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Efficiency optimization of CuInSe2 solar cell

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  • Save American Journal of M aterials Science 2012, 2(5): 160-164 DOI: 10.5923/j.materials.20120205.05 Cuinse2 Solar Cells Efficiency Optimization N. Touafek1,2, M. S. Aida1,*, R. Mahamdi3 1Laboratoire Couches minces et Interface Faculté des Sciences, Université de Constantine, 25000, Algerie 2Département d’Electronique, Faculté des Sciences de l’Ingénieur, Université de Constantine, 25000, Algerie 3Département d’Electronique, Faculté de Technologie, Université Hadj Lakhdar Batna, 05000, Algerie Abstract In the present paper calculation of CuInSe2(CIS)/ CdS solar cell efficiency is presented. The influence of the thickness and the optical band gap of CdS and CIS layers on the solar cell performances are investigated. The thickness of CdS buffer layer, wh ich is generally neglected is taken into account. The solar cells performances calculat ions are based on the calculation, by means of developed software written with Matlab, o f photocurrent from the resolution of the well known three coupling equations: continuity equation for holes in the CdS (n -region) and fo r electrons in the CIS (p- region) and Poisson equation. The obtained results indicated that the solar cell efficiency can be improved by reducing the CdS thickness or by increasing the CIS thickness. The efficiency increasing rates are 0.01 %/ n m and 0.5 %/n m for CdS and CIS layer thickness respectively. The CdS layer alters the shorter wavelength of the solar spectrum, wh ile the CIS layer alters the longer wavelength. CIS layer optical band gap is the most crucial parameter by co mparison to the optical gap of CdS layer. Keywords Solar Cell- Cuinse Photovoltaic-Thin Films 1. Introduction During the last years solar cells based on CuInSe2 (CIS) has emerged as a potential candidate for low cost thin films solar cells production. The 19.5 % efficiency obtained in ZnO/CdS/ CIS[1] hetrostructure is encouraging and is considered as an interesting stimu lant for the development of thin film solar cells technology. Currently CIS solar cells have attracted interest for spatial applications because of the stability of CIS solar cells against electron and proton irradiation then silicon or III-V semiconductors. There is a worldwide effo rt to reduce the material for solar cells production with a higher ratio power on weight. The reduction of different active layers thicknesses will have two direct consequences: (i) the reduction of solar cell cost and (ii) the use of less scare materia l; such as In and harmful and toxic ones such as Se and Cd on the other hand . Thereafter, the thickn ess is an impo rtant parameter to study and to optimize its in fluence on solar cells p ropert ies. Actually most stud ies mad e o n Cd S/ CIS so lar cells are main ly experimental, due to the fact that researchers are interested much more in experimental wo rk to the detriment o f the theoret ical research wh ich remains always insu fficient. However calculation is a simple method, economic and it saves effort and time to optimize solar cells technological * Corresponding author: aida_salah2@yahoo. fr (M. S. Aida) Published online at Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved parameters ( layer thickness, material optical gap, doping ..etc). The merit o f nu merical study is to test and predict the results and the influence of the process parameters on the device without fabrication. The present paper deals with the nu merical study of the influence of CdS layer thickness on the performance of CIS based solar. Generally the absorption in CdS layer is neglected, however it can contribute in short wavelength available in the sun spectrum[2]. Since the optical band gap of the CIS layer can be tailored by controlling the deposition parameters[3-6], the influence of CIS absorber layer and its optical band gap is also investigated. The solar cell efficiency and spectral response dependence on the thickness and optical band gap of both CdS and CIS layers are investigated to determine the factor limit ing the cell performance. 2. Calculation Procedure In this study, the cell is made up of a stacking of thin layers ZnO/CdS/ CuInSe2. ZnO layer is a highly conducting window used as transparent electrode. The heterojunction partner are the n- type CdS layer and the p-type CuInSe2 layer . A schemat ic drawn o f the cross section and the band diagram structure of the device are shown in figure 1. We have developed a software by the Matlab language run on a PC co mputer. The photocurrent is calculated fro m the resolution of the well known three coupling equations: continuity equation for holes in the CdS (n-region) and for electrons in the CIS (p- reg ion) and Poisson equation. 