Design And Synthesis of Band Gap Engineered Conjugated Molecules For Organic Electronics

Abstract

Research in conducting polymers in the area of optoelectronics has expanded enormously and diversified into organic photovoltaics (OPV), organic light?emitting diodes (OLED), organic field?effect transistors (OFET), etc. OPVs have attracted great attention for renewable?energy applications due to their potential for lightweight, paintable large?area devices, and solution processability on flexible substrates, which considerably reduces fabrication cost. Bulk heterojunction (BHJ) solar cells are the most widely used configuration in OPV. A blend of donor and acceptor materials is employed as the active layer in these devices. Conjugated polymers are commonly used as donor materials and fullerene derivatives as acceptors. The design of conjugated polymers for OPV applications is highly significant, as the efficiency of solar cells depends largely on the electronic and optical properties of these polymers. In this thesis, the design and synthesis of several novel conjugated molecules have been carried out using the Donor朅cceptor (D朅) architecture. Chapter I summarizes recent developments in organic photovoltaics. A brief history of organic solar cells and their working principles is explained. Commonly used conjugated polymers such as thiophene, carbazole, fluorene, etc., and their optoelectronic properties are discussed. In addition, the importance of material design in conjugated molecules is described. Chapter II describes the design of a novel conjugated molecule, 7,9?di(thiophen?2?yl)?8H?cyclopenta[a]acenaphthylen?8?one (DTCPA), based on a D朅 architecture using cyclopentadienone as the acceptor and thiophene units as the donor moieties. A homopolymer of DTCPA, poly(DTCPA), was synthesized and characterized. Optical, electrochemical, thermal, and electrical characterizations were carried out for both DTCPA and poly(DTCPA). These materials were used in applications such as OPV and electrochemical sensors. Additionally, DTCPA was subjected to electrospinning to obtain a barbed?wire?like architecture. Chapter III discusses random copolymers of DTCPA. Two acceptor moieties-4,7?di(thiophen?2?yl)benzo[c][1,2,5]thiadiazole (DTBT) and 4,7?bis(3?hexylthiophen?2?yl)benzo[c][1,2,5]thiadiazole (BHTBT)-were used for chemical oxidative copolymerization with DTCPA. Optical, thermal, electrochemical, and structural characterizations of the copolymers were carried out. The influence of alkyl chains on copolymer properties is explained. BHJ solar cells were fabricated using the synthesized copolymers as donors and PC??BM as the acceptor. Chapter IV details the synthesis of an alternating copolymer of DTCPA with benzothiadiazole (BT) moieties via palladium?assisted Stille coupling polymerization. Structural characterization of monomers and the resulting copolymer was performed using Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR). BHJ solar cells were fabricated using PC??BM as the acceptor, and photovoltaic parameters were evaluated. Chapter V presents the synthetic routes for an alternating copolymer of DTCPA with DTBT moieties. This chapter discusses the effect of a thiophene spacer on polymer properties. Stille coupling was used for synthesizing poly(DTCPA?alt?DTBT). A BHJ solar cell constructed using this copolymer with PC??BM as the active material achieved an efficiency of ~2.5%. Chapter VI focuses on the design and synthesis of low?bandgap polymers based on benzodithiophene (BDT) derivatives. Dithieno[2,3?d:2?,3??d?]benzo[1,2?b:4,5?b?]dithiophene (DTBDT) was synthesized using various routes. DTBDT was copolymerized with acceptor moieties such as 4,7?di(thieno[3,2?b]thiophene?2?yl)?2,1,3?benzothiadiazole (DTTBT) and 2?[4?(trifluoromethyl)phenyl]?1H?benzimidazole (CF?). The copolymers were characterized using FTIR, NMR, UV?visible spectroscopy, and cyclic voltammetry. Chapter VII discusses the synthesis and characterization of oligomers. Carbazole?based oligomers were synthesized with BT and bithiophene as comonomers. Nickel?catalyzed Yamamoto coupling reactions were used for synthesizing the co?oligomers. These oligomers were applied in organic solar cells and sensor applications. Chapter VIII describes the synthesis of an acceptor material for organic solar cells. A functionalized fullerene derivative containing a thiophene?based compound (F?DTBT) was synthesized and characterized. When used as an acceptor in BHJ solar cells with P3HT, an efficiency of ~1.1% was achieved.