Description:FIELD OF THE INVENTION
[0001] The present disclosure relates generally to the field of photoactive materials and optoelectronic device fabrication. Particularly, the present disclosure provides a sodium antimony sulfide-based material. Further, the present disclosure also provides a process for synthesis of sodium antimony sulfide-based material. The sodium antimony sulfide-based material of the present disclosure is used in the self-powered photodetector device.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] In recent years, chalcogenides have gained attention because of their physical characteristics and several pathways for creating fascinating new compositions for various technical applications. In general, non-toxic earth-abundant materials with band gaps between 1.1 eV and 3 eV are among the most potential candidates for research because of their low manufacturing costs and high theoretical efficiency [Joseph et al., Mater Res Bull., 2023, 168, 112465; Batabyal et al., Mater Lett. 2004, 58, 1–2; Senthilkumar et al., Journal of Solid State Electrochemistry 2018, 22 (11), 3331–3341]. It has gained huge attention in applications such as solar cells, supercapacitors, batteries, photodetectors, and sensors, etc., Alkali metal chalcogenides fall under the structure of MPnQ where M = Na, K, Cs, Rb, Pn = As, Bi, Sb, and Q = S, Se, Te [Barthwal et al., ACS Applied Energy Materials. 2022; Sophia et al., Advanced Optical Materials. 2022; Donne et al., Frontiers in Chemistry. 2019]. They are very promising for photovoltaic applications due to their earth abundance, and low toxicity. They also possess a mixed ionic and covalent bonding nature, similar to halide perovskites [Xia et al., Chinese Chemical Letters 2017, 28 (4), 881–887]. Some of the reported silver and copper-based ternary metal chalcogenides are AgBiS2, AgBi2S3, AgInS2, AgSbS2, CuSbSe, and CuSbS2 [Padwal et al., Heliyon, 2023, 9 (12)]. However, silver and copper-based materials are less considered potential photo absorbers as they have a significant influence on the band edges. Therefore, sodium-based metal chalcogenides act as a better alternative as they are proven to produce a progressive effect on the electrical performance of an electronic device. Some of Na chalcogenide systems include NaAsSe2, NaSbS2, NaBiTe2, and NaSbTe2 [Liton et al., Results Phys 2023, 55, 107192]. There are a variety of sodium precursors among which Na2S acts as a promising material for the synthesis of many sodium-based metal chalcogens. Sodium-based antimony sulfide has recently attracted attention as a potentially sustainable photo-absorbing material. In general antimony trisulfide is a light-absorbing semiconductor with a suitable bandgap of ~1.7 eV and a highest achieved efficiency of ~10% [Barthwal et al., Optik (Stuttg) 2023, 282; Courel et al., Solar Energy Materials and Solar Cells 2019, 201]. Despite Sb2S3 being a strong photoactive material for solar cells, the incorporation of Na into it has reduced the bandgap to a nearly ideal bandgap of ~1.5 to 1.6 eV which makes it preferable for PV applications. However, sodium antimony sulfide has some challenges as it exists in various phases namely NaSbS2, NaSbS, Na3SbS4, and Na2Sb4S7 etc. Among these NaSbS2 is an emerging semiconductor material discovered by Rahayu et al. [APL Mater 2016, 4 (11); Rasayan Journal of Chemistry 2021, 14 (4)] as an active layer for solar cells yielding an efficiency of 3.18% [Sun et al., IEEE J Photovolt 2018, 8 (4), 1011–1016]. NaSbS2 is proven to act as an ionic-electronic coupled semiconductor for switchable photovoltaic and neuromorphic device applications [Harikesh et al., Advanced Materials 2020, 32 (7)]. It also possesses polymorphism and excellent thermoelectric properties [Hua et al., Chemistry of Materials 2019, 31 (22)]. Still the other phases of sodium antimony sulfide and its controlled deposition remain unexplored. Among the other phases, Na2Sb4S7 is a novel semiconductor material with a direct band gap of ~1.5 to 1.6 eV. Having a bandgap of 1.6 eV, NaSbS2 and Na2Sb4S7 can perform as highly efficient photodetectors. Photodetectors are devices similar to solar cells that convert light energy into electrical energy. However, the development of self-powered nanotechnology that gathers energy from the environment to power electronic devices on their own is highly in demand. Due to their many benefits, self-powered nanodevices and nanosystems have garnered much attention in recent research years [Sun et al., Small 2017, 13 (28), 1–7]. Among the various self-powered devices available, self-powered photodetectors gain much attention due to their applications in remote monitoring, wireless surveillance, weather forecasting, and in power-scarce regions. Their photo-absorbing layers comprise low band gap semiconductors with high optical absorption. However, bulk-heterojunction photodetectors consist of a blend of two or more semiconductors which in turn form a heterojunction interface and produce electron-hole pairs when exposed to light. They are deposited by various solution processing methods namely chemical bath deposition, spray coating, sol-gel, successive ionic layer adsorption and reaction (SILAR), etc., Among these, the SILAR method is employed for thin film deposition due to its stability, uniformity, and controlled film deposition [Madhusudanan et al., Journal of Solid State Electrochemistry 2020, 24 (2), 305–311]. This method helps in the tuning or controlling of the stoichiometric ratios of thin film materials just by altering the cationic and anionic baths. It also allows for a controlled deposition of a pure phase material as well as a bulk heterostructure material in a single SILAR cycle. Moreover a range of fabrication factors such as precursor concentration, number of cycles, and growth temperature impact not only the optical and structural characteristics but also the electrical properties of the thin films. For attaining relevant phases of materials just by tuning the composition of the cationic and anionic precursors under ambient conditions, the SILAR method can be the best option [Ratnayake et al., Small 2021, 17 (49)]. Many complex phase formations of active material can be easily controlled by this method.
[0004] Advanced functional materials are in greater need to keep up with the demands of developing industries and technologies. The ongoing global challenges related to oil, gas, and electricity demand the development of innovative materials to address these critical problems. To solve these urgent issues, it is essential to gain a deeper insight into advanced materials and their diverse chemical properties. The existing cutting-edge energy conversion and storage technologies require novel materials with new features that can enable significant performance improvements.

OBJECTS OF THE INVENTION
[0005] An object of the present disclosure is to provide a sodium antimony sulfide-based material for self-powered photodetector.
[0006] Another object of the present disclosure is to provide a process for synthesis of sodium antimony sulfide-based material for self-powered photodetector.
[0007] Another object of the present disclosure is to provide a self-powered photodetector device.
[0008] Still another object of the present disclosure is to develop a novel bulk heterojunction material using two different phases of sodium antimony sulfide for photovoltaic applications.
[0009] Yet another object of the present disclosure is to provide a simple, cost-effective solution processing methods for the synthesis of a bulk heterojunction (BHJ) material for photovoltaic applications.

