Photonic crystal beam splitter electrode in kesterite tandem solar cells: a numerical approach

1 PHOTONIC CRYSTAL BEAM SPLITTER ELECTRODE IN KESTERITE TANDEM SOLAR CELLS: A NUMERICAL APPROACH Miguel Barragán Sánchez-Lanuza[1][2]*, Isidoro Lillo-Bravo[2], Jose A. Lopez-Alvarez[2], Jose-Maria Delgado-Sanchez[1] [1] Dpt. Applied Physics I – University of Seville, Avda de los Descubrimientos s/n, 41092 Seville (Spain) [2] Dpt. Energy Engineering – University of Seville, Ctra. Utrera km 1, 41013 Seville (Spain) *Corresponding author: M. Barragán Sánchez-Lanuza [email protected] ABSTRACT The majority of numerical research on kesterite tandem solar cells has predominantly focused on a two terminals (2T) configuration that utilizes an ideal tunnel junction. In this study, we investigate the performance of kesterite tandem solar cells by introducing a photonic crystal structure (1DPC) as intermediate layer in both three terminals (3T) and four terminals (4T) configurations. The photonic crystal (1DPC) is designed by stacking ITO and SiO2 layers with the terminal layer consisting of NiO. Optical properties of the 1DPC are modeled. This innovative approach offers several advantages: (1) the 1DPC selectively reflects lower wavelengths, effectively enhancing the short-circuit current density (Jsc) of the top subcell, while the solar irradiance spectrum at higher wavelength optimum for the subcell is not affected; (2) the 1DPC serves as an intermediate electrode, needed for the 3T or 4T configuration; (3) replacing the conventional Mo back contact with a NiO layer significantly boosting the open-circuit voltage (Voc) of the top subcell. Our findings demonstrated that these configurations exhibit higher performance compared to previously reported results. Furthermore, the utilization of 3T and 4T configurations, incorporating the 1DPC as an electrical beam splitter, provides a more effective and accurate design compared to the 2T configuration employing ideal tunnel junctions. 2 Keywords: SCAPS-1D; Kesterite; Thin-film solar cells; Tandem; 1DPC; 3 1. Introduction Kesterite-based materials, such as Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(S,Se)4 (CZTSSe) have emerged as promising p-type absorber layer for thinfilm solar cells. These materials offer the potential to define effective device architectures that reduce costs and allow the exploitation of earth abundance materials [1-5]. The efficiency of this photovoltaic (PV) technology is significantly influenced by its layer structure, typically composed of a back contact electrode, p-type absorber, n-type buffer layer, and one front transparent contact electrode. Optimizing the layer structure is crucial to increase their efficiency. While the Shockley-Queisser limit (~32.23%) sets an upper bound for solar cells efficiencies, Siebentritt [6] compared it with kesterite solar cells to identify the most relevant intrinsic losses mechanisms. Although kesterite solar cells currently achieving efficiencies of 13.6% [7], which are lower than the 23.25% [8] achieved by the chalcopyrite-based solar cells, the latter being the most similar thin-film PV technology available today. And obviously, kesterite record efficiency is still far away from the conventional silicon technology extended used in PV modules. Therefore, despite the fact that kesterite solar cells are often presented as one of the main future options for cost-effective solar cells, it is imperative to improve the efficiency of current developments. Previous studies have indicated that the efficiency of kesterite solar cells is directly related to the composition of the absorber layer: Moholkar et al [9] demonstrated that the efficiency of CZTS absorbers increases from 1.35 to 4.13% as the Cu/(Zn+Sn) ratio increases from 0.8 to 1.1. Chandel et al [10] reported that the bandgap of CZTS films can be tuned in the range of 1.45 to 1.58 eV by adjusting the molar concentration of Zn and Sn. Ayala et al [11] examined the structural optical features of Cu2ZnGeS4 kesterite and found an optical bandgap energy close to 1.8 eV, which is 4 expected to yield a maximum efficiency of 8.5%. The incorporation of Ge increases the bandgap, while the incorporation of Se reduces it for CZTS. However, it