Progresses in dye-sensitized solar cells

Materials Science and Engineering B 161(2009)2–7

Contents lists available at ScienceDirect

Materials Science and Engineering

B

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e

b

Review

Progresses in dye-sensitized solar cells

Hong Lin ?,Xin Li,Yizhu Liu,Jianbao Li

The State Key Laboratory of New Ceramics and Fine Processing,Department of Materials Science and Engineering,Tsinghua University,Beijing 100084,PR China

a r t i c l e i n f o Article history:

Received 12June 2008

Received in revised form 13October 2008Accepted 22December 2008Keywords:

Dye-sensitized solar cells Nano-channel Flexible

a b s t r a c t

Recent progresses on the modi?cation of photoanodes and electrolytes related to the concept of oriented nano-channels,and the fabrication of ?exible cells aiming at practical application in dye-sensitized solar cells are reported.Some mechanisms of material preparation are analyzed,and perspectives are proposed.

?2008Elsevier B.V.All rights reserved.

Contents 1.Introduction............................................................................................................................................22.Titania nanoarrays in DSCs.............................................................................................................................33.

DSCs with composite liquid electrolytes and ionic https://www.360docs.net/doc/353703642.html,posite liquid https://www.360docs.net/doc/353703642.html,posite ionic liquids and corresponding quasi-solid DSCs.................................................................................54.Flexible DSCs with graded TiO 2?lm...................................................................................................................65.

Summary...............................................................................................................................................7Acknowledgements....................................................................................................................................7References .............................................................................................................................................

7

1.Introduction

Dye-sensitized solar cells (DSCs)have been widely investigated due to their low production cost and potentially high conversion ef?ciency ever since its invention almost two decades ago [1].In such a cell,sunlight is harvested by a sensitizer (dye)that is attached to the surface of a wide band gap semiconductor,typically an oxide such as TiO 2?lm.Photo-excitation of the dye results in the injec-tion of electrons into the conduction band of the oxide.The dye is regenerated by hole injection from an organic hole conductor or an electrolyte,such as an ionic liquid containing most frequently the iodide/triiodide couple as a redox system.The redox system is regenerated in turn by the reaction with the electrons at the counter electrode which have passed through the external circuit.The volt-age generated under illumination corresponds to the difference between the quasi-Fermi level of the electron in the oxide and the

?Corresponding author.

E-mail address:hong-lin@https://www.360docs.net/doc/353703642.html, (H.Lin).

redox potential of the electrolyte or the work function of the hole conductor.Thus,the device is regenerative,producing electricity from light without any permanent chemical transformation [2].The TiO 2photoanode and electrolytes play a key role in charge separation and transfer in DSCs,and the advances of these materials may greatly promote the photovoltaic performances by optimizing structural and electrochemical properties.On one hand,oriented nano-structures might provide channels for charge transfer,short-ening their diffusion courses and enhancing the conductivity of the cell.Such oriented nano-structures might not only exist in pho-toanodes as well-known nanowires [3]or nanotubes [4],but also potentially make sense in electrolytes,especially quasi-solid and solid-state ones,to help improve the comparatively lower conduc-tivity.On the other hand,updating of fabrication techniques might contribute to cells with greater practicability,such as ?exible cells.

Here,we report some of our recent work on (1)the modi?-cation of photoanodes and electrolytes related to the concept of oriented nano-channels and (2)the fabrication of cells aiming at practical application.Some mechanisms of material preparation are analyzed,and perspectives are proposed.

0921-5107/$–see front matter ?2008Elsevier B.V.All rights reserved.doi:10.1016/j.mseb.2008.12.030

H.Lin et al./Materials Science and Engineering B 161(2009)2–7

3

Fig.1.(a)Scanning electron microscopy image of the hydrogen titanate (H 2Ti 5O 11·H 2O)nanowire array synthesized by a hydrothermal reaction at 180?C for 4h;(b)transmission electron microscopy image of one nanowire;(c)SAED pattern for the nanowires of the array;(d)high-resolution transmission electron microscopy image of the end of one nanowire.

