# Polymorph Screening and Investigation of Charge Transport of ditBuC6-BTBT

## TL;DR

This study explores how different crystal structures of a new organic semiconductor affect its electrical properties in transistors.

## Contribution

The study identifies four polymorphs of ditBuC6-BTBT and examines their impact on charge transport properties in OFETs.

## Key findings

- Four polymorphs of ditBuC6-BTBT were identified, including three at room temperature and one above 100°C.
- Structural analysis revealed challenges in isolating metastable polymorphs due to seeding of the stable Form II.
- Device performance was influenced by crystal packing, ionization potential, and polymorphic coexistence.

## 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.