(52b) Quantifying Energy Barriers and Elucidating Charge Transport Mechanisms Across Interspherulite Boundaries in Solution-Processed Organic Semiconductor Thin Films

Thin films of triethylsilylethynyl anthradithiophene (TES ADT) exhibit limited order upon spin-coating; subsequent exposure to 1,2-dichloroethane vapor induces growth of TES ADT spherulites. These superstructures are characteristically different from the typical grains observed in molecular semiconductor thin films. Whereas the unit cells within grains adopt a single orientation, the unit cells within spherulites adopt a radial distribution of in-plane orientations. Given the distinct differences in molecular organization within spherulites and grains, we have examined charge transport across TES ADT’s interspherulite boundaries (ISBs) and compared it to charge transport across grain boundaries. We use gated four-probe transistor measurements to quantify the densities and energy levels of shallow traps of devices whose channel is within a single spherulite or spans an ISB. We find that the trap density is 7x10¹⁰ cm^-2 within a single spherulite, and up to 3x10¹¹ cm^-2 across an ISB. The characteristic trap energy level, i.e., the activation energy for charge transport, increases from 34 meV within a spherulite to 50-66 meV across an ISB. Between grains, the molecularly-sharp and terraced boundaries are known to act as bottlenecks to charge transport in thin-film active layers, likely due to submicron crevices at the interface. However, the barrier increase we observe due to the presence of an ISB is much less than that observed at the grain boundaries of conventional thermally-evaporated molecular semiconductors, and is more similar to the modest increase in activation energy observed in charge transport across the crystallite boundaries in polymer semiconductor thin films. TES ADT’s ISBs likely include molecules pinned between the impinging spherulites that can facilitate interspherulitic charge transport. In this manner, TES ADT’s ISBs are more similar to the physically and electrically-connective crystallite boundaries in polymer semiconductor thin films than they are to the creviced grain boundaries in molecular semiconductor thin films.