Electrodeposited ZnO with squaraine sentisizers as photoactive anode of DSCs
Iole Venditti1, Nadia Barbero2, Maria Vittoria Russo1,3, Aldo Di Carlo4, Franco Decker1, IlariaFratoddi1,5*, Claudia Barolo2*, Danilo Dini1*
1 Department of Chemistry, University of Rome “La Sapienza”, p.le Aldo Moro 5, I-00185 Rome,
Italy
2 Department of Chemistry and NIS, Interdepartmental Centre of Excellence, University of Torino,
via Pietro Giuria 7, I-10125 Torino, Italy
3Centro per la Protezione dell’Ambiente e dei Beni Culturali (CIABC), University of Rome “La
Sapienza”, p.le A. Moro 5, I-00185 Rome, Italy
4Center for Hybrid and Organic Solar Energy (CHOSE), University of Rome “Tor Vergata”
via G. Peroni 400/402, I-00131 Rome, Italy
5Center for Nanotechnology for Engineering (CNIS), University of Rome “La Sapienza”, p.le A.
Moro 5, I-00185 Rome, Italy
Corresponding authors email: ilaria.fratoddi@uniroma1.it; claudia.barolo@unito.it; danilo.dini@uniroma1.it
Abstract
The adsorption behavior of symmetrical indolenine-based squaraines, here indicated with VG1-C2 and VG1-C10, respectively, onto electrodeposited mesoporous zinc oxide (ZnO) was studied and compared with that of di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium (II) (N719). The choice of squaraines as dye-sensitizers was motivated by their far-red NIR sensitivity with respect to the traditional Ru complex-based dyes, the easiness of preparation and their higher molar extinction coefficient. This study was carried out with the aim of investigating and evaluating the performance of the dye sensitized solar cells (DSCs) assembled with ZnO anodes sensitized with different squaraine-based organic dyes and N719 metal complex. In the present analysis the process of sensitization of TiO2 with the same set of dyes and the
performance of the corresponding DSCs were also considered for sake of comparison. VG1-C2 sensitizer resulted a particularly effective electron injector in ZnO electrodes giving larger photocurrents in VG1-C2-sensitized ZnO with respect to TiO2. The latter system presented higher
kinetic stability of the photoinjected charges as evinced by the larger photovoltages of TiO2-based
DSCs with respect to ZnO-based devices with the same sensitizer. The long alkyl substituents in squaraine VG1-C10 inhibit electron injection in ZnO and a specific effect of electrical passivation
of the ZnO surface introduced by the bulky apolar groups was claimed. Overall efficiencies in the order of 1% were measured with the ZnO-based DSCs under AM 1.5 solar simulator when the photoactive area and the thickness of ZnO films were 0.5 cm2 and 2 m, respectively. The results
with ZnO here reported were particularly encouraging if one considers that TiO2 electrodes with
larger thickness and smaller areas produced DSCs with comparable performances.
Keywords: ZnO; DSCs, electrodeposition, squaraine, N719, sensitizer
1. Introduction
In the most recent years nanostructured materials have found wide use in a variety of application fields ranging from medicine[1] and photonics[2] to energy conversion.[3] In particular, with reference to the photovoltaic industry, the adoption of nanostructured materials as active components of energy conversion devices has been considered diffusely after Graetzel’s invention of the dye-sensitised solar cell (DSC).[4] The latter represents a low cost alternative to Si-based solar cells and utilizes nanostructured metal oxides with mesoporous features [5] for the efficient sensitization by dyes having appropriate electronic structure.[6] In DSCs the photoactive anode is mainly constituted by TiO2 and it represents the benchmark material for this type of application.[4,
7] As an alternative to TiO2, n-type ZnO has also been considered in anodic DSCs [8] due primarily
to the possibility of obtaining ZnO with a variety of nanostructures.[9] In addition to that, the research on ZnO as electrode material has been motivated also by the fact that it is a cheap and transparent wide bandgap semiconductor (Eg = 3.3 eV), with intrinsic n-type conductivity and high electron mobility in the order of 200 cm2 V-1 s-1 at room temperature.[10] In fact, ZnO is also used
as transparent conducting oxide (TCO) since charge carriers concentration can be increased in ZnO at a very large extent via doping of the oxide structure with chlorine[11] or trivalent atoms like aluminum,[12] gallium,[13] or indium [14] without affecting the transparency of the material. When compared to TiO2 the use of mesoporous ZnO as photoanodic material of a DSC [15] is
generally limited by poor electron injection [16] whereas ZnO presents advantages over TiO2 in
