Accelerated aging test for carbon composite counter electrodes based dye sensitized solar cells

8/13/2019 Accelerated aging test for carbon composite counter electrodes based dye sensitized solar cells

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ACCELERATED AGING TEST FOR CARBON COMPOSITE COUNTER

ELECTRODES BASED DYE SENSITIZED SOLAR CELLS

Authors: Syed Ghufran Hashmi, Janne Halme and Peter Lund

Email: [email protected]

New Energy Technologies Group, Department of Applied Physics, Aalto University, P.O. BOX15100, FI-00076 Aalto (Espoo), Finland

ABSTRACT: We report here an accelerated aging test for carbon composite catalyst layer based

dye sensitized solar cells (DSSCs) that was performed under artificial light intensity (1000W/m

2) equivalent to 1 Sun and at 60 C for 1000 hours in the presence of significant fraction of

UV light intensity. The performance of the reference DSSCs with thermally platinized counter

electrodes degraded almost completely due to severe electrolyte bleaching, which is an expected

result of UV light induced degradation of DSSCs. The performance loss was also accompaniedby an increase in the counter electrode charge transfer resistance (RCT). However, DSSCs with

counter electrodes based on carbon composite catalyst layers exhibited markedly more stable

photovoltaic performance, showed no visible bleaching of the electrolyte, and their catalyticactivity even improved with a gradual decrement in RCT (from 6 cm

2to 2.6 cm

2). The

efficiency of the carbon composite based DSSCs was reduced by 35% from the initial efficiency

due to a slight degradation in N719 dye and the corrosion at the silver contacts which caused a

small increase in the total cell resistance in both types of DSSCs. The resistance of these carboncomposites counter electrode based DSSCs against UV light may potentially reduce the overall

manufacturing cost by partially or completely relaxing the need for UV filters in complete DSSC

assembly, but this hypothesis as well as the physical and chemical origin for the effect should beinvestigated and verified by further studies.

Keywords: Dye sensitized solar cells, stability, carbon composites and counter electrodes.

1. INTRODUCTIONDye sensitized solar cells (DSSCs) propose

variety of cheap materials to be tested in

different combinations[1]

. However, longterm operation of these inexpensive

materials in the cell is critical for the reliable

commercialization of DSSCs[2]

. The

composites of carbon have been tested as analternative to replace the expensive platinum

(Pt) catalyst layer in several studies

[3-7]

.Nevertheless accelerated aging tests for

these composites are rarely reported[8, 9]

andmore precisely, their long term

electrochemical impedance performance is

mostly missing in the literature. In thiscontribution, we present an accelerated

aging test that was performed under artificial

light intensity (1000 W/m2) equivalent to 1

Sun and at 60 C to see the potential of

carbon composites catalyst layers to beimplemented in the durable DSSCs. The

illumination of the solar cells in the aging

test included also significant UV light

component, which is known to be significantaging factor of DSSCs. The performance of

these carbon catalyst layers in an acceleratedaging test was evaluated in terms of

photovoltaic parameters (IV),electrochemical impedance spectroscopy

(EIS measurements) and incident photon to

collected electron efficiency (IPCE) andcompared with reference thermally

platinized counter electrodes (TPCE).

28th European Photovoltaic Solar Energy Conference and Exhibition

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ethanol and acetone solutions (5 min each).

All the substrates were dried via compressed

air before placing them in a UV-O3Chamber (Bio-force nano sciences UV

Ozone Pro Cleaner, 20 minutes) for further

cleaning. After UV-O3 cleaning step, thesubstrates were immersed into a 40 mMTiCl4 solution container box and placed for

30 minutes in a preheated oven at 70 C.

