**4. Conclusion and outlook**

*Integrated Circuits/Microchips*

charge transport properties with the molecular packing structures. The results show that a one-dimensional (1D) electron coupling between adjacent molecules is observed in the α-phase crystal (**Figure 8c**). In comparison, a two-dimensional (2D) electron coupling between adjacent molecules is found in the *β*-phase crystal (**Figure 8d**). Though the values of transfer integrals are close for the two polymorphs, the β-phase polymorph possesses a 2D charge transport network and

*SAED and TEM images of (a) the α-phase and (b) the β-phase crystals (the scale bar is 5 μm). The transfer integrals of (c) the α-phase and (d) the β-phase crystals along the (001) directions. The molecules in panels e and f are colored differently only for clarity purposes [55]. Copyright 2018, American Chemical Society.*

Titanyl phthalocyanine (TiOPC) is a well-known organic semiconductor and photoconductor; however, it exhibits poor solubility in common solvents. In a recent study by Zhang et al., TiOPC crystals were synthesized by physical vapor transport (PVT) technique through a two-zone horizontal tube furnace [6]. Some sheet crystals were obtained at the temperature zone of about 210°C, while some ribbon crystals were grown at the temperature zone of about 180°C. The sheet and ribbon crystals belong to the *α*-phase and *β*-phase polymorphs, respectively. The measurements on single-crystal OFETs of the two polymorphs demonstrated that the *α*-phase crystals exhibit excellent charge transport prop-

, while that of *β*-phase crystals are only

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup>

The crystal structures of the two polymorphs were determined, where the *α*-phase and *β*-phase crystals exhibit a 2D lamellar brick stone motif and an unusual 3D framework, respectively. The main difference in electronic coupling of the two polymorphs was a strong interlayer electronic couplings perpendicular to the current direction in the *β*-phase crystal. The strong interlayer electronic couplings may result in destructive interference effects that remarkably diminish

therefore exhibits higher carrier mobility.

*3.3.9 Titanyl phthalocyanine*

erty with mobility up to 26.8 cm<sup>2</sup>

the charge carrier mobility.

**78**

0.1 cm<sup>2</sup>

**Figure 8.**

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> .

Herein, the polymorphism in organic semiconductors is introduced, including the common strategies for polymorph control and investigations on OFETs from different polymorphs. Polymorphism is proved to be an excellent platform to directly correlate the molecular packing with charge transport for organic semiconductors; such investigations are very limited so far. A main challenge is to precisely tailor thermodynamic and kinetic factors of crystal nucleation and growth for large-area thin films or high-quality single crystals. Among the investigations on polymorphism, several polymorphs with outstanding charge transport performance have been obtained, demonstrating that altering the crystal polymorph structure of organic semiconductors is an efficient strategy to access high-performance OFETs. However, the majority of the high-mobility polymorphs are metastable. Consequently, getting insight into the relationship between molecular structure and crystal polymorph remains an important issue, which is essential for the rational design of molecular structures to further develop the desired crystal polymorphs with outstanding electrical characteristics.
