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J. Phys. Chem. C 2008, 112, 1721-1728

1721

Improved Photon-to-Current Conversion Efficiency with a Nanoporous p-Type NiO Electrode by the Use of a Sensitizer-Acceptor Dyad Ana Morandeira,† Je´ roˆ me Fortage,‡ Tomas Edvinsson,§ Loı1c Le Pleux,‡ Errol Blart,‡ Gerrit Boschloo,§ Anders Hagfeldt,*,§ Leif Hammarstro1 m,*,† and Fabrice Odobel*,‡ Laboratoire de Synthe` se Organique, UMR 6513 CNRS&FR CNRS 2465, UniVersite´ de Nantes, Faculte´ des Sciences et des Techniques de Nantes, BP 92208; 2, rue de la Houssinie` re, 44322 Nantes Cedex 03, France, Chemical Physics, Department of Photochemistry and Molecular Science, Uppsala UniVersity, Box 523, SE-751 20 Uppsala, Sweden, and Center of Molecular DeVices, Department of Chemistry, Royal Institute of Technology, Teknikringen 30, 100 44 Stockholm, Sweden ReceiVed: September 16, 2007; In Final Form: October 26, 2007

A peryleneimide sensitizer and a covalently linked peryleneimide-naphthalenediimide dyad were prepared and characterized by absorption and emission spectroscopies, electrochemistry, and spectroelectrochemistry. These compounds were chemisorbed on nanoporous nickel oxide electrodes and then studied by femtosecond transient absorption spectroscopy in the presence of a redox active electrolyte (I3-/I-). In both compounds, upon excitation of the peryleneimide unit, an electron is efficiently ejected from the valence band of NiO to the dye with an average time constant of approximately 0.5 ps. In the case of the dyad, the excess electron is shifted further onto the naphtalenediimide unit, creating a new charge separated state. The latter exhibits a substantial retardation of the charge recombination between the hole and the reduced molecule compared with the peryleneimide sensitizer. The photoaction spectra of a sandwich dye-sensitized solar cell (DSSC) composed of NiO films and these new dyes were recorded, and the absorbed-photon to current conversion efficiency (APCE) was three times higher with the dyad than with the peryleneimide dye: 45%. The maximum APCE of approximately 45% is the highest value reported for a DSSC based on a nanostructured metal oxide p-type semiconductor.

1. Introduction Dye-sensitized solar cells (DSSCs) are attractive photovoltaic devices for the conversion of solar energy into chemical potential. Generally, these systems are composed of a mesoporous wide band gap n-type semiconductor (SC), such as titanium dioxide, coated with a molecular photosensitizer acting as an electron donor upon light excitation.1 The first and a key step of these devices is the photoinjection of an electron into the conduction band of the SC from the sensitizer excited state. In contrast, there are only a limited number of studies of the sensitization of p-type semiconductors, for which the operation principle is just the inverse scheme and consists of the photoinjection of a hole into the valence band of the semiconductor.2-4 Studies of p-type systems may lead to the development of new types of devices and are particularly vital for the construction of tandem DSSCs in which both the cathode and the anode would be photoactive2 and for photocatalytic oxidations driven by visible light.5 We have shown in our previous studies3 that the fast recombination between the hole in the VB and the reduced sensitizer was a serious drawback in the development of efficient NiO based DSSCs. A possible strategy to tackle this problem is to use as sensitizer a dyad composed of a chromophoric unit, bound to the SC surface, and a secondary electron acceptor unit, located further away from the interface. If the strategy is * Corresponding authors. E-mail: [email protected] (F.O.), [email protected] (L.H.), [email protected] (A.H.). † Uppsala University. ‡ Universite ´ de Nantes. § Royal Institute of Technology.

successful, the chromophore can thus be quickly regenerated after injecting a hole into the SC, and the distance between the hole-electron pair is significantly increased, resulting in a longer-lived charge separated state. This function is analogous to that of sensitizer-donor dyads used on n-type semiconductor systems.6 For the present study, peryleneimide (PI) dyes were chosen as chromophores because of their outstanding chemical and thermal properties and their high molar absorption coefficients in the visible region.7 Moreover, the spectrum of the reduced PI generally exhibits characteristic absorption bands which makes it easy to distinguish from that of its electronic excited states.7 Three OAryl substituents were introduced on the perylene ring to tune its reduction potential, since the unsubstituted PI was a too strong electron acceptor for our application.7a Furthermore, the OAryl substituents could potentially limit the aggregation of dyes, which is known to be an unfavorable factor in DSSCs.8 Naphtalenediimide (NDI) dyes have suitable redox properties to be used as secondary electron acceptors. Also, its absorption spectrum lies in the UV region, far away from that of the PI, and its radical spectrum is distinct.7 These properties enable selective excitation of the PI unit and the detection of electron-transfer products. We report here on the first molecular dyad sensitizer especially designed for the optimization of DSSCs based on a photoactive cathode. The use of a secondary electron acceptor removed from the SC surface results in a significantly longer lived charge separated state and a threefold improved absorbed photon to current conversion efficiency in a DSSC device.

