Coupling of TiO 2 and ZnO with metal sulfides (CuS AND ZnS) for applications in solar cells

TiO 2 and ZnO films were synthesized by the dip-coating method. The coupling of thin films of metal sulfides: CuS and ZnS by thermal evaporation in each semiconductor oxide was analyzed. TiO 2 , CuS/TiO 2 , ZnS/TiO 2 , ZnO, CuS/ZnO, and ZnS/ZnO samples were analyzed by X-ray diffraction, Raman microscopy, atomic force microscopy, UV-Vis spectroscopy, and photoresponse. The optical and morphological analysis revealed that the CuS/TiO 2 and CuS/ZnO samples present better properties than the pristine samples. This was attributed to the fact that their absorption edges move to lower energy regions and their roughness increases refraction, so there is a greater photoelectric response due to the accumulation of photo-injected electrons in the conduction band. (


Introduction
The manufacture of photovoltaic devices using semiconductor materials has had a great impact because it is a highly efficient and environmentally friendly technology [1].Among the various semiconductor materials that are applied to photovoltaic devices are mainly TiO 2 [2] and ZnO [3], which are chemically stable, cheap, non-toxic and easy to manufacture [4], [5], [6].However, the disadvantage of these semiconductors is their high charge carrier recombination [7].Therefore, layers of metal sulfides have been incorporated to improve the optical properties [8].Among the most used metal sulfides as an electron transport layer are: CuS and ZnS whose energy gaps are 2.24 and 3.68 eV, respectively [9], [10].In addition, it has been reported that the incorporation of this sulfide layer provides a significant improvement in corrosion resistance [11].
In the present study the synthesis and deposition of TiO 2 and ZnO films using the dip coating method is reported.After the deposit of the oxides, metallic sulfides of CuS and ZnS were coupled by the evaporation method.With the coupling, the aim is to improve the optical properties of the heterojunctions and thus have an impact on the reduction of the recombination processes of the photogenerated electron/hole pairs (e -/h + ), which affect the efficiency of solar cells.The optical and electrical characterization of thin films allows the analysis of transmittance, refraction, and response to visible light.

Characterization
The materials were characterized using a Rigaku X-ray diffractometer (λ (Cuk_α)=1.54056Å).Raman spectra were obtained with a HORIBA XploRA PLUS spectrometer with a 532 nm laser as Nd-YAG excitation source.An Agilent-8453 UV-Vis spectrophotometer was used to determine optical transmittance.An XE7 Park system atomic force microscope was used to obtain the topography.For the analysis of the photoresponse of the films, an AUTOLAB PGSTAT80N galvanostat potentiostat was used with an applied voltage of 1 V for 200 seconds with 50 W halogen lamp lighting intermittently every 10 seconds.
The diffractograms were compared with JCPDS 06-0464 for CuS and JCPDS 75-1534 for ZnS.It is observed that the incorporation of CuS and ZnS in the TiO 2 and ZnO films causes the shifting of some planes to the left and right, as shown in Figures 1-b), c), e), and f).It is attributed to the likenesses between the positions of their diffraction planes and the electronic electronegativity of the TiO 2 , ZnO, CuS, and ZnS compounds.In addition, because Ti 4+ (0.61 Å), Zn 2+ (0.75 Å), and Cu 2+ (0.69 Å) have different ionic radii, strong TiO 2 and ZnO signals are promoted in the diffractograms [17], [18].The amorphous region in the diffractograms is attributed to the glass substrate [19].
Crystal sizes (D) were calculated using the Scherrer equation [20]: K is a shape factor (0.9), λ is the wavelength, β is the width at half height of the principal plane (101), and θ is the Bragg angle.The values obtained for the TiO 2 and ZnO crystals are summarized in Table 1.The results show changes in the crystallite size and the lattice parameters, attributed to the stacking of the crystals [21].Moreover, the distortion that can occur as a result of the substitution of Ti 4+ ions by Zn 2+ in the structure, which, due to having similar ionic radii, do not change the TiO 2 phase [21], [22].

