Published

2013-07-01

SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS

SÍNTESIS Y CARACTERIZACIÓN DEL COMPUESTO Cu2SnSe3 COMO PRECURSOR DE PELÍCULAS DELGADAS APLICADAS EN CELDAS SOLARES

Keywords:

Cu2SnSe3, Thin Film, Solar Cell, Structural and Optical Properties (en)
Películas delgadas, Cu2SnSe3, celdas solares, propiedades ópticas y estructurales (es)

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Authors

  • Francisco Guzman Grupo de Materiales Semiconductores y Energía Solar, Departamento de Física, Universidad Nacional de Colombia, Bogotá.
  • Robinson Moreno Grupo de Materiales Semiconductores y Energía Solar, Departamento de Física, Universidad Nacional de Colombia, Bogotá.
  • Mikel Hurtado Grupo de Materiales Semiconductores y Energía Solar, Departamento de Física, Universidad Nacional de Colombia, Bogotá.
  • Gerardo Gordillo Grupo de Materiales Semiconductores y Energía Solar, Departamento de Física, Universidad Nacional de Colombia, Bogotá.
The purpose of this paper is to contribute to the development of materials used in the fabrication of solar cells based on Kesterite type compounds which are being widely investigated because they possess excellent photovoltaic properties and their precursor elements are nontoxic, inexpensive and abundant in nature. We present details of the synthesis and optimization of optical and structural properties of the ternary Cu2SnSe3 (CTSe) compound which is used as a precursor in the synthesis of the Cu2ZnSnSe4. The CTSe compound is formed by a solid state reaction of the metal precursors sequentially evaporated in the presence of elemental selenium, in a two stage process. The optimization of the properties of the CTSe was achieved studying the eéct of synthesis parameters on optical, electrical and structural properties through spectral transmittance, electrical conductivity and X-ray diffraction measurements. The results indicated that these compounds grow with cubic structure and have an optical bandgap of 1.6 eV.

El propósito de este trabajo es contribuir al desarrollo de materiales usados en la fabricación de celdas solares basadas en películas en compuestos con estructura tipo kesterita que están siendo ampliamente investigadas debido a que poseen excelentes propiedades fotovoltaicas y sus elementos precursores son no tóxicos, de bajo costo y abundantes en la naturaleza. Se presentan detalles de la síntesis y optimización de las propiedades ópticas y estructurales del compuesto ternario Cu2SnSe3 (CTSe) que es empleado como precursor en la síntesis del compuesto Cu2ZnSnSe4. El compuesto CTSe es formado a través de una reacción en estado sólido de sus precursores metálicos evaporados
secuencialmente en presencia de selenio elemental, en un proceso de dos etapas. La optimización de las propiedades del compuesto CTSe se realizó estudiando el efecto de los parámetros de síntesis sobre las propiedades ópticas, eléctricas y estructurales a través de medidas de transmitancia espectral, conductividad eléctrica y difracción de rayos-x. Los resultados indicaron que este tipo de compuestos crecen con estructura cúbica y presentan un gap óptico de 1,6 eV.

SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS

SÍNTESIS Y CARACTERIZACIÓN DEL COMPUESTO Cu2SnSe3 COMO PRECURSOR DE PELÍCULAS DELGADAS APLICADAS EN CELDAS SOLARES

Francisco E. Guzman, Robinson Moreno, Mikel Hurtado, Gerardo Gordillo

Grupo de Materiales Semiconductores y Energía Solar, Departamento de Física, Universidad Nacional de Colombia, Bogotá.

(Recibido: 05/2013. Aceptado: 11/2013)

Contacto: Francisco E. Guzmán: feguzmanc@unal.edu.co

Cómo citar: Guzman, F.E., Moreno, R., Hurtado, M. & Gordillo, G., Momento 47, 87 (2013)

Abstract

The purpose of this paper is to contribute to the development of materials used in the fabrication of solar cells based on Kesterite type compounds which are being widely investigated because they possess excellent photovoltaic properties and their precursor elements are nontoxic, inexpensive and abundant in nature. We present details of the synthesis and optimization of optical and structural properties of the ternary Cu2SnSe3 (CTSe) compound which is used as a precursor in the synthesis of the Cu2ZnSnSe4. The CTSe compound is formed by a solid state reaction of the metal precursors sequentially evaporated in the presence of elemental selenium, in a two stage process. The optimization of the properties of the CTSe was achieved studying the effect of synthesis parameters on optical, electrical and structural properties through spectral transmittance, electrical conductivity and Xr–ay diffraction measurements. The results indicated that these compounds grow with cubic structure and have an optical bandgap of 1.6 eV.

