Single Crystalline-like and Nanostructured TiO2 Photoanodes for Dye

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Single Crystalline-like and Nanostructured TiO Photoanodes for Dye Sensitized Solar Cells Synthesized by Reactive Magnetron Sputtering at Glancing Angle Pierre-Antoine Cormier, Jonathan Dervaux, Nadine Szuwarski, Yann Pellegrin, Fabrice Odobel, Eric Gautron, Mohammed Boujita, and Rony Snyders J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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The Journal of Physical Chemistry

Single Crystalline-like and Nanostructured TiO2 Photoanodes for Dye Sensitized Solar Cells synthesized by Reactive Magnetron Sputtering At Glancing Angle Pierre-Antoine Cormier1*, Jonathan Dervaux1, Nadine Szuwarski2, Yann Pellegrin2, Fabrice Odobel2, Eric Gautron3, Mohammed Boujtita2 and Rony Snyders1,2

1

Chimie des Interactions Plasma-Surface, Université de Mons, 23 Place du Parc, 7000, Mons,

Belgium 2

Université LUNAM, Université de Nantes, CNRS, Chimie et Interdisciplinarité: Synthèse,

Analyse, Modélisation (CEISAM), UMR 6230,2 rue de la Houssinière, 44322Nantes cedex 3, France 3

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la

houssinière, BP 32229, 44322 Nantes Cedex 3, France 4

Materia Nova R&D Center, Avenue Copernic 1, Mons, Belgium

ABSTRACT The control of the surface area enhancement and the ordering of the mesoporous photoanode is one of the key parameters to overcome the current limits of performance of Dyes Sensitized Solar Cells (DSSCs). These parameters are expected to improve both the concentration of adsorbed dye molecules on the photoanode and the charge collection. In this paper, reactive magnetron sputtering at glancing angle is employed to synthesize nanostructured TiO2 thin films. A post-annealing under ambient atmosphere at 773 K allows a recrystallization of the films to form individual single crystal-like anatase nanocolumns, as shown on a reference structure constituted by well-separated slanted nanocolumns. Even if the best cells provide an open circuit voltage of 0.8 V, a fill factor of 77% and a short circuit current density of 4.6 mA/cm2 (JSC) and permit to reach an overall efficiency up to 2.6%, it does not yet reach the performances of the

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reference TiO2 NPs based cell. This is explained by a poor carrier collection efficiency, as demonstrated by intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. The evaluation of other nanostructures such as zigzag and pillars shows the superiority of these structures on the NPs in terms of charge carrier collection efficiency. Nevertheless, the low penetration of the dye on these structures does not allow to reach as good photovoltaic performances as as those measured in NPs-based DSSCs.

1 Introduction Dye Sensitized Solar Cells (DSSCs) are recognized as a potential low cost photovoltaic technology presenting the great advantages of being lightweight, transparent with aesthetic value and working well in low light conditions1. In typical DSSCs, three key components are involved in the current generation. Briefly, a dye molecule grafted on a mesoporous n-type semiconductor (TiO2 nanoparticles – NPs) absorbs a photon which gives rise to electron injection in the conduction band of the semiconductor. The dye is finally regenerated by a redox mediator solubilized in an electrolyte2. Many efforts have been devoted to modify the characteristics of each component in order to improve the performances of DSSCs. Recently, the co-sensitization of the TiO2 semi-conductor by carboxy-anchor and silyl-anchor dyes permitted to reach a solar conversion efficiency up to 14.3% 3. This high charge collection was mostly explained by a better electron injection into the TiO2 semi-conductor due to the use of multiple dyes. The charge collection efficiency, is also intimately related to the crystalline structure and the morphology of the photoanode since these properties determine the charge transport efficiency and the recombination processes 4. In conventional DSSCs, in which NPs are used as photoanode, electron transport occurs by diffusion due to an electron concentration gradient over the semiconductor material 1. In fact, electrons move by hopping from one crystallite to the next by successive trapping in shallow energy states and detrapping by thermal activation 5. Even if the precise mechanisms are still unclear, it is accepted that the ordering within the photoanode


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material is a key parameter affecting the charge transport. A high degree of ordering would lead to an enhancement of the electron effective diffusion coefficient 6. The charge transport is also largely influenced by the recombination processes. However, in TiO2 NPs based photoanodes the discontinuity in the crystalline lattice implies the presence of shallower defect states acting as charge traps 7, 6. Aiming to ensure a large charge transport while maintaining a high specific surface area (SSA), 1D nanostructures such as nanowires nanostructures

