Semiconducting Thin Films of CuSbS2

In this paper we presenta method to produce polycrystalline CuSbS, thin ?lms through a solid-state reaction at 350 oC and 400 oC involving thin ?lm multilayer ofSb,S, -CuS or Cu,_,Se by chemical bath deposition technique. The formation of the ternary compound was confirmed by X-ray di?raction (XRD). A direct optical band gap of approx. 1.57 eV anda p-type electrical conductivity of 10-3 (O•cmr' were measured. These optoelectronic characteristics show perspective forthe use of CuSbS, as a suitable absorber material in photovoltaic applications. l. lntroduction Many authors have reported antimony sulphide (Sb,S3) thin films obtained by chemical deposition technique since early 1990's [1-4]. Chemical bath deposition (CBD) is a simple and low-cost method to produce thin films of different semiconductor compounds [5-6]. This method has been employed by sorne authors to synthesis temary compounds of antimony chalcogenides involving heat treatments in air or nitrogen atmosphere [7-9]. Rodríguez et al. have reported the formation ofCuSbS, by chemical bath with a p-type electrical conductivity of0.03 (Q.cmr' anda direct optical band gap of 1.52 eV [7]. Subsequently, the same group reported the use of CuSbS, thin films in a p-i-n solar cell structure with an open circuit voltage of 345 m V [10]. Ezugwu et al. employed CBD technique to deposit directly CuSbS, with direct band gaps between 1 .30 and 2.30 eV [11]. lts properties match with the requirement for the photovoltaic materials [12]. Manolache et al. have obtained this material by spray pyrolysis deposition with suitable characteristics for its application in photovoltaic devices [13]. Rabhi et al. have prepared polycrystalline CuSbS, using thermal evaporation method. The films showed direct band gaps at 1.3 and 1. 79 e V after heat treatment at 200 oC in N, [14]. The growing effort to find absorber materials involving copper, is because of the p-type conductivity originating from copper deficiency, which can be utilized to produce p-type absorber films as an altemative to Cu(ln/Ga)(S/Se),. An altemative to replace the CulnS, is CuSbS,; which belongs to the same 1-III-VI, group of semiconductor with the chalcopyrite structure, in which the ionic radius of indium and antimony are almost equal [7]. In this work, we present the formation of CuSbS, thin films of about 600 nm in thickness through the solid state reaction at 350-400 oC of chemically deposited thin films of Sb,S,--CuS or Cu,_,Se. 2. Experimental details 2.1 Sb2S, thin films Thin films of Sb,S3 were deposited on clean microscope glass slides using a chemical bath deposition reported previously by Grozdanov [3] and modified later by Nair et al. as reported in reference [4]. The reaction solution was prepared by dissolving 650 g ofSbCI, in 2.5 mL acetone and 25mL 1 MNa,S 20 3• Thebathwasmaintainedat 1 oCduring6 h. After this time an amorphous Sb,S3 thin film of 600 nm in thickness was obtained. The methodology of deposition has been explained in reference [15). Heat treatment of these films in air at 200 oC during 15 min was necessary in order to give adhesion between the film and the glass substrate. Subsequently, a thin film of CuS was deposited on the preheated Sb,S3 films using the chemical bath reported previously in the reference [16) or chemical bath ofCu,_,Se using the composition reported in reference [ 17]. 2.2 CuS thin film Thin films ofCuS were deposited on the Sb,S3 thin films using a reaction solution containing 1 O mL of0.5 M CuCI,, 8 mL oftriethanolamine (TEA) 50%, 8 mL of 15 M arnmonia (aq.), 10 mL of 1 M NaOH, 6 mL of 1 M thiourea and distilled water to complete a volume of 100 mL. During one hour at 30 oC, a CuS thin film of 120 nm in thickness was deposited on the Sb,S3 films. Toe preheated Sb,S3 films were placed in the CuS bath after 30 min ofthe bath preparation, in order to avoid the peeling of the Sb,S3 films due to the ammonia contained in the CuS bath. Temperature of the bath was maintained at 30 oC. Samples were removed from this bath after 1 h, 2 h and 3 h, rinsed in distilled water and dried in air at room temperature. 2.3 Cu2_,Se thin mm The thin films of Cu,_,Se were deposited on Sb,S3 thin films using reaction solution containing 1 O mL of 0.5 M CuSO., 1.5 mL of ammonia (aq.) 15 M, 12 mL 0.4 M Na,SeSO, solution and distilled water to complete 100 mL volume bath. Substrates with Sb,S3 thin film previously deposited were placed in the Cu,_,Se bath 30 min after preparation. The chemical bath was maintained at 30 oC during 1 h, 2 h and 3 h. Samples were taken out from the bath each hour, rinsed in distilled water and dried in air at room temperature. 2.4 Characterization X-ray diffraction (XRD) pattems were recorded using a Rigaku D-Max 2000 diffractometer using Cu-Ka (~= 1 .5406 Á) radiation in the glazing incidence mode (1.5°). Toe optical transmittance and specular reflectance spectra were measured using a Shimadzu 3100 PC spectrophotometer in the wavelength range of 250 2500 nm. Photocurrent responses of the films were obtained using tungsten-halogen radiation and a computerized measurement system using a Keithley 230 programmable voltage source anda Keithley 619 multimeter. Thickness of the films was measured usingAlpha Step 100 (Tencor, CA). '¡;;' a. .!:!. :z:. ·¡¡; e ~ .E Semiconducting Thin Films of CuSbS, 3. Results and discussion 3.1 X-Ray Diffraction Figure 1 showstheXRDpattemsofSb,S, (600nm)-CuS (120 nm) annealed at 350 oC (figure la) and annealed at400 oC (figure 1 b) in N, at 40 Pa during 1 h. We observed that for the sample heated at 350 oC, the majority ofthe diffraction peaks correspond to the XRD pattem of Sb,S, (PDF 421393). In the case ofthe sample heated at 400 oC, the peaks correspond to the pattem given for CuSbS, (PDF 44-1417). From figure la and lb we may note that the conversion of Sb2S,--CuS film to CuSbS, begins at 350 oC, but a near complete conversion takes place when the films are annealed at 400 oC as reported by Rodríguez et al. [10]. The stoichiometric calculations of these films were obtained from the mass densities and mass formula of the individual layers as suggest in reference [ 1 O]. 250 ,-,--:::--,:--::-::--:-:-:-:-:-:-:-:::-:,:-------------, 1) Sb,S0 • CuS (1h) 350 •e , 0=1.5o


