Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries

Highly crystalline thin films in organic semiconductors are important for applications in high‐performance organic optoelectronics. Here, the effect of grain boundaries on the Hall effect and charge transport properties of organic transistors based on two exemplary benchmark systems is elucidated: (1) solution‐processed blends of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) small molecule and indacenodithiophene‐benzothiadiazole (C16IDT‐BT) conjugated polymer, and (2) large‐area vacuum evaporated polycrystalline thin films of rubrene (C42H28). It is discovered that, despite the high field‐effect mobilities of up to 6 cm2 V−1 s−1 and the evidence of a delocalized band‐like charge transport, the Hall effect in polycrystalline organic transistors is systematically and significantly underdeveloped, with the carrier coherence factor α < 1 (i.e., yields an underestimated Hall mobility and an overestimated carrier density). A model based on capacitively charged grain boundaries explaining this unusual behavior is described. This work significantly advances the understanding of magneto‐transport properties of organic semiconductor thin films. The effect of grain boundaries on the Hall effect in polycrystalline organic transistors is elucidated in two exemplary systems: solution‐coated 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene and thermally evaporated highly crystalline rubrene. It is shown that even when organic field‐effect transistor mobility is as high as ≈5 cm2 V−1 s−1, capacitively charged grain boundaries may lead to an underestimated Hall mobility and an overestimated Hall carrier density.