On the Mass Transport in Apparently Iodine-Free Ionic Liquid

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On the Mass Transport in Apparently Iodine-free Ionic Liquid Polyaniline Coated Carbon Black Composite Electrolyte in Dye-sensitized Solar Cell Henri Johannes Vahlman, Janne Kristian Halme, Juuso T. Korhonen, Kerttu Aitola, and Janne Patakangas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401401t • Publication Date (Web): 25 Apr 2013 Downloaded from http://pubs.acs.org on May 6, 2013

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The Journal of Physical Chemistry

On the Mass Transport in Apparently Iodine-free Ionic Liquid Polyaniline Coated Carbon Black Composite Electrolyte in Dye-sensitized Solar Cell Henri Vahlman, Janne Halme,∗ Juuso Korhonen, Kerttu Aitola, and Janne Patakangas Department of Applied Physics, Aalto University, Espoo E-mail: [email protected]

∗ To

whom correspondence should be addressed

1 ACS Paragon Plus Environment


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Abstract Volatile electrolytes are a stability concern in dye solar cells (DSC) due to their tendency to leakage. A composite electrolyte consisting of iodide based ionic liquid and polyaniline coated carbon black has been previously reported to provide good current transport while being leakage proof due to a quasi-solid structure and absence of volatile constituents. In this paper we investigate the operating principle of this type of electrolyte, and especially its exceptional feature of operating efficiently without added iodine. Absence of additive iodine is significant due to the fact that it is usually required to form the current carrying I− /I− 3 redox couple. We modified an electrolyte mass transport model from the literature to estimate the upper-limit for the charge transport capability of the composite electrolyte. Comparison of experimental results with the estimated upper-limit for the diffusion limiting current density shows clearly that the high current densities observed experimentally with the composite electrolyte can not be explained with normal diffusion even in the case that every feasible source and transport mechanism of free I–3 known until now is considered, including photogeneration − − of I− 3 , shortened diffusion layer thickness, impurity I3 and accumulation of I3 to the photo-

electrode from the counterelectrode pores and electrolyte edge regions. This intriguing result suggests a currently unknown I− 3 source or transport mechanism in this type of DSC.

KEYWORDS: photovoltaics; carbon nanoparticle; quasi-solid electrolyte; extended electron transfer surface

Introduction Dye solar cell (DSC) 1 has a structure and materials that enable fabrication of colorful, semitransparent, flexible and lightweight modules. These traits combined with roll-to-roll adaptability and relatively low investment costs of necessary manufacturing equipment have ensured growing research interest over the past two decades. However, technological and material development is still required, in particular to improve the operating lifetime and reliability of DSC modules. One critical issue with this respect is possible leakage of the volatile liquid electrolyte commonly used in 2 ACS Paragon Plus Environment

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DSC, due, for example, to a sudden sealant failure or a more gradual diffusion through the sealant materials. A possible pathway to solve problems related to electrolyte volatility and leakage is to utilize electrolytes based on ionic liquids (IL) that have practically zero vapor pressure at all conceivable solar cell operating temperatures. Among different classes of ILs, imidazolium iodides are an often preferred choice due to their relatively low viscosity and the fact that they inherently contain high I− concentration which is necessary for fast dye regeneration. Nevertheless, their viscosity still being much higher than that of organic solvents, a relatively high amount of additive I2 is needed to support a diffusion flux of I− 3 from the photoelectrode to the counterelectrode (practically all − additive I2 reacts with electrolyte I− and forms I− 3 ). High I3 concentration, however, has the

downside of increasing recombination losses at the photoelectrode, which tends to decrease the open circuit voltage of the cell. On the positive side, high I− content has been found to give rise to a specific Grotthus-type bond exchange mechanism, which gives imidazolium iodide based ionic liquids better charge transport properties than expected on the basis of their relatively high viscosity. 2 To further improve charge transport properties of ionic liquid electrolytes, several different approaches have been taken. These include mixing imidazolium iodide salts with lower viscosity non-electroactive ionic liquids, 3 mixing different imidazolium iodide salts together in order to form low viscosity eutectic melts, 4 shortening the diffusion path by fabricating straight ion paths, 5,6 and attempting to improve the Grotthus bond exchange by dispersing either conducting, 7 semiconducting 7,8 or insulating nanoparticles 9,10 into the electrolyte. In many cases, the nanoparticle dispersion approach had the additional benefit of quasi-solidifying the electrolyte, thus making it easier to deposit and less prone to escaping from a sealed DSC. A particularly interesting approach in regard to electrolyte gelation with nanoparticles was detailed by Ikeda et al.: 11 the authors made a composite electrolyte using only two electrolyte components; 11 wt.-% of conducting polyaniline-loaded carbon black (PACB) nanoparticles was mixed with 1,3-diethyleneoxide derivative of imidazolium iodide (EOImI), which after grinding



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in an agate mortar formed a highly viscous conductive black electrolyte paste. No added iodine was required for the electrolyte to operate efficiently in a DSC, but formation of triiodide from iodide anions of EOImI under illumination was considered sufficient to drive the device. In fact, added I2 was shown to be even detrimental to the DSC efficiency, for an unknown reason. This is the complete opposite of the situation that usually prevails in the case of ionic liquid electrolytes; normally a high amount of additive I2 is required to remove the I− 3 diffusion limitation, and additional I2 does not begin to deteriorate the cell performance until a relatively high I− 3 concentration is reached. 12 From the point of view of device optimization and improvement, and possible future novel applications of the composite electrolyte concept, it is important to know and understand the mechanisms that, on one hand improve the mass transport such that additive I2 becomes redundant, and on the other hand those that ultimately restrict the conversion efficiency of such an additive I2 free system. In this paper we focus on the first-mentioned point, that is, our objective is to provide clarity for the exceptionally efficient charge transport without added I2 . As to why the ionic liquid - carbon nanoparticle composites work so efficiently as DSC electrolytes without any added I2 , it has been proposed in the literature that a so called extended electron transfer surface (EETS) is formed between the electrodes. 13–15 According to the EETS model, electrons are transferred close to the photoelectrode through ohmic conduction taking place in the conductive and catalytically active carbon material. According to the EETS model, the efficiency enhancement observed in this type of DSC follows from improved charge transport through the electrolyte layer, deriving from the combined effects of photogeneration of I− 3 on one hand, and shortened I− 3 diffusion distance on the other. In the course of this paper, we examine to which extent the above mentioned improvement in electrolyte charge transport derives from these two hypothesis. Moreover, since one of the precursors for the synthesis of imidazolium iodides, iodomethane (CH3 I), usually contains trace amounts of diiodomethane (CH2 I2 ), we take into account the possibility that the ionic liquid contains a non-negligible concentration of impurity I− 3. For theoretical analysis, we utilize a well-known electrolyte diffusion model and modify it such that it can be justifiably used for obtaining a theoretical upper-limit estimation for the diffusion