terms of electron mobility, carrier lifetime,[17] and electron diffusion lengths (> 10 μm).[16] The potential of ZnO as photoactive n-type anode in DSCs was firstly recognized in the late sixties by Gerischer and Tributsch who considered single crystals of the oxide as semiconducting material, and fluorescein and xanthene as dye sensitizers.[18] Later the need of increasing the area of the electroactive surface and, consequently, the extent of sensitization has directed the research towards
the preparation of transparent nanocrystalline structures of ZnO, the electronic structure of which could be polarized[19] and afford larger efficiencies of radiation conversion.[20] Nanostructured ZnO can be formed onto substrates of different nature like fluorine-doped tin oxide (FTO) [9c,d], indium tin oxide (ITO),[21] GaN,[22] Cu(Ga,In)Se2,[23] or Au,[24] with the most reproducible
nanostructures obtained by means of electrochemical deposition[25] among wet methods,[26] or by means of the solvent-free procedure of vapor phase synthesis.[12] Overall efficiencies of about 2.5 % could be obtained at 1 sun when nanoporous ZnO was sensitized with Ru-based Z907[27] or mercurochrome dye.[28] Further performance improvements have been possible when nanoporous ZnO was sensitised with a Ru-based dye {Ru[dcbpy(TBA)2]2(NCS)2} (N719), with DSC
efficiencies ranging from 6.5 [29] to 7.5% [30]. In the framework of DSC technology based on ZnO anodes,[8,9b,9c,28] the electrodeposition of ZnO thin films, made mesoporous by the co-precipitation and successive selective desorption of organic species like eosin Y (EY), is of particular interest [31] because it can be carried out at relatively low temperatures (< 100 °C) [25a] and warrants the property of electrical conduction in the resulting ZnO electrodeposits without any further heat-treatment of sintering above 350 ˚C. This represents an important advantage over the common coating processes from colloids, which require at some stage high-temperature sintering to neck the particles.[4,20,32] Therefore, the electrochemical approach overcomes the limitation of depositing mesoporous electrodes for DSCs exclusively on heat-resistant substrates, and, consequently, can be exploited for the production of DSCs on flexible plastic substrates possessing low softening point (< 150°C).[33,34] Moreover, the electrochemical deposition represents a low-cost synthetic choice due to the drastic reduction of energy consumption required for the elimination of the thermal processing step. In terms of performances the DSCs based on electrochemically deposited ZnO anodes could reach efficiencies as high as 5.6% when the electrodeposited ZnO film was used in combination with the organic dye D149.[25a] These facts motivated us to consider the electrochemical deposition of ZnO thin films with mesoporous and nanostructured morphology (thickness l < 10 m) as method of preparation of photoanodes for DSCs [24,35] utilizing FTO as optical conductive substrate.
The symmetric squaraines VG1-C2 and VG1-C10 (Figure 1) with their red-shifted absorption with respect to traditional Ru-based dyes[36] have been considered as sensitizers because of the easiness of their preparation and the lower costs of purification with respect to their unsymmetrical analogs. [37] In addition to that, the comparison of the DSCs performances based on symmetric and unsymmetrical squaraine sensitizers[38] has revealed that the symmetrical systems impart larger photocurrents and higher overall efficiencies (> 4 % with TiO2 anodes) with respect to the
higher recombination rates which only moderately decrease the photovoltages of the DSCs based on symmetric squaraines.[37a] To our knowledge this study represents one of the few attempts which employ nanoporous ZnO films deposited via an electrochemical route,[39] with organic dyes (squaraines) as sensitizers for DSCs purposes.[40] For the first time it is here utilized with ZnO a series of squarainic dye-sensitizers containing two carboxylic groups that are both electronically conjugated with the chromogenic unit (Figure 1). This is an advancement with respect to the common practice of sensitizing ZnO with squarylium dyes having a single anchoring carboxylic group that does not communicate with the light absorbing moiety of the sensitizer.[25a,40b] The approach here reported is expected to favor the process of electron transfer from the light-absorbing unit to the oxide surface following excitation because of the direct involvement of the anchoring units in the formation of the conjugated path that delivers the photoelectron.
