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Article

TCAD-Based Design and Optimization of Flexible Organic/Si Tandem Solar Cells

by
Marwa S. Salem
1,
Mohamed Okil
2,
Ahmed Shaker
3,
Mohamed Abouelatta
4,
Mostafa M. Salah
5,*,
Kawther A. Al-Dhlan
6 and
Michael Gad
3
1
Department of Computer Engineering, College of Computer Science and Engineering, University of Ha’il, Ha’il 55211, Saudi Arabia
2
Department of Basic Engineering Sciences, Benha Faculty of Engineering, Benha University, Benha 13512, Egypt
3
Engineering Physics and Mathematics Department, Faculty of Engineering, Ain Shams University, Cairo 11535, Egypt
4
Electronics and Electrical Communication Department, Faculty of Engineering, Ain Shams University, Cairo 11535, Egypt
5
Electrical Engineering Department, Future University in Egypt, Cairo 11835, Egypt
6
Department of Information and Computer Science, College of Computer Science and Engineering, University of Ha’il, Ha’il 55211, Saudi Arabia
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(7), 584; https://doi.org/10.3390/cryst14070584
Submission received: 22 May 2024 / Revised: 17 June 2024 / Accepted: 19 June 2024 / Published: 25 June 2024
(This article belongs to the Special Issue Research on Electrolytes and Energy Storage Materials)
Browse Figures
Versions Notes

Abstract

:
In order to surmount the Shockley–Queisser efficiency barrier of single-junction solar devices, tandem solar cells (TSCs) have shown a potential solution. Organic and Si materials can be promising candidates for the front and rear cells in TSCs due to their non-toxicity, cost-effectiveness, and possible complementary bandgap properties. This study researches a flexible two-terminal (2-T) organic/Si TSC through TCAD simulation. In the proposed configuration, the organic solar cell (OSC), with a photoactive optical bandgap of 1.78 eV, serves as the front cell, while the rear cell comprises a Si cell based on a thin 70 μm wafer, with a bandgap energy of 1.12 eV. The individual standalone front and bottom cells, upon calibration, demonstrate power conversion efficiencies (PCEs) of 11.11% and 22.69%, respectively. When integrated into a 2-T organic/Si monolithic TSC, the resultant tandem cell achieves a PCE of 20.03%, indicating the need for optimization of the top organic cell to beat the efficiency of the bottom Si cell. To enhance the performance of the OSC, some design ideas are presented. Firstly, the OSC is designed by omitting the organic hole transport layer (HTL). Consequently, through engineering the front contact work function, the PCE is enhanced. Moreover, the influence of varying the absorber defect density of the top cell on TSC performance is investigated. Reduced defect density led to an overall efficiency improvement of the tandem cell to 23.27%. Additionally, the effects of the variation of the absorber thicknesses of the top and rear cells on TSC performance metrics are explored. With the matching condition design, the tandem efficiency is enhanced to 27.60%, with VOC = 1.81 V and JSC = 19.28 mA/cm2. The presented simulation results intimate that the OSC/Si tandem design can find applications in wearable electronics due to their flexibility, environmentally friendly design, and high efficiency.

