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The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications A. Aissat a,*, M.A. Ghomrani a, W. Bellil a, A. Benkouider b, J.P. Vilcot c LATSI Laboratoire, Faculte de Technologie, Universite de Blida.1, BP270, 09000, Blida, Algeria IM2NP-CNRS (UMR 7334), Aix-Marseille University, 13397, Marseille Cedex 20, France c Institut d'Electronique, de Microelectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Universite des Sciences et Technologies de Lille 1, Villeneuve d'Ascq, BP 60069, 59652, France a
b
article info
abstract
Article history:
In this study, we experimentally elaborated Copper- and Indium-doped Zinc Oxide (Cu:
Received 28 January 2015
ZnO and In: ZnO) thin films at different temperatures (T1 ¼ 480 C and T2 ¼ 520 C), the
Received in revised form
doping ratio were varied between 0% and 8%. Using a low cost solution-based chemical
19 February 2015
deposition, we have developed a ZnO thin film deposition process that offers fine-control of
Accepted 20 February 2015
the surface morphology. It consists in spraying a volatile compound of the material to be
Available online xxx
deposited on a substrate maintained at high temperature to cause a chemical reaction in order to form at least one solid product. Therefore, the proposed ZnO doped layer is highly
Keywords:
promising for applications for the next-generation solar cells.
Thin layer
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
New material Solar cell Photovoltaic
Introduction In the last decades, the development of materials using thin layers has contributed to the expansion of the electronics and optoelectronics performance. Moreover, the new open field of low cost and unbreakable large area photovoltaic devices implies the use of small size devices as an available cheap solution. The thin layers can be elaborated from a wide range of compositions such as conductive materials, insulators, semiconductors and polymers [1e3]. It is found that physical properties are closely related to the deposition parameters, mono or multilayer films can be synthesised with different thicknesses, typically from one monolayer to few hundred micrometres [4,5]. The Zinc Oxide (ZnO) semiconductors were
currently studied; various methods have been used to synthesis ZnO-based thin layers. However, a big progress on the synthesis techniques of thin films and the results indicate the possibility of converting the conductivity of the semiconductor n-type to p-type. The nanotechnology revolution has given him a place among the master race materials for optoelectronic applications; this is due to the multiple benefits that we present in this work. Transparent conductive oxides (TCO) are remarkable materials in many areas. The existence of their dual properties, electrical conductivity and transparency in the visible range, making them ideal candidates for applications in optoelectronics, photovoltaic or electro chromic windows [6e9]. Sponge-like ZnO thin film shows promising prospects as Li-ion battery anode [10]. Dye-sensitized solar cells with an energy storage function are demonstrated
* Corresponding author. Tel.: þ213 772525050; fax: þ213 25433850. E-mail address: [email protected] (A. Aissat). http://dx.doi.org/10.1016/j.ijhydene.2015.02.075 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
by modifying its counter electrode with a poly ZnO nanowire array composite. This simplex device could still function as an ordinary solar cell with a steady photocurrent output even after being fully charged [11]. At the vanguard of these materials, the (ZnO) is a semiconductor oxide which presents very interesting properties. In nature, it is ruby red in colour and is found abundantly in minerals, while artificially prepared is colourless or white. If the properties of natural ZnO have long been known, the researchers focused in recent years on the ZnO obtained artificially. ZnO is a potential candidate for the UV-emitting systems as it possesses at room temperature a large gap (3.37 eV) and a large exciting binding energy (60 meV) [6]. It is also an excellent optics in high performance photovoltaic devices, such as the HIT structure or CIGS based thin layers [1].
Fig. 2 e Schematic illustration of the Sub-Split-Step Fourier method distribution.
