Gel electrolytes containing several kinds of particles used in quasi-solid-state dye-sensitized solar cells

RARE METALS Vol - 2 5 , Spec. Issue , Oct 2006, p .201

Gel electrolytes containing several kinds of particles used in quasi-solid-state dye-sensitized solar cells GENG Yi , SUN Xiaodan , CAI Qiang , SHI Yantao , and LI Hengde Laboratory of Advanced Materials, Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, China (Received 2006-06-30)

Abstract : Composite gel electrolytes containing several kinds of particles used as the quasi-solid-state electrolytes in dye-sensitized solar cells ( DSSCs) were reported. Mesoporous particles ( MCM-41) with unique structures composed of ordered nanochannels were served as a new kind of gelator for quasi-solid-state electrolytes. MCM-41, hydrophobic fumed silica Aerosil R972 and Ti02 nanopatricles P25 were dispersed into gel electrolytes respectively, The solar energy-to-electricity conversion efficiency of these cells is 4.65 % , 6.85 % and 5 ,05 % respectively under 30 mW cm illumination. The preparation methods and the particles sizes exert an influence on the performance of corresponding solar cells. Owing to unique pore structures and high specific BET surface area, mesoporous silica MCM-41 was expected to have the potential to afford conducting nanochannels for redox couple diffusion

-’

.

Key words : dye-sensitized solar cells ; nanocomposite gel electrolyte ; mesoporous ; nanoparticle

[This work was financially supported by the National Natural Science Foundation of China ( N o . 50573043 and 50572046.1

1. Introduction Dye-sensitized nanocrystalline solar cells

( DSSCs) provide a remarkable approach to harness solar energy with inexpensive materials and manufacturing methods [ 1-21. Its present conversion efficiencies are as high as 10% - 12% so that it is considered as a possible substitution to conventional photovolataic devices. Owing to their high energy conversion efficiency and low production cost, they have recently received considerable attention over the past decade. However, in addition to the need of optimization of the structured film electrode and the photosensitizer, the composition of a stable and efficient electrolyte remains one of the key challenges for its practical applications. To overcome the disadvantages of liquid electrolyte in DSSCs, several modifications have been explored, such as room-temperature molten salts, Corresponding author : CAI Qiang

organic hole transport materials, Inorganic ptype semiconductors , ionic conducting polymer electrolytes and solid polymer redox electrolytes [ 3-81 . The major issues in these modifications are focused on improving the overall energy conversion efficiency and the stability in the long-term application of DSSCs . Poly (ethylene oxide) (PEO) has received much attention as an electrolyte medium because it is chemically and mechanically stable [ 91 . On the other hand, in gel polymer quasisolid electrolytes, a framework material is an important component. Several researches have been reported that nano-particles like TiOz, and hydropholic fumed silica ( A200 and A150) were incorporated in polymer matrices as framework materials to obtain the quasi-solid nanocomposite polymer electrolytes. It has been reported that nanoparticles could improve the ionic conductivity of polymer electrolyte [ 10-131. In addition, the electrolytes modified with nan-

E-mail: caiqiang0 mail. tsinghua .edu .cn

202

RARE METALS, Vol. 2 5 , Spec. Issue, Oct 2006

oparticles could exhibit better mechanical strength, higher surface roughness and better interfacial stability. Mesoporous silica is a kind of inorganic particles which bear uniforni orientational channels. Due to the particular pore structures and large surface area, mesoporous silica has received considerable attention in many fields. Recently, Huang[ 141 and Tominaga [ 15 ] have reported their work on the application of mesoporous SRA-15 and MPSi as framework materials of ionic liquid to prepare quasi-solid-state electrolytes. Among the mesoporous material family, the most widely studied material is M C M 4 1 1 6 1 , which possesses large surface area due to the stacking of Si02 channels arranged in hexagonal array with pore size ranging from c a . 2 to 10 nni . Such unique nature is expected to effectively modify the performance of polymer electrolytes by providing conducting channels for redox couple diffusion in its composite with the gel polymer electrolyte host

[ 171. In this paper,

MCM-41, hydrophobic fumed silica Aerosil R972 and TiOz( P 2 5 ) into PEO-based electrolytes were introduced. Photoelectrochemical behaviors of the DSSCs using the obtained composite gel electrolytes were investigated. Effects of several preparation parameters on the performance of the cells were also discussed.