l. lntroduction
Many authors have reported antimony sulphide (Sb,S3) thin films obtained by chemical deposition technique since early 1990's [1][2][3][4]. Chemical bath deposition (CBD) is a simple and low-cost method to produce thin films of different semiconductor compounds [5][6]. This method has been employed by sorne authors to synthesis temary compounds of antimony chalcogenides involving heat treatments in air or nitrogen atmosphere [7][8][9]. Rodríguez et al. have reported the formation ofCuSbS, by chemical bath with a p-type electrical conductivity of0.03 (Q.cmr' anda direct optical band gap of 1.52 eV [7]. Subsequently, the same group reported the use of CuSbS, thin films in a p-i-n solar cell structure with an open circuit voltage of 345 m V [10]. Ezugwu et al. employed CBD technique to deposit directly CuSbS, with direct band gaps between 1 .30 and 2.30 eV [11]. lts properties match with the requirement for the photovoltaic materials [12]. Manolache et al. have obtained this material by spray pyrolysis deposition with suitable characteristics for its application in photovoltaic devices [13]. Rabhi et al. have prepared polycrystalline CuSbS, using thermal evaporation method. The films showed direct band gaps at 1.3 and 1. 79 e V after heat treatment at 200 ºC in N, [14]. The growing effort to find absorber materials involving copper, is because of the p-type conductivity originating from copper deficiency, which can be utilized to produce p-type absorber films as an altemative to Cu(ln/Ga)(S/Se),. An altemative to replace the CulnS, is CuSbS,; which belongs to the same 1-III-VI, group of semiconductor with the chalcopyrite structure, in which the ionic radius of indium and antimony are almost equal [7].
In this work, we present the formation of CuSbS, thin films of about 600 nm in thickness through the solid state reaction at 350-400 ºC of chemically deposited thin films of Sb,S,--CuS or Cu,_,Se.

