- Email: [email protected]
The mixed effect of phthalocyanine and porphyrin on the photoelectric conversion of a nanostructured TiO2 electrode
SYlITHETIC ISTllS ELSEVIER
Synthetic Metals 92 (1998) 269-274
The mixed effect of phthalocyanine and porphyrin on the photoelectric conversion of a nanostructured TiO2 electrode Huihua Deng a,,, Yuming Zhou
b, Haifang Mao c, Zuhong Lu a
National Laboratory of Molecular and Biomolecular Electronics, Southeast UniversiO', Nanjing 210096, China b Department of Chemical Engineering, Southeast University, Nunjing 210096, China c h~stitute of Organic Chemisto,, Academic Sinica. Shanghai 200433, China Received 12 September 1997; revised 23 October 1997; accepted 10 November t997
Abstract The mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion is reported. For the liquid junction solar cell based on the cosensitized TiO2 electrode, cosensitization greatly enhances the photocurrent response at the longer wavelength (the GaTsPc Q band) and markedly decreases the photocurrent response in the Soret band of ZnTsPP. The decrease of the Soret band of ZnTsPP is offset by the improvement of the Q band of GaTsPc, resulting in the mixed effect in the short-circuit photocurrent. This is attributed to the formation of Pc/PP heteroaggregates on the cosensitized TiO2 electrode, resulting in the decreases of the concentration of GaTsPc dimer and the presence of the low-lying charge-transfer state. © 1998 Elsevier Science S.A. Keywords: Charge-transfer states; Cosensitization; Heteroaggregation; Mixed effects; Nanostructured titanium dioxide electrodes
1. Introduction The nanocrystalline solar cell, recently developed by Grfitzel and co-workers [ 1-5], overcomes the low efficiency of conventional photoelectrochemical cells or solid cells due to inefficient separation and transport of the photogenerated charge carriers [6-8] or due to low absorbance [9]. The nanocrystalline solar cell is based on a dye-sensitized colloidal TiO2 film with a very high specific surface area. In our laboratory, the liquid junction solar cells based on transparent and microporous TiO2 electrodes sensitized with phthalocyanines or porphyrins has been fabricated [ 10-12]. Phthalocyanines and porphyrins, whose chemical structures resemble chlorophyll derivatives [4,5], are chosen as the photosensitizer because phthalocyanine shows strong absorption at the longer wavelength, and porphyrin shows very strong absorption and high quantum efficiency in photoelectric conversion in the region of 400-470 nm, which compensates for the lower absorbance and conversion efficiency of ruthenium bipyridine complex at the given region. By the addition of hydroquinone to liquid electrolyte solution that contains 0.1 M Na2SO4, the quantum efficiency of photoelectric conversion of the microporous T i Q electrode, sensitized with zinc phthalocyanine substituted with four carboxyl groups, * Corresponding author. Fax: + 86 25 771 2719; e-mail: [email protected] 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PHS0379-6779 ( 9 7 ) 0 4 1 0 9 - X
reaches about 20% at 690 nm [ 10]. The quantum efficiency for a nanostructured TiO2 electrode sensitized with tetrasulfonated zinc porphyrin reaches 99.4% at 430 nm. But the dimeric absorbance of tetrasulfonated phthalocyanines (MTsPcs; M = Z n , Ga, Co, In, TiO, H2) does not convert into photocurrent [11], due to molecular aggregation and orientation of MTsPcs self-assembled on the positively charged TiO2 electrode via the electrostatic interaction of the negatively charged sulfonate (SO3 - ). The rapid internal conversion in the molecular aggregates of phthalocyanines is believed to be the main deactivated pathway of the excited excitons or photogenerated charge carriers [ 11,13 ]. Decreasing the surface concentration of dimer is probably an efficient means to improving the energy power conversion. In this paper, we report the mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion of the cosensitized TiO2 electrode. The mechanism generating the mixed effect is also discussed.
2. Experimental details TiO2 colloidal solutions were prepared by hydrolysis of tetrabutyl titanate ((C4H90)4Ti) by a procedure similar to
14. Deng et al. / Synthetic Metals 92 (1998) 269-274
270
ZnTsPP in DMSO until the absorbance of the electrode showed no increase• The total concentration of the mixed solution was 5 × 10 -5 M. After the characteristic performances of the cosensitized TiO2 electrode had been measured, GaTsPc and ZnTsPP on the TiO2 electrode were washed away using 0.001 M NaOH solution. The resulting solution was used to analyze the actual content of GaTsPc and ZnTsPP molecules adsorbed on the TiO2 electrode. The liquid junction solar cell for measuring the photocurrent, as shown in Fig. 1 (c), consists of the sensitized or cosensitized TiO2 electrode and a counter electrode with an electrolyte solution in between containing 0.1 M KI and 0.05 M I2 in 0.001 M HCIO4. When light is incident through the working electrode, the counter electrode is an ITO conducting glass, on which a thin layer of platinum has been coated by vapor vacuum deposition at 10- 7 Tort. When light is incident through the counter electrode (the back electrode), an ITO conducting glass is also used as the back electrode. The morphology and the film thickness of the T i Q electrode were examined by scanning electron microscopy (SEM; JEOL, JSM-6300). The absorption spectra were recorded with a Shimadzu UV-2201 UV-Vis spectrometer.
that described in Ref. [ 10]. The colloidal solution on addition of 2 wt.% poly(vinyl alcohol) (PVA) was then concentrated to a desired density through vacuum rotation evaporation. After addition of 1.5 wt.% Triton X-100, the concentrated solution (TiO2 content; 10 wt.%) was spin-coated on a freshly cleaned ITO (indium-tin oxide; sheet resistance of 50 f~ D -~ and transmission of 95% in the visible region) conducting glass substrate. The sheet resistance of the nanostructured T i Q electrode, finally obtained by a heating process similar to that described in Ref. [ 11 ], is 500 1~ [ ] - t Prior to dye sensitization, the nanostructured TiO2 electrodes were soaked in HCI solution (pH = 2) and naturally dried. GaTsPc and ZnTsPP were synthesized according to the methods described in Refs. [14,15]; their molecular structures are shown in Fig. 1 (a) and (b). When sensitized with GaTsPc or ZnTsPP dye molecules alone, the working electrode was obtained by plunging the bare TiO2 electrode into a 5 × 10 5 M solution of GaTsPc or ZnTsPP in dimethyl sulfoxide (DMSO) until there was no increase in the absorbance of the electrode. When cosensitized with GaTsPc and ZnTsPP, the working electrodes were obtained by plunging the bare TiO2 electrode into a mixed solution of GaTsPc and
so~
s o ~ _ r ~
N "
N
N
~
Ca
SO~
so;-
o;
m"-'-'~ N N
(a) SO; (b) sO;
V_q .... co : ele de .......... ........... -liSmd eleetrolTe ..... • '7.. TL..: il..'"'L•"':i ':....'L.."_.7 The sensitized or cosensitized
{
"ggl
(c)
I TiO2 electa°de"
h~,
Fig. 1. Molecular structures of (a) tetrasulfonated gallium phthalocyanine (GaTsPc), (b) tetrasulfonated zinc porphyfin (ZnTsPP), and (c) the structure of the liquid junction cell based on the sensitized or cosensifized TiO2 electrode.