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limiting current density in conditions that correspond closely to the ones present in the ionic liquid - carbon nanocomposite DSC according to the EETS hypothesis. Our results indicate that, although the above mentioned mechanisms can noticeably increase the limiting current density, there still remains an unexplained gap between the low limiting current density predicted by the modified theory and the experimentally observed high short circuit current density in this type of DSC. This unexpected result means that some fundamental aspects related to the operating principle of this type of composite electrolyte in a dye solar cell are not known yet, and an anomalous charge transport mechanism or triiodide source must be assumed in order to account for the exceptionally efficient charge transport.

Theory Our main tool in this paper is the application of a mathematical diffusion model of Papageorgiou et al., 16 which we modified to be able to provide an upper-limit estimation to jlim based upon the presumptions of the EETS-model, namely 1) reduced electrode separation, and 2) crucial role of photogenerated I− 3 in current transport in the electrolyte. The modifications we made to the original model are based on the following simplifying assumptions: A) We hypothesize that the carbon paste functions similar to a porous counter electrode, in which case jlim is not reached until all the carbon pores between the photoelectrode and the counterelectrode glass substrate are completely depleted of triiodide. 17 Moreover, we consider the possibility that I− 3 could accumulate from the electrolyte edge regions outside the photoactive area, into the photoactive area, as suggested by numerical simulations. 18 B) Tortuosity and constrictivity of the porous layers are neglected, and thus the porous structure has no effect on the diffusion coefficients of the redox species. C) Light-absorption profile in the photoelectrode is assumed uniform. We state here that the above assumptions work as to overestimate jlim as compared to reality, meaning that our estimates are valid as upper limits for the true jlim . We clarify this statement and grounds for the above assumptions below in this theory section and further in context with the results.



Photoactivity of the DSC originates from a mesoporous titanium dioxide photoelectrode (PE) stained with a photosensitive dye. Interspace between the photoelectrode and the opposing counterelectrode (CE), including the TiO2 mesopores, is filled with an electrolyte containing a chargemediating redox couple (I− /I− 3 ). In the cross-sectional schematic of a DSC in figure 1(a), we have marked the thickness of the mesoporous layer with the symbol lPE , thickness of bulk electrolyte layer with b, and, adjacent to both of these, we have separately denoted the electrolyte edge region. Depending on light absorbance of the dye, composition of the electrolyte, and mineral form, particle size and morphology of the photoelectrode, the optimal photoelectrode thickness (lPE ) is most often found to vary between 6 µ m and 15 µ m. 19,20 Due to practicalities related to cell sealing, a bulk electrolyte layer (b) of 10-20 µ m commonly exists between the mesoporous TiO2 layer and the counterelectrode catalyst surface. Although the transverse diffusion distance between the counterelectrode catalyst particles and the photoactive dye molecules is no more than a few tens of micrometers, sluggish diffusion in viscous ionic liquids usually becomes a current limiting factor without the addition of a considerable amount of additive I2 . 12,16,21 (a)

(b)

FTO substrate

FTO substrate

lPE

Bulk electrolyte

b

Photoelectrode

lPE

Edge region

Photoelectrode

Edge region

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l

Separator layer SP Porous carbon a counterelectrode

Counterelectrode (planar platinized)

Figure 1: (a): Cross-sectional schematic of a dye solar cell. lPE is the thickness of the mesoporous photoelectrode and b is the bulk electrolyte layer thickness. (b): Schematic of a multilayer cell geometry with a porous counterelectrode of thickness a and a separator layer of thickness lSP . Edge regions are areas that are occupied by the electrolyte but not by the photoelectrode layer. Considering presumption 1) above regarding electrode separation, it is interesting to examine the significance of the gap b in view of current transfer between the electrodes. According to the model derived by Papageorgiou et al., the limiting current density for the cell geometry of figure 1(a) is given by 16 bulk jlim

=

6ϕ FDI− cinit I− 3

lPE

1 + ϕ blPE

3

1 fPE (Aλ )

+ 3ϕ lPE + b

3 2


(

b lPE

)2 ,

(1)

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where ϕPE is the porosity of the mesoporous TiO2 photoelectrode, F is the Faraday constant, DI− 3

is the diffusion constant of triiodide,

cinit I− 3

is the initial open circuit triiodide concentration in the

electrolyte, Aλ is the absorbance of the dyed photoelectrode including electrolyte in the pores, and fPE (Aλ ) is an absorbance dependent factor that takes into account that photon absorption in the photoelectrode follows the Beer-Lambert law. It is to be noted here that the significance of the factor fPE (Aλ ) is related to the simplifying assumption C) regarding the photoelectrode light absorption profile. In the case of nonuniform absorption (Aλ > 0, fPE (Aλ ) < 1), light intensity decays as a function of absorption distance, which means that, on average, dye injection and regeneration take place farther from the counterelectrode than in the case of uniform absorption (Aλ = 0, fPE (Aλ ) = 1) thus increasing the average diffusion distance of the I− 3 ions. Increased average diffusion distance lowers jlim , which in equation (1) is accounted for with a value of the factor fPE (Aλ ) less than unity. 16 Here we use fPE (Aλ ) = 1 since our interest is to estimate the theoretical upper limit for the limiting current density. The next step in our analysis is to link the expression for jlim in eq. (1) to a cell geometry more suitable for a DSC with the ionic liquid-carbon nanocomposite electrolyte. Here we utilize a geometry presented in fig. 1(b), based on three adjacent porous layers; a photoactive mesoporous TiO2 electrode of thickness lPE , a non-sensitized separator layer of thickness lSP , and a porous counterelectrode of thickness a, depicting the carbon nanoparticles contained in the electrolyte formulation. 17 There are varied opinions in the literature as to where the interface between the cathodic structure, i.e. carbon nanoparticles bound by the ionic liquid, and the pure ionic liquid electrolyte medium in the pores of the photoelectrode film actually resides, and correspondingly whether the cathodic charge transfer occurs outside or inside the porous photoactive TiO2 structure. On one hand, the mesopores have been considered too narrow for the carbon nanoparticles to enter, 14 in which case charge transfer would optimally take place at the outer surface of the TiO2 layer. Thereagain, experimental results have been published according to which ionic liquidcovered carbon nanoparticles penetrate deep into the TiO2 mesopores and thus the cathodic charge transfer and dye regeneration would occur in very close proximity to each other. 22 Our theoretical



