Investigation of Temperature and Aging Effects in Nanostructured Dye Solar Cells Studied by...

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2009, Article ID 786429, 15 pagesdoi:10.1155/2009/786429

Research Article

Investigation of Temperature and Aging Effects inNanostructured Dye Solar Cells Studied by ElectrochemicalImpedance Spectroscopy

Minna Toivola, Janne Halme, Lauri Peltokorpi, and Peter Lund

Department of Applied Physics, Advanced Energy Systems, Helsinki University of Technology, P.O. Box 5100, 02015 TKK, Finland

Correspondence should be addressed to Minna Toivola, [email protected]

Received 17 November 2008; Revised 28 May 2009; Accepted 6 August 2009

Recommended by A. Hagfeldt

Effects of aging and cyclically varying temperature on the electrical parameters of dye solar cells were analyzed with electrochemicalimpedance spectroscopy. Photoelectrode total resistance increased as a function of time due to increasing electron transportresistance in the TiO2 film. On the other hand, photoelectrode recombination resistance was generally larger, electron lifetimesin the TiO2 were film longer, and charge transfer resistance on the counter electrode was smaller after the temperature treatmentsthan before them. These effects correlated with the slower deterioration rate of the temperature-treated cells, in comparison to thereference cells.

Copyright © 2009 Minna Toivola et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Nanostructured, dye-sensitized titanium dioxide solar cell(DSC) [1] is a promising, easily manufacturable, and low-cost alternative to the conventional silicon and thin filmphotovoltaic devices. The highest reported energy conversionefficiencies exceed already 10% [2–4] but the long-termstability of the cells and the effects of varying environmentalconditions on them during their operation still requiremore detailed investigation before this technology can fullycompete against the more traditional and established solarelectricity solutions.

Compared to the amount of publications on the devel-opment of new cell materials or manufacturing methods,relatively few have concentrated on the cell aging or theeffects of temperature, air humidity, or other environmentalconditions on the cell performance (e.g., [5–23]). Also, inmany cases the cell function has been characterized mainly byIV-curve measurements. Electrochemical impedance spec-troscopy (EIS) is, however, another well-established andeffective method to probe deeper the fundamental chargetransfer phenomena in the cell components (e.g., [24–37]).Wang et al., for example, have used EIS to identify theorigins of changes induced by thermal aging on the DSC

performance [22]. They found that thermal stress decreasedthe electron lifetimes in the TiO2 film, while improvement inthe counter electrode performance was detected during thefirst few days of aging. Similar behavior was observed alsoby Kern et al. [23], that is, decrease in the electron lifetimesbut no degradation in the counter electrode catalyst duringthermal aging of the cells.

The objective of this study was to gain more profoundunderstanding and verify some previous observations aboutthe DSC’s temperature behavior and aging-induced changeson it. Understanding the temperature behavior of the DSCperformance has significant practical importance because inreal life applications the cells are often subjected to evendrastic changes in temperature, for example, if the modulesare placed on a rooftop and the temporal variation in theambient temperature is large. In our previous research it wasnoticed that repeatedly and regularly varying temperaturehad a regenerative effect on both the photo- and counterelectrode functions in the aged DSCs [38]. In [38] cellperformance was monitored as a function of time with IV-curve measurements while the cell temperature was variedin regular to-and-fro ramps and EIS was used to investigatethe charge transfer on the electrodes before and after everytemperature ramping, while keeping the cells at constant

2 International Journal of Photoenergy

room temperature. In the present study, the cell operatingtemperature was varied in ramps 20◦C→ 40◦C→ 70◦C→40◦C→ 20◦C, and the EIS spectra were measured at allaforementioned temperatures from fresh, 11-12 days, 29days, and 76–78 days, aged cells, after which an extensiveanalysis of the evolution of the impedance parameterswith time and temperature was performed. To follow thetemporal behavior of the overall cell performance, IV-curvemeasurements were also employed before and after everytemperature ramping.

Because temperature affects the open circuit voltageof the cell, another set of temperature-ramped EIS mea-surements was performed later, where the cell voltage wascontrolled and fixed by external polarization. This removedthe temperature-induced variation from the voltage andenabled better analysis of the temperature behavior of theimpedance parameters, such as charge transfer resistance andcapacitance of the electrodes.

2. Experimental

2.1. Cell Preparation. Dye solar cell photoelectrodes wereprepared on conducting glass substrates (Pilkington TEC-15, thickness 2.5 mm, sheet resistance 15Ω/sq., suppliedby Hartford Glass Company, Inc.) with doctor-bladingmethod from 20 wt% suspension of TiO2 nanoparticles(P25, Degussa) in ethanol. Three layers of the suspensionwere spread and compressed in a hydraulic press (125–250 kg/cm2) to improve the interparticle contacts and adher-ence to the substrate. After this half of the electrodes wassintered in 500◦C for 30 minutes, another half was leftnonsintered. The size of the substrates was 16 mm×20 mm,the electrode active area 0.32 cm2 (4 mm × 8 mm), and theelectrode thickness 12–14 μm. Sensitization of the electrodefilms was done by soaking them in 0.32 mM solution of theN-719 dye (cis-bis (isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium, So-laronix SA) in absolute ethanol for ca. 70 hours. Longsensitization time was used to ensure uniform and thoroughadsorption of the dye.

Platinized glass counter electrodes prepared with thestandard thermal platinization method, as described previ-ously [39], were employed in all cells.

