Electrodeposition of CdTe thin films on conductive glass of ...
Electrodeposition of CdTe thin films on conductive glass of manufacturing solar cells of homo-junction A. N. A. Silva1, P. H. F. M. Júnior1, L. M. A. L. Bonfim1, J. H. A. Sousa2, R. C. B. Silva2, F. N. A. Freire1, A. F. L. Almeida1 1 Universidade Federal do Ceará (UFC), Departamento de Engenharia Mecânica, Fortaleza, Ceará, Brazil e-mail: [email protected] In the present study, the potentiostatic electrodeposition of cadmium telluride (CdTe) on conductive glass (SnO2:F) was analyzed. The technique of cyclic voltammetry was used in this study in order to identify a range of potential values in which the electrocrystallization CdTe occurs. Based on this analysis the values of - 288 mV and -576 mV were chosen to implement the potentiostatic electrodeposition. The electrolyte used was an acidic solution, which contained cadmium and tellurium ions. The tests were conducted at room temperature. The technique of scanning (SEM) electron microscopy was used to characterize the morphology of CdTe films obtained. The films electrodeposited - 576 mV are more uniform than the electrodeposited films - 288 mV, but with smaller grain size. The EDS method showed that the films electrodeposited to - 288 mV and - 576 mV have a high ratio of tellurium atoms, i.e, films with p-type conductivity.
Keywords: solar energy, CdTe, electrodeposition, homo-junction. Introduction The CdTe is a semiconductor material of group IIB–VIA, which has a high absorption coefficient of approximately 5.105 cm-1 and a direct optical band gap of 1.5 eV, very close to the maximum energy from solar irradiation (MCCANDLESS and SITES, 2003). Additionally, the cadmium telluride has the characteristic of presenting either p-type conductivity as the n-type depending only on the growth parameters (YANG, CHOU and UENG, 2010). Thus, CdTe thin films are ideal for the manufacture of solar cells de homojunction (HUANG, ZHAO and CAI, 2009). The thin film deposition of CdTe can be made by employing the technique of electrodeposition, which presents low cost and control parameters such as thickness, composition and microstructure. Experimental part CdTe thin film was deposited on conductive glass (SnO2: F) with a electrical sheet resistance in the range 20-50 Ω/□, which was taken by spray pyrolysis method. The electrolyte used was an acidic solution containing a source of cadmium ions (CdSO4.(8/3)H2O from Vetec with purity of 99.97%) and a source of tellurium ions (TeO2 from Aldrich Chemicals with purity of 99.995%). The pH was controlled by addition of sulfuric acid and maintained at about 0.7. The concentration of the solution was 0.25 mol/L for cadmium sulfate hydrate and 0.025 mol/L tellurium dioxide. The technique of cyclic voltammetry was used in this study in order to identify a range of potential values in which the electrocrystallization CdTe occurs significantly. In this method, the potential was swept between - 0.7 V and + 0.7 V at a scan rate of 10 mV/s. The electrodeposition processes were carried out at room temperature for a period of three hours. The techniques of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize thin films of CdTe obtained. Results and discussion It was observed that the potential of 0 mV cathodic current appears on the system, indicating that this potential occurs CdTe deposition. Based on the cyclic voltammogram (Figure 1) the potential values of -288 mV and -576 mV for the testing of electrodeposition were chosen. By means of figures 2 and 3 it is noted that the substrate surface is completely covered by the
film, which consists of grains of variable size. The lowest variation of grain size of the electroplated film to - 576 mV makes the surface more uniform film such that the surface of the film grown at -288 mV.
(a) Fig. 1 - Cyclic voltammogram of the processes of deposition / dissolution of CdTe, obtained 10 mV.s-1.
(b)
Fig. 2 – Micrographs of films grown on: (a) – 288 mV e (b) – 576 mV.
The spectra obtained by EDS technique reveal the existence of cadmium and tellurium, elements which constitute the film. The chemical composition of the electrodeposited films indicated a greater amount of tellurium atoms (Table 1). Given that, films that have higher amount of tellurium atoms exhibit p-type conductivity, both the films are p-type. There is a tendency to increase the proportion of Cd becoming the most negative potential. Thus, it is possible to a value of potential of below - 576 mV, there is the deposition of films with excess cadmium, that is, with n-type conductivity. Table 1 - Chemical composition of CdTe thin films grown at - 288 mV and - 576 mV. Potential (mV) - 288 - 576
atom% Cd 30,230 33,190
atom% Te 69,770 66,810
Conclusions The electrocrystallization of CdTe thin films on conductive glass occurs in regions of negative potential. The conductivity of the films depends on the potential at which the electrodeposition is conducted. The more negative the value of the potential, the greater the fraction of cadmium in the film. However, the more positive the potential value, the higher the fraction of tellurium atoms. Thus, it is possible to deposit films with excess cadmium and, consequently, with n-type conductivity. References [1] MCCANDLESS, B. E.; SITES, J. R. Cadmium Telluride Solar Cells. In: LUQUE, A.; HEGEDUS, S. Handbook of Photovoltaic Science and Engineering. Newark; Fort Colins: John Wiley & Sons, 2003, p. 617-662. [2] HUANG, L.; ZHAO, Y.; CAI, D. Homojunction and heterojunction based on CdTe polycrystalline thin films. Materials Letters, v. 63, p. 2082-2084, 2009. [3] YANG, S. Y.; CHOU, J. C.; UENG, H. Y. Influence of electrodeposition potencial and heat treatment on structural properties of CdTe films. Thin Solid Films, v. 518, p. 4197– 4202, 2010. Acknowledgement: CAPES, CNPq, Universidade Federal do Ceará (UFC).
