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

Cu(In 1-x Ga x )(Se 1-y S y ) 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 CuInSe 2 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 ΔE V (CuInSe 2 /CuInS 2 )= -0.23 eV and ΔE V (CuInSe 2 /CuGaSe 2 )=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.