Curcumin-Assisted Synthesis of MoS2 Nanoparticles as an Electron Transport Material in Perovskite Solar Cells

Recently, two-dimensional (2D) transition metal dichalcogenides (2D TMDs), such as molybdenum sulfide (MoS2) and molybdenum selenide (MoSe2), have been presented as effective materials for extracting the generated holes from perovskite layers. Thus, the work function of MoS2 can be tuned in a wide range from 3.5 to 4.8 eV by adjusting the number of layers, chemical composition, elemental doping, surface functionalization, and surface states, depending on the synthetic approach. In this proposed work, we attempt to synthesize MoS2 nanoparticles (NPs) from bulk MoS2 using two steps: (1) initial exfoliation of bulk MoS2 into few-layer MoS2 by using curcumin-cholesteryl-derived organogels (BCC-ED) and curcumin solution in ethylene diamine (C-ED) under sonication; (2) ultrasonication of the subsequently obtained few-layer MoS2 at 60–80 °C, followed by washing of the above chemicals. The initial treatment with the BCC-ED/C-ED undergoes exfoliation of bulk MoS2 resulted in few-layer MoS2, as evidenced by the morphological analysis using SEM. Further thinning or reduction of the size of the few-layer MoS2 by prolonged ultrasonication at 60–80 °C, followed by repeated washing with DMF, resulted in uniform nanoparticles (MoS2 NPs) with a size of ~10 nm, as evidenced by morphological analysis. Since BCC-ED and C-ED produced similar results, C-ED was utilized for further production of NPs over BCC-ED owing to the ease of removal of curcumin from the MoS2 NPs. Utilization of the above synthesized MoS2 NPs as an ETL layer in the cell structure FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag enhanced the efficiency significantly. The results showed that MoS2 NPs as an ETL exhibited a power conversion efficiency (PEC) of 11.46%, a short-circuit current density of 18.65 mA/cm2, an open-circuit voltage of 1.05 V, and a fill factor of 58.66%, at the relative humidity of 70 ± 10% (open-air conditions) than that of the ED-treated MoS2 devices without curcumin. These results suggest that the synergistic effect of both curcumin and ED plays a critical role in obtaining high-quality MoS2 NPs, beneficial for efficient charge transport, lowering the crystal defect density/trap sites and reducing the charge recombination rate, thus, significantly enhancing the efficiency.


Introduction
Molybdenum disulfide (MoS 2 ) has attracted increasing attention as one of the layered transition metal chalcogenides having S-Mo-S triatomic layers attached to each other via a weak van der Waals attraction force in the bulk.Two-dimensional MoS 2 nanosheets exhibit excellent physical, chemical, optical, and electronic properties [1][2][3]."Mo" atoms are sandwiched between two layers of "S" atoms, forming semiconducting 2H and metallic 1T phases.For example, unique properties of MoS 2 include direct bandgap (~1.8 eV), good mobility (~700 cm 2 V −1 s −1 ), high current on/off ratio of ~10 7 -10 8 , large optical absorption (∼10 7 m −1 in the visible range), and a giant PL arising from the direct bandgap (1.8 eV) in the monolayer; which is distinct from its bulk counterpart (indirect band gap, ~1.2 eV) [4,5].Thus, it has been studied widely for electronics and optoelectronics applications [1,6].
Despite its indirect bandgap, two to several layers of MoS 2 , or "few-layer MoS 2 ", still have attractive properties for various electronic devices because of its semiconductivity.However, the isolation of single-layer MoS 2 and the study of the nature of this atomically thin film have clearly distinguished this mineral from other layered materials because of its unique properties, most notably semiconductivity and valley polarisation.The preparation of 2D MoS2 is, therefore, the first and the most crucial step in revealing and exploring its nature.
Many methods are reported for the isolation of single-layer MoS 2, including mechanical exfoliation [7], chemical exfoliation [5], the lithium intercalation method [8], vaporphase growth, electron beam lithography [9], etc.However, the drawbacks existing in each method, such as low yield and failure to grow large sheets of 2D MoS 2 , have led researchers to pursue the most promising chemical synthesis methods using molybdenum and sulphur precursors.
Among the large number of techniques recently developed to synthesize thin films and single/few-layer structures of MoS 2 , a versatile and industrially scalable one for large-scale production of MoS 2 films is still in demand.For successful exfoliation, the solvation energy should circumvent the van der Waals attraction force between the layers in the bulk.In addition, the exfoliating solvent should have optimum material dispersity, polarity, and H-bonding components.Therefore, in this proposed work, we introduce a curcumin and ethylene diamine (ED)-assisted synthesis of MoS 2 nanoparticles.Here, curcumin solution and organogels in ethylene diamine were selected for the exfoliation of MoS 2 .Curcumin and its derivatives have received considerable attention for their low toxicity and favorable potential in clinical applications, including antioxidant, anti-inflammatory, anti-tumor, anti-mutational, and anti-HIV properties [10,11].In addition, curcumin can also be used as a sensor and reducing agent.Curcumin was reported as a reducing agent [12,13] for the reduction of Fe 3+ to Fe 2+ [12] as well as for AgNO 3 [14].The presence of the diketo moiety in the BCC structure seems to be essential both in redox reactions and in the scavenging of oxygen radicals [12].In general, the organo/hydro gelators can be used as a template or support for the synthesis of nanoparticles such as silver nanoparticles (AgNP), gold nanoparticles (AuNPs), and iron nanoparticles (FeNPs).Since the gelators possess a 3Dnetwork structure, a large number of functional groups can provide space for the nucleation and growth of the noble metal nanoparticles [15,16].
In our earlier report, bis-cholesteryl appended curcumin derivative (BCC), as an organogel in ED, was utilized for the synthesis of silver nanoparticles [17].Therefore, we attempt to use BCC-ED gelators as template/structure directing agents for the synthesis of pure single/few-layer structures of MoS 2 .Moreover, a curcumin solution in ED (C-ED) and ED without the curcumin/curcumin derivative were also studied.The initial treatment using BCC-ED resulted in exfoliation into few-layer structures of MoS 2 .This may be due to the breaking of the intermolecular forces or bonds in between the layers of the bulk MoS 2 , leading to detaching the multi-layers into few-layers.However, further treatment with ultrasonication at heating resulted in nanoparticles because the initiation of the micromechanical acoustic cavitation process by ultrasonication produces microjets, shear forces, and shock waves, which cooperatively work on the breaking of detached layers into smaller crystals and, thus, lead to nanoparticles [18][19][20].Similar results were observed for curcumin-ED (C-ED).Among these, C-ED was selected for further synthesis of MoS 2 -NPs, due to the ease of removal of curcumin from the MoS 2 -NPs.Thus, the obtained nanoparticles are highly pure, and the fabricated device using these high-quality MoS 2 -NPs enhances efficiency by 4.39% (synthesized without curcumin) to 11.46% (synthesized with curcumin).This method offers a low-temperature and uniform-size synthesis of highquality MoS 2 NPs in a shorter time than the reported high-temperature hydrothermal and sol-gel process [21][22][23].The morphological and optical characterization of the synthesized MoS 2 and their corresponding perovskite solar cells are discussed in detail with respect to the method of preparation of MoS 2 .

