J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 Journal of the Nigerian Society of Physical Sciences Numerical Simulation of Copper Indium Gallium Diselenide Solar Cells Using One Dimensional SCAPS Software C. O. Lawania, G. J. Ibeha, O. O. Igea, D. Elia,b,∗, J. O. Emmanuela,c, A. J. Ukwenyaa, P. O. Oyedared aDepartment of Physics, Nigerian Defence Academy, Kaduna, Nigeria bDepartment of Physical Sciences, Greenfield University, Kasarami, Kaduna, Nigeria cDepartment of Basic Science and General Studies, Federal College of Forestry Mechanization, Kaduna, Nigeria dDepartment of Science Laboratory Technology, Federal Polytechnic Ede, Osun State, Nigeria Abstract The effect of multivalent defect density, thickness of absorber and buffer layer thickness on the performance of CIGS solar cells were investigated systematically. The study was carried out using Solar Cells Capacitance Simulator (SCAPS) code, which is capable of solving the basic semicon- ductor equations. Employing numerical modelling, a solar cell with the structure Al |ZnO : Al| In2S 3 |CIGS | Pt was simulated and in it, a double acceptor defect (-2/-1/0) with a density of 1014 cm−3 was set in the absorber in the first instance. This initial device gave a power conversion efficiency (PCE) of 25.85 %, short circuit current density (Jsc) of 37.9576 mAcm−2, Photovoltage (Voc) of 0.7992 V and fill factor (FF) of 85.22 %. When the density of multivalent defect (-2/-1/0) was varied between 1010 cm−3 and 1017 cm−3 the solar cells performance dropped from 26.81 % to 16.87 %.The champion device was with multivalent defect of 1010 cm−3 which shows an enhancement of 3.71 % from the pristine device. On varying the CIGS layer thickness from 0.4 µm to 3.6 µm, an increase in PCE was observed from 0.4 µm to 1.2 µm then the PCE began to decrease beyond a thickness of 1.2 µm. The best PCE was recorded with thickness of 1.2 µm which gave Jsc of 37.7506 mAcm−2, Voc of 0.8059 V, FF of 85.2655 %. On varying the In2S 3 (buffer) layer thickness from 0.01 µm to 0.08 µm, we observed that there was no significant change in photovoltaic parameters of the solar cells as buffer layer thickness increased. DOI:10.46481/jnsps.2021.133 Keywords: SCAPs, CIGS, Multivalent defect, buffer layer, absorber Article History : Received: 30 August 2020 Received in revised form: 02 February 2021 Accepted for publication: 04 February 2021 Published: 29 May 2021 c©2021 Journal of the Nigerian Society of Physical Sciences. All rights reserved. Communicated by: T. Owolabi & B. J. Falaye 1. Introduction CIGS is a quaternary compound semiconductor which is an alloy of CuInS e2 (CIS) and CuGaS e2 (CGS) and comprises four elements namely: Copper, Indium, Gallium and Selenium. It is a direct band gap material [1] whose energy band gap could be varied from 1.06 eV – 1.70 eV by changing the Indium to ∗Corresponding author tel. no: +2348063307256 Email address: danladielibako@gmail.com (D. Eli) Gallium ratio [2]. According to Hamanche [3], CIGS is the most promising candidate for efficient and low-cost solar cells based on the advantages of the optical and electrical properties of the material [3]. In spite of these attractive features of CIGS material, there are factors which could affect the performance of solar cells whose absorbers are made from this material. Such factors include defect density in semiconductor layers and in- terfaces of the solar cell, absorber layer and buffer layer thick- nesses, bandgap of semiconductor materials, working tempera- ture of the solar cell among others. 48 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 49 Multivalent defects are defects caused by transition metals which usually occur as impurities or as part of the structure of some semiconductors [4]. Transition metals are metals which have valence electrons in two shells instead of only one. They therefore exhibit multiple oxidation states and can transit from one charge state to another by accommodating a variable num- ber of electrons in their d-orbitals. When multivalent elements occur as impurities in semiconductors, it is not always clear how many of their valence electrons would be used for charge exchange. For example, if tin (Sn) is doped on divalent zinc (Zn) or Magnesium (Mg) site. We are not sure if it will behave as +IV element or a +II element. There is really the possibil- ity that a multivalent impurity would transit from one oxidation state to