Discerning Variable Extents of Interdomain Orientational and Structural Heterogeneity in Solution-Cast Polycrystalline Organic Semiconducting Thin Films

Abstract

By spatially resolving the polarized ultrafast optical transient absorption within several tens of individual domains in solution-processed polycrystalline smallmolecule organic semiconducting films, we infer the domains' extents of structural and orientational heterogeneity. As metrics, we observe variations in the time scales of ultrafast excited state dynamics and in the relative strength of competing resonant probe transitions. We find that films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) exhibit a much higher degree of both structural and orientational heterogeneity among their domains than do films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSPn), despite the apparent structural similarity between these two small molecules. Since both molecules feature prominently in solution-processed organic transistors, correlating the extent of heterogeneity to bulk transport using our approach will be highly valuable toward determining the underlying design principles for creating high-performing devices. Furthermore, our ability to characterize such variation in heterogeneity will enable fundamental studies of the interplay between molecular dynamics and driving forces in controlling emergent unequilibrated structures. S mall-molecule organic semiconducting thin films serve many applications in solution-processed electronics. In many cases, they form polycrystalline films with a wide range in the size and shape of individual domains. The high degree of intermolecular order in these films predisposes them to applications in solution-processed transistors, where comparatively high charge carrier mobilities can be engineered due to regular π-stacking or other forms of orbital overlap along a particular in-plane direction.

Micro-Raman imaging of isomeric segregation in small-molecule organic semiconductors

“…3c). These results support the anisotropic structure of diF-TES ADT observed in mixed samples using transient absorption microscopy 36 .…”

Spatially-resolved fluorescence-detected two-dimensional electronic spectroscopy probes varying excitonic structure in photosynthetic bacteria

“…For non-fluorescent samples, the SF-2DES spectrometer can be modified for 2D photocurrent detection 68 , or for heterodyne detection by delaying the 4 th pulse and routing it around the sample as the local oscillator 69 . Domain heterogeneity in polymer-fullerene blends, or thin films of perovskites and TIPS-pentacene, could also quench the photoluminescence at certain locations in the film 70 , making the above modification necessary in order to probe the domain-dependent carrier delocalization dynamics in these materials that has been observed in pump-probe microscopy studies 23 , 27 , 28 .…”

“…Similarly, the effect of grain size 25 and chloride content 26 on charge transport in perovskites thin-films is well known, but the connections between layer heterogeneity and charge delocalization is poorly understood. Spatial-resolution provided by pump-probe microscopy has partly addressed these questions 11 , 27 – 29 . However, new spatially resolved tools that can go beyond pump-probe approaches to resolve inhomogeneity and the couplings between excited electronic states directly with high frequency and time-resolution could impact our understanding of a broad range of systems, including emerging 2D materials 30 .…”

Enhancement of charge transfer in thermally-expanded and strain-stabilized TIPS-pentacene thin films

“…A peak shift has been observed for peak 2 and 3 as the polarization is changed from along the short axis of the molecule to the long axis of the molecule. pentacene, but this is a small perturbation that results in weak optical activity for polarization along other directions [42,43]. In Fig.…”