Device Fabrication
The detailed fabrication process of PSCs is given in the Figure 1.Patterned FTO glass was sequentially cleaned with ethanol, isopropanol, and O 2 plasma.Then, 100 µL of the electron transport layer solution (freshly prepared MoS 2 ) was placed on the FTO glass evenly with a micropipette, spin-coated at 4000 rpm for 30 s, and annealed at 150 • C for an hour.The thickness of the spin-coated MoS 2 (obtained from C-ED-2) film was found to be 63 to 68 nm based on the cross-sectional SEM images (given in the Result and Discussion section).CsI (0.087 M), FAI (1.47 M), MABr (0.23 M), PbI2 (1.47 M), and PbBr2 (0.23 M) were mixed in the mixture of DMF, DMSO, and acetone in a volume ratio of 7.2:1.8:1.The perovskite precursor solution (70 µL) was spin-coated onto the MoS 2 substrates at 3000 rpm for 35 s in a chamber with approximately 70% RH.At the 12th s after the spin started, a solution of 0.05 vol.%MA (400 µL) was quickly dropped onto the coating layer.The sample was then annealed at 100 • C for 1 h.Then spiro-OMeTAD (72.3 mg), 4-tBP (28.8 µL), Li-TFSI (17.5 µL, 260 mg•mL −1 in AN), and FK209 (29 µL, 300 mg•mL −1 in AN) were mixed in 1 mL of chlorobenzene, and 70 µL of the Spiro-OMeTAD solution was spin-coated onto the top side of the perovskite layer at 2000 rpm for 30 s. Finally, a top electrode of 100 nm thick silver was deposited using thermal evaporation.
FAI (1.47 M), MABr (0.23 M), PbI2 (1.47 M), and PbBr2 (0.23 M) were mixed in the mixture of DMF, DMSO, and acetone in a volume ratio of 7.2:1.8:1.The perovskite precursor solution (70 µL) was spin-coated onto the MoS2 substrates at 3000 rpm for 35 s in a chamber with approximately 70% RH.At the 12th s after the spin started, a solution of 0.05 vol.%MA (400 µL) was quickly dropped onto the coating layer.The sample was then annealed at 100 °C for 1 h.Then spiro-OMeTAD (72.3 mg), 4-tBP (28.8 µL), Li-TFSI (17.5 µL, 260 mg•mL −1 in AN), and FK209 (29 µL, 300 mg•mL −1 in AN) were mixed in 1 mL of chlorobenzene, and 70 µL of the Spiro-OMeTAD solution was spin-coated onto the top side of the perovskite layer at 2000 rpm for 30 s. Finally, a top electrode of 100 nm thick silver was deposited using thermal evaporation.