10.1021/jp077446n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008

1722 J. Phys. Chem. C, Vol. 112, No. 5, 2008 Experimental Section General Methods. 1H and 13C NMR spectra were recorded on an ARX 300 MHz Brucker spectrometer. Chemical shifts for 1H NMR spectra are referenced relative to residual protium in the deuterated solvent (CDCl3 δ ) 7.26 ppm). Mass spectra were recorded on a EI-MS HP 5989A spectrometer or on a JMS700 (JEOL LTD, Akishima, Tokyo, Japan) double focusing mass spectrometer of reversed geometry equipped with electrospray ionization (ESI) source. MALDI-TOF analyses were performed on an Applied Biosystems Voyager DE-STR spectrometer in positive linear mode at 20 kV acceleration voltage with dithranol as matrix. Thin-layer chromatography (TLC) was performed on aluminum sheets precoated with Merck 5735 Kieselgel 60F254. Column chromatography was carried out either with Merck 5735 Kieselgel 60F (0.040-0.063 mm mesh) or with SDS neutral alumina (0.05-0.2 mm mesh). Air-sensitive reactions were carried out under argon in dry solvents and glassware. Chemicals were purchased from Aldrich and used as received. Compounds N-(2,5-di-tert-butylphenoxy)-perylene-1,6-di(4-tert-butylphenoxy)9-bromo-3,4-dicarboximide 39a and perylene-1,6-di(4-tert-butylphenoxy)-9-bromo-3,4-dicarboxylic acid anhydride 49b were prepared according to literature methods. The electrochemical measurements were performed with a potentiostat-galvanostat MacLab model ML160 controlled by resident software (Echem v1.5.2 for Windows) using a conventional single-compartment three-electrode cell. The working electrode was a Pt disk of 3 mm2, the auxiliary was a Pt wire and the reference electrode was the saturated potassium chloride calomel electrode (SCE). The supporting electrolyte was 0.15 N Bu4NPF6 in dichloromethane, and the solutions were purged with argon before the measurements. All potentials are quoted relative to SCE. In all of the experiments, the scan rate was 100 mV/s for cyclic voltammetry and 15 Hz for pulsed voltammetry. UV-visible absorption spectra were recorded on a UV2401PC Shimadzu or HP8453 diod array spectrophotometer. As the apparent absorption of the NiO film in electrolyte was approximately constant and only approximately 0.1 in the visible region, about 80% of the light was transmitted. The NiO had some color, which means that the remaining 20% represented both scattering and absorption. Thus, the amount of light reaching the detector after multiple scattering can be neglected in calculations of the light-harvesting efficiency. Also, the visible apparent absorption of the dye was low (0.04) so that current generation by scattered light should give a minor effect on the recorded IPCE values. Thus, we estimate that the error in conversion of IPCE to APCE values is less than 10%. Spectroelectrochemical spectra were recorded with the system described above with a quartz UV-visible cell (0.5 mm) surmounted with an electrochemical cell. The working electrode was a platine roast and the other conditions are the same as those for electrochemistry. Fluorescence spectra were recorded on a SPEX Fluoromax fluorimeter and were corrected for the wavelength-dependent response of the detector system. Nanostructured NiO films were prepared on conducting glass (for the solar cell study) and microscope glass (Menzel glass; for the time-resolved studies of the photoinduced dynamics) according to the procedure described in ref 10. The films were 0.8-1.3 µm thick. Dye-sensitization of the NiO films was carried out by soaking the film in a dye solution in CH2Cl2 or in 50:50 (v/v) CH2Cl2/N-methyl-2-pyrrolidinone) overnight. The pyrrolidone was added with the intension of changing the polarity of the dye bath and breaking possible aggregates