161 American Journal of M aterials Science 2012, 2(5): 160-164 The determination of the efficiency of the cell was carried by considering the real electrical model o f a solar cell (as a diode in parallel to a courant generator, series and shunt resistance). The output I current in this model is exp ressed by the following emp irical relation: I = I ph −V + Rs I Rsh − I s exp V + Rs AkbT I   −1  The used semiconductors parameters in this work, gathered from the literature, are summarized in table1. Table 2. comparison between the calculated and measured characteristics of CIS solar cell This work Exp eriment a l results[32] Area( cm2) 0.447 0.447 Voc (v) 0.497 0.491 Jsc (mA/c m2) 40.613 41.1 Fill Fact or (%) 71.58 71.9 Efficiency (%) 14.55 14.5 3. Results and Discussions 3.1. Infl uence of CdS Thickness Layer Figure 1. (a) a schematic cross section and (b) band diagram of the hetrostructure forming the studied CIS solar cell A is the ideality factor of the diode, Iph is the photocurrent , Rs and Rsh are series and shunt resistance respectively. The surface of the cell is of 1cm2. The point of maximu m operation (Im, Vm) wh ich corresponds to the maximal power delivered by the cell is determined by the solution of the non linear current equation by Newton-Raphson method .The conversion efficiency of the cell η is calculated, for standard conditions AM1.5 (100 mW/cm2 of incident illu mination) , according to the relation: η = Vm.Im Pin Vm and Im are respectively the voltage and current of the maximu m operating point , Pin is the incident sun power. Table 1. the values of CdS , CIS layers and solar cell parameters used in the calculation p aramet ers Electron affinity (ev) Sp, Sn (cm/s) Doping (cm-3) Dielectric constant CdS 4,5 4,2.105 8.1015 11.6 CuInSe2 4,3 106 1016 10 Nc, Nv (cm-3) The diffusion length Lp, Ln(µm ) 2.1019 0.46 1.5.1019 2.51 Shunt resist ance (Ω –cm2) Series resist ance (Ω –cm2) 1000 0.8 Figure 2. The variation of CIS solar cell efficiency as a function of the window layer thickness The primary function of CdS layer in the structure is to form the heterojunction with CIS layer. The thickness should be as thin as possible and the bandgap of the CdS layer should be as high as possible for h igh optical throughput with minimal resistive loss and low series resistance. For this reason CdS layer is common ly considered as window layer in CIS solar cell. Figure 2 shows the variation of the CIS solar cell efficiency as a function of CdS layer th ickness. The thickness of the absorber layer is fixed at 2000 n m. The efficiency is reduced with increasing the window thickness. The efficiency is reduced linearly with the thickness increase in the range 100 to 1000 n m. The reduction rate of the efficiency is about 0.01 %/n m .The efficiency is drastically reduced when the CdS thickness exceeds 50 n m. Th is is consistent with the experimental results indicating that the reduction of the CdS thickness lower than 2 n m imp roves the solar cell efficiency[7]. An increase of CdS layer increases the light absorption in this layer on the detriment of CIS absorber layer, thereafter the cell efficiency degradation. The optical band gap of window layer is larger than the CIS, an increase in the window layer alter the absorption in the shorter wavelength range of visible spectrum , as seen in figure 3, showing the variat ion of the solar cell quantum efficiency as a function of CdS thickness. While the larger N. Touafek et al.: Cuinse2 Solar Cells Efficiency Optimization 162 wavelength (in the infrared region) absorption remains unaltered by the CdS thickness. 1,0 0,9 dCdS= 20 nm dCdS= 50 nm 0,8 dCdS= 80 nm dCdS=100 nm 0,7 dCdS=150 nm dCdS=200 nm 0,6 Spectral response 0,5 0,4 0,3 0,2 0,1 0,0 200 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm) Fi gure 3. The variat ion of CIS solar cell quant um efficiency as a funct ion of the incident wavelength for different CdS layer thickness for a fixed absorber layer thickness equal to 2000 nm 20.8 20.6 20.4 Ee=50 nm 20.2 Efficiency (%) 20.0 19.8 19.6 19.4 19.2 19.0 0 5 10 15 20 25 30 CIS layer thickness (nm) Figure 4. The variation of the CIS solar cell efficiency as a function of the absorber layer thickness 1.0 0.9 0.8 0.7 0.6 ddddddECCCCCCbuuuuuu=IIIIIInnnnnnSSSSSS6eeeeee.2222220======m012345......m500000mmmmmmmmmmmm Spectral respose 0.5 0.4 0.3 0.2 0.1 0.0 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm) Fi gure 5. The variat ion of CIS solar cell quant um efficiency as a funct ion of the incident wavelength for different CIS layer thickness for a fixed window layer thickness equal to 50 nm 3.2. The Influence of Cuinse2 For mass production the reduction of the layer thickness reduces the amount of material consumption (especially scarce ones such as In) and then solar cell cost. Current research has focused on reducing the absorber layer thickness without adversely altering the solar cell performances. This explains the importance of the study of influence of absorber thickness upon the solar cell efficiency. In figure 4 we have reported the calculation results of the efficiency as function of absorber layer thickness with a fixed the window layer thickness at 50nm. In the range of 1 to 6000n m CIS thickness, the solar cell efficiency increases linearly with a rate of 0.5% /n m. However further increase in the CIS thickness beyond 6000 nm do not improve the efficiency, this is due to the high absorption coefficient of CIS, 6000 n m is enough to absorb all the solar spectrum incident photons. The influence of the absorber layer co me fro m its influence mainly in the short wavelength region of the visible spectrum, as can deduced fro m the variation of the quantum efficiency reported in figure 5. Th is is due to the low band gap of the CIS material. As seen in figures 3 and 5, the Cd S and CIS thickness layer alter the quantum efficiency of solar cells. CdS layer affects the shorter wavelength of the visible spectrum, wh ile the CIS layer affects the longer wavelength region. It worth noting that the available solar spectrum is not symmetrical, the maximu m intensity is located around 1.8 eV, which is closer to the CIS optical gap than to CdS optical gap. This explains the larger influence of CIS thickness upon the solar cell quantum efficiency. 