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TiO2 thin layers with controlled morphology for ETA (extremely thin absorber) solar cells
Thin Solid Films 511 – 512 (2006) 195 – 198 www.elsevier.com/locate/tsf
TiO2 thin layers with controlled morphology for ETA (extremely thin absorber) solar cells Anca Duta Simona Manolache, Ion Visa Centre for Sustainable Energy, Transilvania University of Brasov, Eroilor 29, 500036, Brasov, Romania and Marian Nanu, Albert Goossens, Joop Schoonman, TU Delft, Julianalaan 136, 2628 Delft, The Netherlands Available online 24 January 2006
Abstract The paper presents the synthesis process of dense and nanoporous TiO2 anatase films obtained via Spray Pyrolysis Deposition (SPD). The deposition of dense and nanostructured TiO2 films uses ethanol solutions of titaniumtetraisopropoxid and acetilacetonate. The influence of the precursor’s concentration and deposition parameters (temperature, pressure of the carrier gas and distance of spraying) in tailoring the TiO2 morphology is presented. The films are tested via X-ray Diffraction and Scanning Electron Microscopy. The photoelectrical properties are tested by current – voltage (I – V) experiments in dark, at room temperature. According to the results, SPD proves to be a reliable technique in obtaining thin layers of TiO2 with controlled morphology. D 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 anatase films; Morphology; Spray pyrolysis deposition; ETA solar cells
1. Introduction Extremely thin absorber layers solar cells (ETA cells) represent a follow up to the new generation of non-silicon based PVs, in the trend firstly stated by Graetzel (1991), [1]. The ETA cells represent solid-state solar cells with an extremely thin absorber layer and with the general structure: transparent n-type semiconductor/n or p-type absorber/transparent p-type semiconductor. In the ETA cell, the light absorber is embedded in a porous and transparent structure enhancing the path of the light through the material and limiting the losses at interface. The n-type semiconductor investigated in our work is TiO2 (anatase) semiconductor. Many applications of TiO2 are based on thin films. Thin films of TiO2 used as optical coatings, integrate circuits, solar cell and electro-chromic windows capacitor dielectrics, heat reflecting layers and waveguides show good corrosion resistance to corrosive and mechanical attack and stability over long time periods. TiO2 has received a great deal of attention due to its chemical stability, non-toxicity, and low cost. TiO2 exists as three polymorphs: anatase, rutile and brookite. The semiconductor properties of the first two polymorphs are presented in Table 1. E-mail address: [email protected]. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.100
To avoid shunts at the front contact interface, the n-type TiO2 semiconductor is formed out of two layers with different morphologies: a dense thin layer with low flexibility and the nanoporous matrix able to be infiltrated with the p-type absorber semiconductor, used as electron conductor in the cell. The dense films generally have superior mechanical, optical and electrical (charge transport) properties. The porous films are used when a large specific surface area is important as needed in the ETA solar cells, [3], Fig. 1. Table 1 Properties of TiO2 Properties
Anatase
Rutile
Bandgap (eV) Density (g/cm3) Dielectric constant Refractive index Heat of formation DH -f ,298.15 (kcal/mol) Absolute entropy S -298.15 (cal/deg/mol) Melting point (-C)
3.26 3.90 55 2.49 – 2.55 218.1
3.05 4.27 170 E//c; 86E//a* 2.61 – 2.90 255.5
11.93
12.01
Phase transition to rutile before melting 5.5 – 6.0
1855
Hardness (Mohs scale)
7.0 – 7.5
* E//c: electrical field parallel to c-axis of unit cell. E//a: electrical field parallel to a-axis, [2].
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A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
Fig. 1. The structure of ETA solar cell.
