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J. Phys. Chem. C 2008, 112, 9530–9537
Coumarin 343-NiO Films as Nanostructured Photocathodes in Dye-Sensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I3-/I- Redox Couple and Mechanism of Photocurrent Generation Ana Morandeira,†,§ Gerrit Boschloo,‡,| Anders Hagfeldt,‡,| and Leif Hammarstro¨m*,† Department of Photochemistry and Molecular Science, The Ångström Laboratories, Uppsala UniVersity, Box 523, SE-751 20 Uppsala, Sweden, and Center of Molecular DeVices, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, Teknikringen 30, 100 44 Stockholm, Sweden ReceiVed: January 25, 2008; ReVised Manuscript ReceiVed: March 25, 2008
Nanoporous, p-type NiO films were sensitized with coumarin 343 (C343), and the photoinduced electron transfer dynamics was studied in the presence of different concentrations of electrolyte (I3-/I- in propylene carbonate). Electron transfer from the valence band of NiO to the excited C343 is very fast, occurring on time scales from hundreds of femtoseconds to a few picoseconds, but also the subsequent recombination is quite rapid, on the time scale of tens of picoseconds. Nevertheless, formation of an intermediate, attributed to I2-| NiO(+), was observed on the picosecond time scale. Simultaneously the reduced dye was converted back to the C343 ground state, indicating that recombination could be intercepted by I3- reduction. Consistent with that interpretation, we observed oxidized NiO and depletion of I3- persisting on the millisecond time scale. Complete dye-sensitized solar cells (DSSCs) with these films as photocathode gave up to 10-11% incident photon to current conversion efficiency at the C343 visible absorption maximum, which is the highest value reported for a p-type DSSC. Our results elucidate the main mechanism for photocurrent generation in this p-type DSSC, which is important for the understanding and development of these rarely studied counterpart of conventional n-type “Gra¨tzel cells”. 1. Introduction It has been shown that sensitized nanostructured NiO, a wide band gap p-type semiconductor, can be used as photoactive electrode (photocathode) in dye sensitized solar cells (DSSCs).1–6 This is an interesting and important finding, as it mirrors the function of the typical n-type DSSCs,7,8 and is a first step toward the development of efficient tandem cells where both anode and cathode are photoactive. A common handicap in all the reported NiO-based DSSC, however, is their low incident photon-tocurrent conversion efficiencies (IPCEs). Indeed, the highest IPCE value reported until now is a mere 6%, for a sandwichtype cell with C343-sensitized NiO as photoactive electrode.2 The fast back electron transfer (BET) from the reduced dye to the oxidized NiO observed in bare C343-sensitized NiO (time constants of 2 and 20 ps9) and phosphorus porphyrin-sensitized NiO (nonexponential recombination ranging from a few picoseconds to a few nanoseconds3) seems to indicate that the low probability of the generated holes to escape recombination with the reduced sensitizer is the reason for the poor IPCE values observed. Indeed, it is almost surprising that any photocurrent at all can be produced with such efficient BET, and it cannot be overlooked that, of the three published reports on the photoinduced dynamics of dye-sensitized NiO,1,9,3 two of them9,3 were carried out in the absence of the redox electrolyte used in * To whom correspondence should be addressed. E-mail:
[email protected]. † Uppsala University. ‡ Royal Institute of Technology. § Present address: Centre for Electronic Materials and Devices, Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London SW7 2AY, UK. | Present address: Department of Physical and Analytical Chemistry, Uppsala University Box 259, SE-751 05 Uppsala, Sweden.