is not only the composition if the absorber layer that poses a limitation; the heterojunction nature of kesterite solar cells also reveals additional challenges: the buffer layer and the absorber layer cannot achieve an optimal p-n junction interface due to the formation of a high concentration of CuZn at the interface, which pins the Fermi level in the middle of the bandgap [12]. Moreover, band-tail states within the CZTS absorber layer contribute to low Voc (open-circuit voltage) [13], the recombination mechanism of the CZTSSe is highly influenced by interface recombination due to a cliff-type band alignment at the buffer-absorber interface. In order to minimize internal losses in carrier transport mechanism and improve solar cell efficiencies, strategies involving graded CZTS absorber layers have been also explored [14,15]. The design of tandem devices (Figure 1) holds particular interest for thin-film technologies [16], because it allows for overcoming practical efficiency limitations by combining different optoelectronic parameters obtained with different bandgaps and spectrum absorption ranges. Several tandem structures based on CZTS/CZSTSSe, as well as combinations with other technologies such as perovskites and silicon have been proposed [17-19]. In 2T tandem cells (Figure 1a), the two subcells are connected in series by an interconnection without any intermediate electrode, with the advantage of having only two terminals, simplifying its integration into the PV module. However, this design requires the incorporation of a tunnel junction layer, which adds complexity to the manufacturing process to achieve the desired specifications: a thin and heavily doped thin film, with high optical transmission, lattice matching, and higher bandgap than the bottom cell to avoid parasitic absorption of the solar radiation. Such specifications often require deposition techniques like epitaxial growth, which are not readily available in 5 conventional thin films manufacturing processes [20]. Alternatively, in 4T tandem solar cells (Figure 1b), the subcells are fabricated independently and finally joined together by mechanical stacking, which requires an optically transparent insulating layer. This configuration provides optical coupling without electrical coupling, eliminating the need for current matching. However, a significant disadvantage of this configuration is the requirement for more than one transparent electrode, resulting in increased parasitic absorption. Additionally, more cable and maximum power point trackers becomes necessary, leading to a negative cost impact [21]. This research study investigates and compares the performance of both 3T and 4T tandem configurations (Figure 1b and 1c, respectively), originally proposed by Nagashima et al [22], with the widely adopted 2T design. The 3T configuration allows for the individual operation of top and bottom subcells in a monolithic device, predicting that the photogenerated current density is similar as 2T design. Furthermore, each subcell can be operated independently at their maximum power point tracker as the 4T design [23]. A key innovation in the proposed 3T and 4T tandem solar cells is the utilization of a photonic crystal (1DPC) as a replacement for the intermediate electrode. This 1DPC is composed of transparent conductive oxides (TCO) materials stacked, providing not only electrical functionality as an electrode but also introducing the novel capability of beam Figure 1. Schematic representation of device configuration: a) 2T tandem solar cell; b) 4T tandem solar cell using a dielectric 1DPC; c) 3T tandem solar cell using a n-type 1DPC 6 splitting to enhance the performance of the tandem solar cell. Moreover, the 1DPC has a layer of NiO on top, acting as a p-type TCO material, forming an ohmic contact with the absorber layer instead of forming a Schottky barrier with detrimental consequences in the solar cell performance. Significantly, unlike previous literature, our proposal approach avoids the requirement of tunnel junctions, which are typically simulated with ideal characteristics to circumvent their inherent challenges. Instead, our research offers a more practical and reliable alternative, which we believe facilitates experimental investigations with improved accuracy and reproducibility. 