2.

Experimental

2.1. Preparation of a cell Optically transparent conducting glass, fluorinedoped SnO, ( C‘TO, 20 fi/O ) over layer, was purchased from 1-aohua Glass Co. Ltd . , Propylene Carbonate ( P C ) and Ethylene Carbonate ( EC ) were purchased from Acros Organics. Polv (ethylene glycol) ( P E O ) ( M ,= 2ooOOOO) were purchased from Aldrich Co. Ltd . , ancl Lithium Iodide Anhydrous, Tert-but!-lpyridine ( TBP ) were from Fluka Co . Ltd . Aerosil K972 ( Degussa AG , Germany, average particle size 16 nm ) and P25 ( Degussa AG, Germany, average particle size 21 nm) powder were donated by Beijing Entrepreneur Science

& Trading Co. Ltd . Iodine was purchased from Alfa Aesar Co. Ltd. All the other solvents and chemicals used in the experiments were purchased from Beijing Chemical Factory, Beijing , China and these chemicals were used as received. Nanocrystalline TiOz films were shaped onto the glass using the doctor-blade technique with a viscous Ti02 powder (degussa P25)paste and then annealing at 450 “c for 30 min in air flow and coated with the dye (cis-dithiocyanatobis [ 2,2’-bipyridyl-4,4’-dicarboxylicacid] ruthenium [ II ] ) solution ( 5 x mol * dm -’) overnight. To assure the effect of dipping and avoid water molecules, the films were immersed into the dye solution at about 80 “c. The Pt counter-electrode was fabricated by three drops of the H2PtC1, solution ( 0 . 005 mol d m - 3 in isopropyl alcohol) spreading uniformly onto the ‘ . conductive glass and then sintering at 380 C The process mentioned above was repeated 4 times to prepare enough Pt on the glass. A spacer (approximately 60 p m ) was placed between the electrodes to infuse electrolyte and avoid short-circuiting. The electrolyte was sandwiched between the photoelectrode and the counter-electrode by firmly press. The liquid electrolyte ( L E ) is composed of 0. 05 mol L - ’ I?. 0. 5 mol L-’ LiI and 0.5 mol.L-‘ terbutylpyridine, in a mixture of PC , EC and acetonitrile (volume ratio 1: 1:25) solution. The particles used to prepare nanocomposite gel electrolytes include mesoporous silica ( MCM-4 1 ) ; fumed silica ( SiOz, Degussa Aerosil R972 ) ; and titanium dioxide ( TiOa, Degussa P25). They were mixed with LE under continuous stirring respectively. MCM-41 was synthesized via an organic template procedure [ I S ] . To introduce more LE into the channels of MCM-41, certain amount of the mesoporous powder was first treated in a vacuum system for an outgassing period, and then LE was introduced into the chamber to immerse the powder for 24 h . Then more LE was added to the LEembedded MCM-4 1 nanocomposite powder with vigorous stimng. Certain amount of PEO was then slowly added with continuous stirring for

Ceng Y .et al . , Gel electrolytes in quasi-solid-statedye-sensitized solar cells

24 h to produce stable polymer electrolytes. For comparison, we also prepared samples containing only LE/PEO without any particles.

2.2. Measurements and characterization To investigate the structure and crystallinity of the samples, the powders were analyzed with an X-ray powder diffractometer, XRD (D/ max-rA Rigaku diffractometer, Cu KR radiation, Japan). The sample was scanned from 0.6" to 10" ( 2 8 ) with a step size of 0.02" and a count time of 1 s at each point. Sample morphology and microstructure were examined by scanning electron microscopy, SEM (SEM, JOEL, LEO-1530, Japan), and by transmission electron microscopy, TEM (TEM , JOEL, JEM-200CX, Japan) . For TEM analysis, specimens were prepared by dispersing the as-obtained powder in alcohol and then placing a drop of the suspension on a copper grid coated with transparent graphite, followed by drying. The I-V characteristics were measured using a KEITHLEY electrometer 4200 controlled by a computer. The samples were illuminated under white light ( Xe lamp, 30 mW .cm-*) through the conducting glass substrate with an illuminated area of 0.25 cm2 and no corrections were made for the reflection and transmission loss in the CTO .

