para-dialkylaminophenyl dyes for efficient nanocrystalline tio2 sensitization in far-red region

Chinese Journal of Chemistry, 2006, 24, 537—545 Full Paper

* E-mail: [email protected]; Tel.: 86-010-64888103 Received June 21, 2005; revised September 8, 2005; accepted December 11, 2006. Project supported by the National Key Project for Basic Research on Photovoltaic Cell (No. G2000028204).

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

para-Dialkylaminophenyl Dyes for Efficient Nanocrystalline TiO2 Sensitization in Far-red Region

LI, Chaoa,b(李超) ZHOU, Jia-Honga,c(周家宏) CHEN, Jing-Ronga(陈景荣) CHEN, You-Shenga,b(陈友生) ZHANG, Xue-Huaa,b(张雪华) DING, Hui-Yinga,b(丁慧颖) WANG, Wei-Boa(王维波) WANG, Xue-Song*,a(王雪松) ZHANG, Bao-Wen*,a(张宝文)

a Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China

c Jiangsu Engineering Research Center for Bio-medical Function Material, Nanjing Normal University, Nanjing, Jiangsu 210097, China

Four para-dialkylaminophenyl (PDAAP1—PDAAP4) bearing carboxyl groups were studied for application to the dye-sensitized solar cells (DSC). It was found the short spacer CH2 between carboxyl and dialkylaminophenyl chromophore in PDAAP3 and PDAAP4 led to highly efficient monochromatic incident photon-to-current conver-sion efficiencies (IPCE), however the long alkyl group C4H9 attached on aniline moieties in PDAAP2 and PDAAP4 favored improvement of open-circuit photovoltage. Thus, the solar cell sensitized by PDAAP4, having both short carboxyl groups CH2COOH and long alkyl groups C4H9, exhibited the IPCE maximum of 73% at 670 nm and over-all energy conversion efficiency η of 3.06%, representing the highest IPCE and η values so far in dialkylamino-phenyl-based organic dye-sensitized semiconductor solar cells. Taking advantage of the highly efficient sensitizing ability of PDAAP4 in far-red region, the data of IPCE above 630 nm of the solar cells were improved greatly by cosensitization with both N3 and PDAAP4. The influences of the TiO2 film thickness and the concentration of 4-tert-butylpyridine (TBP) in electrolyte were also investigated.

Keywords dialkylamineophnenyl, dye, dye-sensitization, solar cell, efficiency

Introduction

Dye-sensitized solar cells (DSC) based on nanocrys-talline oxide semiconductrors (typically TiO2) have been intensively studied and developed over the past 15 years as these unconventional solar cells exhibit high per-formance and have the potential for low cost produc-tion.1-8 The heart of this cell is a photoanode, which is composed of a porous nanocrystalline TiO2 film covered by a monolayer of dye molecules. Upon light excitation, the adsorbed dye molecules inject electrons from their excited states into the conduction band of the semicon-ductor. The electrons are brought back to the oxidized dye molecules through an external circuit, a counter electrode, and a redox system (typically 3I－ /I－).1-3 As for the sensitizing dye, several ruthenium(II) polypyri-dine complexes, such as cis-dithiocyanato bis(4,4'-dicarboxy-2,2'-bipyridyl)ruthenium(II)2 (referred to as N3 dye), trithiocyanato (4,4',4"-tricarboxy-2,2':6',2"-ter- pyridyl)ruthenium(II)4 (referred to as black dye), and cis-dithiocyanato(4,4'-dicarboxy-2,2'-bipyridyl)[4,4'-bis-(3-methoxystyryl)-2,2'-bipyridyl]ruthenium(II)6 (re-ferred to as Z-910), gave the highest photoconversion efficiencies thus far among the examined dyes for DSC.

Ruthenium(II) complexes have many advantageous features for application to DSC.3 First, their broad metal-to-ligand charge transfer (MLCT) absorption bands are overlapped very well with blue and green light of the solar spectrum. Secondly, their ultrafast electron injection on ps or fs timescales and relatively slow electron recombination on µs timescales render highly efficient charge separation on TiO2/dye interface. Thirdly, these dyes are chemically stable for long time exposure to natural sunlight. For the mass production of DSC in practical applications, however, development of alternative dyes is required due to the complicated pro-cedures and expensive cost in preparation and purifica-tion of these ruthenium(II) complexes. Moreover, efforts should be paid to improve the light harvesting efficien-cies of new sensitizing systems in far-red region, in which most ruthenium(II) complexes show very low extinction coefficients.9,10

