R. D. Tomlinson

Structural and optical investigations on CdS thin films grown by chemical bath technique

Cadmium sulphide (CdS) thin films have been prepared by using a chemical bath deposition (CBD) method. Structural analysis (XRD and TEM) revealed that the films showed hexagonal structure with lattice constants a=0.4015 nm and c=0.6545 nm. On annealing, the films exhibited secondary phase of beta -CdS (cubic) along with hexagonal phase, but density of dislocations decreased. The stacking faults have been observed in the crystalline arrangement by high resolution transmission electron microscopy (HRTEM). The direct band gaps of as-grown and annealed films were found to be 2.42 and 2.62 eV with sub band gap of 2.35 eV, respectively, which were confirmed by optical analysis. On annealing, the resistivity of the films decreased from 3 x 10(6) to 1 x 10(3) Omega cm. Multiple Cd and S defects were observed by employing photoluminescence (PL) method. The intensities of emission peaks for annealed films differed with that of as-grown films. As was discussed with giving reasons.

Structure and chemistry of CuInSe2 for solar cell technology: current understanding and recommendations

The results of a workshop on the structure and chemistry of CuInSe2 (CIS), held in Jackson Hole, WY (USA), are presented. The participants are the authors of this paper. Issues in CIS properties and device performance were divided into those dominating common CIS and those relevant to the best CIS. Common CIS in non-equilibrium material containing numerous lattice defects and second phases surrounding the grains or segregated to the surface. Common CIS is capable of producing solar cells yielding in excess of 10% conversion efficiency provided that there is no CuxSe present. The best physical vapor deposited (PVD) CIS is formed at high temperatures, where rapid segregation should minimize the incorporation of dissolved point defects. This CIS has relatively large, strongly (112)-oriented, nearly perfect grains. Growth of the majority of the film under strongly Cu-rich conditions provides the best results, although the final film must be free of CuxSe phases. The differences between common CIS and the best CIS are relatively process-independent (but process-variable-dependent) and tend to limit the open circuit voltage and fill factor of devices. The workshop consensus recommendations were as follows. The selenization reaction must be studied to determine how desirable properties, reproducibly demonstrated in PVD CIS, can be reliably achieved. Controlled experiments should be carried out to determine the effect of extrinsic doping and grain boundaries on minority carrier lifetimes. Finally, a mechanism is needed for analysis of films to show how materials produced by different methods compare.

Optical, structural and electrical properties of tin doped indium oxide thin films prepared by spray-pyrolysis technique

Tin doped indium oxide (In2O3:Sn) or indium tin oxide (ITO) thin films have been successfully deposited by the low cost spray-pyrolysis method. Low sheet resistance and high mobility films were obtained when the films were deposited at the substrate temperature of 793 K. The direct optical bandgaps for the films deposited at 793 (a) and 753 K (b) were found to be 3.46 and 3.40 eV, respectively. Similarly, the indirect bandgaps for a- and b-type films were found to be 3.0 and 2.75 eV, respectively. The Burstein-Moss shift was observed in the films. The refractive index (n) and extinction coefficient (k) were found to be in the range of 2.1 to 1.1 and 0.6 to 0.01, respectively. The various scattering mechanisms such as lattice, ionized impurity, neutral impurity, grain boundary and alloy scattering due to variation of theoretical mobilities with temperature are discussed, in order to compare experimental results. In the lattice scattering mechanism, the quantum size effect phenomena were employed to estimate the energy dilation (EI). The a-type films exhibited SnO2 as secondary phase whereas b-type films showed single phase In2O3:Sn with high sheet resistance. The lattice constants were found to be 10.16 and 10.09 A for a- and b-type films, respectively.

Photoacoustic spectroscopy of defect states in CuInSe2 single crystals

Abstract Photoacoustic spectra have been obtained at room temperature from n and p-type single crystals of CuInSe 2 grown from a near-stoichiometric melt by the vertical Bridgman technique. The spectra have been used to evaluate gap energies and also to establish ionisation energies for several deep defect-related energy states. The data reported here demonstrate that the photoacoustic technique is useful as a contact-less method of appraising the complex intrinsic defect populations which dominate the optoelectronic properties of this semiconducting compound.

