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Article

Investigation of Electron Transport Material-Free Perovskite/CIGS Tandem Solar Cell

by
Mostafa M. Salah
1,*,
Abdelhalim Zekry
2,
Ahmed Shaker
2,
Mohamed Abouelatta
2,
Mohamed Mousa
1 and
Ahmed Saeed
1
1
Electrical Engineering Department, Future University in Egypt, Cairo 11835, Egypt
2
Faculty of Engineering, Ain Shams University, Cairo 11535, Egypt
*
Author to whom correspondence should be addressed.
Energies 2022, 15(17), 6326; https://doi.org/10.3390/en15176326
Submission received: 22 July 2022 / Revised: 11 August 2022 / Accepted: 13 August 2022 / Published: 30 August 2022
(This article belongs to the Special Issue Modeling and Simulation of Solar Cells)
Browse Figures
Versions Notes

Abstract

:
Tandem solar cells have a superb potential to push the power conversion efficiency (PCE) of photovoltaic technologies. They can be also more stable and economical. In this simulation work, an efficient perovskite solar cell (PSC) with Spiro-OMeTAD as a hole transport material (HTM) and with no electron transport material (ETM) to replace the traditional PSC structure is presented. This PSC is then used as a top sub cell together with a copper indium gallium sulfide (CIGS) bottom sub cell to build a tandem cell. The multi-junction solar cell behavior is improved by engineering the technological and physical parameters of the perovskite and HTM. The results show that an n-p heterojunction PSC structure with an ETM free could be a good candidate for the traditional n-i-p structure. Because of such investigations, the performance of the proposed ETM-free PSC/CIGS cell could be designed to reach a PCE as high as 35.36%.

