An A–D–A′–D–A type small molecule acceptor with wide absorption spectrum and near-infrared absorption

Abstract

Small molecule acceptors with wide absorption spectra and small bandgap are very important for the sunlight harvesting of organic solar cells (OSCs). Recently, we have developed A–D–A′–D–A type small molecule acceptors with wide absorption spectra. However, the molecule (IID-IC) suffers from medium bandgap with poor light absorption in the near-infrared (NIR) region. Here, we develop a new A–D–A′–D–A type small molecule acceptor, IID-IC-O, with both wide absorption spectrum and small bandgap. IID-IC-O is designed by replacing the alkyl substituents of IID-IC with alkoxyl substituents. The replacement leads to upshifted HOMO energy level and improved backbone planarity of IID-IC-O. As a result, IID-IC-O exhibits a wide absorption spectrum spanning from 300 nm to 890 nm with a bandgap as small as 1.39 eV. The OSC device of IID-IC-O exhibits a power conversion efficiency of 4.20% with wide photoresponse from 300 nm to 900 nm. These results demonstrate the superior light harvesting capability of rationally designed organic molecules, which may be useful for many solar energy-related applications.

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Introduction

Organic solar cells (OSCs) have recently received great attention because of their solution processing with low cost, flexibility and light weight.^1–5 OSCs use a blend of polymer electron donor and electron acceptor as the active layer to convert solar energy to electricity.^6–18 The development of non-fullerene small molecule acceptors has boosted the power conversion efficiency (PCE) to more than 14%.^19–22 Small molecule acceptors should have strong sunlight harvesting capability, which requires both wide absorption spectra and small bandgap (for absorption in the near-infrared (NIR) region).^23–34 Typical small molecule acceptors are A–D–A type molecules, in which an electron-rich core unit (donor, D) is end-capped by two electron-deficient units (acceptor, A).^35–43 These A–D–A type small molecule acceptors exhibit only one strong absorption band attributed to the intramolecular charge transfer. The full width at half maximum (FWHM) of the absorption band is ca. 100–150 nm. Therefore, these small molecule acceptors suffer from narrow absorption spectra. Recently, we have proposed the A–D–A′–D–A strategy to develop small molecule acceptors by using an extra electron-deficient core unit.⁴⁴ The insertion of the A′ core unit partially disturbs the intramolecular charge transfer effect and leads to wide absorption spectra. However, the A–D–A′–D–A type small molecule acceptor, e.g., IID-IC (see Scheme 1), suffers from medium bandgap (E_g = 1.71 eV) with the onset absorption wavelength of 725 nm, which limits its absorption in the NIR region. Therefore, to maximize the sunlight harvesting capability, it is very challenging to develop small molecule acceptors with both wide absorption spectra and a small bandgap.

Scheme 1 Chemical structures of IID-IC and IID-IC-O, and schematic illustration of the effect of the oxygen atom.

In this manuscript, we report a new A–D–A′–D–A type small molecule acceptor, IID-IC-O, with both wide absorption spectrum and small bandgap. This is achieved by replacing the alkyl substituents of IID-IC with alkoxyl substituents (see Scheme 1). The electron donating oxygen atoms of the alkoxyl substituents increases the HOMO energy level of IID-IC-O and consequently leads to a reduced bandgap.^45,46 The absorption spectrum of IID-IC-O spans effectively from 300 nm to 890 nm with the optical bandgap of only 1.39 eV. The OSC device of IID-IC-O exhibits a PCE of 4.20% with wide photoresponse from 300 nm to 900 nm. These results demonstrate the superior light harvesting capability of rationally designed organic molecules, which may be useful for solar energy-related applications.

Results and discussion

Density functional theory calculations

Theoretical calculations by density functional theory (DFT) at the B3LYP/6-31G(d,p) level were conducted to investigate the influence of alkoxy groups on the molecular configuration and frontier molecular orbitals of IID-IC and IID-IC-O. Fig. 1 shows the optimized molecular geometries of IID-IC and IID-IC-O with the simplified side chains. The dihedral angles between the isoindigo and thiophene unit in IID-IC and IID-IC-O are 35.5° and 19.5°, respectively. The small dihedral angle of IID-IC-O indicates improved backbone planarity, which is due to the smaller steric hindrance of the oxygen atom than that of the methylene group.^47–51 According to the frontier molecular orbitals of IID-IC and IID-IC-O (Fig. S3, ESI†), the introduction of alkoxy groups has little impact on the electron density distributions of the LUMO of the molecule, but the oxygen atoms contribute to the HOMO significantly. Compared with those of IID-IC, the calculated HOMO energy level of IID-IC-O is upshifted by 0.29 eV and the calculated LUMO energy level is nearly unchanged.