the working DSSC. One therefore must be careful when pointing at the recombination of the hole with the reduced dye as the source of the poor performance of the systems. The presence of redox electrolyte will probably affect the valence band edge and, therefore, the number of available donating states in the semiconductor. In fact, the composition of the electrolyte is commonly used to “tune” the performance of common TiO2-based DSSCs and may affect dramatically the electron injection dynamics on dye-sensitized TiO2.10–15 An investigation on the photoinduced dynamics of dye-sensitized NiO in the presence of redox electrolyte should then provide crucial information on the understanding of why the IPCE values of NiO-based DSSC are so low and how to improve them. Such studies may furthermore clarify the mechanism by which the cathodic photocurrent is produced in a complete DSSC. In a recent collaborative effort with Dr. Fabrice Odobel, we studied a peryleneimide dye and a peryleneimide-naphtalenediimide dyad, both linked to NiO films, in the presence of I3-/Ielectrolyte.1 Our transient absorption data showed an ultrafast generation of the reduced dye or dyad, and that the recombination for the dyad was slowed down substantially. This lead to much higher IPCE values than for the single dye, and the data was consistent with a photocurrent generation mechanism via ultrafast interfacial electron transfer and subsequent regeneration of the dye or dyad by the electrolyte. In a former report, we have thoroughly studied the photoinduced dynamics of C343 sensitized NiO.9 A recent report of IPCE measurements of this system with I3-/I- electrolyte, however, suggests that the main mechanism for photocurrent generation is photodissociation of I3- by direct excitation of I3- as well as via energy transfer from coumarin.2 To address the aforementioned questions and investigate the mechanisms
10.1021/jp800760q CCC: $40.75 2008 American Chemical Society Published on Web 05/31/2008
Coumarin 343-NiO Films in DSSCs
Figure 1. (A) Schematic representation of the electron transfer processes thought to occur in a dye-sensitized p-type semiconductor in contact with a redox active electrolyte. First, an electron is transferred from the valence band (VB) of the p-type semiconductor (NiO) to the excited dye (C343). Second, an electron is transferred from the radical anion of the dye to the oxidant species of the redox couple, regenerating the sensitizer. Such a system can be used as active electrode, photocathode, in a DSSC. (B) Some relevant values are: reduction potential of C343 in MeCN, E1/2(C343/C343-) ) -1.2 V vs NHE, 37 energy of the first excited singlet state of C343 in MeCN (calculated from absorption and fluorescence spectra), ES1(C343) ) 2.6 eV, energy of the first triplet state of C343 in MeCN (estimated from the literature,28,29 ET1(C343) ≈ 1.6 eV, valence band (VB) edge of nanostructured NiO measured in water at pH ) 6.8, 16 EVB(NiO) ) 0.4 V vs NHE, and the redox potential of the electrolyte, E(I3-/I-) ) 0.44 V vs NHE.38 Literature redox potentials measured vs different reference electrodes have been converted to NHE to facilitate the comparison.
of photocurrent generation, we now use this material to build a sandwich-type DSSC and study the effect of the redox couple I3-/I- on the photodinduced dynamics of the system and how it relates to the photocurrent generation (Figure 1). It is worth noting that, to the best or our knowledge, this and our previous work with a dyad sensitizer1 are the first studies in which polychromatic transient absorption is used to study the ultrafast dynamics of dye-sensitized mesoporous semiconductors in the presence of redox active electrolytes. 2. Experimental Section 2.1. Samples. Nanostructured NiO films were prepared on conducting glass (for the solar cell study) and microscope glass (Menzel glass; for the time-resolved studies) according to the procedure described in ref 16.16 The films were 1-3 µm thick and gray. The color of the films was attributed to partial oxidation of the semiconductor during the sintering. Dyesensitization of the NiO films was carried out by soaking the film in an ethanol (Kemetyl) solution of coumarin 343 (5 × 10-4 M) overnight. The sensitized films were then rinsed with ethanol and dried at room temperature. After dye loading, the films were of a bright orange color. Electrolytes were made using NaI (Aldrich), I2 (Merck), and propylene carbonate (Aldrich). Those samples where the films are in contact with the electrolyte were prepared in the following
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9531 way. A drop of electrolyte was poured on the film surface. The film was covered with a thin glass cover slide. Because of the cover, the electrolyte drop spread over the surface. The amount of solution was such that the glass cover slide adhered to the film surface by capillarity. 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), it could roughly be estimated that the thickness of the electrolyte layer was about 20 µm. Coumarin 343 was purchased from Aldrich and was used as received for dye sensitization and for femtosecond transient absorbance measurements in solution. All the solvents were of the highest commercially available purity and were used as supplied. 2.2. Measurements. Absorption spectra were recorded on a Hewlett-Packard HP 8453. IPCE was measured in a setup described previously.17 The photoinduced absorption setup has been described elsewhere.18 A blue light-emitting diode (Luxeon Star 1 W, royal blue) was electronically modulated (on/off) to excite the sample. Femtosecond transient absorption measurements were carried out with two different femtosecond laser systems. One of the systems is described in ref 9 and consists of a 1-kHz regenerative amplifier (Quantronix) pumped by a Q-switched frequency doubled Nd:YLF laser (Quantronix) and seeded by a modelocked Ti:sapphire oscillator (Mira, Coherent), the latter pumped by a CW argon-ion laser (Coherent). The other system consists 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). The details of the optical setup and detector system, identical for both laser systems, are described in ref 9. Briefly, 422-nm laser pulses of energy between 0.4-0.8 µJ, obtained by sum frequency generation of the output of an optical parameter amplifier, were used as pump. The probe beam consisted of a white light continuum generated by focusing the 800-nm fundamental output on a moving CaF2 plate. The sample was mounted on a holder which moved up and down with a frequency of about 1 Hz. All measurements were carried out at the magic angle polarization of pump and probe. The spectra 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 about 1 at the excitation wavelength. By convolution of 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 ∼20% larger fwhm values. 3. Results and Discussion To obtain a better understanding on the effect of the redox couple I3-/I- on the dynamics of the sensitized NiO films, we have used different electrolyte mixtures where we have varied the total concentration of ions and/or the relative concentrations of I3- and I- in the propylene carbonate solvent (see Table 1). Hereafter, we will refer to the coumarin sensitized films in the absence of electrolyte as C343|NiO. When in contact with electrolyte, the sensitized films will be named as C343-dil|NiO, C343-el2|NiO, C343-el3|NiO, C343-el4|NiO, or C343-el5|NiO, depending on which electrolyte mixture was used. Finally, we
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TABLE 1: Composition of the Redox Electrolytes and Maximum IPCE Values Obtained for the Corresponding Sandwich-Type DSSCs electrolytea dil el2 el3 el4 el5
[I-]/M
[I2]b/M
IPCE/%
0.05 0.15 0.5 0.5 0.5
0.005 0.015 0.05 0.1 0.25
4.2 5.5 7.7 9.4 10.7
a Propylene carbonate was used as solvent for all the electrolyte mixtures. b In solution, the equilibrium I-+ I2 h I3- is established, and I3- is formed quantitatively.
