The effect of post-processing treatments on inflection points in current–voltage curves of roll-to-roll processed polymer photovoltaics

Inflection point behaviour is often observed in the current–voltage (IV) curve of polymer solar cells. This phenomenon is examined in the context of flexible roll-to-roll (R2R) processed polymer solar cells in a large series of devices with a layer structure of: PET–ITO–ZnO–P3HT:PCBM–PEDOT:PSS–Ag. The devices were manufactured using a combination of slot-die coating and screen printing; they were then encapsulated by lamination using a polymer based barrier material. All manufacturing steps were carried out in ambient air. The freshly prepared devices showed a consistent inflection point in the IV curve and a corresponding poor performance or lack of photovoltaic behaviour. Upon exposure to 1000 Wm−2 illumination at ca. 85 °C and repeated IV scans (photo-annealing) the inflection point gradually disappeared, and performance drastically increased over time. The characteristics and stability of this “photo-annealing” behaviour was further investigated by studying the effects of several key factors: temperature, illumination and atmosphere. The results consistently showed that the inflection point is a dynamic phenomenon which can be removed under specific conditions. Subsequently, chemical characterization of device interfaces was carried out in order to identify possible chemical processes that are related to photo-annealing. A possible mechanism based on ZnO photoconductivity, photooxidation and redistribution of oxygen inside the cell is proposed, and it is anticipated that the findings are applicable to various other device structures based on semi-conducting oxides. The findings may have influences on the possibilities and scale-up of polymer solar technologies.