Probing the Interplay between Mo Back Contact Layer Deposition Condition and MoSe₂ Layer Formation at the CIGSe/Mo Hetero-Interface

The effect of Mo thin film deposition power in DC sputtering on the formation of a MoSe₂ interfacial layer grown via the annealing of CIGSe/Mo precursors in an Se-free atmosphere was investigated. A Mo layer was deposited on glass substrates using the DC magnetron sputtering method. Its electrical resistivity, as well as its morphological, structural, and adhesion characteristics, were analyzed regarding the deposition power. In the case of thinner films of about 300 nm deposited at 80 W, smaller grains and a lower volume percentage of grain boundaries were found, compared to 510 nm thick film with larger agglomerates obtained at 140 W DC power. By increasing the deposition power, in contrast, the conductivity of the Mo film significantly improved with lowest sheet resistance of 0.353 Ω/square for the sample deposited at 140 W. Both structural and Raman spectroscopy outputs confirmed the pronounced formation of MoSe₂, resulting from Mo films with predominant (110) orientated planes. Sputtered Mo films deposited at 140 W power improved Mo crystals and the growth of MoSe₂ layers with a preferential (103) orientation upon the Se-free annealing. With a more porous Mo surface structure for the sample deposited at higher power, a larger contact area developed between the Mo films and the Se compound was found from the CIGSe film deposited on top of the Mo, favoring the formation of MoSe₂. The CIGSe/Mo hetero-contact, including the MoSe₂ layer with controlled thickness, is not Schottky-type, but a favourable ohmic-type, as evaluated by the dark I-V measurement at room temperature (RT). These findings support the significance of regulating the thickness of the unintentional MoSe₂ layer growth, which is attainable by controlling the Mo deposition power. Furthermore, while the adhesion between the CIGSe absorber layer and the Mo remains intact, the resistance of final devices with the Ni/CIGSe/Mo structure was found to be directly linked to the MoSe₂ thickness. Consequently, it addresses the importance of MoSe₂ structural properties for improved CIGSe solar cell performance and stability.