Fig. 1 The geometry-optimized structure based on DFT calculations at the B31LYP/6-31G* level for the model molecules of IID-IC and IID-IC-O with all the long alkyl chains being replaced by methyl groups for simplification.

Synthesis

Scheme 2 shows the synthetic route of IID-IC-O. Starting from 3-methoxythiophene, alkylation, bromination and the subsequent Vilsmeier–Haack reaction afforded 4. The Suzuki coupling reaction between 4 and 5 produced 6. Finally, IID-IC-O was synthesized using the Knoevenagel condensation reaction in 85% yield. The chemical structure of IID-IC-O was fully characterized by ¹H NMR, ¹³C NMR, matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF-MS) and elemental analysis. According to thermogravimetric analysis (TGA), IID-IC-O exhibits excellent thermal stability with a decomposition temperature at 5% weight loss of 342 °C in a nitrogen atmosphere.

Scheme 2 Synthetic route of IID-IC-O. (Reagents and conditions: (i) 2-butyloctan-1-ol, p-toluenesulfonic acid monohydrate, toluene, 115 °C, 70%; (ii) N-bromosuccinimide, N,N-dimethylformamide, 0 °C, 97%; (iii) lithium diisopropylamide, tetrahydrofuran, −78 °C, then N,N-dimethylformamide, −78 °C to r.t., 71%; (iv) tetrakis(triphenylphosphine)palladium, potassium carbonate, tetrahydrofuran, 75 °C, 50%; (v) 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile, pyridine, chloroform, 65 °C, 85%.)

Optical and electrochemical properties

The UV-vis absorption spectra of IID-IC and IID-IC-O in chlorobenzene solution and in thin films are presented in Fig. S5 (ESI†) and Fig. 2a. The characteristics are summarized in Table 1. In solution, IID-IC-O shows two absorption bands with a peak at 684 nm, which is much more bathochromic than that of IID-IC. In a thin film, IID-IC-O exhibits maximum absorption at 805 nm, which is redshifted by ca. 200 nm compared to that of IID-IC. This is attributed to the upshifted HOMO energy level and reduced bandgap of IID-IC-O. According to the onset absorption wavelength, the optical bandgap of IID-IC-O is estimated to be 1.39 eV. IID-IC-O exhibits a wide absorption spectrum spanning from 300 nm to 890 nm, effectively covering the visible region and the NIR region, indicating strong sunlight harvesting capability.

Fig. 2 (a) The UV-vis absorption spectra of IID-IC and IID-IC-O in films; (b) cyclic voltammograms of IID-IC and IID-IC-O.

Table 1 Optical and electrochemical data of IID-IC and IID-IC-O

Molecule	λ ^solmax (nm)	λ ^filmmax (nm)	λ ^filmonset (nm)	E ^optg (eV)	ε (cm^−1)	E ^oxonset (V)	E ^redonset (V)	E HOMO (eV)	E LUMO (eV)
a Data from ref. 44.
IID-IC	501	605	725	1.71	6.86 × 10^4	+1.19	−0.85	−5.99	−3.95
IID-IC-O	684	805	890	1.39	1.06 10^5	+0.94	−0.87	−5.74	−3.93

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According to the DFT results, the wide absorption spectrum of IID-IC-O is ascribed to the overlap of different absorption bands, which mainly come from the transitions of HOMO → LUMO, HOMO−2 → LUMO, HOMO → LUMO+2, HOMO−1→ LUMO+1, HOMO → LUMO+4, HOMO−4 → LUMO and HOMO−3 → LUMO+1 (Fig. S4, ESI†). The maximum extinction coefficient of IID-IC-O is 1.06 × 10⁵ cm⁻¹, which is much higher than that of IID-IC (Table 1).

