 2-bromo3-hexylthiophene or 5'-bromo-3,4'-dihexyl-2,2'bithiophene32 (12) led to the corresponding monoaldehydes (10 and 13, respectively), which were then subjected to Knoevenagel condensation with 3hexylrhodanine (Ro, 8) or 2-(1,1-dicyanomethylene)-3hexyl-rhodanine (CNRo, 9) to yield the acceptorterminated precursors 14–17. Finally, the target compounds 1–4 were synthesized by two-fold Stille coupling reactions between 7 and 14–17, using tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] as the catalyst in the presence of tri(o-tolyl)phosphine. All of these compounds showed good solubility in chlorinated organic solvents such as chloroform and chlorobenzene at room temperature because of the presence of multiple hexyl side chains. The molecular structures of 1–4 were confirmed by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. The detailed synthetic procedures and characterization data can be found in the Supporting Information. Differential scanning calorimetry (DSC) revealed that 1–4 were essentially crystalline materials with high melting temperatures (Tm) of 189–190 °C for 1 and 2, and 210–214 °C for 3 and 4 (Supporting Information). Compared to the Ro-appended 1 and 2, the CNRoappended 3 and 4 showed much sharper endothermic and exothermic transition (i.e., melting and crystallization) peaks at higher temperatures, with larger values of the transition enthalpies. Optical Properties and Electronic Structures. The UV– vis absorption spectra of 1–4 in chloroform solutions and as solid thin films are shown in Figure 2a,b, and the relevant optical data are summarized in Table 1. In dilute solutions, 1–4 exhibited intense and broad absorption bands with peaks (max) at around 530–580 nm, arising from intensive ICT electronic transitions (Figure 2a). These molecules had high maximum molar absorption coefficients (ε) ranging from 59,000 to 74,000 M−1 cm−1, which were comparable to those observed for the previously reported Ro- and CNRo-appended SM donors.16–21,29,30 It is also worth noting that replacing the Ro terminal units of 1 and 2 by the stronger electronwithdrawing CNRo units (for 3 and 4) led to an obvious bathochromic shift of max by 20–30 nm, confirming the intensified ICT effect in the CNRo-appended 3 and 4.17 In comparison with the molecular-dispersed solution states, all of these molecules showed considerably red-shifted and widened absorptions, with an appearance of a distinct low energy shoulder band in their molecularaggregated solid states (Figure 2b). As a result, the absorption edges of 1–4 extended over 720–780 nm in the thin films, and the optical bandgaps (Eg) deduced from the absorption edges decreased from 1.72 eV for 1 to 1.59 eV for 4 (see Table 1). Compounds 2 and 4 with longer conjugation lengths showed comparably smaller Eg values compared with the homologous 1 and 3, respectively.

Figure 2. UV–vis absorption spectra of 1–4 in (a) chloroform solutions and (b) as-spun solid thin films. (c) Energy-level diagram for SM donors 1–4 and PC71BM.

Table 1. Optical Properties of 1–4 compound 1 2 3 4 aMeasured

solutiona εd max −1 cm−1) (M (nm) c

557

7.4 × 104

528

6.4 ×

104

6.7 ×

104

5.9 ×

104

579 557

(nm)

thin filmb αd (cm−1)

Ege (eV)

601

9.1 × 104

1.72

609

8.1 ×

104

1.64

8.7 ×

104

1.63

7.4 ×

104

1.59

max

610 613

c

in chloroform solution (10−5 M) at 300 K. bMeasured as a neat film (ca. 100 nm thickness) spin-coated from a chloroform solution onto a quartz substrate. cAbsorption peak wavelength. dAbsorption coefficient at max. eOptical bandgap derived from the absorption onset of the as-spun neat film.



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Figure 3. (a) Side-view projection of the optimized geometries and (b) frontier molecular orbital distributions, energy levels, and corresponding oscillator strength (f) for 1–4 calculated at the B3LYP/6-31G(d) level. The hexyl chains of 1–4 were replaced by methyl groups to simplify the calculations. The arrows indicate the transition to the S1 state.

