Neelkanth G. Dhere

Preparation and characterization of vacuum deposited CuInSe2 thin films

CuInSe2 is a promising material for photovoltaic solar cells. Thin films of CuInSe2 were prepared by vacuum evaporation of the constituent elements by the “three-temperature” method. Source temperatures rather than the elementary fluxes were controlled. The deposition set-up configuration permitted the simultaneous preparation of films with compositions varying around exact stoichiometry. CuInSe2 thin films with indium concentrations of ±3 at.% around stoichiometry had predominantly chalcopyrite structure with {112} crystallographic texture. These films were p-type, with resistivities of 2 × 10−2 − 27 Ω cm, carrier concentrations of 1 × 1018 − 4 × 1020cm−3 and Hall mobilities of 0.2 – 2 cm2 V−1 s−1. Films with about 5 at.% excess of indium showed mostly the sphalerite phase with {111} texture and a small proportion of the chalcopyrite phase. These films exhibited resistivities in the range 2 − 9 × 103 Ω cm. The films with about 5 at.% excess of copper consisted of berzelianite and/or sphalerite phases. The band gaps of nearly stoichiometric CuInSe2 thin films and those with 5 at.% excess of indium ranged from 0.99 to 1.01 eV.

Structure and morphology of obliquely deposited CdTe films

Structure and morphology of CdTe films deposited in vacuum, on glass substrates, at oblique vapor incidence angles and at substrate temperatures up to 250 °C, have been studied by reflection-electron diffraction and scanning-electron microscopy, so as to help in the understanding of their unusually high photo-electromotive force. Thinner films developed sphalerite type of face centred cubic (c) structure with a0=6.48 A. Thicker films showed additional wurtzite type of hexagonal close packed (h) phase with a0=4.57 A, c0=7.47 A, and a large hexagonal (H) phase, a0=4.57 A, c0=11,27 A, with a corresponding reduction in the proportion of the c phase. For oblique vapor incidence, 1 − d {111} orientation of c phase and 1− d {00.1} orientation of h phase, both being close packed layers showed normal or only slightly oblique growth, while 1 − d {331} orientation of c phase and 1 − d {10.3} orientations of hexagonal phases showed oblique growth towards the vapor direction but not aligned with it. Hillock growth of ...

Recent developments in thin film solar cells

Abstract In recent years, remarkable progress has been made in improving the photovoltaic (PV) conversion efficiencies of thin film solar cells. The best active-area efficiencies (air mass 1.5) of thin film solar cells reported are as follows: polycrystalline CuInSe 2 , 14.1%; CuIn(Ga)Se 2 , 12.9%; CdTe, 12.3%, total area; single-junction hydrogenated amorphous silicon (a-Si:H), 12.0%; multiple-junction a-Si:H, 13.3%; cleaved epitaxial GaAsGa 1− x Al x As, 21.5%, total area. Laboratory methods for preparing small thin film solar cells are evaporation, closed-space sublimation, closed-space vapor transport, vapor phase epitaxy and metallo-organic chemical vapor deposition, while economic large-area deposition techniques such as sputtering, glow discharge reduction, electrodeposition, spraying and screen printing are being used for module fabrication. The following aperture-area efficiencies have been measured, at the Solar Energy Research Institute, for thin film modules: a-Si:H, 9.8%, 933 cm 2 ; CuIn(Ga)Se 2 , 11.1%, 938 cm 2 ; CdTe, 7.3%, 838 cm 2 . The instability issue of a-Si:H continues to be a high priority area. It is necessary to improved the open-circuit voltage of CuIn(Ga)Se 2 cells, which do not seem to exhibit any intrinsic degradation mechanisms. With continued progress and increased production, PV modules are likely to become competitive for medium-scale power requirements in the mid-1990s.

The structure and semiconducting properties of cadmium selenide films

Abstract CdSe films about 1 μm thick were vacuum deposited on unheated glass substrates. Reflection electron diffraction studies showed the growth of a one-dimensional {00.1} texture orientation of the hexagonal phase. Films of lower resistivity were characterized by a larger grain size and better ordering, whilst films of higher resistivity contained amorphous regions and were less ordered. A study of the carrier type, the concentration, the Hall mobility and the variation in the bond length u along the hexagonal c axis of “as-deposited” and heat-treated films showed that the lower resistivity films incorporated a small cadmium excess, which increased with heat treatment, resulting in higher carrier concentrations and a further lowering of the resistivity. Large increases in the resistivities of films deposited at high rates were attributed to the depletion of the small individual grains.

Morphology and semiconducting properties of vacuum-evaporated thick cadmium sulphide films prepared by the hot wall technique

Abstract Cadmium sulphide films of thickness 2–30 μm (useful in the fabrication of solar cells, transducers etc.) were deposited onto glass substrates by the hot wall technique under high ((1−3)×10 -3 Pa) and very high ((1−3)×10 -6 Pa) vacua. Their morphology and semiconducting properties were studied using X-ray diffraction, reflection high energy electron diffraction, scanning electron microscopy, optical absorption spectroscopy and Hall effect measurements. Substrate and wall temperatures of 175–200°C and 350–375°C respectively were found to be suitable for obtaining surface grains of large size (2.0–5.0 μm) and fairly smooth cadmium sulphide films with a one-dimensional {00.1} texture orientation. The growth texture, crystallographic forms and faceting improved with a better vacuum and with deposition rates of about 0.5 μm min -1 . An electron energy level with an activation energy of about 0.22 eV was detected in most films. Films deposited at higher source temperatures showed high electron concentrations and low mobilities which were attributed to excess cadmium in these films.

