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Scientific Reports volume 15, Article number: 34720 (2025) Cite this article
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A novel high-efficiency dual-absorber perovskite solar cell architecture is proposed by integrating a CsPbI₃ wide-bandgap top layer with an EAGeI₃ narrow-bandgap bottom layer, enhanced through nanophotonic and plasmonic engineering. Using advanced three-dimensional simulations based on the Finite Element Method, the optical and electrical characteristics of the device were analyzed under realistic boundary conditions. The inclusion of EAGeI₃ extends the absorption spectrum into the near-infrared region, enabling a power conversion efficiency (PCE) of 23.75%, which is 23.2% higher than the single-junction CsPbI₃ structure. To improve light harvesting and carrier transport, a zigzag nanostructure was introduced at the interface between the two absorbers, raising the PCE to 24.45%. Further enhancement was achieved by embedding silver nanoparticles with cubic geometry, resulting in strong near-field plasmonic effects and efficient light scattering. The final optimized design yielded Jsc = 35.63 mA/cm², Voc = 0.91 V, FF = 85.74%, and PCE = 27.80%, marking a 44.2% increase compared to the SJ reference and an 17% improvement over the flat dual-absorber design. Additionally, the influence of temperature variation, fabrication tolerances, and film quality on device performance was also evaluated to assess theoretical feasibility and preliminary stability. While these simulation results indicate promising potential, experimental validation is required to confirm the robustness, reproducibility, and scalability of this environmentally friendly, lead-reduced PSC.
The growing interest in perovskite solar cells (PSCs) in recent years has been primarily driven by their exceptional and distinctive properties. These include the use of low-cost raw materials, ease of fabrication, high optical absorption coefficients, long carrier diffusion lengths, tunable bandgaps, elevated open-circuit voltages, and compatibility with flexible substrates. Owing to these advantages, PSCs are regarded as promising alternatives to conventional silicon-based solar cells, which currently account for over 90% of the global photovoltaic market share^1,2,3,4,5,6.
Since their inception in 2009, single-junction (SJ) PSCs have witnessed rapid advancements, with power conversion efficiencies (PCEs) increasing from 3.8⁷ to 26.1%⁸ within a relatively short period. Despite this remarkable progress, PSCs remain susceptible to environmental stressors such as humidity, heat, oxygen, and ultraviolet (UV) light, which pose significant stability challenges⁹. Various mitigation strategies have been proposed to address these issues, including interface engineering, encapsulation techniques¹⁰, and improved crystallization processes¹¹. Additionally, the substitution of hybrid organic–inorganic compositions (e.g., MAPbI₃) with fully inorganic perovskites such as CsPbI₃ has shown considerable potential in enhancing stability due to the elimination of volatile organic cations like CH₃NH₃₊, thereby improving both thermal and chemical robustness¹². Notable advancements include a carbon-based CsPbI₃ PSC with a PCE of 9.5%, which retained 90% of its initial performance after 3000 h of operation¹³, and further enhancements by Qiufeng et al., who incorporated phenylethylammonium to achieve a PCE of 17% with 94% stability after 2000 h¹⁴. Experimental optimizations have also yielded a PCE of 19%¹⁵, and the application of acyloin ligands enabled a PCE of 21.15%¹⁶.
However, the theoretical Shockley–Queisser efficiency limit for SJ-PSCs, approximately 31%¹⁷, inherently constrains their performance potential. In order to transcend this limitation, tandem solar cell architectures have been proposed. These consist of multiple sub-cells with distinct bandgaps tailored to different portions of the solar spectrum, commonly comprising narrow-bandgap (NBG) and wide-bandgap (WBG) layers, thereby enabling significant improvements in overall efficiency¹⁸. Despite their efficiency benefits, tandem solar cells are often hindered by complex fabrication requirements and higher production costs¹⁹.
More recently, simplified multi-absorber or half-tandem structures have gained traction as promising alternatives. Unlike traditional tandem configurations, these architectures involve the direct stacking of absorber layers without the inclusion of recombination interfaces or additional electron and hole transport layers (ETL/HTL). Consequently, this design reduces fabrication complexity and cost while eliminating current-matching constraints between sub-cells²⁰. Moreover, these configurations exhibit lower recombination rates and provide additional design flexibility for the incorporation of plasmonic nanoparticles (NPs) or grating-based nanostructures (NSs), which enhance light trapping and optical absorption²¹. These multi-absorber designs may employ perovskite and non-perovskite materials, with their performance highly dependent on several critical factors. These include appropriate bandgap matching for comprehensive spectral coverage, suitable energy-level alignment to ensure efficient charge carrier extraction and minimize recombination at interfacial boundaries, and controlled deposition processes to avoid poor crystallinity or interfacial defects that lead to enhanced recombination and reduced PCE²².
Simulation studies have demonstrated the benefits of such structures. Zandi et al. reported an increase in PCE from 14.32 to 17.5% by incorporating MASnI₃ beneath the MAPbI₃ layer and employing a periodically corrugated back contact²³. Similarly, Solhtalab et al. achieved a PCE increase from 14.37 to 17.62% in a CIGS/MAPbI₃ dual-absorber device, which further improved to 19.29% upon integrating a 60 nm nano-prism structure at the absorber interface²⁰. In another approach, Zhang et al. tuned the I:Br ratio in CsPbIxBr₃₋x/FAPbIyBr₃₋y layers to optimize band alignment and optical absorption, achieving a PCE of 17.48% with enhanced device stability through interface engineering²⁴. Mohandes et al. proposed a CsPbI₃/BaZrSe₃ dual-absorber configuration, extending light absorption and achieving a PCE of 27.52% and a J_sc of 46.33 mA/cm²²⁵.
