The role of gel electrolyte composition in the kinetics and performance of dye-sensitized solar cells

Electrochimica Acta 53 (2008) 7166–7172

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The role of gel electrolyte composition in the kinetics and performance of dye-sensitized solar cells Jilian Nei de Freitas a , Agnaldo de Souza Gonc¸alves a , Marco-Aurelio De Paoli a , ´ James R. Durrant b , Ana Flavia Nogueira a,∗ a b

Instituto de Qu´ımica, Universidade Estadual de Campinas, Campinas, SP, Brazil Centre for Electronic Materials and Devices, Imperial College of Science Technology and Medicine, London SW7 2AY, United Kingdom

a r t i c l e

i n f o

Article history: Received 10 March 2008 Received in revised form 29 April 2008 Accepted 2 May 2008 Available online 9 May 2008 Keywords: Dye-sensitized solar cells DSSC Polymer electrolyte Gel electrolyte Polyiodide

a b s t r a c t Gel-type polymer electrolytes based on the copolymer poly(ethylene oxide-co-epichlorohydrin) and the plasticizer ␥-butyrolactone (GBL) were optimized and applied in dye-sensitized solar cells. The plasticizer added to the electrolyte allowed the dissolution of a higher concentration of salt, reaching conductivity values close to 1 mS cm−1 for the sample prepared with 30 wt% of LiI. Raman spectroscopy confirmed polyiodide formation in the electrolyte when the salt concentration exceeds 7.5 wt%, introducing a significant contribution of electronic conductivity in the electrolyte. The devices were characterized under AM 1.5 conditions and the I–V curves were fitted using a two diode equation. Increasing the concentration of LiI-I2 accelerates dye cation regeneration as measured by transient absorption spectroscopy; however, it also contributes to an increase in the dark current of the cell by one order of magnitude. The best performance was achieved for the solar cell prepared with the electrolyte containing 20 wt% of LiI, with efficiencies of 3.26% and 3.49% at 100 and 10 mW cm−2 of irradiation, respectively. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction ¨ Since Gratzel’s announcement of the first dye-sensitized nanocrystalline solar cell (DSSC) as a promising, low cost, clean and highly efficient device for solar energy conversion, many groups have focused their efforts on improving and comprehending different aspects of this technology. The liquid electrolyte usually employed in this cell is still a drawback for long-term practical operation due to electrolyte leakage or evaporation, and also makes large-scale production difficult. To overcome these problems, many research groups have been searching for alternatives to replace the liquid electrolytes, such as inorganic or organic hole conductors [1–3], ionic liquids [4,5] and polymer [6–8] and gel electrolytes [9–11]. Since 1996, our group has been working on DSSC using polymer electrolytes based on copolymers of poly(ethylene oxide) (PEO) derivatives [12]. The best solar energy conversion efficiency obtained for a solid-state DSSC was 2.6% under 10 mW cm−2 and 1.6% under 100 mW cm−2 [13]. More recently, we demonstrated that the use of a network of TiO2 nanotubes is an alternative to improve the efficiency of such devices [14]. The nanotubes allow

∗ Corresponding author. Tel.: +55 19 3521 3022; fax: +55 19 3521 3023. E-mail address: anafl[email protected] (A.F. Nogueira). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.05.009

the attachment of more dye molecules due to their higher surface area. In addition, the more open structure of this film increases the permeability of the polymer and improves the diffusion of the ionic species [14]. However, even using different anode structures, the overall efficiency has already reached the limit for a system based solely on polymer and salt mixtures. The low ionic mobility of the polymer electrolyte has three primary implications for photovoltaic function: (i) the lower conductivity increases voltage losses arising from charge transport through the electrolyte (i.e., the series resistance of the cell); (ii) the lower ionic mobility is expected to retard the kinetics of dye cation reduction by iodide ions; and (iii) it is also expected to generate concentration gradients in the polymer electrolyte at high current densities. The reduced mobility of I3 − may lead to the buildup of triiodide in the pores relative to the bulk of the electrolyte, leading to increased probability of electron reaction with triiodide (“dark current”). Other factors are involved and must be taken into account; the interface between the TiO2 /dye nanoparticles is of primary importance for the regeneration kinetics (reaction of the dye cation with iodide species). Thus, the nanoporous film must be completely embedded in the polymer electrolyte, and deep penetration is not a simple task to be achieved, being directly related to polymer molecular mass [15]. Recently, Caruso and co-workers [16] demonstrated that, using a vacuum pore filling method, it is possible to achieve a high degree of penetration with a concomitant increase in the

