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Inorganic nanocomposites for the next generation photovoltaics
Materials Letters 60 (2006) 3541 – 3543 www.elsevier.com/locate/matlet
Inorganic nanocomposites for the next generation photovoltaics Sukti Chatterjee a,b,c,⁎, Amita Goyal d , S. Ismat Shah b,d a
b
Centre for Composite Materials, University of Delaware, Newark, Delaware 19716, USA Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716, USA c Material Science Center, Indian Institute of Technology, Karagpur, West Bengal 721302, India d Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA Received 21 September 2005; accepted 5 March 2006 Available online 3 April 2006
Abstract Photovoltaic is an attractive alternative of conventional energy source, but for the limitations of present materials and technology, we need to find out cost effective and environmentally stable new materials. We have synthesized a novel nanocomposite, named titania–germanium (TiO2– Ge). TiO2–Ge is a thermodynamically stable material. Ge nanodots are dispersed in the TiO2 matrix of the nanocomposites. Bohr radius of Ge is relatively large, 24.3 nm, therefore, it is easy to vary the size of Ge nanodots, and consequently the properties (structural, optical and electrical) of TiO2–Ge can be tailored in a wide range just by varying the size and density of Ge nanodots. TiO2–Ge with size gradient of Ge nanodots is a promising active layer of the next generation solar cells. © 2006 Elsevier B.V. All rights reserved.
1. Introduction Solar electric power is an ideal fuel-less energy source. It is no longer just used in terrestrial and space applications or power calculator, emergency telephone, but will become an intimate part of the utility grid in near future. That is why we need inexpensive, environmentally stable semiconductors to fabricate high efficiency photovoltaics. Crystalline/amorphous silicon, gallium arsenide (GaAs), gallium indium phosphide (GaInP2), copper indium gallium selenide (CIGS), cadmium telluride (CdTe) solar cells are the typical semiconductors for photovoltaic applications [1–5]. Crystalline silicon or GaAs/GaInP2 solar cells are highly efficient (20–30%), however their fabrication processes are complex as well as expensive. Relatively inexpensive amorphous silicon (aSi) photovoltaics have a fundamental drawback: light induced degradation of photovoltaic properties. CdTe solar cells suffer from environmental instability. Therefore, conventional photovoltaic devices face new challenges in the coming years. There is an increasing awareness of the possible advantages of nanocrystalline and conducting polymer devices [6] which are relatively cheap to fabricate, can be used on flexible substrates, and can be ⁎ Corresponding author. Centre for Composite Materials, University of Delaware, Newark, Delaware 19716, USA. E-mail address: [email protected] (S. Chatterjee). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.048
shaped or tinted to suit particular devices or architectural or decorative applications. Nanostructured Titania (TiO2) is a promising semiconductor for its photo-activity. It is a good candidate for the active layer of solar cells. Optical gap of TiO2 is 3.2 eV that belongs in the ultraviolet region of a solar spectrum. However, a peak of the solar spectrum is in the visible region. Therefore, to increase the efficiency of each device, TiO2 nanoparticles are customized by different methods, for example, size optimization, doping, various surface modifications, dye sensitization etc. Among the different processes, so far dye sensitization is the most successful way to absorb enough light for achieving useful efficiencies. However, charge transport and instability of the organic complex in the dye are still major issues [7]. In this context, we need a real breakthrough for the further improvement of TiO2 solar cells. In this letter, we propose a nanocomposite called titania– germanium (TiO2–Ge) which will be a stable alternative to dye sensitized TiO2. We will discuss the properties TiO2–Ge and its significant feasibility in solar cells. 2. Experiments To synthesize a TiO2–Ge nanocomposite powder, TiO2 (P25 Degussa, 70% anatase, 99.8% purity, average particle size 30 nm, and specific surface of 50 m2/g) and Ge (− 100 mesh,
