'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.

Novel bulk heterojunction (BHJ) organic photovoltaic (OPV) solar cells have been fabricated by introducing a series of metallophthalocyanines (MPcs) into a blend of a poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C70 butyric...

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%.

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/PC 71 BM 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 1,8diiodooctane, we observe a similar behavior in the timing of the crystallization phase.

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