2.Titania nanoarrays in DSCs [5]

TiO 2(anatase)nanoarrays were obtained by direct hydrother-mal treatment of Ti substrates followed by ion exchange in HCl and post annealing.The as-prepared ?lms were composed of mono-clinic H 2Ti 5O 11·H 2O,which has been demonstrated by SEM images,HRTEM images and SAED patterns (Fig.1).

A postulated mechanism of the nanowire growth is illustrated in the model as shown in Fig.2.At the beginning,the Ti sub-strate reacted with the NaOH aqueous solution and many Na 2Ti 5O 11nanocrystallines were created and covered the plate.The nanocrys-tallines then transformed into a nucleus,from which the nanobelts begin to grow.The growth in the cross direction was inhibited because of the competitive growth from too many nuclei simul-taneously,whereas the growth in the direction perpendicular to the substrate was free.As a result,the nanobelts began to grow vertically on the substrate through a dissolution–recrystallization process,during which the dislocations caused the defect strain in the cross direction and thus increased with the nanobelt growth.When the nanobelt was long enough,the strain was large enough to suppress the nanobelt into a multilayer structure and then the mul-tilayer nanowire was formed.This mechanism was demonstrated in detail by SEM and HRTEM images of ?lms prepared under dif-ferent conditions [5].The Na 2Ti 5O 11nanowire array ?lm was then immersed into an HCl aqueous solution to exchange Na +into H +.The resulting H 2Ti 5O 11·H 2O nanowire array ?lm was postannealed and transformed into a TiO 2nanowire array ?lm.XRD patterns (Fig.3)showed that anatase began to form at 250?C (curve c)and H 2Ti 5O 11phase almost disappeared completely at 450?C (curve

e).

Fig.2.Mechanism of the nanowire growth.

4H.Lin et al./Materials Science and Engineering B 161(2009)

2–7

Fig.3.XRD patterns of TiO 2nanowire array ?lms sintered at different temperatures.(a)H 2Ti 5O 11;(b)200?C;(c)250?C;(d)400?C;(e)450?C;(f)700?

C.

Fig.4.Photocurrent–voltage characteristics of dye-sensitized solar cells prepared using (a)a TiO 2nanowire array synthesized by hydrothermal treatment at 180?C for 4h and postannealing at 450?C for 2h and (b)P25TiO 2powder.

The nanowire thin ?lm was shed from the plate as a membrane,pasted onto a conductive glass and assembled into a DSC.Fig.4shows the photocurrent–voltage (I –V )characteristics of DSCs pre-pared using TiO 2nanowire arrays and commercial TiO 2powder (P25,Degauss,Germany)[6,7].The short-circuit current density (J sc )increased from 13.5to 15.2mA/cm 2on using nanowire arrays with a thickness of 4.1?m as the photoanode compared to the tradi-

tional nanocrystalline ?lm (8?m thick)made of P25TiO 2powder despite of a much smaller thickness.The ef?ciency of DSC was also enhanced from 5.67%to 6.58%,which indicates that the structure of the TiO 2thin ?lm may affect the photocurrent of DSC.

For TiO 2nanoparticles,there are many defects in the bound-aries at the contacts between nanoparticles.These defects may act as electron traps that hamper the movement of electrons [8].How-ever,the oriented one-dimensional TiO 2nanowires may provide channels for fast electron transfer while maintain the merit of high speci?c surface area,which allows adequate loading of sensitizer and thus exhibited even higher energy conversion ef?ciency with thinner ?lms.This is potentially advantageous for quasi-solid and solid-state DSCs where the short diffusion length of injected exci-tons is always problematic due to poor wetting of the electrolyte.3.DSCs with composite liquid electrolytes and ionic liquids https://www.360docs.net/doc/353703642.html,posite liquid electrolytes

As a feature of our effort on electrolyte modi?cation,inor-ganic additives were adopted and their effects on enhancement of electrochemical properties of the electrolytes were investigated concerning with liquid electrolytes [9].Herein,layered ?-zirconium phosphate (?-ZrP)[10]was chosen and added in different weight content to iodine-based electrolyte containing 4-tert-butylpyridine (TBP)as an organic additive which has demonstrated to be able to improve markedly the open-circuit voltage by the suppression of the dark current [11].

