In-Situ Characterization of Film Formation in Solution-Processed Solar Cells

Abstract

Solar cells are an important part for the transition away from fossil fuels. For production of 3rd generation solar cells, like organic and perovskite solar cells, commonly spin coating is used. On the first glance, it is a very easy deposition method. A solution is dripped on a rotating substrate, leading to a thin film. However, if investigating closer, one realizes that many different physical processes take place, which hamper the understanding. To investigate the processing of organic and perovskite layers during spin coating, a special measurement setup is constructed in this thesis. With it, one can measure the permanently changing transmittance of a thin film during spin coating. For that, a special spin coater is constructed which allows for a laser beam to be guided through the motor axis. With the laser, one can measure the complete visible light spectrum simultaneously. A fast spectrometer is used to be able to time resolve the fast spin coating process, which can be shorter than 1s. Using the setup, a simple model for the spin coating process was developed. For that, multiple solution and spin coating parameters are investigated concerning their influence on film formation. The gained results can be transferred to other material systems. Additionally, the state-of-the-art organic material system Y6:PM6 and the solventengineering method for perovskite solar cells is investigated. For Y6:PM6, a longlasting influence of the solvent additive 1-chloronaphtalene is shown. The long-lasting influence is a result of the solubility of Y6 in 1-chloronaphtalene. In addition, it is shown, that methanol is an antisolvent for 1-chloronaphtalene. Solar cells constructed with 5 % 1-chloronaphtalene concentrations and using the solvent-engineering method show the highest reported open-circuit voltage for Y6:PM6 solar cells. For perovskite solar cells, the film formation for different antisolvent application durations is investigated. It is shown that the film formation is clearly dependent on the application duration. In addition, the exposure duration of the antisolvent can be divided into two parts: First, the application duration itself, and second, the time it takes the antisolvent to evaporate. The evaporation is independent of the application duration. These measurements show that a minimum exposure duration of the precursor solution to the antisolvent exists. In addition, a laser interference lithography setup based on a Lloyd’s mirror interferometer is shown, which is also constructed as part of this thesis. Different possible microscopic and macroscopic patterns on different substrates are shown.