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analysis in this paper is based on our own SEM images, presented in the results part, according to which TiO2 pores remain mainly free of carbon nanoparticles and only the ionic liquid penetrates through into the mesoporous TiO2 structure. The expression for the limiting current density in the standard DSC structure (eq. 1 and fig. 1(a)) can be modified to yield an upper limit estimate for the limiting current density in the composSP ite electrolyte DSC in fig. 1(b). Making simplifying assumptions A)-C), and defining cPE, (@ jlim ) I−

3

as the average I− 3 concentration in the redox-active volume of the electrolyte at the limiting current conditions, that is, the PE and the separator layer pores in this case (I− 3 is depleted from CE pores at jlim ), we can rewrite equation (1) for the fig. 1(b) porous multilayer case as

SP

jlim =

SP 6ϕPE FDI− cPE, (@ jlim ) I−

3

3

lPE

SP lSP 1 + ϕϕPE lPE ( )2 , ϕPE lSP 3 lSP 1 + 3 ϕSP lPE + 2 lPE

(2)

where ϕSP and lSP are the porosity and the thickness of the separator layer respectively. Differences in eq. (2) with respect to eq. (1) become apparent if we consider that we have substituted fPE (Aλ ) = 1, and that the bulk electrolyte layer of thickness b and porosity of 1 has been replaced with a separator layer of thickness lSP and porosity of ϕSP . In practice, this means firstly that light absorption in the PE is assumed uniform, and secondly, that the CE pores have been entirely depleted of I− 3 , and the charge transfer current on the whole cathode structure is concentrated in the immediate vicinity of its interface with the separator layer. The last mentioned consideration is motivated by theoretical analysis of porous counterelectrodes, which has shown that the porous − cathode cavities function as a sort of I− 3 reservoir, providing I3 to regions of reduced concentra-

tion i.e. the photoelectrode and separator layers in this case. 17 Note that in the case of a planar CE, limiting current density is reached when I− 3 concentration at the planar electrode surface approaches zero. In the porous CE case, however, zero concentration on the entire cathode surface, corresponding to a fully developed diffusion limitation, requires the depletion of I− 3 from the whole porous structure. Therefore, as stated in the simplifying assumption A) above, our estimations for jlim are based on the presumption that, at the limiting current conditions, the entire supply of I− 3 8 ACS Paragon Plus Environment

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has diffused out of the carbon mesopores, and thus provides an additional contribution to the concentration of I− 3 in the other parts of the cell, in this case, the pores of the separator layer and the mesoporous TiO2 photoelectrode. To calculate an upper-limit estimation for jlim on the basis of eq. (2), we need a credible estimate for the average I− 3 concentration in the redox-active volume. Our estimation for this quantity is based on the following arguments: I) Since the ionic liquid-carbon nanocomposite electrolyte functions without additive triiodide, we must assume that the total concentration of I− 3 originates from either photogenerated or impurity triiodide, or both. II) As stated in simplifying assumption A), we assume that all I− 3 originally present throughout the electrolyte volume, including the edge regions, is accumulated in the PE and separator layers at the limiting current conditions. III) Assuming fast regeneration of oxidized dye molecules (i.e. negligible concentration of oxidized dye at steady state), all photoinjected electrons can be expected to originate from I− anions oxidized to one I− 3 molecule per each pair of injected electrons through reactions injection

γ + D −−−−→ D+ + e− (CB) regeneration

3 I− + 2 D+ −−−−−−→ I− 3 + 2 D,

(3) (4)

where D+ and D are oxidized and neutral dye molecules respectively, γ is an absorbed photon, and e− (CB) are electrons injected into the conduction band (CB) of TiO2 . The exact concentrations − of I− 3 and I thus depend on the illumination-induced total charge temporarily accumulated as

electrons in both trap and conduction band states of the mesoporous PE and the separator layer (in addition to the photoactive layer, electrons diffuse also in the separator layer since we assume here that it is made of semiconducting TiO2 particles). With arguments I) - III), the average I− 3 concentration at the limiting current conditions can be



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written as (valid for both cell geometries in fig. 1) imp SP cPE, (@ jlim ) = cphg − (@ jlim ) + c − (@ jlim ) I− I I3 3 3 ( ) TiO TiO nlight2 − ndark2 [(1 − ϕPE )lPE + (1 − ϕSP )lSP ] = 2NA [ϕPE lPE + ϕSP lSP + b] [ ] ϕPE lPE + ϕSP lSP + b + ϕa a + ϕedgeVedge cinit I− + 3 , [ϕPE lPE + ϕSP lSP + b] TiO

(5)

(6)

TiO

where nlight2 and ndark2 are PE electron densities under illumination and in the dark respectively, lSP and ϕSP are respectively the thickness and the porosity of the separator layer, Vedge and ϕedge are respectively the volume and porosity of edge regions occupied by electrolyte, cphg (@ jlim ) and I− 3

− cimp (@ jlim ) are respectively the average photogenerated I− 3 and the impurity I3 concentrations in I− 3

the redox-active volume at limiting current conditions, and cinit is the initial open circuit (impurity) I− 3

I− 3 concentration in the dark. Here we have assumed for simplicity that the volume of the dye monolayer is negligible. We have also associated the carbon nanocomposite layer with a porosity following from the quasi-solidification of the electrolyte, given by

ϕa =

msol ρsol mdisp ρdisp

sol + mρsol

( =

mdisp ρsol +1 msol ρdisp

)−1 ,

(7)

where msol and ρsol are the mass and the density of the solvent respectively, and mdisp and ρdisp are the mass and the density of the dispersed carbon nanoparticles respectively. To quantify the significance of impurity I− 3 contained in pure PMII we require an estimation for the term cinit in eq. (6). For experimental evaluation of cinit we consider here a cell consisting of two I− I− 3

3

symmetric, parallel planar electrodes and pure PMII as electrolyte. As there is no photoelectrode in this kind of cell, it is clear that there will be no photogeneration of electrons either. Moreover, in eq. (6), all dimensional terms except b will equal to zero. We can thus see that in eq. (6), regardless of the cell voltage, current density or the cell dimensions, the total I− 3 concentration will equal to the initial concentration cinit . For a planar symmetric cell, relationship between jlim and DI− can I− 3

3


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be written as 23 DI− = 3

δdiff jlim , 4zFcI−

(8)

3

where z is the valence number and δdiff is the diffusion layer thickness equaling half the electrode separation. On the other hand, DI− can be expressed with respect to the characteristic frequency of 3

diffusion in a finite layer (ωdiff ) as 23 2 DI− = ωdiff δdiff . 3

(9)

Combining equations (8) and (9) allows us to express the (impurity) I− 3 concentration in a symmetric planar cell as cinit = I− 3

jlim , 4zF ωdiff δdiff

(10)

where z = 2 in this case. An experimental estimate of cinit can be obtained by determining jlim by I− 3

cyclic voltammetry and ωdiff by electrochemical impedance spectroscopy.

