Improvement in Processing CdTe/CdS Thin Film Solar Cells

In order to decrease the thickness of the close-spaced sublimation (CSS) deposited CdTe film, a novelapproach has been used. On top of the CSS-deposited CdTe film, whose thickness was reduced from 6-8 μm to3-4 μm, another CdTe layer was deposited by RF sputtering, with a thickness of 100-200 nm. The purpose of thisapproach was that of filling up the voids, which form when a low thickness CdTe film is deposited by CSS.Surprisingly we observed that solar cells prepared with this CdTe bi-layer exhibit efficiencies higher than 14%. Sincethe sputtered CdTe layer is n-type, while the CSS-deposited one is p-type, we thought to explain these results bysupposing the formation of local p-n junctions at the grain boundaries, which repel the minority carriers in the grains,increasing their lifetime. This approach permitted to reduce the thickness of CdTe films without affecting the cellefficiency. A further improvement in processing CdTe/CdS solar cells has been obtained by making the CdTetreatment in presence of Chlorine with the use of Ar containing 4% HCl mixed with a gas containing Fluorine, suchas CHF3 or C2H2F4.