Recently, types of efficient DSC based on organic dyes, e.g. hemicyanine dyes,11-13 coumarin-dye14-16 and indoline-dye,17,18 were developed. In contrast to ruthe-nium(II) complexes, the merits of organic dyes are ob-vious. (a) Absorption coefficients of organic dyes are generally much higher than those of metal complexes,

538 Chin. J. Chem., 2006, Vol. 24, No. 4 LI et al.


implying thinner semiconductor films and less dye molecules may be applied to DSC. Besides material saving, the use of thinner semiconductor films may fa-vor electron collection in the photoanodes to lead to an improvement in DSC performance. (b) The costs of or-ganic dyes are usually lower than those of ruthenium(II) complexes, and there is no limitation of resources stemming from precious noble metals. (c) The variety of their structures provides possibilities for molecular de-sign and tailoring to achieve desirable photophysical and photoelectrochemical properties for applications to DSC. Moreover, organic dyes to possess strong far-red absorbing features are rich in number, and their utiliza-tion may compensate the lower absorption of ruthe-nium(II) complexes in this region. However, organic dyes have several disadvantages as photosensitizers. First, the lifetime values of their singlet excited states (e.g. 0.4—0.5 ns19-21) are generally shorter than those of MLCT states of ruthenium(II) complexes (50 ns for N32), which demands ultrafast electron injection to compete against their intrinsic fast decay pathways.22-24 Proper anchoring groups such as phosphonate or car-boxylate22,25 and short distance between dyes and semi-conductor surface26,27 can promote electron injection efficiently. Secondly, organic dyes have relatively sharp absorption bands in the visible region, limiting the light harvesting. While dye aggregation including both the blue-shifted H-aggregates and red-shifted J-aggregates has been explored on purpose to broaden the absorption region,27-29 the cosensitization by multiple dyes is also an effective way to resolve problems associated with narrow absorption.30-32

To develop new organic sensitizer dyes for DSC ap-plications, especially those having strong absorption in far-red region, we focused our interest on dial-kylaminophenyl dyes.33 It is well-known that dial-kylaminophenyl dyes exhibit intense absorption in far- red region, and as a result were widely investigated as far-red sensitizers for large bandgap semiconductor in light of solar energy conversion,34-36 and photocurrent quantum yield per absorbed photon of near unity was observed for a dialkylaminophenyl sensitized tin disul-fide single crystals under sufficiently positive bias.37 However, upon photosensitizing nanocrystalline TiO2, low monochromatic incident photon-to-current conver-sion efficiencies (IPCE) and low power conversion effi-ciency η were obtained. The weak coupling between the used dialkylaminophenyls and TiO2 conduction band due to the lacking of carboxyl anchoring groups and the fast interfacial recombination of injected electrons with oxidized, dialkylaminophenyls34,35 may account for their unsatisfactory performance in DSC and should be overcome in such applications. We herein report the application of four carboxyl-containing dialkylaminophenyls (PDAAP1—PDAAP4, Scheme 1) to DSC, where carboxyl anchoring groups may simultaneously promote electron injection and restrict electron recom-bination, and as a result, IPCE and η up to 73%

Scheme 1 Chemical structures of dialkylaminophenyl dyes

(PDAAP4 at 670 nm) and 3.06% (PDAAP4) respec-tively were observed, representing the highest IPCE and η values so far in dialkylaminophenyl-based organic dye-sensitized semiconductor solar cells. Based on the high efficient IPCE of PDAAP4 in far-red region, cosensitization with the combination of N3 and PDAAP4 was also carried out, where the use of PDAAP4 promoted the IPCE in far-red region remarka-bly.

Experimental

Synthesis of dialkylaminophenyl dyes

Five dialkylaminophenyl dyes were synthesized by condensation of the corresponding aniline and squaric acid according to reported procedure.38 All reagents were commercially available and used without further purification. For example, PDAAP1 was synthesized as follows. N-Methyl-N-(3-carboxypropyl)aniline (8 mmol), 2-propanol (25 mL) and tributyl orthoformate (2 mL) were placed in a three-necked, 100 mL flask with a magnetic stirbar. The mixture was stirred and heated to reflux under nitrogen atmosphere. A solution of squaric acid (2 mmol) in 2 mL of DMF was added through a pressure-equalizing funnel over a 3 h period. The mix-ture was kept at reflux for additional 2 h. A blue pre-cipitate was isolated by filtration. After the blue solid was washed with cold 2-propanol and ether, it was vac-

Dialkylaminophenyl Chin. J. Chem., 2006 Vol. 24 No. 4 539


uum dried and recrystallized from DMSO/CH2Cl2 to yield pure PDAAP1.