Figure 3. Photocurrent action spectrum of C343-el5|NiO (solid line) and fraction of photons absorbed as a function of wavelength (1-10-ABS) on C343|NiO (dashed line). Inset: dependence of the IPCE on the concentration of I2 in the electrolyte.
Figure 2. Absorption spectra of nanostructured NiO film (solid line) after sensitization with C343 (C343|NiO; dashed line), just after being put in contact with electrolyte 1 (C343-dil NiO; dashed-dotted line), and one week later, when the solvent of the electrolyte mixture has evaporated (C343-Mat|NiO; dotted line).
have also studied samples where the films were in contact with electrolyte “dil” for a week and the solvent was allowed to evaporate. These “mature” samples will be referred to as C343Mat|NiO. This procedure gave a higher local concentration of electrolyte with only a small increase in optical density. 3.1. Electronic Absorption Spectra. NiO films are slightly gray due to their partial oxidation during the sintering (see Experimental Section). As can be seen in Figure 2, the films present a weak, featureless absorbance in the visible region. After sensitization (C343|NiO), a band centered around 420 nm due to the bound dye appears, but the broad absorbance attributed to the partially oxidized NiO decreases. This decrease is further enhanced when the film is in contact with the redox couple I3-/I- (C343-dil|NiO). The decrease of absorbance can be interpreted as evidence of reduction of the partially oxidized NiO film.16 This interpretation is further supported by the fact that the oxidized form of a very related material, NiOH, is known to rereduce in the presence of alcohols.19 The absorbance in the near UV (around 360 nm) increases due to the presence of I3- . The addition of electrolyte also affects the adsorbed dye band which typically shifts to the red about 5 nm. It is not clear if the shift of the C343 band is due to the formation of an adduct between the coumarin and the electrolyte. In solution, the C343 absorption band does not show a red shift in presence of low concentrations of I3-/I-. In mature samples (C343-Mat|NiO), where the electrolyte solvent has evaporated, the I3- absorbance band at 360 nm is somewhat higher. The same effect is observed in unsensitized NiO films treated in a similar way. The most straightforward explanation is that due to the solvent evaporation; the electrolyte becomes more concentrated, and the formation of I3- is enhanced to restore the equilibrium.
