{"paper_id":"2ecd2f56-c0e9-4733-8abb-d6985bf337bd","body_text":"Optimal PbS quantum-dot loading in composition-graded Bi1-xCaxFeO3 stacks for improved photovoltaic performance | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Optimal PbS quantum-dot loading in composition-graded Bi 1- x Ca x FeO 3 stacks for improved photovoltaic performance Weihao Wu, Zhongxiang Zheng, Xiaoyu Luo, Shubao Yang, Wenchaun Li, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8645878/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Apr, 2026 Read the published version in Journal of Materials Science: Materials in Electronics → Version 1 posted You are reading this latest preprint version Abstract A PbS quantum-dot (QD) interfacial layer was integrated with a composition-graded Bi 1- x Ca ₓ FeO 3 (BCFO) oxide stack to enhance photovoltaic output via coupled optical sensitization and interfacial field regulation. By tuning the PbS precursor concentration, an optimal QD loading window was identified. Increasing PbS loading enhanced optical absorption and interfacial band bending at the PbS/BCFO junction, whereas excessive loading induced aggregation and non-uniform thickening, leading to transport bottlenecks and trap-assisted recombination. Consequently, the photocurrent increased with PbS loading while the power conversion efficiency(PCE) exhibited a volcano-type dependence, peaking at 0.0075 mol L -1 with short circuit current density ( J sc ) = 2431 μA cm -2 , open circuit voltage ( V oc ) = 0.872 V, fill factor ( FF ) = 41.5%, and a maximum PCE of 1.235%, together with stable light on/off switching. These results highlight that controlling QD loading, rather than simply maximizing sensitizer content, is critical for achieving reproducible performance enhancement in QD/oxide composite photovoltaics. ferroelectric-oxide photovoltaics PbS quantum dots composition-graded BCFO band alignment Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Ferroelectric-oxide-based photovoltaic devices have attracted increasing interest because their photoresponse is influenced not only by band-to-band excitation but also by internal fields associated with non-centrosymmetric crystal structures, defect states, and interfacial band bending[ 1 , 2 ]. These internal fields can facilitate carrier separation in oxide thin films, offering opportunities for multifunctional optoelectronics beyond conventional p-n junction concepts[ 3 ]. Nevertheless, the photovoltaic output of most ferroelectric-oxide thin films remains limited in practice, mainly due to insufficient solar light harvesting and severe carrier losses caused by defect-assisted recombination and interfacial extraction barriers[ 4 ]. Therefore, further performance improvement requires approaches that simultaneously enhance optical absorption and establish efficient, directional pathways for carrier separation and collection[ 5 ]. Photovoltaic effects have been explored in a variety of oxide ferroelectrics, including layered Aurivillius phases[ 6 ], tungsten bronze-type ferroelectrics[ 7 ], and perovskite titanates/niobates such as BaTiO 3 and (K,Na)NbO 3 [ 8 , 9 ]. However, many of these oxides exhibit wide bandgaps, which restrict photocarrier generation under AM 1.5G illumination. In contrast, BiFeO 3 (BFO) combines robust room-temperature ferroelectric ordering with a comparatively narrower bandgap among oxide ferroelectrics, making it a representative platform for studying structure/defect/interfacial strategies toward improved photoresponse[ 10 ]. Yet, BFO-based devices often deliver modest photocurrent because recombination and transport losses in polycrystalline films can offset the benefits of internal-field-assisted separation[ 11 ]. To address these limitations, defect/internal-field engineering and sensitization strategies have been widely investigated[ 12 ]. Ca substitution in BFO (Bi 1 − x Ca ₓ FeO 3 , BCFO) can modulate defect chemistry and electronic transport, while composition-graded stacked architectures can introduce an internal potential gradient that reinforces directional carrier separation and suppresses back diffusion[ 13 , 14 ]. Meanwhile, integrating narrow-bandgap quantum dots (QDs) offers an effective route to enhance light harvesting and tailor interfacial band alignment for charge transfer[ 15 ]. Importantly, the benefit strongly depends on QD loading: insufficient coverage provides limited sensitization and interfacial modification, whereas excessive loading may induce aggregation, interfacial traps, and transport barriers that accelerate recombination[ 16 , 17 ]. Here, we demonstrate a PbS quantum-dot-modified, composition-graded BCFO stack on FTO, where the internal field of the gradient architecture cooperates with PbS-derived interfacial band bending to promote carrier separation and extraction. By systematically tuning the PbS precursor concentration, we identify an optimal loading window that maximizes the photoresponse and clarify its physical origin in terms of microstructure-controlled interfacial charge transfer and recombination. This work provides an interfacial-engineering route for improving photovoltaic output in BFO-based oxide stacks and offers practical design guidelines for integrating narrow-bandgap QDs with graded oxide heterostructures governed by internal potential gradients and band alignment. 2. Experimental section 2.1 Preparation of BCFO precursor solutions BCFO precursor sols with Ca contents of x = 0, 0.05, 0.10, and 0.20 (denoted as BFO, BCFO-05, BCFO-10, and BCFO-20) were prepared by a sol-gel method using Bi(NO 3 ) 3 ·5H 2 O, Fe(NO 3 ) 3 ·9H 2 O, and calcium acetate as starting reagents. A 10 mol% excess of Bi(NO 3 ) 3 ·5H 2 O was introduced to compensate for Bi volatilization. The salts were dissolved in a 2-methoxyethanol/glacial acetic acid mixture (3:1, v/v), followed by the addition of citric acid, ethylene glycol, and ethanolamine to stabilize the sol (pH ≈ 1). After stirring for 2 h, the solution was diluted to 100 mL and aged for 24 h to obtain stable precursors, as shown in the Fig. 1 (a). 2.2 Preparation of BCFO multilayer thin film 1 × 1 cm 2 FTO substrates were cleaned (Fig. 2 (a)) and partially masked to define the bottom electrode. BCFO films were spin-coated at 4000 rpm for 20 s (25°C, ~ 50% RH). After each coating, the films were dried at 60, 80, and 100°C for 5 min each and pyrolyzed at 250°C for 10 min, followed by annealing at 600°C for 30 min. The total thickness was kept at ~ 240 nm by adjusting the coating cycles. The composition-graded BCFO-T4 stack was fabricated by sequentially depositing BCFO-20/BCFO-10/BCFO-05/BFO (bottom to top) (Fig. 1 (b)). Finally, an Ag top electrode was applied and annealed at 90°C for 10 min to obtain a conductive contact. 2.3 Preparation of PbS/BCFO-T4 film A PbS quantum-dot overlayer was deposited on the stacked BCFO films by spin-assisted SILAR (Fig. 1 (b)), with the PbS precursor concentration set to 0.005, 0.0075, or 0.01 mol L − 1 . Each cycle consisted of sequential Pb 2+ and S 2− deposition (70 µL each) with intermediate deionized-water rinses, using 1500 rpm for 60 s for the Pb 2+ step and 2000 rpm for 60 s for the S 2− step. The cycle was repeated 20 times to obtain the PbS layer, as shown in the Fig. 2 (b). 