2. Methods 2.1. Device structure Based on Figure 2, the simulated structure of the solar cell comprises a CZTS/CZTSSe tandem junction. The top and bottom subcells consist of CZTS (with a bandgap of 1.45 eV) and CZTSSe (with a bandgap of 1.096 eV), respectively. The design of both architectures is based on the experimental results reported by Shin et al [24] and Wang et al [25]. These architectures serve as the baseline cell for our study, but several modifications have been implemented to explore strategies for increasing the efficiency of the tandem solar cell. The layers of the cell are arranged as follows: AZO/i- ZnO/CdS/CZTS for the top subcell and i-ZnO/CdS/CZTSe/Mo for the bottom subcell. In the top subcell, an aluminum doped ZnO (AZO) layer serves as the transparent front electrode, while molybdenum (Mo) acts as the back contact in the bottom subcell. The ntype buffer layer consists of a cadmium sulfide (CdS) thin film. Table 1 provides further details on the parameters used for each material [26-31]. 7 Parameter TCO ZnO CdS CZTS CZTSSe Thickness (nm) 450 50 50 variable variable Bandgap (eV) 3.3 3.3 2.4 1.45 1.09 Electron affinity (eV) 4.6 4.6 4.5 4.3 4.1 Dielectric permittivity 7.8 7.8 8 10 13.6 Electron effective mass 0.275 0.275 0.28 0.18 Hole effective mass 0.59 0.59 0.70 0.71 Conduction band effective density of states (cm-3) 3.6×1018 3.6×1018 1.8×1018 2.2×1018 2.2×1018 Valence band effective density of states (cm-3) 1.1×1019 1.1×1019 2.2×1019 1.8×1019 1.8×1019 Electron mobility (cm2/Vs) 150 150 160 100 100 Hole mobility (cm2/Vs) 25 25 50 44 44 Donor concentration (cm-3) 5×1020 2×1017 2×1017 0 0 Acceptor concentration (cm-3) 0 0 0 5×1016 2×1016 Table 1. Parameter set for the simulation of the solar cell structures. 2.2. Beam splitter The incoming solar spectrum can be split between the two absorbers selected for the tandem solar cells, for example, by means of a one-dimensional photonic crystal (1DPC) which is placed between the two subcells (Figure 2). Spectrum splitting eliminates the problem of lattice matching in tandem solar cells, at the same time as it separates electrical connections minimizing the problem of the current matching [32]. Solar irradiance energy with low wavelength is mostly absorbed in the top subcell meanwhile higher wavelengths Figure 2. Schematic cross section of the 3T tandem solar proposed and the 1DPC beam splitter as intermediate electrode 8 are absorbed in the bottom subcell. Although there are optical losses in the materials, a portion of the low wavelength energy is not absorbed, but it is reflected from the selective 1DPC surface and directed back to the top subcell passing through it by a second time increasing the optical absorption probability. This reflection does not affect the energy transmitted at high wavelength range suitable for the bottom subcell. Also, unlike solar spectrum splitters made from more conventional but non-conducting transparent optical materials such as silica or titania, these materials can be replaced by transparent conductive oxides (TCO) adding an extra functionality serving as intermediate electrodes in the 3T tandem solar cell. Therefore, the transmitted spectrum (𝐺𝜆) to the bottom subcell, is given by: 𝐺𝜆=𝐺0,𝜆 ·𝑒𝑥𝑝(−∑𝛼𝑖,𝜆𝑑𝑖)·𝑅𝜆.1𝐷𝑃𝐶 [𝐸𝑞.1] where 𝐺0,𝜆 is the incident solar spectrum, 𝛼𝑖,𝜆 is the absorption coefficient of each material of the top solar cell, 𝑑𝑖 is the thickness of each layer forming the top solar cell, and 𝑅𝜆,1𝐷𝑃𝐶 is the reflectivity of the 1DPC intermediate electrode acting as beam splitter. The 1DPC is specifically designed to consist of alternating layers of two transparent materials, enabling the refraction, reflection, and transmission of solar irradiance at each interface of the multilayer. By carefully selecting the thicknesses and refractive indexes of these materials (𝑛1, 𝑛2), constructive interference among the reflected light occurs at a particular wavelength. This phenomenon defines the position of the Bragg peak, which is the reflectance in correspondence of the first order of the multilayer. Bragg peak characteristics can be easily adjusted by proper manipulation of the thickness and optical properties of the multilayer structure achieving forbidden gaps as a minimum in the transmittance spectra. The maximization of reflectance is achieved by ensuring that the periodic thickness of the multilayer satisfies the 𝜆4 ⁄ condition (Eq. 2) based on optical 9 principles. Additionally, the width of the Bragg peak is influenced by the index contrast between the materials employed in the design of the 1DPC, while still conforming to the 𝜆4 ⁄ condition (Eq. 3) [33]. 