another and their transition would appear as deep level inside the band gap of the material. Deep levels which are as- sociated with a change in oxidation state tend to deteriorate the electronic properties of semiconductors. As they form recom- bination centers and carrier traps [4]. We therefore investigate in this work, the effect of multivalent defect density, thickness of absorber and buffer layer on the performance of CIGS solar cells using SCAPS-1D. This software was used to calculate the short circuit current density (Jsc), open circuit voltage (Voc), Fill factor (FF) and efficiency (η) which are photovoltaic pa- rameters used for the assessment of solar cells’ performance. The spectral response of the solar cells in the face of varying defect density, absorber layer thickness and buffer layer thick- ness were also studied. 2. Materials and Methods 2.1. Cell Structure The solar cells simulated have the structure Al|ZnO : Al|In2 S 3|CIGS |Pt| as shown in figure1. The main parts of the cells are CIGS absorber and the In2S 3 buffer layers. The CIGS ab- sorber is responsible for trapping light from the sun. It is con- sidered environmental friendly because of the absence of Cad- mium in its structure. The material has a direct band gap and high absorption coefficient requiring just a few micrometers to absorb the maximum incident photon. The wide band gap of this material has also been found to be variable depending on the composition of the CIGS material [5]. The In2S 3 is cho- sen as the buffer since it is stable, has a wider band gap and is considered nontoxic, when compared with other buffers such as CdS. It is also transparent and photoconductive [6]. A transpar- ent conductive Oxide (TCO) layer (acting as the window) made of ZnO:Al is deposited on top of the buffer layer as it is trans- parent to most of the solar spectrum because of its wide band gap.Although ZnO:B (Boron doped Zinc Oxide) could be more beneficial for the solar cells because it lowers absorption losses leading to an increase in the quantum efficiency of the solar cells[7], ZnO:Al is used in this research because it is a low cost TCO and it is also highly conductive. Front and back contacts usually made of metallic elements are introduced in the cells’ structure for the conduction of photogenerated charge carriers in and out of the solar cells. Al is used as the front contact in this work, because it is lightweight, non-magnetic and corrosion re- sistant. It is a good conductor of electricity; more conductive Figure 1. Model of the simulated solar cell. than Copper and less expensive than silver. A back contact of Pt is preferred to the commonly used Molybdenum since it is non- toxic when compared with Molybdenum and gives a cell with higher efficiency [8]. 2.2. Numerical Modelling The method used for this work is numerical simulation. Nu- merical simulation often gives insight into the interpretation of measurements even as it aids in the assessment of the potential merits of a cell structure [7]. Some softwares among which is SCAPS-1D can be used to analyze the effect of the variation of materials parameters, that is, the presence or absence of par- ticular properties or the varying of all properties in the range of values, for the optimization of solar cells’ efficiencies. This helps the producers of solar cells with the needed insight to ef- fect necessary changes in their production methods in order to improve product performance. SCAPS- 1D simulation results have a good agreement with the results of existing experimental works [9] and this is the major motivating factor for its use in this research. SCAPS computes the steady state band diagram, recombination profile and carrier transport in one dimension, based on Poisson’s equation (equation 1) together with the con- tinuity equations (equations 2a and 2b) for holes and electrons. ∇· �E = q( p − n + N) (1) ∂n(x, t) ∂t = 1 q ∂Jn ∂x + Gn(x, t) − Rn(x, t) (2a) ∂p(x, t) ∂t = 1 q ∂Jp ∂x + G p(x, t) − Rp(x, t) (2b) where E = electric field; � = permittivity of semiconductor; q = electronic charge; n = concentration of electrons; p = con- centration of holes; N = net charge due to dopants and other trapped charges; Jn and J p = current density of electrons and holes; Gn and G p = rates of electron and holes generation in 49 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 50 Figure 2. Simulation procedure. Figure 3. SCAPS solar cell definition panel. the semiconductor device; Rn and Rp are rates of electron and hole recombination in the solar cells. Figure 2 shows the steps that were taken in the simulation, starting with the launch of SCAPS. 