Curcumin-Assisted Synthesis of MoS 2
The schematic representation of initial exfoliation of MoS 2 to few/monolayer MoS 2 using bis-cholesteryl-appended curcumin derivative (BCC) with ethylene diamine (BCC-ED) and further treatment under strong ultrasonication at higher temp, leading to NPs, are shown in the Figure 2.

Synthesis of Bis-Cholesteryl Appended Curcumin (BCC)
BCC was synthesized as described in our earlier report and characterized spectroscopically [17].Typically, about 1 g of curcumin (2.74 mmol) is placed in a 250 mL double neck round bottom flask and dissolved in dichloromethane (50 mL).To this solution, 0.66 g (5.429 mmol) of dimethyl aminopyridine (DMAP) is added and stirred well.To this mixture, the solution of cholesteryl chloroformate (3.65 g; 8.14 mmol) dissolved in dichloromethane is added dropwise over 30 min.The contents of the flask are stirred overnight with constant stirring at room temperature.After completion of the reaction, the contents of the flask are transferred to the separating funnel and washed with dil.HCl, aqueous sodium bicarbonate solution, and water, respectively.Then, dichloromethane was removed by distillation using a rotary evaporator.Thus, the obtained crude product was purified by washing with diethyl ether as well as re-precipitation in ethanol-water FT-IR (KBr, νmax/cm −1 ): 3083 (CH=CH), 2944, 2827 (C-H), 1755 (COO), 1625 (C=O), 1465 (CH2 bending), 1378 (CH3 bending).

Curcumin-Assisted Synthesis of MoS2
The schematic representation of initial exfoliation of MoS2 to few/monolayer MoS2 using bis-cholesteryl-appended curcumin derivative (BCC) with ethylene diamine (BCC-ED) and further treatment under strong ultrasonication at higher temp, leading to NPs, are shown in the Figure 2. The curcumin-based BCC-ED template was prepared by mixing 0.3 g of BCC with 10 mL of ethylene diamine (ED) and heated to obtain a clear solution, which was then allowed to cool for the formation of BCC-ED gel.Similarly, BCC-SS gels were prepared by adding 0.2 g of sodium stearate to 10 mL of dimethyl formamide (DMF), followed by a heating and cooling process, which leads to the formation of the gel.About 0.2 g of bulky MoS2 is added to each gel and further subjected to sonication for 4 h.In addition, curcumin-ED solution was also prepared and utilized for the synthesis The curcumin-based BCC-ED template was prepared by mixing 0.3 g of BCC with 10 mL of ethylene diamine (ED) and heated to obtain a clear solution, which was then allowed to cool for the formation of BCC-ED gel.Similarly, BCC-SS gels were prepared by adding 0.2 g of sodium stearate to 10 mL of dimethyl formamide (DMF), followed by a heating and cooling process, which leads to the formation of the gel.About 0.2 g of bulky MoS 2 is added to each gel and further subjected to sonication for 4 h.In addition, curcumin-ED solution was also prepared and utilized for the synthesis of MoS 2 .During the first 1 h of sonication, only large grains of MoS 2 were observed.Therefore, sonication was continued for 4 h, with a 10 min break every hour.
The resulting solution was centrifuged, and the ED was removed by decantation process and then diluted with 10 mL of DMF.Thus, the obtained mixture was sonicated and then centrifuged at 1000 rpm for 5 min.The bulky MoS 2 was removed and the upper solution was further centrifuged at 3000 rpm.The resulting solution was allowed to sonicate for 30 min in DMF and centrifuged at 6000 rpm for 15 min.In addition, instead of BCC, curcumin was mixed with ED and the resulting mixture was also used to break the bulky MoS 2 .
The BCC + ED and curcumin + ED exhibited similar results in the morphological analysis by scanning electron microscopy (SEM), as shown in Figure 3c,d.These results indicate that curcumin showed a synergistic effect with ED in the synthesis of MoS 2 .Moreover, complete removal of BCC was harder compared to curcumin.Therefore, curcumin mixed with ED was used for the successive synthesis of MoS 2 NPs.
Micromachines 2024, 15, 840 5 of 14 The resulting solution was centrifuged, and the ED was removed by decantation process and then diluted with 10 mL of DMF.Thus, the obtained mixture was sonicated and then centrifuged at 1000 rpm for 5 min.The bulky MoS2 was removed and the upper solution was further centrifuged at 3000 rpm.The resulting solution was allowed to sonicate for 30 min in DMF and centrifuged at 6000 rpm for 15 min.In addition, instead of BCC, curcumin was mixed with ED and the resulting mixture was also used to break the bulky MoS2.
The BCC + ED and curcumin + ED exhibited similar results in the morphological analysis by scanning electron microscopy (SEM), as shown in Figure 3c,d.These results indicate that curcumin showed a synergistic effect with ED in the synthesis of MoS2.Moreover, complete removal of BCC was harder compared to curcumin.Therefore, curcumin mixed with ED was used for the successive synthesis of MoS2 NPs.

