Cu-Doped Sb 2 Se 3 Thin-Film Solar Cells Based on Hybrid Pulsed Electron Deposition/Radio Frequency Magnetron Sputtering Growth Techniques

: In recent years, research attention has increasingly focused on thin-film photovoltaics utilizing Sb 2 Se 3 as an ideal absorber layer. This compound is favored due to its abundance, non-toxic nature, long-term stability, and the potential to employ various cost-effective and scalable vapor deposition (PVD) routes. On the other hand, improving passivation, surface treatment and p-type carrier concentration is essential for developing high-performance and commercially viable Sb 2 Se 3 solar cells. In this study, Cu-doped Sb 2 Se 3 solar devices were fabricated using two distinct PVD techniques, pulsed electron deposition (PED) and radio frequency magnetron sputtering (RFMS). Furthermore, 5%Cu:Sb 2 Se 3 films grown via PED exhibited high open-circuit voltages (V OC ) of around 400 mV but very low short-circuit current densities (J SC ). Conversely, RFMS-grown Sb 2 Se 3 films resulted in low V OC values of around 300 mV and higher J SC . To enhance the photocurrent, we employed strategies involving a thin NaF layer to introduce controlled local doping at the back interface and a bilayer p-doped region grown sequentially using PED and RFMS. The optimized Sb 2 Se 3 bilayer solar cell achieved a maximum efficiency of 5.25%.


Introduction
The development of readily available and non-hazardous materials for low-cost, high-performance optoelectronic devices is crucial to meet the growing demand for environmentally friendly applications, such as energy-efficient light-emitting diodes and photovoltaic cells.The investigation of innovative structures involving thin films holds strategic importance for solar energy conversion, particularly in light of the anticipated scarcity of fossil fuels and the severe environmental consequences of their usage, including pollution and global warming.Currently, the most efficient solar cells, boasting a record efficiency of 47.1% [1], are based on multi-junctions of AlGaInP/AlGaAs/GaAs/GaInAs.However, from a commercial standpoint, they lack competitiveness compared to silicon cells due to their exorbitant costs.Conversely, silicon cells, despite exhibiting lower efficiencies (reaching a maximum of approximately 26% [2]), benefit from significantly lower manufacturing costs, enabling them to dominate the global photovoltaic (PV) market.As a viable alternative to crystalline silicon, thin-film solar cells have gained increasing traction in recent years for building-integrated photovoltaics (BIPV) and product-integrated photovoltaics (PIPV), driven by their reduced material requirements for fabrication, low costs, and potential for integration into flexible devices.Chalcogenide compounds represent some of the most promising thin-film materials for BIPV and PIPV applications.CdTe and Cu(In,Ga)Se 2 (CIGS) stand out among these compounds, achieving the highest photovoltaic conversion efficiencies, exceeding 23% [1].However, due to their reliance on critical raw materials like indium and gallium, chalcogenide PV technologies face resource limitations that are likely to hinder large-scale production in the future [3].To circumvent this challenge, thin films composed of readily available and non-hazardous elements like selenium (Se), antimony (Sb), and sulfur (S) have been extensively investigated over the past decade as absorber layers [4].
Among these materials, antimony selenide (Sb 2 Se 3 ) exhibits properties that make it suitable for physical vapor deposition (PVD) techniques [5] such as a low melting point (608 • C) and a high saturated vapor pressure (22.5 Pa at 400 • C and 3.48 × 10 3 Pa at 600 • C).Moreover, Sb 2 Se 3 is classified as a non-toxic material in practical terms [6], with an optimal optical bandgap of around 1.2 eV and a high absorption coefficient greater than 10 5 cm −1 , which makes it potentially a strong candidate to replace critical absorber layers in photodetectors [7] and in solar cells.In particular, Sb 2 Se 3 solar cells present great room for improvement since their theoretical efficiency of 31.7%, according to Shockley and Queisser [8], is significantly higher than the highest experimental efficiency of 10.57% demonstrated up to now [9].The main reasons for this discrepancy are (i) the intrinsic electrical anisotropy in terms of conduction associated to the difficulty in controlling crystal orientation, (ii) the short carrier lifetime because of the high concentration of intrinsic defects, such as V Se vacancies and Sb Se substitutional defects, (iii) a low hole carrier density, and (iv) the lack of a suitable hole transport layer (HTL) and electron transport layer (ETL) materials.A strong effort is necessary for the study of both the absorber material and the device architecture to improve cell performances.
Sb 2 Se 3 is characterized by the orthorhombic crystal symmetry belonging to the Pbnm space group (JCPDS 15-0861) and lattice parameters a = 11.62 Å, b = 11.77Å, c = 3.962 Å with the presence of covalently bonded [Sb 4 Se 6 ] n ribbons running along the c-axis.Conversely, grain boundaries are formed along the direction of the ribbons, where van der Waals interactions stack the ribbons together.The quasi-one-dimensional structure of Sb 2 Se 3 induces strong anisotropic properties, such as the photocarrier transport that is enhanced along the ribbons and limited towards other directions, since the surfaces parallel to the [001] direction, such as the (110) and (120) planes, have no dangling bonds and consequently reduce non-radiative recombination losses [10,11].Thus, it is crucial for solar cell applications to achieve a preferential alignment of the ribbons perpendicular to the substrate or to align the (hkℓ) directions with non-zero ℓ parallel to the growth direction.
Despite substantial advancements in enhancing solar cell efficiency, several challenges remain to be addressed, including reducing carrier recombination at interfaces, ensuring efficient carrier transfer between layers and improving charge carrier doping in the absorber layer [23].
Sb 2 Se 3 generally presents a low intrinsic p-type doping with an acceptor concentration of around 10 13 cm −3 .A free hole concentration higher than 10 15 cm −3 would be beneficial to improve the electrode/absorber contact quality and, in principle, to increase the open circuit voltage, V OC .However, extrinsic p-type doping represents a significant challenge, as dopants are located preferentially between the 1D ribbons, where they are inert and not in the Sb 2 Se 3 lattice [24].Several works about the extrinsic doping of Sb 2 Se 3 have been reported, using elements like Sn, Cu, Fe, Mg, Sn, Na and I [24][25][26][27][28]. Cu and Pb seem the most promising doping element for p doping, inducing, respectively, an increase in carrier concentrations up to 10 15 cm −3 [25] and a decrease in the resistivity from 2.1 × 10 8 to 2.9 × 10 5 Ωcm [24].Alkaline doping as well seems to have a beneficial effect on the V OC of the cells.Differently, I and Fe have been found to be active like n-doping elements.
In this work, the effect of Na and Cu extrinsic doping on the performances of Sb 2 Se 3based solar cells was investigated.Specifically, Na doping was used at the interface with the back contact, while Cu was introduced directly into the starting Sb 2 Se 3 target with the aim of increasing the acceptor carrier density in the bulk of the film.Two different growth techniques, pulsed electron deposition (PED) and RF magnetron sputtering (RFMS), were used sequentially to grow the Sb 2 Se 3 layer in order to obtain absorber bilayers with different thicknesses, grain orientations, and doping profiles.The optimized bilayer architecture in the substrate configuration exhibited a promising conversion efficiency of 5.25%.

