Polymorph Screening and Investigation of Charge Transport of ditBuC6-BTBT

Abstract

In this study, we investigate the relationship between the polymorphism and crystallographic parameters and the charge transport properties achieved through the fabrication of organic field-effect transistors (OFETs) based on a novel molecular semiconductor, i.e., 2,7-bis(7,7-dimethyloctyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBuC6-BTBT). Four polymorphs of ditBuC6-BTBT were identified: three observed at room temperature (Forms I, Ia, and II), and one appearing above 100 °C (Form III). While cell parameters were measured for all forms, full crystal structures were determined only for Forms Ia and II. Although a direct correlation between molecular packing and charge transport properties could not be established from the present study, the structural analysis of the polymorphs contributes to a broader understanding of the packing motifs in ditBuC6-BTBT. A meticulous examination of the minute discrepancies in the powder patterns substantiated the existence of both the metastable Form I and Form Ia, which became more difficult to isolate due to unintentional seeding of the thermodynamically stable Form II. Nonequilibrium crystallization techniques utilizing thermal gradient and bar-assisted meniscus shearing methods were explored to enhance control over polymorph selection. The intrinsic charge transport properties ruled by the overlap of the frontier orbitals were studied by computing the transfer integrals. Optimized devices fabricated by depositing thin films by solution shearing and vacuum evaporation led to field-effect mobility in the linear regime of ca. 0.05 cm² V^–1 s^–1. The observed device performances were interpreted as a result of the combined effects of crystal packing features, ionization potential values, and polymorphic coexistence, highlighting the challenges in deriving clear structure–property correlations and underscoring the complexities in achieving high-performance organic electronics with this material.

Polymorph screening and OFET fabrication of a novel BTBT derivative (ditBuC6-BTBT) reveal how molecular design and polymorphic control impact charge transport in organic semiconductors.