Using BCC-ED Solution
The synthesized MoS2 were characterized by morphological analysis using SEM, and the morphologies were merged in Figure 3a-d.It was observed that after 1 h of sonication, separation/breaking of the multi-layers of MoS2 to few-layers occurred.Successive sonication with BCC-ED and curcumine-ED for 3 h resulted in a similar morphology (Figure 3c,d).The entangled fibrous structure of BCC-ED gels was observed in the BCC-ED exfoliated MoS2 (Figure 3c).However, pure MoS2 was observed in the curcumine-ED exfoliated MoS2.Moreover, the platelets or nanoparticles of MoS2 were in the size of a few nanometers (approximately 100 to 400 nm).This morphology is in accordance with the reported morphology of MoS2 in the literature [24,25], in which MoS2 was exfoliated using a high-temperature process, whereas we used a room-temperature process with the help of sonication, indicating the successful exfoliation of the MoS2.The synthesized MoS 2 were characterized by morphological analysis using SEM, and the morphologies were merged in Figure 3a-d.It was observed that after 1 h of sonication, separation/breaking of the multi-layers of MoS 2 to few-layers occurred.Successive sonication with BCC-ED and curcumine-ED for 3 h resulted in a similar morphology (Figure 3c,d).The entangled fibrous structure of BCC-ED gels was observed in the BCC-ED exfoliated MoS 2 (Figure 3c).However, pure MoS 2 was observed in the curcumine-ED exfoliated MoS 2 .Moreover, the platelets or nanoparticles of MoS 2 were in the size of a few nanometers (approximately 100 to 400 nm).This morphology is in accordance with the reported morphology of MoS 2 in the literature [24,25], in which MoS 2 was exfoliated using a high-temperature process, whereas we used a room-temperature process with the help of sonication, indicating the successful exfoliation of the MoS 2 .

Using C-ED Solution
Even though we successfully synthesized MoS 2 NPs under room temperature with the help of ultrasonication, the efficiency of the obtained MoS 2 as an ETL layer showed lower power conversion efficiency than expected.The morphological analysis of the MoS 2 -based perovskite layer by SEM analysis revealed rough surfaces with pin holes, which may lead to poor efficiency in the fabricated cells.The pin-hole formation may be due to two reasons.One might be the presence of curcumin (unremoved, if any), which may affect the highquality film formation.Another might be from the various shapes of synthesized MoS 2 , as seen in Figure 3c,d Even though we successfully synthesized MoS2 NPs under room temperature with the help of ultrasonication, the efficiency of the obtained MoS2 as an ETL layer showed lower power conversion efficiency than expected.The morphological analysis of the MoS2based perovskite layer by SEM analysis revealed rough surfaces with pin holes, which may lead to poor efficiency in the fabricated cells.The pin-hole formation may be due to two reasons.One might be the presence of curcumin (unremoved, if any), which may affect the high-quality film formation.Another might be from the various shapes of synthesized MoS2, as seen in Figure 3c,d, such as nanoparticles and nanorods, which affect the uniform coating of the ETL layer.In order to overcome these limitations, we try to reflux the curcumin-ethylene diamine solution containing MoS2 at 60-80 °C overnight, with constant stirring.For comparison, ethylene diamine was used alone without curcumin for the same purpose and then cooled to room temperature.A portion of the resulting solution was taken and centrifuged.The ED and C-ED were removed by decantation and washed with DMF.First-time washing (C-ED-1) resulted in a slightly brown-colored solution, indicating the existence of curcumin.Multiple/repeated washing with DMF resulted in a green-colored solution (C-ED-2).The morphological analysis of the curcumin-ethylene diamine (C-ED) assisted synthesis of MoS2 NPs is shown in Figure 4.In the figure, rough surfaces with random-sized nanoparticles are observed for EDtreated MoS 2 (Figure 4a).However, uniform particles are observed for C-ED-treated samples, indicating that curcumin plays an important role in the exfoliation and further breaking of MoS 2 into NPs (Figure 4b).Hence, the agglomeration or clustering of nanoparticles was observed, which may be due to the presence of excess curcumin or ED as impurities or curcumin-induced aggregation [26][27][28].The repeated washing with excess DMF and the ultrasonication process led to well-organized MoS 2 nanoparticles with a size of approximately 10 nm being observed (Figure 4c-e), indicating the synergistic effect of curcumin and ethylene diamine [26,29].Exfoliation of MoS 2 to single or few-layered nanosheets is expected; however, instead, we observed nanoparticles in all the cases, which may be due to the prolonged ultrasonication causing the breaking of sheets into nanoparticles [1,30].

