CdTe has been regarded as an attractive material for a solar cell because of its optimum band gap (1.4eV) and high optical absorption coefficient (over 105 cm-1 in optical range). Recently CdTe solar cells are growing rapidly at photovoltaic markets because of its moderately high module efficiency of 10% and low-cost competitiveness of 1$/Wp. Thin film CdTe/CdS solar cell had recorded the efficiency of 16.5% and module efficiencies exceeded 10%.
However, CdTe solar cells need higher efficiency to further reduce the module cost and increase the market share. There are several challenging points to im-prove the efficiency such as high-transparency window, low-resistance contacts, and low-defect density junctions, etc. Among these points, back contact is an important issue that affects both efficiency and stability. High work function of CdTe(5.9eV) makes it difficult to form ohmic contacts with low contact resistance. General method to make an ohmic contact is to heavily dope the CdTe surface and by adding dopants such as Cu and making the composition Te rich. But, in this case, Cu is fast diffuser in CdTe layer so Cu moves to CdS/CdTe junction. When Cu piled up CdS/CdTe junction, it makes deep level in CdS/CdTe junction which causes carrier recombination in this region.
In this research, ZnTe is suggested as a back contact of CdTe. ZnTe layer make an ohmic contact with CdTe and acts like a back surface field of CdTe solar cells.
When making a p-type ZnTe, Na is used as a dopant rather than Cu. ZnTe layer is deposited with CSS system.
Using $Na_2Te$, $Na_3PO_4$, NaCl powder, ZnTe doped with Na which has the carrier concentration of ZnTe layer as $10^{18}/cm^3$, $10^{17}/cm^3$ and its resistivity as $0.5\omegacm$, $10 \omegacm$ respectively.
It is expected that the ZnTe:Na can be applied to back contact material for CdTe solar cells with low and stable contact resistances.