Morandeira et al. stabilized by hydrogen bonds. The differences were small, but less dye was adsorbed on the NiO surface while the photoresponse (APCE) was somewhat better compared with sensitization with neat CH2Cl2. The sensitized films were then rinsed with solvent and dried at room temperature. Electrolytes were made using 0.5 M NaI (Aldrich), 0.05 M I2 (Merck) and propylene carbonate (Aldrich). For preparation of films that are in contact with electrolyte, a drop of electrolyte was put on the film surface. The film was then covered with a thin glass slide that made the electrolyte drop spread over the surface. From the difference in optical density with and without electrolyte at 362 nm (absorption maximum for I3-) and assuming that I3- is formed quantitatively from I2 and I- (which is present in large excess), the electrolyte layer thickness was estimated to be about 20 µm. Femtosecond transient absorption experiments were performed on a laser system, which consisted of a 1 kHz regenerative amplifier (Legend HE, Coherent) pumped by a Q-switched frequency doubled Nd:YLF laser (Evolution, Coherent) and seeded by a mode-locked Ti:Sapphire oscillator (Vitesse, Coherent). After compression, the fundamental output (800 nm) was split into a pump and a probe beam (70/30). The green pump light at 540 nm, with an energy of 0.85 µJ/pulse, was obtained by sum frequency generation from an optical parameter amplifier (TOPAS, Light Conversion Ltd). Then, the 1 kHz pump passed a chopper in which every other pulse was blocked before it was focused in the sample. The sample was mounted on a holder which moved up and down with a frequency of about 1 Hz. The probe beam was led through an optical delay line and focused on a moving CaF2 plate where white light (WL) continuum was generated. A beam splitter was used to produce a WL reference beam. The probe and reference beams were focused through the slit of a monochromator and detected by two 512 pixel diode arrays of a detector system constructed by Dr. Torbjorn Pascher, Lund. The diode arrays were read once every laser pulse, and hence the transient absorption was calculated for two directly following pulses. All measurements were carried out at the magic angle. The transient absorption spectra presented here are the average of 5-15 scans with 500-1500 shots at each time step, depending on the quality of the signal. The absorbance of the liquid samples was about 0.2, and that of the sensitized films was about 0.1 at the excitation wavelength. By convoluting the signal with a Gaussian pulse, the instrument response function, measured as the full width at half-maximum (fwhm) of the Gaussian, was obtained. On film samples, the fwhm was estimated to be about 170 fs at 360 nm, 150 fs at 450 nm, and 120 at 600 nm. Measurements on liquid samples (1 mm quartz cell) gave approximately 20% larger fwhm values. 9-(para-Oxybenzoic acid)-O-perylene-N-(2,5-di-tert-butylphenoxy)-1,6-di(4-tert-butylphenoxy)-3,4-dicarboximide (1).

N-(2,5-di-tert-butylphenoxy)-perylene-1,6-di(4-tert-butylphenoxy)-9-bromo-3,4-dicarboximide 3 (46 mg, 52 µmol), 4-hy-

Nanoporous p-Type NiO Electrode droxybenzoic acid (72 mg, 0.52 mmol), and K2CO3 (216 mg, 1.55 mmol) were dissolved in dry dimethylformamide (DMF, 6 mL). The solution was stirred at 110 °C for 20 h and at 140 °C for 20 h. Then, CH2Cl2 was added, and the organic phase was washed with water, dried over MgSO4, and evaporated to dryness. The product was purified by column chromatography over silica gel (CH2Cl2/MeOH: 100/0 f 90/10), to yield a purple solid (16 mg, 33%). 1H NMR δ (300 MHz, CDCl3, 25 °C): 1.26 (s, 9H, tBu), 1.28 (s, 9H, tBu), 1.32 (s, 9H, tBu), 1.34 (s, 9H, tBu), 6.95 (d, 3J ) 2.1 Hz, 1H, H aro′), 7.04-7.16 (m, 8H, 4H phenyl + 4H aro), 7.39-7.43 (m, 5H, 4H phenyl + 1H aro′), 7.54 (d, 3J ) 8.7 Hz, 1H, H aro′), 7.62 (t, 3J ) 8.1 Hz, 1H, H pery), 8.10 (d, 3J ) 8.7 Hz, 1H, H pery), 8.25 (d, 3J ) 8.1 Hz, 1H, H pery), 8.30 (s, 1H, H pery), 8.31 (s, 1H, H pery), 9.34 (d, 3J ) 8.7 Hz, 1H, H pery), 9.45 (d, 3J ) 8.1 Hz, 1H, H pery). UV-vis (CH2Cl2): λ/ (nm/x104 M-1 cm-1); 413 (0.72), 487 (2.12), 518 (3.20). HR-MS (ESI) m/z: [M + Na]+ calcd for C63H59NO7, 964.4189; found, 964.4182. N-Amino-perylene-1,6-di(4-tert-butylphenoxy)-9-bromo3,4-dicarboximide (5).

Perylene-1,6-di(4-tert-butylphenoxy)-9-bromo-3,4-dicarboxylic acid anhydride 4 (304 mg, 0.43 mmol) was dissolved in pyridine (12 mL), and the solution was heated at 130 °C under argon for 10 min. Hydrazine monohydrate (0.41 mL, 8.45 mmol) was added, and the reaction mixture was stirred at 130 °C under argon for 2h. Pyridine was removed under reduced pressure; toluene was added, and the mixture was rotary evaporated to remove all traces of pyridine and hydrazine monohydrate. The reaction mixture was purified by column chromatography over silica gel (CHCl3) to yield 5 as a red solid (97 mg, 31%). 1H NMR δ (300 MHz, CDCl3, 25 °C): 1.35 (s, 18H, H tBu), 5.49 (br, 2H, NH2), 7.03 (m, 4H, H phenyl), 7.42 (m, 4H, H phenyl), 7.67 (t, 3J ) 8.1 Hz, 1H, H pery), 7.85 (d, 3J ) 8.4 Hz, 1H, H pery), 8.26 (2s, 2H, H pery), 8.34 (d, 3J ) 8.1 Hz, 1H, H pery), 9.12 (d, 3J ) 8.4 Hz, 1H, H pery), 9.36 (d, 3J ) 8.1 Hz, 1H, H pery). MALDI-TOF m/z: [M]+ calcd for C42H35BrN2O4, 710.18; found, 710.18. N-Octylnaphtalenetetracarboxylic-1,8-anhydride-4,5acid Monoimide (6).9c