Fabrication of Perovskite Solar Cells Using Synthesized MoS 2 NPs
In order to know the efficiency of the synthesized MoS 2 , it was utilized as electron transport material in the cell with the general structure of glass/FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag, and their photophysical, optical, and power conversion efficiencies were studied.To gain insight into the film crystallinity, crystal size, and lightharvesting ability, the ED and C-ED-treated MoS 2 -based perovskite layers (FTO/MoS 2 /pero vskite) were subjected to scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV−vis absorption.The results are discussed below.
Micromachines 2024, 15, 840 7 of 14 In the figure, rough surfaces with random-sized nanoparticles are observed for EDtreated MoS2 (Figure 4a).However, uniform particles are observed for C-ED-treated samples, indicating that curcumin plays an important role in the exfoliation and further breaking of MoS2 into NPs (Figure 4b).Hence, the agglomeration or clustering of nanoparticles was observed, which may be due to the presence of excess curcumin or ED as impurities or curcumin-induced aggregation [26][27][28].The repeated washing with excess DMF and the ultrasonication process led to well-organized MoS2 nanoparticles with a size of approximately 10 nm being observed (Figure 4c-e), indicating the synergistic effect of curcumin and ethylene diamine [26,29].Exfoliation of MoS2 to single or few-layered nanosheets is expected; however, instead, we observed nanoparticles in all the cases, which may be due to the prolonged ultrasonication causing the breaking of sheets into nanoparticles [1,30].

Fabrication of Perovskite Solar Cells Using Synthesized MoS2 NPs
In order to know the efficiency of the synthesized MoS2, it was utilized as electron transport material in the cell with the general structure of glass/FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag, and their photophysical, optical, and power conversion efficiencies were studied.To gain insight into the film crystallinity, crystal size, and lightharvesting ability, the ED and C-ED-treated MoS2-based perovskite layers (FTO/MoS2/perovskite) were subjected to scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV−vis absorption.The results are discussed below.

