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

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 Tsub,max = 500 °C are characterized by a higher Voc 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 Tsub,max is identified as detrimental on the PV performance when high Na concentration is added, mainly due to a reduced Voc. 12.2% of efficiency (active area) is achieved reducing the Na concentration for the lower Tsub,max. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)