Three types of alkyl-substituted vanadyl phthalocyanines (OVPcs) were synthesized and their organic thin film
transistors (OTFTs) were prepared by solution casting with a bottom gate and top contact device geometry. A peripheral
octaoctyl-substituted OVPc (OVPc-op-C8) showed the mobility in the order of 10-5 cm2/V•s. Both peripheral and nonperipheral
tetraalkyl-substituted OVPcs are a mixture of four isomers, and exhibited the mobility of 10-4-10-2 cm2/V•s,
depending on the length of the alkyl chains. Four isomers of non-peripheral tetraalkyl-substituted OVPcs can be
separated by common lab column chromatography. A mobility of 0.13 cm2/V•s has been demonstrated with a pure
isomer.
There has been a considerable interest on forming ambipolar organic thin film transistors (OTFTs) because such devices
are advantageous for integrated circuits, such as lower power consumption, design and fabrication simplification, and
better immunity. Most recently, Shi et al. observed a substantial mobility improvement in ambipolar OTFTs based on
the heterojunction formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl)
bithiophene (BP2T). Specifically, the hole and electron mobility are improved by 3 and 12 folds from the bulk values,
respectively. We examined the interface formation between F16CuPc and BP2T using ultraviolet photoemission (UPS)
and inverse photoemission spectroscopy (IPES). It is observed that in F16CuPc/BP2T the heterojunction is characterized
by band bending in both materials, while in BP2T/F16CuPc the band bending is confined in BP2T only. For
F16CuPc/BP2T, the band bending of BP2T and F16CuPc are 0.40 and 0.35 eV, respectively. The band bending region is
~15 nm in both materials, from which the Debye lengths of the materials can be deduced. The combination of the band
bending and finite Debye lengths offers an explanation to the observed improvement and thickness dependence of the
mobility in OTFTs based on such heterojunctions.
Behaviors of charge transportation in organic heterojunction consisted of p-type CuPc and n-type F16CuPc have been studied. Conductivity parallel to heterojunction interface is higher than that of each single layer is observed. It has been confirmed that the higher conductivity is originate from accumulation of positive and negative mobile charges. Based on this phenomenon, a novel heterojunction organic field-effect transistor (OFET) having two conducting channels has been demonstrated. The heterojunction OFETs, with either p-type layer or n-type layer connected to the insulator, can operate in mode of normally-on (depletion-accumulation). In addition, ambipolar electrical properties with mobility of 0.04cm2/Vs and balanced charge transportation, and CMOS inverter comprising unique ambipolar transitor have been optimised based on the heterojunction OFET.
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