2.3. Characterization Table 1 Characterization items Structural characterization and performance testing Characteristic curves Instrument and equipment Test parameter Crystal structure XRD patterns XRD(DX-2700) Powder scanning Angle is 20 ~ 70° Surface topography SEM images FESEM(JSM-7800F) - Optical performance Absorbance spectra Puxi T9S-C Spectrophotometer - Photovoltaic performance J - t curve IV Test Station 2000 Keithley 2400 100 mW cm − 2 (AM 1.5G) J - V curve η - V curve 3. Results and discussion 3.1 Microstructure Figure 3 (a) shows the XRD patterns of PbS/BCFO-T4 composite films prepared with different PbS precursor concentrations, together with the device schematic in Fig. 3 (b). In addition to the FTO substrate peaks, three reflections at ~ 26°, ~ 30°, and ~ 43° can be assigned to the (111), (200), and (220) planes of cubic PbS, matching PbS (PDF#99 − 0053). The remaining peaks are consistent with BFO (PDF#74-2016), and no obvious secondary phases are detected, indicating that the perovskite framework is retained after PbS deposition. As the precursor concentration increases from 0.005 to 0.01 mol L − 1 , the PbS peaks intensify, while the diffraction signals from the underlying BCFO stack and FTO weaken. This trend suggests an increased crystalline contribution and coverage of the PbS overlayer, which attenuates the diffraction from the bottom layers, in line with the stacked architecture shown in Fig. 3 (b). The PbS crystallite sizes estimated by the Scherrer equation are 23.691, 25.213, and 30.794 nm for 0.005, 0.0075, and 0.01 mol L − 1 , respectively. Figure 4 (a-c) show the SEM images of PbS/BCFO-T4 composite films prepared with different PbS precursor concentrations. Spherical nanoparticles with evident aggregation are observed, and EDS elemental mapping (Figs. 4 (d,e)) confirms that these particulates are PbS. At 0.005 mol L − 1 , PbS does not fully cover the BCFO surface and appears as sparsely distributed small agglomerates. With increasing concentration, PbS coverage increases and growth on pre-formed PbS nuclei becomes more pronounced, leading to larger clusters and more severe aggregation[ 18 ]. This trend is attributable to coalescence/sintering during repeated annealing, which reduces interparticle spacing and promotes particle merging. Consequently, the surface becomes rougher and less uniform at higher concentration, which can increase interfacial recombination and deteriorate the fill factor. Particle-size statistics (Nano Measure) indicate average PbS sizes of ~ 22.22, 23.65, and 25.73 nm for 0.005, 0.0075, and 0.01 mol L − 1 , respectively. The increase in particle number and size with concentration is consistent with adsorption-limited growth at low concentration, where a higher ion supply promotes nucleation and growth[ 19 ]. 3.2 Optical performance Figure 5 combines the absorption spectra and the proposed band-alignment model to clarify the concentration-dependent optical and carrier-separation behavior of the PbS/BCFO-T4 composites. As shown in Fig. 5 (a), all composite films exhibit strong visible-light absorption, and introducing PbS QDs increases both the absorbance and the absorption window compared with the pristine BCFO stack. With increasing PbS precursor concentration (0.005 → 0.01 mol L − 1 ), the absorbance increases, reflecting higher PbS loading. A noticeable red shift of the absorption edge is also observed, consistent with particle growth at higher concentration. Using the Moreels relation shown in Eq. ( 1 ) and the PbS sizes extracted from SEM[ 20 ], the PbS bandgaps are estimated to be 0.463, 0.458, and 0.452 eV for 0.005, 0.0075, and 0.01 mol L − 1 , respectively. $${E_g}=0.41+1/(0.0252{d^2}+0.283d)$$ 1 The corresponding band alignment for the Ag/PbS/BCFO-T4/FTO architecture is depicted in Fig. 5 (b,c), constructed from literature-reported parameters together with the experimentally estimated PbS bandgap. PbS QDs are commonly reported to be p-type-like with a Fermi level closer to the valence band, whereas BFO/BCFO can exhibit n-type-like behavior associated with oxygen-vacancy-related donor defects, enabling formation of a PbS/BFCO heterojunction[ 21 ]. After contact, Fermi-level equilibration leads to band bending and a built-in electric field at the PbS/BFO interface, which is aligned with the internal field of the composition-graded BCFO stack (Fig. 5 (c)). Under illumination, the coupled fields favor directional carrier separation, driving electrons toward the top Ag electrode and holes toward the FTO side, thereby promoting carrier collection. 3.3 Photovoltaic performance Table 2 Photovoltaic characteristics of PbS/BCFO-T4 composite films with different precursor concentrations Sample J sc (mA/cm − 2 ) V oc (V) FF (%) PCE (%) 0.005 mol/L 0.402 0.699 44.1 0.15212 0.0075 mol/L 2.431 0.872 35.5 1.235 0.01 mol/L 4.073 0.624 24.3 0.619 Figure 6 summarizes the photovoltaic output of the PbS/BCFO composite films, and the extracted parameters are listed in Table 2 . A pronounced precursor-concentration dependence is observed: short circuit current density ( J sc ) ncreases monotonically with increasing PbS precursor concentration, indicating strengthened photogeneration and/or more effective interfacial charge transfer at higher PbS loading. Meanwhile, the overall power conversion efficiency exhibits a volcano-type dependence and reaches a maximum at 0.0075 mol L − 1 , delivering J sc =2.431 mA cm − 2 , open circuit voltage ( V oc ) = 0.872 V, fill factor ( FF ) = 35.5%, and Power Conversion Efficiency (PCE) = 1.235%. The improved output at intermediate loading can be attributed to the formation of a PbS/BCFO-T4 heterojunction, where an interfacial electric field facilitates photocarrier separation and transport, thereby enhancing charge extraction[ 22 ]. In contrast, further increasing the precursor concentration leads to a reduced V oc and a pronounced drop in FF , which can be rationalized by transport and interfacial limitations of the PbS layer. PbS QD films typically possess a limited carrier diffusion/collection length; thus, an excessively thick and/or highly aggregated PbS overlayer can introduce series resistance and trap-assisted recombination, degrading carrier extraction and lowering the overall efficiency[ 23 , 24 ]. Figure 7 shows the time-dependent photocurrent density ( J - t ) responses of the PbS/BCFO composite films under periodic light on/off switching. After introducing the PbS layer on the BCFO stack, the photocurrent density is markedly higher than that of the pristine BCFO film, indicating an enhanced photoresponse. In the dark, the current density remains close to zero, whereas upon illumination it rises rapidly and reversibly, demonstrating a clear photovoltaic/photoconductive response and good switching reproducibility. With increasing PbS precursor concentration, the photocurrent density increases accordingly. This trend