Tuning the Optoelectronic Properties of Vinylene-Linked Donor−Acceptor Copolymers for Organic Photovoltaics

Abstract

Five new donor-acceptor copolymers containing the electron acceptor benzothiadiazole (BTZ) linked to the electron donors fluorene (FL) or cyclopentadithiophene (CPDT) via vinylene units were synthesized to study polymer structure-property relationships in organic photovoltaic devices. Both alternating (P) and random copolymers (P1-P4) were prepared via Suzuki and Stille polycondensations, respectively. The cyclopentadithiophene copolymers (P2 and P4) have smaller electrochemical band gaps (1.79 and 1.64 eV) compared to the fluorene-containing copolymers (2.08 and 1.95 eV for P1 and P3). However, the presence of CPDT raises the electrochemical HOMO energy levels (-4.83 and -4.91 eV for P2 and P4) compared to the FL copolymers (-5.06 and -5.15 eV for P1 and P3) leading to small open circuit voltages (V oc ) in solar cells. The primary solution and thin-film UV-vis absorption peaks of P3 and P4, which do not contain alkylated thiophenes appended to the BTZ unit, are at lower energy and have larger absorption coefficients than their P1 and P2 counterparts. Detailed theoretical analyses of the geometric structure, electronic structure, and excited-state vertical transitions using density functional theory provide direct insight into the interplay between the structural modifications and resulting electronic and optical changes. A high molecular weight (M n = 25 kg/mol) polymer with a large degree of polymerization (DP n = 21) was easily achieved for the random copolymer P1, leading to thin films with both a larger absorption coefficient and a larger hole mobility compared to the analogous alternating polymer P (M n = 22 kg/mol, DP n = 18). An improved short circuit current and a power conversion efficiency up to 1.42% (J sc = 5.82 mA/ cm 2 , V oc = 0.765 V, and FF = 0.32) were achieved in bulk heterojunction solar cells based on P1.

Novel 4,7-Dithien-2-yl-2,1,3-benzothiadiazole-based Conjugated Copolymers with Cyano Group in Vinylene Unit for Photovoltaic Applications

“…The second absorption maxima of the solid thin films exhibit red shifts (26-36 nm) as compared to the corresponding peaks in dilute solutions. The maximum absorption peak of PFVCNDTBT is 120 nm red-shifted as compare to the known poly(fluorenevinylene-alt-4,7-dithien-2-yl-2,1,3-benzothiadiazole) (PFVDTBT), 21 which lacks only the cyano group as compared to our PFVCNDTBT. This big red shift can be attributed to the efficient ICT by the introduction of the cyano group in PFVCNDTBT.…”

Syntheses and Properties of Copolymers with N‐Alkyl‐2,2′‐bithiophene‐3,3′‐dicarboximide Unit for Polymer Solar Cells

“…0 mmol) and zinc powder (2.6 g, 40.0 mmol) in EtOH (16 mL) and DMF (16 mL) was added 5 N NaOH aqueous solution (25 mL), and the mixture was stirred under reflux for 3 h. Afterward, 1-iode-2-octyldodecane (22.3 g, 54.0 mmol) and tetrabutylammoniumbromide (0.9 g, 3.6 mmol) were added to the mixture, and the reaction mixture was further refluxed for 12 h. After extraction of the reaction mixture with ethyl acetate, the organic layer was washed with water and dried over anhydrous MgSO 4 . After removing the solvent under reduced pressure, the residue was purified by flash chromatography to give 4.9 g (35%) of compound 2 as a colorless oil; (3). After adding bromine (0.1 mL, 2.8 mmol) in 5 mL of methylene chloride to a solution of compound 2 (1.0 g, 1.4 mmol) in 10 mL of methylene chloride at 0 C, the reaction mixture was stirred for 12 h at ambient temperature.…”

“…After removing the solvent under reduced pressure, the residue was purified by flash chromatography to give 3.8 g (78%) of compound 3 as a light yellow solid. 0 -dicarboximide (9) (0.2800 g, 0.4157 mmol), P(otolyl) 3 (40 mol%), and Pd 2 (0)(dba) 3 (5 mol%) were mixed with chlorobezene. The mixture was refluxed with vigorous stirring for 8 h under argon atmosphere.…”

“…Two carbonyl groups attached to the thiophene units work as the electron-withdrawing moieties to reduce the bandgap, and the five-membered thiophene units reduce the torsional angles with the adjacent aromatic groups. 3 The fact that the alkyl group on nitrogen atom of the imide is far away from the alkyl groups of the electron-pushing unit is another advantage of the BTI unit to provide better conjugation. [10][11][12] To improve the morphology of the bulk heterojunction (BHJ) using polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM) of PSCs and expand the absorption range of the solar spectrum, copolymerization of the electron-rich and -deficient units with controlled ratios has been reported.…”

Synthesis and Photovoltaic Properties of Quinoxaline-Based Semiconducting Polymers with Fluoro Atoms

“…[1][2][3][4][5][6] Many of the lowbandgap (1.4-1.9 eV) conjugated polymers with excellent efficiencies have electron-deficient heterocycles, such as benzothiadiazole (BT), 11 quinoxaline 12 and benzimidazole, 13 and electron-rich moieties, such as carbazole, 14,15 thiophene, 16 and benzodithiophene. 17 The polybenzodithiophenes have been much emerged for organic solar cells caused by coplanarity, high hole mobility, and side chain patterns for enhanced solubility.…”