13

8, 9

, nanofibers

10

, nanotubes

11, 12

or hierarchical

were suggested as potentially more efficient photoanode material because they

are expected to provide a direct pathway for the electrons to be collected thanks to a significantly lower density of grain boundaries. The diffusion coefficient is in fact enhanced by the presence of single crystalline nano-units along the 1D structures 13, 14, which also offer a better absorption of the light scattering compared to their analogous NPs 4. Various synthesis techniques have been employed to grow these 1D structures such as chemical vapour deposition (CVD) metal-organic or chemical vapour deposition (MOCVD)

17, 18

, electrospinning

19

15, 16

,

, anodic

oxidation 20 and solvothermal methods 21, 22 which recently demonstrated a capability to produce single crystalline photoanode allowing for a efficiency up to ≈ 9%

4, 23

. Nevertheless, this

technique requires the use of toxic solvent and therefore makes necessary the development of cleaner techniques which can be easily transferred to large scale production. In this work, nanostructured TiO2 were synthesized by reactive magnetron sputtering (RMS) at glancing angle (GLAD). RMS process allows an easy control of the chemistry and of the crystalline structure of the film 24, 25. GLAD technique simply consists on tilting the substrate according to the deposition flux that allows the formation of well separated columns due to a ballistic shadowing effect 26. Historically, this technique was associated to evaporation processes,


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but we recently demonstrated that a precise engineering of the nanostructure could be obtained with RMS by tuning the substrate bias voltage, the tilt angle or the substrate rotation speed

27

.

Here, a nanostructured TiO2 film with a slanted columns-based morphology is first grown. We show that a careful annealing post-treatment allows the recrystallization of the film and the formation of individual fully oriented anatase nanocolumns. Then, the influence of the nanostructure (slanted columns, zigzag and pilars), of the thickness and of the deposition angle on the specific surface area, the photovoltaic performances and the electron transport is evaluated.

2 Experimental setup All depositions were carried out in a 60 cm-in-diameter and 42 cm-in-height cylindrical stainless-steel chamber. It was evacuated by a turbo-molecular pump (Edwards nEXT400D 160W), backed by a dry primary pump, down to a residual pressure of 10−4 Pa. A 2-inch in diameter and 0.25-inch thick Ti target (99.99% purity) was placed on unbalanced magnetron cathode installed at the top of the chamber facing the substrate at a distance of 80 mm. The target was sputtered in DC mode using an Advanced Energy MDX 1.5 K power supply with a constant power of 150 W. A mixture of argon (12 sccm) and oxygen (3 sccm) was injected in the chamber in order to grow stoichiometric TiO2 film and the total pressure was maintained at 0.13 Pa. The reactive gas was injected either at the target surface, either at the substrate one whereas the Ar was always injected at the target surface. The substrate was tilted according to the vapor flux (α) in order to generate nanostructured thin films constituted by well separated slanted columns

27

.

Two alpha values were used (80 and 85). The substrate was additionally rotated at a speed of


4

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10°/s to grow TiO2 thin films with a pillars structures, whereas zigzag structures were obtained by successive discrete rotations (φ= +180° or -180°) as described in 28 The morphology and the thickness of the nanostructured films were evaluated by scanning electron microscopy (FEG-SEM Hitachi SU8020). The global crystalline structure was determined by Grazing Incidence X-ray Diffraction (GIXRD) analysis (Panalytical Empyrean) with a Cu Kα 1 source (1.5406 Å) and 8046 keV. The X-ray source voltage was fixed at 45 kV and the current at 40 mA. The X-Ray beam penetrated the film with an incident angle of 0.5°. The nanostructure of the films was then evaluated by transmission electron microscopy using a Hitachi HF2000 TEM at 100 kV. The cross-section lamellae of the untreated nanostructured films were prepared by mechanical polishing and ion milling. Individual columns of the single crystalline thin film was scratched to observe each column separately. Conductive silicon wafers (100) were used as substrate for these characterizations. To determine the concentration of dye molecule adsorbed on the nanostructures per unit film area (dye loading), and thus to evaluate the surface area enhancement (SAE), dye desorption measurements were performed. Ru based dye (N719) was first grafted on the films (7.5 cm2) by immersion during 24 hours. The films were thorough rinsed with acetonitrile before being immersed into an excess KOH solution (3.00 mL, 1.0 mmol KOH) for at least 20 min to desorb and fully deprotonate the dye. The absorbance of the resulting light pink solutions was measured with a UV-vis spectrophotometer. The dye concentration was calculated by using a calibration curve in the range from 5×10−6 to 1×10−6 mol. SAE was calculated from the following equation:

