Damp-heat-induced degradation of layers in CIGS solar cells
article
Investors require a guarantee of a minimum lifetime for PV installations. It is tempting to provide such a guarantee for a longer lifetime simply by specifying test conditions that are more and more severe. In this paper it is argued that, with a more detailed understanding of the basic mechanisms determining cell material behaviour under specific exposure conditions, not only can the inherent lifetime of solar cells and modules be improved, but also the predictive value and effectiveness of lifetime testing. An overview of the literature contributions regarding the influence of damp-heat exposure of the layers in Cu(In,Ga)Se2 (CIGS) solar cells is presented. The material changes, as well as their potential influence on solar cell and module performance, are described. For the molybdenum back contact, it was observed that damp-heat exposure leads to a decrease in conductivity and
reflectivity, most likely caused by molybdenum oxidation. The presence of a selenized-molybdenum (MoSe2) top layer, in combination with the use of low sputtering pressure, resulted in more stable molybdenum films. For the transparent conductive front contact, a comparison of reports in the literature revealed that indium tin oxide (ITO) films are more stable in damp heat than ZnO:Al films. It was also observed that ZnO:Al films degraded as a result of the ingress of water and CO2 via the grain boundaries, thereby lowering the conductivity of the material. The results of damp-heat studies of CIGS absorbers and buffers were difficult to quantify and varied between cases. In many instances, absorber exposure gave rise to the formation of spots on the surface, as well as to sodium migration, whereas buffer exposure often led to interdiffusion and reactions with the absorber and front contact. Literature reports also demonstrated that the Mo/ZnO stack in the P2 scribe is a vulnerable location in CIGS modules.
reflectivity, most likely caused by molybdenum oxidation. The presence of a selenized-molybdenum (MoSe2) top layer, in combination with the use of low sputtering pressure, resulted in more stable molybdenum films. For the transparent conductive front contact, a comparison of reports in the literature revealed that indium tin oxide (ITO) films are more stable in damp heat than ZnO:Al films. It was also observed that ZnO:Al films degraded as a result of the ingress of water and CO2 via the grain boundaries, thereby lowering the conductivity of the material. The results of damp-heat studies of CIGS absorbers and buffers were difficult to quantify and varied between cases. In many instances, absorber exposure gave rise to the formation of spots on the surface, as well as to sodium migration, whereas buffer exposure often led to interdiffusion and reactions with the absorber and front contact. Literature reports also demonstrated that the Mo/ZnO stack in the P2 scribe is a vulnerable location in CIGS modules.
Topics
TNO Identifier
544744
ISSN
1757-1197
Source
Photovoltais International, 31(1), pp. 79-85.
Publisher
Photovoltaics International
Place of publication
Piscataway
Pages
79-85
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