3.2. C343|NiO as Photocathode in DSSCs. Sandwich-type solar cells were assembled using C343 sensitized nanocrystalline NiO as photoactive electrode, platinized conducting glass as counterelectrode and different electrolyte mixtures based on the redox couple NaI/I2 in propylene carbonate (Table 1). I-/I2 (in solution I-/I3-) was chosen as mediator because it was already known to work in other DSSCs based on sensitized nanocrystalline NiO6,5,3,1 and would allow extension to a tandem cell with a n-type dye-sensitized photoanode. No additives were used in the electrolyte. The IPCE as a function of the excitation wavelength was measured for each cell. In contrast to common DSSCs based on sensitized TiO2, the observed photocurrent was cathodic. The photocurrent action spectrum resembled strongly the fraction of photons absorbed as a function of wavelength on C343-sensitized NiO, though slightly shifted to the red (Figure 3). In contrast to ref 2,we did not observe any clear contribution from I3- absorption in the IPCE action spectrum. We suggest that the different solvent employed here, and possibly the different electrolyte cation (Na+ instead of Li+), circumvented this complicating effect.20 It was found that the IPCE values depended strongly on the concentration of iodine (see Table 1 and inset in Figure 3). The highest efficiency, with an IPCE of 10.7% at the maximum, was achieved with the highest concentration of iodine used, 0.25 M (el5). This is, to our knowledge, the highest IPCE published for a DSSC with a photoactive cathode. Since our efforts were mainly directed to obtain a better understanding of the system and no special effort was done to optimize the performance of such a cell, the system seems indeed quite promising. On the basis of the above results and on our former studies on the photoinduced dynamics of sensitized NiO,9,3,1 the following mechanism can be proposed: Upon excitation of the dye, in this case C343
C343|NiO + hν f C343*|NiO
(1)
there is an electron transfer reaction from the semiconductor to the excited bound dye
C343*|NiO f C343•-|NiO(+)
(2)
The reduced dye can then evolve in two ways. It could be reoxidized by the semiconductor, an unwanted path since the photogenerated hole in NiO would be lost,
Coumarin 343-NiO Films in DSSCs
C343•-|NiO(+) f C343|NiO
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9533
(3)
or by the mediator, the desired path for a working solar cell •C343•-|NiO(+) + I3 f C343|NiO(+) + I +I2
(4)
In this latter case, I2•– would be generated, an intermediate which is also found in common, TiO2 -based DSSCs, in the case after oxidation of I- by the Dye+ species.21–24 I2•– can evolve in different ways. It can disproportionate 2I•2 f I +I3
(5)
or be oxidized at the anode I•2 f e +I2
(6)
I2 f I-+I3
(7)
-
and regenerate I3
or it can transfer an electron to the oxidized semiconductor, an unwanted recombination process
I•2 + C343|NiO(+) f I2 + C343|NiO
(8)
I2•-
Since can be detected spectroscopically we used transient absorbance to detect its presence or absence to assess the proposed regeneration mechanism and to measure how fast it occurs. 3.3. Photoinduced Dynamics on the Femtosecond to Nanosecond Time Scales. We carried out pump-probe transient absorbance (TA) measurements on C343-dil|NiO and C343-Mat|NiO. In both samples, the absorbance of I3- at the excitation wavelength was negligible and interference of the well-known photodissociation of I3- 25–27 on the observed signal could be safely discarded. Although the same electrolyte mixture was used to prepare C343-dil|NiO and C343-Mat|NiO, in the case of C343-Mat|NiO the sensitized film was in contact with the electrolyte for a week prior to the measurement and the electrolyte solvent was allowed to evaporate. The straightforward consequence of this treatment is that I3-/I- molecules (and their counterions) were adsorbed onto the nanostructured film and that the local concentration of I3-/I- molecules around the bound dye should be higher than in the case of C343-dil|NiO. For C343-el3|NiO, C343-el4|NiO, and C343-el5|NiO on the other hand, significant photodissociation of I3- was observed due to the high absorbance of I3- at the excitation wavelength in these samples. The strong signal from the excited I3- and its photodissociation products made it impossible to monitor and analyze the photoinduced dynamics of C343 in those conditions. Reference measurements of C343 in solution with and without redox couple were also carried out. No effect on the dynamics of C343 was observed at the studied redox couple concentration, which was however limited to 1.5 × 10 -3 M /1.5 × 10 -4 M for transparency reasons. The best way of analyze and discuss the observed dynamics is to compare them to that of C343 | NiO, alredy reported in a former article.9 Let us first describe and discuss the photoinduced dynamics of C343-dil|NiO (low local concentration of I3-/I-) and then continue with C343-Mat|NiO (high local concentration of I3-/I-). 3.3.1. Low Local I3-/I- Concentration. The transient absorbance spectrum of C343-dil|NiO after excitation and its subsequent evolution resemble very much those measured in C343|NiO in the absence of electrolyte.9 Briefly, if we consider the spectral range from 435 to 755 nm, at short times after excitation there is one positive excited-state absorbance (ESA)
Figure 4. (A-F) Comparison between the transient absorption spectra of C343|NiO (solid line), C343-dil|NiO (dashed line), and C343Mat|NiO (dotted line) at different times after excitation. The ∆A values correspond to the C343|NiO measurements. The C343-dil|NiO and C343-Mat|NiO spectra have been normalized to allow a better comparison.39 In all the measurements the excitation wavelength was 422 nm. See text for more details.