ܵ‫= ܧܣ‬

஼ಿళభవ ௏೏೐ೞ೚ೝ್. ேಲ ௌళభవ ௌೞೠ್ೞ೟ೝೌ೟೐

(1)


5


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where CN719 is the concentration of N719 in the desorbed solution, Vdesorb. the volume of the desorbed solution, NA the Avogadro constant, SN719=2.43×10−18 m2 (0.41 molecule/nm2) 29 is the surface occupied by a single adsorbed N719 molecule, and Ssubstrate the substrate area. Conventional liquid DSSCs were built using the nanostructured thin films as photoanode according to the stacking represented on Figure 1. The electrolyte consisted on a mixture of: Li (1 mol), LiI (0.05 mol), I2 (0.03 mol), GuSCN (0.1 mol), 4-tert-Butylpyridine (TBP) (0.5 mol); in ACN solution. The nanostructured TiO2 films were directly deposited on fluorine-doped SnO2 (FTO) conducting glass electrode covered by a magnetron sputtered 50 nm thick TiO2 layer. TiO2 electrodes (0.25 cm2) were annealed 30 min at 723 K before C106 dye grafting performed by immersion overnight into a solution of acetonitrile and tertbutyl alcohol (volume ratio: 1/1) containing C106 dye sensitizer (0.3 mmol) and 3R,7R-dihyroxy-5-cholic acid (Cheno) (2 mmol) to avoid the dye aggregation. After washing with acetonitrile and drying by air flow, the sensitized TiO2 electrode was assembled with a thermally platinized FTO electrode. The platinized electrodes were fabricated by coating two drops of H2PtCl6 (10 mg/ml) in isopropanol on the substrate and annealed at 648 K during 30 min. Both the photoanode and platinized electrodes were separated by a 25 µm thick thermoplastic sealing film made of Surlyn ® to prevent the electrolyte from leaking. The internal space was filled with a liquid electrolyte by using a vacuum backfilling system. The electrolyte-injecting hole on the counter electrode glass substrate, made with a sand-blasting drill, was sealed with a Surlyn ® sheet and a thin glass cover by heating. The photovoltaic performances of the cells were then measured under simulated AM 1.5 Global spectrum and 100 mW/cm2 (1 sun) illumination. Before proceeding to intensity-modulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) measurements, electrochemical impedance spectra


6

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(EIS) was conducted for checking the DSSC cells, a perturbation of 10mV was applied and impedance spectra were obtained from 100kHz to 50mHz. The electrochemical impedance measurements (EIS) were carried out using a Zahner Electrochemical Station CIMPS-2 system. IMVS under open-circuit conditions and IMPS under short-circuit conditions were performed using the same equipment. The electron transport time (τd) and the recombination time (τn) were estimated from the IMPS and the IMVS plots, respectively. A white-light-emitting diode (WLR02) from Zahner controlled by a PP211 (Zahner) frequency response analyzer was used and provided both the DC and AC components of the illumination. The AC component of the current to the LED generated a modulation (10%) superimposed on the DC light intensity in the range of [20 ; 300] W.m-2 with the frequency range from 100 to 0.010 Hz for IMPS and IMVS.

Figure 1: Sketch of a conventional DSSCs based on a nanostructured TiO2 photoanode 3 Results and Discussions 3.1 Optimization of the crystalline structure


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As a first step, a nanostructured TiO2 film with a slanted columns-based morphology was synthesized without intentional heating (α=85° - Ar and O2 injected at the target surface), and then characterized by GIXRD. The X-Ray diffraction pattern of the figure 2a indicates an anatase structure with a preferential orientation along the [101] direction. The crystallite size was evaluated using the well-known Scherrer formula to a value of ~13 nm, which is considerably smaller than usually obtained by solvothermal methods (few µm). It has to be noted that with an incident X-Ray angle of 0.5 the penetration depth of the beam was estimated higher than 500 nm meaning that the entire depth of the film was analysed. To get more information about the crystallinity inside the columnar structure, TEM was performed (Figure 2(b)). In line with GIXRD results, anatase phase was detected. However, it appears that the film is not homogeneously crystallized and is amorphous at the substrate interface and crystallized near the surface region. The late formation of the crystalline structure in the second half of the film can be the result of a progressive increase of the energy brought to the film due to an intense IR radiation emanating from the target during the deposition 30. This energetic contribution is related to the heating of the target surface and is thus expected to be more intense as the deposition time increases. It has to be noted that some columns are totally amorphous while others are crystallized over a few hundred nanometers from the surface region. This inhomogeneous crystallization of the material is not ideal in view of its application as photoanode in DSSC.