1. Introduction

Solar energy is a key renewable energy source with the potential to meet global energy demands sustainably [1]. To reduce carrier thermalization losses and improve the efficiency of solar cells beyond the theoretical constraints of single-junction solar devices, tandem cells have emerged as an auspicious solution [2]. Tandem cells stack multiple photoactive materials with different bandgaps, enabling broader spectrum absorption and higher efficiencies. Various tandem solar cell (TSC) configurations have been researched both experimentally and numerically in the literature, encompassing both 2-terminal (2-T) and 4-terminal (4-T) designs. While TSCs with 4-T configurations can be easily fabricated using simple mechanical stacking methods, 2-T tandems, which require current matching and more specialized fabrication techniques, offer a more streamlined integration into photovoltaic (PV) systems. Despite the simpler fabrication process of the 4-T setup, it often results in a more complex overall system design because each sub-cell has its electrical output, necessitating additional components and wiring to combine the outputs and manage the power flow. This can increase design complexity and more intricate installation and maintenance processes. Conversely, the 2-T TSC needs only one transparent electrode between the top and bottom cells, which minimizes optical parasitic absorption and reduces overall costs [3]. For both 2-T and 4-T tandems, there is a wide range of potential sub-cell candidates, including silicon-based tandems, such as perovskite/Si [4,5] and Sb2S3/Si [6]. Additionally, CIGS-based tandems have been introduced, such as perovskite/CIGS [7,8], all-polymer/CIGS [9], II–VI/CIGS [10], and organic/CIGS [11]. Other systems comprise all-perovskite [12,13,14] and all-organic tandem solar cells [15,16], in which both top and rear solar cells are made of similar materials with differing energy bandgaps.
When selecting light-absorbing materials for TSCs, various factors, including commercial cost and technical aspects, must be contemplated. Crystalline silicon PV cells are broadly used due to their high efficiency and relatively low fabrication cost, constituting 90% of the PV market. The highest reported efficiency for Si cells is 26.7%, which is close to its theoretical limit [17]. Furthermore, silicon solar cells are widely recognized as excellent candidates for bottom cells in tandem configurations due to their advantageous properties. With a low bandgap of 1.12 eV, silicon efficiently absorbs a substantial portion of solar radiation. Additionally, silicon is abundant and has a well-established manufacturing infrastructure. As a result, extensive research and development efforts have been directed towards optimizing larger bandgap semiconductors to be utilized as top cells in Si-based bottom cell arrangements [18]. Theoretical calculations suggest that the maximum achievable efficiency for such Si-based tandems with a 1.7-eV top cell could reach 43% [19]. In practical implementations, top cells utilizing III–V compounds and perovskite absorbers have been integrated with Si cells in various configurations, demonstrating promising efficiencies in lab-scale devices. For instance, recent studies have reported efficiencies of up to 32.8% with III–V materials/Si [20] and significant advancements in perovskite/Si tandems, with a record efficiency of 33.9% [21].
Organic solar cells (OSCs) provide potential technology for both single-junction and tandem structures thanks to the bandgap tunability of the constituting photoactive organic materials utilized. These materials, typically conjugated polymers or small molecules, offer several advantages, including low-cost production, flexibility, and the potential for lightweight and semi-transparent devices [22]. The semitransparency of the top organic solar cells is crucial for TSCs. Additionally, the integration of a conductive transparent top electrode, which must be processed at low temperatures, is essential to prevent degradation of the top organic and bottom subcells, ensuring durability and cost-effective industrialization [23]. The versatility of organic materials also enables the integration of hybrid TSCs, combining organic layers with other materials such as perovskites or inorganic semiconductors, including silicon and CIGS. Despite their relatively lower efficiencies compared to inorganic counterparts, single-junction and tandem OSCs have seen significant improvements in recent years. Advances in material design and optimization of device architecture have pushed power conversion efficiencies (PCEs) beyond 19% and 20% for single-junction and tandem cells, respectively [24,25]. For instance, in [26], a pseudo-planar heterojunction structure was fabricated based on a binary-dilution strategy. The resulting solar device exhibited a remarkable PCE of 19.32%.
Additionally, one of the main design ideas in OSC development is to control the carrier transportation layers, specifically the electron-transporting layer (ETL) and the hole-transporting layer (HTL). These layers are essential for the effective operation of organic solar cells, as they facilitate the selective extraction and transfer of electrons and holes, respectively, from the active layer to the corresponding electrodes [27]. In this context, G. Zhang et al. introduced the use of the inorganic NiOx material as an HTL. This HTL material offers a potential solution to the stability issues encountered with traditional HTLs, such as PEDOT:PSS. By employing NiOx with a ternary blend system (D18:N3:F-BTA3), a record PCE of 19.18% was achieved [28]. In addition, a ZnO/ZrSe2 composite ETL was fabricated to tune the band alignment in OSCs with PM6:L8-BO as an active layer, accomplishing a PCE of 18.24% [29].
Furthermore, OSCs emerge as an excellent option when considering suitable top cells in TSCs. Some all-organic tandems, structured from polymer donor materials and fullerene acceptors, have achieved an efficiency of 11.6%, while others using polymer small-molecule donors and fullerene acceptors have reached a PCE of 12.7% [30,31]. A tandem OSC using PTB7-Th:BTPV-4F:PC71BM (Eg = 1.61 eV) and PM6:m-DTC-2F (Eg = 1.21 eV) photoactive layers in the top and rear cell, respectively, was introduced, and an efficiency of 16.40% was reported [32]. In [33], the authors fabricated a TSC with D18:FBr-ThCl acting as a top cell and PM6:BTP-4Se:F-2F acting as a top cell. The optimized tandem provided a PCE of 19.55% with a high open-circuit voltage (VOC) of 1.88 V. Recently, a certified efficiency of 20.3% has been reported for an organic TSC based on PFBCPZ:AITC and PM6:AITC:BTP-eC9 as the bottom and top active layers, respectively [34]. Regarding hybrid tandem systems, in which OSCs are utilized as top cells, some studies have been introduced in the literature. In [35], an organic/a-Si hybrid TSC was fabricated. The optimized PCE exhibited 8.32% with VOC = 1.45 V. Despite the advantageous properties that may result from incorporating OSCs with silicon, there have been very few research efforts reported in this area. In [36], the authors presented the first fabricated organic/c-Si TSC, with record PCEs of 8.32% and 15.25% for 2-T and 4-T designs, respectively. The OSC utilized polymer-based materials, while the silicon bottom cell featured an n-type TOPCon structure.
While OSCs are renowned for their flexibility, traditional Si solar cells, which rely on thick wafers, lack the necessary flexibility. However, advancements in thin-film Si technology have enabled researchers to develop high-performance, flexible Si-based solar cells. For instance, in [37], the authors fabricated heterojunction Si solar cells with an efficiency exceeding 22% and a thickness of less than 50 μm, showcasing the significant potential for flexible high-efficiency Si solar cells. More recently, in [38], Si solar cells achieved certified efficiencies of 26.06%, 26.19%, and 26.50% for Si thicknesses of 57 μm, 74 μm, and 84 μm, respectively. These breakthroughs underscore the feasibility of Si solar cells as a viable category of commercial thin-film solar cells with significant flexibility.
While some tandem systems, such as all-organic, perovskite-based, and silicon-based, have gained considerable attention for their progressive efficiencies, organic/Si TSCs have not been fully researched despite the high potential of this promising system, which offers sustainability advantages. Even with these potential attributes, the integration of organic and silicon cells remains uninvestigated, with very few experiments studying their performance. Consequently, there is a notable gap in exploring this specific type of tandem configuration. Compared to other tandems, organic/Si tandems promise superior eco-friendly sustainability due to the non-toxicity and abundance of their constituent materials. Furthermore, their compatibility with flexible substrates positions them as viable candidates for flexible tandems. This research gap highlights the necessity for thorough research into organic/Si TSCs to interpret their prospective. Stimulated by this gap in the literature, this simulation work aims to present a thorough analysis of flexible organic/Si TSCs utilizing thin-film organic and silicon PV cells.
This study introduces a tandem configuration employing flexible organic (with an optical bandgap of 1.78 eV) for the top cell and Si (1.12 eV) for the rear cell, where flexibility for both organic and silicon cells is satisfied. The analyses of this OSC/Si tandem are carried out using Silvaco TCAD simulations. The simulation models and the validation of results are based on published data from various research experimental efforts. The calibrated PCEs for the organic and Si cells are 11.11% and 22.69%, respectively. The verified physical models and material parameters for organic and Si cells are, afterward, retained to form a 2-T organic/Si TSC. The corresponding primary TSC achieves an efficiency of 20.03%. An optimization procedure for the tandem cell begins with an investigation into how the work function of the front contact affects tandem performance after removing the organic PEDOT:PSS layer from the front sub-cell. Subsequently, we investigate the change in the defect concentrations in the organic absorber layer. Likewise, the influence of altering the thickness of both active layers is explored to optimize the TSC efficiency. Ultimately, the suggested organic/Si TSC is constructed at the current matching condition to attain the highest achievable PCE. This device simulation study suggests a potential for further development of the organic/Si TSC, paving the way for its practical development.

