Nandu B. Chaure

Investigation of electronic quality of chemical bath deposited cadmium sulphide layers used in thin film photovoltaic solar cells

The investigation of electronic quality of chemical bath deposited cadmium sulphide (CdS) layers was the main objective of this work. For completeness, the US layers were characterised using X-ray diffraction, atomic force microscopy, optical absorption, photoelectrochemical cell, DC electrical conductivity measurements, current-voltage and capacitance-voltage measurements using Gold/CdS Schottky contacts. It has been found that the US layers grown are hexagonal with (002) preferential orientation. The n-type CdS materials show 1-2 mum clusters consisting of 0.3-0.4 mum size crystallites. The optical band gap is 2.42 eV, which shows a red-shift to 2.25 eV upon heat treatment. Gold Schottky contacts produce large Schottky barriers of 1.02 eV with ideality factors of 1.50, indicating excellent electronic qualities. Schottky-Mott plots indicate a moderate doping concentration of 1.2 X 10(17) cm(-3), suitable for electronic device fabrication. However, the DC electrical conductivity measurements carried out at room temperature indicate a very low electrical conductivity in the range (4-11) X 10(-5) (Omega cm)(-1). This indicates a very low mobility value of (2-5) X 10(-3) cm(2) V-1 s(-1), which are five orders of magnitude below that of single crystal CdS. The way forward for further improvement of the electrical conductivity is discussed

Development of p + , p, i, n, and n + -Type CuInGaSe2 Layers for Applications in Graded Bandgap Multilayer Thin-Film Solar Cells

Copper indium gallium diselenide layers with p(+), p, i, n, and n(+)-type electrical conduction, as predetermined, have been electrodeposited from aqueous solutions in a single bath. The photoelectrochemical cell has been used as the key analytical tool to determine the electrical conduction type, and X-ray fluorescence has been used to determine the stoichiometry of the corresponding layers. Optical absorption, X-ray diffraction, and atomic force microscopy have been used to investigate the bandgap, bulk structure, and surface morphology of the material layers, respectively. It has been found that the bandgaps of these layers can be varied in the range 1.10-2.20 eV. A four-layer n-n-i-p solar cell structure was fabricated and a corresponding energy band diagram for the device constructed. Current-voltage and capacitance-voltage measurements were carried out to assess the devices, and these parameters (V-oc approximate to 570 mV, J(sc)approximate to 36 mA cm(-2), and FF approximate to 0.40) indicate encouraging characteristics enabling further development of multilayer thin-film solar cells based on CuInGaSe2. The addition of a p(+) layer to the structure improved device parameters as expected due to improvements at the metal contact to the p(+) surface of the n-n-i-p-p(+) structure. (c) 2007 The Electrochemical Society.

Optical dispersion in spun nanocrystalline titania thin films

Sol–gel derived titanium dioxide (TiO2) films are prepared on silicon substrates using the method of spin-coating with different speeds from 1000 to 6000 rpm. Spectroscopic ellipsometric measurements provide dispersion of the refractive index and extinction coefficient within the wavelength range of 300–1000 nm. The behaviour is in agreement with the single oscillator model. Optical absorption is believed to be due to allowed indirect transition over the optical gap of approximately 3.2 eV.

Structural and electrical studies on sol-gel derived spun TiO2 thin films

Titanium dioxide thin films were prepared by spin coating of sol precursor onto microscopic glass slides, silicon and indium tin oxide (ITO) coated glass substrates. Spin speed was varied between 1000 and 6000 rpm. From the morphological analysis, it is found that thin films spun with speed ω≤4000 rpm assume higher ordered structure than those spun at a speed higher than 4000 rpm. Conduction at low voltages is believed to be variable range hopping at temperatures T<220 K while the ionization of donors situated at Ea = 0.46 eV below the conduction band edge becomes dominant at temperatures higher than 220 K. At high field charge transport is due to trap-controlled space charge limited mechanism. Traps with a density Nt≈1×1022 m−3 are thought to be situated at energy level Et = 0.3 eV below the conduction band and are associated with film nonstoichiometry and interface states.