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efficiency of a DSSC based on PEO and poly(vinylidene fluoride) (PVDF) polymers. Other components must be added to the polymer electrolyte in order to develop solar cells with an enhanced solar energy conversion. Several groups working in this area have attempted this and the introduction of inorganic nanofillers [17,18], hybrid organic–inorganic electrolytes (i.e., ionic liquids together with nanoparticles) [19–21], oligomers based on ethylene oxide [22,23] and gelators [24,25] have become a common route to elaborate polymer (or gel) electrolytes with improved ionic conductivity. Plasticizers are low molar mass organic molecules, with high boiling points, routinely added to a highly crystalline polymer matrix (i.e., phthalic acid esters added to PVC) to increase the flexibility of the polymer chains. In terms of polymer electrolytes, this additive contributes to an increase in the ionic conductivity of several orders of magnitude [26], since ionic transport occurs in the amorphous phase and is closely associated to polymer segmental motion (i.e., its viscosity) [27]. In fact, DSSC assembled with polymer electrolytes containing plasticizers exhibit much higher efficiencies [15,28–30]. However, the increase in the plasticizer content is always followed by a loss in the mechanical properties. In this report, we summarize our recent experimental efforts to assemble more efficient polymer-gel dye-sensitized solar cells, through optimization of the electrolyte composition and understanding how this composition can affect the kinetics of the cell. 2. Experimental part 2.1. Materials Samples of poly(ethylene oxide-co-epichlorohydrin) were used as received from Daiso Co. Ltd. (Osaka). The ethylene oxide/epichorohydrin ratio in the copolymer was 87/13 and the molar mass was ca. 1 × 106 g mol−1 , according to the supplier. The polymer electrolytes were prepared by the dissolution of the copolymer, NaI or LiI, I2 and the plasticizer ␥-butyrolactone, GBL (Aldrich, 99%) in acetone. In all cases, the copolymer/plasticizer weight ratio was 1:1. Electrolyte solutions were kept under stirring for 1 week before use. 2.2. Ionic conductivity measurements Ionic conductivity measurements were evaluated as a function of salt content for polymer electrolytes prepared with and without the plasticizer. All samples consisted of a polymer electrolyte film (thickness of ∼100 ␮m), obtained by casting electrolyte solutions onto a Teflon disk, under saturated atmosphere conditions. Afterwards, the films were detached from the Teflon by dipping into liquid nitrogen, and further dried under vacuum for 144 h or more. Conductivity measurements were carried out in a MBraun dry box (humidity < 10−4 %, under an argon atmosphere). The films were fixed between two mirror-polished stainless steel disc shaped electrodes (diameter = 12 mm) and the conductivity values were calculated from the data obtained by electrochemical impedance spectroscopy (EIS), using an Eco-Chemie Autolab PGSTAT 12 with FRA module coupled to a computer in the frequency of 106 –10 Hz and amplitude of 10 mV applied to 0 V. The ionic conductivity can be calculated from the bulk electrolyte resistance value obtained from the complex impedance diagram. The experiments were carried our two times to guarantee the accuracy of the results. 2.3. Raman spectroscopy The Raman spectra of the polymer electrolytes were measured by a Raman spectrometer, Micro-Raman Renishaw System 3000,

