Characterization of Thin Films for Polymer Solar Cells : Stability and Response to Concentrated Light

Abstract

The field of polymer solar cells has undergone an extensive development in recent years after the invention of semiconducting polymers in 1991. Efficiencies have gradually increased to above 10 %, and high throughput processing methods such as roll-to-roll coating allow for production of thousands of solar cells with low embedded time, material, and energy consumption as compared to silicon solar cells. Consequently, different demonstration products of small mobile gadgets based on polymer solar cells have been produced, which are fully competitive with conventional energy technologies, illustrating the maturity of the technology. However, a limiting factor in terms of full commercialization is the stability of polymer solar cells. While is has been estimated that 10 years lifetime is needed, existing technologies only provide stabilities up to 1 year. Degradation of polymer solar cell is a multi facetted process where oxygen and water diffusion from the atmosphere, morphology evolution, and photo-bleaching of the polymer are some of the dominant processes. Encapsulation by foils consisting of multi-layer polymer stacks is a conventional way to reduce the diffusion into the solar cell, by which the life time of the cell is highly increased. An alternative approach is to increase the photo stability of the cell components, and especially the light absorbing conjugated polymer has been subject to extensive attention. The photo stability of conjugated polymers varies by orders of magnitude from type to type depending on the chemical structure of the material and consequently, the lifetime is highly influenced by the polymer stability. Photochemical degradation of polymers, i.e. degradation of thin films of polymer in the ambient under light exposure, is a technique normally applied to evaluate polymer stabilities. Hereby, an extensive list of stabilities of different materials has been established providing an understanding of the stability of the individual building blocks of polymer. While being a highly practical tool, no rigorous reports on the photochemical degradation as a technique exist where the technique is validated and different pit-falls identified. Consequently, a rigorous study on the validation and maturing of this technique was performed during this PhD work (Chapter 2). Furthermore, as research gradually increases the lifetime of polymers to months or years, stability evaluations at standard degradation conditions become impractical. Accelerated degradation has been performed with heat and different gases by which the timeframe of stability evaluations have been reduced by up to a factor of 20. However, light, which appears at the most intuitive acceleration condition to conjugated polymers, has until this PhD work not been applied as an acceleration condition to polymer degradation. Light can be concentrated up to thousand of solar intensities by optical components, which has the potential to significantly accelerate polymer degradation. Concentration of light was one of the main topics during this PhD, where the construction and development of light concentrators, both by sun light as well as artificial light, was given extensive attention. This resulted in three different light concentrators, a lens based solar concentrator, a mirror based solar concentrator, and an artificial light concentrator for indoor use (Chapter 3). With these concentration setups, acceleration factors of up to 1200 were obtained for degradation of typical conjugated polymers thus significantly reducing the timeframe of stability evaluations. The potential of this approach is that stability evaluation can become a routine characterization techniquefor novel conjugated polymers, when these are applied to polymer solar cells and their efficiencies are reported. By making the polymer stability practically accessible, development of polymer stability can become significantly more transparent and focused (Chapter 4). A direct alternative application of concentrated light is the application to polymer solar cells. Stabilities of polymer solar cells is within the same order of magnitude of photo chemical stabilities of polymers as the oxidation rate of the polymer in the solar cell is lower, but a large number of additional degradation mechanisms are introduced. Consequently, research in the stability of polymer solar cells is impractical since the extensive timeframe of stability testing reduces the pace of the research. This thesis reports the first results on the response of polymer solar cells to concentrated light, both in terms of performance as well as stability. Additionally, concentrated light was used to study some of the mechanisms governing solar cells operation, which are dominant when currents are very high as a consequence of high photon flux. The response in terms of stability of the cells was found to be highly complex and effects not dominant at 1 sun were observed. Thus, specific knowledge of the response of the different layers to concentrated light is needed to use concentrated light as a valid acceleration parameter (Chapter 5).