Materials and Methods
Sb 2 Se 3 and NaF films were grown via PED using a high vacuum chamber, equipped with a PEBS-20 commercial source (supplied by Neocera Inc., Beltsville, MD, USA).The base pressure was around 2.0 × 10 −4 Pa.The pulsed electron beam was ignited at a discharge voltage of 16 kV, with a pulse repetition rate of 9 Hz.During the deposition process, Ar gas (5N purity) pressure was of about 3.0 × 10 −1 Pa to ensure the stable and controlled emission of electrons towards the target.The used targets were 10 mm thick cylindrical pieces sliced from polycrystalline Sb 2 Se 3 ingots and 5 at.%Cu:Sb 2 Se 3 , synthesized thanks to a customized Czochralski reactor from elemental species (5N purity).The target-substrate distance was maintained at 8 cm.Substrates were first mounted in a load-lock chamber, then transferred inside the main chamber and heated under a graphite susceptor using halogen lamps.The substrate temperature, set to 280 • C for optimal performance, was monitored using a type-K thermocouple and a 2.4 µm IR pyrometer (model Endurance E3M, supplied by Fluke Process Instruments, Berlin, Germany) placed behind a ZnSe-bandpass viewport at an angle of 45 • to the sample surface.
RFMS-grown Sb 2 Se 3 films were grown using a 3 ′′ RF magnetron sputtering cathode (Kenosistec Srl, Milan, Italy) powered at 30 W, using a 4N-purity binary Sb 2 Se 3 target (supplied by Testbourne Ltd., Basingstoke, UK) as the starting material.The base pressure was maintained below 1 × 10 −6 mbar.The target-substrate distance was 8 cm.During deposition, the working pressure was kept at 5.0 × 10 −1 Pa by filling the chamber with 5N-purity Ar gas.The substrate temperature was optimized to 300 • C and monitored using a type-K thermocouple.Substrates consisted of 2.5 × 2.5 cm 2 sized commercial glass sheets coated with fluorine tin oxide (FTO).The substrates were cleaned before PED and RFMS depositions by sequentially rinsing them in acetone, ethanol, and isopropyl alcohol.The top undoped ZnO (UZO) and Al-doped ZnO (AZO) layers were deposited via RF magnetron sputtering (Angstrom Sciences) at room temperature (RT) and 120 W in Ar atmosphere (5.0 × 10 −1 Pa).Structural characterizations of the films, including their crystalline structure and preferred grain orientations, were determined via X-ray diffraction (XRD) using a Siemens D500 (Siemens, Berlin, Germany) diffractometer in Bragg-Brentano geometry.The morphological and compositional characterizations of the samples were obtained using the Zeiss Auriga Compact field-emission scanning electron microscope (FESEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) detector (Oxford).For SEM imaging, the electron beam acceleration was set to 5 kV or 10 kV, while for EDX analysis, it was set to 20 kV.Photovoltaic devices were fabricated according to the following structure (from top to bottom): AZO/UZO/CdS/Sb 2 Se 3 /NaF/FTO/glass.Current densityvoltage (J-V) characteristics were measured using the Keithley 2614B source meter and the ABET SUN 2000 AAB solar simulator under standard test conditions (AM 1.5G spectrum, 1000 W/m 2 irradiance, and a cell temperature of 25 • C).We fabricated RFMS-grown and PED-grown Sb 2 Se 3 -based solar cells respectively with AZO/UZO/CdS/undoped-Sb 2 Se 3 /FTO/glass and AZO/UZO/CdS/Cu:Sb 2 Se 3 /FTO/glass structures.RFMS-grown Sb 2 Se 3 layers exhibited a 2:3 Sb:Se stoichiometry and good homogeneity, as confirmed by EDX microanalysis reported in [17] (Figure S1, see Supplementary Material).In PED-grown Cu:Sb 2 Se 3 layers, the 2:3 Sb:Se stoichiometry and the nominal 5% Cu atomic concentration were confirmed by EDX-SEM analysis both in the target and in the film.J-V curves in Figure 2a illustrate the photovoltaic performance of the sputtered Sb 2 Se 3 solar cell.The incorporation of a NaF layer in the AZO/UZO/CdS/Sb 2 Se 3 /NaF/FTO/glass structure resulted in a significant enhancement of short-circuit current density (J SC ) and open-circuit voltage (V OC ), leading to an efficiency improvement from 1.28% to 3.2%.Solar cells based on PED-grown 5%Cu:Sb2Se3 as the p-type layer with the AZO/UZO/CdS/Cu:Sb2Se3/FTO/glass structure present higher VOC than undoped grown Sb2Se3 (0.35 V-0.51 V as reported also by [25] vs. 0.26 V as reported by [32]).ever, these PED-grown cells also exhibit very low short-circuit currents (JSC) (aroun mA/cm 2 ).The introduction of a NaF interfacial layer leads to a significant tenfold inc in JSC for this type of PED-grown solar cell (Figure 2b).The JSC enhancement induc Na is substantial; however, JSC values for PED-grown Sb2Se3 are generally much Solar cells based on PED-grown 5%Cu:Sb 2 Se 3 as the p-type layer with the same AZO/UZO/CdS/Cu:Sb 2 Se 3 /FTO/glass structure present higher V OC than undoped PEDgrown Sb 2 Se 3 (0.35 V-0.51 V as reported also by [25] vs. 0.26 V as reported by [32]).However, these PED-grown cells also exhibit very low short-circuit currents (J SC ) (around 0.3 mA/cm 2 ).The introduction of a NaF interfacial layer leads to a significant tenfold increase in J SC for this type of PED-grown solar cell (Figure 2b).The J SC enhancement induced by Na is substantial; however, J SC values for PED-grown Sb 2 Se 3 are generally much lower than those for sputtered Sb 2 Se 3 , resulting in a power conversion efficiency (PCE) of only 0.36%.