1. Introduction

Nowadays, the need for energy is increasing rapidly [1,2]. Clean energy, especially produced by solar cells, presents a promising solution [3]. The market for photovoltaics is now dominated by multi-crystalline and monocrystalline solar cells made of crystallized silicon. Solar cells made of crystalline silicon are currently more than 25% efficient [4,5]. Furthermore, thin film solar cells produced from CIGS are extremely competitive and have a PCE of 23.35% [6]. PSCs have also been developed, demonstrating cursory progress, and giving the field of photovoltaics new directions. PSC devices now have an efficiency record of more than 22% [7], and the advancement of PSCs in the last few years shows simulation results that exceed 30% [8]. Perovskite-based cells perform better due to low recombination losses, high extinction coefficients, a long diffusion length, a decent short circuit current, and energy gap adjustability.
ETM-free PSCs are very promising cells because they offer simple architectures, are low-cost, and have a high performance [9]. Because of the imbalanced charge transfer rate and the lack of a permanent built-in field in the absence of an ETL, ETL-free perovskite solar cells suffer from significant hysteresis and unsteady stabilized-power output [10]. The surface modification of an FTO substrate with a self-assembled fullerene monolayer (SAM), on the other hand, can improve device performance and reduce PSC hysteresis [11,12]. FTO effects on cells can be treated using different methods, one of which is produced for PSCs in a simplified configuration of glass/FTO–TMAH/perovskite/spiro-OMeTAD/Au using a modified FTO surface with tetramethylammonium hydroxide (TMAH). The increased device performance may be attributed to the improved charge extraction/transport, lower trap density, and lower charge recombination rate at the FTO/perovskite interface and in the perovskite layer, which displays up to 20.1% efficiency experimentally [13].
In 1961, in a single-junction solar cell with an energy gap of 1.34 eV, a maximum PCE of 33.7% was theoretically reported [14]. As the single junction can absorb photons with the same energy gap or higher of the materials being utilized, the corresponding cells have a constrained performance. The remainder of the incident photons are not absorbed, and the spectrum with higher energy loses the excess energy because of the thermalization process. This limitation could be resolved by tandem solar cells as they are made up of sub cells that perform better than single-junction solar cells and absorb different wavelengths of the light spectrum [15]. The sub cell with a high energy gap is placed at the top as it absorbs the shorter wavelength radiation of the incident solar radiation, while the longer wavelength part of the incident spectrum, which is not absorbed by the top sub cell, will be transferred towards the bottom cell. The maximum theoretical PCE has been found to be more than 68% for infinity sub cells [16].
Tandem cells with two sub cells can be built through various methods; either four-terminal mechanically stacked and spectrum split, three-terminal, or two-terminal monolithic and mechanically stacked devices, as illustrated in Figure 1 [17]. The fundamental electrical constraint in these cells is the current in the two-terminal (2T) tandem cells. The cell performance is restricted by matching the current between the sub cells. In three-terminals (3T) tandem cells, the sub cells can be connected in parallel. The lowest voltage in this pair limits the voltage of this type of tandem cell. In four-terminal (4T) tandem cells, independent sub cells with their anodes and cathodes are connected electrically by an external circuit. The four-terminal spectrum split is still fundamental for the four-terminal device, and the light spectrum is separated using a dichroic filter. The additional expenses of the filter and the four terminals restrict the architecture’s practical potential.
The performance of solar cells can be simulated, assessed, and estimated through the use of simulation tools. Additionally, simulation technologies help designers save time and money by optimizing several architectural and material characteristics. It is well known that SCAPS-1D is a useful simulator for designing different kinds of solar cells [18]. Consequently, it has been used and validated against experimental works [8]. Up to seven layers of solar cells may be simulated using SCAPS-1D [19]. In addition, almost all material parameters, such as energy gaps, affinities, thicknesses, and others, may be tuned [20].
Most works on perovskite/CIGS tandem cells have concentrated on the mechanically stacked design. To enhance the use of the solar spectrum, a semitransparent PSC is placed on top of a CIGS device [21]. Because of the modest capital expenditure required for this technology, current technologies may be upgraded, providing an easy path to commercialization [22]. The four-terminal design is commonly used because it eliminates the requirement for the complicated current matching condition for two-terminal cells. The utilization of two distinct substrates decreases the constraints on the manufacturing process and cell design [21]. A PCE of 15.5% has been reported using solution-processed PSC/CIGS tandem cells [23]. Because mixed-halide perovskites have a poor stability [24], a semi-transparent MAPbI3 cell with 12.7% PCE was layered atop a 17% CIGS cell to generate a PCE of 18.6% efficient tandem cell [22]. The replacement of the ZnO:Al TCO with sputtered In2O3:H resulted in a 20.5% tandem cell [25]. A tandem device with a PCE of 20.7% has been reported [26]. Improved tandem PCEs of up to 22.1% by employing a transparent contact consisting of MoO3 and ZnO:Al was achieved [27]. Recently, Si/CIGS four-terminal tandem solar cells have exceeded a PCE of 25% [28]. According to a comprehensive optical investigation of perovskite/CIGS tandem cells, a PCE of 29% was reported in the literature [29].
In this paper, a PSC/CIGS four-terminal mechanically stacked tandem cell was formed by two independently designed cells, which allowed for a more efficient construction procedure. Each cell created its electrical output, necessitating the need for four-terminal connections. Iodide/chloride mixed halide perovskite MAPbI3−xClx, produced by incorporating Cl into MAPbI3, was used because its films exhibit a higher thermal stability and longer diffusion lengths [8], besides its tunable bandgap energy. In addition, a PSC without an ETM as a top cell replacing the conventional cell was proposed. It demonstrated a better performance. Furthermore, the influence of the thickness and doping of the sub cells’ thin films was investigated in order to increase the PCE. The impact of the defect density and energy gap of the absorber films in the sub cells was studied.
The remainder of this paper is arranged as follows. At the beginning of Section 2, it is discussed how the tandem solar cell was initially conceived and how it has been explored. An n-i-p PSC calibration is shown in Section 2.1, as a CIGS solar cell with experimental cells. In Section 3.1.1, a PSC with ETM free is proposed. Moreover, an optimization of the proposed cell is presented. Section 3.1.2 shows the optimization of the calibrated CIGS cell. Section 3.1.3 compares the calibrated cells and the optimized cells. The optimized tandem device is illustrated in Section 3.2. Furthermore, Section 3.3 gives a comparative study between the suggested tandem cell and recent studies. Finally, Section 4 draws the conclusions.