After that the substrates were again washedwith deionized water (DIW) and ethanol

solution sequentially and dried with

compressed air. Sequential screen printing

steps of commercial nano crystalline TiO2paste (18NR-T Dyesol) and TiO2 scattering

paste (WER2-0 Dyesol) were performed to

get 9-10 m and 2-4 m thick filmsrespectively with mesh T-61. Each layer was

dried at 110C for 5-10 minutes after the

deposition. The TiO2 printed layers were

then sintered at 500 C for 30 minutes andwere cooled down to room temperature in

the oven. After sintering the TiCl4treatment

step was again performed (with abovementioned sequence till the drying with

compressed air) and the substrates were re-

sintered at 500 C (for 30 minutes) and were

again cooled down to room temperature tocomplete the process. These TiCl4 treated

substrates were sensitized in the 0.32 mM

cis bis (isothiocyanato) bis (2, 20-bipyridyl-4, 40-dicarboxylato)-ruthenium (II) bis

tetrabutyl ammonium (N-719, Solaronix)

and ethanol (99.5 wt %) solution for 16hours before the cell assembly.

2.2.3 Cell assembly

Both PE and CE were assembled in

sandwich type fashion by separating them

through a 50 m thick Bynel foil (Dupont).

The (I/I3) redox based electrolyte (HSE-Dyesol) was injected into the cell channel

via drilled holes at PE side. Finally these

cell holes were sealed with a 25 m thickBynel foil (Dupont) and a thin glass cover

piece. The cell contacts were made by

applying the conductive copper tape and

spreading silver ink at the in-active area ofthe cell. The contacts were then protected

with a slow drying epoxy. All the cells were

soaked for 1 day in 1 Sun light intensity(1000W/m2) for initial stabilization before

the initial measurements.

3. RESULTS AND DISCUSSIONS

3.1 Initial photovoltaic performance

Table 1 represents the initial photovoltaicperformance of both thermally platinized

counter electrodes (TPCE) and carbon

composite counter electrodes (CCE) basedDSSCs. The sample to sample variations are

presented in the form of standard deviation

for each type of DSSCs (Table 1). Both

types of DSSCs exhibited almost equalefficiencies (Avg ~ 6.5%) however, the

TPCE based DSSCs showed ~ 8% higher

currents (JSC), 3% higher fill factor (FF) and~ 20% lower cell resistance (RCELL).

Nevertheless the CCE based DSSCs

exhibited 8% higher open circuit voltage

(VOC

). Additionally the high fill factors ineach type of DSSCs suggest good adhesion

of catalyst layers over the substrates.

Table 1:Initial photovoltaic performance of

4-5 cells of each type of DSSC along with

their standard deviations.

Cell type*JSC

(mA/cm2)

VOC

(mV)

FF

(%)

(%)

RCELL

(cm2)

GPEGTPCE15.6 0.2 690 4 61 2 6.6 0.2 11 0.8

GPEGCCE 14.4 0.4 750 14 59 1 6.3 0.2 14 0.4

* GPEGPTCE = Glass photo electrode

Glass thermal platinum counter electrode.GPEGCCE= Glass photo electrode Glass

carbon composite counter electrode.


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3.2 Initial electrochemical impedance

response

Figure 1:Typical EIS spectra of each type

(*GPEGTPCE and **GPEGCCCE) DSSCs

used in this study. a) Nyquist plot, as b)imaginary impedance Z as a function of

frequency f. *GPEGTPCE = Glass photo

electrode Glass thermally platinized

counter electrode. **GPE GCCE = Glassphoto electrode Glass carbon composite

counter electrode.

The initial measurement results regarding

electrochemical impedance response of each

type of DSSCs are summarized in Table 2.

These measurements were performed atopen circuit voltage and in the solar

simulator with same light intensity (1000

W/m2) as was used for measuring the

photovoltaic parameters. The measured

frequency range was from 100 mHz to 100kHz. In this frequency range, the traditional

TPCE exhibits three semi circles[10, 11]

(Figure 1a). Each semicircle can bedistinguished according to its unique peakposition (Figure 1b). The first semicircle for

TPCE is associated with charge transfer

resistance (RCT) which appears at very highfrequency (> 1 kHz) range

[10, 11] (Figure

1b). Additionally the second adjacent

semicircle corresponds to the recombination

resistance of the photo electrode (RPE) andits characteristic frequency appeared around

20-30 Hz[10]

(Figure 1b). The third small

semicircle that appeared at very lowfrequency (~ 1 Hz, Figure 1a-b) is

associated with the diffusion resistance (RD)

of the cell[10, 11]

.

