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10.1021/acsomega.4c04226
Article
Impact of FeS on the TiO2 Layer As Support System in QDSCs
Meyer Edson L. †
https://orcid.org/0000-0002-0434-9635
Agoro Mojeed A. *†‡
† Fort Hare Institute of Technology, University of Fort Hare, Private Bag X1314, Alice, Eastern Cape 5700, South Africa
‡ Department of Chemistry, University of Fort Hare, Private Bag X1314, Alice, Eastern Cape 5700, South Africa
* E-mail: magoro@ufh.ac.za, amodoyin@gmail.com.
25 08 2024
10 09 2024
9 36 3789137900
02 05 2024
14 06 2024
11 06 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

We report on the passivation of titanium oxide with FeS from three molecular precursors with tin sulfide (SnS) photon absorbers that were fabricated and assembled to increase the performance of quantum dot sensitized solar cells (QDSSCs). FeS was loaded on the TiO2 surfaces, and then, SnS photosensitizer was deposited to form a ternary modified device. The morphology, structural structure, size distribution, chemical composition, and conversion efficiency were explored by FE-SEM, XRD, TEM, UV–vis, EDS, EIS, and J–V analysis. The CV, LSV, and stability state were also investigated for migration and separation of photogenerated charge carriers in the as-prepared cells labeled F-S-1, F-S-2, and F-S-3. The FE-SEM image of the F-S-2 cell is composed of FeS interconnected with SnS and FeS, which provided paths for electron movement compared with the F-S-1 and F-S-3 devices. The semicircle for the F/S-1 and F/S-3 solar device diameters illustrates that the high-medium frequency regain is greater than that of the F/S-2 device, implying that both cells have charge-transfer impedances and lower contact. Apparently, the F/S-2 device shows superior catalytic activity, which can be linked to the hybridization of TiO2/FeS/SnS due to the synergistic effect. The F/S-2/S-2l has a maximum efficiency η of 6.73% in comparison to F/S-1 and F/S-3, which have the same conversion efficiency of 3.82%. The results of the F/S-2 device follow a similar trend to the chronoamperometry analysis, CV, and LSV results from this study.

Govan Mbeki Research and Development Centrem, University of Fort Hare 10.13039/100014448 NA National Research Foundation 10.13039/501100001321 137944 National Research Foundation 10.13039/501100001321 118947 Department of Science and Innovation, South Africa 10.13039/100016962 NA document-id-old-9ao4c04226
document-id-new-14ao4c04226
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pmc1 Introductions

Global population growth has led to much concern about utilizing and exploiting innovative renewable energies; this has intensified research on solar cells over the past 30 years. Fabrication of more efficient solar cells that are cost-effective and abundant with outstanding potential, such as dye-sensitized solar cells (DSSCs), could be explored.1−3 The photodegradation issue associated with DSSCs calls for the substitution or discovery of new materials that will improve the device durability and overall performance. The distinct advantages shown by quantum dot-sensitized solar cells (QDSCs), such as easy synthetic route, hot-carrier transfer, optimized optical energy level, low costs, large intrinsic dipole, and high absorption coefficients, place QDSCs as a promising prospective third-generation solar candidate.4−6 The working mechanisms and architecture of QDSCs are very similar to those of ideal DSSCs, with a simple replacement of organic dyes with a quantum dot (QD) material. QDs have a theoretically estimable efficiency of 44% due to their multiple excitons. Though QDSCs’ highest conversion efficiency remains below 13% due to electron loss from photoanode/electrolyte interfaces, transfer at the QD/electrolyte, and charge separation, which is significantly lower than the theoretical estimable value, This is an indication that more study still needs to be done to tackle several factors that influence this poor performance in QDSCs.7,8 To resolve these issues and improve the performance of QDSCs, major cell structural players such as counter electrodes, nanostructured TiO2, electrolyte, and semiconductor QDS sensitizer choices should be well considered.