2. Materials and Methods

In the proposed tandem cell, PSC served as a top sub cell, and a GIGS solar cell served as a bottom sub cell, as illustrated in Figure 2a. The energy diagrams of the two sub cells are illustrated in Figure 2b (for the top cell) and Figure 2c (for the bottom cell). The top sub cell is composed of the perovskite layer stacked between the HTM and ETM. After calibration against an experimental cell, a PSC without ETM will be proposed.
The FTO is an optical transparent cathode, permitting light to cross the layers and transport the electrons produced to the cell’s contacts. In the front of the CIGS absorber material, an n-type window layer consisting of two layers of zinc oxide (ZnO) and Cadmium sulfide (CdS) exists. Because of its characteristics, mainly tunable Eg, CIGS is an efficient absorber in the bottom sub cell. The transferred spectrum of the top cell towards the bottom cell, and the absorption coefficient (α) are given by Equations (1) and (2) [30,31]. The definitions of the parameters of Equations (1) and (2), and their units are listed in Table S1.
S ( λ ) = S o ( λ ) . ∏ x = 1 n e − α x d x   .
α ( E ) = A α h ν − E g  

2.1. Validation of Physical Parameters Selection

A PSC with an ETM was modeled into SCAPS-1D. The results were validated with published experimental PSCs [32]. The design of the fabricated cell was a glass substrate, FTO (work function = 4 eV), titanium dioxide (TiO2) as an ETM, MAPbI3−xCIx as an absorber layer, Spiro-OMeTAD as an HTM, and finally an anode metal contact [32]. The material parameters utilized in the SCAPS-1D simulator are given in Table 1, while the ETM/absorber and absorber/HTM interfacial parameters are recorded in Table S2. To accomplish the practical considerations, a neutral defect with Gaussian distribution was invoked [33]. A 300 K temperature and air mass of 1.5 global (AM 1.5G) as the incident spectrum were utilized for all simulations. Thermal velocities of the carriers of 107 (cm/s) were used [34]. The absorption coefficients (α) were computed using Equation (2).
Figure 3a depicts the J–V characteristics of the experimental PSC with various thicknesses of the perovskite material versus the simulated one, while Figure 3b illustrates the J–V curves of the experimental PSC after optimization against the simulated PSC. The performance parameters (short circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE) of the simulation and the experimental work with different perovskite thicknesses, as well as the champion one, are recorded in Table S3, and Table 2, respectively. The simulation outputs were consistent with the experimentally reported ones.
Furthermore, the CIGS cell was designed as a bottom sub cell of 4T multi-junction solar cells. So, firstly, the CIGS cell model was calibrated with the reported experimental results [35]. The parameters used for the calibration are given in Table 1, while the cell structure is displayed in Figure 2a. The J–V characteristics of both the experimental and simulation CIGS devices are demonstrated in Figure 3c. The output metrics of the calibrated and the experimental cells are given in Table 2. The output parameters of the calibrated device were consistent with the experimentally reported characteristics and cell parameters. The good agreement validates the used physical parameters in the simulation for the two sub cells.
There was little difference between the simulated results and the experimental results in the PSC and CIGS cell. These differences could result from a minimal difference in the layers’ thickness and some physical parameters, which were not given in the experimental work.

3. Results and Discussions

3.1. Optimization of the Sub Cells

In this section, the main material parameters of the top and bottom sub cells as single-junction cells are optimized.

