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Scientific Reports volume 15, Article number: 32054 (2025) Cite this article
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Germanium-based perovskite solar cells (PSCs) have gained attention as a promising alternative to conventional lead-based PSCs due to their environmentally friendly and non-toxic nature. However, their efficiency remains below optimal levels, requiring further exploration to enhance their performance. This study investigates a novel n-i-p structured germanium-based perovskite solar cell using the wxAMPS simulation. The baseline structure—FTO/TiO₂/KGeCl₃/Spiro-OMeTAD/Au—achieved a power conversion efficiency (PCE) of 18.55%. To improve efficiency, various electron transport layer (ETL) materials were evaluated, including TiO₂, IGZO, SnO₂, ZnO, ZnSe₂, WO₃, PCBM, and WS₂ TMDC. The results revealed that the WS₂ emerging as the most suitable candidate. Optimization of key parameters, including the thicknesses of WS₂ ETL (50 nm), Spiro-OMeTAD HTL (30 nm), and the absorber layer KGeCl₃ (600 nm), significantly improved device performance. Additional investigations into defect density, acceptor concentration, electron affinity, and donor concentration further optimized the device’s operation. The study also analyzed the adverse effects of functional temperature, providing insights into stability and efficiency under real-world conditions. The optimized solar cell device demonstrated enhanced performance metrics: V_oc = 1.02 V, J_sc = 25.77 mA/cm², FF = 78.25%, and PCE = 22.98%. These findings highlight the potential of germanium-based perovskite solar cells as a sustainable, lead-free photovoltaic solution. The integration of WS₂ as an ETL paves the way for achieving high-efficiency, environmentally friendly solar cells, with promising implications for advancements in renewable energy solutions.
Metal halide perovskites have established themselves as frontrunners in photovoltaic and optoelectronic applications, thanks to their remarkable characteristics, which include high absorption coefficients, extended carrier diffusion lengths, and adjustable bandgaps. Despite their remarkable performance, the toxicity of lead halide perovskites—the most efficient materials in this category—has spurred an urgent search for environmentally friendly, lead-free alternatives^1,2,3,4. Among the proposed candidates, tin-based and germanium-based iodide perovskites have gained significant attention for their photovoltaic potential. These materials are distinguished by their enhanced anti-bonding characteristics near the highest occupied molecular orbital (HOMO), attributed to the greater reactivity of the Sn 5s and Ge 4s lone pairs compared to the Pb 6s lone pair^5,6,7,8. Furthermore, the strong hybridization between the I 5p orbital and the Sn 5s or Ge 4s orbitals plays a pivotal role in their electronic properties. This hybridization facilitates the oxidation of Sn²⁺ and Ge²⁺ ions, leading to the generation of valence band holes⁹. Consequently, replacing Pb²⁺ with Sn²⁺ or Ge²⁺ not only addresses toxicity concerns but also introduces promising pathways for achieving high-efficiency, next-generation solar cells. This substitution has therefore become a central focus in the development of sustainable and high-performance perovskite materials for future photovoltaic technologies.
Tin-based halide chalcogenides (ASnX₃) have attracted attention for their narrow bandgap and high carrier mobility. However, their performance is significantly hindered by the weak Sn–X bond and the susceptibility of Sn²⁺ to oxidation into Sn⁴⁺, resulting in poor stability in tin-based chalcogenide solar cells^10,11,12,13. In contrast, germanium PSCs present a more stable alternative, addressing the issues of thermal instability and degradation commonly observed in tin perovskites. Research by Qian et al. demonstrated that inorganic CsGeI₃ PSCs outperform traditional organic MAPbI₃ PSCs in both stability and efficiency¹⁴. Additionally, Chen’s design of inorganic perovskite solar cells achieved a PCE of 4.94%¹⁵. Recent first-principles calculations have further expanded the potential of inorganic metal halide cubic perovskites (AGeX₃, where A = K, Cs, Rb; X = F, Cl, Br), revealing their favorable semiconductor properties, direct bandgap energies, and mechanical stability. These materials exhibit high absorption coefficients, exceptional optical conductivity, and low reflectivity, making them highly promising candidates for solar cells and advanced optoelectronic energy devices^{13,16,17,18,19,20,21}.
