posted on 2022-01-06, 20:33authored byT. P. Shalvey, H. Shiel, O. S. Hutter, G. Zoppi, L. Bowen, V. R. Dhanak, J. D. Major
Sodium is a common impurity in CdTe
solar cells, yet there are
relatively few reports investigating its effect on complete device
structures. There is the potential for uncontrolled sodium incorporation,
either from impurities in the CdTe material itself or contaminants
introduced during device processing, which can affect the optoelectronic
properties of CdTe. Therefore, it is important to consider the impact
of sodium incorporation on device performance. In this work, we show
that the deposition of a thin layer of NaF at the back surface of
CdS/CdTe devices prior to metallization is an effective strategy to
form a highly doped back surface and improve contact quality. High
temperature (∼300 °C) annealing is required to effectively
incorporate sodium throughout the device and improve the bulk doping
density; however, this also leads to sodium accumulation in the CdS
layer and the formation of a TeO2 layer at the back surface.
We also find evidence of out-diffusion of sodium from commonly used
TEC glass substrates at typical CdTe processing temperatures, despite
the presence of an alkali diffusion barrier layer. Understanding this
prevalent sodium diffusion in this class of solar cells is vital for
further improvement of CdTe structures.