Quantifying the Energy Barriers and Elucidating the Charge Transport Mechanisms across Interspherulite Boundaries in Solution-Processed Organic Semiconductor Thin Films

Grain boundaries act as bottlenecks to charge transport in devices comprising polycrystalline organic active layers. To improve device performance, the nature and resulting impact of these boundaries must be better understood. The densities and energy levels of shallow traps within and across triethylsilylethynyl anthradithiophene (TES ADT) spherulites are quantified. The trap density is 7 × 10¹⁰ cm^-2 in devices whose channels reside within a single spherulite and up to 3 × 10¹¹ cm^-2 for devices whose channels span a spherulite boundary. The activation energy for charge transport, E_A, increases from 34 meV within a spherulite to 50-66 meV across a boundary, depending on the angle of molecular mismatch. Despite being molecular in nature, these E_A's are more akin to those found for charge transport in polymer semiconductors. Presumably, trapped TES ADT at the boundary can electrically connect neighboring spherulites, similar to polymer chains connecting crystallites in polymer semiconductor thin films. The impact of interspherulite boundaries (ISBs) on charge transport in organic semiconductor thin films is explored using gated four-probe transistor measurements on triethylsilylethynyl anthradithiophene (TES ADT). Quantification of the densities and energy levels of shallow traps at these boundaries suggests TES ADT's ISBs to be akin to the connected boundaries between crystallites in polymer semiconductor thin films.