Design of Ag Injection in Bi2S3 (AgBiS2) Thin Films for Photoelectrochemical Cell and Solar Cell Applications

This study reports the opto-structural, morphological, topological and electrical properties of thermally evaporated Ag x Bi 2-x S 3-y thin lm prepared for various x and y values (x= y= 0, 0.25, 0.50, 0.75 and 1). The lms have cubic structured AgBiS 2 along with orthorhombic structured Bi 2 S 3 as conrmed from X-ray diffraction (XRD) analysis. The lms showed higher optical absorption coecient (10 5 cm -1 ) in the visible region and band gap values are found to be decreased from 2.08 eV to 1.35 eV for Ag x Bi 2-x S 3-y (x= y = 0 to 1) lms. Scanning electron microscope (SEM) images showed the uniform distribution of spherical particles. Carrier concentration of the lms are better than x= y= 0 as observed from Hall effect and Mott-Schottky plots. The FTO/ Ag x Bi 2-x S 3-y (x= y = 1) photoelectrochemical cell yields the photoconversion eciency (PCE) of 7.03 %. The device FTO/ Ag x Bi 2-x S 3-y (x= y = 1) CdS/Ag solar cell has exhibited PCE of 3.26%.


Introduction
of 7.61 mA/cm 2 , an open-circuit voltage (V oc ) of 0.18 V, a ll factor (FF) of 38.6%, and a power conversion e ciency (η) of 0.53% under 1 sun. From the review of related material (AgBiS 2 ), less number of reports is found elsewhere towards the solar cells and photoelectrochemical cell application while their electrical properties are not yet studied widely. The reason is di cult to synthesis ternary semiconducting material using vacuum thermal evaporation technique with phase purity. This is the major challenge to choose this AgBiS 2 thin lm from Bi 2 S 3 and Ag precursors. Hence the aim of the present study is to prepare Ag x Bi 2-x S 3-y thin lms (x= y= 0, 0.25, 0.5, 0.75, and 1) by thermal evaporation method and to study their opto-structural, morphological, and electrical properties and to fabricate FTO/ Ag x Bi 2-x S 3-y (x= y= 0 to 1)/CdS/Ag cells for analyzing photoelectrochemical and solar cell performance.

Experimental Details
Synthesis of Ag x Bi 2-x S 3-y (x= y= 0, 0.25, 0.5, 0.75 and 1) thin lms For the preparation of Ag x Bi 2-x S 3-y (x= y= 0, 0.25, 0.5, 0.75 and 1), the required stoichiometric ratio of metal powders Ag, Bi and S were ground well-using an agate mortar and pestle and placed in a Mo boat for evaporation. The Mo boat is placed in the bell-jar and it is evacuated to 10 -5 mbar with an applied current of 160 Amp. The substrates were kept at a distance of 15 cm from the molybdenum boat. The deposited lms were annealed at 300 ˚C for 1h in the air atmosphere.

Characterization techniques
Phase structure was examined by PANalytical X-Pert Pro' diffractometer and optical properties were investigated by Shimadzu UV-2700 instrument. JEOL -JSM 5610LV Instrument was used to recorded the SEM images. Nanosurf instrument was used to study the surface topology. Hall Effect measurement was studied using Ecopia HMS-7000 Photonic Hall Effect Measurement. PEC and Solar analysis was performed by the Electrochemical Analyzer (CHI604E electrochemical workstation) through Linear Sweep Voltametry technique (LSV) with Xenon lamp source.

Photoelectrochemical cell fabrication
Photoelectrochemical (PEC) measurements were carried out in a standard 3-electrode con guration.
Number of attempts has been made with iodide based (KI+I2) electrolyte with different molar concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 M) for PEC cell setups because; the thin lm working electrodes have high stability in that electrolyte. Better results were obtained in lesser molarity ie., 0.2 M was optimized for Ag x Bi 2-x S 3-y (x= y= 0, 0.25, 0.5, 0.75 and 1) thin lm working electrodes. The photoelectrochemical cell was fabricated with the con guration FTO/ Ag x Bi 2-x S 3-y (x= y= 0, 0.25, 0.5, 0.75 and 1) thin lms acted as working electrode along with Ag/AgCl reference electrode, while Pt wire was used as counter electrode. The PEC cell was illuminated with Xenon lamp with 10 mW cm -2 .
