Charge separation in low-bandgap polymer/fullerene blends

Abstract

This thesis addresses charge separation and charge recombination in different, mainly low bandgap, polymer/fullerene blend films and their relation to device performance. Charge separation and recombination was studied as a function of variables including the difference in the LUMO levels of the polymer and fullerene, the polymer/fullerene blend ratio, the presence of a fluorine atom on the polymer backbone and the use of a bulky fullerene acceptor (Indene-C60-trisadduct, ICTA). A key focus of the thesis is on the impact of film microstructural differences upon charge generation and recombination kinetics. Charge generation and recombination was studied via transient absorption spectroscopy (TAS) with time resolutions from femtoseconds to microseconds. In Chapter 1, an introduction to the field is presented. Basic concepts of polymer solar cells and the steps of light-to-electrical energy conversion are included. The chapter focuses on the current discussions on charge generation, separation and recombination and their relationships with other parameters such as material energetics and morphology. In Chapter 2, the experimental methodologies are presented. A description of the materials used, the techniques used to prepare the samples, and the mainly optical techniques used to study them: steady state photoluminescence (PL), TAS (fs and microsecond), X-ray diffraction (XRD) and device characterization. Chapter 3 to 6 present the results of each project. In Chapter 3, the role of the driving energy for charge separation (ECS) for low bandgap DPP-based polymers is addressed. A s-TAS characterization of DPP-based polymer/fullerene blends is presented, and the yield of charges correlated with the experimentally obtained ECS. The correlation was then extended to other low-bandgap polymers and the trend compared with that obtained for larger bandgap polymers. Chapter 4 deals with the effect of DPPTT-T/PC70BM blend ratio upon the film photophysics. With PL quenching and fs-TAS studies, it is demonstrated that the limitations in the performance of DPPTT-T polymer mainly come from an incomplete exciton quenching for all the compositions studied. The study is in agreement with morphology probes including transmission electron and atomic force microscopies, as well as with the changes in crystallinity, as observed by XRD. Chapter 5 deals with the effect of polymer backbone fluorination on a low-bandgap polymer (PGeDTBT). PL quenching and fs to s TAS data is presented and correlated with structural analyses and theoretical calculations to compare the properties of non-fluorinated and fluorinated version of the polymers. It was found that charge generation seems to be equally efficient, despite the lower driving energy for charge separation (ECS) in the fluorinated polymer. A four-fold slowing down in non-geminate recombination was also observed upon fluorination, correlated with a larger polymer tendency to aggregate, thus demonstrating its multiple effects on material properties and photovoltaic behaviour. Chapter 6 deals with the effect of mixed and “flatter” interfaces upon charge separation. XRD data are presented to show the contrast in intercalation between the polymer and the acceptors (PC70BM and ICTA). These results are correlated with fs-TAS data to show the change in the regime of recombination: while the highly intercalated blends present a high predominance of geminate recombination, the blends with ICTA predominantly present non-geminate recombination. Finally in Chapter 7, the conclusions of every chapter are summarized. A general discussion on the relationship of the conclusions is given and the areas where further research is needed are discussed.