Experimental Cell materials and assembly The substrate material was fluorine-doped tin oxide (FTO) coated glass (Pilkington TEC-15, 15 Ω/sq, Hartford Glass Company, Inc.). Two electrolyte filling holes 1 mm in diameter were drilled in TEC-15 substrates to be used as counterelectrodes in liquid electrolyte DSCs. After rinsing with washing detergent, the substrates were ultrasonicated in ethanol and then in acetone, three minutes in both. The conducting side of the photoelectrode side substrates was then coated with a ∼35 nm overlayer of TiO2 through atomic layer deposition (ALD) to ensure electrical isolation of FTO from the conducting ionic liquid-carbon nanoparticle electrolyte paste. Photoactive mesoporous TiO2 layers were prepared by screen-printing a TiO2 paste (Dyesol 18NR-T) on the center of the ALD-deposited area of the substrates in a rectangular pattern 0.4 cm2



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in area. Two layers ∼ 4µ m thick each were deposited in each printing cycle, followed by a ten minute drying period on a hotplate at 110 ◦ C. The photoactive layers were coated with a ∼ 4µ m thick light-scattering overlayer of porous anatase TiO2 with a particle diameter greater than 100 nm (Solaronix Ti-Nanoxide R/SP) by screen-printing. The printed substrates were sintered for thirty minutes at 450◦ C. The sintered photoelectrodes were placed overnight in a dye bath of 0.32 mM cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)ruthenium(II) bistetrabutylammonium (Dyesol N719, used as received) in AA-grade ethanol (min. 99.5 wt. %). After dyeing, the photoelectrodes were rinsed with A14 grade ethanol (91.2 wt. %) and placed in storage embedded in A14 ethanol. Platinized counterelectrodes (PtCE) were prepared by thermal platinization of the conducting side of a TEC-15 substrate with a 10 mM solution of platinum tetrachloride (PtCl4 , 99.99 %, Aldrich) in 2-propanol (99.99 %, Sigma-Aldrich) at 385 ◦ C for 15 minutes. Organic liquid electrolyte (denoted OLE in the text) composed of 0.05 M iodine (99 %, Merck), 0.5 M N-methylbenzimidazole (99 %, Sigma-Aldrich), 0.5 M 1-propyl-3-methylimidazolium iodide (PMII, >99 %, Iolitec) and 0.1 M Guanidinium thiocyanate (>99 %, Merck) in 3-methoxypropionitrile (MPN, >99 %, Alfa-Aesar) was mixed in a measuring bottle in room air and left enclosed for stirring overnight. Dry ionic liquid electrolyte (denoted ILE (dry) in the text) was prepared such that the ionic liquid (PMII, >98 %, Iolitec) was first dried in a vacuum oven at 100 ◦ C under reduced pressure overnight and moved into a glovebox with a humidity level of less than 10 ppm, after which 0.26 M iodine, 0.5 M N-methylbenzimidazole and 0.1 M Guanidinium thiocyanate were mixed into the dried PMII and the composition was stirred overnight. Electrolyte denoted simply ILE was prepared with similar components in room air and thus contains some water absorbed from ambient air. Ionic liquid-carbon nanocomposite electrolyte (denoted PMII/PACB) was prepared such that 0.5 grams of 20 wt. % polyaniline on carbon black (PACB, Sigma-Aldrich) was dispersed into 4.0 grams of PMII by grinding in an agate mortar for ∼30 minutes in room air to form a black clay-like


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composite shown in figure 2(a). Pure PMII reference was stirred ∼50 minutes in an open container exposed to room air so that the absorbed water contents of the pure PMII and the PMII/PACB composite would approximately concur to each other. (a)

(b)

Figure 2: (a): The PMII/PACB composite prepared by grinding with a viscous paste-like appearance. (b): The PMII/PACB composite used as an electrolyte in a dye solar cell. R Photo- and counterelectrodes were joined by melting a DuPontTM Surlyn⃝1702 frame sealant

between them on a hotplate adjusted to 110 ◦ C. Fabrication of the symmetric platinized counterelectrode (PtCE-PtCE) cells follows the pattern described above with the exception that the photoelectrode was replaced by a second, identical platinized counterelectrode. Thickness of the frame sealants was (24.4 ± 0.6) µ m as measured by profilometry. DSCs were also prepared with a reduced electrode separation, referred to as pure PMII (thin) in the text. These DSCs were prepared with a thinned-down spacer sealant bringing the planar counterelectrode into physical contact with the separator layer, thus reducing the bulk electrolyte layer thickness negligible. For thinning, the Surlyn was heated slightly above its Vicat softening temperature of 65 ◦ C using a hot air gun, and stretched with the help of a bench vice and a chip clip until the thickness of the foil was approximately half of the initial value. The sealant thicknesses were further confirmed by profilometry. To prepare the PMII/PACB DSCs, a layer of the PMII/PACB paste was doctor-bladed on plain FTO-glass using a tape mask. No additional catalyst, e.g. Pt, was used on the FTO surface. A frame foil was then carefully positioned around the deposited PMII/PACB paste while avoiding penetration of the paste between the substrate and the sealant, after which a dyed photoelectrode was pressed against the counterelectrode on a hotplate to close the cell. A picture of a closed PMII/PACB DSC can be seen in fig. 2(b). Liquid electrolyte DSCs, on the other hand, were filled after the frame sealing through two filling holes that were consequently blocked with a 45 mi13 ACS Paragon Plus Environment


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R crometers thick Surlyn⃝1601 cover sealant melted between the backside of the counterelectrode

substrate and a microscope cover glass. Electrical contacts were prepared with copper tape and conducting silver paint. Epoxy was applied on top of the paint to protect the silver layer and to provide mechanical integrity. An opaque black tape mask with an opening of 0.7 cm2 in area was glued on the photoelectrode side substrate to minimize the effect of stray light during a jV -measurement. SEM samples were prepared by opening the cells and cutting the photoelectrode substrate with a glass cutter.

