Role of hydrazine in the enhanced growth of zinc sulfide thin films using chemical bath deposition for Cu(In,Ga)Se2 solar cell application

We investigated the hydrazine effect on the physical characteristics of zinc sulfide (ZnS) thin films on both the soda-lime glass substrate and the Cu(In,Ga)Se-2 (CIGS) absorber layer as a function of its relative content r(c) with respect to ammonia as well as the deposition time t(d) in the chemical bath deposition. As the rc varied from 0 to 1.39, all the deposited ZnS films using hydrazine as a secondary complexing agent showed nearly identical characteristics of amorphous structure with the composition of [S/(S + O)] similar to 0.32 and the direct energy bandgap of E-g = 3.54-3.75 eV. However, they exhibited a strong dependence of light transmission on the longer deposition times td (>= 70 min), due to the enhancement of the areal density of in-plane grains. By applying the ZnS buffer on top of the CIGS, the heterojunction device showed the best solar cell performance with eta=12.03%, V-oc=0.549 V, J(sc)=32.92 mA/cm(2), FF=66.7% from the ZnS buffer grown at r(c)=0.28 and t(d)=30 min. When compared to only ammonia, the relative hydrazine was found to play a key role in the deposition of ZnS to expedite the in-plane grain growth by 6 times accompanied both with a reduction of in-plane grain size and a denser packing density.