The case is different for CCE in which a

large semi-circle results from the

overlapping of RCT and RPE responses atlower frequency (10-20 Hz, Figure 1b)

range. Hence it is difficult to estimate the

exact value of RCTfor CCE. One possibility

is to subtract the RPE

value of TPCE basedDSSC since the same photo-electrode

geometry was employed in fabrication of the

each type of DSSCs[12, 13]

. Based upon thisassumption, the RCT values of CCE are

calculated and presented in Table 2. In

addition to that one small semicircleadjacent to the large semicircle (Figure 1a)

can be seen in the CCE spectrum which

appears at very high frequency (~10 kHz,

Figure 1b) and is distinguished as in-porediffusion resistance (RPORE) or contact

resistance [13, 14]. The value of RPORE was

added in the RCTvalues to calculate the total

charge transfer resistance (RCT-TOTAL) ofCCE

[12, 15]. Table 2 summarizes the EIS

parameters of each type of DSSCs.


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The main conclusion that can be drawn from

initial EIS performance is; almost equal

series resistance (AvgRS= 4.6 0.4 cm2)

for each type of DSSCs due to the sameFTO Glass substrate (RSHEET = 15 /)

however, the TPCE exhibited lowerR

CT(2.2 0.5 cm2) than CCE (6.0 0.2 cm

2).

Table 2: Initial average values of 4-5 cells

of each type along with their standarddeviations representing electrochemical

impedance performances.Cell type* *GPEGPTCE **GPEGCCE

RS(cm2) 4.7 0.6 4.5 0.2

RCT (cm ) 2.2 0.5 5.2 0.5

RREC(cm ) 2.7 0.4

RPORE (cm2) 0.8 0.2

RREC+RCT(cm ) 7.9 0.5RCT-TOTAL(cm ) 6.0 0.2

* GPEGPTCE = Glass photo electrode Glass thermally platinized counter electrode.

** GPE GCCE = Glass photo electrode

Glass carbon composite counter electrode.

3.3 Photovoltaic performance of DSSCs

with accelerated aging

Figure 2 represents the short circuit current

density (J

SC), open circuit voltage (V

OC), fillfactor (FF) and overall efficiency ()recorded for each type of DSSC during this

study (from 0 h to 1000 h). During the entire

period of study, both types of DSSCexhibited different trends compared to each

other. During the whole study, JSCof TPCE

based DSSCs was continuously decreased

(Figure 2a) due to the bleaching of theelectrolyte (Figure 3 b-c). The bleaching of

the electrolyte solution is well known

problem in traditional TPCE based DSSCswhen exposed to real or artificial sunlightand is mainly associated with loss of tri-

iodide ion in the electrolyte solution[16-20]

.

This loss of tri-iodide ion has further beenassociated with a reaction between UV light

and the electrolyte solution that generates an

irreversible reaction for iodine[21, 22]

unless a

UV blocking filter is used to prevent the

reaction. Since in this work no UV blocking

filter was employed we assume the

aforementioned cause is responsible for theloss of tri-iodide ions here as well. Figure 4

shows the photon flux spectrum of the lampsused in the light soaking system (Philips13117), confirming the presence of UV light

(below 400 nm).

Surprisingly, no visible bleaching of

electrolyte occurred in CCE based DSSCs

and theJSCvalues were only fractionally (~

8%, Figure 2 a) decreased from the initialvalue. It should be noted again that in each

type of DSSCs, the same photo electrode

and electrolyte composition was used andthe only difference was in the catalyst layer

i.e. thermal platinum and carbon composite.

This raises a hypothesis that the irreversible

reaction between the electrolyte and UVlight was minimized or stopped by the

carbon composite.

In addition to that, a continuous drop in

open circuit voltage (VOC) for CCE based

DSSCs was observed whereas it was

recovered up to 90% in case of TPCE basedDSSCs (Figure 2b). The drop in open circuit

voltage is mainly associated with the

resistance of photo electrode (RPE) which isinvestigated and further discussed in section

3.4. Nevertheless, over the whole 1000 h

period, the voltage decreased by 13% and7% in case of CCE and TPCE based DSSCs

respectively (Figure 2b).

Also the FF did not remain stable butdecreased by 11% in case of TPCEs and

19% for CCE based DSSCs (Figure 2 c).

