Characterisation of Cu(In,Ga)Se-2-based thin film solar cells on polyimide

Thin films of Cu(In,Ga)Se2 (CIGS) were deposited at temperatures below 450 °C on polyimide (PI) substrates coated with Mo in a roll-to-roll set up by a combination of co-evaporation and ion-beam techniques. Flexible solar cells ITO/i-ZnO/CdS/CIGS/Mo/PI with and without Na incorporation were then fabricated. The films and solar cells were examined by: X-ray fluorescence spectroscopy (XRF) and Auger electron spectroscopy (AES), to determine the elemental composition, as well as by X-ray diffraction for structure- and scanning electron microscopy (SEM) for morphology-analysis. Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 78 K were also used to estimate the band-gap energy of CIGS, examine the electronic properties and defect nature. The aim of this study was to correlate the incorporation of Na with optical and structural parameters of the CIGS layers as well as with the solar cell performance.