The electrodes were sealed together on a hot plate(100◦C) using 50 μm thick Surlyn 1702 thermoplasticpolymer film as a spacer and sealant, and finally thecells were filled with either liquid or semisolid (gel) elec-trolyte. The composition of the liquid electrolyte was0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP)in 3-methoxypropionitrile. The semisolid electrolyte wasmade of the liquid by gelatinizing it with 5 wt% ofPVDF-HFP (polyvinylidenefluoride-hexafluoropropylene),a method described in literature [40]. Table 1 summarizesthe cell types prepared for this study.

2.2. Measurements. EIS spectra were recorded with ZahnerElektrik’s IM6 Impedance Measurement Unit operated withThales software and using a red LED of wavelength 640 nm

Table 1: Cell types.

Cell series Sintered photoelectrode Electrolyte

L-S Yes Liquid

G-S Yes Gel

L-NS No Liquid

G-NS No Gel

Rs

Rct W

TL

CPEce

Figure 1: Equivalent circuit used for the EIS spectra fits. Rs: seriesresistance of the cell. TL : transmission line element, as introducedby Bisquert, Fabregat-Santiago et al. (e.g., [24]), modeling thephotoelectrode and including the photoelectrode resistance Rpe

(consisting of the electron diffusion resistance in the TiO2 film(Rtr) and the recombination resistance on the TiO2-dye-electrolyteinterface (Rrec): i.e., Rpe = Rrec + Rtr) and the constant phaseelement CPEpe, describing the capacitance of the TiO2 film surface.Rct: charge transfer resistance on the counter electrode—electrolyteinterface. CPEce: constant phase element describing the counterelectrode capacitance. W : Warburg element associated with thediffusion in the electrolyte. Constant phase elements were usedinstead of pure capacitances because they describe better a realistic,uneven, and unhomogenous electrode surface [41].

and illumination intensity ca. 8% of the 1 sun irradiance(meaning, the photocurrent measured from the cell being ca.8% of what the cell would give under one sun illumination)as the light source. In the first set of measurements, thespectra were obtained at open circuit voltage with 10 mVvoltage amplitude in the 100 mHz–100 kHz frequency range,first at 20◦C, then at 40◦C and 70◦C, after which thetemperature was lowered and the spectra obtained againat 40◦C and 20◦C. Cell heating/cooling was arranged byplacing the measurement box with the cells and the LEDon a hot plate with internal temperature control by Pt-1000temperature sensors. Before every measurement, the cellswere let to stabilize at a given temperature for 10 minutes.The accuracy of the temperature control was ±2–4◦C.ZPlot/ZView software from Scribner Inc. was used forequivalent circuit fits of the EIS spectra. Figure 1 describesthe model circuit used in the fits.

IV-curves were measured at 20◦C before and after everytemperature ramping with a custom-built solar simulatorconsisting of ten 150 W halogen lamps as the light source, atemperature-controlled measurement plate, and a calibratedmonocrystalline silicon reference cell with which the lampscould be adjusted to provide the standard light intensity of100 mW/cm2. The spectral mismatch factor of the simulator,defined by the reference cell and the measured spectralirradiance of the halogen lamps, was used to make thecurves correspond with the standard AM1.5G equivalentillumination. A few cells of each series were chosen asreference cells which were let to age at constant temperature

International Journal of Photoenergy 3

in the dark. Their EIS spectra and IV-curves were measuredon the same days with the temperature-treated cells.

All measurements described above were performed withfresh, 11-12 days, 29 days (one month), and 76–78 days (twoand half months), aged cells. Between the measurements,the cells were stored in dark and at room temperature (22–24◦C) to provide controlled aging conditions. Aging in thedark was chosen because of the preliminary nature of theexperiments and to distinguish between the changes on thecell performance caused by constant light soaking.

The second set of EIS measurements was performedwith the sintered cells (three liquid and three gel electrolytefilled cells), on one day only, ca. three months after thelast measurements of the first set (the cells were in good,even condition, and their efficiencies still over 4% whichvalidated their use even this long after their preparation).Measurement parameters were the same as in the first set,but instead of obtaining the spectra at open circuit voltage,external bias voltage of −0.6 V, −0.65 V, −0.7 V, and −0.75 Vwas applied on the cell. Temperature rampings similar tothe first set of measurements were performed at everypolarization and the EIS spectra obtained at all temperatures.From here on, the terms “first set” and “second set” are usedto distinguish between these two types of EIS measurements.

3. Results and Discussion

3.1. IV-Curves. The aging and temperature behavior ofthe IV-curves was very similar for all cell types, as alsofound in the previous study [38]. The only differences werethe relatively higher short circuit currents and efficienciesobtained with sintered and liquid electrolyte filled cells incomparison to nonsintered and gel electrolyte cells. This isknown to be due to better TiO2 film quality (interparticleconnections and adhesion to the substrate) in sinteredelectrodes and better penetration of the liquid electrolyteinto the pores of the TiO2 layer.

The most striking feature in all temperature-treated cellswas improvement of their efficiencies during the first coupleof weeks of aging. This was due to increased open circuitvoltages and was observed with both temperature-treatedand reference cells. As depicted in Figure 2 and Supplemen-tary Table 2 (See Supplementary Material available online atdoi: 10.1155/2009/786929.) , the steepest voltage increaseshappened in the first temperature rampings on day 1 afterwhich only very small rise or none at all could be observed.Similar behavior was observed also with the reference cells,indicating that even if the temperature treatments mighthave worked in favor of this effect, they cannot be heldfully responsible of it. For the reference cells, the relativevoltage increase evened out with time also and was the largestbetween the first two consecutive measurement days, causinga simultaneous gain in efficiencies for the most of the cellseries.

