DOI: 10.1002/cphc.201402784
Articles
Ultrafast Charge-Transfer Reactions of Indoline Dyes with Anchoring Alkyl Chains of Varying Length in Mesoporous ZnO Solar Cells Egmont Rohwer,[a] Iulia Minda,[a] Gabriele Tauscher,[a] Christoph Richter,[b] Hidetoshi Miura,[c] Derck Schlettwein,[b] and Heinrich Schwoerer*[a] of the oxidized molecules are dominant spectral features in the transient absorption of the dyes with shorter alkyl chains. A slower picosecond-scale decay proceeds at constant rate for all three derivatives and is assigned to electron transfer into the trap states of ZnO. All assignments are in good agreement with a higher quantum efficiency of charge injection leading to higher short-circuit currents Jsc for dyes with shorter alkyl chains.
Dye-sensitized solar cells based on a mesoporous ZnO substrate were sensitized with the indoline derivatives DN91, DN216 and DN285. The chromophore is the same for each of these dyes. They differ from each other in the length of an alkyl chain, which provides a second anchor to the ZnO surface and prolongs cell lifetime. Ultrafast transient absorption measurements reveal a correlation between the length of the alkyl chain and the fastest electron-injection process. The depopulation of the excited state and the associated emergence
1. Introduction Dye-sensitized solar cells (DSSCs) represent a class of photovoltaic devices that show potential for future commercialization because of their low energy payback times (EPT) and relatively modest material purity requirements when compared to traditional silicon-based solar cells.[1] Despite lower cell efficiencies also when compared with recently reported perovskite solar cells,[2] DSSCs have the benefit of using widely non-toxic and environmentally friendly compounds. A DSSC consists primarily of a wide-bandgap semiconductor, typically a transition metal oxide with a dye attached to the surface that absorbs in the visible or NIR or similarly abundant part of the solar spectrum to provide an efficient light harvesting efficiency of the cell. If there exists sufficient coupling between the photo-excited state of the dye and the conduction band of the semiconductor, the electron can be transferred from the dye to the semiconductor[3] and is harvested at the anode. A redox electrolyte solution or hole conductor completes the cell and transports holes to the cathode and regenerates the neutral state of the dye. Because of the proximity required between dye and semiconductor for coupling of states, only the dye molecules in direct contact with the semiconductor surface transfer electrons with a sufficiently high quantum yield to make a signifi-
cant contribution to the enhanced performance of the cell. Semiconductors with a high surface area are therefore used as photoelectrode materials. TiO2 nanoparticles for example, are typically sintered together at 450 8C to create a porous conducting network with roughness on a nanometer scale.[4] ZnO can serve as an alternative semiconductor with a similar bandgap to TiO2 and higher charge carrier mobility.[5, 6] It can be electrochemically deposited at low temperatures as a mesoporous network with high roughness factors, suitable for use as a DSSC photoelectrode.[7] The fact that it can be produced at low temperatures implies reduced EPTs. Indoline dyes show great potential for ZnO-based dye-sensitized solar cells and amongst organic dyes, the indoline D149 molecule has shown superior conversion efficiency.[8] The I/I3 redox couple is a favorable electrolyte for DSSCs because of its fast action as a reducing agent in regenerating oxidized dye molecules after charge transfer and slow action as oxidizing agent inhibiting recombination; however, it can be highly corrosive and lead to desorption of the dye molecules from the surface of the semiconductor.[9] In studies done by us on D149/ZnO cells,[10] a discolouration of the electrodes was seen as dye molecules were stripped from the semiconductor surface. The ever-reduced cell performance that is measured with time is consistent with this observation. To address this issue, the efficient chromophore used in D149 was synthesized with additional alkyl chains that bond to the surface of the ZnO in an effort to produce more durable cells and extend the lifetime.[9] Electron injection from the excited dye to the semiconductor is an important part of device optimization. Electron injection from various dyes into ZnO has been studied[11–15] and it has been clarified that the injection from the excited dye to
[a] Dr. E. Rohwer, I. Minda, G. Tauscher, Prof. Dr. H. Schwoerer Laser Research Institute, Stellenbosch University Private Bag X1, Matieland 7602 (South Africa) E-mail: [email protected] [b] C. Richter, Prof. Dr. D. Schlettwein Institute of Applied Physics, Justus-Liebig-University Heinrich-Buff-Ring 16, 35392 Gießen (Germany) [c] Dr. H. Miura Chemicrea Co., Ltd. 2-1-6 Sengen Tsukuba, Ibaraki 305-0047 (Japan)
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Articles the ZnO conduction band is slower compared to the injection into TiO2. Detailed studies regarding the electron injection from the excited state of D149 to ZnO[10, 16–21] revealed that the electron injection occurs on a femtosecond to picosecond timescale and that electron injection into ZnO surface states is a non-negligible path that slows down electron injection dynamics.[11, 22] Transient absorption spectroscopy (TAS) measurements performed by us previously on complete D149/ZnO cells with a variety of electrolytes based on the I/I3 redox couple yielded characteristic times for primary charge injection, parasitic recombination, and regeneration rates.[10] However, detailed dynamic studies of indoline derivatives with more than one anchor group necessary for durable cells are not available yet. In this work, complete DSSCs utilizing the new indoline derivatives will be studied by femtosecond transient absorption spectroscopy. We will show that the length of the anchor group significantly influences the charge-injection efficiency into the semiconductor and that an optimized length of the alkyl bridge can lead to further device optimization. Understanding the fundamental charge dynamics following absorption and what role the bonding of the added anchor chains play in these processes is critical for optimization considerations in a trade-off scenario between efficiency and durability.
Figure 2. I–V curves of the various dyes employed in DSSCs. Samples show equal Voc irrespective of the dye used. DN91 shows the highest Jsc and efficiency, DN285 shows the lowest.
manufactured following identical protocols, including ZnO deposition and exposure to dye molecules. The absorption properties of the dye determine how much light is harvested and the quantum efficiency of the charge-transfer process to the semiconductor determines how many of these photo-excited electrons following absorption are injected into the ZnO substrate generating the photocurrent. Steady-state absorption spectra of the DSSCs are presented in Figure 3. The dyes show broad absorption bands corresponding to the S1 S0 transition of the indoline chromophore between 500 and 590 nm, with peaks at 530 nm for DN91,
2. Results and Discussion
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The derivatives are shown in Figure 1, each contains the indoline chromophore identical to that in D149 but differ from each other in the length of the second carboxyl anchor chain R. The different derivatives are labeled DN91, DN216, and DN285 in order of increasing alkyl chain length.