1,4:5,8-Naphthalenetetracarboxylic dianhydride (4 g, 15.1 mmol) was dissolved in dry dimethylformamide (40 mL) and the solution was refluxed. Then, n-octylamine (0.62 mL, 3.73 mmol) in dry dimethylformamide (5 mL) was added dropwise, and the solution was further heated for 15 h. The reaction mixture was cooled to 0 °C and the resulting precipitate was filtered. The filtrate was dissolved in dichloromethane and washed with water. The organic layer was dried over MgSO4

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1723 and rotary evaporated to leave a salmon solid (1.39 g, quantitative).1H NMR (300 MHz, CDCl3): δ ) 8.82 (s, 4H), 4.20 (m, 2H), 1.76 (m, 2H), 1.38 (m, 2H), 1.27 (m, 8H), 0.87 (m, 3H). 13C NMR (75 MHz, CDCl3): δ ) 162.19, 158.83, 133.14, 131.19, 128.86, 127.94, 126.84, 122.80, 41.21, 31.77, 29.23, 29.15, 28.01, 27.04, 22.61, 14.05. EI-MS: [M+] 379.30 (73%), 268.20 (100%). Compound 7.

N-Amino-perylene-1,6-di(4-tert-butylphenoxy)-9-bromo-3,4dicarboximide 5 (50 mg, 70 µmol) and N-octyl-naphthalene1,2-dicarboximide-5,6-dicarboxylic acid anhydride 6 (47 mg, 0.13 mmol) were dissolved in dry pyridine (6 mL). The solution was stirred at 105 °C for 20 h. The pyridine was then removed under reduced pressure, and the reaction mixture was purified by column chromatography over silica gel (petroleum ether/ CH2Cl2, 2/8) to yield 7 as a red solid (54 mg, 72%). 1H NMR δ (300 MHz, CDCl3, 25 °C): 0.88 (m, 3H, CH3), 1.29-1.46 (m, 10H, CH2), 1.33 (s, 18H, tBu), 1.73-1.78 (m, 2H, CH2), 4.17-4.22 (m, 2H, CH2), 7.05 (d, 3J ) 8.7 Hz, 2H, H phenyl), 7.07 (d, 3J ) 8.7 Hz, 2H, H phenyl), 7.42 (d, 3J ) 8.7 Hz, 4H, H phenyl), 7.69 (t, 3J ) 8.1 Hz, 1H, H pery), 7.87 (d, 3J ) 8.7 Hz, 1H, H pery), 8.28 (s, 1H, H pery), 8.31 (s, 1H, H pery), 8.35 (d, 3J ) 8.1 Hz, 1H, H pery), 8.78 (d, 3J ) 7.8 Hz, 2H, H napht), 8.81 (d, 3J ) 7.8 Hz, 2H, H napht), 9.15 (d, 3J ) 8.7 Hz, 2H, H pery), 9.38 (d, 3J ) 8.1 Hz, 2H, H pery). MALDITOF m/z: [M+] calcd for C64H54BrN3O8, 1073.3; found, 1071.3. Compound 8.

The compound 1 (54 mg, 50.4 µmol), benzyl 4-hydroxybenzoate (23 mg, 0.10 mmol), dry Cs2CO3 (33 mg, 0.10 mmol), Pd2dba3‚CHCl3 (11 mg, 10 µmol), and bis(2-diphenylphosphinophenyl)ether (27 mg, 50.4 µmol) were dissolved in dry toluene (6 mL). The solution underwent three “freeze-pump-thaw” cycles and was heated at 100 °C under argon for 22 h. The reaction mixture was diluted with CH2Cl2, and the organic phase was washed with water, dried over MgSO4, and evaporated to dryness. The crude mixture was purified by column chromatography over silica gel (petroleum ether/CH2Cl2, 4/6 f 2/8). The product was then purified by size exclusion column chromatography over Sephadex LH20 (CH2Cl2) and finally by a preparative thin layer chromatography (petroleum ether/CH2Cl2, 2/8), to yield the dyad 8 as a purple solid (24 mg, 39%). 1H NMR δ (300 MHz, CDCl , 25 °C): 0.95 (m, 3H, CH ), 3 3

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Morandeira et al.

SCHEME 1: Synthetic Route for the Preparation of the Dye 1 and the Dyad 2a

a Reagents and Conditions: (a) K2CO3, DMF, 140 °C, 20 h (33%); (b) tertBuOH, reflux, then AcOH (85%); (c) NH2-NH2, H2O, pyridine reflux (31%); (d) pyridine, 105 °C (72%); (e) HO-PhCO2Bn, Cs2CO3, Pd2dba3-CHCl3, toluene, 100 °C (39%); (f) H2 (8 bar), Pd/C, CH2Cl2/MeOH (99%).