Figure 1
2. Experimental Details2.1 Materials and instruments
All of the solvents and chemicals, unless stated otherwise, were purchased from Fluka, pure grade. N719 dye was purchased from Solaronix and used as received. TiO2 films (0.7 x 0.7 cm2; thickness
6 m) on FTO substrates have been prepared via sintering of a commercial paste of titania for screen printing from Solaronix following to the procedure reported in the literature.[41]
Squarainic dyes have been prepared according to literature procedure.[37a] FTIR spectra of the squaraines have been recorded with a Bruker Vertex 70 spectrophotometer with the dyes either mixed in Nujol mulls or casted as films from CH2Cl2 solutions utilizing KRS-5 cells. UV-vis
spectra were taken with a Varian Cary 100 Scan UV-vis spectrophotometer the samples being solutions of the dye in CHCl3 in quartz cuvettes. The molar extinction coefficients (o, L mol-1cm-1)
were measured in the concentration range of 5.0 × 10–6 ~1.8 × 10–4 M, and the values are given with
± 5% experimental error. FE-SEM images have been acquired with the Auriga Zeiss instrument (resolution 1 nm, applied voltage 6-12 kV) on freshly prepared films drop casted from CH2Cl2
Electrochemical experiments were carried out with an Autolab potentiostat/galvanostat driven by the Autolab software Nova 1.9.
2.2 ZnO electrodeposition on FTO
The electrodeposition of ZnO has been carried out according to the following procedure. The glass/FTO substrate (area: 2 x 2 cm2) was cleaned in ultrasonic bath with acetone and ethanol for 15
minutes. ZnO was deposited through the electrolysis of a solution of 5 mM ZnCl2, 0.1M KCl in
distilled water (500 mL) under potential control. The electrolysis has been carried out in the potentiostatic mode at the potential of -1.0 V vs Ag/AgCl for different deposition times, ranging from 900 to 5400 s of electrolysis. The active area of the FTO substrate for electrodeposition was 0.7 x 0.7 cm2. The electrolysis solution was maintained at 80°C under stirring to ensure a
homogeneous concentration of O2 in the order of 10-4 – 10-3 M [42] throughout the electrolyte. The
electrolysis cell had a three-electrode configuration with FTO-covered glass (from Solaronix) as working electrode, a silver coil and Ag/AgCl in 1M KCl solution (+0.24 V vs. NHE) as counter and reference electrode, respectively. Initially a compact ZnO layer was deposited on FTO/glass substrate. After 1/4 of the total electrodeposition time, 50 μM EY saturated with O2 has been added
and a nanoporous ZnO film was formed on top of the compact ZnO layer. After electrodeposition, the ZnO film was rinsed with water at 80°C and then slowly cooled down to room temperature. The EY co-deposited with ZnO was desorbed with a KOH solution 0.1 mM at pH 10 for 24 h. The film was heated at 250 °C in air for 3 h before use.
2.2 Dyes loading
The dye loading has been carried out by dipping the electrodeposited ZnO into dye solution for 2 h at room temperature in the dark. The solutions of VG1-C2, VG1-C10 and N719 at different concentrations (10-4, 10-5 and 10-6 M) in a EtOH solution, have been prepared. Loading has been
calculated by using a desorption procedure described in the following, and using an UV-vis calibration curve for each dye. Molar extinction coefficients (o, L mol-1cm-1 ) at the respective
wavelengths of maximum absorption have been estimated and the values are: o(VG1-C2) = 3.13 ±
0.10 x 105;
o (VG1-C10) = 4.78 ± 0.11 x 105; o(N719) = 5.50 ± 0.11 x 104. The desorption protocol
was optimized for VG1-C2 and VG1-C10 by dipping the dye loaded ZnO substrates in an ethanol solution of KOH 0.01M for 15 min. In the case of N719 desorption procedure, a 0.1 M glycine solution in water was used. Glycine solution was buffered at pH = 10 and the sensitized substrates were immersed in it for 5 min.
2.3 Cell assembly
The fabrication procedure for solar cells, the testing conditions, and the equipment have been reported elsewhere.[43] TiO2 and ZnO samples have been assembled into photovoltaic cells and
characterized at the Center of Hybrid and Organic Solar Energy (CHOSE, Tor Vergata).
