A study for degradation of flexible organic photovoltaic modules via damp-heat test: By accessing individual layers of the module

The stability of slot-die coated flexible organic photovoltaic (OPV) modules with inverted structures of indium tin oxide (ITO)/ZnO/photoactive layer/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag was analyzed under damp heat conditions. The work primarily focused on the understanding of the degradation mechanism of organic photovoltaic modules in very severe operating conditions (85°C/85% relative humidity [rh]). The electrical change of the OPV module was measured as a function of the aging time under damp heat. A rapid drop in the efficiency of the module was observed, mainly caused by the decline of the open-circuit voltage (V_oc) and fill factor (FF). We supposed that the degradation of the modules mainly progressed by the oxygen and moisture penetrating the terminals or at the edges of the barrier films. The physical modification of the layers composing the device by degradation was observed; hence, morphological and chemical analyses were conducted. The analyses revealed that Ag atoms migrated to the interlayers of the cell, resulting in the increase of shunt paths; this was the main reason underlying the reduction of V_oc and FF of the OPV module under damp heat. The ZnO layer was also etched by the acidic molecules from the diffused PEDOT:PSS polymer. In addition, the performance parameters of several OPV modules were evaluated simultaneously under damp heat conditions for 1000 h. The same lifetime patterns were observed although the initial efficiencies of the modules were diverse. The interpretation of the lifetime patterns and a suggestion for an objective comparison between the modules showing different initial efficiencies were also addressed.