| Organic solar cells hold the key for future energy demand due to their unique properties such as low cost, light weight and flexibility. A remarkable amount of effort has been done to improve the power conversion efficiencies in bulk heterojunction solar cells (BHJ). Considerable effort has been done to develop novel donor-acceptor copolymers to improve the absorbance of the copolymer in the visible to near Infrared (IR) regions and to lower the highest occupied molecular orbital (HOMO) energy level of the conjugated polymer as these properties improve the short circuit current density (J SC) and the open circuit voltage (VOC) and hence the power conversion efficiency (PCE) of the BHJ solar cells.;In this dissertation, Chapter 1 describes the synthesis and photovoltaic performance of donor-acceptor semiconducting polymers that have been reported during the last decade. 9,9-Dialkyl-2,7-fluorene (FL), 2,7-carbazole (CZ), cyclopenta[2,1-b:3,4-b']dithiophene (CPDT), dithieno[3,2-b:2',3'-d]silole (DTS), dithieno[3,2- b:2',3'-d]pyrrole (DTP), benzo[1,2-b:4,5- b']dithiophene (BDT), benzo[1,2-b:4,5- b']difurane (BDF) building blocks and their donor-acceptor copolymers and their photovoltaic properties have been discussed in this chapter.;Chapter 2 describes the synthesis and photovoltaic properties of two novel donor-acceptor polymers P1 and P2 containing benzodithiophene with 3,3',5-trihexylbithienyl substituents as the donor unit with Benzo[c][1,2,5]thiadiazole and 5-hexylthieno[3,4- c]pyrrole-4,6-dione as acceptor building blocks. The photovoltaic properties of the synthesized donor-acceptor polymers were investigated in bulk heterojunction solar cells with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor.;Chapter 3 describes the synthesis of three novel donor-acceptor copolymers P3, P4 and P5 via Stille coupling polymerization with bithienyl substituted BDT as the donor with three different acceptors; 5,6-difluorobenzo[c][1,2,5]thiadiazole, 4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole, and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole and their photovoltaic properties. The effect of thiophene space insertion and the fluorine atom insertion on the photovoltaic properties has been investigated by testing the polymer blends with PC71BM in BHJ solar cells and by observing the morphology in TMAFM studies.;Chapter 4 describes the synthesis of three donor-acceptor copolymers P6, P7 and P8 with N,N'-dodecylpyromellitic diimide as the electron acceptor unit with three diethynyl substituted donor monomers; 1,4-diethynyl-2,5-bis(octyloxy)benzene, 2,7-diethynyl-9,9-dioctyl-9H-fluorene and 3,3'-didodecyl-5,5'-diethynyl-2,2'-bithiophene by Sonogashira cross coupling polymerization. The synthesized polymers showed deep HOMO energy levels and larger band gaps (>2.5 eV). These polymers P6, P7 and P8 underwent fluorescence quenching with PCBM, indicating the intermolecular photo-induced charge transfer between the donor polymers and PCBM acceptor. |