3.1.1. Optimization of the Top Sub cell

The doping of the materials in solar devices dramatically impacts the output performance. ETL has a stronger influence on the diffusion or carriers compared with the HTL [46]. Figure 4a illustrates the impact of ETM’s ND on the PCE. In PSCs, ETM doping has a low influence on the output performance when the absorber material has been doped [47]. Consequently, a PSC without ETM has been investigated in order to substitute the traditional PSC design to decrease the number of interfaces and layers. The perovskites could be doped p-type or n-type [48]. The doping density of perovskite could be adjusted between NA of 1014 cm−3 and ND of 7.6 × 1020 cm−3 [48]. After removing the ETM layer, the output parameters were constant in the NA range of 0–1015 cm−3. Figure 4b illustrates the impact of the absorber film’s ND on the PCE with and without ETM. This work proposes a suitable doping level of 1 × 1013 cm−3 for the perovskite active layer’s ND, achieving a PCE of 19.52%.
Because of the higher series resistance for the low doping concentration NA, the performance metrics were low. Upon increasing the doping of HTM, the energy band offset between the perovskite layer and HTM also increased, which caused the diffusion current to rise significantly to the gradient of the doping concentration and caused particles to move according to Brownian motion—random thermal motion—from the regions with the highest to the lowest concentrations. Figure 4c depicted the effect of HTM’s NA on the PCE. The highest PCE was achieved beyond NA of 1019 cm−3. It was preferable to keep NA at a low level to avoid high doping effects, such as band gap narrowing, the generation of deep Coulomb traps, and a reduction in mobility [49]. The output performance factors, PCE, FF, Voc, and Jsc, when NA of HTM was 1019 cm−3 were 20.05%, 77.38%, 1.1 V, and 23.51 mA/cm2, respectively. The effect of HTM thickness was also investigated.
Figure 4d depicts the impact of HTM’s thickness on the PCE. The HTM’s thickness has a minor effect on the performance parameters. It can be observed that as the thickness of HTM grew beyond 50 nm, the PCE slightly dropped. There was no change in PCE for thicknesses lower than 50 nm. Consequently, a layer thickness of 50 nm achieved a PCE of 20.08%, Jsc of 23.51 mA/cm2, and FF of 77.46%, and the same Voc was selected.
Notably, performance characteristics were greatly influenced by the perovskite layer’s defect concentration. The behavior of PSCs was significantly impacted by the quality of the photoactive material [50]. Photoelectrons were generated in the absorber thin film upon illumination. The lower quality of material implied a higher defect density, which resulted in increasing the non-radiative recombination losses [51]. This effect caused a higher SRH recombination rate, which was reflected in a deterioration in Voc [52]. Figure 4e illustrates the fluctuation in PCE against the variation of Nt. Notably, reducing Nt decreased the recombination rates of carriers. As a result, a better performance was achieved at lower values of Nt. It was revealed that an Nt of 109 cm−3 could be obtained [53]. However, as lowering Nt increased the cost, Nt of 1012 cm−3 could be selected as a compromise between a high performance and low cost. When Nt was 1012 cm−3, the key photovoltaic parameters were PCE = 24.64%, FF = 84.17%, Voc = 1.24 V, and, Jsc = 23.61 mA/cm2.

3.1.2. Optimization of the Bottom Sub Cell

The doping concentration ND of the CdS material was tested within the range of 1015–1018 cm−3. The PCE increased with the doping levels up to 1017 cm−3 and then slightly decreased. The PCE of the bottom sub cell with a CdS material doping concentration is shown in Figure 5a. The output parameters, PCE, FF, Voc, and Jsc, when ND of CdS was 1017 cm−3, were 20.14%, 75.39%, 0.72 V, and 37.12 mA/cm2, respectively. Furthermore, the thickness of the CdS layer was tested up until 200 nm. The results showed that the PCE decreased with an increasing thickness, as illustrated in Figure 5b. Consequently, a film thickness of 10 nm was chosen with a PCE, FF, and Jsc, of 20.67%, 76.47%, and 37.52 mA/cm2, respectively, and without variation in Voc of the previous step.
Moreover, the ZnO layer doping concentration was tested up to 1020 cm−3 as the CdS layer. Figure 5c shows the testing results of the bottom sub cell PCE with the ZnO layer doping level. PCE increased with the doping level up to about 1019 cm−3. The doping of the ZnO layer should not be too high to impede the rapid recombination rate. On the other hand, the doping concentration should not be too low to impede decreasing the FF [54]. The performance metrics, PCE, FF, and Jsc, for ND of ZnO of 1019 cm−3, were 20.78%, 76.8%, 0.72 V, and 37.55 mA/cm2, respectively, and without variation in Voc of the previous step. Furthermore, the PCE of the bottom sub cell was analyzed with ZnO film thickness of up to 300 nm, as displayed in Figure 5d. The PCE did not change up to 100 nm, and then decreased by a very small amount up to 200 nm, and was then fixed. So, the ZnO layer thickness variation had almost no effect on the bottom sub cell performance.
Next, the CIGS absorber layer of the bottom sub cell doping level was checked in the range of 1015–1020 cm−3. The PCE of the bottom sub cell with this variation of the doping concentration is illustrated in Figure 6a. The PCE increased with increasing the doping level. The highest PCE was achieved in the highest NA, as shown in Figure 6a. However, as stated before, it was preferable to keep NA at a low level because a high NA creates deep Coulomb traps and, in turn, decreases mobility [49]. The performance key factors, PCE, FF, Voc, and Jsc, for NA of CIGS of 1017 cm−3 were 22.13%, 77.82%, 0.76 V, and 37.31 mA/cm2, respectively. In addition, the CIGS layer thickness of the bottom sub cell was tested up until 5 µm. The PCE increased up to about 2.5 µm and then was almost saturated, as displayed in Figure 6b. Consequently, a thickness of 2.5 µm was selected, with the same performance parameters as the previous step. The last test of the optimization of the bottom sub cell was testing of the cell PCE with a defect level. Figure 6c shows the PCE vs. the variation of the defect density in the range of 1012–1015 cm−3. The PCE decreased by increasing the defect level by a small rate of up to 1014 cm−3 and then dropping by a high rate. Decreasing the defect concentration had a strong dependence on the quality of the fabrication process, which results in increased costs. Consequently, Nt of 1014 cm−3 was chosen to be a practical value with the same performance metrics as the previous step.