2.3. Simulated Parameters The material parameters were selected from experimental results [10,11,12]. Table1 gives a summary of the parameters for each layer in the simulation. Defect parameters in the layers and interfaces were sourced from literatures [1, 13, 14, 15] as shown in table 2. The work functions of the front contact (Al) and back contact (Pt) are 4.26 eV and 5.93 eV, respectively [16]. A working temperature of 300 K, solar spectrum AM1.5 and a Scanning voltage of 0 V - 1.3 V were used for all simulations. The solar cell definition panel shown in figure 3 is the envi- ronment where the physical properties of the various layers of the solar cells were inputted. Figure 4. Energy band diagram in the CIGS solar cell. 3. Results and Discussion 3.1. Performance parameters from initial simulation In the initial device set up for this work, a multivalent defect in the form of a double acceptor (-2/-1/0) defect with a Gaus- sian energy distribution, defect energy level (Et) = {0.1, 0.4} eV above Ev and a concentration of 1.0×1014 cm−3, was introduced into the CIGS (absorber) layer. This defect which is mainly caused by CuI I I (CuI I I is a double acceptor and III represents a group 3 element such as Indium or Gallium) defect is common in CIGS absorbers [17]. Details of other defects set in the sim- ulation are given in Table 2; the resulting performance param- eters of the open- circuit voltage (VOC ), short- circuit current density (JS C ), fill factor (FF) and efficiency determined using J- V characteristics are compared with those derived from exper- imental work [18]. The comparison which is shown in Table 3 reveals that there is a good agreement between data from calcu- lations and those from experiment hence validating parameters used in the simulation. The J-V curve and quantum efficiency curve are also obtained and shown below. The parameters of the different layers used in the simulation are also given in Table 1. In the quantum efficiency (spectral response) curve shown in Figure 6, there is an observed increase of spectral response in the short wavelength between 350 nm(0.35 µm) and 400 nm(0.4 µm). The curve reveals a maximum efficiency of approximately 100 % occurring between 400 nm(0.4 µm) and 1000 nm(1 µm) but this high efficiency begins to fall off after 1000 nm(1 µm). This fall is very likely due to incomplete absorption of the long wave- length photons. This analysis pertaining to the quantum ef- ficiency, agrees very much with those reported in literatures [1,19]. 3.2. Effect of multivalent defect concentration in CIGS (ab- sorber) layer The density of absorber layer defect has a direct effect on photovoltaic cell performance because as the concentration of 50 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 51 Table 1. Materials parameters for CIGS |In2S 3|ZnO : Al| solar cell [10,11,12]. Layer parameter CIGS In2S 3 ZnO : Al Thickness (µm) 2 0.04 1.6 Band gap, Eg(eV ) 1.2 2.5 3.3 Electron affinity, χe(eV ) 4.25 4.25 4.6 Relative permitivity �e 13.6 13.5 9 Nc, effective density of states (1/cm3) 2.2 × 1018 1.8 × 1019 2.2 × 1018 Nv, effective density of states (1/cm3) 1.8 × 1019 4.0 × 1013 1.8 × 1019 Electron mobility, µn(cm2/Vs 100 400 100 Hole mobility, µp(cm2/Vs 25 210 25 Acceptor concentration NA(1/cm3) 1.0 × 1016 0 0 Donor concentration ND(1/cm3) 0 1.0 × 1018 1.0 × 1018 Table 2. Defect parameters of buffer, window and interfaces [1, 13, 14, 15]. Parameters In2S 3 ZnO:Al CIGS/In2S 3 In2S 3/ZnO:Al Interface Interface Defect Type Acceptor 0.04 1.6 Capture cross section for electrons σn(cm2) 1.0 × 10−15 1.0 × 10−12 1.0 × 10−14 1.0 × 10−12 Capture cross section for holes σh(cm2) 5.0 × 10−13 1.0 × 10−12 1.0 × 10−14 1.0 × 10−15 Energetic Distribution Gaussian Gaussian Single Single Energy level with respect to reference (eV) 0.6 above Ev 0.6 above Ev 0.6 above Ev 0.6 above Ev Characteristic Energy level (eV) 0.1 0.1 0.1 0.1 Total density (cm−3) 1.0 × 1014 1.772 × 1016 Concentration (cm−2) 1.0 × 1010 1.0 × 1010 Figure 5. J-V curve of CIGS solar cell with initial parameters. defects increase, the minority charge lifetime reduces. This is evident from τ = 1 σVth Nt (3) Equation 3 above where τ is the minority charge lifetime, σ is electron/hole