band and one strong negative band due to unrelaxed stimulated emission (SE) and ground-state bleach (GB; Figure 4 A). The SE is quickly quenched, and at 6 ps after excitation the positive band is due to the absorbance of the photoproducts, oxidized NiO, and the radical anion of C343 (Figure 4 C). Finally, as we can see in parts D and E of Figure 4, the photoproducts recombine to the groundstate in tens of picoseconds, and the remaining absorbance at 300 ps after excitation is mainly due to a small part of the C343 population which has undergone intersystem crossing to the triplet state: the absorption spectrum of C343 triplet presents a rather sharp band with λmax around 490 nm, 9 see Figure 6 A. Neither the charge separation nor the charge recombination kinetics are much affected by the presence of electrolyte in C343-dil|NiO, as is clear from the data in Figure 5. In C343|NiO we could characterize each of these partly overlapping processes with a sum of two decay time constants, τ ) 200 fs and τ ) 2 ps for charge separation and τ ) 2 ps and τ ) 20 ps for charge recombination.9 Only the latter process seems kinetically affected by the electrolyte, giving a somewhat faster bleach recovery at 440 nm on the 2-20 ps time scale. This could be due to an increased recombination rate or by a new process that also regenerates the dye (see below). However, when taking a very careful look at the spectra, it is possible to appreciate two quantitatively small, but qualitatively very important, differences between the photoinduced dynamics of C343|NiO and C343-dil|NiO. The first one is that the formation of the C343 triplet is somewhat enhanced in the case of C343-dil|NiO. This is most clearly shown in Figure 4C,
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Figure 5. Comparison between the transient absorption traces of C343|NiO (solid circles), C343-dil|NiO (blank circles), and C343Mat|NiO (crosses) at (A) 495 nm, where the main contributions are from SE and the coumarin triplet absorption, and at (B) 440 nm, where the main contribution is from the GB of the coumarin.39 See text for more details.
where the sharp C343 triplet band can already be spotted at 6 ps, and can be attributed to a more efficient intersystem crossing due to the proximity of I-/I3- . The second difference is that at long times after excitation (Figures 4F and 5B) the C343 bleach has disappeared while the triplet absorption band is still present. The triplet spectrum itself shows a marked net bleach in the GB region (Figure 6 A). This is a sign that a new species that absorbs in the GB region has appeared in the C343-dil|NiO sample. 3.3.2. High Local I3-/I- Concentration. If we now compare the photoinduced dynamics of C343-Mat|NiO with those of C343|NiO and C343-dil|NiO, it is clear that the two effects observed in the case of C343-dil|NiO (enhancement of triplet formation and of GB recovery) are now much more pronounced. The further enhancement of the triplet formation, due to the higher concentration of iodine, can be seen in the strong positive band at 495 nm, attributed to the C343 triplet, which has almost peaked already at 1.4 ps after excitation (Figure 4B). Since the singlet-triplet energy difference for similar coumarins is ca. 1.0 eV,28,29 the C343 triplet and C343•–|NiO(+) states lie close in energy according to our estimate (Figure 1). Therefore, there are two possible pathways leading to the formation of the C343 triplet state, namely, (i) S1 to T1 intersystem crossing and (ii) charge recombination of the C343•–|NiO(+) state. Both paths may be enhanced by the presence of iodine. However, since the C343 triplet is formed very quickly, in just a few picoseconds, and the driving force for path (ii) is going to be almost nil, implying a slow charge recombination process, we can most probably discard this path and attribute the formation of C343 triplet to S1 to T1 intersystem crossing, enhanced by the electrolyte. It is important to note that such a fast intersystem crossing competes with the photoinduced electron transfer from NiO to the excited bound dye. Indeed, simple visual inspection
Figure 6. (A) Flash-photolysis spectrum of C343 in MeCN at 1.5 µs after excitation (from ref 9). (B) Right axis, estimation of the transient absorption spectrum of I2•– (solid line) obtained from the difference between the groundstate absorption spectrum of I3- and the absorption spectrum of I2•- obtained by pulse radiolysis.30 Left axis, transient absorption spectrum of I3- in propylene carbonate at 75 ps after excitation (solid circles). The negative band is due to the GB of I3-, the positive bands are attributed to the absorbance of I2•– formed by the photodissociation of I3-. The experimental and the estimated spectra are remarkable similar. The experimental spectrum has been normalized at 725 nm for the sake of comparison.
