Due to inherent advantages, such as low-cost solution-processing on large and flexible areas, conjugated polymers have attracted great attention for the use in organic solar cells (OSCs). The performance of such organic photovoltaic devices is often limited by the (in)compatibility of the solar and the polymer absorption spectra. To achieve high efficiencies the organic semiconductor should feature a low HOMO-LUMO energy gap of < 2 eV. One of the most exercised strategies towards such low band gap polymers is the alternate incorporation of electron-donating (D) and electron-accepting (A) building blocks into the polymer main chain. Lactam and imide groups are strongly electron-withdrawing substituents, so aromatic lactames or imides can be efficiently used as acceptor building blocks.
In this line in chapter 2, a series of alternating DA-copolymers based on phthalimide (PhI), pyromellitdiimide (PyDI), thieno[3,4-c]pyrrole-4,6-dione (TPD), and 3,6-dithiophen-2- yl-pyrrolo[3,4-c]pyrrole-1,4-dione (TDPPT) in combination with different donor units like cyclopenta[2,1-b:3,4-b‘]dithiophene (CPDT) or cyclopenta[1,2-d:4,3-d’]dithiazole (CPDTz) are presented. Their opto-electronic properties and performance in photovoltaic devices depending on the donor building block and the solubilising side chain (alkyl substitution pattern) attached to the nitrogen are investigated. PCEs of up to 4.7 % for bulk heterojunction (BHJ) solar cell devices with the copolymer poly{[5-octylthieno[3,4-c]pyrrole-4,6-dione-1,3- diyl]-alt-[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithien-2,6-diyl]} (PTPD8CPDT) as donor and [70]PCBM as acceptor component could be obtained.
Chapter 3 deals with alternating copolymers with CPDT as donor and two different (di)imide/lactam acceptor building blocks. This combination of different acceptors within one polymer leads to an improved coverage of the solar spectrum. PCEs of 0.6 % in polymerfullerene BHJ solar cells with the statistical copolymer poly{[5-octyldodecylthieno[3,4- c]pyrrole-4,6-dione-1,3-diyl]-co-[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithien-2, 6-diyl]-co-[bis-(2,2’-thienyl)-2,6-naphthalene-1,4,5,8-N,N’-bis(2-oc-tyldodecyl)diimide-5,5’- diyl]} (PTPD8/12-CPDT-TNDIT) as donor component and 0.9 % in all-polymer BHJ solar cells with the polymer as acceptor and P3HT as donor component could be obtained.