The electronic energy levels of the SM donors are crucial for their application in BHJ OSCs. To assess the HOMO energy levels (EHOMO; or ionization potentials), photoelectron yield spectroscopy was performed using thin films of 1–4 (Supporting Information). As schematically shown in Figure 2c, with the increasing number of thiophene rings (or -conjugation length), the EHOMO value increased from −5.50 eV (for 1) to −5.35 eV (for 2), and likewise, from −5.60 eV (for 3) to −5.40 eV (for 4). It was also found that the terminal CNRo units lowered both the EHOMO and LUMO energy level (ELUMO), relative to the Ro units.16,17 In consideration of the EHOMO (−6.1 eV) and ELUMO (−4.3 eV) of an representative acceptor, [6,6]-phenyl-C71-butylic acid methyl ester (PC71BM) in the thin film, 1–4 as SM donors had proper electronic energy levels for the utilization in the PC71BMbased BHJ OSCs. It is known that the energy-level offsets for the HOMOs and LUMOs (EHOMO and ELUMO, respectively) between the donor and acceptor should be larger than the exciton binding energy (ca. 0.1–0.3 eV) for efficient exciton dissociation. For the present 1–4:PC71BM systems, the EHOMO and ELUMO values are estimated to be 0.5–0.7 and 0.3–0.6 eV, respectively, which seem to be sufficient for the exciton dissociation of 1–4 and the photo-induced electron transfer from 1–4 to PC71BM. To gain insight into the fundamental aspects of the molecular geometry and electronic structures of 1–4, density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level. As can be seen from the side-view projection in Figure 3a, all of these molecules had an overall linear and highly coplanar backbone structure owing to the nearly negligible steric hindrance between the intramolecular units, which led to improved molecular packing and thus high hole mobility (as will be discussed later). Figure 3b depicts the electronstate density distributions of the HOMO and LUMO and the calculated EHOMO and ELUMO for 1–4. Importantly, both HOMOs and LUMOs were extended over the entire conjugated backbone including the central TVT core, even though a slightly larger contribution was found on the terminal Ro or CNRo units for the LUMO. Thus, the

transitions to the lowest excited singlet (S1) state, which was dominated by the HOMO → LUMO configuration, exhibited rather large oscillator strengths (f = 2.69–2.98). The trend of variation for the calculated EHOMO and ELUMO values of 1–4 is reasonably consistent with the results obtained from the photophysical studies (see Figure 2c). Photovoltaic Performance. To systematically study the impact of the different backbone structures on the photovoltaic properties, solution-processed BHJ OSCs employing 1–4 as the SM donors and PC71BM as the acceptor were fabricated and evaluated. Here, we used an inverted device structure of indium tin oxide (ITO)/ZnO (30 nm)/donor:PC71BM (70–100 nm)/MoOx (6 nm)/Ag (100 nm) (see Supporting Information for detailed device fabrication procedures). For all devices, the BHJ active layers, composed of a binary blend of 1–4 and PC71BM with an optimal weight ratio at 2:1, were prepared by spincoating their chloroform solutions without using any processing additives. The SM-OSCs fabricated directly after spin-coating (referred to as ‘as-spun devices’) yielded quite low PCEs of 0.3–1.4% in spite of their high Voc of up to 1.0 V (Supporting Information). We then attempted the SVA treatment of these as-spun BHJ active layers in order to control the film morphology and improve the device performance. Tetrahydrofuran (THF) was employed as the annealing solvent because it has a high vapor pressure under atmospheric conditions and 1–4 have medium solubility in THF.61 As shown in Figure 4a and Table 2, compared to the untreated as-spun devices, the SVA-treated devices exhibited remarkable improvements in Jsc and FF, and a slight decrease in Voc. It is worth noting that all 1–4:PC71BM BHJ systems reached their highest PCEs with SVA duration for 30–40 s, and prolonged SVA treatment (typically, beyond 50 s) led to a deterioration of the overall device performance. Figure 4b shows the current density–voltage (J–V) curves for the optimized SM-OSCs based on the 1–4:PC71BM (2:1, w/w) layers with SVA treatment, measured under AM 1.5G illumination (100 mW cm−2). Upon proper SVA treatment, the PCE of the 2-based devices increased from 1.2% to


