Increasing the Carrier Mobility in P3HT by Doping for use in Schottky Barrier TFTs
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Increasing the Carrier Mobility in P3HT by Doping for use in Schottky Barrier TFTs G. C. R. Lloyd*, N. Sedghi*, M. Raja*, R. Di Lucrezia§, S. Higgins§, and W. Eccleston*. * Dept of Electrical Engineering and Electronics, The Univ of Liverpool, Liverpool, UK, § Dept of Chemistry, The Univ of Liverpool, Liverpool, UK ABSTRACT Measurements and theory are presented examining the relationship between mobility and doping in regioregular poly(3-hexylthiophene) (P3HT). Mobility is found to increase superlinearly with doping and is comparable to models reported for other conjugated polymers. Schottky measurements have been used to calculate the doping density and bulk mobility of regioregular P3HT. Aluminium Schottky contacts showed signs of native oxide disrupting current flow through the device. This effect was observed to degrade further with the introduction of dopant into the polymer. Titanium devices show a general shift of the Schottky characteristic to higher current levels with increased dopant. Field effect mobility of P3HT films was also calculated using thin-film transistor (TFT) structures. The field effect mobility values were observed to be more than two orders of magnitude higher than the bulk mobility value. The addition of dopant also increased gate leakage currents in TFT devices. The increased conductivity in doped polymer can increase off currents in the device; this is avoided by using Schottky contacts as the source and drain. Preliminary results on Schottky contact TFTs are also presented as well as a description of the operation of such a device.
INTRODUCTION All conjugated polymer TFTs investigated thus far have used gold as the source and drain electrodes. The Ohmic contact provided by gold does not restrict bulk leakage currents in the TFT; this can cause problems if the conductivity of the semiconductor is high. The problem is particularly relevant due to the large amount of unintentional doping caused by oxidation of the polymer in air. A solution is to use the potential barrier at the interface of a Schottky contact to control this leakage current [1]. The use of a Schottky contact as the source and drain also allows for ultra small channel lengths, as in vertical channel devices, due to the metal-polymer barrier remaining constant as the depletion regions overlap. In gold contact devices the metal-polymer barrier is zero (ohmic contact), therefore as the majority carrier accumulation regions overlap the centre of the channel rises to the potential at the contact edge and becomes highly conducting. With increased doping, the depletion region width can be reduced allowing the channel to remain at the bulk potential thus maintaining control of the channel by the gate. Therefore much smaller channels can be created than with Ohmic contact devices. Previous experiments have doped poly(β ’-dodecyloxy-α,α’,-α’,α’’terthienyl) (PDOT3) with 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) and shown a rapid increase in mobility with doping level [2]. Combining doped polymer with a Schottky contact device
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