The effect of NaF precursors on low temperature growth of CIGS thin film solar cells on polyimide substrates

Abstract The aim of this work is to study the effect of Na on low temperature growth of CIGS solar cells on polyimide (PI) substrates. The Na is introduced via evaporation of a NaF precursor layer prior to the absorber deposition. As reported previously, the devices at nominal T_sub,max = 500 °C are characterized by a higher V_oc when the Na content increases. However, lowering process temperature to nominally 450 °C together with high Na concentration produces a pronounced band gap grading through the absorber layer, as detected by energy dispersive X‐ray spectroscopy (EDX). The absorber layers and the devices were studied by grazing incidence X‐ray diffraction, external quantum efficiency and drive level capacitance profiling. A limit T_sub,max is identified as detrimental on the PV performance when high Na concentration is added, mainly due to a reduced V_oc. 12.2% of efficiency (active area) is achieved reducing the Na concentration for the lower T_sub,max. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)