SUMMARY OF THE INVENTION
[00010] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
[00011] An aspect of the present disclosure provides a sodium antimony sulfide-based material for self-powered photodetector, wherein the material comprises two distinct phases of sodium antimony sulfide wherein the first phase is an intermediary NaSbS2 and the second phase is a bulk-heterojunction comprising of Na2Sb4S7/NaSbS2, wherein the both phases are deposited by a single successive ionic layer adsorption and reaction (SILAR) cycle.
[00012] Another aspect of the present disclosure provides a process for synthesis of sodium antimony sulfide-based material for self-powered photodetector, the process comprising: a) taking a substrate; b) dissolving 0.1 M of SbCl2 in a solvent to obtain a SbCl3 solution as the cationic solution; c) dissolving 0.4 M of Na2S in a solvent to obtain a Na2S solution as the anionic solution; d) taking a rinsing solvent in alternative container; e) immersing the substrate in the cationic solution for time period ranging for 1 to 5 minutes followed by rinsing with rinsing solution for a period ranging from 5 to 20 seconds to obtain a rinsed substrate; f) immersing the rinsed substrate in the anionic solution for a time period ranging from 1 to 5 minutes follow by rinsing with rinsing solution to remove excess unreacted and loosely bound ions for a period ranging from 5 to 20 seconds to obtain a film of heterostructure (Na2Sb4S7/NaSbS2) on substrate; g) immersing the film of heterostructure in the cationic solution again for the second SILAR cycle to produce a NaSbS2 film; and h) repeating the SILAR cycle to obtain desired thickness of the sodium antimony sulfide-based material.
[00013] Yet another aspect of the present disclosure provides a self-powered photodetector device comprising: a sodium antimony sulfide-based material selected from NaSbS₂ or a NaSbS₂/Na₂Sb₄S₇ heterostructure deposited over a FTO substrate and a carbon electrode.
[00014] Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
[00015] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[00016] Figure 1 illustrates schematic representation of the phase-controlled deposition of NaSbS2 and Na2Sb4S7/NaSbS2 heterostructure using cationic and anionic baths.
[00017] Figure 2 illustrates a) XRD pattern of NaSbS2 and Na2Sb4S7/NaSbS2 thin films and powder materials b) FESEM image of NaSbS2 thin film c) FESEM image of Na2Sb4S7/NaSbS2 thin film d) UV-DRS analysis of NaSbS2 and Na2Sb4S7/NaSbS2 thin films e) Kubelka Munk plot for NaSbS2 and Na2Sb4S7/NaSbS2 thin films f) Raman shift of NaSbS2 and Na2Sb4S7/NaSbS2 thin films.
[00018] Figure 3 illustrates XPS Analysis of NaSbS2 and Na2Sb4S7/NaSbS2 thin films a) and d) Sb 3d, b) and e) S 2p, c) and f) Na 1s core levels.
[00019] Figure 4 illustrates HRTEM image of the NaSbS2 a) HRTEM image at a magnification of 200 nm b) HRTEM image of a typical NaSbS2 cube c) SAED pattern d) magnified image of NaSbS2 at 20 nm e) Inverse FFT with lattice fringes f) Plot profile.
[00020] Figure 5 illustrates HRTEM image of the Na2Sb4S7/NaSbS2 a) HRTEM image at a magnification of 100 nm b) HRTEM image of a typical fused sphere c) SAED pattern d) Magnified image at 5nm e) f) and g) Inverse FFT with lattice fringes h) i) and j) corresponding plot profiles.
[00021] Figure 6 illustrates a) and b) Schematic representation of the device structure; c) Current-voltage (IV) characteristics under dark and light conditions of the photodetector device structures fabricated with NaSbS2 (orange) and Na2Sb4S7/NaSbS2 (brown) materials as the photoactive layer respectively.
[00022] Figure 7 illustrates Time-dependent photoresponse at zero voltage and zero current of a) and c) NaSbS2 b) and d) Na2Sb4S7/NaSbS2 at zero volts.
[00023] Figure 8 illustrates a) and b) rise and decay time of both phases NaSbS2 and Na2Sb4S7/NaSbS2 c) and d) intensity variation of both phases NaSbS2 and Na2Sb4S7/NaSbS2.
[00024] Figure 9 illustrates a) Relationship between varying intensities and photocurrent, b) and c) Responsivity, detectivity, and on-off ratio for both phases NaSbS2 and Na2Sb4S7/NaSbS2.
[00025] Figure 10 illustrates Time-dependent photoresponse at different wavelengths a) NaSbS2 b) Na2Sb4S7/NaSbS2 c) comparison voltage bias plot at 455 nm for both NaSbS2 and Na2Sb4S7/NaSbS2 devices.
[00026] Figure 11 illustrates a) and b) time-dependent photocurrent at zero bias under weak light illumination of NaSbS2 and Na2Sb4S7/NaSbS2 devices c) and d) Responsivity and detectivity at low-intensity of NaSbS2 and Na2Sb4S7/NaSbS2.
[00027] Figure 12 illustrates impedance spectroscopy of a) NaSbS2 b) Na2Sb4S7/NaSbS2 c) both NaSbS2 and Na2Sb4S7/NaSbS2 (inset: equivalent circuit).

DETAILED DESCRIPTION
[00028] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[00029] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[00030] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[00031] In some embodiments, numbers have been used for quantifying weights, percentages, ratios, and so forth, to describe and claim certain embodiments of the invention and are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[00032] The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[00033] Unless the context requires otherwise, throughout the specification which follows, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[00034] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[00035] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. Furthermore, the ranges defined throughout the specification include the end values as well, i.e., a range of 1 to 10 implies that both 1 and 10 are included in the range. For the avoidance of doubt, the applicant shall be entitled to any equivalents according to applicable law.
[00036] All methods described herein can be performed in a suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[00037] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.
[00038] The description that follows, and the embodiments described therein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.
[00039] It should also be appreciated that the present disclosure can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.
[00040] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[00041] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[00042] The term “or”, as used herein, is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[00043] The terms “weight percent,” “wt-%,” “percent by weight,” “% by weight,” and variations thereof, as used herein, refer to the concentration of a substance as the weight of that substance divided by the total weight of the composition and multiplied by 100. It is understood that, as used here, “percent,” “%,” and the like are intended to be synonymous with “weight percent,” “wt-%,” etc.
[00044] The term “self-powered photodetector”, as used herein, refer to a type of photodetector that can generate an electrical signal (photocurrent and/or photovoltage) in response to light exposure without the need for an external power supply or bias voltage. These devices typically operate using built-in electric fields, junction potentials, or asymmetrical electrode configurations to separate and transport photogenerated charge carriers.
[00045] The term “bulk-heterojunction”, as used herein, refer to a type of composite structure formed by intimately blending two or more materials with different electronic properties, typically a donor material and an acceptor material, to create a large interfacial area for efficient charge separation and transport.
