Compositional trends of defect energies, band alignments, and recombination mechanisms in the Cu(In,Ga)(Se,S)2 alloy system

Cu(In1-xGax)(Se1-ySy)(2) chalcopyrite thin films are investigated by admittance spectroscopy, current-voltage and capacitance-voltage analysis of ZnO/CdS/chalcopyrite heterojunctions. The energetic depth of the dominant acceptor state in Cu-poor CuInSe2 increases upon alloying with S and remains essentially unchanged under Ga alloying. Using the acceptor energy as a reference, we extrapolate the valence band offsets DeltaE(V)(CuInSe2/CuInS2) = -0.23 eV and DeltaE(V)(CuInSe2/CuGaSe2) = 0.036 eV. The dominant recombination mechanism is mainly driven by changes in the Cu-stoichiometry. Whereas Cu-rich devices are dominated by recombination at the CdS/absorber interface, bulk recombination prevails in Cu-poor devices irrespective of the band gap energy of the absorber. (C) 2003 Elsevier Science B.V. All rights reserved.