The exact explanation for the open circuit voltageincrease remains still open but despite of the long dye-sensitization time, it can be assumed that the final equilib-rium between the dye, TBP, and other molecules adsorbed

on the TiO2 film surface was not yet reached when thefirst measurements were performed. Thus it is possible thatthere were, for example, excess dye molecules only poorlyattached to the film still left during the first measurementsand their desorption with time (possibly favored by thetemperature treatments) made adsorption possible for largeramounts of the other electrolyte species. As TBP is knownto increase the open circuit voltage [28], larger amount ofit on the film surface could at least partially explain thisbehavior. However, verification of this kind of assumptionscalls for different analysis techniques than IV-curve or EISmeasurements only and was outside the scope of this study.

Despite the decreasing overall trend in short circuitcurrents over time, the currents either remained the same orincreased, that is, were regenerated during the temperaturerampings, in most cases both for the fresh and aged cells. Thiswas also responsible for the relatively slower degradation ofthe sintered and temperature-treated cells when comparedto the reference cells in these series. Deterioration in thesintered and temperature-treated cells’ performance was notobserved until the last measurements on days 77-78. For thenon-sintered cells the Voc increase contributed so much tothe evolution of the cell performance over time, also with thereference cells, that their efficiency behavior was the oppositeto the sintered cells, that is, percentually smaller efficiencydegradation for the reference cells during the observationperiod (Supplementary Table 3). However, as it can be seenfrom Figure 2 and Supplementary Table 2, the general trendwas that the temperature treatments led to regeneration ofthe short circuit current also for these cell series. In general,the short circuit current regeneration was relatively largerfor the gel electrolyte-filled cells. This is most likely due tothe lower viscosity of the gel in higher temperatures, whichimproves the gel’s penetration into the pores of the TiO2 filmand its contact with the TiO2 nanoparticles. As an irreversibleeffect this can affect the current generation still when thetemperature is lowered again.

3.2. EIS Measurements

3.2.1. General Trends in the EIS Spectra. Three arcs andthree imaginary impedance peaks can usually be detected inthe Nyquist and Bode plots, respectively, of the impedancespectra of dye solar cells [22–24]. The high-frequency arc(kHz region) can be associated with the charge transfer onthe counter electrode (Rct||CPEce in Figure 1). The middlefrequency arc (ca. 10 Hz) describes the photoelectrodeimpedance (Rtr−Rrec||CPEpe corresponds, in Figure 1, to thetransmission line element TL [24]), where the components ofthe photoelectrode total resistance Rpe are the charge transferresistance for recombination of the TiO2 conduction bandand/or trap state electrons with the electrolyte species (Rrec)and the transport resistance of the electrons in the TiO2

film (Rtr). The low-frequency arc (mHz region) is associatedwith the ionic diffusion in the electrolyte (component Win Figure 1), but in our measurements, the diffusion arcwas either fused with or almost completely masked by thephotoelectrode impedance, which is why it was left out ofthe equivalent circuit fits as well. Figures 3 and 4 present


0 20 40 60 80

Number of days

0.6

0.65

0.7

0.75

Voc

(V)

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Number of days

0.6

0.65

0.7

Voc

(V)

(a)

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Number of days

8

10

12

14

16

18

i sc

(mA

/cm

2)

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10i s

c(m

A/c

m2)

(b)

0 20 40 60 80

Number of days

30

35

40

45

50

55

60

FF(%

)

L-SG-S

L-S referenceG-S reference

0 20 40 60 80

Number of days

45

50

55

60

65

70

FF(%

)

L-NSG-NS

L-NS referenceG-NS reference

(c)

Figure 2: Continued.


0 20 40 60 80

Number of days

2

3

4

5

6

7η

(%)

L-SG-S

L-S referenceG-S reference

0 20 40 60 80

Number of days

0

1

2

3

4

η(%

)

L-NSG-NS

L-NS referenceG-NS reference

(d)

Figure 2: The average IV-parameters as a function of time, before and after every temperature treatment (pairs of two consecutive markers)versus reference cell values. (a) Open circuit voltage Voc, (b) short circuit current density Isc, (c) fill factor FF, and (d) power conversionefficiency η. Standard deviations are not marked in the figures but their values fell on the range 0–0.01 V for the Voc, 0.4–1 mA/cm2 forthe Isc, 1%–4% for the FF, and 0.1%–0.5% for the η (the largest values typical to the most aged nonsintered cells). Small initial standarddeviations indicated even cell quality which is why the final number of temperature-treated cells could be limited to three and that of thereference cells to two.

examples of the sintered and non-sintered cells’ Nyquistplots, respectively.

For the non-sintered cells, photo- and counter electrodeimpedance arcs overlapped in the highest temperature(70◦C) so widely that in the most cases the exact values forRpe and Rct could not be reliably extracted from the data.For the aged (non-sintered) cells this overlapping was oftenstill present when the temperature was lowered from 70◦C to40◦C during the downward sweep of the temperature ramp.In these cases, only the total resistance of the cell and theshape of the photoelectrode impedance arc were monitoredmore closely.

The series resistances of the cells (Rs), caused mostlyby the sheet resistance of the substrate glass, were verysimilar between all cell types. Their values fell mostly on therange 10–15Ω, and no large changes could be observed as afunction of time or temperature.