𝑅1𝐷𝑃𝐶 =1−4(𝑛1 𝑛2)𝑁=1−4(1−Δ𝑛 𝑛2)𝑁 [𝐸𝑞.2] ΔE1𝐷𝑃𝐶 =4𝐸1𝐷𝑃𝐶 𝜋𝑛2−𝑛1 𝑛2+𝑛1=4𝐸1𝐷𝑃𝐶 𝜋Δ𝑛 2𝑛1+Δ𝑛 [𝐸𝑞.3] where E1DPC is the energy of the Bragg peak. Upon examining these equations, it becomes evident that the reflectivity is enhanced as the number of layers stacked increases (𝑁), and the width of the Bragg peak is directly influenced by the contrast in refractive index (Δ𝑛). To serve its designated role as an intermediate beam splitter, the 1DPC has been seamlessly integrated between the sub-cells within the tandem solar cell configuration. Tailored to meet the specific demands of the 3T and 4T setups (Figure 1), the 1DPC has been engineered for selectively transparent with n-type characteristics in the former, and selectively transparent and dielectric performance in the latter. The development of this 1DPC was optimized through the utilization of a Matlab code grounded in the transfer matrix formalism. This computational framework facilitated the optimization of number of layers and thicknesses to achieve a Bragg peak matching with the maximum absorption point of CZTS. The outcomes of our simulations solidified the efficacy of our approach. For the dielectric design of the 1DPC within the 4T tandem solar cell, and alternating pattern 1 2 1 2 1 2 1 was found to be optimal. This design involves the utilization of a 95 nm-thick layer of ZnO (1) with a refractive index of 1.95, coupled with a 57 nm-thick layer of SiO2 (2) possessing a refractive index of 1.45. Conversely, the n-type 1DPC 16 ambiguous to choose the representative value, but the obtained simulations reveal that, independently of the absorber thickness selected, the cutoffs of the EQE do not show any displacement in wavelength. The only difference is observed in the short wavelength region (lower than 500 nm). It is well known that in thin film solar cells, the space region affects significant as the volume of the space charge region and the quasi-neutral regions are comparable, resulting in the potential extension of the depletion region along the entire length of the absorber layer. Under these circumstances recombination in the depletion region limits the photocarriers collected efficiently because of the presence of the high electrical field in this region. Thus, a complex relationship exists between the doping concentration, absorber layer thickness, lifetime, and mobility of minority carrier, and the photocurrent generated. This is the reason that among all the layers considered in the solar cell architecture, the absorber layer plays a crucial role in determining the device’s efficiency. Figure 6 represents the influence of doping concentration and layer thickness on the photocurrent generated. As the absorber layer thickness increases, the simulated value of Jsc enhances from 11 to 22 mA·cm-2 due to a reduction in the recombination rate. With a higher absorber layer thickness, the probability of minority charge carrier recombination decreases [42]. The results demonstrate that while material properties have an impact on Figure 5. External Quantum Efficiency (EQE) as function of the absorber thickness, a) CZTS absorber, and b) CZTSSe absorber. 17 the device performance, the absorber layer thickness exhibits a stronger correlation with overall performance compared to the doping level. The value of acceptor defects, defined by its concentration (NA) is another critical parameter for simulating p-n junction solar cells, and the results observed in Figure 6 determine the values defined in Table 1. CZTS thickness selected must be over 300 nm to ensure high photocurrent density generated. The simulations of single junction kesterite absorbers (Figure 4) do not exhibit large differences in the Voc. Current density is calculated considering the incoming solar spectrum on the top subcell (𝐽𝑠𝑐 (𝑟𝑒𝑓)), the reflected spectrum by the 1DPC to the top subcell (𝐽𝑠𝑐 (𝑟𝑒𝑓)), and the transmitted spectra through the top subcell to the bottom subcell: 𝐽𝑠𝑐 (𝑎𝑏𝑠) =∫𝐸𝑄𝐸(𝜆)·𝐺0(𝜆)𝑑𝜆 [𝐸𝑞.5] 𝐽𝑠𝑐 (𝑟𝑒𝑓)=∫𝐸𝑄𝐸(𝜆)·(1−𝑒−𝛼𝐶𝑍𝑇𝑆(𝜆)𝑡𝐶𝑍𝑇𝑆)𝐺0(𝜆)·𝑅1𝐷𝑃𝐶(𝜆)𝑑𝜆 [𝐸𝑞.6] 𝐽𝑠𝑐 (𝑡𝑟𝑎𝑛𝑠) =∫𝐸𝑄𝐸(𝜆)·(1−𝑒−𝛼𝐶𝑍𝑇𝑆(𝜆)𝑡𝐶𝑍𝑇𝑆 −𝑅1𝐷𝑃𝐶)𝐺0(𝜆)𝑑𝜆 [𝐸𝑞.7] 𝐽𝑠𝑐 𝑡𝑜𝑝 =𝐽𝑠𝑐 (𝑎𝑏𝑠) +𝐽𝑠𝑐 (𝑟𝑒𝑓) [𝐸𝑞.8] Figure 6. Influence of the top absorber layer thickness and doping concentration in the photocurrent generated. 