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Moumita Mahanti, Sutirtha Mukherjee, [Naoto Shirahata](https://orcid.org/0000-0002-1217-7589), Batu Ghosh

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[Design and Simulation of Thermally Stable Lead-Free BaHfSe₃ Perovskite Solar Cells: Role of Interface Barrier Height and Temperature](https://mdr.nims.go.jp/datasets/f3fbb0e0-9719-40c9-a4ab-aedfe11435ea)

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Academic Editor: Qiuwan ShenReceived: 16 October 2025Revised: 10 November 2025Accepted: 12 November 2025Published: 1 December 2025Citation: Mahanti, M.; Mukherjee, S.;Shirahata, N.; Ghosh, B. Design andSimulation of Thermally StableLead-Free BaHfSe3 Perovskite SolarCells: Role of Interface Barrier Heightand Temperature. Eng 2025, 6, 345.https://doi.org/10.3390/eng6120345Copyright: © 2025 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).ArticleDesign and Simulation of Thermally Stable Lead-Free BaHfSe3Perovskite Solar Cells: Role of Interface Barrier Heightand TemperatureMoumita Mahanti 1, Sutirtha Mukherjee 2 , Naoto Shirahata 3,4,* and Batu Ghosh 5,*1 Department of Physics, Raja Narendra Lal Khan Women’s College (Autonomous), Midnapore 721102,West Bengal, India2 Department of Physics, Ram Lakhan Singh Yadav College, Patliputra University, Bakhtiyarpur 803212,Bihar, India3 Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku,Sapporo 060-8628, Japan4 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba 305-0044, Japan5 Department of Physics, Trivenidevi Bhalotia College, Raniganj 713347, West Bengal, India* Correspondence: shirahata.naoto@nims.go.jp (N.S.); batughosh@tdbcollege.ac.in (B.G.)AbstractLead-free chalcogenide perovskites are emerging as promising alternatives to hybrid halideperovskites due to their superior thermal stability, non-toxicity, and strong optical ab-sorption. In this study, the photovoltaic performance of single-junction BaHfSe3-basedperovskite solar cells (PSCs) with the TCO/TiO2/BaHfSe3/Cu2O/Au configuration issystematically investigated using SCAPS-1D simulations. Device optimization identi-fies TiO2 and Cu2O as suitable ETL and HTL materials, respectively. The optimizedstructure—TCO/TiO2 (50 nm)/BaHfSe3 (500 nm)/Cu2O (100 nm)/Au—achieves a powerconversion efficiency (PCE) of 24.47% under standard conditions. Simulation results revealthat device efficiency is influenced by absorber thickness and trap density. A detailedtemperature-dependent study highlights that photovoltaic parameter efficiency is gov-erned by the barrier alignment at the TCO/ETL interface. For lower TCO (TransparentConducting Oxide) work functions (3.97–4.07 eV), PCE decreases monotonically with tem-perature, attributed to the increase in reverse saturation current resulting from a higherintrinsic carrier concentration. By contrast, higher TCO work functions (4.47–4.8 eV) yieldan initial increase in efficiency with temperature, driven by reduced barrier height andfavorable Fermi level shifts before efficiency declines at further elevated temperatures.These insights underscore the promise of BaHfSe3 as a lead-free, environmentally robustperovskite absorber for next-generation PSCs, and highlight the critical importance ofinterface engineering for achieving optimal thermal and operational performance.Keywords: lead-free chalcogenide perovskites; BaHfSe3; perovskite solar cells; SCAPS-1Dsimulation; interface band alignment; temperature-dependent efficiency; device stability1. IntroductionPerovskite solar cells (PSCs) have rapidly emerged as a leading next-generation photo-voltaic technology owing to their remarkable power conversion efficiencies (PCEs), low-costsolution-processable fabrication routes, and highly tunable optoelectronic properties [1–3].Organic–inorganic hybrid perovskites have especially dominated the field with certifiedEng 2025, 6, 345 https://doi.org/10.3390/eng6120345Eng 2025, 6, 345 2 of 20PCEs exceeding 25%, [4] comparable to those of conventional silicon-based solar cells. Morenotably, perovskite/silicon tandem architectures have recently surpassed the Shockley–Queisser limit for single-junction silicon cells, achieving efficiencies above 33% [5]. Suchadvancements underscore the vast potential of PSCs for efficient solar energy conversion.Despite impressive progress, the commercialization of PSCs remains constrained bytheir poor long-term operational stability. Conventional silicon photovoltaic modules arewarranted for 20–25 years, setting a rigorous benchmark for alternative solar technologies.In contrast, PSCs suffer rapid degradation when exposed to environmental factors such asoxygen, moisture, ultraviolet (UV) radiation, temperature stress, and electric fields [6–8].Significant efforts have been made to improve PSC stability through encapsulation,UV filtering, and defect passivation strategies [9]. However, thermal degradation remainsa substantial and unavoidable challenge. During operation, solar modules often experiencetemperature rises of 40–45 ◦C above ambient, with effective operating ranges from −40 ◦Cto +85 ◦C. According to IEC 61646 standards [10], sustaining long-term operational stabilityat 85 ◦C is mandatory for commercial qualification. Elevated temperatures induce complexdegradation pathways, including chemical decomposition of perovskite components, struc-tural phase transitions, morphological changes, and optical deterioration—all of whichcritically shorten device lifetimes.Prior studies have demonstrated that thermal stress significantly affects hybrid PSCperformance. For example, Conings et al. showed that MAPbI3-based devices degradewithin 24 h at 85 ◦C even under inert atmospheres, primarily due to the evaporationof volatile halides and organic cations [11]. Habisreutinger et al. reported that organichole-transport layers (HTLs) such as Spiro-OMeTAD, P3HT, and PTAA worsen thermalinstability, whereas partial enhancements are attainable using inorganic transport layersor molecular passivation [12]. Enhanced thermal resilience is observed in cesium-basedall-inorganic perovskites (CsPbX3; X = Cl, Br, I) and mixed-cation systems; Saliba et al.reported devices preserving 95% of their initial efficiency after 500 h at 85 ◦C [13]. More re-cently, p–i–n PSCs achieved record efficiencies near 24.6%, maintaining up to 