AZO Transparent

ARTICLE pubs.acs.org/JPCC

Dye-Sensitizing of Self-Nanostructured Ti(:Zn)O2/AZO Transparent Electrodes by Self-Assembly of 5,10,15,20-Tetrakis(4carboxyphenyl)porphyrin Giovanna Pellegrino,†,* Guglielmo G. Condorelli,‡ Vittorio Privitera,† Brunella Cafra,§ Silvia Di Marco,§ and Alessandra Alberti† †

CNR-IMM, Zona Industriale Strada VIII, 5-95121 Catania, Italy, Dipartimento di Scienze Chimiche, Universit
a degli Studi di Catania and INSTM UdR di Catania, Via le Andrea Doria, 6-95125 Catania, Italy, § ST Microelectronics, Zona Industriale Stradale Primosole, 50-95121 Catania, Italy ‡

ABSTRACT: Self-nanostructured ZnO:Al conductive layers consisting of [1120] oriented domains were coated by a conformal TiO2 thin film and sensitized by 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) to be used as transparent conductive electrodes in dye-sensitized solar cells. In addition to the higher surface availability due to the nanopatterning, the TCPP surface density increases by 330% (UV-vis) with respect to a flat conventional substrate thanks to a more dense molecular arrangement, as evidenced by combining high-resolution X-ray photoelectron spectroscopy and atomic force microscopy. Furthermore, the presence of zinc atoms in the TiO2 overlayer (Ti(:Zn)O2) crucially influences the electronic properties of the assembled TCPP. As a consequence of the orbitals' rearrangement attributed to the presence of zinc, a significant quenching of luminescence is observed in the emission spectra of TCPP-sensitized Ti(:Zn)O2, suggesting that electrons could be more effectively injected from the molecular orbitals to the conduction band of the semiconductor.

’ INTRODUCTION Hybrid organic/inorganic systems have been attracting great interest for several decades owing to their optical and electronic properties, suitable for photovoltaic application in solar cells as potential low-cost alternatives to inorganic photovoltaic devices.1 Systems constituted by a semiconductive/conductive oxides bilayer modified with a light-harvesting organic element are noted as dye-sensitized solar cells (DSSC). Some of the dyes that have been used as sensitizers are substituted bipyridyl complexes of ruthenium, natural porphyrins, and other natural dyes.2-4 Porphyrins are largely investigated for their characteristic strong absorption in the 400-450 nm range (Soret band) as well as in the 500-700 nm region (Q bands). Once used in a DSSC cell, efficient charge separation can take place due to electron injection from the excited state of the dye to the conduction band of the semiconductor.5-7 Efficient electron transfer from the excited dye to the conduction band requires good electronic coupling between the lowest unoccupied orbital (LUMO) of the dye and the conduction band of the semiconductor.8,9 The photosensitization of wide band gap semiconductors is one of the most focused research subjects, especially since the development of the high-efficiency solar cell is based on dyesensitized TiO2 films.10,11 In 1991 O’Regan and Gr€atzel r 2011 American Chemical Society

introduced porous dye-sensitized TiO2 nanocrystalline-based devices with an overall light-to-electric energy conversion efficiency of 7.1-7.9% in simulated solar light and 12% in diffuse daylight.12 This high quantum efficiency was reached by virtue of the ultrafast charge transfer from the dye to the TiO2 energetic level, as well as by the large increase of the collecting area, allowing the exciton to be collected before recombination. Because of the low mean free path of the exciton, in fact, the use of a large electrode area for the dye functionalization and a relatively short distance between the photoactive moiety of the sensitizer and the semiconductor have to be preferred.13-15 To significantly increase the exposed surface area, nanocrystalline and nanoporous TiO2 films have been extensively investigated, and to maximize the efficiency of the electron transfer, aromatic and well-conjugated molecular systems have been essentially used. Transparent conductive metal oxides are employed as electrodes for accepting and transporting charges. Among them, indium-doped SnO2 (SnO2:In), fluorine-doped SnO2 (SnO2:F), and Al-doped ZnO (ZnO:Al) are the most accredited materials for thin film solar cells because of their low resistivity and high transmittance in the visible region. Received: November 12, 2010 Revised: February 15, 2011 Published: March 28, 2011 7760

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ARTICLE

In the present work, a surface self-nanostructured aluminumdoped ZnO (AZO) layer was covered by a conformal TiO2 sputtered, thin film. A free-base porphyrin bearing carboxylic groups, the 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP), was anchored on the nanostructured TiO2/AZO bilayer as photosensitizer. The chemical properties of the TCPP-modified bilayer were characterized via X-ray photoelectron spectroscopy and compared with those of the TCCP anchored on TiO2 deposited on unstructured glass or on structured AZO layers. The optical properties of the porphyrin-modified bilayers were evaluated by UV-vis and fluorescence spectroscopies to study the effect of the composition of the bilayers on the deexcitation mechanisms related to the electron injection process.

