Polymer field-effect transistors

• Main Author: Georgakopoulos, Stamatis
• Published: University of Surrey 2012
• Subjects: 621.38832
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.582857

High Ionisation Potential (IP) amorphous conjugated polymers are very practical semiconductors and promising candidates for printing applications as they exhibit 1) high air-stability due to the high IP, and 2) reproducible electrical performance due to the uniformity of amorphous morphology. However they generally exhibit low mobilities on the order of 10-3 cm2Ns and below. This work is based mainly on two high-IP amorphous conjugated polymers poly(indenofluorene-triarylamine) (PIFTAA) and poly(indenofluorene- phenanthrene) (PIFPA). The long term ambient stability of PIFTAA and PIFPA with IPs of 5.45 eV and 5.79 eV respectively is characterised in Field-Effect Transistors (FETs) over a period of 4 and 2 months respectively. FET parameters such as the turn-on voltage and subthreshold slope are found to be generally stable, and the charge carrier mobility is found to degrade at an approximate rate of 10% per month, which is amongst the lowest reported values for organic semiconductors. PIFT AA and particularly PIFPA exhibit high field-effect saturation mobilities of 0.03 - 0.04 cm2Ns and 0.2 - 0.3 cm2Ns respectively, which are unusually high for amorphous conjugated polymers. The morphologies are examined by atomic force microscopy, grazing incidence wide angle x-ray scattering, and differential scanning calorimetry, and no evidence of crystallinity is detected, suggesting that the conjugated polymers are indeed amorphous. To investigate charge transport in PIFTAA and PIFPA, FETs of multiple channel lengths are fabricated, providing mobility data for multiple electric fields, and measured over a range of temperatures. In addition to PIFT AA and PIFP A, the measurements are performed on typical amorphous conjugated polymers poly(triarylamine) (PTAA) and poly(indenofluorene-triarylamine-triarylamine) (PIFTAATAA), with mobilities of 0.003 cm2/Vs and 0.004 cm2Ns respectively. The gate voltage dependence of the mobility extracted from FET measurements, as well as the lIT2 fit of the mobility with temperature is consistent with a Gaussian Density of States. The indenofluorene copolymers PIFTAA, PIFTAATAA, and PIFPA exhibit clear negative electric field dependence of the mobility, signature of high spatial disorder in the polymer films. The temperature dependence of the mobility is fed into the Gaussian Disorder Model, which indicates that the source of the high mobility for PIFPA is mainly strong intermolecular coupling indicated by the high pre-factor mobility as well as low energetic disorder along the path of charge flow. These results challenge the widely accepted concept that high crystallinity is a requirement for mobility exceeding 0.1 cm2/Vs in organic semiconductors.