96% and 88%of their performance after 1000 h of continuous operation at 25 ◦C and 75 ◦C, respectively,and surviving rapid thermal cycling between −60 ◦C and +80 ◦C [14]. Nonetheless, out-door field tests reveal efficiency losses exceeding 15% within hours, highlighting ongoingchallenges of thermal degradation [15].These challenges have motivated exploration of thermally robust and environmen-tally benign alternatives to hybrid lead halide perovskites. Transition metal chalcogenideperovskites (TMCPs) with general formula ABX3 (where A = Ca, Sr, Ba; B = Ti, Zr, Hf; andX = S, Se) have emerged as promising candidates [16]. TMCPs are non-toxic, structurallystable, and possess semiconducting properties with tunable direct bandgaps ranging from0.3 to 2.3 eV and high optical absorption coefficients exceeding 105 cm−1, comparable totraditional absorbers like GaAs. Importantly, the large band dispersion in Zr- and Hf-based TMCPs implies high carrier mobilities, essential for efficient charge transport. Acomprehensive computational screening by Sun et al. identified materials such as CaTiS3,BaZrS3, CaZrSe3, and CaHfSe3 as potential solar absorbers with favorable optoelectronicproperties [17].Among these, BaZrS3 has been studied extensively due to its lead-free composi-tion, exceptional environmental stability, and strong light absorption [18,19]. Its highabsorption coefficient facilitates efficient carrier collection in thin-film configurations. Thestructurally analogous BaHfS3 is equally stable but exhibits a relatively wide bandgap(~1.9–2.0 eV), limiting its solar spectrum coverage. To overcome this, anion substitutionstrategies replacing sulfur with selenium yield BaZrSe3 and BaHfSe3, which have reducedbandgaps around 1.5 eV [18,20]. This red-shift originates from selenium’s larger ionic ra-Eng 2025, 6, 345 3 of 20dius and lower electronegativity, enhancing orbital overlap and extending absorption intothe visible–near-infrared range, rendering these selenide variants ideal for single-junctionthermally stable photovoltaic applications. However, experimentally, only BaZrS3-basedsolar cells have been reported, achieving a modest PCE of 0.11% [21], whereas BaZrSe3 andBaHfSe3 remain unexplored experimentally, with existing studies limited to theoretical andsimulation-based investigations.Chalcogenide perovskites such as BaZrS3 and BaHfS3 are known for their excellentthermal and chemical stability, retaining their orthorhombic phase up to ~650 ◦C, farexceeding the stability of halide perovskites like CsPbBr3 (stable up to ~300 ◦C) [22].Although BaHfSe3 has not yet been extensively studied, its close structural similarity toBaHfS3 and BaZrSe3 implies comparable robustness, with an estimated thermal stabilityup to ~500 K. To date, BaHfSe3 has not been experimentally synthesized, and no devicereports exist. However, its predicted narrow bandgap (~1.5 eV) and structural similarity tostable Ba–Zr and Ba–Hf sulfides make it a promising candidate for lead-free and thermallyrobust photovoltaic applications.Temperature-dependent studies have been conducted on Ba–Zr–based chalcogenideperovskites using SCAPS-1D simulations. For instance, Verma et al. observed a reductionin efficiency from 31.14% at 300 K to 29.98% at 500 K in FTO/CdS/BaZrSSe/Cu2O/Audevices [23]. Similarly, Kumar et al. simulated an Au/Cu2O/BaZrSSe3/WS2/FTO systemand noted efficiency decline from 24% at 300 K to 20.6% at 400 K [24]. Mercy et al. reporteda modest decrease from 32.58% to 31.79% when temperature increased from 300 K to 400 Kin an FTO/ZrS2/Ba (Zr0.96Ti0.04)S3/SnS/Pt device [18]. These studies collectively suggestthat the power conversion efficiency of Ba–Zr–based chalcogenide perovskites decreasesmonotonically with temperature.However, the influence of interfacial energy alignment—particularly the barrier heightat the transparent conducting oxide (TCO)/electron transport layer (ETL) interface—plays adecisive role in determining the temperature-dependent photovoltaic behavior. The barrierheight directly affects charge extraction efficiency, interfacial recombination dynamics, andthe variation of open-circuit voltage (Voc) with temperature [25,26]. Despite its potentialimportance, no comprehensive simulation study has yet quantitatively explored howbarrier height modulation impacts the temperature dependence of photovoltaic parametersin chalcogenide perovskite solar cells.In this work, we employ SCAPS-1D simulations to model and optimize the perfor-mance of a BaHfSe3-based single-junction perovskite solar cell (PSC) with the architectureTCO/ETL/BaHfSe3/HTL/Au. Alongside the selection and optimization of suitable ETLand HTL materials, we systematically investigate the combined effects of temperature andinterfacial barrier height variations at the TCO/ETL interface. Furthermore, optimization ofcritical device parameters—including absorber thickness, trap density, and transport layerthickness—has been performed to establish a comprehensive understanding of the coupledthermal and interfacial influences on device performance. The insights derived from thisstudy provide valuable guidance for designing thermally stable, lead-free chalcogenideperovskite solar cells with enhanced operational reliability.2. Materials and MethodsSCAPS-1D has been widely used in the literature for solar cell simulation [27–31]. Inthis work, we employed SCAPS-1D (version 3.3.10), a one-dimensional solar cell simulationprogram developed by Prof. Marc Burgelman and colleagues at the University of Ghent,Belgium, to simulate and optimize the performance of BaHfSe3-based perovskite solarcells (PSCs) [32]. A planar n–i–p heterojunction architecture was adopted, comprising anelectron transport layer (ETL), a BaHfSe3 absorber, and a hole transport layer (HTL), asEng 2025, 6, 345 4 of 20illustrated in Figure 1. In this structure, the BaHfSe3 absorber serves as the intrinsic regionbetween the p-type HTL and n-type ETL. Under illumination, photogenerated electron–hole pairs are separated by the built-in electric field at the heterojunction; electrons drifttoward the ETL, while holes move toward the HTL, resulting in photocurrent generation.BaHfSe3AuETLHTL+MoO3, CZTS, Cu2O, MoS2SnS2, ZnSe, TiO2, ZrS22.35.3MoO34.25.49MoS24.25.7CZTS3.85.3BaHfSe34.266.11SnS 2TiO 23.97.13.25.37Cu2O4.096.9ZnSe4.16.6AuTCO5.1EnergyHOMOLUMOhehνETLs