OligothiopheneâIndandione-Linked Narrow-Band Gap Molecules

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Organic Electronic Devices

Oligothiophene–Indandione-Linked Narrow-Bandgap Molecules: Impact of #-Conjugated Chain Length on Photovoltaic Performance Hideaki Komiyama, Takahiro To, Seiichi Furukawa, Yu Hidaka, Woong Shin, Takahiro Ichikawa, Ryota Arai, and Takuma Yasuda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01233 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Applied Materials & Interfaces

Oligothiophene–Indandione-Linked Narrow-Bandgap Molecules: Impact of π-Conjugated Chain Length on Photovoltaic Performance Hideaki Komiyama,*,† Takahiro To,† Seiichi Furukawa,†,‡ Yu Hidaka,†,‡ Woong Shin,† Takahiro Ichikawa,§ Ryota Arai,‡,|| and Takuma Yasuda*,†,‡ †

INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Department of Biotechnology, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan || Ricoh Co., Ltd., 16-1 Honda-machi, Numazu, Shizuoka 410-8505, Japan ‡

ABSTRACT: Solution-processed organic solar cells (OSCs) based on narrow-bandgap small molecules hold great promise as next-generation energy-converting devices. In this paper, we focus on a family of A−π−D−π−A-type small molecules, namely, BDT-nT-ID (n = 1–4) oligomers, consisting of benzo[1,2-b:4,5-b']dithiophene (BDT) as the central electron-donating (D) core, 1,3indandione (ID) as the terminal electron-accepting (A) units, and two regioregular oligo(3-hexylthiophene)s (nT) with different numbers of thiophene rings as the π-bridging units, and elucidate their structure–property–function relationships. The effect of the length of the π-bridging nT units on the optical absorption, thermal behavior, morphology, hole mobility, and OSC performance were systematically investigated. All oligomers exhibited broad and intense visible photoabsorption in the 400–700 nm range. The photovoltaic performances of bulk heterojunction OSCs based on BDT-nT-IDs as donors and a fullerene derivative as acceptor were studied. Among these oligomers, BDT-2T-ID, incorporating bithiophene as the π-bridging units, showed better photovoltaic performance with a maximum power conversion efficiency as high as 6.9% under AM 1.5G illumination without using solvent additives or post-deposition treatments. These favorable properties originated from the well-developed interpenetrating network morphology of BDT-2T-ID, with larger domain sizes in the photoactive layer. Even though all oligomers have the same A−D−A main backbone, structural modulation of the π-bridging nT length was found to impact their self-organization and nanostructure formation in the solid state, as well as the corresponding OSC device performance. KEYWORDS: organic solar cells, small molecule, oligothiophene, indandione, self-organization, liquid crystals, nanostructure

INTRODUCTION Bulk heterojunction (BHJ) organic solar cells (OSCs), composed of binary blends of p-type (donor) and n-type (acceptor) organic semiconductors, have attracted great interest as next-generation clean energy-converting devices, owing to attractive features such as low-cost fabrication, light weight, and mechanical flexibility.1–3 Recent advances in the development of new π-conjugated polymers4–8 and oligomeric small molecules9–16 as photoactive p-type donor materials have led to considerable improvements in the performance of solution-processed BHJ OSCs, with power conversion efficiencies (PCEs) over 10% for single-junction devices. In comparison with common polymer semiconductors, oligomeric small molecules offer unique advantages, including well-defined structures and molecular weights, excellent reproducibility without batch-to-batch variation, and generally higher open-circuit voltages (Voc) through easier control of energy levels. However, the practical application of these small molecules in BHJ OSCs still require progress in materials design, morphology control, and device engineering to further enhance their short-circuit current density (Jsc) and fill factor (FF), while maintaining high Voc.

Unlike their polymer counterparts, the limited backbone lengths of small molecules restrict the linear extension of πconjugation, which hampers effective photoabsorption in the longer wavelength region, as well as the stabilization of uniform film morphologies. Incorporating π-conjugated electron donor–acceptor (D–A) motifs with intramolecular charge-transfer (ICT) characteristics can thus represent an effective design strategy to tune the energy levels and promote broad and intense visible photoabsorption in small molecules. Bazan and coworkers developed π-conjugated D1–A–D2–A– D1-structured small molecules,17–20 featuring a central dithienosilole (DTS)-based D2 core, benzothiadiazole-derived A units, and terminal oligothiophene D1 units, for application in high-efficiency BHJ OSCs. Over the last few years, extensive efforts have also been devoted to the exploration of narrow-bandgap small molecules based on A–π–D–π–A motifs,9–16,21–42 in which a central electron-donating unit is covalently linked with two terminal electron-accepting units through π-bridging moieties. Recently, Chen and coworkers10– 12 and other groups9,13,16 reported excellent performances of A– π–D–π–A-type small molecules composed of a central benzo[1,2-b:4,5-b']dithiophene (BDT) or oligothiophene D unit and terminal rhodanine-based A units, which yielded high

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PCEs (approaching 10%) in solution-processed OSCs. To date, a variety of central D units, including BDT,9,10,13–16,21–32 oligothiophene,11,12,33–37 DTS,38 and porphyrin,39–42 have been utilized to prepare narrow-bandgap small molecules, by combining them with various terminal A units such as rhodanine, 2-(1,1-dicyanomethylene)rhodanine, and cyanoacetate derivatives. Both the rational design of the D and A motifs and their linking tactics are of vital importance to finely tailor the properties of π-conjugated A–π–D–π–A systems. Although the structure–property relationships of various D–A combinations have been intensively examined, the effects of the conjugation length of the π-bridging moieties on the physicochemical and photovoltaic properties have scarcely been investigated systematically for these promising systems.11,29

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[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptor exhibited considerably high and even tunable Voc values, ranging from 1.06 to 0.88 V, depending on the number of incorporated thiophene rings (1T–4T). Among the four oligomers examined, BDT-2T-ID (incorporating π-bridging bithiophene units) was found to show superior selforganization, charge-transport, and photovoltaic properties, leading to the highest PCEs of up to 6.9% in OSCs, without any processing additives or additional treatments of the active layer.

RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the target BDT-nT-ID (n = 1–4) oligomers is outlined in Scheme 1. Formyl-functionalized intermediates (Br-nT-CHOs) were prepared through regioselective bromination and Vilsmeier– Haack formylation. The subsequent Knoevenagel condensation of Br-nT-CHOs with ID afforded the key Br-nTID precursors. Two-fold Stille cross-coupling reactions between Br-nT-IDs and 2,6-bis(trimethylstannyl)-4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene in the presence of Pd(PPh3)4 catalyst produced the corresponding BDT-nT-IDs in good yields, after sequential purifications by column chromatography and recycling preparative gel permeation chromatography (GPC). The detailed synthetic procedures and characterization data are described in the Experimental Section and Supporting Information (SI). While the BDT-nT-ID oligomers possess different π-conjugation lengths, they all showed good solubility in common organic solvents such as chloroform and chlorobenzene at room temperature. Their chemical structures were thus fully characterized by 1H and 13C NMR spectroscopy, matrixassisted laser desorption ionization time-of-flight (MALDITOF) mass spectrometry, and elemental analysis.

Figure 1. Molecular structures of BDT-nT-IDs (n = 1–4). Their different π-bridging oligo(3-hexylthiophene) units are displayed in red, while the common central D and terminal A units are shown in black and blue, respectively.

In this work, we designed and synthesized a series of narrow-bandgap oligomeric small molecules, BDT-nT-IDs (n = 1–4; Figure 1), based on the A–π–D–π–A motifs. In these systems, the same central BDT-based D unit is linked to the terminal 1,3-indandione (ID) A units by π-bridging regioregular oligo(3-hexylthiophene) moieties with different number of thiophene rings. The effect of varying the πconjugated chain length from monothiophene (1T) to quaterthiophene (4T) on the physicochemical and photovoltaic properties was systematically investigated. The structural homogeneity and simplicity of the present family of BDT-nTIDs make them an ideal platform for analyzing the effect of the π-conjugated chain length on the properties of A–π–D–π– A systems. BHJ OSCs based on BDT-nT-IDs as donors and

Scheme 1. Synthesis of BDT-nT-IDs (n = 1–4).

Molecular Geometry and Electronic Properties. To better understand the effect of the π-conjugated chain length on the molecular geometries and electronic structures, density functional theory (DFT) calculations were performed for the four BDT-nT-ID molecules at the B3LYP/6-31G(d) level. The strategic introduction of regioregular oligo(3-hexylthiophene) units resulted in all BDT-nT-ID molecules to have highly planar, comb-shaped structures, in which the π-conjugated backbones adopt N-shaped geometries, as illustrated in Figure 2a. The bending angles between the longitudinal axis of the oligothiophene-linked BDT moiety (i.e., the π–D–π axis) and those of the peripheral ID units can be used to quantify the


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Figure 2. (a) Space-filling representations (top view) of optimized molecular geometries of BDT-nT-IDs with extended alkyl chains. Atom color code: C, gray; H, white; O, red; S, yellow. (b) Frontier molecular orbital distributions, energy levels, and corresponding oscillator strength (f) for BDT-nT-IDs calculated at the B3LYP/6-31G(d) level. The alkyl chains were replaced by methyl groups to minimize the calculation cost. The arrows indicate the transition to the first singlet excited state (S1).

difference in the backbone curvature of the BDT-nT-ID molecules. The larger bending angles (~135°) measured for BDT-2T-ID and BDT-4T-ID (which contain an even number of thiophene rings) denote more linear π-conjugated backbones compared to those of BDT-1T-ID and BDT-3T-ID, which exhibit smaller bending angles (115–120°). Figure 2b shows the frontier orbital distributions and corresponding energy levels calculated for BDT-nT-IDs. The wave functions of the highest occupied molecular orbitals (HOMOs) are delocalized over the electron-rich BDT core and neighboring oligothiophene moieties. Apparently, such πelectron delocalization leads to a gradual increase in the calculated HOMO energy with increasing number of thiophene rings in the π-bridging units. Meanwhile, the wave functions of the lowest unoccupied molecular orbitals (LUMOs) are mainly distributed on the terminal ID units and some of the outer thiophene rings adjacent to them, resulting in similar LUMO energy levels among the four molecules. The DFT calculation results also suggest that effective intramolecular charge transfer (ICT), primarily dominated by HOMO → LUMO transitions with considerably large oscillator strengths (f), takes place during the electronic excitation process. The UV–vis absorption spectra of BDT-nT-IDs in chloroform solution and as solid thin films are depicted in Figure 3, and the relevant photophysical data are compiled in Table 1. In dilute solutions (Figure 3a), the present molecules exhibit intense and broad ICT absorption bands with peaks (λmax) at similar wavelengths (around 540–550 nm), almost independent of the length of their π-conjugated chain. However, the molar absorption coefficient (ε) is found to increase when the length of the π-bridging oligothiophene units is extended from 1T (ε = 1.04 × 105 M−1 cm−1) to 4T (ε = 1.21 × 105 M−1 cm−1), similar to the trend of the calculated f values. The large ε values (>105 M−1 cm−1) measured for all BDT-nT-IDs can be attributed to the high planarity of their π-

extended backbone, which facilitates the ICT electronic transitions. In neat thin films (Figure 3b), the BDT-nT-ID molecules exhibit considerably red-shifted and broadened absorption profiles with a distinct vibronic shoulder around ~650 nm, relative to those in solution, denoting the formation of intermolecular π-stacked aggregates. However, BDT-1T-ID shows a narrower and weaker photoabsorption in comparison with the other molecules, most likely due to its shorter πconjugation length. As expected, the optical bandgaps (Eg) estimated from the onset wavelength of the thin-film absorption spectra decrease in the order BDT-1T-ID > BDT2T-ID > BDT-3T-ID ≈ BDT-4T-ID (see Table 1).