1.29-1.48 (m, 10H, CH2), 1.33 (s, 18H, tBu), 1.74-1.78 (m, 2H, CH2), 4.18-4.23 (m, 2H, CH2), 5.37 (s, 2H, CH2-aro′′), 7.03-7.17 (m, 8H, 4H phenyl + 4H aro), 7.33-7.45 (m, 9H, 4H phenyl + 5H aro′′), 7.66 (t, 3J ) 8.1 Hz, 1H, H pery), 8.11 (d, 3J ) 8.7 Hz, 1H, H pery), 8.31 (s, 1H, H pery), 8.33 (s, 1H, H pery), 8.34 (d, 3J ) 8.1 Hz, 1H, H pery), 8.78 (s, 4H, H napht), 9.37 (d, 3J ) 8.7 Hz, 2H, H pery), 9.49 (d, 3J ) 8.1 Hz, 2H, H pery). MALDI-TOF m/z: [M+] calcd for C78H65N3O11, 1219.5; found, 1219.9. Dyad 2.

The compound 8 (10 mg, 8.2 µmol) and palladium on charcoal (6 mg, 10% of Pd) were dissolved in a mixture of dichloromethane (1.5 mL) and ethanol (0.7 mL). The solution was carefully degassed at room temperature with argon and then with hydrogen. This mixture was stirred at room temperature under H2 (P ) 8 bar) for 50 h. The crude mixture was filtered over a pad of celite and the organic phase was concentrated to dryness. The product was purified by column chromatography over silica gel (AcOEt/CH2Cl2, 4/6 f MeOH/CH2Cl2, 2/98) to yield the expected dyad 2 as a red solid (9 mg, 99%). 1H NMR δ (300 MHz, CDCl3, 25 °C): 0.95 (m, 3H, CH3), 1.29-1.48 (m, 10H, CH2), 1.33 (s, 18H, tBu), 1.73-1.80 (m, 2H, CH2), 4.18-4.24 (m, 2H, CH2), 7.07 (m, 4H, H phenyl), 7.15 (m, 4H, H aro), 7.33-7.45 (m, 9H, 4H phenyl + 5H aro′′), 7.67 (t,

) 8.1 Hz, 1H, H pery), 8.09 (d, 3J ) 8.7 Hz, 1H, H pery), 8.32 (s, 1H, H pery), 8.34 (s, 1H, H pery), 8.35 (d, 3J ) 8.1 Hz, 1H, H pery), 8.81 (s, 4H, H napht), 9.38 (d, 3J ) 8.7 Hz, 2H, H pery), 9.50 (d, 3J ) 8.1 Hz, 2H, H pery). UV-vis (CH2Cl2): λ/ (nm/x104 M-1 cm-1): 358 (2.7), 379 (3.2), 419 (0.98), 530 (2.9). MALDI-TOF m/z: [M+] calcd for C71H59N3O11, 1130.2; found, 1130.8.

3J

Results and Discussion Synthesis. The preparation of dye 1 and the sensitizer-electron acceptor dyad 2 is depicted in Scheme 1. The synthesis of the perylenemonoimide sensitizer 1 is based on the O-arylation of perylenemonobromide 39 with 4-hydroxy benzoic acid. The preparation of the dyad 2 starts with hydrolysis of the imide group of perylenemonobromide 39a which was then reacted with hydrazine to give perylene 5. The free amino group of 5 was then reacted with N-octylnaphthalene tetracarboxylic anhydride monoimide 6 to give the compound 7. The introduction of the third O-aryl group proved to be more difficult than expected since the Ullmann reaction and various other catalytic systems recently published for this type of transformation failed.11 The utilization of the catalytic system bis(2-diphenylphosphinophenyl)ether (DPEPhos) with palladium (0) allowed us to couple the esterified alcohol (benzyl-4-hydroxybenzoate) to the perylene core of 7.12 The last step of the synthesis involves the hydrogenolysis of the benzyl ester group of 8. Spectral and Electrochemical Properties. The absorption spectra of 1 and 2 in CH2Cl2 (Figure 1) show the strong visible band of the PI unit, with maxima at 515 and 525 nm, respectively (max ) 3.2 × 104 M-1 cm-1). The NDI unit of 2 on the other hand absorbs only in the UV region. The spectrum of 2 is very close to the sum of the spectra for 1 and the NDI reference, showing that the electronic interaction between PI and NDI units in the ground state is weak and that visible excitation of 2 will be localized on the PI unit.

Nanoporous p-Type NiO Electrode

Figure 1. Absorption spectrum of 1 (dashed line) and 2 (solid bold line) along with that of the reference compound N,N′-bis(octyl)-1,8: 4,5-naphthalenetetracarboxydimide (solid thin line), recorded in CH2Cl2.

Figure 2. Molecular structure and frontier orbitals of the PI dye 1 (left) and the dyad 2 (right); see text.