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equipped with a charge couple device (CCD) detector and an Olympus BTH 2 microscope. The excitation laser was set at 632.8 nm. 2.4. Transient absorption spectroscopy Nanosecond to millisecond transient absorption experiments employed a nitrogen laser pumped dye laser as excitation source. Data were collected employing low intensity 530 nm excitation pulses (∼0.1 mJ cm−2 , repetition rate 0.1–0.2 Hz, <1 ns duration). The probe light source was a 150 W tungsten lamp. The monitoring wavelength from the lamp was selected by using a monochromator, typically set to monitor the transient signal at 800 nm, which primarily results form cis-Ru(dcbpy)2 (NCS)2 cation absorption. Appropriate monochromators and/or filters were used to minimize the probe light incident upon the sample. Transient data were collected with a silicon photodiode and digitized using a Tektronix TDS220 digital storage oscilloscope. The time resolution of the apparatus was ∼300 ns. Potential control during the transient measurements was provided by a home-built potentiostat. The data were collected under open circuit conditions. Such experiments were conducted either in the presence or absence of additional white light illumination (∼20 mW cm−2 ,  > 475 nm) generated by a 150 W tungsten lamp. The current drawn by the cell was monitored during all experiments. 2.5. Solar cell assembly and characterization Devices were assembled with 0.20 cm2 of active area. A TiO2 nanoporous film was prepared by doctor blading a small aliquot of a commercial colloidal suspension (Ti-Nanoxide T, Solaronix) onto a TCO substrate (Hartford Glass Co., 8–12 ). The film was heated to 450 ◦ C for 30 min, originating a layer of ∼4 ␮m thickness as measured with a Tecnor Alpha-step 200 profilometer. The electrodes were immersed in a 1.5 × 10−4 mol L−1 solution of the sensitizer cis-bis(isothiocyanato)bis(2,2 -bipyridyl4,4 -dicarboxylate)-ruthenium(II) (Ruthenium 535, Solaronix) in ethanol for 20 h at room temperature. Afterwards, the electrodes were washed with ethanol and dried in air. The electrolyte film was deposited by casting an acetone solution onto the dye-sensitized TiO2 film and placing the substrate onto a hot plate at 60 ◦ C to remove residual acetone. Pt counter electrodes were pressed on the top of the polymer film. I–V curves were obtained in the dark and under standard AM 1.5 conditions using a 150 W Xe lamp as light source and appropriate filters. The polychromatic light intensity at the electrode position was measured with a silicon photodiode from Newport Optical Power Meter, model 1830-C. 3. Results and discussion 3.1. Electrolyte characterization Before any discussion, it is necessary to define the difference between a polymer electrolyte and a gel electrolyte. Although in both cases what matters most is the ionic transport through a polymer matrix, polymer and gel electrolytes are different in their respective constitutions. A polymer electrolyte is better defined as a solid complex formed between a heteroatom coordinating polymer, such as poly(ethylene oxide) and a salt. The interaction between the cations and the heteroatoms is of Lewis acid–base type. The ionic conduction displayed by the majority of the polymer electrolytes occurs in the amorphous phase of the polymer matrix, and the ionic transport is assisted by the segmental motion of the polymer chains that produces free volumes where the cation can easily

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Fig. 1. Effect of salt concentration on conductivity for polymer electrolytes based on P(EO-EPI), I2 and: () LiI + 50 wt% GBL, (䊉) NaI + 50 wt% GBL, () LiI and () NaI.

Fig. 2. Raman spectra of the gel electrolyte samples containing different LiI concentrations: (. . .) 7.5, (– –) 20 and ( ) 30 wt% (excitation laser 632.8 nm).

hop from site to site [26,27]. Anionic motion is believed to follow cation displacement. A gel polymer electrolyte compromises a tri-dimensional polymer network (not necessary a coordinating polymer, in fact most of the gel electrolytes are made of “inert” matrixes), which holds an organic solution of a salt. For an “inert” gel electrolyte, the ionic transport occurs in the organic phase, however, if the polymer employed possess the ability to strongly solvate the cation, then both phases can be responsible for ionic motion. As gel electrolytes usually contain a high fraction of liquid components, their mechanical properties are poorer than those observed for pure polymer electrolyte systems. In our case, considering the high fraction of plasticizer (an organic solvent) the system is more closely defined as a gel-type polymer electrolyte. Fig. 1 presents the plot of ionic conductivity () as a function of salt concentration for the polymer electrolyte based on poly(ethylene oxide-co-epichlorohydrin) (P(EO-EPI)) containing NaI or LiI, with and without the organic solvent GBL (plasticizer). All the samples contain iodine in the molar ratio I− /I2 10:1, to simulate the conditions used in the solar cells. First, the addition of GBL increases the conductivity of the system by one order of magnitude. Second, due to its high lattice energy, LiI is not completely dissolved in the P(EO-EPI) matrix, unless in the presence of GBL. Indeed, it is well known that GBL coordinates Li+ ions [31,32] contributing to salt dissociation. Thus, the gel electrolyte based on LiI presents the highest conductivity, as expected, due to the smaller size of the Li+ cation. Third, it is also observed that for the polymer electrolyte system (without the plasticizer), an increase in salt concentration leads to an increase of  up to 9 wt% of salt, reaching a maximum value of 8.1 × 10−6 S cm−1 for the NaI electrolyte. By increasing salt concentration above this quantity, the conductivity shows a slow decrease due to the formation of ion pairs, multiplets and crosslinking sites that hinder the segmental motion of the polymer chains and, as a consequence, the ionic mobility decreases. However, the conductivity dependence on salt concentration changes remarkably when 50 wt% of GBL is incorporated in the polymer electrolyte. As discussed before, GBL has an ionsolvating ability, allowing the dissolution of higher amounts of salt. Thus, increasing the salt concentration leads to a further increase in the conductivity, reaching  ∼ 5 × 10−4 S cm−1 for the sample prepared with 20 wt% of LiI. Such behavior is the opposite of that usually observed for polymer electrolytes at high salt concentration [33,34]. The conductivity value achieved approaches the value for a liquid electrolyte based on organic solvents. The conductivity for the sample containing 30 wt% of LiI is ∼6 × 10−4 S cm−4 , which is very close to that presented by the sample containing 20 wt%, indicating a saturation in salt dissolution.