Preparation of dye-sensitized TiO2 electrodes

The TiO2 paste (nanoxide-T from Solaronix, Swit-zerland) was coated by means of doctor blade technique using Scotch tape (Sumitomo 3M) as spacer on fluo-rine-doped tin oxide (FTO) conducting glass (transmis-sion＞80%, sheet resistance 20 Ω/cm), followed by sin-tering at 450 in air atmosphere for 30 min.2 Re-peating the above procedures got TiO2 films of different thickness. The films used in this work had the thickness of 2.3, 4.5, 6.9, 9.2 and 13.1 µm. Thickness of TiO2 films was determined by a Dektak 3 profilometer. The TiO2 electrode was soaked in 0.02 mol•L－1 TiCl4 stock solution overnight at room temperature and then washed with distilled water. Finally, it was fired again for 30 min at 450 .2,4 The resulting TiO2 electrodes were dipped in a 1 mmol•L－1 PDAAP solutions in DMSO for 1.5 h at room temperature, rinsed with absolute ethanol and dried under a stream of warm air. For cosensitiza-tion, TiO2 electrodes were socked in the mixed solution of N3 (1 mmol•L－1) and PDAAP4 (1 mmol•L－1) in DMSO for 1.5 h.

Photophysical measurements

Electronic absorption spectra of PDAAP in DMSO solutions, and adsorbed on TiO2 films were measured with a Shimadzu UV-1601PC spectrophotometer. F1uorescence emission spectra of PDAAP in DMSO solutions were recorded on a Hitachi F-4500 fluores-cence spectrophotometer. The steady-state electron spin resonance (ESR) spectra of PDAAP0 upon excitation were obtained on a Bruker ESR-300E spectrometer at room temperature, using a laser Nd:YAG (532 nm, 5—6 ns of pulse width, repetition frequency: 10 Hz, 10 mJ/puse energy) as excitation light source. The ESR instrument settings were 9.8 GHz, X-band with 100 Hz field modulation, microwave power of 10 mW, modula-tion amplitude of 2.0 G, scan width of 300 G, receiver gain of 1×105.

Electrochemical and photoelectrochemical meas-urements

Cyclic voltammetry measurements were carried out on a potentiostat/galvanostat model 283A (EG&G Princeton Applied Research) to determine the oxidation potentials of PDAAP in DMF solutions. A three-electrode cell was composed of a Pt working electrode, a Pt counter electrode and a saturated calomel reference electrode (SCE). Tetrabutyl ammonium perchlorate (0.1 mol•L－1) served as supporting electrolyte and the scan rate was 100 mV/s. This three-electrode system was also used in the in situ ESR measurements of PDAAP0 upon electrochemical oxidation. The in situ UV-visible ab-sorption spectra were recorded on the UV-1601PC spectrophotometer in a thin layer spectroelectrochemi-cal cell, in which a piece of platinum grid was used as working electrode, while the other conditions were the

same as above. A sandwich-type solar cell was constructed by in-

troducing the redox electrolyte containing 0.5 mol•L－1 LiI, 0.05 mol•L－1 I2, 0.6 mol•L－1 1,2-dimethyl-3-n- propylimidazolium iodide (DMPII) and different con-centration of 4-tert-butylpyridine (TBP, ranging from 0.1 to 0.6 mol•L－1) in methoxyacetonitrile (MOAN) between a dye-sensitized TiO2 electrode and a counter electrode of Pt-coated FTO conducting glass. The dye-sensitized TiO2 film was illuminated through the conducting glass support and the effective irradiated area was 0.16 cm2. The photovoltaic performance of the solar cell was taken from a computer-controlled digital sourcemeter (Keithley 2400) under an illumination of AM1.5 Global simulated light (75 mW/cm2) from an Oriel 91192 solar simulator. In the measurements of photocurrent action spectra, the solar cell was irradiated by monochromatic light obtained from a 500 W Xe lamp in combination with a spectrapro-150 monochro-mator (Acton Research Co.), in the range of 400—800 nm at 10 nm interval, and the generated short-circuit photocurrents were recorded on a model 283A potentio-stat/galvanostat. The incident monochromatic light in-tensity was measured with an S370 single-channel op-tometer equipped with a Model 262 flat response sensor head (Graseby Optronics).

