AVS 59th Annual International Symposium and Exhibition
    Energy Frontiers Focus Topic Tuesday Sessions
       Session EN+TF-TuM

Paper EN+TF-TuM10
Selenization of Cu-Ga-In Precursors for Synthesis of CIGS Absorbers: Equilibrium and Kinetic Studies

Tuesday, October 30, 2012, 11:00 am, Room 15

Session: Chalcogenide Solar Cells II
Presenter: C. Muzzillo, University of Florida
Authors: C. Muzzillo, University of Florida
R. Krishnan, University of Florida
W.K. Kim, Yeungnam University, Republic of Korea
E.A. Payzant, Oak Ridge National Laboratory
Y.H. Sohn, University of Central Florida
B. Yao, University of Central Florida
J. Shen, General Research Institute for Non-ferrous Metals of Beijing, China
C. Campbell, National Institute of Standards and Technology
T.J. Anderson, University of Florida
Correspondent: Click to Email
The emerging CuInxGa1-xSe2 (CIGS) PV industry is primarily differentiated on the basis of the process used to synthesize the CIGS absorber. The most common approach is a 2-step metal deposition/selenization with differentiation occurring on the method of metal deposition and Se source for selenization. Typical champion cell efficiencies, however, are slightly lower for 2-step processes as compared to co-evaporation. This has been attributed to difficulty in creating a ‘U-shaped’ Ga profile, void formation near the back contact, and differences in diffusion/reaction rates of Ga and In during synthesis. Furthermore, cost pressures are driving reduction of the selenization time (up to 8 hr) and thinning of the absorber layer. A better understanding of the thermodynamic and kinetic elements of the precursor Cu-Ga-In metal system offers the potential to exploit faster pathways, assist in scale-up, and ensure robust processing of CIGS. This study includes a critical assessment of thermochemical and phase equilibria data of the Cu-Ga-In ternary metal system. The calculated phase diagram includes 11 binary intermetallic phases and no ternary compounds. Of particular interest are the 4 phases which exhibit ternary solubility: α-Cu (fcc), γ-Cu9(GaxIn1-x)4 (sc), and η-Cu16(InxGa1-x)9 (hcp) are all modeled with a sublattice formalism, and an ionic two-sublattice liquid model is employed. Time-resolved in situ high temperature X-ray diffraction data for selenization of metallic precursors have also been collected. Reaction pathways and kinetics of temperature ramp and isothermal anneal experiments have both been examined, and kinetic rate parameters for the Avrami and parabolic growth models have been estimated from the data. As examples, MBE deposited bilayer metal precursor structures (e.g. CuIn/CuGa) and elemental stacked layers (e.g. (Cu/Ga/In)n) were selenized and the Ga distribution measured. The formation of Cu11In9, textured indium, and the solid solution γ-Cu9(GaxIn1-x)4 were also evident as well as formation of CuGaSe2 and CIGS simultaneously for selected structures. Rietveld refinement of temperature ramped selenized samples was performed to estimate the Ga distribution. TEM-EDS results confirm that the Ga distribution after selenization depended on the order of deposition of the precursor structure. The samples were further characterized by SEM (microstructure) and ICP (final composition). The activation energy for formation of CIGS was estimated from isothermal studies, and gave values of 76(±14) for the glass/Mo/CuIn/CuGa and 93(±4) and 101(±9) kJ/mole for the glass/Mo/CuGa/CuIn precursor.