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
The photocurrent was measured with a potentiostat (model CMBP- 1 ). Monochromatic illumination was obtained using a 500 W xenon arc lamp in combination with a grating monochromator (model WPG3D). The light intensity was calibrated using a model LM-5 laser power meter made in the National Institute of Metrology, China. The redox potentials of GaTsPc and ZnTsPP were measured using an electrochemical station (CHI 660; CH Co.).
3. R e s u l t s a n d d i s c u s s i o n
The nanostructured T i Q electrode is composed of interconnected particles (40-60 nm) and pores, and has a thickness of 10 Ixm. The bare nanostructured TiO2 electrode exhibits a fundamental absorption edge at 390 nm of anatase in the UV region. The specific surface area of the nanostructured TiOe electrode is characterized by the dimeric absorbance of GaTsPc molecules in the case of a single sensitization. Assuming that a complete monolayer with a 2 nm 2 area (Sm) for each molecule covers the TiO~ electrode, and that the extinction coefficient (e(A)) of GaTsPc molecules is 3.27 × 107 cm ~ m o l - ] at 620 n m [ 16], the specific surface area (Sv) of the TiO2 electrode of 370 was obtained by the following formula [2]: Sp=(OD×NA×Sm)/(e(A)) in which OD and NA are the optical density and Avogadro constant, respectively. As shown in Fig. 2, the nanostructured TiO~ electrode cosensitized with GaTsPc and ZnTsPP dye molecules reveals the characteristic absorption of both GaTsPc and ZnTsPP, with two maxima at around 430 and 680 nm in the visible region, whereas the nanostructured TiO2 electrode, sensitized with GaTsPc or ZnTsPP alone, reveals only the individual characteristic absorption of GaTsPc or ZnTsPP dye molecules. Therefore, cosensitization extends the absorbance of the electrode into the broader region (400-750 nm) and the ,a.. 4.5
a b e d
~1
42., ~ - - ~
e x=l.00
tq/
~,,
fill
L,
z.00
ft
1.5o
x=0.39 x=0,75 x=0.86 x=0.93
~,.o _ _ ,~ ......... , ~.s ~.o _ _ _
=
~ ~' ,
-~ t.oo
'IL,,,, I:1.,i\l ~
"'
~,,,/:;/',',"., k
_
_
f x=0.09
•
d/'
," ~/
b...,
t l:~'il' i'o° t ! ~l ~ ~, ~'"~~'~'"i~'s:~"'~:~""~:~r:~
~
~/
~ ' ~ -
.--::-..~,~
0.00
300
350
400
450
500
550
Wavelength
600
650
700
750
800
(nm)
Fig. 2. Absorption spectra of the cosensitized TiO2 electrode with GaTsPc and ZnTsPP dyes with the molar ratio of GaTsPc. Inset is the surface concentration of GaTsPc dimer vs. x.
271
photocurrent response in the visible region stems from the absorbance of the dye molecules adsorbed on the TiO2 electrode. In DMSO, the GaTsPc spectrum at a concentration of 5 × 10 -5 M consists of a strong sharp 7r--at* monomeric absorption at 675 nm accompanied by a much weaker vibrationally coupled satellite band at 606 rim. In previous studies, the narrow longer wavelength absorption at around 675 nm has been attributed to a monomeric phthalocyanine compound, and the broad absorption at 606 nm has been assigned as the characteristic absorption of the GaTsPc dimer [ 16,17 ]. As shown in Fig. 2, when adsorbed or coadsorbed on the electrode, the Q band absorption of GaTsPc has a red shift of 5-15 nm relative to 675 nm in DMSO. At the moment, with the increase of molar ratio ( x = [ G a T s P c ] / ( [ G a T s P c ] + [ZnTsPP])) of GaTsPc, a broader and resolved w - v * absorption at 620 nm appears with a markedly increased intensity and a peak absorbance finally occurs in the case of the pure GaTsPc molecules. The absorptions of both the dimer and the monomer of GaTsPc on the electrode have red shifts relative to those in DMSO, indicating that there is strong interaction between GaTsPc and the TiO2 electrode. The 7r-Tr* transition shift with a broader natural bandwidth is a result of molecular association with a cofacial phthalocyanine ring orientation [ 17,18 ] (i.e. face-to-face H-type aggregation). These results indicate the occurrence of molecular higher order aggregation of GaTsPc on the nanostructured TiO2 electrode with increasing molar ratio of GaTsPc. Assuming that there is no higher order aggregate than the dimer, and that the extinction coefficient of the GaTsPc monomer is equal to 1.29 × 108 c m 2 m o l - ~ [ 16 ], it is derived that GaTsPc adsorbed on the nanostructured TiO2 electrode exists mainly as a cofacial dimer. The surface concentration of the dimer, seen in the inset of Fig. 2, is increased with increase of x. Therefore, cosensitization by doping ZnTsPP decreases the surface concentration of the dimer of GaTsPc on the cosensitized TiO2 electrode. In DMSO, the absorption spectrum of ZnTsPP consists of the Soret band with a very strong peak at 423 nm and the Q band with three peaks at 553, 595 and 642 nm. Compared with the absorption of the porphyrin solution in DMSO, that of ZnTsPP on the TiO2 electrode has red shifts at the Soret and Q bands, as shown in Fig. 2. The red shifts are increased with increase of x. But the Soret band of ZnTsPP is blueshifted to 420 nm at x = 0.39. This indicates that there is a strong interaction between GaTsPc and ZnTsPP molecules of the self-assembled film on the TiOz electrode and the occurrence of an extensive orbital mixing. Therefore, the experimental result that the red shifts of the Soret band of ZnTsPP and the Q band of GaTsPc vary with ZnTsPP doping indicates the possible formation of the heteroaggregate between GaTsPc and ZnTsPP molecules. In the previous study, stable heteroaggregates could be obtained with porphyrins and phthalocyanines grafted with ionic substituents of opposite charges [ 19-23 ] and also with anionic porphyrins with different central metals [24]. In the
272
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
former, the main driving force of heteroaggregation is the electrostatic attraction of peripheral substituent, whereas in the latter charge-transfer (CT) interaction of the porphyrin moieties is considered as the main driving force of heteroaggregates composed of anionic zinc and gold porphyrins [ 24]. The van der Waals attraction between the hydrophobic aromatic macrocycles also occurs in holding the individual components together. In addition, the planar geometry of phthalocyanines and porphyrins allows close contact (and therefore extensive orbital overlap) in the face-to-face configuration. In the case of the cosensitization, GaTsPc and ZnTsPP are adsorbed on the nanostructured TiOa electrode through coulombic attraction via sulfonate ( S O ~ ) . The positively charged surface of the nanostructured TiO2 electrode partly shields the electrostatic repulsion between GaTsPc and ZnTsPP molecules. The oxidation potentials of GaTsPc and ZnTsPP [25] are +0.86 and +0.87 V (versus NHE), and the reduction potentials are - 0.98 and - 1.16 V (versus NHE), respectively. This indicates the presence of the CT interaction between GaTsPc and ZnTsPP molecules. Therefore, the CT interaction, together with the shielding interaction and the van der Waals attraction, leads to the formation of the Pc/PP heteroaggregates on the positively charged TiO2 electrode. The interaction between two porphyrin rings or porphyrin and phthalocyanine rings leads to a face-to-face molecular aggregation with their centers offset both in solution and in solid state [23,26]. On this basis, it is concluded that these red shifts are due to the adoption of a face-to-face stacking orientation and the presence of CT interaction in heteroaggregates of GaTsPc and ZnTsPP molecules adsorbed on the nanostructured TiO2 electrode. As a result, an extended and conjugated face-to-face system is formed with ~-orbital overlap. Such an extended and conjugated 'rr system within the molecular heteroaggregates lowers the excitation energy of the "rr-'rr* and n-,rr* electric transition, which results in red shift of the Soret band and Q band absorption, respectively, as shown in Fig. 2. The maximum of the short-circuit photocurrent occurs at x = 0.39 (see Fig. 3) where the molar ratio of GaTsPc to ZnTsPP amounts to 1:1.6 and is close to 1:2. Thus, it is assumed that the PP/Pc/PP heterotrimer is created at x = 0.39. The dependence of the short-circuit photocurrent on molar ratio of GaTsPc, as shown in Fig. 3, reveals the mixed effect for the liquid junction cell based on the cosensitized TiO2 electrode. On illumination at 35.7 mW c m - 2, the short-circuit photocurrent (I~o) of the cosensitized TiO2 electrode is always larger than the linear combined sum of that of GaTsPc and ZnTsPP: /co > x X/GaTsec + ( 1 - x) X/ZnTsPP' The maximum in the short-circuit photocurrent of 771.1 p,A c m - 2 is achieved at x = 0.39. Fig. 4 shows the measured short-circuit photocurrent of the nanostructured TiO2 electrode sensitized with GaTsPc or ZnTsPP alone and cosensitized with GaTsPc (0.39) and ZnTsPP (0.61) dye molecules as a function of wavelength for light incidence through (a) the working electrode and (b) the back electrode. The photocurrent action
Ico 1" x*la=r~*~ + (1-x)*Iz,ra,p ~-"800.0 o k. ~
.~ 600.0
-.