2. TCAD Simulation Methodology and Cell Structures

2.1. Silvaco Atlas Simulation Methodology

Notably, optimizing TSCs through experimental trial and error is not cost-effective and often results in wasted resources. Therefore, simulations are essential for studying and optimizing TSCs prior to fabrication. However, the reliability and validity of simulation results must be confirmed by comparing them with experimental findings. Using real parameters in simulations is crucial for accurate modeling and serves as a useful benchmark. The simulations of the proposed flexible organic/Si TSC are conducted using the Atlas device simulator developed by Silvaco TCAD [39]. Atlas solves the main macroscopic semiconductor transport equations, specifically, the carrier continuity equations (for both electrons and holes) with the drift–diffusion model coupled to Poisson’s equation. Recombination models, such as Shockley–Read–Hall (SRH) and Auger models, are enabled to account for traps inside the energy gaps of various layers and high carrier densities that may occur during the operation of the cells.
In a 2-T TSC, both the top and rear cells must be connected via an interconnection layer. This interconnection must ensure both low electrical resistivity and high optical transmissivity to maintain the overall efficiency of the solar cell. Typically, tunnel junctions or transparent conductive oxides (TCOs) are used to meet these requirements. Also, a thin metallic interlayer, such as silver or gold, can be utilized as the metal. Both techniques have been experimentally confirmed to be optically and electrically effective [40,41]. TCOs are widely used because they offer excellent electrical conductivity and optical transparency, which are essential for allowing light to pass through to the underlying layers while also conducting electrical current with minimal losses. Tunnel junctions, on the other hand, serve as a more sophisticated interconnection method. A tunnel junction is essentially a reversed p-n junction that exists between the top and bottom cells. It facilitates the flow of current from one cell to the next with minimal voltage drop through the process of quantum tunneling. By employing tunnel junctions, the 2-T TSCs can achieve better performance, as these junctions help maintain high electrical conductivity and low resistive losses while also allowing for the necessary optical properties to be preserved.
The physical models implemented in Atlas have been validated through extensive experiments for single and tandem solar cells [42,43,44]. The calculated results presented in this study are supported by experimental data. Many practical factors, such as material interface properties, have been adequately addressed, which is crucial for validating the reliability of the calculations. A schematic representation of a simplified flow chart illustrating the basic elements of Silvaco simulations is displayed in Figure 1. The main equations involved in the solar cell simulation and the corresponding definitions of symbols are given in Supplementary Note S1 and Table S1 (see Supplementary Information file). Upon constructing the device configuration along with the appropriate meshing, Atlas performs optical device simulations. Firstly, the optical intensity is calculated based on the relative dielectric constants (n) utilizing the ray tracing model given an input light source. Secondly, the carrier concentrations are evaluated using the photogeneration model given the extinction coefficients (k). For accurate optical modeling to be performed, both relative dielectric constants and extinction coefficients of all layers have to be inputted from reliable sources, mainly measured data from experimental studies. The n and k values of the distinctive layers of the top and the bottom cells are given in Figures S1 and S2 (see Supplementary Information file). Following these steps, an electrical simulation determines the required terminal currents. This electrical simulation employs the drift–diffusion model to simulate the transport properties. Various output results can be extracted, such as illuminated or dark J–V curves and external quantum efficiency (EQE) spectra. Also, many physical quantities can be extracted, including energy band profiles, electric field distributions, and many others. All of the following TCAD simulations are conducted under AM1.5G (1000 W/m2) illumination conditions.

