Life testing and stability of SnO2/n-Si solar cells

SnO
2
/Si solar cells having 10-12% sunlight engineering efficiencies were fabricated by electron beam evaporation of SnO
2
and by spraying SnCl
4
onto heated Si. The cells were then encapsulated in various ways and the stability and degradation mechanisms for these cells were examined using SIMS depth profiling and accelerated weather testing. Preliminary results indicate an initial efficiency decrease followed by a slower rate of degradation which decreased as a function of time. Depending upon the cell environment, the fill factor remained constant or decreased by a few percent. The short circuit current remained constant and the V
oc
decreased independent of the testing conditions of the cell. A decrease of V
oc
can indicate a change in junction parameters, oxide thickness or surface state density. It has been suggested that the device degradation may be attributed to diffusion of the grid metals. SIMS analysis indicated the Cr and Al used in grids on earlier cells penetrated through the entire SnO
2
layer. It is possible that the back contact is forming a junction with the Si and reduces the open circuit voltage of the cell.