Structural and chemical investigations of CBD- and PVD-CdS buffer layers and interfaces in Cu(In,Ga)Se2-based thin film solar cells

It is known that high-efficiency thin film solar cells based on Cu(In,Ga)Se2 (CIGS) can be obtained using CdS buffer layers grown by chemical bath deposition (CBD). The highest efficiencies achieved with CdS buffer layers produced by physical vapor deposition (PVD) are significantly lower. To find reasons for this difference, structural and chemical properties of CBD- and PVD-CdS buffer layers and their interfaces with CIGS were investigated by means of bright-field (BF-TEM), high-resolution (HR-TEM) and energy-filtered transmission electron microscopy (EF-TEM), and also by energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). PVD-CdS grains were shown to be clearly larger than the CBD-CdS grains. Also, a large defect density was detected at the PVD-CdS/CIGS interface, which is attributed to the larger lattice mismatch than at the CBD-CdS/CIGS interface. Cu diffusion from CIGS into CdS was found for the CBD- and the PVD-CdS sample. The PVD-CdS/CIGS interface turned out to be quite abrupt, whereas the CBD-CdS/CIGS interface is rather diffuse. The differences in efficiencies of solar cells with CBD- and PVD-CdS buffer layers can partly be explained by referring to the higher defect density and the probable absence of an inversion of the near-interface region from p- to n-type at the PVD-CdS/CIGS interface.