ARTICLE pubs.acs.org/JPCC
Enhanced Performance of Flexible Dye-Sensitized Solar Cells: Electrodeposition of Mg(OH)2 on a Nanocrystalline TiO2 Electrode T. A. Nirmal Peiris, S. Senthilarasu, and K. G. Upul Wijayantha* Department of Chemistry, Loughborough University, Loughborough, LE11 3TU, U.K.
bS Supporting Information ABSTRACT:
Nanocrystalline TiO2 photoanodes were prepared on a conductive indiumtin oxide coated polyethylene naphthalate (ITO-PEN) plastic substrate by the doctor-blade method to fabricate flexible dye-sensitized solar cells (DSCs). The surface of the photoanode was coated with Mg(OH)2 by electrodeposition and the deposition time was systematically varied (2, 4, 6, 8, and 10 min). Electrodeposited Mg(OH)2 was confirmed by IR and energy dispersive X-ray (EDX) analysis. The surface morphology was studied by scanning electron microscopy. The internal surface area of TiO2 was studied against the deposition time by taking into account the projected surface area of the photoelectrode and it shows that the internal surface area of the photoelectrode was reduced as the Mg(OH)2 deposition time increased. The performance of flexible DSCs on various deposition times of Mg(OH)2 was evaluated on the basis of their photocurrent densityvoltage characteristics. Among the deposition times, 2 min showed the best performance in Voc on a treated flexible DSC, with resulting 847 mV and a photocurrent density of 7.13 mA/cm2, providing an overall light-toelectricity conversion efficiency of 4.01%. This photovoltage is among the highest attained for a flexible DSC to date. This notable increment in Voc at a thin layer of Mg(OH)2 was attributed to the suppression of recombination of photogenerated electrons via the exposed surface of ITO as well as TiO2 without influencing the internal surface area of the photoanode significantly.
’ INTRODUCTION The solar conversion efficiency of lab scale dye-sensitized solar cell (DSC) is currently in the order of 11%,1 which is significantly lower than the theoretically estimated maximum.2 The difference between theoretical and experimental efficiencies appears to be partly related to the charge loss occurring at the TiO2/electrolyte and conducting F-SnO2 (FTO)/electrolyte interfaces through a process generally referred to as the back reaction.3 According to Cameron et al.,4 there are two possible ways to back transfer electrons to electrolyte in a DSC under open circuit illumination conditions, via TiO2 network and via transparent conducting fluorine-doped tin oxide (FTO) substrate. The back reaction in DSCs considers the generation of iodide species at TiO2/electrolyte and FTO/electrolyte interfaces via the reaction of tri-iodide species and electrons. In a DSC, the porous TiO2 layer is permeated by the tri-iodide/iodide redox liquid electrolyte, so the injected photoelectrons in the conduction band of TiO2 can be transferred to I3 which is mainly responsible for reducing the cell performance.5,6 r 2011 American Chemical Society
During the past decade, many attempts have been made to enhance the overall cell performance of DSCs by retarding back transfer of photoinduced electrons through the TiO2/dye/ electrolyte interface by surface modification of TiO2 using insulating metal oxides7 and hydroxides8 or high band gap semiconductors that form a blocking layer between the dye sensitizer and TiO2 layer to block the back electron flow towards the electrolyte species. For instance, Palomares et al.7 found that the overall efficiency and Voc of Al2O3-coated TiO2 employed in DSC is increased notably compared to the cells made with uncoated TiO2. Kumara et al.9 and Tauguchi et al.10 also reported that a thin layer of MgO on TiO2 suppresses the recombination and hence improves the cell efficiency. Yum et al.8 claimed that the thin layer of Mg(OH)2 functions as the blocking layer at the FTO and TiO2 interfaces, thus improving Voc. In addition to retarding the Received: August 26, 2011 Revised: October 24, 2011 Published: November 18, 2011 1211
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The Journal of Physical Chemistry C recombination, the use of metal oxide based overlayers such as Al2O37 and MgO11 is beneficial in terms of improving the adsorption properties of organic dye molecules, thereby keeping the light-harvesting properties of DSCs. Today DSCs prepared on flexible, thin and lightweight conducting plastic films are receiving wider attention because of their potential for new applications.12 Generally the performance of plastic substrate-based DSCs is relatively lower than their glass substrate-based counterparts. This is mainly due to the poor thermal instability of known plastic conducting substrates (i.e., PET, PEN) which prevents annealing the substrates at the standard sintering temperature (∼450 °C) that is generally required for necking individual TiO2 nanoparticles.13 In addition to that, the poor adhesion between TiO2 nanoparticles and ITO-coated plastic substrates also leads to lower power conversion efficiencies, compared to rigid, glass-based DSCs.12 Although surface coatings have been used as a strategy to retard the back electron transfer occurring in glass substrate based DSCs, this method has yet to be tested for flexible TiO2 electrodes and its DSC performance. Growing a conformal Mg(OH)2 insulating coating by the electrodeposition on the surface of TiO2 at room temperature and evaluating the ability of such an insulating layer to retard interfacial recombination and thereby modulate the performance of flexible DSCs were the main aims of this work. In the present work, we report the first example of surface coating of nanocrystalline TiO2 (prepared on flexible ITO-PEN substrates) with Mg(OH)2 by electrodeposition.