Supplementary Table 4 lists for the sintered cells forwhich proper fits could be obtained at all temperaturesand on all measurement days the average values of thephotoelectrode recombination resistance Rrec, the chargetransfer resistance on the counter electrode Rct, and thephoto- and counter electrode capacitances Cpe and Cce,respectively. The electron lifetimes τe in the TiO2 film arecalculated with the equation

τe = (Rrec · B0)1/β, (1)

where the prefactor B0 and the exponent β describe theimpedance of the constant phase element CPEpe (ω is the

angular frequency) [41]:

ZCPEpe =1B0

(i · ω)−β (2)

and were obtained from the equivalent circuit fits. Since τecan also be interpreted as the time constant of the RC-circuitdescribing the photoelectrode, that is,

τe = Rrec · Cpe, (3)

combining (1) and (3) enables calculation of the photo-electrode capacitance (equation for the counter electrodecapacitance is analogous):

Cpe = (Rrec · B0)1/β

Rrec. (4)

If both Rrec and Rtr can be obtained from the impedance data,their values can be used to determine the diffusion length Leof the electrons in the TiO2 film [35].

Le =√Rrec

Rtr· d, (5)

where d is the film thickness.Some difference in the magnitudes of the photoelectrode

capacitance values can be noticed between liquid and gelelectrolyte cells, the latter having slightly lower Cpe. This canbe explained with the less efficient penetration of the gel intothe pores of the TiO2 layer. If the gel is not properly in touchwith the TiO2 particles, the formation of the electricallycharged double layer on the TiO2 surface is impossible onthose areas.


3 8 13 18 23 28

Z′ (Ωcm2)

−10

−8

−6

−4

−2

0

Z′′

(Ωcm

2)

20◦C40◦C70◦C

40◦C d20◦C d

100 kHz100 mHz

(a)

3 13 23 33 43 53

Z′ (Ωcm2)

−20

−16

−12

−8

−4

0

Z′′

(Ωcm

2)

20◦C40◦C70◦C

40◦C d20◦C d

(b)

Figure 3: Typical Nyquist plots for the sintered cells at different temperatures: cell type L-S, measurement day 1 (a) and 78 (b). Letter “d”after the temperature value indicates the downward sweep of the temperature cycle. Z′ : real impedance; Z′′ : imaginary impedance.

With the first set of EIS measurements, it is importantto notice that the temperature behavior of many impedanceparameters reflects the temperature-induced changes in theopen circuit voltage (i.e., smallerVoc at higher temperatures).Thus, the observed changes in those parameters are notdirectly caused by varying temperature but by varying Voc.While evolution of the parameters and changes in theirsensitivity to temperature variations as a function of timecan be seen from these measurements, detailed analysis of theparameters’ temperature dependence is based on the secondset of EIS data.

EIS parameters obtained from the second set of mea-surements were also plotted as a function of the cellcurrent density i. Since the same current flows throughall cell components, the current density dependence of thedifferential resistance of the cell component x, Rx, can beused to calculate the voltage over that component, Vx,according to [36, 37]

Vx =∫ ii0Rx(i)di +V0,x. (6)

The known reference point, that is, the lower limit ofthe integral, i0, and the corresponding voltage, V0,x (theintegration constant), can be chosen arbitrarily but herethe reference point is open circuit, that is, V0,x = Voc

and i0 = 0. In this way (6) could be used to determinethe photoelectrode voltage Vpe. Plotting the photoelectrodeparameters Rrec, Rtr, Cpe, τe, and Le as a function of Vpe

enabled their analysis independently of the voltage losses inthe other cell components.

3.2.2. Photoelectrode Impedance

A. Photoelectrode Resistance. The main result of the first setof impedance measurements was the Rpe’s steady increasewith time and temperature, though the latter effect is causedby the temperature-induced decrease in the Voc, not directlyby temperature itself (temperature-induced decrease in theVoc of dye solar cells is a well-known effect and it hasbeen reported several times before [15, 16, 18, 38]). Figure 5

presents the behavior of Voc versus T (first set of EIS), andFigure 6 the evolution of Rpe with time and temperature andwith time and Voc. Due to a similar, characteristic behaviorof the impedance parameters between the cell types, cell typeL-S is used as the example in all figures to follow (presentedvalues are averages of all samples measured).

It can be seen from Figure 6 that the rise of the Rpe withtemperature becomes steeper with time. Judged from theshapes of the photoelectrode impedance arcs, exemplified inFigures 3 and 4, the aged cells’ increased sensitivity to thetemperature rise was mostly caused by increased transportresistance, which contributed more to the overall resistancethe more the cells aged and/or the higher the temperaturewas. Figures 3 and 4 clearly show the difference betweensintered and non-sintered cells: for the former, Gerischer-shaped photoelectrode impedance, indicating the conditionLe < d (large transport resistance, high recombination rate)[24], could not be seen until the last measurements onday 78 at the highest temperature (70◦C). Short, straightline with a slope of 45◦ between the photo- and counterelectrode impedance arcs, characteristic of the transportresistance, can also be seen in the aged cell’s 40◦C impedancespectra in Figure 3. For the non-sintered cells, the Gerischer-shape was present already in the first measurements on day1, changing in the last highest temperature measurementsto a large semicircle. In this case it must be noted thatthe increase of Rpe in Figure 4 could also originate fromincreasing Rrec because due to the Gerischer shape of theimpedance arc, the resistance components could not bedetermined separately. However, the data in Figure 3 supportthe opposite conclusion because the increase of the Rtr

can clearly be seen in it (larger Rrec semicircle at elevatedtemperatures is due to the lower Voc). As the cells in Figures3 and 4 were prepared with the exact same methods andmaterials, the only difference being that in the former thephotoelectrodes were sintered, it is a reasonable assumptionthat the time and temperature behavior of both cell types issimilar. For non-sintered cells, the increase of Rtr is likely tohappen even faster, taken that the interparticle contacts andthus the conductivity in a non-sintered TiO2 film are poorerthan those in a sintered one, even in the fresh cells.