Such composite electrolytes were assembled into thin-layer cells and subjected to electrochemical characterizations including electrochemical impedance spectroscopy (EIS)and cyclic voltam-metry (CV).The Nyquist diagram of thin-layer cells (Fig.5(a))showed that with the addition of ?-ZrP,the R ct value of thin-layer cell representing the charge transfer resistance of the counter electrode/electrolyte interface decreased markedly and reached its minimum at 9wt%?-ZrP content relative to the weight of TBP,while the R s value representing the ohmic serial resistance (resistance of electrolyte)had almost no change.This result is supported by cyclic voltammetry characterizations (Fig.5(b))where the diffusion coef-?cient of I 3?,D I 3?,proportional to j lim ,had its maximum value also at 9wt%?-ZrP content.

The enhancement of electrochemical properties was attributed to the intercalation of TBP molecules into ?-ZrP nano-layers and was demonstrated by the XRD patterns (Fig.6(a))of pure ?-ZrP and TBP-doped ZrP (T-ZrP).The (002)diffraction peak shifted from 11.66?to 6.96?,corresponding to the change of the interlayer dis-tance from 7.6to 12.7?(Fig.6(b)),with the difference of 5.1?coinciding with the molecular size of TBP.Meanwhile,the inter-layer gap was enlarged into 6.4?,exceeding both the diameter and length of I 3?ion in acetonitrile solution [12].Thus,I 3?

ions

Fig.5.(a)Nyquist diagram;(b)cyclic voltammogram of thin-layer cells.

H.Lin et al./Materials Science and Engineering B161(2009)2–7

5

Fig.6.(a)The XRD patterns of?-ZrP and T-ZrP;(b)illustration of intercalation of TBP into?-ZrP nano-layers. can transport freely in the interlayer of T-ZrP,which could pile up

and form“bridge”between the two electrodes,and therefore the

electrochemical properties can be greatly enhanced.

https://www.360docs.net/doc/353703642.html,posite ionic liquids and corresponding quasi-solid DSCs

The signi?cant enhancement of electrochemical properties of

liquid electrolytes by layered?-ZrP is highly indicative for initiating

a new?eld of electrolyte modi?cation by inorganic nano-layered

materials,within which the ef?cient modi?cation of ionic liq-

uids that have long been suffering low ion diffusion coef?cient is

expected.Thus,in our study,?-ZrP was also applied to prepare com-

posite ionic liquids and quasi-solid DSCs were assembled thereafter

based on their encouraging electrochemical properties improve-

ments[13].

?-ZrP was?rst added to TBP-containing1-methyl-3-

propylimidazolium dihydrophosphate(MPIDP)[14]ionic liquid

at different weight contents relative to TBP,and either thin-layer

cells or quasi-solid DSCs were fabricated and subjected to EIS and

photovoltaic performance characterizations.The Bode phase plots

of DSCs(Fig.7(a))re?ected the electron lifetime( s)in TiO2?lms

by the equation s=1/ωmid=1/2 f mid,whereωmid represented

the characteristic angle frequency of the midfrequency(f mid)peak.

f mid of the DSC with6%?-ZrP decreased to its minimum of11.13Hz

by a factor of9compared to the DSC without?-ZrP,suggesting

that s is lengthened signi?cantly.Meanwhile,the optimum of

electrochemical properties of composite ionic liquids at6wt%

of?-ZrP was demonstrated by thin-layer cell characterization,

when exchange current density I0and I3?diffusion coef?cient D I

3?

simultaneously reached their maximum respectively(Fig.7(b)). Photovoltaic performances of quasi-solid DSCs(Fig.8)

further

Fig.8.Photovoltaic performances of quasi-solid DSCs.

supported such results,that is,with the increase of?-ZrP content to6wt%,J sc and V oc were both enhanced,reaching their maximum of0.562V and11.38mA/cm2,respectively.An ef?ciencyáof2.61% was achieved,increasing by a factor of more than2compared to a DSC without?-ZrP.