2.2. Calibration of Standalone Organic and Si Cells

Figure 2a represents a schematic figure of the top single-junction OSC structure, demonstrating the basic thin-film layers, while Figure 2b exhibits the band edge levels of the different utilized materials, plotted before contacting. The cell arrangement comprises ITO/PEDOT:PSS/PBDB-TCl:AICT/PNDIT-F3N-Br/Ag. The thickness of the photoactive layer (PBDB-TCl:AICT) is 117 nm, while the thickness of the HTL (PEDOT:PSS) and the ETL (PNDIT-F3N-Br) is 100 nm and 5 nm, respectively. The photoactive organic material is a blend of a polymer donor (PBDB-TCl) and a small molecule acceptor (AICT). The resulting blend optical band gap is about 1.78 eV. These parameters and other key parameters for the top OSC are given in Table S2. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of PBDB-TCl and AICT are estimated by ultraviolet photoelectron spectroscopy (UPS), while the carrier mobilities were estimated by the space–charge limited current technique [34]. In addition, the main defect parameters of the organic blend and the interfaces are addressed in Table S3, where their values are adjusted to obtain the best fit between the simulation and measured data, as will be explained hereafter. Furthermore, the front contact is formed by ITO (with a work function of 4.7 eV), while the back electrode is Ag (with a work function of 3.93 eV) [45].
Alternatively, regarding the bottom Si cell, Figure 3a shows its schematic diagram, demonstrating the various layers. In addition, Figure 3b displays the band edge levels, before contacting, of Si a-Si, as well as front and back contacts. The cell is based on an n-type thin-film Si wafer, whose thickness is 70 μm, which was previously fabricated as given in [38]. Although this thickness is relatively wide, it is flexible, as evident from the experimental work [38]. The main parameters for the bottom Si cell are given in Table S4. The lifetime of the silicon layer is assumed to be 250 μs. The selection of these specific top and bottom cells is based on multiple factors. Primarily, the complementary nature of the bandgaps of the organic and Si materials enables a broader coverage of the illumination spectrum. The organic material’s bandgap is typically tuned to absorb higher-energy photons, while the Si material efficiently absorbs lower-energy photons. Additionally, both organic and thin-film Si cells exhibit excellent flexibility. This combination of complementary spectral absorption and mechanical flexibility makes the chosen materials particularly suitable for advanced thin film, flexible solar cell applications.
Next, our study involves calibrating simulated solar cells against their experimentally fabricated counterparts for the validation of the precision of our simulation model and its reliability. By contrasting PV metrics such as PCE, J–V curves, and EQE spectra, we assess the closeness between measured and simulated data. After incorporating all the material parameters and physical models discussed previously, we simulate the J–V and EQE curves for both the top OSC and the bottom Si cell. The simulated curves, along with extracted measured data, are presented in Figure 4a for J–V characteristics and Figure 4b for EQE characteristics. The defect parameters of the OSC have been meticulously adjusted to achieve the best possible fit between the simulation results and the measured data. The obtained fitting values, listed in Table S3, are in agreement with other similar studies [9,42,46]. Additionally, the quantitative cell parameters of the simulated and experimental OSC and Si cells are detailed in Table 1. The findings presented in Figure 4 and Table 1 notably demonstrate the accomplishment of our calibration procedure, as evidenced by the close agreement between measured and simulated results. This consistency confirms the effectiveness of the simulation model, implemented in Silvaco, in capturing the behavior of practical organic and Si cells. Remarkably, the EQE of the two cells, as indicated in Figure 4b, are complementary, making them highly suitable for use in a tandem configuration.

2.3. Initial Configuration of 2-T Organic/Si Tandem Cell

The proposed structure of the organic/Si tandem is illustrated in Figure 5. The designed structure integrates a wide bandgap organic front cell with a narrow bandgap Si rear cell. In this tandem configuration, the constituting cells are interconnected via an interlayer (Figure 5a) or through a tunnel pn junction (Figure 5b). We investigated both interconnection techniques to demonstrate the difference between them. Figure 6a,b demonstrate the energy band profiles plotted at dark when coupling both top and bottom cells using an interlayer and a tunnel junction, respectively. Besides, Figure 7 displays the simulation results for the J–V curves (Figure 7a) of the top, rear, and tandem cells and their EQE curves (Figure 7b), while Table 2 addresses all cell parameters. The tandem cell factors, in this case, are as follows: JSC = 14.2 mA/cm2, VOC = 1.81 V, FF = 77.54%, and PCE = 20.03%. On the other hand, the J–V and EQE curves for the case of the tunnel junction are illustrated in Figure S3a,b, respectively. The cell parameters are also shown in Table 2, showcasing similar results as in the interlayer case.
It should be pointed out here that when performing the simulation of tunnel-connected cells, the non-local band-to-band tunneling model was incorporated to trace the tunneling between the highly doped pn junction formed between the front and bottom cells in the TSC. A very fine tunneling meshing scheme was also invoked, which significantly increased the simulation time. Thus, in the following simulations, the two cells were combined via an ITO interlayer as a common electrode because there was a minor difference between this technique and the complicated tunnel methodology. To realize smooth current flow between the top and rear cells and to maintain appropriate boundary conditions, a high lumped resistance (1016 Ω) was connected in the simulation [4,47].