To investigate the causes for such high ionic conductivity, as well as the unusual behavior observed as a function of salt concentration, electrolytes consisting of polymer, GBL and LiClO4 were also prepared. For this system, increasing the LiClO4 content after 10 wt%, the ionic conductivity drops significantly (supporting information). Despite the presence of the plasticizer, the gel electrolytes prepared with LiClO4 present a similar behavior to that presented by pure polymer electrolytes containing LiI or NaI (without plasticizer), as opposed to the behavior of gel electrolytes containing iodide salts. Considering this, to explain the increase in the conductivity for highly concentrated iodine/iodide polymer samples, we examined the formation of polyiodide species, such as I3 − and I5 − , in electrolytes containing different concentrations of LiI. It is well known that polyiodides can be formed at high concentrations of iodine in the presence of iodide ions, improving the conductivity by the introduction of an electronic conduction pathway [35–37]. Fig. 2 shows the Raman spectra of the gel electrolytes containing 7.5, 20 and 30 wt% of LiI and iodine. The molar ratio iodide:iodine was kept 10:1 for all samples. A control sample was prepared without iodine. The control sample shows no apparent band in the Raman shift region of 50–250 cm−1 . All the samples present a band around 110 cm−1 , which is assigned to symmetric stretch of I3 − species [38,39]. The band intensity increases as the concentration of iodine/iodide is increased in the gel electrolyte. At high concentrations of these redox species another band around 145 cm−1 is observed, as a shoulder of the main I3 − band. It is assigned to the vibration mode of higher polyiodide species, such as I5 − [40]. The band assigned to molecular iodine (around 180–210 cm−1 ) is not observed in the samples, suggesting that all the iodine introduced into the electrolyte was converted to polyiodide species. The formation of polyiodides in gel electrolytes using the Raman spectra was also shown by Yanagida and co-workers [11]. It is expected that polyiodides present lower limiting molar conductivities than monoiodide because of their large ionic radius [36]. Thus, considering only the diffusion of ionic species as responsible for the conductivity of the electrolytes investigated, a decrease in the conductivity values for samples containing larger concentration of iodine/iodide would be expected. However, it is well known that electron exchange can occur between polyiodides and, in concentrated media, the Grothuss-type charge-transfer mechanism can contribute to the effective conductance of the electrolyte [35,37]. Our results suggest that the polymer electrolyte based on P(EO-EPI), GBL and LiI/I2 could be acting as a mixed conductor, presenting both ionic and electronic conductivity. Further investigations need to be done in order to estimate the contribution of each process (ionic and electronic) to the overall conductivity. Considering the value of the

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Fig. 3. Transient absorption spectra for dye-sensitized TiO2 films covered with gel polymer electrolytes containing different LiI concentrations: (black) control sample (LiClO4 ), (light gray) 7.5 wt%, (gray) 20 wt% and (dark gray) 30 wt% LiI.

conductivity reached (∼10−3 S cm−1 ), we can infer that in our system, the electronic contribution is small but still significant. In other words, the conductivity displayed by our gel polymer electrolyte is high in comparison to materials presenting only ionic conductivity but it is still low in comparison to materials that display solely electronic conductivity. 3.2. Transient absorption spectroscopy In principle, there are two main recombination pathways which can cause loss of efficiency in DSSC:electrons photoinjected into the TiO2 conduction band/trap states can recombine with either dye cations or with the redox electrolyte (kCR1 and kCR2 in Eqs. (1) and (2), respectively). kCR1 : TiO2 (e− ) + dye+ → dye −

kCR2 : TiO2 (e ) + I2 → 2I +

−

−

kRR : dye + 2I → dye + I2

(1) (2)