Keywords: Cu2SnSe3, Thin Film, Solar Cell, Structural and Optical Properties

Resumen

El propósito de este trabajo es contribuir al desarrollo de materiales usados en la fabricación de celdas solares basadas en películas en compuestos con estructura tipo kesterita que están siendo ampliamente investigadas debido a que poseen excelentes propiedades fotovoltaicas y sus elementos precursores son no tóxicos, de bajo costo y abundantes en la naturaleza. Se presentan detalles de la síntesis y optimización de las propiedades ópticas y estructurales del compuesto ternario Cu2SnSe3 (CTSe) que es empleado como precursor en la síntesis del compuesto Cu2ZnSnSe4. El compuesto CTSe es formado a través de una reacción en estado sólido de sus precursores metálicos evaporados secuencialmente en presencia de selenio elemental, en un proceso de dos etapas. La optimización de las propiedades del compuesto CTSe se realizó estudiando el efecto de los parámetros de síntesis sobre las propiedades ópticas, eléctricas y estructurales a través de medidas de transmitancia espectral, conductividad eléctrica y difracción de rayos-x. Los resultados indicaron que este tipo de compuestos crecen con estructura cúbica y presentan un gap óptico de 1,6 eV.

Palabras clave: Películas delgadas, Cu2SnSe3, celdas solares, propiedades ópticas y estructurales

Introduction

Ternary and multinary compound semiconducting materials with direct optical band gap between 1.1 to 1.5 eV are being explored as candidates for absorb layer in thin film solar cells. CuInGaSe2 thin film solar cells achieved at laboratory level and eficiency record of 20.3 % [1] laboratory level. However, the elements In and Ga present in this material are expensive and scarce. Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) have received much attention in recent years as alternative solar cell absorb layers, owing to their suitable properties and non-toxic nature. CZTS based thin film solar cells with laboratory eficiency of 8.4 % [2] have been reported. Cu2ZnSnS4 and Cu2ZnSn(S,Se)4–based solar cells using hydrazine–based solution process have already reached an energy conversion effciency as high as 11.1 % [3], which demonstrates the effectiveness of the solution process in CZTSe–based solar cells.

Because Cu2SnSe3 (CTSe) is an important precursor for the growth of CZTSe via solid state reaction with ZnSe [4], a thorough understanding of the growth and properties of these precursor layers is essential. Techniques like co–evaporation [5], DC sputtering [6, 7] have been earlier used to deposit CTSe thin films. The present paper describes the procedure to prepare CTSe films by means of a solid state chemical reaction between the binary chalcogenide precursors sequentially deposited, followed by annealing in selenium atmosphere at temperatures around 400°C. The optical, structural and electrical properties of CTSe thin films have also been investigated.

Experimental

Thin films of Cu2SnSe3 were grown by means of a solid state chemical reaction between the binary chalcogenide precursors sequentially deposited on a soda lime glass substrate in a two stage process. The synthesis of Cu2ZnSnSe4 is quite diffcult to achieve, however considering the phase diagrams of binary and pseudo–binaries system from the ZnSe–SnSe2–Cu2Se primary sulfides, it is possible to find a route suitable for the synthesis of this compound. Figure 1 shows the phase diagram of the ternary pseudo–binary primary sulfide ZnSe–SnSe2–Cu2Se, where possible routes for obtaining the phase Cu2ZnSnSe4 are appreciated.