’ EXPERIMENTAL METHODS Nanoporous TiO2 thin film electrodes were prepared from a colloidal suspension of TiO2 nanoparticles without any binders. The colloidal suspensions were dispersed on the ITO-PEN substrates (13 Ω/cm2, Peccell Technologies, Inc., Japan) by doctor blade and compression methods.14 TiO2-coated ITO-PEN substrates were then heated at 140 °C for a 30 min period on the surface of a hot plate.15 The Mg(OH)2 coatings were electrodeposited on the TiO2 surface in an aqueous electrolyte solution composed of Mg(NO3)2 3 6H2O (Sigma Aldrich) having a concentration of 0.01 M. The electrodeposition was carried out in the three electrode configuration using the TiO2-coated ITO-PEN substrate as the working electrode with the cathode area of 0.16 cm2, Ag/AgCl electrode and Pt as the reference and counter electrodes, respectively. The working and counter electrodes were placed parallel to each other separated by a distance of approximately 1 cm in the electrodeposition solution. The electrodeposition was conducted at a constant current of 0.6 mA (chronopotentiometry) using a Potentiostat-Galvanostat (Eco Chemie micro-Autolab type III). After the deposition, the films were removed from electrolyte solution, washed with distilled water and allowed to dry at room temperature. The electrodeposition time was varied in such a way that a series of TiO2 films were obtained for 2, 4, 6, 8, and 10 min coatings of Mg(OH)2. The Mg(OH)2-deposited TiO2 films were soaked overnight in an ethanolic solution of 1 106 M N719 (ditetrabutylammonium cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylate) dye (Solaronix SA), sandwiched with a platinized conducting counter electrode prepared on the ITO-PEN substrate using a Surlyn frame (Solaronix SA), filled with the electrolyte through a hole in the counter electrode and sealed. The iodide/tri-iodide
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electrolyte comprising 0.4 M LiI, 0.4 M tetrabutylammonium iodide (TBAI) and 0.04 M I2 dissolved in 0.3 M N-methylbenzimidazole (NMB) in acetonitrile (AN) and 3-methoxypropionitrile (MPN) solvent mixture at a volume ratio of 1:1 was used. The surface morphology of the TiO2 films was studied using a Leo 1530 VP field emission gun scanning electron microscope (FEG-SEM) at an accelerating voltage of 5 kV and a working distance of 5 mm. Steady-state currentvoltage measurements of the cells were carried out using a potentiostat (Eco Chemie micro-Autolab type III), while the cells were illuminated by an AM 1.5 Class A solar simulator (Solar Light 16S 300 solar simulator), at 100 mW cm2 light intensity, calibrated by a silicon pyranometer (Solar Light Co., PMA2144 Class II). Incident photon to electron conversion efficiency (IPCE) measurements were conducted using a 75 W Xenon lamp connected to a monochromator (Bentham, TMc300) and the system was calibrated using a UV-enhanced silicon photodiode (Bentham). The IPCE spectra of DSCs were recorded while the cells were illuminated through the substrate side over the 300800 nm spectral range, using a chopping frequency of 5 Hz. The nature of the TiO2 surface before and after the electrodeposition step was examined by infrared absorption spectroscopy (Spectrum 100, FTIR spectrometer, Perkin-Elmer) and the content of all electrodes was analyzed by the energy-dispersive X-ray (EDX) analysis (Cambridge S-360 SEM). The internal surface area (effective dye adsorption area) of the photoanode was estimated by conducting dye adsorption/desorption measurements.16 The concentration of the desorbed dye in a 0.1 M NaOH solution was recorded by measuring the absorbance (Lambda 35 Perkin-Elmer UVvis spectrometer). Prior to recording the absorbance spectra, the solution was acidified by adding an appropriate amount of 0.1 M aqueous HCl solution.
