Abstract
Large-scale organic electronics manufacturing requires solution processing. For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance. Although understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. Here we use transient absorption microscopy to isolate a unique signature of a hidden interface in a TIPS-pentacene thin film, exposing its exciton dynamics and intermolecular structure. Surprisingly, instead of finding an abrupt grain boundary, we reveal that the interface can be composed of nanoscale crystallites interleaved by a web of interfaces that compound decreases in charge mobility. Our novel approach provides critical missing information on interface morphology necessary to correlate solution-processing methods to optimal device performance.
Highlights
Large-scale organic electronics manufacturing requires solution processing
While it has been repeatedly shown that charge mobility is inversely proportional to the number of grain boundaries between electrical contacts[12,16,18,19], there is still no consensus on the nature of the electronic structure of domain interfaces[20,21], despite it being critical to device performance
If this interface were a molecularly abrupt grain boundary, it would consist of only B1.8 Â 106 interfacial molecules out of the B1.8 Â 109 included in the B2 mm diameter transient absorption (TA) interrogation volume of our 600-nm-thick film (Supplementary Note 3)
Summary
For small-molecule organic semiconductors, solution processing results in crystalline domains with high charge mobility, but the interfaces between these domains impede charge transport, degrading device performance Understanding these interfaces is essential to improve device performance, their intermolecular and electronic structure is unknown: they are smaller than the diffraction limit, are hidden from surface probe techniques, and their nanoscale heterogeneity is not typically resolved using X-ray methods. We observed three dynamic processes, each on a distinct time scale, and proposed a model in which ultrafast relaxation of a hot excited singlet state occurred over tens of femtoseconds, an equilibrium between singlet fission and triplet annihilation occurred on a fewpicosecond time scale, and internal conversion occurred over hundreds of picoseconds This characterization of the exciton dynamics within individual crystalline domains provides a foundation for analyzing the TA signatures measured at their interface, which is the focus of this work. Studying the nature of domain interfaces constitutes a significant contribution towards achieving a complete picture of the correlation between structure and function in organic semiconducting films, and the approach that we demonstrate is applicable to a large variety of small-molecule materials being explored for high-mobility electronic applications
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