Apparatus and method for forming chalcogenide semiconductor absorber materials with sodium impurities

#The disclosure relates, most generally, to the formation of thin films. More particularly, the disclosure relates to producing and using a sodium vapor to form chalcogenide semiconductor materials doped with sodium. The chalcogenide semiconductor materials find application in solar cells and in various other applications.

#Chalcogenide semiconductor materials are used in many applications and their popularity is increasing in recent years. A chalcogenide is a binary compound of a chalcogen and a more electropositive element or radical. Chalcogens are the group XVI elements of the periodic table: oxygen, sulfur, selenium, tellurium, and polonium. One particularly popular chalcogenide semiconductor material is CIGS, copper indium gallium selenide. CIGS materials find use in various applications and are particularly popular as absorber layers for solar cells. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years, increasing the demand for CIGS and other chalcogenide materials. CIGS is a tetrahedrally bonded semiconductor, with a chalcopyrite crystal structure. In addition to the CIGS, copper indium gallium selenide chalcogenide discussed supra, other chalcogenide semiconductor materials include CuInSe2, CuGaSe2, and indium. The aforementioned and other chalcogenide semiconductor materials such as CIGSS, copper indium gallium sulfur-selenide, are semiconductors with a chalcopyrite structure and are therefore often referred to as chalcopyrite-based semiconductor materials or chalcopyrite-structured semiconductor materials. Other chalcogenide materials may also include chalcopyrite crystal structures.

#Solar cells are photovoltaic components that provide for the direct generation of electrical current from sunlight. The absorber layer that absorbs the sunlight that will be converted into electrical current, is therefore of paramount importance. The formation of the absorber layer, the composition of the absorber layer and the placement of the same on a solar cell substrate are therefore each critical matters. The demand for the efficient, accurate and reliable production of an efficient absorber film is of growing and critical importance.

#The present disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing.

#FIG. 1 is a schematic and view of a sodium vaporizing furnace with some components shown in cross-section;

#FIG. 2 is a schematic view showing a system for forming chalcogenide semiconductor absorber materials with some components shown in cross-section, including the sodium vaporizing furnace of FIG. 1;

#The disclosure provides for forming chalcogenide semiconductor absorber materials with sodium dopant impurities. In many embodiments, the chalcogenide semiconductor absorber materials are used in solar cells. In one embodiment, the chalcogenide semiconductor material is CIGS, copper indium gallium selenide, and in another embodiment, the chalcogenide semiconductor material is CIGSS, copper indium gallium sulfur-selenide.

#Sodium doping has been found to be helpful in improving solar cell performance in chalcogenide semiconductor materials such as CIGS and CIGSS. Sodium acts as an acceptor impurity that enhances p-type carrier concentration and eliminates defects particularly at grain boundaries.

#The disclosure provides a method that includes forming metal precursors on a substrate such as a solar cell substrate, then directing reactive gases to a furnace in which the substrate with metal precursors is contained. The reactive gases react with the metal precursors to produce chalcogenide semiconductor materials for use as absorber layers. In some embodiments, the reactive gases include selenium containing gases and in some embodiments, the reactive gases include sulfur containing gases. In some embodiments, a selenization step precedes a sulfurization step and in some embodiments, the sulfurization step follows an annealing step which follows the selenization step. Various different process sequences are used.