’ RESULTS AND DISCUSSION Electrodeposition of Mg(OH)2. Coating the surface of nanocrystalline TiO2 with other suitable metal oxides for DSC application using various thin film coating techniques has received wider attention because of the improvements made in key cell parameters after such coatings.79 Among the coating techniques, the electrodeposition is considered to be a versatile technique for producing surface coatings, owing to its precise controllability, room temperature operation, rapid deposition rates and relatively low cost.17 Therese et al.18 and Dinamini et al.19 have used electrodeposition to prepare Mg(OH)2 thin films on metallic substrates from Mg(NO3)2 and MgCl2 solutions, respectively. In the present work, electrodeposition of Mg(OH)2 was conducted on the surface of nanocrystalline TiO2 films (deposited on ITOPEN flexible substrates) from an aqueous solution of Mg(NO3)2 by following a recipe reported by Zou et al.20 The electrochemical reactions that more likely take place at the cathode are listed below (eqs 1 and 2).
NO3 þ H2 O þ 2e f NO2 þ 2OH
ð1Þ
2H2 O þ 2e f 2OH þ H2
ð2Þ
These reactions cause a steep increase of local pH in the electrodeposition solution close to the cathode, leading to the precipitation of Mg(OH)2 due to the poor solubility of Mg(OH)2 (eq 3) (Ksp of Mg(OH)2 is 1.2 1011 mol3 dm9 at room temperature). The previous in situ measurements have 1212
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shown that the interfacial pH near the cathode surface during the electrodeposition of Mg(OH)2 is around 9.20,21 Mgþ þ 2OH f MgðOHÞ2
ð3Þ
A simple laboratory experiment was conducted to test the hypothesis of precipitation of Mg(OH)2 by adding a known volume of NaOH (hence known ionic strength of OH) to the Mg(NO3)2 electrodeposition solution, which articulated the formation and flocculation of Mg(OH)2 in the solution phase. During the electrodeposition, flocculated Mg(OH)2 in the solution phase heterocoagulated on ITO and TiO2 surfaces. The heterocoagulation of Mg(OH)2 on ITO and TiO2 surfaces can be explained by using the schematic zeta potential curves described in Figure 1. The sign of surface charge flips at a specific pH, i.e., iso-electric point (surface contains no net electric charge) and the electrostatic interaction, is inverted at the iso-electric point. The iso-electric point of Mg(OH)2 is 12,22 whereas it is 6 for ITO23 and 6.511 for TiO2. The zeta potential values of ITO and Mg(OH)2 showed that the ITO was negatively charged in the alkaline pH range and Mg(OH)2 was positively charged below pH 12.22 When the local pH reaches below 12 near the cathode surface where Mg(OH)2 is precipitated, the electrostatic attraction between ITO and Mg(OH)2 likely leads to a coating of Mg(OH)2 on the surface of ITO. At the same time, the net negative charge on the
Figure 1. Schematic representation of the zeta potential of TiO2, ITO and Mg(OH)2 against the solution pH.