Furthermore, dual-absorber configurations have also been shown to improve device longevity. Najafi et al. utilized a 2D/3D perovskite heterostructure, simulated using Drift–Diffusion equations, to achieve a PCE of 28.4% while improving stability and eliminating hysteresis²⁶. Rahman et al. reported a stable device using CsSnI₃ and CsPbI₃ with a CdS ETL, reaching a PCE of 28.74% V_oc of 0.996 V, J_sc of 34.94 mA/cm and a fill factor (FF) of 82.61%²⁷. Noori et al. demonstrated that integrating a 150 nm CsPbI₃ layer on top of a 300 nm MAPbI₃ layer increased the PCE from 15.9 to 20.52%²⁸. A further example by Bhattarai et al. involved a Cs₂BiAgI₆/CIGS dual-absorber structure that reached a PCE of 30.10%, with improved light harvesting at longer wavelengths²⁹.
Several other studies have explored dual-absorber systems employing various perovskite and non-perovskite combinations, achieving PCEs ranging from 18.55 to 31.94%^{30,31,32,33,34,35,36}. These findings collectively underscore the efficacy of multi-absorber architectures in maximizing solar spectrum utilization and advancing high-efficiency photovoltaics.
Among emerging materials, ethylammonium germanium iodide (CH₃CH₂NH₃GeI₃ or EAGeI₃) has recently been identified as a promising lead-free, eco-friendly alternative for photovoltaic applications. Theoretically, EAGeI₃ exhibits a direct bandgap of approximately 1.30 eV, which is well-suited for sunlight absorption. Additionally, it possesses a high absorption coefficient for wavelengths exceeding 600 nm. The replacement of toxic Pb²⁺ with Ge²⁺ and MA⁺ with EA⁺ contributes to both environmental compatibility and enhanced chemical stability. Moreover, its favorable thermoelectric characteristics—such as a Seebeck coefficient exceeding 400 μV/K and a ZT > 0.9—further support its potential use in advanced optoelectronic devices. These attributes make EAGeI₃ a suitable candidate for the NBG layer in high-efficiency perovskite devices³⁷.
Light trapping is a key mechanism for enhancing the efficiency of solar cells. Various strategies utilizing different nanostructures have been developed to improve light trapping, and recent advancements in silicon solar cell technologies have significantly boosted power conversion efficiencies, with architectures such as Passivated Emitter and Rear Cell (PERC) and Tunnel Oxide Passivated Contact (TopCon) leading the progress. PERC technology employs dielectric passivation layers on the rear side of silicon wafers to minimize surface recombination and enhance carrier lifetime, achieving efficiencies exceeding 23%³⁸. TopCon cells integrate an ultrathin tunnel oxide layer with doped polysilicon to provide excellent surface passivation and carrier selectivity, pushing commercial efficiencies beyond 26%³⁹.
In parallel, nanostructure-based designs can complement these architectures by manipulating light at the nanoscale, enhancing absorption in targeted spectral ranges, and further suppressing recombination losses. For instance, metallic nanoparticles, textured interfaces, and quantum dots can induce localized surface plasmon resonances, leading to increased optical path lengths within the absorber layer⁴⁰. Moreover, nanostructured passivation layers and interfaces can reduce surface recombination and improve charge extraction, complementing the functionalities of PERC and TopCon architectures.
Furthermore, core–shell metallic NPs, such as Ag@SiO₂, have attracted considerable attention due to their ability to enhance chemical and thermal stability by isolating the metal core from the reactive perovskite environment⁴¹. However, the dielectric shell may alter plasmonic properties and charge transfer dynamics, making careful structural design essential to balance improved stability with optimal device performance.
In this study, a series of innovative strategies has been proposed to enhance the performance of SJ-PSCs. These strategies include the integration of dual perovskite absorbers—CsPbI₃ and EAGeI₃—interface engineering through zigzag-shaped NSs, and plasmonic enhancement via Ag NPs. The complementary optical properties of CsPbI₃ and EAGeI₃ have been exploited to extend the absorption spectrum and improve the structural and environmental stability of the device. To enhance light trapping and improve Jsc, which is crucial for boosting the PCE in multi-absorber configurations, zigzag nanostructures were introduced at the CsPbI₃/EAGeI₃ interface. These NSs significantly increased Jsc and thereby improved the overall PCE to 24.45%. Furthermore, plasmonic Ag NPs were strategically embedded within the NBG region to harness both near-field electromagnetic enhancement—intensifying local electric fields—and far-field scattering—extending the optical path length. This synergistic plasmonic effect resulted in further absorption enhancement and elevated the PCE to a maximum of 27.80%, representing a relative improvement of over 44.2% compared to the reference SJ configuration. In the following sections, the impacts of temperature variations, fabrication-related imperfections, and perovskite film quality on device performance are systematically examined and discussed, providing preliminary insights into the theoretical stability and potential practical viability of the proposed structure.
In this study, the proposed PSC features a multilayered architecture specifically engineered to enhance light absorption, facilitate efficient charge transport, and improve overall device stability. The designed structure comprises the following sequence of layers: ITO/ZnO/CsPbI₃/EAGeI₃/NiO_x/Au, as illustrated in Fig. 1a and b. Each material has been meticulously selected based on its optical, electronic, and structural properties to ensure optimal performance of the PSC. The topmost layer is composed of indium tin oxide (ITO), with a defined thickness of h_ITO. ITO acts as an anti-reflective coating (ARC) due to its high optical transparency and excellent electrical conductivity. These characteristics minimize photon loss at the front interface and allow efficient extraction of photogenerated charge carriers, thereby enhancing the overall light harvesting and charge collection efficiencies.
(a) Schematic illustration of the proposed dual-absorber PSC structure with a step-by-step structural modification. (b) Cross-sectional view with defined geometrical parameters, along with representations of the employed cubic and spherical NPs. (c) Energy band diagram of the proposed PSC.
