See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/257917841 Fabrication and characteristics of fully-sprayed ZnO/CdS/CuInS2 solar cells Article  in  Journal- Korean Physical Society · November 2012 DOI: 10.3938/jkps.61.1494 CITATION READS 50 authors, including: Thai Tran Thanh Luu Lan Anh Quy Nhon University VNU University of Science PUBLICATIONS   4 CITATIONS    PUBLICATIONS   1 CITATION    SEE PROFILE SEE PROFILE Pham Phi Hung Vo Thach Son VNU University of Science VNU University of Science PUBLICATIONS   6 CITATIONS    PUBLICATIONS   4 CITATIONS    SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Influence of surface treatment and annealing temperature on the recombination processes of the Quantum dots Solar cells View project Investigration of GaN/AlGaN/high-k entilities in high electron mobility transistors (HEMT) for high frequency, high power electronic applications View project All content following this page was uploaded by Thai Tran Thanh on 11 May 2018 The user has requested enhancement of the downloaded file Journal of the Korean Physical Society, Vol 61, No 9, November 2012, pp 1494∼1499 Fabrication and Characteristics of Fully-sprayed ZnO/CdS/CuInS2 Solar Cells Tran Thanh Thai,∗ Nguyen Duc Hieu, Luu Thi Lan Anh, Pham Phi Hung and Vo Thach Son School of Engineering Physics, Hanoi University of Science and Technology, Hanoi 84-04, Vietnam Vu Thi Bich Center for Quantum Electronics, Institute of Physics, Vietnam Academy of Science and Technology (VAST), Hanoi 84-04, Vietnam (Received December 2011, in final form 23 June 2012) This paper reports the successful fabrication of glass/ZnO/CdS/CuInS2 solar cells with a superstrate structure deposited using full spray pyrolysis deposition (FSPD) The structure, and the optical and electrical properties of the constituent layers are investigated The CuInS2 (CIS) film deposited from a starting solution with [Cu]/[In] = 1.1 and the Al-doped CuInS2 (CIAS) film deposited from a solution with [Cu]/[In] = 1.0 and [Al]/[In] = 0.12, and using a sulfurization process, is observed to exhibit the best crystallites with tetragonal structures The optical band-gap of the CIAS film is obtained as 1.49 eV Moreover, physical properties of both the ZnO and the CdS thin films are also studied The obtained parameters of the cells are an open-circuit voltage of VOC = 425 mV, a short circuit current density of JSC = 14.02 mA/cm2 , a fill factor of FF = 28.75%, and a conversion efficiency of η = 1.71% The results in our experiment show that FSPD is a potential technique for preparing solar cells based on CIS absorbers in a superstrate structure with low cost and high performance PACS numbers: 81.15.Rs, 79.60.Dp, 85.60.Bt Keywords: Solar cells, CuInS2 , Ultrasonic spray pyrolysis DOI: 10.3938/jkps.61.1494 I INTRODUCTION Cu-chalcopyrite semiconductor materials have emerged for decades as promising candidates for absorber layers in heterojunction solar cells [1–5] Recently, a high conversion efficiency of 20.3% has been achieved with CuInSe2 (CISe) solar cells [3] However, these CISe technologies still have some problems They include the adjustment of stoichiometry, and the use of other elements (Ga, Na, S) at increased production costs in order to achieve high efficiencies [1] Moreover, the presence of selenium and the steps of selenization are also problematic from an environmental view due to the potential toxicity [1,4] For these reasons, today, solar cells based on a CuInS2 (CIS) absorption layer have attracted much attention from research groups [1,2,4,5] CIS has several advantages with respect to other chalcopyrite-based materials First of all, the bandgap of 1.53 eV is ideal for optimal absorption of solar radiation Second, unlike CISe or CIGSe, it does not contain the poisonous element Se Third, CIS as an absorber layer material for thin-film solar cells has the highest theoretical conversion efficiency of 30.5% among Cu-chalcopyrite absorber ∗ E-mail: thaittdhqn@gmail.com; Fax: +84-4-3869-3498 materials [1,2,4,5] In the case of CIS absorber-layer-based substratestructure solar cells, the most successful techniques for absorber preparation have been the multi-sourceevaporation and the two-step (sulfurization of metal precursor films) processes [1,4,5] Cells deposited by using vacuum-based techniques have reached a confirmed total area efficiency of 11.4% [4] However, they were vacuum techniques that had some disadvantages including the high-cost and low-speed [2,5] Meanwhile, non-vacuum techniques such as ion layer gas reaction spray pyrolysis and electro-deposition can generate low-cost, highspeed photovoltaic production [2,4,5] Therefore, nonvacuum techniques have received extensive consideration from research groups concerned with the aim to reduce the production costs Among these methods, spray pyrolysis supports a superstrate structure design, which is used for full-layer-sprayed solar cells (full spray pyrolysis depostion-FSPD) [2,5] However, the efficiency of CIS cells deposited by using FSPD is quite low compared with that of CIS cells prepared by using a vacuum-based method Thus, this deposition method must be developed more in order to enhance the conversion efficiency In this study, we develop ultrasonic spray pyrolysis (USP) deposition of semiconductor thin films to produce solar cell structures The USP technique is chosen be- -1494- Fabrication and Characteristics of Fully-sprayed ZnO/CdS/CuInS2 Solar Cells – Tran Thanh Thai et al cause it is known as a simple, low-cost, unlimited-area method [6] Our experience with these layers shows that with USP, the physical properties of the ZnO, CdS, and CIS films can be controlled in order to achieve high efficiency while making solar cells based on these materials We successfully fabricate glass/ZnO:In/CdS/CuInS2 /metal solar cells with efficiencies of 1.71% by using FSPD as a preliminary step Even though the efficiency is low, we consider this to be the beginning of a challenging work The structure, and the optical and electrical properties of the CIS, ZnO:In and CdS films are also investigated in our work -1495- Fig (Color online) SEM cross-sectional image and photograph of a glass/IZO/CdS/CIS solar cells fabricated by using the FSPD technique Measurement Details II EXPERIMENTS AND DISCUSSION Experimental Details Substrates: We used glass slides (Germany) (20 mm × 10 mm of mm (thickness)) to deposit the constituent films for study their characteristics and deposit the cell structures The substrates were cleaned in a neutral detergent solution, rinsed in de-ionized water and dried prior to depositing the constituent films and cell structures Window layer: In-doped ZnO (IZO) films of 200 – 250 nm in thickness were deposited onto the glass substrates The deposition was performed by using the USP method with Zn(CH3 COO)2 dissolved in deionizer water The concentration of Zn(CH3 COO)2 in the spray solution was 0.2 mol/l Indium was added from InCl3 in a molar ratio [In]/[Zn] = 0.012, and the deposition temperature was 420 ± 0.2 ◦ C The starting solution was atomized at a frequency of 130 KHz by using an ultrasonic nebulizer The solution flow rate was kept at 1.0 ml/min, and nitrogen was used as a carrier gas The no-zzle-substrate distance was maintained at about 12 cm Buffer layer: CdS thin film of 120 – 130 nm in thickness was directly grown on top of the IZO layer and glass substrate by using the USP method at a substrate temperature of 380 ± 0.2 ◦ C The spray solution contained 0.05 M CdCl2 and 0.1 M (NH2 )2 CS solutions The solution flow rate was kept at 0.8 ml/min, and the other deposition parameters were kept constant as the window layer was deposited Absorber layer: CIS and CIAS films were created on the buffer layer and glass substrate CIS films with thicknesses of 0.5 and 1.5 µm and CIAS films with a thickness of 1.5 µm were prepared by using the ultrasonic repeated spray pyrolysis (URSP) technique