Purity and morphology of aluminium films

Abstract A continuous wire feed source of aluminium of 99.999% purity in conjunction with a composite ceramic boat has been used to deposit aluminium films of high purity in vacuum. The impurity content was analysed by proton-induced X-ray analysis with a Van de Graaff accelerator, supplemented by X-ray fluorescence, optical emission and Auger electron spectroscopy. The low impurity content achieved (∼ 50 atomic ppm) is attributed to the small quantity of molten aluminium maintained in the boat at one time. The morphology of the Al films was studied by reflection electron diffraction, X-ray diffraction and scanning electron microscopy. Thick Al films grew with {111} and {311} texture orientations. The grain size variation with thickness generally followed the known variation with the deposition rate.

Composition and temperature coefficient of resistance of Ni-Cr thin films

Abstract Thin films of Ni-Cr have previously been found to have an anomalous maximum in the temperature coefficient of resistance (TCR) values in the thickness range 100–150 A. The region of high TCR on the higher thickness side (170–230 A) of this maximum was studied. Films were evaporated from a given amount of the evaporant material under high vacuum ((1–8) × 10 −4 Pa) in series of depositions at 0.5–1.5 A s −1 . Their thickness-composition profiles were analysed using Auger electron spectroscopy in conjunction with sputtering and their TCR and electrical resistivity values were compared. It was shown that under certain conditions the anomalous TCR maximum could be avoided. Thinner films and films deposited at slower rates had higher resistivities and higher total contents of oxygen and carbon. Ni-Cr films of low (less than 100 ppm °C −1 ) TCR were obtained at all thicknesses studied when the sum of the total atomic contents of chromium, oxygen and carbon reached 50–55%. This effect is attributed to potential barriers due to chromium oxide and carbide in the grain boundary region.

Composition and structure of CuInSe2 thin films prepared by vaccum evaporation of the constituent elements

Abstract Further compositional and structural studies of CuInSe 2 thin films deposited by co-evaporation of the constituent elements in high vacuum are presented. The morphology, composition and crystallographic structure of films grown at a substrate temperature of 350°C and with thicknesses in the range 0.15–1.0 μm are discussed. Characterization includes Auger electron spectroscopy, X-ray and electron diffraction and optical absorption spectroscopy. Stoichiometric CuInSe 2 thin films are obtained with the chalcopyrite structure and with grain sizes in the range 0.2–0.6 μm, band gaps near 1.02–1.04 eV and absorption coefficients which exceed 1 × 10 5 cm −1 .

The morphology of thick evaporated aluminium films and their purity as determined by proton-induced x-ray analysis

Abstract Aluminium films of thickness 7–65 μm were vacuum deposited using a 99.999% purity aluminium wire, a composite ceramic boat and a continuous wire feed source. The morphology and the purity of the films were studied at various deposition and wire feed rates. X-ray diffraction and scanning electron microscopy investigations showed that the films grew mostly in a one-dimensional {311} texture orientation and that the surface grain size first increased with deposition rates up to about 100 kA min-1 and then decreased. The purity of the aluminium films was studied mainly by proton-induced X-ray analysis with a 4 MeV Van de Graaff accelerator. A total impurity content of about 10 atomic ppm, composed mainly of iron, copper and zinc and in some cases lead, was observed at low and medium wire feed rates. In addition, of the light elements, magnesium was qualitatively detected.

Present status of the development of thin-film solar cells

Abstract Thin-film solar cells have undergone continuous, steady progress over the last several years. The principal types of thin-film solar cells are based on single-junction and multi-junction hydrogenated amorphous silicon (aSi:H), copper indium diselenide (CulnSe 2 ) and cadmium telluride (CdTe). Impressive gains in the performance of these cells have been reported in recent months. The problem of contacts to CdTe cells has been circumvented, resulting in the development of stable 11% efficient nCdS/iCdTe/pZnTe heterostructure solar cells. Total small-area (1 cm 2 ) efficiencies which have been measured at SERI under standard conditions (global AM 1.5) are as follows: single-junction aSi:H, 11.5–12%; triple-junction aSi:H:F, 12.4% (active area 13.3%); CuGalnSe 2 , 14.1% (active area); CdTe, 10–11%; and CulnSe 2 aSi:H cascade cells, 14.6%. Hydrogenated amorphous silicon solar-cell panels are being commercialized in several countries. Small-scale production of CdTe panels has also been undertaken. Recently, a CulnSe 2 module with the worlds highest thin-film module efficiency, 11.1%, an aperture area of 938 cm 2 and a total power of 10.4 W, has been tested at SERI. These developments make it possible to envisage the achievement of the revised programme goals of achieving 15% single-junction or 20% two-junction solar-cell efficiencies for large-scale production in the not-too-distant future.