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5.8%, with a significantly improved Jsc from 4.4 to 15.0 mA cm−2. Among the SVA-treated devices, the highest PCE of 6.0% was achieved for the SVA-treated 4-based device, with Jsc of 12.1 mA cm−2, Voc of 0.83 V, and FF of 60%. The 4-based device had a higher Voc (0.83 V) than that of the optimized 2-based one (0.80 V) because of the deeperlying HOMO energy level of 4 that is induced by the stronger electron-withdrawing CNRo units. It is anticipated that the crystallites of SM donors grew during

the SVA process, but prolonging the duration of the SVA treatment, led to over-growth of the domains and resulted in the reduction of Jsc and/or FF to a certain degree (Figure 4a). Consequently, the PCEs of the devices with proper SVA treatment increased to 4–6 times that of the initial as-spun devices. Figure 4c represents the incident photon-to-current conversion efficiency (IPCE) spectra for the optimized SVA-treated devices. Upon proper SVA treatment for the

Figure 4. (a) Relationship between the key photovoltaic parameters (PCE, Voc, Jsc, and FF) and SVA duration time. (b) J–V characteristics under one sun illumination (100 mW cm−2) and (c) IPCE spectra for the optimized SM-OSCs based on 1– 4:PC71BM (2:1, w/w) blend films with SVA treatment using THF. (d) UV–vis absorption spectra of the 1–4:PC71BM blend films before (dashed lines) and after SVA treatment for 30–40 s (solid lines).

Table 2. Photovoltaic Parameters for SM-OSCs Based on 1–4:PC71BM Blends with and without SVAa donor

condition

thickn essc (nm)

Jsc (mA cm−2)

Jd (mA cm−2)

Voc (V)

FF (%)

PCEe (%)

μ+f (cm2 V−1 s−1)

μ−g (cm2 V−1 s−1)

1

As-spun

93

1.5 ± 0.1

1.8

0.76 ± 0.06

24 ± 1

0.3 ± 0.1 (0.4)

5.4 × 10−3 (5.2 × 10−3)

1.0 × 10−4

SVAb

97

10.0 ± 0.3

11.1

0.71 ± 0.01

37 ± 1

2.7 ± 0.2 (2.9)

6.5 × 10−3 (5.0 × 10−3)

1.0 × 10−4

As-spun

99

4.4 ± 0.1

4.5

0.80 ± 0.04

32 ± 1

1.1 ± 0.1 (1.2)

5.6 × 10 (1.2 × 10 )

1.7 × 10−4

SVA

97

15.0 ± 0.4

15.1

0.74 ± 0.04

48 ± 2

5.5 ± 0.3 (5.8)

4.5 × 10 (1.1 × 10 )

2.0 × 10−4

As-spun

86

3.1 ± 0.1

5.1

0.98 ± 0.02

32 ± 1

1.0 ± 0.1 (1.1)

2.1 × 10−3 (2.1 × 10−2)

1.3 × 10−4

SVA

99

8.2 ± 0.1

8.9

0.94 ± 0.01

52 ± 1

4.0 ± 0.1 (4.1)

6.7 × 10 (1.4 × 10 )

1.3 × 10−4

As-spun

80

4.0 ± 0.1

5.9

0.86 ± 0.02

39 ± 1

1.3 ± 0.1 (1.4)

8.1 × 10−3 (7.7 × 10−3)

2.5 × 10−4

SVA

77

11.9 ± 0.2

12.8

0.83 ± 0.01

58 ± 2

5.8 ± 0.2 (6.0)