3.3. Infl uence Of The CIS and Cds Optical Gaps The optical gaps of CdS and CIS can be varied when prepared as thin films. Severa l techniques have been used to prepare CdS thin films namely : sputtering[8] , thermal evaporation[9] electrodeposition[10], spray pyrolysis[11] and chemical bath deposition (CBD)[12]. Each technique have its specification but as well as for other physical properties, the optical gap can be tailored with varying the deposition parameters (substrate temperature, power, temperature, mo larity or pH of solution). CIS thin film can be also prepared by a variety of different fabrication techniques, such as vacuum evaporation[13], flash evaporation[14], mo lecular beam epitaxy [15], sputtering [16–21], electrodeposition[22] and selenizat ion of Cu–In precursors[23,24]. As well as for CdS its optical gap can be controlled by the deposition parameter and also by adding amount of Ga and S. A lloying with Ga[25], Al[26] or S[27] increases the bandgap of CuInSe2 and makes it mo re suitable for high-efficiency devices. Solar cells are fabricated using CIS alloys cousin such as: Cu(In,Ga)Se2 (CIGS) and Cu(In,Ga)(Se,S)2, which have h igher band gaps (to about 1.2 eV co mpared to 1.04 e V for CIS) . An increase in the band gap resulted in the production of high-performance solar cells with efficiencies of 19.2% for small-area[28]. The relat ionship between absorber energy 163 American Journal of M aterials Science 2012, 2(5): 160-164 gap and the open-circuit voltage and device efficiency for a large variat ion in Ga content show that open circuit voltage Voc scales linearly with optica l gap Eg over a wide range (up to 1.4eV) of Ga concentration, but the efficiency decreases when Eg is greater than 1.25 eV[25]. We have investigated the influence optical band gap of both window and absorber layers. In figures 6.a and b we have reported the variation of the solar cell efficiency as a function of CdS ( figure 6.a) and CIS optical gaps (figure 6.b). We have considered a thickness equal to 50 n m fo r the front layer and to 2000 n m for the absorber layer. We have varied the CdS layer band gap in the range (2,42 -2,54 eV) and CIS band gap layer in the range (0,95 -1,20eV) . These ranges are chosen after a synthesis of compiled data fro m the literature [29, 30]. As can be see, the influence of CdS optical gap on the efficiency is insignificant. However the optical gap of the absorber layer is more decisive. The efficiency increases linearly with the optical gap of CIS layer. It reaches the value of 27% for a gap of 1.20 eV. It approaches the theoretical optimu m for the conversion of the solar energy[31] . quantum efficiency of a CIS solar cell respectively . The experimental data are taken fro m reference[32]. As seen in these figures and in the table II resuming the ext racted solar cell parameters. The calculated characteristics fit well with the experimental results. This clearly indicates the validity of the used model and the calculat ion procedure. Fi gure 7. Calculat ion result s of current -volt age charact erist ic of CIS solar cell compared with experimental results Fi gure 8. Calculated quantum efficiency of CIS solar cell compared with experimental results 4. Conclusions Figure 6. Variation of CIS solar cell efficiency as a function of (a) CdS layer opt ical band gap and (b) CIS opt ical band gap In order to validate our study a consistent comparison of our numerica l study with e xperimental one is necessary. For this purpose, we have drawn in figures 7 and 8 the experimental results and calculated I-V characteristic and a In the present work we have carried a numerical investigation of the thicknesses and optical gap of CdS and CIS layers used in CdS/ CIS solar cell . The obtained results indicates that the reduction in CdS window layer thickness enhances the solar cell efficiency with a rate of 0.01% /n m. The cell efficiency is mo re sensitive to the absorber layer thickness, an increase in CIS th ickness enhances the efficiency with the rate 0.5%/ n m . However, the cell efficiency saturates beyond 6000 n m CIS thickness. The CdS thickness alters only the short wavelength response of the solar cell, wh ile the absorber layer alters the longer wavelength region. We have concluded also that CdS optical gap influence on cell efficiency is insignificant. A slight decrease in the cell efficiency follows an increase in CdS N. Touafek et al.: Cuinse2 Solar Cells Efficiency Optimization 164 optical. The influence of CIS optical gap is mo re crucial. The solar cell efficiency increases linearly with the optical gap. The calculated results are in god concordance with the experimental ones. 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