The deposition of thin films with specific properties is a challenge for the researchers. Many deposition techniques were used in obtaining TiO2 anatase layers: Chemical Vapor Deposition (CVD), [4] doctor blade, [5], Spray Pyrolysis Deposition (SPD), [6,7], etc. The aim of our research is to develop both dense and nanoporous layers by SPD. SPD is a low cost deposition method for large area thin films and it is economically more attractive than other techniques that have been used until this moment being also suitable for up-scaling. The SPD technique was proven to by suitable for the deposition of the dense TiO2 films, but literature references on the deposition of the nanoporous films is scarce, [8]. In tailoring the thin films morphology two important factors must be considered: the nucleation and the particles’ growth. For dense layers the nucleation rate must be higher than the growth rate while in the nanoporous layers growth, these must be reversed. These two rates and, consequently, the size of the particles formed and the morphology of the resulting films are strongly dependent on the precursors (especially the complexation agents), the substrate temperature, the pressure of the carrier gas, the time between the sprayed layers, and the spraying distance.
Fig. 2. X-ray Diffraction of dense TiO2 (anatase) film.
ratio of 22.5 : 1 : 1.5. The TCO substrate was cleaned before using by successive immersion in ethanol and acetone in an ultrasonic bath and dried in a nitrogen flow. The deposition is
2. Experimental The precursors’ solutions for the TiO2 layers deposition are obtained using absolute ethanol, EtOH (J.T. Baker) solutions of titanium(IV)isopropoxid, TTIP (99.99%, Sigma-Aldrich), and acetilacetonate, AcAc (2,4 pentadione 99+%, Aldrich) used as complexation agent. The dens TiO 2 layer was deposited via SPD, [4] on the top of a TCO glass (transparent conducting oxide, F doped SnO2 coated glass, Libbey Owens Ford-TEC 20/2.5 mm) using absolute ethanol solutions of TTIP and AcAc in a volumetric
Fig. 3. SEM picture of dense and homogenous anatase TiO2.
Table 2 The parameters varied in the deposition of nanoporous TiO2 layers by SPD Tests
Temperature (-C)
TTIP : AcAc : EtOH
Substrate
H SPD (cm)
P carrier (bar)
1 (A) 2 (A)
400*
1.3 : 1 : 20.8
TCO TCO
25, 30, 35 30
TCO/dense TiO2 anatase
30
1.2 0.8, 1.0, 1.2, 1.4 1.2
3 (B)
gas
Fig. 4. X-ray diffraction of nanoporous TiO2 (anatase) film. The other peaks represent the TCO substrate.
A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
197
Fig. 5. SEM picture of porous TiO2 films deposited at: (a) 25 cm high and (b) 30 cm high ( p = 1.2 bar).
done on the heated TCO substrate using the optimum conditions reported by Nanu et al. [8], at 350 -C in open atmosphere, with the pressure of the carrier gas at 1.2 bars. A 17 cm distance from the nozzle to the substrate and 60 s breaks between the spraying sequences were considered as optimal. After spraying, the samples are annealed in air at 450 -C, for 2 h followed by cooling on the heater, down to the room temperature.
The nanoporous layers are deposited (A) on the top of TCO substrate and (B) on TCO covered with dense TiO2 layer. For all the testing sets, the deposition parameters (the TTIP : AcAc : EtOH ratio, the sprayed height and pressure of carrier gas) were varied as presented in Table 2. After the deposition the films are annealed in air for 6 h at 500 -C. The films are tested via X-ray Diffraction (XRD, Bruker D8 Advance Diffractometer), Scanning Electron Microscopy
Fig. 6. SEM picture of porous TiO2 films deposited at: (a) 0.8 bar, (b) 1.0 bar, (c) 1.4 bar; h = 30 cm.
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A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
Current Density (mA/cm2)
2 0 -2
(c) -4
(b)
-6
(a)
-8 -10 -1.0
-0.5
0.0
0.5
1.0
Voltage (V) Fig. 7. I – V dark curves of (a) dense TiO2 layer; (b) nanoporous TiO2 anatase films; (c) TCO/dens TiO2/nanoporous TiO2 structure.