Figure 3. UV–vis absorption spectra of BDT-nT-IDs in (a) chloroform solutions and (b) as-spun thin films. (c) Photoelectron yield spectra of BDT-nT-IDs in thin films measured in air.

Photoelectron yield spectroscopy measurements were then performed to determine the HOMO energy levels (or



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Table 1. Thermal and Optical Properties of BDT-nT-IDs (n = 1–4)

compound BDT-1T-ID

Tma (°C)

∆Hb (kJ mol−1)

∆Sb (J mol−1 K−1)

229

54.9

109

solutionc

λmaxe (nm) 550

thin filmd

εf (M−1 cm−1) 1.04 × 105 5

BDT-2T-ID

222

92.1

186

545

1.09 × 10

BDT-3T-ID

218

54.9

112

537

1.16 × 105

539

5

BDT-4T-ID

226

58.8

118

1.21 × 10

λmaxe (nm) 582, 632

αf (cm−1) 1.11 × 105

HOMOg (eV)

LUMOh (eV)

Egh (eV)

−5.23

−3.43

1.80

600, 650

1.33 × 10

5

−5.13

−3.41

1.72

606

1.49 × 105

−5.03

−3.38

1.65

605

5

−4.99

−3.35

1.64

1.55 × 10

a

Melting temperature determined by DSC. bMelting enthalpy (∆H) and entropy (∆S) estimated for the Cr → Iso phase transition upon heating in DSC. cMeasured in chloroform solution (10−5 M) at 300 K. dMeasured in a neat thin film (ca. 100 nm) spin-coated from a chloroform solution onto a quartz substrate. eAbsorption peak wavelength. fAbsorption coefficient at λmax. gDetermined using photoelectron yield spectroscopy in a neat film. hLUMO = HOMO + Eg, where Eg is the optical energy gap derived from the absorption onset of the neat film.

ionization potentials) of BDT-nT-IDs in the solid state. As shown in Figure 3c, when the number of thiophene rings is increased from one to four, the HOMO energy increases from −5.23 eV (for BDT-1T-ID) to −4.98 eV (for BDT-4T-ID), in agreement with the DFT calculations (see Figure 2b). We anticipate that the low-lying HOMO energy levels of these molecules (particularly BDT-1T-ID) acting as donors will potentially generate a high Voc in BHJ OSCs incorporating PC71BM as acceptor. This is because the Voc value strongly depends on the energy difference between the HOMO of the donor and the LUMO of the acceptor.43 The LUMO energy levels of BDT-nT-IDs were estimated to be approximately −3.4 eV, which is sufficiently higher than that of PC71BM (ca. −4.0 eV) to provide sufficient driving force for effective exciton dissociation (or electron transfer) at the donor/acceptor interface.44 Thermal Behavior. The thermotropic phase-transition behavior of BDT-nT-IDs was examined by combining differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and temperature-dependent X-ray diffraction (XRD) measurements. As can be seen from Figure 4a, all four materials underwent direct phase transition from crystalline (Cr) to isotropic (Iso) phases at similar high melting temperatures (Tm) of 218–229 °C during heating. Intriguingly, BDT-1T-ID, BDT-2T-ID, and BDT-3T-ID exhibited monotropic liquid-crystalline (LC) mesomorphism upon cooling, whereas no obvious LC behavior was observed for BDT-4T-ID, which contained the longest π-bridging units. Note that the temperature range of the LC phases was reduced upon elongation of the π-bridging oligothiophene units from 1T to 3T, presumably because of the increased conformational fluctuations of the mesogenic backbones. Based on the typical Schlieren textures45 observed under POM (Figure 4b) and the very small transition enthalpies (∆H) obtained by DSC, the LC phases of both BDT-1T-ID and BDT-3T-ID, which were formed after cooling from the Iso melt, can be assigned as nematic (N) phases. Moreover, a markedly different mesomorphic behavior was observed for BDT-2T-ID, in which two distinct LC phases, corresponding to the lessordered N and layer-ordered smectic (Sm) phases, were specifically formed prior to crystallization upon cooling. In this case, the ∆H value for the N → Sm phase transition was determined to be 24.7 kJ mol−1, which is much larger than that of the Iso → N phase transition (1.2 kJ mol−1), but smaller than the crystallization enthalpy (58.5 kJ mol−1). The POM observation of BDT-2T-ID further supported the characterization of its phase-transition sequence as Iso → N

→ Sm → Cr; Schlieren and fan-like birefringent textures were successively observed in the N and Sm phases, respectively (Figure 4c,d). To date, only a few studies have reported photovoltaic LC materials for efficient BHJ OSCs with PCEs over 4%.13,46–48

Figure 4. Left panels (a): DSC thermograms of BDT-nT-IDs measured at a scanning rate of 10 °C min−1 under N2. Abbreviations: Cr, crystalline; Sm, smectic; N, nematic; Iso, isotropic. Right panels: POM images of (b) N phase of BDT-1TID, (c) N and (d) Sm phases of BDT-2T-ID, taken upon cooling from the Iso phase. The arrows in each POM image indicate the directions of polarizer and analyzer axes.