Measurements and equipment jV -curves were measured with a non-commercial solar simulator utilizing ten 150 W halogen lamps. The measured current densities were corrected for a spectral mismatch factor of 0.94. Irradiation power was adjusted to 1000 W/m2 with the help of a silicon calibration cell to correspond to AM 1.5G conditions. The jV -behavior of an illuminated DSC was measured with a Keithley 2420 3A SourceMeter. Electrochemical impedance spectroscopy (EIS) response of the symmetric PtCE-PtCE cells was measured with a Zahner IM6 Impedance Measurement Unit with an R-MUX multiplexer card. The frequency range used was from 10 mHz to 100 kHz. Amplitude of voltage modulation was 10 mV, and bias voltage was fixed to 0 V. Zview 2 software by Scribner was used for equivalent circuit fitting. Screen printed thick-film thicknesses were measured using a Dektak 6M stylus profiler profilometer by Veeco Instruments. Viscosity measurements were performed with a Physica MCR 301 rheometer (Anton Paar GmbH). Inbuilt temperature control system was used to determine the temperature dependence of viscosity. All reported temperatures excluding the viscosity measurement were measured with a type K thermocouple attached to a Testo 925 thermocouple thermometer. Scanning electron microscopic (SEM) images were taken with a Zeiss Sigma VP. Acceleration voltage was 10 kV in the case of figures 4(a), 4(b) and 4(d), and 1 kV in the case of fig. 4(c). 14 ACS Paragon Plus Environment

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The reported experimental results are an average over a minimum of three samples. We have used the standard error of the mean to estimate component errors, and the total differential for estimating the maximum error of functions of several component variables.

Results and discussion To obtain a good picture of the overall operation of the ionic liquid-carbon nanoparticle (PMII/PACB) electrolyte DSC, we first compare it with chosen reference cases, namely pure PMII and standard MPN based electrolytes, in terms of current-voltage characteristics and especially the temperature behavior of jsc . The experimental jsc values are then compared with modeled upper-limit estimates for jlim revealing the main result of the paper.

Photovoltaic performance and its temperature dependence The ionic liquid-carbon nanocomposite electrolyte (PMII/PACB) greatly outperformed reference pure PMII electrolyte in its current carrying ability at room temperature, which can be seen in table 1 listing the photovoltaic parameters of our cells. Here we note that in our case the performance difference between the PMII/PACB electrolyte and pure PMII was strongly temperature dependent, in particular when it comes to jsc . This can be seen in fig. 3(a) illustrating the evolution of the current-voltage behavior of pure PMII DSCs during several consecutive back and forth voltage sweeps under the heat irradiance of the solar simulator lamps (without cooling). In addition to the factor of six increase in the short circuit current density, the open circuit voltage dropped by ∼50 mV when the photoelectrode substrate outer surface temperature (TS ) increased from ∼30 ◦ C to ∼50 ◦ C. The current density increase can be linked to exponentially declining viscosity of pure PMII as a function of temperature as shown in figure 3(b), and a consequent enhancement in the limiting current density. As seen in fig. 3(b), the viscosity of pure PMII is decreased by a factor of ∼5 with temperature climbing from 25 ◦ C to 50 ◦ C, in fair agreement with the observed increase in the short circuit current density.



Table 1: Photovoltaic parameters measured close to room temperature, including the efficiency (η ), the short circuit current density ( jsc ), the open circuit voltage (Voc ) and the fill factor (FF) of DSCs with a carbon nanoparticle-ionic liquid composite, a pure PMII or an OLE electrolyte. Neither efficiency nor fill factor of the pure PMII cells were determined due to insufficient ambient temperature control in the solar simulator for these samples.

η jsc (%) (mA cm−2 ) PMII/PACB 1.0 ± 0.2 2.8 ± 0.3 pure PMII 0.5 ± 0.2 pure PMII (thin) 0.76 ± 0.05 ILE 1.5 ± 0.2 4.2 ± 0.4 OLE 4.2 ± 0.7 9.0 ± 0.7 electrolyte

(a) pure PMII

dynamic viscosity (cp)

current density (mA cm-2)

TS = 54 °C

Voc FF (mV) 506 ± 9 0.69 505 ± 6 493 ± 9 581 ± 9 0.62 722 ± 7 0.64

(b) pure PMII viscosity

increasing temperature: voltage sweep direction:

TS = 34 °C voltage (V)

temperature (°C)

TS = 29 °C

(c) PMII/PACB

TS = 53 °C

increasing temperature:


increasing temperature: voltage sweep direction:


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voltage (V)

TS = 37 °C

(d) MPN

TS = 51 °C

voltage (V)

Figure 3: Initial jV -curves at 25-35 ◦ C and the effect of solar simulator lamp-induced cell surface temperature (TS ) increase for DSCs with different electrolytes. Temperature dependence of the dynamic viscosity (µ ) of pure PMII is shown in (b).


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On the other hand, the temperature behavior of PMII/PACB, as seen in fig. 3(c), differed notably from that of pure PMII. As the cells heated up, instead of increasing such as in the pure PMII case, jsc of the PMII/PACB cells decreased to about 80 % of the starting value. Simultaneously, Voc dropped ∼50 mV similar to what was observed with pure PMII. Based on an expected Arrheniustype exponential temperature dependence of the recombination rate constant we assume here that the decline in both jsc and Voc with increasing temperature derives from magnified recombination losses with a possible contribution from a downward shift of the TiO2 conduction band energy. A similar phenomenon was also visible in the reference MPN cells (fig. 3(d)), although in much smaller scale due to considerably lower recombination current density to begin with. Finally, we note that at ca. 54 ◦ C, jsc was higher with pure PMII than with PMII/PACB. Without a possibility for a detailed analysis, here we tentatively assign the lower jsc in the PMII/PACB cells to recombination losses due to direct contact of the semiconducting spacer layer with the PACB particles that can catalyze the I− 3 reduction reaction. The above results give support to earlier publications 14,22,24,25 in the sense that considerably higher photocurrents were obtained with an electrolyte where an ionic liquid (here PMII) was quasi-solidified with carbon nanoparticles ( jsc ≈ 3 mA/cm2 ), rather than merely using pure PMII as electrolyte ( jsc ≈ 0.5 mA/cm2 ), close to room temperature (∼30◦ C). Minimizing the bulk electrolyte layer in the pure PMII (thin) case narrowed the difference only slightly ( jsc ≈ 0.76 mA/cm2 ). These results strongly suggest that quasi-solidification of pure PMII with carbon nanoparticles, albeit raising an issue of significant current loss through recombination, expedites charge transfer to such a degree that diffusion limitation observed with pure PMII is removed without the need for additive iodine. Moreover, minimizing the bulk electrolyte layer thickness improved the photocurrent only slightly in the pure PMII case, which indicates that the two main hypothesis for the PMII/PACB electrolyte operation, namely photogeneration of I− 3 and shortened diffusion layer thickness, are not sufficient to explain the high jsc of the PMII/PACB DSCs. In this work, efficiencies were considerably lower than in the earlier publications where corresponding electrolyte compositions of ionic liquid and carbon nanoparticles were used, and ef-