capture cross section, Vth is thermal velocity of electron/holes and Nt is total density of defects but σVth = c and c is the capture constant of electron/holes so τ = 1cNt mean- ing that the life time τ is inversely proportional to the product of defect density and capture constant of the charge carrier. With Table 3. Results from initial simulation compared with experimental data. VOC JS C FF Efficiency (V ) (mA/cm2) (%) (%) Experimental [18] 0.7410 37.8000 80.60 22.60 Simulation 0.7992 37.9576 85.22 25.85 Figure 6. Quantum efficiency curve with initial parameters. reduction in life time, the diffusion length of electrons and holes reduces. Diffusion length is the average distance a charge car- rier can travel in a semiconductor material before it recombines. Di f f usion length, L = √ Dτ (4) 51 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 52 meaning that if defect density increases, τ reduces and diffu- sion length reduces also. In Equation 4, D is the diffusion coef- ficient for electrons/holes With a reduction in diffusion length, the probability of collection of photogenerated charge carriers at the terminals reduces; this in turn lowers the photocurrent for the solar cell, and increase the chances of recombination hence increasing the recombination loss in the absorber [20]. Defects could be introduced either intentionally or uninten- tionally into semiconductors during the growth process, during processing of the device or from the working environment [21]. Theoretical studies which are confirmed by results of measure- ment show that most of the existing defects in chalcopyrite so- lar cells, are multivalent in nature [22, 23]. In this study, the impact of varying multivalent defect concentration is observed by choosing the values of the defect density in the range of 1010 cm−3–1017 cm−3. Table 4 gives the performance parame- ters of the CIGS solar cells with various values of multivalent defect density in the absorber. It would be observed that the Table 4. Dependence of cells performance on multivalent defect density in CIGS absorber layer. Multivalent VOC JS C FF η defect (V ) (mA/cm2) (%) (%) density (cm−3) 1010 0.82086 37.96297 86.0388 26.8116 1011 0.82083 37.96297 86.0371 26.8102 1012 0.82059 37.96292 86.0196 26.7967 1013 0.81785 37.96244 85.8998 26.6700 1014 0.79924 37.95760 85.2210 25.8537 1015 0.74993 37.90711 83.6588 23.7824 1016 0.70338 37.29294 80.0979 21.0106 1017 0.67284 33.33359 75.2251 16.8716 solar cells’ performance does not change much when the defect density is below 1014 cm−3. This result tallies with the find- ing of similar study [24] in this regard. Figure 7 shows that all electrical performance parameters start degrading at a de- fect density of ≈ 1015 cm−3. VOC goes down from 0.82086 V to 0.67284 V , representing a decrease of 22.38 %. JS C falls from 37.96297 mA/cm2 to 33.33359 mA/cm2 corresponding to a decrease of 13.89 %. These drops may be attributed to re- combination within localized energy levels created by defects which cause current leakage [25]. As a result, the conversion efficiency goes down from 26.8116 % to 16.8716 % represent- ing a decrease of 58.92 %. Since solar cell efficiency is the amount of energy in the form of sunlight that can be converted into electricity by a solar cell, this 58.92 % decrease in conver- sion efficiency brought about by an increase in concentration of multivalent defect in the absorber layer poses a disadvantage to the functioning of the solar cell. According to a study [26], an efficient solar cell will have a high short circuit current density Jsc, a high open circuit Voc and a fill factor as close as possible to 1 (or 100 %). The fill factor is a measure of the ideality of a solar cell. In Figure 7, the fill factor is observed to depart more from its ideal value of 100 % with an increase in the density of multivalent defect in the absorber leading to less efficiency and ideality of the solar cells. The multivalent defect density which produces an optimum performance of the solar cells is 1010/cm3 at an open circuit voltage Voc of 0.8209 V , short circuit current density Jsc of 37.96300 mA/cm2, fill factor FF of 86.0388 % and conversion efficiency η of 26.8116 % (as shown in Figure 8). This implies that multivalent defect densities (double acceptors, in this case) in CIGS solar cells should be controlled in such a way that they do not exceed this value. Figure 9 shows the quantum efficiency (QE) as a function of wavelength for different values of defect density in the CIGS layer. When the wavelength is in the range of 300 nm(0.3 µm) – 1200 nm(1.2 µm) the absorption efficiency decreases with in- creased multivalent defect density in the CIGS layer. This is because as defect density increases the recombination (which causes loss of charge carriers) phenomena becomes more pro- nounced and since quantum efficiency is the ratio of the number of carriers collected by the solar cell to the number of incident photons [19], quantum efficiency drops. 