Efficiency Improvement of DSSC Photoanode by Scandium Doping of

Article pubs.acs.org/JPCC

Efficiency Improvement of DSSC Photoanode by Scandium Doping of Mesoporous Titania Beads Alessandro Latini,* Carmen Cavallo, Fadi Kamal Aldibaja, and Daniele Gozzi Dipartimento di Chimica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy

Daniela Carta and Anna Corrias Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Complesso Universitario, S.S. 554 bivio per Sestu, 09042 Monserrato, CA, Italy

Laura Lazzarini and Giancarlo Salviati IMEM − CNR , Viale G. Usberti 37/A, 43124 Parma, Italy ABSTRACT: Solid solutions of scandium in anatase as semiconductor material for DSSC photoanodes were prepared by the controlled hydrolysis of titanium(IV) isopropoxide and scandium(III) isopropoxide in hydroalcoholic medium. The final powder was constituted by mesoporous anatase beads doped with Sc. A superstructure characterizes the beads, which are spherical at the microscopic level (≈650 nm) and rice-grain-shaped at the nanoscopic level (≈20 nm). The BJH pore size distribution and BET surface area of the powder beads were found depending from the Sc content ranging the peak of the former between 7 and 25 nm and between 65 and 128 m2 g−1 the latter. Data obtained by XRD and EXAFS confirm that we are dealing with real solid solutions with ScTi substitution defects. The electronic properties of the synthesized semiconductor material as a function of Sc doping were investigated by the measure of the flat band potential, band gap, and deep levels. In the range 0.0−1.0 at. % of Sc, the flat band energy changes from −4.15 to −4.07 eV, whereas the band gap height increases by 0.03 eV. The presence of Sc modifies heavily the cathodoluminescence spectrum of anatase at the lowest concentration too. Several DSSCs with photoanodes at different Sc doping were tested both under solar simulator and in the dark. The maximum efficiency of 9.6% was found at 0.2 at. % of Sc in anatase that is 6.7% higher with respect to the DSSCs with pure anatase.

1. INTRODUCTION The Grätzel et al. paper1 in 1991 fixes conventionally the birth date of the dye sensitized solar cells (DSSCs). The overall lightto-electric energy conversion yield claimed at that time was 7.1−7.9% in simulated solar light and 12% in diffuse daylight. Since then, many papers2−6 were published to understand the mechanisms related to the operation of DSSCs that are photoelectrochemical cells where several branches of chemistry play all together a fundamental role: electrochemistry, photochemistry, nanochemistry, solid state chemistry, as well as physics of semiconductors and nanotechnology. A lot of work has been done to pursue the increase of the efficiency that had a first significant jump up to 11.1% in 2006, as claimed by the Sharp Co. researchers,7 and more recently8 up to 12.3% where the dye was a porphyrin and the I−/I3− redox couple was replaced by the Co2+/Co3+ couple. With the photoanode semiconductor material fixed, the dye and electrolyte composition, there are some main factors affecting the DSSC efficiency: (i) the electron trapping in the photoanode; (ii) the electron recombination at the semiconductor/electrolyte interface; (iii) the nanostructure of the © 2013 American Chemical Society

semiconductor; (iv) the cell assembly. Each one of these factors is further due to the combination of processes in competition and sometimes not well understood yet. The trapping is an important effect affecting the electron lifetime, τe, in the semiconductor, which reduces significantly the electron diffusion length, Le. In order to have a quantitative collection of electrons at the anode, the condition Le > d should be satisfied, with d being the semiconductor film thickness, which in turn has the constraint d > δ(λ), with δ being the light absorption length of the dye. Our recent paper9 showed that Le can be increased by a given quantity of functionalized multiwalled carbon nanotubes (MWCNTs) coated by nanostructured TiO2−anatase. In fact, at 0.1 wt % of MWCNTs, a 55% increase of efficiency with respect to DSSCs without MWCNTs has been obtained. This is due to the spontaneous formation of Schottky-type rectifying junctions between multiwalled carbon nanotubes and anatase nanoparticles and Received: August 10, 2013 Revised: November 8, 2013 Published: November 12, 2013 25276

dx.doi.org/10.1021/jp409813c | J. Phys. Chem. C 2013, 117, 25276−25289

The Journal of Physical Chemistry C

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the best collection of electrons by percolation among MWCNTs that are good electronic conductors. One of the key parameters affecting the incident photon-tocurrent conversion efficiency, IPCE(λ), is the quantum yield for electron injection, ϕinj, from the excited dye in the conduction band of the semiconductor, mostly TiO2−anatase. ϕinj is a dimensionless quantity, which would be close to unity only if the rate constants of all deactivation processes, kdeact, were negligible with respect to the injection rate constant, kinj. The value of the latter quantity increases as the energy difference, Δε, between the LUMO level of the dye and the conduction band edge of the semiconductor increases. With the dye and the semiconductor fixed, the unique chance to increase Δε is a targeted doping of the semiconductor. Some works10−14 explored this way by doping TiO2−anatase, for example, with Zr4+,15 Nb5+,11 and W6+,16 and satisfactory results were obtained even if the different experimental conditions adopted each time do not allow one to determine if the efficiency changes were effectively related to the doping. In principle, without considering the electron trapping, two are the factors produced by the doping on the electronic behavior of a semiconductor: the widening or narrowing of the band gap and the shift of the conduction band edge and both play a role in the DSSC photoanode. The band gap height affects the rate of the electron−hole recombination process, while the position of the conduction band changes the kinj value and the value of the open circuit voltage, OCV. The scope of the present work is to test the effect on the DSSC efficiency and its whole operation when Sc(III) dopes the TiO2−anatase photoanode via the formation of a real and well-characterized substitutional solid solution. The choice of Sc was based essentially on two reasons: (i) The size of Sc3+ is quite close to the size of Ti4+. This is to favor the formation of a solid solution in which the ScTi defects are the prevailing defective species. (ii) The substitution of Sc3+ in the site of Ti4+ generates holes in the valence band. To understand how this acts on the height of the band gap and how it moves in the energy scale, it was necessary to measure the flat band potential or more correctly the potential of the conduction band edge17 by the Mott−Schottky technique and band gap by diffuse reflectance UV−vis spectroscopy.