’ EXPERIMENTAL SECTION Preparation of AZO Layers. ZnO:Al films were deposited on 1737 Corning glass (thickness 0.7 mm and roughness ∼1 nm) by DC pulsed sputtering (Symmorphix 1600 PVD system) from an aluminum zinc oxide target (98 wt % ZnO and 2 wt % Al2O3). During the deposition, the susceptor temperature was fixed at 200
C, the sputtering power was 6000 W, and the Ar gas flow was 80 sccm. With these parameters, a deposition rate as high as 1.5 nm/s was achieved, which allows growth of highly faceted columnar AZO domains. The sheet resistance of the AZO film, 900 nm thick, is 28.8 Ω/sq. The optical properties of the AZO layer were determined by measuring the optical transmittance and reflectance of the layer from 350 to 1100 nm (Perkin-Elmer Lambda 900 UV/vis/NIR spectrophotometer). The mean transmittance in the 400-1100 nm range is 78%. Preparation of TiO2 Layers on AZO and Corning Glass. TiO2 thin films were deposited on AZO/1737Corning glass and on 1737Corning glass by DC reactive magnetron sputtering from a pure titanium target in ambient oxygen, changing the O2 flow rate in the range from 5 to 45 sccm and the flow rate of Ar from 25 to 45 sccm. The sputtering power was maintained at 600 W, and the susceptor temperature changed from room temperature to 250
C. A gas flow ratio of 5 sccm O2/45 sccm Ar was used to guarantee the right stoichimetry (investigated by Rutherford backscattering spectroscopy) and a conformal coverage on the nanostructured AZO surface, (revealed by transmission electron microscopy (TEM)). Adsorption of Dye onto the Oxide Layers. The TiO2/AZO bilayer and the three reference samples consisting of (1) pure Corning glass, (2) TiO2, and (3) AZO deposited on Corning glass were immersed into 2.5
10-5 M THF solutions of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) for 4 h at room temperature. The sensitized layers were withdrawn from the solution and immersed in pure THF, rinsed and sonicated for 10 min to remove any physysorbed dye. All the chemical reagents, (that is, 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, tetra(p-hydroxyphenyl)porphyrin (THPP), and THF) were commercially available and were used as received. Characterization. TEM analyses in cross section were performed using a JEOL JEM 2010 microscope operating at 200 KV. X-ray diffraction (XRD) was used to analyze the structures of the TiO2 and AZO substrates. XRD patterns were carried out using a D8-Discover Bruker AXS diffractometer equipped with a Cu X-ray source. The measurement data were acquired in locked coupled and in grazing incidence geometies by using an optical setup consisting of a Goebel mirror, slit 0.1 mm, and axial Soller

Figure 1. TEM images of (a) ZnO:Al (AZO) nanostructured layer and (b) TiO2 deposited on AZO. The sputtered TiO2 layer is conformal to the substrate. The inset in Figure 1 a shows the Bragg X-ray diffraction pattern and the schematic representation of the AZO hexagonal structure showing the preferential [1120] growth direction.