[00046] The term “heterostructure”, as used herein, refer to a material system composed of two or more layers or phases of different materials or different crystal structures, typically with dissimilar electronic, optical, or chemical properties, that are joined together to form a single functional structure.
[00047] Various terms are used herein to the extent a term used is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[00048] The present disclosure overcomes the limitations of the prior art by providing a bulk heterostructure comprising two different phases of sodium antimony sulphide first time for self-powered photodetector applications. A bulk-heterojunction consisting of two different phases of sodium antimony sulfide Na2Sb4S7 and NaSbS2 was deposited by the SILAR method. The formation of the BHJ was achieved in a single SILAR cycle. The changes in the structural and optical properties of this BHJ semiconductor material were studied elaborately. The material was fabricated into self-powered photodetectors using cost-effective carbon electrodes (FTO/ {NaSbS2/ Na2Sb4S7} /Carbon). To determine the working of the photodetector, electrical characteristics were measured under dark and light conditions.
[00049] An embodiment of the present disclosure provides a sodium antimony sulfide-based material for self-powered photodetector, wherein the material comprises two distinct phases of sodium antimony sulfide wherein the first phase is an intermediary NaSbS2 and the second phase is a bulk-heterojunction comprising of Na2Sb4S7/NaSbS2, wherein the both phases are deposited by a single successive ionic layer adsorption and reaction (SILAR) cycle.
[00050] In some embodiment, the NaSbS2 exhibits an orange color having an X-ray powder diffraction pattern (CuKα) comprising peaks at 2θ = 27.42°, 31.78°, 45.5° and 56.5° and shows a cube like morphology whereas the Na₂Sb₄S₇/NaSbS₂ heterostructure exhibits a brown color having an X-ray powder diffraction pattern (CuKα) comprising peaks at 2θ = 22.05°, 26.72°, 30.87°, 31.78°, 37.1°, 38.9°, 44.3°, 45.5°, 52.40° and 59.3° and shows a mixture of small fused spheres and large cubes like morphology.
[00051] In some embodiment, the NaSbS₂ has a band gap ranging from 1.9 eV to 2.1 eV and the Na₂Sb₄S₇/NaSbS₂ heterostructure has a band gap ranging from 1.5 eV to 1.7 eV.
[00052] Another embodiment of the present disclosure provides a process for synthesis of sodium antimony sulfide-based material for self-powered photodetector, the process comprising: a) taking a substrate; b) dissolving 0.1 M of SbCl2 in a solvent to obtain a SbCl3 solution as the cationic solution; c) dissolving 0.4 M of Na2S in a solvent to obtain a Na2S solution as the anionic solution; d) taking a rinsing solvent in alternative container; e) immersing the substrate in the cationic solution for time period ranging for 1 to 5 minutes followed by rinsing with rinsing solution for a period ranging from 5 to 20 seconds to obtain a rinsed substrate; f) immersing the rinsed substrate in the anionic solution for a time period ranging from 1 to 5 minutes follow by rinsing with rinsing solution to remove excess unreacted and loosely bound ions for a period ranging from 5 to 20 seconds to obtain a film of heterostructure (Na2Sb4S7/NaSbS2) on substrate; g) immersing the film of heterostructure in the cationic solution again for the second SILAR cycle to produce a NaSbS2 film; and h) repeating the SILAR cycle to obtain desired thickness of the sodium antimony sulfide-based material.
[00053] In some embodiment, the substrate is selected from a group comprising of fluorine doped tine oxide (FTO), glass slide and indium tin oxide (ITO). Preferably, the substrate is FTO.
[00054] In some embodiment, the solvent in step b) and step c) is selected from a group comprising of ethanol, methanol, isopropyl alcohol and combination thereof. Preferably, the solvent in step b) and step c) is ethanol.
[00055] In some embodiment, the rinsing solvent in step d) is selected from a group comprising of ethanol, methanol, isopropyl alcohol and combination thereof. Preferably, the rising solvent is ethanol.
[00056] In some embodiment, the deposition ends after rinsing at the second container in step e) to obtain a single-phase NaSbS2 film.
[00057] In some embodiment, the deposition is halted after rinsing at the fourth container in step g) to obtain Na2Sb4S7/NaSbS2 film.
[00058] Another embodiment of the present disclosure provides a self-powered photodetector device comprising: a sodium antimony sulfide-based material selected from NaSbS₂ or a NaSbS₂/Na₂Sb₄S₇ heterostructure deposited over a FTO substrate and a carbon electrode.
[00059] The present disclosure focuses on synthesizing a Bulk Heterojunction (BHJ) material for photovoltaic applications via simple, cost-effective solution processing methods. Bulk heterojunction (BHJ) solar materials are a type of photovoltaic materials that feature a blended donor-acceptor active layer, allowing for efficient charge separation and transport. Unlike traditional planar heterojunctions, where charge separation occurs at a single interface, BHJ structures maximize the donor-acceptor interface by intermixing the two materials at the nanoscale, thereby increasing the charge carrier concentration. The formation of pure NaSbS2 nanoparticles was achieved only after annealing at 350ºC for 50 minutes. Whereas the present disclosure provides a controlled formation of two different phases of sodium antimony sulfide - NaSbS2 and a heterostructure of NaSbS2/Na2Sb4S7 achieved in a single Successive Ionic Layer Adsorption and Reaction (SILAR) cycle without annealing procedures. Both phases are formed in two distinct colors namely orange (NaSbS2) and brown (Na2Sb4S7/NaSbS2) and found to be two different material with different electronic properties. The band gap for both the phases was calculated to be 2.0 eV and 1.6 eV which lies in the ideal band gap region for a solar absorber. The reduction in band gap makes this material more ideal for solar cell applications. The material was fabricated into self-powered photodetectors using cost-effective carbon electrodes (FTO/ {NaSbS2/ Na2Sb4S7} /Carbon). Therefore, the material is novel compared to all the previously published articles.
[00060] Two photodetectors (FTO/NaSbS2/Carbon), (FTO/{NaSbS2/ Na2Sb4S7}/Carbon) were fabricated where both phases acted as the active layer with fluorine-doped tin oxide (FTO) and carbon as the other two electrodes. Both devices produced an outstanding photocurrent and photovoltage at zero bias conditions proving to work as excellent self-powered photodetectors. The devices were tested under 455 nm, 525 nm, 632 nm, and white light emitting diode (LED) light illuminations. The rise time and fall time under light irradiations were as rapid as 380 ms and 480 ms for the NaSbS2 device and 370 ms and 420 ms for the Na2Sb4S7/NaSbS2 device. The responsivity and detectivity for both the photodetectors at low intensities were found to be 0.89 mA/W and 3.5 mA/W and 8.8 × 109 Jones and 4.7 × 1010 Jones respectively.