Study of the optical properties TCO ZnO is transparent in the visible and near infrared field. This later is considered as “twin” of gallium nitride. It can be used for potential applications in the fields of photovoltaic, light emitting diodes for illumination, transparent conductive oxides, photonics or sensors (Fig. 1.) The main advantage of ZnO-based devices is due to its components which are nontoxic and very abundant on earth. This is an important advantage because it reduces production costs. It can also be found in nature in the form of powder or solid crystal. It is in the form of mineral [12]. It was shown that is possible to have an n-type ZnO with many elements such as Al [1], Ga [2] In [3], etc. orp-type doping which remains, meanwhile, still controversial. The advent of p-type ZnO will open the door to transparent electronics the formation's reaction of stoichiometric ZnO [13]. 1 Znþþ þ 2e þ O2 0ZnO 2
light produced by the source passes first through the sample and the sphere harvest as well as the light that was transmitted through the sample. The sphere harvests as well as the light that was transmitted through the sample. The reflectance measuring R is done by placing the sample in place of a small square of the sphere wall, located as opposed to the entrance of this one (position 2 in Fig. 2). It is important to note that these measurements of T and R were performed on ZnO films deposited on a glass substrate. The light when it passes through the sample, it passes through several media having different refractive indices, in the following order: air/ZnO/glass/air. We can grossly evaluate the proportion of light that is reflected at these interfaces, using equation (2). R¼
(1)
The measure of transmission (T) is done by placing the sample at the entrance to the sphere (position 1 in Fig. 2). The
2 n2 n1 n2 þ n1
(2)
R: part of the reflected light when it passes between two media of refractive index n2 and n1. If we consider that the reflections that occur when the light passes in each medium, and we take nair ¼ 1, nglass ¼ 1.5 and nZnO ¼ 2, the part of the light reflected to different interfaces is about 17%. This calculation does not take into account the different roughness of the interfaces, which may also influence R. Therefore the R and T measurements performed with the spectrometer to the ZnO layers deposited on a glass substrate does not fully correspond to values of the transmitted and reflected light in a solar cell.
Optical transmission
Fig. 1 e Structure of a solar cell made of material and the ZnO thin layers.
The optical characterizations were based on the transmission spectra in the visible-UV. Indeed, as has been detailed in the previous section, the operation of the spectra allows us to calculate the optical gap. Figs. 3e6 combined transmission spectra in the range of 250e1000 nm, the films prepared with the two dopants Cuand In. Even though the general shape of the spectra is the same, they are composed of two parts:
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Fig. 3 e Variation of the transmittance of ZnO films dopped indium in function of the wavelength at T ¼ 480 C.
a e A region of high transparency: between 400 and 1000 nm, the transmission value is of the order of 75e85%. This value is reported by several authors [14], it gives the character of transparency in the visible to the ZnO thin layers doped and undopped. In this wavelength range, of the interference fringes is observed in the case of certain developed films. These fringes, characterized by undulating curves are due to the multiple reflection of radiation on two interfaces of the film. b e A region of strong absorption: This region corresponds to the fundamental absorption (l < 400 nm) in the ZnO films doped and undopped. This absorption is due to the electronic band gap transition. The change of the transmission in this region is used for determining the optical gap. This last one is estimated from the intersection of the curve giving (ahy)2 ¼ f (hy) with the abscissa axis. On the other hand, a shift of absorption threshold to the major energies is mainly due to the increase in concentration of free carriers in the material [15]. The shift in the absorption threshold is also equal to the gap variation DEg which is expressed by the following relation [16e18].
Fig. 4 e Variation of the transmittance of ZnO films dopped indium in function of the wavelength at T ¼ 520 C.
3
Fig. 5 e Variation of the transmittance of copper dopped ZnO films in function of the wavelength at T ¼ 480 C.
DEg ¼
23 h2 3n=p * 8m
(3)
Where h is Planck's constantm * the effective mass of carriersn the concentration of free electrons This relation shows that the variation of the gap is mainly caused by the concentration of free electrons. Fig. 3 represents the variation of the transmittance of the ZnO films undoped and those In-doped 2%, 4%, 6% and 8% as function of the wavelength, by setting the temperature at T ¼ 480 C. Fig. 4 represents the variation of the transmittance of the ZnO films undoped and those In-doped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 520 C. The Fig. 5 represents the variation of the transmittance of the non-doped ZnO films and those Cudoped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 480 C. Fig. 6 represents the variation of the transmittance of the nondoped ZnO films and those Cu-doped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 520 C. We note that there is no remarkable
Fig. 6 e Variation of the transmittance of copper dopped ZnO films in function of the wavelength at T ¼ 520 C.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Table 1 e Minimum and maximum values for optical transmission of Indium doped ZnO thin layers and those copper dopped.