3.

Results and discussion

The structure of the synthesized calcined MCM-41 sample was confirmed by small angle

203

X-ray diffraction. The XRD pattern of the sample in Fig. 1 shows four distinguishable peaks including a strong peak around 2.48" and three weak peaks at 4.24", 4.87", and 6.45", which are indexed as [ 1001, [ 1101, [ 2001, and [210] of the hexagonal arrangement in mesoporous MCM-41, suggesting fine long-range order in this material. The pore diameter is calculated to be 3.13 nm, which is enough for liquid electrolyte molecules to penetrate into the nanopores of the mesoporous silica. The BrunanerEmmett-Teller specific surface area of the sample was measured to be as high as 965 m2.g-'

+q-%

riooi

[I101

4.24

dinin 3.58 2.09 1.82

138

2

3

4

5

2 0 I(

6

7

8

9

)

Fig. 1. X-ray diffraction pattern of MCM-41 samples obtained after calcination at 823 K .

The morphologies and microstructures of the obtained MCM-41 samples and fumed silica (Aerosil R972) are clearly revealed by TEM and SEM shown in Fig. 2 . Inset HRTEM image of MCM-41 in Fig. 1( a ) illustrates the existence

Fig. 2. TEM and SEM photographs of particles : ( a ) TEM of MCM41( inset is HRTEM image of sample) ; (b) SEM of fumed silica.

204

RARE

of highly ordered hexagonal arrays in the particles. The SEM image of the fumed Aerosil R972 is shown in Fig. 1 ( b ) , in which Si02 nanoparticles are about 30 nm in diameter, while the original particle size is 16 nm according to the Degussa Co. Ltd. This reveal is that the conglomeration has occurred among the minute particles. Fig. 3 shows the photocurrent-voltage curves of the DSSCs containing the different particles in gel composite electrolytes obtained at 30 m l V - cm-'. From the characteristics of the solar cells summarized in Table 1, it is clearly seen that their performance vanes significantly. The best performance of these solar cells comes from the one using the R972/gel electrolyte which demonstrates a conversion efficiency of 6.85% . The conversion efficiency of the solar cells using R972/gel, Ti02/gel and MCM4l/gel electrolytes are all higher than the cell without any particles in the gel electrolyte, which is consistent with early reports that nanoa R972/gelelectrotyte b P2Wgel eledsdyte c MCM4llgel decl~dyte d gddatmtyte

E

- y \f

0

200

400

600

860

VoltagcmV

Fig. 3. Photocurrent-voltage curves for dye-sensitized solar cells, assembled with R972/gel electrolyte ( a ) , and €%/gel electrolyte( b) , MCM4Ygel electrolyte (c), and gel electrolyte ( d ) , at 30 mW-cm-2.

Table 1. Photo-electrochemical properties of DSSCs using gel electrolytes" Electrolyte V,/mV

J,l(mA-crn-2)

v/%

FF

Gel 619 2.95 0.76 4.62 R972/gel 660 4.19 0.74 6.85 TiOzlgel 611 3.77 0.66 5.05 601 3.18 0.73 4.65 SiO2/gel * The experimental conditions: under 30 m W . c m - 2 illumination; cell active area = 0.25 cm2

METALS, Vol. 2 5 , Spec. Issue , Oct 2006

particle plays a favorable role in promoting ion transport of the electrolyte for DSSCs. Fig. 3 shows the photocurrent-voltage curves of the DSSCs containing the different particles in gel composite electrolytes obtained at 30 mW cm-'. From the characteristics of the solar cells summarized in Table 1 , it is clearly seen that their performances vary significantly. The best performance of these solar cells comes from the one using the R972/gel electrolyte which demonstrates a conversion efficiency of 6.85 % . The conversion efficiency of the solar cells using R972/gel, TiOJgel and MCM-4l/gel electrolytes are all higher than that of the cell without any particles in the gel electrolyte, which is consistent with early reports that nanoparticles play a favorable role in promoting ion transport of the electrolyte for