[00061] Antimony Trichloride (98.5% Loba Chemie), Sodium sulfide flakes purified (Na2S.xH2O) (Merck), Distilled ethanol, n-methyl-2-pyrrolidone extra pure (NMP) (99.5% - SRL), Carbon (Sigma Aldrich), polyvinylidene fluoride (PVDF) (99% - Sigma Aldrich), Graphite ( > 20 μm - Sigma Aldrich), FTO slides (Sigma Aldrich), Double distilled (DD) water was used throughout the entire study.
[00062] The antimony trichloride and sodium sulfide solution act as the cationic and anionic precursors for the film deposition. The presence of excess sodium or antimony ions may affect the relative ratios of elements in the deposited thin films, leading to the formation of different phases - NaSbS2 and Na2Sb4S7/NaSbS2. This deposition method involves a complex interplay of chemical reactions, structural reconstruction, and ionic adsorption. In this work, the deposition to favor two different phases was carried out by halting the deposition process after the cationic bath to obtain NaSbS2 and later ending the deposition process after the anionic bath to obtain Na2Sb4S7/NaSbS2 film. Generally, coating thin films by SILAR method deals with the adsorption and desorption of ions. However, in this case, structural rebuilding of two different phases of sodium antimony sulfide occurs due to the chemical reactions that take place during the deposition. The cationic or Sb-rich precursor tends to favor a single-phase NaSbS2 and higher ratios of sodium precursor promote the formation of a heterostructure Na2Sb4S7/NaSbS2. The cationic bath is rich in Sb3+ ions and the anionic bath is rich in Na+ and S2- ions. In the SILAR deposition, during the first immersion, a very fine transparent layer forms on the substrate surface by the deposition of antimony cations. A positively charged layer is formed by cations interacting with the substrate surface. This gives rise to a Helmholtz electric double layer when a secondary anion layer builds on top of the cation layer. Following this, the double layer is reduced by the next rinse process, which removes surplus ions from the surface to a hypothetical positively charged monolayer of the cationic precursor. While this would be the ideal result, it is more likely that some of the counter ions (Cl-) from the first bath will remain on the substrate and enter the anionic bath as contaminants.34 The reaction between the Sb3+/Cl- ions and Na⁺/S²⁻ ions takes place at the interface between the substrate and the liquid. The excess Cl- ions that enter the anionic precursor react with Na+ and form NaCl.
[00063] During each cycle, some of the sodium ions from the adsorbed layer on the substrate surface may exchange with antimony ions from the newly adsorbed layer, resulting in the integration of sodium into the antimony sulfide film. During this reaction, sodium can be incorporated into the antimony sulfide lattice via chemical reactions. A thin layer of brown product, which is insoluble in the solvent, is deposited over the substrate. However, there’s a possibility for the presence of loose products, byproducts, and residual anions after the last rinsing step.
[00064] To explain more elaborately to obtain a NaSbS2 film, the substrate is first immersed in the cationic solution rich in Sb3+ ions. When immersed in the anionic precursor, the adsorbed Sb3+ ions from the SbCl3 solution react with the S2- ions from the Na2S solution and form an Sb-S bond to which Na+ ions can be incorporated. Alongside, the Cl- ions react with the Na+ to form NaCl.
----------------------- (1)
-------------------------------(2)
The above equations (1) and (2), prove the formation of the orange phase NaSbS2.
[00065] During the second SILAR cycle, upon immersion, ions from the antimony-rich solution begin to exchange with ions present in the brown film. This process involves the structural reconstruction across the film-solution interface driven by concentration gradients. Moreover, in the second SILAR cycle, the excess Sb3+ ions and S2- ions can also react to form an intermediate SbS33-. This orange film when immersed into the Na2S solution again can form Na2Sb4S7 on the surface.
--------------- (3)
-----------------------(4)
[00066] As seen in equations (3) and (4), the formation of Na2Sb4S7 is feasible on the surface of NaSbS2 when there is an excess of Na ions. This can lead to a heterostructure formation of Na2Sb4S7/NaSbS2. This is visually seen as the orange color of NaSbS2 that forms in every SILAR cycle when immersed in the cationic precursor. Subsequently, after washing when immersed in the anionic precursor rich in Na+ ions, it undergoes structural reconstruction and the brown color of Na2Sb4S7/NaSbS2 is achieved. This structural reconstruction and switching between two phases occur in every SILAR cycle. When the substrate is immersed into the anionic solution, along with the formation of Na2Sb4S7/NaSbS2, the counter ions from the cationic beaker Cl- also react with the Na+ ions and form NaCl crystals. The NaCl crystals do not hinder the formation of both orange and brown films. Finally, the dried thin film is washed with double distilled water to remove the NaCl crystals. The sequential deposition of two different semiconductor materials onto the substrate facilitates the formation of a pure phase and a heterostructure. This interplay of the Na-rich and Na-deficit antimony sulfide alters the structural, morphological, and optoelectronic properties. To understand more about the NaSbS2 and Na2Sb4S7/NaSbS2 phases, the thin films were subjected to material characterizations.
[00067] Equation (5) is used to determine the light-to-dark current ratio under zero bias based on the rapid increase in photocurrent.
RON/OFF = (Iph - Id) / Id ---------------- (5)
Where Iph and Id are the photocurrent and dark current respectively.
[00068] The responsivity of the device was calculated for both the NaSbS2 and Na2Sb4S7/NaSbS2 devices using the equation (6),
R = (Iph – Id) / (P * A) ------------ (6)
where Iph and Id are the currents under light and dark conditions, P is the input power and A is the active area of the material that is exposed to light.
[00069] From equation (7), the detectivity values are calculated using the formula,
Ds = , ---------------- (7)
where A is the effective device area exposed to light, ‘e’ is the charge of the electron, and R is the responsivity of the device.
[00070] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
EXAMPLES
[00071] The present invention is further explained in the form of following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention.
Example 1
(a) Method of preparation of sodium antimony sulfide-based material by SILAR method
[00072] The FTO/glass slides were ultra-sonicated for thorough cleansing of the substrates. The substrates were cleaned using soap water, double distilled water, acetone, and isopropyl alcohol (IPA) each for 15 minutes. After cleaning, the slides were dried at 80 °C for half an hour.
[00073] Using the SILAR technique, two phases of sodium antimony sulfide were deposited onto the substrates. 0.1 M of SbCl3 and 0.4 M of Na2S dissolved in distilled ethanol were used as the precursor solutions. The first beaker containing antimony trichloride was used as the cationic solution, and the third beaker containing the sulfur source was used as the anionic solution. Alternate beakers containing distilled ethanol were used as rinsing solutions. Firstly, the cleaned substrate was immersed in the cationic solution for a minute and later rinsed in the second beaker for 10 seconds. The rinsed substrate was then immersed in the third beaker containing anionic solution for a minute. Finally, the excess unreacted and loosely bound ions were rinsed in the fourth beaker. A heterostructure of undissolved material Na2Sb4S7/NaSbS2 forms on the substrate. This film was again immersed in the cationic precursor for the second SILAR cycle, producing a NaSbS2 film. To obtain a Na2Sb4S7/NaSbS2 film, the deposition is halted at the fourth beaker, and for the single-phase NaSbS2, the deposition ends after rinsing at the second beaker. This process was repeated for 10 SILAR cycles to achieve the desired thickness of the material. The deposited NaSbS2 and Na2Sb4S7/NaSbS2 films appeared in orange and brown color in every SILAR cycle. Figure 1 shows the steps involved in the deposition of the NaSbS2 and Na2Sb4S7/NaSbS2 films by the SILAR method.
