Investigation of junction properties of CdS/CdTe solar cells and their correlation to device properties

Secondary-ion mass spectrometry analysis of the CdS/CdTe interface shows that S diffusion in CdTe increases with substrate temperature and CdCl
2
heat treatment. There is also an accumulation of Cl at the interface for CdCl
2
-treated samples. Modulated photo-reflectance studies shows that devices with CdCl
2
heat treatment and open-circuit voltage (V
oc
) of 835 mV have a distinct high electric-field region in the layer with bandgap of 1.45 eV. Electron-beam induced current measurements reveal a one-sided junction for high V
oc
devices. The nature of the junction changes with processing. For heterojunction devices, the depletion region includes the highly defective CdS/CdTe interface, which would increase the recombination current and consequently the dark current, leading to lower V
oc
. In the case of CdCl
2
-treated cells, the n
⁺
-p junction and its high electric-field results in the junction between structurally compatible CdTe and the Te-rich CdSTe alloy, and thus, in higher V
oc
.