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S. Chatterjee et al. / Materials Letters 60 (2006) 3541–3543
99.99% pure) powders were mixed thoroughly in different Ge weight concentrations (20%, 33%, 43%, 50%, 57%, 66% and 80%); then the mixtures of TiO2 and Ge powders were annealed at 550 °C under Ar atmosphere. TiO2–Ge nanocomposite thin films were synthesized by RF magnetron sputtering. Base pressure of the deposition chamber was 5 × 10− 7 Torr. Two composite targets (T1 and T2) were used for nanocomposite films preparation. Weight ratios of TiO2 and Ge in T1 and T2 are 10:1 and 2:1, respectively. The sputtering was carried out in an inert atmosphere of 6 mTorr Argon. Substrate temperature and RF power were 600 °C and 200 W, respectively. For the structural cauterizations of TiO2–Ge nanocomposites, we extensively exploited X-ray diffraction (XRD), X-ray photo-thermal spectroscopy (XPS), and Transmission electron microscopy (TEM). The XRD data of the nanocomposite films were collected by using a Rigaku D-Max B horizontal diffractometer. A SSI-M probe XPS was used employing Al kα (hν = 1486.6 eV) exciting radiation to study the chemical state of Ge and Ti as well as to obtain the cation composition of the films. JEM 2010F and JEM 2000FX TEM were used to examine the microstructure of the films as well as to obtain the particle size, film composition, and crystal structure. Optical Transmission and reflection data in the wavelength range 200 to 2600 nm were taken from UV–VIS-NIR Perkin Elmer spectrophotometer. The dark and photoconductivities were measured in an environmental test chamber. The photoconductivity was measured under 50 mW cm− 2 white light from a tungsten halogen lamp. 3. Results and discussion TiO2–Ge is a novel nanocomposite which includes the uniform distribution of Ge nanodots in a TiO2 matrix. The following questions become obvious regarding new TiO2–Ge. • Is there any thermodynamic stability in TiO2–Ge? • Is it feasible to synthesize TiO2–Ge? • What are the roles of Ge in TiO2?
Fig. 1. Z-contrast imaging, generated by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) of TiO2–Ge with Ge concentration of 50%.
Fig. 2. XRD patterns in the 2Θ range of (a) 26° to 28° and (b) 53° to 55° of a TiO2–Ge nanocomposite for various Ge concentrations.
Ti and Ge interact with each other at temperatures above 450 °C and form various Ti–Ge alloys [8]. Based on this observation, we have carried out experiments to anneal the mixtures of TiO2 and Ge powders with various Ge concentrations at 550 °C, under Ar atmosphere. We observed uniform distribution of Ge clusters (quantum dots) in the TiO2 matrix via Z-contrast imaging by Transmission Electron Microscope (TEM) [Fig. 1] and X-ray energy dispersive spectroscopy (XEDS) analysis. We obtained a strong Ge signature in our annealed TiO2–Ge samples by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). For evidence, we display XRD spectra of four TiO2–Ge samples in Fig. 2. Fig. 2 has two parts: (a) and (b) represent the spectra in the 2Θ range of 26–28 and 53–55, respectively. Bragg peak positions of Ge (111), rutile TiO2 (110) [Fig. 1(a)], Ge (311 kα1), Ge (311 kα2) and rutile TiO2 (211) [Fig. 2(b)] are indicated by arrows. Besides those peaks, a peak in Fig. 2(a) as well as one peak in Fig. 2(b) is not yet identified. These peaks are related with neither Ge nor TiO2. Most probably, these peaks are for Ti–Ge metastable phases. The XRD and TEM results suggest complete interaction of the mixed powders and the formation of non-percolating Ge regions within the TiO2 matrix. It is important to point out that the formation of GeO2 within the
Fig. 3. UV–VIS spectra of a TiO2–Ge nanocomposite for various Ge concentrations.