TiO2 surface causes a formation of Mg(OH)2 coating on TiO2 as well. Electrode Surface Morphology. The surface topographic FEG-SEM images of bare and Mg(OH)2-treated TiO2 electrodes appeared distinctly different, as shown in Figure 2. The SEM images shown in Figure 2 describe the morphology changes that occurred as the deposition time is gradually increased. Figure 2 (t = 0 min) shows the topography of a bare nanocrystalline TiO2 electrode forming an interconnected 3-D nanoparticle matrix that is typically used to construct DSCs. The image confirms the suitability of the low-temperature method presented here to make nanoparticle TiO2 electrodes on flexible plastic substrates such as PEN. Once the electrodeposition is started, it is more likely that the low resistive exposed ITO surface as well as the TiO2 surface in the vicinity are coated with Mg(OH)2 first. At that stage, the deposition of Mg(OH)2 on ITO may even be marginally favored as the iso-electric point of ITO is slightly less than that of TiO2 (i.e., electrostatic interaction may be slightly stronger between ITO and Mg(OH)2 than that between TiO2 and Mg(OH)2). However, as the reaction proceeds (i.e., deposition time increases), the alkaline pH boundary extends further away from the interior ITO surface, and the precipitation and subsequent coating may occur on the nanocrystalline TiO2 surface further away from the substrate. FEG-SEM images of Figure 2 show the gradual increase of the Mg(OH)2 growth outward from the substrate. It is evident that the growth of Mg(OH)2 has resulted in a gradual reduction of internal surface area of the electrode (as described in the latter part of this work, reduction of internal surface area was confirmed by dye adsorption/desorption studies). The schematic representation in Figure 3 illustrates the cross-sectional profile of the TiO2 electrode as the Mg(OH)2 growth time is gradually varied. Also, it appears from Figure 2 that the bare nanocrystalline TiO2 electrode (without Mg(OH)2 coating) is more likely to contain an ITO surface that was exposed to the redox electrolyte than coated films when employed in the electrochemical cell configuration. The nature of Mg(OH)2 growth within the interior of the TiO2 matrix was studied by recording the cross-sectional FEG-SEM image of the
Figure 2. Surface topographic FEG-SEM images of TiO2 electrodes untreated and treated with Mg(OH)2 at different electrodeposition times. 1213
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TiO2 film coated with Mg(OH)2 for 10 min (Figure 4). It is evident from the figure that: (i) Mg(OH)2 deposition has taken place on
both ITO as well as TiO2 surfaces and (ii) the growth has occurred uniformly within the interior of the TiO2 matrix after 10 min. Characterization of Mg(OH)2. To confirm the growth of Mg(OH)2 on the photoelectrode surface, the treated films were analyzed by IR spectra. Figure 5 shows the IR spectra of untreated and Mg(OH)2 deposited TiO2 electrodes at various deposition times. On the basis of analysis of IR spectra, the well-known OH stretching peak (∼3400 cm1) and OH bending peak (∼1600 cm1)24 were assigned. The spectra show a clear increase of OH groups as the Mg(OH)2 electrodeposition time is gradually extended from 2 to 10 min. Both the OH stretching and bending peaks were increased gradually as the deposition time was extended. We attribute this behavior to the increase of Mg(OH)2 coating on the TiO2 surface as the deposition time was increased.8 Furthermore, an EDX analysis has been done to confirm the Mg(OH)2 content on the TiO2 electrodes. The increase of atomic
Figure 3. Schematic representation of Mg(OH)2 growth in the TiO2 matrix as the deposition time is gradually varied.
Figure 5. Attenuated total reflectance infrared spectra of TiO2 electrodes untreated and treated with Mg(OH)2.
Figure 4. FEG-SEM cross-section of the 10 min electrodeposited Mg(OH)2 on TiO2 film. 1214
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Table 1. Summary of Estimated Internal Surface Area of All Flexible TiO2 Electrodes (Deposited on ITO-PEN Substrates, Untreated and Treated with Mg(OH)2) and the Key DSC Parameters Recorded by Employing them in Cells ED time (min)
estimated surface area (103 cm2)
Voc (mV)
Jsc (mA cm2)
ff
η (%)
0
12.04
744
7.94
0.63
3.76
2
10.55
847
7.13
0.66
4.01
4
6.47
839
6.19
0.68
3.57
6
5.49
805
4.75
0.64
2.46
8
5.36
791
4.29
0.55
1.87
10
2.35
714
2.95
0.55
1.16