(b) Synthesize the NaSbS2 and Na2Sb4S7/NaSbS2 powder
[00074] To synthesize the NaSbS2 and Na2Sb4S7/NaSbS2 powder samples, 0.1 M of SbCl3 and 0.4 M of Na2S in 25 ml of distilled ethanol were stirred separately. To prepare the orange powder, 5 ml of Na2S was added to 25 ml of SbCl3 solution, and to prepare the brown powder, 5 ml of SbCl3 was added to 25 ml of Na2S solution. Both the solutions were stirred for half an hour, and the orange and brown precipitate formed was washed thoroughly with double distilled water, centrifuged, and dried at 60 °C. To prepare a thin film using the powder materials, both the orange and brown color powders were made into a paste using NMP as the solvent and PVDF as the binder. The paste was doctor-bladed onto a pre-cleaned FTO substrate.
(c) Carbon paste fabrication
[00075] To prepare a carbon paste for the electrode material in the photodetector devices, polyvinylidene fluoride, conducting carbon, and graphite were taken in a ratio of 0.1:2:1. A homogenous solution of PVDF in NMP was prepared to which the carbon graphite mixture was added. This mixture was grinded thoroughly into a fine paste which was then coated on top of the photo-active material.
(d) Characterizations
[00076] To determine the different phases of the materials, the X-RAY Diffraction was analyzed with an Ultima - IV Model X-Ray diffractometer Rigaku with Cu kα irradiation. The surface morphologies of the SILAR-deposited NaSbS2 and Na2Sb4S7/NaSbS2 films were identified by Carl Zeiss SEM 300, Germany. The UV - diffused reflectance spectroscopy (UV-DRS) to determine the reflectance and the band gap of the material was observed from JASCO UV-DRS V-750. The X-ray photoelectron spectroscopy analysis was done by PHI VersaProbe lll instrument. The FEI Tecnai F20 Transmission Electron Microscope (FEI Company, USA) was used for TEM analysis. All the electrical measurements for the photodetector were carried out with a Keithley - 2450 source measuring unit.
[00077] The crystalline structure of both the brown and orange materials has been studied using XRD. Figure 2a shows the XRD pattern of NaSbS2 (orange) and Na2Sb4S7/NaSbS2 (brown) thin films and powder samples. The pattern of the sample with the orange film shows intense peaks at 2θ = 27.36°, 31.74° and 45.4° and the brown film at 2θ = 22.08°, 26.7°, 30.9°, 31.6°, 44.39°, 45.55° with monoclinic crystal structures. Both the phases match perfectly with the JCPDS card no. 00-010-0023 and 01-071-0498. The XRD pattern of only powder samples with lattice plane indexed showed 2θ 27.42, 31.78, 45.5, 56.5 for orange powder and 2θ = 22.05, 26.72, 30.87, 31.78, 37.1, 38.9, 44.3, 45.5, 52.40, 59.3 for brown powder. The orange powder was obtained by mixing a higher concentration of SbCl3 and a lesser concentration of Na2S and to obtain the brown powder, the mixing was carried out vise versa. The precipitate was collected, thoroughly washed, and dried before characterization. As the present disclosure focused on the thin film samples, further characterizations were carried out on thin film samples. The crystallite size D was determined using the Debye-Scherrer formula, D = kλ/(βcosθ), where k is the shape factor 0.9, the wavelength of the X-ray source is λ = 1.54 Å, and θ is Bragg's angle. The crystallite size (D), dislocation density (ẟ), and microstrain (ε) for both NaSbS2 and Na2Sb4S7/NaSbS2 films were determined using D = kλ/(βcosθ), δ=1/D2 and ε = β/(4tanθ), where k is the shape factor 0.9, D is the crystallite size and β is the full-width half maximum of the peaks. The crystallite size for NaSbS2 ranges between 24 to 48 nm and for the heterostructure Na2Sb4S7/NaSbS2 the size reduces to 8 to 21 nm.
[00078] FESEM analyses were conducted to understand the morphology of both the NaSbS2 and Na2Sb4S7/NaSbS2 thin films. As seen from Figure 2b, the NaSbS2 film shows a very defined cubes-like morphology. The Na2Sb4S7/NaSbS2 film shows a mixture of small fused spheres and large cubes proving the heterostructure formation as shown in Figure 2c. However, the film comprised mainly of small fused spheres offering a high surface area compared to the pure phase NaSbS2. The surface reconstruction occurred in every SILAR cycle during the cationic-anionic reaction process and thereby control of the desired phase can be easily achieved. The morphological variations seen between the two samples are most likely caused by a combination of parameters, which include composition, ionic interactions, crystal structure, surface energy, nucleation, and growth kinetics. The EDS spectra prove the presence of sodium in both phases which makes up to ~2.5 % in NaSbS2 and ~25% in Na2Sb4S7/NaSbS2 films respectively. On contrary to the thin film, the morphologies of powder samples differed hugely. The NaSbS2 powder samples showed a mixture of flakes and clustered spheres-like morphology. The Na2Sb4S7/NaSbS2 powder samples showed a mixture of clustered spheres and some presence of NaCl crystals. The presence of NaCl crystals can be due to the residual crystals even after thorough washing with distilled water. The morphologies of powder materials differed from thin films due to the instant precipitation during powder material synthesis. However, in the case of thin films, the reaction process is slow in SILAR method. From the EDS spectra, the atomic percentages of Na, Sb, and S for Na2Sb4S7/NaSbS2 powder samples were 57%, 14%, and 28%. For the NaSbS2 powder samples the atomic percentages of Na, Sb, and S were 37%, 28%, and 33%.