Sb2S, thin films
Thin films of Sb,S3 were deposited on clean microscope glass slides using a chemical bath deposition reported previously by Grozdanov [3] and modified later by Nair et al. as reported in reference [4]. The reaction solution was prepared by dissolving 650 g ofSbCI, in 2.5 mL acetone and 25mL 1 MNa,S 20 3 • Thebathwasmaintainedat 1 ºCduring6 h. After this time an amorphous Sb,S3 thin film of 600 nm in thickness was obtained. The methodology of deposition has been explained in reference [15). Heat treatment of these films in air at 200 ºC during 15 min was necessary in order to give adhesion between the film and the glass substrate. Subsequently, a thin film of CuS was deposited on the preheated Sb,S3 films using the chemical bath reported previously in the reference [16) or chemical bath ofCu,_,Se using the composition reported in reference [ 17].

CuS thin film
Thin films ofCuS were deposited on the Sb,S3 thin films using a reaction solution containing 1 O mL of0.5 M CuCI,, 8 mL oftriethanolamine (TEA) 50%, 8 mL of 15 M arnmonia (aq.), 10 mL of 1 M NaOH, 6 mL of 1 M thiourea and distilled water to complete a volume of 100 mL. During one hour at 30 ºC, a CuS thin film of -120 nm in thickness was deposited on the Sb,S3 films. Toe preheated Sb,S3 films were placed in the CuS bath after 30 min ofthe bath preparation, in order to avoid the peeling of the Sb,S3 films due to the ammonia contained in the CuS bath. Temperature of the bath was maintained at 30 ºC. Samples were removed from this bath after 1 h, 2 h and 3 h, rinsed in distilled water and dried in air at room temperature.

Cu2_,Se thin mm
The thin films of Cu,_,Se were deposited on Sb,S3 thin films using reaction solution containing 1 O mL of 0.5 M CuSO., 1.5 mL of ammonia (aq.) 15 M, 12 mL 0.4 M Na,SeSO, solution and distilled water to complete 100 mL volume bath. Substrates with Sb,S3 thin film previously deposited were placed in the Cu,_,Se bath 30 min after preparation. The chemical bath was maintained at 30 ºC during 1 h, 2 h and 3 h. Samples were taken out from the bath each hour, rinsed in distilled water and dried in air at room temperature.

Characterization
X-ray diffraction (XRD) pattems were recorded using a Rigaku D-Max 2000 diffractometer using Cu-Ka (~= 1 .5406 Á) radiation in the glazing incidence mode (1.5°). Toe optical transmittance and specular reflectance spectra were measured using a Shimadzu 3100 PC spectrophotometer in the wavelength range of 250 -2500 nm. Photocurrent responses of the films were obtained using tungsten-halogen radiation and a computerized measurement system using a Keithley 230 programmable voltage source anda Keithley 619 multimeter. Thickness of the films was measured usingAlpha Step 100 (Tencor, CA).  Figure 1 showstheXRDpattemsofSb,S, (600nm)-CuS (120 nm) annealed at 350 ºC (figure la) and annealed at400 ºC (figure 1 b) in N, at 40 Pa during 1 h. We observed that for the sample heated at 350 ºC, the majority ofthe diffraction peaks correspond to the XRD pattem of Sb,S, (PDF 42-1393). In the case ofthe sample heated at 400 ºC, the peaks correspond to the pattem given for CuSbS, (PDF 44-1417). From figure la and lb we may note that the conversion of Sb2S,--CuS film to CuSbS, begins at 350 ºC, but a near complete conversion takes place when the films are annealed at 400 ºC as reported by Rodríguez et al. [10]. The stoichiometric calculations of these films were obtained from the mass densities and mass formula of the individual layers as suggest in reference [ 1 O]. There is a notable dissolution ofthe Sb,S, films during the deposition of the subsequently CuS !ayer. This was confirmed by the thickness measurements of the as-prepared Sb,S, (300 nm) thin films and the final thickness after the CuS deposition. In table 1 these measurements are given. However, the thin film of CuS grew quickly on the Sb,S, films heated at 200 ºC in air during 15 min. Also we found that the Sb,S, losses can be avoided if a chemical bath of Cu,_,Se is used instead of the CuS bath. The thickness measurements of the as -prepared films of Sb,S, after the Cu,_,Se deposition are also given in table 1.  Figure 2 shows the XRD pattems ofthe Sb,S, (300 nm) + Cu,_,Se (100 nm) layers after heat treatment in: a) N, atmosphere at 350 ºC during lh. b) 350 ºC in air during 5 min and c) 400 ºC in air during 5 min. In these systems we found temary compounds ofCu,SbS, and Cu,SbSe, for the sample heated in N, at 350 ºC during 1 h, dueto the excess of copper in the samples. A rapid thermal treatment in air during 5 min was made in order to avoid the losses of sulfur or selenium, as well as, to do the heat treatment easier for large area applications. The formation of a solid solution is expected from figure 2a, 2b and 2c because the position ofthe XRD peaks are between the peaks for Cu3SbS3 -Cu,SbSe3 and Cu,SbS, -Cu,SbSe, due to the presence of selenium in the reaction solution for the deposition ofCu,_,Se.

