S. Raj Mohan

Negative electric field dependence of mobility in TPD doped Polystyrene

Abstract A total negative field dependence of hole mobility down to low temperature was observed in N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped in Polystyrene. The observed field dependence of mobility is explained on the basis of low values of energetic and positional disorder present in the sample. The low value of disorder is attributed to different morphology of the sample due to aggregation of TPD. Monte Carlo simulations were also performed to understand the influence of aggregates on charge transport in disordered medium with correlated site energies. The simulation supports our experimental observations and justification on the basis of low values of disorder parameters.

Electron irradiation effects on TGA-capped CdTe quantum dots

8MeV electron irradiation effects on thioglycolic acid (TGA)-capped CdTe quantum dots (QD) are discussed in this study. CdTe QDs were characterized using x-ray diffraction (XRD), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS). Steady-state and time-resolved emission spectroscopy and UV-visible absorption spectroscopy were performed before and after irradiation with 8MeV electrons. XRD and TEM confirm the growth of TGA-capped CdTe QDs. The photoemission wavelength, intensity and lifetimes were found to vary with electron dose. At lower doses, they were found to be increasing (red-shift of photoluminescence (PL) peak and intensity) while the intensity decreased at higher electron doses. The observed changes in PL property, XPS and XRD analysis suggest possible epitaxial growth of the CdS shell on the CdTe core. This work demonstrates electron beam induced formation of the CdS layer on the CdTe core, which is a key step towards growth of the water soluble CdTe/CdS core-shell structure for biomedical labelling applications.

Electrical and optical characterization of photooxidized TPD

Changes in current–voltage characteristics of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′diamine (TPD) dispersed in polymer films are studied when solutions of TPD are exposed to UV light. Higher current density is observed in films cast with solutions exposed under UV for an optimum time. Observations are rationalized on the basis of the photoinduced oxidation of TPD when chlorinated solvents are used. Photoinduced generation of the TPD cation (TPD+) is confirmed using linear absorption spectroscopy when compared with salt-based generation (using CuCl2) of TPD+. Results suggest another possible route for generating TPD+ and the possibility of unintentional oxidation of TPD when chlorinated solvents like chloroform or dichloromethane are used for making devices.

Band offset at TiO2/MDMO PPV and TiO2/PEDOT PSS interfaces studied using photoelectron spectroscopy

We report band alignment and band offset studies across the interfaces of hetero-structures of TiO2 with poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)-p-phenylene vinylene] (MDMO PPV) and poly(styrenesulfonate) doped poly(3,4-ethylenedioxythiophene) (PEDOT PSS) using photoelectron spectroscopy. In both the cases the band alignment was found to be type II with significant band bending in the range 0.2 to 0.3 eV. In the case of the TiO2/MDMO PPV hetero-structure, the valance band offset (VBO)/conduction band offset (CBO) were found to be 2.68 ± 0.1 eV/1.68 ± 0.1 eV, while the TiO2/PEDOT PSS hetero-structure exhibited VBO/CBO as 2.3 ± 0.1 eV/0.7 ± 0.1 respectively. Based on these results, schematic band diagrams for these organic/inorganic hetero-structures were proposed. Our studies are important for the optimum design of various photovoltaic devices based on these organic/inorganic hetero-junctions.

Synthesis and Characterization of Linear Polymers for Non-Linear Optics through Click Chemistry Route

Click chemistry is used for synthesizing polymers for second order NLO study. The molecular weights found by gel-permeation chromatography (GPC), were in the range of 7000-55000 g/mol. Differential scanning calorimetry shows glass transition temperature (Tg) above 120 oC. From electronic spectra order parameter of the poled films were calculated to be 0.1-0.5. The change in surface morphology after poling was checked by atomic force microscopy. By using a pulsed Nd:YAG laser (1064nm), the second harmonic generation (SHG) intensity was measured. The SHG intensity was also studied as a function of against temperature and time respectively.

Monte Carlo simulation of charge transport in disordered organic thin films: Applicability of Meyer-Neldel rule for extracting energetic disorder

Multi-carrier Monte Carlo simulation of charge transport is employed to test the suitability of Meyer-Neldel rule (MNR) in extracting energetic disorder from homogeneous organic thin films in diode geometry. Studies validate the use of MN rule for extracting energetic disorder from homogeneous organic thin films.

Negative field dependence of mobility in organic thin films

Negative field dependence of mobility (NFDM) at low field regime due to the influence of non‐equilibrium charge transport is observed. The observed NFDM is found to be independent of the model used for simulation.