According to the phase diagram of the ternary pseudo–binary ZnSe–SnSe2–Cu2Se system, several routes can lead to the formation of Cu2ZnSnSe4 compound; however we have grown this compound by a solid state chemical reaction of the ZnSe and Cu2SnSe3

Figure 1. Phase diagram of the ternary pseudo–binary primary sulfide ZnSe — SnSe2 — Cu2Se, which provide information on the molar proportions required to obtain the Cu2ZnSnSe4 phase. [8]

Figure 2. System used to prepare Cu2SnSe3 thin films by sequential deposition of CuSe/SnSe thin films followed by annealing in Se atmosphere.

precursors, where the ternary precursor is obtained by a solid state reaction of the binary Cu2Se and SnSe2 compounds sequentially deposited in a two stage process. This process was performed in a vacuum cabinet (Figure 2), in the first stage, the Cu2Se and SnSe2 layers are sequentially deposited by co–evaporation of its elemental precursors at 250°C. In the second stage the CTSe compound is formed by annealing the stacked CuSe/SnSe system at a temperature of 400°C, in a Se atmosphere to evaporate Cu and Sn, and an effusion cell to evaporate Se.

The background pressure of the vacuum chamber was around 1 × 10—5 mbar. The temperature of the effusion cell is controlled using PID temperature controller. The deposition rates were monitored with a thickness monitor which uses a quartz sensor. In order to find the growth conditions of the CTSe films containing predominantly the Cu2SnSe3 phase, a broad number of samples were deposited on glass substrates under different sequences (CuS/SnSe, SnSe/CuSe) and varying the main deposition parameters (post deposition annealing temperature, deposition rate, mass ratio of the elemental precursors) in a wide range. XRD measurements carried out to each one of the prepared samples allowed us to find the sequence and deposition parameters which lead to the growth of Cu2SnSe3 thin films. The study revealed that CTSe films can be grown using the CuSe/SnSe sequence and the deposition routine displayed in Fig. 3.

The electrical properties of the CZTS films were studied through temperature dependent conductivity measurements and the optical properties through transmittance measurements carried out on a Oriel VIS–NIR spectrometer. Further characterization involved X–ray diffraction on a Shimadzu–6000 diffractometer. The film thickness was determined using a Veeco Dektak 150 surface profile.

Figure 3. Routine used to prepare Cu2SnSe3 thin films

Figure 4. Comparison of the diffractogram of a Cu2SnSe3 thin film with those of thin films of CuSe, SnSe deposited under the conditions described previously

Results and Discussion

Structural characterization

Figure 4, shows the XRD pattern of a CTSe thin film prepared by sequential deposition of CuSe and SnSe thin films, using a preparation routine like that plotted in Fig. 3 and evaporated masses of Cu and Sn of 0.01 and 0.07 g respectively. The Figure 4 shows the XRD for films of CuSe and SnSe. These are compared with the CTSe diffractogram in order to identify with a greater degree of accuracy the reflections corresponding to secondary phases in the CTSe thin film.

It is observed that the diffractogram of the CTSe film shows reflections corresponding to the Cu2SnSe3 phase (4PDF cart #03–065–7524) and to the Cu2Se phase (PDF cart #03–065–7737). The diffractograms of copper selenide thin films present only the Cu2Se phase (PDF cart #03–065–7737), whereas the diffractogram of tin selenide indicates a mixture of both SnSe3 (PDF cart

Figure 5. Comparison of the transmittance of a typical Cu2SnSe3 thin film with those of thin films of CuSe and SnSe

#03–065–7524( and SnSe2 (PDF cart #00–23–0602) phases.

From this study can be established that the sequential evaporation of Cu and Sn at 250°C in presence of elemental selenium followed by annealing at 400°C results mainly in the formation of a mixture of Cu2SnSe3 and Cu2Se. The XRD study also revealed that the co–evaporation of Cu and Se at 250°C followed by annealing at 400°C leads to the formation of a single phase of Cu2Se, whereas the co–evaporation of Sn and Se at 250°C followed by annealing at 400°C leads to the formation of a mixture of SnSe3 and SnSe2 phases.

Optical characterization

In Fig. 5 is compared the transmission spectrum of a Cu2SnSe3 thin film with those of CuSe and SnSe thin films, which were prepared as described previously.

