An Effective Area Approach to Model Lateral Degradation in Organic Solar Cells

In standard unencapsulated poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells exposed to humid air, the oxidation of the aluminum cathode is known to be a key degradation mechanism. Water that enters the device at the edges and through pinholes diffuses to the organic-electrode interface. The forming oxide acts as a thin insulating layer that gives rise to an injection/extraction barrier and leads to a loss in the device current. In order to understand this behavior in detail various steady-state, transient, and impedance measurement techniques are performed in combination with drift-diffusion simulations. With this combinatorial approach the dominant degradation mechanism is confirmed to be the development of a blocking interface layer. This layer grows laterally leading to a loss in effective area due to the rapid local oxidation of the aluminum layer. Thus by combining multiple electrical techniques and optoelectrical simulations the dominant degradation mechanism can be evaluated. The same methodology is also beneficial for more stable and efficient novel solar cells.