as received with the exception of 4-tert-butylpyridine, which was vacuum distilled before use. 3 mm thick FTO glass slides with sheet resistances of 10 and 15 Ω/□ for the photoanode and cathode, respectively, were purchased from XOP Fisica (Spain). Silver loaded conductive paint was purchased from RS Components. Surlyn (Dupont) hot melt thermoplastic was used to seal the cells (25 μm thick) and the holes for the insertion of the electrolyte (60 μm thick); in this last case, it was covered with a thin microscope glass slide. Cerasolzer CS186-150 soldering alloy was purchased from MBR Electronics (Switzerland). Kynar PVDF 502-CUH-HC film was used as an antireflection and UV blocking layer (98%) was purchased from Iolitec (Germany). All the chemicals were used 25277



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At the Sc K-edge, the highest accessible wave vector k value is 1 nm−1 due to the presence of the Ti K-edge. 2.1.1.2.3. FEG-SEM. Scanning electron microscope images of the beads were obtained by using a Zeiss Auriga FESEM microscope. 2.1.1.2.4. TEM. TEM analyses were performed by a JEOL JEM 2200FS field emission transmission electron microscope operating at 200 kV (resolution: 0.19 nm) equipped with an Ωtype energy filter. Samples for imaging were prepared by an ultrasonic dispersion of small amounts of powders in isopropanol and then putting a drop of the dispersion on holey carbon-coated copper grids. 2.1.1.2.5. Porosimetry/Surface Area. The Micromeritics Accelerated Surface Area and Porosimetry System (ASAP 2020) was utilized for the determination of the specific surface area of the synthesized powders by the method of Brunauer− Emmet−Teller (BET) and pore size distribution by the method of Barret−Joyner−Halenda (BJH) which covers only the range of the mesopores and small macropores. 2.1.1.2.6. Dye Loading Determination. The amount of dye absorbed by the anatase film was measured as described elsewhere.9 2.1.1.3. Electronic Characterization. 2.1.1.3.1. Flat-Band Determination. The flat band potential, Vfb, of the nanocrystalline TiO2 films was obtained from the Mott−Schottky capacitance analysis using the potentiostat/galvanostat 1286 coupled with frequency response analyzer 1260, both the instruments from Solartron Analytical, U.K., operated by the Full Combo ZPLOT/CorrWare software by Scribner Associates Inc., USA. The measurements were performed using a Pt counter electrode and an SCE reference electrode with 0.1 M Na2SO4 (aq) as a supporting electrolyte, which was degassed by ultrapure Ar for 15 min before measurements. The frequency was fixed at 100 Hz for all the films under a sinusoidal signal of 10 mV amplitude. 2.1.1.3.2. Band Gap Measurement. Diffuse reflectance of the nanostructured TiO2−anatase pellet samples was measured over a wavelength range of 220−800 nm (5.64−1.55 eV), using a double-beam spectrophotometer (Shimadzu, Japan, model UV2600) with a diffuse reflectance integrating sphere accessory. Baseline spectra were collected using pressed BaSO4 powder compacts that were placed in the sample and reference beams. Data were collected at a scan rate set in slow mode and a slit width of 0.5 nm. To find the band gap value, the Tauc plot23 with the Kubelka−Munk function was adopted considering the indirect interband transition. 2.1.1.3.3. Deep Levels Determination. The cathodoluminescence (CL) spectroscopy has been performed with a commercial Gatan monoCL system, fitted onto an S360 Cambridge SEM. A holographic grating and a multialkali photomultiplier sensitive in the range 350−830 nm (3.6−1.5 eV) are the main components of the system with a spectral resolution of 0.1 nm. The sample temperature can be changed from 300 to 6 K. The energy of the electron beam has been kept at 20 keV and the beam current at 25 nA. The irradiated area of the samples was approximately 4 × 10−5 cm2, and the specimen temperature was 300 K. The samples were positioned inside the vacuum chamber of the SEM at about 10−5 mbar. 2.1.2. Device Preparation. 2.1.2.1. Photoanode and Cathode. FTO slides (2.5 × 2.5 cm2) were used for both the anode and the cathode. On the cathode, a 1 mm diameter hole was drilled by using a diamond drill bit.

