chapter  5
Degradation and Its Characterization in Organic Solar Cells
Pages 38

Silicon (Si) solar cells presently dominate the photovoltaic (PV) market, but some of their inherent limitations and high cost make them less attractive for cost-effective energy production. Researchers are trying to find ways for cost-effective and efficient energy production by reducing the cost of Si PVs or using alternate materials for PV applications. Organic solar cells (OSCs) could be an effective alternative to Si PVs, as they could reduce the energy production cost from PV modules. Additionally they are thin, lightweight, semitransparent, and mechanically flexible giving them a freedom of incorporation into different surfaces making new types of solar-energy-collecting shapes and designs. But OSCs suffer from the flaws of lower efficiency and deterioration in performance with time. Like efficiency, cost, and processing, stability is another decisive parameter for success and real-life application of OSC technology. The lifetime of Si PV devices ranges up to 25 years or more, whereas the lifetime of OSCs ranges from months to years. Si PV companies are still trying to improve the lifetime up to 40 years. Therefore to compete with conventional PV technologies, OSCs need to be improved tremendously. OSCs of high efficiency with long operational lifetime are still to be realized and it is a real challenge for this technology. Unlike inorganic semiconductors, organic semiconductors are by nature very susceptible to oxygen-and moisture-induced degradation, and degradation happens throughout the device from the top electrode to the bottom electrode. Oxygen and moisture may get introduced during device preparation or even after preparation and react with device components. Understanding and preventing degradation is highly important for the success of this emerging technology. There have been a number of studies on degradation that show that it is a complicated phenomenon and not yet completely understood; however, there is a rough classification between chemical and physical degradation. Chemical degradation includes formation of organic molecules with inferior electrical and optical properties due to reaction of original organic molecules with oxygen, moisture, and electrode materials [1-4], whereas physical degradation is because of delamination of the top electrode, change in the active layer morphology, diffusion of electrode materials into organic layers, and degradation of different interfaces [5-7]. Photoelectrochemical reactions, photooxidation, and oxidation of low work function electrodes are other important chemical degradation processes [8-10]. Degradation happens in both the dark and illumination; however, under illumination the degradation is expected to accelerate due to heating and ultraviolet (UV) light exposure. It is important to note the intensity and spectrum of incident light, the way of device illumination, device temperature, and surrounding conditions, as the rate of

degradation and degradation mechanisms may be different for different test conditions. Therefore, where different materials and different test conditions are employed, direct comparison of stability becomes very difficult. In this chapter, I shall discuss how diversified the degradation in OSCs is. The variety of tools and techniques used for characterization of degradation are also discussed.