The melting enthalpies ∆H and the corresponding entropy changes ∆S (= ∆H/Tm) of BDT-nT-IDs, estimated from DSC, are listed in Table 1. It is worth noting that the ∆S value of BDT-2T-ID (186 J mol−1 K−1) is significantly larger than those measured for the other oligomers (109–118 J mol−1 K−1). This result indicates the presence of greater intermolecular forces in the more ordered Cr phase of BDT-2T-ID. The selforganization (or crystallization) behavior of the comb-shaped


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BDT-nT-ID molecules can be affected by both the overall shape (bending angles) of their centrosymmetric π-conjugated backbone and the relative size of their rigid backbone and flexible chains. Photovoltaic Performance. To investigate the photovoltaic properties of BDT-nT-IDs as donors, we fabricated solutionprocessed BHJ OSCs with an inverted device structure of indium tin oxide (ITO)/ZnO (30 nm)/donor:PC71BM (80–110 nm)/MoOx (10 nm)/Ag (100 nm).49,50 For all devices, the BHJ photoactive layer, consisting of a BDT-nT-ID:PC71BM binary blend, was prepared by spin-coating from their CHCl3 solutions, without any solvent additives or post-deposition treatments such as thermal and solvent vapor annealing. The OSC devices were first optimized by varying the D/A blend ratio (see SI for details). Upon increasing the D/A ratio from 1:1 to 3:1 (w/w), both Jsc and FF rapidly increased, resulting in enhanced PCEs. However, further increasing the donor

content to give a D/A ratio of 4:1, the PCE values slightly decreased. Overall, for all four donor oligomers, the best photovoltaic performances (i.e., the highest PCEs) were attained with the same D/A blend ratio of 3:1 and the active layer thickness of 80–110 nm. Figures 5a and 5b show representative current density– voltage (J–V) curves under simulated AM 1.5G illumination (100 mW cm−2) and the incident photon-to-current conversion efficiency (IPCE) spectra under monochromatic light, respectively, for optimized BHJ OSCs based on BDT-nTID:PC71BM (3:1, w/w) layers. Figure 5c shows the relationship between the key photovoltaic parameters and the length of the π-bridging oligothiophene units in BDT-nT-IDs, while Table 2 summarizes the corresponding photovoltaic data. The highest PCEs of up to 6.9% (6.7% on average) were achieved for the BDT-2T-ID-based devices, with Jsc = 14.2 mA cm−2, Voc = 0.97 V, and FF = 50%. Comparably high PCEs (>6%) were also found for the BDT-3T-ID-based

Figure 5. (a) J–V characteristics under one sun illumination at 100 mW cm−2 (inset: dark J–V curves) and (b) IPCE spectra for optimized BHJ OSCs based on as-deposited BDT-nT-ID:PC71BM (3:1, w/w) blend films. (c) Relationship between key photovoltaic parameters (PCE, Voc, Jsc, and FF) and number of thiophene rings (n) incorporated in BDT-nT-IDs.

Table 2. Optimized Photovoltaic Parameters for OSCs Based on BHJ Blends of BDT-nT-IDs and PC71BMa donor BDT-1T-ID

D/A ratio (w/w)

thickness (nm)

Jscb (mA cm−2)

Jc (mA cm−2)

Vocb (V)

FFb (%)

PCEb,d (%)

µD/Ae (cm2 V−1 s−1)

µDe (cm2 V−1 s−1)

3:1

107 ± 1

11.0 ± 0.1

12.1

1.06 ± 0.01

49 ± 1

5.7 ± 0.1

1.7 × 10−3

2.0 × 10−3

−3

5.7 × 10−2 2.2 × 10−2

BDT-2T-ID

3:1

93 ± 1

14.0 ± 0.2

13.8

0.96 ± 0.02

49 ± 1

6.7 ± 0.2

7.1 × 10

BDT-3T-ID

3:1

82 ± 2

13.5 ± 0.2

13.4

0.93 ± 0.01

49 ± 1

6.1 ± 0.1

4.2 × 10−3 −3

BDT-4T-ID 3:1 80 ± 1 11.3 ± 0.1 11.5 0.88 ± 0.01 51 ± 1 5.0 ± 0.1 5.4 × 10 1.9 × 10−2 Device structure: ITO/ZnO (30 nm)/donor:PC71BM (80–110 nm)/MoOx (10 nm)/Ag (100 nm); in the case of the optimized BDT-1T-ID and BDT-2T-ID-based devices, the ZnO layer was pretreated with a solution of 4-dimethylaminobenzoic acid in ethanol before depositing each active layer. bAverage value obtained from four individual devices. cCalculated by integrating the IPCE spectra. dAveraged power conversion efficiency (PCE) calculated as PCE = (Jsc × Voc × FF)/P0, where P0 is the incident light intensity (100 mW cm−2). eHole mobilities for the donor:PC71BM (3:1, w/w) blend film (µD/A) and pristine donor neat film (µD) determined by the SCLC technique. a



devices. On the other hand, the photovoltaic performance of the longest oligomer, BDT-4T-ID, was inferior to that of the shorter ones, mainly due to lower Jsc and Voc values. Among the optimized devices, the BDT-1T-ID-based one yielded a noticeably high Voc of ~1.06 V. As the π-bridging oligothiophene moieties were extended, the Voc of the OSCs decreased from 1.06 V (for BDT-1T-ID) to 0.88 V (for BDT4T-ID), which is consistent with the trend of the HOMO energy levels of the donor materials (see Figure 3c). In the IPCE spectra, all the BDT-nT-ID-based OSCs exhibited strong photoresponses, covering a broad UV-to-visible wavelength range of 300–750 nm, roughly consistent with the absorption spectra of the blend films (see SI). While the BDT-nT-ID donors contributed to the intense photoabsorption in the longer wavelength region, the main photoabsorption of PC71BM was located in the complementary shorter wavelength region. Notably, for the BDT-2T-ID-based devices, the maximum IPCE reached 70% within the 500–600 nm region, thus, resulting in the highest current density (J) of 13.8 mA cm−2, as estimated from the integration of the IPCE spectrum (see Table 2). This strong IPCE response suggests that the BHJ structure consisting of BDT-2T-ID and PC71BM can dissociate excitons and extract charge carriers more effectively, as we will discuss below. In addition, the BDT-2T-ID-based device exhibited a smaller series resistance (10.1 Ω cm2) and a larger shunt resistance (436 Ω cm2) than the other devices. The present systematic investigation thus clearly demonstrates that a judicious selection of the chain length of the π-bridging units is essential for the advanced design of high-performance donor molecules. Charge recombination is a critical factor that considerably affects the photovoltaic performance of BHJ OSCs. To gain insight into the charge recombination behavior of these systems, we examined the variation of Jsc and Voc as a function of illumination intensities (P) ranging from 1 to 100 mW cm−2 (i.e., 0.01–1 sun), as depicted in Figure 6. In general, the relationship between Jsc and P is described by the following power-law equation:51–54 Jsc ∝ Pα

