Comparative study of optimised molybdenum back-contact deposition with different barriers (Ti, ZnO) on stainless steel substrate for flexible solar cell application

Abstract

In this study, we optimised the molybdenum (Mo) back-contact layer for solar cell applications on stainless steel substrates using direct-current (dc) sputtering with varying sputtering powers (100 W to 500 W) and pressures (5 mTorr to 20 mTorr). We comparatively analysed the effectiveness of titanium (Ti) layer deposited using e-beam evaporation deposition and zinc oxide (ZnO) layer deposited using radio-frequency (RF) sputtering for barrier application with Mo. Structural characterisation of the optimised Mo films was carried out using XRD studies confirmed the (110) plane corresponding to the body-centred cubic (bcc) structure. Estimated Mo film parameters for films deposited on barrier layers were compared against films deposited on SS substrate without any barriers as these properties influence the prospective diffusion of Fe and Cr into the absorber layer. Surface characterisation of the deposited films was carried out using a scanning electron microscopy (SEM) to study the morphology of films, and energy-dispersive X-ray (EDX) to identify elemental presence to confirm the blockage of the impurities atoms through the film. Secondary ion mass spectroscopy (SIMS) was employed to study the depth profiles of films while atomic force microscopy (AFM) was used to characterise the topographical properties from the sputtered Mo film and analyse the grain properties of the films. A low resistivity value of 0.511 × 10–6 Ω m for Mo films on the reference glass substrate and 0.625 × 10–6 Ω m for the Mo film on ZnO barrier was measured using the four-point probe. We observed a further 40% reduction in impurities using annealed ZnO barrier combined with an optimised Mo layer.