3.1.3. The Initial Sub Cells vs. the Optimized Sub Cells

Lowering the recombination rates at the ETM/CH3NH3PbI3−xCIx/HTM interfaces is a significant method to efficiently extract the carriers. The optimum valence band offset (VBO) and conduction band offset (CBO) to be 0–0.2 eV lower than and 0–0.3 eV higher than the VBO and CBO of the CH3NH3PbI3−xCIx, for HTM and ETM, respectively [55,56]. VBO and CBO are given by (3) and (4), respectively [38,55,56].
VBO = ( χ H T L + E g . H T L − χ p e r o . − E g . p e r o . )
CBO = ( χ p e r o . − χ ETM )
where χ H T L is the affinity of the HTM and χ p e r o . is the absorber layer affinity. Considering the equation and perovskite affinity of 3.9 eV, and the Eg is 1.55 eV, the optimum VBO of the HTM lies between the values of 5.15 eV and 5.45 eV. This condition is satisfied with Spiro-OMeTAD as HTM. The optimum CBO of the ETM lies in the range of 3.9–4.2 eV because of the previous equation and the perovskite affinity. This condition is satisfied with the TiO2 affinity, which is 4 eV. In addition, CBO is in the optimum range when removing the ETM as the affinity of FTO is 4 eV.
To give supplemental physical reasoning for the enhancement of the output parameters, Figure 7a,b illustrates the energy band profiles of the initial PSC and those of the optimum PSC with ETM free, respectively. As it can be inferred from these figures, the electrostatic potential between CH3NH3PbI3−xCIx and FTO films is better than the electric potential between CH3NH3PbI3−xCIx and TiO2. This improvement can speed up the separation of photon-generated carriers, lowering the recombination rate and improving the PSC metrics.
The variation of the performance factors could be clarified by observing the recombination rates of the initial and optimized cells. Figure 7c demonstrates the recombination rate (in cm−3·s−1) variations along the PSC distance. It is obvious from the figure that the recombination rate is high at the interface between perovskite/ETM, which can be prevented in the optimized cell. Furthermore, the recombination rate at the interface between HTM/perovskite after optimization is also lower than before optimization. In addition, the recombination rate in the absorber layer is reduced. The output performance trend supports the idea of removing the ETM from the PSC and showing our optimization’s importance. Table 3 lists a comparison of PCEs for single-junction PSCs of calibrated PSC, optimized PSC without ETM, and other investigations.
For a more supplemental physical reasoning for the enhancement of the output parameters, Figure 8a,b illustrates the energy band diagrams of the initial CIGS cell and the optimized one. It can be inferred from these figures that the spike-like CBO effect, which resulted in lowering the FF, was solved after optimization. This improvement could speed up the separation of photogenerated carriers, reducing the recombination rate and enhancing the performance parameters. The improvement of the performance parameters could be justified by the observation of the recombination rates before and after optimization. In this regard, Figure 8c demonstrates the recombination rate variations along the CIGS cell distance. As demonstrated, the recombination rate was very high at the ZnO/CdS interface, which was decreased after optimization.