3 13 23 33 43 53

Z′ (Ωcm2)

−20

−15

−10

−5

0

Z′′

(Ωcm

2)

20◦C40◦C70◦C

40◦C d20◦C d

(a)

0 100 200 300 400

Z′ (Ωcm2)

−200

−150

−100

−50

0

Z′′

(Ωcm

2)

20◦C40◦C70◦C

40◦C d20◦C d

(b)

Figure 4: Typical Nyquist plots for the nonsintered cells at different temperatures: cell type L-NS, measurement day 1 (a) and 76 (b). Letter“d” after the temperature value indicates the downward sweep of the temperature cycle. Z′ : real impedance; Z′′ : imaginary impedance.

0 20 40 60 80

T (◦C)

0.4

0.45

0.5

0.55

0.6

0.65

0.7

−Voc

(V)

Day 1Day 12

Day 29Day 78

Figure 5: Temperature dependence of the open circuit voltage: firstset of EIS measurements.

The large semicircle in the aged non-sintered cells’ high-temperature spectra, indicating exceptionally poor conduc-tivity in the TiO2 film, is typical for low Voc (at low opencircuit voltages, the electron concentration in the TiO2 filmis smaller, hindering efficient charge transport and resultingin higher Rtr). This radical drop in the photoelectrode’sperformance was reversible, however, indicating no perma-nent damage to the film by high temperature. This effectimplies that non-sintered TiO2 films are less suitable forhigh-temperature operating conditions than sintered films.

The second set of impedance measurements providedmore information about the direct temperature behavior ofthe Rpe components. Unlike in the first set, where eitherthe characteristic of the Rtr was overlapped by the counter

electrode impedance or the photoelectrode arc took theshape of Gerischer impedance, rendering extraction of Rtr

impossible, here the Rpe components were distinguishableenough to be obtained separately from the equivalent circuitfits. The current density dependence of Rrec, Rrec(i), alongwith the bias voltage dependence of i, i(V), both illustrated inFigure 7, could now be used to calculate the photoelectrodevoltage Vpe, according to (6). Exponential functions of thetype

Rrec(i) = A · eB·i (7)

were fitted to the Rrec versus i data at different temperatures,and second-order polynomials, from which the Voc valuesneeded in (6) could be obtained, to the i versus V data. Bothsets of fits are included in Figure 7. After inserting (7) into (6)and integrating, Vpe could be obtained from the expression

Vpe = A

B

(eB·i − 1

)+Voc. (8)

Figure 8 presents the temperature dependence of Rrec and Rtr

at different photoelectrode voltages. In the rest of this paper,all other photoelectrode parameters obtained from the dataof the second set of EIS measurements are similarly presentedas a function of Vpe.

The temperature behavior of the Rpe components pre-sented in Figure 8 is partially in agreement with the regen-eration of the short circuit current after the temperatureramping, which was observed in several of our previousmeasurements. When the temperature is increased, both Rtr

and Rrec decrease, but when it is lowered again, the effect isovercompensated by both resistances reaching higher valuesthan those before the ramp. Higher Rrec inhibits recombina-tion and through (1) and (5), respectively, can contributeto a longer electron lifetime and diffusion length. Longerdiffusion length, in turn, increases the probability that theelectrons reach the current collector before recombination,thus resulting in higher short circuit photocurrent.

According to the (simplified) equation

vrec = krec(n) · n, (9)


0 20 40 60 80

T (◦C)

0

10

20

30

40

50

60R

pe

(Ωcm

2)

Day 1Day 12

Day 29Day 78

(a)

0.4 0.5 0.6 0.7

−Voc (V)

0

10

20

30

40

50

60

Rp

e(Ω

cm2)

Day 1Day 12

Day 29Day 78

(b)

Figure 6: Photoelectrode (total) resistance as a function of temperature (a) and open circuit voltage (b) on different measurement days; firstset of EIS measurements.

where vrec is the recombination rate, krec the recombinationrate constant, and n the electron concentration in the TiO2

film (including, here, both the conduction band and bandgap trap state electrons), the decrease in Rrec at elevatedtemperatures and higher voltages can be explained withthe temperature-induced increase in krec and larger electronconcentration in the TiO2 film, respectively [15, 18]. Fasterrecombination at elevated temperatures is responsible alsofor the smaller Voc. Similarly, the decrease of Rtr at highertemperatures and voltages can be deduced to originate fromincreased electron concentration. Higher temperature liftselectrons from the band gap trap states to the conductionband, thus increasing the conductivity of the TiO2 film[15, 18].

The temperature behavior of Rrec and Rtr is in agreementwith literature also [35, 36]. Both resistances have exponen-tial voltage dependence:

Rrec = R0 exp

[−βkBT

(EFn − Eredox)

], (10)

Rtr = Rt0 exp[ −ekBT

(V +

Eredox − Ecb

e

)], (11)

where R0 and Rt0 are constants, β the transfer coefficient forButler-Volmer type charge transfer reaction, e the elementarycharge, T the absolute temperature, kB the Boltzmannconstant, EFn the position of the Fermi level of the electrons,Ecb the energy of the conduction band, and Eredox the redoxFermi level of the electrolyte (EFn − Eredox = eVpe). Theslope d(logRrec/tr)/dV changes with temperature, which canbe seen in Rtr in Figure 8 (with Rrec the change in the slope is

not so evident, but this can be due to a rather small amountof measurements points and a narrow voltage range).