Fabrication of FTO/ Ag x Bi 2-x S 3-y /CdS/Ag devices The following steps involved in the fabrication of solar cell Absorbing layer Ag x Bi 2-x S 3-y was deposited on the conducting layer (FTO).
CdS used as an n-type layer was deposited over the absorber layer Ag paste was used for metallic contacts.
The J-V plot was recorded using the CH60E Instrument with Xenon lamp illumination of 10 mW cm -2 .
In the present research work CdS was used as an n-type layer. Preparation of CdS thin lm was already reported by the same authors [T. Daniel et al.,21].
When introducing Ag (x= 0.25) and reducing S (y = 0.25) into Ag x Bi 2-x S 3-y lm, a high intense prominent peak is observed at 2θ = 31.6° due to the re ection of (2 0 0) plane belongs to the formation of cubic structured AgBiS 2 (JCPDS 893672), with existing peaks of orthorhombic structured Bi 2 S 3 (at 2θ=28.6°). A peak shift is observed (from 2θ=28.7° to 31.6°) towards the higher Bragg angle than that of lm Ag x Bi 2x S 3-y (x= y= 0), which con rms the incorporation of Ag into Bi 2 S 3 lattice due to partial replacement of Bi ion (radius of 1.40 Å) by Ag ion (radius of 1.15 Å), which makes lattice distortion since both ions have various ionic radii [86]. When increasing the amount of (x = 0.50) Ag and reduced the amount of S (y = 0.50) into Ag x Bi 2-x S 3-y , the position of the prominent peak does not vary but its FWHM gets narrower than lm prepared at x = y = 0.25. It is also observed the presence of Bi 2 S 3 at 2θ = 44.5° (4 4 0).
The improvement in average crystallite size, in comparison to x = y = 0, leads to a decrease in the grain boundaries which may restrict the trapping of electrons during PV process [28]. The decrease in dislocation density indicates the formation of better crystallites and improved crystallinity [29]. Changes in unit cell volume are attributed to the incorporation of Ag into Bi 2 S 3 lattice.
It is noticed from the XRD analysis that the deposited lms have a change in peak position, variation in intensity, presence of impurity phases (for lms Ag x Bi 2-x S 3-y (x = y = 0.25) and Ag x Bi 2-x S 3-y (x = y = 0.50), changes in crystallite size and dislocation density with the variation of Ag and S.
The prepared Ag x Bi 2-x S 3-y (x = y= 0 to 1) lms were subjected into UV-Visible spectroscopy technique and their results are shown in gure 2. It can be seen from the spectra that the Ag x Bi 2-x S 3-y (x = y = 0) lm shows good optical absorption in the visible region. While introducing of Ag and reducing S content (x = y = 0.25) into Ag x Bi 2-x S 3-y lm , the optical absorption get enhanced than the Bi 2 S 3 lm and the result is continued upto x = y = 0.50. By comparing the UV-Visible spectra, the Ag x Bi 2-x S 3-y lms prepared at x = y = 0.25 and 1 have exhibited higher optical absorption than the Ag x Bi 2-x S 3-y lms prepared for x = y = 0.75 and 0.50 attributed to phase pure nature, higher lm thickness values with wavy shaped which indicates homogeneity of the lms [30]. The lowering of optical absorbance may be due to the presence of Bi 2 S 3 secondary phases (evident from XRD).
The optical absorption coe cient (α) vs wavelength plots of Ag x Bi 2-x S 3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin lms are shown in gure 3. The 'α' value is found to be 10 5 cm -1 for all the lms except Ag x Bi 2-x S 3-y (x= y= 0) lm. Due to the phase pure nature, Ag x Bi 2-x S 3-y (x = y = 0.75) and Ag x Bi 2-x S 3-y (x = y = 1) lms having higher 'α' value than others . This result suggests that all the lms have direct band gap energy and this phenomenon in uenced the solar conversion e ciency [31]. In order to study the PEC cells and PV device performance, determination of bandgap energy is one of the most important parameters, because it only gives the insight about the lm material's region transparency. Based on this aspect, it is important to estimate the bandgap energy of Ag x Bi 2-x S 3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) lms using Tauc's relation [32]. Figure 4 shows the optical band gap energy 'E g ' ( (αhυ) 2 vs hυ) of Ag x Bi 2-x S 3-y (x = y = 0 to 1) thin lms.