3.3. Effect of varying In2S 3 (buffer) layer thickness The influence of the thickness of In2S 3 buffer layer on per- formance of the photovoltaic cell is shown in Figure 10. The thickness of the buffer was varied from 0.01 µm through 0.08 µm. Although the variation of all photovoltaic parameters with in- creasing buffer thickness is not very significant, a reduction in the efficiency of the solar cells with increasing thickness of the buffer was noticed, in line with the findings in literatures [27, 28, 29]. This is caused by absorption of some photons in this layer [27] as a large number of short-wave length photons are absorbed before reaching the absorber layer while photons hav- ing wavelengths greater than that associated with the band gap of the buffer cannot generate electron-hole pairs and are there- fore lost as heat. Whereas a thin buffer layer means majority of photons can pass through the buffer into the absorber without being absorbed, increasing the buffer layer thickness causes a drop in efficiency of the solar cells. This is due to photon loss occurring inside the buffer layer. When a smaller number of photons make it through the buffer, less electron-hole pairs are created and this means less electricity is generated. This agrees with the findings [29]. The observed reduction in JS C is caused by less production of electron-hole pair as a smaller number of electron- hole pairs can reach the absorber layer with increase in buffer layer thick- ness. A decreased short circuit current means that less photo- generated carriers are produced and this lowers the efficiency of the solar cells. Apart from the very little initial decrease in Voc when buffer layer thickness is increased from 0.01 µm to 0.02 µm, the open circuit voltage remains constant showing that the buffer layer thickness has little or no effect on Voc. From Figure 10, there is a slight increment in fill factor when buffer layer thickness is increased from 0.02 µm to 0.03 µm thereafter, FF begins to drop again. These changes must have resulted from valence band discontinuities at the interfaces that appear as spikes [14]. The best efficiency of the solar cells after vari- ation of the buffer thickness is 25.9813 % and this is achieved 52 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 53 Figure 7. Variation in performance of CIGS solar cells with multivalent defect density. Figure 8. J-V curves of CIGS solar cells with various values of multi- valent defect density. for a thickness of 0.01 µm at an open circuit voltage Voc of 0.8030 V , Jsc of 37.9591 mA/cm2 and fill factor of 85.2329 % (as shown in Figure 11). Figure 12 shows the quantum efficiency (QE) as a func- tion of wavelength for different values of buffer (In2S 3) layer Table 5. Dependence of solar cells’ performance on buffer layer thickness. Thickness VOC Jsc FF η of buffer (V) (mA/cm2) % % (µm) 0.01 0.8030 37.9591 85.2329 25.9813 0.02 0.7997 37.9586 85.2183 25.8698 0.03 0.7993 37.9582 85.2211 25.8556 0.04 0.7992 37.9576 85.2210 25.8537 0.05 0.7992 37.9569 85.2206 25.8529 0.06 0.7992 37.9560 85.2202 25.8521 0.07 0.7992 37.9549 85.2198 25.8512 0.08 0.7992 37.9536 85.2193 25.8502 53 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 54 Figure 9. Quantum efficiency as a function of wavelength for different values of defect density in CIGS layer. thickness. When the wavelength is in the range of 300 nm (0.3 µm) – 1200 nm (1.2 µm), we observed that the spectral response curves overlap because the absorption efficiency re- mains constant for all values of buffer layer thickness. This spectral response in relation to varying buffer layer thickness further proves that buffer layer thickness has little or no effect on CIGS solar cells investigated. 