6.2 × 10 (1.1 × 10 )

1.8 × 10−4

2 3 4

b

b

b

aDevice

−3

−3

−2

bSolvent

−2

−2

−2

−1

−1

structure: ITO/ZnO (30 nm)/donor:PC71BM (2:1, w/w)/MoOx (6 nm)/Ag (100 nm). vapor annealing using THF with optimal duration for 30 s (1 and 4) or 40 s (2 and 3). cActive layer thickness for the optimum devices. dCalculated by integrating the IPCE spectra. eAverage power conversion efficiency (PCE) calculated using 4 individual devices by PCE = (Jsc × Voc × FF)/P0, where P0 is the incident light intensity (100 mW cm−2); the best PCEs are given in parentheses. fHole mobilities for the donor:PC71BM (2:1, w/w) blend films evaluated by using the SCLC technique; the values in parentheses are hole mobilities obtained for the pristine donor neat films. gElectron mobilities for the donor:PC71BM (2:1, w/w) blend films.



2-based device, its conversion efficiency was drastically improved. Additionally, high IPCE responses exceeding 60% were achieved over a broad wavelength range from 400 to 700 nm, which is consistent with the increase in Jsc. To further understand the effect of SVA treatment on the photovoltaic properties, the absorption spectra of the 1– 4:PC71BM blend films processed with or without SVA were examined. As can be seen from Figure 4d, the photoabsorption in the range of 500–750 nm were obviously intensified after the SVA treatment, and at the same time, a distinct low energy shoulder peak appeared at around 670–700 nm, which should originate from the enhanced – stacking of the more ordered aggregates. Therefore, the increment of Jsc in the SVA-treated devices is attributable to a combined effect of improved photoabsorption and enhanced charge extraction at short-circuit conditions. Charge-Transport Properties. The effect of the SVA treatment on the charge-transport properties was verified by measuring the hole mobilities, using the space-charge limited current (SCLC) method.62–65 The hole and electron mobilities (μ+ and μ−, respectively) obtained for the 1– 4:PC71BM blend films with and without SVA are included in Table 2, and the corresponding dark J–V curves are given in the Supporting Information. All as-spun blend films had similar μ+ values on the order of 10−3 cm2 V−1 s−1. After the SVA treatment, the hole mobilities for the 3:PC71BM and 4:PC71BM blend films were found to increase by one order of magnitude, reaching as high as 6.7 × 10−2 and 6.2 × 10−2 cm2 V−1 s−1, respectively. These μ+ values were substantially higher than those of the SVAtreated 1:PC71BM and 2:PC71BM blend films. The improved hole mobilities could be ascribed to a higher degree of molecular ordering of the CNRo-appended SM donors within the SVA-treated blend films (as will be discussed later). In contrast, μ− for the 1–4:PC71BM blend films did not change significantly upon SVA treatment, retaining the values on the order of 10−4 cm2 V−1 s−1. As a result, it was expected that the photo-generated charge carriers could be effectively transported and collected at the electrodes in the devices, as evidenced by the relatively higher FF values of the SVA-treated 4- and 3-based devices.

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bright and dark regions in each image correspond to the SM donor-rich and PC71BM-rich domains, respectively. With SVA treatment for 30–40 s, more clearly phasesegregated and slightly larger domains were formed within the blend films. In particular, fibrous interpenetrating networks were evident in the SVAtreated 4:PC71BM blend film (Figure 5h). Such fine fibrous nanostructures are beneficial for exciton diffusion/dissociation and charge transport, leading to higher Jsc and FF of the 4-based device (see Figure 4 and Table 2). Using power spectral densities calculated from two-dimensional fast Fourier transform (2D FFT) analysis,32,66,67 average domain sizes (D), which correspond to the periodicity of the phase-segregated structures, could be estimated (see Figure 5 and also the Supporting Information). It is noteworthy that relatively smaller D values of 15 and 20 nm were obtained for the SVA-treated 2:PC71BM and 4:PC71BM blend films, respectively. In contrast, the SVA-treated 1:PC71BM and 3:PC71BM blend films containing shorter-length SM donors exhibited relatively larger phase-segregated domains (D = 31 and 28 nm, respectively). Considering the short exciton diffusion length of organic semiconductors (typically ~10 nm), their larger domains would be unfavorable for charge generation at the donor/acceptor interfaces, which were actually responsible for the smaller Jsc values in the 1- and 3-based OSC devices.