have direct influence on the anatase growth, especially in the nucleation step. The best films, with high porosity, are obtained at 30 cm spraying height and at the pressure of the carrier gas of 1.2 bar, as the SEM pictures in Figs. 5 and 6 show. The electrical measurements (current – voltage curves) of the dense and nanoporous films are registered in dark; the two TiO2 layers will be further used as deposition substrate for the p-type layers necessary to develop an ETA cell. The current– voltage (I –V) dark curves of the TCO/dense, TCO/nanoporous and TCOdens/nanoporous TiO2 anatase layers (Fig. 7) show the diode behavior of the films confirming that homogenous films, free of pinholes are obtained. The differences in the I – V response can be correlated with the amount of defects that is differently modified during the growth and annealing steps, according to the porosity of the thin layer. 4. Conclusions
(SEM, Jeol JSM-5800LV model), and current– voltage dark measurements (Keithley DC source, model 2400). 3. Results and discussions At the fixed precursors’ concentration ratio (EtOH : TTIP : AcAc = 22.5 : 1 : 1.5) and deposition parameters the Xray diffraction measurements for the dense films reveal an anatase crystalline structure, Fig. 2. The SEM picture of the film shows the formation of a dens and homogeneous film with a grain size of about 150 nm, Fig. 3. The donor free carrier concentration existent in the dense TiO2 determines the conduction properties of the films. To measure the donor density, the impedance spectrum is recorded as a function of a DC bias using the Mott – Schottky formula. Preliminary results show a donor density of about 1.1610 defects/cm3 in the dense film, [8]. Varying the deposition parameters (the sprayed height and the pressure of the carrier gas), the chemical composition of the final product is not changed but the morphology of the films can be tailored, for obtaining nanoporous films. The X-ray diffraction measurements done on the nanoporous TiO2 films confirm the formation of an anatase crystalline structure, Fig. 4, according to JCPDS 73-1764. The SEM picture of the TiO2 films deposited at different spraying heights and different carrier gas pressures revealed the formation of nanostructured grains assembled at micro level. The temperature above the heating plate increases fast when approaching the deposition surface and induces different vaporization; consequently, the distance of spraying represents a tool, able to control the precursors’ composition in the aerosol. The same effect can be related to the pressure of the carrier gas which modifies the droplets dimensions and their speed towards the deposition surface; hence, both parameters
The aim of this paper was to present the results that prove that the SPD technique is suitable for the deposition of the TiO2 (anatase) films with controlled morphology (dense and nanoporous) used in ETA solar cells. If the deposition of dense TiO2 anatase films was already reported, for the nanoporous TiO2 films the research is in the beginning. The control of the spraying parameters allows the obtaining of nanoporous semiconducting films, homogenous, without shunts. Acknowledgements All the authors thank for the entire support given by the Inorganic Chemistry Department in the Delft University of Technology, the Netherlands, in developing the experiments that made possible this paper and also to the Leonardo da Vinci projects RO/02/B/F/PP-141026 and RO/02/91181S. References [1] B. O’Reagan, M. Gratzel, Nature 353 (1991) 737. [2] C. Zilverentant, Hybrid Solar Cells of Titanium Dioxide Sensitized with Organic Semiconductors, Techniquel University of Delft, Delft, Netherlands, 2003. [3] M. Nanu, Towards a 3D Solar Cell Based on TiO2/CuInS2 Heterojunction, Report, Brasov, Romanian, 2003. [4] V.G. Bessergeneva, I.V. Khmelinskiia, R.J.F. Pereiraa, V.V. Krisukb, A.E. Turgambaevab, I.K Igumenovb, Vacuum 64 (2002) 275. [5] V. Shkolover, M.K. Nazeeruddin, S.M Zakeeruddin, C. Barbe, A. Key, T. Haibach, S. Steurer, R. Hermann, H.U. Nissen, M. Gratzel, Chem. Mater. 9 (1997) 430. [6] A. Duta, I. Visa, M. Nanu, ISES World Congress, Goteborg, Sweden, 2003. [7] O. Masayuki, N. Koji, O. Daisuke, N.G.R. Takafumi, K. Asoka, K. Shoji, J. Photochem. Photobiol., A Chem. 164 (2004) 167. [8] A. Duta and M. Nanu, Eurosun 2004, Freiburg, 2 (2004) 737-742, ISBN 39809656-2-7.