Therefore, passivation or combination of new material on the surface layer of TiO2 could enhance the electrical equilibrium and the rate of generation of electron holes, resolve QD chemical corrosion, and improve cell stability.2 Optimization of the metal sulfide band gap using molecular precursors is a promising class of materials for the passivation of the TiO2 surface layer, which could yield high cell performance. Materials such as SnS2,9 FeS, and FeS210,11 have a prominently large surface area with a high performance rate and specific capacities. Iron sulfide is an emerging, newly promising anode material recently considered by several scientists due to its high theoretical capacity, earthly abundance, pollution-free nature, and cost-effectiveness.12 Semiconductors FeS and FeS2 have a narrow band gap of 0.95 eV and a high absorption coefficient (α > 6 × 105 cm–1 for hv > 1.3 eV) as candidates for photosensitization.13 Song et al.14 report on photon-induced properties of FeS2 with TiO2 nanotube as a photoanode with enhanced photodegradation, and the visible light response of TiO2 was greatly improved after FeS2 sensitization.

A substantial study has been piloted to strengthen the photoochemical properties of the transport layer electrode to form heterojunctions via electron transport through doping and the separation of electron–hole and metal nanoparticle loading.14−16 Among these numerous techniques, metal nanoparticle loading engineering has effectively proven ways to delay the recombination rate and further the transportation of electrons to the participating semiconductor.14,16 This implies that the choice of absorber materials as a cooperative barrier layer to suppress surface charge recombination is of great importance.17 A semiconductor photon absorber such as SnS has a high light absorption coefficient that can absorb the entire visible solar spectrum due to its controllable band gap; thus, sensitization of SnS with TiO2/FeS or TiO2/FeS2 having better morphology and optical properties could be the game changer for QDSCs.14,18 To the best of the authors’ knowledge, the metal nanoparticle loading passivation of TiO2/FeS/SnS in QDSCs has not been extensively studied.

2 Experimental Section

2.1 Materials and Methods

Complete testing kits from Solaronix containing platinum substrate fluorine-doped tin oxide (FTO), TiO2 substrate (FTO), HI-30 electrolyte iodide, gaskets, masks, and hot seals were purchased. Water, FeS and SnS quantum dots from primary amines Butyldtc and Dodecyldtc Sn(II) complexes and secondary amines form N-piperldtc and N-anildtc Fe(II) complexes.

2.2 Synthesis, Passivation of FeS, and Assembling of QDSCs

FeS, FeS2, and FeS metal sulfides were prepared from (N-anil-N-piperldtc) Fe1), (N-piperldtc) Fe2), and (N-anildtc) Fe3) as previously reported as FeS#1, FeS#2, and FeS#3.10 While SnS was formed from [Sn(Dodecbutyldtc)], [Sn(Dodecyldtc)], and [Sn(Butyldtc)]. [Sn(Dodecyldtc)] is labeled as SnS1, SnS2, and SnS3 as seen in Figure 1, as potential photon absorbers, as reported in our previous study.3 The coating of both materials on the TiO2 was done by combining the as-prepared FeS#1 with SnS1 and labeling them as F/S-1, FeS#2 and SnS2 as F/S-2, and FeS#3 and SnS3 as F/S-3, respectively. FeS and SnS were used for the passivation and sensitization of the new modified devices in the present study. The TiO2 substrate was first immersed in 0.1 g of FeS mixed with distilled water for 10 h, then rinsed with distilled water and dried in the air. Subsequently, the coated photoanode substrate was immersed in a 0.1 g SnS solution for 10 h to allow proper sensitization of the TiO2/FeS.6 Finally, the coated photoanode substrates were rinsed with distilled water and dried in the air. The photoanodes and the Pt electrodes were sandwiched together using 60 mm thick transparent Surlyn film, and the internal space of the cells was filled with an electrolyte called HI-30. The active area of the solar cells was 0.25 cm2.