The bandgap values of Ag x Bi 2-x S 3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin lms are found to be decreased with respect to Ag and S content as 2.08 eV, 1.68 eV, 1.55 eV, 1.37 and 1.35 eV for x = y = 0, 0.25, 0.50, 0.75 and 1 respectively and these values are comparable with the earlier reports on AgBiS 2 thin lms [3,33].
By comparing the E g values, the higher E g values are observed for the lms prepared at x = y= 0.25 and x = y = 0.50 than the remaining lms (x = y= 0.75 and x = y = 1), be due to the formation of secondary phases in the lms as evident from the XRD analysis [29,34]. The lowering of band gap energy is due to more amount of Ag and S into the Ag x Bi 2-x S 3-y system and the localized defect states promote to extend the tail of optical absorption into direct gap, hence the band gaps get decreased (1.37 eV and 1.35 eV) [35] with suitable lm thickness [36,37]. Besides, the optical band gap energy values mostly depend on the crystalline nature of the lms, incorporation of precursors [38] and the method of lm deposition [39].
However, these values lie in the range of ideal region for PV applications [40]. The optical absorption, 'αá nd E g of Ag x Bi 2-x S 3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin lms have met the requirements for the use of solar absorber in PV and PEC cells.
The surface morphology of thin lm absorbing layer plays a vital role in the performance of the PV devices and PEC cells [41]. Figure 5 (a) and (b) show the SEM images of Ag x Bi 2-x S 3-y (x = y = 0) thin lm with two different magni cations (10k and 30k). The surface of the lm shows uniformly distributed grains and average grain sizes are measured to be lesser than 500 nm as clearly reveals in the higher magni cation image (30k).
When Ag and S (x= y= 0.25) are introduced into Ag x Bi 2-x S 3-y system, the surface seems to be dense in nature as clearly displayed in gure 6. The deposited particles are uniformly distributed on the entire surface and individual grains with grain boundaries are visualized in the higher magni cation (50k). The average grain size of the nanospheres is found to be less than 100 nm.
The surface morphologies of Ag x Bi 2-x S 3-y (x = y= 0.50 to 1) thin lms are shown in shown in gures 7 to 9 with different magni cations (10k (a) 30k (b) and 50k). It can be seen from the images (30k and 50k) that well de ned spherical grains are uniformly distributed on the lm surface with an average grain size measured to be 100 nm to 150 nm. The observation of spherical shapes without voids can make the surface smooth and compact. These spherical shaped grains will reduce the recombination of electronhole pairs during the photovoltaic process [30,42].  Figure 10 shows the AFM images of Ag x Bi 2-x S 3-y (x = y= 0) thin lm which exhibits the growth of irregular polycrystalline shaped particles as clearly visible on the lm surface. Surface roughness is measured to be 30 nm.
When introducing x= y= 0.25 of Ag and S into Ag x Bi 2-x S 3-y lm, there are well de ned polygonal-shaped grains are seen in the 2D image ( gure 11 (a)) while pyramid-like structure is clearly visible in the 3D image ( gure 11 (b)). Surface roughness is reduced to of 26 nm. When increasing the amount of x = y = 0.25 to 0.50 into Ag x Bi 2-x S 3-y lm, there are spherically grown grains ( gure 12 (a)) observed in the 2D image while the 3D image shows cylindrical pillars shaped grown rich grains ( gure.12 (b)). The surface roughness of the lm gets reduced to 24 nm. AFM scan of Ag x Bi 2-x S 3-y (x = y= 0.75) thin lm is shown in gure 13. The entire surface of the lms is denser and occupied by spherical shaped grains (2D image) as supported by the SEM results whereas the 3D image shows hillocks shaped grains that having circular top surface shaped grains whose surface roughness is again reduced to 20 nm. Figure 14 shows the AFM scans of Ag x Bi 2-x S 3-y (x = y= 1) thin lm. The image visualized with spherical grains distributed uniformly in the entire surface ( gure 14 (a)) whose roughness is reduced to 17 nm and the 3D image shows well-grown grains with a denser surface ( gure 14 (b)). The result of AFM images indicates the growth of grains along the c axis of all the lms. It is interesting to note that the AFM scans of Ag x Bi 2-x S 3y (x = y= 0.50, 0.75 and 1) thin lms are having almost similar topology features to that of SEM images.