ZnO ZnO ZnO ZnO
:In at T ¼ 480 C :In at T ¼ 520 C :Cu at T ¼ 480 C :Cu at T ¼ 520 C
Transmission min (%)
Transmission max (%)
70 70 72 73
85 88 86 88
difference between the values of transmission for all films doped (In) and those doped (Cu) in the visible range. These values vary between 70% and 88%, which leads us to conclude that the optical transmission does not vary by inter changing these two dopants. We also note that all the deposited layers have a high optical transmission in the visible range at approximately 85%, and we note that the layers doped with indium and copper at 2% and 4% concentration present greater transparency in the visible for deposition temperatures 520 C and 480 C, respectively. As against the highest value of transmission for the In-doped layer and deposited at 480 C is attributed to the 4% concentration. The study of the
Fig. 8 e (a, b) Variation of the Optical reflection films Copper dopped ZnO according to the wavelength at T ¼ 480 C and T ¼ 520 C.
influence of deposition temperature on the values of the optical transmission shows an increase of the latter by about 3% in the visible range, for In-doped ZnO thin films than for those Cu-doped, as seen in Table 1 that gives the minimum and maximum values of the optical transmission.
Optical reflection
Fig. 7 e (a, b) Variation of the Optical reflection films of indium dopped ZnO according to the wavelength at T ¼ 480 C and T ¼ 520 C.
The optical reflection spectra of thin layers of ZnO undoped and In- and Cu-dopped were developed for both deposition temperatures T ¼ 480 C and T ¼ 520 C and measured by UV spectrophotometry, Visible and Infrared. Figs. 7(a, b) and 8(a, b) represent the reflection spectra of the layers of ZnO. We note that for low doping with In (2% and 4%) the deposition temperature reduces the reflection of about 50%, while for the strong doping (6% and 8%) it increases. For thin layers of Cudoped ZnO we noticed that the deposition temperature has not changed the values of reflections with the exception of the films doped with 4% who experienced a decrease in the percentage of reflection. The effect of doping is clearly visible in the reflection spectra. This effect is expressed by a variation of values of the latter depending on the wavelength.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Conclusion [7]
In conclusion, we successfully prepared Cu- and In-doped ZnO films using the ultrasonic spray deposition method. We show the influence of doping and temperature on the different optical properties of thin layers of ZnO. The method of hot tip confirmed that the In-doped layers are of type n as against to the Cu-doped type p, a result that is very interesting and promising for applications in solar cells. ZnO thin films increases as the conductivity and the transmission to 90%. Then we can improve the efficiency of a multijunction solar cell by introducing ZnO thin layers.
[8]
[9]
[10]
[11]
references [12] [1] Yousfi EB, Weinberger B, Donsanti F, Cowache P, Lincot D. Atomic layer deposition of zinc oxide and indium sulfide layers for Cu(In,Ga)Se2 thin-film solar cells. Thin Solid Films 2001;387(1e2):29e32. [2] Makhseed S, Samuel J. The synthesis and characterization of zinc phthalocyanines bearing functionalized bulky phenoxy substituents. Dyes Pigments 2009;82:1e5. [3] Fattakhova-Rohlfing D, Brezesinski T, Rathousky J, Feldhoff A, Oekermann T, Wark M, et al. Transparent conducting films of indium tin oxide with 3D mesopore architecture. Adv Mater 2006;182:980e2983. ndez de [4] Alonso JL, Ferrer JC, Cotarelo MA, Montilla F, Ferna Avila S. Influence of the thickness of electrochemically deposited polyaniline used as hole transporting layer on the behaviour of polymer light-emitting diodes. Thin Solid Films 2009;517:2729e35. [5] Ma QB, Ye ZZ, He HP, Zhu LP, Huang JY, Zhang YZ, et al. Influence of annealingtemperature on the properties of transparent conductive and near-infrared reflective ZnO: Ga films. Scr Mater 2008;58(1):21e4. [6] Cho S, Ma J, Kim Y, Sun Y, Wong GKL, Ketterson JB. Photoluminescence and ultraviolet lasing of polycrystalline
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ZnO thin films prepared by the oxidation of the metallic Zn. Appl Phys Lett 1999;75(18):2761e3. Rekioua D, Matagne E. Optimization of photovoltaic power systems and control. Green Energy Technol 2012:102. Cauda V, Pugliese D, Garino N, Sacco A, Bianco S, Bella F, et al. Multi-functional energy conversion and storage electrodes using flower-like zinc oxide nanostructures. Energy 2014;65(C):639e46. Aissat A, El bey M, Bestam R, Vilcot JP. Modeling and simulation of AlxGayIn1xyAs/InP quaternary structure for photovoltaic. Int J Hydrogen Energy 2014;39(27):15287e91. Garino N, Lamberti A, Gazia R, Chiodoni A, Gerbaldi C. Cycling behaviour of sponge-like nanostructured ZnO as thin-film Li-ion battery anodes. J Alloys Compd 2014;615:454e8. Zhang Xi, Huang Xuezhen, Li Chensha, Jiang Hongrui. Dyesensitized solar cell with energy storage function through PVDF/ZnO nanocomposite counter electrode. Adv Mater 2013;25(30):4093e6. Corea-Lozano B, Comninellis Ch, De battisti A. Preparation of SnO2eSb2O5 films by the spray pyrolysis technique. J Appl Electrochem 