A hidden bulk polymorph governs charge transport dimensionality in an organic semiconductor

TL;DR

This study uncovers a previously unknown bulk polymorph of DNTT, revealing how polymorphism influences charge transport dimensionality and mobility in organic semiconductors, with implications for flexible optoelectronic devices.

Contribution

The discovery and structural resolution of a new blue DNTT polymorph demonstrate its impact on charge transport properties, highlighting polymorphism as a tool for tuning organic semiconductor performance.

Findings

Blue DNTT is the thermodynamically stable polymorph coexisting with green DNTT.

Blue DNTT exhibits three-dimensional charge transport with electron mobility exceeding twice hole mobility.

Polymorphism alters phonon landscapes and transfer-integral networks, affecting charge transport pathways.

Abstract

Organic semiconductors (OSCs) are widely explored for flexible optoelectronic technologies, with performance governed not only by molecular design, but also by solid-state packing, which can give rise to polymorphism. Dinaphthothienothiophene (DNTT) is a benchmark OSC that has long been considered monomorphic. Here, we discover, isolate, and resolve the crystal structure of a previously unrecognised bulk polymorph of DNTT, termed blue DNTT owing to its characteristic blue emission. Coexisting with the well-known (green) DNTT in commercial powders, yet previously overlooked, blue DNTT represents the thermodynamically stable form. By combining X-ray diffraction, Raman, and THz spectroscopy with simulations, we demonstrate that polymorphism in DNTT reshapes the low-frequency phonon landscape and transfer-integral network, impacting charge transport. While green DNTT exhibits two-dimensional charge transport with holes more mobile than electrons, blue DNTT shows charge transport along all crystallographic directions enabled by a distinct herringbone packing. Electron mobility along the crystallographic a and b-axes in blue DNTT exceeds twice the hole mobility in the green phase. To our knowledge, this is the first reported acene-based semiconductor exhibiting three-dimensional charge transport. Polymorphism emerges as a key lever to tune charge transport dimensionality and carrier efficiency in organic semiconductors.