TiO2 thin layers with controlled morphology for ETA (extremely thin absorber) solar cells Anca Duta Simona Manolache, Ion Visa Centre for Sustainable Energy, Transilvania University of Brasov, Eroilor 29, 500036, Brasov, Romania and Marian Nanu, Albert Goossens, Joop Schoonman, TU Delft, Julianalaan 136, 2628 Delft, The Netherlands Available online 24 January 2006
Abstract The paper presents the synthesis process of dense and nanoporous TiO2 anatase films obtained via Spray Pyrolysis Deposition (SPD). The deposition of dense and nanostructured TiO2 films uses ethanol solutions of titaniumtetraisopropoxid and acetilacetonate. The influence of the precursor’s concentration and deposition parameters (temperature, pressure of the carrier gas and distance of spraying) in tailoring the TiO2 morphology is presented. The films are tested via X-ray Diffraction and Scanning Electron Microscopy. The photoelectrical properties are tested by current – voltage (I – V) experiments in dark, at room temperature. According to the results, SPD proves to be a reliable technique in obtaining thin layers of TiO2 with controlled morphology. D 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 anatase films; Morphology; Spray pyrolysis deposition; ETA solar cells
1. Introduction Extremely thin absorber layers solar cells (ETA cells) represent a follow up to the new generation of non-silicon based PVs, in the trend firstly stated by Graetzel (1991), [1]. The ETA cells represent solid-state solar cells with an extremely thin absorber layer and with the general structure: transparent n-type semiconductor/n or p-type absorber/transparent p-type semiconductor. In the ETA cell, the light absorber is embedded in a porous and transparent structure enhancing the path of the light through the material and limiting the losses at interface. The n-type semiconductor investigated in our work is TiO2 (anatase) semiconductor. Many applications of TiO2 are based on thin films. Thin films of TiO2 used as optical coatings, integrate circuits, solar cell and electro-chromic windows capacitor dielectrics, heat reflecting layers and waveguides show good corrosion resistance to corrosive and mechanical attack and stability over long time periods. TiO2 has received a great deal of attention due to its chemical stability, non-toxicity, and low cost. TiO2 exists as three polymorphs: anatase, rutile and brookite. The semiconductor properties of the first two polymorphs are presented in Table 1. E-mail address: [email protected]. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.100
To avoid shunts at the front contact interface, the n-type TiO2 semiconductor is formed out of two layers with different morphologies: a dense thin layer with low flexibility and the nanoporous matrix able to be infiltrated with the p-type absorber semiconductor, used as electron conductor in the cell. The dense films generally have superior mechanical, optical and electrical (charge transport) properties. The porous films are used when a large specific surface area is important as needed in the ETA solar cells, [3], Fig. 1. Table 1 Properties of TiO2 Properties
Anatase
Rutile
Bandgap (eV) Density (g/cm3) Dielectric constant Refractive index Heat of formation DH -f ,298.15 (kcal/mol) Absolute entropy S -298.15 (cal/deg/mol) Melting point (-C)
3.26 3.90 55 2.49 – 2.55 218.1
3.05 4.27 170 E//c; 86E//a* 2.61 – 2.90 255.5
11.93
12.01
Phase transition to rutile before melting 5.5 – 6.0
1855
Hardness (Mohs scale)
7.0 – 7.5
* E//c: electrical field parallel to c-axis of unit cell. E//a: electrical field parallel to a-axis, [2].
196
A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
Fig. 1. The structure of ETA solar cell.
