A.P. Samantilleke

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.

Comparison of electrodeposited and sputtered intrinsic and aluminium-doped zinc oxide thin films

Intrinsic zinc oxide (i-ZnO) and aluminium-doped ZnO (ZnO:Al) are components of high-efficiency copper indium gallium diselenide solar cells. This paper examines both of these materials grown by two different techniques, namely radio frequency sputtering and electrodeposition (ED) for comparison and a better understanding. X-ray diffraction showed all materials to be polycrystalline and hexagonal (wurtzite) ZnO. Scanning electron microscopy indicated crystallites with different orientations for ED materials compared to agglomerated nanocrystallites of the sputtered layers. The band-gap energy was determined to be in the range 3.27-3.45 eV. The transmission was 85% for both ED materials and 95% for the sputtered layers. Glass/FTO/i-ZnO/Al structures were rectifying, and glass/FTO/ZnO:Al/Al contacts were ohmic for both ZnO:Al layers. Addition of Al decreases the bulk resistivity for both i-ZnO layers by 1-2 orders of magnitude. The photovoltage response to pulsed illumination showed a slow relaxation hysteresis, and all materials showed n-type electrical conduction.

Sulphidation of electrodeposited cuprous oxide thin films for photovoltaic applications

Abstract Electrodeposited cuprous oxide thin films on indium-doped tin oxide (ITO) substrates were sulphided by exposing them to a spray of aqueous solution of sodium sulphide or to a mixture of hydrogen sulphide and nitrogen gases. Both methods produced light darker and darker films having different photovoltaic characteristics in a solar cell structure. The photovoltages produced by the light darker films under AM 1.5 illumination was negative as compared to the positive photovoltages produced by the darker films. Spectral response measurements revealed that most of the light darker films produced positive photovoltages in the shorter wavelengths and negative photovoltages in the longer wavelengths. However, some of the light darker films produced only the negative photovoltage for the entire spectral range and their photovoltaic properties were comparatively better. Darker films resulted in only the positive photovoltages in the entire spectral range. As a result of the sulphidation, the bulk crystal structure of the cuprous oxide thin films was not changed, however, the interfacial characteristics of the solar cell structure were modified.

Growth of n-type and p-type ZnSe thin films using an electrochemical technique for applications in large area optoelectronic devices

ZnSe layers have been grown by a low temperature (∼65 °C) electrochemical deposition technique in an aqueous medium. The resulting thin films have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive analysis by X-rays (EDAX), glow discharge optical emission spectroscopy (GDOES) and X-ray fluorescence (XRF) for bulk material properties. A photo-electrochemical (PEC) cell and an optical absorption method have been used for determination of the electrical and optical properties of the thin films. XRD patterns indicate the growth of ZnSe layers with (1 1 1) as the preferred orientation. The XPS spectra are similar to those of commercially available ZnSe and the EDAX, GDOES and XRF also indicate the presence of Zn and Se in the layers. PEC studies show p-type semiconducting properties for the as deposited layers and n-type ZnSe can be produced by appropriate doping. Optical absorption is maximum around 460 nm indicating a band gap of 2.7 eV. Annealing at 200 °C for 15 mins improves both the crystallinity of the layers and the photoresponse of the electrolyte/ZnSe liquid/solid Schottky junction.

Electrodeposited p-type and n-type ZnSe layers for light emitting devices and multi-layer tandem solar cells

Zinc selenide layers have been grown on glass/conducting glass substrates using a low temperature (∼65°C) electrochemical technique, and characterized using X-ray diffractions (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and photo electrochemical cell (PEC) techniques. XRD shows that the material growth is highly preferential with (1 1 1) orientation. XPS work indicates that this material has a chemical and stoichiometric nature similar to that grown by molecular beam epitaxy. Annealing at ∼250°C for 15 min improves the crystallinity of the layers. PL studies indicate the presence of a low number of defect levels which causes radiative transitions within the energy region 0.7–1.4 eV below the conduction band, in the case of electrodeposited ZnSe when compared to MBE grown ZnSe. Optical properties of the thin films were characterized using a PEC cell arrangement and both n- and p-doping of the materials has been achieved successfully using Ga and As, respectively. The use of crystalline ZnSe layers in both simple p-n junctions and multi-layer solar cell structures shows encouraging results.

Electrodeposition of n-type and p-type ZnSe thin films for applications in large area optoelectronic devices

ZnSe layers have been grown by a low temperature (∼65 °C) electrochemical deposition technique in an aqueous medium. The resulting thin films have been characterized using X-ray diffraction (XRD) and a photoelectrochemical (PEC) cell for determination of the bulk properties and electrical conductivity type. XRD patterns indicate the growth of ZnSe layers with (1 1 1) as the preferred orientation. PEC studies show p-type semiconducting properties for the as deposited layers and n-type ZnSe can be produced by appropriate doping. Annealing at 250 °C for 15 min improves the crystallinity of the layers and the photoresponse of the ZnSe/electrolyte junction.

Development of opto-electronic devices using electrochemically grown thin ZnSe layers

AbstractZnSe thin films of both p and n conducting types were successfully deposited on transparent conducting glass substrates using an electrochemical deposition technique. Thin films were deposited from an aqueous acidic electrolyte containing

Reply to Comment on 'New ways of developing glass/conducting glass/CdS/CdTe/metal thin-film solar cells based on a new model', by Dharmadasa et al 2002 Semicond. Sci. Technol. 17 1238–48

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