When ln(Rtr) was plotted as a function of 1/T at fixedvoltage, however, deviation from linear dependence couldbe observed between 40◦C and 70◦C (data not shown).According to (11), this indicates temperature dependence ineither Rt0 or (Eredox − Ecb).

B. Photoelectrode Capacitance. Photoelectrode capacitanceis plotted in Figure 9 as a function of time, temperature,and Voc (first set of EIS). Cpe decreased with time andtemperature, the latter effect reflecting again the decrease ofthe Voc as a function of T . On the voltage range used in thisstudy, Cpe consists of the capacitance on the TiO2-electrolyteinterface, Cdl, and the chemical capacitance of the TiO2 film,Cμ, which exhibits exponential voltage dependence [35, 36]:

Cμ = e2

kBTexp[

α

kBT

(V +

Eredox − Ecb

e

)]. (12)

α = T/T0, where T0 is a parameter (in temperatureunits) which determines the depth of the band gap states’distribution versus the lower edge of the conduction band.Due to the narrow voltage range in our measurements,however, the exponential shape of the curve cannot clearlybe seen and the dependence appears to be linear.

Figure 10 presents the direct temperature dependence ofthe Cpe (second set of EIS measurements). An increase inCpe at elevated temperatures can be seen, overcompensated,at the higher photoelectrode voltages during the downwardtemperature sweep, by its decrease to values lower than thosebefore the temperature ramping (i.e., opposite effect to that


−10 −8 −6 −4 −2 0 2

I (mA/cm2)

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40

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200R

rec

(Ωcm

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20◦C40◦C70◦C

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20◦C d

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i(m

A/c

m2)

20◦C40◦C70◦C

40◦C d20◦C d

(b)

Figure 7: Recombination resistance as a function of current density (a) and current density as a function of cell voltage (b), at differenttemperatures; second set of EIS measurements. Letter “d” after the temperature value indicates the downward sweep of the temperaturecycle.

0.55 0.6 0.65 0.7 0.75

−Vpe (V)

1

10

100

1000

Rre

c(Ω

cm2)

20◦C40◦C70◦C

40◦C d20◦C d

(a)

0.55 0.6 0.65 0.7 0.75

−Vpe (V)

0.1

1

10

100

Rtr

(Ωcm

2)

20◦C40◦C70◦C

40◦C d20◦C d

(b)

Figure 8: Recombination resistance (a) and transport resistance (b) as a function of photoelectrode voltage, at different temperatures; secondset of EIS measurements. Letter “d” after the temperature value indicates the downward sweep of the temperature cycle.

of the Rpe components). This seems to be in disagreementwith [35], in which Cpe remained practically independentof temperature between 0◦C and 60◦C, and also with (12),according to which an increase in T should result in decreasein Cμ, unless temperature-induced shifts in (Eredox − Ecb)

happen. The slope d(logCpe)/dV appears independent ontemperature, though, and is thus in agreement with thepreviously observed behavior [35]. Our results are also inagreement with [21], where the Cpe clearly increased withtemperature.


0 20 40 60 80

T (◦C)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Cp

e(m

F/cm

2)

Day 1Day 12

Day 29Day 78

(a)

0.4 0.5 0.6 0.7

−Voc (V)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Cp

e(m

F/cm

2)

Day 1Day 12

Day 29Day 78

(b)

Figure 9: Photoelectrode capacitance as a function of temperature (a) and open circuit voltage (b); first set of EIS measurements.

0.55 0.6 0.65 0.7 0.75 0.8

−Vpe (V)

0.1

1

10

Cp

e(m

F/cm

2)

20◦C40◦C70◦C

40◦C d20◦C d

Figure 10: Photoelectrode capacitance as a function of photo-electrode voltage, at different temperatures; second set of EISmeasurements. Letter “d” after the temperature value indicates thedownward sweep of the temperature cycle.

Cpe was also plotted as a function of 1/T , keeping thevoltage fixed (data not shown). Analogously to Rtr, deviationfrom linear dependence was observed at high temperatures.This indicates again temperature-induced changes in eitherT0 or (Eredox − Ecb). Based on the earlier observation,however, that the Cpe’s increase with temperature contradicts

the behavior predicted by (12), unless temperature-inducedshifts in (Eredox − Ecb) happen, a reasonable assumption isthat the deviation from linearity when Cpe is plotted against1/T is, indeed, caused by temperature-induced variation inEcb and/or Eredox. This would be in agreement also with [21].

Direct proof of the origin of the Cpe’s contradictorytemperature behavior could not be obtained from our EISmeasurements, but temperature is likely to affect the chem-ical composition of the TiO2 surface, that is, concentrationsof the electrolyte additives, dye, and possible impurities onit. Increased adsorption of ions at elevated temperaturescould result in larger Cpe values, and correspondingly, theirdesorption when the temperature is lowered again, to smallerCpe. One reason for the inconsistency of our result and [21]with [35] could also be in lighting conditions—in [21], themeasurements were made under illumination, like in ourstudy, whereas in [35], in the dark. Basically, lighting con-ditions only affect the “direction” from which the electronsare fed into the TiO2 film—under illumination, they comefrom the dye and in the dark, from the external circuit, theirtotal amount in the film then setting the photoelectrodevoltage which, in turn, directly affects the photoelectrodecapacitance. The fact that there is temperature dependence inthe capacitance when the electrons are fed to the TiO2 fromthe dye could thus reflect temperature-dependent changesin the dye-TiO2 interface or the chemical structure of thedye.