Photo-physical properties of the MoS2 NPs
Figure 5a shows the XRD patterns of the glass/FTO/MoS2/perovskite. The peaks at 13.82°, 31.5°, and 40.39°, which correspond to the (002), (100), and (103) planes of the MoS2 nanosheet, respectively, and the diffraction pattern matched with the standard diffraction pattern of 2H-MoS2 [25].The peaks at 19.7°, 23.8°, 28.1°, and 42.9° corresponded to the (200), ( 211), (220), and (314) planes for the perovskite, respectively [31].However, the PbI2 (004), (008), and (0012) peaks located at 12.58°, 25.81°, and 35.5°, respectively, can be observed in all the samples.The coexistence of the two CH3NH3PbI3 and PbI2 phases was observed in the perovskite layers (CsFAMAPbI3−xBrx).This is due to the post-annealing process leading to the thermal decomposition of MAI and the formation of the PbI2 phase.In addition, (105), (110), and (008) planes of bulk MoS2 were absent, indicating the successful preparation of NPs [1].Since the impurities could affect the light absorption of perovskite thin films due to the lower crystallite size in the perovskite layer, the light absorption of the perovskite thin films with different MoS 2 were investigated through UV−vis spectroscopy and the results are given in Figure 5b.From the figure, it was observed that C-ED-treated MoS 2 exhibited an absorption peak at ~750 nm, which is similar to the case of MoS 2 treated with the BCC-ED template.These results suggest that the C-ED did not affect the light absorption properties of the perovskite layer.In addition, the C-ED-2 (multiple washing with DMF) showed better absorption than C-ED-1 (one-time washing), indicating that the presence of curcumin impurities affects the performance of the cell.Repeated washing leads to the complete removal of curcumin, ED, or any other impurities, which enhances the quality or crystal growth (large crystallites) in the perovskite layer.This can be attributed to the higher crystallinity of the high-quality perovskite film, supported by high-quality MoS 2 .All the spectrum shows a major peak around 740 nm.The results are in accordance with the reported literature [24], indicating the successful synthesis of MoS 2 .
In general, MoS 2 can absorb visible light to some extent due to its 1.8 eV band gap and large optical absorption coefficient and, thus, has the possibility of affecting the light absorption of perovskite.However, the MoS 2 can cover the visible spectrum only near to 400 nm, and it will not cover the entire region of the visible spectra (up to the wavelength of 700 nm) [32].Moreover, once exfoliated, the magnitude of the peak for few-layer MoS 2 is lower than multi-layer MoS 2 , which means multi-layer MoS 2 absorbs light better than mono/few-layer MoS 2 at 400 nm wavelength.In addition, MoS 2 acts as a defect passivating layer, which somewhat lowers the charge carrier recombination.Therefore, exfoliated MoS 2 is used as a dopant to passivate the metal surfaces such as Cu 2 O, Si 2 O, ZnO, etc. [33][34][35][36][37].In our case, the MoS 2 NPs enhanced the power conversion efficiency than the bulk or multi-layer MoS 2 , suggesting that the synthesized MoS 2 NPs have very little effect or negligible effect than bulk towards the light absorption of perovskite.
To figure out the photoconversion process/charge transfer process associated with the quality of MoS 2 in the perovskite films, the photoluminescence was recorded for the above said samples, and the results are merged in Figure 6.Here, the excitation wavelength of 400 nm is used, which is the specific excitation wavelength of perovskite (CsFAMAPbI 3−x Br x ).From the figure, it can be seen that the photoluminescence spectrum of perovskite generates energy around the emission wavelength of 750 nm.The PL intensity of the perovskite thin film sample prepared from the MoS 2 ETL separated by C-ED-2 showed a significant decline than that of C-ED1, suggesting that C-ED-2 promotes electron extraction and transport capacity at the MoS 2 /perovskite interface through multiple interaction systems.
Since the impurities could affect the light absorption of perovskite thin films due to the lower crystallite size in the perovskite layer, the light absorption of the perovskite thin films with different MoS2 were investigated through UV−vis spectroscopy and the results are given in Figure 5b.From the figure, it was observed that C-ED-treated MoS2 exhibited an absorption peak at ~750 nm, which is similar to the case of MoS2 treated with the BCC-ED template.These results suggest that the C-ED did not affect the light absorption properties of the perovskite layer.In addition, the C-ED-2 (multiple washing with DMF) showed better absorption than C-ED-1 (one-time washing), indicating that the presence of curcumin impurities affects the performance of the cell.Repeated washing leads to the complete removal of curcumin, ED, or any other impurities, which enhances the quality or crystal growth (large crystallites) in the perovskite layer.This can be attributed to the higher crystallinity of the high-quality perovskite film, supported by high-quality MoS2.All the spectrum shows a major peak around 740 nm.The results are in accordance with the reported literature [24], indicating the successful synthesis of MoS2.
In general, MoS2 can absorb visible light to some extent due to its 1.8 eV band gap and large optical absorption coefficient and, thus, has the possibility of affecting the light absorption of perovskite.However, the MoS2 can cover the visible spectrum only near to 400 nm, and it will not cover the entire region of the visible spectra (up to the wavelength of 700 nm) [32].Moreover, once exfoliated, the magnitude of the peak for few-layer MoS2 is lower than multi-layer MoS2, which means multi-layer MoS2 absorbs light better than mono/few-layer MoS2 at 400 nm wavelength.In addition, MoS2 acts as a defect passivating layer, which somewhat lowers the charge carrier recombination.Therefore, exfoliated MoS2 is used as a dopant to passivate the metal surfaces such as Cu2O, Si2O, ZnO, etc. [33][34][35][36][37].In our case, the MoS2 NPs enhanced the power conversion efficiency than the bulk or multi-layer MoS2, suggesting that the synthesized MoS2 NPs have very little effect or negligible effect than bulk towards the light absorption of perovskite.
To figure out the photoconversion process/charge transfer process associated with the quality of MoS2 in the perovskite films, the photoluminescence was recorded for the above said samples, and the results are merged in Figure 6.Here, the excitation wavelength of 400 nm is used, which is the specific excitation wavelength of perovskite (CsFAMAPbI3−xBrx).From the figure, it can be seen that the photoluminescence spectrum of perovskite generates energy around the emission wavelength of 750 nm.The PL intensity of the perovskite thin film sample prepared from the MoS2 ETL separated by C-ED-2 showed a significant decline than that of C-ED1, suggesting that C-ED-2 promotes electron extraction and transport capacity at the MoS2/perovskite interface through multiple interaction systems.The SEM morphology of the FTO/MoS 2 /perovskite were analyzed and merged in Figure 7. From the figure, it was observed that the morphology of the ED-treated MoS 2 -based device showed very small and random-sized perovskite crystals (Figure 7a).However, the BCC-ED-treated MoS 2 -based device showed better crystalline than that of ED (Figure 7b).But still seems to be low quality when compared with C-ED-treated devices (Figure 7c-e).It may be due to the unremovable BCC existing as impurities, which may affect the coating as well as the crystallization of perovskites.
Micromachines 2024, 15, 840 9 of 14 The SEM morphology of the FTO/MoS2/perovskite were analyzed and merged in Figure 7. From the figure, it was observed that the morphology of the ED-treated MoS2-based device showed very small and random-sized perovskite crystals (Figure 7a).However, the BCC-ED-treated MoS2-based device showed better crystalline than that of ED (Figure 7b).But still seems to be low quality when compared with C-ED-treated devices (Figure 7ce).It may be due to the unremovable BCC existing as impurities, which may affect the coating as well as the crystallization of perovskites.Obviously, bigger-sized crystalline textures were observed for C-ED-2 than for C-ED-1, suggesting that curcumin interferes in the formation/crystallization of perovskites and, thus, needs to be washed thoroughly before being utilized as ETL in the fabrication PSCs.These results indicate that multiple washing (removal of curcumin) can effectively repair pin-hole issues.The cross-sectional SEM image of the C-ED-2-treated MoS 2 -based device is given in Figure 7f, which showed a thickness of 659 nm.