Under the ITO, a ZnO layer of thickness h_ZnO is utilized as the ETL. ZnO is selected due to its high electron mobility, appropriate conduction band alignment with the perovskite absorber, and superior optical transparency. The high mobility of ZnO enables swift carrier extraction and reduces charge recombination losses. Its transparency allows efficient photon transmission to the absorber layer, which contributes to elevated photocurrent generation. Moreover, ZnO offers enhanced thermal and chemical stability compared to conventional ETLs such as TiO₂, thereby improving the long-term durability of the PSC. Importantly, ZnO can be synthesized at relatively low temperatures using scalable methods such as solution processing, making it highly attractive for low-cost fabrication processes^42,43.
The light-absorbing region of the device comprises a bilayer configuration formed by CsPbI₃ and EAGeI₃ perovskites. CsPbI₃, with a thickness of h_WBG, serves as a WBG absorber layer and contributes significantly to high-energy photon absorption and efficient charge transport. In tandem, EAGeI₃, with a thickness of h_NBG, acts as a NBG absorber and complements CsPbI₃ in terms of both optical absorption and structural compatibility. The dual-absorber configuration not only broadens the absorption spectrum but may also provide improved operational stability, as Ge-based perovskites are highly vulnerable to moisture and oxygen, and the top CsPbI₃ layer can partially shield the underlying EAGeI₃ from direct environmental exposure^44,45. To ensure efficient charge separation and extraction, all materials within the device were selected to maintain favorable energy level alignment, thereby preventing potential energy barriers at the interfaces. The corresponding energy band diagram of the proposed structure is presented in Fig. 1c.
To further enhance light absorption and carrier collection, a zigzag-shaped NS with height h_NS is introduced at the interface between the CsPbI₃ and EAGeI₃ layers. This interfacial engineering approach improves internal light scattering, reduces carrier recombination losses at the junction, and compensates for the V_oc reduction that are typically observed in multi-absorber configurations. The resulting increase in internal photon recycling and light trapping contributes to a significant enhancement in PCE.
In addition, cubic Ag NPs with side length a_cubic are embedded within the EAGeI₃ layer to introduce plasmonic enhancement. These NPs exploit both near-field and far-field plasmonic effects: near-field enhancement increases the local electromagnetic field intensity near the NPs, while far-field scattering extends the optical path length of incident light. This dual mechanism significantly improves absorption in the NBG region, leading to greater photocurrent generation and improved device performance.
NiO_x, with a thickness of h_NiOx, is employed as the HTL. NiO_x is chosen for its suitable energy level alignment with the valence band of the perovskite, high chemical stability, and facile fabrication through low-temperature processes, and its electrical properties can be enhanced through higher doping levels. Its robustness against moisture and oxygen exposure contributes to enhanced device longevityFurthermore, NiO_x exhibits low toxicity and excellent environmental compatibility, making it an eco-friendly HTL candidate. It has also been reported to suppress hysteresis and improve operational stability in perovskite-based devices⁴⁶. The back contact layer is composed of Au, with a thickness of h_Au. Owing to its high electrical conductivity and chemical inertness, the Au layer ensures efficient hole extraction and long-term device stability.
The thicknesses of all individual layers used in the proposed PSC structure are summarized in Table 1. This architecture has been modeled using a three-dimensional Finite Element Method (3D-FEM), with an optimized meshing scheme applied to ensure high spatial accuracy in the simulation of optical and electrical behavior. The combination of bilayer absorbers, plasmonic enhancement, and nanostructured interfaces results in a synergistic improvement in light harvesting, carrier transport, and device stability, ultimately contributing to a high-efficiency and reliable PSC design.
To experimentally realize the simulated nanostructured ITO/ZnO/CsPbI₃/EAGeI₃/NiO_x/Au solar cell, we propose a combination of nanostencil hard-mask lithography and low-damage anisotropic dry etching.
Nanostencil lithography provides solvent-free, reusable masking suitable for defining the zigzag perovskite interface and embedded nanopatterns.
While nanosphere lithography (NSL) and nanoimprint lithography (NIL/SCIL) offer cost-effective large-area nanostructuring, they require either solvent lift-off or physical stamping, making them less compatible with fragile all-inorganic perovskites unless combined with protective hard masks.
For etching, low-power fluorine-based RIE allows selective patterning of the EAGeI₃ layer while preserving the underlying CsPbI₃, minimizing plasma-induced defects^47,48.
To accurately simulate the optical behavior of the proposed PSC and emulate real-world illumination conditions, a plane wave light source based on the standard AM1.5G solar spectrum was utilized⁴⁹. The solar irradiance spectrum was interpolated over the wavelength range of 300–1100 nm to cover the most significant portion of sunlight reaching the Earth’s surface. To minimize computational complexity and processing time, periodic boundary conditions (PBC) were applied along the lateral (x and y) directions. Perfectly matched layers (PMLs) were incorporated in the vertical (z) direction, aligning with the direction of incident light, to absorb outgoing waves and prevent artificial reflections at the domain boundaries, thereby ensuring accurate wave propagation modeling. The optical field distribution was determined by solving the Helmholtz equation, which is derived from Maxwell’s equations in the frequency domain. The Helmholtz equation for the electric field E is expressed as follows⁵⁰:
where (E) is the electric field vector, (k_{0}) is the wave number in vacuum, and (n(lambda )) and (k(lambda )) represent the real and imaginary parts of the refractive index, respectively, as functions of the wavelength. After obtaining (E(r,lambda )) across all regions, the local absorption (A(r,lambda )) at each specific location within the cell can be calculated using the following equation⁵¹:
where (omega) is the angular frequency, (varepsilon_{0}) is the vacuum permittivity, and ({text{Im}} varepsilon (r,omega )) is the imaginary part of the relative permittivity. The input light intensity, (P_{in}), is assumed to be 1000 W/m², based on the standard AM1.5G solar spectrum. According to Eqs. (3 and 4), the total absorption within the absorber layers, (A_{tot – wbg}) and (A_{tot – nbg}), can then be calculated as follows⁵²:
where the parasitic absorption in the NP is calculated and subtracted from the total absorption of the cell. (a_{cubic}) represents the side length of the cubic nanoparticle, and (w_{h}),(h_{nbg}),(h_{wbg}) correspond to the period of the unit cell, the thickness of the NBG layer, and the thickness of the WBG layer, respectively. The generation rate is dependent on the light absorption in the absorber layers, which is calculated using the transfer matrix method. By employing this method, the optical generation rate Gopt​ could be determined as described in Eqs. (5,6) ⁵¹:
In this equation, (hbar) is Planck’s constant, the total generation rate (G_{tot}) is obtained by integrating (G_{opt}) as described in the following equations⁵¹:
The wavelength range for the calculations is set from 300 to 1100 nm with steps of 2 nm. This range is chosen because the majority of the solar spectrum reaching the Earth’s surface starts from 300 nm. The resulting (G_{tot}) from Eq. (7 and 8) is then used as an input in the electrical model. The wavelength-dependent real (n) and imaginary (k) parts of the refractive index for each material layer used in the optical simulations are presented in Fig. 2, thereby accounting for both dispersion and absorption losses in the materials.These values were extracted from previously validated experimental studies and were obtained from previous studies^{21,37,53,54,55}.