In the present study, the standard procedure on Hall effect measurement for the prepared Ag x Bi 2-x S 3-y (x= y= 0 to 1) thin lms was carried out and their results are tabulated in Table 2. It is clearly evoked that all the lms have a positive Hall coe cient value, which con rms the lms possessed 'p' type charge carriers except Ag x Bi 2-x S 3-y (x= y= 0) thin lm.
In the case of carrier concentration (Table 2) In the case of resistivity, the 'ρ' value gets a decreasing trend for all the lms, and its range is measured to be 400 Ω -cm to 8.52 Ω -cm for FTO/ Ag x Bi 2-x S 3-y (x = y = 0 to 1) thin lms. This may be attributed to the higher mobility of the charge carriers (evidenced from table 2) and the reduction of potential barriers generated by the grain boundaries. Besides, it is also related to the signi cant crystallite growth, closer packing of crystallites on the surface of the substrate and the formation of spherical particles (from SEM analysis) [43][44][45].
Generally, spherical particles without any voids or cracks lead to avoid the trapping, scattering and recombination of the photogenerated charges [46]. The mobility of charge carriers of FTO/ Ag x Bi 2-x S 3-y (x = y = 0, 0.25 and 0.50) thin lms are lower than FTO/ Ag x Bi 2-x S 3-y (x = y = 0.75 and 1) lms, which may be attributed to the presence of secondary phase Bi 2 S 3 and AgBiS 2 (evidenced from the XRD analysis Fig. 1) in the lms. Among the lms, Ag x Bi 2-x S 3-y (x = y = 0.75 and 1) thin lms possessed higher mobility which related to higher lm thickness (which shortening the effective mean free path of the carriers [45] and which will reduce the recombination of charge carriers and leads to obtain better e ciency of photovoltaic devices [47]. Overall, the lms Ag x Bi 2-x S 3-y (x = y = 0.75 and 1) have carrier concentration, lower resistivity (higher conductivity), higher mobility due to better crystallites, improved crystallinity, dense and compact lm surface with defect free nature which increases the lifetime of the charge carriers [48].
In the present study, 0.2 M of (KI + I 2 ) is used as a redox electrolyte. M-S plot of the prepared FTO/ Ag x Bi 2x S 3-y (x= y= 0, 0.25, 0.50, 0.75 and 1) lms are given in gure 15. In the M-S plot, FTO/ Ag x Bi 2-x S 3-y (x= y= 0) thin lm shows a positive slope which con rms the 'n' type conductivity of the lm [49] whereas the rest of the lms exhibited negative slopes which con rmed the prepared lms are in 'p' type conducting nature as coinciding well with the observations made from Hall-effect analysis (table 2). The reason for the conversion of conductivity type in the lms (except x= y = 0) is attributed to the formation of acceptor defect between Ag and Bi. The 'p' type conductivity of the Ag x Bi 2-x S 3-y thin lm was also reported by T.
Manimozhi et al [2] and L. Hu et at. [37]. It can be seen from the gure 16 that the presence of accumulation region, depletion region and inversion region in FTO/ Ag x Bi 2-x S 3-y (x= y= 0, 0.25, 0.50, 0.75 and 1) /electrolyte. The values of these regions are tabulated in Table 3.  Table 3. The carrier concentration of FTO/Ag x Bi 2-x S 3-y (x= y = 0) lm is found to be 6.23 x10 19 cm -3 which is well accordance with Hall effect analysis (table 2). Interestingly, the carrier concentrations for the remaining lms under liquid medium lie in the optimal range about 7.39 x 10 17 cm -3 , 7.21 x10 17 cm -3 , 6.57 x10 17 cm -3 and 6.11 AgBiS 2 is a p-type semiconductor [2], hence the prepared FTO/ Ag x Bi 2-x S 3-y (x= y= 0.25, 0.50, 0.75 and 1) thin lms were subjected to analyze the PEC performance under both dark and illumination condition. The measured results are given in gure 16. Since Ag x Bi 2-x S 3-y (x= y = 0) possessed n-type conductivity, so we didn't concentrate the lm towards on both PEC and PV cells. In the PEC analysis, 0.2 M of KI+I 2 redox electrolyte solution was used for maintaining the stability of working electrodes (Ag x Bi 2-x S 3-y (x= y= 0.25, 0.50, 0.75 and 1)) as hole scavenger. Current density versus voltage (J -V) plots of the FTO/ Ag x Bi 2-x S 3-y (x= y= 0.25, 0.50, 0.75 and 1) thin lms are shown in gure 16.