The HOMO/LUMO energy levels of IID-IC-O and IID-IC were estimated by cyclic voltammetry. The cyclic voltammograms are presented in Fig. 2b and the characteristics are summarized in Table 1. The HOMO and LUMO energy levels of IID-IC-O are estimated to be −5.74 eV and −3.93 eV, respectively. Compared with IID-IC, the HOMO energy level of IID-IC-O is up-shifted by 0.25 eV while the LUMO energy level is similar. The upshifted HOMO energy level is attributed to the electron-donating properties of the alkoxyl substituents. These results are consistent with the DFT calculation results. Fig. 3c compares the LUMO/HOMO energy levels of IID-IC-O and a typical polymer electron donor, poly[4,8-bis(5-(dodecylthio)thiophen-2-yl)-benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-5,6-difluoro-2-(2-hexyldecyl)-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (J61).⁵² The LUMO/HOMO energy levels of IID-IC-O are lower than those of J61 by 0.85 eV and 0.42 eV, respectively, indicating that IID-IC-O can work as an electron acceptor in OSCs.

Fig. 3 (a) OSC device structure; (b) chemical structure of polymer donor J61; (c) energy level alignment for J61, IID-IC and IID-IC-O.

Carrier mobility

We measured the electron mobility of IID-IC-O using a space charge limited current (SCLC) method based on the current density–voltage (J–V) curve of the electron-only device (Fig. S6, ESI†). The electron mobility of IID-IC-O is estimated to be 3.89 × 10⁻⁵ cm² V⁻¹ s⁻¹, which is nearly three times higher than that of IID-IC (1.04 × 10⁻⁵ cm² V⁻¹ s⁻¹).⁴⁴ The improved charge transport ability of IID-IC-O is possibly due to its more planar molecular backbone. However, the electron mobility of IID-IC-O is moderate, mainly due to the lack of ordered π–π stacking of the molecule backbone, which can be seen from the grazing incident X-ray diffraction (GI-XRD) pattern of the IID-IC-O film (Fig. S9a, ESI†).

Photovoltaic properties

To evaluate the photovoltaic performance of IID-IC-O, we used J61 as the polymer donor and IID-IC-O as the electron acceptor to fabricate the OSC device. The device architecture is ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/J61:IID-IC-O or IID-IC/Ca/Al (Fig. 3a). The current density–voltage (J–V) curves of the devices based on J61:IID-IC and J61:IID-IC-O are shown in Fig. 4a and the corresponding photovoltaic parameters are listed in Table 2. The device of J61:IID-IC-O exhibits a PCE of 4.20%, with an open-circuit voltage (V_OC) of 0.78 V, a short-circuit current density (J_SC) of 10.56 mA cm⁻² and a fill factor (FF) of 0.51. In comparison, the device of IID-IC shows a PCE of 2.82%, with a V_OC of 0.83 V, a J_SC of 6.36 mA cm⁻² and an FF of 0.53.⁴⁴ The IID-IC-O-based device exhibits a slightly lower V_OC but much higher J_SC than those of the IID-IC-based device. As the LUMO energy level of the two acceptors is similar, the different V_OC is maybe ascribed to the different blend morphology of the two systems. The photon energy loss (the difference between the E^opt_g and eV_OC) of the J61:IID-IC-O device is as small as 0.61 eV, indicating appropriate energy level alignment between J61 and IID-IC-O.⁵³Fig. 4b shows the external quantum efficiency (EQE) curves of the two devices. The device of J61:IID-IC-O displays a broad photoresponse in the range of 300–900 nm with the maximum EQE value of 40% at 820 nm. The EQE response of the J61:IID-IC-O device is stronger and wider than that of the J61:IID-IC device. The integrated photocurrents from the EQE spectra are 6.63 and 10.96 mA cm⁻² for J61:IID-IC and J61:IID-IC-O, respectively, which are consistent with the J_SC values obtained from the J–V measurements.

Fig. 4 (a) J–V curves and (b) EQE spectra of the IID-IC-based and IID-IC-O-based OSC devices.