3. Results and Discussions

This section addresses the optimization steps for the organic/Si thin-film tandem device. Initially, we investigate how the front contact work function affects tandem performance after removing the organic PEDOT:PSS layer from the front OSC. Subsequently, we assess how changing the defect density of the top organic film influences the overall PV factors. The effect of varying the absorbers’ thicknesses is also investigated. Lastly, we examine the current matching condition, which is crucial for optimizing the efficiency of a TSC.

3.1. Impact of Front Contact Work Function

To improve the stability of the top OSC, it is recommended to alleviate the stability issues related to the HTL material. This can be done by either replacing the PEDOT:PSS material with inorganic candidates or by completely removing it, thereby forming an HTL-free structure [6]. Upon removing the HTL, it becomes challenging to maintain the energy level alignment between the photoactive absorber and the front electrode. To address this effect, the work function of the front electrode should be engineered. It was shown experimentally that the ITO work function can be tuned by over 1 eV through O2 plasma treatment on ITO modified with an organic interlayer material (PEIE). Specifically, the work function of the PEIE-modified ITO increases continuously with longer plasma treatment durations [48]. Moreover, it was confirmed that the ITO work function can be expanded up to 5.75 eV by spin-coating cesium-fluoride on the top of the ITO electrode [49]. Motivated by this experimental evidence, we simulated the tandem structure with an HTL-free top OSC. The effect of changing the front contact work function on the tandem output metrics is explained in Figure 8, where the work function ranges from 4.7 to 5.7 eV. The figure demonstrates that the PCE increases steadily upon expanding the work function up to 5.4 eV, after which the PCE almost saturates (the same trend holds true for the other cell parameters). Thus, we can select a work function of 5.4 eV as a design threshold. For this criterion, the resulting cell parameters are JSC = 14.64 mA/cm2, VOC = 1.81 V, FF = 85.90%, and PCE = 22.76%.
To offer a physical explanation for the trend in PCE relative to variations in the work function, we plotted the energy band diagrams for three different work function values. As shown in Figure 9a, as the work function increases, the energy bands exhibit greater bending. This increased bending indicates a higher electric field within the device. This relationship is further corroborated by the electric field distribution, which is depicted in Figure 9b. In the energy band diagrams, we can observe that with a higher work function, the conduction and valence bands bend more significantly near the interface. This bending enhances the built-in electric field, which is crucial for the separation of photogenerated electron–hole pairs. The increased electric field facilitates more efficient charge carrier extraction, lowering recombination losses and thus boosting the overall PCE of the device. Furthermore, the electric field distribution figure illustrates that as the work function is tuned upwards, the magnitude of the electric field across the active layer becomes stronger. This enhanced field effectively drives the charge carriers toward their respective electrodes, ensuring a more efficient charge collection process. Consequently, the higher electric field resulting from the increased work function directly contributes to the improved PV performance of the cell, as evidenced by the higher PCE values.

3.2. Impact of Defect Density of the Top Organic Absorber

The performance of OSCs is substantially affected by the quality of the absorber layer, which plays a crucial role in the overall PCE of the solar cell. Among the various factors that affect the performance of OSCs, bulk defects in the absorber material are particularly critical. Bulk defects, which include various types of structural imperfections, such as vacancies, interstitials, and impurities, act as recombination centers for charge carriers. These defects can severely hamper the efficiency of OSCs by increasing the recombination rates of photoexcited electron–hole pairs, thereby reducing the effective carrier lifetime and diffusion lengths. Figure 10a illustrates how the PV parameters of the TSC change with variations in the bulk defect density of the front OSC absorber. The defect level ranges from 1 × 1010 to 1 × 1014 cm−3. As depicted in the figure, all PV factors present a consistent trend, remaining relatively stable up to a defect density of approximately 1 × 1012 cm−3, beyond which they start to degrade gradually. A value of 1 × 1011 cm−3 can be chosen as a design threshold. Here, the subsequent metrics are JSC = 14.64 mA/cm2, VOC = 1.82 V, FF = 87.54%, and PCE = 23.27%.
The substantial cell enhancement when reducing defects is mainly attributed to higher carrier diffusion lengths and, as a result, lower recombination rates, as illustrated in Figure 10b, which displays the recombination rates inside the absorber layer regarding different values of defect densities. These simulations are performed in short-circuit and illumination conditions. As anticipated, the recombination rate decreases intensely with lower trap densities, indicating higher performance. The observed trends in the performance parameters underscore the critical effect of defect density of the top cell on the efficiency of TSCs. Lower defect densities lead to reduced recombination rates, enabling better charge separation and collection, and thus, higher power conversion efficiencies. This highlights the importance of advanced fabrication techniques aimed at minimizing defects to achieve optimal performance in tandem cells.