(a) The real part of the refractive index of materials. (b) Imaginary part of the refractive index of materials.
In this section, by inserting the values of (G_{tot}) into the Poisson and continuity equations, and solving these equations according to relations (9–12), the electron and hole current densities (J_{n}) and (J_{p}) are calculated⁵⁰:
where, (phi) represents the electrostatic potential, and (varepsilon) is the permittivity. The (n) and (p) refer to the electron and hole concentrations, respectively, while (N_{A}) and (N_{D}) represent the concentrations of acceptor and donor densities. Additionally, (R) and (G) denote the recombination and generation rates, respectively. (D_{n}) and (D_{p}) are the diffusion coefficients for electrons and holes, respectively, and (mu_{n}) and (mu_{p}) represent their mobilities. In order to account for non-radiative recombination losses in the absorber layers of the solar cell, the Shockley–Read–Hall (SRH) recombination model is employed. This model, as defined in Eq. (13), allows for the calculation of the carrier recombination rate and plays a crucial role in determining the V_oc⁵⁶.
In this equation, (n_{1}) and (p_{1}) are electron and hole densities, respectively. (tau_{n}) and (tau_{p}) are the lifetimes of electrons and holes, respectively. In this analysis, the front contact (ITO) and back contact (Au) are modeled as Schottky contacts with work functions of 4.5 eV and 5.1 eV, respectively. Surface recombination velocities are also considered for each contact, with values of Vs,n = 1 × 10⁵ cm/s and Vs,p = 1 × 10⁷ cm/s for Au, and Vs,n = 1 × 10⁷ cm/s and Vs,p = 1 × 10⁵ cm/s for ITO⁵⁷. All parameters related to the contacts are listed in Table 2. The PCE of the solar cell can be obtained using the following relation⁵⁰:
where, (J_{sc}), (V_{oc}), and (FF) represent the short-circuit current density, open-circuit voltage, and fill factor, respectively. To calculate the fill factor of the cell, we will use the following relation ⁵⁰:
In this equation, (V_{mp}) and (J_{mp}) represent the voltage and current density at the maximum power point, which are obtained from the J–V curve. It is worth mentioning that all the electrical parameters of the materials used in these equations, have been gathered from previous studies and are presented in Tables 3 and 4 for all the materials used in this simulation ^{21,32,57,58,59,60,61,62,63,64}.
The first step in the simulation process involved validating the accuracy and reliability of the simulated results. This validation is essential to enable the development of a high-precision model and to ensure that its behavior closely replicates that of the corresponding experimentally fabricated solar cell. To achieve this, the simulated structure was constructed based on a previously reported single-absorber solar cell, which comprises FTO/SnO₂/CsPbI₃/Spiro-OMeTAD/Au layers¹⁵. In this experimental study, the all-inorganic CsPbI₃ solar cell was fabricated using a blade-coating method under ambient conditions at temperatures lower than 100 °C. Without encapsulation, the samples were reported to retain their performance with only a 2% drop after 700 h of operation.
To validate the simulated structure, a detailed optical and electrical analysis was performed. The results for both the simulated and the experimental structures are presented in Table 5, alongside their respective percentage variation. The differences in V_oc, J_sc, PCE, and FF were found to be 0%, − 2.933%, 0.694%, and 4.133%, respectively. Furthermore, Fig. 3 shows a comparison of the J–V curves for both structures, demonstrating that the two are in strong agreement. These results collectively confirm the accuracy of the simulation framework and provide a reliable basis for proceeding with further investigations.
J–V curve comparison of the simulated and experimental PSCs.
To establish a reliable reference for comparing the performance and results of the dual-absorber perovskite solar cells, a SJ-PSC was first designed and simulated by modifying the previously simulated structure through appropriate adjustments in the materials and thicknesses of its constituent layers. The structure comprises an 80 nm indium tin oxide (ITO) layer, which serves as an anti-reflective coating (ARC), a 52 nm zinc oxide (ZnO) as ETL, a 570 nm CsPbI₃ perovskite absorber layer, a 60 nm nickel oxide (NiO_x) as HTL, and an 80 nm Au back contact. The period of the unit cell, denoted as W_h, was set to 200 nm. ZnO and NiO_x were used due to their favorable electronic properties, stability, and ease of fabrication, as previously discussed in “Device architecture and functional layer design” section.
