Oligomer studies on polymer photovoltaics

Abstract

Solar cells based on polymers are an attractive alternative to silicon-based photovoltaics, because of their low cost and processing advantages. To increase the efficiency of polymer solar cells, however polymers are required that absorb light also in the near infrared part of the solar cells. The research described in this thesis aims to address some fundamental questions related to these so-called small band gap polymers. The method followed consists of the synthesis of small molecular model systems and detailed investigation of their properties by a variety of spectroscopic and electrochemical methods. In chapter 2, oligomers consisting of two cyclopentadithiophenes and different acceptor units are described. These oligomers were synthesized to investigate the influence of the type of acceptor unit on the band gaps and energy levels of small band gap polymers. It was found that oligomers having thiophene-based acceptors generally have lower HOMO and LUMO levels than oligomers having benzene-based acceptors. This will ultimately lead to lower voltages when these acceptor systems are applied in solar cells. No clear correlation was found between the acceptor strength and the singlet–triplet splitting energy. Chapters 3 to 5 deal with several series of donor–acceptor oligomers consisting of thiophene and thieno[3,4-b]pyrazine moieties. The series described in chapter 3 consists of oligomers formed by one, two or three thienopyrazines end capped with thiophene units. The effect of increasing the chain length of the systems with one or two thienopyrazines in the acceptor block is described in chapters 4 and 5. The optical and electrochemical properties of the series are evaluated both experimentally and theoretically. It is found that the dependence of these properties on the chain length is identical in all series. In literature, a number of possible causes for the small band gaps in this kind of systems are given, e.g. donor–acceptor effects and induction of a quinoid structure in the polymer chain. In the work described in this thesis, it is concluded that the main cause for the small band gaps in these systems is none of the previously mentioned possibilities. Rather, the strong acceptor and donor character of thienopyrazine (usually only regarded as a strong acceptor), combined with strong interactions between the neighboring thienopyrazine units, explains the observed small band gaps. Besides light absorption, charge separation and recombination processes are of crucial importance to photovoltaic cells. A detailed study of these processes in small band gap oligomer – fullerene triads is described in chapters 6 and 7. Triads containing oligomers using the thienopyrazine unit, described in previous chapters, are presented in chapter 6. In these systems very fast charge separation takes place close to the Marcus optimal region, followed by fast recombination in the inverted regime. Because of the short lifetime of the charge separated state, no recombination into triplet states could be observed. Systems using the diketopyrrolopyrrole unit in the oligomer part of the triads are described in chapter 7. Charge separation in these systems takes place in the Marcus normal regime, followed by recombination in the inverted regime. As the energy of the charge separated state is higher in these systems than in systems using the thienopyrazine unit, the lifetime of this state is long enough to allow intersystem crossing to the triplet state. Clear evidence for triplet recombination was observed in these systems. In the last chapter, the use of diketopyrrolopyrrole containing oligomers as acceptor materials in solar cells is explored. Although diketopyrrolopyrrole-containing polymers are normally used as donor materials, acceptor behavior was also present and solar cells were prepared consisting of polythiophene as the donor material and the oligomers as acceptor materials. The best device shows a power conversion efficiency of 0.31% in simulated solar light, with a photon to electron conversion efficiency of ~10% up to 700 nm. The efficiency seems to be limited by the coarse morphology of the blend.