Before use, the glass slides were ultrasonicated for 15 min in a 1% Liquinox solution in deionized water, followed by rinse with deionized water and two more ultrasonication processes in Milli-Q water for 5 min. Then, the slides were treated with RCA cleaning solution A (Milli-Q water, ammonium hydroxide solution, hydrogen peroxide solution, volume ratio 5:1:1), rinsed with Milli-Q water, treated with RCA cleaning solution B (Milli-Q water, hydrochloric acid, hydrogen peroxide solution, volume ratio 6:1:1), and rinsed again in Milli-Q water. Both the RCA treatments were performed at 75 °C for 10 min. RCA treatments were used in order to remove any traces of organic and inorganic contaminants from the glass. FTO glass slides for the photoanodes were eventually treated with 40 mM TiCl4 in Milli-Q water at 70 °C for 30 min and then rinsed with Milli-Q water and ethanol. The photoanodes of DSSCs (about 3.5 × 3.5 mm2) were prepared by screen printing ethyl cellulose-based pastes in terpineol by using a manual screen printing table (model 60-90, Mismatic, Italy) equipped with a 34T polyester mesh screen. The preparation of the pastes and the printing process were performed according to the procedures developed by Ito et al.24 The printing process was repeated in order to get a thickness of 10−15 μm. After printing, the films were thermally treated for a first time, treated again with 40 mM TiCl4 in Milli-Q water at 70 °C for 30 min, rinsed with Milli-Q water and ethanol, and thermally treated again. The thermal treatments were the same reported in the literature.24 The photoanodes were sensitized for 15 min by using a 20 mM solution of N719 with 25 mM tetrabutylammonium hydroxide 30-hydrate in acetonitrile, according to the procedure of Nazeeruddin et al.25 The area of each photoanode was accurately measured by using the method reported in the literature.24 The cathodes were prepared by brushing the appropriate area of the hole drilled FTO slides with 50 mM hydrogen hexachloroplatinate(IV) solution in absolute ethanol, and then thermally treating the slides at 500 °C for 30 min in air. 2.1.2.2. Nanostructured Film Profilometry. The thickness of the photoanodes was measured by using a stylus profilometer (model MAP3D-25, A.P.E. Research, Italy) with a nominal resolution of 10 nm. 2.1.2.3. Cell Assembly. The sensitized photoanode and the cathode were sandwiched together with a gasket of 25 μm thick Surlyn, tightly kept together and sealed by heating to 110 °C for the time necessary to completely melt the gasket. After cooling to room temperature, the electrolyte was inserted through the 1 mm hole by using a Solaronix Vac’n’Fill syringe. The electrolyte composition was 0.6 M DMPII, 0.1 M LiI, 0.03 M I2, and 0.5 M TBP in acetonitrile. The hole was then sealed with a 60 μm thick Surlyn gasket covered with a thin microscope glass slide by melting the gasket with a hot solder tip. Copper wires were fixed on both electrodes with Cerasolzer soldering alloy, and the contact areas were covered with silver loaded conductive paint in order to reduce the contact resistances. Kynar antireflection and UV-blocking film was applied over the cell on the photoanode side by fixing it using 3M biadhesive tape. At least six cells were tested for each Sc composition. 2.1.3. Device Test. The photocurrent−photovoltage curves, open circuit voltage decay (OCVD), and dark currents were recorded by the potentiostat/galvanostat 1286 from Solartron Analytical, U.K., using the Full Combo ZPLOT/CorrWare software by Scribner Associates Inc., USA. I−V curves were acquired at a 10 mV/s scan rate. An Asahi Spectra HAL-320, 25278



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AM 1.5 G class A solar simulator was utilized for determining the conversion efficiency. Before the tests, a calibrated Asahi Spectra Sun Checker was used to check the intensity of the radiation to be within ±1% of 1 sun. The incident photon to current conversion efficiency (IPCE) was measured in DC mode without light bias by a custom-made apparatus assembled according to the design reported in the literature.26 It is constituted by a 250 W halogen lamp (Newport-Oriel), a Spectral Products CM110 1/8 m monochromator with variable resolution slits (in all the experiments, a 4 nm resolution slit was adopted), a Newport broadband beam splitter, a calibrated poly-Si detector, and all the necessary lenses for collimating and focusing the light beam. Both the devices under test and the detector were connected to a transimpedance system to avoid polarization effects. Data acquisition was performed with a National Instruments NI 9219 24-bit universal analog input, and the control software was written in LabView. All the measurements were performed in the wavelength range 400−1100 nm with a scan interval of 5 nm.

3. RESULTS AND DISCUSSION 3.1. Characterization of Sc Doped TiO2−Anatase Nanoparticles. 3.1.1. Structure and Morphology. 3.1.1.1. XRD. A widespread and careful work was done to establish that the Sc doped TiO2−anatase nanostructured powders were true solid solutions, i.e., just a two-component single phase. Within the detection limits of XRD, this was proved unambiguously for all the tested compositions, as shown in Figure 1 where the powder doped with 1 at. % of Sc is reported as an example. The figure shows the reference XRD spectra of tetragonal TiO2−anatase27 and cubic Sc2O328 from the JCPDS database together with the spectra of the doped (top panel) and undoped (second panel from the bottom) synthesized anatase powders. No features of Sc2O3 are present in the spectrum of the doped sample. The inset in the top panel is a magnified view of the most intense (101) peaks of doped and undoped anatase where the shift at lower angles of the doped sample proves the enlargement of the TiO2 lattice due to the presence of scandium. The measured 2θ shift corresponds to ∼0.015° which gives a lattice expansion of about 0.09% in the same direction. About the Sc site, substitutional or interstitial position, XRD cannot give any information and the synchrotron radiation spectroscopy measurements were performed at this scope (see below). The results of the Rietveld analysis performed on all the samples in the whole range of composition studied are reported in Figures 2 and 3. The dependency on the composition of the a and c (bottom right inset) axes of anatase as well as the cell volume (top left inset) are shown in Figure 2. As expected, all the trends are increasing with the composition and they are well fit by quadratic equations. The particle size distribution calculated on the whole range of composition shows an almost Gaussian shape in which the 77% of nanoparticles is concentrated in the range from 16.6 to 18.1 nm. The inset in Figure 3 shows that the nanoparticle size changes linearly with the Sc level doping. The error bar on the composition axis in the above figures was likely overestimated due to the way to maximize the error at the lowest concentrations and reducing it as the concentration increases. Due to the synthesis procedure of small quantities of product, which requires at the lowest concentrations the addition of very low amounts of SIP, we do

Figure 1. XRD spectra at the Cu Kα source (λ = 0.154184 nm) of synthesized pure anatase and solid solution of 1.0 at. % of Sc in anatase. The bar line panels show the XRD spectra of scandia and anatase as given in the JCPDS database. The inset in the top panel displays the shift of the main peak of anatase when doped (black curve) with respect to the pure one (red curve).

not expect a negligible error with respect to the nominal concentration. 3.1.1.2. EXAFS. The k3χ(k) quantity at the Ti K-edge of the undoped TiO2−anatase (red curve) and samples doped with 0.7, 1, 2, 5 and 10% Sc are shown in Figure 4A. They are all very similar, indicating that the Sc doping, in agreement with the XRD results, does not modify the TiO2−anatase lattice. The corresponding Fourier transforms, FTs, which are reported in Figure 4B show a broadening of the peaks as the doping level increases, in particular for samples with 5 and 10% at doping. This cannot be due to the variation in particle size with the doping level, which would give rise to the opposite trend, since broadening of the peaks should be lower for bigger nanoparticles. Therefore, the increase in the broadening of the peaks must be interpreted as an increase in the disorder of the Ti sites 25279



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Figure 2. Trend of the a-axis of anatase as a function of the atomic fraction of Sc in the range 0−10 at. %. The insets in the bottom right and top left corners show the trends of the c-axis and cell volume of anatase in the same range of Sc concentration.