−

(3)

In a liquid electrolyte DSSC, rapid re-reduction of dye cations by the redox electrolyte (kRR , Eq. (3)) competes effectively with kCR1 , and therefore charge recombination to the redox electrolyte, kCR2 , is the primary recombination loss pathway limiting device efficiency [41,42]. However, the low ionic conductivity of polymer electrolyte systems introduces the possibility that dye cation re-reduction by the electrolyte may no longer compete effectively with recombination pathway kCR1 . As a consequence, charge recombination to dye cations may become critical in limiting device efficiency. In an earlier work, the data collected with transient absorption kinetics provided clear evidence that kinetic competition between charge recombination to dye cations and dye cation re-reduction by the polymer electrolyte limited the performance of DSSC employing polymer electrolytes [43]. For electrolytes prepared with P(EO-EPI) and NaI without the plasticizer, kCR1 exhibited a half time of ∼2 ms [43]. In liquid electrolytes, kRR usually exhibits a half time of ∼1 ␮s, and therefore competes effectively with charge recombination to the redox electrolyte [41,44]. Fig. 3 shows the transient absorption kinetics observed at 800 nm for dye-sensitized TiO2 films in the presence of a non-redox active polymer electrolyte (P(EO-EPI)/LiClO4 plus GBL) and in the presence of electrolytes containing 7.5, 20 or 30 wt% of LiI/I2 plus GBL. In all cases, the transient shows a fast, instrument-response limited absorption increase which can be assigned to dye cation absorption arising from photoinduced electron injection from the dye excited state into the TiO2 conduction band [45]. In all cases, decay of the induced absorption signal is not monoexponential. In

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Fig. 4. Transient absorption spectra under white light illumination for dyesensitized TiO2 films covered with gel polymer electrolytes containing different LiI concentrations: 7.5 wt% (black), 20 wt% (light gray) and 30 wt% (gray) LiI.

the absence of any redox active electrolyte, the cation excited state is reduced by charge recombination with electrons occupying conduction band/trap states of the TiO2 (kCR1 ). This process is relatively slow and can be roughly quantified by the time needed to reach half of the initial absorbance,  1/2 , equal to ∼140 ␮s. This charge recombination half time is similar to that we have reported previously for dye-sensitized TiO2 films in non-redox active liquid electrolytes in the absence of external applied bias [41]. In the presence of iodide ions cation decay becomes faster, resulting from re-reduction by iodide ions, kRR . For the sample with a moderate concentration of iodide (7.5 wt% LiI), the dye cation signal exhibits a faster decay with  1/2 ∼ 20 ␮s. This acceleration of dye cation decay is assigned to the regeneration reaction kRR . We note that for this iodide concentration, this regeneration reaction is only marginally faster than kCR1 , even under these dark conditions, suggesting that kinetic competition between kCR1 and kRR may be a significant issue under device operating conditions (where kCR1 accelerates due to enhanced electron density in the TiO2 film, see below). Adding more iodide (20 and 30 wt%), kRR becomes clearly predominant and  1/2 values are approximately 1 ␮s in both cases, similar to that reported previously for acetonitrile based electrolytes [44]. The decay profiles with 20 and 30 wt% LiI are very similar, suggesting that kRR is no longer limited by iodide concentration or diffusion, at least, for these salt concentrations. However, this could also be related to the limited time resolution of the experiment, because part of the regeneration might be occurring in less than the instrument response (as can be seen from the difference in dye cation yield in Fig. 3). The long-lived tail of the absorption transients observed in the presence of LiI is assigned, as previously [42,43] to TiO2 (e− )/I2 − absorption. This tail is less pronounced than in our previous studies [43] tentatively assigned to the presence of iodine in the electrolyte (1/10 in relation to the iodide). Fig. 4 shows absorption transients recorded at 800 nm for samples under white light illumination, collected at open circuit conditions. Under illumination, the electron density in the TiO2 film is increased, shifting the Fermi level to more negative potentials. As a result, an acceleration of kCR1 is expected. In fact, the decay dynamics are accelerated for the sample P(EO-EPI) + 7.5 wt% LiI (from 20 to ∼1 ␮s). A faster decay for this sample under white light illumination indicates that at least under this condition, kCR1 is faster than kRR , resulting in a low regeneration efficiency. For 20 and 30 wt% LiI samples, the transients were less dependent upon white light illumination ( 1/2 is approximately ∼1 ␮s for both samples, with and without illumination). These results indicate that for high

