High Performance Cyanine Solar Cells

Today a plethora of soluble cyanine dyes absorbing from the ultra-violet to the near infrared domain are available owing to more than a century of research and development, mostly in photographic industry. Numerous properties of cyanine dyes suggest that this material class would be interesting for organic solar cell applications. Most importantly the unparalleled absorption coefficients allow using very thin films for harvesting the solar photons. Cyanines also own favourable redox potentials making it possible to use them as electron donors and acceptors in organic heterojunction solar cells. With respect to crystallinity, these polymethine dyes tend to form aggregates where charge and excited states are delocalized over hundreds of molecules. Furthermore, cyanines are cationic polymethine dyes, offering the possibility to tune the materials by choosing the counter-anion. In this thesis work, the limiting factors for efficient power conversion in thin solid cyanine solar cells were studied. The mechanisms of charge separation and extraction in pristine and doped trimethine cyanine (Cy3) solar cells were investigated with various methods including ultrafast time resolved experiments and electrochemical impedance spectroscopy. A rather fast charge collection as well as a slow recombination rate could be revealed. Together with the strong absorption and large exciton diffusion length in cyanine films it could be shown that the bulk heterojunction film architecture of donor and acceptor phases, which is predominantly used in polymer solar cells, was no longer required for achieving high efficiency. Instead, the much simpler planar bilayer architecture proved to be valuable for high extinction dyes such as Cy3 as well as squaraine dyes. The device performance was further optimized by introducing appropriate charge injecting layers at the electrodes. On the anode side, a conductive polyaniline layer was applied that made it possible to reach an efficiency of over 3%, despite the rather narrow absorption band of Cy3 dyes. Oxidative doping of the cyanine layer by various agents was also investigated carefully. A simple solution doping method using a nitrosyl salt was developed that allowed to carefully investigate the effect of cyanine layer conductivity on device performance. At the optimum doping concentration, the maximum external quantum efficiency reached 80%, which corresponds to an internal quantum efficiency of almost unity, when optical losses are considered. Thus it could be demonstrated that organic solar cells based on high extinction dyes processed from solution achieve high device performance, comparable to the one of polymeric bulk-heterojunction cells.