is mainly attributed to the higher PbS loading, which improves surface coverage and effective thickness, thereby strengthening light harvesting and facilitating interfacial charge transfer. Within an appropriate concentration range, the increased number of PbS quantum dots and their enlarged domains can provide more percolation pathways for carrier transport and promote charge collection, leading to a higher steady-state photocurrent[ 25 ]. To benchmark the device performance, the photovoltaic parameters of the present PbS/BCFO-T4 device are compared with previously reported ferroelectric-and QD/oxide-based photovoltaic structures in the literature (Table 3 ). The comparison indicates that the achieved output is competitive within this material class, particularly in terms of J sc and the overall PCE under comparable measurement conditions. Table 3 Comparison of photovoltaic parameters in this study with those reported in the literature Device Structure Fabrication method V oc (V) J sc (µA cm − 2 ) PCE (%) Light source Ref. P 0 (mW cm − 2 ) Spectrum AZO/BFO/FTO Sol-gel 0.63 130 2.08×10 − 2 100 AM 1.5G [ 26 ] FTO/TiO 2 /PbS/Au - 0.27 3170 0.24 100 AM 1.5 [ 27 ] (FTO)/BFO/PEDOT:PSS/Au CSD 0.32 1080 0.086 100 AM1.5G [ 28 ] Ag/BNFCO/FTO Sol-gel 0.80 1600 - 100 - [ 29 ] SiO 2 /Si/multilayer BFO/BTO thin films/Au PLD o.4 384 0.098 160 - [ 30 ] TO/AuNPs/BLFO/FTO Sol-gel 0.4 20 - 100 AM1.5G [ 31 ] Ag/PbS/BFO/CdS/FTO Sol-Gel 0.13 239.6 7.65×10 − 3 100 AM1.5G [ 32 ] Ag/PbS/BFO/ZnO/FTO Sol-Gel 0.11 0.61 1.63×10 − 5 100 AM1.5G [ 32 ] ITO/Co‑doped BFO/Al CSD 0.72V 93 - - AM 1.5G [ 33 ] FTO/Ca doped BFO/Al Sol-gel 0.446 42.2 - - - [ 34 ] ITO/Mn doped BFO/SRO PLD 0.16 0.16 0.0022 280 White light [ 35 ] ITO/Ca and Mn co-doped BFO/SRO PLD 0.29 0.26 0.0075 280 White light [ 35 ] Pt/Ti-doped BFO/ITO Sol-Gel 0.48 380 - 100 AM 1.5G [ 36 ] ITO/La doped BFO/HOPG PLD 0.57 350 - 100 AM 1.5G [ 37 ] Ag/PbS/BFO/CdS/FTO PLD 0.13 239.6 0.00765 100 AM 1.5G [ 38 ] FTO/ZnO/BFMO/NiO/Au Sol-gel 1.4 360 - 100 - [ 39 ] Ag/PbS/BCFO-T4/FTO Sol-Gel; spin-assisted SILAR 0.872 2431 1.235 100 AM 1.5G This work 4. Conclusions In conclusion, this work establishes an optimal PbS loading window for enhancing photovoltaic output in a composition-graded BCFO-T4 oxide stack via quantum-dot interfacial engineering. The results reveal a fundamental trade-off between photogeneration and carrier extraction: moderate PbS deposition strengthens light harvesting and promotes field-driven carrier separation at the PbS/BFO interface in concert with the internal potential gradient of the graded stack, whereas excessive loading leads to aggregation and non-uniform thickening that introduce trap-assisted recombination and transport barriers, thereby reducing the fill factor and overall efficiency. Consequently, an intermediate PbS precursor concentration of 0.0075 mol L − 1 provides the best balance, delivering J sc = 2431 µA cm − 2 , V oc =0.872 V and a maximum PCE of 1.235%. From an application standpoint, the identified loading-window guideline provides a simple and reproducible process lever for QD/oxide integration and, together with the stable light-switching response, is well suited for scalable thin-film optoelectronics such as self-powered photodetectors and light-responsive sensors. Declarations Author contributions This article represents a collaborative effort. Below is a detailed breakdown of the contributions from each author: [Weihao Wu]: Conceptualization; Data curation; Methodology; Validation; Writing-original draft; [Zhongxiang Zheng]: Formal Analysis; Investigation; Visualization; [Xiaoyu Luo] [Shubao Yang]: Writing-review & editing; [Wenchuan Li]: Project administration; Resources; Supervision; [Rongli Gao], [Xiaoling Deng], [Wei Cai], [Chunlin Fu]: Writing-original draft preparation. Acknowledgments The present work has been Sponsored by Chongqing Natural Science Foundation Joint Fund for Innovation and Development (Grant No. CSTB2025NSCQ-LZX0097) and the Innovation Project of Chongqing University of Science and Technology (Grant No. YKJCX2520308) Data availability The data presented in this study are available from the corresponding author upon reasonable request. Declarations The authors declare that they have no fnancial and personal relationships with other people or organizations that can inappropriately infuence our work. References Gao C, Li W, Jing L, et al. Enhanced photovoltaic and piezo-photovoltaic effects in flexible oxide ferroelectric film directly coated on polyimide substrate[J]. Nano Energy, 2023, 117: 108839. You L, Zheng F, Fang L, et al. Enhancing ferroelectric photovoltaic effect by polar order engineering[J]. Science advances, 2018, 4(7): eaat3438. Frye M B, Garten L M. Reaching the potential of ferroelectric photovoltaics[J]. 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Cite Share Download PDF Status: Published Journal Publication published 19 Apr, 2026 Read the published version in Journal of Materials Science: Materials in Electronics → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-8645878\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":584849973,\"identity\":\"2b177bd6-aa6d-48f4-8bbe-fc930a6dec3d\",\"order_by\":0,\"name\":\"Weihao Wu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"College of Materials and New Energy, Chongqing University of Science and 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Gao\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7klEQVRIiWNgGAWjYBADGTb2xoYDHyok5OSJ1cLDxnP44MMZZyyMDRuI1cIgkZZszNtWkchwgIBSeffDxz583FHLw8eQYyY5c55EAmMD88NHN/BoMTyTljxz5pnjPGwMZ8wkPm6TyGNnYDM2zsGnpSHHmJm37RgPG2MP0JZtEsWMDTxs0ni19L+BamHmMZPmnSOR2HCAgBZ5CbAtNTxsbGxA7zcQocVA4lky48w2oDIeZmAgH5MwNmwm4Bf5/uTDDB/b6uTk5z8ERmUNkMHe/PAxXlsOgKnDSELMeJSDbWkAU3UElI2CUTAKRsGIBgArs0j2Z4eqFQAAAABJRU5ErkJggg==\",\"orcid\":\"\",\"institution\":\"College of Materials and New Energy, Chongqing University of Science and Technology\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Rongli\",\"middleName\":\"\",\"lastName\":\"Gao\",\"suffix\":\"\"},{\"id\":584849991,\"identity\":\"046d188b-b372-400c-ab64-e682b5c2170a\",\"order_by\":6,\"name\":\"Xiaoling Deng\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"College of Materials and New Energy, Chongqing University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Xiaoling\",\"middleName\":\"\",\"lastName\":\"Deng\",\"suffix\":\"\"},{\"id\":584849992,\"identity\":\"39ad6b2f-0f5b-46f9-8671-5d2ba61eead0\",\"order_by\":7,\"name\":\"Wei Cai\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"College of Materials and New Energy, Chongqing University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Wei\",\"middleName\":\"\",\"lastName\":\"Cai\",\"suffix\":\"\"},{\"id\":584849993,\"identity\":\"cb1575df-855f-4cb2-bec3-758bc3f70074\",\"order_by\":8,\"name\":\"Chunlin Fu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"College of Materials and New Energy, Chongqing University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Chunlin\",\"middleName\":\"\",\"lastName\":\"Fu\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2026-01-20 06:55:57\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-8645878/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-8645878/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1007/s10854-026-17284-y\",\"type\":\"published\",\"date\":\"2026-04-19T15:59:41+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":101941368,\"identity\":\"5ceeb76f-c769-46cf-a355-746e90554a83\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:21:16\",\"extension\":\"jpg\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":112692,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e(a) Spin-coating process of single-layer BCFO films; (b) Spin-coating process of PbS/BCFO-T4 multilayer thin film\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"1.