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Fig. 5. J–V characteristics (symbols) for the DSSC assembled with electrolytes containing different LiI concentrations: () 7.5 wt%, () 20 wt% and () 30 wt% LiI, under 100 mW cm−2 . Fits to the two diode model (solid lines) are also shown.

iodide concentrations kRR competes more effectively with kCR1 (e.g., kRR is faster than kCR1 ), indicating a more efficient regeneration. 3.3. Current/voltage characteristics Fig. 5 exhibits the current–voltage curves for dye-sensitized solar cells assembled with the gel polymer electrolytes containing different concentrations of iodide and iodine under AM 1.5 conditions (intensity of 100 mW cm−2 ). The corresponding values for short-circuit current (JSC ), open circuit potential (VOC ), fill factor (FF) and overall efficiency () are summarized in Table 1. We also present the results obtained for devices under illumination of 10 mW cm−2 . The short-circuit current of the devices increases when the salt concentration is increased from 7.5 to 20 wt% LiI, and this effect is more evident at 100 mW cm−2 . Devices prepared with the electrolyte containing 30 wt% of LiI presented lower short-circuit current values compared to solar cells prepared with 20 wt% of LiI. This result is in agreement with the transient absorption data, where no changes were observed in the regeneration reaction when adding more than 20% of iodide in the electrolyte (at least on the time scale of the measurement), meaning that no improvements are obtained from the addition of more salt. Besides, adding more salt, the formation of ion clusters and ion pairs can hamper ionic transport. We also observed that, with increasing salt concentration, VOC decreases. Two factors can be considered to affect VOC : (i) the excess of Li+ cations that adsorb/intercalate in the TiO2 film, shifting the conduction band downwards [46,47]; and (ii) an increase in rate of

Table 1 Short-circuit current (JSC ), voltage (VOC ), fill factor (FF) and efficiency () for dyesensitized solar cells assembled with polymer electrolytes based on P(EO-EPI), GBL and I2 with different LiI concentrations, under different illumination conditions (Irr) LiI (wt%)

Irr (mW cm−2 )

JSC (mA cm−2 )

VOC (V)

FF (%)

 (%)

7.5

100 10

6.3 0.6

0.71 0.65

48 58

2.1 2.3

20

100 10

9.1 1.0

0.67 0.61

54 60

3.3 3.5

30

100 10

8.3 0.9

0.61 0.55

50 60

2.5 3.1

7.5 (without plasticizer)

100 10

4.2 0.4

0.83 0.76

40 62

1.3 1.9

For comparison, the electrical characterization for a solar cell assembled with polymer electrolyte without plasticizer is also shown.

the dark reaction. This effect on dark current will be discussed in more details in the next section. All solar cells present a dependence of the efficiency on the illumination, and the best values were obtained under diffuse light (10 mW cm−2 ). An optimum performance is obtained for the devices assembled with the electrolyte containing 20 wt% LiI, yielding efficiencies of 3.3% (100 mW cm−2 ) and 3.5% (10 mW cm−2 ). Probably at this concentration of iodide/iodine, the balance between short-circuit and open-circuit values leads to more efficient cells. In other words, increasing the conductivity of the electrolyte increases the photocurrent output but the high concentration of polyiodide species contributes in the opposite direction, decreasing VOC . The performance of a solar cell prepared with polymer electrolyte without plasticizer is also shown in Table 1. It is clear that both current and efficiency of this cell are lower than the ones obtained with devices containing a gel electrolyte. This can be related to the low ionic conductivity of the non-plasticized electrolyte (8 × 10−6 S cm−2 ). On the other hand, values of VOC are much higher. High values of VOC were observed before for a dye-sensitized solar cell assembled with a polymer electrolyte based solely on poly(ethylene oxide) and salt [43], and this can be attributed to the effect of the basicity of this polymer which is “diluted” when replacing the polymer by the plasticizer. Although the results presented here are comparable to the results reported so far for dye-sensitized solar cells assembled with gel-type polymeric electrolytes, it is worth noting that the fill factor (FF) values are very poor. One reason might be the adhesive tape introduced between the working electrode and the platinum electrode to avoid short-circuits at high iodide concentrations (20% or 30%). This tape increases the thickness between these two electrodes up to ∼40 ␮m. The shortcircuits may be associated to polyiodide species in the electrolyte, inserting an electronic pathway between the two electrodes. Thus, increasing the thickness of the electrolyte layer prevented the short-circuits, but it is also possible that in these conditions the ionic transport is more probably limiting the efficiency of the devices. Second, FF is limited by the increase in the rate of the dark current due to high concentration of iodide/iodine. A clear evidence for this limiting step is addressed below. It was attempted to fit the data with the simple one-diode model (i.e., by setting k = 0 in Eq. (4)). This simplified model was found to give fits with ideality factor values much higher than 2 (see supporting information), which indicated that the charge transport is still dominated by interface recombination [51]. Thus, the current–voltage curves were fitted using a two diode model [43]:



