AVS 53rd International Symposium
    Thin Film Wednesday Sessions
       Session TF-WeA

Paper TF-WeA2
Effect of Na Quantity on the Morphology of the Absorber Layer in the CIGSS/CdS Thin Film Solar Cell

Wednesday, November 15, 2006, 2:20 pm, Room 2022

Session: Thin Films for Energy Applications in Photovoltaics, Fuel Cells, Hydrogen Storage & Batteries
Presenter: V.V. Hadagali, Florida Solar Energy Center, University of Central Florida
Authors: V.V. Hadagali, Florida Solar Energy Center, University of Central Florida
S.A. Pethe, Florida Solar Energy Center, University of Central Florida
N.G. Dhere, Florida Solar Energy Center, University of Central Florida
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The highest efficiencies of CIGS thin-film solar cells have been obtained by using soda lime glass as substrate material. Soda lime glass contains significant amounts of sodium in the form of NaO2. The presence of Na during growth of the CIGS absorber layer is beneficial. Na promotes increase in grain size and preferred (112) orientation of CIGS films. The overall effect of Na incorporation during grain growth is an increase in efficiency by enhancements of fill factor and open circuit voltage. This paper presents the effect of the Na quantity on the morphology of CIGSS thin film solar cells. Among the available sodium containing precursor, NaF is non-hygroscopic, stable in air and evaporates stoichiometrically. NaF of various thicknesses ranging from 30-120 Å is deposited by thermal evaporation on a Mo coated glass. The CIGSS is grown in a two stage process. The Cu-In and Ga are deposited by sputtering and the selenization / sulfurization process in a conventional furnace. The absorber layer was deposited on the substrates with various NaF quantities in the same process. Various morphological effects such as hillocks and valleys have been observed due to the presence of Na. The area density of hillocks and valleys varied with the quantity of NaF. Selenization of the film containing Na results in the formation of NaSex compounds. In some cases, absorber layer peeled immediately after deposition or during chemical bath deposition of CdS heterojunction layer. The peeling effect could be attributed to the non-optimum sodium and selenium contents. SEM, AES and Optical microscopy techniques were used to understand morphology.