The deposition of thin films with specific properties is a challenge for the researchers. Many deposition techniques were used in obtaining TiO2 anatase layers: Chemical Vapor Deposition (CVD), [4] doctor blade, [5], Spray Pyrolysis Deposition (SPD), [6,7], etc. The aim of our research is to develop both dense and nanoporous layers by SPD. SPD is a low cost deposition method for large area thin films and it is economically more attractive than other techniques that have been used until this moment being also suitable for up-scaling. The SPD technique was proven to by suitable for the deposition of the dense TiO2 films, but literature references on the deposition of the nanoporous films is scarce, [8]. In tailoring the thin films morphology two important factors must be considered: the nucleation and the particles’ growth. For dense layers the nucleation rate must be higher than the growth rate while in the nanoporous layers growth, these must be reversed. These two rates and, consequently, the size of the particles formed and the morphology of the resulting films are strongly dependent on the precursors (especially the complexation agents), the substrate temperature, the pressure of the carrier gas, the time between the sprayed layers, and the spraying distance.
Fig. 2. X-ray Diffraction of dense TiO2 (anatase) film.
ratio of 22.5 : 1 : 1.5. The TCO substrate was cleaned before using by successive immersion in ethanol and acetone in an ultrasonic bath and dried in a nitrogen flow. The deposition is
2. Experimental The precursors’ solutions for the TiO2 layers deposition are obtained using absolute ethanol, EtOH (J.T. Baker) solutions of titanium(IV)isopropoxid, TTIP (99.99%, Sigma-Aldrich), and acetilacetonate, AcAc (2,4 pentadione 99+%, Aldrich) used as complexation agent. The dens TiO 2 layer was deposited via SPD, [4] on the top of a TCO glass (transparent conducting oxide, F doped SnO2 coated glass, Libbey Owens Ford-TEC 20/2.5 mm) using absolute ethanol solutions of TTIP and AcAc in a volumetric
Fig. 3. SEM picture of dense and homogenous anatase TiO2.
Table 2 The parameters varied in the deposition of nanoporous TiO2 layers by SPD Tests
Temperature (-C)
TTIP : AcAc : EtOH
Substrate
H SPD (cm)
P carrier (bar)
1 (A) 2 (A)
400*
1.3 : 1 : 20.8
TCO TCO
25, 30, 35 30
TCO/dense TiO2 anatase
30
1.2 0.8, 1.0, 1.2, 1.4 1.2
3 (B)
gas
Fig. 4. X-ray diffraction of nanoporous TiO2 (anatase) film. The other peaks represent the TCO substrate.
A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
197
Fig. 5. SEM picture of porous TiO2 films deposited at: (a) 25 cm high and (b) 30 cm high ( p = 1.2 bar).
done on the heated TCO substrate using the optimum conditions reported by Nanu et al. [8], at 350 -C in open atmosphere, with the pressure of the carrier gas at 1.2 bars. A 17 cm distance from the nozzle to the substrate and 60 s breaks between the spraying sequences were considered as optimal. After spraying, the samples are annealed in air at 450 -C, for 2 h followed by cooling on the heater, down to the room temperature.
The nanoporous layers are deposited (A) on the top of TCO substrate and (B) on TCO covered with dense TiO2 layer. For all the testing sets, the deposition parameters (the TTIP : AcAc : EtOH ratio, the sprayed height and pressure of carrier gas) were varied as presented in Table 2. After the deposition the films are annealed in air for 6 h at 500 -C. The films are tested via X-ray Diffraction (XRD, Bruker D8 Advance Diffractometer), Scanning Electron Microscopy
Fig. 6. SEM picture of porous TiO2 films deposited at: (a) 0.8 bar, (b) 1.0 bar, (c) 1.4 bar; h = 30 cm.
198
A. Duta / Thin Solid Films 511 – 512 (2006) 195 – 198
Current Density (mA/cm2)
2 0 -2
(c) -4
(b)
-6
(a)
-8 -10 -1.0
-0.5
0.0
0.5
1.0
Voltage (V) Fig. 7. I – V dark curves of (a) dense TiO2 layer; (b) nanoporous TiO2 anatase films; (c) TCO/dens TiO2/nanoporous TiO2 structure.