I = IL − I0 exp



qVj m1 kB T







− 1 − kIL exp



qVj m2 kB T





−1

(4)

where IL is the light intensity dependent short-circuit current, kB the Boltzmann’s constant, T the temperature and I0 , k, m1 and m2 are fitting constants related to the dark current and the ideality factor. The bias drop across the internal junction, Vj can be related to the externally applied bias, V, through Vj = V + IRS

(5)

where RS is the series resistance of the system. The first two terms on the right of Eq. (4) represent a voltage independent photocurrent, IL , and a light independent recombination or “dark current”. Together they compose the usual non-ideal one diode current–voltage characteristics for a solar cell [48]. The final term in Eq. (4) is a light dependent recombination current, and is required to describe adequately the observed behavior for the cells assembled with the gel-type polymer electrolyte. In an earlier work, we demonstrated the need for the second term in the diode

J.N.de. Freitas et al. / Electrochimica Acta 53 (2008) 7166–7172 Table 2 Parameters obtained using a two diode model (Eq. (4)) to fit the J–V curves under 100 mW cm−2 illumination for dye-sensitized solar cells with electrolytes based on P(EO-EPI), GBL and I2 with different salt concentrations LiI (wt%)

RS ()

I0 (10−7 A)

k (10−3 )

m

7.5 20 30

718 227 467

0.6 1.3 3.0

5.9 45.2 16.7

2.9 2.7 3.0

equation in order to fit the current–voltage curves for a DSSC based on polymer electrolyte (salt and polymer only). In that case, the light dependent recombination term was introduced because of the high rate observed for kCR1 due to the low ionic conductivity of the electrolyte [43]. Eqs. (4) and (5) provide a reasonable fit to the data collected for devices prepared with electrolytes containing 7.5, 20 or 30 wt% of LiI, for standard AM 1.5 conditions. For the fitting parameters, we assumed m1 = m2 . I0 was obtained from fitting the J–V curves under dark conditions. The values of I0 , m, k and RS are summarized in Table 2. The ideality factor of ∼3 obtained for all the devices is close to the ideality factor of ∼2 reported for dye-sensitized cells employing liquid electrolytes [49,50]. For the gel polymer electrolyte described here, the need of this light dependent term seems to be no longer because of the charge recombination (kCR1 ) and differs from what was observed in the DSSC using polymer electrolyte (without plasticizer and with low concentration of iodide, ∼9%) [43]. Here, the use of this second term might be a consequence of the increase in dark current values when we employed gel electrolytes with high concentrations of iodine/iodide. From Table 2, it can be seen that I0 increases from 0.06 to 0.3 ␮A when the concentration of LiI is increased from 7.5 to 30 wt%. This is most probably related to the high concentration of iodine, increasing the dark current of the solar cell. On the other hand, the value of I0 for the cells prepared with 20 or 30 wt% of LiI presents only a slightly increase. This data suggest that for the sample containing 30 wt % LiI, the drop on VOC is more influenced by the increase in Li+ concentration at TiO2 surface, rather than by the dark current loss. The decrease in RS from ∼718 to ∼227  when the concentration of LiI is increased from 7.5 to 30 wt% is more straightforward since is directly related to the increase in the conductivity of the electrolyte. When the concentration of iodide is increased to 30 wt%, RS increases to 467 . The increase in the RS is a consequence of the high concentration of ionic species and formation of ion pairs and clusters in the electrolyte which hamper ionic transport inside the TiO2 pores. These results are in agreement with the transient absorption data where it was shown that there is no significant difference in the dye cation regeneration rate in the dark for the samples with 20 and 30 wt% LiI. The fact that, under white light, the dye cation regeneration rate accelerates for the sample with 7.5 wt% of LiI but does not for the samples with 20 and 30 wt% LiI means that the competition between kCR1 and kRR is no longer the limiting step. In other words, the performance at high concentrations of iodide/iodine is no longer dominated by the conductivity of the medium or by the low rate of dye cation regeneration; instead, the efficiency of this polymer gel DSSC might be dominated by the recombination reactions at the interfaces (mainly kCR2 or dark current), and the low values of VOC , due to the high concentration of Li+ ions. Although the efficiency for the cell containing the electrolyte with 20 wt% of LiI is comparable to quasi-solid solar cells reported in the literature, one must notice that the series resistance inside the cell is still very high. Despite the very high conductivity measured for this electrolyte, RS is almost four times greater than the value

