 Advances in Technology Innovation, vol. 2, no. 3, 2016, pp. 95 - 98 95 Zinc Sulfide Buffer Layer for CIGS Solar Cells Prepared by Chemical Bath Deposition Rui-Wei You, Yen-Pei Fu* Department of Materials Science and Engineering, National Dong-Hwa University, Hualien, Taiwan Received 21 March 2016; received in revised form 28 May 2016; accepted 02 June 2016 Abstract In this study, ZnS thin films were success- fully synthesized by chemical bath deposition (CBD) with starting materials of NH2-NH2, SC(NH2)2, and ZnSO4‧7H2O. ZnS thin films were deposited with different time on glass substrates by CBD at 80 o C and pH=9. Based on X-ray diffraction (XRD) patterns, it is found that the ZnS thin films exh ibit cubic polycrystalline phase. It was found that the optimum deposition time is 90 min for preparing ZnS thin film that is suitable as buffer layer for CuIn1-xGaxSe2 solar cells. The thin film deposited for 90 min has high transmittance up to 80% in the spectra range from 350 nm to 800 nm, and the optical band gap is about 3.59 eV. Keywords : ZnS, buffer layer, chemical bath deposition, thin film solar cells , optical property 1. Introduction Zinc sulfide is a wide-band-gap semi- con- ductor with a range of potential applications in optoelectronic devices. Generally, in thin film so lar cells based on CuInS2, CuInSe2 , Cu(In,Ga)Se2, Cu(In,Ga)(SSe)2, (CIGSSe), the buffer layer is mainly the II-VI type semiconductor, such as cadmium sulfide (CdS) and zinc sulfide (ZnS) thin film [1]. The buffer layer with II-VI type semiconductor is a direct gap semiconductor. The band gap of cadmium sulfide is 2.26~2.5 eV, and zinc sulfide is larger than 3.5 eV [2]. The highest efficiency is up to 19.9%, if the CuIn1-xGaxSe2 thin film solar cell combined with cadmium sulfide buffer layer at present [3]. For fear of cadmium (Cd) toxicity damages our en- vironment, we have to choose a free-cadmium process for preparation of buffer layer. We utilize zinc sulfide as buffer layer and the efficiency of CuIn1-xGaxSe2 thin film solar cell with zinc sul- fide buffer layer is up to 18.6% [4]. Consequently, there is no toxicity in the use of zinc sulfide and that can lower and lighten the influence on the environment. The buffer layer located between ZnO win- dows layer (n-type semiconductor) and CuIn1 -xGaxSe2 absorp t ion layer (p -type semiconductor) is able to eliminate the band discontinuity. The ZnS buffer layer requires high optical transmission and allows photon to reach absorption layer to excite electron, and then electron-hole pairs (EHP) are generated. If the buffer layer is too thick, photoelectron can’t pass through the layer and reach to electrodes. On the contrary, the layer is too thin that couldn’t separate absorption layer and transition conduction electric layer (TCO). For better performance, the thickness of thin film should be controlled in the range of 30-50 nm. Chemical bath deposition (CBD) method has been used for many years to prepare ZnS large-area and uniform thin film, and it can be prepared under room temperature [5]. Utilizing the CBD method produces nano-structure thin film of zinc sulfide with smooth surface and uniform composition, and it improves the effi- ciency of CuIn1-xGaxSe2 thin film solar cell. In this study, we attempt to prepare the uniform zinc sulfide thin film by CBD technique using NH3OH, NH2-NH2, SC(NH2)2, and ZnSO4•7H2O as s tart ing materials , and invest igate its characterizations such as structural, compositional and optical properties. 2. Method In this study, ZnS thin film buffer layer prepared by chemical bath deposition process . Zinc su lfate (ZnSO4 •7H2 O) and th iourea (SC (NH2)2) were used as the source of zinc ions and sulfide ions, respectively. The reaction s o- * Corresponding author, Email: ypfu@mail.ndhu.edu.tw Advances in Technology Innovation, vol. 2, no. 3, 2016, pp. 95 - 98 96 Copyright © TAETI lu t ion was obtained by mixing 0.1M ZnSO4•7H2O, 0.3M SC(NH2)2 , and 1.5M N2H4, whereby hydrazine (N2H4) was used as a com- plex agent. The pH of the reaction solution was adjusted to 9 by ammonia (NH3OH), reaction solution temperature was controlled at 80 o C and the rotational