[00079] Sodium antimony sulfide is a material that offers excellent optical properties. Therefore, to determine the band gap and the optical nature of both phases, UV-diffused reflectance spectroscopy was recorded. Figures 2d and 2e show the reflectance spectra and Kubelka Munk plot of both the NaSbS2 and Na2Sb4S7/NaSbS2 thin films. The reflectance spectra and the band gap variations were analyzed in both films. The orange and brown films showed two different absorption peaks at 510 and 605 nm, respectively, proving that both phases offer different optical behavior. The band gap of the two materials was calculated using Kubelka Munk theory. The optical band gap was found to be 2.09 eV and 1.65 eV for NaSbS2 and Na2Sb4S7/NaSbS2 semiconductor materials. Conversely, the NaSbS2 powder materials showed similar absorption peaks at 510 nm but the Na2Sb4S7/NaSbS2 powder had different absorption peaks at 583 nm compared to the thin film. The band gap of the NaSbS2 powder sample remained the same at 2.09 eV but the band gap of the Na2Sb4S7/NaSbS2 sample increased slightly to 1.7 eV from 1.65 eV. As these materials fall into the category of an ideal bandgap material for PV applications, they can act as an efficient active layer for visible light photodetectors. Figure 2f shows the Raman spectroscopy analysis for NaSbS2 and Na2Sb4S7/NaSbS2 thin films. It can be seen that from Figure 2f the peaks located at 291.89 cm-1 and 302.42 cm-1 correspond to the NaSbS2 and Na2Sb4S7/NaSbS2 materials. The slight peak shift to a higher wavenumber from 291 cm-1 in NaSbS2 to 302 cm-1 in Na2Sb4S7/NaSbS2 may imply the complex bonding structure and the influence of the additional sulfur and antimony atoms in the Na2Sb4S7/NaSbS2 material. They possibly reflect more stronger or constrained Sb-S and S-S bonds in both compounds. In the case of powder materials, there is a slight shift in the peaks from 302.42 cm-1 to 306.59 cm-1 in the NaSbS2 powder sample and from 291.89 cm-1 to 295.36 cm-1 in the Na2Sb4S7/NaSbS2 powder samples as seen from Figure S5c. The shift in peaks also highlights the transformations in bonding environments and crystal structures between the two materials.
[00080] The samples were subjected to X-ray photoelectron Spectroscopy analysis as seen in the survey spectra in Figure 3 to understand the chemical structure and oxidation states of the thin film samples. In NaSbS2, the elemental spectra of Sb 3d, S 2p, and Na 1s are seen in Figures 3a, 3b, and 3c. The Sb0 3d3/2 and Sb0 3d5/2 peaks are at 539.91 eV and 530.01 eV. In elemental spectra, S 2p1/2 and S 2p3/2 doublet are seen at 162.23 eV and 161.22 eV. O 1s peak was found at 530.93 eV due to the surface interaction. Na 1s peak found at 1072.28 eV confirms the presence of sodium in the material. In the case of Na2Sb4S7/NaSbS2 the elemental spectra of Sb 3d, S 2p, and O 1s are observed as seen in Figures 3d, 3e, and 3f. The peaks of Sb0 3d3/2 and Sb0 3d5/2 are seen at 539.86 eV and 534.38 eV. Also, Sb3+ 3d3/2 and Sb3+ 3d5/2 peaks are evident at 537.79 eV and 528.67 eV. The Sb3+ doublet peak is seen at 536.51 eV and 528.64 eV. The S 2p1/2 and S 2p3/2 peaks are seen at 162.57 eV and 161.23 eV. O 1s surface oxidation peak is seen at 530.55 eV. S-Ox is seen at 168.97 eV due to surface oxidation. The sodium presence is very evident and confirms that excess sodium is present in sample Na2Sb4S7/NaSbS2.
[00081] The HRTEM images of NaSbS2 from Figure 4a confirm the cube-like morphology. Figure 4b shows the magnified image of the NaSbS2 cube at 10 nm which confirms the lattice fringes with a d-spacing of d = 0.2857 nm corresponding to the (002) plane of the NaSbS2 phase indicating its prominent crystalline structure. Figure 4c presents the SAED pattern confirming the crystal structure of the orange phase where the diffraction point patterns index to (002), (-202), and (-223) planes of the NaSbS2 respectively. Moreover, the SAED pattern also proves that the orange phase is attributed to a single crystalline structure. Figure 4d shows a magnified image of the material at 20 nm. Figures 4e and 4f show the lattice fringe pattern considered for FFT and the corresponding plot profile.
[00082] Unlike the HRTEM images of NaSbS2, the morphology of Na2Sb4S7/NaSbS2 was rather different. Figure 5a confirms the formation of fused spheres at a magnification of 100 nm. Figure 5b shows the magnified image of a fused sphere at 5nm which resolves the lattice fringes of both Na2Sb4S7 and NaSbS2 confirming the formation of a heterostructure. The lattice fringe pattern matches with the d-spacing values of d = 0.2857 nm, 0.2972 nm, and 0.21 nm respectively. The SAED point pattern in Figure 5c further confirms the formation of a heterostructure between NaSbS2 and Na2Sb4S7. The SAED patterns index to the planes (002) and (-223) of Na2Sb4S7 and NaSbS2 respectively prove to form a polycrystalline heterostructure material. Figure 5d shows a magnified image of the fused sphere at 5 nm magnification. Figures 5e, 5f, and 5g show the lattice fringe pattern considered for FFT, and Figures 5h, 5i, and 5j show its corresponding plot profiles.
(E) Device
[00083] To determine the electrical characteristics of NaSbS2 and Na2Sb4S7/NaSbS2 phases of sodium antimony sulfide, the materials were deposited over a fluorine-doped tin oxide (FTO) glass substrate with carbon as the other electrode. Considering the variation of work functions of the asymmetric electrodes to the active material as an advantage, the generation of charge carriers due to the in-built potential can be achieved. Two photodetectors were fabricated with NaSbS2 (orange) and Na2Sb4S7/NaSbS2 (brown) materials as the photoactive layer. The photo-generated charge carriers produced during light irradiation were collected by FTO and carbon to produce photocurrent. FTO acts as the electron-collecting layer and carbon as the hole-collecting layer. Figures 6a and 6b show the schematic representation of the device structures. The sequential deposition of the cationic and anionic precursors as layers results in the formation of interfaces or junctions where the characteristics of the materials abruptly change. These junctions are crucial for photodetection because they operate as active sites where charge carriers are generated and separated when exposed to light. The electrical performance of the self-powered photodetector devices was evaluated under ambient conditions.