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/152dd2c9f986bd97c24467c1.jpg\"},{\"id\":101941371,\"identity\":\"1e1e2bbc-d97a-4c26-9144-932b04774ac1\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:21:16\",\"extension\":\"jpg\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":48680,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e(a) Cleaning procedure for FTO substrates; (b) Preparation of PbS thin film using spin-assisted-SILAR method\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"2.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/fe3a2d707cf195ea244652f9.jpg\"},{\"id\":101941369,\"identity\":\"27351b62-b997-4633-a501-eee30e9c3acb\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:21:16\",\"extension\":\"jpg\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":47273,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003ePbS/BCFO-T4 composite films: (a) XRD pattern; (b) Schematic diagram of device structure\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/a1e06c42ba43c8e9d06bb87d.jpg\"},{\"id\":101941374,\"identity\":\"db415e68-4574-480e-b4dc-e22ec032e244\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:21:16\",\"extension\":\"jpg\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":96574,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSEM and element analysis of PbS/BCFO-T4 composite films\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"4.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/41e76e197503cccf9d9a43d4.jpg\"},{\"id\":101943561,\"identity\":\"8cb12acf-82b6-468e-870d-c7a155f48be8\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:42:21\",\"extension\":\"jpg\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":61779,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e(a) Absorption spectra of PbS/BCFO-T4\\u003csub\\u003e \\u003c/sub\\u003ecomposite films; schematic diagram of energy band of PbS/BCFO composite film: (b) before contact; (c) after contact\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"5.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/d4e2880af886066829a423be.jpg\"},{\"id\":101943564,\"identity\":\"1f83dbd2-0652-4de7-9819-d5df9fb90f35\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:42:22\",\"extension\":\"jpg\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":66012,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003ePbS/BCFO-T4 composite films: (a) \\u003cem\\u003eJ\\u003c/em\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e curve; (b) \\u003cem\\u003eη\\u003c/em\\u003e-\\u003cem\\u003eV\\u003c/em\\u003ecurve\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"6.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/fce447a4cbae6dacebce55c3.jpg\"},{\"id\":101941372,\"identity\":\"66feff8e-6308-408e-99a2-46cc84807f80\",\"added_by\":\"auto\",\"created_at\":\"2026-02-05 09:21:16\",\"extension\":\"jpg\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":69213,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cem\\u003eJ\\u003c/em\\u003e-\\u003cem\\u003et\\u003c/em\\u003ecurve of PbS/BCFO-T4 composite films\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"7.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/ee071dc82c65784246301f98.jpg\"},{\"id\":107351082,\"identity\":\"16b9b44b-197f-4953-9d6f-d7e52d557d4c\",\"added_by\":\"auto\",\"created_at\":\"2026-04-20 16:09:09\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":952621,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-8645878/v1/96cff99a-2935-4450-9359-eb0f7acde37e.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"\\u003cp\\u003e\\u003cstrong\\u003eOptimal PbS quantum-dot loading in composition-graded Bi\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e1-\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003csub\\u003e\\u003cem\\u003e\\u003cstrong\\u003ex\\u003c/strong\\u003e\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eCa\\u003c/strong\\u003e\\u003csub\\u003e\\u003cem\\u003e\\u003cstrong\\u003ex\\u003c/strong\\u003e\\u003c/em\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eFeO\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e3\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e stacks for improved photovoltaic performance\\u003c/strong\\u003e\\u003c/p\\u003e\",\"fulltext\":[{\"header\":\"1. Introduction\",\"content\":\"\\u003cp\\u003eFerroelectric-oxide-based photovoltaic devices have attracted increasing interest because their photoresponse is influenced not only by band-to-band excitation but also by internal fields associated with non-centrosymmetric crystal structures, defect states, and interfacial band bending[\\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2\\u003c/span\\u003e]. These internal fields can facilitate carrier separation in oxide thin films, offering opportunities for multifunctional optoelectronics beyond conventional p-n junction concepts[\\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e]. Nevertheless, the photovoltaic output of most ferroelectric-oxide thin films remains limited in practice, mainly due to insufficient solar light harvesting and severe carrier losses caused by defect-assisted recombination and interfacial extraction barriers[\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e]. Therefore, further performance improvement requires approaches that simultaneously enhance optical absorption and establish efficient, directional pathways for carrier separation and collection[\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e].\\u003c/p\\u003e \\u003cp\\u003ePhotovoltaic effects have been explored in a variety of oxide ferroelectrics, including layered Aurivillius phases[\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e], tungsten bronze-type ferroelectrics[\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e], and perovskite titanates/niobates such as BaTiO\\u003csub\\u003e3\\u003c/sub\\u003e and (K,Na)NbO\\u003csub\\u003e3\\u003c/sub\\u003e[\\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e]. However, many of these oxides exhibit wide bandgaps, which restrict photocarrier generation under AM 1.5G illumination. In contrast, BiFeO\\u003csub\\u003e3\\u003c/sub\\u003e(BFO) combines robust room-temperature ferroelectric ordering with a comparatively narrower bandgap among oxide ferroelectrics, making it a representative platform for studying structure/defect/interfacial strategies toward improved photoresponse[\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e]. Yet, BFO-based devices often deliver modest photocurrent because recombination and transport losses in polycrystalline films can offset the benefits of internal-field-assisted separation[\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e].