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estimated in a previous report (RS ∼60 ) for a polymer electrolyte (without plasticizer) which presented an ionic conductivity of only 1.5 × 10−5 S cm−1 [43]. This is probably related to the thickness of the gel electrolyte layer employed in this work. Investigations are under development in our lab to introduce an optimized blocking layer beneath the TiO2 nanoporous film in order to eliminate the need for the adhesive tape. These changes are expected to reduce the series resistance and increase FF, allowing the assembly of more efficient gel-type polymeric electrolyte solar cells. 4. Conclusions Gel polymer electrolytes based on the copolymer poly(ethylene oxide-co-epichlorohydrin) and the plasticizer ␥-butyrolactone were optimized and successfully applied in dye-sensitized solar cells. The plasticizer added to the electrolyte allowed the dissolution of higher concentrations of salt, and remarkable improvements in the conductivity were observed at high concentration of iodide/iodine. The conversion of iodine to polyiodide species when the salt concentration exceeds 7.5 wt% was confirmed by Raman spectroscopic measurements, and the charge transport in these electrolytes was rationalized to occur through both ionic transport and a Grothuss-type mechanism. The high concentration of iodide in the electrolyte contributed to accelerate dye cation regeneration as measured by transient absorption spectroscopy, but also increased the dark current of the cell by one order of magnitude. In these cells, differently from what was observed previously for a DSSC assembled with a polymer electrolyte (polymer and salt only), it is not the competition between kCR1 and kRR that limits the efficiency; instead, kCR2 or dark current, together with the low VOC caused by the excess Li+ cations in the TiO2 surface seems to be playing the main role in the efficiency determination. The best performance was achieved when the solar cell was prepared with the electrolyte containing 20 wt% of LiI, with efficiencies of 3.3 and 3.5% at 100 and 10 mW cm−2 of irradiation, respectively. The use of a TiO2 blocking layer beneath the titania film, the introduction of a Al2 O3 layer in the nanoparticles, the increase of film thickness, and the use of a vacuum technique during electrolyte deposition [16] are important steps to push the efficiencies of these cells above 6%. Acknowledgments The authors acknowledge FAPESP (fellowships 05/56924-0 and 04/14829-8) and CNPq for financial support, Daiso Co. Ltd., Osaka, Japan, for providing the copolymer and Prof. Carol H. Collins for English revision. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2008.05.009. References [1] Q.B. Meng, K. Takahashi, X.T. Zhang, I. Sutano, T.N. Rao, O. Sato, A. Fujishima, H. Watanabe, T. Nakamori, Langmuir 19 (2003) 3572. [2] J.E. Kroeze, N. Hirata, L. Schmidt-Mende, C. Orizu, S.D. Ogier, K. Carr, M. Gratzel, J.R. Durrant, Adv. Fucnt. Mater. 16 (2006) 1832. [3] S.X. Tan, J. Zhai, B.F. Xue, M.X. Wan, Q.B. Meng, Y.L. Li, L. Jiang, D.B. Zhu, Langmuir 20 (2004) 2934. ¨ [4] P. Wang, S.M. Zakeeruddin, J.E. Moser, M. Gratzel, J. Phys. Chem. B 107 (2003) 13280. [5] N. Yamanaka, R. Kawano, W. Kubo, N. Masaki, T. Kitamura, Y. Wada, M. Watanabe, S. Yanagida, J. Phys. Chem. B 111 (2007) 4763. [6] G.P. Kalaignan, M.S. Kang, Y.S. Kang, Solid State Ionics 177 (2006) 1091. [7] M.S. Kang, J.H. Kim, J. Won, Y.S. Kang, J. Photochem. Photobiol. A 183 (2006) 15.

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