speed of stirrer was controlled in 50 rpm. Soda-lime g lass substrates were used as substrates for the deposition of ZnS films. Be- fore deposition, the substrates were ultrasoni- cally cleaned with acetone, rinsed with deion- ized water and dried in air. To investigate the effect of deposition time on properties of ZnS, the substrates were collected every 30 minutes in which the deposition-time is set in the range of 30 to 180 min. Then these samples were cleaned by deionized water and dried with a N2 gas stream. In order to obtain crystalline ZnS, these as-deposited ZnS specimens required a post-annealing in a tube furnace for l h under Ar atmosphere at 300 o C. The average roughness of the ZnS films was investigated by surface profile measuring sys- tem (alpha-step, Veeco Dektak 3 ST). The co m- position of ZnS thin films was analyzed by en- ergy dispersive spectrometer (EDS, Horiba NORAN instrument). The crystalline phase of the annealed ZnS films were characterized by X-ray diffractometer (XRD, Rigaku D/ MAX-2500 V) with a wavelength of 1.5406Å from the CuKα radiation with 2θ ranging from 20 o to 60 o . Op- t ical p ropert ies o f th in films were charac- terized by an UV-vis spectrometer (Jasco V-650 spectrophotometer). The band gaps (Eg) of ZnS films were determined by the relat ionship of the transmittance and thickness of the film. 3. Results and Discussion Fig. 1 shows X-rays diffraction of the zinc sulfide thin film annealed at 300 o C for 1 h under argon atmosphere. ZnS exists with two struc- tures, one is cubic with zinc-blende type the other is hexagonal with wurtzite type. ZnS thin films prepared via the chemical bath deposited are highly disordered, but it can be transformed into a wurtzite-2H phase by annealing [6]. Göde et al. reported that an amorphous ZnS film ob- tained at bath temperature of 60–70 o C and a wurtzite-2H phase acquired at 80 o C [7]. In this study, ZnS thin film prepared by CBD is zinc-blend type with cubic structure being in correspondence with JCPDS card no. 79-043. The XRD pattern reveals a wide diffraction peak from 25 o to 30 o . There are three d iffraction peaks corresponding to (111), (220) and (311); how- ever, the diffract ion peaks in (200) and (311) are not clear indicating the zinc sulfide thin film with low crystallization. Fig. 1 X-rays diffraction pattern of ZnS thin film at deposition time of 90 min, then an- nealed at 300 o C for 1h under Ar atmos- phere To understand the effect of the deposition time on the composition of films, the energy dispersive spectrometer (EDS) was used to an- alyze the atomic rat io of Zn/S listed in Table 1. It is found that the ratio of Zn/S is close to 1 for films with various deposition times. Table 1 The composition ratio of Zn/S at different deposited time Composi- tion Deposition time (min) 30 60 90 120 150 180 Zn (Atom %) 50.4 51.9 50.3 51.2 50.8 49.8 S (Atom %) 49.6 48.1 49.7 48.8 49.2 50.2 Fig. 2 The Thickness of ZnS films as a function of deposition time, after annealing at 300 o C for 1 h under Ar atmosphere Advances in Technology Innovation, vol. 2, no. 3, 2016, pp. 95 - 98 97 Copyright © TAETI Fig. 2 shows the ZnS film thickness as a function of deposition time. Apparently, the relation between thickness and deposition time is divided into two stages, the first stage is the linear growth for thin film and the thickness grows from 124 nm to 252 nm in the period of 30 ~ 120 min. However, the second stage is the exponential growth; the thickness significantly increases from 352 nm to 750 nm for deposition time from 120 to 180 min. At the second stage, the ZnS film undergoes homogeneous nuclea- tion. As deposition time increasing above 150 min, homogeneous particles begin to deposit on the substrate leading the significant enhance- ment in g rowth rate of thin film. Tab le 2 is ZnS thin film thickness and roughness as function of deposition time. The average roughness of film for deposition time of 30 min is significantly high due to the facts that the heterogeneous deposition on substrate is still not uniform at the initia l stage. As deposition time from 30 to 120 min, the roughness is relatively lower and the thin