HTLsZrS 23.97To 4.8(a) (b) Figure 1. (a) Proposed n-i-p structure of the PSC with different layers. (b) Energy band diagram ofthe PSC structure (combined) with bandgap denotation for absorber and different possible chargetransport layers.Four ETLs (TiO2, SnS2, ZnSe, ZrS2) and four HTLs (MoO3, MoS2, Cu2O, and CZTS)were examined in the Transparent conducting oxide/ETL/BaHfSe3/HTL/Au configura-tion to identify the most efficient material combination. The effects of absorber thicknessand defect densities (bulk and interfacial) were systematically optimized. All simulationswere performed under AM 1.5 G illumination (100 mW/cm2, 1 sun) at 300 K. As trans-parent conducting oxide (TCO) Fluorine doped tin oxide (FTO) has been used. All inputparameters for the device layers were obtained from previously reported studies and aresummarized in Table 1, while the electrode parameters are provided in Table 2.Eng 2025, 6, 345 5 of 20Table 1. Parameters for the different layers of proposed solar cell.ETL BaHfSe3[20] HTLMaterial TiO2 [19] ZrS2 [18] ZnSe [33] SnS2 [34] Cu2O [19] MoO3 [34] CZTS [18] MoS2 [34]Thickness (µm) 0.04 0.04 0.04 0.04 0.5 0.1 0.1 0.1 0.1Band gap (eV) 3.2 2.5 2.81 1.85 1.5 2.17 3 1.5 1.29Electron affinity (eV) 3.9 4.1 4.09 4.26 3.8 3.2 2.3 4.2 4.2Dielectricpermittivity(relative)9 16.4 8.6 17.7 11 7.11 18 10 3CB density of states(1/cm3) 1.0 × 1021 2.2×1018 2.2×1018 7.32×1018 2.2×1018 2.02×1017 1.0×1019 2.2×1018 2.2×1018VB density of states(1/cm3) 2.0×1020 1.8×1019 1.8×1018 1.0×1019 1.8×1019 1.1×1019 2.2×1018 1.8×1019 1.9×1019Electron mobility(cm/s) 20 2.3×103 4×102 50 9.4×10 −2 2.0×102 210 1.0×102 100Hole mobility(cm/s) 10 1.3×103 1.1×101 25 3.5×102 80 210 25 150Donor density(1/cm3) 2.0×1019 1.0×1015 1.0×10 18 9.85×10 19 0 0 0 0 0Acceptor density(1/cm3) 0 0 0 0 1.0×1018 1.0×10 18 1.0×10 18 1.0×10 17 1.0×10 17Table 2. Contact parameters in the simulation.Contact/Parameter Front Contact (FTO) Back Contact (Au)Metal work function,Φm (eV) 4.07 [35] 5.1 [36]Electron thermal velocity 1.0 × 107 1.0 × 107Hole Thermal Velocity 1.0 × 107 1.0 × 107SCAPS-1D numerically solves the Poisson, carrier continuity, and drift–diffusionequations self-consistently to obtain current–voltage (J–V) characteristics, from which Voc,Jsc, FF, and power conversion efficiency (η) are determined. The governing equations are:d2ψdx2 = ∂E∂x = − ρε = qε[p − n + N+D − N−A](1)∂Jn∂x = q(Gn − Rn), (2)∂Jp∂x= q(Gp − Rp)(3)Jn = qnµnE + qDndndx , (4)Jp = qpµpE − qDpdpdx(5)Here, ψ is the electrostatic potential, E is the electric field, ρ is the charge density, pand n are hole and electron concentrations, µn and µp are mobilities, and Dn and Dp arethe diffusion coefficients.The Einstein relation, linking mobility and diffusion, is given by:D(n,p) =kBTq µ(n,p) (6)Hence, the diffusion coefficients vary proportionally with temperature, while carriermobilities remain constant unless manually varied.Eng 2025, 6, 345 6 of 20The temperature-dependent performance of the device was investigated over the rangeof 270–520 K. To examine the influence of the front contact on thermal behavior, simulationswere performed using six different work function (WF) values (3.97, 3.98, 4.07, 4.47, 4.58and 4.8 eV) for the TCO electrode. Varying the TCO work function effectively altersthe interfacial energy alignment and the barrier height at the TCO/ETL interface, whichcritically governs carrier injection, extraction, and recombination processes. This approachenables a detailed understanding of how contact energetics modulate the temperaturedependence of key photovoltaic parameters such as Voc, Jsc, FF, and overall device efficiency.In SCAPS-1D, only a few parameters such as conduction band density of states, valenceband density of states, and thermal velocity of charge carriers vary intrinsically withtemperature, expressed as:Nc = Nc0(T300)3/2, Nv = Nv0(T300)3/2, vth = vth,0(T300)1/2. (7)These variations influence the intrinsic carrier concentration:ni =√NcNvexp(− Eg2kBT)(8)All other quantities—including band gap (E9), carrier mobilities, and defectparameters—remain fixed unless explicitly modified. The thermal velocities of elec-trons and holes were set to 107 cm/s at 300 K.Although both band gap and mobility are generally temperature-dependent in semi-conductors, no experimental or theoretical data are currently available for BaHfSe3. Giventhe chemical similarity to BaZrSe3, which exhibits a decreasing band gap with increas-ing temperature, [37] BaHfSe3 is expected to follow a similar trend. Therefore, while thetemperature-dependent simulations maintained a fixed band gap, a separate analysis (seeSupporting Information Figure S2) examined efficiency variation by changing the band gapfrom 1.5 eV to 1.4 eV and mobility.3. Results and Discussion3.1. Optimization of ETL and HTL MaterialThe conduction band minimum (CBM) of BaHfSe3 is positioned at 3.8 eV belowthe vacuum level, while its valence band maximum (VBM) lies near 5.3 eV. In designingefficient solar cells based on BaHfSe3 absorbers, the selection of appropriate electrontransport layer(ETL) and hole transport layer (HTL) materials is crucial to enable effectivecharge extraction and minimize interfacial recombination losses. Specifically, for efficientelectron transport, the ETL’s CBM should be aligned below (i.e., at a lower energy than) theBaHfSe3 CBM (value is greater than 3.8 eV since the energy is measured from vacuum, i.e.,zero) to facilitate energetically favorable electron transfer toward the front contact or FTOelectrode. Similarly, the HTL’s valence band edge should closely match or lie slightly abovethe BaHfSe3 VBM (around or less than 5.3 eV) to ensure efficient hole extraction toward theAu back electrode.Guided by these criteria, several inorganic ETL and HTL materials were selected basedon their reported band edge positions that closely satisfy these alignment conditions. TheETL candidates include TiO2, ZrS2, ZnSe, and SnS2, all possessing conduction band minimaideally placed relative to BaHfSe3. For the HTL, materials such as MoS2, Cu2O, Cu2ZnSnS4(CZTS), and MoO3 were chosen for their suitable valence band maxima favorable for holetransport (Figure 1b). Different combinations of ETL and HTL materials were used inthe device configuration FTO/ETL (50 nm)/BaHfSe3 (400 nm)/HTL (100 nm)/Au, andEng 2025, 6, 345 7 of 20their photovoltaic performances were systematically simulated using SCAPS-1D to get thebest efficiency.The simulation results, summarized in Figure 2, reveal that the device incorporatingTiO2 as the ETL and Cu2O as the HTL demonstrates the highest power conversion efficiency,reaching 24.15%. This superior performance is attributed to the optimal band alignmentprovided by TiO2 and Cu2O with respect to BaHfSe3, enabling efficient charge separationand extraction while minimizing recombination at the interfaces. Other ETL/HTL combi-nations exhibited lower efficiencies, reflecting less favorable band alignment or increasedinterfacial losses. These findings highlight the critical importance of carefully matching ETLand HTL band edges with the absorber’s electronic structure to maximize device efficiencyin BaHfSe3-based perovskite solar cells. 