3.4. Effect of varying CIGS (absorber) layer thickness An important parameter which also affects the performance of CIGS solar cells is the thickness of the absorber layer. The effect of thickness of the absorber layer on the solar cell’s per- formance parameters VOC , JS C , FF, PCE is seen in Figure 13. When the thickness of CIGS absorber layer was varied from 0.4 µm to 3.6µm, the solar cell’s efficiency was seen to increase from 23.96 % to 25.94 % for an increase in thickness, of 0.4 µm – 1.2µm respectively. This increase is attributable to absorption of more photons as absorber layer thickness increases. This translates to the production of a significant number of electron- hole pairs which then leads to an improvement in efficiency of the solar cells [12]. This is good for the performance of the solar cells since it means that more of the sun’s energy would be converted to electricity in the solar cells. Beyond an ab- sorber thickness of 1.2µm, the efficiency begins to drop due to decreased collection of photo-generated charge carriers which is caused by charge recombination. This tallies with similar finding [24] JS C increases with increasing absorber thickness since longer wavelength photons are absorbed in thicker layers of the absorber and they enhance the amount of photo-generated carriers which in turn boosts efficiency and therefore produces solar cells which perform better. Fill factor FF remains nearly constant but VOC kept decreas- ing as a result of recombination of charge carriers [20] which increases in thicker layers of the absorber. this is not good for the performance of the solar cell as the efficiency of the solar cell has a direct dependence on open circuit voltage V oc. For optimum performance, the absorber thickness of the CIGS solar cells should be kept at 1.2 µm. The photovoltaic cell parameters corresponding to this optimum value are V oc of 0.8059 V , Jsc of 37.7506 mA/cm2, FF of 85.2655 % and conversion efficiency of 25.9403 % (as shown in Figure 14). Table 6. Dependence of solar cells’ performance on absorber layer thickness. Thickness VOC Jsc FF η of absorber (V) (mA/cm2) % % (µm) 0.4 0.8126 34.5688 85.2896 23.9577 0.8 0.8097 37.2321 85.2886 25.7114 1.2 0.8059 37.7506 85.2655 25.9403 1.6 0.8024 37.9045 85.2379 25.9258 2.0 0.7992 37.9576 85.2210 25.8537 2.4 0.7961 37.9805 85.2398 25.7724 2.8 0.7933 37.9930 85.2467 25.6934 3.2 0.7909 38.9921 85.2450 25.6128 3.6 0.7887 38.0000 85.2375 25.5452 Figure 15 shows the quantum efficiency (QE) as a func- tion of wavelength for different values of absorber (CIGS) layer thickness. When the wavelength is in the range of 300 nm (0.3 µm) – 1200 nm (1.2 µm) the absorption efficiency increases with increased absorber (CIGS) layer thickness. This is be- cause as absorber (CIGS) layer thickness increases the number of absorbed photons increases consequently, a higher number of electron-hole pairs are produced and the quantum efficiency increases [1]. For all values of absorber (CIGS) layer thickness, it is observed that the spectral response curves show a decrease of the long wave length collection. This is most likely due to incomplete absorption of the long wavelength photons [30]. 3.5. Performance of optimized parameters Based on the optimized multivalent defect density, absorber layer thickness and buffer layer thickness, an efficiency of 27 %, current density of 37.75 mA/cm3, voltage of 0.829 V and fill factor of 86.26 % were obtained as depicted in Figure 16. Compared with the experimental data obtained by Jackson et al.[18] were an efficiency of 22.6 % was reported, the opti- mized cell in this work shows an improvement of 16.30 % in efficiency. Aside the alkali Post Deposition Treatment (PDT) done on CIGS absorbers which were used in the experimental solar cells referred to in Table 8 their efficiencies could be im- proved upon by carefully controlling the concentration of mul- tivalent defect in their absorbers as this form of defect is preva- lent in chalcopyrite materials. Table 7. Optimized parameters of the device. Optimized Parameters Absorber Buffer Thickness (µm) 1.2 0.01 Multivalent defect density (cm3) 1010 – 54 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 55 Figure 10. Variation in performance of CIGS solar cell with buffer layer thickness. Figure 11. J-V curves of CIGS solar cells with various values of buffer layer thickness. 