Results and discussion

Photophysical and electrochemical properties of dialkylaminophenyl dyes

Photophysical properties of the dialkylaminophenyl dyes in DMSO, including the absorption and emission maxima (λab and λem), the molar extinction coefficients at absorption maxima (εmax) and the 0-0 transition ener-gies (E0-0) deduced from the intersect wavelength of normalized absorption and emission spectra, are sum-marized in Table 1. PDAAP0—PDAAP4 all exhibited a sharp and intense absorption band in DMSO solution with maximum of 653.0, 651.5, 655.5, 643.0 and 647.5 nm, respectively (Figure 1, with the absorption spec-trum of PDAAP0 omitted for clarity). The s0—s1 elec-tronic transition of dialkylaminophenyl dyes might in-volve a charge transfer process which is primarily con-fined to the central cyclobutane ring, from each oxygen atom to the four-membered ring with a small degree of charge transfer from the anilino moiety to the central C4O2 unit.39 The minor involvement of aniline moiety in s0—s1 electronic transition could make the absorption spectra of dialkylaminophenyl dyes dependant on the electron donating or withdrawing features of the sub-stituents on the nitrogen atom of aniline moiety.39 Thus, the absorption maximum blue shift of PDAAP3 vs. PDAAP1 and PDAAP4 vs. PDAAP2 can be attributed to the stronger electron withdrawing ability of short carboxyl group CH2COOH than long carboxyl group (CH2)3COOH. The molar extinction coefficients at λmax were 3.24×105, 3.77×105, 3.21×105, 1.99×105,



2.99×105 L•mol－1•cm－1 for PDAAP0-PDAAP4 re-spectively, one order of magnitude larger than those of Ru(II) polypyridyl complexes.

Table 1 Photophysical and electrochemical properties of Dial-kylamineophnenyl dyes

Dyes λmaxa/nm

εmaxa/(10－5

L•mol－1•cm－1) λem

a/nm E0-0a/eVEox

b/V *

oxE /V

PDAAP0 653.0 3.24 685 1.85 0.69 －1.16

PDAAP1 651.5 3.77 680 1.86 0.71 －1.15

PDAAP2 655.5 3.21 684 1.85 0.71 －1.14

PDAAP3 643.0 1.99 670 1.88 0.80 －1.08

PDAAP4 647.5 2.99 673 1.87 0.80 －1.07 a In DMSO solution; b In DMF solution.

Figure 1 Absorption spectra of PDAAP1—PDAAP4 (5.0× 10－5 mol•L－1) in DMSO.

Listed in Table 1 are also the ground state oxidation peak potentials Eox of PDAAP0—PDAAP4 in DMF measured by cyclic voltammetry and their excited state oxidation potentials *

oxE calculated by subtracting individual E0-0 from the oxidation potentials. The ground state and excited state oxidation potentials of PDAAP3 and PDAAP4 were shifted anodically than those of PDAAP1 and PDAAP2, suggesting the stronger electron-withdrawing ability of the short car-boxyl group CH2COOH than the long carboxyl group (CH2)3COOH, in line with the substituent effect exhib-ited in their absorption spectra. The ground state oxida-tion potentials of these dyes were positive with respect to 3I－ /I－ redox potential (0.24 V vs. SCE3) and the excited state oxidation potentials of them were negative with respect to the conduction band potential of TiO2 (ca. －0.7 V vs. SCE3), respectively, making both elec-tron injection from excited dialkylaminophenyl dyes into TiO2 conduction band and reduction of oxidized dialkylaminophenyl dyes by I－ ions thermodynamically possible.