Photoconversion Efficiency of the Fabricated Devices
The J-V curves of the perovskite solar cell with MoS 2 as an electron transport material, prepared in different ways, are shown in Figure 8 and Table 1.It can be seen that ED-treated MoS 2 solar cells exhibited low efficiency (4.39 ± 0.81) in the structure of FTO/ETL/perovskite/spiro-OMeTAD/Ag, and the curves were also very poor, which is due to the uneven particle size and the uneven surface of the electron transport layer, resulting in poor crystallization of the perovskite layer.Moreover, the MoS 2 layer was not completely covered by the perovskite layer, and only moderate crystal growth of the perovskite layer was observed, as evidenced by SEM.Moreover, the surface of the perovskite layer was seemingly rough, with irregular packing of crystals, which may be due to the rough surface of the MoS 2 layer.From the morphology of neat MoS 2 , it can be seen that the surface is not smooth, which may lead to poor packing of the perovskite layer, thus causing poor PCE.In the case of BCC-ED-treated MoS 2 -based devices, which exhibited better performance (PCE: 9.46) than ED, indicating that BCC improved the quality of the perovskite film by reducing the defects significantly.Interestingly, C-ED-treated MoS 2 cells (C-ED-1 and C-ED-2) enhanced efficiency significantly because the smooth and uniform particles of MoS 2 improved the quality of the thin film.Repeated washing or removal of excess curcumin improved the fill factor (FF) from 56 to 58%, as well as the open circuit voltage (V oc ) from 0.94 to 1.05, revealing that C-ED-2 significantly enhanced the efficiency by reducing the recombination rate.This indicates that the quality of MoS 2 plays a crucial role in the PCE of the devices.The high-quality MoS 2 offers the best platform for perovskite film formation as well as a template for crystal growth, which is supported/consistent with SEM results.Therefore, any impurities like curcumin/ED that exist reduce the crystallinity of perovskite and increase the defects leading to poor efficiency.Because a larger crystal size also means lower crystal defect density and trap sites existing in the thin films, which is beneficial for efficient charge transport and reduced charge recombination.The study of the stability of the fabricated devices and other characterizations is in progress.We also compared the efficiency of our synthesized MoS 2 NPs with the reported MoS 2 as ETL, shown in Table 2. From the table, it is observed that there are some reports that showed significantly higher efficiencies than our fabricated devices.However, these reported MoS 2 were obtained by a high-temperature process or by using microwave or other commercially available methods.In addition, in some cases, MoS 2 was used as a bilayer by combining it with other compounds such as graphene, TiO 2 , etc.Moreover, gold is used as the back contact in most of the reported structures, whereas silver is used in our fabricated devices.However, the efficiencies of our fabricated devices are comparable with the reported devices in the literature.For further insights regarding the improved performance, especially the low voltage loss, the effect of MoS 2 on the energy levels of the perovskite films was examined with ultraviolet photoemission spectroscopy (UPS).The UPS experiments probe the vacuum, Fermi, and valence energy levels near the perovskite layer, where electrons are extracted in the working solar cells.The value of the work function was calculated using the equation: WF = 21.2 − E cut-off in which E cut-off is the energy of emitted secondary electrons and 21.2 eV is according to the He laser UV light source [41].
The secondary electron cut-off energy of this material was 17.0 eV (Figure 9b).Therefore, the work function (WF) and valance band maximum (VBM) were calculated to be 4.21 eV and 7.24 eV, respectively.Additionally, from the low energy inverse photoelectron spectra (LEIPS), the vacuum energy level (EVAC) and electron affinity (EA) were obtained, and then conduction band maximum (CBM) could be calculated (EVAC-EA) as 4.16 eV.The energy differences between the perovskite and ETL layer is small, which is beneficial for charge transportation and enhanced efficiency.The energy-level scheme based on UPS is given in Figure 10.

Conclusions
In this study, the high-quality MoS2 were synthesized by using curcumin and ethylenediamine-based compounds C-ED and BCC-ED.The repeated washing with DMF (C-ED-2) under ultrasonication produced impurities (curcumin and ED) free high-quality nanoparticles with an average size of 10 nm as evidenced by the morphological analysis by SEM.Utilization of the C-ED-2-treated MoS2 nanoparticles as electron transport layer (ETL) in the cell structure FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag enhanced the photoelectric conversion efficiency (PCE) from 4.39 to 11.46%, short-circuit current density increased from 9.3 mA/cm 2 to 18.6 mA/cm 2 , open circuit voltage increased from 0.8 V to 1.05 V, and fill factor 57.4 to 58.6% that that of ED-treated MoS2 devices with a relative humidity of 70 ± 10%, indicating the synergistic effect of curcumin and ethylene diamine.

Conclusions
In this study, the high-quality MoS2 were synthesized by using curcumin and ethylenediamine-based compounds C-ED and BCC-ED.The repeated washing with DMF (C-ED-2) under ultrasonication produced impurities (curcumin and ED) free high-quality nanoparticles with an average size of 10 nm as evidenced by the morphological analysis by SEM.Utilization of the C-ED-2-treated MoS2 nanoparticles as electron transport layer (ETL) in the cell structure FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag enhanced the photoelectric conversion efficiency (PCE) from 4.39 to 11.46%, short-circuit current density increased from 9.3 mA/cm 2 to 18.6 mA/cm 2 , open circuit voltage increased from 0.8 V to 1.05 V, and fill factor 57.4 to 58.6% that that of ED-treated MoS2 devices with a relative humidity of 70 ± 10%, indicating the synergistic effect of curcumin and ethylene diamine.