S. Chatterjee et al. / Materials Letters 60 (2006) 3541–3543 Table 1 Opto-electronic properties of TiO2–Ge nanocomposites
3543
4. Conclusion
Sample number
TiO2–Ge ratio in sputtering target
Optical gap (eV)
Dark Conductivity (S cm− 1)
Photo Conductivity (S cm− 1)
TG1 TG2
10:1 2:1
2.22 0.76
6.9 × 10− 7 3.9 × 10− 3
1.4 × 10− 6 4.1 × 10− 3
TiO2 matrix is thermodynamically unfavourable. The electronegativity of Ti (1.54 Pauling) is lower than that of Ge (2.01 Pauling) [9] and TiO2 is a much more stable compound than GeO2. The enthalpies of formation (ΔHf°) for TiO2 and GeO2 are −944.0 and − 580 kJ/mol, respectively [10]. This provides a simple way of introducing Ge quantum dots in the TiO2 matrix and an easy way of sensitizing TiO2 by the formation of stable TiO2–Ge nanocomposites. Relatively large exciton effective Bohr radius [11,12] of Germanium (24.3 nm) is helpful to tailor the size of Ge nanodots. Varying the density and size of Ge nanodots, it is easy to change the structural, optical and electronic properties of TiO2–Ge. For example, Fig. 3 demonstrates the red shift of UV–VIS spectra with the increase of % of Ge in TiO2–Ge. Like Germanium, silicon is also another common low band gap semiconductor. Its electronegativity and enthalpy for the formation of SiO2 are 1.90 Pauling and 905.49 kJ/mol respectively. Therefore, in principle it is possible to fabricate Si nanodots in TiO2 matrix. However, the effective Bohr radius of the excitons in Ge is larger than that in Si (4.9 nm), since Si has larger electron and hole effective masses and a lower dielectric constant than Ge. This implies that the Ge nanodots show a larger shift of an optical band gap (blue shift) than the Si nanodots. In other words, Ge nanodots would be more efficient than Si nanodots to tailor the properties. Therefore we favour to synthesize TiO2–Ge instead of TiO2–Si. Like optical properties, tailoring of electrical properties of TiO2–Ge nanocomposites is promising too. To check the variation of transport properties in TiO2–Ge nanocomposites, we deposited two TiO2–Ge films (TG1 and TG2) by RF sputtering using two different sputtering targets (T1 and T2). Thicknesses of TG1 and TG2 are 0.8 and 2.2 μm, respectively. Opto-electronic properties of two sputtered films are displayed in Table 1. Optical gaps of TG1 and TG2 determined from Tauc's plot [13] are 2.22 and 0.76 eV, respectively. TG2 is a Ge rich film and dark and photoconductivity of TG2 is 3 orders higher than those of TG1. Therefore, we can tailor several orders of magnitude of electrical properties of TiO2–Ge with Ge atom concentration in the films.
In short, TiO2–Ge nanocomposite is a promising cost effective and environmentally stable material for solid state solar cells as well as hydrogen energy generation. Synthesis of TiO2–Ge nanocomposites is simple. Direct deposition at high temperature (N 450 °C) is an option to synthesize TiO2–Ge nanocomposites. TiO2–Ge nanocomposites can absorb a wide range of solar spectrum from UV via VIS to IR just by the introduction of different sizes of Ge nanodots in the TiO2 matrix. Bohr radius [11,12] of Ge is relatively large, 24.3 nm, therefore, it is easy to make size gradients of Ge nanodots in the TiO2 matrix. Electronic properties of TiO2–Ge can be tailored in several orders of magnitude by varying the size and density distribution of Ge dots. Acknowledgement The authors acknowledge NSF (Grant No. DMR-0441619) for the financial support of this work. References [1] M.A. Green, Sol. Energy 76 (2004) 3. [2] A. Goetzberger, C. Hebling, H.-W. Schock, Mater. Sci. Eng., R Rep. 40 (2003) 1. [3] K.A. Bertness, Sarah R. Kurtz, D.J. Friedman, A.E. Kibbler, C. Kramer, J.M. Olson, Appl. Phys. Lett. 65 (1994) 989. [4] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovolt. Res. Appl. 7 (1999) 311. [5] T. Aramoto, S. Kumazawa, H. Higuchi, T. Arita, A. Hanafusa, M. Murozono, Proc. 26th IEEE PVSC, Anaheim, 1997, p. 343. [6] M. Grätzel, Nature 421 (2003) 586–587. [7] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [8] Q.Z. Hong, K. Barmak, F.M. d'Heurle, Appl. Phys. Lett. 62 (1993) 3435. [9] W.W. Porterfield, Inorganic Chemistry, a Unified Approach, Addison Wesley Publishing Co., Reading Massachusetts, USA, 1984. [10] J.D. Cox, D.D. Wagman, V.A. Medvedev, CODATA Key Values for Thermodynamics, Hemisphere Publishing Corp., New York, 1984, p. 1. [11] L.E. Brus, J. Chem. Phys. 79 (1983) 5566. [12] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [13] J. Tauc, Optical Properties of Solids, Plenum, New York, 1974, p. 159.
