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 analyze the impact of a notch or a linear Ga-profile on the CIGS electronic energy levels structure and subsequent dominant recombination channels. Notwithstanding, two radiative transitions involving fluctuating potentials were observed for each sample, and significant differences in the luminescence resulting from the two Ga-profiles were identified. For the CIGS absorber with a notch Ga-profile, two tail-impurity radiative transitions involving equivalent donor clusters and the same deep acceptor level were ascribed to the CIGS/CdS interface region and to the notch region. The probability of radiative recombination in these 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 V Cu defect were ascribed to the CIGS/CdS interface region. Our results show that the dominant acceptor defects are dependent on the Ga-profile, and they also highlight the complexity of the radiative and nonradiative recombination channels revealed by the tight control of the parameters in the experiment.

Front passivation of Cu(In,Ga)Se2 solar cells using Al2O3: Culprits and benefits

“…9 a). The energy range and shape of the luminescence are close to observations in previous studies for Cu-poor CIGS samples with a linear Ga-profile, despite the front layer used over the CIGS [48,80]. The PL spectra measured with 16 mW are dominated by a radiative transition at approximately 1.04 -1.05 eV and a transition at approximately 1.00 eV with a lower relative intensity.…”

“…In this study, the CIGS layer has a linear Ga profile, which is commonly associated with reducing rear interface recombination, allowing for a higher impact of the front passivation layer in the recombination mechanisms [48]. The CIGS thickness is 2.0 μm with [Cu]/([Ga] + [In]) = 0.92 ± 0.01 (CGI) and [Ga]/([Ga] + [In]) = 0.41 ± 0.02 (GGI) as measured by X-ray fluorescence.…”

High‐Performance and Industrially Viable Nanostructured SiOx Layers for Interface Passivation in Thin Film Solar Cells

“…Therefore, the lower V oc deficit of the passivated devices compared with the Ref device cannot be fully explained by its N cv value, highlighting that the lower V oc deficit is likely due to the passivation effects. In Figure 4, it is not considered the V oc losses in our devices that are related to 1) potential fluctuations; [ 84-89 ] 2) the used Ga profile; [ 52,53,90-92 ] 3) the lack of PDT; [ 93-98 ] and 4) the lack of a front passivation layer, [ 70,99-102 ] to name but a few potential V oc losses. However, these losses should be roughly the same for the three devices.…”

Encapsulation of Nanostructures in a Dielectric Matrix Providing Optical Enhancement in Ultrathin Solar Cells

“…Other than light trapping, other strategies can be implemented to improve even more the J sc value, such as an improvement of the rear passivation, the addition of front passivation, or even the use of Ga‐graded CIGS absorbers. [ 49–52 ] The verified optical enhancement was attributed mainly to two factors: 1) an improvement of the solar cell's anti‐reflection properties and 2) the enhanced rear scattering. However, the optical improvement was also accompanied by a drop of the device FF and V oc .…”