A solution processable fluorene-benzothiadiazole small molecule for n-type organic field-effect transistors

We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5] thiadiazol-4-yl) methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 degrees C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4 X 10(-3) cm(2)/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric. (C) 2011 American Institute of Physics. [doi:10.1063/1.3569818

Three-dimensional carbazole-based dendrimers: model structures for studying charge transport in organic semiconductor films

We report the synthesis and charge transport properties of a series of three-dimensional dendrimers up to the third generation that have a 9,9'-spirobifluorene core, carbazole-based dendrons and di-n-propylfluorene surface groups. The dendrimers can all be spin-coated to form good quality amorphous films. The charge carrier mobility of the dendrimers was measured by two different methods; in an organic field-effect transistor (OFET) architecture, and by Charge Extraction by Linearly Increasing Voltage (CELIV). In the OFET configuration the first generation dendrimer had a maximum mobility of 4.1 x 10(-4) cm(2) V-1 s(-1) and an ON/OFF ratio of 1.1 x 10(5). Unexpectedly, in spite of the third generation dendrimer having a volume approximately six times that of the first generation, the mobility was found to decrease by only an order of magnitude. A similar trend in mobility was seen in the CELIV results. Photoluminescence (PL) measurements in solution showed that the first generation dendrimer was comprised of non-interacting chromophores, while the second and third generation dendrimers had substantial intra-dendrimer interchromophore interactions. In the solid-state, PL measurements showed that for the first generation dendrimer there were clear inter-dendrimer interchromophore interactions with little change for the second and third generations. Comparison of the dendrimer molecular volumes in solution and the solid-state showed that in the latter, the dendrimers took up a smaller volume suggesting that there was interdigitation of the dendrons. For the first generation dendrimer the interdigitation leads to trap sites for charge transport, with the small decrease in mobility in moving from the first to the second and third generation being due to the extra intra-dendrimer interchromophore interactions. Model dendritic systems such as these can be used to gain significant insight into the subtly of charge transport phenomena in solution processable macromolecular organic semiconductors, since they offer a level of molecular control that is difficult to achieve with polymers

A flexible n-type organic semiconductor for optoelectronics

n-Type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes, and field effect transistors (FETs). In spite of this clear motivation there has been significantly less development of n-type compounds relative to p-type systems. We have developed a simple, small molecule n-type material, 2-[(7-{9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12), that can be processed either by spin-coating from solution or evaporation in vacuum. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation). The electron mobility measured in a FET configuration was of the order of 10(-3) cm(2) V-1 s(-1) for films prepared via either processing method whilst Photoinduced Charge Extraction in Linearly Increasing Voltage (PhotoCELIV) gave a mobility of order 10(-4) cm(2) V-1 s(-1)

The effect of dendronisation of arylamine centred chromophores on field effect transistor performance

We report the design and synthesis of new dendronised hole transport materials and their first use in organic field effect transistors (OFETs) as solution processed amorphous channels. The dendrimers were comprised of a triphenylamine centre, core chromophores containing one or two thiophene units per arm, and first generation biphenyl dendrons with 2-ethylhexyloxy surface groups attached. The dendronised materials were found to be more easily processed than their non-dendronised equivalents. Top contact OFETS were fabricated and found to have hole mobilities in the saturated regime of 1.7 x 10(-6) cm(2) V(-1) s(-1) and 1.1 x 10(-5) cm(2) V(-1) s(-1) for dendrimers with one and two thiophene units in each arm of the chromophore, respectively. The devices had threshold voltages of around 10 V and ON/OFF ratios in the range 10(2) to 10(3). The OFET results demonstrate that for amorphous films it is important that the chromophore is as large as possible to allow for maximal intermolecular interactions