Figure 1. Chemical structure of the indoline derivative dye molecules studied in this work. From top to bottom: D149 (no second anchor) and DN91, DN216 and DN285 in order of increasing anchoring alkyl chain length. Figure 3. Steady-state absorption spectra of three indoline derivatives: DN91, DN216, and DN285 adsorbed to ZnO in a fully operational cell.
The I–V curves of the DN91-, DN216-, and DN285-sensitized solar cells are presented in Figure 2. The open-circuit potential Voc and the short circuit current density Jsc both increase in the order DN91 > DN216 > DN285. It is evident that the DSSC employing the dye with the shortest chain (DN91) generates the highest power, followed by the dye with an intermediate chain length (DN216), whilst the longest chain length (DN285) generates the smallest power. For a high Jsc, which is the most prominent difference among the cells, the packing density of dye molecules on the porous ZnO surface, the electronic coupling to the conduction band, and the photophysical properties of the dyes are important. All samples have been carefully
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DN216, and DN285. To obtain a measure for the relative quantum efficiency in the different electrodes, we divided JSC by the absorptance (1-transmittance) maxima in the optical spectra, which is justified by the spectral similarity of the three dyes. Values of relative absorbed photon conversion efficiencies of 1.0 (set for DN91), 0.91 (DN216), and 0.68 (DN285) were thus obtained. Aside from possible differences in the quantum efficiency of electron injection from the excited state, which would be a very important optimization parameter, the dyes 2
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Articles may accidentally have different packing density and molecular orientations on the inner surface of the highly porous ZnO and this alone may explain the observed differences in Jsc. To elucidate the role of the molecular processes after absorption, a time-resolved technique is required to separately study the injection efficiency of each dye. The photo-induced dynamics of DSSCs containing either DN91, DN216 or DN285 as sensitizer were measured in the visible spectral range with our TAS setup on a fs to ps timescale and compared to previous data of indoline D149 not having the second carboxyl anchor to ZnO.[10] In Figure 4 the transient
scale(s) the intramolecular and charge-transfer dynamics responsible for that spectral signature evolve on. In the following we discuss the temporal characteristics of different processes and spectral components. The ESA centred around 680 nm is partially assigned to the population of the dye’s S1 state. However, the oxidized dye molecule also absorbs in this wavelength region.[24, 25] Therefore, this signature must be interpreted as a sum of contributions from both states of the molecule, the S1 of the dye and the molecular ground state of the oxidized molecule (OX). For the sake of brevity we will call it the ESA + OX decay. The absorption band seen below 480 nm is dominated by absorption by the dye in the oxidized state as this band absorbs strongly in the later time window as compared to the band at 650 nm when the dye is measured in the context of a solar cell. When the dye is measured in solution, where no oxidation occurs, the relative absorption strength of the two bands is reversed. The temporal evolution of the ESA + OX signal is summarized in Figure 5 for the DN91 sample. The trace can very well be described by four exponential decays with significantly different time constants t1 to t4 ranging from 150 fs to ns, and corresponding relative amplitudes A1 to A4. Data and analytical fits are normalized to the fit value at time of pumping (t = 0 fs) for correction of experimental temporal resolution of 150 fs (the first process is faster than we can resolve, therefore significant data start only after about 150 fs). The interpretation of the four processes, further substantiated below, is the following: the three fastest processes are all dominated by ESA of the dye molecule, and their decay is respectively due to direct electron injection from the excited dye to the ZnO conduction band (t1), injection into trap states on the ZnO surface (t2), and radiationless relaxation of the excited dye back to its ground state (t3). Only the final process, occurring on a nanosecond scale, too long to be determined accurately with our setup, is due to the final reduction of oxidized dye molecules via the electrolyte (t4). For comparison of the three different dyes with two carboxyl anchor groups and D149 with only one anchor, we present the time traces on a short and a long timescale separately in Figures 6 and 7.
Figure 4. fs-TAS measurement on a DN91 solar cell showing the typical spectral signatures of indoline dyes in the visible: GSB in a broad region between 500 and 600 nm, ESA + OX from 600 to 700 nm, and OX below 480 nm. The change in absorption represented by the color map is scaled on the color bar in increments of 10 mOD.
absorption spectrum of a typical indoline sensitizer (DN91) adsorbed to ZnO is shown for the first few picoseconds. The cell has been excited at its S1 S0 band around 530 nm.[23] Wavelength is plotted on the vertical axis, time on the horizontal axis, and the change in optical density DOD with respect to an unexcited sample as a color code. A negative DOD signal (blue) due to a decrease in absorption is observed in the S1 S0 range around 530 nm as a result of ground-state bleaching (GSB). This indicates the fraction of molecules in our sample volume that has been excited from the ground state by the pump laser pulse. However, in the wavelength range of the pump pulse (530 25) nm, the signal is obscured by scattering of the pump laser pulse off the sample into the detector. GSB can result from any excitation out of S0. Positive DOD signals (red) are observed in a band centred at 680 nm and below 480 nm. These correspond to an increased absorption at these wavelengths due to the population of excited or transition states (excited state absorption—ESA). No (stimulated) fluorescence S1!S0 is detectable in the transient spectra. As time after excitation increases, all signals approach zero (green) until all molecules in the sample volume return to the ground-state electron configuration. Figure 5. Time traces at (680 5) nm (short-time window) and (675 5) nm (long-time Horizontal lineouts through a specific spectral signa- window) for the full ESA + OX signal of a DN91 DSC. Four time constants and correture yield a decay trace indicating on what time- sponding relative amplitudes are obtained from the exponential decay fits. !
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Articles probe experiments at perylene-sensitized ZnO.[22] Its longer time constants t2 of 1.5 to 2 ps for the dyes with two anchors and of about 3 ps for D149 and amplitudes A2 of 15–25 % speak in favour of rather weak coupling between the dyes and trap states, which is less influenced by the COOH anchors. Charge dynamics in cells with D149, having only one COOH anchor, closely resemble those in DN285, indicating that the second carboxyl anchor on the long chain does not lead to significantly closer proximity of chromophore and semiconductor and hence does not significantly facilitate electron injection, but serves rather as mechanical and chemical stabilizer. Figure 7 displays the ESA + OX signal in a longer time window, measured at intervals of 1 ps over 0.4 ns. Bi-exponential decays deliver a satisfactory description of all the traces with time constants t3 and t4 and amplitudes A3 and A4. The faster process consistently reveals decay constants of t3 = (30 10) ps for all samples, considering the rather low signal/noise in the time window. This contribution is assigned to the S0 S1 relaxation. This interpretation is supported by the fact that the GSB signal, see Figure 8, does not show the two ultrafast com-
Figure 6. ESA decays at (680 5) nm for DN91, DN216, DN285, and D149 in ZnO DSSCs. The faster component of the bi-exponential fit is assigned to direct electron injection to ZnO. The slower component is due to charge injection into surface states.
