Moumita Mukherjee

Control over Multifunctionality in Optoelectronic Device Based on Organic Phototransistor

Highly stable, reproducible, photosensitive organic field-effect transistors based on an n-type organic material, copper hexadecafluorophthalocyanine, and two different polymeric gate dielectrics has been reported and their performances have been compared by evaluating the surface/interface properties. The devices produced a maximum photocurrent gain (I(light)/I(dark)) of 79 at V(G) = 7 V and showed the potentiality as multifunctional optoelectronic switching applications depending upon the external pulses. The switching time of the transistor upon irradiation of light pulse, i.e., the photoswitching time of the device, was measured to be approximately 10 ms. On the basis of optical or combination of optical and electrical pulses, the electronic/optoelectronic properties of the device can be tuned efficiently. The multifunctions achieved by the single device can ensure very promising material for high density RAM and other optoelectronic applications. Furthermore, as the device geometry in the present work is not limited to rigid substrate only, it will lead to the development of flexible organic optoelectronic switch compatible with plastic substrates.

Organic phototransistors based on solution grown, ordered single crystalline arrays of a π-conjugated molecule

High quality, single crystalline, ordered arrays of a π-conjugated organic molecule, N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), were grown by solution processing and used to fabricate a low-cost, high-performance organic phototransistor (OPT). The single crystalline nature of the microstructure was investigated using 2D-GIXD measurement. The organic field-effect transistor fabricated using periodic arrays of elongated crystals exhibited a photoresponsivity (P) of ca. 1 A W−1 and a photo to dark current ratio (Ion/Ioff) of 2.5 × 103 at VG = 12 V and a maximum P of ca. 7 A W−1 at the high gate bias regime (VG = 50 V) with an optical power of ca. 7.5 mW cm−2. With polymeric gate dielectric, the OPT exhibited very stable n-type characteristics both in the dark and under light illumination and showed reproducible photo-switching behavior. The dependence of the photocurrent on the gate/drain voltage and on illumination intensity provided an effective way to control the number of photo-carriers generated in the active material, enabling the precise tuning of the devices performance. Performance comparison between OPTs with ordered crystal arrays and thin films of PTCDI-C8 confirmed that the materials intrinsic properties were better realized in the crystalline device, presumably because of higher charge carrier mobility and better charge transport capability. This one-step, solution-based, self-assembly fabrication of multifunctional (photodetection, photoswitching, signal amplification) optoelectronic devices has potential to aid the development of organic semiconductors with high-quality micro/nanostructures for large-scale application and low-cost optoelectronic devices.

Nonvolatile memory device based on Ag nanoparticle: Characteristics improvement

A single layer memory device based on silver nanoparticles has been fabricated. The device exhibits electrical bistability and nonvolatile memory phenomenon. The performance of the device improved (in terms of On/Off ratio, switching cycles, and retention time) when an additional polymer (PMMA) layer was deposited prior to nanoparticles deposition. The retention time and switching cycles of the device improved a lot and on/off current ratio of the device increased by more than three orders of magnitude. The ability to write, erase, read, and refresh the electrical states of the polymer-nanoparticle composite fulfills the functionality of a dynamic random access memory.

Solution processed, aligned arrays of TCNQ micro crystals for low-voltage organic phototransistor

A simple approach is developed to prepare periodic arrays of large, elongated crystals of π-conjugated organic molecules, viz, 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) through a solution based method. The ordered crystalline array is successfully used to fabricate low-voltage organic phototransistors (OPT). The OPT with polymeric gate dielectric exhibits very stable n-type characteristics with a low threshold voltage (<0.5 V). Under illumination, the devices produce a current gain (Ilight/Idark) of 31 at VG = 0.3 V. Photoswitching occurs within 10 ms and photosensitivity is greater than 1 mA W−1 at low driving voltages and low optical powers. The drain current increases gradually with increasing the illumination intensity resulting in typical output FET characteristics. The one-step, solution-based, self-assembly method for highly ordered organic crystals in large area could have significant potential for future large-scale and low-cost optoelectronic devices. In addition, this study aims to aid the development of organic semiconductor materials with high quality crystalline structures for various optoelectronic applications.

High Performance Organic Thin Film Transistors with Solution Processed TTF-TCNQ Charge Transfer Salt as Electrodes

Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the source-drain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as source-drain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic-organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of all-organics and plastic electronics devices with high throughput and low-cost processing.

Periodic arrays of organic crystals on polymer gate dielectric for low-voltage field-effect transistors and complementary inverter

Periodic arrays of highly oriented 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals, directly grown on a polymeric gate dielectric through a solution process, are used for the fabrication of a low-voltage organic field-effect transistor (OFET). Consequently, an organic complementary inverter using the TCNQ periodic array (n-channel) and pentacene (p-channel) is also reported. The TCNQ-based n-channel OFET exhibited very stable field-effect characteristics with low operational (2 V) and threshold voltages (<0.5 V). The highest field-effect carrier mobility in the saturation region was found to be 0.03 cm2 V−1 s−1. Furthermore, the organic complementary inverter showed good response characteristics in the low-voltage regime. The swing range of VOUT is same as VDD, ensuring “zero” static power consumption in the digital logic circuit. For the inverter with VDD = 2 V, the noise margin for low and high voltages are 1.0 V and 0.3 V, respectively. The logic threshold (VIN = VOUT) is 1.3 V and the maximum gain (−dVOUT/dVIN) of 4 is obtained at VIN = 1.3 V.