wt % of magnesium in the electrode against the electrodeposition time further supports the IR data. The results are given in the Supporting Information (Figure S1, Figure S2, and Table S1). Electrode Internal Surface Area. The effective dye adsorbed area of the photoanode (internal surface area) was estimated by conducting N719 dye adsorption/desorption measurements.25 On the basis of the optical absorbance data of desorbed dye solution, the desorbed dye concentration was evaluated with the aid of BeerLambert’s law (the extinction coefficient of the N719 dye at 525 nm, ε525 = 11 400 M1 cm1).25 Then, the total number of adsorbed dye molecules was calculated by taking into account the optical absorbance of desorbed dye solution at 525 nm. The surface area occupied by dye on a given photoelectrode was determined considering the area occupied by each dye molecule, 1.46 nm2.25 The internal surface area of TiO2 was then estimated by taking into account the projected surface area of the photoelectrode, 0.16 cm2. The calculated internal surface area of each electrode is listed in Table 1. As the Mg(OH)2 and dye form strong chemical bonding, we believe that there is a small effect on the area of TiO2 estimated by dye adsorption/desorption measurements. The calculated values are comparable with the internal surface area estimated for similar electrodes reported earlier.16 The analysis shows that the internal surface area of the photoelectrode has reduced as the Mg(OH)2 deposition time increased. This also suggests that the Mg(OH)2 coating is not a conformal thin layer but a thick enough layer that fills the nanoscale voids in the nanocrystalline TiO2 matrix and reduces the internal surface area as described in the schematic representation in Figure 3. The surface topographic FEG-SEM images of TiO2 electrodes described in Figure 2 also confirm it. DSC Characterization. The influence of the Mg(OH)2 coating on ITO and TiO2 surfaces was evaluated by constructing flexible DSCs employing a series of such electrodes (subjected to coating for 0, 2, 4, 6, 8, and 10 min) and then studying the JV characteristics. Figure 6 illustrates the variations acquired in open-circuit voltage (Voc) against the Mg(OH)2 deposition time. The cell employing an untreated TiO2 electrode yielded a Voc of 744 mV and a short-circuit current density (Jsc) of 7.94 mA/cm2 under 1 sun illumination, which correspond to a light-toelectricity conversion efficiency of 3.76%. The performance of other DSCs shows that both Voc and Jsc have been clearly influenced by the systematic growth of Mg(OH)2 on ITO and TiO2 surfaces. As indicated by the summarized data in Table 1, the DSCs constructed by employing Mg(OH)2 treated flexible electrodes up to 8 min showed higher Voc than that made with the untreated TiO2 electrode. This indicates that the electrodeposition of Mg(OH)2 on the ITO and TiO2 surface over a relatively low deposition time results in an increase of the Voc. It is also noticeable
Figure 6. JV characteristics of flexible DSCs constructed by all flexible TiO2 electrodes (deposited on ITO-PEN substrates, untreated and treated with Mg(OH)2) under 1 sun illumination with a deposition time of Mg(OH)2.
that the Jsc has systematically decreased as the Mg(OH)2 coating time varied from 0 to 10 min, suggesting the blocking of porous TiO2 (Figure 3). The cell prepared with the 2 min Mg(OH)2 electrodeposited electrode showed the highest Voc of 847 mV, which was associated with dropping the Jsc down to 7.13 mA/cm2. The corresponding light-to-electricity conversion efficiency of 4.01% is the highest recorded in the present work. Referees pointed out the worthiness of studying the JV characteristics for DSCs when electrodeposition of Mg(OH)2 was conducted for even lesser times than 2 min, as the best device performance could even exist within that window. Therefore, cells were constructed with TiO2 electrodes that had Mg(OH)2 coating for 10, 30, 60, and 90 s. The JV plots of them are given in the Supporting Information (Figure S3 and Table S2). The summary of all DSC characteristics was shown in Figure 7. This confirmed our initial conclusion that the best performance was shown when the cell was prepared with the 2 min Mg(OH)2 electrodeposited TiO2 nanoparticle electrode. As discussed, it is reasonable to assume that the Mg(OH)2 deposition takes place on ITO and TiO2 in the vicinity at relatively short deposition times such as 10, 30, 60, and 90 s. The DSC photovoltage gradually increased peaking at 2 min, suggesting the retardation of charge recombination through the ITO/electrolyte interface and TiO2/ electrolyte interface (in the vicinity of ITO) (Figure 7c). It is more likely that the coverage of Mg(OH)2 on TiO2 (in the vicinity of ITO) prevents dye adsorption on TiO2 which was evident by the small reduction of photocurrent density up to 2 min. The trend was continued as the electrodeposition time further increased (Figure 7a). The effect of Mg(OH)2 coating just on the ITO layer on cell performance was also studied by preparing a series of Mg(OH)2-coated ITO layers (at 10 s, 30 s, 60 s, 90 s, 2 min, 4 min, 6 min, 8 min, and 10 min of electrodeposition times) and employing them in DSCs to study JV characteristics. A rough estimation (assuming that all OH groups generated by the cathodic reactions contribute for Mg(OH)2 formation) shows that the thickness of the Mg(OH)2 coating could be as high as 430 nm for 90 s electrodeposition. The corresponding JV plots were given in the Supporting Information (Figure S4) and the summary of cell performance was included in Figure 7 (Figure 7b and 7d). When the nanoparticle TiO2 paste is doctor bladed onto 1215
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Figure 7. Comparison of Jsc and Voc against electrodeposition times (i) when Mg(OH)2 was electrodeposited on the ITO/nanoparticle TiO2 electrode (a and c) and (ii) when DSCs were constructed on ITO/Mg(OH)2 layers (b and d).
