(5do) Pathways to Cost-Effective, High Efficiency Cu(InGa)Se2 Solar Cell Absorbers

In addition to high cell efficiency (19.9 %), Cu(InGa)Se2 thin-film solar cells exhibit outstanding long-term outdoor stability, excellent radiation hardness, and the potential for low-cost, large-area production on glass or flexible substrates. The reaction time for synthesis of this absorber, however, needs to be significantly shortened to reduce the manufacturing costs.

Reaction pathways and kinetics of chalcopyrite CuInSe2, CuGaSe2 and Cu(InGa)Se2 formation from various precursor structures were systematically investigated using time-resolved, in situ high-temperature X-ray diffraction. Stacked binary bilayers (e.g., GaSe/CuSe and In2Se3/CuSe) and metallic (e.g., CuIn, CuGa and CuGaIn) precursor films were deposited on glass substrates in a migration enhanced epitaxial deposition system. These films were then temperature ramp annealed or isothermally soaked under inert or Se ambient while monitoring the phase evolution by X-ray diffraction. Parabolic and Avrami diffusion controlled reaction models represented the isothermal experimental data very well over the entire soak temperature range yielding an activation energy in the range 66 to 162 kJ/mol depending on the precursor structure. TEM-EDS analysis supported the interfacial reaction of binary bilayer diffusion couples as suggested by the parabolic growth model. The results of this study suggest precursor structures and pathways to synthesize high performance Cu(InGa)Se2 absorber at low temperature for use with flexible polymer substrates (T_sub < 450 deg.C), and at a high rate for a high-throughput rapid thermal process. Another approach to reduce time and temperature for the formation of high performance Cu(InGa)Se2 is to use post-deposition thermal or laser annealing. Co-evaporated Cu(InGa)Se2 absorber films annealed using pulsed Nd:YAG laser beam with a 532 nm wavelength improved photovoltaic efficiency and re-orientation of chalcopyrite Cu(InGa)Se2 phases, e.g., (220)-->(112).