Elucidating Processing-Structure-Function Relationships in Solution-Processed, Organic-Semiconductor Thin Films for Transistor Applications

Abstract

Solution-processed, organic-semiconductor thin films comprising spherulites are structurally complex. Low-angle grain boundaries exist within spherulites to accommodate a large distribution of molecular orientations about the radial axis. Interspherulite boundaries (ISBs) also exist where two spherulites impinge. To determine how these structural heterogeneities affect charge transport, we first developed a fundamental understanding of the structural development of solution-processable triethylsilylethynyl anthradithiophene (TES ADT) organic-semiconductor thin films. We then developed methods to control the structure of TES ADT thin films in order to elucidate structure-function relationships in organic thin-film transistors (OTFTs).

To control the nucleation density in TES ADT thin films, fractional amounts of small-molecule additives were used to seed TES ADT crystallization. By increasing the concentration of small-molecule additives in TES ADT thin films, the average spherulite diameter was varied from 3 mm to 30 μm. After nucleation, spherulites propagate radially outwards at a constant rate until neighboring spherulites impinge. The growth rate of TES ADT spherulites can vary by a factor of three depending on the surface energy of the underlying substrate. By selectively patterning the underlying substrate to have regions of different surface energies, we imposed differential growth rates of TES ADT to guide crystallization along pre-specified, nonlinear patterns. This method of guided crystallization was used to engineer ISBs having low and high angles of molecular orientation mismatch.

The ability to specify the nucleation density and control the direction of crystallization in TES ADT thin films were then employed to decouple how the structure of spherulites affects charge transport in the active layers of OTFTs. By testing arrays of transistors within a single spherulite, intraspherulite charge transport was found to be independent of the general molecular orientation, but dominated by low-angle intraspherulite grain boundaries. By measuring charge transport across single engineered ISBs, high-angle ISBs were found to act as significant barriers to charge transport. Small-molecule additives were also incorporated into the active layers of TES ADT OTFTs to vary the number of ISBs in the active channel. OTFT device performance degraded with increasing number of ISBs in the active channel, since charge transport is dominated by high-angle ISBs.