0510152025Efficiency (%)HTL/ETLMoS2/TiO2CZTS/ZrS2CZTS/TiO2Cu 2O/TiO2MoO3/ZnSeMoO3/SnS2Cu 2O/ZnSeAu/HTL/BaHfSe3/ETL/FTOFigure 2. Efficiencies with different HTL and ETL combination.3.2. Optimization of Absorber Layer ThicknessAfter selecting the optimal device structure, the thickness of the BaHfSe3 absorberlayer was systematically optimized, as it critically influences the photovoltaic performanceof PSCs. An appropriate absorber thickness must balance efficient light absorption andeffective charge carrier collection. An absorber that is too thin results in incompletephoton absorption, reducing the short-circuit current density ( Jsc) and overall efficiency.Conversely, excessively thick layers contribute to increased bulk recombination, whichdegrades device performance.Our simulations reveal that Jsc increases steadily with the absorber thickness, reachinga saturation point near 500 nm where the efficiency peaks at 24.47% (Figure 3a). Theefficiency remains relatively stable between 500 nm and 700 nm, indicating an optimalthickness range where the benefit of increased photon absorption offsets recombinationlosses. Beyond 700 nm, efficiency declines progressively, reaching 23.09% at 1500 nmthickness (Figure 3b). This degradation is attributed to elevated recombination rates withinEng 2025, 6, 345 8 of 20the thicker absorber, which increase the dark saturation current and subsequently reducethe Voc as demonstrated in Figure 3a.Figure 3. Variation of (a) short-circuit current density, open-circuit voltage, (b) fill factor, and efficiencywith absorber layer thickness.The gradual decrease in fill factor (FF) observed with increasing thickness furthersupports the presence of enhanced carrier recombination and series resistance. The com-bined effect of saturation in Jsc, rising recombination, and increasing saturation currentexplains the observed efficiency trend. Our results indicate that the absorber thicknessrange of 500–700 nm provides the best trade-off between photon absorption and chargeextraction, maximizing power conversion efficiency before performance deteriorates athigher thicknesses.3.3. Optimization of ETL and HTL Layers ThicknessOptimization of the ETL and HTL layers was also carried out by varying their thick-nesses from 10 nm to 500 nm. The corresponding results are provided in the SupportingInformation (Figure S1). The variation in photovoltaic parameters with layer thickness wasfound to be negligible, indicating that both ETL and HTL layers primarily serve chargetransport and selective contact roles rather than contributing significantly to optical absorp-tion. Therefore, an ETL thickness of 50 nm and an HTL thickness of 100 nm were selectedfor the optimized device structure.The HTL thickness was chosen slightly higher to ensure complete coverage of theabsorber surface, thereby minimizing interfacial recombination and improving hole extrac-tion uniformity. A thicker HTL also helps in reducing shunt pathways and enhancing theoverall stability of the device without significantly affecting series resistance. This thickness-dependent behavior aligns well with previous theoretical and experimental observations inperovskite solar cells, underscoring the importance of tailoring absorber dimensions foroptimized device operation [38,39].The optimized BaHfSe3-based perovskite solar cell, structured as TCO/TiO2(50 nm)/BaHfSe3 (500 nm)/Cu2O (100 nm)/Au, achieves a short-circuit current density(Jsc) of 22.61 mA/cm2, an open-circuit voltage (Voc) of 1.25 V, a fill factor (FF) of 86.29%,and a power conversion efficiency (PCE) of 24.47% under standard testing conditions(trap density of 1014 cm−3, room temperature 300 K). This performance highlightsthe exceptional promise of lead-free chalcogenide perovskites for highly efficient andstable solar cell applications, with device metrics closely comparable to the leadingresults in the contemporary perovskite research field.Eng 2025, 6, 345 9 of 203.4. Evaluation of the Shockley–Queisser Efficiency Limit and the Role of Radiative RecombinationTo benchmark the simulated performance of the BaHfSe3-based perovskite solarcell, the theoretical Shockley–Queisser (SQ) efficiency limit was estimated based on itsoptical bandgap (E9 = 1.50 eV). Under standard AM1.5G illumination (ASTM G-173 [40]),the corresponding SQ limit is approximately 31–32%, which represents the maximumattainable efficiency when radiative recombination is the only loss mechanism.In practical photovoltaic devices, however, radiative recombination contributes sig-nificantly to carrier losses and reduces the overall efficiency. Since BaHfSe3 is a relativelynew and less-explored lead-free perovskite, there are currently no reported theoretical orexperimental values for its radiative recombination coefficient (B). To address this uncer-tainty and to assess its potential influence on device performance, a systematic simulationstudy was performed by varying B over a wide range from 10−9 to 10−13 cm3 s−1 using theSCAPS-1D framework.The results of this parametric study are summarized in Supplementary Material(Figure S2). A clear trend was observed where the power conversion efficiency (PCE)decreases with increasing radiative recombination rate. Specifically, the efficiency dropsfrom 24.77% for B = 0 cm3 s−1 (no radiative recombination) to about 19% for B = 1013 cm3s−1, and further down to approximately 5% for B = 109 cm3 s−1. This pronounced reductiondemonstrates the detrimental