3.3. Impact of Thicknesses of the Two Absorber Layers

In this simulation run, we vary the thicknesses of both absorber layers to examine their impact on the tandem functioning. The thickness of the top photoactive layer (ttop) is varied from 150 to 300 nm, while the bottom photoactive layer (tbot) is varied from 50 to 90 µm. Figure 11 illustrates the dependence of the TSC efficiency on the thickness of both active films. As shown in the figure, increasing the thickness of the bottom absorber layer from 70 to 90 µm has a negligible effect on the PCE. However, when the rear absorber thickness drops below 70 µm, the PCE significantly decreases for all values of the top absorber thickness. To balance performance and maintain flexibility, optimal thicknesses of 250 nm for the front organic layer and 70 µm for the n-type Si layer can be selected. The TSC parameters for these selected values are as follows: JSC = 19.22 mA/cm2, VOC = 1.81 V, FF = 79.56%, and PCE = 27.60%. These results demonstrate that the selected thicknesses provide a good compromise between efficiency and flexibility, ensuring that the TSC achieves high performance while maintaining structural integrity. This optimization highlights the importance of carefully balancing absorber layer thicknesses to maximize the overall efficiency and practicality of TSCs.

3.4. Current Matching Condition

Finally, we investigated the current matching condition by varying the top absorber thickness from 230 to 280 nm while keeping the rear absorber thickness fixed at 70 µm, as demonstrated in the preceding subsection. Figure 12a shows the dependence of the JSC for both organic and Si cells on the top absorber thickness. Increasing ttop notably results in greater photon absorption in the top OSC, thereby lowering the amount of light reaching the rear Si cell. Consequently, an increase in ttop leads to a higher JSC for the top OSC and a lower one for the bottom Si cell, as approved by the results in Figure 12a. It is evident from the figure that the current matching condition occurs at ttop = 254 nm. The TSC was simulated under these specifications, and Figure 12b presents the illuminated J–V characteristic curves for the TSC and its forming organic and Si cells. The corresponding PV factors are detailed in Table 3. The TSC realizes a maximum JSC of 19.28 mA/cm2, a VOC of 1.81 V, and a tandem efficiency of 27.60%. The effective operation of the recombination junction is demonstrated by the fact that the tandem VOC (1.81 V) is the sum of the VOC of the front cell (1.08 V) and that of the rear cell (0.73 V). Additionally, the EQE spectra of the TSC and its cells are shown in Figure 12c. The EQE of the rear c-Si cell surpasses 80% at a wavelength of around 800 nm, while the EQE of the front OSC exceeds 90%, indicating efficient photon absorption and carrier generation in both the front and bottom cells. These outcomes underscore the weight of optimizing the top absorber thickness to reach current matching and maximize the overall efficiency of the TSC.

3.5. State-of-the-Art Comparison

Table 4 displays a state-of-the-art comparative analysis between our optimized 2-T organic/Si TSC and other tandem cells reported in the literature. The table provides numerous experimental and numerically based tandems. On the experimental basis, the PCE of a 2-T polymer-based/Si tandem has been restricted to 9% thus far. Although lead-based perovskite tandems attain higher PCEs than our proposed flexible tandem cell, their toxicity presents a significant concern that limits their applicability. Regarding simulation studies, little effort has been devoted to polymer-based solar cells. In [50], a polymer-based front cell and a thin Si bottom cell were interconnected by an ITO interlayer to form a 2-T TSC. The polymer solar cell was an n-i-p heterojunction featuring a blend of polymer (PDTBTBz-2F) and fullerene (PC71BM), while the Si cell thickness was 20 μm. A PCE of 28.41% was obtained after a series of optimization routines, including adjustment of band alignment, defect density, and varying top and bottom thickness. In our study, an organic/Si is designed in a 2-T configuration, presenting two possible connection techniques, either by a tunnel junction or via an ITO interlayer. Band-to-band tunneling is utilized to model the tunneling through the tandem cell. Our simulated organic/Si tandem exhibits favorable characteristics for both methods of interconnection, including relatively high VOC and PCE. These attributes suggest that with further development, organic/Si TSCs could become competitive alternatives to the existing flexible TSCs. A broader comparison, including more tandem systems, is illustrated in Table S5 and Figure S4.
Table 4. A state-of-the-art comparison between tandem cell parameters of organic/Si TSC and other reported tandems.
Table 4. A state-of-the-art comparison between tandem cell parameters of organic/Si TSC and other reported tandems.
Front CellRear CellMethodJsc
(mA/cm2)		Voc
(V)		FF
(%)		PCE
(%)		Ref.
PBDB-T-2F:Y6SiExp. (2-T)15.811.0855.578.32[36]
PBDB-T:ITICSiExp. (4-T)---15.25[36]
Lead-based PerovskitePBDB-T-2F:Y6:P71CBMExp. (2-T)14.562.1375.6023.40[51]
Lead-based PerovskiteSiExp. (2-T)20.241.97981.2032.50[52]
Sb2S3SiSim. (2-T)18.041.6482.4124.34[6]
Lead-free PerovskiteSiSim. (2-T)16.011.7686.7024.40[53]
PDTBTBz-2F:PC71BMSiSim. (2-T)16.432.0484.8128.41[50]
PBDB-TCl:AICTSiSim. (2-T)19.281.8179.3127.60This
work
One of the main outcomes of this study is the insight that when designing TSCs incorporating thin-film PV cells, it is not preferable to pair a high-efficiency cell with a significantly lower-efficiency cell. Instead, it is advisable to initiate the design with two cells of relatively close performance levels, with the bottom cell having higher efficiency but not excessively so. This approach ensures better overall performance for the TSC. Thus, a practical suggestion for combining organic materials and Si in TSCs is to design the Si bottom cell as a thin-film cell with efficiencies of around 20% rather than using high-efficiency commercial cells like TOPCon or other advanced Si cells. High-efficiency cells such as TOPCon are designed to maximize their performance as standalone units and may not perform as effectively in a tandem configuration involving low-efficiency thin-film cells. This strategy helps in maintaining a higher combined efficiency for the tandem cell. It also takes advantage of the cost and material benefits associated with thin-film Si cells, which are generally more flexible and lighter.