(1)

where α is a parameter indicating the extent of bimolecular recombination. A value of α = 1 denotes absence of photocurrent loss, arising from minimal bimolecular recombination in the OSCs. By fitting the double logarithmic plots of Jsc versus P using eq. 1, α values of 0.92, 0.96, 0.94, and 0.91 were obtained for the OSCs containing BDT-nT-IDs with n = 1–4, respectively (Figure 6a). Thus, the BDT-2T-IDbased device demonstrated a more efficient charge sweep-out under short-circuit conditions, with a lower extent of bimolecular recombination than the other devices. To further examine the extent of bimolecular and monomolecular recombination processes in the OSCs, we analyzed the variation of Voc as a function of P (Figure 6b). This analysis can provide independent and complementary information on charge recombination under open-circuit conditions, in which all photogenerated charge carriers should recombine within the device (i.e., J = 0). In this case, the relationship between Voc and P can be described as:52–55

ln

(2)

where Egap is the energy difference between the HOMO of the donor and the LUMO of the acceptor, q is the elementary

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charge, k is the Boltzmann constant, T is temperature, PD is the dissociation probability of electron–hole pairs, γ is the Langevin recombination constant, NC is the density of states, and G is the generation rate of bound electron–hole pairs. Because G is directly proportional to the intensity of the incident light, the slope of Voc versus the logarithm of P provides insight into the degree of trap-assisted monomolecular charge recombination. In principle, a slope equal to kT/q indicates that bimolecular recombination is dominant. However, for trap-assisted monomolecular recombinations, a much stronger dependence of Voc on P with a slope of ~2kT/q can be observed.52–55 As shown in Figure 6b, the slopes of the fitted lines for the BDT-nT-ID-based OSCs range from 1.24kT/q to 1.48kT/q. Therefore, under opencircuit conditions, bimolecular recombination is dominant in the present systems, and trap-assisted monomolecular recombination is comparatively less relevant.

Figure 6. Dependence of (a) Jsc and (b) Voc on the incident light intensity for BHJ OSCs based on BDT-nT-ID:PC71BM (3:1, w/w) blend films. Solid lines represent linear fits to the respective data.

Film Morphology and Charge-Transport Properties. The structural ordering and orientation of the donor molecules have a significant impact on the exciton diffusion/dissociation and charge-transport characteristics of the BHJ photoactive layer, and hence on the photovoltaic performance. Grazing incidence X-ray diffraction (GIXD) was used to characterize the self-organized nanostructure and morphology of thin films made from pristine BDT-nT-IDs and BDT-nT-ID:PC71BM blends, fabricated by spin-coating (Figure 7 and SI). In addition to the two-dimensional (2D) diffraction images, the corresponding out-of-plane (qz-scan) and in-plane (qxy-scan) line-cut profiles are also shown. The neat films of pristine BDT-1T-ID, BDT-2T-ID, and BDT-3T-ID exhibited an intense (100) reflection together with multiple higher-order (h00) reflections in the out-of-plane direction, along with an evident (010) π–π stacking reflection in the in-plane direction. These results indicate that the three shorter oligomers are capable of forming lamellar structures with preferential edgeon molecular orientation and long-range ordering onto the substrates. On the other hand, in the case of the neat film of BDT-4T-ID, the (010) π–π stacking reflection and the (100) lamellar reflection mainly located in the out-of-plane and inplane directions, respectively (see SI); hence, with the extension of the π-conjugated backbone, the spontaneous


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molecular orientation switched from edge-on to face-on in the neat films.

Figure 7. (a) 2D GIXD images of the BDT-nT-ID:PC71BM (3:1, w/w) blend films. (b) Out-of-plane (qz-scan) and (c) in-plane (qxyscan) line-cut profiles of the GIXD of the blend films.

In the BHJ blend films with D/A ratio of 3:1 (Figure 7),

however, all four BDT-nT-ID oligomers were found to preferentially adopt an edge-on packing mode, with high crystallinity but a slight azimuthal orientation distribution. The (100) reflection peaks for the blend films of BDT-nT-IDs (n = 1–4) were clearly detected at 0.321, 0.327, 0.329, and 0.340 Å−1 in the out-of-plane direction, corresponding to nearly identical lamellar periodic distances of 19.6, 19.2, 19.1, and 18.5 Å, respectively. Moreover, the π–π stacking distances of BDT-nT-IDs (n = 1–4) along the in-plane direction were 3.68, 3.61, 3.68, and 3.68 Å, respectively. It is important to note that all lamellar and π–π stacking distances observed for BDT-nTIDs are comparable to those reported for the widely used regioregular poly(3-hexylthiophene) (ca. 16–17 and 3.8 Å, respectively).56–58 This implies that the π-bridging oligo(3hexylthiophene) units can control the overall self-organization of the BDT-nT-ID molecules through their laterally extended multiple alkyl chains. Using the Scherrer’s equation,49,59 the correlation lengths (or crystallite sizes) of the lamellar stacked assemblies within the blend films were estimated to be 21, 32, 18, and 24 nm (corresponding to 9–17 molecular stacks) for BDT-1T-ID through BDT-4T-ID, respectively. Although these crystallite sizes were smaller than those estimated for the corresponding neat films (31–45 nm; see SI), the lamellar stacked structures of BDT-nT-IDs were retained even after blending with PC71BM. The morphologies of the BHJ photoactive layers were further characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM topographic images (Figure 8a–d) revealed that the BDT-nTID:PC71BM blend films had relatively smooth and uniform surface morphologies, with root-mean-square (RMS) surface roughnesses ranging from 1.2 to 2.2 nm. The TEM images of the blend films (Figure 8e–h) highlighted nanoscale phase segregation and the spontaneous formation of bicontinuous D/A interpenetrating networks. The bright and dark regions in each image correspond to donor-rich and PC71BM-rich domains, respectively, because PC71BM has higher electron density than the donor oligomers. In particular, the BDT-2TID:PC71BM blend film exhibited a well-developed interpenetrating network morphology with larger domains (ca. 15 nm in size) composed of donor crystallites and acceptor

