Pentacene performance

The Infineon group reported a pure polymer dielectric which has also been shown to improve pentacene performance, both as an unmodified polymer, and with a silane treatment performed on it [7b]. In addition, Samsung SDI has recently reported a proprietary polymer dielectric which enables them to achieve high mobility in pentacene TFTs [12]. Table 2.2 emphasizes some of the reports of increasing pentacene mobility. 

Silinsh et al. (1989) applied their thermalization procedure to naphthalene and anthracene at low temperatures, 35 K or less. A stationary state was envisaged in the presence of an external field. Calculations have been performed for the saturation drift velocity, friction coefficients, and effective mass as functions of the external field. The conclusions are almost the same as for pentacene. 

Afzali, A. Dimitrakopoulos, C. D. Breen,T. L. 2002. High-performance, solution processed organic thin film transistors from a novel pentacene precursor. J. Amer. Chem. Soc. 124 8812-8813. 

When the solvent evaporation is performed in a well-controlled way, sufficiently large single crystals can be obtained. This is the case for pentacene, where single crystals are grown from solution in TCB by slowly evaporating the solvent over a period of four weeks at 450 K, under a stream of ultrapure N2 gas. 

As an example let us consider the pentacene/samarium interface (Koch et al, 2002). Samarium has a low work function ((/>m — 2.7 eV), which is comparable to E a from pentacene (— 2.7 eV). Thus, if A 0, the condition Ep, should provide efficient electron injection because in this case fp and LUMO are nearly aligned. In order to avoid contamination that may alter the instrinsic m, h and homo values, such heterostructures have to he prepared in ideally clean conditions, imposing the use of UHV. The UPS experiments performed with synchrotron radiation are shown in Fig. 4.24. After measuring (pM of the clean samarium surface (2.7 eV) as described above, increasing amounts of pentacene are controllably deposited onto the samarium surface. The survey spectra of the valence states and a close-up view of the energy region near E are shown in Figs. 4.24(a) and (b). 

Because molecules located in the same layer are much closer to each other than those situated in different layers, charge transport is expected to be much more efficient in the direction along the layers than across them. This has largely been confirmed by X-ray diffraction measurements on sexithiophene-based devices [27, 28], which indicated that the highest performance is attained when molecules are standing upright on the insulator. Similar behavior was found for pentacene [29, 30]. This is illustrated in Fig. 1.7, in which the molecules are depicted as short rods. 

This chapter will give an overview of methods used to achieve higher mobilities in pentacene devices, will point out potential progress in understanding the nature of exceptional mobility in these devices, and will comment on possible routes to stabilize device performance. 

The consistent use of top-contact architecture with development of a variety of surface modifications to alter the interface where the semiconductor is deposited, detailed below, have contributed to reports of steadily increasing performance in pentacene and other organic materials. 

Fig. 2.6. Some representative methods for effective surface-treatment preparation of both SAMs and polymers to improve the performance of pentacene TFTs.

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