NaF Layer Effect on
The trap-assisted tunneling effect induced by Na at the FTO/ Sb 2 Se 3 interface is supported by studying the effects of annealing treatments.As shown in Figure S2, the V OC of the PED-grown Cu:Sb 2 Se 3 cell increases after different annealing cycles at 175 • C in air, and conversely, J SC is reduced.During the annealing process, Na atoms from the NaF layer are expected to diffuse from the interface to the Sb 2 Se 3 bulk, passivating the GBs and other compensating defects [29,30].However, as the defect density at the interface decreases, the tunneling through localized states is also reduced, resulting in a lower photocarrier extraction and, consequently, in a lower J SC .A recent study [33] also reported similar effects on improved electrical performance, particularly V OC , due to the inclusion of a NaF layer between the back contact and Sb 2 Se 3 .The study suggests that the diffusion of Na ions in the Sb 2 Se 3 absorber layer could positively passivate defects at the bulk and heterojunction interface, thereby reducing defect-assisted recombination within the bulk.
The Cu:Sb 2 Se 3 layer deposited via PED shows a free carrier density larger by two orders of magnitude with respect to undoped Sb 2 Se 3 films, but it shows a very limited J SC .This observation could be attributed to a potential barrier that Cu ions create for photocarrier extraction at either the FTO/Cu:Sb 2 Se 3 or Cu:Sb 2 Se 3 /CdS interfaces.In the former case, Cu ions may raise the minimum conduction band energy (since the minimum conduction band of CuSbSe 2 is at −4.07 eV [34]), thereby enhancing the band offset with FTO and impeding the extraction of photogenerated charge carriers.Alternatively, the non-perpendicular orientation of [Sb4Se6]n ribbons within the PED-grown 5%Cu:Sb 2 Se 3 layer could hinder photocarrier conduction.Despite the restricted J SC , PED-grown Sb 2 Se 3 solar cells consistently exhibit higher V OC and fill factor (FF) values, indicating reduced non-radiative recombination and overall superior crystal quality.