Figure 1 Fe(II) complexes (a–c), synthesis of nanoparticles (d), passivation of synthesized FeS, and assembly of DSSCS (e) for F-S-1, F-S-2, and F-S-3 devices.

2.3 Physical Characterizations

X-ray diffraction (XRD) patterns of as-prepared devices were identified by using a Bruker D8 Advance A25 system with Cu Kα1+2 (λ = 0.154184 nm) radiation at 50 kV and were matched to the reference powder diffraction files (PDFs). The infrared spectra were obtained from the PerkinElmer FT-IR spectrometer at room temperature from a wavenumber of 300 to 4000 cm–1. The optical properties of the three samples were measured through a UV–visible spectrometer via PerkinElmer-Lamda 45. The morphology and size of the fabricated cells were analyzed by a field-emission scanning electron microscope (SEM) using a Nova Nano SEM 430 system and high-resolution transmission electron microscopy (HRTEM) by the JEM-2100F system at 200 kV. The photochemical response of the devices was evaluated by current–voltage (J–V) using a simulated AM 1.5 G solar light radiated by a 550-W xenon lamp solar simulator with incident light intensity at 100 mW/cm2. The electrochemical impedance spectroscopy (EIS) was analyzed using the Gamry 1010E/ZRA Reference 3000 by a three-electrode standard arrangement consisting of working electrodes, a reference electrode using a saturated calomel electrode, and Pt as the counter electrode. Cyclic voltammetry and chronoamperometry were studied to evaluate the effect of the heterostructure materials on the fabricated devices. Linear scanning voltammetry (LSV) was performed at a scan rate of 10 mV s–1.

3 Results and Discussion

3.1 XRD

The XRD patterns shown in Figure 2a for the prepared F-S-1, F-S-2, and F-S-3 devices from primary and secondary amine molecular precursors were recorded. The XRD pattern for the three samples matches well with hexagonal FeS (ICDD no. 96–150–4401) and orthorhombic SnS (ICDD no. 96–810–4251). The diffraction peaks could be assigned to (101), (201), (210), (211), (410), (610), and (222) for SnS and (101) for FeS. The parameters of the three samples with passivative layer and photon are tabulated in Table 1, which are in good agreement with previous studies by Malek et al.19−21 for Fe-doped SnS orthorhombic phase and hexagonal FeS.

Table 1 XRD Parameters for F/S-1, F/S-2, and F/S-3 Devices, Where the Area of Phase A is the (SnS) Peak and the Peak Area of Phase B is (FeS)

no.	2theta [deg]	Miller indices [hkl]	d [Å]	fwhm	matched	D size [nm]	
 	F/S-1	
1	25.49	(201)	3.4919	0.4200	A	20.26	
2	26.51	(210)	3.3593	0.3675	 	23.21	
3	33.70	(101)	2.6571	0.3150	B	27.53	
4	37.75	(410)	2.3813	0.3675	A	23.87	
5	48.22	(220)	1.8857	0.3675	A	24.75	
6	51.53	(610)	1.7722	0.3675	A	25.08	
7	65.52	(222)	1.4235	0.3150	A	31.34	
 	 	 	 	 	 	Average 25.14	
 	F/S-2	
1	20.32	 	4.3675	1.3650	 	6.18	
2	20.89	 	4.2481	0.6825	 	12.37	
3	21.79	(101)	4.0760	4.1475	A	2.04	
4	25.57	(201)	3.4813	0.2625	A	32.43	
5	26.67	(210)	3.3398	0.3150	A	27.08	
6	33.86	(211) (101)	2.6451	0.2625	A,B	33.05	
7	37.88	(410)	2.3733	0.2625	A	33.43	
8	51.63	(610)	1.7688	0.2625	A	35.13	
9	65.62	(222)	1.4215	0.4200	A	23.52	
 	 	 	 	 	 	Average 22.80	
 	F/S-3	
1	25.41	(201)	3.5025	0.2625	A	32.42	
2	26.56	(210)	3.3528	0.2625	A	32.49	
3	33.76	(101)	2.6531	0.2100	B	41.31	
4	37.77	(410)	2.3797	0.2100	A	41.78	
5	51.55	(610)	1.7713	0.2625	A	35.12	
6	65.52	(222)	1.4235	0.2100	A	47.01	
 	 	 	 	 	 	Average 38.35	

Figure 2 X-ray diffraction patterns (a) and Raman spectra (b) of F-S-1, F-S-2, and F-S-3 thin films.

