Growth kinetics during kesterite coevaporation

Kesterite solar cells have been considered as earth-abundant substitute to chalcopyrites. NREL's 9.2% co-evaporated kesterite solar cell is inspired by the copper-rich growth of co-evaporated chalcopyrites. The excess Cu
_x
Se
_y
is believed to conduct liquid-phase assisted grain growth and hence improves the device performance. The effect of the deposition sequence on film growth, morphology, and device performance, are explored in this study. At high deposition temperature, the expected binary precursors of Cu
₂
ZnSnSe
₄
include Cu
_x
Se
_y
and ZnSe but not SnSe
_x
. Because SnSe
_x
is volatile, the sticking of Sn occurs only if the adsorbed Sn encounters Cu
_x
Se
_y
and ZnSe and the formation of kesterite takes place. Otherwise, SnSe
_x
will be re-evaporated. Here we designed deposition recipes to create precursor films with different ratio between ZnSe/Cu
_x
Se
_y
/as-formed kesterite in the first stage, and end the deposition with the same end-point composition. First, it is of interest if the existence of Cu
_x
Se
_y
phase provides the opportunity of grain growth analogous to co-evaporating CIGS. Second, by observing the evolution of the substrate temperature during deposition, the reaction progression may be better realized. Third, this series of depositions with different sequence also tells us the saturated Zn level relative to Cu and Sn, and where the excess portion of Zn stays in the film. Finally, the device performance above 9% is briefly presented.