The optical study of the single-junction structure was performed by solving the corresponding equations for transmission, reflection, generation rate, and absorption spectrum over a wavelength range of 300–1100 nm. The results are presented in Fig. 4a–d, where the blue curve denotes the SJ-PSC structure. The absorption spectrum shows maximum absorption within the 380–600 nm range, which contributes to a higher generation rate, as illustrated in the generation profile in Fig. 4c. Beyond a wavelength of 600 nm, absorption gradually decreases.
(a) Transmission, (b) reflection, (c) generation rate, and (d) absorption profiles of the proposed PSC structures.
The electrical study revealed the following key performance parameters for the SJ-PSC: PCE = 19.28%, V_oc = 1.02 V, J_sc = 22.52 mA/cm², and FF = 83.93%. These results indicate a 21.6% improvement in PCE in comparison to the experimental reference structure, which was achieved by carefully optimizing the layer thicknesses and selecting appropriate ETL and HTL materials. To enable a clearer visualization of the electrical performance of the proposed solar cell, the power–voltage (P–V) and current density–voltage (J–V) characteristics are presented in Fig. 5a and b, respectively. In these graphs, the blue curve corresponds to the SJ-PSC structure.
(a) P–V curve and (b) J–V curve of the proposed PSC structures.
In the subsequent step, a dual-absorber structure was designed in order to enable more efficient utilization of the solar spectrum. The first absorber layer, CsPbI₃ (with a thickness of 350 nm), serves as the WBG absorber, while the second absorber layer, EAGeI₃ (with a thickness of 220 nm), operates as the NBG absorber. This configuration allows for enhanced absorption of solar radiation across a broader range of wavelengths: CsPbI₃ predominantly absorbs photons within the 300–600 nm range, while EAGeI₃ extends absorption toward the 600–1100 nm range. Figure 6a and b illustrate the absorption spectra for the WBG and NBG layers, respectively. This complementary absorber configuration results in a significant improvement in light harvesting, yielding greater electron–hole pair generation and, ultimately, enhanced solar cell performance. The purple curves in Figs. 4 and 5 correspond to the dual-absorber structure. As shown in Fig. 4d, absorption is notably enhanced in the 550–1100 nm range, reflecting the additional absorption provided by the NBG layer.
Absorption profiles of the proposed PSC structures: (a) for the WBG layer and (b) for the NBG layer.
Electrical stimulation of the dual-absorber structure revealed a PCE of 23.75%, a V_oc of 0.90 V, a J_sc of 30.70 mA/cm², and a FF of 85.95%. This corresponds to a 23.2% improvement in PCE over the modified SJ-PSC structure. The drop in V_OC can be attributed to the lower bandgap of the NBG absorber; in tandem configurations, V_OC is typically governed by the narrower bandgap material. To enable smooth carrier transport and minimize recombination, careful energy level alignment was implemented across the stack⁶⁵. As a result, the V_OC of the designed solar cell is maintained without significant reduction. Furthermore, the optimization of energy levels facilitates the efficient extraction of photogenerated carriers from both absorber layers toward the ETL and HTL, thereby contributing to an increase in J_SC.
During the fabrication process, careful consideration must be given to the lattice matching between the two perovskite materials. Proper lattice matching serves to minimize surface defect density and reduce carrier recombination rates. Conversely, lattice mismatch may increase recombination, leading to reductions in both the V_OC and the PCE of the device. Hence, close lattice matching between the two perovskite materials (with lattice constants a_EAGeI3 = 6.387 Å and a_CsPbI3 = 6.2015 Å^37,66)—yielding a lattice mismatch of approximately 2.99%—helps reduce defect formation and further restrict recombination, thereby preserving high PCE.
To enhance light absorption, the initially flat interface between the two absorber layers was modified into a zigzag NS geometry. The period of the zigzag NS was set to 200 nm, based on the consideration that reducing the structural period below the shortest incident wavelength (300 nm) improves light absorption. Simulations were repeated for various NS heights (h_NS) to determine the optimal configuration, with a height of 210 nm found to yield the best performance. This optimized height was subsequently applied in all subsequent designs.
The zigzag NS enhances solar cell efficiency through two primary mechanisms. First, the refractive index difference between the two absorber materials causes incident light to scatter at their interface⁶⁷. This scattering effect increases the optical path length within the active layers, which, according to the Beer–Lambert law⁶⁸, improves photon absorption and consequently enhances the generation rate of electron–hole pairs, leading to higher PCE. Second, the presence of the zigzag NS causes the direction of photon propagation and carrier transport to become nearly orthogonal²⁰. This geometric configuration facilitates carrier extraction, reduces the carrier recombination rate, and thereby increases both the J_sc and PCE of the device.
As illustrated in Fig. 7a and b, both the absorption and PCE increase with the height of the zigzag NS, reaching a maximum at 210 nm. This improvement is primarily attributed to enhanced light scattering within the absorber layers, which increases light absorption in the perovskite region. However, when the height exceeds 210 nm, although overall absorption continues to rise, device efficiency begins to decline. This behavior can be explained by the additional absorption occurring predominantly in the 970–1100 nm wavelength range, while a slight reduction in absorption is observed in the 850–970 nm region. Given the AM1.5G solar spectrum, photon absorption below 1000 nm plays a more critical role in carrier generation, resulting in a net decrease in carrier generation and consequently a reduction in cell efficiency. Moreover, excessively tall nanostructures may cause strong light scattering at unfavorable angles, leading to optical losses due to light escaping from the active layer or being reflected out of the cell, thereby diminishing device performance. As illustrated in Fig. 7b, increasing the height of the zigzag NS also leads to a reduction in the V_OC of the solar cell. This decrease is primarily attributed to the dominant increase in the volume of the NBG absorber relative to the WBG absorber. Since the NBG absorber has a smaller bandgap, the overall V_OC of the designed solar cell is reduced.
(a) Absorption profiles and (b) Electrical parameters of the PSCs for different amounts of hNS.
