2023, Raab, Timo

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.

2017, Schmidt, Jan, Raab, Timo, Oelmann, Jannis, Seletskiy, Denis V.

Upon excitation of a material below its fundamental transition, cooling of the lattice results if the subsequent emission is predominantly radiative. Despite overwhelming experimental success, it remains a challenge to understand the microscopic nature of detrimental processes that can even prevent cooling. We apply ultrafast spectroscopy to resolve the laser refrigeration cycle in the time domain. Strong evidence for lattice cooling on picosecond timescales in bulk GaAs/InGaP double-heterostructures and GaAs/AlGaAs quantum wells establishes the non-local nature of the parasitic mechanisms. Further precision measurements investigating long-time dynamics are currently underway to resolve detrimental heating in bulk GaAs for the first time.

2023, Raab, Timo, Seewald, Tobias, Kraner, Stefan, Schmidt-Mende, Lukas

Additives, like 1-chloronaphtalene (CN), are commonly used in Y6:PM6 solar cells as they lead to an increased power conversion efficiency. In this work, we investigate the influence of CN during spin coating of Y6:PM6 dissolved in chloroform via an in situ transmission setup. We show that, in the presence of CN, the film formation of Y6:PM6 can be divided into two parts: one related to the evaporation of chloroform and one related to the evaporation of CN. This is mostly related to Y6 being dissolved in CN. We find that even for low CN concentration, the film formation is not completed for several minutes after the spin coating process. Furthermore, the removal of CN is needed to achieve a smooth film surface. We demonstrate that this fast removal can be achieved by spin coating the electron transport layer PDINN from methanol. The methanol is acting as an anti-solvent for the CN, leading to its removal from the film. Using this approach, solar cells fabricated with a high CN concentration of 5% feature a comparable performance to ones with more common concentrations between 0.5% and 1%.

2016, Xu, Weigao, Liu, Weiwei, Schmidt, Jan, Zhao, Weijie, Lu, Xin, Raab, Timo, Diederichs, Carole, Gao, Weibo, Seletskiy, Denis V., Xiong, Qihua

'Blinking', or 'fluorescence intermittency', refers to a random switching between 'ON' (bright) and 'OFF' (dark) states of an emitter; it has been studied widely in zero-dimensional quantum dots and molecules, and scarcely in one-dimensional systems. A generally accepted mechanism for blinking in quantum dots involves random switching between neutral and charged states (or is accompanied by fluctuations in charge-carrier traps), which substantially alters the dynamics of radiative and non-radiative decay. Here, we uncover a new type of blinking effect in vertically stacked, two-dimensional semiconductor heterostructures, which consist of two distinct monolayers of transition metal dichalcogenides (TMDs) that are weakly coupled by van der Waals forces. Unlike zero-dimensional or one-dimensional systems, two-dimensional TMD heterostructures show a correlated blinking effect, comprising randomly switching bright, neutral and dark states. Fluorescence cross-correlation spectroscopy analyses show that a bright state occurring in one monolayer will simultaneously lead to a dark state in the other monolayer, owing to an intermittent interlayer carrier-transfer process. Our findings suggest that bilayer van der Waals heterostructures provide unique platforms for the study of charge-transfer dynamics and non-equilibrium-state physics, and could see application as correlated light emitters in quantum technology.

2022, Raab, Timo, Mayer, Tim, Seewald, Tobias, Schmidt-Mende, Lukas

Spin coating is one of the most common techniques for the production of thin films in laboratories. In this work, we investigate spin coating of a P3HT/PC₇₁BM blend while in situ measuring the change in transmission to gain a general understanding of the film-forming-process. We show that spin coating can be divided into three phases: first, the distribution phase; second, a film thinning phase; and finally, film crystallization. The final morphology of the crystalline film at the end of the spin coating process is almost entirely influenced by the crystallization phase, whereas the other phases do not have the same influence on the final film quality. We found that processing conditions, such as decreasing the solution temperature, decreasing the spin speed, or increasing the solution concentration, increase the crystallinity of the film. This is always related to an increase in crystallization phase duration. When using additives in the solution, such as 1,8-diiodooctane, we observe a similar behavior in the timing of the crystallization phase.