In Ge-based perovskite solar cells (Ge-PSCs), enhancing device performance is often achieved by incorporating suitable carrier transport materials (CTMs) into the device structure^22,23. CTMs are classified into two categories: hole transport materials (HTMs) and electron transport materials (ETMs)²⁴. These layers serve dual purposes: they separate the photo-generated charge carriers in the active layer and facilitate their transport to the electrodes, while also acting as buffer layers that minimize resistance between the absorber surfaces and the electrodes^25,26,27. Raoui et al. demonstrated that the use of different ETLs in lead-based PSCs led to significant enhancements in PCE, ranging from 19.70 to 26.74%²⁸. Similarly, Khattak et al. reported comparable improvements in PCE for tin-based PSCs through modifications to the CTMs²⁹. Among various emerging Pb-free perovskite materials, KGeCl₃ has emerged as a strong contender for optoelectronic devices. The main feature of this material that its energy band gap depends on the structure. The trigonal structure has a band gap of 2.70 eV, while the cubic and tetragonal counterparts have energy band gap of 0.80 eV and 1.20 eV respectively. Additionally, KGeCl₃ exhibits a broad absorption spectrum that spans the UV–Vis region, making it a promising candidate for photovoltaic applications³⁰.
Despite several studies investigating the physical properties of KGeCl₃, there remains a limited number of publications addressing its efficacy as a solar absorber material^30,31,32. For instance, Siddique et al. conducted SCAPS-1D simulations of KGeCl₃ as a solar absorber layer, identifying optimal carrier transport layers³¹. Furthermore, KGeCl₃ is a relatively new and rare material, and its experimental feasibility or challenges have not been extensively studied compared to more well-known compounds. Most recent studies have focused on mathematical analysis such as DFT or numerical analysis on this material. However, based on the chemistry of similar compounds and the general behaviour of halides and germanium-based materials, several potential challenges can be identified when working with KGeCl₃ and mainly^33,34
The uncertainty of crystal Structure: The crystallographic properties of KGeCl₃ are not as well-characterized as some other halides. Determining the exact crystal structure and understanding its electronic properties would require detailed X-ray diffraction (XRD) studies, which could be challenging if the crystal growth is difficult, or the sample is impure.
Electronic Band Gap and Conductivity: As KGeCl₃ is a semiconductor, understanding its band gap, electrical conductivity, and potential applications in electronics would be critical. Experimental challenges here include measuring these properties accurately and investigating how different preparation methods affect the compound’s performance.
The scarcity of research on this material has motivated us to undertake this study. In this study, we investigate various configurations of KGeCl₃-based solar cells using the wxAMPS simulator, integrating several ETLs, including indium-gallium-zinc-oxide (IGZO), tin dioxide (SnO₂), tungsten disulfide (WS₂), titanium dioxide (TiO₂), zinc oxide (ZnO), tungsten trioxide (WO₃), poly(phenylene vinylene) (PCBM), and zinc selenide (ZnSe₂). Spiro-OMeTAD serves as the HTL. The focus of this research is to evaluate the impact of different ETLs, HTL thicknesses, and electron affinity on the performance of solar cells. Our findings successfully identify the most effective ETL material for enhancing the photovoltaic performance of KGeCl₃-based PSCs, along with optimal values for their properties.
Numerical analysis on solar cells has been recognized as an effective method in solar energy and photovoltaic research. These computational techniques allow researchers to analyze and optimize various critical parameters that significantly influence solar cell performance, thereby facilitating in identifying the most effective photovoltaic devices. These parameters include layer thickness, energy band gap, electron affinity, …etc. Such modeling allows researchers to optimize solar cell designs without the high costs, labor intensity, and long durations associated with experimental methods. Several software packages have been created and made accessible to the research community, including PC-1D, SCAPS, wxAMPS, and AMPS-1D. These tools have played a significant role in promoting the study and development of solar cells by providing robust platforms for simulation and analysis³⁵.
In the numerical simulation of wxAMPS, the formula for Poisson’s field in one-dimensional (1D) space is given by Eq. (1). The delocalized conduction band states given by Eq. (2) contain the unpaired electrons. More specifically, the continuity equation for the delocalized free holes in the valence band has the form given by Eq. (3). The net direct recombination rate is defined by Eq. (4). It is possible to locate the notation for every symbol used in Eqs. (1–4) elsewhere^{36,37,38,39,40}.
n and p are the electron and hole band carrier density in the device, respectively. Furthermore, the bandgaps of the cell under investigation define the proportionality constant. G(x) is the rate of carrier generation as a function of x resulting from external light supply in the continuous equation. It is necessary to mention here that even though wxAMPS showed a good agreement between the simulation and experimental results in several studies, it has some limitations, which must be considered in any numerical analysis studies: wxAMPS can only simulate devices in one dimension. This imply to not be able to model lateral effects such as edge recombination, non-uniform illumination, or grain boundaries in 2D/3D structures. Also, it does not have built-in modelling of ion migration or Ge²⁺ oxidation kinetics or redox chemistry.