at the primary optics and Soller 0.35
before the scintillator as secondary optics. X-ray reflectivity (XRR) curves were carried out with the same equipment used for XRD. The setup for the analysis consists of a Goebel mirror, 1 mm slits at the primary optics and 0.5 mm slit, and a scintillator as detector at the secondary optics. Step size was fixed at 0.01
, and scan speed, at 0.5 s per step. AFM images were obtained in tapping mode by a NT-MDT instrument. The noise level before and after each measurement was 0.01 nm. XPS spectra were recorded with a PHI ESCA/SAM 5600 Multy technique spectrometer equipped with a monochromatized Al KR X-ray source. Unless otherwise noted, the analyses were carried out at 45
photoelectron takeoff angle relative to the sample surface with an acceptance angle of 7
. The binding energy (BE) scale was calibrated by centering the adventitious/ hydrocarbon carbon C 1s at 285.0 eV.16 Experimental uncertainties in binding energies lie within (0.45 eV. UV-vis measurements were carried out on a UV-vis V-650 Jasco spectrophotometer, and the spectra were recorded with a (0.2 nm resolution. The temperature was kept at 25
C, and measurements were repeated using five different monolayers. Fluorescence spectra were recorded with a Cary Eclipse Varian spectrofluorimeter. The spectra were collected using for all the samples the same operative conditions: exit slit and emission slit of 5 nm and high PMT voltage.

’ RESULTS TEM Analyses. Figure 1a shows the cross-sectional TEM micrograph of the as-deposited ZnO:Al film. The film is a polycrystal with columnar grains having thickness of ∼900 nm and presents a very compact structure (see Figure 1a). The 7761

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The Journal of Physical Chemistry C

Figure 2. XRR curves of (I) TiO2 deposited on Corning glass, (II) AZO deposited on Corning glass, and (III) TiO2 deposited on AZO/Corning glass. The TiO2/AZO profile has two different critical angles due to TiO2 and AZO, respectively; the slope of the curve reflects the roughness of the nanostructured substrate (AZO).

related X-ray diffraction pattern indicates that the AZO layer has the hexagonal structure of the zincite with the [1120] axis perpendicular to the growth direction (see the inset in Figure 1a). Most of the AZO grains are, indeed, [1120]-oriented, with a small amount that are [0002]-oriented. The intensity ratio between the [0002] and [1120] XRD peaks is as low as ∼0.5:1 if compared with that theoretically expected (∼1.4:1).17 The pyramidal tips visible in the TEM cross-sectional micrograph (Figure1a), correspond to the facets of the hexagon (see the inset in Figure 1a). The typical cusps of such a 1120-oriented substrate have a characteristic ratio between the apothem (30-32 nm) and radius (20-25 nm) of 1.4. On the basis of these geometric considerations, it is possible to evaluate the increment of the available surface due to the nanostructuring. By considering the AZO cusps as cones, in fact, the lateral surface (Sl) is found to exceed by 40% the area of the base (Sb), being Sl/Sb = πar/πr2 equal to the apothem/radius ratio. Figure 1b shows the cross-sectional TEM micrograph of the TiO2 layer sputtered on the faceted surface of the AZO layer. The layer, having a thickness of 28 ( 2 nm, appears conformal to the substrate so as to preserve the nanostructured features. TEM and X-ray diffraction patterns indicate the presence of TiO2 nanocrystals (anatase) in a TiO2 amorphous matrix. XRR Measurements. Figure 2 shows the XRR curves related to (I) TiO2, (II) AZO, and (III) TiO2/AZO layers deposited on Corning glass. In the XRR profile of the TiO2 film on Corning glass, the phenomenon of total external reflection causes the X-ray beam to be fully reflected for incident angles smaller than θc = 0.29
(in Figure 2, θ = 0.58
). As the incident angle crosses over the critical angle, the X-ray beam gradually penetrates into the TiO2 film until it meets the interface with the substrate (Corning glass). Due to the different electronic density of the medium, the beam is reflected from that inner interface and interferes with that already reflected from the TiO2 surface, giving rise to Kiessig fringes;18 the periodicity of the fringes depends on the film thickness and results in a 30 ( 1-nm-thick layer. The density of TiO2 is obtained from the position of critical angle,19,20 and the results are as high as 4.05 ( 0.5 g/cm3. The decreasing of the intensity in the reflected beam is due to the diffuse scattering from discontinuities, and from the slope, a low roughness (around 1.0 ( 0.2 nm) was estimated, indicating that on Corning glass, TiO2 films are flat on the nanometer scale.