Film Morphology and Nanostructure Characterization. The surface morphologies of the 1–4:PC71BM blend films with and without SVA treatment were analyzed using atomic force microscopy (AFM). Before SVA, all of the asspun blend films had very smooth surfaces with small root-mean-square (RMS) roughness of 0.3–1.2 nm in the AFM topographic images (Supporting Information), indicating the good miscibility of 1–4 with PC71BM. After SVA for 30–40 s, their surfaces became slightly coarser, with an increased RMS roughness of 0.7–1.4 nm, and aggregation features associated with the reorganization of the SM donors were apparent. High-resolution transmission electron microscopy (TEM) measurements were performed for characterizing the interior morphologies of the blend films with and without SVA treatment, as presented in Figure 5. The


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Figure 5. TEM images of BHJ active layers composed of (a,b) 1:PC71BM, (c,d) 2:PC71BM, (e,f) 3:PC71BM, and (g,h) 4:PC71BM (2:1, w/w) blends before (left panels) and after SVA (right panels) using THF solvent for optimal duration. The D values represent the average domain sizes calculated by 2D FFT.

Grazing incidence X-ray diffraction (GIXD) was used to further investigate the effect of SVA treatment on the nanostructures (molecular ordering/orientation) and crystallinity within the active layers. Figure 6a–d shows the two-dimensional (2D) diffraction images for the blend films with different SVA duration times, revealing a systematic growth in the crystallinity. The corresponding out-of-plane (qz-scan) and in-plane (qxy-scan) line-cut profiles are presented in Figure 6e and 6f, respectively. In the as-spun blend films, weak (100) reflections originating from lamellar structures of 1–4 were observed at d100 = 1.70–1.90 nm. After SVA treatment for 30–40 s, more intense (100) reflections (d100 = 1.70–1.81 nm) together with multiple higher-order (h00) reflections were detected in the out-of-plane direction (Figure 6e), indicative of enhanced molecular ordering and crystallinity. Using the Scherrer’s equation,68,69 the mean crystallite sizes (or crystal coherence lengths) were estimated to be 15, 10, 18, and 20 nm for 1–4, respectively, in their SVA-treated blend films. Another important feature is that the (001) reflections originating from π–π stacking became noticeable after SVA treatment. For the blend films of 3

and 4, this π–π stacking reflection was more pronounced in the out-of-plane direction after SVA, indicating that there existed a preferential face-on crystalline orientation. Additionally, the π–π stacking distances for 3 and 4 (d001 = 0.36–0.37 nm) were slightly shorter than those of 1 and 2 (d001 ≈ 0.38 nm), implying the existence of strong intermolecular interactions in the CNRo-appended SM donors. This propensity is favorable for charge transport along the out-of-plane direction. With further prolonged SVA (for 60 s), the 2D GIXD patterns exhibited sharp scattering spots, which were indicative of an extended long-range order similar to crystals even in the thin films. Thus, it was concluded that proper SVA treatment was highly effective for 1–4 to induce long-range order and crystallinity, as well as better nano-segregation between 1–4 and PC71BM with more crystalline domains in their BHJ blend films, which ultimately led to enhanced carrier mobilities and PCEs of the OSC devices.