1996:83e9. NI J, Zhao X, Zheng X, Zhao J, Liu B. Electrical, structural, photoluminescence and optical properties of P-type conducting, antimony-doped SnO2 thin films. ACTA Mater 2009;57(1):278e85. € r M, Fischer CH-H. Optical Rebien M, Henrion W, Ba properties of ZnO thin films: ion layer gas reaction compared to sputter deposition. Appl Phys Let 2002;80:3518. Nunes P, Fortunato E, Tonello P, Braz Fernandes F, Vilarinho P, Martins R. Effect of different dopant elements on the properties of ZnO thin films 2002;64(3e4):281e328. Mass J, Bhattacharya P, Katiyar RS. Effect of high substrate temperature on Al-doped ZnO thin films grown by pulsed laser deposition. Mater Sci Eng B 2003;103(1):9e15. Mondal S, Bhattacharya S, Mitra P. Structural morphological and LPG sensing properties of Al-doped ZnO thin film prepared by silar. Adv Mater Sci Eng 2013;2013:382380e6. Hafdallah A, Ynineb F, Daranfed W, Attaf N, Aida MS. Structural, optical and electrical properties of ZnO thin films: Al prepared by ultrasonic spray. Revue Nat Technol 2012:25e7.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications A. Aissat a,*, M.A. Ghomrani a, W. Bellil a, A. Benkouider b, J.P. Vilcot c LATSI Laboratoire, Faculte de Technologie, Universite de Blida.1, BP270, 09000, Blida, Algeria IM2NP-CNRS (UMR 7334), Aix-Marseille University, 13397, Marseille Cedex 20, France c Institut d'Electronique, de Microelectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Universite des Sciences et Technologies de Lille 1, Villeneuve d'Ascq, BP 60069, 59652, France a
b
article info
abstract
Article history:
In this study, we experimentally elaborated Copper- and Indium-doped Zinc Oxide (Cu:
Received 28 January 2015
ZnO and In: ZnO) thin films at different temperatures (T1 ¼ 480 C and T2 ¼ 520 C), the
Received in revised form
doping ratio were varied between 0% and 8%. Using a low cost solution-based chemical
19 February 2015
deposition, we have developed a ZnO thin film deposition process that offers fine-control of
Accepted 20 February 2015
the surface morphology. It consists in spraying a volatile compound of the material to be
Available online xxx
deposited on a substrate maintained at high temperature to cause a chemical reaction in order to form at least one solid product. Therefore, the proposed ZnO doped layer is highly
Keywords:
promising for applications for the next-generation solar cells.
Thin layer
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
New material Solar cell Photovoltaic
Introduction In the last decades, the development of materials using thin layers has contributed to the expansion of the electronics and optoelectronics performance. Moreover, the new open field of low cost and unbreakable large area photovoltaic devices implies the use of small size devices as an available cheap solution. The thin layers can be elaborated from a wide range of compositions such as conductive materials, insulators, semiconductors and polymers [1e3]. It is found that physical properties are closely related to the deposition parameters, mono or multilayer films can be synthesised with different thicknesses, typically from one monolayer to few hundred micrometres [4,5]. The Zinc Oxide (ZnO) semiconductors were
currently studied; various methods have been used to synthesis ZnO-based thin layers. However, a big progress on the synthesis techniques of thin films and the results indicate the possibility of converting the conductivity of the semiconductor n-type to p-type. The nanotechnology revolution has given him a place among the master race materials for optoelectronic applications; this is due to the multiple benefits that we present in this work. Transparent conductive oxides (TCO) are remarkable materials in many areas. The existence of their dual properties, electrical conductivity and transparency in the visible range, making them ideal candidates for applications in optoelectronics, photovoltaic or electro chromic windows [6e9]. Sponge-like ZnO thin film shows promising prospects as Li-ion battery anode [10]. Dye-sensitized solar cells with an energy storage function are demonstrated
* Corresponding author. Tel.: þ213 772525050; fax: þ213 25433850. E-mail address: [email protected] (A. Aissat). http://dx.doi.org/10.1016/j.ijhydene.2015.02.075 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
by modifying its counter electrode with a poly ZnO nanowire array composite. This simplex device could still function as an ordinary solar cell with a steady photocurrent output even after being fully charged [11]. At the vanguard of these materials, the (ZnO) is a semiconductor oxide which presents very interesting properties. In nature, it is ruby red in colour and is found abundantly in minerals, while artificially prepared is colourless or white. If the properties of natural ZnO have long been known, the researchers focused in recent years on the ZnO obtained artificially. ZnO is a potential candidate for the UV-emitting systems as it possesses at room temperature a large gap (3.37 eV) and a large exciting binding energy (60 meV) [6]. It is also an excellent optics in high performance photovoltaic devices, such as the HIT structure or CIGS based thin layers [1].