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ficiencies up to 6.37 % have been reported. 11,13–15,22,24–27 This is most likely due to the lack of TiCl4 treatment on the sintered photoelectrodes and/or thinner ∼ 8 µ m photoelectrodes in the present case. However, as will be discussed below, our experimental jsc values are sufficiently high compared to the theoretical upper limit for jlim in order to confirm the incapability of previously presented rationalizations (photogeneration of I− 3 and shortened diffusion layer thickness) or newly suggested in this paper (impurity I− 3 ) in explaining the operation of the PMII/PACB DSC. Considering that our jsc values were comparably low, the above conclusion should also be valid regarding the results of the earlier publications where higher values of jsc were obtained and the discrepancy between the modified model used here and experimental results is even greater. In the following, we seek further understanding on this result by estimating the upper limit of jlim enhancement deriving from various sources described in the theory section. First, however, we confirm through SEM imaging that the actual layer structure of the PMII/PACB DSCs is in agreement with the structure assumed in the theoretical modeling.

Supporting evidence for the modeled multilayer cell geometry from SEM imaging Scanning electron microscopic (SEM) images provided confirmation that the multilayer structure depicted in fig. 1(b) is in good correspondence with the actual structure of the PMII/PACB cells. In view of modeling considerations, it was important to rule out the possibility that the PACB particles penetrate into the separator layer, and possibly even further into the mesoporous photoelectrode layer. Should this mixing of the layers occur, the carbon material, functioning as a cathodic structure in our model, would penetrate even closer to dye molecules, and eq. (2) expression would no longer represent an upper limit for jlim . The acquired SEM images strongly suggest that PACB particles do not penetrate farther than one or two micrometers into the separator layer, and that the rest of the spacer layer and the mesoporous photoelectrode layer are PACB-free. Therefore we can conclude that, from the diffusion modeling viewpoint, fig. 1(b) schematic is a valid depiction of the PMII/PACB DSC. 18 ACS Paragon Plus Environment

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(a)

FTO

(b)

lPE

lPE 5 µm

lSP lSP

0.5 µm

(c)

(d)

lSP a 10 µm 5 µm

a

Figure 4: (a): SEM image of the photoelectrode with the FTO glass on top, the mesoporous layer composed of ∼ 20 nm TiO2 particles in the middle, and the separator layer composed of large > 100 nm TiO2 particles undermost. The PMII/PACB layer is not visible below the separator layer due to the last mentioned being cracked during microscopic sample preparation. (b): Interface between the mesoporous TiO2 and the separator layer with high magnification. Neither of the layers show any sign of PACB penetrating into the pores. (c): SEM image of the PMII/PACB layer. A portion of the cracked spacer layer is visible at the top of the image, into which the PMII/PACB electrolyte has penetrated a short distance (about a micrometer thick white stripe). (d): Magnification of the interface between the PMII/PACB layer (bottom) and the separator layer (top). The layers can be clearly distinguished by their colour. Judging by the clearly porous structure of the topmost area depicting the cracked separator layer, PACB seems not to have penetrated more than 1-2 µ m into the pores.



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− Estimation of impurity I− 3 and photogenerated I3 concentrations at the limit-

ing current conditions In the expression for the average redox-active volume I− 3 concentration in eq. (6), impurities and photogeneration were taken into account as possible sources of non-additive I− 3 . Here we firstly estimate the impurity I− 3 concentration experimentally by using a combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques as described in the theory section. Secondly, assessing the amount of photogenerated I− 3 on the basis of eq. (6) requires knowledge about the photoelectrode electron density under operating conditions, which we have approximated here based on the literature. init Open circuit I− 3 concentration cI− of pure PMII was measured from symmetric PtCE-PtCE 3

cells at Vbias = 0 V by determining ωdiff using EIS and jlim through CV, and substituting both quantities into eq. (10). Table 2 shows the above mentioned quantities for different electrolytes, and in addition, diffusion coefficients calculated on the basis of eq. (9). Triiodide concentrations of OLE and ILE (dry) electrolytes obtained with the above described method correspond well with their respective additive iodine concentrations of 50 mM and 260 mM. Additive I2 being the only significant source of I− 3 in the OLE, the good correspondence between the added and the measured I− 3 concentrations shows that the method of eqs. (8)-(10) gives reasonably accurate estimates for − the free I− 3 concentration. The method gives relatively low but significant free I3 concentration of

(8 ± 2) mM for the pure PMII, which in the absence of additive iodine corresponds fully to the − impurity I− 3 . It is noteworthy that this impurity concentration is only 3 % of the additive I3 in the

ILE. Photoelectrode electron density has been investigated earlier in the literature through time integration of photocurrent decay transients. 28,29 In this method, a DSC is initially illuminated from the photoelectrode side for 5 s with a red light emitting diode at open circuit. After a short voltage decay period in the dark, the residual charge is extracted from the photoelectrode at short circuit conditions. Varying the decay period a voltage-charge relationship is obtained. 29 It has been found that while the solvent type has a subtle effect on the electron density at a given photovoltage, 28 20 ACS Paragon Plus Environment