3. Results and Discussion
In order to optimize the electrochemical deposition of zinc oxide onto a FTO glass substrate, a cyclic voltammetry of the electrolysis cell has been carried out (see Supporting Information). Cyclic voltammetry allows the determination of the potential value at which O2 is reduced to OH- and
triggers the precipitation of Zn(OH)2 on the FTO cathode surface (see Supporting Information).
This determination is crucial since the reduction of Zn2+ to metallic Zn might interfere with that of
O2 when electrolysis is carried out at pH > 0.[42] Actually the reduction of Zn2+ constitutes an
unwanted process during zinc oxide precipitation because it diminishes the concentration of Zn2+
cations at the electrode surface and prevents the formation of thick and well adherent zinc oxide deposits.[42] In the adopted experimental conditions the onset of O2 reduction is located at -0.4 V
vs Ag/AgCl whereas the cathodic reaction starting at – 1 V vs Ag/AgCl corresponds to the reduction of Zn2+ to metallic Zn (see Supporting Information). This implies that the potential Eappl of
electrolysis must be controlled carefully within the window -1 ≤ Eappl ≤ -0.4 V vs Ag/AgCl for achieving zinc oxide electrodeposits with high efficiency of charge-to-deposit conversion. The unambiguous assignment of the reduction processes observed in the cyclic voltammetries of the electrolysis cell has been possible because of the disappearance of the cathodic wave at -0.4 V vs Ag/AgCl when the electrolysis cell was kept in Ar atmosphere (see oxygen-free electrochemical experiment in Supporting Information). The complex mechanism of ZnO electrodeposition involves first the reduction of O2 according to the reaction
O2 + 2H2O + 4e- 4OH- {1}
which brings about a local increase of pH at the electrode/electrolyte interface.[44] Reaction {1} is followed by the formation of zinc hydroxide
Zn2+ + 2OH- Zn(OH)
and by the successive precipitation of zinc oxide on the electrode as the product of zinc hydroxide de-hydration [45]
Zn(OH)2 ZnO + H2O {3}
For the formation of ZnO electrodeposits with nanoporous structure Eosin Y was added in the electrolyte.[46] The temperature of the electrolytic bath was maintained at 80°C in order to ensure the deposition of ZnO and avoid the formation of the hydroxide as a by-product of precipitation. [47] Potentiostatic conditions have been used in order to achieve the sole nucleation of ZnO on the FTO susbtrate.[42,48] The duration of the electrodeposition process ranged between 900 and 5400 min. The maximum value of the electrodeposited charge density was 8.34 C cm-2, which
corresponded to the growth of a 2.5-3.0 m thick film. Table 1 reports the data correlating the parameters of ZnO electrochemical deposition with ZnO film properties when the oxide was grown in absence of EY.
Table 1
When ZnO electrodeposition is carried out in presence of EY the charge of electrolysis is partially consumed by the process of EY reduction, which brings about the adsorption of the anions (EY)
-and (EY)2- on the surface electrode. These adsorbed anions combine with the Zn2+ cations at the
electrode/electrolyte interface, and favor the further formation of ZnO.[49]
The successive desorption of EY from ZnO precipitate is carried out by immersing the hybrid film ZnO/EY in 0.1 mM KOH solution for 24 h. This step has been conducted with particular care in order to avoid also the partial dissolution of the ZnO layer in the alkaline solution.[50]
3.1 Mesoporous ZnO on FTO
The morphology of the electrodeposited ZnO samples has been determined by means of FE-SEM analysis. The films obtained with different durations of deposition presented a thickness ranging between 1.0 and 3.0 m (Table 1). Figure 2 shows the FE-SEM images of the samples obtained at the deposition times 900 and 2700 s, and the profile of the ZnO film deposited at 2700 s. The ZnO sample deposited in 900 s presents scarce adhesion on the FTO substrate due to the presence of
voids at the ZnO/FTO interface (Figure 2a). This feature can give rise to the undesired effect of shunting which consists of the direct contact between the FTO substrate and the electrolytic solution. Electrodeposited films grow generally with non uniform thickness (Table 1). For example, the thickness of the ZnO layer grown for 900 s is comprised in the range 0.8-1.4 m (Table 1). Upon prolongation of the electrodeposition time, the adhesion of the resulting ZnO films improves as verified with the sample obtained after 2700 (Figure 2c) and 5400 s (not shown) of electrolysis. In these latter cases the ZnO layers also present more uniform thickness than the samples grown with shorter deposition times (Table 1). All ZnO samples here considered present an extended surface area as proved by the presence of cracks and nanopores with diameters in the order of 10-20 nm (Figures 2b and d). These morphological features are a direct consequence of the removal of the EY template,[25a,36] and will result useful for the efficacious sensitization of the ZnO layer through the surface adsorption of an opportune dye-sensitizer (vide infra). The profilometer trace of the ZnO sample grown with 2700 s of electrolysis allows the direct evaluation of the film thickness (2.0 m, Figure 2e). The active surface area of mesoporous ZnO films increases linearly with thickness roughly by a factor of 150 per every micron of electrochemically grown film within the thickness range 2-10 m,[51] and corresponds approximately to 1000 times the geometrical area of the ZnO electrode due to the presence of pore and cracks as verified with FE-SEM imaging (Figure 2).
