Correlation between preparation parameters and properties of molybdenum back contact layer for CIGS thin film solar cell

Molybdenum (Mo) thin film back contact layers for thin film CuIn
_(1-x)
Ga
_x
Se
₂
(CIGS) solar cells were deposited onto soda lime glass substrates using a direct current (DC) planar magnetron sputtering deposition technique. Requirements for the Mo thin film as a back contact layer for CIGS solar cells are various. Sheet resistance, contact resistance to the CIGS absorber, optical reflectance, surface morphology, and adhesion to the glass substrate are the most important properties that the Mo thin film back contact layer must satisfy. Experiments were carried out under various combinations of sputtering power and working gas pressure, for it is well known that mechanical, optical, morphological, and electrical property of a sputter-deposited Mo thin film are dependent on these process parameters. Sheet resistances were measured using a four-point probe equipment and minimum value of 0.25 Ω/sq was obtained for the 0.6 μm-thick Mo film. Average surface roughnesses of each Mo film as measured by Dektak profilometer ranged from 15 to 26 Å. Minimum resistivity of 11.9 μΩ·cm was obtained with the Mo thin film deposited at 0.1 mTorr and 250 W. A residual stress analysis was conducted with a bending beam technique with very thin glass strips, and maximum tensile stress of 358 MPa was obtained; however, films did not exhibit a compressive stress. Adhesive strengths were examined for all films with a "Scotch-tape" test, and all films showed a good adhesion to the glass substrate. Sputter-deposited Mo thin films are commonly employed as a back contact layer for CIGS and CuInSe
₂
(ClS)-based solar cells; however, there are several difficulties in fabricating a suitable back contact layer. Therefore, it has been a practice to employ multi-layered Mo thin film back contact layers to achieve the properties of good adhesion to the glass substrate and low resistivity simultaneously. However, compared to single layer processes, multi layer processes have a lower throughput and higher fabricating cost, and require more elaborated equipment, which are not desirable from the industrial point of view. Hence experiments were conducted to find out the mechanisms which influence the properties of Mo thin films by changing the two process parameters of working gas pressure and sputtering power individually. The relationships between process parameters and above mentioned properties were studied and explained. It was found that by selecting the process parameters properly, less resistive and highly adhesive single layer Mo thin films with appropriate morphology for CIGS solar cells can be achieved.