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Figure 7. Decay trace of the transient absorption signal centered at (675 5) nm due to ESA and absorption of oxidized molecules for DSSCs containing DN91, DN216, DN285, and D149 fitted with a bi-exponential fit function.
Figure 8. Decay trace of the transient absorption signal centered at (570 5) nm due to GSB for DSSCs containing DN91, DN216, and DN285 fitted with a bi-exponential fit function.
ponents t1 and t2, but only t3 and t4 (see below), indicating that regeneration of the neutral molecule only occurs via S0 S1 relaxation (3) and via reduction of OX by the electrolyte (4). The differences between the dyes in the decay traces are manifested in the relative amplitudes A3 : the intramolecular relaxation (process 3) has a significantly smaller relative amplitude in the two short chain dyes DN91 and DN216 than in the DN285 and D149, contrariwise indicating a bigger portion of electron injection into ZnO and generation of oxidized molecules. On the longest timescale t4 of hundreds of picoseconds to nanoseconds the absorption of the oxidized dye molecule dominates. The time window is insufficiently long to reliably fit the ns component resulting from the decay of the oxidized molecules’ absorption. All decays are about equal except DN91 being longer. This may indicate that DN91 tightly adsorbed to ZnO is more difficult to reduce by the redox couple compared with the other sensitizers. The fast rise and ns decay of the oxidized molecules’ absorption is also reflected in the TAS signal
In the short time window, the decay of the ESA is recorded in 20 fs increments of pump–probe delay for each of the sensitizer molecules, see Figure 6. Here, it is evident that the molecule with the shortest carboxyl chain, DN91, injects fastest into the ZnO conduction band (t1 = 175 10 fs), followed by the DN216 sensitizer molecule with an intermediate anchor chain length (t1 = 195 15 fs). This ultrafast time constant speaks for an open injection channel via orbital overlap between dye and ZnO at the COOH anchors. The ultrafast growth of the OX absorption band below 480 nm confirms this interpretation, see Figure 9. DN285 with the longest anchor chain has the slowest injection time (t1 = 260 20 fs) similar to but slightly longer than D149 (t1 = 230 20 fs). Accordingly, the relative amplitudes A1 decrease with longer chain length towards the value also observed for D149 (no second anchor). The second process, which we assigned to electron injection into ZnO surface trap states, is largely independent of the anchoring of the dye molecule. Such surface traps have been detected in photoelectrochemical experiments[26] and also from infra-red pump– ChemPhysChem 0000, 00, 0 – 0
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Articles longer injection times, therefore smaller OX signals with DN216 injecting faster than DN285 due to the shorter anchor chain of DN216. The spectroscopically observed charge dynamics in DN285 solar cells are very close in characteristic times and amplitudes to those of the D149 cell, which does not possess a second anchor group. Therefore, we conclude that while the second anchor improves electron injection if a short enough alkyl chain is used to decrease the distance between the indoline functional unit and the ZnO surface, it loses this additional benefit if an overly long chain is used, but still provides better stability and longevity of the cell.
3. Conclusions
Figure 9. Exponential growth trace of the transient absorption signal centered at (480 5) nm due to absorption of oxidized molecules for DSSCs containing DN91 and DN216 fitted with a bi-exponential fit function. The peak at time zero (overlap of pump and white-light probe pulses) is an artefact, not representing charge dynamics.
We investigated photo-induced charge dynamics in dye-sensitized solar cells that were systematically altered in their use of organic dye molecules: indoline dyes were anchored to a highly porous ZnO surface with either one anchor only or with a second anchor with different alkyl chain length. Cells below 480 nm at the short wavelength edge of our TAS specwith D149 providing only one carboxyl anchor group were tra (Figure 9). compared to cells with DN91, DN216, and DN285 with two All four processes of charge transfer and relaxation combine COOH anchors of increasing alkyl chain length. The two hyto a consistent picture, as illustrated in Figure 10, and the conpotheses for this approach were a longer lifetime of the cells due to improved fixation by strengthened bonding of the dyes to the semiconductor and an improved electron injection rate mediated by a closer proximity of the chromophore to the semiconductor enforced by the second bonds. In all solar cells we observed an ultrafast electron injection directly into ZnO (< 300 fs) and indirectly via surface trap states (< 3 ps), a molecular relaxation loss channel on a 30 ps scale, and a nanosecond-scale reduction of the oxidized dye to regenerate the sensitizer. SucFigure 10. Summary of photo-induced charge-transfer processes assigned to the ESA + OX signal of the TAS measurements of DN91, DN216, DN285, and D149 in a ZnO cessful electron transfer and the generation of oxiDSSC. The blue arrows indicate the processes directly observed here, the grey arrows are dized dyes was systematically faster and showed processes without spectral signatures in our measurements. a higher yield with shorter alkyl chain length of the second anchor group. For the two shorter chain declusions drawn from the transient absorption measurements rivatives, DN91 and DN216, these values were superior to the regarding efficiencies agree well with the photoelectrochemioriginal one-anchor dye, D149, and also translated into superical measurements: first of all, a second COOH anchor between or cell characteristics based on an improved quantum efficienthe indoline molecule and the ZnO surface improves the stabilcy. These results indicate that the dye anchoring to the ZnO ity of the cells against desorption of dye molecules into the surface via the alkyl chains was successfully altered. Additionalelectrolyte; furthermore, it contributes to the direct electron inly, the new DSSCs showed a prolonged longevity of more than jection from the excited dye into the ZnO conduction band. It a year compared to only a few weeks in the case of D149 provides a faster injection time and in total higher yield, less DSCs. In conclusion, the photoelectrochemical properties of losses through S1!S0 relaxation, and consequently also the dye-sensitized solar cells have been improved in two critia higher short-circuit current Jsc. The DSSC containing DN91, cal areas; durability and injection efficiency due to chemical the derivative with the shortest anchor chain, indeed showed