from an aqueous solution of CuCl2 , InCl3 , (NH2 )2 CS and Al(NO3 )3 , as described in detail in Refs and Figure shows a SEM cross-sectional image and photograph of a glass/IZO/CdS/CIS solar cells fabricated by using the FSPD technique The optical transmission of the films was measured by using a Carry 100, Spectrophotometer in the wavelength range 300 to 800 nm The thicknesses of the films were measured using an alpha-step IQ profilometer The structures of the films were recorded using a Siemens D5005 diffractometer with Cu-Kα radiation (λ = 1.54056A◦ ) The conductivity type and the resistivity of the IZO, CdS, CIS and CIAS films were determined by using Hall measurement system (7600 Series, Lakeshore, USA) The photovoltaic performance of the solar cells was determined by examining the current versus voltage characteristics obtained under simulated AM 1.5 illumination (100 mW/cm2 ) (Keithley 4200-SCS, Saigon Hi-tech Park R&D Center, Vietnam) Discussion Figure shows the XRD patterns of the layers in the cell structures The results correspond to IZO films having a hexagonal wurtzite structure, and the diffraction picks were there for ZnO (JCPDS no 36-1451) as reported in Ref (see in Fig 2(a)) The average crystallite diameter for IZO was about 12 nm, as estimated from the (002) peak In Fig 2(b), the XRD pattern of the CdS layer is presented The CdS films can be seen to exhibit a preferential orientation along the (101) plane, which correspond to the hexagonal phase (JCPDS no 06-0314) [6] The hexagonal CdS film is preferred for solar-cell applications thanks to its excellent stability The average diameters of CdS crystallites were about 15 nm when the CdS thin films were deposited with 120 to 130 nm thicknesses According to the patterns diffraction (Figs 2(c) and 2(d)), the crystalline CIS and CIAS films were characterized by peaks corresponding to (112), (200, 004), (220, 204), and (312, 116) orientations Previously reported in Refs and 8, those main peaks were related to the tetragonal phase (JCPDS no 27-0159) With a CIS film of 2.2 µm thickness, the average diameter of a CIS crystallite was about 95 nm Figure shows plots of transmissions T of the window layer, of the buffer layer and of the absorber layers Figure 3(a) presents transmission spectrum of the IZO thin -1496- Journal of the Korean Physical Society, Vol 61, No 9, November 2012 Fig XRD patterns of (a) IZO, (b) CdS, (c) CIS, and (d) CIAS Fig Plots of (αhν)2 versus hν for calculating the optical band gaps the of the layers (Fig 3) by using the following equation [8,9,11]: α=− ln T , d (1) where d is the film thickness and T is the transmittance It is now clear that the IZO, CdS, CIS, and CIAS films are direct band-gap semiconductors The absorption coefficient is related to the optical band gap (Eg ) according to the equation [8,9,11] (αhν)2 = A(hν − Eg ), Fig Optical transmittance spectra of (a) IZO, (b) CdS, (c) CIS, and (d) CIAS film and shows an average transmission of about 80% in the visible region The result displays that the IZO film had a sharp absorption at about 350 nm For the CdS buffer layer shown in Fig 3(b), the spectrum showed an average transmission of about 78% in the visible region The CdS films clearly strong absorption in the region around 500 nm, which might have a serious effect on the cells performance The optical transmissions of the CIS and the CIAS films are shown in Figs 3(c) and 3(d) The transmissions clearly decrease sharply the near IR region due to the band-gap absorption In order to indicate the band gap, we calculated the absorption coefficient (α) from the transmission data (2) where A is a constant and h is the Planck constant The band-gap of the films is estimated from plots of (αhν)2 vs hν by extrapolating the straight line portion of the plot and finding it intersection with the abscissa Figure exhibits the optical band-gap of layers in cell structures We can see that the IZO thin film had an optical band-gap