The crystal sizes were calculated using the Debye–Scherrer formula:1

where D is the crystal size, k is a constant typically 0.9, λ = 0.15406 nm is the wavelength of the X-ray, β is the fwhm, and θ is the diffraction angle. The average size was 25.14, 22.80, and 38.35 nm for F-S-1, F-S-2, and F-S-3, respectively.

3.2 Raman Analysis

For phase analysis of the F-S-1, F-S-2, and F-S-3 as-prepared electrodes described herein, Raman spectra were obtained and are displayed in Figure 2b. The three prepared cells reveal Raman modes at 158, 204, 260, 415, and 603 cm–1. The modes at positions 158 and 196 cm–1 are similar to the previous studies on hexagonal SnS,22,23 which established that the modified cells have hexagonal structures. The peaks at 260 and 415 cm–1 relate to the characteristic Raman active mode for pyrite (FeS). The vibrational peak at 603 cm–1 is assigned to the Ti–O vibrational band contribution. These results prove that the prepared F-S-1, F-S-2, and F-S-3 cells are hexagonal, which could be linked to the miscibility of the metal sulfide at optimized concentration conditions and deposition temperature.24,25 To identify the possible structural units of the modified cells, FT-IR analysis was used to obtain information about the three samples, as depicted in Figure S1. The broad peaks at 3313 cm–1 are observed due to the N–H stretching vibrations. Three bands from 1633–1471 cm–1 correspond to the C–N vibration, and the bands at 720 cm–1 can be allocated to the C–S vibration. The peaks at 2928 and 2919 cm–1 are assigned to the C–H vibration. The M-S bands for the modified cell are found at 580 and 413 cm–1 due to Sn–S and Fe–S.10,26 The presence of all functional groups associated with the metal sulfide confirms the purity of the cells, which correlates with the EDS results.

3.3 TEM, HRTEM, and SAED Analysis

As shown in Figure 3, both HRTEM images (Figure 3c,f,i) and the SAED images (Figure 3b,e,h) do not show clear, sharp diffraction rings. The ring matches well to the indices corresponding to the (101) hexagonal FeS phase, which is similar to the observation by Malek et al.19 and the (101) orthorhombic SnS phase, as reported by Ning et al.,27 which correlate with the XRD observation in Figure 2a for the three devices. The diameters of the three samples have a uniform size distribution between 4.12 and 7.82 nm with interplanar distances of 0.33 0.27, and 0.33 nm, which affirms the lattice fringe of (101) hexagonal FeS and (101) orthorhombic SnS.

Figure 3 TEM (a–g) and HRTEM (c–i) and SAED (b–h) of F-S-1, F-S-2, and F-S-3 thin films.

3.4 SEM and EDS Mapping Analysis

The surface morphology of passivized TiO2/FeS/SnS modified solar cells derived from molecular precursors was studied using SEM. The SEM images obtained from F-S-1 exhibited spherical and clustered crystallites consisting of irregular nanoparticles, as seen in Figure 4a, and were in good agreement with previous literature19 for FeS nanoparticles. The F-S-2 metal sulfide is composed of FeS nanoparticles with interconnected SnS balls (Figures 4d,f). Herein, FeS nanoparticles are surrounded by interconnected SnS balls, which are expected to provide paths for electron movement, which cements the strong superiority of the F/S-2 device, which is supported by the study from Li et al.28 for metal sulfide from molecular precursor. Figure 3g shows that F-S-3 particles are agglomerated and tend to form a rectangular morphology with a nonuniform size distribution. The elemental mapping as shown in Figure 4 (b,c,e,f,h,i-l) confirms the presence of C, O, S, Fe, Sn, and Ti in all three samples, which is similar to the report by Lenus et al.29 for the elemental confirmation of Fe.