Fabrication of Bi-Layered PED/Sputtering Sb 2 Se 3 Solar Cells
Since carrier recombination is significantly reduced in PED-grown Sb 2 Se 3 , while RFMS enhances the alignment of ribbons and minimizes the photocurrent barrier, we deposited Sb 2 Se 3 bilayers sequentially using both PED and RFMS techniques.This hybrid approach was effectively investigated in a recent literature study [35], in which two other techniques were utilized to fabricate graded Sb 2 (Se,S) 3 multilayers, taking advantage of the distinct properties of each technique.Our hypothesis suggests that optimizing the electrical parameters (V OC and FF in the PED layer, J SC in the RFMS layer) in the PED-RFMS bilayers can enhance the overall efficacy of the solar cell.The thicknesses and deposition order of the two layers were systematically varied to investigate their impact in the structure and their behavior with the FTO and CdS interfaces.The various bilayer architectures are presented in Figure 3.The sample types and corresponding thicknesses are detailed in Table 1 for PED-on-RFMS bilayers and in Table 2 for RFMS-on-PED bilayers.Table 1 contains a list of all the samples (A-series) in which the first layer is deposited with RFMS, followed by the PED film (PED-on-RFMS) (Figure 3c).Sample A1 corresponds to a single thick sputtered Sb 2 Se 3 layer (Figure 3a).Table 2 lists all samples (B-series) in which the first layer is deposited with PED, followed by a sputtered layer (RFMS-on-PED) (Figure 3d).Sample B1 corresponds to a single thick layer deposited via PED (Figure 3b).

Structural Analysis
XRD patterns of Sb 2 Se 3 -based solar cells were collected to determine the preferential orientation of these films (Figure 4), using the Sb 2 Se 3 orthorhombic phase as the reference (Ref.JCPDS 15-0681).The relative texture coefficient, TC (hkℓ), was calculated to compare ribbon orientation in all the layers and bilayers.TC (hkℓ) is the ratio between the measured (hkℓ) peak intensity and the intensity of the same peak for the reference randomly-oriented powder, weighted as a percentage on the summation of the same value for all chosen peaks [36,37], as expressed by the following formula: where I 0 (hkℓ) is the relative intensity of the reflection with (hkℓ) Miller indices reported in the JCPDS card, and I (hkℓ) is the net intensity measured by the experimental XRD patterns after the background subtraction.A good crystalline quality can be observed for all the films, as testified by the low full-width half-maximum values of the reflections.In most of the cases, the (hk0) are the preferred crystal orientations, with the ribbons lying on the surface, already seen for this kind of growth [32].However, (hkℓ) orientations with ℓ ̸ = 0, especially the (041), ( 061) and (141) ones, are more intense or comparable to the (hk0) peaks in some bilayers such as in the A3, A4, A5 and B3 samples.In general, the (041), ( 061) and (141) reflections become predominant when the sputtered layer is thicker.Figure 4b presents the TC values for the principal crystal orientations of the A-series samples.Similarly, Figure 5b depicts the TC values for the B-series samples.Based on its definition in Equation (1), when the texture coefficient (TC) exceeds 10%, a preferred orientation of grains in Sb 2 Se 3 films along one or more of the 10 detected crystallographic directions is observed.Table 3 presents the values of ΣTC(ℓ ̸ = 0), representing the sum of all TC values for hkl reflections with ℓ ̸ = 0 for each sample analyzed in this study.most of the cases, the (hk0) are the preferred crystal orientations, with the ribbons lying on the surface, already seen for this kind of growth [32].However, (hkℓ) orientations with ℓ ≠ 0, especially the (041), ( 061) and (141) ones, are more intense or comparable to the (hk0) peaks in some bilayers such as in the A3, A4, A5 and B3 samples.In general, the (041), ( 061) and (141) reflections become predominant when the sputtered layer is thicker.Figure 4b presents the TC values for the principal crystal orientations of the A-series samples.Similarly, Figure 5b depicts the TC values for the B-series samples.Based on its definition in Equation ( 1), when the texture coefficient (TC) exceeds 10%, a preferred orientation of grains in Sb2Se3 films along one or more of the 10 detected crystallographic directions is observed.Table 3 presents the values of ΣTC(ℓ ≠ 0), representing the sum of all TC values for hkl reflections with ℓ ≠ 0 for each sample analyzed in this study.