The effect of?-ZrP on composite ionic liquids can be interpreted in a similar way with that in?-ZrP doped liquid electrolytes.How-ever,the optimal content of?-ZrP here is6wt%,rather than9wt%in composite liquid electrolytes,which is mainly due to the difference in the dispersion of?-ZrP in acetonitrile and ionic

liquids.

Fig.7.(a)Bode phase plots of DSCs;(b)dependence of exchange current density I0and I3?diffusion coef?cient D I

3

?on weight content of?-ZrP.

6H.Lin et al./Materials Science and Engineering B161(2009)

2–7

Fig.9.Photovoltaic performances of DSC with?-ZrP doped MPII.

Besides MPIDP ionic liquid,several other ionic liquids were also doped with?-ZrP to prepare various composite ionic liquids,among which?-ZrP-doped1-methyl-3-propylimidazolium iodide(MPII) exhibited4.7%light-to-energy conversion ef?ciency(Fig.9).

4.Flexible DSCs with graded TiO2?lm[15]

Three types of particles with average diameters of12,28,and 100nm were used to prepare graded TiO2pastes.The100nm-diameter large particles with pure anatase structure were used as the scattering particles,referred to as the large particles.The 28nm-diameter P25particles were accepted as the skeleton of the binary or ternary graded particles.The12nm-diameter small parti-cles obtained by hydrothermal methods[16]were also accepted as one part of the binary or ternary graded particles.Graded TiO2?lms with different component ratios were deposited on either rigid ITO-coated glass or?exible ITO-coated PEN substrate by simple doctor-blade technique,and were annealed under different temper-atures.They were then assembled into DSCs using N719sensitizer and4-tert-butylpyridine-doped LiI/I2electrolyte.

The optimal weight ratio of P25particles,the small particles, and the large particles in the graded?lm was determined as5:2:2 according to theoretical calculations combined with investigations of its in?uences on light-to-energy conversion ef?ciencies(Fig.10). To study the effect of heat-treatment temperature on cell perfor-mances,such a graded?lm was deposited on ITO-coated glass, calcined under different temperatures with otherwise same but unoptimized parameters and assembled into DSCs.The ef?ciency of the cell with150?C treated graded?lm was up to4.1%,which was just a little lower than that of the DSCs with a450?C treated

?lm.Fig.11.Dependence of the ef?ciency and the current density on the heat-treatment temperature.The line with solid circle symbols represents the ef?ciency and the line with triangle symbols represents the current

density.

Fig.12.I–V curves of the typical DSCS with the photoanode?lms treated at150?C: (1)DSC with the optimum graded structure?lm on an ITO/glass substrate;(2)DSC with the optimum graded structure?lm on an ITO/PEN substrate.

Even without any heat-treatment,the cell still showed an ef?ciency of2.6%(Fig.11).

To assemble a?exible DSC,the graded structure?lm was deposited on an ITO/PEN substrate treated at150?C.Fig.12shows the photovoltaic properties of the optimized DSCs with the graded structure?lm on an ITO/glass substrate and an ITO/PEN substrate. Under standard testing conditions,the cell based on the ITO/glass substrate gave a photocurrent density of J sc=11.68mA/cm2,an open-circuit voltage of V oc=0.73V,and a?ll factor of ff=

0.668 Fig.10.Dependence of light-to-energy conversion ef?ciencies on weight content of(a)the small particles and(b)the large particles.

H.Lin et al./Materials Science and Engineering B161(2009)2–77

yielding a 5.69%conversion ef?ciency as shown in curve(1). Meanwhile the cell based on the ITO/PEN substrate gave a pho-tocurrent density of J sc=9.8mA/cm2,an open-circuit potential of V oc=0.735V,and a?ll factor of ff=0.63,yielding a4.54%conver-sion ef?ciency as shown in curve(2).The ef?ciency of the former is just25.3%higher than that of the latter.

