DOI: https://doi.org/10.29363/nanoge.inform.2019.042
Publication date: 8th January 2019
Aggregates – that is short-ranged ordered moieties in the solid-state of π-conjugated polymers – play an important role in the photophysics and performance of various optoelectronic devices. It is often not clear what controls their formation during the spin-coating process of a film. Previously, we have shown that many polymers change from a disordered to a more ordered conformation when cooling a solution below a characteristic critical temperature. Here, I adress how we can use this knowledge to control the formation of aggregates during the deposition of films.
Using in situ time-resolved absorption spectroscopy on the prototypical semiconducting polymers P3HT, PFO, PCPDTBT, and PCE11 (PffBT4T-2OD), we show that spin-coating at a temperature below the critical temperature enhances the formation of aggregates with strong intra-chain coupling. An analysis of their time-resolved spectra indicates that the formation of nuclei in the initial stages of film formation for substrates held the critical temperature is responsible for this. We observe that the growth rate of the aggregates during film formation is thermally activated with an energy that exceeds the activation energy of the solvent viscosity . From this we conclude that the rate controlling step is the planarization of a chain that is associated with its attachment to a nucleation center.
Further, it is well known that the film formation process during spin-coating as well as the subsequent long-time film drying process differ significantly when DIO is added to a solution of P3HT. Here we show how the addition of DIO increases the time until the disorder-order transition sets in, yet it accelerates the actual transition, which impacts on the nature of the resulting aggregates. Moreover, aggregates form through an all-over solidification throughout the entire film rather than the spreading of a solidfication front in the absence of DIO in the solution.
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