Figure 8. Tapping-mode AFM height images (top panels) and TEM images (bottom panels) of BHJ active layers based on (a,e) BDT1T-ID:PC71BM, (b,f) BDT-2T-ID:PC71BM, (c,g) BDT-3T-ID:PC71BM, and (d,h) BDT-4T-ID:PC71BM (3:1, w/w). (i) PSD profiles of the blend films obtained from FFT analysis of the corresponding TEM images.



agglomerates. This represents a desirable feature for achieving efficient charge separation and transport, and can be the source of the high Jsc and PCEs in the corresponding OSCs. However, less defined, much finer phase-segregated morphologies were found for the BDT-1T-ID and BDT-3T-ID systems, in agreement with the GIXD data (Figure 7). Such marked differences in the length scale of phase segregation (or domain size) within the blend films thus originate from the different intrinsic crystallinity of BDT-nT-IDs, rather than their miscibility with PC71BM. As presented in Figure 8i, the periodicity of each quasi-periodic phase-segregated structure was further quantified using the power spectral densities (PSDs) obtained from 2D fast Fourier transform (FFT) analysis of the TEM images.60,61 Based on the PSD peak positions, the mean periods, which correspond to the averaged widths of the D/A networks, were 5, 11, 6, and 8 nm for the blend films of BDT-nT-IDs with n = 1–4, respectively. The appropriate domain size should be small enough for efficient exciton diffusion/dissociation, matching the short exciton diffusion length, and also large enough for creating effective percolation channels of charge transport. The size of BDT-2TID domains formed in the BHJ blend film seemed to match these criteria. The hole mobility of the BDT-2T-ID:PC71BM blend film measured by the space-charge limited current (SCLC) method was 7.1 × 10−3 cm2 V−1 s−1 (see Experimental Section and SI for details), which is clearly higher than those obtained for the blend films of BDT-1T-ID (1.7 × 10−3 cm2 V−1 s−1), BDT-3TID (4.2 × 10−3 cm2 V−1 s−1), and BDT-4T-ID (5.4 × 10−3 cm2 V−1 s−1), in agreement with the GIXD and TEM results. Additionally, in its pristine neat films, BDT-2T-ID exhibited a rather high hole mobility of 5.7 × 10−2 cm2 V−1 s−1, approximately three times higher than those of BDT-3T-ID and BDT-4T-ID (which possess even longer π-conjugated backbones), and also more than one order of magnitude higher than that of BDT-1T-ID (see Table 2). The superior chargetransport ability of BDT-2T-ID suggests that the incorporation of regioregular bithiophene as π-bridging units in the BDTnT-ID system leads to improved molecular ordering and intermolecular electronic couplings in the condensed state.

CONCLUSIONS To explore the effective design for photovoltaic donor materials, in this study we have designed and synthesized a family of oligomeric BDT-nT-IDs (n = 1–4) with π-bridging oligo(3-hexylthiophene) segments of different length. We have shown that the number of thiophene rings incorporated in the π-conjugated backbone has a marked impact on the optical, electronic, and morphological properties of BDT-nT-IDs, and thus on their photovoltaic performance in OSCs. Among the four oligomers investigated, BDT-2T-ID (incorporating bithiophene units) adopts an optimal BHJ morphology when blended with PC71BM, involving bicontinuous interpenetrating networks with a size of ~10 nm, close to the exciton diffusion length. Consequently, the optimized BHJ OSCs based on the BDT-2T-ID:PC71BM blend films exhibited the highest PCEs, up to 6.9%, with IPCEs exceeding 70%. Compared to the other shorter and longer BDT-nT-ID systems, the superior photovoltaic properties of BDT-2T-ID can be primarily ascribed to its specifically high self-organization ability, as evident from the DSC, GIXD, TEM, and SCLC measurements. The versatile nature of these structures suggests that there is

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still scope for producing further improved A–π–D–π–A molecular systems for efficient BHJ OSCs, through the structural engineering of their π-bridging moieties. The present results thus provide a simple yet effective approach for further developing high-performance photovoltaic materials and devices.