Optical properties of high-quality CuInSe2 single crystals

CuInSe2 single crystals have been studied employing photoluminescence (PL), optical reflection (OR), optical absorption (OA) and wavelength derivative reflection (WDR) techniques at temperatures from 4.2 to 300 K. Exciton-related peaks were observed in the near-band-edge region of the PL spectra: several narrow lines, with full width at half maximum (FWHM) of about 0.3 meV, and two wider peaks (FWHM about 0.7 meV) at 1.0414 (A) and 1.0449 eV (B). The A and B peaks were also observed in the OR and OA spectra and identified as A and B free excitonic states. The narrow lines were attributed to bound exciton recombination on intrinsic defects. A third exciton resonance (C) was observed in the WDR spectra at 1.2779 eV. The crystal-field and spin-orbit splittings were derived to be 5.3 and 234.7 meV, respectively.

Band-gap narrowing in n-type CuInSe2 single crystals

Abstract The gap energies at room temperature of n -type CuInSe 2 single crystals are found to decrease with increasing electron and ionized impurity concentrations in the samples. It is shown that the observed gap energy decrease can be explained in terms of screening-induced band-gap narrowing if the contributions of both free carriers and ionized impurities are included in evaluating the screening length. A value of E g = (1.010 ± 0.001) eV is found for the true gap energy of CuInSe 2 at 300 K.

The growth and doping of single crystals of CuInTe2

Abstract The results of some crystal growth experiments with the semiconducting compound CuInTe 2 are reported. A programmed directional freezing technique is described which facilitated the production of large, single phase, p type crystals. Data concerning the electrical doping characteristics are also described and observed changes in resistivity during heat treatment are related directly to the partial vapour pressure parameters for molecular dissociation.

The effect of sulfur concentration on the properties of chemical bath deposited CdS thin films

We study the structural, surface morphology and optical properties of chemical bath deposited (CBD) cadmium sulfide (CdS) thin films under the effect of variation of S/Cd ratio. CdS thin films have been successfully deposited by CBD technique with solutions containing S/Cd ionic concentration ratio of 5.0, 2.5, 1.0, 0.5 and 0.25. Single phase CdS, with a hexagonal structure, is observed for the concentration of S/Cd = 5.0, 2.5, 1.0 and 0.5 films while for the ratio of 0.25, the films exhibited a partially amorphous nature. These have been confirmed by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) analyses. The band gaps of the films obtained by transmission and photoacoustic spectra are found to be in the range of 2.40 to 3.26 eV. The large variation of band gaps of the films with composition is discussed by employing quantum size effect phenomena. The transition levels of CdS are also studied using photoacoustic spectroscopy.

A microstructural and compositional analysis of CuInSe2 ingots grown by the vertical Bridgman technique

Abstract An in-depth microstructural study of CuInSe2 ingots grown by the vertical Bridgman technique has been carried out. Electron probe microanalysis (EPMA) has been used to analyse the matrix composition and second phase distribution as a function of distance along the Bridgman boule. By systematically varying the composition of the initial charge, we identified the optimum conditions for producing large regions of nearly stoichiometric single crystal CuInSe2, free from second phase inclusions. We found that additions of small amounts of excess copper aided the formation of the “ideal” crystal. Compositional variations along the length of the Bridgman boules have been identified and qualitatively explained in terms of the ternary composition field for the Cu, In, Se system. Furthermore, a number of characteristic second phases have been identified and their presence correlated with whether or not the initial charge was nominally stoichiometric, Cu-rich or In-rich. The effects of these second phases in terms of the microstructural defects they induce in the neighbouring CuInSe2 matrix has also been critically assessed.

Changes in the opto-electronic properties of CuInSe 2 following ion implantation

The development of efficient thin-film solar cells based on CuInSe2 absorber layers has encouraged fundamental research on both thin films and single crystals of this chalcopyrite semiconducting compound. The resistance to radiation and ion bombardment is of technical importance particularly for a material which could find future applications in space photovoltaic power systems. In this paper results are described for an ion implantation study using CuInSe2 single crystal substrates. Oxygen, helium and neon implantations have produced significant changes in surface resistivity and photoconductivity. Also the near-surface regions ofn-type crystals have been type-converted top-type following ion implantation. It is apparent that the ion implantation process creates defects which affect surface state densities and recombination probabilities. In the case of oxygen there is an additional doping effect caused either by the introduction of acceptor states or by the reduction of the existing donor state population. Following implantation there appears to be an overall decrease in carrier recombination at the surface which leads to an enhanced photoconductive response.