The fluorescence of the dye 1 is strong, with a maximum at 592 nm, while that of 2 is very weak, because of quenching by the NDI unit (see below). The energy of the lowest singlet excited state was estimated from the average of the absorption and fluorescence maxima as E00 ) 2.24 eV for 1. As the emission from 2 was very weak, the 0-0 energy was estimated to shift by the same amount as the absorption maximum, relative to that in 1, resulting in a value of E00 ) 2.20 eV. Information on the molecular and electronic structure was obtained from DFT calculations perfomed with Gaussian 0313 using the 1996 gradient-corrected correlation functional of Perdew, Burke, and Ernzerhof (PBE) and the 6-31G(d) basis set.14 Figure 2 represents the optimized geometrical structure and frontier molecular orbitals. The NDI and PI units in 2 orient with their planes perpendicular to one another, certainly to limit

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1725

Figure 3. Spectral changes recorded upon electrolytic reduction of the peryleneimide 1 in dichloromethane with 0.15 M of Bu4NPF6. The arrows indicate the changes observed from the initial spectrum.

the steric hindrance and the electrostatic repulsions of the oxygens of the two nearest bisimide groups of each unit. The HOMO of both compounds is distributed evenly over the aromatic core of the PI unit. Also the LUMO of 1 is located evenly over the PI unit, suggesting negligible charge-transfer character of the lowest excited state. In contrast, the LUMO of the dyad 2 is located entirely on the NDI unit. As usually observed for this type of compound, there are HOMO and LUMO nodes on the nitrogen of the bisimide groups for both PI and NDI.15 Together with the perpendicular orientation of the PI and NDI planes, this imparts a weak electronic coupling between the two units, which explains the localized nature of the frontier orbitals. This results in a dyad whose ground state properties are essentially the sum of those of each unit. The redox potentials of interest in 1 and 2 were determined by cyclic voltammetry in dichloromethane (Figure S1). The PI unit gave a reversible reduction process at -1.05 and -0.90 V versus SCE in 1 and 2, respectively, whereas its oxidation took place respectively at 1.12 and 1.18 V. The NDI unit in 2 is reduced at a more positive potential (-0.51 V), which makes it a suitable secondary electron acceptor with respect to the PI sensitizer. The spectrum of the radical anion of 1 was investigated by spectroelectrochemistry since there is little data for trisubstituted PI. The spectrum of the neutral PI and its electrochemically generated species are given in Figure 3. Upon reduction, the intensity of the main visible absorption band decreases to leave a new red-shifted transition located at 606 nm. The stability of the radical anion is demonstrated by the reversible recovery of the absorption spectrum of the initial neutral dye by reversing the potential of the electrode. Furthemore, the electrochemical transformation gave rise to isosbestic points indicating the direct interconversion of one species in another and therefore the absence of any coupled chemistry. Compared with the unsubstituted PI published previously,7a the radical anion of 1 exhibits a characteristic absorption band at a significantly lower energy. These changes most probably reflect the higher energy of the HOMO orbital in 1•- owing to the presence of O-aryl substituents. The electrochemical and spectroscopic data were used to build an energy diagram showing the pertinent excited states of dyad 2 with respect to the valence band of NiO (Scheme 2). The standard Weller approach16 was used to calculate the free energy relative to the ground state. From the electrochemical data (E0) and the 0-0 energy (E00) of the lowest excited state, the excitedstate potential was calculated as E0(*PI/PI-) ) E0(PI/PI-) +

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SCHEME 2: Free Energy Diagram of the Light-Induced Dynamics of Sensitized NiO and Structures of the Molecular Photosensitizers Employeda

Figure 4. Transient absorption spectra of 1 on NiO films with electrolyte after excitation at 540 nm with a 120 fs laser pulse: ∆t ) 0 ps (solid line),15 ps (dashed) and 2500 ps (dotted). The initially formed PI* state converts in a few ps to PI•- by electron transfer from the NiO. Inset: Transient absorption trace at 578 nm, which is near an isosbestic point for the ground and excited states, showing the formation and decay of the PI•- anion. Note that the time axis is linear from 0 to 2 ps and then logarithmic.

a

See text for more details.

E00/e and E0(PI+/*PI) ) E0(PI+/PI) - E00/e, where e is the elementary charge. The potential for the NiO valence band was obtained from our previous studies.10 The reaction free energy was estimated, neglecting Coulombic work terms, as ∆G0 ) e(E0donor - E0acceptor). Femtosecond Spectroscopy and Photoelectrochemistry. Nanoporous NiO electrodes on ITO were sensitized by soaking in a dye bath (0.1 mM in CH2Cl2 or in 50:50 (v/v) CH2Cl2/Nmethyl-2-pyrrolidinone) overnight. The absorption spectrum of the sensitized films shows the visible band of the PI unit at 525 nm (Figure 5, inset). The modest absorption shows that the amount of bound dye is lower than typical for this type of system.1 Femtosecond transient absorption spectroscopy was performed on the sensitized films with a drop of 0.5 M NaI/0.05 M I2 electrolyte covered with a thin glass plate. This electrolyte is standard in n-type DSSCs1 and not optimized for the present system. In this study, it served as a model electrolyte for interception of the recombination process. Excitation at 540 nm (120 fs, 0.85 µJ/pulse) of 1 generates a transient absorption spectrum characteristic for the excited dye 1 (Figure 4), with a maximum at 660 nm and a bleach around 525 nm. This is very similar to the transient spectra for 1 in CH2Cl2 (Figure S2). On NiO, the spectrum converts on a time scale of a few picoseconds into a spectrum with a maximum around 605 nm, attributable to the PI•- anion of 17a; see Figure 4. A fit of the trace at 578 nm, where the initial amplitude of the signal is zero, to a sum of exponentials shows that the anion is formed with an average time constant of 0.5 ps and decays with an average time constant of 170 ps, while a fraction (30%) remains at the end of the experimental time window (∆t ) 2.5 ns). Thus, ultrafast interfacial electron transfer occurs from the NiO film to the excited state of the attached dye 1, mainly on the subpicosecond time scale (eq 1). Note that the extinction coefficient for oxidized NiO is small and is not expected to give a distinguishable spectral signature.10 This undoubtedly shows that the main deactivation process of the excited PI is a hole photoinjection into the valence band of NiO (eq 1). The distinct spectral