Fig. 2 e Schematic illustration of the Sub-Split-Step Fourier method distribution.
Study of the optical properties TCO ZnO is transparent in the visible and near infrared field. This later is considered as “twin” of gallium nitride. It can be used for potential applications in the fields of photovoltaic, light emitting diodes for illumination, transparent conductive oxides, photonics or sensors (Fig. 1.) The main advantage of ZnO-based devices is due to its components which are nontoxic and very abundant on earth. This is an important advantage because it reduces production costs. It can also be found in nature in the form of powder or solid crystal. It is in the form of mineral [12]. It was shown that is possible to have an n-type ZnO with many elements such as Al [1], Ga [2] In [3], etc. orp-type doping which remains, meanwhile, still controversial. The advent of p-type ZnO will open the door to transparent electronics the formation's reaction of stoichiometric ZnO [13]. 1 Znþþ þ 2e þ O2 0ZnO 2
light produced by the source passes first through the sample and the sphere harvest as well as the light that was transmitted through the sample. The sphere harvests as well as the light that was transmitted through the sample. The reflectance measuring R is done by placing the sample in place of a small square of the sphere wall, located as opposed to the entrance of this one (position 2 in Fig. 2). It is important to note that these measurements of T and R were performed on ZnO films deposited on a glass substrate. The light when it passes through the sample, it passes through several media having different refractive indices, in the following order: air/ZnO/glass/air. We can grossly evaluate the proportion of light that is reflected at these interfaces, using equation (2). R¼
(1)
The measure of transmission (T) is done by placing the sample at the entrance to the sphere (position 1 in Fig. 2). The
2 n2 n1 n2 þ n1
(2)
R: part of the reflected light when it passes between two media of refractive index n2 and n1. If we consider that the reflections that occur when the light passes in each medium, and we take nair ¼ 1, nglass ¼ 1.5 and nZnO ¼ 2, the part of the light reflected to different interfaces is about 17%. This calculation does not take into account the different roughness of the interfaces, which may also influence R. Therefore the R and T measurements performed with the spectrometer to the ZnO layers deposited on a glass substrate does not fully correspond to values of the transmitted and reflected light in a solar cell.
Optical transmission
Fig. 1 e Structure of a solar cell made of material and the ZnO thin layers.
The optical characterizations were based on the transmission spectra in the visible-UV. Indeed, as has been detailed in the previous section, the operation of the spectra allows us to calculate the optical gap. Figs. 3e6 combined transmission spectra in the range of 250e1000 nm, the films prepared with the two dopants Cuand In. Even though the general shape of the spectra is the same, they are composed of two parts:
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Fig. 3 e Variation of the transmittance of ZnO films dopped indium in function of the wavelength at T ¼ 480 C.
a e A region of high transparency: between 400 and 1000 nm, the transmission value is of the order of 75e85%. This value is reported by several authors [14], it gives the character of transparency in the visible to the ZnO thin layers doped and undopped. In this wavelength range, of the interference fringes is observed in the case of certain developed films. These fringes, characterized by undulating curves are due to the multiple reflection of radiation on two interfaces of the film. b e A region of strong absorption: This region corresponds to the fundamental absorption (l < 400 nm) in the ZnO films doped and undopped. This absorption is due to the electronic band gap transition. The change of the transmission in this region is used for determining the optical gap. This last one is estimated from the intersection of the curve giving (ahy)2 ¼ f (hy) with the abscissa axis. On the other hand, a shift of absorption threshold to the major energies is mainly due to the increase in concentration of free carriers in the material [15]. The shift in the absorption threshold is also equal to the gap variation DEg which is expressed by the following relation [16e18].
