New Molecular Donors with Dithienopyrrole as the Electron-Donating Group for Efficient Small-Molecule Organic Solar Cells

Abstract

Six new molecules with donor−acceptor−acceptor (D−A−A) configuration bearing coplanar electron-donating dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or aryl-substituted DTP as the D unit and the electron-withdrawing pyrimidine−dicyanovinylene or benzothiadiazole−dicyanovinylene as the A−A block are synthesized. The introduction of aryl groups (p-tolyl or p-anisolyl) onto the α position of DTP is found to greatly benefit the chemical stability and extend the molecular conjugation of the DTP-based D−A−A molecules. The observed broad absorption spectra and anisotropic molecular orientation in the thin films allow for these new D−A−A molecules to perform good capability of light harvesting. These new D−A−A donors are subject to the fabrication of vacuum-processed small-molecule organic solar cells (SMOSCs). The results showed that the structural modulation on the central A block (pyrimidine versus benzothiadiazole) as well as the terminal substitution (p-tolyl versus p-anisolyl)give an evident trade-off between the open circuit voltage (V oc ) and short circuit current density (J sc ) values. Among them, SMOSC-adopted D−A−A molecule TDPM composed of a p-tolyl terminal group and a D−A−A configuration of DTP− pyrimidine−dicyanovinylene as the electron donor combined with C 70 as the electron acceptor shows a J sc of 11.34 mA/cm 2 , a V oc of 0.94 V, and a fill factor (FF) of 0.52, giving a best power conversion efficiency (PCE) as high as 5.6%.

Tuning the Optoelectronic Properties of Nonfullerene Electron Acceptors

“…This simple analysisd oes not take into account the cavity optics effects that are present in such thin-film solarc ells, but to first order this analysis can be used to understand the origin of the photocurrent. [18] For 18,t here is clear photocurrent generation beyondt he absorption edge of P3HT (the onset is 850 nm), which can only come from absorption of the acceptor followed by hole transfer to the polymer (the Channel II mechanism). However,t he shape of the EQE at shorter wavelengths suggests that the light absorption by 6 contributes to the photocurrent.…”

“…pound 10 has been recently reported as ad onor materiali n blends with PCBM. [18] For 18,t here is clear photocurrent generation beyondt he absorption edge of P3HT (the onset is 850 nm), which can only come from absorption of the acceptor followed by hole transfer to the polymer (the Channel II mechanism). Compared to 6, 10,a nd 14,asignificantly higher EQE was measured for 18.T he optimum annealing temperature for 18 was slightly highert han that for the other compounds;i t was found to be in the range of 120-140 8C.…”

“…After tetrakis(triphenylphosphine)palladium(0) (4.5 mg, 0.004 mmol) was added, the mixture was deoxygenated af urther five times, and then heated at reflux for 20 h. After cooling to room temperature, the organic layer was separated and the aqueous layer was extracted with dichloromethane (3 5mL). [1,2,5]thiadiazol-4-yl)methylene]malononitrile (10) [18] Am ixture of 9 (450 mg, 1.0 mmol), malononitrile (1.3 g, 20 mmol) and pyridine (0.5 mL) in anhydrous toluene (20 mL) was deoxygenated five times by placing under vacuum (100 mBar) and backfilling with argon, and then heated at 70 8Cf or 16 h. After cooling to room temperature, the solvent was removed and the residue was purified by column chromatography over silica with ad ichloromethane/n-hexane mixture ( [22] (380 mg, 1.44 mmol) in anhydrous tet-rahydrofuran (40 mL) that had been cooled in an acetone/dry ice bath under argon. The solvent was removed under reduced pressure and the residue was purified by column chromatography over silica using ad ichloromethane/nhexane (80:2 0) mixture as the eluent to give 5 as an orange gum (45 mg, 78 %).…”

“…1 (6) As olution of 5 (135 mg, 0.31 mmol), malononitrile (202 mg, 3.1 mmol), and pyridine (0.5 mL) in chloroform (13 mL) was deoxygenated five times by placing under vacuum (100 mBar) and backfilling with argon, and then heated at reflux for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure and the residue was purified by column chromatography over silica using ad ichloromethane/n-hexane mixture ( [21] (735 mg, 2.52 mmol) in anhydrous tetrahydrofuran (50 mL) that had been cooled in an acetone/dry ice bath under argon. [1,2,5]thiadiazol-4-yl)methylene]malononitrile (18) As olution of 17 (271 mg, 0.51 mmol), malononitrile (670 mg, 10.1 mmol), and pyridine (0.7 mL) in anhydrous toluene (15 mL) was deoxygenated five times by placing under vacuum (100 mBar) and backfilling with argon. The reaction mixture was stirred with acetone/dry ice bath cooling for an additional 1h,a nd then allowed to warm to room temperature, and stirred overnight.…”

“…2-[(7-{6-(4-Fluorophenyl)-4,4-di-n-propyl-4 H-silolo[3,2-b:4,5b']dithien-2-yl}benzo[c] [1,2,5]thiadiazol-4-yl)methylene]malononitrile (18) As olution of 17 (271 mg, 0.51 mmol), malononitrile (670 mg, 10.1 mmol), and pyridine (0.7 mL) in anhydrous toluene (15 mL) was deoxygenated five times by placing under vacuum (100 mBar) and backfilling with argon. Then the mixture was heated at 70 8Cf or 20 h. After cooling to room temperature, the solvent was removed and the residue was purified by column chromatography over silica with ad ichloromethane/n-hexane mixture (7:3) as the eluent to give 18 as ab lack solid (280 mg, 94 %).…”