Nanostructured Interfaces in Hybrid Solar Cells

Abstract

Excitonic solar cells are an emerging technology which holds the great promise of generating clean and sustainable photovoltaic power at lower cost than conventional silicon solar cells. In excitonic solar cells, the light is absorbed by organic semiconductors and dye molecules, which typically exhibit higher exciton binding energies than inorganic semiconductors. Therefore, free charge carriers can be generated only at interfaces between donor and acceptor materials. These interfaces can provide sufficient energy to overcome the exciton binding energy, resulting in free charge carriers, which then have to be transported towards the external electrodes. Since typical exciton diffusion lengths in organic materials do not exceed 10 nm, a sophisticated design of the internal morphology of the photoactive layer is necessary in order to allow loss-free diffusion of excitons to the separating interface while simultaneously providing consistent pathways for charge transport.This requirement can be met when employing metal oxide semiconductors like TiO2 as acceptor materials in combination with absorbing organic donors in so-called hybrid solar cells. TiO2 is an abundant, non-toxic, and cheap material and there are several well-established strategies to cover large areas with TiO2 nanostructures. In hybrid solar cells, these structures are decorated with self-assembled monolayers of dye molecules and infiltrated with conducting polymers. This results in a nano-phase separated donor-acceptor structure, which provides short exciton diffusion pathways towards interfaces but works as a consistent charge transport network. A general overview of different types of excitonic solar cells is given in Chapter 2, while working mechanisms of hybrid solar cells and fabrication routes for TiO2 nanostructures are described in more detail in Chapter 3.Two of the main challenges in hybrid solar cell research are the optimization of 1) the interface between organic and inorganic compounds and 2) the nano-geometry of the metal oxide electrode, i.e., the internal morphology of the active film. Topic 1) is adressed in the first part of this thesis, where the impact of interfacial properties on the mechanisms of charge separation, collection, and recombination is investigated for hybrid solar cells based on TiO2 as the metal oxide electrode and polythiophene as the organic hole transporter. The introduction of a conducting polymer between photoactive film and metal top contact as interfacial layer is discussed in Chapter 5. This coating establishes an Ohmic contact between organic semiconductor and metal electrode, which is favorable for charge collection. Three different self-assembled monolayers of dye molecules and a thin coating of Sb2S3 as modifiers at the interface between TiO2 and polythiophene are presented in Chapter 6. Choice of the modifier allows to partly control charge recombination kinetics at the hybrid interface. In Chapter 7 fine-tuning of the properties of the TiO2-dye-polymer interface with pyridine derivatives is discussed. It is found that a combination of 4-tert-butylpyridine and 4-mercaptopyridine enhances photocurrent and photovoltage simultaneously.The second part of the thesis adresses topic 2). As a model system for excitonic photoactive layers with different donor-acceptor morphologies polymer:fullerene bulk heterojunction solar cells are investigated in Chapter 8. In this chapter, the kinetics of charge recombination and extraction are analyzed as a function of the internal morphology of the devices. Chapter 9 presents a synthesis route for large area fabrication of TiO2 nanotube arrays on transparent conducting oxides with good control over nanostructure dimensions like tube spacing, diameter, and wall thickness. These structures are highly interesting for applications in hybrid solar cells since they allow a donor-acceptor nano-geometry with fine phase separation, enabling efficient exciton separation by still providing direct pathways for charge transport.