Table 2 The device parameters of OSCs based on J61:IID-IC and J61:IID-IC-O under the illumination of AM 1.5G, 100 mW cm⁻²

Acceptor	V OC (V)	J SC (mA cm^−2)	FF	PCE (%)
a Data from ref. 44.
IID-IC	0.83	6.36	0.53	2.82
IID-IC-O	0.78	10.56	0.51	4.20

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Charge transport behavior in OSCs

We measured the charge mobilities of the two active layers by an SCLC method with the J–V curves of the electron-only and hole-only devices (Fig. S7, ESI†). The device structures are ITO/PEIE/J61:IID-IC-O/Ca/Al and ITO/PEDOT:PSS/J61:IID-IC-O/MoO₃/Al, respectively. The electron mobility (μ_e) and hole mobility (μ_h) of the J61:IID-IC-O film are estimated to be 6.47 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 1.62 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. These values are higher than those of the J61:IID-IC film (μ_e = 4.18 × 10⁻⁵ cm² V⁻¹ s⁻¹ and μ_h = 1.94 × 10⁻⁴ cm² V⁻¹ s⁻¹).⁴⁴ The improved electron mobility should contribute to the high J_SC of the IID-IC-O-based device. However, the electron/hole mobilities of the J61:IID-IC-O film are relatively low, which may be ascribed to the less ordering in the blend (Fig. S9b, ESI†). The relatively low mobilities in the blend may be one of the reasons for the moderate PCE.

Charge generation, collection and recombination behaviors

To investigate the charge generation, collection and recombination behaviors, we measured the photocurrent density (J_ph) versus the effective voltage (V_eff) and light density dependent J–V characteristics of the IID-IC and IID-IC-O based devices. As shown in Fig. 5a, the IID-IC-O-based devices show much higher J_ph than IID-IC-based devices, which is consistent with the improved photon harvesting ability of IID-IC-O. As the V_eff rises, the J_ph increases and does not saturate for both the devices. The J_ph,SC/J_ph,sat value (J_ph,SC is the J_ph under short-circuit condition, J_ph,sat is the J_ph at saturation) is used to evaluate the charge collection process in the OSC device.⁵⁴ The J_ph,SC/J_ph,sat values (the J_ph at V_eff of 7 V was selected as J_ph,sat here) for the IID-IC and IID-IC-O based devices are 60.6% and 69.5%, respectively.⁴⁴ This suggests that the charge collection is enhanced for the IID-IC-O-based device. The relationship between J_SC and light intensity (P) could be described as J_SC ∝ P^α, where α is the power-law exponent. If all the charges are swept out and efficiently collected by the electrodes, α should be equal to 1, while α < 1 means the existence of bimolecular recombination. As shown in Fig. 5b, the α value for the IID-IC-O-based device is 0.99, which is higher than that (α = 0.94) for the IID-IC-based device.⁴⁴ It suggests that the bimolecular recombination is suppressed for the IID-IC-O-based device compared to the IID-IC-based device. The improved charge generation and collection, and reduced charge recombination agree well with the higher device performance of the IID-IC-O-based device.

Fig. 5 (a) J_phversus V_eff plots; (b) dependence of J_SC on light intensity for OSCs based on J61:IID-IC and J61:IID-IC-O.

Film morphology

The morphology of the J61:IID-IC-O and J61:IID-IC blends was studied by atomic force microscopy (AFM). The height images of the two blends are shown in Fig. S10a and c (ESI†). The J61:IID-IC blend film shows small domains with a low root-mean-square (RMS) roughness of 1.98 nm,⁴⁴ while the J61:IID-IC-O blend film exhibits relative larger domains and a higher RMS roughness of 3.97 nm. This is consistent with the larger phase separation size in the phase images (Fig. S10b and d, ESI†).⁴⁴ The increased phase-separation may be due to the increased aggregation of IID-IC-O caused by the more planar molecular backbone, and this aggregation also facilitates the electron transport in the active layer. Photoluminescence (PL) spectroscopy was employed to study the exciton dissociation behavior in the blends (Fig. S11, ESI†). For the blend films, the PL quenching efficiency of IID-IC is 90% while the PL quenching efficiency of IID-IC-O is 84% when using the photo excitation at 600 nm. The reduced PL quenching efficiency in the J61:IID-IC-O blend film indicates the formation of a coarser morphology with increased phase separation, which are coincident well with those of AFM tests.