\\u003c/p\\u003e \\u003cp\\u003eTo address these limitations, defect/internal-field engineering and sensitization strategies have been widely investigated[\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e]. Ca substitution in BFO (Bi\\u003csub\\u003e1\\u0026thinsp;\\u0026minus;\\u0026thinsp;\\u003cem\\u003ex\\u003c/em\\u003e\\u003c/sub\\u003eCa\\u003cem\\u003eₓ\\u003c/em\\u003eFeO\\u003csub\\u003e3\\u003c/sub\\u003e, BCFO) can modulate defect chemistry and electronic transport, while composition-graded stacked architectures can introduce an internal potential gradient that reinforces directional carrier separation and suppresses back diffusion[\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e]. Meanwhile, integrating narrow-bandgap quantum dots (QDs) offers an effective route to enhance light harvesting and tailor interfacial band alignment for charge transfer[\\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e]. Importantly, the benefit strongly depends on QD loading: insufficient coverage provides limited sensitization and interfacial modification, whereas excessive loading may induce aggregation, interfacial traps, and transport barriers that accelerate recombination[\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e].\\u003c/p\\u003e \\u003cp\\u003eHere, we demonstrate a PbS quantum-dot-modified, composition-graded BCFO stack on FTO, where the internal field of the gradient architecture cooperates with PbS-derived interfacial band bending to promote carrier separation and extraction. By systematically tuning the PbS precursor concentration, we identify an optimal loading window that maximizes the photoresponse and clarify its physical origin in terms of microstructure-controlled interfacial charge transfer and recombination. This work provides an interfacial-engineering route for improving photovoltaic output in BFO-based oxide stacks and offers practical design guidelines for integrating narrow-bandgap QDs with graded oxide heterostructures governed by internal potential gradients and band alignment.\\u003c/p\\u003e\"},{\"header\":\"2. Experimental section\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e2.1 Preparation of BCFO precursor solutions\\u003c/h2\\u003e \\u003cp\\u003eBCFO precursor sols with Ca contents of \\u003cem\\u003ex\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0, 0.05, 0.10, and 0.20 (denoted as BFO, BCFO-05, BCFO-10, and BCFO-20) were prepared by a sol-gel method using Bi(NO\\u003csub\\u003e3\\u003c/sub\\u003e)\\u003csub\\u003e3\\u003c/sub\\u003e\\u0026middot;5H\\u003csub\\u003e2\\u003c/sub\\u003eO, Fe(NO\\u003csub\\u003e3\\u003c/sub\\u003e)\\u003csub\\u003e3\\u003c/sub\\u003e\\u0026middot;9H\\u003csub\\u003e2\\u003c/sub\\u003eO, and calcium acetate as starting reagents. A 10 mol% excess of Bi(NO\\u003csub\\u003e3\\u003c/sub\\u003e)\\u003csub\\u003e3\\u003c/sub\\u003e\\u0026middot;5H\\u003csub\\u003e2\\u003c/sub\\u003eO was introduced to compensate for Bi volatilization. The salts were dissolved in a 2-methoxyethanol/glacial acetic acid mixture (3:1, v/v), followed by the addition of citric acid, ethylene glycol, and ethanolamine to stabilize the sol (pH\\u0026thinsp;\\u0026asymp;\\u0026thinsp;1). After stirring for 2 h, the solution was diluted to 100 mL and aged for 24 h to obtain stable precursors, as shown in the Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e(a).\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec4\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e2.2 Preparation of BCFO multilayer thin film\\u003c/h2\\u003e \\u003cp\\u003e1 \\u0026times; 1 cm\\u003csup\\u003e2\\u003c/sup\\u003e FTO substrates were cleaned (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e(a)) and partially masked to define the bottom electrode. BCFO films were spin-coated at 4000 rpm for 20 s (25\\u0026deg;C, ~\\u0026thinsp;50% RH). After each coating, the films were dried at 60, 80, and 100\\u0026deg;C for 5 min each and pyrolyzed at 250\\u0026deg;C for 10 min, followed by annealing at 600\\u0026deg;C for 30 min. The total thickness was kept at ~\\u0026thinsp;240 nm by adjusting the coating cycles. The composition-graded BCFO-T4 stack was fabricated by sequentially depositing BCFO-20/BCFO-10/BCFO-05/BFO (bottom to top) (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e(b)). Finally, an Ag top electrode was applied and annealed at 90\\u0026deg;C for 10 min to obtain a conductive contact.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec5\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e2.3 Preparation of PbS/BCFO-T4 film\\u003c/h2\\u003e \\u003cp\\u003eA PbS quantum-dot overlayer was deposited on the stacked BCFO films by spin-assisted SILAR (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e(b)), with the PbS precursor concentration set to 0.005, 0.0075, or 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e. Each cycle consisted of sequential Pb\\u003csup\\u003e2+\\u003c/sup\\u003e and S\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e deposition (70 \\u0026micro;L each) with intermediate deionized-water rinses, using 1500 rpm for 60 s for the Pb\\u003csup\\u003e2+\\u003c/sup\\u003e step and 2000 rpm for 60 s for the S\\u003csup\\u003e2\\u0026minus;\\u003c/sup\\u003e step. The cycle was repeated 20 times to obtain the PbS layer, as shown in the Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e(b).\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec6\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e2.3. Characterization\\u003c/h2\\u003e \\u003cp\\u003e \\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e \\u003ccaption language=\\\"En\\\"\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003eCharacterization items\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"4\\\"\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eStructural characterization and performance testing\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eCharacteristic curves\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003eInstrument and\\u003c/p\\u003e \\u003cp\\u003eequipment\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003eTest parameter\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eCrystal structure\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eXRD patterns\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003eXRD(DX-2700)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003ePowder scanning Angle is 20\\u0026thinsp;~\\u0026thinsp;70\\u0026deg;\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSurface topography\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eSEM images\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003eFESEM(JSM-7800F)\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eOptical