CONCLUSIONS A series of TVT-based narrow-bandgap small molecules, 1–4, with different -conjugation lengths and different electron-withdrawing terminal units were synthesized and systematically studied. These compounds exhibited favorable optical, electronic, and self-organization properties for use as donor materials in solutionprocessed BHJ OSCs. Rapid SVA treatment using THF as an annealing solvent was surprisingly effective for

Figure 6. 2D GIXD images for (a) 1:PC71BM, (b) 2:PC71BM, (c) 3:PC71BM, and (d) 4:PC71BM (2:1, w/w) blend films before and after SVA treatment. (e) Out-of-plane (qz-scan) and (f) in-plane (qxy-scan) line-cut profiles of the GIXD patterns of the 1– 4:PC71BM blend films upon SVA treatment for 30–40 s.



enhancing the photovoltaic performance of the BHJ OSCs based on 1–4:PC71BM active layers. With proper SVA treatment, PCEs of up to 6.0% and 5.8% were attained for the 4- and 2-based OSCs, respectively. The film morphology, self-organized nanostructures, crystallinity, and charge-transport properties of the 1–4:PC71BM blend films were investigated by AFM, TEM, GIXD, and SCLC measurements. Our results revealed that the blend films exhibited an increased crystallinity, better phasesegregation, and higher hole mobility after SVA treatment for 30–40 s. This study offers a valuable insight into the structure–property relationship for photovoltaic small molecules, and also an effective design strategy for further developing high-performance TVT-based photovoltaic materials and devices.

EXPERIMENTAL SECTION Materials and Synthesis. All commercially available reagents and solvents were used without further purification unless otherwise noted. PC71BM was purchased from Frontier Carbon Corp., and used for the device fabrication without further purification. All reactions were carried out under N2 atmosphere using standard Schlenk techniques. The detailed synthetic procedures for the intermediates are described in the Supporting Information. Synthesis of 1: To a stirred solution of 7 (0.480 g, 0.70 mmol) and 12 (0.698 g, 1.47 mmol) in dry toluene (20 mL) were added Pd2(dba)3 (0.027 g, 0.03 mmol) and P(o-tol)3 (0.040 g, 0.13 mmol). The mixture was stirred overnight at 100 °C. After cooling to room temperature, the reaction mixture was added into water and extracted with chloroform. The combined organic layers were washed with water and dried over anhydrous Na2SO4. After filtration and evaporation, the product was purified by silica gel column chromatography (eluent: hexane/chloroform = 6:4, v/v), recrystallized from chloroform/methanol, and dried under vacuum to afford 1 as a black solid. This material was further purified by recycling preparative GPC (eluent: chloroform) prior to use (yield = 0.203 g, 25%). 1H NMR (400 MHz, CDCl3): δ 7.76 (s, 2H), 7.22 (s, 2H), 7.05 (s, 2H), 7.00 (s, 2H), 4.10 (t, J = 7.7 Hz, 4H), 2.82 (t, J = 7.9 Hz, 4H), 2.70 (t, J = 7.7 Hz, 4H), 1.76-1.60 (m, 12H), 1.46-1.27 (m, 36H), 0.95-0.86 (m, 18H). 13C NMR (100 MHz, CDCl3): δ 192.26, 167.53, 142.48, 141.09, 139.99, 138.08, 137.50, 135.09, 132.40, 129.57, 124.90, 120.41, 119.40, 44.87, 31.67, 31.60, 31.35, 30.78, 30.11, 29.49, 29.18, 29.03, 28.46, 26.96, 26.46, 22.62, 22.61, 22.51, 14.13, 14.08, 14.00. MS (MALDI-TOF): m/z calcd 1146.45 [M]+; found 1146.64. Anal. calcd (%) for C62H86N2O2S8: C 64.88, H 7.55, N 2.44; found: C 64.97, H 7.49, N 2.51. Synthesis of 2: This compound was prepared by a method similar to that of 1, using 7 (0.480 g, 0.70 mmol), 16 (0.942 g, 1.47 mmol), Pd2(dba)3 (0.038 g, 0.042 mmol), and P(o-tol)3 (0.043 g, 0.14 mmol). The product was purified by silica gel column chromatography (eluent: hexane/chloroform = 6:4, v/v), recrystallized from chloroform/methanol, and dried under vacuum to afford 2 as a black solid. This material was further purified by recycling preparative GPC (eluent: chloroform) prior to use (yield = 0.153 g, 15%). 1H NMR (400 MHz, CDCl3): δ 7.78 (s, 2H), 7.23 (s, 2H), 7.11 (s, 2H), 6.99 (s, 2H), 6.95 (s, 2H), 4.14-4.07 (4H), 2.82 (t, J = 7.8 Hz, 8H), 2.69 (t, J = 7.7 Hz, 4H), 1.75-1.62 (m, 16H), 1.45-1.28 (m, 48H), 0.94-0.87 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 192.31, 167.56, 141.84, 140.97, 140.28, 139.76, 137.42, 136.91, 135.03, 132.99, 132.58, 132.53, 130.02, 128.68, 124.96, 120.39, 119.01, 44.87, 31.69, 31.65, 31.62, 31.36, 30.82, 30.41, 30.18, 29.52, 29.42, 29.26, 29.18, 29.05, 28.48, 26.97, 26.46, 22.64, 22.51, 14.11, 14.01. MS (MALDI-TOF): m/z calcd 1478.61 [M]+; found 1478.84. Anal.

