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My prime area of research interest is the study of physics of organic semiconductors. I am interested in the device physics of organic thin film transistors (OTFT), organic solar cells and organic light emitting diodes(OLED). Nonvolatile memory elements using OFETs, ambipolar transport in FETs and light emitting FETs and depletion mode FETs are areas related to OFET where I have interesting research plans.

I am interested in studying the properties of blend organic solar cells fabricated by co-evaporation of donor and acceptor materials. The modification of electrodes using nanoparticles also is another area.

I am interested to do some work in fluorescent OLEDs. Flexible and top emitting OLED appears to be a challenging area considering the requirements of future plastic light-weight displays.

Organic Field-Eeffect Transistors

Non-volatile memory elements

The ferroelectric field-effect is the modulation of the surface potential of a semiconductor by the spontaneous polarization of a ferroelectric, which is in intimate contact with the semiconductor. We have used a ferroelectric polymer, polyvinyledene fluoride-trifluoroethylene as the gate dielectric of a pentacene FET. Storing information is possible by the polarization of the gate dielectric. During the writing process, some of the carriers generated in the semiconductor could be trapped near the FE/semiconductor interface. Even when the gate field is withdrawn, these charges remain trapped because of the polarization electric field. Thus the traps at the interface being inactive, establishment of a channel becomes easier and the charges generated by the polarization field constitute a drain current in the absence of any gate field. The memory remains as long as the polarization lasts.

Depletion mode FETs

One of the most important applications of the depletion mode OFETs is as the load device of the inverter due to the symmetrical charging and discharging time constants, much faster than the inverter with enhancement mode transistors. Thus the depletion mode OFETs can be used in organic integrated circuits. However reports on depletion mode OFETs are scarce in literature. We have fabricated a depletion mode pentacene FET with a polymeric gate dielectric, which is an acidic polyelectrolyte. Absorption of water creates a net negative surface charge density in the in the dielectric/semiconductor interface. Due to this, the transistor is normally ON. So application of positive gate biases was seen to deplete the channel very efficiently.

Ambipolar FETs

Organic field-effect transistors were fabricated where the active semiconductor layer consisted of a co-evaporated film of pentacene and a perylene derivative, N, N' –ditridecylperylene -3, 4, 9, 10 -tetracarboxylic diimide3. The device characteristics were evaluated in an oxygen-free environment. The field-effect transistor showed excellent ambipolar operation with field-effect hole mobility of 0.09 cm2 V-1s-1 and field-effect electron mobility of 9.6 ´ 10-3 cm2 V-1s-1. The threshold voltage for p-channel operation was –18V and the same for n-channel operation was 15V. This ambipolar device could be a building block to form flexible integrated circuits with low-power consumption and ease of design.

N- channel FETs

N-channel semiconductors are important components of p-n junction diodes, bipolar transistors, and complementary circuits. Nevertheless most of the studies on OFETs have been done with p-type materials. We are studying some important perylene derivatives and we have obtained the third highest electron mobility so far reported for any perylene derivatives with N, N' –ditridecylperylene -3, 4, 9, 10 -tetracarboxylic diimide. The low voltage operation is a significant characteristic of this device. A mobility value of 1.7 ´ 10-2 cm2 V-1s-1 was estimated from the transistor characteristics. The threshold voltage was 0.2V and the inverse subthreshold slope was 7.6V. At low gate voltages and at high drain voltages, the device exhibits injection of carriers from the Al electrode. This was rectified by using LiF as a buffer layer between Al and (PTCDI-C13H27). Such modified devices exhibited mobility 1.2 ´ 10-2 cm2 V-1s-1, threshold voltage -0.3V, and inverse subthreshold swing of 5V.

Dependence of transistor performance on characteristics of polymer dielectrics

Though several organic dielectrics have been employed as the gate material, most of the studies on organic FETs are still made with SiO2 as the gate dielectric. However to realize low cost and large area electronics, using organic semiconductors, it is highly desirable that SiO2 may be substituted with an organic dielectric. We evaluated the performance of five organic dielectrics, namely cyanoethylpullulan (CyEP), poly (methylmethacrylate) (PMMA), parylene-C, poly (4-vinyl phenol) (PVP), and polyimide in an organic FET with pentacene as a model active material.