UV-visible absorption and ESR measurements of dialkylaminophenyl dyes cations

Electron injection from squaraines to TiO2 conduc-

tion band could lead to the formation of dialkylamino-phenyl dyes cations, therefore a lot of work was focused on the properties of squaraine cations.40-42 For example, the cations of some crown-ether dialkylaminophenyl dyes generated by flash photolysis, electrochemical oxidation, and pulse radiolysis were characterized by their absorption features.40 In this study the UV-visible and steady state ESR measurements of the cations of PDAAP0 generated by electrochemical oxidation or photolysis were carried out. Figure 2 displays the ab-sorption spectrum changes of the PDAAP0 solution in dichloromethane at applied potential of 0.9 V vs. SCE. The 0.9 V bias ensured one electron oxidation of PDAAP0. The absorption intensity at 637 nm was de-creased, while at the same time, a new absorption band (λmax＝665 nm) appeared. The isobestic point located at 655 nm suggests the formation of one and only one new species, which can be reasonably ascribed to PDAAP0＋• produced by one electron oxidation. When this elec-trolysis process was performed in ESR detection cavity, an ESR signal was detected (Figure 3a). Before elec-trolysis no ESR signal was observed. Thus, this ESR signal can be attributed to PDAAP0＋ •. The radical cation formation of a crown-ether dialkylamino-phenyl dyes upon excitation was probed by Kamat with 532 nm laser pulses in time-resolved absorption measurements, and attributed to the result of the multiphotonic excita-tion of the dye at high laser intensities. In our experi-ments, no ESR signal was found when PDAAP0 solu-tions in dichloromethane or ethanol were irradiated by 532 nm laser pulses (Figure 3b), suggesting the concen-tration of PDAAP0＋• in solution, if any, should be too low to detect. However, if PDAAP0 was included in a TiO2 colloidal ethanol solution and irradiated by 532 nm laser, an ESR signal (Figure 3c) with the same features as that in Figure 3a was observed. Irradiation of blank TiO2 colloidal ethanol solution did not produce any sig-nals. Obviously, PDAAP0＋• was generated by electron injection from the excited PDAAP0 to the conduction band of TiO2 particles, implying the potential applica-tion of these dialkylaminophenyl dyes to DSC. The poor solubility of PDAAP1-PDAAP4 in ethanol limited the examination of their radical cations formation in TiO2 colloidal solutions through ESR measurements.

Absorption properties of dialkylaminophenyl dyes adsorped on TiO2 films

Except for PDAAP0, PDAAP1—PDAAP4 can be adsorbed onto TiO2 films readily from the DMSO solu-tions with the help of strong anchoring group of car-boxyl. Dye adsorption was completed within tens of minutes. The absorption spectra of PDAAP1—PDAAP4 on TiO2 films were broadened remarkably (Figure 4) compared with those in DMSO (Figure 1). The absorp-tion peak of PDAAP1 was blue shifted from 651.5 nm in DMSO solution to 635 nm on TiO2 film. Similar blue shifts of ca. 15 nm were also observed for PDAAP2—PDAAP4 upon adsorption on TiO2 films. While the ab-sorption maximum blue shifts may be the result of



Figure 2 Absorption spectra changes of PDAAP0 (8.0×10－6 mol•L－1) in CH2Cl2 upon electrolysis at 0.9 V vs. SCE with the electrolysis time of 0 (a), 1 (b), 2 (c), 3 (d), 4 (e), 5 (f) and 6 min (g).

Figure 3 ESR spectra of PDAAP0 solution in CH2Cl2 during electrolysis at 0.9 V vs. SCE (a), CH2Cl2 or C2H5OH upon excita-tion by 532 nm laser (b) and TiO2 colloidal ethanol solution upon excitation by 532 nm laser (c).

Figure 4 Absorption spectra of PDAAP1—PDAAP4 on TiO2 films (4.5 µm thick).

either interactions between dialkylaminophenyl dyes and TiO2 or polarity changes of the microenvironment of dialkylaminophenyl dyes on TiO2 films with respect to in DMSO solutions, and such spectrum broadening

suggests the aggregation of dialkylaminophenyl dyes on TiO2 films, especially for PDAAP3 and PDAAP4. A new absorption band in the region of 400—500 nm originating from H-type aggregation29 appeared in PDAAP3 or PDAAP4 sensitized TiO2 films, suggesting short carboxyl anchoring groups favor interactions be-tween dyes and/or between dye and TiO2.

Effect of TiO2 film thickness on solar cell perform-ances

As the TiO2 film thickness could strongly affect the photocurrent action spectrum and the overall energy conversion efficiency, the effect of film thickness on the performances of PDAAP4-sensitized solar cells was investigated as a representative. Table 2 collects the photovoltaic performances of five PDAAP4-sensitized solar cells with varied TiO2 film thickness from 2.3 to 13.1 µm. With the increase of the TiO2 film thickness, the short-circuit photocurrent Isc was increased from 2.44 to 10.99 mA/cm2, but the open-circuit photovoltage Voc decreased from 0.44 to 0.37 V. Meanwhile, the fill factor FF declined from 0.68 to 0.52, which is defined as Eq. 1:

FF＝VmaxImax/VocIsc (1)

where Vmax and Imax are the photovoltage and photocur-rent of the solar cell at its maximum power output re-spectively. Thus, the overall energy conversion effi-ciency η of the solar cell, defined as Eq. 2.

