N. Padma

A study on threshold voltage stability of low operating voltage organic thin-film transistors

A low operating voltage (<2 V) organic field-effect transistor (OFET) using phenylhexyltrichlorosilane (PTS) self-assembled monolayer (SAM) dielectric and copper phthalocyanine (CuPc) as semiconductor with improved mobility (0.035 cm2 V−1 s−1) and threshold voltage stability was demonstrated. This device showed better performance when compared to an OFET with octyltrichlorosilane (OTS-8) SAM dielectric. The improved mobility was attributed to the 2D growth mode of CuPc on PTS SAM because of surface energy matching between the two, whereas CuPc film on OTS-8 showed a 3D growth mode with larger grain boundary density. The higher threshold voltage stability of OFETs on PTS SAM was attributed to the efficient coverage and screening of trap centres at dielectric/semiconductor interface due to stronger intermolecular linking and formation of closely packed surface by the bulky phenyl end groups. Decrease in grain boundaries offered by 2D growth of CuPc for electron and hole trapping was also found to be another reason for improved threshold voltage stability. The results indicated that the nature of the end group of SAM dielectric, surface chemistry of dielectric and initial growth mode of semiconductors are all responsible for improvement in threshold voltage stability and enhanced performance of OFET.

Probing molecular packing at engineered interfaces in organic field effect transistor and its correlation with charge carrier mobility.

Surface engineering of SiO2 dielectric using different self-assembled monolayer (SAM) has been carried out, and its effect on the molecular packing and growth behavior of copper phthalocyanine (CuPc) has been studied. A correlation between the growth behavior and performance of organic field effect transistors is examined. Depth profiling using positron annihilation and X-ray reflectivity techniques has been employed to characterize the interface between CuPc and the modified and/or unmodified dielectric. We observe the presence of structural defects or disorder due to disorientation of CuPc molecules on the unmodified dielectric and ordered arrangement on the modified dielectrics, consistent with the high charge carrier mobility in organic field effect transistors in the latter. The study also highlights the sensitivity of these techniques to the packing of CuPc molecules on SiO2 modified using different SAMs. Our study also signifies the sensitivity and utility of these two techniques in the characterization of buried interfaces in organic devices.

Photovoltaic Properties Of ZnO Nanoparticle Based Solid Polymeric Photoelectrochemical Cells

In this work, we report optical and photovoltaic properties of ZnO based solid polymeric photoelectrochemical (PEC) cell. In this device construction, film of ZnO nanoparticle (NP) has been sandwiched between ITO and LiClO4‐PEO solid electrolyte. ZnO NP has been used as photo‐anode and the electrolyte LiClO4 has been dissolved in a solid solution polyethylene oxide that makes the device lightweight and portable. The charge transfer is performed by Li+ and ClO4− ions which serve the purpose of a redox couple in a conventional dye sensitize solar cell (DSSC). Short Circuit Current (JSC) shows maximum when the ZnO/ITO interface has been illuminated by a UV light (350±15 nm) passing through the transparent ITO glass substrate. JSC and Open Circuit Voltage (VOC) of the best performing device are found to be 7 μA/cm2 and 0.24 V respectively under white light of intensity 15 mW/cm2.

PERFORMANCE COMPARISON OF p–n JUNCTION DIODES USING ZINC OXIDE AND COPPER PHTHALOCYANINE HYBRID NANOCOMPOSITES AND BILAYER HETEROSTRUCTURES

Nanocomposites of n-type Zinc Oxide (ZnO) and p-type copper phthalocyanine (CuPc) were synthesized using solution route and the electrical properties of heterojunctions of ZnO and CuPc in the nanocomposite film was studied. For comparison, electrical properties of bilayer heterojunction devices using ZnO nanostructure drop cast film and thermally evaporated CuPc were also studied. Rectification ratio (RR) of about 28 and 5.5 was obtained at 4 V for devices with nanocomposite film and bilayer heterojunctions indicating improved formation of p–n junction characteristics for nanocomposite films. Values of ideality factor, barrier height at the p–n junction interface and series resistance were estimated using different methods like semilog plots, Cheung and Nordes methods. Ideality factor estimated from semilog plots and Cheungs methods were found to be higher than unity indicating deviation from ideal diode behavior. Barrier height estimated from different methods was about 0.7 eV. Series resistance measur...

Effect of post deposition annealing on the performance of copper phthalocyanine based organic thin film transistor

The electrical performance of copper phthalocyanine (CuPc) based OFETs on SiO2 dielectric was studied with and without post deposition annealing of CuPc films. Field effect mobility of holes and the drain current modulation (Ion/Ioff) was found to increase by one order for devices with annealed films as compared to that with as deposited film. This is attributed to well connected grains and increased crystallinity of CuPc film. Subthreshold slope (SS) was found to be reducing with increase in annealing temperature and was minimum for the device with CuPc film annealed at 225 °C, implying lesser traps affecting the mobility of charge carriers.

Effect of polymer modification of SiO2 dielectric on the performance of copper phthalocyanine based organic thin film transistor

Effect of polystyrene (PS) modification of SiO2 dielectric on the performance of copper phthalocyanine (CuPc) based OFETs was studied. Field effect mobility of holes was found to increase by one order for polystyrene modified dielectric as compared to that on unmodified SiO2. This is attributed to well connected grains and increased crystallinity of CuPc film. Significantly reduced hysteresis was found in the devices with PS/SiO2 dielectric due to almost complete absence of hydroxyl groups at the dielectric/semiconductor interface and hence reduced trap sites.

EFFECT OF GATE INSULATOR ON THE PERFORMANCE OF COPPER PHTHALOCYANINE-BASED ORGANIC THIN FILM TRANSISTORS

Effect of the gate dielectric on the performance of Copper phthalocyanine (CuPc) based top contact organic field effect transistors (OFET) has been studied using thermally grown SiO2 and sputtered HfOx films with dielectric constants of 3.9 and 12.5 respectively. Operating voltages of the devices on SiO2 and HfOx were found to be 10–50 V and 2–3 V, respectively. The lower operating voltage for HfOx is attributed to the higher dielectric constant. Devices on SiO2 and HfOx were found to have field effect mobilities of 0.01 and 3.5 × 10-3 cm2/Vs and drain current modulation of 103 and 102, respectively. Scanning Electron Microscopy showed widely scattered nanowires on HfOx and densely packed nanofibers on SiO2. X-ray diffraction studies showed better crystallinity of films on SiO2. The results show that operating voltage of devices can be reduced by using higher dielectric constant material while mobility and FET characteristics depend on structure of CuPc that in turn is influenced by the dielectric.