films become uniform gradually. When deposition time for 180 min, the rate of homo- geneous deposition on substrate increased, and the average roughness of ZnS film is increased up to 94 nm. Table 2 The thickness and average roughness for ZnS films with different deposition time Deposition time (min) 30 60 90 120 150 180 Thickness (nm) 124 160 249 252 358 750 Average roughness (nm) 87 30 58 34 32 94 Fig. 3 shows the optical transmittance in the wavelength range of 300 - 800 nm for the ZnS films deposited on the sodium glass substrates as a function of different deposition time. The ZnS films reveled h igh optical trans mittance in the range of 70 ~ 80% at visib le wavelength; therefore, the films are suitable as buffer layers in CIGS-based solar cells. The h ighest trans- mittance of 85% is located about wavelength of 425 nm for deposition time of 90 min. As depo- sition time increased from 90 to 150 nm, the absorption edge shifted gradually from 400 nm to 500 nm. The sharp absorption feature is due to the uniform ZnS thin films and the low concen- tration of defects in the films. However, as the deposition time increased, th ickness became th icker, and more homogeneous particles deposited on the glass substrate leading the optical transmittance lower than 80% for films that deposition time is above 90 min. Based on optical results, the film deposited for 90 min exhibited good optical propert ies, and the shortest wavelength of adsorption edge, which could make CIGS solar cell with a higher short circuit density (Vsc) [8]. Fig. 3 Transmission spectra of ZnS thin films at different deposition time on g lass sub- strates. Bath conditions: [ZnSO4] =0.1M, [SC(NH4)]=0.3M, [NH2-NH2]=1.5M, pH=9 The optical band gap of ZnS films could be obtained using the Tauc relationship revealed as follows [9]. αhν= A(hν-Eg) n (1) where A is a constant, h is Planck’s constant, v is the photon frequency, Eg is the optical band gap energy, and n is 1/2, respectively. The band gap value was determined from the intercept of the straight-line portion o f the (αhν) 2 against the graph on the hν-axis. The band gap values of the ZnS thin films prepared by CBD with various deposition time are revealed in Table 3. The band gap values are greater than 3.5 eV for the films deposited from 30 to 120 min. Higher band gap values could match well with CIGS solar cell, and the solar cells gain better quantum efficiency [10]. Table 3 The band gap for ZnS thin films with various deposition time Deposition time (min) 30 60 90 120 150 180 Band gap (eV) 2.57 3.53 3.59 3.54 3.45 3.30 Advances in Technology Innovation, vol. 2, no. 3, 2016, pp. 95 - 98 98 Copyright © TAETI 4. Conclusions In this study, the ZnS thin films with sphal- erite structure prepared by chemical bath depo- sition using zinc sulfate, thiourea, and complex of hydrazine as starting materials. The thickness of ZnS thin film varies with deposition time from 124 nm of 30 min to 750 nm of 180 min. The ZnS films reveled h igh optical transmittance in the range of 70 ~ 80% at visible wavelength. The band gap values for ZnS films are in the range of 2.57 ~ 3.61 eV. Based on the results, the following bath conditions, [ZnSO4]=0.1M, [SC(NH4)]=0.3M, [NH2-NH2] [ =1.5M, pH=9 and T=80 o C deposited for 90 min, revealed the optimum ZnS film properties, which is suitable as buffer layer for CuIn1-xGaxSe2 solar cells, and the properties of the film are described as follows. (1). XRD pattern reveals a cubic zinc b lend structure with the typical composition rat io of Zn/S = 50.3:49.7, which is very close to the stoichiometry of the ZnS compound. (2). The highest transmittance of 85% is located about wavelength of 425 nm and the optical band gap is about 3.59 eV. References [1] A. Wei, J. Liu, M. Zhuang, and Y. Zhao, “Preparation and characterizat ion of ZnS thin films prepared by chemical bath depo- sition,” Materials Science in Semiconductor Processing, vol. 16, pp. 1478-1484, De- cember 2013. [2] B. G. Streetman and S. K. Banerjee, “Solid state electron devices,” 6th edition, Pearson Prentice Hall, New Jersey, pp. 158-208, 2006. [3] I. Repins, M. A. 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