It is observed that the studied samples exhibit low transmittance at wavelengths longer than that corresponding to the cut–off wavelength, indicating that the CTSe, SnSe and CuSe films grow with a high density of native defects, that generate absorption centers within the energy gap which contribute to the photon absorption. The presence of native defects on Cu2ZnSnSe4 have been identified by other authors [9] on the basis of a structural analysis of neutron powder diffraction data. This compound was

Figure 6. curves of α vs λ and (αhυ)2 vs. hυ, corresponding to the Cu2SnSe3 film.

found to crystallize in the kesterite type structure, but with a disorder within the CuZn layers at z 1/4 and 3/4. The latter causes CuZn and ZnCu anti–site defects, whose concentration depends on the sample growth conditions.

In particular, the transmittance of the CuSe films decreases strongly near infrared (NIR) region apparently due to the formation of a very high density of shallow defects. It is also observed that the transmittance curve of the SnSe film has a very small slope. This behavior seems to be caused by absorption of photons in shallows centers generated by structural defects.

The optical gap of the CTSe thin film (Fig. 6) was determined by extrapolation of the (αhυ(2 vs hυ; curve with the axe. The absorption coeffcient α was estimated using the relation T(λ) = (1 — R(λ))2 exp(—αd) [10] , where T(λ) is the spectral transmittance, R(λ) the spectral reflectance and d the film thickness which in this case corresponds to a sample 0.8µm thick. An Eg value of 1.6 eV was found for the CTSe films.

Electrical characterization

It is observed in Fig. 7 that the conductivity increases as the temperature increases indicating a typical behavior of semiconductor materials. The behavior of the ln σ vs 1000/T curve plotted in Fig. 8 indicates that the conductivity of the CTSe films can be expressed by the relation: σ = σ0 • exp[—ΔE/kT] where E is the activation energy. This result shows that the conductivity

Figure 7. Curve of σ vs T for a CTSe film grown as described in section 2

Figure 8. Curve of ln σ vs 1000/T, for a CTSe film grown as described in section 2.

is predominantly affected by free carrier transport in states of the valence band [11]. Considering that the mobility decreases with increasing temperature (from 11 to 9.5 cm2/Vs in the studied temperature range), the increase in conductivity with increasing temperature is mainly caused by an increase in the density of charge carriers apparently generated by native acceptor impurities.

Considering that the activation energy in the range of high temperatures T > 550 K is signiffcantly higher than that observed in the low temperature range, the increase of σ at T > 550 K could be attributed to an increase of the carrier density coming from deep acceptor impurities presents in the Cu2SnSe3 compound, whereas the change of σ observed in the range of low temperatures (T < 350K) can be attributed to a change of the carrier density coming from shallow acceptor impurities associated to the binary phases. The two slopes identified in the ln σ vs 1000/T curve indicates that the conductivity of the CZTS films is affected by two different types of impurities, which could be vacancies and antisite defects that have been reported elsewhere.[12]

Conclusions

Cu2SnSe3 thin films were grown using a method based on sequential evaporation of thin films of CuSe, and SnSe in a two stage process. Characterization performed by XRD gave evidence of the formation of a compound containing predominantly the Cu2SnSe3 phase; however the sequence in which the binary precursors are evaporated and the preparation parameters, significantly affects the phase as well as the structural, optical and electrical transport properties of the CTSe films. Optical characterization performed by spectral transmittance measurements revealed that the CTSe films have low transmittance and poor crystallographic quality, probably associated to structural and native defects, indicating that further studies must be done to improve the properties of the CTSe films. The results also revealed that the Cu2SnSe3 films are characterized to get p–type conductivity and an energy band gap Eg of about 1.6 eV.

Conductivity measurements on temperature dependence revealed that conductivity of the CTSe films is predominantly affected by free carrier transport in states of the valence band. In the range of high temperatures (T > 550K), the increase of σ could be attributed to an increase of the carrier density coming from deep acceptor impurities, whereas the change of σ observed in the range of low temperatures (T < 350K) can be attributed to a change of the carrier density coming from shallow acceptor impurities associated to secondary phases.

References

[1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, Prog. Photovoltaics 19, 894 (2011).

