The preparation and properties of CuInSe¦2 thin films and CuInSe¦2/CdS thin film solar cells

Abstract

CuInSe2 and the related I-III-VI2 ternary chalcopyrite compounds and alloys are semiconductors suitable for use as absorbers in thin film solar cells. A three source co-evaporation system for the growth of CuInSe2 films, a hot wall evaporation system for CdS films, and a radio frequency sputtering system for Mo films are described. Demonstration Mo/CuInSe2/CdS solar cells are produced to verify that the Mo, CuInSe2 and CdS films are of acceptable photovoltaic quality. CuInSe2 film samples are deposited on Corning 7059 glass for a detailed study of their properties. The Se is evaporated at a rate about three times the rate required for stoichiometry, so that the films are fully selenized. The Cu and In sources are positioned so that the Cu/In ratio varies by about 10% across the film area. This allows many of the properties to be measured as a function of the Cu/In ratio, on one substrate. Most other parameters which can effect film properties, such as film thickness, substrate temperature during deposition, film growth rate, and impurity concentrations, are the same for a film on a given substrate.

CuInSe2 films grown as a single layer at a substrate temperature of 350oC with constant Cu and In deposition rates are compared with films grown by the two layer technique, in which the first layer is Cu-rich and deposited at 350oC substrate temperature, and the second layer is In-rich and deposited at 450oC. It is found that the measured film conductivity is much less sensitive to the incident Cu/In atomic flux ratio in the case of the two layer films. The two layer films also generally show greatly increased photoluminescence intensity and photoconductivity gain, when compared to the single layer films. Scanned beam specular reflectance imaging is a sensitive indicator of film surface roughness, which is affected by film composition and substrate material. It is shown by optical beam induced current (OBIC) imaging that solar cells made with two layer films have reduced sensitivity to variations in the Cu/In ratio, compared to those based on single layer films.

Sodium is introduced into some of the films by depositing a thin layer of NaOH on one half of a sodium-free 7059 glass substrate before starting the CuInSe2 film growth. The other half of the substrate is used as an otherwise identical sodium-free reference films. The sodium has a significant effect on the x-ray diffraction pattern, specular reflectance, transmittance, photoluminescence, and photoconductivity of the films.

The properties of typical CdS window layer films and Mo electrode films are presented. The problems of poor Mo adhesion to the glass substrate and poor CuInSe2 adhesion to the Mo are discussed. Argon ion beam bombardment during the CuInSe2 film nucleation appears to improve adhesion to Mo.

The energy efficiency of the best solar cells is measured in bright sunlight. No attempt is made to select small high quality regions from a larger cell in order to obtain a higher maximum efficiency. The optical and electrical properties of some solar cells are presented.