Small-molecule organic semiconductors

Fig. 2.2. Examples of herringbone and herringbone-like packing motifs for several rigid-backbone, small molecule, organic semiconductors A) pentacene [54a] B) pyrene [54b] C) perylene [54c]...

An alternative route to solution-processible organic semiconductors is to use precursors to small molecule semiconductors, such as pentacene [59] or tetrabenzo-porphyrin [60], which can be converted into their fully conjugated, insoluble form by thermal or irradiative [61] treatment on the substrate. Pentacene precursors have been shown to yield field-effect mobilities of 0.01-0.1 cm2 V-1 s 1 [62], and 0.1-0.8 cm2 V-1 s 1 [63] after thermal conversion at 150-200 °C. Small molecule organic semiconductors can also be rendered solution processible by attachment of flexible side chains [64-66], Due to the relatively low solubility of these molecules the growth of uniform thin films of these molecules remains challenging, however. 

INTRODUCTION THE FIELD EFFECT IN SMALL-MOLECULE ORGANIC SEMICONDUCTORS... 

Side-chain substitution Small molecule organic semiconductors can also be rendered solution processable by attachment of flexible side chains. The substitution pattern needs to be designed carefully so that the side chains, which are needed to impart adequate solubility and film forming properties, do nof inferfere with the ability of the molecule to Jt-stack. Katz reported side-chain-substituted small molecule semiconductors such as dihexylanthradithiophene [47,48] that can be solution-deposited with mobilities of 0.01-0.02 cmWs. In bis(hexyl-bithiophene)benzene solution-cast onto a heated Si02 substrate, mobilities of up to 0.03 cmWs were reported [49]. Due to the relatively low solubility of these molecules, growth... 

As with polymeric organic semiconductor materials, the variety of available small molecule materials is almost infinite. Many materials are chain or fused units of the fundamental building blocks shown in Fig. 3.1. Small molecules are light and can usually be purified and deposited through thermal evaporation processes. Many small molecule organic semiconductors have also been functionalized to be soluble and can be deposited from solution. 

Thermal evaporation in vacuum allows for deposition and purification of small molecule organic semiconductors. The material is placed into a vacuum and heated. Once the vapor pressure of the heated material exceeds the background pressure of the material in the chamber, the material evaporates and condenses on cooler surfaces that it lands on. High molecular weight organic semiconductors cannot be deposited this way they are too heavy to evaporate and decompose instead. 

The ability to purify and crystallize material using this class of techniques and readily available equipment is an advantage of small molecule organic semiconductors over polymeric materials. 

A. Mishra, P. Bauerle, Small molecule organic semiconductors on the move promises for future solar energy technology. Angew. Chem. Int. Ed. 51, 2020-2067 (2010)... 

Small molecule organic semiconductors and promises for future solar energy technology 12AG(E)2020. 

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