Investigation into the degradation mechanisms affecting long term stability of hybrid and organic photovoltaic devices

In this thesis work, the attention is focused on the deep comprehension of the degradation mechanisms affecting the stability of both hybrid and organic photovoltaic devices. One of the most representative architecture for the selected PV device families (namely solution based dye sensitized (DSCs) and thermal evaporated small molecule based solar cells) is here designed, realized, optimized in term of performance and subjected to different real working conditions. In the first part of this thesis work, high vacuum evaporated solar cells based on zinc-phthalocyanine – ZnPc and Buckminster fullerene - C60 are realized and optimized. The innovative in vacuum encapsulation technique developed via calcium test procedure, allows for separating the different impact of the external aging agents (temperature and humidity) to the overall device efficiency losses. The use of several complementary characterization techniques results in the discovery of the predominant degradation mechanism affecting the acceptor material (C60) while undergoing prolonged thermal stress. In particular, the coarsening of C60 grains induced by prolonged thermal stress (@85°C) is the responsible for the well-known burn-in phenomenon, consisting in a degradation of the initial value device electrical parameters before to reach a stabilized value. The second part of this thesis work consists in the investigation of the degradation mechanism occurring in a DSC when undergoing reverse bias (RB) stress. The main findings clarify some strange degradation phenomena often observed but never completely understood until now. In particular, the slowing-down of the charge dynamic within the cell is here explained by the discovery of polyiodide formation in the iodide (I-)/(I3-) triiodide redox couple based commercial electrolyte and elucidated by Raman and fluorescence spectroscopy. Furthermore, the unequivocally observed fluorescence emission from stressed electrolyte solution is finally explained and ascribed to a complex interaction between imidazolium cations ever present in the most common electrolyte formulation. Finally, Raman images combined with the absorption spectra unequivocally identify the chemical modifications suffered by dye molecules, when RB stress induces bubbling phenomena within the electrolyte solution, leading to the final breakdown of the device.