4. Conclusions

Organic solar cells, known for their light weight, flexibility, and simple fabrication processes, can offer significant advantages in tandem structures. Despite their potential, organic materials have yet to be widely applied as top-cell materials in Si-based tandems. This paper displayed the results of simulations accomplished on a 2-T monolithic organic/Si TSC through the utilization of the Atlas device simulator. In the proposed configuration, the organic solar cell (OSC), with a photoactive optical bandgap of 1.78 eV, serves as the front cell, while the bottom cell comprises a Si cell with a bandgap of 1.12 eV. After the validation of the simulation physical models and input material parameters, the organic/Si monolithic TSC was integrated, and the resulting tandem cell achieved a PCE of 20.03%, indicating the need for optimization of the top OSC to beat the efficiency of the bottom Si cell. Then, we optimized the TSC by designing the cell without an HTL and by adjusting the front contact work function. Furthermore, the impact of altering the organic defect density and the thickness of both absorber layers on TSC performance was examined to obtain the maximum attainable PCE. At the matching designed condition, the PV metrics were as follows: JSC = 19.28 mA/cm2, VOC = 1.81 V, FF = 79.31%, and PCE = 27.60%.
It should be mentioned here that the flexible organic/Si system represents a novel tandem approach that has not been explored in the existing literature. Consequently, there is a lack of experimental data specifically pertaining to our proposed stacked structure, which utilizes organic materials in conjunction with thin-film Si solar cells. Despite this, our simulations predict an efficiency exceeding 27%, suggesting that experimental validation may achieve efficiency levels greater than 20%. Advanced fabrication techniques and precision control over manufacturing conditions will likely lead to further improvements in efficiency.
The TCAD device simulations conducted in our study reveal a promising avenue for enhancing the performance of organic/Si TSCs by optimizing crucial properties of the top cell. This research effort offers valuable insights that can serve as design guidelines for the development of organic/Si tandem cells. These enhancements are clearing the path for practical applications in portable electronics and flexible solar panels. Furthermore, the convergence of industry-standard Si technology and low-cost printed organic PV technologies holds significant potential, both industrially and societally.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst14070584/s1, Figure S1: Absorption coefficients deduced from available experimental data for the bottom Si cell: (a) c-Si [54] and (b) a-Si layers [55]; Figure S2: Absorption spectrum of (a) PBDB-TCl:AITC absorber layer [34], (b) UV-vis absorption spectrum of PNDIT-F3N-Br layer [45], (c) Real, and (d) imaginary refractive index Refractive indices deduced from available experimental data for PEDOT:PSS layer [56]; Figure S3: (a) Illuminated J–V curves, and (b) EQE spectra of TSC and its cells, coupled using a tunnel junction; Figure S4: Efficiency over time for various tandem solar cell configurations from 2020 to 2024; Table S1: Definition of physical quantities used in Atlas device simulator; Table S2: Main parameters of various layers of organic solar cell [34,45,57]; Table S3: Defect parameters in OSC; Table S4: Main factors of various layers of thin Si cell [4,38,50,58]; Table S5: A state-of-the-art comparison between tandem cell metrics for a wide range of tandem systems [4,6,7,9,36,42,50,52,53,59,60,61,62,63,64,65,66,67,68,69,70,71].

Author Contributions

Conceptualization, M.S.S., A.S. and M.A.; methodology, M.S.S., M.O., A.S., M.G. and M.M.S.; software, M.S.S., M.O. and A.S.; validation and formal analysis, M.O., M.M.S. and K.A.A.-D.; visualization, M.G., K.A.A.-D. and M.M.S.; investigation, M.S.S., M.M.S. and M.G.; writing—original draft preparation, M.S.S., A.S., M.A. and M.M.S.; writing—review and editing, A.S., K.A.A.-D., M.M.S. and M.G.; supervision, A.S. and M.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