tuning of the indoline dyes on porous ZnO. The results open the shortest injection time of t1 = 150 fs, resulting in the highthe way for further understanding of the charge-transfer proest relative amplitude A1 = 65 %, the lowest amplitude A3 for cess at the ZnO/dye/electrolyte interface. radiationless dye relaxation, and the highest short circuit current of Jsc = 8.5 mA cm2. For every injected electron, a dye Experimental Section molecule is oxidized. It follows that the DN91 sample will have the largest signal at the oxidized molecule’s absorption band, The electrodeposition of ZnO thin films is a well-established techseen in the later part of the long decay trace (Figure 6) when nique[7, 27] and was done on activated FTO (fluorine-doped tin molecular processes associated with the excited dye molecule oxide) covered glass slides. Activation of the FTO was carried out have run their course. DN216 and DN285 have successively by submerging the glass substrates in nitric acid for 2 min. Before ChemPhysChem 0000, 00, 0 – 0
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Articles the activation process, the glass slides (3 cm x 2.5 cm) were washed in a soap solution, acetone, and isopropanol and dried with N2 gas. The glass substrates were then placed in a sample mount, contacted with copper tape, and sealed with insulating tape. The active area to be used for electrodeposition was first stamped out of the insulating tape with the use of a 1 cm radius circular stamp, exposing only a p cm2 area of FTO to the deposition bath. The samples were then submerged in a 70 8C deposition bath consisting of 0.1 m KCl aqueous solution and rotated at 500 rpm by means of a rotating disk electrode. The environment was oxygenated before and during the electrodeposition by pumping of oxygen gas at 400 mL min1 through the deposition bath. The FTO (working electrode) was activated in the KCl bath with a Pt wire as the counter electrode for 15 min by applying a potential of 1160 mV versus the Ag/AgCl reference electrode. The electrodeposition of the ZnO blocking layer was carried out for 10 min at an applied potential of 1060 mV (vs. Ag/AgCl) with a Zn wire as the counter electrode after adding sufficient ZnCl2 aqueous solution to the deposition bath to achieve a concentration of 5 mm ZnCl2. A porous ZnO layer was deposited for 5 min at 960 mV (vs. Ag/AgCl) by adding 300 mm eosin Y. The samples were then washed with deionized water and dried with N2 gas before being submerged in a pH 10.5 KOH aqueous solution to desorb the eosin Y and realize a porous ZnO thin film. The ZnO thin films were heated at 150 8C for 30 min to remove adsorbed water from the pores. After cooling, they were placed under a UV lamp for 30 min to create hydroxyl groups on the ZnO surface for improved dye adsorption. Following this, the samples were immersed for 1 min in the dark in one of the three dye solutions (D149, DN285, DN216, and DN91)—all made up as 0.5 mm dye in 1 mm lithocholic acid. The sensitized thin films were then washed with ethanol and dried with N2 gas. Sandwich cells were made by sealing a sensitized ZnO electrode to a Pt-coated FTO counter electrode together at 120 8C with a hot melt-sealing foil. The FTO counter electrodes were coated with Pt by drop-casting two drops of a 5 mm ethanoic solution of hexachloroplatinic acid after washing the slides with soap solution, acetone, and isopropanol; and being baked at 450 8C for 30 min. The cells were filled with 0.5 m 1methyl-3-propylimidazolium iodide and 0.05 m I2 in acetonitrile as the redox electrolyte through one of two predrilled holes in the counter electrode. The holes were sealed with two pieces of glass by sealing foil.
changed due to excitation. The overall temporal resolution of the setup is better than about 150 fs.
Acknowledgements This work is based upon research supported by the South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation. Keywords: dye-sensitized solar cells · indoline dyes · injection rate · pump–probe spectroscopy · zinc oxide [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595 – 663. [2] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395 – 398. [3] R. A. Marcus, N. Sutin, Biochim. Biophys. Acta 1985, 811, 265 – 322. [4] B. O’Regan, M. Gratzel, Nature 1991, 353, 737 – 740. [5] D. C. Look, Mater. Sci. Eng. B 2001, 80, 383 – 387. [6] S. Pearton, Prog. Mater. Sci. 2005, 50, 293 – 340. [7] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauport, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wçhrle, K. Funabiki, M. Matsui, H. Miura, H. Yanagi, Adv. Funct. Mater. 2009, 19, 17 – 43. [8] S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska, P. Comte, P. Pchy, M. Grtzel, Chem. Commun. 2008, 5194 – 6. [9] J. Falgenhauer, C. Richter, H. Miura, D. Schlettwein, ChemPhysChem 2012, 13, 2893 – 2897. [10] E. Rohwer, C. Richter, N. Heming, K. Strauch, C. Litwinski, T. Nyokong, D. Schlettwein, H. Schwoerer, ChemPhysChem 2013, 14, 132 – 9. [11] A. Furube, R. Katoh, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2003, 107, 4162 – 4166. [12] A. Furube, R. Katoh, T. Yoshihara, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 12583 – 12592. [13] R. Katoh, A. Furube, K. Hara, S. Murata, H. Sugihara, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2002, 106, 12957 – 12964. [14] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 4818 – 4822. [15] T. Yoshihara, R. Katoh, A. Furube, M. Murai, Y. Tamaki, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 2643 – 2647. [16] G. Burdzinski, J. Karolczak, M. Ziolek, Phys. Chem. Chem. Phys. 2013, 15, 3889 – 3896. [17] A. El-Zohry, A. Orthaber, B. Zietz, J. Phys. Chem. C 2012, 116, 26144 – 26153. [18] A. M. El-Zohry, B. Zietz, J. Phys. Chem. C 2013, 117, 6544 – 6553. [19] M. Fakis, P. Hrobrik, E. Stathatos, V. Giannetas, P. Persephonis, Dye. Pigment. 2013, 96, 304 – 312. [20] P. W. Lohse, J. Kuhnt, S. I. Druzhinin, M. Scholz, M. Ekimova, T. Oekermann, T. Lenzer, K. Oum, Phys. Chem. Chem. Phys. 2011, 13, 19632 – 19640. [21] K. Oum, P. W. Lohse, O. Flender, J. R. Klein, M. Scholz, T. Lenzer, J. Du, T. Oekermann, Phys. Chem. Chem. Phys. 2012, 14, 15429 – 15437. [22] C. Strothkmper, A. Bartelt, P. Sippel, T. Hannappel, R. Schtz, R. Eichberger, J. Phys. Chem. C 2013, 117, 17901 – 17908. [23] T. Le Bahers, T. Pauport, G. Scalmani, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 2009, 11, 11276 – 84. [24] U. B. Cappel, S. M. Feldt, J. Schçneboom, A. Hagfeldt, G. Boschloo, J. Am. Chem. Soc. 2010, 132, 9096 – 9101. [25] A. Fattori, L. M. Peter, H. Wang, H. Miura, F. Marken, J. Phys. Chem. C 2010, 114, 11822 – 11828. [26] M. Rudolph, T. Yoshida, D. Schlettwein, J. Electroanal. Chem. 2013, 709, 10 – 18. [27] J. Zhang, L. Sun, K. Ichinose, K. Funabiki, T. Yoshida, Phys. Chem. Chem. Phys. 2010, 12, 10494 – 10502.