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calcd (%) for C82H114N2O2S10: C 66.53, H 7.76, N 1.89; found: C 66.55, H 7.70, N 1.93. Synthesis of 3: This compound was prepared by a method similar to that of 1, using 7 (0.412 g, 0.60 mmol), 15 (0.638 g, 1.26 mmol), Pd2(dba)3 (0.033 g, 0.036 mmol), and P(o-tol)3 (0.037 g, 0.12 mmol). The product was purified by silica gel column chromatography (eluent: chloroform/hexane = 3:1, v/v), recrystallized from chloroform/methanol, and dried under vacuum to afford 3 as a black solid. This material was further purified by recycling preparative GPC (eluent: chloroform) prior to use (yield = 0.460 g, 63%). 1H NMR (400 MHz, CDCl3): δ 7.98 (s, 2H), 7.29 (s, 2H), 7.13 (s, 2H), 7.03 (s, 2H), 4.21 (t, J = 7.9 Hz, 4H), 2.85 (t, J = 7.9 Hz, 4H), 2.72 (t, J = 7.7 Hz, 4H), 1.80-1.63 (m, 12H), 1.49-1.29 (m, 36H), 0.95-0.87 (m, 18H). 13C NMR (100 MHz, CDCl3): δ 166.05, 165.63, 142.79, 141.38, 141.26, 138.93, 138.73, 133.75, 132.00, 130.36, 128.59, 119.62, 113.43, 113.29, 112.34, 45.38, 31.65, 31.58, 31.26, 30.90, 30.12, 29.55, 29.19, 29.06, 28.79, 28.50, 25.63, 22.64, 22.61, 22.44, 14.13, 14.08, 13.95. MS (MALDI-TOF): m/z calcd 1210.51 [M]+; found 1210.66. Anal. calcd (%) for C68H86N6O2S6: C 67.40, H 7.15, N 6.94; found: C 67.49, H 7.11, N 7.00. Synthesis of 4: This compound was prepared by a method similar to that of 1, using 7 (0.343 g, 0.50 mmol), 17 (0.706 g, 1.05 mmol), Pd2(dba)3 (0.018 g, 0.020 mmol), and P(o-tol)3 (0.027 g, 0.089 mmol). The product was purified by silica gel column chromatography (eluent: chloroform/hexane = 7:3, v/v), recrystallized from chloroform/methanol, and dried under vacuum to afford 4 as a black solid. This material was further purified by recycling preparative GPC (eluent: chloroform) prior to use (yield = 0.551 g, 71%). 1H NMR (400 MHz, CDCl3): δ 7.99 (s, 2H), 7.29 (s, 2H), 7.18 (s, 2H), 6.99 (s, 2H), 6.97 (s, 2H), 4.21 (t, J = 7.9 Hz, 4H), 2.84 (t, J = 7.9 Hz, 8H), 2.69 (t, J = 7.7 Hz, 4H), 1.801.60 (m, 16H), 1.51-1.28 (m, 48H), 0.96-0.85 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 166.08, 165.67, 141.94, 141.19, 141.13, 140.51, 138.91, 137.09, 133.69, 133.67, 132.45, 132.02, 130.73, 128.86, 128.68, 119.06, 113.43, 113.22, 112.36, 55.61, 45.37, 31.69, 31.62, 31.60, 31.26, 30.84, 30.51, 30.14, 29.55, 29.47, 29.30, 29.17, 29.11, 29.05, 28.79, 28.48, 25.63, 22.66, 22.63, 22.44, 14.12, 14.10, 13.95. MS (MALDITOF): m/z calcd 1542.68 [M]+; found, 1543.14. Anal. calcd (%) for C88H114N6O2S8: C 68.44, H 7.44, N 5.44; found: C 68.54, H 7.44, N 5.47. Fabrication and Evaluation of OSC Devices. Prepatterned ITOcoated glass substrates were cleaned by sequentially sonicating in detergent solution, deionized water, acetone, and