The modelled KGeCl₃ perovskite solar cell and its band diagram is illustrated in Fig. 1. This device features an n-i-p planar heterojunction structure, where the n region is the ETL, the i region is the perovskite layer, and the p region is the HTL. The input parameters of the primary structure of the perovskite based solar cell are summarized in Table 1. The primary cell configuration uses TiO₂ as the ETL and Spiro-OMeTAD as the HTL. The simulation is performed under the AM1.5G solar spectrum with an incident power density of 100 mW/cm² at 300 K. The input parameters utilized in this simulation are sourced from references^{36,41,42,43,44,45,46,47,48}.
The configuration of the simulated perovskite solar cell, along with the band diagram of the n-i-p perovskite solar cell at equilibrium.
In PSCs, several parameters can be optimized to improve the device performance, with the choice of the ETL being particularly critical. The role of the ETL in establishing optimal energy band alignment with the absorber layer significantly influences charge transport and overall device efficiency. In this study, we conducted a systematic evaluation of various ETL materials through simulation to identify the most effective option for improving device performance. The ETLs examined include a 100 nm layer of the organic fullerene derivative[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as well as IGZO, SnO₂, WS₂, WO₃, ZnSe₂, and ZnO. These materials were compared to the conventional TiO₂ ETL. The input parameters for the ETL materials were obtained from the literature and are listed in Table 2^{36,41,42,43,44,45,46}.
Figure 2a,b illustrate the J–V characteristics and quantum efficiency (QE) of the evaluated solar cell configurations, respectively. As indicated in the figures, the organic fullerene derivative PCBM exhibited the lowest J_sc at 21.50 mA/cm², whereas WS₂ achieved the highest current density of 25 mA/cm². The quantum efficiency for all ETLs remained relatively stable across the wavelength range of 300 nm to 740 nm. However, a significant decline in QE is observed beyond 740 nm, culminating in a complete disappearance at 820 nm. The QEs recorded were as follows: 83.70% for PCBM, 86.60% for WO₃, 88.20% for TiO₂, 94.80% for ZnSe₂, 95.30% for both ZnO and SnO₂, 96.40% for IGZO, and 97.90% for WS₂. Notably, PCBM demonstrated the lowest QE among the materials tested. This underperformance can be attributed to its limited electron and hole mobilities of 0.01 cm²/V s, which impede effective charge collection.
(a) The IV-Curve and (b) the QE (rescaled range) of the simulated cell architecture for different ETLs.
It is important to mention here that the overall performance for the QE well-aligned with the recent optical investigation on KGeCl₃. The optical constants of KGeCl₃ such as the refractive index (n(omega )) and the extinction coefficient (k(omega )) have been reported recently by W. Azeem et. al. The refractive index is found to vary from 1.5 to 2.25 within the visible wavelength range and display a peak at 330 nm, and the extinction coefficient is observed to display a peak near 280 nm having a value of 1. Furthermore, the material showed low reflectivity within the visible light range, which supports the obtained QE behavior. In addition, it was found that the material is highly active to the light absorption within a broad range of spectrum including the visible light range⁴⁹. Within the range 300 – 400 nm, the absorption coefficient is found to be 7 × 10⁴ cm⁻¹, and at low wavelength beyond 100 nm, the absorption coefficient is around 22 × 10⁴ cm⁻¹.
Figure 3 presents the extracted optoelectronic parameters for the various ETLs investigated within the device structure. Among all the ETLs assessed, WS₂ emerged as the most effective candidate, achieving a power conversion efficiency PCE of 20.51%, an open circuit voltage V_oc of 1.0217 V, a short circuit current density J_sc of 25.02 mA/cm², and a fill factor FF of 80.61%. Notably, both ZnO and SnO₂ demonstrated comparable enhancements in efficiency, exhibiting high quantum efficiencies as well. The device performance metrics for ZnO and SnO₂ were nearly identical, with PCEs of 19.97% and 19.96%, respectively. They also displayed similar electrical outputs, featuring a V_oc of 1.20 V, J_sc of 24.37 mA/cm², and FF of 80.30%. The comparable performance of ZnO and SnO₂ indicates that these materials also present viable options as effective ETL candidates.