η＝VocIscFF/Pin (2)

where Pin is the power of incident white light, increased from 1.02% at 2.3 µm of TiO2 to the highest value of 3.06% at 9.2 µm of TiO2, and then decreased with fur-ther increase of TiO2 film thickness, suggesting the im-portance of the optimization on TiO2 film thickness. It is noteworthy that the photocurrent underwent a sudden increase when TiO2 film thickness was changed from ca. 7 to ca. 9 µm (the reproducible phenomena), although the underlying mechanism was yet unknown. The thicker TiO2 film can load more dye molecules and hence may generate higher short-circuit photocurrent. However, the thicker TiO2 film will increase the recom-bination probability of the injected electrons with both oxidized dyes and 3I－ , since the injected electrons have, on average, to pass more colloidal particles and grain boundaries before collected into external circuit, resulting in the decrease of open-circuit photovoltage.43 Moreover, in the case of illumination through the photoanode support, dye molecules adsorbed far from FTO glass will have less and less photon flux available for absorption with increasing film thickness due to the inner filter effect of the dye molecules adsorbed adja-cently to FTO glass, leading to the photocurrent satura-tion. Balancing all of the factors, 9.2 µm thick TiO2 film was selected to carry out all of the experiments unless otherwise specified.



Table 2 Thickness effect of TiO2 films on the performances of the PDAAP4-sensitized solar cellsa

Film thickness/µm Isc/(mA•cm－2) Voc/V FF Η/%

2.3 2.44 0.44 0.68 0.97

4.5 3.24 0.41 0.63 1.12

6.9 3.91 0.39 0.63 1.28

9.2 10.5 0.39 0.56 3.06

13.1 10.99 0.37 0.52 2.82

13.1 10.99 0.37 0.52 2.82 a Electrolyte containing 0.5 mol•L－1 LiI, 0.05 mol•L－1 I2, 0.6 mol•L－1 DMPImI and 0.1 mol•L－1 TBP in MAC.

Effect of 4-tert-butylpyridine on solar cell perform-ances

Treatment of the ruthenium(II) complex2 or certain organic dye11,14 sensitized TiO2 electrodes with proper concentrations of TBP added in electrolyte was found to improve both Voc and η. The suppressed charge recom-bination between injected electrons and triiodide ions and the negative movement of conduction band edge, both as the result of TBP adsorption on TiO2 surface sites not occupied by sensitizer dyes, are generally be-lieved to be responsible for the Voc improvement.44 The effect of TBP concentration in electrolyte on the per-formance of the solar cells based on PDAAP4 is sum-marized in Table 3. The Voc and FF were remarkably improved, whereas the Isc was decreased with increasing TBP concentration. Figure 5 exhibits the dark current plots of the PDAAP4-sensitized solar cell as the func-tion of applied bias voltages. It is clear that the dark current was reduced significantly at higher concentra-tion of TBP, indicating that TBP can restrict the recom-bination of conduction band electrons with triiodide ions. The best efficiency was obtained at 0.1 mol•L－1 TBP for PDAAP4 sensitized solar cells. Similar phe-nomena were observed for solar cells based on PDAAP1—PDAAP3.

Figure 5 Dark current plots of the PDAAP4-sensitized solar cells with different concentrations of TBP: 0.1 mol•L－1 (solid curve) and 0.6 mol•L－1 (dash curve) in the electrolyte.

Table 3 Concentration effect of TBP on the performances of the PDAAP4-sensitized solar cellsa

TBP/(mol•L－1) Isc/(mA•cm－2) Voc/V FF η/%

0 13.3 0.3 0.44 2.34

0.1 10.5 0.39 0.56 3.06

0.15 7.31 0.39 0.51 1.94

0.2 4.85 0.4 0.61 1.58

0.3 4.34 0.4 0.56 1.30

0.4 4.09 0.42 0.63 1.44

0.6 3.70 0.43 0.64 1.36 a Electrolyte containing 0.5 mol•L－1 LiI, 0.05 mol•L－1 I2, 0.6 mol•L－1 DMPImI and varied concentrations of TBP in MAC

Relationship between photoelectrochemical proper-ties and chemical structures for dialkylaminophenyl dyes

The monochromatic incident photon-to-current con-version efficiency, defined as the number of electrons generated by light in the external circuit divided by the number of incident photon, can be calculated with Eq. 3.

