**4.1. The stability and the lifetime issue**

The stability issue of the light harvesting material in the solar cell device should be brought into discussion as the OPV technology is aimed to generate electricity from sunlight for a long period of service time. The materials used to construct the OPV module, which include the active layer, the electrode materials and the encapsulating materials, should be robust under the outdoor condition and the performance of the OPV module should be maintained to ensure the power generation efficiency of the technology. The currently known degradation mecha‐ nisms of the solar cell device, including morphology degradation, photo-oxidation, interface degradation, physical and mechanical degradation, have been well discussed in a number of review articles [137-139]. It should be highlighted that the active layers and the metal electrode materials are especially prone to degrade upon contact with water and oxygen. Therefore, in real practice the encapsulation of the device is mandatory to guarantee the long-term stability of the device. The water and oxygen transmission properties of the encapsulant materials are thus essential to ensure the stability of the OPV module [29]. The growing concerns over the stability issue on OPV technology and fulfillment of the promise of OPV as a renewable energy technology has initiated the "International Summits on Organic Photovoltaic Stability" (ISOS) [140] to stimulate the research effort to address these issues.

**Figure 9.** Typical decay curve of a polymer solar cell employing a standard device architecture. The lifetime is defined by the point at which the efficiency has dropped by 20% from the start of the linear decay period. [141] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The degradation profile of an OPV module typically follows a trend shown in Figure 9 [141]. The device suffers a *burn-in* degradation process at the early working life which is character‐ ized by an exponential loss in efficiency and then a linear decay process. The lifetime of the device is defined as the time at which the efficiency drops to 80% of the efficiency after *burnin* process. By appropriate encapsulation, solar cell devices based on poly[9'-hepta-decan‐ yl-2,7-carbazole-alt-5,5-(4',7'-di-thienyl-2',1',3'-benzothiadiazole)] and PC71BM blend have demonstrated a lifetime of ca. six years [141]. This lifetime is marvelous in reported lifetime of OPV solar cells [142]. However, such lifetime is still considered insufficient if OPV technology is aimed to compete with the mainstream Si-based PV technology. A silicon based solar cells typically lasts on the order of 25 years and much higher PCE (Figure 1). In this regard, there is still a huge space for OPV technology to improve in order to survive in the PV market.
