CdTe solar cells are leading candidate for cost-effective photovoltaics due to near-optimum band gap og 1.4eV, high absorption coefficient, and low cos manufacturability. However, CdTe has a large work function to form a schottky contact with metal electrode instead of ohmic contact. To achieve a high and stable efficiency for the CdS/CdTe solar cells, the formation of ohmic contact to p-type CdTe film is essential. In search of a new ohmic contact material, this thesis proposes $Cu_2Te$ because it can be a Cu source for p+ tunneling ohmic contact and be a metallic electrode with a good lattice match to CdTe.
In this work, CdS films were prepared by a chemical bath deposition (CBD) method on ITO/glass substrate. CdTe films were deposited on CdS/ITO/glass substrate by a close spaced sublimation (CSS) method with screen printed CdTe layer as a source at the source temperature of 650 ℃ and at the substrate temperature of 590 ℃. These films were annealed at 400 ℃ with $CdCl_2$ powder on the CdTe film. A $Cu_2Te$ layer was deposited oc the CdTe film by evaporating $Cu_2Te$ powder. Then the samples were annealed at 100, 150, 200 and 250 ℃ for 5 min to for p+ ohmic contact. Au was deposited on the $Cu_2Te$ and annealed at 100℃ for secondary electrode.
Transmission electron microscope (TEM) analysis showed that an amorphous layer with a thickness of about 2 nm was formed at the CdTe/$Cu_2Te$ interface. As the $Cu_2Te$ annealing temperature increased, the thickness of amorphous layer was decreased. And the amorphous layer was completely crystallized when the annealing temperature was 200 ℃. X-ray diffraction (XRD) analysis showed that both the hexagonal and orthorhombic $Cu_2Te$ was formed in as - deposited. But more hexagonal $Cu_2Te$ was grown at and above 200 ℃ annealing. From the TEM images and XRD patterns, we think that the orthorhombic $Cu_2Te$ phase near the CdTe interface was transformed into the hexagonal $Cu_2Te$ by the good lattice match between $Cu_2Te$ (0001) plane and CdTe (111) planes.
A good p+ ohmic contact was achieved when the annealing temperature was between 180 ℃ to 200 ℃. The series resistance of the CdS/CdTe cell was reduced to $0.5 Ω·cm^2$. Best cell efficiency of 10.9% was obtained when post annealing temperature was 200℃ for 5 min.
Thermal aging test of the CdS/CdTe cells with carbon back contact showed that the $Cu_2Te$ contact was stable at 50 ℃ in $N_2$ and was slowly degraded at 100 ℃ in $N_2$. In comparison to the conventional carbon contact, the $Cu_2Te$ contact showed a better thermal stability.
