Showing papers by "Junsin Yi published in 2006"

Characteristics of Polymer Light Emitting Diodes with the LiF Anode Interfacial Layer

Abstract: Electrical and electroluminescent characteristics of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)-based polymer light emitting diodes (PLEDs) with the LiF anode interfacial layer were investigated. Current–voltage measurement and impedance analysis showed that LiF insertion affected the electrical properties of PLEDs notably. The maximum luminance efficiency of the PLED device with a LiF anode interfacial layer of 1-nm-thick was 3.0 lm/W, which is higher than 1.97 lm/W of the PLED device without a LiF layer. By inserting LiF as the anode interfacial layer, excess injection of holes from indium tin oxide anode can be reduced and hence the balance between electrons and holes can be increased in the emitting layer to improve device efficiency.

Direct and High Frequency Alternating Current Conduction Mechanisms in Solution Cast Polyaniline Films

Abstract: Frequency dependent (ac) and independent (dc) conductivity measurements have been carried out on polyaniline (PANI) films deposited by solution casting technique. Under low electric field (1 × 103 V/cm) condition, the dc conductivity measured in the temperature range of 173–303 K obeys the three-dimensional variable range hopping (3D VRH) formalism. The Mott parameters such as localization length (α−1 ≈ 7 A), density of states [N(EF) = 1.04 × 1019 states/eV cm3], hopping range (Rhop = 60 A) and hopping energy (Whop = 0.38 eV) are computed. The ac conductivity measured in the frequency range 10 kHz–5 MHz and in the temperature range 150–380 K follow a power-law dependence σac ∼ ωs, typical for charge transport by hopping or tunnelling processes. Therefore, the experimental results are analyzed with reference to various theoretical models based on quantum-mechanical tunnelling and classical hopping over barriers. The observed minimum in the temperature dependence of the frequency exponent s strongly suggests that tunnelling of large polarons is the dominant transport process. The polaron radius (rp ≈ 25 A) and barrier height for infinite site separation (WHO ≈ 0.22 eV) are evaluated. The density of states [N(EF)] and tunnelling distances (Rω) are estimated and discussed in terms of frequency and temperature.

Study of process induced variation in the minority carrier lifetime of silicon during solar cells fabrication

Abstract: A systematic study of the variation in the minority carrier effective lifetime in silicon associated with the different solar cell processing steps in a conventional industrial production line has been carried out using the microwave photoconductive decay (μ-PCD) technique. The solar grade silicon wafers used for this study presented bulk carrier lifetime of ∼10 μs and resistivity 0.5–3 Ω cm. Alkali texturing, phosphorus diffusion using POCl3, thermal oxide growth for surface passivation, plasma etching for edge isolation, and APCVD of TiO2 for surface passivation and antireflection coating were the major steps taken into consideration. The results clearly showed that the lifetime increased as the fabrication process proceeds from the bare wafer with the exception of the step associated to plasma edge isolation. The effective lifetime of the bare wafer was 4.04 μs, which increased to 16.67 μs after the antireflection coating and surface passivation with TiO2. The results of a systematic study of the effective minority carrier lifetime of silicon due to different surface passivation processes are also reported. The results obtained are useful for the design and implementation of proper measures for minority carrier lifetime enhancement during silicon solar cell fabrication at the industrial scale.

Study of High-Density Helicon-Plasma Generation and Measurement of the Plasma Parameters by Using a Frequency-Compensated Langmuir Probe