performance\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eAbsorbance spectra\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003ePuxi T9S-C Spectrophotometer\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e-\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\" morerows=\\\"2\\\" rowspan=\\\"3\\\"\\u003e \\u003cp\\u003ePhotovoltaic performance\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eJ\\u003c/em\\u003e-\\u003cem\\u003et\\u003c/em\\u003e curve\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\" morerows=\\\"2\\\" rowspan=\\\"3\\\"\\u003e \\u003cp\\u003eIV Test Station 2000\\u003c/p\\u003e \\u003cp\\u003eKeithley 2400\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\" morerows=\\\"2\\\" rowspan=\\\"3\\\"\\u003e \\u003cp\\u003e100 mW cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;2\\u003c/sup\\u003e(AM 1.5G)\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eJ\\u003c/em\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e curve\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eη\\u003c/em\\u003e-\\u003cem\\u003eV\\u003c/em\\u003e curve\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003c/tbody\\u003e \\u003c/colgroup\\u003e \\u003c/table\\u003e\\u003c/div\\u003e \\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"3. Results and discussion\",\"content\":\"\\u003cdiv id=\\\"Sec8\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e3.1 Microstructure\\u003c/h2\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e(a) shows the XRD patterns of PbS/BCFO-T4 composite films prepared with different PbS precursor concentrations, together with the device schematic in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e(b). In addition to the FTO substrate peaks, three reflections at ~\\u0026thinsp;26\\u0026deg;, ~\\u0026thinsp;30\\u0026deg;, and ~\\u0026thinsp;43\\u0026deg; can be assigned to the (111), (200), and (220) planes of cubic PbS, matching PbS (PDF#99\\u0026thinsp;\\u0026minus;\\u0026thinsp;0053). The remaining peaks are consistent with BFO (PDF#74-2016), and no obvious secondary phases are detected, indicating that the perovskite framework is retained after PbS deposition.\\u003c/p\\u003e \\u003cp\\u003eAs the precursor concentration increases from 0.005 to 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, the PbS peaks intensify, while the diffraction signals from the underlying BCFO stack and FTO weaken. This trend suggests an increased crystalline contribution and coverage of the PbS overlayer, which attenuates the diffraction from the bottom layers, in line with the stacked architecture shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e(b). The PbS crystallite sizes estimated by the Scherrer equation are 23.691, 25.213, and 30.794 nm for 0.005, 0.0075, and 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, respectively.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e (a-c) show the SEM images of PbS/BCFO-T4 composite films prepared with different PbS precursor concentrations. Spherical nanoparticles with evident aggregation are observed, and EDS elemental mapping (Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e(d,e)) confirms that these particulates are PbS. At 0.005 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, PbS does not fully cover the BCFO surface and appears as sparsely distributed small agglomerates. With increasing concentration, PbS coverage increases and growth on pre-formed PbS nuclei becomes more pronounced, leading to larger clusters and more severe aggregation[\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e]. This trend is attributable to coalescence/sintering during repeated annealing, which reduces interparticle spacing and promotes particle merging. Consequently, the surface becomes rougher and less uniform at higher concentration, which can increase interfacial recombination and deteriorate the fill factor. Particle-size statistics (Nano Measure) indicate average PbS sizes of ~\\u0026thinsp;22.22, 23.65, and 25.73 nm for 0.005, 0.0075, and 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, respectively. The increase in particle number and size with concentration is consistent with adsorption-limited growth at low concentration, where a higher ion supply promotes nucleation and growth[\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e].\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec9\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e3.2 Optical performance\\u003c/h2\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e combines the absorption spectra and the proposed band-alignment model to clarify the concentration-dependent optical and carrier-separation behavior of the PbS/BCFO-T4 composites. As shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e(a), all composite films exhibit strong visible-light absorption, and introducing PbS QDs increases both the absorbance and the absorption window compared with the pristine BCFO stack. With increasing PbS precursor concentration (0.005 \\u0026rarr; 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e), the absorbance increases, reflecting higher PbS loading. A noticeable red shift of the absorption edge is also observed, consistent with particle growth at higher concentration. Using the Moreels relation shown in Eq.\\u0026nbsp;(\\u003cspan refid=\\\"Equ1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e) and the PbS sizes extracted from SEM[\\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e], the PbS bandgaps are estimated to be 0.463, 0.458, and 0.452 eV for 0.005, 0.0075, and 0.01 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, respectively.\\u003cdiv id=\\\"Equ1\\\" class=\\\"Equation\\\"\\u003e\\u003cdiv format=\\\"TEX\\\" class=\\\"mathdisplay\\\" id=\\\"FileID_Equ1\\\" name=\\\"EquationSource\\\"\\u003e\\n$${E_g}=0.41+1/(0.0252{d^2}+0.283d)$$\\u003c/div\\u003e\\u003cdiv class=\\\"EquationNumber\\\"\\u003e1\\u003c/div\\u003e\\u003c/div\\u003e\\u003c/p\\u003e \\u003cp\\u003eThe corresponding band alignment for the Ag/PbS/BCFO-T4/FTO architecture is depicted in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e(b,c), constructed from literature-reported parameters together with the experimentally estimated PbS bandgap. PbS QDs are commonly reported to be p-type-like with a Fermi level closer to the valence band, whereas BFO/BCFO can exhibit n-type-like behavior associated with oxygen-vacancy-related donor defects, enabling formation of a PbS/BFCO heterojunction[\\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e]. After contact, Fermi-level equilibration leads to band bending and a built-in electric field at the PbS/BFO interface, which is aligned with the internal field of the composition-graded BCFO stack (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e(c)). Under illumination, the coupled fields favor directional carrier separation, driving electrons toward the top Ag electrode and holes toward the FTO side, thereby promoting carrier collection.