Figure 8. Schematic cross-sectional representations of (a) the electrodeposited Mg(OH)2 layer on ITO, (b) after doctor-blading TiO2 nanoparticles onto ITO/Mg(OH)2 and (c) after compression of the electrode and dye adsorption.
the ITO/Mg(OH)2 layer and then compressed, the electrical link between TiO2 and ITO may be established as some TiO2 nanoparticles squeeze through the Mg(OH)2 layer and then are pressed onto the ITO surface. An unwelcome consequence of the compression is that the TiO2 nanoparticles pressed interior to the Mg(OH)2 layer lose direct contact with the liquid phase, thereby preventing (i) dye adsorption and (ii) direct contact with the electrolyte under DSC operational conditions. The result is
the presence of a significant amount of bare TiO2 nanoparticles (without dye coverage) close to the ITO layer without direct contact with iodide/tri-iodide electrolyte. This situation is schematically illustrated in Figure 8. This has a direct consequence in DSC operation. As the dielectric constant of Mg(OH)2 is as low as 3.8,26 electron transport across those bare TiO2 nanoparticles to the ITO layer is difficult, as there is no charge screening (by electrolyte species). This leads to an accumulation of electrons at 1216
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Figure 9. IPCE spectra of flexible DSCs prepared from treated with Mg(OH)2 for 2 min and untreated TiO2 film.
the boundary of electrolyte and the Mg(OH)2 layer, resulting in a significant recombination. The drop of photovoltage until 2 min provides evidence for this (Figure 7d). The photovoltage drops to 500 mV as the Mg(OH)2 layer becomes further thick, suggesting the formation of a static junction that prevents charging the ITO. It suggests that formation of a thick Mg(OH)2 barrier later on ITO has no benefit for the DSC cell operation. Therefore, cells prepared on ITO/Mg(OH)2 were not considered for the performance comparison with untreated electrodes. Only the cells made with Mg(OH)2 electrodeposited onto ITO/ nanoparticle TiO2 electrodes were considered for comparison of the performance. The JV characteristics on the treated flexible DSCs compared to the untreated can be explained by considering the porosity changes that may takes place in the nanocrystalline TiO2 film. The evidence for the changes of the film porosity with the deposition of Mg(OH)2 is obtained from the FEG-SEM images shown in Figure 2. The coverage of TiO2 nanoparticles with Mg(OH)2 influences the flexible DSC parameters due to its thickness as well as the changes in internal surface area that occur in the TiO2 matrix. In DSCs made with Mg(OH)2 coating, the photogenerated electrons (injected from the dye molecules attached to the Mg(OH)2 surface) have the capability to tunnel across the Mg(OH)2 barrier if the coating is thin enough. Although thicker coatings may screen electron leakage effectively, the charge tunnelling probability falls exponentially against the barrier width.27 Hence, thick Mg(OH)2 coatings on the electrode surface reduce the photocurrent significantly. 28 The isoelectric point of Mg(OH)2 is pH 1222 compared to pH 6.5011 for TiO2, therefore, the Mg(OH)2 surface is more basic and forms strong bonding with dye. The photogenerated electrons in DSCs may undergo direct recombination with I3 ions via the conduction band of TiO2 as well as the substrate ITO, resulting in significant losses.29 The increase of Voc up to 8 min coating of Mg(OH)2 suggests that both recombination pathways may have been suppressed by the Mg(OH)2 coating. In the present work, the highest Voc was recorded when the TiO2 electrode had a 2 min Mg(OH)2 coating. This indicates the suppression of recombination of photogenerated electrons via the surface of ITO as well as TiO2, as only a thin layer of Mg(OH)2 could be deposited at relatively