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Table 2: Diffusion-related parameters for the studied electrolytes (from PtCE-PtCE-cells). Characteristic frequency of diffusion (ωdiff ) was determined through EIS at Vbias = 0 V. Limiting current density ( jlim ) was measured through CV and substituted along with ωdiff into eq. (10) to obtain the concentration of I–3 . The triiodide diffusion coefficient DI− was calculated 3

from eq. (9). cadd corresponds to the concentration of additive I− 3. I− 3

ωdiff (EIS)

electrolyte OLE ILE (dry) pure PMII

DI− (eq. (9)) 3

[s−1 ] [cm2 s−1 ] 2.7 ± 0.3 (4.0 ± 0.6) · 10−6 0.12 ± 0.02 (1.7 ± 0.3) · 10−7 0.090 ± 0.004 (1.3 ± 0.2) · 10−7

jlim (CV)

cadd I− 3

[mA cm−2 ] [mM] 38 ± 2 50 7±1 260 0.160 ± 0.002 0

(eq. (10)) cinit I− 3

[mM] 60 ± 20 260 ± 90 8±2

certain additives such as TBP1 can reduce the charge density up to a factor of five. 29 Based on the figure 3(a) of Paulsson et al. 28 and figure 2 of Boschloo et al., 29 we estimated the difference TiO2 2 between electron densities under illumination (nTiO light ) and in the dark (ndark ), i.e. the photoinjected TiO

TiO

electron density, to vary between nlight2 − ndark2 ∼ 1018 cm−3 ...1019 cm−3 in the relevant DSC operating voltage range of ∼ 0.4V...0.6V. From the viewpoint of our effort to find an upper limit for TiO

TiO

jlim , it is crucial not to underestimate the value of this quantity. Therefore, nlight2 − ndark2 = 1019 cm−3 was used in our calculations, which, substituted into eq. (6), gives an I− 3 concentration ranging from few to few tens of mM depending on cell dimensions, which is in the same order of magnitude as the impurity I− 3 concentration of pure PMII.

Theoretically predicted upper-limits for jlim and comparison to experimental results Redox-active volume average I− 3 concentration (see eq. (6)) and consequently the theoretical upper-limit estimation for jlim (see eq. (2)) were modeled separately with two different presumptions; in the first we consider that all I− 3 in the DSC edge regions (see fig. 1) accumulates into the redox-active volume as suggested by numerical simulations, 18 whereas in the second we omit the edge region entirely to get an idea of the theoretical significance of this accumulation phenomenon on jlim . 1 tert-butylpyridine



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SP Comparison of modeled jlim values in table 3 with experimental results in table 1 shows that,

while experimental behavior of all-liquid electrolytes is within reasonable consistency with the model, the PMII/PACB electrolyte exhibits anomalously high experimental jsc with respect to the SP diffusion model. For the pure PMII electrolyte, the model yielded a jlim of 0.27-0.57 mA cm−2 ,

coinciding fairly well with the experimentally observed jsc of 0.5 ± 0.2 mA cm−2 . For the organic SP liquid electrolyte (OLE), the model reproduced a jlim of 50-118 mA cm−2 , whereas in the dry

ionic liquid electrolyte (ILE (dry)) case it was 8.9-22 mA cm−2 , both of which are, as expected, considerably higher than jsc (4.2 ± 0.7 mA cm−2 with OLE and 1.5 ± 0.2 mA cm−2 with ILE). SP of 0.85On the other hand, in the case of the PMII/PACB electrolyte, the model yielded jlim

1.7 mA cm−2 which is one-and-a-half to three times lower than the experimentally observed jsc of 2.8 ± 0.3 mA cm−2 . The difference between the theory and experiments is emphasized when we consider that jsc values of over 9 mA cm−2 have been reported 13,14 for this type of electrolyte with very similar materials and layer thicknesses as in our case, when in fact, according to the modified model, jlim should not exceed 1.4-2.9 mA cm−2 in the literature case (without better knowledge we assumed the volume of the edge region in ref. 14 to be the same as in our case, see table 3 for parameter values in this case). Based on the experimental results in table 1 we already concluded that most of the anomalous jsc increase has its origins in the properties of the PMII/PACB layer other than its effect of reducing the diffusion layer thickness. Modeling considerations support this conclusion; according to the SP model, the jlim of pure PMII cells with a thin sealant resides at 0.42-0.88 mA cm−2 , which is in the

same order of magnitude as the experimental value of jsc 0.76 ± 0.05 mA cm−2 for the pure PMII SP (thin) cells. Comparing these values to the modeled jlim of 0.27-0.57 mA cm−2 and experimental

jsc of 0.5±0.2 mA cm−2 for the pure PMII cells with a sealant of normal thickness, we can see that minimizing the bulk electrolyte layer brought approximately a one-and-a-half fold jlim increase in both modeled and experimental results. However, as already mentioned, this improvement was not enough to explain the high jsc in the PMII/PACB cells. We are thus led to conclude that despite the model being valid for all-liquid electrolytes, it does not explain jsc of the PMII/PACB


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Table 3: Modeled jlim and redox-active volume average I− 3 concentrations at limiting current conditions, and the various parameters used in the modeling of the different electrolytes. We assumed that in the PMII/PACB electrolyte the impurity I− 3 concentration was 8 mM such as in pure PMII. Literature 14 values correspond to layer thicknesses reported in conjuction with an achieved jsc of over 9 mA cm−2 with a PMII/PACB type of DSC (other parameters assumed the same as in this work). Thin sealant (as opposed to normal sealant) corresponds to minimized bulk electrolyte layer thicknesses in the pure PMII (thin) cells (see table 1). In the case of the PMII/PACB and the pure PMII electrolytes, depletion of impurity I− 3 from SP the porous CE causes a significant increase in cPE, (@ j ) as compared to the initial open lim I− 3

circuit concentration cinit , and is further magnified if we take accumulation of I− 3 from the I− 3 edge regions into account. lPE

lSP

b

a

[µ m] [µ m] [µ m] [µ m] PMII/PACB This work Literature 14 pure PMII normal sealant thin sealant reference ILE (dry) OLE

{

7.6 { 7.6 10 10 {

7.6 { 7.6 7.6 7.6 {

7.6 { 7.6 7.6 7.6

Vedge

ϕPE

ϕSP

ϕa

ϕedge

TiO

DI−

cinit I− 3

SP cPE, (@ jlim ) I−

SP jlim

28,29 refs [ −3 ] cm

from [ 2eq.−1(9) ] cm s

from eq. (8) [mM]

from eq. (6) [mM]