EXPERIMENTAL SECTION Materials and Methods. 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 key intermediates, including Br-nT-CHOs and Br-nT-IDs, are reported in the SI. NMR spectra were recorded on an Avance III 400 spectrometer (Bruker) using tetramethylsilane (δ = 0.00) as an internal standard. MALDI-TOF mass spectra were collected on an Autoflex III spectrometer (Bruker Daltonics) using dithranol as a matrix. Elemental analyses were carried out using an MT-5 analyzer (Yanaco). UV–vis absorption spectra were measured with a V-670 spectrometer (Jasco). Photoelectron yield spectra were recorded on an AC-2 ultraviolet photoelectron spectrometer (Riken-Keiki). DSC measurements were performed on a DSC7000X system (Hitachi High-Tech Science) at a scanning rate of 10 °C min−1 under N2 atmosphere. Optical textures were inspected using a BX53 polarizing optical microscope (Olympus) equipped with an HS82 hot stage (Mettler Toledo) and an Infinity1-3C digital camera (Lumenera). The DFT calculations were performed with the Gaussian 09 program package, using the B3LYP functional with the 6-31G(d) basis set. Synthesis of BDT-1T-ID: To a stirred mixture of Br-1T-ID (0.42 g, 1.05 mmol) and 2,6-bis(trimethylstannyl)-4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene (0.45 g, 0.50 mmol) in dry dimethylformamide (DMF, 15 mL) was added Pd(PPh3)4 (24 mg, 0.02 mmol). The mixture was stirred overnight at 85 °C. After cooling to room temperature, the reaction mixture was poured into methanol. The formed precipitate was collected by filtration, and washed with methanol, ethyl acetate, and acetone. The product was purified by column chromatography on silica gel (eluent: CHCl3) and dried under vacuum to afford BDT-1T-ID as a black solid (yield = 0.48 g, 78%). This material was further purified by recycling preparative GPC (eluent: CHCl3) prior to use. 1H NMR (400 MHz, CDCl3): δ 7.98-7.96 (m, 4H), 7.90 (s, 2H), 7.89 (s, 2H), 7.86 (s, 2H), 7.79-7.78 (m, 4H), 7.39 (d, J = 3.5 Hz, 2H), 6.96 (d, J = 3.5 Hz, 2H), 2.90 (t, J = 7.5 Hz, 8H), 1.75-1.70 (m, 6H), 1.50-1.25 (m, 28H), 0.98 (t, J = 7.5 Hz, 6H), 0.92-0.88 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 190.21, 189.61, 146.38, 145.03, 144.53, 142.43, 142.07, 140.53, 139.73, 137.33, 136.76, 136.32, 136.03, 135.53, 135.05, 134.83, 128.13, 125.71, 124.45, 124.16, 123.73, 123.02, 122.84, 41.45, 34.34, 32.58, 31.62, 30.24, 29.47, 29.25, 28.96, 25.71, 23.06, 22.64, 14.19, 14.11, 10.90. MS (MALDI-TOF): m/z calcd 1222.42 [M]+; found 1222.22. Anal. calcd (%) for C74H78O4S6: C 72.63, H 6.42; found: C 72.52, H 6.47. Synthesis of BDT-2T-ID: This compound was synthesized according to the same procedure described above for BDT-1T-ID, except that Br-2T-ID (0.60 g, 1.05 mmol) was used as the reactant instead of Br1T-ID. BDT-2T-ID was obtained as a black solid (yield = 0.63 g, 81%). 1H NMR (400 MHz, CDCl3): δ 7.97-7.91 (m, 4H), 7.85 (s, 2H), 7.77-7.74 (m, 6H), 7.71 (s, 2H), 7.38 (d, J = 3.2 Hz, 2H), 7.30 (s, 2H), 6.95 (d, J = 3.2 Hz, 2H), 2.90-2.82 (m, 12H), 1.76-1.68 (m, 10H), 1.47-1.30 (m, 40H), 0.98-0.88 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 190.30, 189.69, 146.06, 145.31, 144.80, 142.07, 141.90, 140.95, 140.55, 139.17, 137.02, 136.80, 136.59, 135.65, 134.92, 134.68, 134.34, 133.65, 130.97, 127.91, 125.53, 123.89, 123.54, 122.93, 122.76, 122.02, 41.53, 34.37, 32.62, 31.66, 31.65, 30.58, 30.03, 29.68, 29.35, 29.18, 28.98, 25.79, 23.06, 22.66, 22.62, 14.17, 14.12, 14.09, 10.93. MS (MALDI-TOF): m/z calcd 1554.59 [M]+;


Page 9 of 12 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


found 1555.05. Anal. calcd (%) for C94H106O4S8: C 72.54, H 6.87; found: C 72.47, H 6.82. Synthesis of BDT-3T-ID: This compound was synthesized according to the same procedure applied for BDT-1T-ID, using Br-3T-ID (1.03 g, 1.40 mmol), 2,6-bis(trimethylstannyl)-4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene (0.60 g, 0.66 mmol), and Pd(PPh3)4 (31 mg, 0.027 mmol). BDT-3T-ID was obtained as a black solid (yield = 0.66 g, 53%). 1H NMR (400 MHz, CDCl3): δ 7.98-7.94 (m, 4H), 7.89 (s, 2H), 7.79-7.76 (m, 6H), 7.69 (s, 2H), 7.37 (d, J = 3.6 Hz, 2H), 7.31 (s, 2H), 7.06 (s, 2H), 6.93 (d, J = 3.6 Hz, 2H), 2.89-2.80 (m, 16H), 1.75-1.67 (m, 14H), 1.49-1.31 (m, 52H), 0.98-0.85 (m, 30H). 13C NMR (100 MHz, CDCl3): δ 190.37, 189.74, 145.92, 145.45, 145.09, 142.04, 141.52, 140.75, 140.62, 140.52, 139.03, 136.94, 136.75, 135.70, 134.91, 134.70, 134.62, 134.48, 133.40, 133.01, 131.50, 130.85, 129.31, 127.79, 125.45, 123.70, 123.34, 122.91, 122.73, 121.60, 41.50, 34.32, 32.57, 31.66, 30.56, 30.45, 30.04, 29.63, 29.48, 29.37, 29.29, 29.26, 29.21, 28.95, 25.75, 23.05, 22.63, 14.17, 14.11, 10.92. MS (MALDI-TOF): m/z calcd 1886.75 [M]+; found 1887.39. Anal. calcd (%) for C114H134O4S10: C 72.49, H 7.15; found: C 72.65, H 7.02.

