Cyclopentadithiazole-Based Monomers and Alternating Copolymers

Conjugated, alternating copolymers composed of electron-rich (so-called “donor”) and electron-poor (so-called “acceptor”) building blocks are in the focus of very intense research activities both on the academic and industrial scale. Main driving force for this research is their application as hole-generation/hole-transport material (donor material) in bulk heterojunction-type organic solar cells (BHJ-OSCs). [There is some confusion in the use of the term “donor”. “Donor” is both used for the electron-rich building blocks of the alternating copolymers and the copolymer material itself in its function as hole-generating/hole-transport component of the active polymer/fullerene blend in bulk heterojunction-type organic solar cells (BHJ-OSCs).] The number of published copolymer structures is currently rapidly increasing. A couple of examples allow, in combination with soluble fullerene acceptors (PC61BM, PC71BM), for power conversion efficiencies (PCEs) of >5.5%. Unfortunately, reliable design rules in the selection of the electron-rich and electron-poor building blocks of the alternating copolymers are not available until now. Nevertheless, a few donor and acceptor building blocks have proved to be effective toward maximum PCEs including N-alkylcarbazole-2,7-diyl and 4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl (CPDT) as “donor” and benzo-2,1,3-thiadiazole-4, 7-diyl, 4,7-dithienyl-2,1,3-benzothiadiazole-50,50 0-diyl,9,10 and thiophene-3,4-dicarbimide-2,5-diyl as “acceptor” units. The combination of 4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl and benzo-2,1,3-thiadiazole-4,7-diyl building blocks leads to the well-known poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b0]dithiophene2,6-diyl-co-benzo-2,1,3-thiadiazole-4,7-diyl) PCPDTBT copolymer (introduced by Konarka) for which a PCE of 5.5% in alternating copolymer/fullerene BHJ-OSCs has been reported after a careful optimization of blend composition and processing conditions by Peet et al. One shortcoming in the application of PCPDTBT and related copolymers in donor-acceptor couples with soluble fullerene acceptors (PC61BM, PC71BM) is the relatively low open circuit voltage of the resulting BHJ-OSCs with a Voc of ca. 0.6-0.7 V. An increase of the Voc value in related alternating copolymers can be expected for a decreased HOMO energy level of the alternating copolymer. Hereby, the energetic position of the copolymer HOMO level is mainly determined by the structure of the electronrich (“donor”) building block of the alternating copolymer. The favorable downshift of the HOMO energy level can be accomplished by a successive replacement of aromatic (sp) carbons of the thiophene unit by sp nitrogens. Following this idea we have focused our synthetic efforts on 7,7-dialkyl-cyclopenta[1,2-d:4,3d0]dithiazole-2,5-diyl-type monomers as target for further condensation with suitable electron-poor “acceptor” moieties. Results and Discussion