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(a)

(b)

Figure 2 : Evaluation of the crystalline structure of a nanostructured TiO2 films with a slanted column-based morphology: X-Ray pattern obtained by GIXRD with an incident X-Ray angle of 0.5 (2(a)) and TEM micrographs and selected area electron diffraction (SAED) pattern (2(b)). In the red square is shown the interface between the substrate and the thin film at high magnification. Indexed rings corresponding to the anatase structure (grey circles) were added to show that the experimental diffraction spots (from the columns included in the green circle) can be indexed with this structure.

The crystalline structure of TiO2 films is directly related to the energy brought to the growing film 31. In thin film science, conventional strategies to improve the crystallization of the deposited films, consist to bombard the growing film by the plasma using a controlled substrate bias voltage and/or to increase the growth temperature

32

. Nevertheless, we previously showed

that even if these parameters permitted an enhancement of the film crystallization, they led to its densification and the growth of a smaller number of thicker columns, respectively 27. Therefore,


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these parameters are expected to decrease the specific surface area which would be detrimental for the charge collection in the DSSC. To enhance the crystallite size, the as deposited films were post annealed at 773 K during 2 hours under ambient atmosphere. It has to be noted that for the following set of experiments, the films were deposited by injecting O2 at the substrate surface and Ar at the target one to limit its poisoning, and thus to increase the deposition rate

33

. The X-Ray diffractogramm presented

on Figure 3(a) clearly indicates a high level of crystallization in the anatase phase with a preferential orientation along the [101] direction. The chosen annealing temperature allows the atom diffusion leading to the material recrystallization by a local reorganization minimizing the system energy without affecting the film morphology at larger scale 34 while annealing at higher temperature (> 973 K) would allow to recrystallize the film in rutile phase 35. In order to confirm the high level of crystallinity inside the columns, TEM imaging and SAED were performed (see Figure 3(b)). The film was scratched with a diamond tip and a column was isolated. Several SAED patterns were acquired along this entire column. They evidence that the column material is crystallized in the anatase phase and fully oriented along the [101] direction. The crystal orientation is in line with the GIXRD results. Similar observations were made on other columns from the sample. We can therefore conclude that the post-annealing step allows the entire crystallization of the porous films, the columns being constituted by a pure anatase phase fully oriented in the [101] direction. As a consequence, all films used as photoanode for the solar cell fabrication were post treated according to the same procedure.


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(a)

(b)

Figure 3 : Evaluation of the crystalline structure of a nanostructured TiO2 films with a slanted column-based morphology post-annealed at 773 K during 2 hours: X-Ray pattern obtained by GIXRD with an incident X-Ray angle of 0.5 (3(a)) and TEM micrographs and SAED patterns (3(b)). The dotted circles denote the area of the electron beam spot for the corresponding SAED patterns. The simulated pattern of anatase structure with [-111] zone axis evidences that the growth direction of the column is [101].

3.2 Evaluation of the surface area enhancement Once the crystalline structure is optimized for DSSCs applications, the surface area enhancement (SAE) is evaluated by dye desorption. This value corresponds to the surface area per unit substrate area and represents a good approximation of the effective specific surface area 26, 36

. SAE evolution of nanostructured films with slanted columns, zigzag and pillars

morphologies is represented on figure 4 according to their thickness and for tilts angles, α, of 80° and 85°. It clearly appears that SAE rises linearly with the thickness of the slanted columns regardless α. This result is in line with the work of Krause et al. 36 and indicates that the column broadening


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has no impact in our particular experimental conditions. This effect appears for extremely thick films and is related to the column extinction as the film grows up, leading to unstable local shadowing 26. Also in line with the work of Krause et al., the SAE decreases for higher α values which could be due to an extreme shadowing regime. The latter involves a drastic change in the film morphology which consists on few large columns separated by a large inter-columnar space 26

. The dye loading evolves in line with the SAE from 0.3×10−8 to 1.3×10−8 mol/cm2

corresponding to the thinner and thicker films, respectively. This is in line with the values reviewed by Chen et al. for several TiO2 nanostructures (nanorods, nanowires, nanotubes and NPs): from 2×10−8 mol/cm2 to 9×10−8 mol/cm2 for photoanode thicknesses ranging from 2 to 11 µm 13. The higher values were obtained for film thicknesses higher than 8 µm. Khan et al. 4 also reported TiO2 NPs based films with dye loading capability of about 13 mol/cm2, which, when used as photoanode provided a conversion efficiency up to 8.1%. In this case, the photoanode was also considerably thicker than our film, i.e. 18 µm, which could explain the lower dye loading measured in our case. For a given thickness, SAE is not significantly affected by the modification of the nanostructure, i.e. comparing slanted columns, pillars and zigzag nanostructures. This is a surprising result since pillars and zigzag structures leads to an enhancement of the effective porosity as predicted by Dervaux et al.