SC

in

1240IPCE 100%

I

λP＝ ×

(3)

where Isc is the short-circuit photocurrent to take µA/cm2 generated by the monochromatic light with wavelength of λ (nm) and intensity of Pin (W/m2). Fig-ure 6 shows the IPCE action spectra of the DSC sensi-tized by PDAAP1—PDAAP4, respectively. There are close similarities between the IPCE action spectra (Fig-ure 6) and the absorption spectra of PDAAP1—PDAAP4 on TiO2 films (Figure 4), in which thinner films of 4.5 µm were used, indicating that the photocur-rent originated from the excitation of both dial-kylaminophenyl dyes monomers and aggregates. Dial-kylaminophenyl dyes bearing short carboxyl groups (PDAAP3 and PDAAP4) exhibited much higher IPCE in the whole examined spectrum region than dial-kylaminophenyl dyes bearing long carboxyl groups (PDAAP1 and PDAAP2), and the IPCE at 670 nm reached 73% for PDAAP4. The short carboxyl anchor-ing groups of PDAAP3 and PDAAP4 seem to play mul-tiple roles. On one hand, they might enhance the inter-action of dyes with TiO2 by shortening their distance, and therefore promoting electron injection. On the other hand, they raise the oxidation potential of dyes and enlarge the energy gap between oxidized PDAAP and I－/ 3I－ couples, as a result to speed up the reduction of oxidized PDAAP by I－ and restrict the recombination of injected electrons with oxidized dyes. Moreover, short carboxyl anchoring groups could favor the forma-tion of H-aggregate, enhance the light harvesting over the blue and green light region, and consequently im-prove the photoelectric responses of the solar cells in this region of 400—550 nm. These reasons can lead to the efficient sensitization of PDAAP4 upon TiO2 with



Figure 6 Photocurrent action spectra of the dye-sensitized solar cells based on PDAAP1—PDAAP4.

the IPCE maximum of 73%, which is so far the highest IPCE value in dialkylaminophenyl-dyes-based organic dye-sensitized semiconductor solar cells, and higher than that of N3 in this region (ca. 60% at 670 nm2).

The photocurrent-photovoltage characteristics of dialkylaminophenyl-dyes-based DSC are shown in Fig-ure 7 and related parameters listed in Table 4. On virtue of their more efficient IPCE spectra, solar cells based on PDAAP3 and PDAAP4 exhibited much higher short- circuit photocurrents than on PDAAP1 and PDAAP2, illustrating again the superiority of CH2COOH to (CH2)3COOH as anchoring groups. It is interesting to note that the long alkyl group C4H9 favored the im-

Figure 7 Photocurrent-photovoltage curves of the solar cells based on PDAAP1—PDAAP4.

Table 4 Photovoltaic performances of the solar cells based on PDAAP1—PDAAP4a

Dyes Isc/(mA•cm－2) Voc/V FF η/%

PDAAP1 4.46 0.36 0.53 1.13

PDAAP2 4.61 0.39 0.57 1.37

PDAAP3 10.4 0.36 0.51 2.53

PDAAP4 10.5 0.39 0.56 3.06 a Electrolyte used was the same as in Table 2.

provement of open-circuit photovoltages if the open- circuit photovoltages of the PDAAP4- and PDAAP2- sensitized solar cells were compared with those of PDAAP3- and PDAAP1-sensitized ones. Similar to other organic dye-sensitized solar cells,45 the PDAAP1—PDAAP4 based solar cells showed lower open-circuit photovoltages than ruthenium(II) complex based solar ones. The higher dark currents of organic dye-sensitized solar cells were invoked to explain their lower open-circuit photovoltages.45 Thus, optimization of or-ganic dye-sensitized solar cells should focus more atten-tion on the control of dark current. The improved open-circuit photovoltages of PDAAP4- and PDAAP2- based cells may result from the blocking of triiodide ions to TiO2 surface by C4H9 groups, leading to reduced dark currents as shown in Figure 8, in which PDAAP2- and PDAAP4-sensitized solar cells showed diminished dark currents with respect to PDAAP1- and PDAAP3- sensitized ones at the same biased potentials. Therefore, PDAAP4 with both short carboxyl group and long alkyl group become the best sensitizer dye in DSC applica-tions among the four dialkylaminophenyl dyes, showing short-circuit photocurrent of 10.5 mA•cm － 2, open- circuit photovoltage of 0.39 V, fill factor of 0.56, and an overall energy conversion efficiency of 3.06%.