[00084] The device’s current-voltage characteristics in the dark and under 100mW/cm2 white LED irradiation at a voltage sweep of -1 to +1 V are shown in Figure 6c. On comparing the currents in the dark and light, illumination causes an increase in the currents. At a voltage sweep of -1 V to +1 V, the photocurrent obtained was 1.2 mA and 4 mA for NaSbS2 and Na2Sb4S7/NaSbS2 films. There is certainly some asymmetry and nonlinearity between the forward and reverse biases. In the case of the powder material device, at a voltage sweep of -1 to +1 V, the photocurrent obtained was 0.3 mA and 0.6 mA for NaSbS2 and Na2Sb4S7/NaSbS2. According to the literature, if a photon with energy higher than the active material's band gap strikes the material's surface of a photodetector, it gets absorbed and produces electron-hole pairs with an exciton binding energy greater than hundreds of meV. When light strikes the NaSbS2 and Na2Sb4S7/NaSbS2 films, electrons are excited from the valence band into the conduction band, leaving positively charged holes in the valence band. Heterostructures are formed when two or more distinct semiconductors or materials with different properties are deposited in a controlled manner. Here, the consecutive deposition of two different phases results in the formation of a bulk heterojunction comprising two phases of NaSbS2 and Na2Sb4S7. These phases differ in their electrical properties and band gaps. The interface between these semiconductor materials possesses unique electronic and optical properties that do not match with the individual materials. Due to the formation of a heterojunction between two phases in the case of Na2Sb4S7/NaSbS2 film, there occurs an electric field gradient i.e., variation in the electric field strength across the interface of the different semiconductor materials. This gradient is essential for determining photo-generated charge carriers, which enhances the efficiency of a photodetector. Generally, in self-powered heterojunction photodetectors, the space charge electric field across the interface junction provides the driving force to separate the photogenerated excitons to electron and hole transport layers. The built-in electric field forms at the heterojunction due to differences in band alignment between the materials. This field aids in separating electron-hole pairs, generating a measurable photocurrent without an external power source. The band alignment for the junction depends on the materials used, typically resulting in a type-II heterojunction. Here, one material has a narrower bandgap and another wider bandgap, creating a staggered alignment of band edges. This built-in electric field facilitates efficient charge separation, improving photodetector performance in Na2Sb4S7/NaSbS2 films.
[00085] Under unbiased conditions, a device that works in the presence of light to produce photocurrent due to its in-built potential is a self-powered device. A self-powered device’s efficiency relies on two important parameters namely repeatability and operational stability. To further analyze the transient response characteristics of the self-powered PDs, the time-dependent current characteristic is recorded at regular switching on and off light. Figures 7a and 7b display the time-dependent photocurrent response of the detector using a 10 s square pulse of the white light and an intensity of 100 mW/cm2. Although the material's current-boosting abilities were cut off due to the absence of external bias, the photodetector still produced a current of 20 µA for the Na2Sb4S7/NaSbS2 device and 1.5 µA for the NaSbS2 device. In the absence of external bias, the powder sample devices also showed a photocurrent of 225 nA and ~ 0.9 µA proving that the powder samples also exhibit light response. The current in the powder sample devices was compromised due to the addition of a binder. Moreover, the devices behaved like a photovoltaic system by producing a voltage of 11 mV and 1 mV at zero current as seen in Figures 7c and 7d. It can be seen that the photocurrent and photovoltage for the Na2Sb4S7/NaSbS2 dominate the NaSbS2 material. When the light illumination is switched on and switched off, the photogenerated current and voltage instantly increase to a stable value and decrease quickly proving its excellent stability and repeatability. The I-V characteristics, the time-dependent photocurrent, and the photovoltage of only carbon are provided, it can be seen that the carbon shows some photocurrent and photovoltage which is very negligible compared to the current obtained from the photodetector devices. Moreover, the response time for only carbon devices is very huge and is incomparable to the original photodetector devices. Therefore, it is very prominent that the photocurrent and voltage obtained in the photodetector devices at zero bias conditions were the cause of the active layers of NaSbS2 and Na2Sb4S7/NaSbS2.
[00086] The time-dependent measurements that determine the slow response decrease were observed in both the photocurrent and photovoltage. From the ON/OFF curves seen in Figure 7, the shape of the photocurrent or photovoltage transients can be broken down into four distinct parts; a rapid increase in the photocurrent or photovoltage in the order which is less than one second, a slow exponential decrease under constant irradiation i.e., the slow response current (SRC), and quick decay of less than one second that appears as a negligible negative value when the illumination is turned off and then gradually approaches the dark-current value. During illumination, the photocurrent shoots rapidly as a result of the instantaneous generation of the photocarriers, its separation, and collection at either of the electrodes. The photocurrent and the photovoltage gradually drop with time under illumination following the first sharp spike; this area of the transient response is the SRC which can be due to the interface properties rather than the bulk of the active material.
[00087] Another crucial parameter for a device to behave as a photodetector is its response time. The response time is the rise and fall time for the device to achieve 90% of its saturated photocurrent and the time it takes for the device to elapse from 10% of its current to reach 90% of its current under dark conditions. Figures 8a and 8b show the rise and decay time for both NaSbS2 and Na2Sb4S7/NaSbS2 devices. The rise time and fall time calculated for the Na2Sb4S7/NaSbS2 film are 0.37 s and 0.42 s, whereas, for the NaSbS2 film, the rise and fall time is slightly lower than that of the Na2Sb4S7/NaSbS2 film, which is 0.38 s and 0.48 s.
[00088] Time-dependent photocurrent measurements were recorded to determine the behavior of both films under varying illuminations. The intensity of the LED light was varied between 100 mW/cm2 to 21 mW/cm2. It can be seen from Figures 8c and 8d that the photocurrent at zero bias increases with intensity for both the photodetectors. When the photocurrent is plotted against the varying intensity, it can be seen that they are proportional to each other. Figure 9a shows the relationship between time-dependent photocurrent and varying intensity. As light intensity increases photocurrent also increases.
[00089] Figures 9b and 9c show the plot between irradiance vs responsivity, detectivity, and ON-OFF ratio. The calculated ON/OFF ratio for both the NaSbS2 and Na2Sb4S7/NaSbS2 films are 900 and 1600. Among these two, the Na2Sb4S7/NaSbS2 device shows a higher ratio compared to NaSbS2.
[00090] A photodetector's other crucial parameters are responsivity (R) and detectivity (D). Responsivity is the ratio of the photocurrent generated by the device to the incident optical power. The calculated responsivity for both devices is 24 µA/W and 205 µA/W. From the obtained photocurrent values, the values of the Na2Sb4S7/NaSbS2 device are higher than the NaSbS2 devices which is reflected in the responsivity and detectivity plot as the R-value depends on the photocurrent. Another crucial parameter for a photodetector is its Detectivity (D) which is a measure of detecting weak minor optical signals in the presence of noise.
[00091] The estimated D values for both NaSbS2 and Na2Sb4S7/NaSbS2 films are 4 × 108 and 3.2 × 109 Jones. From Figures 9b and 9c, it can be seen that the R and D values increase as the incident light power density increases in the case of the Na2Sb4S7/NaSbS2 device. This can be attributed to the increase in the density of photo-generated charge carriers that are effectively separated by the in-built potential at the interface junctions. However, in the case of NaSbS2 films, the presence of trap states in the interface junctions and photoactive layer may lead to the recombination of photo-generated excitons which results in the gradual decrease of R and D values as light intensity increases.
[00092] To determine the working of the photodetectors at different wavelengths of light, the devices were subjected to 455 nm, 525 nm, and 632 nm irradiations. Figures 10a and 10b show the time-dependent photocurrent at red, green, and blue wavelengths of light. Interestingly, it can be seen that the devices produced photocurrent under RGB light wavelengths proving to work under different wavelength conditions without any external bias. The devices show a better response at the lower wavelength of 455 nm (blue) compared to the higher wavelength of 632 nm (red). Figure 10c shows the photoresponse comparison graph of only 455 nm of both the NaSbS2 and Na2Sb4S7/NaSbS2 devices.
