**4.2. Processing technique**

turing and product development has also been witnessed for OPV technology. With the fast performance improvement of OPV cells in research labs, the application of OPV technology as a renewable energy source has become more appealing. However, it is never trivial to translate a lab-based technology into a large volume production process. A large number of difficulties and problems have to be overcome to ensure the successful commercialization of the technology. In this session, we will highlight some aspects related to the industrialization

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)

**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

of OPV technology.

372 Solar Cells - New Approaches and Reviews

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

[140] to stimulate the research effort to address these issues.

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

As one of the potential advantages, solution based processing methods, such as roll-to-roll printing and ink-jet printing, are suitable for organic solar cell materials [143-145]. So far in the lab scale, the BHJ solar cells are typically fabricated by spin-coating method. As spin-coating turns out to be suitable for the reproducible formation of homogeneous thin films, it is difficult to scale up and a large amount of inks is wasted during the spin-coating process. Wet-printing with a roll to roll production process is a more favorable processing technique for large scale OPV module fabrication. These printing techniques include gravure printing, flexographic printing, screen printing, rotary screen-printing, knife coating, slot die coating, and so on. More details regarding these printing techniques can be found in [144, 145]. Demonstration of large scale printing of OPV modules has been done by Krebs et al. [146] and currently OPV based solar parks have been established to explore the potential this new energy technology. One issue related to the printing technology is the uniformity and reproducibility of the final OPV module since the OPV performance of BHJ blend is very sensitive to the morphology of the thin film whereas the morphology is very sensitive to the processing condition. The materials used and the processing technique should be able to provide an OPV module with lifetime > 10 years with an average power conversion efficiency > 10% to compete with the mature inorganic PV products in the mainstream market. Even though the current start-of-the-art efficiency can reach >10% for small devices in the research lab, the efficiencies of the large area devices by solution processing methods are still low (< 3.5%) [144]. Further optimization of both the materials and the processing methods is urgently needed to fulfill all the advanta‐ geous claims of OPV technology.
