Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells

Solution-processable small molecules for organic solar cells have attracted intense attention for their advantages of definite molecular structures compared with their polymer counterparts. However, the device efficiencies based on small molecules are still lower than those of polymers, especially for inverted devices, the highest efficiency of which is <9%. Here we report three novel solution-processable small molecules, which contain π-bridges with gradient-decreased electron density and end acceptors substituted with various fluorine atoms (0F, 1F and 2F, respectively). Fluorination leads to an optimal active layer morphology, including an enhanced domain purity, the formation of hierarchical domain size and a directional vertical phase gradation. The optimal morphology balances charge separation and transfer, and facilitates charge collection. As a consequence, fluorinated molecules exhibit excellent inverted device performance, and an average power conversion efficiency of 11.08% is achieved for a two-fluorine atom substituted molecule.

fitting of the in-plane GIWAXS peaks.

Supplementary Tables
Supplementary

Supplementary Discussion
Evidence of the role of surface enrichment and vertical phase separation: Conventional photovoltaic device performance and related characterization.
After fluorination, the lateral morphologies (molecular packing, TEM, domain size, domain purity) changed in the same tendency whether on ZnO or PEDOT: PSS ( Supplementary Fig. 7); while in the vertical direction, electron-blocking layer formed on the active layer and the hole transported path from the active layer to surface is only favored by inverted devices. This could explain the strange phenomenon that BTID-2F obtained a higher inverted device performance while BTID-0F obtained a higher conventional device performance. Hence, the surface enrichment and vertical phase separation played an important role to enhance the inverted device performance based on BTID-2F.

Evidence of the role of hierarchical morphology
To certify the influence of hierarchical morphology on device performance, we increased the substrate temperature from 28 o C (normal) to 40 o C (hot), because increasing the substrate temperature was to shorten the film forming time which would probably eliminate the hierarchical morphology. To our expectation, the hierarchical morphology disappeared on the hot substrate, as shown in the TEM images and RSoXs images ( Supplementary Fig. 9a). Consequently, the main domain spacing was decreased from 53 to 17 nm. Though the smaller domain spacing facilitated charge separation, however, the device performance based on the cool substrate (the hierarchical morphology) was higher than that on the hot substrate (Supplementary Table. 6). In addition, the domain purity and the molecular packing was similar ( Supplementary Fig. 9b-c). Hence, the hierarchical morphology would reduce the recombination loss in our materials system.

Characterization methods
(1) Molecular structure characterization and calculation (NMR, MS spectra, DFT). 1 H NMR (400 MHz) and 13   where ∆ 12 is the difference in electron density between the two phases, i is the material volume fraction of each domain and is the illuminated volume. Domain contrast affect the TSI and thus can be used as a measure of domain purity. In resonant soft x-rays region, can be extended as: K is shape factor (0.9 is used here), ∆ is FWHM of the peak. The example of fitting of FWHM has been shown in Supplementary Fig. 11b. where J is the current density (mA cm -2 ), L is the film thickness of the active layer (cm), μ is the hole or electron mobility, is the relative dielectric constant of the transport medium, ε 0 is the permittivity of free space (8.85 × 10 −14 F cm −1 ), and V is the internal voltage in the device. V = V appl − V r − V bi , where V appl is the applied voltage to the device, V r is the voltage drop owing to contact resistance and series resistance across the electrodes, and V bi is the built-in voltage owing to the relative work function difference of the two electrodes.

Materials and synthesis
All reagents and chemicals were purchased from Aldrich, Alfa and used as received. Solvents and other common reagents were obtained from the Beijing Chemical Plant. Toluene, chloroform and THF were freshly distilled prior to use. Other materials were used without further purification.