Inorganic nanocomposites for the next generation photovoltaics Sukti Chatterjee a,b,c,⁎, Amita Goyal d , S. Ismat Shah b,d a
b
Centre for Composite Materials, University of Delaware, Newark, Delaware 19716, USA Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716, USA c Material Science Center, Indian Institute of Technology, Karagpur, West Bengal 721302, India d Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA Received 21 September 2005; accepted 5 March 2006 Available online 3 April 2006
Abstract Photovoltaic is an attractive alternative of conventional energy source, but for the limitations of present materials and technology, we need to find out cost effective and environmentally stable new materials. We have synthesized a novel nanocomposite, named titania–germanium (TiO2– Ge). TiO2–Ge is a thermodynamically stable material. Ge nanodots are dispersed in the TiO2 matrix of the nanocomposites. Bohr radius of Ge is relatively large, 24.3 nm, therefore, it is easy to vary the size of Ge nanodots, and consequently the properties (structural, optical and electrical) of TiO2–Ge can be tailored in a wide range just by varying the size and density of Ge nanodots. TiO2–Ge with size gradient of Ge nanodots is a promising active layer of the next generation solar cells. © 2006 Elsevier B.V. All rights reserved.
1. Introduction Solar electric power is an ideal fuel-less energy source. It is no longer just used in terrestrial and space applications or power calculator, emergency telephone, but will become an intimate part of the utility grid in near future. That is why we need inexpensive, environmentally stable semiconductors to fabricate high efficiency photovoltaics. Crystalline/amorphous silicon, gallium arsenide (GaAs), gallium indium phosphide (GaInP2), copper indium gallium selenide (CIGS), cadmium telluride (CdTe) solar cells are the typical semiconductors for photovoltaic applications [1–5]. Crystalline silicon or GaAs/GaInP2 solar cells are highly efficient (20–30%), however their fabrication processes are complex as well as expensive. Relatively inexpensive amorphous silicon (aSi) photovoltaics have a fundamental drawback: light induced degradation of photovoltaic properties. CdTe solar cells suffer from environmental instability. Therefore, conventional photovoltaic devices face new challenges in the coming years. There is an increasing awareness of the possible advantages of nanocrystalline and conducting polymer devices [6] which are relatively cheap to fabricate, can be used on flexible substrates, and can be ⁎ Corresponding author. Centre for Composite Materials, University of Delaware, Newark, Delaware 19716, USA. E-mail address: [email protected] (S. Chatterjee). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.03.048
shaped or tinted to suit particular devices or architectural or decorative applications. Nanostructured Titania (TiO2) is a promising semiconductor for its photo-activity. It is a good candidate for the active layer of solar cells. Optical gap of TiO2 is 3.2 eV that belongs in the ultraviolet region of a solar spectrum. However, a peak of the solar spectrum is in the visible region. Therefore, to increase the efficiency of each device, TiO2 nanoparticles are customized by different methods, for example, size optimization, doping, various surface modifications, dye sensitization etc. Among the different processes, so far dye sensitization is the most successful way to absorb enough light for achieving useful efficiencies. However, charge transport and instability of the organic complex in the dye are still major issues [7]. In this context, we need a real breakthrough for the further improvement of TiO2 solar cells. In this letter, we propose a nanocomposite called titania– germanium (TiO2–Ge) which will be a stable alternative to dye sensitized TiO2. We will discuss the properties TiO2–Ge and its significant feasibility in solar cells. 2. Experiments To synthesize a TiO2–Ge nanocomposite powder, TiO2 (P25 Degussa, 70% anatase, 99.8% purity, average particle size 30 nm, and specific surface of 50 m2/g) and Ge (− 100 mesh,