TAS experiments were conducted with the same pump–probe setup described in a previous paper:[10] Complete ZnO solar cells sensitized with either Indoline DN91, DN216, DN285 or D149 and coupled with an iodide tri-iodide redox solution were excited along the S1 S0 transition around 530 nm with 100 nJ, 50 fs pulses generated in a non-collinearly phase-matched optical parametric amplifier (NOPA). The transient absorption spectrum was measured by a supercontinuum (450–700 nm) pulse generated in a 3 mm sapphire crystal and recorded by a 1024 pixel linescan camera with a 1 KHz readout rate (Ingenieurbro Stresing) to match the repetition rate of the Clark-MXR laser system that powers the setup. A beam chopper was incorporated into the pump beam path and blocked every other pulse. A software recording the absorption spectrum of the sample from the readout of the pixel array in the focal plane of the spectrometer that the probe pulse was coupled into binned every alternate pulse spectrum into a pumped (beam block open) and unpumped (beam block closed) spectrum. The software then divided these spectra by each other to elucidate the spectral signatures that had !
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ARTICLES Pump–probe spectroscopy studies in the ps to sub-ps regime are performed to reveal the role of different anchor groups at indoline-type sensitizer molecules in the charge-carrier dynamics of ZnO-based dye-sensitized solar cells.
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E. Rohwer, I. Minda, G. Tauscher, C. Richter, H. Miura, D. Schlettwein, H. Schwoerer* && – && Ultrafast Charge-Transfer Reactions of Indoline Dyes with Anchoring Alkyl Chains of Varying Length in Mesoporous ZnO Solar Cells
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Articles
Ultrafast Charge-Transfer Reactions of Indoline Dyes with Anchoring Alkyl Chains of Varying Length in Mesoporous ZnO Solar Cells Egmont Rohwer,[a] Iulia Minda,[a] Gabriele Tauscher,[a] Christoph Richter,[b] Hidetoshi Miura,[c] Derck Schlettwein,[b] and Heinrich Schwoerer*[a] of the oxidized molecules are dominant spectral features in the transient absorption of the dyes with shorter alkyl chains. A slower picosecond-scale decay proceeds at constant rate for all three derivatives and is assigned to electron transfer into the trap states of ZnO. All assignments are in good agreement with a higher quantum efficiency of charge injection leading to higher short-circuit currents Jsc for dyes with shorter alkyl chains.
Dye-sensitized solar cells based on a mesoporous ZnO substrate were sensitized with the indoline derivatives DN91, DN216 and DN285. The chromophore is the same for each of these dyes. They differ from each other in the length of an alkyl chain, which provides a second anchor to the ZnO surface and prolongs cell lifetime. Ultrafast transient absorption measurements reveal a correlation between the length of the alkyl chain and the fastest electron-injection process. The depopulation of the excited state and the associated emergence
1. Introduction Dye-sensitized solar cells (DSSCs) represent a class of photovoltaic devices that show potential for future commercialization because of their low energy payback times (EPT) and relatively modest material purity requirements when compared to traditional silicon-based solar cells.[1] Despite lower cell efficiencies also when compared with recently reported perovskite solar cells,[2] DSSCs have the benefit of using widely non-toxic and environmentally friendly compounds. A DSSC consists primarily of a wide-bandgap semiconductor, typically a transition metal oxide with a dye attached to the surface that absorbs in the visible or NIR or similarly abundant part of the solar spectrum to provide an efficient light harvesting efficiency of the cell. If there exists sufficient coupling between the photo-excited state of the dye and the conduction band of the semiconductor, the electron can be transferred from the dye to the semiconductor[3] and is harvested at the anode. A redox electrolyte solution or hole conductor completes the cell and transports holes to the cathode and regenerates the neutral state of the dye. Because of the proximity required between dye and semiconductor for coupling of states, only the dye molecules in direct contact with the semiconductor surface transfer electrons with a sufficiently high quantum yield to make a signifi-
cant contribution to the enhanced performance of the cell. Semiconductors with a high surface area are therefore used as photoelectrode materials. TiO2 nanoparticles for example, are typically sintered together at 450 8C to create a porous conducting network with roughness on a nanometer scale.[4] ZnO can serve as an alternative semiconductor with a similar bandgap to TiO2 and higher charge carrier mobility.[5, 6] It can be electrochemically deposited at low temperatures as a mesoporous network with high roughness factors, suitable for use as a DSSC photoelectrode.[7] The fact that it can be produced at low temperatures implies reduced EPTs. Indoline dyes show great potential for ZnO-based dye-sensitized solar cells and amongst organic dyes, the indoline D149 molecule has shown superior conversion efficiency.[8] The I/I3 redox couple is a favorable electrolyte for DSSCs because of its fast action as a reducing agent in regenerating oxidized dye molecules after charge transfer and slow action as oxidizing agent inhibiting recombination; however, it can be highly corrosive and lead to desorption of the dye molecules from the surface of the semiconductor.[9] In studies done by us on D149/ZnO cells,[10] a discolouration of the electrodes was seen as dye molecules were stripped from the semiconductor surface. The ever-reduced cell performance that is measured with time is consistent with this observation. To address this issue, the efficient chromophore used in D149 was synthesized with additional alkyl chains that bond to the surface of the ZnO in an effort to produce more durable cells and extend the lifetime.[9] Electron injection from the excited dye to the semiconductor is an important part of device optimization. Electron injection from various dyes into ZnO has been studied[11–15] and it has been clarified that the injection from the excited dye to
[a] Dr. E. Rohwer, I. Minda, G. Tauscher, Prof. Dr. H. Schwoerer Laser Research Institute, Stellenbosch University Private Bag X1, Matieland 7602 (South Africa) E-mail: [email protected] [b] C. Richter, Prof. Dr. D. Schlettwein Institute of Applied Physics, Justus-Liebig-University Heinrich-Buff-Ring 16, 35392 Gießen (Germany) [c] Dr. H. Miura Chemicrea Co., Ltd. 2-1-6 Sengen Tsukuba, Ibaraki 305-0047 (Japan)
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Articles the ZnO conduction band is slower compared to the injection into TiO2. Detailed studies regarding the electron injection from the excited state of D149 to ZnO[10, 16–21] revealed that the electron injection occurs on a femtosecond to picosecond timescale and that electron injection into ZnO surface states is a non-negligible path that slows down electron injection dynamics.[11, 22] Transient absorption spectroscopy (TAS) measurements performed by us previously on complete D149/ZnO cells with a variety of electrolytes based on the I/I3 redox couple yielded characteristic times for primary charge injection, parasitic recombination, and regeneration rates.[10] However, detailed dynamic studies of indoline derivatives with more than one anchor group necessary for durable cells are not available yet. In this work, complete DSSCs utilizing the new indoline derivatives will be studied by femtosecond transient absorption spectroscopy. We will show that the length of the anchor group significantly influences the charge-injection efficiency into the semiconductor and that an optimized length of the alkyl bridge can lead to further device optimization. Understanding the fundamental charge dynamics following absorption and what role the bonding of the added anchor chains play in these processes is critical for optimization considerations in a trade-off scenario between efficiency and durability.
