Structural and microchemical characterization of Cu(In,Ga)Se2 solar cells with solution-grown CdS, Zn(O,S), and Inx(O,S)y buffers

Abstract

Thin films of Cu(In,Ga)Se2 (CIGS) with a [Ga]/([Ga] + [In]) ratio of about 0.3 were deposited on Mo-coated soda-lime glass substrates by co-evaporation of the elements. In combination with three different solution-grown buffer layers, namely CdS, Zn(O,S), and Inx(O,S)y, and sputtered ZnO/ZnO:Al or Zn0.75Mg0.25O/ZnO:Al double layers as the front contact they were completed to three different types of CIGS-based solar cells. The as-fabricated cell efficiencies are 16.9%, 14.7%, and 14.6%, respectively. The microstructure of all the three devices was analyzed by transmission electron microscopy (TEM). Pores occur at the interface between CIGS absorber and Mo back electrode, and stacking faults and microtwins are observed in the CIGS absorber. Zn(O,S) and Inx(O,S)y buffers are nanocrystalline in contrast to larger grain sizes in the CdS buffer. The structure of the CIGS/CdS interface is frequently coherent, while that of the CIGS/Zn(O,S) and CIGS/Inx(O,S)y interfaces is incoherent. High-resolution TEM imaging demonstrates that CdS buffer occurs in the cubic zincblende and hexagonal wurtzite structure with a high stacking fault density in zincblende CdS. The difference in crystalline quality of the buffers and CIGS/buffer interfaces might contribute to the different efficiencies of the investigated CIGS-based thin-film solar cells. Chemical analyses by energy-dispersive X-ray spectroscopy show interdiffusion and chemical inhomogeneities in Zn(O,S) and Inx(O,S)y buffers.