Conclusions

In summary, we have developed a new A–D–A′–D–A type small molecule acceptor, IID-IC-O, with alkoxyl substitutes as the side chains. Compared with the control molecule with alkyl substitutes, IID-IC-O exhibits greatly upshifted HOMO energy level and improved backbone planarity, and consequently an obvious redshift of absorption spectrum. IID-IC-O shows a wide absorption spectrum spanning from 300 nm to 890 nm and a small bandgap of 1.39 eV. The OSC device of IID-IC-O exhibits a PCE of 4.20% with wide photoresponse from 300 nm to 900 nm. Our results indicate superior light harvesting capability of rationally designed organic molecules, which may be applicable for many solar energy-related applications beyond OSCs.

Experimental

Materials and instruments

All chemicals and reagents were purchased from commercial sources and used as received unless otherwise mentioned. Toluene, tetrahydrofuran and chloroform were dried using sodium or calcium hydroxide before use. ¹H and ¹³C NMR spectra were measured using a Bruker AV-400 MHz NMR spectrometer. Mass spectra were measured on a Bruker Daltonics Flex matrix-assisted laser desorption ionization time of flight mass spectrometer (MALDI-TOF-MS). Elemental analysis was recorded on a VarioEL elemental analyzer. Thermal analysis was performed on a Perkin–Elmer 7 instrument under nitrogen flow at a heating rate of 10 °C min⁻¹. UV-vis absorption spectra were measured with a Shimadzu UV-3600 spectrometer in spectral grade solvent. Cyclic voltammetry (CV) was performed on a CHI660a electrochemical workstation using Bu₄NClO₄ (0.1 mol L⁻¹) in acetonitrile as electrolyte solution and ferrocene as an internal reference at a scan rate of 100 mV s⁻¹. The CV cell consisted of a Pt wire counter electrode, a glassy carbon electrode, and a standard calomel reference electrode. The small molecules were cast on the working electrode for measurements. The ferrocene/ferrocenium (Fc/Fc⁺) couple was used as an internal standard, which was assigned an absolute energy of −4.80 eV. The HOMO/LUMO energy levels of the material were estimated by the equations: E_HOMO/LUMO = −(4.80 + E^ox_onset/E^red_onset) eV. Atomic force microscopy (AFM) was performed with an SPA300HV (Seiko Instruments, Inc., Japan) in tapping mode. The thickness of various layers was measured by a step profiler with a Dektak 6M Stylus Profile. The out-of-plane grazing incidence X-ray diffraction (GI-XRD) was measured on a Rigaku SmartLab X-ray diffractometer with an X-ray generation power of 40 kV tube voltage and 30 mA tube current. Photoluminescence (PL) was measured using an FL3-KIT (Horiba Instruments Inc., USA).

3-((2-Butyloctyl)oxy)thiophene (2). 2-Butyloctan-1-ol (7.35 g, 39.42 mmol) was added to a mixed solution of 3-methoxythiophene⁵⁵ (2.25 g, 19.71 mmol) and p-toluenesulfonic acid monohydrate (0.37 g, 1.97 mmol) in anhydrous toluene (20 mL) under an argon atmosphere. The mixture was stirred at 115 °C for 12 h. After cooling, the mixture was poured into water and extracted three times with dichloromethane. The organic layer was washed with water and then dried over anhydrous Na₂SO₄ and the solvent was then removed by rotary evaporation. The crude product was purified by silica gel chromatography to give 2 (3.70 g, yielded 70%) as a colorless liquid. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.16 (dd, J = 8.0 Hz, J = 4.0 Hz, 1H), 6.75 (d, J = 5.1 Hz, 1H), 6.21 (d, J = 1.4 Hz, 1H), 3.81 (d, J = 5.6 Hz, 2H), 1.82–1.69 (m, 1H), 1.50–1.18 (m, 16H), 0.96–0.81 (m, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 158.37, 124.39, 119.66, 96.83, 73.23, 38.03, 31.88, 31.40, 31.09, 29.70, 29.10, 26.86, 23.07, 22.69, 14.11, 14.09.