Figure 2. I–V curves of the various dyes employed in DSSCs. Samples show equal Voc irrespective of the dye used. DN91 shows the highest Jsc and efficiency, DN285 shows the lowest.
manufactured following identical protocols, including ZnO deposition and exposure to dye molecules. The absorption properties of the dye determine how much light is harvested and the quantum efficiency of the charge-transfer process to the semiconductor determines how many of these photo-excited electrons following absorption are injected into the ZnO substrate generating the photocurrent. Steady-state absorption spectra of the DSSCs are presented in Figure 3. The dyes show broad absorption bands corresponding to the S1 S0 transition of the indoline chromophore between 500 and 590 nm, with peaks at 530 nm for DN91,
2. Results and Discussion
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The derivatives are shown in Figure 1, each contains the indoline chromophore identical to that in D149 but differ from each other in the length of the second carboxyl anchor chain R. The different derivatives are labeled DN91, DN216, and DN285 in order of increasing alkyl chain length.
Figure 1. Chemical structure of the indoline derivative dye molecules studied in this work. From top to bottom: D149 (no second anchor) and DN91, DN216 and DN285 in order of increasing anchoring alkyl chain length. Figure 3. Steady-state absorption spectra of three indoline derivatives: DN91, DN216, and DN285 adsorbed to ZnO in a fully operational cell.
The I–V curves of the DN91-, DN216-, and DN285-sensitized solar cells are presented in Figure 2. The open-circuit potential Voc and the short circuit current density Jsc both increase in the order DN91 > DN216 > DN285. It is evident that the DSSC employing the dye with the shortest chain (DN91) generates the highest power, followed by the dye with an intermediate chain length (DN216), whilst the longest chain length (DN285) generates the smallest power. For a high Jsc, which is the most prominent difference among the cells, the packing density of dye molecules on the porous ZnO surface, the electronic coupling to the conduction band, and the photophysical properties of the dyes are important. All samples have been carefully
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DN216, and DN285. To obtain a measure for the relative quantum efficiency in the different electrodes, we divided JSC by the absorptance (1-transmittance) maxima in the optical spectra, which is justified by the spectral similarity of the three dyes. Values of relative absorbed photon conversion efficiencies of 1.0 (set for DN91), 0.91 (DN216), and 0.68 (DN285) were thus obtained. Aside from possible differences in the quantum efficiency of electron injection from the excited state, which would be a very important optimization parameter, the dyes 2
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Articles may accidentally have different packing density and molecular orientations on the inner surface of the highly porous ZnO and this alone may explain the observed differences in Jsc. To elucidate the role of the molecular processes after absorption, a time-resolved technique is required to separately study the injection efficiency of each dye. The photo-induced dynamics of DSSCs containing either DN91, DN216 or DN285 as sensitizer were measured in the visible spectral range with our TAS setup on a fs to ps timescale and compared to previous data of indoline D149 not having the second carboxyl anchor to ZnO.[10] In Figure 4 the transient
scale(s) the intramolecular and charge-transfer dynamics responsible for that spectral signature evolve on. In the following we discuss the temporal characteristics of different processes and spectral components. The ESA centred around 680 nm is partially assigned to the population of the dye’s S1 state. However, the oxidized dye molecule also absorbs in this wavelength region.[24, 25] Therefore, this signature must be interpreted as a sum of contributions from both states of the molecule, the S1 of the dye and the molecular ground state of the oxidized molecule (OX). For the sake of brevity we will call it the ESA + OX decay. The absorption band seen below 480 nm is dominated by absorption by the dye in the oxidized state as this band absorbs strongly in the later time window as compared to the band at 650 nm when the dye is measured in the context of a solar cell. When the dye is measured in solution, where no oxidation occurs, the relative absorption strength of the two bands is reversed. The temporal evolution of the ESA + OX signal is summarized in Figure 5 for the DN91 sample. The trace can very well be described by four exponential decays with significantly different time constants t1 to t4 ranging from 150 fs to ns, and corresponding relative amplitudes A1 to A4. Data and analytical fits are normalized to the fit value at time of pumping (t = 0 fs) for correction of experimental temporal resolution of 150 fs (the first process is faster than we can resolve, therefore significant data start only after about 150 fs). The interpretation of the four processes, further substantiated below, is the following: the three fastest processes are all dominated by ESA of the dye molecule, and their decay is respectively due to direct electron injection from the excited dye to the ZnO conduction band (t1), injection into trap states on the ZnO surface (t2), and radiationless relaxation of the excited dye back to its ground state (t3). Only the final process, occurring on a nanosecond scale, too long to be determined accurately with our setup, is due to the final reduction of oxidized dye molecules via the electrolyte (t4). For comparison of the three different dyes with two carboxyl anchor groups and D149 with only one anchor, we present the time traces on a short and a long timescale separately in Figures 6 and 7.