Figure 4 SEM images (a,d,g) and EDS mapping (b,c,e,f,h,i) and spectra (j–l) of F-S-1, F-S-2, and F-S-3 thin films.

3.5 UV Spectrum and Band Gap Analysis

The optical band gap of the modified cell devices (see Figure 5b) was calculated from the UV–vis data using the formula given below:2

where A′ is a constant, Eg is the band gap energy, E is the photon energy, α is the absorption coefficient, and n is the power factor of the optical transition mode in a semiconductor, i.e., direct transition (n = 1/2). The band gaps of the modified cell devices were obtained by extrapolating the linear portion of the (αhν)2 vs E plot for direct transition.

Figure 5 UV–vis absorption (a) and Taucs plot (b) of F-S-1, F-S-2, and F-S-3 thin films.

The band gap energy Eg of F/S-1, F/S-2, and F/S-3 cell devices was valued at 3.2, 2.9, and 3.1 eV, respectively. The reduction of Eg implies that there is a reduction in the recombination rate under solar light. The UV–vis spectrum of the three samples, as shown in Figure 5a, shows absorption within the range of 350–400 nm. The F/S-2 heterostructure device displayed significantly enhanced absorption in the visible region compared with the other two samples. These absorptions in visible areas are due to 2(6A1) → (4T1) ligand field transitions of the metal, which collaborate with findings in the literature by Huang et al.30 for improved photochemical activity and enhanced photogenerated carriers. Tauc plot results are supported by the J–V for the F/S-2 modified device with narrow band gap energy and better conversion output, as also reported by Bootluck et al.31

3.6 EIS Nyquist and Bode Plot Analysis

In order to identify the electrochemically prominent properties of F/S-1, F/S-2, and F/S-3 solar cells, readings from electrochemical impedance spectroscopy (EIS) were taken, as illustrated in Figure 6a–c. The fluffy model of the equivalent circuit, signifying the internal resistance of the test cells, was also inserted in Figure 6a–c. The model adopted for the extraction of impedance and the capacitive value are obtained using eq 1, according to the literature by Olayiwola et al.32,33 This concept is commonly used to explain the occurrence of metal oxide electrodes surface passivation layers, where current must pass through the outer layer before reaching the underlying electrode.

Figure 6 EIS analysis (a–c) and Bode plot (d) of F-S-1, F-S-2, and F-S-3 thin films and the Equivalent circuit model (e).

From the Nyquist plots shown in Figure 6, the overall impedance, ZT, of the devices can be generally considered to be of the form given by eq 3.32,333

Where RS is the series resistance resulting from the electrode-wafer interface and estimated from the horizontal shift of the impedance from the “origin” = 0 ohms. Z1 represents R1 ∥ ZCPE1 i.e., impedance from the high frequency semicircle of the impedance plot. “CPE” term is used to represent constant phase element for representing an electrochemical element with phase shift starting from +90° to −90°. Thus, it can be used to describe impedance resulting from capacitors (capacitance), resistors (resistance), inductors (inductance), and any other impure element which falls in-between. This is evident as the capacitance measured from the Bode plot is not pure capacitance. However, this value can be converted into a capacitance value and used to obtain the electronic charge-carrier time constant, τcharge carries.