ARTICLE

The AZO XRR curve shows total external reflection for incident angles smaller than θc = 0.35
, corresponding to a density of 5.61 g/cm3. Fringes are not visible due to the high roughness of the layer, estimated to be ∼10 ( 2 nm, as confirmed by TEM analyses. The XRR curve of the TiO2/AZO bilayer shows two different plateaus with critical angles of 0.28
and 0.35
due to TiO2 and AZO, respectively. The roughness is estimated to be ∼7-8 nm, as expected because of the conformal coverage on the nanostructured AZO surface. Kiessig fringes are not visible because of the high roughness. The thickness of the TiO2 layer deposited on AZO, as estimated by TEM (28 nm), is close to that found by XRR on TiO2/Corning glass. A similar TiO2 growth rate is, in fact, expected on glass and AZO substrates when the same deposition parameters are used. X-ray Photoelectron Spectroscopy. Table 1 shows the relative percentage of the elements as measured by X-ray photoelectron spectroscopy before and after the anchoring of TCPP; the data related to the adsorption of THPP, bearing OH moieties instead of carboxylic groups, are also shown for comparison. The increment of the C 1s and N 1s atomic percentage after the TCPP anchoring is a reliable indicator of the adsorption of the molecules on the substrate. The amount of nitrogen (which is strictly associated with the porphyrin presence) is notably less in the dye-sensitized flat TiO2 than in the nanostructured substrates. 1s The intensity ratio ITiO2N 1s/IN n-struct results are close to 1:3, where N 1s is the nitrogen intensity substrates normalized to the I intensity of the substrates. Note that THPP adsorption on the surface is negligible, thus proving the role of carboxylic group for the tethering process. High resolution XPS spectra (detection angle 45
) of (I) AZO, (II) TiO2/AZO and (III) TCPP/TiO2/AZO relative to the zinc region are shown in Figure 3. In the pure AZO substrate (Figure 3I) the signal of the Zn 2p3/ 2 is centered at 1021.4 eV. After TiO2 deposition, the XPS spectra of the TiO2/AZO bilayer still show a certain amount of zinc at the surface (Figure 3II), and the Zn/Ti ratio is ∼1:5. The presence of zinc at the surface (XPS sampling depth is ∼12 nm) may be explained by two different effects: (i) diffusion of zinc atoms into the TiO2 layer and (ii) not complete coverage of TiO2 on AZO, eventually due to some ZnO high cusps emerging from TiO2. To understand the role of the surface morphology, the TiO2 layer was deposited on a different AZO layer, without cusps, obtained at a lower sputtering deposition rate. XPS spectra show, also in this case, an amount of zinc comparable to that observed in the faceted AZO. This indicates, hence, that the effect of the cusps is not relevant. Furthermore, energy-filtered TEM chemical maps indicate the presence of zinc in the TiO2 layer (not shown). This evidence suggests that Zn atoms diffuse into the TiO2 layer during TiO2 deposition, thus determining the presence of Zn on the sample surface. This hypothesis is further supported by the energy shift (∼0.6 eV) of the Zn 2p band in the TiO2/AZO layer (Figure 3 II) compared with pure AZO films, which suggests a different chemical environment of the surface Zn atoms in AZO and in TiO2/AZO (Ti(:Zn)O2) layers. Figure 3III shows that the XPS Zn band position of Ti (:Zn)O2samples after TCPP adsorption is further shifted to 1022.3 eV. A similar shift (∼0.4 eV) toward higher binding energy is observed in the Ti 2p3/2 signal after TCPP chemical adsorption21 (Table 2). This evidence suggests a strong interaction between TCPP and the Ti(:Zn)O2/AZO layer. In pure 7762

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Table 1. XPS Percentage of the Elements of the Layers before and after the Dye Functionalization before sensitizing TiO2

AZO

after sensitizing

Ti(:Zn)O2

TCPP/TiO2

TCPP/AZO

TCPP/Ti(:Zn)O2

THPP/AZO

THPP/Ti(:Zn)O2

N 1s

0.2

0.2

0.3

1.1

2.8

2.6

0.5

0.5

C 1s

25.8

26.0

28.8

40.2

51.8

48.3

28.7

30.6

O 1s

49.1

44.3

51.5

43

33

34.6

45.5

48.9

Ti 2p

19.4

16.3

15.6

Zn 2p

28.6

Al 2p

0.9

3.4

Figure 3. XPS spectra of the Zn 2p3/2 region related to (I) AZO, (II) Ti(:Zn)O2/AZO, and (III) TCPP/Ti(:Zn)O2/AZO. The Zn 2p3/2 peak shifts to higher binding energy after the TiO2 deposition and successively after the TCPP absorption.