Fill factor ((V max J max /V oc J sc ) x 100)) and photoconversion e ciency ((η (%) = (V oc J sc x FF x 100) / P in )) are calculated using the formulae [50] and the values are given in table 4. From gure 16 it is observed that all the lms exhibited cathodic photocurrent which leads to the reduction of the hole scavenger at the surface of the working electrode that con rms that the prepared Ag x Bi 2-x S 3-y (x= y= 0.25, 0.50, 0.75 and 1) thin lms are 'p' type in nature [2,51,37].
The values of V oc , J sc and FF for Ag x Bi 2-x S 3-y (x= y= 0.25) lms are found to be 0.29 V, 1.26 mA/cm 2 and 18% respectively. The lm has exhibited e ciency about 0.65%. While increasing the Ag (x=0.50) and reduce the S (y=0.50) into Ag x Bi 2-x S 3-y lm, the V oc , J sc and FF values are enhanced upto 0.35 V, 1.97 mA/cm 2 , 23% respectively and it is found to be 1.57%. Further the values of V oc , J sc and FF get increased about 0.55 V, 3.37 mA/cm 2 , and 25% respectively for x= y= 0.75 lm. E ciency is found to be 4.78 %. In the case of x= y= 1, the V oc , J sc and FF is found to be 0.58 V, 4.62 V and 26% respectively and the e ciency get increased to 7.03% for Ag x Bi 2-x S 3-y (x=y= 1) lm. This may be attributed to the reduction in dislocation density (table 1), optimum E g value, higher 'α' value, optimal carrier concentration range and enhanced mobility of charge carriers which reduces the recombination loss and increases the e ciency of the cells. (Table 2 and 3). Based on the results, the performances of FTO/ Ag x Bi 2-x S 3-y thin lms can be written in the order of x= y= 1 > x= y= 0.75 > x= y= 0.50> x= y= 0.25.
For fabrication of solar cell, FTO coated Ag x Bi 2-x S 3-y (x = y= 0.25 to 1) thin lm used as an absorbing 'p' layer and CdS was used as an 'n' type layer and the device structure is FTO/ Ag x Bi 2-x S 3-y (x = y= 0.25 to 1)/CdS/Ag. LSV technique is adopted to investigate the photovoltaic performance (correlation between the current density and applied voltage of the FTO/ Ag x Bi 2-x S 3-y /CdS/Ag (x = y = 0.25, 0.5, 0.75 and 1) of the prepared device under dark and illumination condition. J -V analysis of FTO/ Ag x Bi 2-x S 3-y /CdS/Ag (x= y= 0.25 to 1) is shown in gure 17. The device is exposed in front of the light that exhibits solar cell behaviour. The observation con rmed that the prepared devices are photoconductive in nature [38,52].
Under illumination, the photons are allowed to fall on the CdS layer and the energy of incident photon is enough to break the covalent bond, which results the creation of free electron -hole pairs current conduction [53]. Table 5 shows the solar cell parameters of V oc , J sc , FF and η. FTO/ Ag x Bi 2-x S 3-y (x = y=0.25) /CdS/Ag device produced the V oc , J sc and FF of 0.42 V, 0.58 mA/cm 2 and 21% respectively whose e ciency is found to be 0.53%. FTO/ Ag x Bi 2-x S 3-y (x = y = 0.50) /CdS/Ag device produced higher e ciency about 1.06% compared to the lm prepared for x = y=0.25 whose V oc , Jsc of FF are found to be 0.36 V, 1.26 mA/cm 2 and 23% respectively. While the e ciency of FTO/ Ag x Bi 2-x S 3-y (x = y = 0.75) /CdS/Ag and of FTO/ Ag x Bi 2-x S 3-y (x = y = 1) /CdS/Ag devices have higher than x = y=0.50 and it is found to be 2.23% and 3.26 % respectively and the both device possessed same FF of 21%.