3.2. The Final Tandem Cell

Based on the preceding outcomes, the top sub cell used a glass substrate, TCO, an absorber layer of N-type CH3NH3PbI3−xCIx, and an HTM of Spiro-OMeTAD. The bottom sub cell that was used had CIGS as an absorber layer. The CIGS employed in the bottom sub cell exhibited a high current density, which influenced the tandem cell’s overall behavior in terms of current density and PCE. The energy gap of the perovskite material could be tuned from 1.5 to 1.63 eV [62,63], while the energy gap of CIGS could be tuned from 1.04 to 1.67 eV [43,44,45]. Decreasing the energy gap increased the amount of the absorbed spectrum. Figure 9a illustrates the PCE of the N-CH3NH3PbI3−xCIx (without ETM)/CIGS tandem solar cell with various energy gaps of the absorbers of the sub cells. The top sub cell had a PCE of 27.74% when the absorber’s Eg was 1.5 eV, and its thickness was 0.9 µm. The bottom sub cell had a PCE of 20.04% when the absorber’s Eg was 1.04 eV with the transmitted spectrum of the top sub cell. The PCE of the tandem solar cell with these energy gaps was 34%. It can be concluded from Figure 9b that the highest PCE was at a 1.1 um thickness of the active thin film of the top sub cell. However, a thickness of 0.9 um was chosen with a PCE of 35.36% so as to be of more practical value with a low-cost fabrication [64,65]. Figure 9c shows the final tandem solar cell’s structure in detail, including each layer’s thickness and doping concentration.
The current density as a function of the voltage of the sub cells as an incident spectrum was AM 1.5G and of the bottom sub cell, as illuminated by the transmitted spectrum from the top sub cell, are illustrated in Figure 9d. Figure 9e shows the QE characteristics of the sub cells and the tandem cell. Figure 10a shows the materials used in the tandem cell absorption coefficients. At the same time, Figure 10b shows the absorbed spectrum of both sub cells with the incident AM 1.5G spectrum. According to the results, CIGS can absorb most of the transmitted spectrum up to 1200 nm when used as a bottom sub cell. This is larger than most of the other substances that can serve as bottom sub cell absorbers. This explains why the behavior of the tandem cell has improved.

3.3. Comparison with the State of Art Published Results

The PCE of some recent tandem solar cells during the past several years is described in this section. In this context, Table 4 compares the results of this study and other published results. Even among tandem cells with more than two connections and double junction tandem cells, the suggested tandem cell has one of the greatest efficiencies.

4. Conclusions

The tandem multi-junction solar cells’ performance exceeds single-junction cells because of their capability to absorb a broader spectrum. Here, we reported using a tandem solar cell that is based on perovskite and CIGS as top and bottom sub cells, respectively. The simulation of perovskite solar cells is firstly presented with TiO2 as an ETM and Spiro-OMeTAD as an HTM. Then, a PSC without an ETM is proposed in order to achieve more stability and reduce the cost of the top sub cell. This structure has a simple design and shows superior output metrics. The main material parameters of the suggested Spiro-OMeTAD/MAPbI3−xClx perovskite solar cell without an HTM are studied in order to enhance the photovoltaic device’s behavior. The doping, thickness of all layers, perovskite layer’s defect concentration, and energy gap are investigated. The output performance parameters after optimization, for PCE, FF, Voc, and Jsc, are 27.74% 80.71%, 1.23 V, and 28 mA/cm2, respectively.
Moreover, a CIGS sub cell is calibrated with a practical cell. In addition, the doping and the thickness of all layers, besides the CIGS layer’s defect concentration are studied. After optimization, the CIGS cell has performance metrics when illuminated by the transmitted spectrum of the top sub cell, for PCE, FF, Voc, and Jsc, which are 20.04%, 76.34%, 0.57 V, and 18.31 mA/cm2, respectively. This CIGS cell is utilized as a bottom sub cell of the tandem cell. All of the applied improvements on the sub cells, besides the energy gaps of the absorbers, reflect on the PCE of the suggested tandem cell to be 35.36%. This design shows a higher performance than many more complex tandem cells. The tandem cell that has been proposed here exhibits one of the tandem cells’ finest performances, despite being simpler.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en15176326/s1. Table S1: Physical parameters of the incident, transmitted spectrum definitions, and their units; Table S2: Parameters of defect interfaces; Table S3: Comparison of experimental PSC data [32], and simulations utilizing SCAPS-1D with different perovskite thicknesses. Reference [71] is cited in the Supplementary Materials.

