The hunt for the third acceptor in CuInSe₂ and Cu(In,Ga)Se₂ absorber layers

The model for intrinsic defects in Cu(In,Ga)Se₂ semiconductor layers is still under debate for the full range between CuInSe₂ and CuGaSe₂. It is commonly agreed by theory and experiment, that there are at least one shallow donor and two shallow acceptors. Spatially resolved photoluminescence on CuGaSe₂ previously revealed a third acceptor. In this study we show with the same method that the photoluminescence peak at 0.94 eV in CuInSe₂, previously attributed to a third acceptor, is a phonon replica. However another pronounced peak at 0.9 eV is detected on polycrystalline CuInSe₂ samples grown with high copper and selenium excess. Intensity and temperature dependent photoluminescence measurements reveal that this peak originates from a DA-transition from a shallow donor (<8 meV) into a shallow acceptor A3 (135 [Formula: see text] 10) meV. The DA3 transition has three distinct phonon replicas with 28 meV spectral spacing and a Huang Rhys factor of 0.75. Complementary admittance measurements are dominated by one main step with an activation energy of 125 meV which corresponds well with the found A3 defect. The same defect is also observed in Cu(In,Ga)Se₂ samples with low gallium content. For [Ga]/([Ga]  +  [In])-ratios of up to 0.15 both methods show a concordant increase of the activation energy with increasing gallium content shifting the defect deeper into the bandgap. The indium vacancy [Formula: see text] is discussed as a possible origin of the third acceptor level in CuInSe₂ and [Formula: see text] in Cu(In,Ga)Se₂.