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Two carboxylate-functionalized wide band gap polymers, 2TC-TT-BDTFT and 2T-TTC-BDTFT, which feature a fluorinated benzodithiophene (BDTFT)-alt-2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene (2T-TT) ba...

A new building block, 1,4-di(3-alkoxy-2-thienyl)-2,5-difluorophenylene (DOTFP) with several desirable features such as high backbone planarity, suitably lying highest occupied molecular orbital (HOMO), and good solubility, was developed by inserting an electron-deficient difluorophenylene into the 3,3′-dialkoxy-2,2′-bithiophene (BTOR) unit. Three regioregular D-A1-D-A2 type polymers based on DOTFP and benzothiadiazole (BT) derivatives were synthesized and characterized by comparing with a D–A type BTOR-based polymer. The content of highly electron-rich alkoxythiophene is reduced by half in the DOTFP-based polymers versus that of the BTOR-based polymer analogue, which results in a deeper HOMO level and benefits high open-circuit voltage (Voc) in polymer solar cells (PSCs). Consequently, the DOTFP-ffBT-based solar cells exhibited a significantly improved power conversion efficiency (PCE) of 8.7% and an increased Voc of 0.84 V compared to the BTOR-ffBT-based solar cells with a PCE of 2.6% and a Voc of 0.49 V...

Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S···O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3′-alkoxy-2,2′-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to first-generation 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)S···O(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer π-stacking, and bett...

High degree of polymer backbone planarity is achieved by incorporating intramolecular noncovalent sulfur···oxygen interaction, which also affords good solubility by attaching alkoxy chains at the 3,3′-positions of bithiophene. However, the applications of the resulting polymers are plagued by their high-lying HOMOs due to the strong electron-donating alkoxy chains, resulting in small open-circuit voltages (Vocs) in polymer solar cells. Herein, a novel head-to-head linkage containing 3,3′-dialkoxy-4,4′-dicyano-2,2′-bithiophene (BTCNOR) is invented. Single-crystal X-ray diffraction shows direct evidence of noncovalent sulfur···oxygen interaction and coplanar backbone of BTCNOR. Very low-lying HOMOs (−5.5 to −5.6 eV) of the corresponding polymers with high degree of backbone planarity and good solubility are achieved by introducing strong electron-withdrawing cyano group onto the dialkoxybithiophene. The cyano offsets the effect of the electron-donating alkoxy chain, rendering the new BTCNOR as a weak donor ...