Film thicknesses of AgxBi2-xS3-y (x= y =0), AgxBi2-xS3-y (x= y =0.25), AgxBi2-xS3-y (x= y =0.50), AgxBi2-xS3-y (x= y =0.75), and AgxBi2-xS3-y (x= y =1) thin films were measured using Swanepoel method [22] and the calculated values are given in table 1.
Figure 1 shows the XRD patterns of thermally deposited AgxBi2-xS3-y (x= y= 0 to 1) thin films annealed at 300 °C. The XRD pattern of AgxBi2-xS3-y (x= y= 0) shows a number of polycrystalline peaks at 2θ=28.7°, 31.8°, 45.6°, 48.3° and 59° which are corresponding to the (hkl) planes of (2 1 1), (2 2 1), (4 4 0), (0 6 0) and (2 4 2) respectively of orthorhombic structured Bi2S3, matched well with the standard JCPDS card number 170320. The result well coincides with earlier reports on Bi2S3 by X. H. Liao et al [23], F. Ding et al [24] and V. Stavila et al [25].
When introducing Ag (x= 0.25) and reducing S (y = 0.25) into AgxBi2-xS3-y film, a high intense prominent peak is observed at 2θ = 31.6° due to the reflection of (2 0 0) plane belongs to the formation of cubic structured AgBiS2 (JCPDS 893672), with existing peaks of orthorhombic structured Bi2S3 (at 2θ=28.6°). A peak shift is observed (from 2θ=28.7° to 31.6°) towards the higher Bragg angle than that of film AgxBi2-xS3-y (x= y= 0), which confirms the incorporation of Ag into Bi2S3 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 AgxBi2-xS3-y, the position of the prominent peak does not vary but its FWHM gets narrower than film prepared at x = y = 0.25. It is also observed the presence of Bi2S3 at 2θ = 44.5° (4 4 0).
Further, increase the amount of Ag and reducing S from 0.50 to 0.75 (AgxBi2-xS3-y (x = y = 0.75)), the film has phase pure cubic structured AgBiS2 (2θ= 31.6° (2 0 0) and 66.1° (4 0 0)). The similar result is continued in x = y = 1 ie., AgxBi2-xS3-y film also and the result is well accordance with earlier studies on AgBiS2 [2, 3,17,26,27].
The average crystallite size (D), strain (ε) and dislocation density (δ) calculated using the following formulae (equations 1,2 and 3) [2]
D = 0.9 λ/ β cos θ ----- (1)
ε = β / tan θ ----- (2)
δ = 1 / D2 m-2 -----(3)
where ‘λ’ is the wavelength of incident X-ray beam (1.5406 Å), ‘β’ is the full width at half maximum value, ‘θ’ is the angle of diffraction. The calculated values are listed in table 1. Lattice parameter (a) can be calculated using the formula, a = d [(h2+k2+l2)]1/2. It is observed that average crystallite size is found to be increased in comparison to x = y = 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 Bi2S3 lattice.
It is noticed from the XRD analysis that the deposited films have a change in peak position, variation in intensity, presence of impurity phases (for films AgxBi2-xS3-y (x = y = 0.25) and AgxBi2-xS3-y (x = y = 0.50), changes in crystallite size and dislocation density with the variation of Ag and S.
The prepared AgxBi2-xS3-y (x = y= 0 to 1) films were subjected into UV-Visible spectroscopy technique and their results are shown in figure 2. It can be seen from the spectra that the AgxBi2-xS3-y (x = y = 0) film shows good optical absorption in the visible region. While introducing of Ag and reducing S content (x = y = 0.25) into AgxBi2-xS3-y film, the optical absorption get enhanced than the Bi2S3 film and the result is continued upto x = y = 0.50. By comparing the UV-Visible spectra, the AgxBi2-xS3-y films prepared at x = y = 0.25 and 1 have exhibited higher optical absorption than the AgxBi2-xS3-y films prepared for x = y = 0.75 and 0.50 attributed to phase pure nature, higher film thickness values with wavy shaped which indicates homogeneity of the films [30]. The lowering of optical absorbance may be due to the presence of Bi2S3 secondary phases (evident from XRD).