As illustrated in Figs. 4 and 5, the key optical and electrical parameters for this structure are represented by the green curves. As shown in Fig. 4b, the incorporation of the zigzag structure clearly results in a significant reduction in reflection from the PSC. This enhancement is attributed to the improved light-trapping effect provided by the zigzag design at the absorber-interface, which consequently increases optical absorption and, in turn, the PCE of the device.
The proposed dual-absorber PSC with a 210 nm grating achieved PCE = 24.45%, V_oc = 0.89 V, J_sc = 32.60 mA/cm², and FF = 84.26% in electrical simulations, demonstrating a notable 26.9% improvement in PCE over the modified SJ-PSC structure.
One effective mechanism for enhancing light trapping in solar cells is the incorporation of metal NPs within the absorber layer. To investigate this approach, Ag NPs were introduced into the previously developed zigzag configuration, and they were positioned on the surface of the NBG layer. At this stage, two distinct NP geometries (cubic and spherical) were individually integrated into the structure, as depicted in Fig. 1b, and their respective optical and electrical simulation results were subsequently obtained and analyzed.
Silver was selected as the plasmonic material due to its cost-effectiveness, superior optical properties, and higher plasmonic enhancement effects in comparison with gold. Notably, both spherical and cubic NPs were designed with equal volumes in order to enable a fair and meaningful comparison between the two cases.
The black and gold curves in Figs. 4 and 5 represent the key optical and electrical characteristics of the dual-absorber structures incorporating spherical and cubic NPs, respectively. As shown in Fig. 4b, the incorporation of Ag NPs into the previously designed structure results in a significant reduction in reflection within the 1000–1100 nm wavelength range. This reduction, in turn, enhances light absorption and carriers’ generation in this spectral region, ultimately improving the PCE of the solar cell. Figure 8 further compares the External quantum efficiency (EQE) of the dual-absorber PSC with cubic NPs and the SJ CsPbI₃ PSC, highlighting the enhanced spectral response achieved through the plasmonic design.
External quantum efficiency (EQE) spectra of CsPbI₃ SJ PSC (red line) and dual-absorber PSC with cubic NP (black line).
As presented in Table 6, a noticeable enhancement in both and PCE is observed following the incorporation of Ag NPs. For the structure with spherical Ag NPs, PCE increases to 26.81%, while for the structure with cubic Ag NPs, PCE reaches 27.80%. These values represent a significant 44.2% improvement in comparison with the reference SJ structure. This improvement can be attributed to two primary mechanisms⁶⁹.
Near-field effect: LSPRs in metallic NPs strongly enhance the local electric fields in their vicinity, thereby increasing light absorption within a few nanometers of their surface.
Far-field effect: In addition, metallic NPs efficiently scatter incident light, altering its trajectory and extending its optical path length within the absorber layer. This phenomenon further contributes to improved light absorption and carrier generation. Far-field effects typically become significant over distances ranging from several hundred nanometers to a few micrometers.
As illustrated in Fig. 9a, the underlying mechanisms responsible for these effects are depicted, all of which collectively contribute to the enhanced solar cell efficiency. The sharp corners and edges of cubic NPs enable the near-field to become significantly more intense. As shown in Fig. 9b, the electric field at the sharp edges of the NPs is several times stronger than that in the surrounding areas, while spherical NPs exhibit a softer, more uniform field distribution. Furthermore, cubic NPs scatter light more strongly than spherical ones, thereby extending the optical path within the absorber layer and increasing the probability of photon absorption.
(a) Illustration of near-field and far-field effects of plasmonic NPs in the PSC. (b) Electric field distribution of the PSC with Cubic NPs at a Wavelength of 750 nm.
It is worth noting that the NPs’ placement, as shown in Fig. 1b, is precisely beneath the apex of the zigzag nanostructure. This positioning is crucial because, according to Snell’s law⁷⁰, incident light at the interface between two media with different refractive indices is redirected toward the center of the cell—directly toward the NPs—thereby enhancing light scattering and absorption and, in turn, improving solar cell performance.
To enhance the accuracy of the optical analysis, the total absorption of the solar cell was calculated exclusively based on the absorption occurring within the perovskite layer. The absorption taking place within the metallic NPs was deliberately excluded from the photogeneration rate calculations. Instead, this absorption was computed separately and presented as parasitic absorption, as illustrated in Fig. 4d.
As demonstrated in Figs. 5a and b, the integration of zigzag nanostructures along with various NP geometries leads to a significant enhancement in both power output and current density of the dual-absorber PSC compared to the SJ structure. These improvements confirm the effectiveness of enhanced light trapping and charge extraction mechanisms.
Further insights into the device performance can be observed in Fig. 10a–c, where the total generation rate is compared across three configurations: the SJ structure, the dual-absorber configuration without NPs, and the dual-absorber configuration incorporating cubic Ag NPs. The generation rates were evaluated at five selected wavelengths ranging from 450 to 1000 nm. As shown in the top row of Fig. 10, the SJ structure exhibits high generation near the surface, which rapidly diminishes at longer wavelengths, signifying limited light absorption beyond the visible spectrum. By incorporating EAGeI₃ as a secondary absorber, a more uniform and extended generation profile is achieved, particularly within the 600–900 nm wavelength range. This redistribution of generation highlights improved internal light redirection due to the additional absorber layer.
3D schematic of the generation rate at different wavelengths for (a) the SJ structure, (b) the dual-absorber structure, and (c) the dual-absorber structure with cubic NPs.
The most notable enhancement is observed in the third structure, which combines the dual-absorber configuration with cubic Ag NPs. At longer wavelengths (900 and 1000 nm), not only is the generation rate markedly increased, but the photogeneration also penetrates deeper into the absorber layers. This behavior is indicative of stronger light trapping and pronounced plasmonic effects induced by the metallic NPs. These results suggest that the nanostructured design effectively extends the optical path length, thereby enhancing absorption in spectral regions where the material intrinsically exhibits weaker absorptance. Overall, the comparison of the three structures underscores the critical importance of light management strategies, particularly under near-infrared illumination, in improving device performance.
