Research Information System
8th World Conference on Photovoltaic Energy Conversion, Milan, Italy, 26 - 30 September 2022, pp.1
One of the goals of the German joint research project EFFCIS-II is to gain a better
physical and chemical understanding of devices with (Ag,Cu)(In,Ga)Se2 (ACIGS) and widegap
ACIGS absorbers with increased bandgap energy Eg > 1.5 eV as a result of an increased
Ga/(Ga+In) (GGI) ratio. Especially the decrease of the open-circuit voltage (VOC) deficit, the
influence of alkali metal post-deposition treatments (PDT) on performance and properties of
ACIGS and wide-gap ACIGS solar cells, as well as the nature of the ACIGS/buffer interface
are in the focus of the project.
The depth profile of the GGI ratio in ACIGS absorbers is often very shallow compared
to Ag-free Cu(In,Ga)Se2 (CIGS) absorbers, which exhibit pronounced single or double GGI
gradients. This difference was found for ACIGS with a GGI of 0.3 and wide-gap absorbers with
a GGI of 0.8. Furthermore, Ag addition could help to decrease the VOC deficit of corresponding
wide-gap solar cells.
Atom probe tomography reveals the distribution of both, alkali elements and Ag, in the
ACIGS bulk and at grain boundaries on the nanometer scale. For ACIGS absorbers grown with
Ag precursors, Ag segregation was detected at grain boundaries by scanning transmission
electron microscopy combined with energy-dispersive X-ray spectroscopy, whereas ACIGS
deposited by coevaporation exhibits a homogeneous Ag distribution. Electroreflectance showed
an increase of Eg in ACIGS with increasing Ag content, but this effect is superimposed by
changes in the Cu concentration and GGI. For ACIGS with Ag precursors, increasing Ag
incorporation leads to a smaller inhomogeneous broadening of the absorber resonance,
suggesting reduced composition fluctuations on a nanometer scale. Using in-situ energydispersive
X-ray diffraction, the time-dependent formation of crystal phases during ACIGS
growth processes was assessed and compared with those in Ag-free CIGS absorbers.
The combination of soft and hard X-ray photoelectron spectroscopies revealed a change
of the chemical environments of absorber elements, as well as Rb and Na, and a decrease of the
valence band maximum at the ACIGS surface. Ab-initio density functional theory calculations
for ACIGS indicate smaller Ag jump barriers compared with Cu jump barriers.