DSSCs . As the additives of electrolyte, the particles were expected to have relatively smaller sizes so that the composite gel electrolyte can penetrate into the Ti02 film easily to make a good contact with nanocrystalline Ti02and triiodidehodide ions can transfer freely through the open channels made of the nanoparticles in the gel electrolyte. Comparing with the sizes of R972 ( 16 nm) and P25 (25 nm) , the MCM-41 particles with 1 p m average diameter are huge, while the conversion efficiency of the solar cell with MCM-4l/gel electrolyte is still higher than that of the cells applying gel electrolyte without any particles and close to that of cells with TiOJgel electrolyte . These facts indicate that the ordered channels observed in MCM-41 (Fig. 2 ) might provide a possibility to offer unique passages for redox couple to move freely and optimized the performance of solar cells. Changing the preparation methods for introducing the LE into the channels of MCM-41 results in different photovoltaic device performance. As seen from Fig. 4 , the outgassing process on MCM-41 has a significant effect on Z-V characteristics of the solar cells. The energy conversion efficiency of cell using the electrolyte dealt with outgassing process was 4 . 6 5 % . While that of the cell using the elec-

Ceng Y .et al . , Gel electrolytes in quasi-solid-statedye-sensitized solar cells

trolyte without outgassing process, decreased to be 3. 46% . It is supposed that more liquid electrolyte could be absorbed into the channels of MCM-41 after the outgassing process so that the redox couple ( triiodide and iodide) could move freely both in the open channel of MCM41 and in the matrix of gel electrolyte, resulting in an enhanced performance of the DSSCs consequentially.

205

spectively, while the conversion efficiency is reduced from 6.85 % to 5.87 % . The reduction in the conversion efficiency when the light intensity is increased may be due to a limitation in mass transport between the photoelectrode and counterelectrode in the DSSCs, since large current will pass through the cell at high light intensity[ 201 . 14

-,

With outgassing period

.

WR

0

H

100

I

200

500

400

300

100

200

400

500

600.

700

t 0

VoltageN

700

600

300

Voltage/V

Fig. 4. Photocurrent-voltage curves for dye-semithed cells assembled with MCM4l/gel polymer electrolytes. Curves obtained at different preparamethod.

Fig. 6. Photocurrent-voltage curves for dye-sensitized cells assembled with RW2/gel polymer electrolytes. Curves obtained at different light intensities and treatment.

650

4.

€40 630

>

620

Lz

E

\

610

600

0.01, 0

I

,

10

.

I

20

.

I

30

.

I

40

.

!

50

[MCM-.ll]/wt.% LiI

Fig. 5. Short-circuit photocurrent J , ( 0 ) and open-circuit voltages V,(*) as a function of concentration of MCM-41.

The current-voltage plots of the solar cell with the R972/PEO gel electrolyte under 30 and 100 mW ~ c m simulated - ~ sunlight are presented in Fig. 6. At an irradiance of 100 mW cm-2, which approximates to the standard AM1.5 irradiance, the values of J , and V , are increased to 12.69 mW * cm-’ and 715 V , re-

Conclusions

When SiOz nanoparticles ( R972 ) , Ti02 nanoparticles ( P25 ) and mesoporous silica (MCM-41) particles are added into the PEO gel electrolyte, the conversion efficiency of the DSSCs increases from 4. 62% to 6. 8 5 % , 5.05% and 4. 65% respectively under 30 mW*cm-’ illumination. The preparation methods and particle sizes exert an influence on the performance of corresponding solar cells. Mesoporous silica particles with ordered nanochannels seem to be a potential filler to fabricate gel electrolyte. Systematical researches on the mechanism about the effects of the nanaoparticles on the gel, electrolyte and the corresponding performance of DSSCs are being carried out, and will be reported in another paper later. Acknowledgements: Special thanks to Prof. Wang Liduo and Wu Xueming in Chemistry Department, Tsinghua University given for their useful discussion.

RARE METALS, Vol. 2 5 , Spec. Issue, Oct 2006

206

[ 101 Swierczynski D . , Zalewska A . , and Wierzorek

References

.