Figure 4. fs-TAS measurement on a DN91 solar cell showing the typical spectral signatures of indoline dyes in the visible: GSB in a broad region between 500 and 600 nm, ESA + OX from 600 to 700 nm, and OX below 480 nm. The change in absorption represented by the color map is scaled on the color bar in increments of 10 mOD.
absorption spectrum of a typical indoline sensitizer (DN91) adsorbed to ZnO is shown for the first few picoseconds. The cell has been excited at its S1 S0 band around 530 nm.[23] Wavelength is plotted on the vertical axis, time on the horizontal axis, and the change in optical density DOD with respect to an unexcited sample as a color code. A negative DOD signal (blue) due to a decrease in absorption is observed in the S1 S0 range around 530 nm as a result of ground-state bleaching (GSB). This indicates the fraction of molecules in our sample volume that has been excited from the ground state by the pump laser pulse. However, in the wavelength range of the pump pulse (530 25) nm, the signal is obscured by scattering of the pump laser pulse off the sample into the detector. GSB can result from any excitation out of S0. Positive DOD signals (red) are observed in a band centred at 680 nm and below 480 nm. These correspond to an increased absorption at these wavelengths due to the population of excited or transition states (excited state absorption—ESA). No (stimulated) fluorescence S1!S0 is detectable in the transient spectra. As time after excitation increases, all signals approach zero (green) until all molecules in the sample volume return to the ground-state electron configuration. Figure 5. Time traces at (680 5) nm (short-time window) and (675 5) nm (long-time Horizontal lineouts through a specific spectral signa- window) for the full ESA + OX signal of a DN91 DSC. Four time constants and correture yield a decay trace indicating on what time- sponding relative amplitudes are obtained from the exponential decay fits. !
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Articles probe experiments at perylene-sensitized ZnO.[22] Its longer time constants t2 of 1.5 to 2 ps for the dyes with two anchors and of about 3 ps for D149 and amplitudes A2 of 15–25 % speak in favour of rather weak coupling between the dyes and trap states, which is less influenced by the COOH anchors. Charge dynamics in cells with D149, having only one COOH anchor, closely resemble those in DN285, indicating that the second carboxyl anchor on the long chain does not lead to significantly closer proximity of chromophore and semiconductor and hence does not significantly facilitate electron injection, but serves rather as mechanical and chemical stabilizer. Figure 7 displays the ESA + OX signal in a longer time window, measured at intervals of 1 ps over 0.4 ns. Bi-exponential decays deliver a satisfactory description of all the traces with time constants t3 and t4 and amplitudes A3 and A4. The faster process consistently reveals decay constants of t3 = (30 10) ps for all samples, considering the rather low signal/noise in the time window. This contribution is assigned to the S0 S1 relaxation. This interpretation is supported by the fact that the GSB signal, see Figure 8, does not show the two ultrafast com-
Figure 6. ESA decays at (680 5) nm for DN91, DN216, DN285, and D149 in ZnO DSSCs. The faster component of the bi-exponential fit is assigned to direct electron injection to ZnO. The slower component is due to charge injection into surface states.
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Figure 7. Decay trace of the transient absorption signal centered at (675 5) nm due to ESA and absorption of oxidized molecules for DSSCs containing DN91, DN216, DN285, and D149 fitted with a bi-exponential fit function.
Figure 8. Decay trace of the transient absorption signal centered at (570 5) nm due to GSB for DSSCs containing DN91, DN216, and DN285 fitted with a bi-exponential fit function.
ponents t1 and t2, but only t3 and t4 (see below), indicating that regeneration of the neutral molecule only occurs via S0 S1 relaxation (3) and via reduction of OX by the electrolyte (4). The differences between the dyes in the decay traces are manifested in the relative amplitudes A3 : the intramolecular relaxation (process 3) has a significantly smaller relative amplitude in the two short chain dyes DN91 and DN216 than in the DN285 and D149, contrariwise indicating a bigger portion of electron injection into ZnO and generation of oxidized molecules. On the longest timescale t4 of hundreds of picoseconds to nanoseconds the absorption of the oxidized dye molecule dominates. The time window is insufficiently long to reliably fit the ns component resulting from the decay of the oxidized molecules’ absorption. All decays are about equal except DN91 being longer. This may indicate that DN91 tightly adsorbed to ZnO is more difficult to reduce by the redox couple compared with the other sensitizers. The fast rise and ns decay of the oxidized molecules’ absorption is also reflected in the TAS signal
In the short time window, the decay of the ESA is recorded in 20 fs increments of pump–probe delay for each of the sensitizer molecules, see Figure 6. Here, it is evident that the molecule with the shortest carboxyl chain, DN91, injects fastest into the ZnO conduction band (t1 = 175 10 fs), followed by the DN216 sensitizer molecule with an intermediate anchor chain length (t1 = 195 15 fs). This ultrafast time constant speaks for an open injection channel via orbital overlap between dye and ZnO at the COOH anchors. The ultrafast growth of the OX absorption band below 480 nm confirms this interpretation, see Figure 9. DN285 with the longest anchor chain has the slowest injection time (t1 = 260 20 fs) similar to but slightly longer than D149 (t1 = 230 20 fs). Accordingly, the relative amplitudes A1 decrease with longer chain length towards the value also observed for D149 (no second anchor). The second process, which we assigned to electron injection into ZnO surface trap states, is largely independent of the anchoring of the dye molecule. Such surface traps have been detected in photoelectrochemical experiments[26] and also from infra-red pump– ChemPhysChem 0000, 00, 0 – 0
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Articles longer injection times, therefore smaller OX signals with DN216 injecting faster than DN285 due to the shorter anchor chain of DN216. The spectroscopically observed charge dynamics in DN285 solar cells are very close in characteristic times and amplitudes to those of the D149 cell, which does not possess a second anchor group. Therefore, we conclude that while the second anchor improves electron injection if a short enough alkyl chain is used to decrease the distance between the indoline functional unit and the ZnO surface, it loses this additional benefit if an overly long chain is used, but still provides better stability and longevity of the cell.
3. Conclusions
Figure 9. Exponential growth trace of the transient absorption signal centered at (480 5) nm due to absorption of oxidized molecules for DSSCs containing DN91 and DN216 fitted with a bi-exponential fit function. The peak at time zero (overlap of pump and white-light probe pulses) is an artefact, not representing charge dynamics.