[2] B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Prog. Photovoltaics 21, 72 (2013).

[3] T. K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, and D. B. Mitzi, Adv. Energy Mater. 3, 34 (2013).

[4] P. U. Bhaskar, G. S. Babu, Y. K. Kumar, and V. S. Raja, Appl. Surf. Sci. 257, 8529 (2011).

[5] G. S. Babu, Y. K. Kumar, Y. B. K. Reddy, and V. S. Raja, Mater. Chem. Phys. 96, 442 (2006).

[6] H. Yoo, R. Wibowo, A. Holzing, R. Lechner, J. Palm, S. Jost, M. Gowtham, F. Sorin, B. Louis, and R. Hock, Thin Solid Films 535, 73 (2013).

[7] P. Salome, P. Fernandes, and A. da Cunha, Thin Solid Films 517, 2531 (2009), thin Film Chalogenide Photovoltaic Materials (EMRS, Symposium L).

[8] H. Wang, Int. J. Photoenergy 2011, 801292 (2011).

[9] S. Schorr, Sol. Energ. Mat. Sol. C. 95, 1482 (2011).

[10] J. Pankove, Optical Processes in Semiconductors, Dover books in physics (Dover, 1971).

[11] M. H. Brodsky, Amorphous Semiconductors, Topics in Applied Physics, Vol. 36 (Springer Berlin Heidelberg, 1986).

[12] S. Chen, J.–H. Yang, X. G. Gong, A. Walsh, and S.–H. Wei, Phys. Rev. B 81, 245204 (2010).

How to Cite

APA

Guzman, F., Moreno, R., Hurtado, M. and Gordillo, G. (2013). SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS. MOMENTO, (47), 87–97. https://revistas.unal.edu.co/index.php/momento/article/view/45071

ACM

[1]
Guzman, F., Moreno, R., Hurtado, M. and Gordillo, G. 2013. SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS. MOMENTO. 47 (Jul. 2013), 87–97.

ACS

(1)
Guzman, F.; Moreno, R.; Hurtado, M.; Gordillo, G. SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS. Momento 2013, 87-97.

ABNT

GUZMAN, F.; MORENO, R.; HURTADO, M.; GORDILLO, G. SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS. MOMENTO, [S. l.], n. 47, p. 87–97, 2013. Disponível em: https://revistas.unal.edu.co/index.php/momento/article/view/45071. Acesso em: 28 mar. 2024.

Chicago

Guzman, Francisco, Robinson Moreno, Mikel Hurtado, and Gerardo Gordillo. 2013. “SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS”. MOMENTO, no. 47 (July):87-97. https://revistas.unal.edu.co/index.php/momento/article/view/45071.

Harvard

Guzman, F., Moreno, R., Hurtado, M. and Gordillo, G. (2013) “SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS”, MOMENTO, (47), pp. 87–97. Available at: https://revistas.unal.edu.co/index.php/momento/article/view/45071 (Accessed: 28 March 2024).

IEEE

[1]
F. Guzman, R. Moreno, M. Hurtado, and G. Gordillo, “SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS”, Momento, no. 47, pp. 87–97, Jul. 2013.

MLA

Guzman, F., R. Moreno, M. Hurtado, and G. Gordillo. “SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS”. MOMENTO, no. 47, July 2013, pp. 87-97, https://revistas.unal.edu.co/index.php/momento/article/view/45071.

Turabian

Guzman, Francisco, Robinson Moreno, Mikel Hurtado, and Gerardo Gordillo. “SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS”. MOMENTO, no. 47 (July 1, 2013): 87–97. Accessed March 28, 2024. https://revistas.unal.edu.co/index.php/momento/article/view/45071.

Vancouver

1.
Guzman F, Moreno R, Hurtado M, Gordillo G. SYNTHESIS AND CHARACTERIZATION OF Cu2SnSe3 THIN FILMS COMPOUND USED IN THE FABRICATION OF SOLAR CELLS. Momento [Internet]. 2013 Jul. 1 [cited 2024 Mar. 28];(47):87-9. Available from: https://revistas.unal.edu.co/index.php/momento/article/view/45071

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