Absorber and Interface Properties of (Ag,Cu)(In,Ga)Se2 Solar Cells: Results of the EFFCIS-II Project

Witte W., Hariskos D., Maiberg M., Ağca S., Hauschild D., Blankenship M., ...More

8th World Conference on Photovoltaic Energy Conversion, Milan, Italy, 26 - 30 September 2022, pp.1

  • Publication Type: Conference Paper / Summary Text
  • City: Milan
  • Country: Italy
  • Page Numbers: pp.1
  • Ankara Yıldırım Beyazıt University Affiliated: Yes


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.