Abstract: Institute for Plasma Research, Bhat, Gandhinagar, India 382-428(Received 13 September 2005, in final form 6 March 2006)The present work is an experimental study of the various parameters of a high density plasmaproduced by using helicon waves in a toroidal structure using ordinary Langmuir probe and anRF compensated Langmuir probe. Helicon breakdown and the parameters were investigated in thetoroidal vessel was observed with a right helical antenna (m = +1) at a frequency of 32 MHz. Duringthe experiment, transition from a capacitive mode to a helicon mode of discharge was observed.This work also takes a step toward a realization of the necessity for a compensated Langmuir probein the study of an RF plasma, especially in the frequency range of tens of MHz. A toroidal magneticfield of 600G (max) is applied at the central axis of a toroidal vessel with an aspect ratio equal to3. The plasma is generated by using a helical antenna to form and sustain a plasma using hydrogengas. The power is delivered from an RF generator at 32 MHz via an L-Type matching network.A comparative study has been done using compensated and uncompensated Langmuir probes tomeasure various plasma parameters. A nonlinear increase in the plasma density with input RFpower and with the magnetic field is observed.

Exciton luminescence from silicon nanocrystals embedded in silicon nitride film

Abstract: Plasma-enhanced chemical vapour deposition (PECVD) method was used for silicon nanostructure synthesis in silicon nitride film. After deposition, films were annealed in vacuum at temperatures ranging from 600 to 900 °C for 10 min using rapid thermal annealing system. Exciton luminescence from quantum silicon structures was observed in annealed films by low temperature photoluminescence at a temperature of 11 K. Indirect momentum-conserving phonon-assisted exciton luminescence peaks observed give the evidence that silicon nitride matrices have silicon nanostructures in crystalline form.

Passivation of Silicon Oxide Film Deposited at Low Temperature by Annealing in Nitrogen Ambient

Abstract: Poly silicon TFT requires high quality dielectric film; conventional method of growing silicon dioxide needs highly hazardous chemicals such as silane. We have grown high quality dielectric film of silicon dioxide using non-hazardous chemical such as TFOS and ozone as reaction gases by APCVD. The films grown were characterized through C-V curves of MOS structures. Conventional APCVD requires high temperature processing where as in the process of current study, we developed a low temperature process. Interface trap density was substantially decreased in the silicon surface coated with the silicon dioxide film after annealing in nitrogen ambient. The interface with such low trap density could be used for poly silicon TFT fabrication with cheaper cost and potentially less hazards.

Synthesis and characterization of Ce-doped BZT thin films deposited by a RF magnetron sputtering method

Abstract: We investigated the structural and electrical properties of the 0.5% Ce-doped Ba(ZrxTi1-x)O3 (BZT) thin films with a mole fraction of x=0.2 and a thickness of 150 nm for the MLCC (Multilayer Ceramic Capacitor) application. Ce-doped BZT films were prepared on Pt/Ti/SiO2/Si substrates by a RF magnetron sputtering system as a function of Ar/O2 ratio and substrate temperature. X-ray diffraction patterns were recorded for the samples deposited with three different substrate temperatures. The thickness and the surface roughness of the films deposited with different Ar/O2 ratios were measured. The oxygen gas, which was introduced during the film deposition, had an influence on the growth rate and the roughness of the film. The surface roughness and dielectric constant of the Ce-doped BZT film varied with Ar to O2 ratios (5:1, 2:1, and 1:1) from 1.21 nm to 2.33 nm and 84 to 149, respectively. The Ce-doped BZT film deposited at lower temperature has small leakage current and higher breakdown voltage.

Admittance Spectroscopic Analysis of Polymer Light Emitting Diodes with the LiF Cathode Buffer Layer

Abstract: Admittance Spectroscopic analysis was applied to study the effect of LiF buffer layer and to model the equivalent circuit for poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)-based polymer light emitting diodes (PLEDs) with the LiF cathode buffer layer The single layer device with ITO/MEH-PPV/Al structure can be modeled as a simple combination of two resistors and a capacitor Insertion of a LiF layer at the Al/MEH-PPV interface shifts the lowest unoccupied molecular orbital (LUMO) level and the vacuum level of the MEH-PPV layer as a result of which the barrier height for electron injection at the Al/MEH-PPV interface is reduced The admittance spectroscopic analysis of the devices with the LiF cathode buffer layer shows reduction in contact resistance (RC), parallel resistance (RP) and increment in parallel capacitance (CP)