Compound 3 4-hexyl-5-(6-hexylthieno[3,2-b]thiophen-2-yl)thiophene-2-carbaldehyde
Under an Ar atmosphere, Pd(PPh 3 ) 4 (198 mg, 2% mmol) was added to the solution of compound 1 (3 g, 8.57 mmol), compound 2 (2.5 g, 9.12 mmol), and NaHCO 3 (2.16 g, 25.7 mmol) in a mixed solvent of 48 mL of THF, 16 mL of distilled water, and 16 mL of toluene. Then, Ar gas was bubbled for 20 min, and the mixture was heated to 85 °C and maintained at the temperature for 48 h. After the mixture was cooled to ambient temperature, the solvent was diluted in THF; the organic layer was washed with water for three times and then dried over MgSO 4 . After concentration, the crude product was purified with column chromatography on silica gel with a mixture of petroleum and dichloromethane (1:1) as eluent. The product was obtained as a yellow solid (3 g, 84%

Compound 4 5-(5-bromo-6-hexylthieno[3,2-b]thiophen-2-yl)-4-hexylthiophene-2-carbaldehyde
In an ice bath, NBS (1.28 g, 7.2 mmol) was added in portion to the solution of compound 3 (3 g, 7.2 mmol) in a mixed solvent of 50 mL of chloroform and 50 mL of acetic acid. After addition, the mixture was warmed to ambient temperature and left undisturbed overnight. The whole mixture was poured into 50 mL of chloroform: then, the organic layer was washed with water, saturated NaHCO 3 , and water for three times and then dried over MgSO 4 . After concentration, the crude product was purified with column chromatography on silica gel, with petroleum and dichloromethane (3:2) as eluent, and the product was obtained as a yellow solid (3 g, 84.3%

Compound 6 4-fluoro-1H-indene-1, 3(2H)-dione
A stirred solution of 4-fluoroisobenzofuran-1,3-dione (2 g, 12 mmol) and acetic anhydride (11 mL, 120 mmol) containing TEA (8.6 mL, 24 mmol) was added with tert-butyl acetoacetate (2.1 mL, 13 mmol). After being stirred for 6 h at room temperature, the reaction system was poured into a flask containing ice, and 5 N HCl (25 mL) was added drop-wise. The resulting mixture was stirred for 5 min, and then the flask was placed in an oil bath at 5 °C for 5 min. The flask was then cooled to room temperature, and the reaction was extracted several times with DCM (3×50 mL). The organic layers were combined, dried (sodium sulfate), filtered, and concentrated to yield a yellow solid at 60% (

Compound 8 4,7-difluoro-1H-indene-1,3(2H)-dione
The procedure was the same as that for compound 6.  compound 9 (496 mg, 1 mmol) in 40 mL of toluene. Then, Ar gas was bubbled for 20 min; then, the mixture was heated to 100 °C and maintained at the temperature for 12 h. After being cooled to ambient temperature, the mixture was evaporated, and the crude product was purified using column chromatography on silica gel with petroleum and dichloromethane (2:3); the product was given as a

BTID-0F
Under Ar protection, five drops of piperidine was added into the mixture of compound 12 (200 mg, 0.14 mmol) and 1H-indene-1,3(2H)-dione (207 mg, 1.4 mmol). After being stirred for 24 h at ambient temperature, the mixture was poured into water and extracted using CHCl 3 . The organic layer was washed with brine and water and then dried over MgSO 4 . After being concentrated, the crude product was purified by using column chromatography on silica gel, with a mixture of CHCl 3 and petroleum (2:3) as eluent, and then recrystallized with chloroform and hexane to yield the target

BTID-1F
Under the protection of Ar, five drops of TEA was added in the mixture of compound 10 (200 mg, 0.13 mmol) and 4-fluoro-1H-indene-1, 3(2H)-dione (215 mg, 1.3 mmol). After being stirred for 24 h at ambient temperature, the mixture was poured into water and extracted with CHCl 3 . The organic layer was washed with brine and water and then dried over MgSO 4 . After concentration, the crude product was purified with column chromatography on silica gel, with a mixture of CHCl 3 and petroleum

BTID-2F
The synthesized process was the same with compound 1F.