Heterointerfaces TEM characterization of buffer layers in KF treated CIGS solar cells. Towards a new buffer layer?

Abstract

Cu(In,Ga)Se 2 –based (CIGS) solar cells are thin film devices achieving nowadays the highest conversion efficiency (22.3% at the laboratory scale [1]), surpassing any other thin‐film or multi‐crystalline silicon technology. A heterojunction is obtained in these solar cells by depositing a cadmium sulfide (CdS) buffer layer (n‐type semiconductor) on the CIGS absorber layer (p‐type semiconductor). The strong conversion efficiency increase over the past three years is due to the introduction of a potassium fluoride treatment (KF evaporation) between the CIGS coevaporation stage and the CdS chemical bath deposition stage [2]. The understanding of the KF deposition stage on the solar cell performance is still under debate, due at least partly to the lack of material characterization at the nanometer scale. We first present the characterization of two samples with the same CIGS absorber, with and without KF treatment. Transmission electron microscopy (TEM) conducted on cross section lamellae prepared by focused ion beam (FIB) allowed us to evidence several material changes induced by the KF treatment: compared to the untreated sample, the KF‐treated sample shows a depletion of Cu, Ga, and Se and a segregation of K and O at grain boundaries close to the CIGS/CdS interface (figure 1). Without KF the CdS/CIGS interface is abrupt and homogenous in the longitudinal direction. The KF treated sample exhibits a 5 nm‐thick layer containing mainly Cd, S, In, Se and O at the CdS/CIGS interface, as well as few 50 nm large CdSe particles (figure 2). This ultra‐thin interface layer could result in p‐CIGS/n + ‐CdIn 2 (S,Se) 4 type/n‐CdS heterojunction, explaining the possibility to reduce the CdS thickness without loss of performance, as already reported in [2]. To go deeper in the understanding of the benefit of such a layer, two other samples were synthesized: both with the same CIGS layer but either with a CdS or a CdIn 2 S 4 buffer layer. The CdS layer was obtained by chemical bath deposition while the CdIn 2 S 4 layer was deposited by physical vapor deposition (PVD). TEM results concerning the sulfur diffusion in CIGS and the crystalline structure of CdIn 2 S 4 layer will be presented [3].