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“Polymer based hybrid materials for photovoltaic applications: a morphological and electro-optical approach”

In the scope of reducing the cost of the photovoltaic electricity, the polymer based solar cells are considered as a potential alternative to the expensive conventional silicon solar photovoltaic devices. While reducing the production costs, polymer based solar cells have to maintain the stability and efficiency of the silicon based devices. To realize this, it is necessary to understand and improve the currently limiting factors of the polymer based solar cells. The study of the morphology of the active layers is a key issue to be considered for further optimisation of polymer based solar cells and therefore is defined as the primary goal of this thesis. The polymer based solar cells of interest are the fully organic polymer:PCBM (PCBM= (6,6)-phenyl C61-butyric acid methyl ester) and the hybrid polymer:metal oxide (MOx) bulk heterojunction (BHJ) solar cells. Besides a morphology study using transmission electron microscopy and X-ray diffraction also the preparation methods to construct the hybrid polymer:MOx BHJ active layers are explored. Both the current and promising methods of mixing the polymer with the MOX nanoparticles and filling a porous MOx structure with the polymer are used and investigated.

The second goal of this thesis includes the study of the fundamental processes of charge separation and photocurrent generation in the polymer based solar cells. In the case of the fully organic polymer:PCBM solar cells these processes are already extensively studied. The fundamental electronic interactions between the polymeric donors and the fullerene acceptors and their influence on the device performance are derived. In this thesis, this fundamental study is extended to the polymer:MOx BHJ solar cells providing insight into the interfacial charge-transfer states present in such unique material system. For the first time, the existence of a ground-state charge-transfer complex in conjugated polymer (P3HT):titanium dioxide BHJ photovoltaic cells was demonstrated by using Fourier Transform photocurrent spectroscopy. The excited CTC acts as an intermediate state in the charge separation and recombination processes. Therefore, the presence of sub-bandgap absorption by CTCs in organic and hybrid photovoltaic devices is considered as a prerequisite for efficient charge carrier generation. Hence, the presence of a CTC is an important parameter in the choice and optimization of the donor:acceptor material systems in polymer based BHJ solar cells.

Finally the last goal of this thesis is to demonstrate that, besides improving the efficiency of solar cells, it is also possible to provide an environmentally friendly processing in solar cells. A water soluble polythiophene is introduced in order to avoid the use of toxic organic solvents. In addition an aqueous synthesis procedure is used for the porous TiO2 layer illustrating the possibility to develop ‘green’ solar cells using water as a solvent. The water-soluble polythiophene seems a promising polymer for further developments towards ‘green’ solar cells.