Universiteit Hasselt - Knowledge in action


"Synthesis and characterization of novel donor-acceptor isoindigo-based conjugated copolymers and small molecules and their integration in organic photovoltaics."

The conversion of solar energy into electricity is an environmentally friendly, safe and lowcost way of renewable energy production. Among the different photovoltaic technologies, organic photovoltaics (OPV) have particular assets in terms of aesthetics, flexibility and low-cost large area coverage. Nevertheless, the moderate OPV efficiencies (˞11%) and lack of durability (< 10 years) strongly limit their large-scale exploitation in particular consumer goods. The main goal of this thesis is to deal with these two drawbacks by designing novel conjugated polymers and small molecules with broad absorption in the visible range (i.e. low band gap), and by setting up strategies to improve the (thermal) stability of the photovoltaic cells. To this extent, novel push-pull type organic semiconducting materials have been synthesized, with different architectures and composed of electron-poor isoindigo building blocks alternating with electron-rich moieties, presenting a favorable spectral overlap with the solar emission. The optical properties of the novel materials were generally investigated by UV-visible absorption spectroscopy, while cyclic voltammetry was implemented to estimate the frontier orbital (HOMO/LUMO) energy levels. Relationships between the chemical nature and architecture of the push-pull systems and their absorption spectra and HOMO-LUMO energy levels have been pursued. Finally, the photovoltaic performances of the new materials have been evaluated in conventional bulk heterojunction organic solar cells using methanofullerene acceptor materials. Correlations between the molecular and photovoltaic parameters have been established.

Ultimately, cross-linkable diblock copolymers based on poly(3-hexylthiophene) (P3HT) have been synthesized to improve the long-term stability of P3HT/PC61BM photovoltaic cells. Bulk heterojunction polymer solar cells have been prepared and their stability has been evaluated by accelerated ageing experiments and compared with standard P3HT-based devices.

Photovoltaic technologies represent evidence of sustainable energy production based on solar energy, the most plentiful and widely distributed renewable energy source. Operating under solar irradiation, photovoltaic devices produce electricity that can be directly consumed or stored by chemical (battery) or mechanical means (e.g. flywheels). Organic photovoltaic (OPV) devices convert solar energy directly into electrical energy using carbon-based semiconducting materials which exhibit favourable light absorption and charge generation properties. OPV devices can be fabricated by low-cost solution processes such as inkjet printing and roll-to-roll coating techniques, compatible with flexible plastic substrates or even paper and with potential large area applications. To enhance the moderate power conversion efficiency (PCE) of organic solar cells (~11%) and their durability (< 10 years), new materials and device optimization are continuously investigated. The first part of this introductory chapter focuses on the different resources that are currently used to produce (renewable) energy, and their corresponding advantages and disadvantages. In the second part, the donor-acceptor (D-A) approach to prepare low band gap copolymers and small molecules used in OPV devices and organic field-effect transistors (OFETs) is discussed. In particular, the structure and properties of isoindigo (IID) and the progress in the development of IID-based copolymers and small molecules are reviewed, as IID is a key building block applied in the presented thesis work. Additionally, the long-term stability of organic solar cells represents an important issue towards the commercialization of OPV technology. The last part of the chapter therefore presents an overview of some of the novel concepts to prepare OPV materials and devices with long-term stability