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shorter electrodeposition times, without compromising the internal surface area in great deal. The increase of electrodeposition time leads to further growth of Mg(OH)2 within the TiO2 matrix, resulting in gradual reduction of internal surface area of the electrode. The systematic reduction of photocurrent density in DSCs against the Mg(OH)2 electrodeposition time suggests: (i) a decrease of available TiO2 surface area for dye adsorption (hence lowering light harvesting efficiency) and (ii) the ability of relatively thick enough Mg(OH)2 layers (that may have deposited on certain parts of the TiO2 surface) to act as a barrier for the charge injection from excited dye molecules to the conduction band of TiO2, despite that dye adsorption still may take place on the surface of Mg(OH)2. This hypothesis is also supported by the systematic reduction of Voc against the Mg(OH)2 deposition time beyond 2 min. Therefore, it is reasonable to conclude that the relatively thin Mg(OH)2 coating that was obtained over a 2 min deposition time still permits the injection of the majority of photogenerated charges from excited dye molecules to the conduction band of TiO2 via tunneling. This could be the reason for the observation of only a small reduction of photocurrent density for the DSC that corresponds to 2 min Mg(OH)2 electrodeposition time, compared to that of the DSC made with untreated TiO2 electrode. Incident Photon to Electron Conversion Efficiency (IPCE). Figure 9 compares the IPCE of DSCs prepared with untreated and 2 min Mg(OH)2 electrodeposited nanocrystalline TiO2 electrodes. The external quantum efficiency of DSC made with N719 sensitized untreated TiO2 electrodes is superior to that of the DSC prepared with N719 sensitized 2 min Mg(OH)2 deposited TiO2 electrodes. The maximum IPCEs of 27% and 22% were observed for flexible cells made with untreated and 2 min Mg(OH)2 deposited TiO2 electrodes, respectively, at 525 nm. As described in our earlier reports,30 the Jsc was estimated to be 7.51 mA cm2 by taking into account the integrated IPCE and light absorbance for DSC prepared from an untreated nanocrystalline TiO2 electrode. This value is very close to the recorded Jsc of 7.94 mA cm2 for the same cell under AM 1.5 simulated light (Figure 6).
’ CONCLUSIONS Electrodeposition of Mg(OH)2 was conducted on the surface of nanocrystalline TiO2 coated ITO-PEN flexible substrates. The effect of electrodeposition time (i.e., 0, 2, 4, 6, 8, and 10 min) on the performance of flexible DSCs was evaluated on the basis of their key cell parameters. The deposited Mg(OH)2 was confirmed by IR and EDX analysis. The surface topographic FEGSEM images of bare and Mg(OH)2-treated TiO2 electrodes illustrated the distinct difference in the film morphology. The effective dye adsorbed area of the photoanode was deduced to find out the effect of the internal surface area of photoelectrode against Mg(OH)2 the deposition time by conducting N719 dye adsorption/desorption experiments and subsequent optical absorbance measurements. It was found that the photovoltaic performance of flexible DSC was improved for the deposition time as low as 2 min, thus giving the highest Voc reported for a flexible DSC to date. It was evident that the photovoltaic performance was decreased as the electrodeposition time was further increased, which is attributed to the reduction of internal surface area as well as the decrease of the photogenerated charge transfer across the Mg(OH)2 barrier. The increase of Voc was attributed to the suppression of the back reaction of electrons 1217
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The Journal of Physical Chemistry C with I3 species in the electrolyte via the surface of ITO as well as TiO2 under the operational conditions. Our findings suggest that the electrodeposition of a thin coating of Mg(OH)2 on a flexible TiO2 electrode suppresses the recombination via the ITO and TiO2 surface, thus improving the flexible DSC efficiency.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures S1S4 and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by UK EPSRC, DSTL, Johnson Matthey Plc, and Department of Chemistry, Loughborough University. The authors would like to thank all the members of the Renewable Energy Group of the Chemistry Department at Loughborough University for their assistance. We would like to thank Prof. Laurie Peter at University of Bath for stimulating discussions.
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