Morphological Analysis
As shown in SEM cross-sectional micrographs (Figures 6b and 7b), the layers grown via PED and RFMS in the bilayer architecture exhibit no morphological differences, indicating a well-defined crystal structure throughout the active region of the cells.However, planar images reveal a distinct morphological contrast between the RFMS-grown layer in the B4 cell (Figure 6a) and the PED-grown layer (Figure 7a).The former layer exhibits a flatter surface morphology, while the PED-grown layer exhibits a needle-like morphology.

Solar Cell Performance and Characterization
The electrical performances of the fabricated devices are plotted in Figures 8 and 9.The A samples, corresponding to the bilayer PED-on-RFMS architecture, are compared to the corresponding cells fabricated with a single layer of RFMS-grown (A1) and PED-grown (B1) Sb 2 Se 3 .As mentioned earlier, the B1 device is used as the reference for the highest values of the V OC and FF of a single layer, while A1 is the reference for the maximum J SC .As one can see in Figure 8, in the bilayer PED-on-RFMS architecture, V OC and FF increase with increasing PED layer thickness.In contrast, J SC decreases significantly for thicker PED layers, leading to lower overall solar cell efficiencies.In this set of samples, the electrical quantities exhibit a monotonic trend, transitioning from the characteristics of a single RFMS cell to those of a single PED cell and vice versa.When the RFMS layer is thicker, the bilayer cell exhibits properties similar to the A1 cell, characterized by high J SC but lower V OC and FF, while the performance of the bilayer cell approaches that of the B1 cell with increasing thickness of the upper PED layer.Despite the increase in V OC with thicker PED layers, no bilayer configuration surpasses the efficiency of the A1 cell with a single RFMS layer.

Morphological Analysis
As shown in SEM cross-sectional micrographs (Figures 6b and 7b), the layers grown via PED and RFMS in the bilayer architecture exhibit no morphological differences, indicating a well-defined crystal structure throughout the active region of the cells.However, planar images reveal a distinct morphological contrast between the RFMS-grown layer in the B4 cell (Figure 6a) and the PED-grown layer (Figure 7a).The former layer exhibits a flatter surface morphology, while the PED-grown layer exhibits a needle-like morphology.

Solar Cell Performance and Characterization
The electrical performances of the fabricated devices are plotted in Figures 8 and 9.The A samples, corresponding to the bilayer PED-on-RFMS architecture, are compared to the corresponding cells fabricated with a single layer of RFMS-grown (A1) and PEDgrown (B1) Sb2Se3.As mentioned earlier, the B1 device is used as the reference for the highest values of the VOC and FF of a single layer, while A1 is the reference for the maximum JSC.As one can see in Figure 8, in the bilayer PED-on-RFMS architecture, VOC and FF increase with increasing PED layer thickness.In contrast, JSC decreases significantly for

Morphological Analysis
As shown in SEM cross-sectional micrographs (Figures 6b and 7b), the layers grown via PED and RFMS in the bilayer architecture exhibit no morphological differences, indicating a well-defined crystal structure throughout the active region of the cells.However, planar images reveal a distinct morphological contrast between the RFMS-grown layer in the B4 cell (Figure 6a) and the PED-grown layer (Figure 7a).The former layer exhibits a flatter surface morphology, while the PED-grown layer exhibits a needle-like morphology.