Optical Properties
The optical transmittance T (%) and specular reflectance R (%) spectra of the films of approximately 600 nm in thickness obtained from Sb,S3-CuS heated in N, at 350 ºC and 400 ºC and from Sb,S,-Cu,_,Se of 400 nm in thickness heated in N, at 350 ºC were recorded to evaluate the absorption coefficient (a) of the films considering multiple reflections [18]: The optical band gap of the material was obtained from the intercepts of plots of (o.hv)' or (o.hv) ' A straight line was observed in the plot of ( ahv )' vs. hv for the samples showed in figure 3a and 3b which indicates the presence of a direct optical band gap.
To obtain the value of E,, an extrapolation ofthe plot to the photon energy axis was made. For the sample annealed at 350 ºC (figure 3c) E, equals to 1.79 eV. This value corresponds to that reported for crystalline Sb,S3 [19] as observed in the XRD patterns showed in figure la. For the sample annealed at 400 ºC the energy gap is located in 1.57 eV, which corresponds to that value reported for CuSbS, suggesting a total conversion ofthe stack films [1 O].
In both cases the straight line indicates the presence of a directbandgap. Forthe sample Sb,S3+Cu,_xSe (figure 3c) the straight line can be seen in the plot of ( ahv ) 213 vs. hv which suggests the presence of a direct band gap with forbidden transitions with E,= 1.43 e V as expected for this material due to the presence of selenium in the film.

Electrical properties
The photocurrent response of the CuSbS, thin films obtained from: a) Sb,S,+CuS annealed at 350 ºC and b) Sb,S,+CuS annealed at 400 ºC in N, are given in figure 4. A bias, 1 O V has been applied in each case. The electrical conductivity of the films in the dark is in the range of 10-3 (Q.cm)"'. U pon illumination, there is an increase in the conductivity by almost an order of magnitude, but the films annealed at temperature 400 ºC have more conductivity. P-type conductivity was confirmed by the hot-probe method.
The photo-response ofthe samples obtained by annealing of the Sb,S, +Cu,_xSe was negligible, hence this response is omitted in figure 4, and the formation of CuSbS, was observed only in the samples with heat treatrnent of Sb,S, +CuS thin films. The very small effect of illumination in these samples is similar to those presented in degenerate semiconductors materials. 16

Conclusions
Thin films of SbS, were deposited by chemical bath deposition technique on glass substrates. It has been demonstrated that the obtained films must be annealed in vacuum at temperature of 400 ºC for an almost total conversion. For the films ofSb,S,+CuS annealed at 400 ºC, an optical direct band gap was observed at 1.57 eV which correspond to the reported for CuSbS,. For the films heated at 350ºC the energy band gap was observed at 1. 79 e V which corresponds to Sb,S,. For the films obtained by annealing of Sb,S,+Cu,.xSe a direct band gap was observed at 1.43 eV, however, involves forbidden transitions. The p-type conductivity ofthe samples was confirmed by the hot-probe measurements. Dark conductivity in the order of 1 O-' (Q.cmr' for CuSbS, thin films matches well with previous reports for this material, but no effect of illumination was observed in the samples with Cu,_,Se.
Octubre-Diciembre de 2011 T he dissolution of Sb2S, thin film in the CuS bath was avoided by pre-heating the Sb2S, films in air during 15 min before the deposition of CuS or by using a chemical bath of Cu2.xSe, which was demonstrated by the thickness measurements of the films. The optical and electrical properties ofthe thin films presented here show its suitable characteristics for application in photovoltaic devices. Further work on the optimization on the film thickness in the stack films of Sb2S, -Cu,_,Se and heat treatments are necessary to produce CuSbSe2•