 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; accept ed 02 June 2016 Abstract In this study, ZnS thin films were success- fully synthesized by chemica l 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 e xh ibit cubic polycrystalline phase. It was found that the optimu m deposition time is 90 min for preparing ZnS thin film that is suitable as buffer layer for CuIn 1-xGaxSe2 solar cells. The thin film deposited for 90 min has high transmittance up to 80% in the spectra range fro m 350 n m to 800 n m, and the optica l band gap is about 3.59 eV. Ke ywor ds : ZnS, buffer layer, che mical 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 la r c e lls base d on Cu In S2, Cu InSe2 , Cu(In,Ga)Se2, Cu(In,Ga)(SSe)2, (CIGSSe), the buffer layer is mainly the II-VI type semiconductor, such as cadmiu m sulfide (CdS) and zinc su lfide (ZnS) thin film [1]. The buffer layer with II-VI type semiconductor is a direct gap semiconductor. The band gap of cadmiu m sulfide is 2.26~2.5 e V, and zinc sulfide is larger than 3.5 e V [2]. The highest efficiency is up to 19.9%, if the CuIn1-xGaxSe2 thin film solar cell combined with cadmiu m sulfide buffer layer at present [3]. For fear of cadmiu m (Cd) toxicity damages our en- vironment, we have to choose a free-cadmiu m 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 Cu In1 - xGa xSe2 a bso rp t io n lay e r (p -t yp e semiconductor) is able to eliminate the band discontinuity. The ZnS buffer layer requires high optical transmission and allows photon to reach absorption layer to e xc ite electron, and then electron-hole pairs (EHP) a re generated. If the buffer layer is too thic k, 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 thic knes s of thin film should be controlled in the range of 30-50 nm. Che mical 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 te mperature [ 5]. Utilizing the CBD method produces nano-structure thin film of zinc sulfide with smooth surface and uniform co mposition, and it improves the effi- ciency of CuIn1-xGa xSe2 thin film solar ce ll. 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 ta rt in g ma te ria ls , a nd inv est igat e its characterizations such as structural, compositional a n d optical properties. 2. Method In this study, ZnS thin film buffe r layer prepared by chemical bath deposition process . Z in c su lfa te (Z n SO4 •7H2 O) an d th iou re a (SC (NH2)2) were used as the source of zinc ions and sulfide ions, respectively. The reaction s o - * Corresponding aut hor, Email: ypfu@mail.ndhu.edu.tw Advances in Technology Innovation , vol. 2, no. 3, 2016, pp. 95 - 98 96 Copyright © TAETI lu t io n was o bt a in ed b y mi xin g 0.1M ZnSO4•7H2O, 0.3M SC(NH2)2 , and 1.5M N2H4, whereby hydrazine (N2H4) was used as a com- ple x 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 rp m. Soda-lime g lass substrates were used as substrates for the deposition of ZnS films. Be - fore deposition, the substrates were ultrason i- cally c leaned with acetone, rinsed with deio n- ized water and dried in a ir. 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 c leaned by deionized water and dried with a N2 gas stream. In order to obtain crystalline ZnS, t hese 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 De ktak 3 ST ). The co m- position of ZnS thin films was analyzed by e n- 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 wave length of 1.5406Å fro m the Cu Kα radiation with 2θ ranging fro m 20 o to 60 o . Op - t ic a l p ro p e rt ies o f th in fil ms we re 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 e xists with two stru c- tures, one is cubic with zinc-blende type the other is hexagonal with wurtzite type. ZnS thin films prepared via the che mica l bath deposited are highly disordered, but it can be transformed into a wurt zite-2H phase by annealing [ 6]. Göde et al. reported that an a morphous ZnS film o b- 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 fro m 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) a re not clear indicating the zinc sulfide thin film with low crystallization. Fig. 1 X-rays diffraction pattern of ZnS thin film at deposition time o f 90 min, then an- nealed at 300 o C for 1h under Ar at mos- 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 ato mic 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- t ion Deposit ion t ime (min) 30 60 90 120 150 180 Zn (At om %) 50.4 51.9 50.3 51.2 50.8 49.8 S (At om %) 49.6 48.1 49.7 48.8 49.2 50.2 Fig. 2 The Thic kness 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 thic kness grows fro m 124 n m to 252 n m in the period of 30 ~ 120 min. However, the second stage is the e xponential growth; the thickness significantly increases from 352 n m to 750 n m for deposition time fro m 120 to 180 min. At the second stage, the ZnS film undergoes homogeneous nuclea- tion. As deposition time increasing above 150 min, ho mogeneous 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 thic kness 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 depos ition time fro m 30 to 120 min, the roughness is relative ly lower and the thin films become uniform gradually. When deposition time for 180 min, the rate of ho mo- 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 diffe rent deposition time Dep osition 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 n m for the ZnS films deposited on the sodium glass substrates as a function of diffe rent 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 n m for deposition time of 90 min. As depo- sition time increased fro m 90 to 150 n m, the absorption edge shifted gradually fro m 400 n m to 500 n m. The sharp absorption feature is due to the uniform ZnS thin films and the lo w concen- tration of defects in the films. Ho wever, as the deposition time increased, t h ic kn e ss b e ca me t h ic ke r, a n d mo re ho mogeneous particles deposited on the glass substrate leading the optical transmittance lo wer than 80% for films that deposition time is above 90 min. Based on optical results, the film deposited for 90 min e xhibited good optical propert ies , and the shortest wavelength of adsorption edge, which could ma ke CIGS solar ce ll with a higher short circuit density (Vsc) [8]. Fig. 3 T ransmission spectra of ZnS thin films at diffe rent 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 fro m 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 a re revea led in Table 3. The band gap values are greater than 3.5 e V for the films deposited fro m 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 Dep osition 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 chemica l bath depo- sition using zinc sulfate, thiourea, and co mp le x of hydrazine as starting materia ls. The thic kness of ZnS thin film varies with deposition time fro m 124 n m of 30 min to 750 n m of 180 min. The ZnS films reveled h igh optical transmittance in the range of 70 ~ 80% at visible wave length. The band gap values for ZnS films are in the range of 2.57 ~ 3.61 e V. 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 optimu m ZnS film properties, which is suitable as buffe r layer for CuIn1-xGaxSe2 solar ce lls, and the properties of the film are described as follo ws. (1). XRD pattern reveals a cubic zinc b lend structure with the typical co mposition rat io of Zn/S = 50.3:49.7, which is very c lose to the stoichiometry of t he ZnS compound. (2). The high est transmittance of 85% is located about wavelength of 425 n m and the optical band gap is about 3.59 eV. References [1] A. Wei, J. Liu, M. Zhuang, and Y. 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