Long term stability of organic solar cells Zusammenfassung: Organic solar cells bear the potential to be used as flexible light weight and cheap source of renewable energy. Efficiency long lifetime and low cost are the important factors for the successful application of this technology. Significant improvement in the power conversion efficiency in the past few years brought lifetime of organic solar cells into focus. Encapsulation assists to achieve long lifetime by protecting organic solar cells against different aging factors like oxygen and moisture. Understanding of degradation mechanisms further helps to develop different strategies to prolong the lifetime.The experimental work of this thesis is divided into two parts. The first part deals with the evaluation of various encapsulation and barrier materials under different aging conditions such as elevated temperature continuous illumination UV stress damp heat and outdoor exposure. In the second part detailed investigations of degradation mechanisms under different aging conditions (mainly under UV stress and damp heat) and strategies how to minimize the degradation are presented.A promising lifetime of more than 12 000 hours both under elevated temperature of 85°C ambient air in the dark and continuous illumination of 1000 W m² was demonstrated with less than 10 % degradation for encapsulated devices. Similar devices showed less than 20 % degradation after one complete year of outdoor exposure. Further completely flexible encapsulated devices also exhibited > 95 % of their initial device performance after 1000 hours of aging under damp heat successfully fulfilling the standard test condition set by the IEC61646 under 85°C 85 % rh.In contrast devices exhibited comparably fast degradation in presence of UV radiation resulting in more than 50 % degradation within 1000 hours of aging. The observed degradation under UV radiation was found to be characterized by a drop in fill factor and short circuit current density. Additional experiments revealed an increase in the sheet resistance of the PEDOT:PSS hole transport layer. Numerical simulations based on an effective semiconductor model using experimentally observed sheet resistance values were carried out. Simulation results were found to be in good agreement with experimental observations. The use of high conductive or UV stable PEDOT:PSS formulations high post annealing temperature ensuring better removal of moisture or an additional UV filter were proven as possible routes for retar ...