These drops in FFs are resulted due to an

increase in total cell resistance (RCELL 74%increase for TPCE and 26% increase for

CCE based DSSCs) of the cells (Figure 5 a).

The RCELLdepends upon various factors forinstance charge transfer resistance at counter


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electrode (RCT), ideality factor of photo

electrode, contact resistance of the contacts,

sheet resistance (RSH) of the substrates and

electrolyte diffusion. These parameters areinvestigated in detail in section 3.4. Due to

the decrease inFF

andV

OC, the efficiency ofCCE based DSSCs was reduced from 6.3 0.2% to 4.1 0.2 (i.e. by 35 %) whereas the

efficiency of the TPCE based cells dropped

linearly by 85% mainly due to the UVinduced, electrolyte bleaching reaction

(Figure 2 d).

Figure 2:Photovoltaic parameters of *GPE GTPCE (red squares) and **GPE GCCE

(black circles) cells. a) Short circuit current

density (JSC) (b) Open circuit voltage (VOC)

(c) Fill factor (FF) and (d) Efficiency () of

the cells. *GPE GTPCE = Glass photoelectrode Glass thermally platinized

counter electrode. **GPE

GCCE = GlassPhotoelectrode Glass carbon compositecounter electrodes.

Figure 3: Bleaching of the electrolyte in

*TPCEs based DSSCs was observed duringthe entire period. a) Fresh electrolyte with

dark yellow color. b) Semi bleached

electrolyte losing yellow color. c)

Completely colorless/bleached electrolyte.* Thermally platinized counter electrodes.

Figure 4: Spectrum of lamps used in the

light soaking system.

3.4 Electrochemical performance of

DSSCs with accelerated aging

Some of the critical factors which determinethe performance of counter electrodes are

also investigated with electrochemical

impedance spectroscopy (EIS). Theseparameters are presented in Figure 5 b-d. As

discussed in earlier section, both types of

DSSCs exhibited almost equal initial seriesresistance (RS). However, the TPCE based


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- visible bleaching, the carbon composite

counter electrodes exhibited improving

catalytic performance: their RCT gradually

decreased (down to 58%) throughout thestudy (Figure 5 c).

In addition to that, gradual increase in theresistance of photo electrode (RPE) has also

been observed (Figure 5 d). This confirms

the reason for the degradation in the opencircuit voltage (VOC) that was discussed in

section 3.2). The possible factor of these

deviations in the RPEcan be the degradation

in the photo electrode. This hypothesis wasfurther studied with incident photon to

collected electron efficiency (IPCE)

measurements and discussed in the nextsection.

3.5 Incident photon to collected electron

efficiency (IPCE) of DSSCs.

Figure 6:Average IPCE spectra of *TPCE

and *CCE based DSSCs. *Thermally

platinized counter electrodes. **Carbon

composite counter electrode.

The in-situ IPCE measurements of TPCE

and CCE based DSSCs was also performed

to investigate the possibility of degradationin N719 dye used in this work. The initial

and final average IPCE spectra of CCE

based DSSCs are presented in Figure 6whereas it was not possible to present the

final IPCE spectrum of TPCE based DSSCs

due to the complete bleaching of the

electrolyte as shown in Figure 3 b-c.

However an idea can be obtained from the

initial IPCE spectrum of TPCE basedDSSCs since the same photo electrodegeometry was used in each type of DSSCs.

Both types of DSSCs revealed almost equal

(~ 72 - 74%) initial peak IPCE valueshowever these efficiencies were reduced to

~ 65% confirming degradation in photo

electrodes. The possible hypothesis can

either be desorption of N719 dye moleculeswhich has been observed at high (60-80 C)

temperatures[2, 27]

or penetration of water

inside the cell. Here the aging wasperformed at 60 C and 20% humidity and

N719 dye is known to be hydrophilic dye[2]

.

3.6 Short comparison of thermally platinizedcounter electrodes and carbon composite

counter electrodes based DSSCs.

An interesting comparison between TPCE

and CCE based DSSCs has been obtained at

the end of the study. The reference TPCEbased DSSC exhibited higher initial

performance in terms of ~ 8% higher JSC,3% higher FF, ~ 5% higher efficiency and270% lower RCT than CCEs based DSSCs.