Figure 4. EXAFS spectra at the Ti k-edge (panel A) and related Fourier transforms (panel B) at various compositions of Sc in anatase. There is no evidence of perturbation of the local structure in the Ti lattice with respect to pure anatase.

is consistent with the formation of a substitutional solid solution. In Figure 5B, the corresponding FTs are reported. The FTs of the samples are very similar to each other, indicating that the Sc environment is always the same, and it is very different from the Sc environment in Sc2O3. 3.1.1.3. FEG-SEM and HR-TEM. The morphology obtained by the FEG-SEM analysis of the beads of synthesized Sc doped anatase is shown in Figure 6. A superstructure characterizes the beads, which is spherical at microscopic level and rice-grainshaped at nanoscopic level. This appears clearly in the central part of the figure where a 670 nm diameter bead particle doped with 0.25% of Sc is constituted by a large number of nanoparticles randomly oriented but organized as a sphere to minimize the surface energy. The insets A and B on the upper corners of the figure show beads doped with Sc at 1 and 10%, respectively. The surface of the beads at higher Sc concentration is rougher, and no significant size difference between the two compositions of the beads appears. The nanoparticles in the beads have uniform size along the three directions when the size is less than 10 nm. The TEM photos A and B of Figure 7 refer to compositions 0.2 and 1.0 at. % of Sc, respectively. The red arrows point spotted zones that in TEM mode could be cavities. The observation in STEM-HAADF (high angle annular dark field) mode shows those areas darker (panel C), which confirms that the smallest particles are porous in nature with cavities ≤2 nm (panel D). These findings are consistent with the BJH measurements below. 3.1.1.4. Porosimetry and Surface Area Measurements. The determination of the surface area and porosity of the synthesized Sc doped anatase powders is fundamental to

Figure 3. Nanoparticle size distribution by Rietveld analysis of the Sc doped anatase powders in the whole range of composition explored. The inset shows a linear relationship between the nanoparticle size and Sc concentration in anatase.

due to the increasing substitution of titanium cations with scandium cations. Similarly to Figure 4, the k3χ(k) at the Sc K-edge of the samples doped with Sc at 1, 2, 5, and 10% are shown in Figure 5A together with the curve of pure Sc2O3. The k3χ(k) of the samples have a kmax of about 1 nm−1, due to the presence of the Ti K-edge, that limits the information that can be achieved. However, it is quite clear that the oscillations in all the samples are very similar to each other, apart from the noise which is higher at low doping concentration and which limited the samples that could be investigated to those with at least 1 at. % doping. The oscillations are instead very different from those of Sc2O3, confirming that a single-phase solid solution is obtained. The oscillations are in phase with those at the Ti K-edge, which 25280



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Figure 7. TEM photos A and B refer to compositions 0.2 and 1.0 at. % of Sc, respectively. The red arrows point out spotted zones that in TEM mode could be cavities. The observation in STEM-HAADF (high angle anular dark field) mode shows those areas darker (panel C), which confirms that the smallest particles are porous in nature with cavities ≤2 nm (panel D).

Figure 5. EXAFS spectra at the Sc k-edge (panel A) and related Fourier transforms (panel B) at various compositions of Sc in anatase. There is clear evidence of perturbation of the local structure of Sc in anatase with respect to pure scandia.

the pore size distribution moves toward higher diameter, and its height decreases as far as the Sc concentration in the powders increases up to 2 at. %. Further increases of Sc concentration produce an abrupt change from this apparently regular behavior. The shape of the distribution, the peak intensity, and position change significantly. This behavior matches also with the trend of the BET data against the Sc concentration as reported in Figure 9. As expected, the shift of the peak

understand how these quantities play a role in changing the amount of dye chemisorbed and how the latter influences the DSSC performances. The BJH pore size distribution is shown in Figure 8 for some Sc doped anatase powders. The inset is a magnified view in the pore size range 0−40 nm. The peak of

Figure 6. FEG-SEM image of a synthesized 670 nm diameter bead particle with 0.25 at. % of Sc in anatase. The bead is constituted by a large number of nanoparticles randomly oriented but organized as a sphere to minimize the surface energy. The insets A and B on the upper corners of the figure show beads doped with Sc at 1 and 10%, respectively. 25281



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morphology of the beads at this composition, as shown in inset B of Figure 6, agrees well with those findings. 3.1.1.5. Dye Loading. The measured loading values of the cells reported in Table 1 were found ranging around 0.10 mg cm−2 practically independent from the Sc content. This result should be considered as expected because the cells differ by the Sc content only. Furthermore, it is worth noticing that the photoanode surface area (see Table 1), BET surface area of doped anatase (see Figure 9 at Sc content within 1 at. %), and thickness (see Figure 10) do not change significantly for the tested cells.

Figure 8. The BJH pore size distribution is shown for some Sc doped anatase powders. The inset is a magnified view in the pore size range 0−40 nm. The peak of the pore size distribution moves toward higher diameter, and its height decreases as far as the Sc concentration in the powders increases up to 2 at. %. Further increases of Sc concentration produce an abrupt change from this apparently regular behavior.

Figure 10. Typical curves of profilometry of the semiconductor layer as deposited and sintered on FTO glass substrates.