The photovoltaic parameters of the simulated cell architecture (a) J_sc, (b) V_oc, (c) FF, and (d) PCE of the CFTS-based solar cells for TiO₂, IGZO, SnO₂, ZnO, ZnSe₂, WO₃, PCBM, and WS₂ TMDC ETLs.
For further analysis, the energy band alignment between the KGeCl₃ perovskite and all evaluated ETLs is illustrated in Figure 4. The electron affinity and bandgap of the materials employed in the solar cell structure are critical in determining the band alignment between the layers. A favorable alignment, with the conduction and valence band edges of the ETL closely aligned with those of KGeCl₃, facilitates the efficient transport of charge carriers generated in the absorber layer. The figure clearly indicates that WS₂ is the most effective ETL among the tested materials, exhibiting optimal energy band alignment with the KGeCl₃ absorber, thereby corroborating the performance results presented in Figure 3. In the case of ZnO and SnO₂, the difference in their bandgap is minimal, with SnO₂ having a slightly wider bandgap compared to ZnO. The observed improvements in efficiency are likely attributed to the superior mobility values of these materials, as detailed in Table 2. It important to note here that in solar cell devices, the band alignment between the ETL and absorber layer is one of several critical factors that affect the device performance. In our study, while the conduction band alignment between WS₂ and KGeCl₃ may not be optimal, the high mobility of WS₂ for charge transport as depicted clearly in Table 2, tunable bandgap, favorable work function alignment, reduced recombination losses, and optical transparency all contribute to the enhanced performance of WS₂-based devices. These factors help mitigate the impact of the unfavorable band alignment and enable superior performance in devices that use WS₂ as an electron transport layer (ETL). Several studies have shown how the abovementioned factors plays positively in enhancing the device performance of the Perovskite-WS2 based solar cells^50,51,52.
Corresponding band energy diagrams of planar perovskite solar cells using Spiro-OMeTAD as HTM and different ETMs.
To examine the impact of ETL thickness on device performance, the thickness of all ETLs was varied from 50 to 300 nm. The results are depicted in Fig. 5 from (a) to (d). As depicted in Fig. 5a, no significant effect on the J_sc was observed across the range of ETL thicknesses. A slight impact on the V_oc was noted within the thickness range of 250 nm to 300 nm, with an average change of approximately 0.01 V. The FF and PCE exhibited similar behavior for all ETLs, as depicted in Fig. 5c,d, remaining largely unchanged except for PCBM. In the case of PCBM, both FF and PCE decreased noticeably as the thickness increased from 50 to 300 nm. Notably, WS₂ exhibited the highest values for J_sc, V_oc, and PCE, measuring approximately 25.02 mA/cm², 1.02 V, and 20.50%, respectively, while the lowest performance was recorded for PCBM.
The obtained photovoltaic parameters versus the different ETL thicknesses with (a) J_sc, (b) V_oc, FF, and (d) PCE.
The electron affinity is a critical factor in determining the Conduction Band Offset (CBO) and the Valence Band Offset (VBO) at the heterojunction between the charge transport layer (CTL) and the perovskite material, which are key to the efficiency and performance of PSCs⁵³. The CBO and the VBO between the absorber layer KGeCl₃ and the ETL affect greatly the transport process of the photogenerated charge carriers. In general, existence a difference in the conduction band levels between the ETL and the absorber leads to a formation of intermediate energy band which affects the transport process of the charge carriers. This intermediate energy band can be in a form of cliff shape, flat, and spike-like shape. The CBO and VBO can be calculated by the equations^54,55:
where (chi) refers to the electron affinity and ({E}_{g}) refers to the energy band gap. When the energy difference is zero, then the shape is nearly flat, which means that there is no band offset and consequently no barrier for the transport of charge carriers. The band alignment of the ETL/absorber exhibits a spike-like appearance for positive CBO or negative VBO values. This situation prevents the recombination at the interface and facilitate the charge carriers’ flow. When CBO is negative and VBO is positive, the ETL/absorber band alignment is cliff-like, facilitating increased and faster recombination at the interface. Thus, a reduction in the device efficiency. Table 3 shows the calculated CBO and VBO for all utilized ETL in this study. While TiO₂ and ZnO have zero values of CBO and negative VBO values, their PCE values are less than the PCE for WS₂. This can be the low VBO for WS₂ in comparison to the other ETLs and the large electron and hall mobilities, which contributed effectively to having higher efficiency in WS₂/KGeCl₃ solar cell device. In PCMB, its low PCE could be related to its small values of the electron and hall mobilities, which impacted significantly even though its energy band at the interface demonstrate a spike-like appearance.