AZO films after TCPP anchoring, a 0.6 eV shift of the Zn band was also observed (Table 2), thus suggesting a similar strong interaction of the TCPP with the surface atoms. Figure 4 shows high-resolution XPS C 1s regions on Ti(: Zn)O2/AZO films before and after TCPP adsorption. Similar C 1s spectra (not shown) have been observed for AZO films before and after TCPP sensitizing. Before sensitizing, C 1s spectra of both substrates consist of two contributions: a main signal, due to adventitious carbon centered at 285.0 eV; and a broad and low band around 288.6 eV, which is associated with the presence of carbonate species. After the TCPP-sensitizing (Figure 4II, III, and IV), the intensity of the main C 1s component at 285.0 eV is enhanced due to the carbon atoms of the porphyrin. A new component at high binding energy (CCOOH = 289.1 eV) consistent with the presence of unreacted -COOH moieties of the porphyrin and a small component at 288.3 eV (CCOO-) due to the carboxilate COO- bridge interacting with the surface are present. An additional component centered at 287.0 eV must be added to optimize fitting due to C1þ of TCPP (C-N) and oxidized carbon contamination.22 Note that analogue components are present in the C 1s spectra of TCPP-sensitized TiO2/Corning glass, even though the relative band intensities are different compared with Ti(:Zn)O2- and AZO-sensitized films. The intensity ratio between CCOOH and CCOO- bands in Ti(:Zn)O2 and AZO is much higher than in the TiO2 substrate, indicating that the number of unreacted COOH moieties per molecule is higher in the nanostructured layers with respect to the flat surface. In particular, the ICOOH/ICOO- ratio for AZO and for Ti(: Zn)O2 is ∼1.9 and ∼1.7, suggesting that TCPP is anchored via

12 12.2

2.5

16.5 26.3

3.5

0.2

one (ICOOH/ICOO- = 3:1) or two (ICOOH/ICOO- = 1:1) carboxilate bridges (Scheme 1). Instead, for the TiO2 layer deposited on Corning glass, the ratio is ∼0.7, indicating that the interaction with the surface occurs mostly via two or, in some cases, three COO- groups (ICOOH/ICOO- = 1:3). The N 1s spectral region of Ti(:Zn)O2/AZO before the TCPP absorption presents a small amount of adsorbed nitrogen centered at 400.2 eV (Figure 5I).23 After the dye anchoring, compared with the substrate, the N 1s spectral region shows two separate features (Figure 5II). The main peak is centered at 400.4 eV, and the shoulder is located at 398.4 eV, indicating the presence of at least two species of nitrogen. The observed binding energy values lie in the range reported in the literature for pyrrolic (-NH-) and iminic (Nd) nitrogen species of the porphyrinic tetrapyrrolic ring.24,25 The XPS N 1s region of TCPP-modified AZO (not shown) is very similar to that of TCPP-TiO2/AZO. Note, finally, that the surface treatment with THPP does not lead to any change either in the shapes of the C 1s and N 1s XPS bands or in the BE positions of Zn 2p and Ti 2p peaks (Table 2). These observations strongly indicate that anchoring takes place via a carboxylate bridge and that TCPP surface interaction modifies the involved energy levels of the substrate. UV-Vis Measurements. To study the optical response of dyesensitized layers, UV-vis electronic spectra were collected. Figure 6I shows the electronic spectrum of a 2.5
10-5 mol/L TCPP THF solution taken as reference. The spectrum gives evidence of the presence of a strong Soret band centered at 418.5 nm and less intense red-shifted Q bands at 514, 548, and 590 nm. Figure 6 shows the UV-vis spectra of TCPP-sensitized (II) AZO, (III) Ti(:Zn)O2 and (IV) TiO2 on Corning glass substrates. The Soret band of all porphyrin modified layers is characterized by a red shift with respect to both TCPP-THF solution and to TCPP adsorbed on Corning glass (CG) (Table 3). In particular, the TCPP-sensitized Ti(:Zn)O2/AZO layer have a larger shift compared with TCPP-sensitized AZO and TiO2 on CG.26Moreover, a larger FWHM value is observed in the TCPP-Ti(:Zn)O2/AZO with respect to AZO and TiO2 layers. The presence of broader bands for TCPP adsorbed on Ti(:Zn)O2 can be explained by a more complex environment experienced by the molecules adsorbed on a zinc-titanium-containing layer. In the Ti(:Zn)O2/AZO-sensitized surface, the less intense bands associated with TCPP Q bands, are observed at ∼520, 556, and 606 nm, slightly red-shifted with respect to the TCPP solution. The intensities of the Soret band of the porphyrin-sensitized AZO and Ti(:Zn)O2/AZO layers are much higher with respect to that of the flat TiO2 layer grown on Corning glass (the intensity ratio is ∼4.5:1, see Table 3). This evident absorbance increase cannot be explained only by the increased surface area 7763