3542
S. Chatterjee et al. / Materials Letters 60 (2006) 3541–3543
99.99% pure) powders were mixed thoroughly in different Ge weight concentrations (20%, 33%, 43%, 50%, 57%, 66% and 80%); then the mixtures of TiO2 and Ge powders were annealed at 550 °C under Ar atmosphere. TiO2–Ge nanocomposite thin films were synthesized by RF magnetron sputtering. Base pressure of the deposition chamber was 5 × 10− 7 Torr. Two composite targets (T1 and T2) were used for nanocomposite films preparation. Weight ratios of TiO2 and Ge in T1 and T2 are 10:1 and 2:1, respectively. The sputtering was carried out in an inert atmosphere of 6 mTorr Argon. Substrate temperature and RF power were 600 °C and 200 W, respectively. For the structural cauterizations of TiO2–Ge nanocomposites, we extensively exploited X-ray diffraction (XRD), X-ray photo-thermal spectroscopy (XPS), and Transmission electron microscopy (TEM). The XRD data of the nanocomposite films were collected by using a Rigaku D-Max B horizontal diffractometer. A SSI-M probe XPS was used employing Al kα (hν = 1486.6 eV) exciting radiation to study the chemical state of Ge and Ti as well as to obtain the cation composition of the films. JEM 2010F and JEM 2000FX TEM were used to examine the microstructure of the films as well as to obtain the particle size, film composition, and crystal structure. Optical Transmission and reflection data in the wavelength range 200 to 2600 nm were taken from UV–VIS-NIR Perkin Elmer spectrophotometer. The dark and photoconductivities were measured in an environmental test chamber. The photoconductivity was measured under 50 mW cm− 2 white light from a tungsten halogen lamp. 3. Results and discussion TiO2–Ge is a novel nanocomposite which includes the uniform distribution of Ge nanodots in a TiO2 matrix. The following questions become obvious regarding new TiO2–Ge. • Is there any thermodynamic stability in TiO2–Ge? • Is it feasible to synthesize TiO2–Ge? • What are the roles of Ge in TiO2?
Fig. 1. Z-contrast imaging, generated by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) of TiO2–Ge with Ge concentration of 50%.
Fig. 2. XRD patterns in the 2Θ range of (a) 26° to 28° and (b) 53° to 55° of a TiO2–Ge nanocomposite for various Ge concentrations.
Ti and Ge interact with each other at temperatures above 450 °C and form various Ti–Ge alloys [8]. Based on this observation, we have carried out experiments to anneal the mixtures of TiO2 and Ge powders with various Ge concentrations at 550 °C, under Ar atmosphere. We observed uniform distribution of Ge clusters (quantum dots) in the TiO2 matrix via Z-contrast imaging by Transmission Electron Microscope (TEM) [Fig. 1] and X-ray energy dispersive spectroscopy (XEDS) analysis. We obtained a strong Ge signature in our annealed TiO2–Ge samples by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). For evidence, we display XRD spectra of four TiO2–Ge samples in Fig. 2. Fig. 2 has two parts: (a) and (b) represent the spectra in the 2Θ range of 26–28 and 53–55, respectively. Bragg peak positions of Ge (111), rutile TiO2 (110) [Fig. 1(a)], Ge (311 kα1), Ge (311 kα2) and rutile TiO2 (211) [Fig. 2(b)] are indicated by arrows. Besides those peaks, a peak in Fig. 2(a) as well as one peak in Fig. 2(b) is not yet identified. These peaks are related with neither Ge nor TiO2. Most probably, these peaks are for Ti–Ge metastable phases. The XRD and TEM results suggest complete interaction of the mixed powders and the formation of non-percolating Ge regions within the TiO2 matrix. It is important to point out that the formation of GeO2 within the
Fig. 3. UV–VIS spectra of a TiO2–Ge nanocomposite for various Ge concentrations.