The optical absorption coefficient (α) vs wavelength plots of AgxBi2-xS3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin films are shown in figure 3. The ‘α’ value is found to be 105 cm-1 for all the films except AgxBi2-xS3-y (x= y= 0) film. Due to the phase pure nature, AgxBi2-xS3-y (x = y = 0.75) and AgxBi2-xS3-y (x = y = 1) films having higher ‘α’ value than others. This result suggests that all the films have direct band gap energy and this phenomenon influenced the solar conversion efficiency [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 film material’s region transparency. Based on this aspect, it is important to estimate the bandgap energy of AgxBi2-xS3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) films using Tauc’s relation [32].
Figure 4 shows the optical band gap energy ‘Eg’ ( (αhυ)2 vs hυ) of AgxBi2-xS3-y (x = y = 0 to 1) thin films. The bandgap values of AgxBi2-xS3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin films 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 AgBiS2 thin films [3, 33].
By comparing the Eg values, the higher Eg values are observed for the films prepared at x = y= 0.25 and x = y = 0.50 than the remaining films (x = y= 0.75 and x = y = 1), be due to the formation of secondary phases in the films 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 AgxBi2-xS3-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 film thickness [36,37]. Besides, the optical band gap energy values mostly depend on the crystalline nature of the films, incorporation of precursors [38] and the method of film deposition [39]. However, these values lie in the range of ideal region for PV applications [40]. The optical absorption, ‘α´ and Eg of AgxBi2-xS3-y (x = y = 0, 0.25, 0.50, 0.75 and 1) thin films have met the requirements for the use of solar absorber in PV and PEC cells.
The surface morphology of thin film 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 AgxBi2-xS3-y (x = y = 0) thin film with two different magnifications (10k and 30k). The surface of the film shows uniformly distributed grains and average grain sizes are measured to be lesser than 500 nm as clearly reveals in the higher magnification image (30k).
When Ag and S (x= y= 0.25) are introduced into AgxBi2-xS3-y system, the surface seems to be dense in nature as clearly displayed in figure 6. The deposited particles are uniformly distributed on the entire surface and individual grains with grain boundaries are visualized in the higher magnification (50k). The average grain size of the nanospheres is found to be less than 100 nm.
The surface morphologies of AgxBi2-xS3-y (x = y= 0.50 to 1) thin films are shown in shown in figures 7 to 9 with different magnifications (10k (a) 30k (b) and 50k). It can be seen from the images (30k and 50k) that well defined spherical grains are uniformly distributed on the film 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 electron-hole pairs during the photovoltaic process [30, 42].
Figures 10 to 14 show the AFM images of AgxBi2-xS3-y thin films for various x & y concentrations (x=0, 0.25, 0.50, 0.75 and 1). Figure 10 shows the AFM images of AgxBi2-xS3-y (x = y= 0) thin film which exhibits the growth of irregular polycrystalline shaped particles as clearly visible on the film surface. Surface roughness is measured to be 30 nm.
When introducing x= y= 0.25 of Ag and S into AgxBi2-xS3-y film, there are well defined polygonal-shaped grains are seen in the 2D image (figure 11 (a)) while pyramid- like structure is clearly visible in the 3D image (figure 11 (b)). Surface roughness is reduced to of 26 nm. When increasing the amount of x = y = 0.25 to 0.50 into AgxBi2-xS3-y film, there are spherically grown grains (figure 12 (a)) observed in the 2D image while the 3D image shows cylindrical pillars shaped grown rich grains (figure.12 (b)). The surface roughness of the film gets reduced to 24 nm. AFM scan of AgxBi2-xS3-y (x = y= 0.75) thin film is shown in figure 13. The entire surface of the films 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 AgxBi2-xS3-y (x = y= 1) thin film. The image visualized with spherical grains distributed uniformly in the entire surface (figure 14 (a)) whose roughness is reduced to 17 nm and the 3D image shows well-grown grains with a denser surface (figure 14 (b)). The result of AFM images indicates the growth of grains along the c axis of all the films. It is interesting to note that the AFM scans of AgxBi2-xS3-y (x = y= 0.50, 0.75 and 1) thin films 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 AgxBi2-xS3-y (x= y= 0 to 1) thin films was carried out and their results are tabulated in Table 2. It is clearly evoked that all the films have a positive Hall coefficient value, which confirms the films possessed ‘p’ type charge carriers except AgxBi2-xS3-y (x= y= 0) thin film.