impact of radiative losses and provides a realistic projectionof device efficiency under practical conditions, where radiative recombination cannot befully suppressed.All final simulations presented in this manuscript were carried out using B = 0 cm3 s−1to represent the upper theoretical limit of performance for the optimized device structure.Importantly, even under this idealized condition, the obtained efficiency (≈24.77%) re-mains well below the SQ limit of 31–32%, confirming that the chosen material and deviceparameters are physically reasonable and do not involve any overestimation. The variationof efficiency with B further illustrates the performance degradation expected under real-istic recombination scenarios, offering valuable insight into the potential efficiency rangeachievable in experimental BaHfSe3-based solar cells.3.5. Effect of Absorber Thickness and Trap Density on Device PerformanceOur SCAPS-1D simulations systematically elucidate the critical interplay betweenabsorber thickness d and bulk trap density Nt in determining perovskite solar cell efficiency.Key trends emerge from defect-related recombination physics and optical absorptionconsiderations. The findings are shown in Figure 4.• Low trap density regime (Nt ≤ 1013 cm−3): The minority carrier diffusion length Lis much greater than d (L ≫ d), enabling nearly all photogenerated carriers to becollected efficiently. Increasing d improves light absorption and thus short-circuitcurrent density Jsc, resulting in monotonically increasing power conversion efficiency(PCE). Efficiency tends to saturate at very high thicknesses (> 1500 nm) as absorptionapproaches completeness and marginal gains diminish.• Moderate trap density regime (Nt ∼ 1014 cm−3): The diffusion length becomes com-parable to d (L ∼ d), yielding an optimal absorber thickness around 500–700 nm.Thinner devices suffer from insufficient photon absorption causing low Jsc, whilethicker films experience pronounced Shockley-Read-Hall (SRH) recombination losses,which reduce carrier collection efficiency and degrade performance beyond ∼ 800 nm.• High trap density regime (Nt ∼ 1017 cm−3): SRH recombination dominates; carrierlifetimes and diffusion lengths shrink drastically (L ≪ d), causing rapid recombina-tion before carriers reach contacts. Here, Jsc and overall efficiency become largelyEng 2025, 6, 345 10 of 20independent of thickness and converge to a low value (~3.7%), reflecting severerecombination losses regardless of d. Figure 4. Variation of power conversion efficiency (PCE) with total defect density for differentabsorber layer thicknesses, as specified in the legend.These behaviors can be quantitatively described by the SRH carrier lifetime τ anddiffusion length L, which link defect density directly to transport and collection efficiency:τ =1σ vth Nt(9)L =√Dτ (10)where σ = capture cross-section of traps, vth = carrier thermal velocity, D = diffusion coefficient.Higher Nt reduces τ, shortening L and impairing collection efficiency, affecting Jsc,Voc, and FF.3.6. Role of Transparent Conducting Oxide (Front Contact) Work FunctionThe work function (ΦTCO) of transparent conducting oxides (TCOs) plays a pivotal rolein governing the performance of optoelectronic devices, including organic and perovskitesolar cells (PSCs). It critically determines the energy band alignment at the TCO/electrontransport layer (ETL) interface, thereby influencing charge extraction efficiency, carrierrecombination dynamics, and ultimately the open-circuit voltage. Several studies havedemonstrated that surface modification of TCOs can substantially enhance device per-formance. For instance, Lee et al. reported a 3.6% improvement in power conversionefficiency (PCE) by introducing a MoO3-graded ITO anode [41] while Lei et al. achieved animprovement PCE of 7.4% by incorporating a CuS interlayer on ITO [42].A systematic simulation-based investigation of device behavior as a function of TCOwork function is therefore essential—not only for optimizing photovoltaic performanceEng 2025, 6, 345 11 of 20but also for interpreting temperature-dependent trends in PSCs. Such computationalinsights can guide targeted experimental strategies for TCO surface engineering, enablingenhanced interfacial alignment and superior thermal stability in next-generation perovskite.Indium tin oxide (ITO), a commonly used TCO, has a tunable WF that can be controlled viaoxidative treatments such as oxygen plasma, UV-ozone exposure, or chemical oxidation.These treatments alter surface dipoles and electronic states, allowing ΦITO to vary typicallybetween 3.95 eV and 5.11 eV, as reported earlier [43].In this work, we systematically varied the ITO WF from 3.97 to 4.8 eV and examinedits influence on key photovoltaic parameters: short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η). As illustrated inFigure 5a,b, all these parameters decrease as the value of TCO WF increases (i.e., shiftsdeeper below the vacuum level). Specifically, Voc remains nearly constant at approximately1.25 V for ΦTCO = 3.97 − 4.07 eV, but declines beyond this range, reaching approximately0.99 V at ΦTCO = 4.8 eV. Correspondingly, FF and η follow a similar downward trend,implying increased series resistance and reduced charge extraction efficiency. Jsc alsodecreases with increasing ΦTCO, likely due to enhanced interfacial barriers causing minortransport limitations. 4.8eVITO3.97eVITOAfter ContactAfter ContactETL(TiO2)ETL(TiO2)FavorableHigh EfficiencySchottky BarrierLow EfficiencyETL(TiO2)ETL(TiO2)(c) (d)Figure 5. Variation of (a) open-circuit voltage (Voc), and short-circuit current density (Jsc), (b) fillfactor (FF), and power conversion efficiency (PCE) with different TCO work functions. Band diagram(c) before contact band diagram (d) after contact with TCO of WF 3.97 eV and 4.8 eV.Eng 2025, 6, 345 12 of 20This behavior is explained by the variation in the interfacial barrier height (ΦB) formedbetween ITO and the ETL (typically TiO2). The barrier height is defined as:ΦB = ΦTCO − χETL (11)where ΦTCO is the TCO WF and χETL is the electron affinity (conduction band minimum,CBM) of the ETL.Figure 5c illustrates the energy level alignment of the ITO work function (WF), con-duction band minimum (CBM), valence band maximum (VBM), and the Fermi level of theETL layer before contact. When the ITO work function is relatively low (e.g., 3.97 eV), it liesabove the Fermi level of the n-type TiO2 layer. Upon contact, electrons transfer from theITO electrode to the TiO2 layer until Fermi level equilibrium is achieved. This electron flowleads to the accumulation of negative charge on the TiO2 side, resulting in a downwardband bending at the TCO/ETL interface, as shown in Figure 5d.Conversely, when the ITO work function is higher (e.g., 4.8 eV) and lies deeper thanthe Fermi level of TiO2, electrons move from the TiO2 layer to the ITO during equilibration.This process leaves behind positively charged donor ions near the TiO2 interface, leadingto an upward band bending. These two distinct interfacial alignments crucially affect thebarrier height, carrier transport, and open-circuit voltage (Voc) behavior, which are furtherdiscussed in the following section.