Figure 6 illustrates the variation of χ for the tested ETLs and its impact on device performance. The electron affinity was varied from 3.6 to 4.5 eV to evaluate its influence. The results reveal that for PCBM, an optimal electron affinity of 3.8 eV ensures ideal band alignment, promoting efficient charge transfer while minimizing recombination losses. However, increasing the electron affinity beyond this threshold introduces a steeper CBO at the heterojunction, leading to increased recombination and reduced performance. Conversely, for the other ETLs examined, variations in electron affinity had negligible impact on device performance. This stability suggests that optimizing electron affinity is particularly critical for materials like PCBM, whereas other ETLs demonstrate robust performance across a range of electron affinities.
The simulated photovoltaic results versus the ETL electron affinity with (a) J_sc, (b) V_oc, (c) FF, and (d) PCE.
To deepen the understanding of device performance using different ETLs, analysis on the lattice mismatch is included. Lattice mismatch is an important parameter in solar cells to verify the layers compatibility. A significant lattice mismatch can undermine interface stability, resulting in inefficient charge carrier collection and decreased overall performance. To enhance solar cell efficiency, it is crucial to assess the lattice mismatch between the ETL and absorber using the equation⁵⁶:
where δ is the lattice mismatch. ({a}_{s}) and ({a}_{e}) are the substrate and epitaxial thin film lattice constants, respectively. Table 4 presents the calculated lattice mismatch between KGeCl₃ and selected ETLs, including WS₂ assuming supercell matching. The lattice constants for these materials were derived from the lattice parameters reported in previous studies^31,57,58,59.
Table 4 shows that the lattice mismatch is minimum when the ETL is WS₂ with a value of 16.58%. This result confirms the obtained results for the device performance using different ETLs.
Figure 7 illustrates the impact of the doping concentration of various ETLs on solar cell performance. The doping concentration of each ETL varied from 10¹⁴ to 10²¹ cm⁻³. The results demonstrate that all optoelectronic parameters improve with increasing ETL doping concentration up to a critical value. Initially, the parameters remain constant up to 10¹⁶ cm⁻³, followed by a significant improvement in performance in a step-like manner. Beyond 10¹⁷ cm⁻³ up to 10²¹ cm⁻³, the parameters plateau and remain unchanged up to 10²¹ cm⁻³. This behavior can be attributed to the increased electron conductivity at higher doping levels, which reduces resistance to electron flow from the perovskite absorber layer, driven by the strong built-in electric field. The plateau at higher doping concentrations suggests that further increases in doping levels do not contribute significantly to performance enhancement. At an optimal doping concentration of 10¹⁷ cm⁻³, WS₂ consistently outperforms the other ETLs, achieving the highest values for key performance metrics: V_oc at 1.05 V, J_sc at 25.10 mA/cm, FF at 80.05%, and PCE at 20.21% at a doping concentration of 10¹⁷ cm⁻³. These results reaffirm WS₂ as the most effective ETL among those examined, offering superior photovoltaic performance under optimal doping conditions.
The photovoltaic parameters as a function of the donor concentrations of the ETL with (a) J_sc, (b) V_oc, (c) FF, and (d) PCE.
