**1.2 The role of polymorphism in OFET**

The charge transport property of organic semiconductors is sensitive to the molecular packing, where a slight change in molecular packing may result in huge difference in charge carrier mobility [23]. The side-chain engineering, which is efficient in tailoring the molecular packing, has been extensively applied to develop high-performance organic semiconductors [24–27]. However, the introduction of side chains alters the molecular structure, which makes the investigation on relationship between molecular packing and charge transport very complex.

Polymorphism offers an opportunity to tailor the molecular packing of a material, without affecting its chemical components. For example, rubrene can crystallize into three crystalline polymorphs, including an orthorhombic, a triclinic, and a monoclinic phase (**Figure 1a**) [28–30]. Taking advantage of polymorphism, it is possible to fabricate OFETs from the same organic semiconductor but with different polymorphs, hence, with different properties (**Figure 1b**). Importantly, by

#### **Figure 1.**

*(a) Molecular structure and the crystal phases of rubrene. (b) Schematic diagram for the fabrication of the OFETs from microcrystals of different polymorphs.*

**67**

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

measuring charge transport performance in OFETs from different polymorphs, a direct relationship between molecular packing and charge transport can be established. Thus, many investigations on charge transport in different polymorphs have been performed in thin-film transistors, including some benchmark organic semiconductors like pentacene derivatives [15, 31, 32], rubrene [28–30], sexithiophene (6T) [33–36], and [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives [10, 37, 38]. However, the different factors affecting the device performance, including crystallinity, grain size, and grain boundaries, are difficult to be eliminated in thin-film transistors. Thus, tremendous efforts have been paid to manufacture OFETs from highly crystalline films even single crystals [39–44]. Especially, single-crystal OFETs from different polymorphs have attracted increasing attentions [45]. Compared to polycrystalline films, single crystals have the advantages of high molecular ordering and minimal grain boundaries, and its structure is much

There is no doubt that high-performance OFETs can be obtained by tailoring molecular packing motifs in the active layer. Among the investigations on polymorphism of organic semiconductors, several high-mobility OFETs have been manufactured by polymorph control, such as the TIPS-pentacene [46], C8-BTBT [4], C6-DBTDT [47], and TiOPC [48]. Tailoring polymorphs has become an emerging

Though polymorphism is observed on many organic semiconductors, the fabrication of each polymorph with high purity is very difficult. For instance, even for the extensively studied organic semiconductors like pentacene and BTBT derivatives, only part of their polymorphs have been useful to establish the correlation between the material molecular packing and its charge transport properties [17]. The difficulties for the investigations on polymorphism include the fabrication of pure polymorphs and the determination of their crystal structures, where the polymorph control is fundamental. Some of the polymorph control methods most commonly applied to organic semiconductors are

Solution process is important for the fabrication of organic semiconductor devices, which has the advantages of low-cost and large-area fabrication. In solution processes, solvent-induced polymorphism has been frequently observed in organic semiconductors such as DB-TTF [49], DT-TTF [50], TIPS-pentacene [51], and so on. Consequently, the solvent of choice for solution-processed organic semiconductors has become a commonly practiced method for highthroughput polymorph screening. For example, the triethylsilylethynyl anthradithiophene (TES-ADT) films can crystallize into two polymorphs from different solvents [52–54]. The polymorph selectivity for solution processes mostly relates to the polarity of solvents, while the concentration can also induce polymorphism [47, 55]. For instance, the C6-DBTDT molecules can crystallize into the α-phase and β-phase crystals from high concentration and dilute chlorobenzene solutions, respectively [47]. At the molecular level, the specific interactions between semiconductor and solvent molecules in the solution can induce the nuclei formation in a particular polymorph and therefore result in polymorph

selectivity as a function of the solvent or the concentration [56].

design strategy for high-performance organic electronics.

**2. Methods of polymorph control in organic semiconductors**

*DOI: http://dx.doi.org/10.5772/intechopen.91905*

easier to determine.

discussed below.

**2.1 Solvent control**

#### *Crystal Polymorph Control for High-Performance Organic Field-Effect Transistors DOI: http://dx.doi.org/10.5772/intechopen.91905*

measuring charge transport performance in OFETs from different polymorphs, a direct relationship between molecular packing and charge transport can be established. Thus, many investigations on charge transport in different polymorphs have been performed in thin-film transistors, including some benchmark organic semiconductors like pentacene derivatives [15, 31, 32], rubrene [28–30], sexithiophene (6T) [33–36], and [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives [10, 37, 38]. However, the different factors affecting the device performance, including crystallinity, grain size, and grain boundaries, are difficult to be eliminated in thin-film transistors. Thus, tremendous efforts have been paid to manufacture OFETs from highly crystalline films even single crystals [39–44]. Especially, single-crystal OFETs from different polymorphs have attracted increasing attentions [45]. Compared to polycrystalline films, single crystals have the advantages of high molecular ordering and minimal grain boundaries, and its structure is much easier to determine.

There is no doubt that high-performance OFETs can be obtained by tailoring molecular packing motifs in the active layer. Among the investigations on polymorphism of organic semiconductors, several high-mobility OFETs have been manufactured by polymorph control, such as the TIPS-pentacene [46], C8-BTBT [4], C6-DBTDT [47], and TiOPC [48]. Tailoring polymorphs has become an emerging design strategy for high-performance organic electronics.