Z2, represents R2 ∥ ZCPE2 or ZW depending on the Nyquist plot. For cell types F/S-1, F/S-2, and F/S-3, Z2≡R2 ∥ ZCPE provides a better description of the chemical process. It should be noted that Zw describes a much slower charge carrier diffusion as indicated by the arc starting at an approximate angle of 45°. This region of the Nyquist plot is commonly referred to as the Warburg region. The overall diameter of the F/S-3 solar device is significantly larger and illustrates high impedance to carrier mobility compared to that of F/S-1 and F/S-2 device. The values of the intrinsic resistance parameters obtained from equivalent circuit fit using Gamry software are as follows: RS, R1, and R2.RS 22.42, 16.41, 20.32 Ω, R1 801.2, 4.225, 896.4 Ω, and R2 12.15, 6.783, 23.71 Ω. F/S-2 shows both a higher capacitive effect at the low frequency regions of the Bode plot in Figure 6d as well as lower charge delay dynamics at the high frequency region by its lower effective resistance. The behavior of the F/S-2 electrode in the iodolite HI-30 electrolyte depends mainly on the catalytic activity and capacitance of both FeS and SnS synergistic effect on electrode TiO2/FeS/SnS/iodolyte HI-30. High currents and catalytic electrodes provide low resistance. These results corroborate that the passivation of TiO2 with the introduction of FeS can improve the electrical conductivity of the overall solar cell TiO2/FeS/SnS/iodolyte HI-30/Pt for the F/S-2. Figure 6d clearly shows that the introduction of FeS as a passivation layer further decreases the charge transfer resistance and diffusion resistance, which connotes that the ion mobility and conductivity are increasing. Therefore, the electrochemical properties of F/S-2.

3.7 CV Curve and LSV Analysis

The CV curve of the F/S-2 solar cell shows a slight difference in the integrated area compared to the F/S-1 and F/S-3 devices, which connotes the superior electrochemical activity of the F/S-2 modified device, as seen in Figure 7a. This implies that there is a possibility of attaining better photochemical cells through the passivation of TiO2/FeS/SnS modified devices due to the beneficial synergistic effect. The peak current value for both F/S-1 and F/S-3 modified devices has a capacitance nature.34 The slight current increase in the F/S-2 device with the injection of HI-30 electrolyte shows a coupled cycling component present (there is a sharp oxidation peak with a significant trailing and a broad reduction peak).35 Linear sweep voltammetry (LSV) curves representative of F/S-1, F/S-2, and F/S-3 devices are shown in Figure 7b. Apparently, the F/S-2 solar device shows superior catalytic activity compared with the F/S-1 and F/S-3 electrodes, illustrating poor catalytic activity. The improved catalytic performance can be linked to the hybridizing of TiO2/FeS/SnS for F/S-2 due to the synergistic effect, which is supported by Luan et al.36 and the results of ELS from this study.

Figure 7 CV curve (a), LSV curves (b), chronoamperograms (c), and I–V curve (d) of F-S-1, F-S-2, and F-S-3 thin films.

3.8 Stability Test and I–V Curve Analysis

Figure 7c shows the chronoamperometry analysis for the modified hybridized TiO2/FeS/SnS as F/S-1, F/S-2, and F/S-3 electrode devices using the Cottrell eq 4.4

Where n is number of electrons transferred, F is Faraday’s constant, A is surface area of electrode, C0 is initial concentration, D is diffusion coefficient, and t is time (s). The oxidation current for the F/S-1 and F/S-3 devices decreases rapidly during the initial 300–400 s compared to the F/S-2 within the 100–200 s, which is likely due to the relaxation effects of concentration on the account of diffusional transport of hybridizing SnS/FeS toward the TiO2 surface before reaching their steady state. The results of the F/S-2 device follow a trend similar to the EIS, CV, and LSV results from this study, with better oxidation current, which coincided with other voltammetry analyses. This further demonstrates the stability of F/S-2 in enhancing the conversion efficiency of the cell. The decrease in F/S-1 and F/S-3 current values could be due to decay caused by the instability of the catalyst material and the poisoning of active surface sites. The slow number of active sites accounts for the continuous, slow current time decay. Figure 7d and Table 2 show the I–J characterization performance parameters, namely fill factor (FF), short circuit current (Isc), open circuit voltage (Voc), and conversion efficiency (η), which were measured and computed according to the eqs and 56.375

