Effect of oxygen and water on the lifetime of organic solar cells

Ageing of organic solar cells is still a problem to resolve before these devices can reach the market. This question can be addressed using different strategies including encapsulating materials, device design and buffer layers, with the latter being addressed in this paper. The shelf life of un-encapsulated small-molecule organic solar cells, (ITO/PEDOT-PSS/CuPC/C₆₀/buffer layer/Al), have been studied under 100 mW/cm² continuous illumination in open air with different thin buffer layers of bathocuproine (BCP), tris-8-hydroxy-quiolinato-aluminum (Alq3), 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBI) and lithium fluoride (LiF). Changes in the main photovoltaic parameters upon ageing (power conversion efficiency, fill factor, open circuit voltage, short circuit current density, series and shunt resistances) have been monitored. In all cases two ageing mechanisms with two different time constants have been demonstrated and attributed to a rapid degradation of cathode contact followed by contamination of the active layer.