However in the accelerated aging test, TPCE

based DSSCs were unable to maintain theabove mentioned performance and their

efficiency dropped below that of CCE based

cells due to almost complete failure via

electrolyte bleaching. Despite of lowerinitial performance, the CCE based DSSCs

were able to retain ~ 92% of initial JSC, 87%

VOC, 81% FFand 65% overall efficiency andthus showed clear superiority over TPCEbased DSSCs in this aging experiment.

Interestingly, the RCT also was remarkably

decreased from 6 0.2 cm2 to 2.6 0.1

cm2.


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The key observation of this study was the

absence of electrolyte bleaching in the CCE

based DSSCs under halogen lamp light

without a UV filter. At the same time,equally prepared but TPCE based DSSCs

degraded due to complete loss of tri-iodide.It is also noteworthy that the CCE showedstable catalytic performance. In fact, their

RCTwas significantly reduced from its initial

value which means no degradation at thesecounter electrodes. The study raises

interesting questions such as why bleaching

did not occur in CCE based DSSCs. It is

important to verify and investigate it withmore sophisticated techniques such as

camera imaging[16]

, limiting current density

analysis

[28]

and selective application of UVfilters. The degradation at the photo

electrode was established here through IPCE

measurement; however, more sophisticated

setup for instance Raman Spectroscopy[2]

orIR spectroscopy

[2]is suggested for a future

study.

4. CONCLUSIONS

Stability of carbon composite based catalystlayers was tested under artificial light

intensity (1000 W/m2) equivalent to 1 Sun at60 C. The efficiency of these carboncomposite based dye sensitized solar cells

were only reduced by 35% from the initial

efficiency compared to the referencethermally platinized counter electrodes

based DSSCs which completely degraded

due to severe bleaching of the electrolyte.

On the other hand, no bleaching of theelectrolyte occurred in carbon composite

counter electrode based DSSCs. The

catalytic activity of the carbon compositecounter electrodes was also improved with agradual decrement in their charge transfer

resistance (from 6 0.2 cm2

to 2.6 0.1

cm2). Moreover, the critical reasons for

35% reduction of efficiency in CCE based

DSSCs was not the carbon composite

counter electrodes themselves but a slight

degradation in the N719 dye at the

photoelectrode and the corrosion at the

silver contacts which occurred in both types

of DSSCs. These results speak for theviability of inexpensive carbon composite as

an alternative to expensive platinum catalystlayer for stable high performance dyesensitized solar cells.

5. ACKNOWLEDGEMENT

Ghufran Hashmi thanks Fortum Foundation

and Tekniikan edistmissti (Finnish

Foundation for Technology Promotion) fortravel grants.

6. REFERENCES

[1] G. Hashmi, K. Miettunen, T. Peltola, J.

Halme, I. Asghar, K. Aitola, M. Toivola,and P. Lund, Renew. Sust. Energ. Rev 15

(2011) 3717-3732.

[2] M. I. Asghar, K. Miettunen, J. Halme, P.

Vahermaa, M. Toivola, K. Aitola, Energy

Environ Sci 3(2010) 418-426.

[3] Y. Jo, J. Y. Cheon, J. Yu, H. Y. Jeong,

C. H. Han, Y. Jun, S. H. Joo, Chem.Commun 48(2012) 8057-8059.

[4] S. J Thompson, J. M. Pringle, X. L.

Zhang, Y. B. Cheng,J. Phys. D: Appl. Phys46 (2013) 024007.

[5] D. Y. Kim, J. Kim, J. Kim, A. Y. Kim,

G. Lee, M. Kang, J. Indus. Eng. Chem 18(2012) 1-5.

[6] X. Miao, K. Pan, Q. Pan, W. Zhou, L.Wang, Y. Liao, G. Tian, G. Wang,Electrochimica Acta96(2013) 155-163.

[7] C. T. Hsieh, B. H. Yang, W. Y. Chen,Int. J. Photo. Energ, (2012)

doi:10.1155/2012/709581


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Accelerated aging test for carbon composite counter electrodes based dye sensitized solar cells