C. Electron Lifetime and Diffusion Length. According to (1)and (5), the behavior of τe and Le reflects the behavior ofRrec, Cpe, and Rtr. Figures 11 and 12 present τe as a functionof time, temperature, and Voc, and as a function of pho-toelectrode voltage at different temperatures, respectively.


0 20 40 60 80

T (◦C)

0

10

20

30

40

50

60

70

80Te

(ms)

Day 1Day 12

Day 29Day 78

(a)

0.4 0.5 0.6 0.7

−Voc (V)

0

10

20

30

40

50

60

70

80

Te

(ms)

Day 1Day 12

Day 29Day 78

(b)

Figure 11: Electron lifetimes in the TiO2 film, as a function of temperature (a) and open circuit voltage (b); first set of EIS measurements.

0.55 0.6 0.65 0.7 0.75

−Vpe (V)

1

10

100

1000

Te

(ms)

20◦C40◦C70◦C

40◦C d20◦C d

Figure 12: Electron lifetimes in the TiO2 film, as a function ofphotoelectrode voltage, at different temperatures; second set of EISmeasurements. Letter “d” after the temperature value indicates thedownward sweep of the temperature cycle.

In Figure 13, Le is plotted as a function of photoelectrodevoltage, at different temperatures also.

The seeming contradiction between the voltage depen-dence of τe in Figures 11 and 12 can be explained withthe different nature of the measurements. In Figure 11,the measured voltage is that generated by the cell at open

circuit, the points of higher voltage representing lowertemperatures. At lower temperature, slower recombinationresults in longer electron lifetimes and the cell can sustainhigher (open circuit) voltage. In Figure 12, the voltage isset externally, that is, larger electron concentration in thefilm results in faster recombination, thus lowering the τe aswell.

Despite that Cpe increases as a function of temperature,this effect was not large enough to affect the electron life-times, which were shorter at elevated temperatures followingdirectly the trend in Rrec, even to the overcompensatingeffect during the downward sweep of the ramp. However,the τe in Supplementary Table 4 are not, in general, andwithin the limits of the measurement accuracy, higher afterthe temperature rampings than before those, especially inthe case of the aged cells. The major increase seems tohappen between the consecutive measurement days (i.e., theτe in the end of the previous ramp are much lower than inthe beginning of the next). This indicates that the overallefficiency regeneration is not solely caused by the beneficialeffects of the temperature rampings on the photoelectrodecharacteristics but also caused by other factors such asimprovement in the counter electrode function. In any case,even with the aged cells, the rampings, and aging in general,did not result in noticeable decrease in τe as can be seen fromFigure 11.

The parallel changes in Rrec and Rtr with temperatureas discussed in Section 3.2.2(A) are directly reflected inthe temperature behavior of Le (Figure 13). In the limitsof measurement accuracy, electron diffusion length showsvery small and irregular variation with temperature onthe observed voltage range. This is, interestingly, also indisagreement with [35] in which clear, regular temperature


0.55 0.6 0.65 0.7 0.75

−Vpe (V)

0

20

40

60

80

Le

(μm

)

20◦C40◦C70◦C

40◦C d20◦C d

Figure 13: Electron diffusion lengths in the TiO2 film, as a functionof photoelectrode voltage, at different temperatures; second set ofEIS measurements. Letter “d” after the temperature value indicatesthe downward sweep of the temperature cycle.

dependence was observed. As a function of Vpe Le increaseslinearly, indicating relatively larger drop in Rtr in comparisonto Rrec with increasing voltage.

3.2.3. Counter Electrode Impedance. Since the same type ofcounter electrode (platinized glasses prepared in the samebatch) was employed in all cells, the time and temperaturebehavior of the charge transfer resistance on it followedvery regular and similar pattern for all cell types, the onlynoticeable difference being relatively larger Rct values for thenon-sintered cells. Figures 14 and 15 present Rct as a functionof time, temperature, and Voc (first set if EIS measurements),and as a function of bias voltage at different temperatures(second set of EIS measurements), respectively.

Rct exhibited the most regular temperature dependenceof all the impedance parameters. Linear decrease wasobserved as a function of temperature, which can be easilyunderstood by the more efficient function of the Pt catalystat elevated temperatures. During the downward sweep of thetemperature ramp, the Rct values set on slightly lower levelsthan what they had reached during the upward sweep, bothfor the fresh and aged cells (Figure 14), which suggests thatthe gain in the catalytic efficiency caused by the temperatureincrease had at least temporary permanence. This canpartially explain the regenerating effect of the temperaturerampings on the overall cell efficiencies and is in agreementwith previous results [38].

The voltage dependence of the Rct seems to be oppositein Figures 14 and 15, but, as in the case of τe, is causedby different experimental conditions. In Figure 14, the linesconnect points measured at different temperatures whereas

in Figure 15 they display the voltage dependence at fixedtemperature.

Only slight increase in Rct or none as a function of timecould be detected. This indicates stable counter electrodequality in general, that is, no significant detachment of thePt catalyst or other degradation mechanisms affecting theelectrode performance.

Counter electrode capacitance Cce was also obtainedfrom the equivalent circuit fits. Slight decrease with timecould be observed for the Cce but no clear, regular temper-ature dependence could be seen (data not shown).