400.0 0 0 200.0
%
0.0 0.00
0,20
0,40
0.60
0.80
GaTsPc m o l a r
1.00
ratio x
Fig. 3. Dependence of the short-circuit photocurrent on the molar ratio (x) of GaTsPc molecules for light incidence through the working electrode on illumination at 35.7 m W cm ~.
(a)
3000
.........
2500
/
I
--~2000
'' ',, <~-------.].--.-~:~-~,",,, 11
1500
'
i.
1000
, ',
0 500 ", 350
400
45o
500
500
I I
400
I
300
/
:..)
I
200
i , I\ ,'
~.i,,,. 4
-I
// "
/
:- 6 0 0
II
GaTsPc
. . . . . ZnTsPP _ _ GaTsPc+ZnTsPP(0.39:0.61)
L, /", / ,,
/
~ -, ~
550
600
C 100
\
650
700
750
0--
Wavelength ( n m ) 1400
''''1'"11''''1"
I
......... GaTsec
_ __ ZnTsPP GaTsPc+ZnTsPP(O. 39:0.61 )
i',
'~ 1 2 0 0
~
"l'"'P''"l""r'"'
(b)
250
1000
C
800
iI
s ,'
600
j
15o ~
/', ' ,
/ 100
o
400
~
2 /I
200 0
~'
350
;
400
,.
450
!'.\,'
500
- /,£
550
600
50
650
700
o ~
750
Wavelength ( n m ) Fig. 4. Photocurrent action spectra of the liquid junction cell based on the TiO2 electrode sensitized with a single dye or cosensitized with GaTsPc (0.39) and ZnTsPP (0.61) dye for light incidence through (a) the working electrode and (b) the back (counter) electrode.
spectra have been corrected for the absorption of incident light by the ITO conducting glass substrate. The photocurrent for light incidence through the working electrode is generally larger than that for light incidence through the back electrode, due to the light multi-reflection of the Pt-coated ITO counter electrode in the former, and the longer distance across which the photogenerated charge carriers transport towards the TiO2 electrode in the latter. For light incidence through the working electrode or the counter
273
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
electrode, the photocurrent action spectrum is different from the absorption spectrum of the working electrode. However, the photocurrent action spectrum in the former is always 'inphase' with that in the latter whether for sensitization alone or cosensitization, indicating that the adsorbed dye molecules exist as a monolayer on the nanostructured TiO2 electrode or at least the thickness of the adsorbed or coadsorbed layer is smaller than the diffusing length (e.g., 15 nm for phthalocyanine material ] 27 ] ) of the photogenerated charge carriers or excitons if dye molecules exist as a multilayer. Otherwise, the 'in-phase' relation will not be held according to the exciton dissociation theory [27,28] that excitons must diffuse into the junction and are dissociated into free charge carriers by a built-in field at the junction region. Therefore, it is reasonably believed that the sensitized dye molecules exist as a monolayer self-assembled on the nanostructured TiO2 electrode. In addition, the 'in-phase' relation between photocurrent action spectra in the above two cases excludes the possibility that the above action spectrum in the former results from the filter effect of the adsorbed multilayer of dye molecules. As shown in Fig. 4, for sensitization with GaTsPc dye alone, the absorbance in the region of 430-640 nm, in which the absorbance of the GaTsPc dimer is included, does not convert into a photocurrent, while only that in the region of 640-760 nm generates a photocurrent with a maximum of 70 nA cm -2 and an incident-photon-to-current conversion efficiency ( I P C E ) maximum of 0.86% at 680 nm. I P C E = 1243 × lsc × 100% / (A × Pi~), in which lsc and P~, are the short-circuit current density and the incident light power corrected for the absorbance and scattering of the substrate at the monochromatic wavelength, respectively. This result is in agreement with our previous observation [ 11 ], which is due to the rapid internal conversion of the photogenerated excitons resulting from molecular dimers or aggregates of phthalocyanine and the anisotropy in the charge carrier transport for the phthalocyanine compounds [29]. For sensitization with ZnTsPP dye alone, the absorbance of the ZnTsPP Soret band in the region of 400-470 nm generates the strong photocurrent response with a photocurrent maximum of 1990 nA cm -~- and an I P C E maximum of 99.4% at 430 nm and that of the ZnTsPP Q band in the region of 500-640 nm shows the weaker photocurrent response with 190 nA c m - 2 at 620 nm (2.1% for I P C E ) . In contrast, at 430 nm, the photocurrent of 1990 nA cm -2 is reduced to 280 nA cm -2 and an I P C E value of 99.4% is reduced to 11.9% by the eight-fold decrease. However, the photocurrents of 450 nA cm 2 at 680 nm and 620 nA cm 2 at 700 nm are obtained with six-fold and 20-fold improvement, respectively. It is noted that the I P C E value (15.6%) at 680 nm is almost 20 times as large as that (0.86%) for sensitization with the pure GaTsPc molecules, and that an I P C E (20.5%) at 700 nm is 50 times as large as that (0.4%) at x = 1.0. Because the absorbance of ZnTsPP molecules at 680 nm does not convert into a photocurrent, the photocurrent response at 680 nm is attributed to the absorbance of the