of about 3.3 eV, the CdS thin film had one of about 2.42 eV, the CIS film had one of about 1.45 eV, and the CIAS film had one of about 1.49 eV The resistivity, carrier concentrations and mobility of the IZO, CdS, CIS, and CIAS films were determined using Hall measurements, and the results are presented in Table The IZO thin film was found to have n-type conductivity and a low resistivity (10−3 Ωcm) The CdS thin film exhibited n-type conductivity Both the CIS and the CIAS films were found to have p-type conductivity and carrier densities of about 6.04 × 1016 – 3.34 × 1017 cm−3 Fabrication and Characteristics of Fully-sprayed ZnO/CdS/CuInS2 Solar Cells – Tran Thanh Thai et al -1497- Table Hall measurement results Sample IZO CdS CIS CIAS Resistivity Mobility (Ω·cm) (cm2 /V·s) 1.2 × 10−3 423.7 19.3 63.8 15.7 41.8 11.3 23.8 Carrier Type Density Conductivity (cm−3 ) 1.92 × 1019 n n 8.23 × 1013 P 3.34 × 1017 p 6.04 × 1016 Table Summary of J-V parameters for the solar cells depicted in Fig Cell S-1 S-2 S-3 VOC (mV) 385 395 257 JSC (mA/cm2 ) 6.24 7.01 1.61 FF (%) 22.12 26.27 20.46 η (%) 0.53 0.74 0.09 RS (Ω) 24.1 20.3 32.2 Rsh (Ω) 72.8 115.3 40.2 Fig (Color online) Characteristic J-V curves of three cells: S-1, S-2, and S-3 Solar cell parameters depend on many variables, each of which needs control to optimize its value The properties of the sprayed CIS absorber layer, the most important part of the solar cell, are mainly adjusted by using the molar ratio [Cu]/[In] in the starting solution Thus, a set of full-sprayed glass/IZO/CdS/CIS/metal solar cells was deposited using various starting solution compositions [Cu]/[In] to grow the absorber layer, keeping other deposition parameters and underlayers invariable In this study, CuInS2 films with [Cu]/[In] ratios at 1.0, 1.1, and 1.2 and with constant [S]/[Cu] ratio (= 5) were deposited on glass/IZO/CdS substrates by using the URSP method The cells were named S-1, S-2, and S-3, respectively Characteristic J-V curves of the three cells are given in Fig 5, and the cell dependences of the outputs on the [Cu]/[In] ratio in solution are presented in Table In addition, RS and Rsh were calculated from the illuminated J-V characteristics in Fig Fig Plot of ln (J) versus V for determining the diode quality factor of solar cell S-2 A comparison of the data in Table shown that an rise in the [Cu]/[In] ratio in the starting solution from 1.0 to 1.1 increased the cells output characteristics Cell S-2 was observed to exhibit the best parameters However, cell S-3 based on the CIS absorber prepared with a [Cu]/[In] ratio of 1.2 exhibited decreased open-circuit voltage (VOC = 257 mV), short-circuit current density (JSC = 1.61 mA/cm2 ) and efficiency (η = 0.09%) As in our previous report in Ref 12, the Cux S phase was present in the CIS film deposited at a [Cu]/[In] ratio of 1.2 Therefore, the presence of a high-conductivity Cux S phase at the CuInS2 /CdS heterojunction is thought to generate shunt paths, leading to reduced open-circuit voltages and short-circuit current densities The diode quality factor calculated from the dark characteristics of the best cell (S-2) by plotting ln (J) versus V was 3.4, as shown in Fig Cell S-1 or S-2 had higher diode quality factors This indicates that as reported in Ref 1, the dominating generation-recombination process might be caused by the interface states In this case, the series resistance was high (RS = 20.3 Ω), and the shunt resistance was low, as indicated by the low fill factor In order to reduce the series resistance, to increase the shunt resistance, and to improve the diode quality factor, we made attempts to fabricate solar cells with bilayer CIS absorber layers CdS/CIS heterojunctions were prepared using a bilayer structure for the CIS The preparation process for the junction was as follows: the IZO and CdS films were deposited on glass substrates by using the USP method Next the CIS layer was deposited in two stages by using the