4. Summary and Conclusions

We have used electrochemical impedance spectroscopy toinvestigate the origins of time- and temperature-inducedchanges in the dye solar cell performance. The cells weresubjected to temperature variations in repeated and regularto-and-fro ramps between 20◦C and 70◦C during theiraging, and their EIS spectra were recorded as a function oftemperature along every ramp after which extensive analysisof the time and temperature behavior of the impedanceparameters was performed. IV-curves of the cells werealso obtained in a solar simulator before and after thetemperature rampings to monitor the evolution of the cellperformance parameters with time.

Linearly decreasing charge transfer resistance on thecounter electrode as a function of temperature was verifiedand it can be associated with the temperature-dependentactivity of the Pt catalyst. This effect was reversible, thoughslight hysteresis was observed, that is, lower Rct values duringthe downward sweep of the ramp, compared to those in itsupward sweep. It also showed short-term permanence (i.e.,the Rct values stayed on lower level several hours after theramping), which can partially explain the slower deterio-ration rate of the temperature-treated cells in comparisonto the reference cells aged at constant temperature. Nosignificant increase in the Rct could be observed as a functionof time either, indicating good and stable counter electrodequality in general.

Photoelectrode resistance increased with both time andtemperature, the latter effect reflecting the lower Voc atelevated temperatures. The central trend in the Rpe evolutionwith time was a gradual increase of the electron transportresistance in the TiO2 film, especially in the case of non-sintered cells and favored by temperature elevation.

The direct temperature behavior of the Rpe componentscould be obtained from the second set of EIS measure-ments, where the cell voltage was controlled with externalpolarization. When the current flowing through the cell wasknown, the photoelectrode voltage could be obtained and theimpedance parameters plotted as its function, which enabledtheir analysis independently of the voltage losses on the othercell components. Both Rrec and Rtr decreased with temper-ature, and hysteresis was observed in this case also; that is,both components reached higher than original values duringthe downward sweep of the temperature ramp. The exactreason for the hysteresis remains open this far, though the


0 20 40 60 80

T (◦C)

0

1

2

3

4

5

6R

ct(Ω

cm2)

Day 1Day 12

Day 29Day 78

(a)

0.4 0.5 0.6 0.7

−Voc (V)

0

1

2

3

4

5

6

Rct

(Ωcm

2)

Day 1Day 12

Day 29Day 78

(b)

Figure 14: Charge transfer resistance on the counter electrode as a function of temperature (a) and open circuit voltage (b). First set of EISmeasurements.

0.55 0.6 0.65 0.7 0.75 0.8

−Vcell (V)

1

10

Rct

(Ωcm

2)

20◦C40◦C70◦C

40◦C d

20◦C d

Figure 15: Charge transfer resistance on the counter electrode as afunction of cell voltage; second set of EIS measurements. Letter “d”after the temperature value indicates the downward sweep of thetemperature cycle.

behavior of Rrec could, through longer Le (5), correlate to theisc regeneration observed after several temperature rampings.Higher diffusion length results in more electrons reachingthe current collector before recombination—however, theproblem with this kind of analysis is that the voltage drop

inside the TiO2 layer (which affects the Le) is unknown atshort circuit. Temperature-induced variation in Le was alsoquite small and irregular in this study, contradicting theresults in [35].

Photoelectrode capacitance decreased with time butincreased as a function of temperature. The behavior of Cpe

exhibited slight hysteresis too, resulting in lower capacitancevalues in the end of the temperature ramp, compared tothose before it. The behavior of Cpe, as observed here, isalso somewhat contradictory compared to earlier studies—in [35], Cpe was found to be independent on temperature,whereas in [21], similar behavior to this research was seen.The behavior of Cpe as a function of 1/T at fixed voltageindicated temperature-induced shifts in the conduction bandenergy Ecb and/or redox Fermi level Eredox, also in agreementwith [21].

Electron lifetime in the TiO2 film decreased as a functionof temperature, reflecting the behavior of Rrec. During aging,no noticeable deterioration could be observed for τe.

An interesting feature in the IV-curve measurementswas the increase in the Voc in the beginning of the agingperiod. This was accompanied with slight enhancement inthe τe as well. The exact reason for this behavior remainsunresolved at this point, though slow adjustment of thefinal equilibrium between the dye, TBP, and other adsorbentspecies on the TiO2 film surface can be assumed to be partlyresponsible of it. Otherwise the aging behavior of the cellswas normal, that is, after the initial rise slow degradationin all parameters; especially in the non-sintered cells. Inmost cases the short circuit currents were regenerated inthe temperature treatments, however, especially for the


gel electrolyte filled cells, which slowed down the overallperformance degradation.

In general, the cells in this study exhibited good long-term stability. With the best cells only slight decrease inefficiency or not at all was detected in the last measurementson days 76–78, indicating that instead of being detrimentalto the cell operation, repeatedly and regularly varyingtemperature could even improve the long-term stability ofthe conventional, iodine electrolyte-filled DSC, taken thatthe cell temperature does not exceed ca. 60◦C for extendedperiods of time.

EIS proved to be an efficient tool to extract detailedinformation about the charge transfer/accumulation pro-cesses in the cells, and how they relate to the behavior ofthe IV-curves as a function of time and temperature. Thebeneficial effect of the temperature rampings on the celllong-term performance was verified, though other types ofmeasurements, for example, spectroscopic techniques, couldstill be useful in clarifying the physicochemical processes onthe TiO2-electrolyte interface.

Acknowledgment

Financial support from the National Technology Agency ofFinland (Tekes) is gratefully acknowledged.

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