of the temporal evolution of the TA spectrum (parts A-F of Figure 4) indicates that a significant part of the excited dye population does not undergo electron transfer but relaxes to the triplet state. From the perspective of a potential use of the system as photocathode, this is an unwanted effect of the presence of I3-/I-. The other important consequence of the higher concentration of I3-/I- is the different dynamics in the GB range, around 440 nm, observed in C343-Mat|NiO when compared to C343|NiO and C343-dil|NiO. While in C343|NiO and C343-dil|NiO GB recovery occurs in the nanosecond time scale, in C343-Mat|NiO the signal goes from negative to positive in just a few picoseconds (Figures 4C and 5B). The observed dynamics cannot be attributed predominantly to a faster GB recovery due to an enhanced charge recombination of C343•–|NiO(+) to the groundstate. This is because a significant part of the initially excited dye is populating the C343 triplet state that in itself shows a strong GB. Therefore, the observed dynamics have to be at least partially attributed to the formation of a new species absorbing in the coumarin GB region. The nature of this new species is discussed in the next section. 3.3.3. Identification of the New Species. We have observed in C343-Mat|NiO and C343-dil|NiO the appearance of a new
Coumarin 343-NiO Films in DSSCs species absorbing in the 435-465 nm region, absent in the case of plain C343|NiO. This new species has to be related to the presence of the I3-/I- redox couple. The question that must now be discussed is if this new band can be attributed to the formation of I2•–, the product of the dye regeneration mechanism proposed in section 3.2. From the literature, we know that the absorption spectrum of I2•– in water has two bands in the UV-vis region with maxima around 375 (ε ) 15600 M-1 cm-1) and 725-750 nm.30,31 Moreover, we know the molar absorptivity of I3- in propylene carbonate at 363 nm, ε ) 24100 M-1 cm-1.32 Consequently, assuming that the absorption spectra of I2•– in water and propylene carbonate are similar, we can make an estimate of the TA spectrum of I2•– . Another way to obtain this spectrum is to measure the photoinduced dynamics of I3in propylene carbonate. When excited, I3- dissociates generating I2•– and I• . Since solvated I• in polar solvents is known to absorb weakly and only in the UV,33,34 the TA spectrum of I3- in the visible region can be safely attributed to the absorbance of I2•– and the GB of I3- . Figure 6 B presents a comparison of both the estimated and experimental spectra. The estimated spectrum presents three bands. The negative band in the near UV is due to GB of the I3-, and the net absorption bands around 415 and 730 nm are due to I2•– . The shape of the experimental spectrum is very similar to the estimated one, but one of the positive bands, the one in the blue region of the spectrum, is significantly shifted to the red, with maximum around 450 instead of 415 nm. This is most probably due to the different solvent environment (water vs propylene carbonate). We will assume that the TA spectrum of I2•– generated by the reoxidation of the bound dye is similar to that generated by photodissociation. Note that NiO(+) gives a featureless absorption in the visible,16 which increases absorption and reduces the net bleach but does not otherwise alter the shape of the expected transient spectrum. From the above we can conclude that the TA spectrum of I2•– shows a positive signal in the 435-465 region, where the new species appears. Its absorbance is comparatively weak and rather featureless, and it does not present any sharp or strong band that can facilitate the identification of the transient species in Figure 4. Observation of the appearance of the 725-750 nm band will be hindered by the fact that the radical anions of 7-aminocoumarins also absorb in this region and with a similar absorption coefficient.35 Thus, the growth of the I2•– absorption in the red will be hidden by the decrease of the radical anion absorption. In the region 435-460 nm we should observe both GB recovery due to the dye regeneration and formation of I2•–|NiO(+). Consequently, in the presence of the redox couple, the bleach should recover faster, and if the formation of I2•– occurs with a high yield, as may be expected with a high concentration of I3-, the signal should become positive. This is, indeed, what we observe when we compare the kinetics and transient absorption magnitudes of C343|NiO, C343-dil|NiO, and C343-Mat|NiO at 440 nm (Figure 5B). Therefore, the experimental data support the dye regeneration mechanism proposed in section 3.2. 3.4. Photoinduced Dynamics on the Millisecond Time Scale. To complete our study on the photoinduced dynamics of C343|NiO in the presence of redox-active electrolyte, we have monitored C343-dil|NiO and C343-el4|NiO on a longer time scale. Nanosecond laser-flash techniques gave immediate photobleaching of the sample under conditions where we could study typical dye|TiO2 samples without significant degradation. Instead, we used photoinduced absorption (PIA) with an intensity-
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9535
Figure 7. Comparison between the imaginary (out of phase) PIA spectra of C343-dil|NiO (solid line), C343|NiO (dashed line), and dil|NiO (dotted line).