isopropyl alcohol for 10 min each, and then subjected to UV/ozone treatment for 15 min. A thin layer (~30 nm) of ZnO was prepared by spin-coating (at 5000 rpm for 60 s) a precursor solution of zinc acetate (1.00 g) and ethanolamine (0.28 g) in 2methoxyethanol (10 mL) through a 0.20 µm polyethylene membrane filter, followed by baking at 200 °C for 10 min under air. The photoactive layer was then deposited by spin-coating (at 1000 rpm for 30 s) from a chloroform solution containing a donor (9.3 mg mL−1) and PC71BM (4.6 mg mL−1) after passing through a 0.45 µm poly(tetrafluoroethylene) membrane filter. The thickness of the photoactive layer was ca. 70–100 nm, as measured with a profilometer. For SVA in an N2-filled grove box, the as-spun films were placed in a glass Petri-dish (10 cm diameter) treatment container containing dry THF (5 mL), and kept inside for a certain duration time (0–60 s) with a lid closed. After SVA, the films were quickly taken out and loaded into an E-200 vacuum evaporation system (ALS Technology). Finally, 6nm-thick MoOx and 100-nm-thick Ag layers were sequentially vacuum-deposited on top of the photoactive layer under high vacuum (< 5.0 × 10−4 Pa) through a shadow mask, defining an active area of 0.04 cm2 for each device. The current density– voltage (J–V) characteristics and IPCE spectra of the fabricated OSCs were measured with a computer-controlled Keithley 2400


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source measure unit in air, under simulated AM 1.5G solar illumination at 100 mW cm−2 (1 sun) conditions, using an Xe lamp-based SRO-25GD solar simulator and IPCE measurement system (Bunko Keiki). The light intensity was calibrated using a certified Si photodiode.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental procedures and characterization data, additional figures (NMR, DSC, OSC, SCLC, AFM, TEM, and GIXD data) (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] (T.Y.)

ORCID Takuma Yasuda: 0000-0003-1586-4701

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for JSPS KAKENHI Grant No. JP18H02048 (T.Y.). The GIXD measurements were performed at the BL-40B2 beamline of SPring-8, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2016B1118 and 2017A1119). The computations were primarily performed using the computer facilities at the Research Institute for Information Technology, Kyushu University. The authors are grateful for the support of the Cooperative Research Program "Network Joint Research Center for Materials and Devices". S.F. acknowledges the support from the Leading Graduate Schools Program of "Advanced Graduate Course on Molecular Systems for Devices" by MEXT, Japan.

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