Figure 8 Dark current plots of the solar cells based on PDAAP1—PDAAP4.

Cosensitization of TiO2 electrodes by N3 and PDAAP4

An ideal sensitizer dye should have a wide electronic absorption band matching the solar spectrum over the whole visible and near infrared region of 400—920 nm,4 unfortunately, unlike the narrow bandgap semi-conductor materials such as Si and GaAs, no organic or inorganic dyes can possess such a wide and strong ab-sorption, as a result the cosensitization approach re-ceived increased interest in which two or more kinds of dyes were utilized simultaneously to broaden the light harvesting region.30-32,36 In order to improve the light harvesting efficiency of N3 over far-red region, PDAAP4 was incorporated with N3 to sensitize nanocrystalline TiO2 electrodes. Figure 9 displays the absorption spectra of TiO2 films sensitized by N3 and



PDAAP4, respectively, suggesting their cooperation possibility in TiO2 sensitization on virtue of their com-plementary features with light absorption.

Figure 9 Absorption spectra of N3 and PDAAP4 on TiO2 film of 4.5 µm thickness.

The photocurrent action spectra of the solar cells based on N3, PDAAP4, and both N3 and PDAAP4 clearly show that in the cosensitized system both N3 and PDAAP4 contribute to the photocurrent (Figure 10). N3-sensitized solar cell exhibited lower IPCE over far- red region, while in the cosensitized solar cell the IPCE in this region of 630—730 nm was improved greatly with the help of PDAAP4. However, the IPCE of the cosensitized cell in the region of 400—630 nm was smaller than those of the N3-sensitized one, which ac-counts for the lower overall energy conversion effi-ciency of the cosensitized cell (6.2%) with respect to the N3-sensitized one (7.5%). The diminished IPCE of the cosensitized solar cell in 400—630 nm region may re-sult from several reasons. First, N3 and PDAAP4 were adsorbed on the same TiO2 electrode surface, and therefore the amount of the adsorbed N3 was decreased by ca. 40%, leading to a reduction of photocurrent con-tributed from N3, which is a superior sensitizer over

Figure 10 Photocurrent action spectra of the solar cells based on N3, PDAAP4 and both N3 and PDAAP4 (refered to as N3PDAAP4).

PDAAP4 in the region of 400—630 nm. Secondly, the dark current in the cosensitized solar cell was increased compared with N3-sensitized one (Figure 11), which may take some negative effects on IPCE. As mentioned above, dark current was generally associated with low open-circuit photovoltages in organic dye-sensitized solar cells.45 Bearing these considerations in mind, much attention was focused on the construction of dye multilayer on nanocrystalline TiO2 surface with ruthe-nium(II) complexes and organic dyes such as PDAAP4 as inner and outer layers, respectively. Dye multilayer sensitization approach has achieved limited success re-cently,31,32 and its applications was expected to be able to overcome problems encountered in single layer cosensitization.

Figure 11 Dark current plots of the solar cells based on N3 and both N3 and PDAAP4 (refered to as N3PDAAP4).

Conclusion

Comparison of photovoltaic performances of PDAAP1—PDAAP4 in dye-sensitized solar cells re-vealed that the linkage manner by which the anchoring group of carboxyl was attached onto dialkylamino-phenyl dyes is crucial. Short spacer CH2 between car-boxyl and dialkylaminophenyl dyes chromophore in PDAAP3 and PDAAP4 not only led to higher IPCE in far-red region, where dialkylaminophenyl dyes mono-mer served as the main light absorber, but also gave rise to efficient IPCE in blue and green light region, where dialkylaminophenyl dyes H-type aggregate dominated light harvesting, reflecting that the short carboxyl an-choring groups CH2COOH may enhance both the inter-actions of dye molecules with TiO2 and those of dye molecules themselves. Moreover, long alkyl group C4H9 attached on aniline moieties in PDAAP2 and PDAAP4 favored improvement of open-circuit photovoltage, probably by restricting recombination between injected electrons and triiodide ions. Taking advantage of the highly efficient sensitizing ability of PDAAP4 in far-red region, the data of IPCE above 630 nm of the solar cells were improved greatly by cosensitization with both N3 and PDAAP4.



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(E0506213 ZHAO, C. H.)

para-dialkylaminophenyl dyes for efficient nanocrystalline tio2 sensitization in far-red region

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