Akanksha Sharma

Capacitance-voltage characteristics of organic Schottky diode with and without deep traps

Capacitance based spectroscopic techniques have been used to characterize defects in organic Schottky diode based on copper phthalocyanine. Deep traps in organic thin films introduced by varying growth conditions have been identified and characterized by voltage and temperature dependence of capacitance. These results are interpreted using a consistent modelling of capacitance of organic Schottky diode with and without deep traps.

Single electron transfer-driven multi-dimensional signal read-out function of TCNQ as an "off-the-shelf" detector for cyanide.

Herein we report the first applications of TCNQ as a rapid and highly sensitive off-the-shelf cyanide detector. As a proof-of-concept, we have applied a kinetically selective single-electron transfer (SET) from cyanide to deep-lying LUMO orbitals of TCNQ to generate a persistently stable radical anion (TCNQ(•-)), under ambient condition. In contrast to the known cyanide sensors that operate with limited signal outputs, TCNQ(•-) offers a unique multiple signaling platform. The signal readability is facilitated through multichannel absorption in the UV-vis-NIR region and scattering-based spectroscopic methods like Raman spectroscopy and hyper Rayleigh scattering techniques. Particularly notable is the application of the intense 840 nm NIR absorption band to detect cyanide. This can be useful for avoiding background interference in the UV-vis region predominant in biological samples. We also demonstrate the fabrication of a practical electronic device with TCNQ as a detector. The device generates multiorder enhancement in current with cyanide because of the formation of the conductive TCNQ(•-).

Defect states and their energetic position and distribution in organic molecular semiconductors

Energetic position and distribution of defect states due to structural disorder in pentacene and copper phthalocyanine have been obtained by capacitance based spectroscopic techniques. It has been shown that capacitance-frequency and capacitance-voltage characteristics exhibit Gaussian distribution of traps with an energetic position at around 0.5 eV above the highest occupied molecular orbital level of the pentacene and CuPc. These traps have been created by varying growth conditions and almost identical trap parameters in pentacene and copper phthalocyanine indicate that similar structural disorder is responsible for these traps.

Effect of deep traps on small molecule based thin film transistors

The influence of the traps on the carrier transport has been investigated in two and three terminal devices using current-voltage and capacitance based spectroscopic methods. The electrical characteristics of organic thin film transistors fabricated on thin films with and without deep traps are compared. Vast improvement in the different parameters which define the performance of organic thin film transistors is observed in devices fabricated on thin films without structural disorder induced deep traps.

Organic transistor and inverter based on assembly of organic nanowires achieved by optimizing surface morphology

We have demonstrated that assemblies of organic nanowires can be grown on Si and SiO2 substrates by controlling growth parameters. At higher growth temperatures, anisotropic growth dominates over isotropic growth, resulting in surface morphologies consisting of nanowire-like elongated grains. These elongated grains provide better π-π stacking, leading to higher carrier mobility and better performance of organic transistors. Using this approach, we have demonstrated organic inverter using complementary semiconducting materials, p-type copper phthalocyanine and n-type copper hexadecafluoro phthalocyanine. These results indicate that small organic molecule-based nanowires are promising candidates for future organic based microelectronics.

Current Transport Mechanism in ZnO and Metal Phthalocyanine Based Inorganic/Organic Hybrid p – n Junction Diodes

Here we report an investigation on ZnO and metal phthalocyanine (MePc) based heterostructures for inorganic/organic hybrid p–n junction diodes using temperature dependent current–voltage (J–V) and capacitance–voltage (C–V) characteristics. The J–V characteristics of the ZnO/MePc’s based hybrid p–n junction diode are asymmetrical with a rectification ratio of 104 indicating the formation of a depletion layer, which has also been corroborated by C–V characteristics. The mechanism of device operation has been revealed by the temperature dependence of the ideality factor and saturation current–density. These results indicate that operation of a hybrid p–n junction diode can be described by tunneling enhanced bulk recombination.

Charge transport mechanism in copper phthalocyanine thin films with and without traps

We investigate the charge transport mechanism in copper phthalocyanine thin films with and without traps. Previously, charge transport in polycrystalline thin films has been widely described by the multiple trapping and release (MTR) model, without emphasizing the origin of the traps. In this work, polycrystalline organic thin films with and without traps have been grown by engineering different growth conditions. We find that the density of interface states at the grain boundaries can decide the mechanism of charge transport in organic thin films and completely different charge transport mechanisms can be observed in thin films with and without traps.

Determination of diffusion coefficient in disordered organic semiconductors

Charge carrier transport in organic semiconductors is dominated by positional and energetic disorder in Gaussian density of states (GDOS) and is characterized by hopping through localized states. Due to the immobilization of charge carriers in these localized states, significant non-uniform carrier distribution exists, resulting diffusive transport. A simple, nevertheless powerful technique to determine diffusion coefficient D in disordered organic semiconductors has been presented. Diffusion coefficients of charge carriers in two technologically important organic molecular semiconductors, Pentacene and copper phthalocyanine (CuPc) have been measured from current-voltage (J-V) characteristics of Al/Pentacene/Au and Al/CuPc/Au based Schottky diodes. Ideality factor g and carrier mobility μ have been calculated from the exponential and space charge limited region respectively of J-V characteristics. Classical Einstein relation is not valid in organic semiconductors due to energetic disorders in DOS. Using g...

Charge carrier transport in polycrystalline organic thin film based field effect transistors

The charge carrier transport mechanism in polycrystalline thin film based organic field effect transistors (OFETs) has been explained using two competing models, multiple trapping and releases (MTR) model and percolation model. It has been shown that MTR model is most suitable for explaining charge carrier transport in grainy polycrystalline organic thin films. The energetic distribution of traps determined independently using Mayer-Neldel rule (MNR) is in excellent agreement with the values obtained by MTR model for copper phthalocyanine and pentacene based OFETs.

Surface Morphology of Pentacene Thin Film

Growth mechanism of Pentacene thin film has been investigated with a combination of atomic force microscopy measurements and numerical modeling. Initially Pentacene grows as a monolayer fractal islands caused by DLA mechanism and evolves into compact islands as the surface coverage increases. As the branches of the first layer are widened, the monomer density on the top of the first layer increases and eventually becomes large enough to form critical islands for the growth of second layer. This phenomenon is mainly attributed to a growth instability caused by the Schwoebel barrier that prevents the newly landing molecules from hopping down the edge. Simulations based on a simple model of film growth are found to agree with experimental observations.