Among the devices, the FTO/ Ag x Bi 2-x S 3-y (x=y=1) /CdS/Ag device has higher e ciency is attributed to the better crystallinity (from XRD), smooth and void/crack free surface (from SEM) [41], lower surface roughness, optimum carrier concentration and mobility of charge carriers (evidenced from Hall parameters table 2) that reduces the recombination of electron and hole which leads to higher e ciency than other devices. Besides, thickness [54] and bandgap energy of the absorbing layer leads to enhance the generation of photogenerated carriers and hence higher photoconversion e ciency [55]. For an ideal solar cell, the carrier concentration must be in the order of 10 16 to 10 17 cm -3 this leads to getting appreciable PCE in both PEC and PV devices. The solar cell performances of FTO/ Ag x Bi 2-x S 3-y/CdS/Ag devices can be written in the order of x= y= 1 > x= y = 0.75 > x= y= 0.50 > x= y= 0.25 respectively. Solar performance of the present result is compared with previous reports and is given in table 6. From the table 6, it is noted that the device FTO/ AgBiS 2 /CdS/Ag has higher e ciency than that of AgBiS 2 /PTB7 [56], AgBiS 2 QDSSC [9], ITO/ZnO/AgBiS 2 /P3HT/MoO 3 /Al [17] devices whereas slightly lower than ITO/ZnO/AgBiS 2 /P3HT/Au [37] and ITO/ZnO/ AgBiS 2 /PTB7/MoO 3 /Ag [11,12] devices. By comparing with higher performance devices, the present study used AgBiS 2 coated on FTO as 'p' layer, CdS as 'n' layer and Ag metallic contact whereas in the previous investigations, the authors had used polymer layer for conducting enhancement, window layer for avoiding the loss of incident photons and Mo as metallic contact.

Conclusion
The opto-structural, morphological, topological and electrical properties of thermally evaporated Ag x Bi 2x S 3-y (x= y= 0, 0.25, 0.50, 0.75 and 1) thin lms are studied. The following conclusions are drawn from the studies. The XRD analysis con rms the formation of cubic structured AgBiS 2 and orthorhombic structured Bi 2 S 3 . Besides, changes in peak position, peak shift, variation in intensity, presence of impurity phases (for samples Ag x Bi 2-x S 3-y (x = y = 0.25) and Ag x Bi 2-x S 3-y (x = y = 0.50), changes in crystallite size and dislocation density are also observed from the XRD analysis. Ag x Bi 2-x S 3-y exhibited strong optical absorption in the visible region whose optical absorption coe cient is found to be 10 5 cm -1 . The E g values are found to be decreasing from 2.08 eV to 1.35 eV for Ag x Bi 2-x S 3-y (x= y = 0 to 1) lms. SEM images showed the uniform distribution of spherical particles of Ag x Bi 2-x S 3-y (x= y = 0 to 1) lms. Surface roughness values are found to be decreased with increase the x and y values. From the Hall effect measurement, carrier concentration, improved mobility and p-type electrical conductivity (except Ag x Bi 2x S 3-y (x= y = 0 ) thin lm) were observed. Among the PEC cells, the FTO/Ag x Bi 2-x S 3-y (x= y = 1)/electrolyte cell exhibited highest photoconversion e ciency of about 7.03% attributed to reduction in dislocation density, optimum E g value, higher 'α' value, optimal carrier concentration range and enhanced mobility of charge carriers which reduces the recombination loss and increases the e ciency of the cells. In the case of solar cell performance, FTO/ Ag x Bi 2-x S 3-y (x= y = 1) /CdS/Ag cell showed highest photoconversion e ciency of 3.26% than other systems attributed to the better crystallinity (from XRD), smooth and void/crack free surface (from SEM), lower surface roughness, optimum carrier concentration and mobility of charge carriers.