Hybrid Cycloalkyl‐Alkyl Chain‐Based Symmetric/Asymmetric Acceptors with Optimized Crystal Packing and Interfacial Exciton Properties for Efficient Organic Solar Cells

Abstract Hybrid cycloalkyl‐alkyl side chains are considered a unique composite side‐chain system for the construction of novel organic semiconductor materials. However, there is a lack of fundamental understanding of the variations in the single‐crystal structures as well as the optoelectronic and energetic properties generated by the introduction of hybrid side chains in electron acceptors. Herein, symmetric/asymmetric acceptors (Y‐C10ch and A‐C10ch) bearing bilateral and unilateral 10‐cyclohexyldecyl are designed, synthesized, and compared with the symmetric acceptor 2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐butyloctyl)‐3,9 bis(ethylhexyl)‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″′:4′,5′]thieno[2′,3′:4,5] pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐ diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (L8‐BO). The stepwise introduction of 10‐cyclohexyldecyl side chains decreases the optical bandgap, deepens the energy level, and enables the acceptor molecules to pack closely in a regular manner. Crystallographic analysis demonstrates that the 10‐cyclohexyldecyl chain endows the acceptor with a more planar skeleton and enforces more compact 3D network packing, resulting in an active layer with higher domain purity. Moreover, the 10‐cyclohexyldecyl chain affects the donor/acceptor interfacial energetics and accelerates exciton dissociation, enabling a power conversion efficiency (PCE) of >18% in the 2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (Y6) (PM6):A‐C10ch‐based organic solar cells (OSCs). Importantly, the incorporation of Y‐C10ch as the third component of the PM6:L8‐BO blend results in a higher PCE of 19.1%. The superior molecular packing behavior of the 10‐cyclohexyldecyl side chain is highlighted here for the fabrication of high‐performance OSCs.


Materials and Methods.
All the reagents, unless otherwise specified, were purchased from Sigma-Aldrich Co., J&K, and Tokyo Chemical Industry Co., Ltd., and were used without further purification. PM6 was purchased from Solarmer Materials Inc. All air and water-sensitive reactions were carried out under N 2 . Toluene and THF were dried by Na and then freshly distilled before use. The detailed synthetic procedures are as follows.
Scheme S1. The synthetic routes of Y-C10ch and A-C10ch.

Synthesis of compound 1
Under the protection of N 2 atmosphere, magnesium powder (4.7 g, 196 mmol), catalytic amount of I 2 (20 mg) and 10 mL of anhydrous THF were added into a three-necked round flask. The solution of bromomethylcyclohexane (24.8 g, 140 mmol) in THF (50 mL) was slowly dropped in the flask. The reaction mixture was refluxed for 4 h until the magnesium was fully consumed. After the reaction was cooled down to ambient temperature, the corresponding Grignard reagent was prepared.
tBuMgCl (100 mL, 1M in THF) was added into a two-necked round flask that contained a 10-bromodecanoic acid (25.1 g, 100 mmol), NMP (9.9 g, 100 mmol), and THF (150.0 mL) at -78 ℃ and the mixture was stirred for 10 min under N2 atmosphere. The Grignard reagent that was prepared, 1,3-butadiene (50 mL, 2M in THF), and NiCl 2 (65 mg) were added at the same temperature and the mixture was stirred with cooling in an ice-bath for 1 h. The reaction was quenched by the addition of aqueous HCl and the resulting solution was extracted with CH 2 Cl 2 . The combined extract was concentrated under reduced pressure, and purified by column chromatography on silica gel to yield 1 as a white solid (24.5 g, 91.3%). 1

Synthesis of compound 2
To a solution of compound 1 (24.5 g, 91.3 mmol) in CH 2 Cl 2 (150 mL) was added oxalyl chloride (34.8 g, 274 mmol) at 0 ℃. The mixture was stirred for 5 h at room temperature.
After removal of the solvents and remaining oxalyl chloride, the crude product was then used directly for the next step.
To a stirring mixture of 3-bromothiophene (14.9 g, 91.3 mmol) and the prepared acid chloride (91.3 mmol) in dichloromethane, AlCl 3 (12.2 g, 91.3 mmol) was added in portions over 30 min. The resulting solution was stirred for 4 h at room temperature. Cold water (300 mL) was then slowly added to quench the reaction. The crude product was extracted from the mixture with CH 2 Cl 2 . The combined extract was dried over Na 2 SO 4 and concentrated under reduced pressure, and purified by column chromatography on silica gel to yield 2 as orange liquid (20.8 g, 55% yield). 1

Synthesis of compound 3
Compound 2 (20.6 g, 50 mmol) and K 2 CO 3 (13.8 g, 100 mmol) were mixed with DMF (120 mL). To this mixture, ethyl thioglycolate (6.0 g, 50 mmol) was added dropwise under N2 atmosphere at 60 °C. The mixture was stirred overnight and poured into water (200 mL). The organic material was extracted with CH 2 Cl 2 . The combined organic layers were washed with brine and dried over anhydrous Na 2 SO 4 . The crude product of compound 3 was obtained by removing organic solvents and used directly for the next step without further purification.
To a mixture solution of compound 3 in ethanol (160 mL), NaOH (4.0 g, 100 mmol) was added. The resulting solution was refluxed overnight. After cooling to room temperature, the liquid was acidified with concentrated HCl. A yellow solid was collected by filtration and washed several times with water. The solid was then washed three times with petroleum ether to give 3 as a light pink solid (9.8 g, 48% yield). 1