• Case 1: ITO Work Function ΦITO = 3.97 − 4.07 eV (Lower Work Function)In this case, the ITO Fermi level lies above the TiO2 CBM at 3.9 eV (shallower energy level), re-sulting in a minimal or negligible barrier height: ΦB = 3.97− 3.9 = 0.07 eV → no effective barrier.Electrons in TiO2 can move “downhill” energetically toward the ITO electrode withvery little resistance. This favorable alignment results in downwards band bending onthe TiO2 side at the interface, promoting efficient electron extraction and low recom-bination losses as shown in Figure 5a. Consequently, devices show stable and higherJsc, FF, Voc and η in that range.• Case 2: ITO Work Function ΦITO = 4.7 − 4.8 eV (Higher Work Function)In this case, the Fermi level of ITO lies below the conduction band minimum (CBM)of TiO2, forming a Schottky barrier at the TCO/ETL interface. The barrier height canbe estimated as ΦB = 4.8 − 4.2 = 0.6 eV. This results in upward band bending on theTiO2 side near the interface, as illustrated in Figure 5b. Such an interfacial energy barrierhinders electron transfer from TiO2 to ITO, thereby enhancing interface recombination andincreasing series resistance. Consequently, electron extraction efficiency is reduced. As theITO work function increases, the corresponding barrier height also increases, leading to amonotonic decline in Voc, FF, and overall power conversion efficiency (η).3.7. Temperature Dependence of Photovoltaic ParametersThe temperature dependence of photovoltaic parameters strongly influences theoperational performance of solar cells. With increasing temperature, band gap narrowing,carrier mobility degradation, and interface barrier modification collectively affect the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversionefficiency (PCE). Among these factors, the work function (WF) of the transparent conductingoxide (TCO) layer is particularly critical, as it determines the built-in potential and barrierheight at the front interface. Moreover, tuning the TCO work function is experimentallymore feasible than modifying intrinsic semiconductor parameters, making it a practicalroute for optimizing temperature-dependent performance.Since 1D SCAPS simulations do not account for temperature-induced changes in bandgap and carrier mobility, these parameters were independently varied to understand theirEng 2025, 6, 345 13 of 20qualitative effects on efficiency. For BaHfSe3, the band gap was varied from 1.5 eV to 1.4 eV,corresponding to a temperature range of 270–500 K. The PCE increased from 24.47% to25.77%, indicating that band gap narrowing exerts a positive influence as it approachesthe optimal single-junction band gap. The variation of the parameters is shown in Supple-mentary Materials (Figure S3). Conversely, assuming carrier mobility follows µ ∝ T−0·5,efficiency decreased from 24.47% at 300 K to 17.73% at 500 K, showing a negative temper-ature effect. The detailed results are provided in the Supporting Information (Figure S4).Overall, band gap narrowing slightly enhances efficiency, while mobility degradationdominates, leading to a net decrease in performance with temperature.After confirming these trends, we systematically examined the impact of TCO workfunction variation on temperature-dependent device characteristics. The WF of TCOwas varied from 3.97 eV to 4.80 eV, and the corresponding Voc, Jsc, FF, and PCE valueswere analyzed with temperature. As shown in Figure 6, Voc and PCE exhibit similartemperature-dependent behavior, while Jsc increases monotonically. These results con-firm that careful tuning of the TCO work function—being relatively easier to modifyexperimentally—can effectively mitigate adverse thermal effects and enhance the tempera-ture stability of BaHfSe3-based solar cells.Figure 6. Variation of (a) open-circuit voltage (Voc), (b) short-circuit current density (Jsc), (c) fillfactor (FF), and (d) power conversion efficiency (PCE) with temperature for six different TCO workfunctions, as specified in the legend.Eng 2025, 6, 345 14 of 20(a) Open Circuit Voltage (Voc) BehaviorFor TCOs with lower work functions (3.97 eV, 3.98 eV, and 4.07 eV), Voc decreasesmonotonically with increasing temperature. However, for deeper work functions (4.47 eV,4.58 eV, and 4.80 eV), Voc initially increases with temperature, reaches a maximum at acharacteristic temperature, and then gradually decreases. Notably, the peak Voc shiftstoward higher temperatures as the TCO work function deepens, suggesting a temperature-dependent Fermi-level alignment effect at the TCO/ETL interface.At 270 K, for WF = 4.47 eV, Voc = 1.28 V and decreases linearly to 0.99 V at 520 K. ForWF = 4.58 eV, Voc rises from 1.18 V at 270 K to a peak at 300–330 K before decreasing to0.99 V at 520 K. Similarly, for WF = 4.80 eV, Voc increases from 0.96 V to 1.10 V at 400 K,then decreases beyond 420 K.The temperature dependence of Voc can be described by:Voc =nkTqln(1 +JphJ0)(12)where k is the Boltzmann constant, T is absolute temperature, q is the electronic charge, n isthe ideality factor, Jph is the photocurrent, and J0 is the reverse saturation current density.For a one-sided p–n junction, J0 is expressed as:J0 = q(Dnn2iNALn+Dpn2iNDLp)(13)where Dn and Dp are the electron and hole diffusion coefficients, Ln and Lp are the cor-responding diffusion lengths, and NA, ND are acceptor and donor doping densities. Theintrinsic carrier concentration ni follows:n2i = NcNv e−Eg/(kT) (14)Thus,J0 = q(DnNcNvNALn+DpNcNvNDLp)e−EgkT (15)Since Nc, Nv ∝ T3/2, J0 can be approximated asJ0 ∝ T3e−EgkT (16)The monotonic increase in J0 with temperature leads to a decrease in Voc due toenhanced recombination [23,44].However, for deeper WF (≥4.58 eV), the initial increase in Voc can be explained bytemperature-induced reduction in the interfacial barrier height caused by Fermi-level shifts.The Fermi level in an n-type semiconductor depends on the intrinsic carrier concentrationand donor density as:EF = Ei + kT ln(NDni)(17)where Ei is the intrinsic Fermi level. As temperature increases, ni increases exponentially,leading to a downward shift in EF. For TCOs with deeper work functions, this downwardshift of the ETL Fermi level reduces the TCO/ETL barrier height, temporarily improvingband alignment and increasing Voc. Beyond a critical temperature, however, the expo-nential rise in J0 dominates, resulting in a decrease in Voc. Consequently, the temperaturecorresponding to maximum Voc shifts toward higher values for higher TCO work func-Eng 2025, 6, 345 15 of 20tions, reflecting the prolonged influence of Fermi-level realignment before J0-dominateddegradation sets in.(b) Short-Circuit Current Density (Jsc) BehaviorIn contrast to Voc, the short-circuit current density ( Jsc) increases monotonicallywith temperature for all TCO work functions, indicating that its temperature depen-dence is largely insensitive to interfacial barrier variations. In perovskite solar cells, Jscis primarily governed by bulk photogeneration and collection processes rather than byinterface energetics.The photocurrent under short-circuit conditions is given by:Jsc = q∫ λg0Φ(λ) [1 − R(λ)] ηcol(λ, T) dλ (18)where Φ(λ) is the photon flux, R(λ) is the reflectance, and ηcol(λ, T) is the carrier collectionefficiency, which can vary weakly with temperature.With increasing temperature, the intrinsic carrier concentration ni rises due to en-hanced thermal excitation of carriers across the band gap:ni =√NcNve−Eg/(2kT) (19)This increased ni enhances the photogenerated carrier population, improving carriercollection efficiency and leading to a higher Jsc.(c) Fill Factor BehaviorThe fill factor (FF) exhibits a complex and work-function-dependent temperaturebehavior in the simulated devices. For lower TCO work functions (3.97, 3.98, and 4.07 eV),FF decreases monotonically with increasing temperature, primarily due to enhanced re-combination and increased series resistance effects at higher temperatures.At a moderate work function of 4.47 eV, FF shows a non-monotonic trend—initiallyincreasing as moderate temperature rise improves carrier mobility and reduces interfacialresistance, followed by a decrease as thermal effects such as increased recombination andtrap-assisted losses dominate.For deeper work functions (4.58 eV and 4.80 eV), the FF demonstrates more intricatebehavior. At 4.58 eV, FF first decreases, then increases due to the interplay between barrierheight reduction and improved charge extraction efficiency at moderate temperatures,before decreasing again as thermal degradation and recombination intensify. For 4.80 eV,FF initially decreases, then increases, and within the simulation temperature range (up to520 K) shows no subsequent decrease; however, a decline beyond 520 K is plausible basedon the observed trends and underlying physics.This complex FF variation arises from competing effects: temperature-driven improve-ments in carrier transport and interface charge extraction coexist with degradation fromenhanced recombination, ion migration, and interface state activation. The barrier heightsignificantly modulates these competing processes, thereby shifting the temperature rangeover which FF maxima occur.Such non-monotonic temperature dependence of FF highlights the critical role ofTCO/ETL band alignment and interface engineering in optimizing device performanceacross operating temperatures.Eng 2025, 6, 345 16 of 20(d) Efficiency (η) BehaviourThe power conversion efficiency is given by:η =Voc Jsc FFPin(20)• Low WF (3.97–4.07 eV): Both Voc and FF decrease monotonically, while Jsc increasesslightly. The net effect is a monotonic decrease in efficiency.• Moderate WF (4.47 eV): Despite a monotonically decreasing Voc, the initial increase inFF causes the efficiency to first rise at low-to-intermediate temperatures. Beyond thetemperature where FF peaks, efficiency decreases as recombination dominates.• Deep WF (4.58–4.80 eV): Both Voc and FF initially increase due to barrier reduction,leading to an initial rise in efficiency. At higher temperatures, increased recombinationreduces Voc and eventually saturates FF, producing a peak efficiency at intermediatetemperature, similar to the Voc behavior. Behaviors are summarized in the Table 3.Table 3. Summarization table of the trend of four photovoltaic parameter.WF (eV) Voc Trend FF Trend Jsc Trend η Trend3.97–4.07 ↓ ↓ ↑ ↓4.47 ↓ ↑→↓ ↑ ↑→↓4.58 ↑→↓ ↓→↑→↓ ↑ ↑→↓4.80 ↑→↓ ↓→↑ ↑ ↑→↓Figure 7 illustrates the temperature-dependent energy band diagrams of the BaHfSe3-based device for two representative TCO work functions (4.07 eV and 4.8 eV), simulatedat 300 K, 400 K, and 500 K using SCAPS-1D. The band alignment at various interfaces—including TCO/ETL, ETL/absorber, and absorber/HTL—plays a crucial role in determin-ing charge transport, carrier extraction, and overall device performance [45–47]. In thepresent study, different ETL and HTL materials were systematically screened to achieveoptimal energy level matching with the BaHfSe3 absorber, thereby identifying the mostefficient device configuration.The present analysis focuses specifically on the TCO/ETL interface to elucidate therole of TCO work function and temperature on device behavior. Experimentally, thework function of TCOs such as ITO or FTO can be tuned without altering the materialcomposition through surface treatments like oxygen plasma exposure, UV–ozone treatment,or controlled annealing. Therefore, the simulated variation of work function in this work isboth physically meaningful and experimentally feasible.It should be noted that in this temperature-dependent study, the TCO work functionitself is not assumed to vary with temperature. Instead, it was systematically varied toexamine its indirect influence on temperature-dependent photovoltaic performance. Theenergy band diagrams