Raman spectroscopy
Figure 2-a) shows vibrational bands located at 144 cm -1 and 638 cm -1 of the E g ( 1) and E g (3) modes of the anatase phase of TiO 2 .The 399 cm -1 peak is assigned to the B 1g (1) mode and is the symmetric bending vibration of the O-Ti-O bonds; and the 515 cm -1 peak assigned to the A 1g +B 1g mode; the A 1g mode antisymmetric bending vibration of O-Ti-O bonds [23].In CuS/TiO 2 and ZnS/TiO 2 samples, signal displacements are observed due to a change in symmetry associated with the chemical interaction between ZnS, CuS and TiO 2 , respectively [24].
Figure 2-b) shows the characteristic vibrational modes of the hexagonal wurtzite structure of ZnO [13].The E 2 (low) vibrational mode located at ~100 cm -1 is associated with the Zn 2+ sublattice [25].The E 2 (high) -E 2 (low) mode at 330 cm -1 is associated with the optical transversal mode of the lattice phonons.The E 2 (high) mode signal is related to high crystallinity in the samples and is located at ~440 cm -1 .The band at 573 cm -1 is associated with the E 1 (LO) of the optical longitudinal mode of the phonons [26], [27].The shifting of the characteristic peaks of semiconductor oxides incorporating sulfides is related to substituting Cu and S ions in the crystalline structure [28], corroborating the X-ray observations.Figure 3 shows the FWHM behavior of the Raman spectrum in the preferential direction of TiO 2 (144 cm -1 ) and ZnO (440 cm -1 ).Since the values decrease, the heterojunctions have better crystallinity than the oxide films [12].

UV-Vis spectroscopy
In the UV-Vis spectra shown in Figure 4, it is observed that CuS/TiO 2 extends the absorption range to the visible light region since CuS nanoparticles contribute to the displacement due to band doubling [29].Figure 4 shows that ZnO transmits 73-87% in the visible region, and the transmittance of CuS/ZnO decreases concerning ZnO (52-73%).In addition, a displacement in the absorption of ZnS/TiO 2 attributed to the doubling of bands in the composite material is observed [24].The ZnS/ZnO spectra show a slight displacement in the absorption caused by energetic states generated with the coupling (83% in visible).The bandgap of the films was calculated using the Tauc model (Eq.2), extrapolating the straight (ℎ)  vs ℎ [30].
where k is Planck's constant, hv is the photon energy (eV),  = 1/2 y  = 2, for ZnO and TiO 2 , respectively.The decrease in the bandgap of the couplings for the pure compounds is attributed to the incorporation of energetic states [31].Table 2 summarizes the refractive index values and the porosity calculated using the Wolfe method.The results for the compounds with CuS show that the refractive index decreases when the porosity of the films increases (See Figure 5) [30], [32].

Atomic force microscopy (AFM)
The roughness of the samples is observed in the AFM micrographs (Figure 6).The roughness values are 1,507 nm, 2,262 nm, and 30 nm for TiO2, ZnS/TiO 2 , and CuS/TiO 2 , respectively.The difference in the roughness values is attributed to the nucleation of the particles and is related to the results obtained for the refractive index (Figure 5) [33], [34], [35], [36].For the samples with ZnO, the roughness obtained is similar to that shown by the films with TiO2: CuS/ZnO > ZnO > ZnS/ZnO.

Photoelectrochemical evaluation
Figure 7 shows the photoelectric response of films under visible light irradiation.The results show a weak TiO₂ signal attributed to the band gap.The CuS/TiO₂ film has higher electrical conduction than TiO 2 due to the combination of bands that induce photo-induced charge separation [37].However, the CuS/ZnO sample exhibits the most increased electrical conduction due to decreased film porosity.In addition, this film presents a decrease in resistivity due to the incorporation of CuS, improving the current collection in the electrodes.Therefore, CuS nanoparticles enhance the separation of photo-induced electron-hole pairs [38].

Conclusions
Experimental results obtained showed that the fabrication of thin films with CuS and ZnS supports by metal oxides (TiO 2 y ZnO) are benefited by the whole samples, indicating a decrease of energy gaps due the size change of crystals.As wel as, increase a considerable way of absorb the visible spectrum because of the new creations of energetics levels that helps enhancing factors like electrical and optical.This as a result of an excellent homogeneity of the material roughness, besides all that has CuS present a major roughness, generating a better refraction and a photoelectric response due to the accumulation of photo-injected electrons in the conduction band.The response to light to conduct electricity of the composites coupled with CuS is greater than those coupled with ZnO.
Meanwhile, these results establish that the dense, smooth and homogeneity obtained by Dip-Coating and evaporation were such effective in order to decrease considerably the resistance to charge transfer.

Fig. 5 .
Fig. 5. Variation of refractive index and porosity films based on TiO 2 y ZnO.

Table 1 .
Structural Patterns of TiO 2 and ZnO Crystal.

Table 2 .
Optical and Surface properties of oxides and compositos samples.