Solar Cell Performance and Characterization
The electrical performances of the fabricated devices are plotted in Figures 8 and 9.The A samples, corresponding to the bilayer PED-on-RFMS architecture, are compared to the corresponding cells fabricated with a single layer of RFMS-grown (A1) and PEDgrown (B1) Sb2Se3.As mentioned earlier, the B1 device is used as the reference for the highest values of the VOC and FF of a single layer, while A1 is the reference for the maximum JSC.As one can see in Figure 8, in the bilayer PED-on-RFMS architecture, VOC and FF increase with increasing PED layer thickness.In contrast, JSC decreases significantly for The electrical performances of the B-series solar cells (RFMS-on-PED) are depicted in Figure 9, compared to the reference cells made using single sputtered-grown (A1) and single PED-grown (B1) absorber layers.Consistent with the earlier results, the V OC and FF tend to increase with thicker PED-grown layers, while J SC decreases only for a PED-grown Cu:Sb 2 Se 3 layer thicker than 100 nm.The B4 solar cell, consisting of a 100 nm PED-grown Cu:Sb 2 Se 3 layer and a 1.1 µm sputtered-grown Sb 2 Se 3 layer, exhibits the highest efficiency among the investigated devices.
Table 4 summarizes the average and record values for all the obtained devices.Furthermore, 20 solar cells with an area of 6 mm 2 each were made and measured for each sample.It can be observed that samples with thicker Cu:Sb 2 Se 3 PED-grown layers also generally show a narrower data spread, especially for J SC and PCE.In addition, they display higher R sh , primarily caused by lower saturation currents, J 0 , which result in ideality factors, n, close to 1 and a higher FF.J 0 values two-three orders of magnitude lower, especially for B-series cells in comparison with A1 and A2, indicate reduced bulk carrier recombination in PED-grown Cu:Sb 2 Se 3 p-type layers.Since for Cu:Sb 2 Se 3 PED-grown layers > 100 nm (B1, B2, B3, A4, A5, A6) J SC suffers from a dramatic decrease, one can argue that the presence of the PED Cu:Sb 2 Se 3 bottom layer impedes photocarrier extraction.However, when this layer is < 100 nm, it does not appear to significantly block the photogenerated holes from Sb 2 Se 3 to FTO, but it does reduce leakage currents (leading to higher FF and R sh values).In contrast, for Cu:Sb 2 Se 3 PED-grown layers exceeding 100 nm, a negative blocking effect becomes dominant.The non-ideal ribbon orientation (TC with (hkℓ) orientations ℓ ̸ = 0 is only 35% for the B1 cell) could only partially explain the blocking behavior of this layer, since A1 shows only a slightly larger TC with respect to B1, but no photocurrent barrier seems to exist.Other factors, such as band misalignment at the FTO/Cu:Sb 2 Se 3 interface, likely contribute to this blocking effect, creating an energy barrier that impedes the flow of holes from Sb 2 Se 3 to FTO.Additionally, a comparison of the A-and B-series cells with similar thicknesses (i.e., A6, with a record PCE = 0.73% with B2, with PCE = 1.41%, or A2, with PCE = 2.04%, with B4, with PCE = 3.85%, or also A3 with B3), reveals that the B-series cells, with a RMFS layer on the absorber surface before the CdS, generally exhibit a superior performance compared to the A-series cells.A superior interface quality between the second RFMS-grown Sb 2 Se 3 layer and CdS is likely responsible for these results.As evident from the SEM top-view images in Figures 6a and 7a, the rougher PED-grown surface likely contributes to the junction deterioration, since the UZO/AZO layers appear to penetrate more deeply into the PED-grown layer and degrade the CdS/Sb 2 Se 3 interface, leading to a significantly leakier junction.Table 4 reveals a wide distribution of cell parameters.Therefore, more precise measurements were conducted on the samples using Ag paste and wire bonding techniques.The samples from A1 to B3 exhibit no significant improvement.The B4 cell demonstrates an absolute increase in efficiency of approximately 1.5%, representing a significant relative 50% improvement.Figure 10 compares the J-V curves of the best cells (A1 and B4) after enhancing the top and bottom contacts using Ag paste and wire bonding.
Solar 2024, 4, FOR PEER REVIEW 12 thicker PED layers, leading to lower overall solar cell efficiencies.In this set of samples, the electrical quantities exhibit a monotonic trend, transitioning from the characteristics of a single RFMS cell to those of a single PED cell and vice versa.When the RFMS layer is thicker, the bilayer cell exhibits properties similar to the A1 cell, characterized by high JSC but lower VOC and FF, while the performance of the bilayer cell approaches that of the B1 cell with increasing thickness of the upper PED layer.Despite the increase in VOC with thicker PED layers, no bilayer configuration surpasses the efficiency of the A1 cell with a single RFMS layer.The electrical performances of the B-series solar cells (RFMS-on-PED) are depicted in Figure 9, compared to the reference cells made using single sputtered-grown (A1) and single PED-grown (B1) absorber layers.Consistent with the earlier results, the VOC and FF tend to increase with thicker PED-grown layers, while JSC decreases only for a PED-grown Cu:Sb2Se3 layer thicker than 100 nm.The B4 solar cell, consisting of a 100 nm PED-grown Cu:Sb2Se3 layer and a 1.1 µm sputtered-grown Sb2Se3 layer, exhibits the highest efficiency  Table 4 summarizes the average and record values for all the obtained devices thermore, 20 solar cells with an area of 6 mm 2 each were made and measured for sample.It can be observed that samples with thicker Cu:Sb2Se3 PED-grown layer generally show a narrower data spread, especially for JSC and PCE.In addition, the play higher Rsh, primarily caused by lower saturation currents, J0, which result in id factors, n, close to 1 and a higher FF.J0 values two-three orders of magnitude lower, cially for B-series cells in comparison with A1 and A2, indicate reduced bulk carri combination in PED-grown Cu:Sb2Se3 p-type layers.Since for Cu:Sb2Se3 PED-grown l > 100 nm (B1, B2, B3, A4, A5, A6) JSC suffers from a dramatic decrease, one can argu the presence of the PED Cu:Sb2Se3 bottom layer impedes photocarrier extraction.ever, when this layer is < 100 nm, it does not appear to significantly block the phot erated holes from Sb2Se3 to FTO, but it does reduce leakage currents (leading to high and Rsh values).In contrast, for Cu:Sb2Se3 PED-grown layers exceeding 100 nm, a neg blocking effect becomes dominant.The non-ideal ribbon orientation (TC with (hkℓ) o tations ℓ ≠ 0 is only 35% for the B1 cell) could only partially explain the blocking beh of this layer, since A1 shows only a slightly larger TC with respect to B1, but no phot rent barrier seems to exist.Other factors, such as band misalignment at the FTO/Cu:S interface, likely contribute to this blocking effect, creating an energy barrier that imp the flow of holes from Sb2Se3 to FTO.Additionally, a comparison of the A-and Bcells with similar thicknesses (i.e., A6, with a record PCE = 0.73% with B2, with P 1.41%, or A2, with PCE = 2.04%, with B4, with PCE = 3.85%, or also A3 with B3), re that the B-series cells, with a RMFS layer on the absorber surface before the CdS, gen exhibit a superior performance compared to the A-series cells.A superior interface q   The best solar cell consisting of a AZO/UZO/CdS/Sb 2 Se 3 (RFMS)/Cu:Sb 2 Se 3 (PED)/ NaF/FTO/glass structure, where the Cu:Sb 2 Se 3 PED grown layer is 100 nm and the sputtering layer is 1100 nm, exhibited PCE = 5.25%, V OC = 343 mV, J SC = 31.4mA/cm 2 and FF = 0.49 (red line).All the key electrical parameters of this cell are upgraded in comparison with those of the sputtered A1 solar cell (black line).The improvement is attributed to enhanced FF and V OC , thanks to the thin Cu:Sb 2 Se 3 layer grown via PED.The remarkable PCE = 5.25% achieved by the bilayer cell beats previous records for devices employing single and undoped Sb 2 Se 3 absorbers.This impressive feat surpasses the previous benchmark of 2.1% for a PED-grown Sb 2 Se 3 cell reported in [32], as well as the 1.28% and 2.36% efficiencies obtained for RFMS-Sb 2 Se 3 cells on FTO and CdS/FTO substrates, respectively [17].The highest reported efficiency for an RFMS-grown Sb 2 Se 3 cell is 6.06%, but this required an additional post-selenization step [38].