Figure 5. Incident photon-to-current conversion efficiency as a function of wavelength for a DSSC with a 800 nm thick, nanoporous NiO film as photocathode, sensitized with 1 (dashed), 2 (solid), or unsensitized bare NiO electrode (dotted). Inset: the visible absorption spectrum of the film sensitized with 2 after substraction of the NiO contribution due to scattering.

features of the different PI states is a clear advantage compared to our previous studies, with porphyrin on NiO in particular,3b but also compared with coumarin.3a The ultrafast interfacial electron transfer is followed by charge recombination on time scales of 100 ps to >10 ns (eq 2). This is very similar to our previous results with porphyrin or coumarin on NiO3 but is very different from n-type TiO2/dye systems where recombination occurs mainly on time scales of nano- to milliseconds, depending on the conditions.1 The rapid recombination leads to current losses in DSSCs, as was shown in the case of porphyrin on NiO.3b Experiments with 1 in the presence and absence of electrolyte gave very similar transient kinetics, which is a first indication that charge recombination is much faster than dye regeneration by the electrolyte.

1*/NiO f 1•-/NiO(+)

(1)

1•-/NiO(+) f 1/NiO

(2)

DSSCs were prepared with a platinized conducting glass as counter electrode and 0.5 M NaI/0.3 M I2 in 3-metoxypropi-

Nanoporous p-Type NiO Electrode

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1727

Figure 6. Transient absorption spectra of 2 in CH2Cl2 solution after excitation at 540 nm with a 120 fs laser pulse: ∆t ) 0-40 ps.

onitrile as electrolyte. Figure 5 shows the incident photo-tocurrent conversion efficiency (IPCE) as a function of wavelength for the DSSCs. The current is cathodic, consistent with electron transfer to the excited dye and subsequent dye regeneration by the electrolyte. For 1 on NiO, the effect of dye sensitization is clear in the visible, although the yield of electrons collected as current is low. An estimate correcting for the incomplete absorption of photons gives an absorbed photon-to-current conversion efficiency (APCE) value of 15% at the visible maximum (see below). On the basis of the transient absorption results above, we conclude that the rapid charge recombination is the main reason for the low APCE value observed, as was the case also for a porphyrin-NiO system.3b For the dyad 2, the results are different. After selective excitation of the PI unit of 2 in homogeneous solution, the local PI* excited state converts to the charge separated NDI•--PI•+ state in a few picoseconds (Figure 6). The NDI•- bands at 475 nm ( ) 26000 M-1 cm-1) and 605 nm and (7200 M-1 cm-1)7a are clearly seen, as is the 570 nm band attributed to PI•+. On a longer time scale, this charge separated state recombines to reform the ground state reactants. A fit to single-wavelength traces gave time constants of 2 ps for the forward electron transfer and 20 ps for the subsequent recombination. For 2 on NiO films, the initial spectrum after excitation of the PI unit is very similar to those for 1 on NiO and for 2 in a homogeneous CH2Cl2 solution, again attributable to the initially excited PI* unit. After 3 ps, the features of the reduced NDI unit have grown in, with peaks at 475 and 605 nm, while much of the PI* state has disappeared. After 2.5 ns, no features of PI* remain while most of the NDI•- remains. A fit to the kinetic trace at 466 nm (Figure 7b) shows a rise with τ ) 1 ps, followed by a biphasic decay with a fast component (τ ) 55 ps) and a long-lived plateau (τ > 50 ns; 50% of the initial signal). Note however that also the PI* unit of 2 absorbs around 460 nm (Figure 6), more than does the PI* state in 1, which accounts for much of the 466 nm decay on the 100 ps time scale. This is consistent with the simultaneous decay of the PI* features at 565 and 720 nm. Thus, we found that most of the NDI•- initially formed remains on the ns time scale, in gratifying contrast to the rapid decay observed with 1 and in our previous studies with dye-sensitized NiO. We note that the multi-phasic decay of the PI* state is typical for interfacial electron transfer in dyesensitized semiconductor films.1,3,17 As neither a PI•- nor a PI•+ intermediate is seen, it is not clear if NDI•- is formed via initial interfacial electron transfer from NiO to PI* (solid arrows in Scheme 2), as for 1/NiO (τ ≈ 0.5 ps), or via initial intramolecular charge separation generating a NDI•--PI•+ state (dotted arrows), as seen for 2 in CH2Cl2 (τ ) 2 ps). Obviously, the time constants for these