Based on the prior investigations, WS₂ is identified as the optimal ETL for the device structure. To further refine device performance, an extended study was conducted to evaluate the combined effects of WS₂ thickness and absorber layer thickness on solar cell performance. The absorber layer thickness is a critical parameter that influences photon absorption efficiency and must strike a balance between maximizing light capture and minimizing reverse saturation current. The contour plot in Fig. 8 illustrates the effect of absorber thickness (horizontal axis) and WS₂ thickness (vertical axis) on the photovoltaic performance of the solar cell. The absorber layer thickness was varied from 300 to 900 nm, while the WS₂ thickness ranged from 30 to 110 nm. The results indicate that the optimal efficiency of 20.72% is achieved at an absorber layer thickness of 600 nm and a WS₂ thickness of 30 nm. The V_oc remains largely unaffected by changes in WS₂ thickness, demonstrating consistent values across the range. For instance, at an absorber thickness of 300 nm, V_oc is consistently 1.03 V. The J_sc, however, exhibits a notable increase as the absorber layer thickness is increased, rising from 24.50 mA/cm² at 300 nm to 26.21 mA/cm² at 900 nm. Meanwhile, the FF remains stable at any given ETL thickness, further confirming the robustness of WS₂ as an ETL. PCE improves as the WS₂ thickness is maintained at 30 nm while varying the absorber thickness from 300 to 900 nm, increasing from 20.20 to 20.72%. These findings highlight the importance of optimizing both the ETL and absorber layer thicknesses to achieve maximum photovoltaic performance, with WS₂ playing a pivotal role in enabling high-efficiency solar cells.
(Color online) Contour mapping of the optoelectronic parameters for the KGeCl₃ absorber solar cell device thickness and WS₂ ETL thickness.
To further optimize the modeled solar cell, the physical parameters of the KGeCl₃ absorber layer and the HTL were analyzed to determine their optimal values. Figure 9 presents a contour plot of the optoelectronic parameters, showing the relationship between absorber layer thickness (horizontal axis) and defect density (vertical axis). The absorber thickness varied from 300 to 900 nm, while the defect density ranged from 10¹⁴ to 10¹⁹ cm⁻³. The results indicate that while increasing the absorber thickness enhances device performance, a higher defect density significantly deteriorates it. At a fixed absorber thickness, the J_sc remains stable at 25.65 mA/cm², regardless of the defect density, indicating that J_sc is not directly influenced by defect density changes. In contrast, the open-circuit voltage V_oc, FF, and PCE are highly sensitive to increases in defect density. For instance, V_oc and PCE remain relatively stable up to a defect density of 10¹⁷ cm⁻³. Beyond this threshold, a sharp decline is observed, with V_oc dropping from 0.99 to 0.87 V and PCE decreasing from 20.40% to 18.22% as the defect density increases from 10¹⁷ to 10¹⁹ cm⁻³. The FF is notably influenced by both absorber thickness and defect density. The optimal FF of 80% is achieved with an intermediate defect density of 10¹⁷ cm⁻³and an absorber thickness of 450 nm. Overall, the findings emphasize that high defect densities adversely affect solar cell performance, underscoring the importance of using high-quality materials with minimal defects during fabrication. Achieving low defect densities is essential for producing efficient and stable perovskite solar cells^47,59,60,61.
(Color online) Contour mapping of the cell performance of thickness and defect density of the KGeCl₃ absorber layer.
Absorber doping density significantly influences solar cell performance⁶². At moderate doping levels, studies have shown improved carrier concentration and conductivity, which can enhance device efficiency. However, excessive doping density can reduce charge carrier lifetime, increase recombination rates, and degrade material performance due to scattering and the introduction of defects. Thus, optimizing doping density is essential for achieving superior performance in absorber materials used in photovoltaics. In this investigation, the doping density and thickness of the absorber layer were varied from 10¹⁵ to 10¹⁹ cm⁻³ and 400 nm to 900 nm, respectively. Figure 10 illustrates that the doping density exhibits a weak impact on device performance at any given absorber thickness. However, all parameters improve significantly within the higher doping density range of 10¹⁸ to 10¹⁹ cm⁻³. The optimal device performance is achieved at an absorber thickness of 450 nm and a doping density of 10¹⁹ cm⁻³. At these conditions, the solar cell exhibits a PCE of 21.94%, V_oc of 26.50 V, a FF of 83.65%, and a J_sc of 25.30 mA/cm². These findings highlight the importance of carefully controlling the doping density to balance carrier concentration and minimize recombination losses, thereby optimizing the performance of perovskite solar cells.
(Color online) Contour mapping of the solar cell performance of thickness and doping density of the absorber layer.
For further optimization of device performance, the thickness of the Spiro-OMeTAD HTL was studied by varying it from 30 nm to 80 nm. As illustrated in Figure 11, it was observed that V_oc​ and J_sc remain unaffected by changes in HTL thickness, maintaining average values of 25.77 mA/cm² and 1.016 V, respectively. However, the FF and PCE demonstrated a negative correlation with increasing HTL thickness. This decline is attributed to higher lateral resistance within the junction as the HTL becomes thicker. The results indicate that an optimal HTL thickness of 30 nm achieves the best performance, with a FF of 87.25% and a PCE of 22.84%. This underscores the importance of optimizing the HTL thickness to minimize resistance losses and maximize overall solar cell efficiency.
