Organic field effect transistors (OFETs)
The field effect transistor (FET) is the simplest transistor, it is the cornerstone of digital electronics. An important aim of the research of many groups working on organic electronics is, therefor, to make FETs using organic semiconductors. One of the important advances made by our group in recent years has been the development of novel high capacitance gate insulators, prepared by anodisation of valve metals (for example, aluminium or titanium). This technique allows us to produce efficient OFET devices that can be operated at low gate voltages, compatible with conventional electronic circuitry. [1 & 2]
Above: A photograph of OFET devices designed and built by the group, alongside a graph showing the final device characteristics. Note the low operating voltages attained by the use of novel gate insulators [1 & 2]
Other developments are conventional polymer insulators filled with nanoparticles for higher capacitance [3], single transistor memory using a polymer insulator with remanent polarisation [4] (see graph below)and the modification of OFET contacts by doping [5], electroplating [6], or electropolymerisation [7].
Right: A memory effect organic field effect transistor. [3]
Recent research investigates the use of organic transistors as sensors for airborne pollutants, e.g. NO2, a common environmental pollutant released by motor vehicles. For this work, we collaborate closely with Dr Tim Richardson' s group.
It is known that many organic semiconductors change their conductivity when exposed to vapours, e.g. solvents; this is the working principle of simple so- called 'chemiresistor' sensors. However, organic transistors allow for more sensitive, and more selective, sensors, because systematic characterisation reveals a number of properties - charge carrier mobility, threshold voltage, and conductivity - which may respond more strongly to pollutants than conductivity of a chemiresistor, or different properties may change under exposure to different pollutants, giving selectivity.
Our work is concerned with the preparation and testing of sensor transistors, as well as the development of simple, portable characterisation methods that can work in the field, rather than just in the laboratory. For method development, we rely on our previous work on anodised Aluminium insulators [1,2], which enable low voltages transistors that can be characterised with conventional, OpAmp- based electronics [8].
Above: A sensor OFET in a test chamber for vapour exposure, and the electronic circuit we developed for 'gain method' characterisation [8]. The graph shows OFET characteristics prior to exposure (blue), during exposure (red), and during recovery after exposure (grey) [9].
The graph shows the response of an organic transistor using a fluorinated poly(triaryl amine) as organic semiconductor. Data were recorded with the 'gain method' we have developed for portable sensor applications [8]. Theory predicts data points shall fall onto straight lines, with the line's slope proportional to carrier mobility, and intercept equal to threshold voltage. Both mobility and threshold respond to NO2 exposure [9], i.e. 'multiparameter sensing'.
Recently we have implemented low cost, portable and fast multiparameter data acquisition for organic transistor sensors [10]. Real-time data acquisition from multi-channel OFETs are demonstrated in the video (right). This video shows data collection from two separate channels only. In fact, the present configuration can interface a total of 16 channels.
References
[1] Majewski et al, J. Phys. D: Appl. Phys. 37 (2004) 3367.
[2] L A Majewski, R Schroeder, M Grell, Adv. Mater. 17 (2005), 192.
[3] R Schroeder, L A Majewski, M Grell, Adv. Mater. 17 (2005) 1535.
[4] Schroeder et al IEEE Electron Device Letters 26 (2005) 69.
[5] R Schroeder. L A Majewski, M Grell, Appl. Phys. Lett. Vol 84 (2004), 1004
[6] L A Majewski, R Schroeder, M Grell, APL 85, (2004) 3620.
[7] R Schroeder, L A Majewski, M Grell, J Manoury, J Gautrot, P Hodge, M Turner, APL 87 (2005), 113501.
[8] R Dost, A Das, M Grell, J. Phys. D: Appl. Phys. Vol. 40 (2007), 3563
[9] A Das, R Dost, M Grell, T Richardson, J Morrison, M L Turner, accepted for Adv. Mater.
[10] A. Das et. al, Sensor and Actuator B : Chem (2009) [ DOI:10.1016/j.snb.2009.01.006]
Links
Tim Richardson: Nanomaterial Engineering Group