Synthesis of compound 4
A mixture of compound 3 (9.5 g, 23.4 mmol), copper power (1.5 g) and quinolone (100 mL) was heated under N2 atmosphere at 260 °C in a Woods-metal bath. After 3 h, the mixture was cooled to room temperature and hexane (200 mL) was added to the quinoline mixture.
This mixture was then washed with HCl. The organic layer was dried over anhydrous Na 2 SO 4 and the solvent was removed. Compound 4 was obtained by chromatography on silica gel, eluting with petroleum ether (7.9 g, 93.1% yield). 1

Synthesis of compound 5
To a solution of compound 4 (7.8 g, 21.5 mmol) in tetrahydrofuran (120 mL), 2.4 M nbutyllithium (9.0 mL, 21.5 mmol) was added dropwise under N2 atmosphere at -78 °C. After stirring at -78 °C for 1 h, tributylstannyl chloride (7.0 g, 21.5 mmol) was added to the mixture at -78 °C, and the mixture was gradually warmed to room temperature. After stirring overnight, the reaction was quenched with water (100 mL). The mixture was extracted with petroleum ether, and the organic layer was dried over Na 2 SO 4 . Removing the solvent under reduced pressure gave the crude compound 5. Without any further purification, the product was used into the following reaction.

Synthesis of compound 7
In a 50 mL two-neck round-bottom flask, compound 6 (0.32 g, 0.338 mmol), triethyl phosphite (3.5 mL), and 1,2-dichlorobenzene (o-DCB, 1.0 mL) were added, and then evacuated and back-filled with N2 three times. The resulting reaction mixture was refluxed overnight under N2 atmosphere. After cooling to room temperature, the mixture was quenched with saturated aqueous solution of ammonium chloride and then extracted with dichloromethane. The organic layers were combined and dried over anhydrous Na2SO4. After removal of the solvent, the crude product was obtained as red oil and could be directly used

Synthesis of compound 8
To a solution of compound 7 (0.16 g, 0.144 mmol) in freshly distilled 1,2-dichloroethane (10 mL), the mixture of freshly distilled DMF (3.0 mL) and POCl3 (1.0 mL) was added dropwise under N2 atmosphere at 0 °C. After addition, the cooling bath was removed and the reaction mixture was refluxed at 80 °C for 4 h. The mixture was quenched with saturated aqueous solution of sodium acetate, stirred for another 1 h, and the reaction mixture was then extracted with dichloromethane. The organic layers were combined and washed with saturated brine solution and dried over anhydrous Na2SO4. After removal of the solvent, the crude product was purified by silica gel column chromatography, eluting with petroleum ether/dichloromethane (1:1) to obtain the product as orange solid (0.12 g, 71.6%). 1

Synthesis of compound 12
To

Measurements and characterization.
Nuclear magnetic resonance (NMR): 1H and 13C NMR were measured on a Bruker AV-500 MHz spectrometer in deuterated solvents at room temperature. Chemical shifts were recorded with tetramethylsilane (TMS) as the internal reference.

UV-vis absorption spectra:
The UVvis absorption spectra of solutions and films were recorded using a Hitachi U-4100 spectrophotometer.

GIWAXS and GISAXS characterization:
Grazing incidence wide-angle X-ray scattering (GIWAXS) and grazing incidence small-angle X-ray scattering (GISAXS) patterns were acquired by beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF). [1] The Xray wavelength was chosen as 0.124 nm (E = 10 keV), and the incidence angle was set to 0.15 degree. For GIWAXS and GISAXS experiments, the sample to detector distance was 276.26 mm and 5520.51 mm.
Single Crystal X-Ray Diffraction: Single crystals data collection of Y-C10ch, A-C10ch were performed at 193 K on a Bruker D8 Venture. Single crystals data of L8-BO are from CCDC No. 2005533. [2] All calculations were performed using the SHELXL and the crystallographic software package.
The High Sensitive EQE measurements: The High Sensitive EQE was measured by using an integrated system (LST-QE), where the photocurrent was amplified and modulated by a lock-in instrument. [3] Electroluminescence (EL) measurements: Electroluminescence (EL) quantum efficiency (EQE EL ) measurements were performed by applying external voltage/current sources through the devices (LST-QE). where J stands for current density, ε o is the permittivity of free space, ε r is the relative dielectric constant of the transport medium, µ is the hole mobility, V is the voltage drop across the device (V = V appl -V bi -V RS , where V appl is the applied voltage to the device, V bi is the built-in voltage due to the difference in work function of the two electrodes, and V RS is the voltage drop due to series resistance across the electrodes), and L is the thickness of the active layer.