GaTsPc monomer. The absorbance at 620 nm also generates the considerably large photocurrent with 150 n A c m 2 and an I P C E value of 2.7%, which compensates for the very low conversion efficiency of the absorbance of the GaTsPc dimer. Therefore, cosensitization markedly decreases the monochromatic photocurrent response in the Soret band of ZnTsPP dye and at the moment greatly enhances the monochromatic photocurrent response in the region of 640-760 nm, resulting in the mixed effect. This is attributed to the formation of heteroaggregates (e.g. heterodimer or heterotrimer) between GaTsPc and ZnTsPP molecules adsorbed on the cosensitized TiO2 electrode with the positively charged surface. However, in general, cosensitization leads to the achievement of the photocurrent response in the broader region (400-760 nm) and improves the photoelectric conversion of the liquid junction cell based on the cosensitized TiO2 electrode. On the one hand, the formation of Pc/PP heteroaggregates prevents GaTsPc molecules from forming homodimers (e.g. self-aggregation), in which most of the excited excitons or photogenerated charge carriers are quenched by the rapid internal conversion, and results in the strong enhancement of photocurrent response in the GaTsPc Q band. On the other hand, the formation of Pc/PP heteroaggregates results in an extended and conjugated face-to-face system formed with rrorbital overlap between GaTsPc and ZnTsPP, and aids the generation of the low-lying CT state during the cosensitization. The low-lying CT state was reported to exist in the heterodimer composed of tetra (4-carboxyphenyl) porphyrin and tetra(N-methylpyridyl) porphyrin [30]. The oxidation and reduction potentials of GaTsPc and ZnTsPP show that there is a low-lying CT state in the Pc/PP heteroaggregate. As shown in Fig. 5, the low-lying CT state, the CT energy of which is equal to 1.85 eV from the relation EcT = EI/2, o~(ZnTsPP)-Ej/2,~(GaTsPc) [30], locates under the lowest singlet state (2.05 eV) of ZnTsPP and above the lowest singlet state ( 1.83 eV) of GaTsPc (energy levels of the lowest singlet state and triplet state of GaTsPc and ZnTsPP are shown in Fig. 5). The low-lying CT state leads to the strong quenching of the photocurrent in the 400-600 nm, especially for in the Soret band of ZnTsPP. However, the decrease of the Soret band of ZnTsPP is offset by the striking improvement of the Q band of GaTsPc. Therefore, the mixed effect in the photoelectric conversion of the cosensitized TiO2 ZnTsPP
PctPP
GaTsPc
2.05 eV Sl ~ ~:~:~:~~
1.85 eV
1.83 eV
To
1.61 eV
/ So
1.49 eV
No
Fig. 5. Energy level scheme of ZnTsPP/GaTsPc heteroaggregateson the TiO2electrode. S~, the lowestsinglet state; So, groundstate; To,triplet state; CT, charge-transferstate. Ec~= El/2.,,~( ZnTsPP) - E~/2~d(GaTsPc).
274
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
electrode with GaTsPc and ZnTsPP molecules can be understood from the formation of heteroaggregates between GaTsPc and ZnTsPP molecules.
4. Conclusions A mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion of the cosensitized TiO2 electrode occurs. Cosensitization greatly enhances the photocurrent response at the longer wavelength (GaTsPc Q band of 640-760 nm), with 20- and 60-fold improvement in the quantum efficiency at 680 and 700 nm, and at the moment markedly decreases the photocurrent response in the Soret band of ZnTsPP with 8-fold drop in the quantum efficiency at 430 nm. The decrease of the Soret band of ZnTsPP is offset by the improvement of the Q band of GaTsPc, resulting in the mixed effect of the short-circuit photocurrent with the maximum of 771.1 IzA cm -2 at x = 0.39 on illumination at 35.7 mW cm-2. This is attributed to the formation of Pc/PP heteroaggregates on the cosensitized TiO2 electrode, resulting in the decrease of concentration of GaTsPc dimer and the presence of the low-lying CT state ( 1.85 eV).
Acknowledgements This work was supported by the National Natural Science Foundation of China.
References [ 1] B. O'Regan, J. Moser, M. Anderson and M. Gr~itzel, J. Phys. Chem., 94 (1990) 8720. [2] B. O'Regan and M. Gr/itzel, Nature, 253 ( 1991 ) 737.
[3] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am. Chem. Soc., 115 (1993) 6382. [4] A. Kay and M. Gr~.tzel, J. Phys. Chem., 97 (1993) 6272. [ 5 ] A. Kay, R. Humphry-Baker and M. Gr~itzel, J. Phys. Chem., 98 (1994) 952. [6] G.D. Shatrma, S.C. Mathur and D.C. Dube, J. Mater. Sci., 26 ( 1991 ) 6547. [7] W.A. Nevin and G.A. Chamberlain, J. Appl. Phys., 69 ( 1991 ) 4324. [8] H. Gerischer, in M. Schiavello (ed.), Photoelectrochemistry, Photocatalysis and Photoreaction, Reidel, Dordrecht, 1985. [9] W.A. Nevin and G.A. Chamberlain, J. Appl. Phys., 68 (1990) 5247. [ 10] Y.C. Shen, L. Wang, Z.H. Lu, Y. Wei, Q.F. Zhou, H.F. Mao and H.J. Xu, Thin Solid Films, 257 (1995) 144. [ 111 H. Deng, H.F. Mao, BJ. Liang, Y.C. Shen, Z.H. Lu and H.J. Xu, J. Photochem. Photobiol. A, Chem., 99 (1996) 71. [ 12] H. Deng, Z.H. Lu, H.F. Mao and H.J. Xu, J. Photochem. Photobiol. A, Chem., (1997) to be published. [ 13] E. Reddi and G. Jori, Rev. Chem. Intermediates, 10 (1988) 241. [ 14] J.H. Weber and D.H. Bush, Inorg. Chem., 4 (1965) 469. [ 15] E.D. Fleischer, J.M. Palmer and T.S. Svivastara, J. Am. Chem. Soc., 93 (1971) 3162. [ 16] Y.C. Yang, J.R. Ward and R.P. Seider, Inorg. Chem., 24 (1985) 1765. [ 17] A,W. Snow and N.L. Jarvis, J. Am. Chem. Soc., 106 (1984) 4706. I 18 ] A,R. Kane, J.F. Sullivan, D.H. Kenny and M.E. Kenney, lnorg. Chem., 9 (1970) 1455. [ 19] T. Shimidzu and T. Iyoda, Chem. Lett., ( 1981 ) 853. [20] E. Ojadi, R. Scizer and H. Linschitz, J. Am. Chem. Soc., 107 (1985) 7783. [21 ] H. van Willigen, U. Das, E. Ojadi and H. Linschitz, J. Am. Chem. Soc., 107 (1985) 7784. [22] D.K. Geiger and C.A. Kelly, Inorg. Chim. Acta, 154 (1988) 137. [23] J.F. Lipskier and T.H. Tran-Hi, Inorg. Chem., 32 (1993) 722. [24] H. Segawa, H. Nishino, T. Kamikana and K. Honda, Chem. Lett., (1989) 1917. [25] K. Kalyanasundaram and M. Neumann-Spallart, J. Phys. Chem., 86 (1982) 5163. [26] C.A. Ltuate and K.M. Sanders, J. Am. Chem. Soc., 112 (1990) 5525. [27] A.K. Ghosh, D.L. Morel, T. Feng, R.S. Shaw and C.A. Rowe, J. Appl. Phys., 45 (1974) 230. [28] A.K. Ghosh and T. Feng, J. Appl. Phys., 49 (1978) 5982. [29] C.J. Schramm, R.P. Scaringe, D.R. Stojakovic, B.M. Hoffman, J.A. Ibers and T.J. Maekers, J. Am. Chem. Soc,, 102 (1980) 6702. [30] U. Hofstra, R.B.M. Koehorst and T.J. Schaafsma, Chem. Phys. Lett., 130 (1986) 555.
