Charge transfer complexes at various donor-acceptor nanostructures in organic based solar cells

Progress in organic photovoltaics requires the development of efficient and stable materials. To achieve this goal, an understanding of the physical processes occurring at the organic/organic interface and involved in the production of photovoltage and photocurrent is needed. The present work has investigated charge transfer (CT) complex formation at organic based donoracceptor interfaces. This thesis aimed to investigate whether the CT complex is a “universal” property of organic interfaces and explored the correlation between the charge transfer state energy and the nanostructures at the interface of the donor and acceptor materials. The presence of the CT complex was probed by Fourier-Transform Photocurrent Spectroscopy and its energy (ECT) was evaluated and correlated with the morphological and chemical properties of the investigated materials. The work was divided in two main sections: the first one (paper A and B) investigates the presence of the CT complex at organic-organic interfaces alternative to the polymer:fullerene bulk heterojunction. The second part (paper C and D) investigates the morphology of the fullerene in BHJ and its correlation with the energy of the CT state. In paper A the presence of the CT complex at a planar interface between evaporated small molecules is reported, additionally, the photocurrent origination from direct excitation into the triplet state of copper phthalocyanine (CuPc) was detected at photon energies below the CuPc singlet and CT state energy. This allowed the comparison between the exciton dissociation yields from states with different driving energies. In paper B the direct excitation and dissociation of the CT state in polymer:polymer blends is reported. We observed that the field dependence of the CT dissociation remains invariant irrespective if the CT state is populated by polymer exciton dissociation (above the ECT) or by direct excitation from the CT ground state (below the gap of the two polymers). The fullerene nano-morphology in polymer:fullerene bulk heterojunctions is investigated in the paper C. The crystallinity of the fullerene blended with a polymer was systematically investigated in function of the fullerene weight fraction in the blend. The organization of the fullerene was evaluated by means of solid state NMR and was established that the increased fullerene organization can give rise to a reduction of the energy of the CT state as well as the reduction of the open-circuit voltage in the device. Finally in paper D the influence of sunlight exposure on fullerene crystallization is reported. When fullerene containing films are exposed to light before or during thermal annealing, fullerene crystallization induced phase separation can be reduced or even completely prevented. Based on optical microscopy, transmission electron microscopy (TEM), UV-VIS absorption and Raman measurements, the inhibition of fullerene crystallization is attributed to the light-induced dimerization of the fullerenes. In this thesis the interfacial charge transfer complex has been observed to be a property of several organic interfaces strictly correlated with the morphology of the layer.

Publication year:2013

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