One major challenge in embedding metallic NPs within perovskite absorbers is their degradation due to the chemically reactive environment. To mitigate this, an ultrathin dielectric shell, such as SiO₂, is often employed as a chemical and thermal barrier. Core–shell structures like Ag@SiO₂ enhance thermal stability by minimizing direct contact between metal NPs and the corrosive perovskite, thereby prolonging device lifetime⁴¹. However, for efficient hot-electron transfer, the dielectric shell thickness must remain below the electron tunneling distance (~ 2–5 nm).
Without such protection, LSPR induces NP heating, accelerating degradation. Nevertheless, core–shell designs influence LSPR and hot electron dynamics in complex ways that can reduce device efficiency. Specifically, the dielectric shell confines and attenuates the electric field at the metal–absorber interface, weakening optical coupling and LSPR intensity, especially with increasing shell thickness. Additionally, the shell hinders direct hot electron injection, requiring quantum tunneling, which significantly slows transfer rates if the shell thickness is not kept sufficiently thin. Therefore, careful design of the core–shell structure is crucial to prevent these issues and optimize device performance^71,72.
In this work, Ag cubic NPs were coated with a 5 nm thick SiO₂ layer to improve stability and device architecture. While stability may be improved, a slight efficiency loss was observed. The optical and electrical performance results of this configuration are presented in Figs. 4 and 5 (pink curves). As observed in Fig. 4d, the absorption spectrum of the core–shell structure shows a slight decrease compared to the bare NPs. Correspondingly, electrical measurements indicate a modest drop in performance compared to bare NPs: PCE of 26.19%, Jsc of 33.75 mA/cm², and Voc of 0.91 V.
A common fabrication-related issue in PSCs is the poor crystallinity of the perovskite film. Inadequate crystallization of the absorber layer leads to several detrimental effects⁷³. First, the number of grain boundaries increases, which act as non-radiative recombination centers. Second, the density of intrinsic crystal defects—such as vacancies and interstitials—rises, creating numerous charge-trapping sites. Third, the carrier mobility is reduced, thereby limiting the efficient transport of charge carriers toward the electrodes. These factors collectively shorten the carrier lifetime and increase the rate of recombination before charge extraction, ultimately degrading both the FF and PCE of the device.
As a result, the overall performance of PSCs is highly dependent on the carrier lifetime within the perovskite absorber layer. In the present model, sensitivity to carrier lifetime is evident in both the WBG and NBG sub-cells. However, the NBG layer demonstrates a greater dependence due to its primary role in photon absorption and charge carrier generation.
To examine the influence of crystallinity-related variations in carrier lifetime, a series of simulations was conducted in which the carrier lifetime of the NBG layer was varied. Figure 11a presents the resulting changes in key photovoltaic parameters, including PCE, V_oc, J_sc, and FF, as functions of carrier lifetime. As the carrier lifetime decreases from 25 to 5 ns, a significant decline in all parameters is observed. This trend is attributed to the increased charge carrier recombination rate in the absorber layer caused by lower material quality.
Electrical parameters of the proposed dual-absorber PSC: (a) as a function of carrier lifetime in the NBG layer, and (b) under different operating temperatures.
Conversely, improving the crystallinity of the perovskite films extends the carrier lifetime and thereby enhances device performance. These findings emphasize the critical importance of fabrication quality in determining solar cell efficiency. Techniques such as stepwise thermal annealing, two-step crystallization processes, grain growth enhancement, and precise control over the crystallization process have been shown to significantly improve crystallinity. These methods are therefore essential for achieving longer carrier lifetimes and higher PCE in the industrial production of PSCs⁷⁴.
In industrial solar cell fabrication, temperature is a critical factor that significantly influences device efficiency. To expand the scope of the current analysis, the impact of ambient temperature variations on device performance was investigated. Simulations were conducted not only at room temperature but also across a broader temperature range. The results, presented in Fig. 11b, show the variations in V_oc, J_sc, PCE, and FF under different thermal conditions.
The analysis reveals that as the temperature increases from − 20 °C to 60 °C, the efficiency of the solar cell steadily declines from 29.83 to 25.49%. This decrease is attributed entirely to a reduction in V_oc, while J_sc remains relatively unaffected by temperature changes. The decline in V_oc can be primarily attributed to two physical mechanisms. First, elevated temperatures enhance non-radiative recombination, resulting in increased thermal losses, which in turn reduce both V_oc and PCE. Second, thermal expansion of the perovskite crystal lattice occurs at higher temperatures, leading to a decrease in the bandgap and a reduction in crystalline stability. These changes further diminish V_oc and device efficiency. Notably, the long-term instability caused by reduced crystallinity also poses a significant threat to the operational stability of the solar cell.
Therefore, a comprehensive understanding of the physical mechanisms governing the temperature-dependent behavior of PSCs is essential for optimizing their design and accurately forecasting their performance under real-world operating conditions. Addressing these thermal effects is vital for improving both the efficiency and long-term stability of future PSC technologies.
Recent advancements in deposition techniques, including vacuum deposition and roll-to-roll processing, have enabled nanometer-scale precision in solar cell fabrication. Despite these technological breakthroughs, certain fabrication challenges persist. Among the most prominent issues are minor deviations in structural parameters such as layer thicknesses, geometric dimensions, and NP sizes. Since such variations are inevitable in practical manufacturing, it is essential that the device performance remains relatively insensitive to small perturbations in these parameters. A robust solar cell design must therefore exhibit minimal sensitivity to minor fabrication inconsistencies⁷⁵.