2-Bromo-3-((2-butyloctyl)oxy)thiophene (3). To the solution of 3-((2-butyloctyl)oxy)thiophene (2) (0.84 g, 3.13 mmol) in N,N-dimethylformamide (DMF) (6 mL), N-bromosuccinimide (0.56 g, 3.13 mmol) was added at 0 °C in the dark. The mixture was stirred at 0 °C for 1 h exactly. Then, the reaction mixture was poured into water and extracted with dichloromethane. The organic phase was washed with water three times, dried over anhydrous Na₂SO₄, filtered and concentrated. The crude product was purified by silica gel chromatography to give 3 (1.05 g, yielded 97%) as a colorless oil. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.18 (d, J = 5.9 Hz, 1H), 6.74 (d, J = 5.9 Hz, 1H), 3.90 (d, J = 5.7 Hz, 2H), 1.80–1.70 (m, 1H), 1.47–1.21 (m, 16H), 0.95–0.84 (m, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 154.85, 124.07, 117.54, 91.56, 75.13, 38.20, 31.88, 31.20, 30.89, 29.70, 29.06, 26.82, 23.08, 22.71, 14.14, 14.11.

5-Bromo-4-((2-butyloctyl)oxy)thiophene-2-carbaldehyde (4). Lithium diisopropylamide (LDA) (1.20 mL, 2.00 mol L⁻¹) was added dropwise to a solution of 2-bromo-3-((2-butyloctyl)oxy)thiophene (3) (0.80 g, 2.30 mmol) in anhydrous THF (10 mL) at −78 °C under an argon atmosphere. The mixture was kept at −78 °C for 2 h before N,N-dimethylformamide (DMF) (0.53 mL, 6.90 mmol) was added slowly, then the mixture was allowed to warm and kept at room temperature for 6 h. The reaction was quenched by water and extracted by dichloromethane three times. The organic phase was washed with water three times, and dried over anhydrous Na₂SO₄ and the solvent was then removed by rotary evaporation. A purified product was obtained by silica gel chromatography as a yellow oil (0.61 g, yielded 71%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.70 (s, 1H), 7.37 (s, 1H), 3.96 (d, J = 5.6 Hz, 2H), 1.84–1.72 (m, 1H), 1.52–1.22 (m, 16H), 0.98–0.81 (m, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 181.30, 155.80, 140.07, 123.59, 105.58, 75.40, 38.06, 31.82, 31.14, 30.83, 29.63, 29.00, 26.77, 23.01, 22.67, 14.10, 14.03.

IID-Th-O-CHO (6). Potassium carbonate (2.00 mL, 2.0 mol L⁻¹ in H₂O) was added to a mixture of 6,6′-(N,N′-2-butyloctyl)-pinacoldiboronisoindigo (5)⁴⁴ (0.46 g, 0.54 mmol), 5-bromo-4-((2-butyloctyl)oxy)thiophene-2-carbaldehyde (4) (0.50 g, 1.33 mmol), Pd(PPh₃)₄ (16 mg, 0.01 mmol), and THF (10 mL), and the mixture was stirred at 75 °C for 12 h. Then the reaction mixture was poured into water, and the organic phase was extracted by dichloromethane three times. The organic phase was dried over anhydrous Na₂SO₄ and the solvent was then removed by rotary evaporation. Purification by silica gel chromatography afforded the product 6 (0.32 g, yielded 50%) as a brown black solid. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.82 (s, 2H), 9.20 (d, J = 8.5 Hz, 2H), 7.57 (s, 2H), 7.54 (d, J = 8.5 Hz, 2H), 7.31 (s, 2H), 4.06 (d, J = 5.3 Hz, 4H), 3.71 (d, J = 7.4 Hz, 4H), 2.03–1.93 (m, 2H), 1.90–1.81 (m, 2H), 1.51–1.18 (m, 64H), 0.98–0.75 (m, 24H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 181.93, 168.50, 155.22, 145.59, 138.00, 136.05, 132.34, 131.21, 130.04, 124.12, 121.74, 121.13, 106.60, 75.14, 44.77, 38.24, 36.34, 31.82, 31.66, 31.36, 31.04, 29.73, 29.65, 28.99, 28.70, 26.75, 23.11, 23.00, 22.62, 14.01. Anal. calcd for C₇₄H₁₁₀N₂O₆S₂: C, 74.83; H, 9.33; N, 2.36. Found: C, 74.66; H, 9.42; N, 2.31.