S. Chatterjee et al. / Materials Letters 60 (2006) 3541–3543 Table 1 Opto-electronic properties of TiO2–Ge nanocomposites
3543
4. Conclusion
Sample number
TiO2–Ge ratio in sputtering target
Optical gap (eV)
Dark Conductivity (S cm− 1)
Photo Conductivity (S cm− 1)
TG1 TG2
10:1 2:1
2.22 0.76
6.9 × 10− 7 3.9 × 10− 3
1.4 × 10− 6 4.1 × 10− 3
TiO2 matrix is thermodynamically unfavourable. The electronegativity of Ti (1.54 Pauling) is lower than that of Ge (2.01 Pauling) [9] and TiO2 is a much more stable compound than GeO2. The enthalpies of formation (ΔHf°) for TiO2 and GeO2 are −944.0 and − 580 kJ/mol, respectively [10]. This provides a simple way of introducing Ge quantum dots in the TiO2 matrix and an easy way of sensitizing TiO2 by the formation of stable TiO2–Ge nanocomposites. Relatively large exciton effective Bohr radius [11,12] of Germanium (24.3 nm) is helpful to tailor the size of Ge nanodots. Varying the density and size of Ge nanodots, it is easy to change the structural, optical and electronic properties of TiO2–Ge. For example, Fig. 3 demonstrates the red shift of UV–VIS spectra with the increase of % of Ge in TiO2–Ge. Like Germanium, silicon is also another common low band gap semiconductor. Its electronegativity and enthalpy for the formation of SiO2 are 1.90 Pauling and 905.49 kJ/mol respectively. Therefore, in principle it is possible to fabricate Si nanodots in TiO2 matrix. However, the effective Bohr radius of the excitons in Ge is larger than that in Si (4.9 nm), since Si has larger electron and hole effective masses and a lower dielectric constant than Ge. This implies that the Ge nanodots show a larger shift of an optical band gap (blue shift) than the Si nanodots. In other words, Ge nanodots would be more efficient than Si nanodots to tailor the properties. Therefore we favour to synthesize TiO2–Ge instead of TiO2–Si. Like optical properties, tailoring of electrical properties of TiO2–Ge nanocomposites is promising too. To check the variation of transport properties in TiO2–Ge nanocomposites, we deposited two TiO2–Ge films (TG1 and TG2) by RF sputtering using two different sputtering targets (T1 and T2). Thicknesses of TG1 and TG2 are 0.8 and 2.2 μm, respectively. Opto-electronic properties of two sputtered films are displayed in Table 1. Optical gaps of TG1 and TG2 determined from Tauc's plot [13] are 2.22 and 0.76 eV, respectively. TG2 is a Ge rich film and dark and photoconductivity of TG2 is 3 orders higher than those of TG1. Therefore, we can tailor several orders of magnitude of electrical properties of TiO2–Ge with Ge atom concentration in the films.
In short, TiO2–Ge nanocomposite is a promising cost effective and environmentally stable material for solid state solar cells as well as hydrogen energy generation. Synthesis of TiO2–Ge nanocomposites is simple. Direct deposition at high temperature (N 450 °C) is an option to synthesize TiO2–Ge nanocomposites. TiO2–Ge nanocomposites can absorb a wide range of solar spectrum from UV via VIS to IR just by the introduction of different sizes of Ge nanodots in the TiO2 matrix. Bohr radius [11,12] of Ge is relatively large, 24.3 nm, therefore, it is easy to make size gradients of Ge nanodots in the TiO2 matrix. Electronic properties of TiO2–Ge can be tailored in several orders of magnitude by varying the size and density distribution of Ge dots. Acknowledgement The authors acknowledge NSF (Grant No. DMR-0441619) for the financial support of this work. References [1] M.A. Green, Sol. Energy 76 (2004) 3. [2] A. Goetzberger, C. Hebling, H.-W. Schock, Mater. Sci. Eng., R Rep. 40 (2003) 1. [3] K.A. Bertness, Sarah R. Kurtz, D.J. Friedman, A.E. Kibbler, C. Kramer, J.M. Olson, Appl. Phys. Lett. 65 (1994) 989. [4] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovolt. Res. Appl. 7 (1999) 311. [5] T. Aramoto, S. Kumazawa, H. Higuchi, T. Arita, A. Hanafusa, M. Murozono, Proc. 26th IEEE PVSC, Anaheim, 1997, p. 343. [6] M. Grätzel, Nature 421 (2003) 586–587. [7] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [8] Q.Z. Hong, K. Barmak, F.M. d'Heurle, Appl. Phys. Lett. 62 (1993) 3435. [9] W.W. Porterfield, Inorganic Chemistry, a Unified Approach, Addison Wesley Publishing Co., Reading Massachusetts, USA, 1984. [10] J.D. Cox, D.D. Wagman, V.A. Medvedev, CODATA Key Values for Thermodynamics, Hemisphere Publishing Corp., New York, 1984, p. 1. [11] L.E. Brus, J. Chem. Phys. 79 (1983) 5566. [12] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [13] J. Tauc, Optical Properties of Solids, Plenum, New York, 1974, p. 159.