The effect of anneal, solar irradiation and humidity on the adhesion/cohesion properties of P3HT:PCBM based inverted polymer solar cells

We use a thin-film adhesion technique that enables us to precisely measure the energy required to separate adjacent layers in OPV cells. We demonstrate the presence of weak interfaces in prototypical inverted polymer solar cells, either prepared by spin, spray or slot-die coating, including flexible and non flexible solar cells. In all cases, we observed adhesive failure at P3HT:PCBM/PEDOT:PSS interface, indicating the intrinsic material dependence of this mechanism. The impact of temperature, solar irradiation and humidity on the adhesion and cohesion properties of this particular interface is discussed. First, we have found that post-deposition annealing increases the adhesion significantly. Annealing changes the morphology in the photoactive layer and consequently alters the chemical properties at the interface. Second, solar irradiation on fully encapsulated solar cells has no damaging but in contrast an enhancing effect on the adhesion properties, due to the heat generated from IR radiation. Finally, the synergetic effect of stress and an environmental species like moisture greatly accelerates the decohesion rate in the weak hygroscopic PEDOT:PSS layer. This results in a loss of mechanical integrity and device performance. The insight into the mechanisms of delamination and decohesion yields general guidelines for the design of more reliable organic electronic devices.