O‘Regan B . and Gratzel M . , .4 low-cost, highefficiency solar-cell hayed on dye-sensitized colloidal Ti02 films. . V a t w e , 1991, 353(24) : 737. Nazeeruddin M . K., Kay A . , Rodicio I . , et a l . , Conversion of light to electricity by cisX2Bis ( 2 . 2’-hipyridyl-4, 4’-dicarboxylate ) ruthenium ( 0 ) charge-transfer sensitizers ( X = C1- , Br- , I - CN- , and SCN- ) on nanocrystalline Ti02 electrodes. J . A m . Chem. S o c . , 1993. 115( 1 4 ) : 6382. Meng Q . B . , Takahashi K . . Zhang X . T . , et 0 1 . . Fabrication of an effirient solid-state dyesensitized solar cell. h n g m u i r , 2003, 19( 9 ) : 3572. Tennakonr K . Perera V . P . S. , Kottegoda I . R . M . et a1 . , Dye-sensitized solid state photovoltaic cell based on composite zinc oxide/tin ( I V ) oxide films. J . Phys. D : .Appl. P h y s . . 1999, 3 2 ( 4 ) : 374. Bach U . , Lupo D . , Comte P . et a l . , Solidstate dye-sensitized mesoporous TiOz solar cells with high photon-to-electron conversion efficiencies. .Vature , 1998, 395(6702) : 583. Wang P . , Zakeeruddin S . M . , MOSER J . E . , el a1 . 4 stable quasi-solid-state dye-sensitizedsolar cell with an amphiphilir rutheniumsensitizer and polymer gel electrolyte. il;at. M n l e r , , 2003. 2 ( 6 ) : 402. Haque S.A. Palomares E . Upadhyaya H . M . , et a / . , A flexihle dye sensitised nanocrystalline semironductor solar cells . Chern . Commun . , 2003. 24: 3008. An H . L . , Xue B . F . , Li D . M . , et d . , Environmentally friendly LiUethanol based gel electrolyte for dye-sensitized solar cells. Electrochem . C o m m u . n . . 2006. 8 ( 1 ) : 170. Kim J . H . Kang M. S., Kim Y . J . , et a l . , Dye-sensitized nanocrystalline solar cells based on composite polymer electrolytes containing fumed silica nanoparticles . Chem . Commun . , 2004. 14: 1662.

.

.

.

.

.

.

K‘. , Composite .polymeric electrolytes from the . PEODME-LiC104-Si02 system. Chem . Mater . , 2001, 1 3 ( 5 ) : 1560. Yang H . , Cheng Y . F . , Li F . Y . . et a l . , Quasi-solid-state dye-sensitized solar cells based on mesoporous silica SBA- 15 framework materials . Chin. Phys . L e t t . , 2005, 2 2 ( 8 ) : 2116. Tominaga Y . , Asai S., Sumita M . , et a ] . , A novel composite polymer electrolyte : Effect of mesoporous SiOz on ionic conduction in poly (ethylene oxide)-LiCF3S03 complex. J . Power Sources, 2005, 146( 1 - 2 ) : 402. Katsaros G . , Stergiopoulos T . , Arabatzis I . M . , ef ~ l, .A solvent-free composite polymer/inorganic oxide electrolyte for high efficiency solidstate dye-sensitized solar cells. J . Phoroch . Pholobio. A , , 2002, 149( 1 - 3 ) : 191. Usui H . , Matsui H . , Tanabe N . , et a l . . lmproved dye-sensitized solar cells using ionic tiauocomposite gel electrolytes. J . Photoch . Photobio. A . , 2004. 1 6 4 ( 1 - 3 ) : 97. Kato T . . Kado T . , Tanaka S., et a l . , Quasisolid dye-sensitized solar cells containing nanoparticles modified with ionic liquid-type niolecules. J. Electrochem. S o c . , 2006, 153(3): A626. Kresge C .T. , l~onowiczM . E. , Roth W. J . , et a l . , Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. N a t u r e , 1991. 359(22): 710. Chu P. P . , Reddy M . J . and Kao H . M . , Novel composite polymer electrolyte comprising mesoporous structured SiOz and PEO/Li. Solid State fonics, 2003, 156( 1 - 2 ) : 141. Cai Q . , Luo Z . S . , Pang W . Q . et a l . , Dilute solution routes to various controllable morphologies of MCM-41 silica with a basic medium. Chem . Mnter . , 2001, 13(2 ) : 258. Kim D . W . , Jeong Y . B . , Kim S. H . , et a / . , Photovoltaic performance of dye-sensitized solar cell assembled with gel polymer electrolyte. J. Power Sources , 2005, 149: 112.

.