In the case of carrier concentration (Table 2), the FTO/ AgxBi2-xS3-y (x = y = 0) film has the highest value in the order of 1019 cm-3. When introducing x = y = 0.25 of Ag and S into the AgxBi2-xS3-y film, the carrier concentration level get decreased (8.34 x 1017 cm-3) and it slightly increased to 8.78 x 1017 cm-3, 9.01 x 1017 cm-3 and 9.58 x 1017 cm-3 for FTO/ AgxBi2-xS3-y (x = y = 0.50), FTO/ AgxBi2-xS3-y (x = y = 0.75) and FTO/ AgxBi2-xS3-y (x = y = 1) films respectively. This may be attributed to the reduction of surface roughness of the film (evidenced from AFM analysis). Importantly, all the films (except the film FTO/ AgxBi2-xS3-y (x = y = 0) ) showed the carrier concentration in the order of 1017 cm-3 which are having considerable range to meet the optimum level for solar cell application (~1017 cm-3). T. Manimozhi [2] et al., have also reported the same results of carrier concentration in the order of 1017 cm-3.
In the case of resistivity, the ‘ρ’ value gets a decreasing trend for all the films, and its range is measured to be 400 Ω -cm to 8.52 Ω -cm for FTO/ AgxBi2-xS3-y (x = y = 0 to 1) thin films. 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 significant crystallite growth, closer packing of crystallites on the surface of the substrate and the formation of spherical particles (from SEM analysis) [43-45].
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/ AgxBi2-xS3-y (x = y = 0, 0.25 and 0.50) thin films are lower than FTO/ AgxBi2-xS3-y (x = y = 0.75 and 1) films, which may be attributed to the presence of secondary phase Bi2S3 and AgBiS2 (evidenced from the XRD analysis Fig. 1) in the films. Among the films, AgxBi2-xS3-y (x = y = 0.75 and 1) thin films possessed higher mobility which related to higher film 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 efficiency of photovoltaic devices [47].
Overall, the films AgxBi2-xS3-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 film surface with defect free nature which increases the lifetime of the charge carriers [48].
In the present study, 0.2 M of (KI + I2) is used as a redox electrolyte. M-S plot of the prepared FTO/ AgxBi2-xS3-y (x= y= 0, 0.25, 0.50, 0.75 and 1) films are given in figure 15. In the M-S plot, FTO/ AgxBi2-xS3-y (x= y= 0) thin film shows a positive slope which confirms the ‘n’ type conductivity of the film [49] whereas the rest of the films exhibited negative slopes which confirmed the prepared films 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 films (except x= y = 0) is attributed to the formation of acceptor defect between Ag and Bi. The ‘p’ type conductivity of the AgxBi2-xS3-y thin film was also reported by T. Manimozhi et al [2] and L. Hu et at. [37]. It can be seen from the figure 16 that the presence of accumulation region, depletion region and inversion region in FTO/ AgxBi2-xS3-y (x= y= 0, 0.25, 0.50, 0.75 and 1) /electrolyte. The values of these regions are tabulated in Table 3.
FTO/ AgxBi2-xS3-y (x= y= 0, 0.25, 0.75 and 1) samples having accumulation, depletion and inversion regions whereas the FTO/ AgxBi2-xS3-y (x= y= 0.50) sample shows accumulation and depletion regions only. It is also seen that the accumulation, depletion and inversion region of FTO/ AgxBi2-xS3-y films varied with x and y values. The carrier concentration of the FTO/AgxBi2-xS3-y (x= y = 0 to 1) thin films was calculated from the slope of the Mott-Schottky equation [50] and it is presented in Table 3. The carrier concentration of FTO/AgxBi2-xS3-y (x= y = 0) film is found to be 6.23 x1019 cm-3 which is well accordance with Hall effect analysis (table 2). Interestingly, the carrier concentrations for the remaining films under liquid medium lie in the optimal range about 7.39 x 1017 cm-3, 7.21 x1017 cm-3, 6.57 x1017 cm-3 and 6.11 x1017 cm-3 for FTO/AgxBi2-xS3-y (x= y = 0.25 to 1) films respectively. Overall, the carrier concentration values are almost similar to the values determined from the Hall-effect analysis (table 2).