Conclusions
The results presented in this work show simple strategies of interfacial engineering for Sb 2 Se 3 solar cells, which clearly improve their performance, and suggest viable architectures for Sb 2 Se 3 -based devices.The deposition of a NaF interface layer between the FTO back contact and Sb 2 Se 3 p-region can improve J SC : Na, migrating from the NaF layer to Sb 2 Se 3 can both passivate grain boundaries and favor carrier extraction through defect-assisted tunneling.The properties of the bi-layered absorber based on sputtered undoped Sb 2 Se 3 and PED-grown Cu:Sb 2 Se 3 sequentially grown in different thicknesses have also been analyzed.In particular, the RFMS-on-PED structure leads to a valuable maximum efficiency value = 5.25% when a very thin (maximum 100 nm) PED-grown layer is deposited onto NaF.The presence of this thin Cu-doped Sb 2 Se 3 layer favors the enhancement of V OC and FF, in comparison to a solar cell with a completely sputtered Sb 2 Se 3 p-region.While thicker PED-grown layers degrade the J SC , introducing a thin current blocking layer, a hole-electron charges separation limiting the photocarrier recombination and current leakage is obtained.
Sputtered and PED Sb 2 Se 3 Based Solar Cells In order to enhance photocarrier extraction at the FTO/Sb 2 Se 3 interface, a 10 nm NaF thin layer was deposited via PED on the FTO/glass surface prior to Sb 2 Se 3 deposition to examine its influence on photocarrier extraction.A schematic representation of the band diagram for the Sb 2 Se 3 -based solar cell with the NaF interlayer is shown in Figure 1.The enhanced p-doping effect of Na is attributed to (i) the substitution of Na for Sb at the interface (as Na presents a +1 oxidation state, while Sb +3); and (ii) the passivation of grain boundaries (GBs) and the associated defect states, as demonstrated in CIGS-based solar cells [29,30].According to several models, Na effectively removes donor defects at grain boundaries, thereby reducing recombination traps and enhancing the carrier concentration in their vicinity.The NaF layer also introduces a controlled level of local doping at the interface, facilitating carrier tunneling through localized defect states.This strategy has been previously employed in bifacial CIGS-based solar cells, where a NaF layer was inserted at the CIGS/AZO interface to enhance carrier extraction at the transparent back contact [31].

