Improved field‐effect mobility in short oligothiophenes: Quaterthiophene and quinquethiophene

Abstract

Organic field-effect transistors (OFETs) made of conjugated oligomers (oligothiophenes, pentacene), which were first fabricated in this laboratory eight years ago, ['] have now been reproduced by several group^,[^-^^ and proved to be promising devices with a field-effect mobility pFE well into the cm2V-' s-' range and an on/off current ratio of up to lo5. Enhanced performance of the sexithiophene (6T) OFET has been achieved by improving the ordering of the evaporated film, from a disordered three-dimensional(3D) structure to a well-ordered two-dimensional organization, where all the molecules align along a direction nearly perpendicular to the film and form layers parallel to it, with a preferential charge transport along the direction of the layers.r61 6T belongs to the family of the oligothiophenes (nT, where n stands for the number of a-linked thiophene rings). Early results on shorter (3T to 5T)[2371 and longer (8T)171 oligomers than 6T revealed a very steep increase of pFE-by up to two order of magnitude for each additional ring-from 3T to 6T, followed by a slight decrease from 6T to 8T. The latter feature was attributed to the poor structural organization of evaporated films of longer oligomers, due to the lower purity of the raw material and a partial decomposition during the evaporation. The former behavior, namely the huge increase of pFE with n, is still unexplained. It is at variance with what has been observed for a long time on another intensively studied family of molecular crystals, the polyacenes, where the time-offlight (TOF) mobility at room temperature is found to lie in the range 0.1-1 cm2 V-' s-', whatever the length of the molecule, from naphthacene to tetracene.['] (We note that, to the best of our knowledge, field-effect devices with these well-known molecular crystals have only been realized with polycrystalline penta~ene. [~,~]) Charge injection in organic semiconductors has long been shown to be a crucial problem in organic-based devices, especially for large-bandgap compounds. This could

Organic heterostructures in organic field-effect transistors

“…Hajlaoui et al inserted a thin layer of tetracyanoquinodimethane (TCNQ) between the electrodes and the semiconducting layer, resulting in better carrier injection and improved mobility [73]. A CuPc/F 16 CuPc heterojunction with high conductivity has also been used as a buff er layer in OFETs to improve the contact between metal and organic semiconductors (Figure 9(a)) [74], improving the electron fi eld-eff ect mobility from 4.2 × 10 -2 to 6.6 × 10 -2 cm 2 V -1 s -1 .…”

Exciton emission in PTCDA films andPTCDA∕Alq3multilayers

“…8 However, despite recent improvements in device performance, the microscopic understanding of recombination processes in organic crystals and films is still rather limited.…”

“…8,[25][26][27][28][29] An interesting organic molecule for multilayer investigations is tris (8-hydroxy) quinoline aluminum ͑Alq 3 ͒ that has attracted much attention as a superior material for OLEDs. 1,3 Alq 3 serves as the electron transporting layer and is responsible for the emission of light.…”

Nano-scale interface controls for future plastic transistors

“…Optimization of the channel/electrode interface is therefore another crucial issue in improving the performance of OTFTs. Structural modifications and doping control around the electrodes will also contribute to the enhancement of OTFT characteristics [15][16][17].…”