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec10\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003e3.3 Photovoltaic performance\\u003c/h2\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003e \\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab2\\\" border=\\\"1\\\"\\u003e \\u003ccaption language=\\\"En\\\"\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 2\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003ePhotovoltaic characteristics of PbS/BCFO-T4 composite films with different precursor concentrations\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"5\\\"\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e \\u003cthead\\u003e \\u003ctr\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eSample\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e (mA/cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;2\\u003c/sup\\u003e)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e (V)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e\\u003cem\\u003eFF\\u003c/em\\u003e (%)\\u003c/p\\u003e \\u003c/th\\u003e \\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003ePCE (%)\\u003c/p\\u003e \\u003c/th\\u003e \\u003c/tr\\u003e \\u003c/thead\\u003e \\u003ctbody\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003e0.005 mol/L\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e0.402\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.699\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e44.1\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.15212\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003e0.0075 mol/L\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e2.431\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.872\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e35.5\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e1.235\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003e0.01 mol/L\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003e4.073\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.624\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e24.3\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e0.619\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003c/tbody\\u003e \\u003c/colgroup\\u003e \\u003c/table\\u003e\\u003c/div\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure \\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e summarizes the photovoltaic output of the PbS/BCFO composite films, and the extracted parameters are listed in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e. A pronounced precursor-concentration dependence is observed: short circuit current density (\\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e) ncreases monotonically with increasing PbS precursor concentration, indicating strengthened photogeneration and/or more effective interfacial charge transfer at higher PbS loading. Meanwhile, the overall power conversion efficiency exhibits a volcano-type dependence and reaches a maximum at 0.0075 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e, delivering \\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e=2.431 mA cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;2\\u003c/sup\\u003e, open circuit voltage (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e)\\u0026thinsp;=\\u0026thinsp;0.872 V, fill factor (\\u003cem\\u003eFF\\u003c/em\\u003e)\\u0026thinsp;=\\u0026thinsp;35.5%, and Power Conversion Efficiency (PCE)\\u0026thinsp;=\\u0026thinsp;1.235%. The improved output at intermediate loading can be attributed to the formation of a PbS/BCFO-T4 heterojunction, where an interfacial electric field facilitates photocarrier separation and transport, thereby enhancing charge extraction[\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e]. In contrast, further increasing the precursor concentration leads to a reduced \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e and a pronounced drop in \\u003cem\\u003eFF\\u003c/em\\u003e, which can be rationalized by transport and interfacial limitations of the PbS layer. PbS QD films typically possess a limited carrier diffusion/collection length; thus, an excessively thick and/or highly aggregated PbS overlayer can introduce series resistance and trap-assisted recombination, degrading carrier extraction and lowering the overall efficiency[\\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e].\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFigure\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e shows the time-dependent photocurrent density (\\u003cem\\u003eJ\\u003c/em\\u003e-\\u003cem\\u003et\\u003c/em\\u003e) responses of the PbS/BCFO composite films under periodic light on/off switching. After introducing the PbS layer on the BCFO stack, the photocurrent density is markedly higher than that of the pristine BCFO film, indicating an enhanced photoresponse. In the dark, the current density remains close to zero, whereas upon illumination it rises rapidly and reversibly, demonstrating a clear photovoltaic/photoconductive response and good switching reproducibility. With increasing PbS precursor concentration, the photocurrent density increases accordingly. This trend is mainly attributed to the higher PbS loading, which improves surface coverage and effective thickness, thereby strengthening light harvesting and facilitating interfacial charge transfer. Within an appropriate concentration range, the increased number of PbS quantum dots and their enlarged domains can provide more percolation pathways for carrier transport and promote charge collection, leading to a higher steady-state photocurrent[\\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e].\\u003c/p\\u003e \\u003cp\\u003eTo benchmark the device performance, the photovoltaic parameters of the present PbS/BCFO-T4 device are compared with previously reported ferroelectric-and QD/oxide-based photovoltaic structures in the literature (Table\\u0026nbsp;\\u003cspan refid=\\\"Tab3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e). The comparison indicates that the achieved output is competitive within this material class, particularly in terms of \\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e and the overall PCE under comparable measurement conditions.