Paper TF-ThP25
Effect Of Sodium Addition On Cu-Poor CIGS2 Thin Film Solar Cells

Thursday, October 18, 2007, 5:30 pm, Room 4C

Copper-Indium-Gallium Sulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of 1.5 eV. Recent research in the CIGS2 thin film photovoltaic community is directed towards thinner films because the availability and cost of Indium are limiting factors. The required amounts of metals can be lowered by using thinner films. Initially, small sized grains are formed during the film growth. With continuing growth to larger thicknesses, more favorably oriented grains grow faster at the expense of others and coalesce to form compactly packed large-grain morphology. Hence certain thickness of a thin film is necessary so that the grains start coalescing. The solar cell performance in smaller grained chalcopyrite absorber deteriorates due to detrimental effects of the larger fraction of grain boundaries. It is essential to hasten the grain growth through coalescence to retain high-quality properties even in thinner films. Sodium may play a vital role by enhancing the atomic mobility and improving the coalescence even in thinner films. It will be interesting to study if sodium can play a role in Copper-poor CIGS2 absorbers. In Copper-rich CIGS2 precursors, increased mobility and consequently absorber phase formation is obtained due to pseudo-binary phase region. Hence Sodium may not play a vital role in Copper-rich precursors. Also Copper-rich precursors for this purpose are prepared with 40 percent excess Copper. After sulfurization, the absorber film is etched to remove the excess Copper. This may or may not lead to an exact stoichiometric Cu/In+Ga ratio of 0.92 for the absorber in addition making the film rough. Copper-poor absorber has a comparatively smooth film and better morphology after etching. This work represents a study of morphology and device properties of CIGS2 thin films with Copper-poor absorbers after Sodium is introduced on the substrate in the form of Sodium Fluoride before sputtering Copper, Indium and Gallium.