To account for realistic manufacturing conditions, a set of simulations was performed by introducing slight variations—ranging from 1 to 10%—around the optimal values of selected parameters. The parameters examined in this study include the width of the zigzag structure (wh), the side length of the cubic NPs (a_cubic), and the thicknesses of both the WBG and NBG layers (h_WBG and h_NBG, respectively). It is worth mentioning that variations in the cubic NP dimension were applied isotropically along all three spatial axes.
Figure 12a–d presents the impact of these deviations on the photovoltaic parameters PCE, V_oc, and J_sc. For a clearer comparison and improved interpretability, the percentage changes in PCE resulting from each structural deviation are summarized in Tables 7 and 8.
Variation of the electrical parameters of the proposed dual-absorber PSC due to fabrication errors: (a) W_h, (b) a_cubic, (c) h_WBG, and (d) h_NBG.
The simulation results show that the maximum change in cell efficiency is only − 0.61%, which occurs due to deviations in the NP side length (a_cubic). These findings indicate a high theoretical tolerance of the proposed solar cell structure to fabrication-induced errors, suggesting potential robustness and reliability of the design from a simulation perspective. Consequently, if the proposed model is fabricated in practical settings, it may exhibit a reasonable degree of reproducibility and performance consistency; however, experimental validation is necessary to confirm these aspects^76,77.
It is worth mentioning that, improvements in PCE directly contribute to increased energy yield and enhanced economic viability of photovoltaic modules throughout their operational lifetime. Achieving higher efficiencies while ensuring cost-effectiveness remains a key challenge in solar cell technology. The nanostructured designs presented in this work provide promising pathways for performance enhancement. Future studies should focus on exploring scalable and cost-efficient fabrication techniques, such as solution processing and roll-to-roll nanoimprint lithography, to evaluate their impact on balancing efficiency gains with manufacturing feasibility and cost.
It is important to note that the extinction coefficient values obtained from the referenced optical dataset^37,53 do not vanish abruptly at the nominal band edge. Instead, a finite absorption tail extends into the near-infrared region, leading to a non-zero absorption coefficient below the nominal bandgap. Although the magnitude of this sub-bandgap absorption is smaller than that of the above-bandgap region, it is not negligible. Such sub-bandgap absorption (commonly described as an Urbach tail) is a well-known and physically justified phenomenon in perovskite and related semiconductors, arising from disorder, defect states and phonon-assisted transitions⁷⁸. In addition, our optimized structure incorporates plasmonic nanoparticles that enhance the local optical field intensity within the absorber layer. This enhancement is particularly effective in spectral regions where the intrinsic absorption is weak, including the sub-bandgap tail. Consequently, the photon absorption in these spectral regions is enhanced. As a result, a measurable fraction of photogenerated carriers originates from this spectral tail, which explains why the simulated Jsc remains close to the Shockley-Queisser limit (35.82 mA/cm²), even when realistic optical and recombination losses are considered in the model.
The comparative simulations further confirm this effect: when the absorption tail is included, the Jsc reaches 35.63 mA/cm² with a PCE of 27.80%, whereas under the cut-off condition (extinction coefficient set to zero above the bandgap), the Jsc decreases to 31.38 mA/cm² and the PCE drops to 24.44%. These results clearly demonstrate that the near-SQ performance originates from the combined contribution of the intrinsic absorption tail and plasmonic enhancement.
This study presents a high-efficiency dual-absorber PSC design that combines CsPbI₃ and EAGeI₃ as WBG and NBG layers, respectively. Through comprehensive 3D optical and electrical simulations, it was demonstrated that the integration of EAGeI₃ effectively extends the absorption spectrum into the near-infrared region, compensating for the spectral limitations of CsPbI₃ and thereby enhancing carrier generation. Furthermore, the incorporation of zigzag nanostructures at the interface between the two absorbers improved light trapping, resulting in more directional absorption and facilitating efficient charge transport. The addition of Ag NPs with cubic geometry further improved near-field plasmonic effects, producing significant optical field enhancement. Consequently, the optimized device achieved a J_sc of 35.63 mA/cm², an V_oc of 0.91 V, a FF of 85.74%, and a PCE of 27.80%. This corresponds to a 44.2% enhancement over the SJ reference structure, a 13.7% increase compared to the dual-absorber zigzag structure without NPs, and a cumulative improvement of approximately 17% over the flat dual-absorber design without gratings and NPs. Beyond optical and electrical improvements, performance gains were also attributed to reduced recombination losses enabled by enhanced crystallinity and extended carrier lifetime within the perovskite layers. The improved film quality decreased trap state densities and grain boundaries, thereby lowering the probability of non-radiative recombination and contributing to higher V_oc and FF values. Moreover, simulation results suggest tolerance to fabrication parameter variations and thermal stability, indicating potential robustness under realistic operating conditions Overall, this work establishes a comprehensive and scalable design approach for achieving high-performance, lead-reduced, and environmentally friendly PSCs, advancing the development of next-generation photovoltaic technologies. Future studies should focus on experimental validation of the proposed structures, long-term operational stability under real environmental conditions, and further optimization of interface engineering to enhance charge extraction and reduce hysteresis effects. Such investigations will be crucial to facilitate the practical deployment of these promising perovskite solar cells.
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Quantum Photonics Research Lab (QPRL), Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran
Majid Najarpour & Samiye Matloub
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S. Matloub conceptualized the study and provided the supervision throughout the research process. M. Najarpour performed the simulations. Both authors wrote the manuscript.
Correspondence to Samiye Matloub.
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Najarpour, M., Matloub, S. Efficiency enhancement of dual-absorber EAGeI₃/CsPbI₃ perovskite solar cells via nanostructured interfaces and plasmonic nanoparticles. Sci Rep 15, 34720 (2025). https://doi.org/10.1038/s41598-025-18330-1
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Received: 25 June 2025
Accepted: 01 September 2025
Published: 06 October 2025
Version of record: 06 October 2025
DOI: https://doi.org/10.1038/s41598-025-18330-1
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