6

Table 2 I–V Parameters for F/S-1, F/S-2, and F/S-3 Devices and Previous Studies of QDSCs Passivated Layersi

samples	VOC (V)	JSC (mA/cm2)	FF	η (%)	year/ref.	
CuInS2/ZnS/SiO2	0.603	12.83	0.598	4.63	201838	
NiS2 (550◦C)	0.45	12.81	42.72	2.25	201839	
CuS/NiS	0.45	13.09	0.44	2.56	201840	
CuS/CoS	0.56	19.96	0.47	5.22	201840	
CuS(1)/CdS(7)	0.65	11.77	0.47	3.60	201541	
TiO2/CdS	0.54	16.07	0.43	3.7	201742	
CdS0.12Se0.88	0.56	20.8	0.53	6.14	201843	
TiO2/AISe/CdS/ZnS	0.65	18.27	0.52	6.27	201844	
TiO2/CdS/CdSe	0.62	23.74	0.48	7.16	201845	
TiO2/Cu–In–S	0.64	26.5	0.46	8.0	201846	
TiO2/CdS	0.70	14.05	0.64	6.37	201947	
TiO2/CdSe	0.58	11.83	0.36	2.72	201948	
ZnO/TiO2/CdS	0.46	7.8	0.68	2.44	202049	
TiO2/CdSe	0.48	18.80	0.54	4.88	202150	
NiAl2O/CdS/ZnS	0.79	28.22	0.71	15.14	202251	
F/S-1	0.57±0.01	10±0.01	0.67±0.02	3.82±0.01	PS	
F/S-2	0.66±0.01	12±0.0	0.85±0.02	6.73±0.01	PS	
F/S-3	0.58±0.01	10±0.02	0.66±0.0	3.82±0.01	PS	
i Present study = PS.

Table 1 shows that the F/S-2 solar cell has a maximum efficiency η of 6.73% with an increase in the corresponding parameter: Voc at 0.66 V, Jscat 12 mA/cm2, and FF at 0.85. Performances of F/S-1 and F/S-3 in Table 2 show that both solar cells are low, having the same conversion efficiency of 3.82%. The poor performance of both devices with semiconductor quantum dot photosensitizers has been attributed to poor interaction passivation of FeS between the TiO2 surface and the photosensitizer.37

4 Conclusions

In conclusion, the passivated TiO2/FeS/SnS heterojunctions coupled with a Pt counter electrode were successfully fabricated and assembled in this study in DSSCs. The F/S-3 heterostructure device achieved a significantly enhanced absorption in the visible region. The inclined line from the low frequency affirmed that the diffusion process of the F/S-1 and F/S-3 devices is less difficult, confirming the benefit of passivation on TiO2 substrate layers with FeS. F/S-2 device’s superior catalytic activity was linked to the hybridization of TiO2/FeS/SnS due to the synergistic effect with maximum efficiency η of 6.73% in comparison to F/S-1 and F/S-3 with 3.82% conversion efficiency. In addition, the chronoamperometry, CV, and LSV results affirm the F/S-2 device as a better photochemical cell.

Author Contributions

The authors contributed to the conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing of the original draft, review and editing, supervision, and funding acquisition. All authors have reviewed and approved the manuscript.

The authors acknowledge the financial support received from the PV Spoke National Energy Research Programme of the Department of Science and Innovation, the National Research Foundation (GUN: 137944 and 118 947), and the Govan Mbeki Research and Development Centre (GMRDC) at the University of Fort Hare, South Africa.

The authors declare no competing financial interest.

Acknowledgments

The authors appreciate the support from Dr. I.O. Olayiwola, at the Institute for Safe Autonomy University of York, United Kingdom for the EIS data analysis, visualization, review, and editing in the manuscript.
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