IID-IC-O . IID-Th-O-CHO (6) (0.21 g, 0.17 mmol) and 1,1-dicyanomethylene-3-indanone (0.17 g, 0.88 mmol) were dissolved in anhydrous CHCl₃ (50 mL), and then pyridine (0.8 mL) was added to the solution. Then, the mixture was allowed to stir at 65 °C overnight. After cooling, the reaction mixture was poured into water and extracted three times with chloroform. The organic phase was washed with water three times, and dried over anhydrous Na₂SO₄ and the solvent was then removed by rotary evaporation. The crude product was purified by silica gel chromatography and recrystallization in n-hexane/dichloromethane/chloroform to give IID-IC-O (0.23 g, yielded 85%) as a black solid. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 9.20 (d, J = 8.5 Hz, 2H), 8.72 (s, 2H), 8.70 (d, J = 7.2 Hz, 2H), 7.94 (d, J = 6.7 Hz, 2H), 7.82–7.72 (m, 4H), 7.71 (s, 2H), 7.67 (d, J = 8.6 Hz, 2H), 7.41 (s, 2H), 4.11 (d, J = 5.1 Hz, 4H), 3.75 (d, J = 7.1 Hz, 4H), 2.07–1.96 (m, 2H), 1.95–1.86 (m, 2H), 1.50–1.17 (m, 64H), 0.97–0.78 (m, 24H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 187.77, 168.37, 160.30, 156.37, 145.69, 140.03, 138.17, 137.01, 136.72, 135.99, 135.18, 134.58, 133.33, 132.40, 130.33, 130.30, 130.13, 130.10, 125.35, 123.98, 123.84, 122.52, 121.74, 114.47, 106.77, 75.18, 69.94, 44.79, 44.73, 38.28, 36.41, 31.84, 31.80, 31.74, 31.41, 31.09, 29.77, 29.69, 29.00, 28.75, 26.77, 26.52, 23.14, 23.04, 22.65, 22.64, 14.05, 14.02, -0.04. MS (MALDI): m/z 1538.9 (M⁺). Anal. calcd for C₉₈H₁₁₈N₆O₆S₂: C, 76.42; H, 7.72; N, 5.46. Found: C, 76.30; H, 7.75; N, 5.58.

OSC device fabrication and measurement

The device architecture was ITO/PEDOT:PSS/J61:IID-IC-O/Ca/Al. IID-IC-O was used as an electron acceptor. J61 was purchased from Solarmer materials, Inc, with the number-average molecular weight (M_n) of 24900 and polydispersity index of 2.37. Indium tin oxide (ITO) glass substrates were cleaned by sequential ultrasonication in deionized water, acetone, and isopropyl alcohol, followed by drying at 120 °C for 0.5 h and treatment with UV-ozone for 0.5 h. Then PEDOT:PSS (Baytron PVP Al 4083) was spin-coated on the ITO glass substrates at 5000 rpm for 40 s to give a thickness of 40 nm, followed by baking at 120 °C for 0.5 h. The substrates were transferred to a nitrogen-filled glove box. The blend was spin-coated onto the PEDOT:PSS layer to produce the active layer. The blend ratio was 1 : 1.3 (w : w) for J61:IID-IC-O in o-DCB solution. The active layer of the J61:IID-IC-O blend was annealed at 160 °C for 10 min. Finally, the active layer was transferred to a vacuum chamber, and Ca (20 nm) and Al (100 nm) were deposited by thermal evaporation at the pressure of about 2 × 10⁻⁴ Pa. The active area of each device was 8 mm². The current density (J–V) curves of the OSC devices were measured using a computer-controlled Keithley 2400 source meter under 100 mW cm⁻² AM 1.5G simulated solar light illumination provided by an XES-40S2-CE Class Solar Simulator (Japan, SAN-EI Electric Co., Ltd). The EQE spectra were measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd). The light intensity at each wavelength was calibrated using a calibrated monosilicon diode.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support by the National Key Basic Research and Development Program of China (973 program, Grant No. 2015CB655001) Founded by MOST, the National Natural Science Foundation of China (No. 21625403, 21875244, 51603203, and 21875241), the Jilin Scientific and Technological Development Program (No. 20170519003JH), the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB12010200) and a project supported by State Key Laboratory of Luminescence and Applications (SKLA-2016-02).

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR, TGA measurement, DFT calculations, charge mobility measurements, film morphology and PL measurements. See DOI: 10.1039/c8qm00441b

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