MoSe2 formation during fabrication of Cu(In, Ga)Se2 solar cell absorbers by using stacked elemental layer precursor and selenization at high temperatures

Cu(In, Ga)Se2 solar cell absorbers are prepared on a Mo-coated glass substrate by using a sequential process consisting of a sputter deposition of an In/CuGa/In metal precursor, subsequent PVD deposition of a Se layer and annealing in N2 atmosphere. The Se concentration in the final layer stack was found to be relatively up to 20 % higher than expected. The excess Se is bound in a MoSe2 layer with laterally varying thickness, between the absorber and the Mo back contact. Such a layer can lead to an increase in the series resistance of the completed solar cells. By pumping at a specific time, we were able to reduce the Se partial pressure selectively during the selenization. For a constant annealing time, we find that the MoSe2 thickness increases with the time in which a high Se partial pressure is maintained, i.e., the time before the selective pumping. A significant reduction of the Se partial pressure after half the annealing time led to solar cells with the smallest series resistance and overall best conversion efficiency. We further found that the addition of NaF before the annealing led to comparatively thin MoSe2 layers. This suggests that the Na incorporation from the glass substrate in our process is too small to hinder the MoSe2 growth. A more specific control of the Na supply is required in our process to manipulate the MoSe2 growth and the doping density in the absorber.