18 𝐽𝑠𝑐 𝑏𝑜𝑡𝑡𝑜𝑚 =𝐽𝑠𝑐 (𝑡𝑟𝑎𝑛𝑠) [𝐸𝑞.9] where 𝐺0(𝜆), 𝑅1𝐷𝑃𝐶(𝜆), 𝛼𝐶𝑍𝑇𝑆(𝜆) and 𝑡𝐶𝑍𝑇𝑆 are the incident solar irradiance spectrum, the reflectivity response of the 1DPC, absorption coefficient and thickness of the CZTS absorber layer, respectively. Applying [Eq. 8] and [Eq-9] the density current for both subcells as function of their thicknesses is calculated, considering that the AM 1.5G solar spectrum received by the bottom cell is reduced due to the top cell absorption, and the solar irradiance incoming in the top cell is also enhanced due to the reflectivity of the 1DPC placed between both subcells. Looking the current matching conditions (Table 3) between both subcells, the top CZTS solar cell should be thinner than the conventional design (typically 1.5 to 2.0 µm), and the optimum thickness for both subcells forming the tandem device is determined by the intersection showed in Figure 7. Figure 7 shows how the photocurrent generated by the bottom subcell increases with the CZTSSe thickness, but it is also damped as the CZTS thickness increases enhancing the absorption effect on the solar radiation available on it. Maximum value simulated through [Eq. 7] is 32.4 mA·cm-2, which is lower than the champion solar cell fabricated by Wang et al [25] with the same absorber material, due to the reduction of solar irradiance received as consequence of the top subcell absorption. However, this Jsc obtained is close to similar approaches reported by Ferhati et al [17] and Saha et al [30]. The photocurrent generated by the top subcell increases with the CZTS absorber thickness, and the maximum value obtained (24.97 mA·cm-2) is higher than previously reported [17, 19, 26, 30] due to the optical consequences of the intermediate 1DPC multilayer [Eq. 8], despite the absorber layer thickness selected in this simulation is similar to other research works. 19 As the Voc does not show significant differences, the overall performance in both 3T and 4T tandem configurations, is summarized in Table 4. The J-V characteristics of 3T (subcells connected in series) and 4T tandem solar (subcells connected in parallel) is illustrated in Figure 8. Each J-V characteristic curve (single subcells, 3T and 4T tandem solar cells) is simulated independently using SCAPS. The 3T tandem solar cell exhibits a 22.97% performance, meanwhile 4T tandem configuration reduce the performance to 21.82%, but in both cases is higher than any single junction solar cell kesterite-based. The 3T tandem has the minimum Voc of both subcells and Jsc is the sum of each individual subcell, as the equivalent electrical circuit is represented by two subcells connected in parallel. The 4T tandem has the opposite configuration: the Jsc is the minimum value corresponding to the top subcell, and the Voc is the sum of both single Voc. Our work represents a higher achievement compared to the literature reported values for kesterite/kesterite double junction 2T tandem solar cells (Table 5). Some of these works employ an ideal tunnel junction [17] eliminating any electrical resistance nor optical loss, between the top and bottom subcells. With these results it has been demonstrated the benefits of using one 1DPC intermediate optical layer, with the additional advantage that Figure 7. Current density as function of both CZTS top and CZTSSe bottom absorber thickness layer 20 it serves as positive and negative electrode to avoid the sophisticate tunnel junction in favor of 3T or 4T tandem solar cells. 