4. Conclusion In this work, we undertook numerical simulation to inves- tigate multivalent defects and the influence of absorber layer thickness and buffer layer thickness on Al|ZnO : Al|In2S 3|CIGS Figure 12. Quantum efficiency as a function of wavelength for different values of buffer (In2S 3) layer thickness. |Pt| structured solar cells using SCAPS code. The efficiency of the initial device which was found to be 25.85 % with a mul- tivalent defect density of 1014 cm−3 experienced a boost to 27 % when the solar cell was optimized with an absorber layer thickness of 1.2 µm, buffer layer thickness of 0.01 µm and a 55 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 56 Figure 13. Variation in performance of CIGS solar cells with absorber layer thickness. Figure 14. J-V curves of CIGS solar cell with various values of absorber layer thickness. multivalent defect density of 1010 cm−3 in the CIGS absorber layer. The results obtained revealed that when the density of multivalent defect in the absorber was varied from 1010 cm−3 through 1017 cm−3, the efficiency of the CIGS photovoltaic cells dropped from 26.81 % to 16.87 % representing a decrease of Figure 15. Quantum efficiency as a function of wavelength for different values of absorber (CIGS) layer thickness. 58.92 %. This result clearly shows how detrimentally multi- valent defects can affect the performance of CIGS solar cells. As expected, increasing the absorber layer thickness caused an increase in efficiency until an optimal thickness of 1.2 µm was achieved while increase in buffer layer thickness from 0.01 µm 56 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 57 Table 8. Photovoltaic parameters corresponding to optimized parameters of the CIGS solar cells compared with those of experimental researches. Simulation VOC Jsc FF η (V) (mA/cm2) % % Initial 0.7992 37.9576 85.2200 25.8500 Optimized absorber layer thickness (µm) 0.8059 37.7506 85.2655 25.9403 Optimized buffer layer thickness (µm) 0.8030 37.9591 85.2329 25.9813 Optimized multivalent defect density (cm3) 0.8209 37.9630 86.0388 26.8116 Final Optimization 0.8290 37.7541 86.2600 27.0000 Experimental data 0.7570 34.8000 79.1000 20.8000 [31] Experimental data 0.7440 36.7000 80.5000 22.0000 [32] Experimental data 0.7410 37.8000 80.6000 22.6000 [18] Figure 16. J-V curve of CIGS solar cell with optimized parameters. through 0.08 µm caused a slight decrease of 0.51 % in effi- ciency. Acknowledgments The authors wish to express their gratitude to Prof. Marc Burgelman and his colleagues at the University of Gent for pro- viding SCAPS-1D software which was used in this work. We also thank the referees for the positive enlightening comments and suggestions, which have greatly helped us in making im- provements to this paper. References [1] M. Hasan, M. Islam & A. Khan, “Comprehensive analysis of CdS/In2S 3/ZnS buffer layers on performance of CIGS solar cell”, (Bachelor’s Degree project, Military institute of science and technology, Bangladesh (2017). [2] P. Lynn, “Electricity from sunlight: An introduction to photovoltaics”, John Wiley and sons (2010). [3] A. Hamanche, “Study of the type inversion of the semiconductor in ir- radiated solar cells”, Doctoral Dissertation, Mohamed Khider University, Democratic and popular republic of Algeria (2018). [4] K. Biswas, S. Lany & A. Zunger, “The consequences of mul- tivalent elements in inorganic solar absorber: Multivalency of Sn in Cu2ZnSnS4”. Applied physics letters, 96 (2010) 201902. http://dx.doi.org/10.1063/1.3427433. [5] S. Banerjee, Y. Ojha, K. Vikas & A. Kumar, “High efficient CIGS based thin film solar cell performance optimization using PCID”, International research journal of Engineering and technology (IRJET), 3 (2016). [6] M. Mathew, “Engineering the properties of Indium Sulphide for thin film solar cells by doping” (Doctoral dissertation). Cochin University of Sci- ence and Technology, India, (2009). [7] S. Ouedraogo, F. Zougmore, & J. Ndjaka, “Numerical analysis of Copper- Indium-Gallium-Diselenide based solar cells by SCAPS”.International Journal of photoenergy, (2013). http://dx.doi.org/10.1155/2013/421076. [8] A. Adekoya, A. Alabi, & A. Oni, “Performance of CIGS solar cell with changes in Absorber layer thickness and back contacts”, CARD interna- tional journal of science and advanced innovative research (IJSAIR), 2 (2017). [9] A. Niemegeers, S. Gillis & M. Burgleman, “A user program for realis- tic simulation of polycrystalline heterojunction solar cells; SCAPS-1D”, Proceedings of the 2nd world conference on photovoltaic energy conver- sion, Wein (1998). [10] N. Khoshsirat, & N. Yunus, “Copper- Indium- Gallium- Dise- lenide (CIGS) nanocrystalline bulk semiconductor as the absorber layer and its current technological trend and optimization”, (2016), http://dx.doi.org/10.5772/64166. [11] J. Keller, M. Knipper, J. Parisi, I. Riedel, T. Dalibor & A. Avellan, “Im- pact of thickness variation of the ZnO: Al window layer on optoelec- tronic properties of CIGSSe solar cells”. Mater. Res. Soc. Symp. Proc 1324 (2011), http://dx.doi.org/10.1557/opl.2011.1058. [12] A. Kotbi, B. Hartiti, A. Batan, S. Fadili, A. Ridah & P. Thevanin, “The effect of several parameters on the performance of CuInS2 – based so- lar cells using SCAPS-1D software”, Journal of fundamental and applied science, (2019), http://dx.doi.org/10.4314/jfas.v11i2.11. [13] R. Scheer & H. Schock, “Chalcopyrite photovoltaics: physics, technolo- gies and thin film devices”. Wiley- VCH, (2011). [14] M. Ali, M. Hossain, M. Biswas, K. Alam & M. Khan, “Numerical sim- ulation and observation the characteristics of CIGS thin film solar cells using SCAPS-1D”. American Journal of Engineering Research (AJER), 7 (2018), pp176-182. [15] M. Soce, M. Dieng, K. Ehemba, D. Diallo & I. Wade, “Influence of dop- ing of the absorber and the charged defects on the electrical performance of CIGS solar cells”. International Journal of scientific and research pub- lications, 5 (2015). [16] W. Haynes, “CRC handbook of chemistry and physics (97th ed)”. New York: CRC press, (2017). [17] S. Wei, S. Zhang & A. Zunger, “Effects of Ga addition to CuInSe2on its electronic, structural and defect properties”, Applied Physics letters, 72 (1998) pp3199-3201. [18] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte & M. Powalla, “Effects of heavy alkali elements in Cu (In, Ga) Se2 solar cell with efficiencies up to 22.6 %”, Physica Status Solidi (RRL) (2016), http://dx.doi.org/10.1002/pssr.201600199. [19] S. Dabbabi, T. Nasr & N. Kamoun-Turki, “Parameters optimization of CIGS solar cell using 2D physical modeling”. Results in physics, (2017), https://doi.org/10.1016/j.rinp.2017.06.057. [20] Y. Khattak, “Modelling of high-power conversion efficiency thin film solar cells” (Doctoral Dissertation), Universitat Politecaica de Valencia, (2019). 57 Lawani et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 48–58 58 [21] T. Duc, “Electronic properties of intrinsic defects and impurities in GaN”. (Doctoral Dissertation), Linkoping University, Sweden, (2015). [22] S. Sienbentritt & U. Rau, “Wide gap chalcopyrites”, Springer – verlag, Berlin Heidelberg, (2006). [23] ] S. Zhang, S. Wei & A. Zunger, “Stabilization of energy compounds via ordered arrays of defect pairs”, Physical review letters 78 (1997). [24] M. Wanda, S. Ouedrago, F. Tchoffo, F. Zougmore & J. Ndjaka, “Numerical investigation and analysis of Cu2ZnSnS4 based solar cells by SCAPS-1D”, International Journal of Photoenergy, (2019), https://doi.org/10.1155/2016/2152018. [25] S. Oyedele, B. Soucase & B. Aka, “Numerical simulation and perfor- mance optimization of Cu (In, Ga) Se2 solar cells”, IOSR Journal of ap- plied physics (JOSR-JAP), 18 (2016) pp1-11. [26] J. Gray, “The physics of the solar cell”. In A. Luque & S. Hegedus (Eds), Handbook of photovoltaic science and engineering, John wiley and sons, (2003). [27] A. Chadel, B. Benyoucef, & M. Chadel, “A comparative study of CIGS solar cells based on Zn (O, S) buffer layers and CIGS solar cells based on CdS buffer layers”, Optoelectronics and advanced materials- rapid com- munications, 9 (2015) pp653-656. [28] A. Benmir & M. Aida, “Analytical modelling and simulation of CIGS solar cells”, Energy procedia, 36 (2013) pp618-627, https://doi.org/10.1016/j.egypro.2013.07.071. [29] M. Hossain, P. Chelvanathan, M. Zaman, M. Karim, M. Alghoul & N. Amin, “Prospects of Indium Sulphide as an Alternative to Cadmium Sul- phide Buffer Layer in CIS Based Solar Cells From Numerical Analysis”, Chalcogenide Letters, 8 (2011). [30] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann & M. Powalla, “Com- positional investigation of potassium doped Cu (In, Ga) Se2solar cells with efficiencies up to 20.8 %”, Physica Status Solidi (RRL), 8 (2014), https://doi.org/10.1002/pssr.201409040. [31] ZWS, “Thin film photovoltaics success story continues: ZWS sets European record of 22 percent for CIGS cells”, htt ps : //www.zswbw.de/ f ileadmin/useru pload/PDF s/Pressemitteilungen /2016/pi07 − 2016 − ZS W − CIGS 22Percent − en.pd f , (2016). 58