[00093] To examine the working of the self-powered photodetector under very low light illuminations, both devices were subjected to very low intensities. Figures 11a and 11b show the varying time-dependent photoresponse at zero volts for NaSbS2 and Na2Sb4S7/NaSbS2 devices. At low intensities of 12 mW/cm2 to ~120 µW/cm2, the devices produced photocurrent without any external bias proving that the devices work as self-powered photodetectors even at very low intensities of light. In this case, as the input optical signal is feeble, the generated electron-hole pairs produce a shallow electrical signal. However, the response time was not compromised even at very low intensities of light. Though the responsivity and detectivity values for higher intensities as seen in Figures 9b and 9c had different increasing and decreasing trends, at lower intensities it was seen that both the devices showed a decreasing trend with increasing light intensities. The calculated responsivity and detectivity values for both devices are 0.89 mA/W, 3.5 mA/W, 8.8 × 109, and 4.7 × 1010 Jones. In Figures 11c and 11d, it is determined that the responsivity and detectivity values decrease with an increase in irradiance at very low intensities. All other antimony compounds are established with an electron and a hole transport layer (ETL and HTL) to enhance their efficiency. Therefore, incorporating ETL and HTL layers can also increase the efficiency of sodium antimony sulfide materials.
[00094] An impedance analysis was recorded to determine the electrical behavior of both devices. Figures 12a and 12b show the solid-state impedance analysis of NaSbS2 and Na2Sb4S7/NaSbS2 under both dark and light conditions. The shunt resistance Rs was calculated after the equivalent circuit fitting for the devices. Interestingly, both NaSbS2 and Na2Sb4S7/NaSbS2 photodetector devices exhibited similar equivalent circuits. Figure 12c shows the difference between the impedance curves of both devices under dark and light conditions. The inset in Figure 12c shows the equivalent electrical circuit model. The shunt resistance Rs was calculated to be 204 Ω in the dark and 175 Ω in the light conditions for the NaSbS2 device. However, Rs values change in the case of Na2Sb4S7/NaSbS2 device. The device exhibits a shunt resistance of 127 Ω in the dark condition and 123 Ω in the light condition. There is a drop in the resistance values in both photodetectors in the light condition due to the generation of photogenerated charge carriers which contributes to the overall photocurrent.
[00095] The foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.

ADVNATEGES OF THE INVENTION
[00096] Two phases of sodium antimony sulfide NaSbS2 and Na2Sb4S7/NaSbS2 were deposited in a single SILAR cycle.
[00097] These phases possessed a band gap that lay near the ideal band gap of a solar absorber, these materials were fabricated into two photodetectors with FTO and Carbon as the other two electrodes.
[00098] These photodetectors showed excellent photocurrent and photovoltage in the absence of an external electric field proving to work as a self-powered photodetector.
[00099] Under white light, 455 nm, 525 nm, and 632 nm illumination, the NaSbS2 and Na2Sb4S7/NaSbS2 photodetectors showed a quick rise and decay time as rapid as 380/480 ms and 370/420 ms. The heterostructure showed an on-off ratio of 1600 which is much higher compared to the on-off ratio of NaSbS2. The responsivity and detectivity for both the photodetectors at low intensities were found to be 0.89 mA/W and 3.5 mA/W and 8.8 × 109 Jones and 4.7 × 1010 Jones.
, Claims:1. A sodium antimony sulfide-based material for self-powered photodetector, wherein the material comprises two distinct phases of sodium antimony sulfide wherein the first phase is an intermediary NaSbS2 and the second phase is a bulk-heterojunction comprising of Na2Sb4S7/NaSbS2, wherein the both phases are deposited by a single successive ionic layer adsorption and reaction (SILAR) cycle.
2. The material as claimed in claim 1, wherein the NaSbS2 exhibits an orange color having an X-ray powder diffraction pattern (CuKα) comprising peaks at 2θ = 27.42°, 31.78°, 45.5° and 56.5° shows a cube like morphology whereas the Na₂Sb₄S₇/NaSbS₂ heterostructure exhibits a brown color having an X-ray powder diffraction pattern (CuKα) comprising peaks at 2θ = 22.05°, 26.72°, 30.87°, 31.78°, 37.1°, 38.9°, 44.3°, 45.5°, 52.40° and 59.3° and shows a mixture of small fused spheres and large cubes like morphology.
3. The material as claimed in claim 1, wherein the NaSbS₂ has a band gap ranging from 1.9 eV to 2.1 eV and the Na₂Sb₄S₇/NaSbS₂ heterostructure has a band gap ranging from 1.5 eV to 1.7 eV.
4. A process for synthesis of sodium antimony sulfide-based material for self-powered photodetector, the process comprising:
a) taking a substrate;
b) dissolving 0.1 M of SbCl2 in a solvent to obtain a SbCl3 solution as the cationic solution;
c) dissolving 0.4 M of Na2S in a solvent to obtain a Na2S solution as the anionic solution;
d) taking a rinsing solvent in alternative container;
e) immersing the substrate in the cationic solution for time period ranging for 1 to 5 minutes followed by rinsing with rinsing solution for a period ranging from 5 to 20 seconds to obtain a rinsed substrate;
f) immersing the rinsed substrate in the anionic solution for a time period ranging from 1 to 5 minutes follow by rinsing with rinsing solution to remove excess unreacted and loosely bound ions for a period ranging from 5 to 20 seconds to obtain a film of heterostructure (Na2Sb4S7/NaSbS2) on substrate;
g) immersing the film of heterostructure in the cationic solution again for the second SILAR cycle to produce a NaSbS2 film; and
h) repeating the SILAR cycle to obtain desired thickness of the sodium antimony sulfide-based material.
5. The method as claimed in claim 4, wherein the substrate is selected from a group comprising of fluorine doped tine oxide (FTO), glass slide and indium tin oxide (ITO).
6. The method as claimed in claim 4, wherein the solvent in step b), step c) and step d) is selected from a group comprising of ethanol, methanol, isopropyl alcohol and combination thereof.
7. The method as claimed in claim 4, wherein the rinsing solvent in step d) is selected from a group comprising of ethanol, methanol, isopropyl alcohol and combination thereof.
8. The method as claimed in claim 4, wherein the deposition ends after rinsing at the second container in step e) to obtain a single-phase NaSbS2 film.
9. The method as claimed in claim 4, wherein the deposition is halted after rinsing at the fourth container in step g) to obtain Na2Sb4S7/NaSbS2 film.
10. A self-powered photodetector device comprising: a sodium antimony sulfide-based material selected from NaSbS₂ or a NaSbS₂/Na₂Sb₄S₇ heterostructure deposited over a FTO substrate and a carbon electrode.