The change of the Spiro-OMeTAD HTL thickness with (a) J_sc, (b) V_oc, (c) FF, and (d) PCE. The thickness changes from 30 to 80 nm.
While the model predicts a PCE approaching ~23% under idealized conditions, we acknowledge that achieving this performance in practical devices will depend on several critical factors, including material quality, interface engineering, charge transport layers, and fabrication reproducibility. Our model assumes optimized parameters such as minimal non-radiative recombination losses, ideal energy level alignment, and efficient charge extraction. These conditions represent a theoretical upper limit under near-ideal circumstances. However, recent experimental advances in similar systems have reported PCEs exceeding 16.2%⁶³, indicating that the ~23% target, while ambitious, is within reach with continued improvements in materials processing and device architecture.
The effect of operating temperature on the solar cell performance was investigated by varying the temperature from 280 to 500 K. Figure 12 illustrates the variation of key cell parameters as a function of temperature. The results show a continuous decrease in all parameters, except for the J_sc, which remains constant throughout the entire temperature range. While an increase in temperature typically enhances J_sc​ due to an elevated electron–hole pair generation rate and a reduction in the energy bandgap, the constant J_sc​ observed in this study suggests that the rate of recombination increases proportionally with the temperature, offsetting the benefits of higher carrier generation. The efficiency PCE decreased significantly from 22.98% at 280 K to 18.6% at 500 K. This decline in efficiency corresponds with reductions in both the FF and V_oc, both of which follow a similar decreasing trend as PCE. As expected, elevated operating temperatures negatively impact solar cell performance, emphasizing the importance of thermal management and the use of heat-resistant materials in the design and deployment of solar cells.
Influence of the operating temperature of the KGeCl₃ Perovskite solar cells (a) J_sc, (b) V_oc, (c) FF, and (d) PCE.
This study explores various materials, including WO₃, WS₂, PCBM, TiO₂, IGZO, SnO₂, ZnO, and ZnSe₂, as potential ETLs for perovskite solar cells. The results revealed that PCBM exhibited the lowest Jsc of 21.50 mA/cm² and PCE of 17.11% among the tested materials, along with the lowest QE. In contrast, zinc oxide (ZnO) and tin dioxide (SnO₂) demonstrated notable and comparable efficiencies of 19.98% and 19.97%, respectively, along with high QE. Among all candidates, WS₂ emerged as the most promising ETL. Performance optimization was further achieved by fine-tuning the thicknesses of WS₂ (ETL), Spiro-OMeTAD (HTL), and the absorber layer to 50 nm, 30 nm, and 600 nm, respectively. Additionally, the study analyzed the influence of temperature on device performance across a range of 280 K to 500 K. The findings indicate that device parameters significantly deteriorated with increasing temperature, primarily due to changes in resistance, bandgap, and the mobility of charge carriers at elevated temperatures.
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
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Department of Physics, College of Science, The University of Jordan, Amman, 11942, Jordan
Z. Abu Waar
Department of Physics, School of Sciences and Engineering, The American University in Cairo, New Cairo, 11835, Egypt
A. Abd El-Samad, H. Zeenelabden, M. Swillam & M. Moustafa
College of Integrative Studies, Abdullah Al Salem University, Khaldiya, Kuwait
S. Yasin
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Z. W. Validation, Visualization, Writing—review & editing. A. A.: Conceptualization, Methodology, Software, Writing—original draft. H. Z.: Software, Validation. S. Y.: Methodology, Writing—original draft. M. S. : Software, Validation, Writing—original draft, M. M.: Methodology, Validation, Writing—review & editing, Visualization. All authors provided critical feedback and approved the final version.
Correspondence to S. Yasin or M. Moustafa.
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Waar, Z.A., El-Samad, A.A., Zeenelabden, H. et al. Computational study of KGeCl₃ perovskite solar cells toward high efficiency via electron transport innovation. Sci Rep 15, 32054 (2025). https://doi.org/10.1038/s41598-025-00822-9
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Received: 23 January 2025
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Published: 01 September 2025
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DOI: https://doi.org/10.1038/s41598-025-00822-9
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