3.1.2. Profilometry. The thickness of the semiconductor of the photoanode and its uniformity along the whole layer deposited on the FTO glass were routinely checked because the reproducibility of the cell performances is strongly dependent on this parameter. Figure 10 shows typical profile curves of our photoanode semiconductor layers where the average thickness is around 14 μm, namely, 14 ± 2 and 14 ± 1 μm for pure anatase and 0.2 at. % Sc doped, respectively. It is widely recognized in the literature that the optimum thickness is within 10−20 μm range. Higher thicknesses would be beneficial for increasing the amount of dye adsorbed, but this would increase the electrical resistance as well as decrease the transparency of the semiconductor layer. 3.1.3. Electronic Characterization. 3.1.3.1. Flat Band Potential. The flat band potential, Vfb, is defined as the potential at which the semiconductor energy bands are “flat”, leading up to the solution junction. This definition implies the

Figure 9. BET surface area as a function of the atomic fraction of Sc in anatase.

distribution at smaller pore sizes is in general compatible with the increase of the specific surface area, but in the present case, we have to consider the effect of the change in composition which tends clearly to produce opposite effects. At 10 at. % of Sc, the BJH peak is 4.6 times lower than the peak of pure anatase. On the other hand, the visible change of surface

Table 1. Experimental Parameters of DSSCs as a Function of the Sc Doping of the Anatase Photoanode under AM 1.5 G Class A Solar Simulatora at. % Sc

A (cm2)

JSC (mA cm−2)

VOC (V)

FF

ηb (%)

Rsh (kΩ)

Rs (Ω)

0.0 0.1 0.2 0.3 0.5 1.0

0.099 0.096 0.098 0.101 0.112 0.110

16.25 17.70 19.10 10.40 9.52 6.21

0.795 0.749 0.752 0.707 0.714 0.717

0.705 0.704 0.675 0.769 0.778 0.770

9.0 9.3 9.6 5.6 5.3 3.4

0.95 0.93 980 10 43 5.7

6.0 6.0 4.9 7.5 7.7 11

The tabulated values refer to the active area, A, of the photoanode, short circuit current density, JSC, and open circuit voltage, VOC. FF is the fill factor and η the efficiency. Rsh and Rs are respectively the shunt and series resistances obtained by fitting the curves of Figure 15 at bias values close to 0 and OCV, respectively. The standard deviation of the efficiency values is within ±0.2% for each composition. bOn at least six cells for each composition. a

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band bending. This is negligible only for very small particles as for TiO2 particles with size less than 6 nm,17 so band bending can be possible for particles with dimensions in the range of tens of nanometers as in the present case (see section 3.1.1.1). Several papers10,16,29 dealt with the determination of the flat band potential in nanocrystalline electrodes where the nanoparticle size was decidedly greater than 6 nm. We agree with the definition given by Grätzel et al.17 as “the potential of the conduction band edge” under accumulation regime instead of “flat band potential” when nanocrystalline materials are considered. Thus where the above conditions hold, for a n-type semiconductor, SC, the depletion layer capacitance, CSC, as a function of the applied potential, V, can be evaluated by the Mott−Schottky equation: 1/CSC 2 =

⎡ kBT ⎤ 2 ( ) − − V V ⎢ ⎥ fb e0 ⎦ NDe0ε0εsA2 ⎣

(1)

where ND, e0, ε0, and εs are the carrier density, elementary charge, vacuum permittivity, and relative static dielectric constant of the semiconductor. kB is the Boltzmann constant, T the absolute temperature, and A the geometric area of SC. Equation 1 can be rewritten in the form CSC−2 = a + bV

Figure 11. Mott−Schottky plots at various compositions of Sc in anatase for determining the flat band potential against SCE, as the reference electrode. The Mott−Schottky plot for the bare FTO (GFTO) is also reported for comparison purposes. The measurements were performed with a sinusoidal signal at 100 Hz and a 10 mV amplitude in 0.1 M aqueous solution of Na2SO4. The inset is a magnification around the linear extrapolation zone.

(2)

where a = −(2(Vfb + kBT/e0))/(NDe0ε0εsA ) and b = 2/ (NDe0ε0εsA2). The extrapolated straight line at CSC−2 = 0 implies that 2

Vfb = Vextr − kBT /e0

(3)

and ND =

2 be0ε0εsA2

(4)

being Vextr = −a/b, the ratio between the intercept and slope of the extrapolated straight line. The systematic maximum error on Vfb and ND was taken by the errors on a and b associated with the linear fit of the extrapolated straight line. The value of εs was assumed equal to 55.30 This assumption is almost rough because it is expected that εs is composition dependent. Unfortunately, no data can be found to estimate, even qualitatively, how the relative static dielectric constant of anatase solid solutions changes with Sc content. A series of Mott−Schottky plots are reported in Figure 11 for some compositions studied together the bare FTO substrate indicated in the figure as G-FTO. The Vfb and ND values of G-FTO were found as −1.01 ± 0.01 V and (1.1 ± 0.2) × 1021 cm−3, respectively. These values are in agreement with the literature.31 The inset in the figure shows a magnification of the extrapolation region. Figure 12 shows the trend of the flat band energy in the vacuum scale as a function of the Sc doping level in anatase. The estimation of the error bar on the composition was done as described before. The insets in the same figure show the values of Vfb vs SCE (top corner) and the carrier density (bottom corner), calculated according to eq 4, against the Sc concentration. It is worth noticing that the Sc doping is not beneficial for increasing the density of carriers. Besides, the edge of the conduction band moves up in the absolute energy scale and this reduces the energy difference with the LUMO level of dye. This result is opposite to the result found by doping anatase with W.16 This behavior is explained by the compensation between the electrons in the CB, due to the

Figure 12. Flat band energy reported in the vacuum scale as a function of the atomic fraction of Sc in anatase. The insets in the top left and bottom right corners show the flat band potential vs SCE and carrier concentration, respectively. The latter quantity was calculated according to eq 4. The systematic maximum error on Vfb and ND was taken by the errors on a and b associated with the linear fit of the extrapolated straight line (see eq 2).

donor levels associated with the oxygen vacancies, and holes in VB generated by the substitution of Ti4+ with Sc3+. 3.1.3.2. Band Gap. The effect of the substitution of Sc in the site of Ti produces a modest dilatation/deformation of the tetragonal lattice of anatase with a simultaneous formation of an electron vacancy, i.e., a hole in the valence band. These combined actions influence the width of the band gap, as Figure 13 demonstrates. The band gap value increases with Sc 25283