Synthetic Metals 92 (1998) 269-274
The mixed effect of phthalocyanine and porphyrin on the photoelectric conversion of a nanostructured TiO2 electrode Huihua Deng a,,, Yuming Zhou
b, Haifang Mao c, Zuhong Lu a
National Laboratory of Molecular and Biomolecular Electronics, Southeast UniversiO', Nanjing 210096, China b Department of Chemical Engineering, Southeast University, Nunjing 210096, China c h~stitute of Organic Chemisto,, Academic Sinica. Shanghai 200433, China Received 12 September 1997; revised 23 October 1997; accepted 10 November t997
Abstract The mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion is reported. For the liquid junction solar cell based on the cosensitized TiO2 electrode, cosensitization greatly enhances the photocurrent response at the longer wavelength (the GaTsPc Q band) and markedly decreases the photocurrent response in the Soret band of ZnTsPP. The decrease of the Soret band of ZnTsPP is offset by the improvement of the Q band of GaTsPc, resulting in the mixed effect in the short-circuit photocurrent. This is attributed to the formation of Pc/PP heteroaggregates on the cosensitized TiO2 electrode, resulting in the decreases of the concentration of GaTsPc dimer and the presence of the low-lying charge-transfer state. © 1998 Elsevier Science S.A. Keywords: Charge-transfer states; Cosensitization; Heteroaggregation; Mixed effects; Nanostructured titanium dioxide electrodes
1. Introduction The nanocrystalline solar cell, recently developed by Grfitzel and co-workers [ 1-5], overcomes the low efficiency of conventional photoelectrochemical cells or solid cells due to inefficient separation and transport of the photogenerated charge carriers [6-8] or due to low absorbance [9]. The nanocrystalline solar cell is based on a dye-sensitized colloidal TiO2 film with a very high specific surface area. In our laboratory, the liquid junction solar cells based on transparent and microporous TiO2 electrodes sensitized with phthalocyanines or porphyrins has been fabricated [ 10-12]. Phthalocyanines and porphyrins, whose chemical structures resemble chlorophyll derivatives [4,5], are chosen as the photosensitizer because phthalocyanine shows strong absorption at the longer wavelength, and porphyrin shows very strong absorption and high quantum efficiency in photoelectric conversion in the region of 400-470 nm, which compensates for the lower absorbance and conversion efficiency of ruthenium bipyridine complex at the given region. By the addition of hydroquinone to liquid electrolyte solution that contains 0.1 M Na2SO4, the quantum efficiency of photoelectric conversion of the microporous T i Q electrode, sensitized with zinc phthalocyanine substituted with four carboxyl groups, * Corresponding author. Fax: + 86 25 771 2719; e-mail: [email protected] 0379-6779/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved PHS0379-6779 ( 9 7 ) 0 4 1 0 9 - X
reaches about 20% at 690 nm [ 10]. The quantum efficiency for a nanostructured TiO2 electrode sensitized with tetrasulfonated zinc porphyrin reaches 99.4% at 430 nm. But the dimeric absorbance of tetrasulfonated phthalocyanines (MTsPcs; M = Z n , Ga, Co, In, TiO, H2) does not convert into photocurrent [11], due to molecular aggregation and orientation of MTsPcs self-assembled on the positively charged TiO2 electrode via the electrostatic interaction of the negatively charged sulfonate (SO3 - ). The rapid internal conversion in the molecular aggregates of phthalocyanines is believed to be the main deactivated pathway of the excited excitons or photogenerated charge carriers [ 11,13 ]. Decreasing the surface concentration of dimer is probably an efficient means to improving the energy power conversion. In this paper, we report the mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion of the cosensitized TiO2 electrode. The mechanism generating the mixed effect is also discussed.
2. Experimental details TiO2 colloidal solutions were prepared by hydrolysis of tetrabutyl titanate ((C4H90)4Ti) by a procedure similar to
14. Deng et al. / Synthetic Metals 92 (1998) 269-274
270
ZnTsPP in DMSO until the absorbance of the electrode showed no increase• The total concentration of the mixed solution was 5 × 10 -5 M. After the characteristic performances of the cosensitized TiO2 electrode had been measured, GaTsPc and ZnTsPP on the TiO2 electrode were washed away using 0.001 M NaOH solution. The resulting solution was used to analyze the actual content of GaTsPc and ZnTsPP molecules adsorbed on the TiO2 electrode. The liquid junction solar cell for measuring the photocurrent, as shown in Fig. 1 (c), consists of the sensitized or cosensitized TiO2 electrode and a counter electrode with an electrolyte solution in between containing 0.1 M KI and 0.05 M I2 in 0.001 M HCIO4. When light is incident through the working electrode, the counter electrode is an ITO conducting glass, on which a thin layer of platinum has been coated by vapor vacuum deposition at 10- 7 Tort. When light is incident through the counter electrode (the back electrode), an ITO conducting glass is also used as the back electrode. The morphology and the film thickness of the T i Q electrode were examined by scanning electron microscopy (SEM; JEOL, JSM-6300). The absorption spectra were recorded with a Shimadzu UV-2201 UV-Vis spectrometer.
that described in Ref. [ 10]. The colloidal solution on addition of 2 wt.% poly(vinyl alcohol) (PVA) was then concentrated to a desired density through vacuum rotation evaporation. After addition of 1.5 wt.% Triton X-100, the concentrated solution (TiO2 content; 10 wt.%) was spin-coated on a freshly cleaned ITO (indium-tin oxide; sheet resistance of 50 f~ D -~ and transmission of 95% in the visible region) conducting glass substrate. The sheet resistance of the nanostructured T i Q electrode, finally obtained by a heating process similar to that described in Ref. [ 11 ], is 500 1~ [ ] - t Prior to dye sensitization, the nanostructured TiO2 electrodes were soaked in HCI solution (pH = 2) and naturally dried. GaTsPc and ZnTsPP were synthesized according to the methods described in Refs. [14,15]; their molecular structures are shown in Fig. 1 (a) and (b). When sensitized with GaTsPc or ZnTsPP dye molecules alone, the working electrode was obtained by plunging the bare TiO2 electrode into a 5 × 10 5 M solution of GaTsPc or ZnTsPP in dimethyl sulfoxide (DMSO) until there was no increase in the absorbance of the electrode. When cosensitized with GaTsPc and ZnTsPP, the working electrodes were obtained by plunging the bare TiO2 electrode into a mixed solution of GaTsPc and
so~
s o ~ _ r ~
N "
N
N
~
Ca
SO~
so;-
o;
m"-'-'~ N N
(a) SO; (b) sO;
V_q .... co : ele de .......... ........... -liSmd eleetrolTe ..... • '7.. TL..: il..'"'L•"':i ':....'L.."_.7 The sensitized or cosensitized
{
"ggl
(c)
I TiO2 electa°de"
h~,
Fig. 1. Molecular structures of (a) tetrasulfonated gallium phthalocyanine (GaTsPc), (b) tetrasulfonated zinc porphyfin (ZnTsPP), and (c) the structure of the liquid junction cell based on the sensitized or cosensifized TiO2 electrode.
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
The photocurrent was measured with a potentiostat (model CMBP- 1 ). Monochromatic illumination was obtained using a 500 W xenon arc lamp in combination with a grating monochromator (model WPG3D). The light intensity was calibrated using a model LM-5 laser power meter made in the National Institute of Metrology, China. The redox potentials of GaTsPc and ZnTsPP were measured using an electrochemical station (CHI 660; CH Co.).