3T tandem 4T tandem Top subcell CZTS thickness (nm) 300 Jsc (mA·cm-2) 24.97 Voc (mV) 813 FF (%) 69.1  (%) 14.03 % Bottom subcell CZTSSe thickness (nm) 2000 Jsc (mA·cm-2) 32.41 Voc (mV) 572 FF (%) 67.4  (%) 12.49% Tandem CZTS/CZTSSe thickness (nm) 300 2000 Jsc (mA·cm-2) 57.37 24.97 Voc (mV) 572 1385 FF (%) 69.2 66.7  (%) 22.71% 23.07% Table 4. Comparison of numerical optoelectronic parameters with the doublejunction kesterite based solar cell using a 3T or 4T electrical configuration. Figure 8. J-V curve single junction top and bottom subcells, and 3T/4T tandem configurations 21 From Table 4 it is observed that the current matching condition can be achieved by utilizing either the JMPP or JSC values, resulting in similar efficiencies. This similarity arises due to the comparable fill factors of the top and bottom subcells, which are commonly observed in III-V tandem solar cells. Some previous studies only considered the JSC value to define the current matching condition [31, 44-45]. However, when the bottom subcell exhibits a significantly higher fill factor than the top subcell, it is more advantageous to match the current density using JMPP values [46]. In the current study, employing the current matching conditions outlined in Table 4, no significant disparities are observed in terms of the overall performance of the tandem solar cell. The results presented in Table 4 align well with the findings depicted in Figure 3. The incorporation of the 1DPC as a beam splitter resulted in a 18% improvement of the solar irradiation on the CZTS top subcell. This increase is consistent with the observed enhancement in the Jsc values during the model calibration process (19.9 mA·cm-2) against the top subcell (24.97 mA·cm-2). Conversely, the Jsc obtained for the CZTSSe bottom subcell (32.41 mA·cm-2) is slightly lower than the calibrated value (35.7 mA·cm- 2), as illustrated in Figure 3. This discrepancy can be attributed to the influence of the 1DPC, which affects the transmission of the solar irradiance spectrum incident upon it. The results obtained from the design of a 3T/4T tandem solar cell are compared with prior existing data reported in the literature (Table 5). When comparing the 3T and 4T configurations with the 2T configuration featuring an ideal tunnel junction, it becomes evident that the intermediate electrode with advanced optical functionalities (1DPC) provides significant benefits. The selective reflection of the 1DPC structure allows the top subcell to generate a high current density, despite employing a thinner absorber layer to minimize optical absorption losses in the bottom subcell. Furthermore, the combination of both subcells, considering the previously defined matching criteria, yields higher 22 efficiencies compared to previous studies. Additionally, it has been reported [20] that 3T/4T design offers a reproducible manufacturing device, avoiding the constrains associated with the use of tunnel junctions. Previous works often assumed the ideal characteristics of the tunnel junction due to the challenges in simulating their characteristics accurately. However, our design overcomes these limitations, providing a practical and feasible solution for tandem solar cell manufacturing. Structure Voc (mV) Jsc (mA·cm-2) FF (%)  (%) Refs. 2T CZTS/CZTSSe 1420 19.8 71.5 20.1 [17] 2T CZTS/ACZTSe 1370 17.3 68.3 16.2 [26] 2T CZTS/CZTSSe 1100 20.4 81.3 18.2 [31] 2T CZTS/CZTSe 1143 19.17 72.6 19.86 [48] 2T CZTS/CZTSe 1320 20.98 78.2 20.87 [13] 2T CZTGS/CZTS 1350 18.53 62.2 17.51 [14] 2T CZTS/Si 1400 19.38 83.5 22.9 [43] 2T CZTS/Perovskite 1250 15.80 81.5 19.80 [32] 3T CZTS/CZTSSe 572 57.37 69.2 22.71 Our work 4T CZTS/CZTSSe 1385 24.97 66.7 23.07 Our work Table 5. Comparison of similar works in the literature data and our work A second advantage of the 1DPC designed, used as a beam splitter to establish electrical connections between the top and bottom subcells, is the incorporation of NiO as a high doped p-type semiconductor layer. NiO possesses a high working function of 5.3 eV and a bandgap of approximately 3.8 eV [47]. In contrast, conventional kesterite-based solar cells typically employ Mo as the back contact electrode, which could result in the formation of a n-type MoS2 layer. This layer introduces a high series resistance, leading to a decrease in FF and an undesirable junction with p-type absorber layer. Furthermore, the work function of Mo back contact (4.5 eV) compared to CZTS (5 eV) creates a Schottky barrier that reduces Voc [48]. By implementing NiO as