modulated blue light-emitting diode excitation and a lock-in amplifier.18 This is a very sensitive method and also allowed us to study the concentrated electrolyte samples. Most of the I2•- formed by photodissociation of I3- disappears in less than 1 ns (geminate recombination) and should not interfere with our measurements. Figure 7 shows the imaginary (out of phase) PIA spectrum of C343-dil|NiO after 440 nm excitation, modulated with a frequency of 9 Hz. A very similar spectrum, but with a higher amplitude, was recorded with C343-el4|NiO. Note that reference experiments with C343|NiO (no redox couple), dil|NiO, and el4|NiO (no sensitizer) showed no signal. The observed species is very long-lived, decaying with a time constant of seconds at 800 nm (measured time constant values from 1 to 10 s). The spectrum of Figure 7 is very broad and rather featureless, in good agreement with that of NiO(+).16 Toward the blue, the spectrum also shows the GB of I3- . The I2•- intermediate present on shorter time scales is expected to decay by disproportionation to I- and I3- (eq 5), thus reforming half of I3-, or by recombination with NiO(+) (eq 8). Although the spectrum peaks at roughly the same wavelength as that of C343 triplet, the presence of this latter species on this time scale can be ruled out. The measurements were carried out at room temperature and in presence of O2 . Moreover, flash-photolysis measurements of C343 in Ar-purged MeCN solution (cf. Figure 6A) provided a triplet lifetime of about 2.5 µs. As significant NiO(+) absorption and I3- bleach signals remain on this long time scale, a substantial fraction of the charge separation must escape recombination and may give photocurrent in the complete DSSC device. These results indicate that, once the sensitizer is regenerated and I2•- has disproportionated, the lifetime of the hole in the semiconductor VB is long, meaning that losses due to recombination of the hole with I - seem not to be significant. 3.5. Mechanism of Photocurrent Generation and Photocurrent Losses. As discussed above, the I2•- intermediate appears in the transient spectra on a picosecond time scale, simultaneous with C343•– decay and C343 ground-state recovery. Thus, it seems clear that it forms by electron transfer from C343•– to I3-, in competition with NiO(+)-C343•– recombination. This shows that photocurrent generation in the present case is indeed photovoltaic, via ultrafast electron transfer from NiO to the excited dye, followed by very rapid dye regeneration by I3- on the sub-nanosecond time scale. The enhanced C343 triplet formation on a picosecond time scale gives independent support for a close proximity of the dye and electrolyte on the NiO
9536 J. Phys. Chem. C, Vol. 112, No. 25, 2008 surface, which allows these reactions to compete with excitedstate decay and NiO(+)-C343•– recombination. The photovoltaic mechanism does not exclude the possibility of photogalvanic currents in other cases, e.g., when the dye is not bound to the NiO and direct reactions with the electrolyte may compete with hole injection.2 Judging from the transient absorption signals and extinction coefficients for the initial excited state and the I2•- intermediate, it is clear that the latter is formed on the picosecond time scale in a sufficient yield to explain the observed photocurrent. In fact, our rough estimate suggests that approximately 45% of the excitations results in formation of I2•- and NiO(+) after 1 ns in C343-mat|NiO. The corresponding estimate for C343dil|NiO is about 5%, closely corresponding tot the IPCE maximum of 4% for this electrolyte concentration. The uncertainty is large, but it is possible that all photocurrent losses occur in the triplet formation from the 1C343 state and the initial C343•–|NiO(+) recombination in competition with electrolyte reduction. This would mean that I2•- recombination with NiO(+) is not significant and that the I2•--NiO(+) pairs once formed give quantitative photocurrent generation. We emphasize that the I2•- yield estimate is very approximate, however, and we cannot exclude significant losses on longer time scales. Photocurrent generation from direct I3- sensitization of NiO was reported in ref 2. In the present study it is clear from the comparison of the photocurrent action spectrum with the C343 and I3- absorption spectra that there is no detectable contribution from direct I3- sensitization. As noted above, the present study used a different solvent and electrolyte cation, which may possibly explain the different results.20 Finally, there is a possibility that additional photocurrent is generated via the triplet C343. Energy transfer to the electrolyte followed by its dissociation can be excluded as the triplet C343 lies at ca. 1.7 eV (Figure 1), which is well below the lowest excited state of I3-.36 Electron transfer from the triplet C343 can also be excluded on energetic grounds. Electron transfer to the triplet C343 from the I3-/I- couple may be nearly isoenergetic. This is a two-electron potential, however, and it is the less favorable I2•-/I- potential that is kinetically relevant for the single electron reduction of C343.24 We note that the maximum IPCE value of C343-dil|NiO is already 4%, almost half of the highest value measured, although the triplet signal in Figure 4 is about five times smaller than for the sample with the highest electrolyte concentration. Thus, the IPCE value does not scale with initial triplet yield. To conclude, although we cannot exclude some contribution to the photocurrent from reactions via the triplet C343, it seems unlikely to play an important role under the present conditions. Figure 8 summarizes the main processes involved in a working DSSC based on C343sensitized NiO. We can only hypothesize why the addition of Na+, as part of the electrolyte, has no significant effect on the photoinduced dynamics of C343|NiO in contrast with dye-sensitized TiO2 .10 In general, the presence of protons (or small cations) affects the flat band potential of metal oxide semiconductors following the Nernst equation. A typical example is the frequent addition of Li+ to pull down the CB edge of TiO2 and increase, sometimes dramatically, both the electron injection rate and short circuit photocurrent in common DSSCs.12,14 Since the addition of small cations will have a similar effect upon the VB edge (the optical band gap is conserved), one would expect that, in the case of NiO, the addition of Na + will lead to slower hole injection dynamics. We should, however, consider two facts. First, Na+, due to its higher charge-to-radius ratio, is expected
Morandeira et al.