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Figure 13. Band gap of anatase by increasing the Sc doping up to 10 at. %. The experimental data were obtained by diffuse reflectance in the range 220−800 nm of sample powder pellets. To find the band gap value, the Tauc plot with the Kubelka−Munk function was adopted.

concentration, and the highest measured value is 3% greater than the value of pure anatase. In the range 0.0−1.0 at. % of Sc, which is the useful range for a satisfactory efficiency of our DSSCs, the increase of the band gap is just 1%. The knowledge at each doping level of the potential of the conduction band edge and band gap allows us to fix the position of the semiconductor band structure in the vacuum scale of energy by taking into account the related shifts due to the fact that the flat band potentials are measured with respect to SCE (ESCE = −4.5 − eVSCE = −4.741 eV, where −4.5 eV is the energy of NHE and VSCE = 0.241 V is the standard potential of SCE with respect to NHE). Therefore, by increasing the Sc doping level, there is a modest widening of the band gap, which shifts upward in the energy scale. 3.1.3.3. Deep Levels. The CL emissions of two anatase Sc doped samples (0.05 and 1.0 at. %) are shown in Figure 14 (panel A) together with the spectra of pure anatase and pure scandia. More information on the spectra of the pure compounds anatase32 and scandia33 can be found in the literature. Panels B and C of Figure 14 show that the spectrum of each sample is constituted by two distinct transitions: 632 and 735 nm for the sample in panel B and 498 and 556 nm for the sample in panel C. Comparing the CL spectra with 0.05 and 1% of Sc3+ with those reported of the pure compounds (panel A), it is apparent that, by increasing the Sc3+ concentration inside the TiO2 nanoparticles, the integrated intensities of the two CL bands peaked at 632 and 735 nm with a remarkable decrease in intensity up to disappearance (see panel C) for 1% of Sc3+ with a consequent blue shift of the center of mass of the spectrum. There are almost no literature data on optical transitions in the TiO2:Sc system, and the assignation of the bands is based only on absorption studies.34 The authors report that TiO2 anatase doped with 2 at. % of Sc3+ and 2 at. % of V5+ ions exhibit red shifts and weak wide absorptions in the visible region 400−600 nm with respect to pure anatase. Considering that our samples are doped only with Sc3+ ions, it is possible to tentatively assign the bands from 498 to 632 nm as being due to deep levels inside the TiO2 band gap induced by the Sc3+ ions in the TiO2 matrix, while the peak at 735 nm is most likely due to the radiative recombination of the excited donor−acceptor pairs Sc3+−O2− as for pure Sc2O3. The study of the nature of the deep levels is beyond the purpose of this paper and will be the object of further more accurate investigations. As a final

Figure 14. Deep levels determination by cathodoluminescence spectroscopy. The emissions of two anatase Sc doped samples (0.05 and 1.0 at. %) are shown in panel A together with the spectra of pure anatase and pure scandia. Panels B and C show that the spectrum of each sample is constituted by two distinct transitions 632 and 735 nm for the sample in panel B and 498 and 556 nm for the sample in panel C.

comment on the CL results, we can state that the dispersion of Sc3+ ions in the TiO2 matrix induces the formation of deep levels in the TiO2 energy gap with emissions in the visible range. On the contrary, as shown by our diffusion reflectance measurements, the band gap energy value is negligibly affected at the Sc3+ concentrations investigated by CL. 3.1.4. Test and Characterization of DSSCs. 3.1.4.1. Polarization Curves. The DC polarization curves of some DSSCs at different doping levels of Sc are reported in Figure 15, and related data are given in Table 1. The curves were obtained under illumination by a AM 1.5 G class A solar simulator. The fitting the curves at bias values close to 0 and OCV, respectively, gave an estimation of the shunt resistance and series resistance. The respective values are reported in Table 1. The polarization procedure was repeated for obtaining the dark current curves starting from ∼0.0 mV and increasing the voltage up to 0.75 V at 1 mV s−1. The corresponding curves are given in the bottom part of Figure 15. It appears clearly that the efficiency as a function of Sc doping passes through a maximum, which is at 0.2 at. % of Sc. Higher doping levels decrease significantly the efficiency. The corresponding increase of FF is to be attributed mainly to the decrease of JSC. The 25284



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Figure 16. Dark curves of Figure 15 plotted according to eq 5, i.e., ln|Jdark| vs V. The figure shows the portion of the plot at the highest voltage where the fit of the linear part was performed.

Figure 15. J−V polarization curves of DSSCs at different Sc doping of the photoanode semiconductor under illumination by a AM 1.5 G class A solar simulator. The efficiency as a function of Sc doping passes through a maximum, which is at 0.2 at. % of Sc. The light gray bottom part of the plot displays the curves in the dark for the same DSSCs.

ϕB =

(6)

where A* = 4πem*kB2/h3 is the Richardson constant with m* and h the effective electron mass which is reported36 equal to 1200 A cm−2 K−2 for TiO2 and the Planck constant, respectively. A trend with Sc doping appears having its maximum around the highest DSSC efficiency observed. The higher the barrier height, the better the rectifying behavior of the Schottky diode FTO/TiO2, which implies low back current, i.e., better collection of electrons. The exchange current density of this process corresponds to J0, the average value of which agrees well in the range 0.01−1 nA cm−2 reported in the literature.17 The value of ϕB is comparable with barriers of nanostructured anatase with noble metals such as Au,37 Pt,38 and Pd,39 where ϕB is 0.9, 1.0, and 0.85 eV, respectively. Just to check the consistency within the present findings, Table 2 compares the calculated values of VOC by the diode equation40

standard deviation of the efficiency values is within ±0.2% for each composition. The dark current, Jdark, is the current passing through the interface when a forward bias is applied between the terminals of the DSSC under test. It acts in the opposite sense to the short circuit photocurrent. This is in fact the process that limits the performance of any solar cell. By the dark current curves, it is possible to obtain some information on the rate of the recombination processes which in DSSCs should be mainly attributed to the reduction of the oxidized species (I3− in the present case, I3− + 2e(TiO2)cb = 3I−) by the injected electrons in the anatase conduction band. Jdark can be treated as the current in a rectified junction and can be approximated by the form Jdark = J0 (eeV / mkBT − 1)

kBT A*T 2 ln e J0

(5)

where J0 is the intrinsic current density flowing in the cell at temperature T and at bias V = 0. J0 is a quantitative measure of the extent of rectification of a given interphase; the smaller the J0, the better is the rectification. m is the ideality factor which normally lies in the range 1 ≤ m ≤ 2. Both of the above parameters are related to the mechanism for the charge transport and recombination. Table 2 gives the J0 and m values as a function of Sc doping level in anatase taken up respectively from the intercept and slope of the linear part of the ln|Jdark| vs V curve at sufficiently high voltage values where Jdark ≈ J0eeV/mkBT (see Figure 16). With J0 being known, the potential barrier height, ϕB, can be calculated by using the equation35