from eq. (2)] [ mA cm−2

0.91 0.91 -

1019 1019 1019 1019

0.13 · 10−6 0.13 · 10−6 0.13 · 10−6 0.13 · 10−6

8 8 8 8

67 33 78 38

1.7 0.85 2.9 1.4

[µ l] 0.5 0.91 0.5 0.91 - 0.91 - 0.91

TiO

nlight2 − ndark2

3

3

3.8 3.8 0 0

0 0 0 0

13 13 15 15

1.1 0 1.1 0

0.5 0.5 0.5 0.5

3.8 3.8 3.8 3.8

13 13 0 0

0 0 0 0

1.1 0 1.1 0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

-

1 1 -

1019 1019 1019 1019

0.13 · 10−6 0.13 · 10−6 0.13 · 10−6 0.13 · 10−6

8 8 8 8

22 11 34 16

0.57 0.27 0.88 0.42

3.8 3.8 3.8 3.8

13 13 13 13

0 0 0 0

1.1 0 1.1 0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

-

1 1 -

1019 1019 1019 1019

0.17 · 10−6 0.17 · 10−6 4.0 · 10−6 4.0 · 10−6

260 260 60 60

636 263 149 63

22 8.9 118 50



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cells, not even when all impurity and photogenerated I− 3 contained in the electrolyte is assumed to accumulate to the redox-active region and contribute to the charge transport. The above considerations suggest that charge transport in the PMII/PACB electrolyte cannot be explained by normal diffusion given the amount of I− 3 deriving from feasible known sources. To account for the exceptional charge transport behavior requires assuming either a considerably shorter diffusion layer thickness than expected, an in-pore diffusion coefficient notably exceeding that of the bulk electrolyte, or an unidentified I− 3 source. However, as was evident based on the fig. 4 SEM images, the PACB particles are not present in the TiO2 pores and thus it is unlikely that the diffusion layer thickness could be shorter than our optimistic estimation. Regarding inpore diffusion coefficient, charge carrier diffusion in organic liquid electrolytes has been verified to slow down in nanoporous layers. 30,31 In the case of ionic liquids, charge carrier diffusion has been reported to decrease in nanoporous materials, 32 although experimental evidence of accelerated in-pore diffusion has also been recently found in the case of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid in nanoporous silica. 33 Nevertheless, since diffusion limited jsc of the thin reference cells with pure PMII corresponded well with the modified diffusion model, it is unlikely that deviation from the model observed in the case of the PMII/PACB electrolyte is explained by accelerated in-pore charge transport in the PE and separator layer pores. Thus, within the framework of the theory, the most credible explanation for the high jlim in the PMII/PACB − − DSC is an unexpected I− 3 source in addition to the photogenerated I3 and impurity I3 that were

quantified here. As the PMII/PACB DSCs apparently seem to contain more I− 3 than expected, we consider here the concentration of I− 3 that would be required in order for the diffusion model and the experiments to agree. We calculated a minimum "apparent" I− 3 concentration required under illumination at limiting current conditions for the model to allow the experimentally measured jsc by solving for SP SP cPE, (@ jlim ) in eq. (2). Substituting jlim with the jsc of 2.8 mA cm−2 experimentally observed I−

3

in this work gives a minimum I− 3 concentration of 110 mM in the redox-active volume (PE and separator layer pores), which is 1.6 to 3.3 times higher than the estimated upper-limit concentra-


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tion (see table 3). Similar analysis for the literature 14 jsc of 9.2 mA cm−2 gives an apparent I− 3 concentration of 240 mM in the redox-active volume, which is 3.1 to 6.4-fold compared to the value determined in this work. We refrain here to speculate the possible origins for this high apparent extra I− 3 concentration. Instead, we settle for concluding that the high jsc observed with the PMII/PACB electrolyte cannot be explained by the combined effect of all the hitherto known mechanisms: photogeneration of − − I− 3 , shortened diffusion distance, impurity I3 from the PMII and accumulation of I3 from the edge

region to the active region. Whether the missing explanation is an unknown I3− source or a yet unidentified charge transport mechanism inside the mesoporous photoelectrode, requires further research.

Conclusions Improvement of short circuit current density of dye-sensitized solar cells (DSC) upon mixing polyaniline coated carbon black (PACB) in a pure 1-propyl-3-methylimidazolium iodide (PMII) ionic liquid electrolyte could not be explained by the previously proposed extended electron transfer surface (EETS) model even when all the following possible hitherto known sources and trans− port mechanisms for free I− 3 ions were taken into account: photogeneration of I3 , shortened diffu− sion distance, impurity I− 3 from PMII and accumulation of I3 from the edge region to the redox-

active volume of the electrolyte. Extraordinarily high jsc , unaccountable for by a diffusion model for liquid electrolyte DSCs, suggests the existence of an unknown triiodide source or an unidentified charge transport mechanism inside the photoelectrode pores. This means that further research is necessary to fully understand the electrochemical operating principle of the PMII/PACB electrolyte in the DSC.



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Acknowledgement The authors thank T. Vainikka and D. Lloyd for help with viscosity measurements and glovebox operation, K. Miettunen for expertise and assistance regarding substrate treatment and manuscript proofreading, Beneq Oy for ALD-coating the TiO2 compact layer, and M.I. Asghar for assistance with SEM imaging. This work was funded partially by Multidisciplinary Institute of Digitization and Energy (MIDE) of Aalto University (project CNBe), and partially by Tekes - the Finnish Funding Agency for Technology and Innovation, under the project Robust dye solar cells printed on metal (KesMPV), number 2928/31/2010.

References (1) O’Regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films. Nature 1991, 353, 737–40. (2) Kawano, R.; Watanabe, M. Anomaly of charge transport of an iodide/tri-iodide redox couple in an ionic liquid and its importance in dye-sensitized solar cells. Chemical communications (Cambridge, England) 2005, (16), 2107–2109. (3) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Grätzel, M. A New Ionic Liquid Electrolyte Enhances the Conversion Efficiency of Dye-Sensitized Solar Cells. Journal of Physical Chemistry B 2003, 107, 13280–13285. (4) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Grätzel, M.; Wang, P. Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes. The Journal of Physical Chemistry C 2008, 112, 13775–13781. (5) Kato, T.; Hayase, S. Quasi-Solid Dye Sensitized Solar Cell with Straight Ion Paths. Journal of the Electrochemical Society 2007, 154, B117–B121. (6) Kogo, T.; Hayase, S.; Kaiho, T.; Taguchi, M. Quasi-solid Dye Sensitized Solar Cells Having Straight Ion Paths. Journal of the Electrochemical Society 2008, 155, K166–K169. 26 ACS Paragon Plus Environment

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Graphical TOC Entry ALD-coated TiO2 thin layer

FTO substrate -

3I

photoelectrode short I3- diffusion distance

separator layer

ionic liquid carbon nanoparticle electrolyte paste

2 e-

I3I3- photogeneration impurity I3- accumulation ohmic conduction


On the Mass Transport in Apparently Iodine-Free Ionic Liquid

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