Figure 8. Suggested mechanistic scheme for photocurrent generation in the DSSCs. The scheme illustrates the main reaction pathways, as discussed in the text. All unproductive recombination steps are represented with vertical arrows. The main sources of photocurrent loss are assigned to the first two steps, where the competing reactions are 3 C343 formation and C343-| NiO(+) recombination, respectively, while our data suggests that the extent of I2•--NiO(+) recombination is small. Note that we do not exclude the possibility that some of the 3C343 leads to charge separation.
to have a smaller effect in the energetics than Li+ . Second, in optimized, common DSSCs (TiO2-based) the injecting state typically lies close to the edge of the conduction band to minimize energy losses. Around the semiconductor band edge the density of states is low, and a small shift of the band position can change dramatically the availability of acceptor states and therefore the kinetics of the reaction. In our case, however, the driving force for the photoinduced charge separation of C343|NiO is estimated to be rather large (ca. 1 eV), so that the hole injection may occur deep into the valence band with a high density of electron donating states. It is then reasonable to expect that the effect of a band shift on the hole injection rate will be smaller when the injection is deep into the band, as has been shown for electron injection in n-type semiconductors.14 4. Conclusion The present study provides an insight on the mechanism and kinetics involved in the function of a DSSC based on sensitized nanocrystalline NiO, a p-type semiconductor. Comparison of the photoinduced dynamics of C343|NiO in air with those of C343|NiO in contact with 0.05 M NaI/0.005 M I2 in propylene carbonate show that charge separation (*C343 + NiO f C343•+ + NiO(+)) and the corresponding charge recombination kinetics are affected only to a small extent by the presence of electrolyte. The hole injection is very fast, occurring on time scales from hundreds of femtoseconds to a few picoseconds, but also the recombination is quite rapid, on time scales of tens of picoseconds. Only a small part of the injected NiO(+) holes will escape the back electron transfer, due to the reoxidation of the dye radical anions by the mediator (I3-) which generates in the process I2•-. These surviving holes are, however, very longlived (lifetime of a few seconds), indicating that recombination of the injected holes with the reductor species of the mediator is inefficient. A DSSC based on the system described above gives, in spite of the fast recombination, a maximum IPCE value of 4%. This value is increased to almost 11% by using a more concentrated electrolyte (0.5 M NaI/0.25 M I2), thus increasing the efficiency of the dye radical anion regeneration. This is the highest value published this far for a p-type DSSC. The presence of I3-/I-, however, brings an enhancement of C343 S1 f T1
Coumarin 343-NiO Films in DSSCs intersystem crossing and at high electrolyte concentrations the latter process is fast enough to compete with hole injection. Acknowledgment. We acknowledge Leonard Csenki for preparation of the NiO films and Tomas Edvinsson for assistance with the IPCE measurements. A.M. gratefully acknowledges Staffan Wallin and Håkan Rensmo for inspiring discussions. This work was supported by The Swedish Energy Agency, The K&A Wallenberg Foundation, The Swedish Research Council, and The Swedish Foundation for Strategic Research. Supporting Information Available: Electronic absorption spectra and time-resolved dynamics of C343 in solution in the ·presence of I-/I3 and estimate of the yield of I2 formation. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Morandeira, A.; Fortage, J.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarström, L.; Odobel, F. J. Phys. Chem. C 2008, 112, 1721–1728. (2) Zhu, H.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 17455–17458. (3) Borgström, M.; Blart, E.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A.; Hammarström, L.; Odobel, F. J. Phys. Chem. B 2005, 109, 22928–22934. (4) Nakasa, A.; Usami, H.; Sumikura, S.; Hasegawa, S.; Koyama, T.; Suzuki, E. Chem. Lett. 2005, 34, 500–501. (5) He, J.; Lindström; Hagfeldt, A.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 2000, 62, 265. (6) He, J.; Lindstrm, H.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1999, 103, 8940–8943. (7) Grätzel, M. Inorg. Chem. 2005, 44 (20), 6841–6851. (8) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737. (9) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. J. Phys. Chem. B 2005, 109, 19403–19410. (10) Watson, D. F.; Meyer, G. J. Coord. Chem. ReV. 2004, 248, 1391– 1406. (11) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 3480–3487. (12) Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R. J. Am. Chem. Soc. 2005, 127, 3456–3462. (13) Watson, D. F.; Meyer, G. J. Annu. ReV. Phys. Chem. 2005, 56 (1), 119–156. (14) Anderson, N. A.; Lian, T. Annu. ReV. Phys. Chem. 2005, 56 (1), 491–519. (15) Kroeze, J. E.; Hirata, N.; Koops, S.; Nazeeruddin, M. K.; SchmidtMende, L.; Grätzel, M.; Durrant, J. R. J. Am. Chem. Soc. 2006, 128 (50), 16376–16383.
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