3. R e s u l t s a n d d i s c u s s i o n
The nanostructured T i Q electrode is composed of interconnected particles (40-60 nm) and pores, and has a thickness of 10 Ixm. The bare nanostructured TiO2 electrode exhibits a fundamental absorption edge at 390 nm of anatase in the UV region. The specific surface area of the nanostructured TiOe electrode is characterized by the dimeric absorbance of GaTsPc molecules in the case of a single sensitization. Assuming that a complete monolayer with a 2 nm 2 area (Sm) for each molecule covers the TiO~ electrode, and that the extinction coefficient (e(A)) of GaTsPc molecules is 3.27 × 107 cm ~ m o l - ] at 620 n m [ 16], the specific surface area (Sv) of the TiO2 electrode of 370 was obtained by the following formula [2]: Sp=(OD×NA×Sm)/(e(A)) in which OD and NA are the optical density and Avogadro constant, respectively. As shown in Fig. 2, the nanostructured TiO~ electrode cosensitized with GaTsPc and ZnTsPP dye molecules reveals the characteristic absorption of both GaTsPc and ZnTsPP, with two maxima at around 430 and 680 nm in the visible region, whereas the nanostructured TiO2 electrode, sensitized with GaTsPc or ZnTsPP alone, reveals only the individual characteristic absorption of GaTsPc or ZnTsPP dye molecules. Therefore, cosensitization extends the absorbance of the electrode into the broader region (400-750 nm) and the ,a.. 4.5
a b e d
~1
42., ~ - - ~
e x=l.00
tq/
~,,
fill
L,
z.00
ft
1.5o
x=0.39 x=0,75 x=0.86 x=0.93
~,.o _ _ ,~ ......... , ~.s ~.o _ _ _
=
~ ~' ,
-~ t.oo
'IL,,,, I:1.,i\l ~
"'
~,,,/:;/',',"., k
_
_
f x=0.09
•
d/'
," ~/
b...,
t l:~'il' i'o° t ! ~l ~ ~, ~'"~~'~'"i~'s:~"'~:~""~:~r:~
~
~/
~ ' ~ -
.--::-..~,~
0.00
300
350
400
450
500
550
Wavelength
600
650
700
750
800
(nm)
Fig. 2. Absorption spectra of the cosensitized TiO2 electrode with GaTsPc and ZnTsPP dyes with the molar ratio of GaTsPc. Inset is the surface concentration of GaTsPc dimer vs. x.
271
photocurrent response in the visible region stems from the absorbance of the dye molecules adsorbed on the TiO2 electrode. In DMSO, the GaTsPc spectrum at a concentration of 5 × 10 -5 M consists of a strong sharp 7r--at* monomeric absorption at 675 nm accompanied by a much weaker vibrationally coupled satellite band at 606 rim. In previous studies, the narrow longer wavelength absorption at around 675 nm has been attributed to a monomeric phthalocyanine compound, and the broad absorption at 606 nm has been assigned as the characteristic absorption of the GaTsPc dimer [ 16,17 ]. As shown in Fig. 2, when adsorbed or coadsorbed on the electrode, the Q band absorption of GaTsPc has a red shift of 5-15 nm relative to 675 nm in DMSO. At the moment, with the increase of molar ratio ( x = [ G a T s P c ] / ( [ G a T s P c ] + [ZnTsPP])) of GaTsPc, a broader and resolved w - v * absorption at 620 nm appears with a markedly increased intensity and a peak absorbance finally occurs in the case of the pure GaTsPc molecules. The absorptions of both the dimer and the monomer of GaTsPc on the electrode have red shifts relative to those in DMSO, indicating that there is strong interaction between GaTsPc and the TiO2 electrode. The 7r-Tr* transition shift with a broader natural bandwidth is a result of molecular association with a cofacial phthalocyanine ring orientation [ 17,18 ] (i.e. face-to-face H-type aggregation). These results indicate the occurrence of molecular higher order aggregation of GaTsPc on the nanostructured TiO2 electrode with increasing molar ratio of GaTsPc. Assuming that there is no higher order aggregate than the dimer, and that the extinction coefficient of the GaTsPc monomer is equal to 1.29 × 108 c m 2 m o l - ~ [ 16 ], it is derived that GaTsPc adsorbed on the nanostructured TiO2 electrode exists mainly as a cofacial dimer. The surface concentration of the dimer, seen in the inset of Fig. 2, is increased with increase of x. Therefore, cosensitization by doping ZnTsPP decreases the surface concentration of the dimer of GaTsPc on the cosensitized TiO2 electrode. In DMSO, the absorption spectrum of ZnTsPP consists of the Soret band with a very strong peak at 423 nm and the Q band with three peaks at 553, 595 and 642 nm. Compared with the absorption of the porphyrin solution in DMSO, that of ZnTsPP on the TiO2 electrode has red shifts at the Soret and Q bands, as shown in Fig. 2. The red shifts are increased with increase of x. But the Soret band of ZnTsPP is blueshifted to 420 nm at x = 0.39. This indicates that there is a strong interaction between GaTsPc and ZnTsPP molecules of the self-assembled film on the TiOz electrode and the occurrence of an extensive orbital mixing. Therefore, the experimental result that the red shifts of the Soret band of ZnTsPP and the Q band of GaTsPc vary with ZnTsPP doping indicates the possible formation of the heteroaggregate between GaTsPc and ZnTsPP molecules. In the previous study, stable heteroaggregates could be obtained with porphyrins and phthalocyanines grafted with ionic substituents of opposite charges [ 19-23 ] and also with anionic porphyrins with different central metals [24]. In the
272
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
former, the main driving force of heteroaggregation is the electrostatic attraction of peripheral substituent, whereas in the latter charge-transfer (CT) interaction of the porphyrin moieties is considered as the main driving force of heteroaggregates composed of anionic zinc and gold porphyrins [ 24]. The van der Waals attraction between the hydrophobic aromatic macrocycles also occurs in holding the individual components together. In addition, the planar geometry of phthalocyanines and porphyrins allows close contact (and therefore extensive orbital overlap) in the face-to-face configuration. In the case of the cosensitization, GaTsPc and ZnTsPP are adsorbed on the nanostructured TiOa electrode through coulombic attraction via sulfonate ( S O ~ ) . The positively charged surface of the nanostructured TiO2 electrode partly shields the electrostatic repulsion between GaTsPc and ZnTsPP molecules. The oxidation potentials of GaTsPc and ZnTsPP [25] are +0.86 and +0.87 V (versus NHE), and the reduction potentials are - 0.98 and - 1.16 V (versus NHE), respectively. This indicates the presence of the CT interaction between GaTsPc and ZnTsPP molecules. Therefore, the CT interaction, together with the shielding interaction and the van der Waals attraction, leads to the formation of the Pc/PP heteroaggregates on the positively charged TiO2 electrode. The interaction between two porphyrin rings or porphyrin and phthalocyanine rings leads to a face-to-face molecular aggregation with their centers offset both in solution and in solid state [23,26]. On this basis, it is concluded that these red shifts are due to the adoption of a face-to-face stacking orientation and the presence of CT interaction in heteroaggregates of GaTsPc and ZnTsPP molecules adsorbed on the nanostructured TiO2 electrode. As a result, an extended and conjugated face-to-face system is formed with ~-orbital overlap. Such an extended and conjugated 'rr system within the molecular heteroaggregates lowers the excitation energy of the "rr-'rr* and n-,rr* electric transition, which results in red shift of the Soret band and Q band absorption, respectively, as shown in Fig. 2. The maximum of the short-circuit photocurrent occurs at x = 0.39 (see Fig. 3) where the molar ratio of GaTsPc to ZnTsPP amounts to 1:1.6 and is close to 1:2. Thus, it is assumed that the PP/Pc/PP heterotrimer is created at x = 0.39. The dependence of the short-circuit photocurrent on molar ratio of GaTsPc, as shown in Fig. 3, reveals the mixed effect for the liquid junction cell based on the cosensitized TiO2 electrode. On illumination at 35.7 mW c m - 2, the short-circuit photocurrent (I~o) of the cosensitized TiO2 electrode is always larger than the linear combined sum of that of GaTsPc and ZnTsPP: /co > x X/GaTsec + ( 1 - x) X/ZnTsPP' The maximum in the short-circuit photocurrent of 771.1 p,A c m - 2 is achieved at x = 0.39. Fig. 4 shows the measured short-circuit photocurrent of the nanostructured TiO2 electrode sensitized with GaTsPc or ZnTsPP alone and cosensitized with GaTsPc (0.39) and ZnTsPP (0.61) dye molecules as a function of wavelength for light incidence through (a) the working electrode and (b) the back electrode. The photocurrent action
Ico 1" x*la=r~*~ + (1-x)*Iz,ra,p ~-"800.0 o k. ~
.~ 600.0
-.
