Directional Crystallization from the Melt of an Organic p‑Type and n‑Type Semiconductor Blend

Directional crystallization from the melt has been used as a tool to grow parallel crystalline stripes of p- and n-type molecular semiconductors. To start, the phase behavior of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8): tetracyanoquinodimethane (TCNQ) blends (molar ratio from 20:1 to 1:1) has been investigated by variable-temperature X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy. The partial charge transfer between the C8-BTBT-C8 donor and the TCNQ acceptor as a function of temperature has been studied. Blends of 10:1 and 20:1 have been selected for directional crystallization because they show similar thermotropic and phase behavior comparable to that of pure C8-BTBT-C8. Directional crystallization results suggest that moderate cooling rates (6 and 12 °C min-1) leads to a digitated growth mode that gives rise to parallel crystalline stripes of C8-BTBT-C8 and C8-BTBT-C8-TCNQ charge-transfer complexes (C8-BTBT-C8-TCNQ CT), as confirmed by confocal Raman imaging. X-ray diffraction reveals high preferential orientation and good in-plane alignment for both C8-BTBT-C8 and C8-BTBT-C8-TCNQ CT crystallites.