\\u003c/p\\u003e \\u003cp\\u003e \\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab3\\\" border=\\\"1\\\"\\u003e \\u003ccaption language=\\\"En\\\"\\u003e \\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 3\\u003c/div\\u003e \\u003cdiv class=\\\"CaptionContent\\\"\\u003e \\u003cp\\u003eComparison of photovoltaic parameters in this study with those reported in the literature\\u003c/p\\u003e \\u003c/div\\u003e \\u003c/caption\\u003e \\u003ccolgroup cols=\\\"8\\\"\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e \\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e \\u003cdiv 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colname=\\\"c8\\\"\\u003e \\u003cp\\u003e[\\u003cspan citationid=\\\"CR39\\\" class=\\\"CitationRef\\\"\\u003e39\\u003c/span\\u003e]\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003ctr\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e \\u003cp\\u003eAg/PbS/BCFO-T4/FTO\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e \\u003cp\\u003eSol-Gel;\\u003c/p\\u003e \\u003cp\\u003espin-assisted SILAR\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e \\u003cp\\u003e0.872\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e \\u003cp\\u003e2431\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e \\u003cp\\u003e1.235\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e \\u003cp\\u003e100\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c7\\\"\\u003e \\u003cp\\u003eAM 1.5G\\u003c/p\\u003e \\u003c/td\\u003e \\u003ctd align=\\\"left\\\" colname=\\\"c8\\\"\\u003e \\u003cp\\u003eThis work\\u003c/p\\u003e \\u003c/td\\u003e \\u003c/tr\\u003e \\u003c/tbody\\u003e \\u003c/colgroup\\u003e \\u003c/table\\u003e\\u003c/div\\u003e \\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"4. Conclusions\",\"content\":\"\\u003cp\\u003eIn conclusion, this work establishes an optimal PbS loading window for enhancing photovoltaic output in a composition-graded BCFO-T4 oxide stack via quantum-dot interfacial engineering. The results reveal a fundamental trade-off between photogeneration and carrier extraction: moderate PbS deposition strengthens light harvesting and promotes field-driven carrier separation at the PbS/BFO interface in concert with the internal potential gradient of the graded stack, whereas excessive loading leads to aggregation and non-uniform thickening that introduce trap-assisted recombination and transport barriers, thereby reducing the fill factor and overall efficiency. Consequently, an intermediate PbS precursor concentration of 0.0075 mol L\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e provides the best balance, delivering \\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e = 2431 \\u0026micro;A cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;2\\u003c/sup\\u003e, \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e=0.872 V and a maximum PCE of 1.235%. From an application standpoint, the identified loading-window guideline provides a simple and reproducible process lever for QD/oxide integration and, together with the stable light-switching response, is well suited for scalable thin-film optoelectronics such as self-powered photodetectors and light-responsive sensors.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAuthor contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis article represents a collaborative effort. Below is a detailed breakdown of the contributions from each author: [Weihao Wu]: Conceptualization; Data curation; Methodology; Validation; Writing-original draft; [Zhongxiang Zheng]: Formal Analysis; Investigation; Visualization; [Xiaoyu Luo] [Shubao Yang]: Writing-review \\u0026amp; editing; [Wenchuan Li]: Project administration; Resources; Supervision; [Rongli Gao], [Xiaoling Deng], [Wei Cai], [Chunlin Fu]: Writing-original draft preparation.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgments\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe present work has been Sponsored by Chongqing Natural Science Foundation Joint Fund for Innovation and Development (Grant No. CSTB2025NSCQ-LZX0097) and the Innovation Project of Chongqing University of Science and Technology (Grant No. YKJCX2520308)\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eData availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe data presented in this study are available from the corresponding author upon reasonable request.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eDeclarations\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare that they have no fnancial and personal relationships with other people or organizations that can inappropriately infuence our work.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eGao C, Li W, Jing L, et al. 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Ceramics International, 2025.\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"ferroelectric-oxide photovoltaics, PbS quantum dots, composition-graded BCFO, band alignment\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-8645878/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-8645878/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eA PbS quantum-dot (QD) interfacial layer was integrated with a composition-graded Bi\\u003csub\\u003e1-\\u003c/sub\\u003e\\u003csub\\u003e\\u003cem\\u003ex\\u003c/em\\u003e\\u003c/sub\\u003eCa\\u003cem\\u003eₓ\\u003c/em\\u003eFeO\\u003csub\\u003e3\\u003c/sub\\u003e(BCFO) oxide stack to enhance photovoltaic output via coupled optical sensitization and interfacial field regulation. By tuning the PbS precursor concentration, an optimal QD loading window was identified. Increasing PbS loading enhanced optical absorption and interfacial band bending at the PbS/BCFO junction, whereas excessive loading induced aggregation and non-uniform thickening, leading to transport bottlenecks and trap-assisted recombination. Consequently, the photocurrent increased with PbS loading while the power conversion efficiency(PCE) exhibited a volcano-type dependence, peaking at 0.0075 mol L\\u003csup\\u003e-1\\u003c/sup\\u003e with short circuit current density (\\u003cem\\u003eJ\\u003c/em\\u003e\\u003csub\\u003esc\\u003c/sub\\u003e) = 2431 μA cm\\u003csup\\u003e-2\\u003c/sup\\u003e, open circuit voltage (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eoc\\u003c/sub\\u003e) = 0.872 V, fill factor (\\u003cem\\u003eFF\\u003c/em\\u003e) = 41.5%, and a maximum PCE of 1.235%, together with stable light on/off switching. These results highlight that controlling QD loading, rather than simply maximizing sensitizer content, is critical for achieving reproducible performance enhancement in QD/oxide composite photovoltaics.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Optimal PbS quantum-dot loading in composition-graded Bi1-xCaxFeO3 stacks for improved photovoltaic performance\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2026-02-05 09:21:11\",\"doi\":\"10.21203/rs.3.rs-8645878/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"ef8bf17d-9af5-432f-bb41-c06142001680\",\"owner\":[],\"postedDate\":\"February 5th, 2026\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[],\"tags\":[],\"updatedAt\":\"2026-04-20T16:07:06+00:00\",\"versionOfRecord\":{\"articleIdentity\":\"rs-8645878\",\"link\":\"https://doi.org/10.1007/s10854-026-17284-y\",\"journal\":{\"identity\":\"journal-of-materials-science-materials-in-electronics\",\"isVorOnly\":false,\"title\":\"Journal of Materials Science: Materials in Electronics\"},\"publishedOn\":\"2026-04-19 15:59:41\",\"publishedOnDateReadable\":\"April 19th, 2026\"},\"versionCreatedAt\":\"2026-02-05 09:21:11\",\"video\":\"\",\"vorDoi\":\"10.1007/s10854-026-17284-y\",\"vorDoiUrl\":\"https://doi.org/10.1007/s10854-026-17284-y\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-8645878\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-8645878\",\"identity\":\"rs-8645878\",\"version\":[\"v1\"]},\"buildId\":\"XKTyCvWXoU3ODBz1xrDgd\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}