angle of 0.1°. AFM measurements were carried out using a Dimension Icon scanning probe microscope (Bruker) in tapping mode in air. TEM measurements were performed using a JEM-2010 transmission electron microscope (Jeol) at an accelerating voltage of 120 kV. The spin-coated thin films on mica were peeled from the substrates by soaking in water and then transferred onto copper grids for the TEM observations. Carrier Mobility Measurements. The mobility measurements on the pristine neat films and BHJ blend films were carried out using the following diode structure: ITO/MoO3 (1 nm)/BDT-nT-ID or BDT-nTID:PC71BM layer/MoO3 (10 nm)/Ag (100 nm) for hole mobility; dark J–V curves were recorded over a voltage range of 0–8 V (see SI). The charge carrier mobilities were calculated using the SCLC model: J = (9/8)ε0εrµ(V2/L3), where ε0 is the permittivity of free space (8.85 × 10– 14 C V–1 cm–1), εr is the relative dielectric constant of the transport medium (assumed to be 3.0), µ is the hole mobility, and L is the thickness of the active layer.

ASSOCIATED CONTENT

Synthesis of BDT-4T-ID: This compound was synthesized according to the same procedure applied for BDT-1T-ID, using Br-4T-ID (1.20 g, 1.33 mmol), 2,6-bis(trimethylstannyl)-4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b']dithiophene (0.57 g, 0.63 mmol), and Pd(PPh3)4 (29 mg, 0.025 mmol). BDT-4T-ID was obtained as a black solid (yield = 1.05 g, 75%). 1H NMR (400 MHz, CDCl3): δ 7.97-7.94 (m, 4H), 7.88 (s, 2H), 7.80-7.76 (m, 6H), 7.68 (s, 2H), 7.37 (d, J = 3.6 Hz, 2H), 7.31 (s, 2H), 7.03 (s, 2H), 7.02 (s, 2H), 6.92 (d, J = 3.6 Hz, 2H), 2.91-2.79 (m, 20H), 1.77-1.67 (m, 18H), 1.46-1.33 (m, 64H), 0.98-0.85 (m, 36H). 13C NMR (100 MHz, CDCl3): δ 190.44, 189.80, 145.87, 145.57, 145.20, 142.02, 141.41, 140.70, 140.51, 140.43, 140.27, 138.97, 137.16, 136.98, 135.76, 134.94, 134.73, 134.40, 133.56, 133.30, 132.79, 131.14, 130.89, 129.13, 128.94, 127.74, 125.44, 123.61, 122.92, 122.73, 121.44, 41.48, 34.29, 32.54, 31.67, 30.58, 30.44, 30.04, 29.61, 29.45, 29.36, 29.29, 29.21, 28.94, 25.71, 23.05, 22.64, 14.18, 14.12, 10.91. MS (MALDI-TOF): m/z calcd 2218.91 [M]+; found 2219.92. Anal. calcd (%) for C134H162O4S12: C 72.45, H 7.35; found: C 72.33, H 7.29.

Supporting Information

Fabrication and Evaluation of OSC Devices. Pre-patterned ITOcoated glass substrates were cleaned by sequentially sonicating in detergent solution, deionized water, acetone, and isopropanol for 15 min each, and then subjected to UV/ozone treatment for 30 min. A thin layer (~30 nm) of ZnO was deposited by spin-coating (5000 rpm) a precursor solution of zinc acetate dehydrate (0.50 g) and ethanolamine (0.14 g) in 2-methoxyethanol (5 mL), followed by baking at 200 °C for 10 min under air. A BHJ photoactive layer was then prepared by spin-coating from a chloroform solution containing the BDT-nT-ID donor (10 mg mL−1) and a corresponding amount of PC71BM acceptor, after passing through a 0.45 µm PTFE membrane filter. The thickness of each photoactive layer was measured with an ET150 surface profilometer (Kosaka Laboratory). The thin films were then loaded into an E-200 vacuum evaporation system (ALS Technology). Finally, 10-nm-thick MoO3 and 100-nm-thick Ag layers were sequentially deposited on top of the photoactive layer under high vacuum through a shadow mask, defining an active area of 0.04 cm2 for each device. The J–V characteristics and IPCE spectra of the fabricated BHJ OSCs were measured with a computer-controlled Keithley 2400 source measure unit in air, under simulated AM 1.5G solar illumination at 100 mW cm−2 (1 sun) conditions, using a Xe lamp-based SRO-25 GD solar simulator and IPCE measurement system (Bunko-Keiki). The light intensity was calibrated using a certified silicon photovoltaic reference cell. For testing the OSCs under various light intensities (0.01–1 sun), the intensity of the light was modulated with a set of neutral density filters.

ACKNOWLEDGMENT

Thin-Film Characterization. GIXD experiments were conducted at the SPring-8 synchrotron radiation facility (Japan) using the BL45XU and BL40B2 beamlines. The samples were prepared on Si substrates in the same manner as actual devices, and then irradiated with an X-ray energy of 12.39 keV (λ = 1.0 Å) at a fixed incidence

The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis of intermediates, additional OSC, GIXD, and SCLC data, and NMR spectra (PDF)

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

ORCID Takuma Yasuda: 0000-0003-1586-4701

Notes The authors declare no competing financial interest.

This work was partially supported by Grants-in-Aid for Scientific Research (Grant No. JP16K21218 (H.K.)) from JSPS, the Izumi Science and Technology Foundation (H.K.), the ATI Research Grants 2016 (H.K.), and the KDDI Foundation (T.Y.), and the Canon Foundation (T.Y.). The authors are grateful for the support of the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, The GIXD measurements were performed at the SPring-8 using BL45XU and BL40B2 beamlines, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2015B1305 and 2016A1081).

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