the back contact of the top-subcell, several benefits can be achieved. NiO work function minimizes the Schottky 23 barrier for the majority of the carriers (holes) at the back contact, facilitating an ohmic contact between the electrode and the absorber layer (Figure 9). Figure 9 depicts the band diagram of conventional kesterite-based solar cell employing Mo as the back contact (represented by the red line), along with the impact of replacing Mo with NiO in the top-subcell (represented by the blue line). The energy levels of the CZTS shift upwards as the work function increases, ultimately achieving an optimum band alignment when NiO is incorporated into the kesterite hetero-structure. The transport of a hole carrier is described as follows: photogenerated holes within the absorber layer migrate towards the back contact electrode and recombine with incoming electrons from the external circuit. The introduction of a NiO thin film layer alters the nature of the contact between the electrode and the p-type layer, shifting it towards a Schottky or an ohmic contact, thereby minimizing recombination losses. Consequently, it is observed that the Voc increases with the work function of the material selected as back contact in the solar cell. This outcome explains why Voc values obtained in the present study for the top subcell (as presented in Table 5) are higher than those reported in the previous literature. Figure 9. (a) Dependence of open circuit voltage with the back contact work function; (b) Band diagram of the kesterite solar cell for two different work functions using Mo (red) and NiO (blue) as back contact electrode 24 4. Conclusions This article presents a numerical investigation using SCAPS software to investigate tandem kesterite solar cells in 3T/4T configuration, incorporating a novel 1DPC multilayer as an intermediate electrode. The 1DPC, composed of stacked ITO and SiO2 layers with a NiO layer on top, offers a promising and reliable configuration for a beam splitter, effectively boosting the open circuit voltage of the top subcell. The research study began with the calibration of the modeling parameters to accurately reproduce the performance of both CZTS and CZTSSe absorber layers as achieved experimentally. Subsequently, the thicknesses of the top and bottom subcells were optimized, taking into account the modified irradiance spectrum due to the optical properties of the 1DPC. The 1DPC reflects lower wavelengths to improve the Jsc in the top subcell, while considering the absorption spectra of the 1DPC and CZTS to calculate the Jsc in the bottom subcell. Once the optimal thicknesses were determined, the performance of the tandem solar cell was evaluated in both 3T and 4T configurations, achieving efficiencies of 22.71% and 23.07%, respectively. These results were compared with previous reports in the literature, mainly focusing on the 2T configuration assuming an ideal tunnel junction layer. Furthermore, this research highlights that replacing the conventional Mo electrode with NiO as the back contact for the top subcell significantly boosted the Voc, thereby enhancing the overall performance of the device. This improvement can be attributed to the analysis of the and energy levels in the hetero-structure of the solar cell, demonstrating that NiO promotes ohmic contacts instead of Schottky barriers, effectively reducing carrier recombination. 25 Overall, the present proposal overcomes the challenges associated with using tunnel junction layers in tandem solar cells, offering a better understanding and improving accuracy for these devices when experimentally tested. Our results will enable a comprehensive understanding of the implications and advantages associated with utilizing a 1DPC as an interconnection component within 3T or 4T multi-terminal tandem solar cell configurations. It is recommended that forthcoming research endeavors focus on assessing the challenges encountered during the experimental realization of 3T and 4T tandem solar cells, employing a single 1DPC as an intermediate electrode. These experimental results should be systematically examined and compared against the outcomes of fabricating similar 2T tandem solar cells equipped with a tunnel junction.