Semiconductors poly derivatives

Several attempts to use otganic polymeric semiconductors as the active component in photovoltaic devices have been reported during the last two decades. Interest in the photovoltaic properties of conjugated polymers like polyacelylcne, various derivatives of polythiophenes and poly(para-phenylene vinylene)s arose from... 

Through reduction or oxidation of the molecule by a dopant molecule. Atoms or molecules with high electron affinity, such as iodine, antimony pentafluoride (SbCls), or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), may oxidize a typical organic semiconductor such as poly(p-phenylene) derivatives, leaving them positively charged. Reduction, i.e., addition of an electron, may be obtained by doping with alkali metals. 

Attempts to dope organic semiconductors have been made very early in the field, motivated by the prospect of possibly reaching metallic conductivities [108, 109]. These synthetic metals, however, have not been realized. While p-type doping could be obtained, for example, with iodine gases for poly-p-phenylene vinylene (PPV) derivatives, and n-type doping was demonstrated with sodium for a cyano-derivative of PPV, the doping levels obtained were not stable with time. The dopant molecules readily diffused into the organic semiconductor, yet also out of it. Due to the lack of stability, these approaches were not suitable for commercial applications. 

Research Focus Synthesis of poly(4,8-didodecyl-2,6-bis-(3-methyl-thiophen-2-yl)-benzo[l,2-b 4,5-b ]dithiophene) derivatives for use as semiconductors in... 

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

The first example of using the [2+2] cycloaddition for crosslinking of organic semiconductor layers was reported in 1997 by Remmers et al. [10]. It was a derivative of poly-p-phenylene (PP) deposited by the Langmiur-Blodgett (LB) technique (Fig. 9.5(a)). Polarized absorption and fluorescence of the films was reported, but no OLED devices were fabricated. 

A variety of substituted parylene derivatives have been used as gate dielectrics for TFTs in a control study to investigate the effect of surface energy on channel characteristics and mobility [55]. It was found that the field effect mobility for an amorphous polymer semiconductor (MEH-PPV) increases with increasing surface hydrophobicity (a result that has been seen with amorphous poly(triarylamine)s [PTAAs] also) [5]. 

By 1995 one finds that the main actors are different polymers, a change which can be partly attributed to the recent interest in semiconductor properties, especially light emission. In the field of conduction, polyacetylene has given its place to polyaniline, a polymer with a fascinating rich chemistry and a promise of good processability. In the semiconductor arena, the most studied polymer of the past decade is polythiophene, or rather its soluble derivatives which lend themselves very well to various fabrication processes. Another polymer, one which was only mentioned briefly in the 1986 Handbook, is competing with polythiophene for application in semiconductor devices poly(pura-phenylene vinylene). These three polymers received very little interest before 1985 no crystallographic studies had been published at that time. 

The enhanced flow of current under the influence of an apphed electric field that occurs when a semiconductor is exposed to visible light or other electromagnetic radiation is known as photoconduction. Poly (N-vinyl carbazole) (VII) and various other vinyl derivatives of polynuclear aromatic compounds such as poly(2-vinyl carbazole) (Vin) and poly(vinyl pyrene) (DC) have high photo-conductive efficiencies. The excellent photoconductivities of these polymers are believed to be due to their helical condormation with successive aromatic side chins lying parallel to each other in a stack along which electron transfer takes place relatively easily. 

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