VOC =

⎞ kBT ⎛ JSC ln⎜⎜ − 1⎟⎟ e ⎝ J0 ⎠

(7)

where JSC is the short circuit current density (third column in Table 1). The values are comparable, and the minimum difference is just 12 mV at 0.2 at. % of Sc. 3.1.4.2. Open Circuit Voltage Decay (OCVD) Curves. The lifetime, τn, of electrons inside DSSCs with the photoanode at different levels of Sc doping is reported in Figure 17 against the OCV values. This quantity was calculated according to the following equation:41

Table 2. Experimental Parameters of DSSCs as a Function of the Sc Doping of the Anatase Photoanode in the Darka at. % Sc 0.0 0.1 0.2 0.3 0.5 1.0

J0 (nA cm−2) 0.64 0.24 0.49 0.50 0.73 1.23

± ± ± ± ± ±

0.02 0.01 0.01 0.02 0.03 0.03

m 1.63 1.49 1.70 1.69 1.75 1.92

± ± ± ± ± ±

0.04 0.03 0.01 0.05 0.06 0.03

ΦB (V)

Vcal OC (V)

Vexp OC (V)

± ± ± ± ± ±

0.713 0.694 0.764 0.730 0.736 0.761

0.795 0.749 0.752 0.707 0.714 0.717

1.019 1.044 1.026 1.026 1.016 1.002

0.008 0.006 0.004 0.001 0.001 0.005

a

J0 and m are the current density in the dark extrapolated at null bias and ideality factor as given in eq 5, respectively. Both of these parameters were found by fitting the linear part of the experimental curves ln|Jdark| vs V at the highest bias values (see Figure 16). The height of the potential barrier, ϕB, was calculated according to eq 6 and VOC through eq 7. The uncertainties were evaluated by error propagation law. 25285



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Table 3. Experimental Parameters of DSSCs as a Function of the Sc Doping of the Anatase Photoanode in OCV Decay Experimentsa at. % Sc 0.0 0.1 0.2 0.3 0.5 1.0

τ1 (s) 5.93 7.92 6.68 6.48 8.88 8.05

± ± ± ± ± ±

0.04 0.04 0.04 0.04 0.05 0.05

τ2 (ms) 391 400 319 316 313 488

± ± ± ± ± ±

6 3 4 4 4 5

Ln (μm) 54.5 62.3 64.0 63.4 74.5 56.9

± ± ± ± ± ±

0.5 0.3 0.5 0.5 0.5 0.4

De (cm2 s−1) (×10−5) 7.7 5.63 7.7 7.3 7.3 4.94

± ± ± ± ± ±

0.1 0.05 0.1 0.1 0.1 0.06

The time constants τ1 and τ2 were obtained by fitting the electron lifetime, τn, curves, calculated according to eq 8, with the linear combination of two exponentials (see eq 9). The diffusion length, Ln, and diffusion coefficient, De, of electrons in the semiconductor were calculated by eqs 10 and 11, respectively. a

(see Figure 10) for all the DSSCs tested. The diffusion coefficient, De, of electrons in the semiconductor is given by

Figure 17. The lifetime, τn, of electrons in the DSSC photoanodes at different levels of Sc doping reported against the OCV values. The experimental data were obtained by acquiring the decay curve of OCV after switching off the solar simulator. The lifetime curves were calculated according to eq 8 and fitted by a linear combination of two exponentials (eq 9) that allow determination of the time constants τ1 and τ2 (see Table 3). The insets in the top right and bottom left corners show a magnification in the OCVD range where the effect of the Sc concentration is more evident and the trends of the τ1 and τ2 time constants as a function of the Sc concentration.

τn = (kBT /e)

⎛ dV ⎞−1 ⎜ ⎟ ⎝ dt ⎠

De = Ln 2 /τn

with τn being the lifetime of electrons at a given OCVD (see Figure 17). The evaluation of De is reported in Table 3, where τn was taken at OCV = 0.3 V. At this value, the lifetime shows a marked dependency from the Sc doping. It is worth noticing that within the error the values of Ln and De are practically independent from the Sc concentration. The calculated values of Ln are more than 3 times the semiconductor layer thickness L. Other authors41 found Ln > L. This result, which is a favorable condition implying in principle that the electrons are not trapped and they could be all collected at the FTO/SC interface, provided that no recombination occurred at the SC/ electrolyte interface. Since Ln > L was found for all the tested doping levels, other phenomena should be responsible for the efficiency trend. It is reported45 that substitution of Ti4+ with trivalent ions such as Y3+ and Ga3+ increases the electron lifetime but reduces the transport rate. The charge collection efficiency, ηCE, is given by the ratio of the transport rate and the sum of transport and recombination rate. Therefore, the balance between the electron lifetime and transport rate drives ηCE, and thus the DSSC efficiency. The efficiency maximum at a certain doping is consequently a compromise between these two opposing quantities. The average De value is very close to the values available in the literature.42 The increase of the size of nanoparticles (see the inset of Figure 3) together with the almost random pore size distribution (see Figure 8) as the Sc concentration increases both play an important role in controlling the transport properties within the semiconductor layer. 3.1.4.3. IPCE. Figure 18 shows normalized IPCE curves for the devices under test. The normalization was done in order to make possible a more clear comparison of the curves. It is obvious that the area under the curve for non-normalized IPCE curves is very different for each device, being