28

for Ti material. This could be explained by a more

difficult access to the pores in the case of these structures because of the tortuosity of the architecture.


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Figure 4: Evolution of the specific surface enhancement (SAE) and the dye loading according to the thickness and the deposition angle of slanted column, zigzag and pillars based TiO2 films. These parameters were evaluated by dye desorption.

3.3 Photovoltaic performances of DSSCs based on optimized films The single crystalline and nanostructured TiO2 films were then used as photoanodes in conventional DSSCs. The three nanostructures (slanted columns, zigzag and pillars) synthesized for different α (80 and 85°) and thicknesses (from 1.25 to 4.3 µm) were investigated. The corresponding photovoltaic parameters, i.e. the fill factor (FF), the open circuit voltage (VOC), the short circuit current density (JSC) and the efficiency (η) are represented on Figure 5(a). Concerning the slanted columns-based photoanodes, the overall efficiency linearly increases with their thickness from 1 to 2.6 % and is not significantly affected by the value of α. This variation seems to be correlated to the JSC value since this parameter exhibits the same evolution and that the VOC and the FF marginally vary around ≈800 mV and 72%, respectively. The VOC is determined by the relative position of the conduction band edge of the TiO2 photoanode to the redox potential of the electrolyte and the charge recombination reactions 1. All photoanodes


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composing the tested cells were post-annealed under an identical procedure, and are therefore expected to exhibit the same crystalline structure. As this parameter likely mainly governs the conduction band position, a constant VOC for each cell is not surprising. On the other hand, the FF mainly depends on the quality of the interface between: i) the TiO2 and the FTO electrodes and ii) the grafted dye on TiO2 and the Pt electrode which is conditioned by the electrolyte diffusion between them. We have shown that by combining MS and GLAD, well-adherent and well-organized sculptured TiO2 films are deposited on the FTO layer providing a good contact 27. Moreover, the open surfaces generated by the columnar morphology ensures an efficient electrolyte diffusion throughout the thickness of the film. As expected, the higher JSC values (4.6 mA/cm2) have been obtained for the cells built using the thicker photoanode thanks to the larger dye density that can adsorb on the TiO2 surface. Indeed, the JSC value is directly correlated to the amount of light absorbed by the dye and converted into electric work by the cell. Similarly to the dye, the electrolyte would not penetrate in the zigzag and pillars structures as efficiently as in between the slanted columns, which could explained the slightly lower η and JSC values measured for these structures. In addition to the specific area, it has to be noted that the crystalline structure of the photoanode can also drastically influence the JSC value by affecting the charge transport and thus the charge collection efficiency. Nevertheless, as mentioned above this parameter should not significantly influence the measured values.


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(a)

(b)

Figure 5 : (a) Photovoltaic performances of liquid DSSCs integrating a slanted columns, zigzag and pillars based TiO2 film as photoanode and according to the thickness of the latter. (b) Plot of the cell effciency according to the corresponding JSC

The measurements of the photovoltaic parameters reveal that the overall efficiency of the designed DSSCs are mainly determined by the quantity of dye adsorbed on the photoanode through the corresponding JSC. It is demonstrated in Figure 5(b) since there is an almost linear correlation between the photocurrent density and the surface area whatever the type of nanostructure. η increases monotonically with the TiO2 film thickness up to 2.6% for the 4.3 µm thick film. Our best cell was compared to a reference DSSCs using the same architecture, but integrating a screen printed 9.5 µm thick layer of TiO2 NPs. This reference cell presents the following performances: η = 10.7 %, JSC = 20 mA.cm-², VOC = 752 mV and FF = 69%. The ~4 times higher efficiency of the NPs-based DSSC can directly be related to the ~4 times higher current density since the VOC and FF values are very close. Similar conclusions can be drawn by comparing our DSSCs to other devices based on TiO2 nanowires synthesized by solvothermal


