Recombination channels in Cu(In,Ga)Se2 thin films: impact of the Ga-profile

Abstract

Depth bandgap profiles via a [Ga]/([Ga]+[In]) variation in the Cu(In,Ga)Se2 (CIGS) absorber layer have been implemented as a strategy to enhance the performance of CIGS solar cells. Since the [Ga]/([Ga]+[In]) determines to a large extent the position of the conduction band minimum, different Ga-profiles lead to different electronic energy levels structures throughout the CIGS layer. In this paper, from the investigation of the dependence of the photoluminescence (PL) on excitation power and temperature, we critically analyse the impact of a notch or a linear Ga-profiles on the CIGS electronic energy levels structure and subsequent dominant recombination channels. Notwithstanding two radiative transitions involving fluctuating potentials were observed for each sample, significant differences in the luminescence resultant from the two Ga-profiles were identified. For the CIGS absorber with a notch Gaprofile, two tail-impurity radiative transitions involving equivalent donors cluster and the same deep acceptor level were ascribed to the near CIGS/CdS interface and the notch regions. The probability of radiative recombination in the two regions is discussed. For the CIGS absorber with a linear Ga-profile, two band-impurity radiative transitions involving an acceptor, with an ionization energy compatible with the VCu defect were ascribed to the region near the CIGS/CdS interface. Our results shows that the dominant acceptor defects are dependent on the Ga-profile and they also highlight the complexity of the radiative and non-radiative recombination channels revealed by a tight control of the parameters in the experiment.