The good performance of the designed graded?lms at a low tem-perature may be attributed to the chemical sintering process[17] during the low-temperature treatment.The small particles were synthesized by hydrothermal method in an acid environment.The pH value of the resulting sols was ca.1[16],and there were many OH groups in the surfaces of the small particles.Once P25particles were mixed with the precursor,OH groups would be adsorbed by the sur-faces of P25particles.When such a graded structure was heated at a low temperature,OH groups on the small particles and P25particles would lead to the reaction of dehydration,as shown below:

Ti OH+HO Ti→Ti O Ti+H2O

The dehydration would increase the physical and chemical con-nections not only between the small particles and P25particles,but also between the small particles in the graded?lm,and then reduce the internal resistance of the?lm.This de?nitely improves the photovoltaic performance of the DSCs even with low temperature-treated photoanode?lms.At the same time,with the binding effect of the small particles and the appropriate pileup by the optimal ratio,the mechanical strength should be improved.That is to say, the concept of graded?lms opens a window for low-temperature treatment of TiO2photoanodes.This is especially crucial for the fab-rication of ef?cient?exible cells,and is believed to greatly enhance both their researches and commercialization.

5.Summary

One-dimensional oriented TiO2nanoarray photoanodes were prepared by simple hydrothermal oxidation of Ti substrates.DSCs assembled with such photoanodes exhibited signi?cantly promoted photovoltaic properties,which was attributed to the nano-channels resulted from the oriented nano-structures.A simi-lar concept was employed in the modi?cation of liquid electrolytes and ionic liquids with?-ZrP,which also constructed nano-channels facilitating I3?ion diffusion and promoted the ionic liquids to quite a practical level.Besides,the low-temperature sintering problems for?exible cells were overcome by the introduction of graded TiO2 photoanodes,which structurally solidi?ed the?lms and chemically enhanced the connection for reliable electron transfer. Acknowledgements

This work was?nancially supported by the National High Tech-nology Research and Development Program of China(863Program, 2006AA03Z218),NSF of China(50572051,50672041),and Beijing Natural Science Foundation(2062013).

References

[1]B.O’Regan,M.Gr?tzel,Nature353(1991)737–740.

[2]M.Gt?tzel,Prog.Photovolt.:Res.Appl.14(2006)429–442.

[3]https://www.360docs.net/doc/353703642.html,w,et al.,Nat.Mater.4(2005)455–459.

[4]K.Varghese,et al.,J.Mater.Res.18(2003)156–165.

[5]W.Wang,H.Lin,et al.,J.Am.Ceram.Soc.91(2008)628–631.

[6]A.Hagfeldt,M.Gr?tzel,Chem.Rev.95(1995)49–68.

[7]N.Wang,et al.,Appl.Phys.Lett.89(2006)194104.

[8]M.Adachi,et al.,J.Electrochem.Soc.150(2003)G488–G493.

[9]N.Wang,H.Lin,et al.,https://www.360docs.net/doc/353703642.html,mun.8(2006)946–950.

[10]A.Clear?eld,Prog.Inorg.Chem.Res.47(1998)371.

[11]S.A.Haque,M.Gr?tzel,et al.,J.Phys.Chem.B104(2000)538.

[12]H.Sakane,et al.,J.Synchrotron Radiat.8(2001)674.

[13]N.Wang,H.Lin,et al.,Appl.Phys.Lett.89(2006)194104.

[14]Y.Wang,et al.,https://www.360docs.net/doc/353703642.html,mun.17(2004)1938.

[15]X.Li,H.Lin,et al.,J.Photochem.Photobiol.A:Chem.195(2008)247–253.

[16]C.J.Barbe,et al.,J.Am.Ceram.Soc.80(1997)3157–3171.

[17]N.G.Park,et al.,Adv.Mater.17(2005)2349–2353.