Transparent Ohmic Contact for CIGS Solar Cells Based on p-Type Aluminum Copper Sulfide Material Synthesized by Atomic Layer Deposition

A p-type transparent conducting material was developed to be used as transparent back-contact for the next generation of Cu(In,Ga)Se₂ (CIGS) based solar cells. Cu-S:Al-S thin films were synthesized by atomic layer deposition at temperature T_dep = 150 °C, by combining Cu_xS and Al₂S₃ cycles obtained from copper acetylacetonate (Cu(acac)₂), trimethylaluminum (TMA), and hydrogen sulfide (H₂S). Thin films were characterized notably by X-ray diffraction, scanning electron microscopy, and UV-vis spectrophotometry. This synthesis strategy allowed a tuning of the film compositions and optoelectronic properties. Films have a high oxygen content, and their growth mechanisms as well as their fine chemistries were explored by X-ray photoelectron spectroscopy chemical profiling studies. The layers appeared as intermixed copper sulfide and aluminum oxide networks with some Cu-O and Al-S chemical bonds. Finally, best candidates were successfully applied as transparent ohmic contacts with CIGS with contact resistance between 0.1 and 0.7 ω ·cm^-2.