Author Contributions

Conceptualization, M.M.S., M.A., A.S. (Ahmed Shaker), M.M., and A.S. (Ahmed Saeed); methodology, M.M.S., M.A., A.S. (Ahmed Shaker), M.M., and A.S. (Ahmed Saeed); validation and formal analysis, M.M.S., M.A., A.S. (Ahmed Shaker), M.M., and A.S. (Ahmed Saeed); visualization, M.M.S. and M.M.; investigation, all authors; writing—original draft preparation, M.M.S., M.A., A.S. (Ahmed Shaker), M.M., and A.S. (Ahmed Saeed); writing—review and editing, all authors; supervision, A.Z., M.A., A.S. (Ahmed Shaker), and A.S. (Ahmed Saeed). All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing does not apply to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Tandem solar cell types: (a) two terminals monolithic, (b) two terminals mechanically stacked, (c) three terminals monolithic, (d) four terminals mechanically stacked (used in this work), and (e) four terminals spectrum-split.
Figure 1. Tandem solar cell types: (a) two terminals monolithic, (b) two terminals mechanically stacked, (c) three terminals monolithic, (d) four terminals mechanically stacked (used in this work), and (e) four terminals spectrum-split.
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Figure 2. (a) The proposed perovskite/CIGS tandem device configuration and the energy levels of (b) the top cell and (c) the sub cell.
Figure 2. (a) The proposed perovskite/CIGS tandem device configuration and the energy levels of (b) the top cell and (c) the sub cell.
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Figure 3. The J–V characteristics of the proposed photovoltaic devices vs. the experimental cells: (a) PSC with different thicknesses [32], (b) the champion one of the PSCs [32], and (c) CIGS [35].
Figure 3. The J–V characteristics of the proposed photovoltaic devices vs. the experimental cells: (a) PSC with different thicknesses [32], (b) the champion one of the PSCs [32], and (c) CIGS [35].
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Figure 4. Output PCE variation of the top sub cell depends on (a) ND of the ETM, (b) ND of the perovskite material with and without the ETM, (c) NA of the HTM, (d) thickness of the HTM, and (e) Nt of the perovskite.
Figure 4. Output PCE variation of the top sub cell depends on (a) ND of the ETM, (b) ND of the perovskite material with and without the ETM, (c) NA of the HTM, (d) thickness of the HTM, and (e) Nt of the perovskite.
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Figure 5. (a) Photovoltaic output parameters of the bottom sub cell depend on (a) ND of the CdS layer, (b) thickness of the CdS material, (c) ND of the ZnO layer, and (d) thickness of the ZnO layer.
Figure 5. (a) Photovoltaic output parameters of the bottom sub cell depend on (a) ND of the CdS layer, (b) thickness of the CdS material, (c) ND of the ZnO layer, and (d) thickness of the ZnO layer.
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Figure 6. (a) NA of the CIGS layer, (b) thickness of the CIGS layer, and (c) Nt of CIGS.
Figure 6. (a) NA of the CIGS layer, (b) thickness of the CIGS layer, and (c) Nt of CIGS.
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Figure 7. Energy band diagrams of the PSC: (a) before the optimization of doping, thickness, defect density; (b) after the optimization; and (c) the recombination rates of the initial and the optimized cells.
Figure 7. Energy band diagrams of the PSC: (a) before the optimization of doping, thickness, defect density; (b) after the optimization; and (c) the recombination rates of the initial and the optimized cells.
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Figure 8. Energy band diagrams of the CIGS solar cell: (a) before the optimization of doping, thickness, defect density; (b) after the optimization; and (c) the recombination rates of the initial and the optimized cells.
Figure 8. Energy band diagrams of the CIGS solar cell: (a) before the optimization of doping, thickness, defect density; (b) after the optimization; and (c) the recombination rates of the initial and the optimized cells.
