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In CIGS solar cells, Chemical Bah Deposited (CBD) CdS buffer layer is usually used to obtain high conversion efficiency [1]. ZnS thin film is promising candidate for the Cd-free buffer layer for CIGS cells.
All previous reports on ZnS thin film preparation by CBD method are based on the use of ammonia as a complexing agent for zinc ions [1-3].In present investigation, we report on, for the first time, a new method without using ammonia based on a new complexing agent for zinc ions that eliminates the problems of ammonia volatility and toxicity, besides of the elimination of toxic hazards related to CdS layer.
We prepared ZnS thin films on commercial glass substrates by ammonia-free CBD using thioacetamide and EDTA in the bath solution. The obtained ZnS films were homogenous, hard, specularly reflecting, and had very good adhesion on to the glass substrate. The film thickness can be controlled from 18 to 450 nm either by reaction duration or by multiple depositions.
The effect of film thickness and annealing temperatures, on structure, optical properties, band gap energy, and grain size of nanocrystals were studied. The X-ray diffraction analysis showed a cubic zinc blend structure and a diameter of about 2-5 nm for ZnS nanocrystals depending upon annealing temperature. No diffraction peaks of impurity phases such as ZnO and Zn(OH)2 detected in the XRD patterns. The FT-IR spectrum of ZnS films on quartz substrates revealed no impurity peaks such as Zn-O, Zn-O-H, or –N=C=S.
The ZnS films had more than 70% transmittance in visible region.We observed a red shift in band gap energy with increasing thickness and annealing temperature due to the coarsening of grains, as expected. The direct band gap of as-deposited films ranged from 3.68 to 3.78 eV depending upon their thickness and those for annealed films varied from 3.60 to 3.70 eV depending upon annealing temperature.
Keywords: ZnS, chemical bath deposition, thin film, buffer layer
References:
[1]D.A.Johnston,M.H.Carletto,K.Reddy,I.Forbes,Thin Solid Film 403-404(2002)102
[2]T.Nakada,M.Mitzutani,Y.Hagiwara,AKunioka,Sol.En.Mater.Sol.Cells67(2001)255
[3]T.Ben Nasr,N.Kamoun,C.Guasch, Mater.Chem.Phys 96(2006)84-89
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