Figure 2 .
Figure 2. Illuminated current density-voltage (J-V) characteristics of (a) an RFMS-grown Sb2S lar cell with and without NaF interlayer; (b) a PED-grown Cu:Sb2Se3 solar cell with and withou interlayer.

Figure 2 .
Figure 2. Illuminated current density-voltage (J-V) characteristics of (a) an RFMS-grown Sb 2 Se 3 solar cell with and without NaF interlayer; (b) a PED-grown Cu:Sb 2 Se 3 solar cell with and without NaF interlayer.

Solar 2024, 4 ,Figure 4 .
Figure 4. (a) XRD patterns of the A-series solar cells based on PED-on-RFMS Sb2Se3 bilayer grown on FTO/soda-lime glass substrates; (b) histogram of the TC values of the fabricated samples for different (hkℓ) reflections.

Figure 4 . 10 Figure 5 ., 4 91Figure 5 .
Figure 4. (a) XRD patterns of the A-series solar cells based on PED-on-RFMS Sb 2 Se 3 bilayer grown on FTO/soda-lime glass substrates; (b) histogram of the T C values of the fabricated samples for different (hkℓ) reflections.

Figure 5 .
Figure 5. (a) XRD patterns of the B-series solar cells based on RFMS-on-PED Sb 2 Se 3 bilayer grown on FTO/soda-lime glass substrates; (b) histogram of the T C values of the fabricated samples for different (hkℓ) reflections.

Figure 6 .
Figure 6.(a) SEM micrograph of the Cu:Sb2Se3 surface of an RFMS-on-PED bilayer Sb2Se3 solar cell and (b) the corresponding cross-sectional image.

Figure 7 .
Figure 7. (a) SEM micrograph of the Sb2Se3 surface of a PED-on-RFMS bilayer Sb2Se3 solar cell and (b) the corresponding cross-sectional image.

Figure 6 .
Figure 6.(a) SEM micrograph of the Cu:Sb 2 Se 3 surface of an RFMS-on-PED bilayer Sb 2 Se 3 solar cell and (b) the corresponding cross-sectional image.

Figure 6 .
Figure 6.(a) SEM micrograph of the Cu:Sb2Se3 surface of an RFMS-on-PED bilayer Sb2Se3 solar cell and (b) the corresponding cross-sectional image.

Figure 7 .
Figure 7. (a) SEM micrograph of the Sb2Se3 surface of a PED-on-RFMS bilayer Sb2Se3 solar cell and (b) the corresponding cross-sectional image.

Figure 7 .
Figure 7. (a) SEM micrograph of the Sb 2 Se 3 surface of a PED-on-RFMS bilayer Sb 2 Se 3 solar cell and (b) the corresponding cross-sectional image.

Figure 10 .
Figure 10.The current density-voltage (J-V curve) of the best Sb 2 Se 3 cells obtained entirely via sputtering (Sample A1, black curve) and the bilayered RFMS-on-PED B4 structure (red curve).

Table 1 .
List of bilayered samples with PED-on-RFMS Sb2Se3 architecture and their corresponding layer thickness.

Table 2 .
List of bilayered samples with RFMS-on-PED Sb2Se3 architecture and their corresponding layer thickness.

Table 1 .
List of bilayered samples with PED-on-RFMS Sb 2 Se 3 architecture and their corresponding layer thickness.

Table 2 .
List of bilayered samples with RFMS-on-PED Sb 2 Se 3 architecture and their corresponding layer thickness.

Table 4 .
Average and record (between parentheses) device parameters of the different Sb 2 Se 3 -based solar cells.