400.0 0 0 200.0
%
0.0 0.00
0,20
0,40
0.60
0.80
GaTsPc m o l a r
1.00
ratio x
Fig. 3. Dependence of the short-circuit photocurrent on the molar ratio (x) of GaTsPc molecules for light incidence through the working electrode on illumination at 35.7 m W cm ~.
(a)
3000
.........
2500
/
I
--~2000
'' ',, <~-------.].--.-~:~-~,",,, 11
1500
'
i.
1000
, ',
0 500 ", 350
400
45o
500
500
I I
400
I
300
/
:..)
I
200
i , I\ ,'
~.i,,,. 4
-I
// "
/
:- 6 0 0
II
GaTsPc
. . . . . ZnTsPP _ _ GaTsPc+ZnTsPP(0.39:0.61)
L, /", / ,,
/
~ -, ~
550
600
C 100
\
650
700
750
0--
Wavelength ( n m ) 1400
''''1'"11''''1"
I
......... GaTsec
_ __ ZnTsPP GaTsPc+ZnTsPP(O. 39:0.61 )
i',
'~ 1 2 0 0
~
"l'"'P''"l""r'"'
(b)
250
1000
C
800
iI
s ,'
600
j
15o ~
/', ' ,
/ 100
o
400
~
2 /I
200 0
~'
350
;
400
,.
450
!'.\,'
500
- /,£
550
600
50
650
700
o ~
750
Wavelength ( n m ) Fig. 4. Photocurrent action spectra of the liquid junction cell based on the TiO2 electrode sensitized with a single dye or cosensitized with GaTsPc (0.39) and ZnTsPP (0.61) dye for light incidence through (a) the working electrode and (b) the back (counter) electrode.
spectra have been corrected for the absorption of incident light by the ITO conducting glass substrate. The photocurrent for light incidence through the working electrode is generally larger than that for light incidence through the back electrode, due to the light multi-reflection of the Pt-coated ITO counter electrode in the former, and the longer distance across which the photogenerated charge carriers transport towards the TiO2 electrode in the latter. For light incidence through the working electrode or the counter
273
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
electrode, the photocurrent action spectrum is different from the absorption spectrum of the working electrode. However, the photocurrent action spectrum in the former is always 'inphase' with that in the latter whether for sensitization alone or cosensitization, indicating that the adsorbed dye molecules exist as a monolayer on the nanostructured TiO2 electrode or at least the thickness of the adsorbed or coadsorbed layer is smaller than the diffusing length (e.g., 15 nm for phthalocyanine material ] 27 ] ) of the photogenerated charge carriers or excitons if dye molecules exist as a multilayer. Otherwise, the 'in-phase' relation will not be held according to the exciton dissociation theory [27,28] that excitons must diffuse into the junction and are dissociated into free charge carriers by a built-in field at the junction region. Therefore, it is reasonably believed that the sensitized dye molecules exist as a monolayer self-assembled on the nanostructured TiO2 electrode. In addition, the 'in-phase' relation between photocurrent action spectra in the above two cases excludes the possibility that the above action spectrum in the former results from the filter effect of the adsorbed multilayer of dye molecules. As shown in Fig. 4, for sensitization with GaTsPc dye alone, the absorbance in the region of 430-640 nm, in which the absorbance of the GaTsPc dimer is included, does not convert into a photocurrent, while only that in the region of 640-760 nm generates a photocurrent with a maximum of 70 nA cm -2 and an incident-photon-to-current conversion efficiency ( I P C E ) maximum of 0.86% at 680 nm. I P C E = 1243 × lsc × 100% / (A × Pi~), in which lsc and P~, are the short-circuit current density and the incident light power corrected for the absorbance and scattering of the substrate at the monochromatic wavelength, respectively. This result is in agreement with our previous observation [ 11 ], which is due to the rapid internal conversion of the photogenerated excitons resulting from molecular dimers or aggregates of phthalocyanine and the anisotropy in the charge carrier transport for the phthalocyanine compounds [29]. For sensitization with ZnTsPP dye alone, the absorbance of the ZnTsPP Soret band in the region of 400-470 nm generates the strong photocurrent response with a photocurrent maximum of 1990 nA cm -~- and an I P C E maximum of 99.4% at 430 nm and that of the ZnTsPP Q band in the region of 500-640 nm shows the weaker photocurrent response with 190 nA c m - 2 at 620 nm (2.1% for I P C E ) . In contrast, at 430 nm, the photocurrent of 1990 nA cm -2 is reduced to 280 nA cm -2 and an I P C E value of 99.4% is reduced to 11.9% by the eight-fold decrease. However, the photocurrents of 450 nA cm 2 at 680 nm and 620 nA cm 2 at 700 nm are obtained with six-fold and 20-fold improvement, respectively. It is noted that the I P C E value (15.6%) at 680 nm is almost 20 times as large as that (0.86%) for sensitization with the pure GaTsPc molecules, and that an I P C E (20.5%) at 700 nm is 50 times as large as that (0.4%) at x = 1.0. Because the absorbance of ZnTsPP molecules at 680 nm does not convert into a photocurrent, the photocurrent response at 680 nm is attributed to the absorbance of the
GaTsPc monomer. The absorbance at 620 nm also generates the considerably large photocurrent with 150 n A c m 2 and an I P C E value of 2.7%, which compensates for the very low conversion efficiency of the absorbance of the GaTsPc dimer. Therefore, cosensitization markedly decreases the monochromatic photocurrent response in the Soret band of ZnTsPP dye and at the moment greatly enhances the monochromatic photocurrent response in the region of 640-760 nm, resulting in the mixed effect. This is attributed to the formation of heteroaggregates (e.g. heterodimer or heterotrimer) between GaTsPc and ZnTsPP molecules adsorbed on the cosensitized TiO2 electrode with the positively charged surface. However, in general, cosensitization leads to the achievement of the photocurrent response in the broader region (400-760 nm) and improves the photoelectric conversion of the liquid junction cell based on the cosensitized TiO2 electrode. On the one hand, the formation of Pc/PP heteroaggregates prevents GaTsPc molecules from forming homodimers (e.g. self-aggregation), in which most of the excited excitons or photogenerated charge carriers are quenched by the rapid internal conversion, and results in the strong enhancement of photocurrent response in the GaTsPc Q band. On the other hand, the formation of Pc/PP heteroaggregates results in an extended and conjugated face-to-face system formed with rrorbital overlap between GaTsPc and ZnTsPP, and aids the generation of the low-lying CT state during the cosensitization. The low-lying CT state was reported to exist in the heterodimer composed of tetra (4-carboxyphenyl) porphyrin and tetra(N-methylpyridyl) porphyrin [30]. The oxidation and reduction potentials of GaTsPc and ZnTsPP show that there is a low-lying CT state in the Pc/PP heteroaggregate. As shown in Fig. 5, the low-lying CT state, the CT energy of which is equal to 1.85 eV from the relation EcT = EI/2, o~(ZnTsPP)-Ej/2,~(GaTsPc) [30], locates under the lowest singlet state (2.05 eV) of ZnTsPP and above the lowest singlet state ( 1.83 eV) of GaTsPc (energy levels of the lowest singlet state and triplet state of GaTsPc and ZnTsPP are shown in Fig. 5). The low-lying CT state leads to the strong quenching of the photocurrent in the 400-600 nm, especially for in the Soret band of ZnTsPP. However, the decrease of the Soret band of ZnTsPP is offset by the striking improvement of the Q band of GaTsPc. Therefore, the mixed effect in the photoelectric conversion of the cosensitized TiO2 ZnTsPP
PctPP
GaTsPc
2.05 eV Sl ~ ~:~:~:~~
1.85 eV
1.83 eV
To
1.61 eV
/ So
1.49 eV
No
Fig. 5. Energy level scheme of ZnTsPP/GaTsPc heteroaggregateson the TiO2electrode. S~, the lowestsinglet state; So, groundstate; To,triplet state; CT, charge-transferstate. Ec~= El/2.,,~( ZnTsPP) - E~/2~d(GaTsPc).