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Table 2. Chemical Shift in the Zn 2p3/2 XPS Spectral Region TiO2

TCPP/TiO2

458.4

458.7

Zn 2p3/2 Ti 2p3/2

AZO

TCPP/AZO

TiO2/AZO

TCPP/TiO2/AZO

THPP/TiO2/AZO

1021.4

1022.0

1022.0

1022.3

1021.3

458.4

459.0

458.4

Figure 4. XPS spectra of the C 1s region of (I) the Ti(:Zn)O2/AZO surface before the anchoring process, (II) TiO2/ Corning glass, (III) Ti(:Zn)O2/AZO, and (IV) AZO after TCPP anchoring. The COOH/ COO- intensity ratio gives information on the TCPP arrangement on the surface.

Figure 5. XPS spectra of the N 1s region (I) before and (II) after the TCPP anchoring on Ti(:Zn)O2/AZO. The N region is largely modified by the molecular assembly on the surface.

Scheme 1. Schematic Representation of (a) TCPP Arrangement on Ti(:Zn)O2 Layer Deposited on Æ11-20æ Oriented AZO Showing TCPP Anchoring by One or Two COOGroups (Vertical Configuration) and (b) TiO2 Grown on Corning Glass Showing TCPP Anchoring by More than Two COO- groups (Planar Configuration).a

Figure 6. UV-vis spectra of (I)TCPP 2.5
10-5 M THF solution and TCPP-modified (II) AZO, (III) Ti(:Zn)O2/AZO, and (IV) TiO2. The Soret band of the TCPP anchored on the nanostructured layers (II and III) is notably more intense than that on TiO2 deposited on the Corning glass surface (IV). The larger red shift is registered for Ti(:Zn)O2/AZO (∼7 nm).

a The different binding geometry is due to the different morphology of the substrate at molecular scale.

(∼40%). It is, rather, likely due to the different arrangements of TCCP on flat and nanostructured surfaces, as suggested by XPS results.

Using the Lambert-Beer law, it is possible to estimate the surface molecular density, dsurf (number of molecules per cm2), as dsurf = Aε-1
N, where A is the absorbance, ε is the absorption coefficient, and N is the Avogadro number.13 Assuming that ε is constant for the different substrates and approximately equal to 4.0
105 M-1 cm-1, as reported for THF, the estimated density for TiO2 on CG and for both the pure AZO and the T(:Zn)O2/AZO are 1
1013 and 4.5
1013 molecules/cm2, respectively. However, the effective surface in the nanostructured substrate is higher (40% more) than the nominal area (1.4 cm2 of effective area available in a square of 1 cm2). For this reason, the effective surface density of the molecules considering all the available surface is 4.5
1013 molecules/1.4 cm2 = 3.3
1013 molecules/cm2. Since the surface density obtained for the flat surface is 1.0
1013 molecules/cm2, the surface density ratio 7764

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Table 3. UV-Vis Parameters of the Soret Band Measured in TCPP 2.5
10-5 M THF Solution and Grafted on the Surfaces

position (nm) absorbance fwhm (nm)

2.5
10-5in THF

TCPP/CG

418.5

418.5 ( 0.2

7.1 14

0.002 22.5 ( 3

TCPP/TiO2 421 ( 1 0.007 ( 0.001 22.5 ( 2

TCPP/AZO

TCPP/Ti(:Zn)O2/AZO

422 ( 0.5

425 ( 2

0.028 ( 0.02

0.031 ( 0.03

23.5 ( 2

26 ( 2

Figure 7. Phase (left) and height (right) AFM images of 1 μm
1 μm surface of (1) TiO2/Corning glass, (2) AZO/Corning glass, and (3) Ti(:Zn)O2/ AZO on Corning glass. The TiO2 layer deposited on AZO reproduces the morphology of the substrate at the subnanometer scale since its roughness (along the facets) is