Small molecule semiconductors

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. 

TABLE 8.1 Optoelectronic Properties of Common Inorganic, Polymer, and Small-Molecule Semiconductors... 

An alternative route to solution-processable organic semiconductors is to use small molecule semiconductors that have been designed to be compatible with solution deposition. 

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... 

Small molecule semiconductors for high-efficiency organic photovoltaics 12CSR4245. 

Vapor-deposited polycrystalline pentacene has become a widely studied small molecule semiconductor approaching the molecular crystal mobility limit with mobilities as high as 5 cm [65]. 

Small molecule semiconductors for bulk heterojunction organic solar cells are attractive because of their advantages over their polymer counterparts, which include well-defined molecular structure, definite molecular weight, and high purity without batch-to-batch variations. 

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

Gaunt, J. A. Knight, A. E. Windsor, S. A. and Chechik, V. (2005). Stability and quantum yield effects of small molecules additives on solutions of semiconductor nanoparticles. /. Colloid and Interface Science, 290 (2), 437-443. 

We know that not all solids conduct electricity, and the simple free electron model discussed previously does not explain this. To understand semiconductors and insulators, we turn to another description of solids, molecular orbital theory. In the molecular orbital approach to bonding in solids, we regard solids as a very large collection of atoms bonded together and try to solve the Schrodinger equation for a periodically repeating system. For chemists, this has the advantage that solids are not treated as very different species from small molecules. 

Two types of OLEDs are being developed small-molecule [45] and polymer [46], Most work has been done on small-molecule OLED. Representative small-molecule hole-transport [47] (p-type) and electron-transport (n-type) organic semiconductors are shown in Figure 6.14. Not surprisingly, the hole-transport molecules are identical or very similar to those used in electrophotography. 

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