Keywords: solar energy, CdTe, electrodeposition, homo-junction. Introduction The CdTe is a semiconductor material of group IIB–VIA, which has a high absorption coefficient of approximately 5.105 cm-1 and a direct optical band gap of 1.5 eV, very close to the maximum energy from solar irradiation (MCCANDLESS and SITES, 2003). Additionally, the cadmium telluride has the characteristic of presenting either p-type conductivity as the n-type depending only on the growth parameters (YANG, CHOU and UENG, 2010). Thus, CdTe thin films are ideal for the manufacture of solar cells de homojunction (HUANG, ZHAO and CAI, 2009). The thin film deposition of CdTe can be made by employing the technique of electrodeposition, which presents low cost and control parameters such as thickness, composition and microstructure. Experimental part CdTe thin film was deposited on conductive glass (SnO2: F) with a electrical sheet resistance in the range 20-50 Ω/□, which was taken by spray pyrolysis method. The electrolyte used was an acidic solution containing a source of cadmium ions (CdSO4.(8/3)H2O from Vetec with purity of 99.97%) and a source of tellurium ions (TeO2 from Aldrich Chemicals with purity of 99.995%). The pH was controlled by addition of sulfuric acid and maintained at about 0.7. The concentration of the solution was 0.25 mol/L for cadmium sulfate hydrate and 0.025 mol/L tellurium dioxide. The technique of cyclic voltammetry was used in this study in order to identify a range of potential values in which the electrocrystallization CdTe occurs significantly. In this method, the potential was swept between - 0.7 V and + 0.7 V at a scan rate of 10 mV/s. The electrodeposition processes were carried out at room temperature for a period of three hours. The techniques of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize thin films of CdTe obtained. Results and discussion It was observed that the potential of 0 mV cathodic current appears on the system, indicating that this potential occurs CdTe deposition. Based on the cyclic voltammogram (Figure 1) the potential values of -288 mV and -576 mV for the testing of electrodeposition were chosen. By means of figures 2 and 3 it is noted that the substrate surface is completely covered by the
film, which consists of grains of variable size. The lowest variation of grain size of the electroplated film to - 576 mV makes the surface more uniform film such that the surface of the film grown at -288 mV.
(a) Fig. 1 - Cyclic voltammogram of the processes of deposition / dissolution of CdTe, obtained 10 mV.s-1.
(b)
Fig. 2 – Micrographs of films grown on: (a) – 288 mV e (b) – 576 mV.
The spectra obtained by EDS technique reveal the existence of cadmium and tellurium, elements which constitute the film. The chemical composition of the electrodeposited films indicated a greater amount of tellurium atoms (Table 1). Given that, films that have higher amount of tellurium atoms exhibit p-type conductivity, both the films are p-type. There is a tendency to increase the proportion of Cd becoming the most negative potential. Thus, it is possible to a value of potential of below - 576 mV, there is the deposition of films with excess cadmium, that is, with n-type conductivity. Table 1 - Chemical composition of CdTe thin films grown at - 288 mV and - 576 mV. Potential (mV) - 288 - 576
atom% Cd 30,230 33,190
atom% Te 69,770 66,810
Conclusions The electrocrystallization of CdTe thin films on conductive glass occurs in regions of negative potential. The conductivity of the films depends on the potential at which the electrodeposition is conducted. The more negative the value of the potential, the greater the fraction of cadmium in the film. However, the more positive the potential value, the higher the fraction of tellurium atoms. Thus, it is possible to deposit films with excess cadmium and, consequently, with n-type conductivity. References [1] MCCANDLESS, B. E.; SITES, J. R. Cadmium Telluride Solar Cells. In: LUQUE, A.; HEGEDUS, S. Handbook of Photovoltaic Science and Engineering. Newark; Fort Colins: John Wiley & Sons, 2003, p. 617-662. [2] HUANG, L.; ZHAO, Y.; CAI, D. Homojunction and heterojunction based on CdTe polycrystalline thin films. Materials Letters, v. 63, p. 2082-2084, 2009. [3] YANG, S. Y.; CHOU, J. C.; UENG, H. Y. Influence of electrodeposition potencial and heat treatment on structural properties of CdTe films. Thin Solid Films, v. 518, p. 4197– 4202, 2010. Acknowledgement: CAPES, CNPq, Universidade Federal do Ceará (UFC).
