Low Band-Gap Donor Polymers for Organic Solar Cells

Abstract

One of the key challenges of organic solar cells is their relatively low power conversion efficiency. One way to improve the efficiency of these cells is to develop donor materials with improved photon harvesting capabilities, well-located highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) energy levels, good hole transport characteristics and good processability. In this thesis, the design, synthesis and characterization of fifteen low band gap donor-acceptor type polymers are described. Two different acceptor moieties, 3,6- bis(thien-2-yl)-2,5-di-N-alkylpyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and 2,1,3- benzothiadiazole (BT) were used in our polymer designs and the polymers were synthesised using the palladium-catalysed Stille cross-coupling method. The first series of polymers were random co-polymers of DPP and dithienothiophene. By tuning the solubility and absorption characteristics of the polymers, we achieved a polymer that gave power conversion efficiencies of up to 4.85 % when applied in solar cells. Low open-circuit voltages were obtained for these cells, hence the next series of polymers was designed with the aim of improving the open-circuit voltages. Although the lower HOMO levels of these polymers resulted in higher open-circuit voltages when applied in solar cells, the low hole mobility of the polymers and poor morphology of the polymer:fullerene films resulted in low solar cell power conversion efficiencies. Finally, a series of benzothiadiazole-oligothiophene polymers were synthesised. These polymers had high hole mobilities and wide absorption spectra. When these polymers were applied in organic thin-film transistors, good hole mobilities of up to 0.20 cm2/Vs were achieved, and when applied in solar cells, power conversion efficiencies of up to 6.2 % were achieved. These results show that benzothiadiazoleoligothiophene systems are promising candidates for both transistor and solar cell applications.