Conclusions
In this study, the high-quality MoS 2 were synthesized by using curcumin and ethylened iamine-based compounds C-ED and BCC-ED.The repeated washing with DMF (C-ED-2) under ultrasonication produced impurities (curcumin and ED) free high-quality nanoparticles with an average size of 10 nm as evidenced by the morphological analysis by SEM.Utilization of the C-ED-2-treated MoS 2 nanoparticles as electron transport layer (ETL) in the cell structure FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag enhanced the photoelectric conversion efficiency (PCE) from 4.39 to 11.46%, short-circuit current density increased from 9.3 mA/cm 2 to 18.6 mA/cm 2 , open circuit voltage increased from 0.8 V

Figure 1 .
Figure 1.The schematic representation of fabrication of perovskite solar cells (a) deposition of MoS2; (b) annealing process; (c) coating of perovskite absorber; (d) annealing process; (e) doposition of hole trasport material; (f) drying process at room temperture and (h) coating of Silver as back contact.

Figure 1 .
Figure 1.The schematic representation of fabrication of perovskite solar cells (a) deposition of MoS 2 ; (b) annealing process; (c) coating of perovskite absorber; (d) annealing process; (e) doposition of hole trasport material; (f) drying process at room temperature, (g,h) coating of Silver as back contact.

Micromachines 2024 ,
15,  x FOR PEER REVIEW 4 of 15 doposition of hole trasport material; (f) drying process at room temperture and (h) coating of Silver as back contact.

Figure 2 .
Figure 2. (a) Structure of the gel components, schematic representation of (b) exfoliation of MoS2 and then to NPs, and (c) process involved for the synthesis of high-quality MoS2 NPs.

Figure 2 .
Figure 2. (a) Structure of the gel components, schematic representation of (b) exfoliation of MoS 2 and then to NPs, and (c) process involved for the synthesis of high-quality MoS 2 NPs.

Figure 3 .
Figure 3.The SEM images of the exfoliated MoS2 (a) at initial exfoliation, (b) after 1 h of sonication with BCC-ED, (c) after 4 h of sonication with BCC-ED, and (d) after 4 h of sonication with curcumin-ED.

Figure 3 .
Figure 3.The SEM images of the exfoliated MoS 2 (a) at initial exfoliation, (b) after 1 h of sonication with BCC-ED, (c) after 4 h of sonication with BCC-ED, and (d) after 4 h of sonication with curcumin-ED.

3 .
Results and Discussion 3.1.Morphological Analysis of MoS 2 NPs 3.1.1.Using BCC-ED Solution , such as nanoparticles and nanorods, which affect the uniform coating of the ETL layer.In order to overcome these limitations, we try to reflux the curcuminethylene diamine solution containing MoS 2 at 60-80 • C overnight, with constant stirring.For comparison, ethylene diamine was used alone without curcumin for the same purpose and then cooled to room temperature.A portion of the resulting solution was taken and centrifuged.The ED and C-ED were removed by decantation and washed with DMF.First-time washing (C-ED-1) resulted in a slightly brown-colored solution, indicating the existence of curcumin.Multiple/repeated washing with DMF resulted in a green-colored solution (C-ED-2).The morphological analysis of the curcumin-ethylene diamine (C-ED) assisted synthesis of MoS 2 NPs is shown in Figure4.

Figure 5 .
Figure 5. X-ray diffraction patterns (a) and the absorption spectra (b) of the perovskite layer consisting of MoS2 NPs, prepared at different conditions.

Figure 5 .
Figure 5. X-ray diffraction patterns (a) and the absorption spectra (b) of the perovskite layer consisting of MoS 2 NPs, prepared at different conditions.

Figure 6 .
Figure 6.The emission spectra of the perovskite layer consisting of MoS2 prepared by C-ED with different washings.

Figure 6 .
Figure 6.The emission spectra of the perovskite layer consisting of MoS 2 prepared by C-ED with different washings.

Figure 8 .
Figure 8. Photocurrent-voltage curves of perovskite solar cells with different electron transport layers.

Figure 10 .
Figure 10.Schematic representation of (a) the energy-level diagram (b) HOMO-LUMO band diagram of the proposed MoS2.

Figure 10 .
Figure 10.Schematic representation of (a) the energy-level diagram (b) HOMO-LUMO band diagram of the proposed MoS2.

Figure 10 .
Figure 10.Schematic representation of (a) the energy-level diagram (b) HOMO-LUMO band diagram of the proposed MoS 2 .

Table 2 .
Comparison of reported MoS 2 , cell structure, and efficiencies with respect to ETL.