Sol?gel derived nanocrystalline titania thin films on silicon

Electrical characteristics of sol–gel derived titanium dioxide (TiO2) as insulating layers were investigated by making capacitance and leakage current measurements in metal–insulator–semiconductor configurations. The structure was fabricated by depositing 37 nm thick anatase TiO2 films on p-type silicon (p-Si) substrates. The frequency dispersion of capacitance was attributed to the leaky behaviour of the TiO2 dielectrics. Using an equivalent circuit, values of the frequency-independent dielectric constant, interfacial surface density and threshold voltage were estimated to be 13, 3 × 1014 m−3 and −0.085 V, respectively. The carrier diffusion was found to be primarily responsible for the diode leakage current at room temperature but the increase in the ideality factor with lowering temperature was believed to be due to fluctuations of barrier height at the TiO2/p-Si interface.

High-mobility solution-processed copper phthalocyanine-based organic field-effect transistors

Abstract Solution-processed films of 1,4,8,11,15,18,22,25-octakis(hexyl) copper phthalocyanine (CuPc6) were utilized as an active semiconducting layer in the fabrication of organic field-effect transistors (OFETs) in the bottom-gate configurations using chemical vapour deposited silicon dioxide (SiO2) as gate dielectrics. The surface treatment of the gate dielectric with a self-assembled monolayer of octadecyltrichlorosilane (OTS) resulted in values of 4×10−2 cm2 V−1 s−1 and 106 for saturation mobility and on/off current ratio, respectively. This improvement was accompanied by a shift in the threshold voltage from 3 V for untreated devices to -2 V for OTS treated devices. The trap density at the interface between the gate dielectric and semiconductor decreased by about one order of magnitude after the surface treatment. The transistors with the OTS treated gate dielectrics were more stable over a 30-day period in air than untreated ones.

In situ Sb-doped CdTe films

This paper presents the first report on in situ antimony doping for growing p-CdTe films using electrodeposition. The verification of p-doping of CdTe during growth was ascertained by means of steady state cathodic polarization under chopped illumination. Proton induced x-ray emission spectroscopy (PIXE) was used to verify the incorporation of antimony. It is further shown that Sb impurities promote preferential orientation of the low index (111) plane of CdTe films. The morphology of the deposit was studied using atomic force microscopy (AFM).

Electrodeposition of ZnTe films from a nonaqueous bath

The electrodeposition of ZnTe films on an ITO-coated glass substrate in a bath containing M/10 , M/2000 and M/3.3 KI dissolved in ethylene glycol is described. Cyclic voltammetry in the dark and under illumination was used to identify the potential for ZnTe deposition. It is shown that cubic ZnTe can be grown at -0.95 V. Film characterization by x-ray diffraction and scanning electron microscopy is also discussed.

Effect of Cu-doping on the morphology of ZnTe films electrodeposited from nonaqueous bath

Electrodeposition of Cu-doped ZnTe has been studied in a bath containing ZnCl2, TeCl4, KI and CuCl2 at 160°C using cyclic voltammetry in the dark and under illumination. X-ray diffraction as well as scanning electron microscopy techniques have been employed to investigate the influence of deposition potential on the structure and surface morphology of the electrodeposit. It is shown that a mixed cubic as well as hexagonal ZnTe film is electrodeposited even at a potential lower than the equilibrium potential of Zn electrodeposition. Intermediate electrodeposition potential (−0.7 V to −0.75 V) favours growth of a mixed Te and ZnTe phase. Compact, globular and single phase ZnTe can be grown at −0.95 V. The atomic force microscopic examination of the ZnTe films deposited at −0.95 V indicated compact and close packed granular growth.

Enhanced Oxygen Reduction at Composite Electrodes Producing a Large Magnetic Gradient

A platinum-coated electrode is designed to produce a magnetic field with ∇B 2 in excess of 10 5 T 2 m -1 at its surface in an applied field of 1 T. It is based on an array of cobalt nanowires embedded in an anodized alumina membrane. Performance for a series of electrochemical measurements on a model oxygen reduction reaction in an alkaline medium is compared with those of control electrodes with no cobalt, or with a continuous cobalt film behind the platinum surface. The measurements include rotating disk electrode cyclic voltammetry, steady-state polarization, and chronoamperometry. The limiting current for the field gradient electrode is enhanced by more than an order of magnitude compared to the controls, and the enhancement is retained when the electrode rotates. The effect is attributed to a concentration of paramagnetic species, HO 2 - and O 2 , at the surface of the field gradient electrode. The results demonstrate the potential of using magnetic field gradient electrodes for electrochemical reactions involving paramagnetic species.