Figure 7. (a) Transient absorption spectra of 2 on NiO films with redox active electrolyte after excitation at 540 nm with a 120 fs laser pulse; (b) traces corresponding to panel (a) at 466 nm (solid line), 565 nm (dashed), 610 nm (dotted), and 720 nm (dotted-dashed).

processes in the reference systems are very similar, and they are furthermore likely to be affected by the electric field of the semiconductor/electrolyte interface; therfore, we cannot exclude a mixture of these pathways based on kinetic comparisons. In any case, as no intermediate is seen, the subsequent charge shift to generate the fully charge separated state NDI•-PI-NiO(+) is even faster. Thus, the important result is that NDI•- is formed very rapidly and that recombination with the NiO(+) is slowed down substantially compared with the case of 1 on NiO. The longer charge separation lifetime also resulted in higher IPCE values in DSSCs made with 2 (Figure 5). At the visible maximum, the IPCE is 4.0% for 2, whereas it was 1.3% for 1. The value is still low, however, but this is mainly because of the poor dye loading, leading to incomplete light absorption. The current quantum yield or absorbed photon-to-current conversion efficiency (APCE) is a better measure of the competing processes and intrinsic light conversion efficiency. The absorption 0.04 at 525 nm (Figure 7, inset) for the dyad 2 gives a light harvesting efficiency (LHE) of only 1 × 10-A ) 1 × 10-0.04 ) 0.088. The absorbed photon-to-current efficiency is defined as IPCE/LHE. Thus for 2, with 4.0% IPCE maximum, the APCE is 45%, while corresponding calculations for the PI dye 1 gives an APCE of 15%. The visible apparent absorption (scattering and absorption) by unsensitized NiO in electrolyte was only 0.1, and the total apparent absorbance was low (less than 0.2), which means that effects of scattering can to a good approximation be neglected (see Experimental Section). We thus estimate the error in converting values of IPCE to APCE is less than 10%. In these measurements, the NiO film thickness was 0.8 µm. For comparison, experiments were made with 1.3 µm films, with a higher internal surface area and thus higher dye loading. The maximum IPCE at 525 nm was somewhat higher with the thicker film: 4% with 1 and 7% with 2. The APCE is not

1728 J. Phys. Chem. C, Vol. 112, No. 5, 2008 improved, however, since more photons were absorbed (e.g., APCE ) 44% with 2). In summary, in a DSSC made of the dyad 2 on NiO, the quantum efficiency of the device is about three times higher than that of a device made of the dye 1 on NiO (Figure 7) reaching a promising APCE value of approximately 45%. This is the highest value reported for a DSSC based on a nanoporous, p-type semiconductor. Conclusions In this work, we prepared new peryleneimide (PI) sensitizers that were tested in DSSCs based on NiO, a nanoporous wide band gap metal oxide p-type semiconductor. The femtosecond transient absorption spectroscopy study clearly showed, for the first time, that the major deactivation process of the PI excited state consists in hole injection into NiO. As already published with sensitized NiO,3 but contrary to what is typically observed with TiO2 photoelectrodes,1 the charge recombination reaction is very fast and certainly limits to a large extent the electron collection efficiency at the counter electrode and the sensitizer regeneration via the redox mediator. To slow down the charge recombination process, we functionalized the PI sensitizer with a naphthalenediimide unit that acts as an electron acceptor. This makes the charge separated state sufficiently long-lived so that the electron transfer to the electrolyte and hole collection at the photocathode back contact occurs with high efficiency. This strategy shows for the first time with a p-type DSSC that the kinetics of charge recombination can be controlled by appending a secondary electron acceptor to the dye. This corresponds to the approach with electron donor-dye systems on n-type TiO2, although the effect on the already good overall performance in that case is not dramatic.6 In the present case, however, this approach also gives a substantial increase of the absorbed photon-to-current conversion yield, lifting it from poor to promising and pointing toward efficient p-type DSSCs and devices. Acknowledgment. We thank Dr. Amanda Smeigh for comments on the manuscript. This work was supported by The Swedish Foundation for Strategic Research, The K&A Wallenberg Foundation, The Swedish Research Council, the French Ministry of Research (ACI jeune chercheur and ANR “PhotoCumElec”), Re´gion Pays de la Loire (PERLE Project) and COST D35. Supporting Information Available: Cyclic volatammogram of 2, transient absorption spectra of 1 in CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (b) Kalyanasundaram, K.; Gra¨tzel, M. Coord. Chem. ReV. 1998, 177, 347414. (c) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269-277. (d)

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