Figure 2
3.2 Squaraine dyes characterizationWhen compared to Ru-polypyridyl complexes, organic dye-sensitizers for DSC electrodes present several advantages like higher molar extinction coefficients in the visible range, larger spectral sensitivity including the NIR region, easier procedures of purification and disposal, and lower costs of large-scale production. Among organic dyes, squaraines are well known for their photothermal stability and the tunable optical absorption in the red and NIR regions.[37,38] The two symmetrical squaraines VG1-C2 and VG1-C10 here considered (Figure 1) are characterized by the presence of two carboxylic anchoring group and differ in the length of the alkyl chain.[37a,52] FTIR spectra of the VG1-C2 and VG1-C10 dyes confirmed the reported structures, with the stretching bands of the carboxyl and carbonyl moieties at about 1724 and 1701 cm-1. The aryl system shows bands at 1607
UV-vis spectra of VG1-C2 and VG1-C10 in EtOH solution present the most intense absorption peaks at 643 and 646 nm (Figure 3), with corresponding ε0 values 3.13 and 4.78 x 105 L mol-1cm-1,
respectively. The cyclic voltammetries of VG1-C2 and VG1-C10 dissolved in acetonitrile (ACN) electrolyte are reported in the Supporting Information. The one-electron oxidation of VG1-C2 and VG1-C10 presented non reversible features with the oxidation peaks located at 0.12 and 0.73 V vs Fc+/Fc for VG1-C2 and VG1-C10, respectively. Such a difference cannot be correlated with the
different electron-releasing properties of the ethyl and decyl chains in the squaraines under investigation. Therefore, the formation of intermolecular aggregates for the squaraine with the larger substituent and, eventually , the occurrence of specific interactions between VG1-C10 and the bulky alkylammonium cation of the electrolyte have to be taken into account for explaining the observed differences in the electrochemical behaviors of the two differently substituted dyes.
3.2 Dye loading on ZnO photoanodes
The control of the conditions under which dye adsorption is carried out is crucial for the effective sensitization of mesoporous ZnO for DSCs.[15a] The first part of this study was aimed to the comparative evaluation of dye-loading on electrochemically grown ZnO when VG1-C2, VG1-C10 and N719 were used. The procedure has been carried out on electrodeposited ZnO and TiO2
photoanodes having similar thickness. The adsorption procedure consists in the immersion of the ZnO (or TiO2) substrate in solution of the dye in ethanol for 2 h. The sensitized samples were
characterized with UV-vis spectroscopy (Figure 3), and the absorption properties of VG1-C2- and VG1-C10-sensitized ZnO were compared with those of VG1-C2 and VG1-C10 in solution.
Figure 3
When squaraine dyes are adsorbed onto a ~ 2 m thick film of ZnO, the UV-vis spectra of the sensitized system present a wide enlargement of the main absorption peak due to aggregation phenomena that alter the electronic structure of the dyes.[53] Moreover, the electronic interactions between the functional carboxylic groups of squaraines and the anchoring sites of the ZnO surface constitute another cause of UV-vis spectral alteration in passing from the anchored to the dispersed state of the dye in solution, as verified with sensitized TiO2 photoanodes.[38a,54]
Successively, the desorption procedure has been followed (vide supra), and the UV-vis spectra of the solutions of the desorbed dyes have been recorded in order to evaluate the amount of loaded dye. The concentration of dye-sensitizer in the solutions of desorption has been determined in a
quantitative way by means of the calibration curve obtained from the absorption spectra of solutions with known concentration of the sensitizers. The amount of dye in the desorption solutions was utilized to calculate the concentration of sensitizer in the electrode by taking into account the volume of the electrodeposited ZnO (VZnO = 1x10-4 cm3). This was determined through the
profilometric analysis of the ZnO samples employed in the DSCs (vide infra), and the FE-SEM pictures of the cross-sections of the same samples (Figure 2). The volume of the TiO2
photoelectrodes taken as terms of comparison was 3x10-4 cm3 (VTiO2 = 0.7 cm x 0.7 cm x 6x10-4 cm).