Fig. 4 e Variation of the transmittance of ZnO films dopped indium in function of the wavelength at T ¼ 520 C.
3
Fig. 5 e Variation of the transmittance of copper dopped ZnO films in function of the wavelength at T ¼ 480 C.
DEg ¼
23 h2 3n=p * 8m
(3)
Where h is Planck's constantm * the effective mass of carriersn the concentration of free electrons This relation shows that the variation of the gap is mainly caused by the concentration of free electrons. Fig. 3 represents the variation of the transmittance of the ZnO films undoped and those In-doped 2%, 4%, 6% and 8% as function of the wavelength, by setting the temperature at T ¼ 480 C. Fig. 4 represents the variation of the transmittance of the ZnO films undoped and those In-doped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 520 C. The Fig. 5 represents the variation of the transmittance of the non-doped ZnO films and those Cudoped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 480 C. Fig. 6 represents the variation of the transmittance of the nondoped ZnO films and those Cu-doped with 2%, 4%, 6% and 8% as function of the wavelength, and the temperature was fixed at T ¼ 520 C. We note that there is no remarkable
Fig. 6 e Variation of the transmittance of copper dopped ZnO films in function of the wavelength at T ¼ 520 C.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Table 1 e Minimum and maximum values for optical transmission of Indium doped ZnO thin layers and those copper dopped.
ZnO ZnO ZnO ZnO
:In at T ¼ 480 C :In at T ¼ 520 C :Cu at T ¼ 480 C :Cu at T ¼ 520 C
Transmission min (%)
Transmission max (%)
70 70 72 73
85 88 86 88
difference between the values of transmission for all films doped (In) and those doped (Cu) in the visible range. These values vary between 70% and 88%, which leads us to conclude that the optical transmission does not vary by inter changing these two dopants. We also note that all the deposited layers have a high optical transmission in the visible range at approximately 85%, and we note that the layers doped with indium and copper at 2% and 4% concentration present greater transparency in the visible for deposition temperatures 520 C and 480 C, respectively. As against the highest value of transmission for the In-doped layer and deposited at 480 C is attributed to the 4% concentration. The study of the
Fig. 8 e (a, b) Variation of the Optical reflection films Copper dopped ZnO according to the wavelength at T ¼ 480 C and T ¼ 520 C.
influence of deposition temperature on the values of the optical transmission shows an increase of the latter by about 3% in the visible range, for In-doped ZnO thin films than for those Cu-doped, as seen in Table 1 that gives the minimum and maximum values of the optical transmission.
Optical reflection
Fig. 7 e (a, b) Variation of the Optical reflection films of indium dopped ZnO according to the wavelength at T ¼ 480 C and T ¼ 520 C.
The optical reflection spectra of thin layers of ZnO undoped and In- and Cu-dopped were developed for both deposition temperatures T ¼ 480 C and T ¼ 520 C and measured by UV spectrophotometry, Visible and Infrared. Figs. 7(a, b) and 8(a, b) represent the reflection spectra of the layers of ZnO. We note that for low doping with In (2% and 4%) the deposition temperature reduces the reflection of about 50%, while for the strong doping (6% and 8%) it increases. For thin layers of Cudoped ZnO we noticed that the deposition temperature has not changed the values of reflections with the exception of the films doped with 4% who experienced a decrease in the percentage of reflection. The effect of doping is clearly visible in the reflection spectra. This effect is expressed by a variation of values of the latter depending on the wavelength.
Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e5
Conclusion [7]
In conclusion, we successfully prepared Cu- and In-doped ZnO films using the ultrasonic spray deposition method. We show the influence of doping and temperature on the different optical properties of thin layers of ZnO. The method of hot tip confirmed that the In-doped layers are of type n as against to the Cu-doped type p, a result that is very interesting and promising for applications in solar cells. ZnO thin films increases as the conductivity and the transmission to 90%. Then we can improve the efficiency of a multijunction solar cell by introducing ZnO thin layers.
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Please cite this article in press as: Aissat A, et al., The doping effect on the properties of zinc oxide (ZnO) thin layers for photovoltaic applications, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.075