Brief Introduction To Molybdenum Electrode

Molybdenum (Mo) is commonly used as back contacts for Cu(In,Ga)Se2(CIGS) thin film solar cells because it has the good electrical properties of an ohmic contactand is an inert, mechanically durable substrate duringthe absorber film growth. In general, Mo is deposited bydc sputtering onto soda-lime glass substrates. The elec-trical and the mechanical properties of sputtered refrac-tory metal films are known to vary with the working gaspressure . 

CIGS thin film growth occurs at tempera-tures > 500  and at these temperatures, the thermallyinduced extrinsic stresses in the glass-Mo structure maycause bending or mechanical distortion of substrate .During the formation of CIGS films, Na ions diffuse fromthe soda-lime glass substrate through the Mo back con-tact into the absorber layer. The diffusion of Na into theabsorber film depends on the deposition conditions of theMo back contact .

Molybdenum Electrodes

Molybdenum films were deposited on soda-lime glass substrates by using DC magnetron sputtering. Their structural and morphological properties and their electrical resistivities were analyzed with respect to the working gas (Ar) pressure. The electrical resistivity of the Mo films monotonically increased as the gas pressure was increased. The films showed a (110) preferred orientation,regardless of the working gas pressure. The surface morphology of the films changed from an elon-gated grain structure at lower working pressures to a porous nodular shape at higher pressures.The surface roughness of the films increased with the working gas pressure. Secondary ion mass spectrometry data showed that Na diffusion from the soda lime glass substrate through the Moback contact was high, for the film sputtered at 5 mTorr. Cu(InGa)Se2 solar cells fabricated on Mofilms sputtered at 5 mTorr showed the best conversion efficiencies.