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

Acknowledgments

This research has been funded by the Scientific Research Deanship at the University of Ha’il, Saudi Arabia, through project number RG-23 069.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Silvaco flow chart, highlighting the basic steps and equations involved in simulations.
Figure 1. Silvaco flow chart, highlighting the basic steps and equations involved in simulations.
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Figure 2. (a) Basic cell arrangement and (b) energy band diagram before contact of organic cell [34].
Figure 2. (a) Basic cell arrangement and (b) energy band diagram before contact of organic cell [34].
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Figure 3. (a) Basic cell arrangement and (b) energy band diagram before contact of Si cell [38].
Figure 3. (a) Basic cell arrangement and (b) energy band diagram before contact of Si cell [38].
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Figure 4. (a) Illuminated J–V curves and (b) EQE spectra of simulated organic [34] and Si [38] cells against measured data.
Figure 4. (a) Illuminated J–V curves and (b) EQE spectra of simulated organic [34] and Si [38] cells against measured data.
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Figure 5. Tandem structure showing main layers, coupling two sub-cells using (a) an interlayer and (b) a tunneling pn junction.
Figure 5. Tandem structure showing main layers, coupling two sub-cells using (a) an interlayer and (b) a tunneling pn junction.
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Figure 6. Energy band profiles after contact plotted under dark of the tandem structure, coupling two cells by means of (a) an interlayer and (b) a tunneling pn junction.
Figure 6. Energy band profiles after contact plotted under dark of the tandem structure, coupling two cells by means of (a) an interlayer and (b) a tunneling pn junction.
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Figure 7. (a) J–V curves and (b) EQE spectra of the TSC and its cells coupled using an interlayer.
Figure 7. (a) J–V curves and (b) EQE spectra of the TSC and its cells coupled using an interlayer.
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Figure 8. Dependency of TSC output parameters on the front contact work function.
Figure 8. Dependency of TSC output parameters on the front contact work function.
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Figure 9. (a) Energy band profiles after contact, plotted at dark, through 115 nm thickness of the top cell for different values of ITO work functions. (b) Electric field distribution through the top cell with three distinct work functions.
Figure 9. (a) Energy band profiles after contact, plotted at dark, through 115 nm thickness of the top cell for different values of ITO work functions. (b) Electric field distribution through the top cell with three distinct work functions.
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Figure 10. (a) Dependency of TSC output parameters on defect density of the top absorber layer. (b) Recombination rates inside the top absorber under illumination.
Figure 10. (a) Dependency of TSC output parameters on defect density of the top absorber layer. (b) Recombination rates inside the top absorber under illumination.
Crystals 14 00584 g010
Crystals 14 00584 g011
Figure 11. Dependency of tandem PCE on thicknesses of absorber layers.
Figure 11. Dependency of tandem PCE on thicknesses of absorber layers.
Crystals 14 00584 g011
Crystals 14 00584 g012
Figure 12. (a) JSC of organic and Si cells vs. front absorber thickness. (b) J–V and (c) EQE characteristics of the organic/Si TSC and its constituting cells regarding the current matching condition.
Figure 12. (a) JSC of organic and Si cells vs. front absorber thickness. (b) J–V and (c) EQE characteristics of the organic/Si TSC and its constituting cells regarding the current matching condition.
Crystals 14 00584 g012
Table 1. Cell parameters of experimental and simulated organic and Si cells.
Table 1. Cell parameters of experimental and simulated organic and Si cells.
PV ParametersJSC (mA/cm2)VOC (V)FF (%)PCE (%)
Organic cellExperimental [34]14.001.0972.6011.10
This work 14.041.0972.6811.11
Thin Si cellExperimental [38]38.200.74379.8022.60
This work38.200.74679.6222.69
Table 2. Cell parameters of simulated organic/Si tandem cells with distinctive coupling methods.
Table 2. Cell parameters of simulated organic/Si tandem cells with distinctive coupling methods.
PV ParametersJSC (mA/cm2)VOC (V)FF (%)PCE (%)
InterlayerTop sub-cell14.151.0872.2611.01
Bottom sub-cell22.340.7380.4813.20
Tandem cell14.221.8177.7520.03
Tunnel junctionTop sub-cell14.321.0872.2311.14
Bottom sub-cell23.510.7480.3713.93
Tandem cell14.131.8477.9520.20
Table 3. Cell parameters of organic/Si TSC and its constituting cells involving CMP.
Table 3. Cell parameters of organic/Si TSC and its constituting cells involving CMP.
PV ParametersJSC (mA/cm2)VOC (V)FF (%)PCE (%)
Top sub-cell19.281.0879.0116.41
Bottom sub-cell19.280.7380.6511.30
Tandem cell19.281.8179.3127.60
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Salem, M.S.; Okil, M.; Shaker, A.; Abouelatta, M.; Salah, M.M.; Al-Dhlan, K.A.; Gad, M. TCAD-Based Design and Optimization of Flexible Organic/Si Tandem Solar Cells. Crystals 2024, 14, 584. https://doi.org/10.3390/cryst14070584

AMA Style

Salem MS, Okil M, Shaker A, Abouelatta M, Salah MM, Al-Dhlan KA, Gad M. TCAD-Based Design and Optimization of Flexible Organic/Si Tandem Solar Cells. Crystals. 2024; 14(7):584. https://doi.org/10.3390/cryst14070584

Chicago/Turabian Style

Salem, Marwa S., Mohamed Okil, Ahmed Shaker, Mohamed Abouelatta, Mostafa M. Salah, Kawther A. Al-Dhlan, and Michael Gad. 2024. "TCAD-Based Design and Optimization of Flexible Organic/Si Tandem Solar Cells" Crystals 14, no. 7: 584. https://doi.org/10.3390/cryst14070584

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