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methods 23. The latter cells are composed by very thick photoanode (9.5 and 44 µm, respectively) offering a high dye loading, which leads to very high JSC, which seems to be the main parameter determining the overall efficiency. To get a better insight in the charge transfer dynamics, IMPS and IMVS measurements were used to determine the electron transport time (τtrans) and the recombination time (τrec,). The charge collection efficiency, representing the number of electrons collected at the transparent electrode per photogenerated electrons, was estimated according to ηcc=1-(τtrans/τrec)

37

. Three

slanted columns based DSSCs were characterized to evaluate the influence of the photoanode thickness and they were also compared to pillar, zigzag and NPs based DSSCs (Figure 6). First, for all the devices the transport time and recombination time become faster as the incident light power increases. This result stems from the fact that the number of injected electrons increases with light power and consequently raises the conductivity (more charge carriers in the conduction band) but also the probability of charge recombination of the injected electrons with the electrolyte. Interestingly, the NPs based devices exhibit shorter transport time, but also faster charge recombination time than pillar, zigzag and columns based cells. It is clear that the increase of the slanted columns-based photoanode thickness enhances the electron transport time. Indeed, the long travel distance of the photo-generated electrons to reach the FTO electrode naturally increases the probability of interfacial charge recombination with the electrolyte. Moreover, the charge collection efficiency of DSSCs based on the slanted column structures is lower than for the NPs based ones. In fact, as described by Anta, the ordering of the photoanode material is beneficial to the charge carrier collection efficiency only if a good balance in the improvement of the transport time and the recombination rate is found [5]. In the case of slanted columns structures, the increase of the photoanode thickness is clearly not the best strategy to


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improve the overall efficiency of the DSSCs even if a higher number of electrons can be generated, as shown by the increase of JSC. Nevertheless, it is interesting to note that pillar and zigzag structures offer a better carrier collection efficiency than NPs in spite of the longer transport time. This indicates that the photovoltaic performances of the DSSCs based on these structures, reported on Figure 5, are mainly limited by the quantity of grafted dyes. Therefore, by improving the dye impregnation, which does not penetrate in the entire film depth, one could expect a significant enhancement of the overall cell efficiency without increasing further the photoanode thickness.


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(a)

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(b)

(c) Figure 6: (a) Electron transport time, (b) recombination time and (c) charge collection efficiency of liquid DSSCs integrating slanted columns, zigzag and pillars based TiO2 films as photoanode and according to the thickness of the latter. They are compared to a conventional DSSCs integrating a 12µm thick NPs photoanode.

4 Conclusion


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In summary, nanostructured TiO2 photoanodes were synthesized by reactive magnetron sputtering at glancing angle followed by a post-annealing at ambient atmosphere. As shown by HRETM and electron diffraction on a reference nanostructure, i.e well separated slanted columns this strategy allowed to obtained individual fully oriented anatase nanocolumns. Different structures were then generated such as slanted columns, pillars and zigzag, and successfully integrated as photoanode in conventional liquid dye sensitized solar cells. The best cell was based on a slanted columns photoanode and exhibited an open circuit voltage of 0.8 V, a fill factor of 77% and a short circuit current density of 4.6 mA/cm2 (JSC) permitting to reach an overall efficiency up to 2.6%. This efficiency was considerably lower than the one of reference DSSCs based on a thick TiO2 NPs (DyeSol) photoanode, attributed to a higher quantity of dye molecules grafted on NPs.

IMPS and IMVS measurements clearly demonstrated that the

photovoltaic performances of the slanted columns based DSSC were not limited by the dye concentration, but by an unsuitable balance between charge transport and recombination. The latter measurements also proved the superiority of the zigzag and pillars based photoanode on the NPs ones, and this even the dye penetration into the nanostructures was not optimal as shown by dye desorption measurements.

AUTHOR INFORMATION Corresponding Author *Pierre-Antoine Cormier, [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Pierre-Antoine Cormier would thank the financial support of the Walloon Region through the Diag&Growth project. REFERENCES 1

Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-sensitized solar cells.

Chem. Rev. 2010, 110, 6595-6663 2

O’regan, B.; Gratzell, M. A low-cost, high-efficiency solar cell based on dye-sensitized.

Nature 1991, 353, 737–740. 3

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TOC Graphic

TOC Graphic: Working principle of a Dye Sensitized Solar Cells composed by a TiO2 and nanostructured photoanode.


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Single Crystalline-like and Nanostructured TiO2 Photoanodes for Dye

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