In Table 2 the average values of dye loading are reported for electrodeposited ZnO and screen-printed TiO2 sensitized with VG1-C2, VG1-C10 and N719. The evaluation of dye loadings was
accomplished on the same type of photoanodes utilized in the DSCs (vide infra). The sensitization of ZnO and TiO2 anodes has been carried out at the two different concentrations of sensitizer 10-4
and 10-5 M in ethanol (Table 2).
Table 2
The amount of sensitizer loaded on the ZnO electrodes was in the order of 10-5 moles per cubic
centimeter of oxide when the sensitizing solution had a dye concentration of 10-4 M. Upon decrease
of sensitizer concentration ([Dye] = 10-5 M) the amounts of VG1-C2 and VG1-C10 loaded by ZnO
diminished of about one order of magnitude (Table 2). On the other hand, the upload of N719 sensitizer on ZnO is rather insensitive to the concentration of the dye and indicates the higher reactivity of N719 with respect to VG1-C2 and VG1-C10 dyes in the reaction of ZnO surface esterification[15a] when more diluted conditions are adopted. The uptake of dye-sensitizer per volume unit of electrode is generally higher for TiO2 than for ZnO, which is probably due to the
higher porosity of screen-printed TiO2 layers vs electrodeposited ZnO.[37b]
3.3 DSCs with squaraine-sensitized ZnO
Anodic DSCs have been assembled with electrochemically grown ZnO (thickness: 2 m), which has been sensitized with the organic dyes VG1-C2, VG1-C10 (Figure 1), and Ru-based N719 chosen as a comparative term. The results (Figure 4) have been compared with those of the DSCs derived from 6 m thick films of TiO2 sensitized with the same set of dyes (Table 3). The
discrepancy between the thickness values of electrodeposited ZnO and screen-printed TiO2 is
directly related to the method of photoanode deposition and cannot be overcome. In fact, the electrochemical method used for ZnO growth cannot afford nanostructured layers in a reproducible
way when ZnO thickness gets larger than 2.5-3 m, [35b] whereas screen-printing is generally not suitable for the deposition of uniform TiO2 layers thinner than 5-6 m.[42]
Figure 4
Table 3
Electrochemically grown ZnO presents a relatively large short-circuit current density, JSC, with respect to TiO2 when the two differently prepared oxides are sensitized with VG1-C2 (Table 3).
This is because JSC values are comparable (JSC = -3200 and -3600 A m-2 for VG1-C2-sensitized
ZnO and TiO2, respectively) despite the fact that ZnO thickness is about three times smaller than
that of TiO2. This behavior reflects the larger electronic conductivity of ZnO with respect to TiO2
and a remarkable efficiency of injection of the VG1-C2 dye in ZnO.[10,17] On the other hand, the larger photovoltage of VG1-C2-sensitized TiO2 with respect to a thinner ZnO anode sensitized with
the same dye (596 vs 416 mV, see Table 3), indicates that TiO2 has considerably less
recombination of the photoinjected electrons with respect to ZnO. In passing from C2 to VG1-C10 sensitizer (Figure 1) TiO2 electrodes of titania display analogous properties of electron
photo-injection, recombination and charge transport as proved by the similar values of VOC and JSC, respectively (Table 3). This is not the case of ZnO electrodes since the VG1-C10 sensitization introduces an evident effect of electrical passivation of the ZnO surface, which is ascribable to the long alkyl chains of the dye as demonstrated by the dramatic decrease of Jsc (JSC = -3200 and -730 A m-2 for VG1-C2- and VG1-C10-sensitized ZnO, respectively, Table 3). The bulky substituents
of VG1-C10 would prevent the injection of electrons from the light absorbing moiety of the excited dye to the empty electronic states of ZnO.[55] Such a dramatic effect of passivation by alkyl groups specifically in ZnO might be a consequence of a considerably different surface polarity of the mesoporous ZnO obtained via electrochemical growth with respect to that of screen-printed TiO2.