show that, with increasing temperature, all interfaces except theTCO/ETL junction exhibit nearly parallel shifts, indicating that the band edges of the ETL,absorber, and HTL layers move uniformly. Consequently, their relative alignments remaineffectively unchanged, and they contribute similarly to device performance across thetemperature range.Eng 2025, 6, 345 17 of 20 0.60−3−2−101Energy(eV)Distance (μm) 300K 400K 500K Work Function = 4.07eV0.59 0.60 0.61 0.62 0.63 0.64−3−2−101Energy (eV)Distance (μm) 300K 400K 500K Work Function 4.8eV0.0 0.1 0.2 0.3 0.4 0.5 0.6−3−2−1012Energy (eV)Distance (μm) 300K 400K 500K 0.0 0.1 0.2 0.3 0.4 0.5 0.6−3−2−1012Energy(eV)Distance (μm) 300K 400K 500K (a) (b)(c) (d)Work Function 4.8eV Work Function 4.07eVFigure 7. Energy band diagrams of the BaHfSe3-based device at different temperatures for (a) TCOwork function of 4.80 eV and (b) TCO work function of 4.07 eV. Panels (c,d) show the correspondingzoomed-in views of the TCO/ETL interface regions for (a,b), respectively, highlighting the variationin interface barrier alignment with temperature.A distinct behavior is observed at the TCO/ETL interface. For the higher TCO workfunction (4.8 eV), the interfacial barrier height decreases with increasing temperature,as evident from Figure 6. This reduction enhances carrier extraction and results in aninitial increase in both open-circuit voltage (Voc) and short-circuit current density (Jsc) atmoderate temperatures. However, as the temperature continues to rise, the increase inreverse saturation current and recombination processes becomes dominant, leading to asubsequent decrease in these parameters. In contrast, for the lower TCO work function(4.07 eV), the barrier height remains nearly constant with temperature, resulting in amonotonic decrease of Voc, Jsc, and overall efficiency.Therefore, the observed non-monotonic variation of device parameters (initial increasefollowed by a decrease) can be comprehensively explained by the temperature-dependentband alignment and barrier height evolution at the TCO/ETL junction, while the otherinterfaces remain relatively unaffected.4. ConclusionsThis study demonstrates the promising potential of lead-free chalcogenide per-ovskite BaHfSe3 as an efficient and environmentally stable absorber in perovskite solarcells. Through comprehensive SCAPS-1D simulations, the optimized device architecture—TCO/TiO2 (50 nm)/BaHfSe3 (500 nm)/Cu2O (100 nm)/Au—achieves excellent photo-voltaic performance metrics, with a power conversion efficiency of 24.47% under standardEng 2025, 6, 345 18 of 20conditions. The device performance is shown to be sensitive to absorber thickness anddefect density, reinforcing the importance of material quality and controlled fabrication.The temperature-dependent analysis reveals that the photovoltaic parameters, es-pecially Voc, FF, and efficiency, exhibit distinct behaviors governed by the TCO workfunction and corresponding interface barrier height. For lower TCO work functions, ef-ficiency and Voc decrease monotonically with temperature due to increases in saturationcurrent driven by intrinsic carrier concentration. In contrast, higher work functions pro-duce a non-monotonic temperature dependence, where initial efficiency improvementsarise from barrier height reduction and favorable Fermi level shifts before declining atelevated temperatures.These findings underscore the critical role of interface band alignment and defectmanagement in optimizing device stability and performance over operational tempera-ture ranges. Moreover, BaHfSe3, with its suitable bandgap, strong absorption, and ro-bustness, emerges as a viable lead-free alternative to conventional hybrid perovskites,advancing the prospects of thermally stable, high-efficiency, and environmentally friendlyphotovoltaic technologies.Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/eng6120345/s1, Figure S1: (a) Variation of Voc and Jsc (b) Variationof FF and efficiency with ETL thickness (c) Variation of Voc and Jsc (d) Variation of FF and efficiencywith HTL thickness. Figure S2: (a) Simulated J–V characteristics of the BaHfSe3-based perovskitesolar cell for different values of the radiative recombination constant. (b) Corresponding variation ofpower conversion efficiency (PCE) as a function of the radiative recombination constant, illustratingits influence on overall device performance. Figure S3: (a) Variation of Voc and Jsc with band gap(b) variation of efficiency and FF with band gap. Figure S4: Variation of PCE with variation inelectron mobility.Author Contributions: Conceptualization, B.G.; Data curation, M.M.; Formal analysis, M.M. andS.M.; Funding acquisition, N.S.; Methodology, M.M.; Project administration, B.G.; Software, M.M.;Supervision, N.S.; Writing—original draft, B.G.; Writing—review and editing, S.M. and N.S. Allauthors have read and agreed to the published version of the manuscript.Funding: This research is funded by the WPI program, ARIM of MEXT (JPMXP1225NM5200), JSPSKAKENHI grant (24K01462 and 24K21720), and the Hosokawa Powder Technology Foundation(HPTF24111).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.Data Availability Statement: The original contributions presented in this study are included in thearticle/Supplementary Material. Further inquiries can be directed to the corresponding authors.Acknowledgments: The authors thank the Marc Burgelman, University of Gent, Belgium for the1D SCAPS software (Version: 3.3.09). M.M thanks Arjun Mukhopadhyay for useful discussion. BGthanks Asim Guchhait and Ipsita Saha for valuable discussions.Conflicts of Interest: The authors declare no conflicts of interest.References1. Hamukwaya, S.L.; Hao, H.; Zhao, Z.; Dong, J.; Zhong, T.; Xing, J.; Hao, L.; Mashingaidze, M.M. A Review of Recent Developmentsin Preparation Methods for Large-Area Perovskite Solar Cells. Coatings 2022, 12, 252. [CrossRef]2. Afre, R.A.; Pugliese, D. Perovskite Solar Cells: A Review of the Latest Advances in Materials, Fabrication Techniques, and StabilityEnhancement Strategies. Micromachines 2024, 15, 192. [CrossRef] [PubMed]3. Roy, P.; Ghosh, A.; Barclay, F.; Khare, A.; Cuce, E. Perovskite Solar Cells: A Review of the Recent Advances. Coatings 2022, 12, 1089.[CrossRef]Eng 2025, 6, 345 19 of 204. 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