(8)

where e and V are the elementary charge and the OCV of the cell after the switch off of the solar simulator. The lifetime value is strongly dependent on the OCV value with an overall variation, which covers more than 2 orders of magnitude. The best-fit curve of the experimental OCVD data is represented by the linear combination of two exponential functions as y = m1 + m2e−M0 / m3 + m4 e−M0 / m5

(9)

where the coefficients m3 and m5 are the time constants τ1 and τ2, respectively, and they are plotted against the Sc atomic fraction in the bottom left inset of Figure 17. The top right inset in the same figure shows that, at a given OCV, the electron lifetime increases significantly passing from pure anatase to anatase with the highest Sc content. For instance, at 0.35 V, the τn value of the DSSC with 1.0 at. % of Sc is the double of pure anatase. The so different values of τ1 and τ2 (see also Table 3) suggest two different processes, one about 10 times faster than the other. The faster one is related to the diffusion of electrons within the semiconductor with time constant τ2. The other one, τ1, can be attributed to the reduction of I3−, which implies a loss of injected electrons at the semiconductor/ electrolyte interface. Both of these values are within the wide range of the data reported in the literature.16,42,43 As a first approximation, the diffusion length, Ln, can be calculated as44

⎛ τ1 ⎞1/2 Ln = L⎜ ⎟ ⎝ τ2 ⎠

(11)

JSC =

∫wavelengths (np)λ (IPCE)λ e dλ

where np is the number of photons occurring in the wavelength interval dλ and e the elementary charge. The value of the integral for each cell corresponds to its Jsc value within ±5%. This effect can be explained considering that increasing the content of Sc3+ in anatase lowers its surface acidity, which affects the spectral properties of the adsorbed dye. It is in fact

(10)

where L is the thickness of the semiconductor layer. The values of Ln reported in Table 3 were calculated assuming L = 14 μm 25286



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within ±0.2%. This figure does not agree with the differences among the measured efficiencies. The value of the efficiency vs the Sc concentration passes through a maximum, and it decreases quickly by further increases. Two prevailing effects explain this behavior at least: • The conduction band edge moves up as found by the measure of the flat band potential. This reduces the difference between the LUMO energy level of the dye and the bottom of the conduction band of the semiconductor, which produces a decrease of the rate of the electron injection. Jointly, an increase of OCV is expected, which is not found. The OCV values reported in Table 1 show a decreasing trend. Part of this decrease could be justified by the increase of the voltage drop through the series resistance that increases with the Sc concentration. • The substitution of Sc on the Ti site causes the increase of the atomic and electronic disorder. The atomic disorder implies morphological changes of nanoparticles such as size and pore size distribution. The electronic disorder is mostly caused by formation of holes in the valence band that counterbalance the donor levels due to the oxygen vacancies of anatase. The measured decrease (see the bottom inset of Figure 12) of the charge carriers as a function of Sc concentration gives a proof of this. The peculiar effects on electron lifetime and transport constant in doped anatase with trivalent ions already shown in the literature further support the beneficial role, within certain limits, of forming holes in the valence band. In conclusion, the whole picture emerging from the experimental results of the present work indicates that the DSSC performances can be improved through a careful dosage of Sc in anatase together with a strict control of each step of the cell assembly by bothering about the chemistry of materials.

Figure 18. Normalized IPCE curves for DSSCs at different Sc doping levels of the photoanode semiconductor. The vertical dotted line marks the region where the blue shift is more evident, due to the lowering of surface acidity caused by the increase of scandium content in anatase.

well-known46 that the N719 dye absorption spectrum is characterized by a marked blue shift with increasing pH. The whole picture that emerges from all the characterizations is that, whereas the flat band potential increases with increasing Sc concentration, its effect is not detectable in the OCV of the cells probably because of the increased series resistance of the cells and the less efficient electron injection process from the dye LUMO level of the dye and the bottom of the CB of the semiconductor which is nearer in energy. On the other side, the Jsc value increases with increasing Sc concentration; it passes through a maximum at 0.2 at.% and then decreases substantially with further concentration increase. This behavior can be justified by the beneficial effect of Sc in electron lifetime but its detrimental effect on the transport rate. Evidently, the best efficiencies represent the best compromise between these opposite phenomena that happen with increasing Sc concentration in the anatase lattice: increasing flat band energy and decreasing transport rate on one side and increasing electron lifetime on the other one. Considering eventually the FF values, they are all quite high, and the lower values for the most efficient cells can be attributed to the higher power losses they experience due to their internal series resistance, related to their substantially higher produced photocurrents.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 39 06 4991 3161. Fax: 39 06 4991 3161. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for funding provided by the Ministero dell’Istruzione dell’Università e della Ricerca (MIUR) through PRIN 2009 Project No. 2009N4BJ4J and in part by the Università di Roma “La Sapienza”. Antonella Iadecola is kindly acknowledged for assistance during XAFS data collection. The authors wish to thank Arkema, U.S., for kindly supplying a free sample of their Kynar PVDF antireflection, UV blocking film.

4. CONCLUSIONS The planned scope of this work was to obtain a detailed and almost complete characterization of synthesized solid solutions of scandium in anatase to be utilized as new semiconductor materials for DSSC photoanodes. Obviously, the final scope was to obtain devices with improved performances with respect to the classical DSSC with pure anatase. This objective has been reached because at 0.2 at. % of Sc we obtained the maximum efficiency of 9.6%, which was found to be 6.7% greater than the efficiency of DSSCs with pure anatase. It is useful to remind that among the cells tested (more than 30, at least 6 for each composition) the unique changing parameter was the doping Sc concentration in anatase. Since the materials composing the cells belonged to the same batch, the same protocol for their assembly was carefully followed; the different performances of the cells could be attributed to some uncontrolled parameter and/or operator errors, but the variability of the efficiencies obtained for each composition is

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REFERENCES

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Efficiency Improvement of DSSC Photoanode by Scandium Doping of

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