"Charge transfer complexes at various donor-acceptor nanostructures in organic based solar cells."
Organic solar cells contain a lot of technological potential. For progress to be made, it is essential that new materials are developed with higher efficiencies and longer lifetimes. In order to achieve this goal, it is crucial to the physical processes that take place in the generation of the photocurrent -en voltage, fully to understand. The present work analyzes the formation of the charge transfer (CT) complex at the interface between donor and acceptor materials. It is also examined whether the CT complex is a universal property (own organic materials) and explores the correlation between the energy of the charge transfer state (ECT) and nanostructures at the interface between donor and acceptor materials. The presence of the CT complex was examined by Fourier-Transform photocurrent spectroscopy, wherein the energy of the complex is correlated to the morphological and chemical properties of the materials studied.
This thesis consists of two main parts: the first section (paper A and B) investigates the presence of the CT complex in polymer: polymer thin films, which offer an alternative to the conventional polymer: fullerene thin films. The second part (paper C and D) deals with the morphological study of the fullerene derivative in a bulk heterojunction solar cell and the corresponding correlation with ECT.
Paper A explores the presence of the CT complex for a planar interface with vacuum-deposited small molecules. In addition, the photo-current, which is caused by direct excitation of the ground state to the triplet state of copper phthalocyanine (CuPc), detected at photon energies below the energy level of the singlet excited state, and ECT. This allows a comparison to be made between the exciton dissociation proceeds for states with different energies.
Paper B discusses the direct excitation and dissociation of the CT state in polymer: polymer film. Here we have observed that the field-dependence of the dissociation CT remains the same regardless of whether the state CT filled by exciton dissociation of the polymer is (> ECT) or by direct excitation of the CT ground state (<band gap of the two polymers).
Paper C explores the nano morphology fullereenacceptor in polymer: fullerene bulk heterojunction solar cells. The crystallinity of the fullerene derivative (mixed with a polymer) in a systematic manner was studied as a function of the weight fraction fullerene. The local organization of the fullerene derivative was evaluated by means of solid state NMR of which was found to be that an increased fullerene organization can bring about a reduction of ECT as well as a reduction of the open circuit voltage of a full working solar cell.
Paper D, finally, deals with the influence of sunlight on the crystallization behavior of the fullerene derivative. When fullerene-containing thin films are exposed to light before or during a thermal treatment, then the phase separation, which is induced by crystallization, can be reduced or completely avoided. The cause of this is attributed to dimerization of fullerene molecules, in which use was made of optical microscopy, transmission electron microscopy (TEM), UV-vis absorption measurements, and Raman spectroscopy to argue this proposition.
From this thesis it appears that the charge transfer complex is an inherent property of many organic interfaces where there is a strong correlation with the morphology of the organic layer.