Crystal Polymorph Control for High-Performance Organic Field-Effect Transistors

*Zhi-Ping Fan and Hao-Li Zhang*

## **Abstract**

Organic molecules are assembled together by weak non-covalent intermolecular interactions in solid state. Multiple crystalline packing states (crystal polymorphism) have commonly existed in the active layer for organic field-effect transistors (OFETs). Different polymorphs, even with the slightest changes in their molecular packing, can differ the charge transport mobility by orders of magnitude. Therefore, accessing new polymorphs can serve as a novel design strategy for attaining high device performance. Here, we review the state of the art in this emerging field of crystal polymorph control. We firstly introduce the role of polymorphism and the methods of polymorph control in organic semiconductors. Then we review the latest studies on the performance of polymorphs in OFET devices. Finally, we discuss the advantages and challenges for polymorphism as a platform for the study of the relationship between molecular packing and charge transport.

**Keywords:** organic field-effect transistors, organic semiconductors, polymorphism, structure-property relationship, carrier mobility

#### **1. Introduction**

An organic field-effect transistor (OFET) is a transistor using an organic semiconducting thin film as the active layer in its channel [1, 2]. Charge carriers are transported in the OFET active layer under the electric field. Through the design of new materials and the improvement of fabrication processes, many impressive developments in the field of OFETs have been achieved [3–6]. It has long been realized that the morphology of the active layer has a crucial impact on its charge transport properties. Tremendous efforts have been devoted to fabricate highly ordered crystalline films to achieve high device performance, including introduction of self-assembled monolayers [7–9], annealing [10, 11], off-center spin coating [4, 12, 13], and solution shearing [14, 15]. However, the lack of knowledge on the intrinsic properties of organic semiconductors remains the barrier for high-performance materials being efficiently developed.

Polymorphism of organic semiconductors has recently received much attention in the field of OFETs [16–18]. Different polymorphic crystals have the same molecular structure but a different molecular arrangement, which can be used as an ideal platform to correlate charge transport with respect to molecular arrangement. Through investigating OFETs with different polymorphs, the relationship between molecular packing and charge transport can be obtained. Recently, some ultra-high-mobility

OFETs have been obtained by controlling the polymorphic structures of organic semiconductors, revealing that the crystal polymorph control has become an efficient strategy for the manufacture of high-performance OFETs [4, 17].