[56] Moreover, the Van der Waals interaction between VG1-C10 alkyl chains and eventual residuals of co-deposited EY still anchored on ZnO after alkaline treatment cannot be completely excluded in the present analysis of ZnO surface insulation. There is also an indication that the absorbed decyl groups bring about a prolongation of photoinjected electrons lifetime in ZnO by reducing the rate of electron recombination with oxidizing species in the electrolyte. This is because Voc does not decrease as dramatically as JSC when the squaraine sensitizer on ZnO has a longer alkyl substituent (VOC = 416 and 355 mV for VG1-C2- and VG1-C10-sensitized ZnO, respectively, Table
3)[55] The DSCs that differ solely for the nature of the sensitizer present offer comparable performances when VG1-C2 and N719 are compared. In general N719-sensitized electrodes produce DSCs with higher open circuit photovoltages and larger overall efficiencies in comparison to the DSCs with VG1-C2-sensitized photoanodes irrespective of the nature of the n-type oxide (Table 3). This common trend is a consequence of the higher rate of electron transfer between the oxidized version of N719 dye and the reduced form I- of the redox shuttle with respect to oxidized
VG1-C2.[57] The more rapid neutralization of the dye prevents more efficaciously the back diffusion of the photoinjected electrons from the anode to the oxidized dye and originates DSCs with relatively larger photovoltages/fill factors as verified with both N719-sensitized cells.
The overall efficiencies of the DSCs we have tested are affected by the utilization of photoactive electrodes having a relatively larger active area (0.50 cm2) with respect to that of the photanodes
commonly utilized in the best performing DSCs (< 0.3 cm2).[37b] In fact, the area of the
photoelectrode affects heavily the performance of the corresponding DSCs with a rapid decay of the photo-generated power per area unit with increasing device area.[58] In addition to that, the thickness of our electrodeposited ZnO sample was 2 m, i.e. thinner than the oxides sensitized with the squaraines reported so far.[37e] Therefore, the results here presented can be reasonably considered as a progress for the development of DSCs with alternative anodic materials and organic dyes with convenient procedures of deposition/preparation.
4. Conclusions
This work has proved that electrochemically grown ZnO possesses mesoporous features useful for its exploitation in DSCs as competitive anodic material alternative to TiO2 when squaraines are the
dye-sensitizers. This is particularly evident with VG1-C2 sensitizer which injects photoelectrons with higher efficiency in electrodeposited ZnO with respect to TiO2. This finding is partially offset
by the higher kinetic stability of the photoinjected charges in VG1-C2-sensitized TiO2 with respect
to ZnO as proved by the larger photovoltages of TiO2-based DSCs. Interestingly, we also found that
the presence of a long alkyl chain in the squaraine-based sensitizer VG1-C10 gives much poorer injection in ZnO whilst the performance of the DSC assembled with VG1-C10-sensitized TiO2
results practically unaffected by the variation of the squaraine substituent. This was attributed to the occurrence of electrical passivation of the ZnO electrode due to a specific interaction between the decyl groups of VG1-C10 and the surface of electrochemically grown ZnO. The present study also shows that there is room for a further improvement of the anodic DSCs that utilize electrodeposited ZnO as photoactive electrode. For example, the electrical contact between the TCO substrate and
the mesoporous ZnO grown with the electrochemical procedure could be ameliorated by using a TCO based on doped ZnO oxide.[51] This will be warranted by the much better matching of the crystalline cell parameters of doped ZnO (the substrate) and electrodeposited ZnO (the photoactive layer), and would lead to a general increase of the current density in DSCs devices based on ZnO photoanodes.
Acknowledgements
This research project was supported by Regione Lazio and CHOSE. The authors acknowledge the financial support of Ateneo Sapienza 2011/VG1-C26A11PKS2. This work was partially supported by the Dipartimento di Chimica, Sapienza Università di Roma through the Supporting Research Initiative 2013.
NB and CB gratefully acknowledge financial support by DSSCX project (PRIN 2010-2011,
20104XET32) from Ministero dell’Istruzione, dell’Università e della Ricerca and the University of Torino (Ricerca Locale ex-60%, Bando 2012). N.B. thanks MIUR for partial financial support of her Research grant.