274
H. Deng et al. / Synthetic Metals 92 (1998) 269-274
electrode with GaTsPc and ZnTsPP molecules can be understood from the formation of heteroaggregates between GaTsPc and ZnTsPP molecules.
4. Conclusions A mixed effect of tetrasulfonated gallium phthalocyanine (GaTsPc) and tetrasulfonated zinc porphyrin (ZnTsPP) on the photoelectric conversion of the cosensitized TiO2 electrode occurs. Cosensitization greatly enhances the photocurrent response at the longer wavelength (GaTsPc Q band of 640-760 nm), with 20- and 60-fold improvement in the quantum efficiency at 680 and 700 nm, and at the moment markedly decreases the photocurrent response in the Soret band of ZnTsPP with 8-fold drop in the quantum efficiency at 430 nm. The decrease of the Soret band of ZnTsPP is offset by the improvement of the Q band of GaTsPc, resulting in the mixed effect of the short-circuit photocurrent with the maximum of 771.1 IzA cm -2 at x = 0.39 on illumination at 35.7 mW cm-2. This is attributed to the formation of Pc/PP heteroaggregates on the cosensitized TiO2 electrode, resulting in the decrease of concentration of GaTsPc dimer and the presence of the low-lying CT state ( 1.85 eV).
Acknowledgements This work was supported by the National Natural Science Foundation of China.
References [ 1] B. O'Regan, J. Moser, M. Anderson and M. Gr~itzel, J. Phys. Chem., 94 (1990) 8720. [2] B. O'Regan and M. Gr/itzel, Nature, 253 ( 1991 ) 737.
[3] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, J. Am. Chem. Soc., 115 (1993) 6382. [4] A. Kay and M. Gr~.tzel, J. Phys. Chem., 97 (1993) 6272. [ 5 ] A. Kay, R. Humphry-Baker and M. Gr~itzel, J. Phys. Chem., 98 (1994) 952. [6] G.D. Shatrma, S.C. Mathur and D.C. Dube, J. Mater. Sci., 26 ( 1991 ) 6547. [7] W.A. Nevin and G.A. Chamberlain, J. Appl. Phys., 69 ( 1991 ) 4324. [8] H. Gerischer, in M. Schiavello (ed.), Photoelectrochemistry, Photocatalysis and Photoreaction, Reidel, Dordrecht, 1985. [9] W.A. Nevin and G.A. Chamberlain, J. Appl. Phys., 68 (1990) 5247. [ 10] Y.C. Shen, L. Wang, Z.H. Lu, Y. Wei, Q.F. Zhou, H.F. Mao and H.J. Xu, Thin Solid Films, 257 (1995) 144. [ 111 H. Deng, H.F. Mao, BJ. Liang, Y.C. Shen, Z.H. Lu and H.J. Xu, J. Photochem. Photobiol. A, Chem., 99 (1996) 71. [ 12] H. Deng, Z.H. Lu, H.F. Mao and H.J. Xu, J. Photochem. Photobiol. A, Chem., (1997) to be published. [ 13] E. Reddi and G. Jori, Rev. Chem. Intermediates, 10 (1988) 241. [ 14] J.H. Weber and D.H. Bush, Inorg. Chem., 4 (1965) 469. [ 15] E.D. Fleischer, J.M. Palmer and T.S. Svivastara, J. Am. Chem. Soc., 93 (1971) 3162. [ 16] Y.C. Yang, J.R. Ward and R.P. Seider, Inorg. Chem., 24 (1985) 1765. [ 17] A,W. Snow and N.L. Jarvis, J. Am. Chem. Soc., 106 (1984) 4706. I 18 ] A,R. Kane, J.F. Sullivan, D.H. Kenny and M.E. Kenney, lnorg. Chem., 9 (1970) 1455. [ 19] T. Shimidzu and T. Iyoda, Chem. Lett., ( 1981 ) 853. [20] E. Ojadi, R. Scizer and H. Linschitz, J. Am. Chem. Soc., 107 (1985) 7783. [21 ] H. van Willigen, U. Das, E. Ojadi and H. Linschitz, J. Am. Chem. Soc., 107 (1985) 7784. [22] D.K. Geiger and C.A. Kelly, Inorg. Chim. Acta, 154 (1988) 137. [23] J.F. Lipskier and T.H. Tran-Hi, Inorg. Chem., 32 (1993) 722. [24] H. Segawa, H. Nishino, T. Kamikana and K. Honda, Chem. Lett., (1989) 1917. [25] K. Kalyanasundaram and M. Neumann-Spallart, J. Phys. Chem., 86 (1982) 5163. [26] C.A. Ltuate and K.M. Sanders, J. Am. Chem. Soc., 112 (1990) 5525. [27] A.K. Ghosh, D.L. Morel, T. Feng, R.S. Shaw and C.A. Rowe, J. Appl. Phys., 45 (1974) 230. [28] A.K. Ghosh and T. Feng, J. Appl. Phys., 49 (1978) 5982. [29] C.J. Schramm, R.P. Scaringe, D.R. Stojakovic, B.M. Hoffman, J.A. Ibers and T.J. Maekers, J. Am. Chem. Soc,, 102 (1980) 6702. [30] U. Hofstra, R.B.M. Koehorst and T.J. Schaafsma, Chem. Phys. Lett., 130 (1986) 555.