AgBiS2 is a p-type semiconductor [2], hence the prepared FTO/ AgxBi2-xS3-y (x= y= 0.25, 0.50, 0.75 and 1) thin films were subjected to analyze the PEC performance under both dark and illumination condition. The measured results are given in figure 16. Since AgxBi2-xS3-y (x= y = 0) possessed n-type conductivity, so we didn’t concentrate the film towards on both PEC and PV cells. In the PEC analysis, 0.2 M of KI+I2 redox electrolyte solution was used for maintaining the stability of working electrodes (AgxBi2-xS3-y (x= y= 0.25, 0.50, 0.75 and 1)) as hole scavenger. Current density versus voltage (J – V) plots of the FTO/ AgxBi2-xS3-y (x= y= 0.25, 0.50, 0.75 and 1) thin films are shown in figure 16.
Fill factor ((VmaxJmax/VocJsc) x 100)) and photoconversion efficiency ((η (%) = (Voc Jsc x FF x 100) / Pin)) are calculated using the formulae [50] and the values are given in table 4. From figure 16 it is observed that all the films exhibited cathodic photocurrent which leads to the reduction of the hole scavenger at the surface of the working electrode that confirms that the prepared AgxBi2-xS3-y (x= y= 0.25, 0.50, 0.75 and 1) thin films are ‘p’ type in nature [2, 51, 37].
The values of Voc, Jsc and FF for AgxBi2-xS3-y (x= y= 0.25) films are found to be 0.29 V, 1.26 mA/cm2 and 18% respectively. The film has exhibited efficiency about 0.65%. While increasing the Ag (x=0.50) and reduce the S (y=0.50) into AgxBi2-xS3-y film, the Voc, Jsc and FF values are enhanced upto 0.35 V, 1.97 mA/cm2, 23% respectively and it is found to be 1.57%. Further the values of Voc, Jsc and FF get increased about 0.55 V, 3.37 mA/cm2, and 25% respectively for x= y= 0.75 film. Efficiency is found to be 4.78 %. In the case of x= y= 1, the Voc, Jsc and FF is found to be 0.58 V, 4.62 V and 26% respectively and the efficiency get increased to 7.03% for AgxBi2-xS3-y (x=y= 1) film. This may be attributed to the reduction in dislocation density (table 1), optimum Eg value, higher ‘α’ value, optimal carrier concentration range and enhanced mobility of charge carriers which reduces the recombination loss and increases the efficiency of the cells. (Table 2 and 3). Based on the results, the performances of FTO/ AgxBi2-xS3-y thin films 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 AgxBi2-xS3-y (x = y= 0.25 to 1) thin film used as an absorbing ‘p’ layer and CdS was used as an ‘n’ type layer and the device structure is FTO/ AgxBi2-xS3-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/ AgxBi2-xS3-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/ AgxBi2-xS3-y /CdS/Ag (x= y= 0.25 to 1) is shown in figure 17. The device is exposed in front of the light that exhibits solar cell behaviour. The observation confirmed 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 Voc, Jsc, FF and η. FTO/ AgxBi2-xS3-y (x = y=0.25) /CdS/Ag device produced the Voc, Jsc and FF of 0.42 V, 0.58 mA/cm2 and 21% respectively whose efficiency is found to be 0.53%. FTO/ AgxBi2-xS3-y (x = y = 0.50) /CdS/Ag device produced higher efficiency about 1.06% compared to the film prepared for x = y=0.25 whose Voc, Jsc of FF are found to be 0.36 V, 1.26 mA/cm2 and 23% respectively. While the efficiency of FTO/ AgxBi2-xS3-y (x = y = 0.75) /CdS/Ag and of FTO/ AgxBi2-xS3-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/ AgxBi2-xS3-y (x=y=1) /CdS/Ag device has higher efficiency 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 efficiency 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 efficiency [55]. For an ideal solar cell, the carrier concentration must be in the order of 1016 to 1017 cm-3 this leads to getting appreciable PCE in both PEC and PV devices. The solar cell performances of FTO/ AgxBi2-xS3-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/ AgBiS2/CdS/Ag has higher efficiency than that of AgBiS2 /PTB7 [56], AgBiS2 QDSSC [9], ITO/ZnO/AgBiS2/P3HT/MoO3/Al [17] devices whereas slightly lower than ITO/ZnO/AgBiS2/P3HT/Au [37] and ITO/ZnO/ AgBiS2 /PTB7/MoO3/Ag [11, 12] devices. By comparing with higher performance devices, the present study used AgBiS2 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.