have direct influence on the anatase growth, especially in the nucleation step. The best films, with high porosity, are obtained at 30 cm spraying height and at the pressure of the carrier gas of 1.2 bar, as the SEM pictures in Figs. 5 and 6 show. The electrical measurements (current – voltage curves) of the dense and nanoporous films are registered in dark; the two TiO2 layers will be further used as deposition substrate for the p-type layers necessary to develop an ETA cell. The current– voltage (I –V) dark curves of the TCO/dense, TCO/nanoporous and TCOdens/nanoporous TiO2 anatase layers (Fig. 7) show the diode behavior of the films confirming that homogenous films, free of pinholes are obtained. The differences in the I – V response can be correlated with the amount of defects that is differently modified during the growth and annealing steps, according to the porosity of the thin layer. 4. Conclusions
(SEM, Jeol JSM-5800LV model), and current– voltage dark measurements (Keithley DC source, model 2400). 3. Results and discussions At the fixed precursors’ concentration ratio (EtOH : TTIP : AcAc = 22.5 : 1 : 1.5) and deposition parameters the Xray diffraction measurements for the dense films reveal an anatase crystalline structure, Fig. 2. The SEM picture of the film shows the formation of a dens and homogeneous film with a grain size of about 150 nm, Fig. 3. The donor free carrier concentration existent in the dense TiO2 determines the conduction properties of the films. To measure the donor density, the impedance spectrum is recorded as a function of a DC bias using the Mott – Schottky formula. Preliminary results show a donor density of about 1.1610 defects/cm3 in the dense film, [8]. Varying the deposition parameters (the sprayed height and the pressure of the carrier gas), the chemical composition of the final product is not changed but the morphology of the films can be tailored, for obtaining nanoporous films. The X-ray diffraction measurements done on the nanoporous TiO2 films confirm the formation of an anatase crystalline structure, Fig. 4, according to JCPDS 73-1764. The SEM picture of the TiO2 films deposited at different spraying heights and different carrier gas pressures revealed the formation of nanostructured grains assembled at micro level. The temperature above the heating plate increases fast when approaching the deposition surface and induces different vaporization; consequently, the distance of spraying represents a tool, able to control the precursors’ composition in the aerosol. The same effect can be related to the pressure of the carrier gas which modifies the droplets dimensions and their speed towards the deposition surface; hence, both parameters
The aim of this paper was to present the results that prove that the SPD technique is suitable for the deposition of the TiO2 (anatase) films with controlled morphology (dense and nanoporous) used in ETA solar cells. If the deposition of dense TiO2 anatase films was already reported, for the nanoporous TiO2 films the research is in the beginning. The control of the spraying parameters allows the obtaining of nanoporous semiconducting films, homogenous, without shunts. Acknowledgements All the authors thank for the entire support given by the Inorganic Chemistry Department in the Delft University of Technology, the Netherlands, in developing the experiments that made possible this paper and also to the Leonardo da Vinci projects RO/02/B/F/PP-141026 and RO/02/91181S. References [1] B. O’Reagan, M. Gratzel, Nature 353 (1991) 737. [2] C. Zilverentant, Hybrid Solar Cells of Titanium Dioxide Sensitized with Organic Semiconductors, Techniquel University of Delft, Delft, Netherlands, 2003. [3] M. Nanu, Towards a 3D Solar Cell Based on TiO2/CuInS2 Heterojunction, Report, Brasov, Romanian, 2003. [4] V.G. Bessergeneva, I.V. Khmelinskiia, R.J.F. Pereiraa, V.V. Krisukb, A.E. Turgambaevab, I.K Igumenovb, Vacuum 64 (2002) 275. [5] V. Shkolover, M.K. Nazeeruddin, S.M Zakeeruddin, C. Barbe, A. Key, T. Haibach, S. Steurer, R. Hermann, H.U. Nissen, M. Gratzel, Chem. Mater. 9 (1997) 430. [6] A. Duta, I. Visa, M. Nanu, ISES World Congress, Goteborg, Sweden, 2003. [7] O. Masayuki, N. Koji, O. Daisuke, N.G.R. Takafumi, K. Asoka, K. Shoji, J. Photochem. Photobiol., A Chem. 164 (2004) 167. [8] A. Duta and M. Nanu, Eurosun 2004, Freiburg, 2 (2004) 737-742, ISBN 39809656-2-7.