Design and synthesis of efficient polymers for photovoltaic applications

Harvesting energy directly from sunlight is an environmentally friendly method of converting energy from one type to another. Indeed, inorganic solar cells have been commonly used over the last few decades and are now evidencing good power conversion efficiencies, but the cost of this type of solar cells is high. The cheaper alternative to inorganic solar cells is organic conjugated polymers. These are basic materials which take advantage of the single-double bond alternation along the polymer’s backbone. Electronic transitions occur within the electronic levels of the different species of the conjugated polymers, especially between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). These transitions represent the basic work of the conjugated polymer-based solar cells. In this project, we present our work on the preparation of furan based donor-acceptor (D-A) conjugated polymers based on benzothiadiazole (BTD) with alkoxy substitution as an acceptor with donor moieties of fluorene and carbazole using Suzuki cross coupling reaction polymerisation. In addition, conjugated polymers based on benzothiadiazole (BTD) with fluorine atoms substituents as an acceptor with donor moieties of fluorene and carbazole were copolymerised via direct hetero arylation (DHA). Also in this project, we synthesised acceptor-donor-acceptor (A-D-A) conjugated polymers based on (IDIC-C16) as an acceptor with donor units of benzothiadiazole flanked by thiophenes, bithiophene and fullerene through the Stille cross coupling reaction. Impressively, the results from UV-visible and cyclic voltammetry both achieved low bandgap polymers regardless of the type of donor units used in the conjugated polymers synthesised. Chapter 2 in this thesis presents work synthesising three novel conjugated polymers P1, P2 and P3 based on 4, 7- bis (5-bromofuran-2-yl)-5, 6-bis (octyloxy)benzo[c][1,2,5] thiadiazole (as an acceptor) with fluorene, fluorene flanked by furan, and carbazole respectively. The optical studies presented bandgaps of P1 and P3 which were slightly above 2.01 eV and 2.04 eV. P2 had a lower bandgap than those of P1 and P3 (1.93 eV), which was attributed to the extra furan in the backbone of the polymer. In chapter 3, two novel polymers were synthesised and characterised (P4 and P5) based on benzothiadiazole with fluorine atoms at the 5,6-positions flanked by furan as an acceptor, with fluorene and carbazole as donors respectively. The optical properties displayed bandgaps of 1.91 and 1.92 eV for P4 and P5 respectively. The electrochemical bandgaps were slightly higher due to barriers to oxidation and reduction between polymer films and electrodes. In the last chapter, three novel polymers (A-D-A) were prepared and characterised (P6, P7 and P8) based on (IDIC-C16) as an acceptor with donor units such as benzothiadiazole flanked by thiophenes, bithiophene and fluorene through the Stille cross coupling reaction. The optical properties showed bandgaps of 1.55, 1.54 and 1.65 eV for P6, P7 and P8 respectively. The electrochemical bandgaps were higher at 1.58, 1.78 and 1.82 eV for P6, P8 and P8 respectively. All polymers were characterised by Proton Nuclear Magnetic Resonance (1H NMR) and Gel Permeation Chromatography (GPC). The results showed each polymer had a different molecular weight depending on factors such as the chemical structure of the repeat moiety, the solubility in organic solvents, and the fraction in which the polymer was collected using the Soxhlet extraction method. In addition, UV-visible absorption spectroscopy, Cyclic Voltammetry (CV), Thermal Gravimetric Analysis (TGA) and powder X-Ray diffraction were also used to analyse the properties of the polymers. The optical bandgaps were in the range of 1.54 eV to 2.07 eV. Thermal Gravimetric Analysis revealed that all the polymers possessed excellent thermal stability. Finally, powder X-Ray diffraction showed that all polymers were, in general, amorphous.