Diffusion barrier properties of molybdenum back contacts for Cu(In,Ga)Se2 solar cells on stainless steel foils

Abstract

Flexible Cu(In,Ga)Se2 (CIGS) solar cells on stainless steel foils face the problem of efficiency deterioration when iron impurities diffuse into the absorber layer. The influence of the magnetron sputtering conditions and the design of Mo-based back contacts on the property of the diffusion barrier against iron is reported here for high efficiency CIGS solar cells grown at low substrate temperatures (Tmax = 475 °C). The overall material density of the Mo back contact was identified as the dominant parameter for the impurity diffusion barrier performance. It was found that this is also true for Mo bilayer contacts, which show enhanced film densities at low residual stress. The iron diffusion profile in the back contact and CIGS was measured by secondary ion mass spectroscopy, where a linear decrease in the iron impurity concentration in the CIGS towards the CdS buffer layer was found. Furthermore, this iron distribution in CIGS and its consequences on the solar cell efficiency is discussed, supported by defect analysis measurements and photovoltaic device simulations. With a stress-free ∼500 nm thick Mo bilayer back contact, best solar cell efficiencies above 15% were achieved with antireflection coating. © 2013 American Institute of Physics.