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Figure 9. Dependence of PCE of the tandem cell on (a) the energy gaps of the absorbers, (b) the thickness of the top sub cell absorber, (c) the final perovskite/CIGS tandem cell, (d) the output performance curves of the sub cells, tandem cell, J–V curves, and (e) QE curves.
Figure 9. Dependence of PCE of the tandem cell on (a) the energy gaps of the absorbers, (b) the thickness of the top sub cell absorber, (c) the final perovskite/CIGS tandem cell, (d) the output performance curves of the sub cells, tandem cell, J–V curves, and (e) QE curves.
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Figure 10. (a) Absorption coefficients of the materials used in the final tandem cell, and (b) AM 1.5G spectrum with the absorbed spectrum of the sub cells.
Figure 10. (a) Absorption coefficients of the materials used in the final tandem cell, and (b) AM 1.5G spectrum with the absorbed spectrum of the sub cells.
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Table
Table 1. The SCAPS-1D top and bottom sub cells’ material parameters.
Table 1. The SCAPS-1D top and bottom sub cells’ material parameters.
ParametersFTO [36]MAPbI3−xCIx [37,38]Spiro-OMeTAD [10,39,40]TiO2 [38,41,42]CIGS [35,43,44,45]CdS [44]ZnO [44]ZnO-Al [44]
Band gap energy Eg (eV)3.51.55331.04–1.672.453.303.30
Thickness (nm)200Fitting200502000–300050100400
Relative permittivity ɛr96.531013.610.09.09.0
Electron affinity χ (eV)43.92.3544.54.44.64.6
Effective conduction band density Nc (cm−3)2.2 × 10183 × 1018
Electron mobility µe (cm−2 V−1 s−1)2022 × 10−420100
Donor concentration ND (cm−3)2 × 10190010180102010171020
Effective valence band density Nv (cm−3)18 × 10171.8 × 10191.7 × 1019
Hole mobility µp (cm−2 V−1 s−1)1022 × 10−41025252531
Acceptor concentration NA (cm−3)002 × 101802×1016000
Defect density Nt (cm−3)10152 × 10141015101510141014–5 × 101610141014–1016
Thermal velocity of electrons vth,n (cm/s)1072.4 × 107
Thermal velocity of holes vth,p (cm/s)1.3 × 107
Table
Table 2. Comparison of the experimental PSC data [32], CIGS data [35], and simulations utilizing SCAPS-1D.
Table 2. Comparison of the experimental PSC data [32], CIGS data [35], and simulations utilizing SCAPS-1D.
AbsorberTypePCE (%)Jsc (mA/cm2)Voc (V) FF (%)
PerovskiteExperimental19.4123.801.0975.16
Simulation19.4123.501.0975.55
CIGSExperimental20.1036.300.7276.80
Simulation20.0836.220.7277.05
Table
Table 3. Comparison of PCEs of the latest PSCs.
Table 3. Comparison of PCEs of the latest PSCs.
ETM/HTMPCE (%)
TiO2/CuI [57]21.32
TiO2/CuI [58]21.76
ZnOS/CuI [58]26.11
PEDOT:PSS/HATNT [59]18.10
PEDOT:PSS/TDTP [60]18.20
TiO2/- [61]25.15
nanoparticle based SnO2/Spiro-OMeTAD [28]17.90
TiO2/Spiro-OMeTAD [32]19.41
-/Spiro-OMeTAD [optimized]27.13
Table
Table 4. A comparison of the suggested tandem cell with the state-of-the-art of published data.
Table 4. A comparison of the suggested tandem cell with the state-of-the-art of published data.
PCE (%)The Material of Top/Bottom Sub CellsRef.-Year
25Perovskite/CIGS[28]-2020
25.7Perovskite/C-Si[28]-2020
29.8Perovskite/Si[66]-2022
30.2Se/CZTSSe[67]-2019
30.5MAPbI3/CIGS[68]-2020
31.1GaInP/Si[69]-2020
33.3Triple-junction (Ga0.51 In0.49 P/GaAs/Si)[67]-2018
35.36MAPbI3−xClx without ETM/CIGSThis work
35.9Triple-junction (GaInP/GaAs/Si)[70]-2017
39.2Six-junction (monolithic)[34]-2017
30.2Se/CZTSSe[67]-2019
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Salah, M.M.; Zekry, A.; Shaker, A.; Abouelatta, M.; Mousa, M.; Saeed, A. Investigation of Electron Transport Material-Free Perovskite/CIGS Tandem Solar Cell. Energies 2022, 15, 6326. https://doi.org/10.3390/en15176326

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Salah MM, Zekry A, Shaker A, Abouelatta M, Mousa M, Saeed A. Investigation of Electron Transport Material-Free Perovskite/CIGS Tandem Solar Cell. Energies. 2022; 15(17):6326. https://doi.org/10.3390/en15176326

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Salah, Mostafa M., Abdelhalim Zekry, Ahmed Shaker, Mohamed Abouelatta, Mohamed Mousa, and Ahmed Saeed. 2022. "Investigation of Electron Transport Material-Free Perovskite/CIGS Tandem Solar Cell" Energies 15, no. 17: 6326. https://doi.org/10.3390/en15176326

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