URSP technique First, a layer of high-resistance CIS (0.5 µm) was deposited while maintaining the Cu/In ratio in the starting solution at 1.0 Second, a layer of low-resistance CIS (1.5 µm) was deposited from a solution with a [Cu]/[In] ratio of 1.1 or a CIAS layer (1.5 µm) was prepared from a solution with a [Cu]/[In] ratio of 1.0 and an [Al]/[In] ratio of 0.12 by using the sulfurization process The CIAS layer was deposited over the high-resistance CIS layer The cells were name D-1 (glass/IZO/CdS/CIS (high-resistance)/CIS -1498- Journal of the Korean Physical Society, Vol 61, No 9, November 2012 Table Summary of J-V parameters for the solar cells depicted in Fig Cell D-1 D-2 VOC (mV) 340 425 JSC (mA/cm2 ) 6.65 14.02 FF (%) 27.39 28.75 η (%) 0.85 1.71 RS (Ω) 18.7 10.4 Rsh (Ω) 95.5 180.1 Fig Characteristic J-V curves of two cells: D-1, and D-2 (low-resistance)/metal) and D-2 (glass/IZO/CdS/CIS (high-resistance)/CIAS (low-resistance)/metal) From the J-V characteristics (Fig 7), the cell parameters were obtained, and the values are given in Table Among these two types of cells, a better result was obtained for cell D-2 From the comparison in Fig 7, the solar cell with bilayer CIS absorber layers clearly showed a significant improvement in the efficiency conversion, and cell D-2 using a bilayer CIS/CIAS absorber layer exhibited the best solar-cell performance Its efficiency was 1.71%, which is higher than that of cell S-2 using a single-layer CIS absorber layer (0.74%) or that of cell D-1 using a bilayer CIS/CIS absorber layer Here, the open-circuit voltages and the short-circuit current density can also be seen to have improved strongly (Table 3) On the other hand, the diode quality factor for cell D-2 was 2.5, which was lower than that of cell S-2 We consider that VOC improvement happened because the optical band gap of CIAS (1.49 eV) was higher than that of CIS (1.45 eV), thus enabling a higher Vbi and VOC In order to understand the influence of the bilayer structure of the solar cells based on the CIS absorber layer, we numerically calculated the band diagrams for cell S-2 and cell D-2 by using the SCAPS-1D software [13] The structure of the modeled solar cell was depicted schematically in Fig The band bending in the absorber determined the indicated effective barrier (Φpb ) for interface recombination In the case of the dominating interface recombination, the activation energy (EA ) was equal to the interface barrier for holes (see in Fig 8), which normally is lower than the band gap [1,14] It fol- Fig Band diagrams for (a) single-layer CIS and (b) bilayer CIS/CIAS based solar cells lows from Fig that Φpb of CIAS (∼500 meV) was higher than that of CIS (∼380 meV) This is remarkable here, as the change in the single-layer CIS absorber layer one to the bilayer CIS/CIAS absorber layer increased the activation energy of the dominant interface recombination (cell D-02) This is also one of the reasons for the increase in the p-n junction barrier height and led to the improved cell parameters III CONCLUSION The important conclusion of this study is that glass/ZnO/CdS/CIS/CIAS/metal solar cells with a superstrate structure can be deposited by using full spray pyrolysis deposition (FSPD) We consider that the bilayer CIS/CIAS absorber layers might increase the activation energy of the dominant interface recombination This is one of the reasons for the increase in the p-n junction barrier and leads to increases in the solar-cell parameters The parameters of the solar cell obtained are VOC = 425 mV, JSC = 14.02 mA/cm2 , FF = 28.75%, and η = 1.71% These results show that the FSPD method is a very promising technique for fabricating solar cells based on CIS absorbers in a superstrate structure at low cost and with high efficiency Fabrication and Characteristics of Fully-sprayed ZnO/CdS/CuInS2 Solar Cells – Tran Thanh Thai et al ACKNOWLEDGMENTS The authors gratefully 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