Applied Science and Convergence Technology 2019; 28(6): 229-233
Published online November 30, 2019
Copyright © The Korean Vacuum Society.
Department of Energy Convergence, Cheongju University, Cheongju 28403, Republic of Korea
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We investigate the effect of hydrazine on the growth and physical characteristics of ZnS thin films deposited on both soda–lime glass (SLG) and copper indium gallium selenide (CIGS)/Mo/SLG substrates as a function of the relative molar ratio of hydrazine to ammonia reagent,
Keywords: ZnS film, Hydrazine, Chemical bath deposition, Soda-lime glass, CIGS solar cell
Chemical bath deposition (CBD) is currently the most commonly used simple, low-cost approach for fabricating large-area semiconductor thin films for application to solar cells . In particular, CdS and ZnS thin films form the most commonly used n-type buffer layers in the fabrication of copper indium gallium selenide (CIGS) [2,3], Cu2ZnSnS4 , and CdTe  solar-cell devices. Unlike physical vapor deposition methods such as sputtering and e-beam evaporation, CBD involves the deposition of layers from an alkaline mixture solution of metal ions and a chalcogenide source with an appropriate complexing agent. Thus, it is crucial to optimize CBD parameters such as the precursor ratio, pH [6,7], and bath temperature  for preparing high-quality thin films.
In the CBD process, the relation between the complexing agent and the film growth rate depends on whether the reaction path chosen is heterogeneous or homogeneous precipitation route . CBD-ZnS thin films are preferred to CBD-CdS thin films because of the non-toxicity of the CBD-ZnS process and the improved light transmission of the resulting films in the wavelength range of 300–500 nm . However, CBD-ZnS films require a longer deposition time, which is a drawback when compared with CBD-CdS films. This increased deposition time can be attributed to hydroxyl adulteration and poor control of the complexing agent with regard to the release of metal ions in the solution . In this context, the use of thioacetamide (TAA) with ammonia in association with nitrilotriacetic acid trisodium salt (Na3NTA) has been reported to improve the CBD-ZnS growth rate .
Meanwhile, it has been reported that the CBD-ZnS thin-film quality can be improved when a second ligand (hydrazine, triethanolamine (TEA), ethanolamine) is present [9,12,13]. Among these ligands, hydrazine (N2H4), as a potential reagent during ZnS deposition, can act as a bridging ligand to reduce the free-metal-ion concentration in the solution. This in turn can increase the deposition rate of ZnS along with the formation of a smooth and homogeneous layer on the substrates [14,15]. In this regard, previous studies have reported that the use of hydrazine in the CBD process can enhance the growth rate of ZnS thin films by a factor of nearly 6 .
Against this background, here we study and compare the effects of hydrazine on the growth and structural characteristics of ZnS thin films deposited on both soda–lime glass (SLG) and CIGS/Mo/SLG substrates as functions of the relative ratio of hydrazine to ammonia and the deposition time of the CBD process. Furthermore, the ZnS films fabricated with and without hydrazine are compared for solarcell performance via the fabrication of CIGS solar cells.
We used the standard CBD method  to prepare ZnS films deposited both on SLG and CIGS/Mo/SLG substrates with dimensions of 25 × 25 × 1.1 mm. Prior to deposition, the SLG substrates were sequentially ultrasonically cleaned with acetone, ethanol, and deionized (DI) water for 10 min followed by drying with N2 gas. The metal ion and sulfur ion precursors used in the reaction bath included 50 ml zinc sulfate heptahydrate (0.019 M) and 50 ml thiourea (0.17 M). The complexing agents employed as the primary and secondary base ligands were ammonia (NH3) and hydrazine (N2H4), respectively. We prepared two sets of samples using 1) only ammonia and 2) hydrazine along with ammonia. We varied the molar concentration ratio,
The crystallographic analysis of the ZnS films was performed with use of high-angle and low-angle (
In the beginning of CBD process, the pH value of the solution bath dropped rapidly to 9.7 from over 10.5 for the first 10 min and it subsequently linearly decreased to 9.0 in the interval of 10 min ≤
Figure 1 shows the in-plane SEM images of the ZnS thin films deposited on SLG substrates at
In contrast, we also note that the oval-shaped grains reduce both in shape and height when prepared in the mixed solution of ammonia and hydrazine (Fig. 1). However, the surface coverage by the grains is significantly improved relative to the case of deposition without hydrazine. In the figure, we note that the ZnS layer prepared with
Optical transmission through the ZnS thin films was measured using a UV-Vis-NIR spectrophotometer. Figure 2 plots the average light transmission (Avg. Tr) evaluated in the wavelength range of λ = 400–1100 nm as a function of the deposition time
Next, we estimated the optical energy bandgap Eg of the ZnS films using the relation (
Figure 4 shows the plots of (a) the ZnS film thickness deposited for
To study the growth characteristics of the ZnS grains on the CIGS/Mo/SLG substrate, we acquired high-resolution SEM images of the sample surfaces. Figure 5 shows SEM surface morphology of ZnS on CIGS/Mo/SLG grown at
Figure 6 depicts the XRD spectra of the ZnS films deposited on the SLG substrate at
Next, to investigate the effectiveness of hydrazine as an additive complex agent in conjunction with ammonia, we fabricated CIGS solar cells with various ZnS buffers, which showed similar optical qualities corresponding to Avg. Tr = 93 %. With the optical quality of samples being maintained as in Fig. 2, we prepared ZnS buffer layers with hydrazine ratios of
We investigated and compared the effect of hydrazine on the growth and physical characteristics of ZnS thin films deposited on both SLG and CIGS/Mo/SLG substrates as a function of the relative molar ratio of hydrazine,
This work was supported by a research grant from Cheongju University (2018.03.01 ~ 2020.02.28.)