We investigated photo-induced charge dynamics in dye-sensitized solar cells that were systematically altered in their use of organic dye molecules: indoline dyes were anchored to a highly porous ZnO surface with either one anchor only or with a second anchor with different alkyl chain length. Cells below 480 nm at the short wavelength edge of our TAS specwith D149 providing only one carboxyl anchor group were tra (Figure 9). compared to cells with DN91, DN216, and DN285 with two All four processes of charge transfer and relaxation combine COOH anchors of increasing alkyl chain length. The two hyto a consistent picture, as illustrated in Figure 10, and the conpotheses for this approach were a longer lifetime of the cells due to improved fixation by strengthened bonding of the dyes to the semiconductor and an improved electron injection rate mediated by a closer proximity of the chromophore to the semiconductor enforced by the second bonds. In all solar cells we observed an ultrafast electron injection directly into ZnO (< 300 fs) and indirectly via surface trap states (< 3 ps), a molecular relaxation loss channel on a 30 ps scale, and a nanosecond-scale reduction of the oxidized dye to regenerate the sensitizer. SucFigure 10. Summary of photo-induced charge-transfer processes assigned to the ESA + OX signal of the TAS measurements of DN91, DN216, DN285, and D149 in a ZnO cessful electron transfer and the generation of oxiDSSC. The blue arrows indicate the processes directly observed here, the grey arrows are dized dyes was systematically faster and showed processes without spectral signatures in our measurements. a higher yield with shorter alkyl chain length of the second anchor group. For the two shorter chain declusions drawn from the transient absorption measurements rivatives, DN91 and DN216, these values were superior to the regarding efficiencies agree well with the photoelectrochemioriginal one-anchor dye, D149, and also translated into superical measurements: first of all, a second COOH anchor between or cell characteristics based on an improved quantum efficienthe indoline molecule and the ZnO surface improves the stabilcy. These results indicate that the dye anchoring to the ZnO ity of the cells against desorption of dye molecules into the surface via the alkyl chains was successfully altered. Additionalelectrolyte; furthermore, it contributes to the direct electron inly, the new DSSCs showed a prolonged longevity of more than jection from the excited dye into the ZnO conduction band. It a year compared to only a few weeks in the case of D149 provides a faster injection time and in total higher yield, less DSCs. In conclusion, the photoelectrochemical properties of losses through S1!S0 relaxation, and consequently also the dye-sensitized solar cells have been improved in two critia higher short-circuit current Jsc. The DSSC containing DN91, cal areas; durability and injection efficiency due to chemical the derivative with the shortest anchor chain, indeed showed tuning of the indoline dyes on porous ZnO. The results open the shortest injection time of t1 = 150 fs, resulting in the highthe way for further understanding of the charge-transfer proest relative amplitude A1 = 65 %, the lowest amplitude A3 for cess at the ZnO/dye/electrolyte interface. radiationless dye relaxation, and the highest short circuit current of Jsc = 8.5 mA cm2. For every injected electron, a dye Experimental Section molecule is oxidized. It follows that the DN91 sample will have the largest signal at the oxidized molecule’s absorption band, The electrodeposition of ZnO thin films is a well-established techseen in the later part of the long decay trace (Figure 6) when nique[7, 27] and was done on activated FTO (fluorine-doped tin molecular processes associated with the excited dye molecule oxide) covered glass slides. Activation of the FTO was carried out have run their course. DN216 and DN285 have successively by submerging the glass substrates in nitric acid for 2 min. Before ChemPhysChem 0000, 00, 0 – 0
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Articles the activation process, the glass slides (3 cm x 2.5 cm) were washed in a soap solution, acetone, and isopropanol and dried with N2 gas. The glass substrates were then placed in a sample mount, contacted with copper tape, and sealed with insulating tape. The active area to be used for electrodeposition was first stamped out of the insulating tape with the use of a 1 cm radius circular stamp, exposing only a p cm2 area of FTO to the deposition bath. The samples were then submerged in a 70 8C deposition bath consisting of 0.1 m KCl aqueous solution and rotated at 500 rpm by means of a rotating disk electrode. The environment was oxygenated before and during the electrodeposition by pumping of oxygen gas at 400 mL min1 through the deposition bath. The FTO (working electrode) was activated in the KCl bath with a Pt wire as the counter electrode for 15 min by applying a potential of 1160 mV versus the Ag/AgCl reference electrode. The electrodeposition of the ZnO blocking layer was carried out for 10 min at an applied potential of 1060 mV (vs. Ag/AgCl) with a Zn wire as the counter electrode after adding sufficient ZnCl2 aqueous solution to the deposition bath to achieve a concentration of 5 mm ZnCl2. A porous ZnO layer was deposited for 5 min at 960 mV (vs. Ag/AgCl) by adding 300 mm eosin Y. The samples were then washed with deionized water and dried with N2 gas before being submerged in a pH 10.5 KOH aqueous solution to desorb the eosin Y and realize a porous ZnO thin film. The ZnO thin films were heated at 150 8C for 30 min to remove adsorbed water from the pores. After cooling, they were placed under a UV lamp for 30 min to create hydroxyl groups on the ZnO surface for improved dye adsorption. Following this, the samples were immersed for 1 min in the dark in one of the three dye solutions (D149, DN285, DN216, and DN91)—all made up as 0.5 mm dye in 1 mm lithocholic acid. The sensitized thin films were then washed with ethanol and dried with N2 gas. Sandwich cells were made by sealing a sensitized ZnO electrode to a Pt-coated FTO counter electrode together at 120 8C with a hot melt-sealing foil. The FTO counter electrodes were coated with Pt by drop-casting two drops of a 5 mm ethanoic solution of hexachloroplatinic acid after washing the slides with soap solution, acetone, and isopropanol; and being baked at 450 8C for 30 min. The cells were filled with 0.5 m 1methyl-3-propylimidazolium iodide and 0.05 m I2 in acetonitrile as the redox electrolyte through one of two predrilled holes in the counter electrode. The holes were sealed with two pieces of glass by sealing foil.
changed due to excitation. The overall temporal resolution of the setup is better than about 150 fs.
Acknowledgements This work is based upon research supported by the South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation. Keywords: dye-sensitized solar cells · indoline dyes · injection rate · pump–probe spectroscopy · zinc oxide [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595 – 663. [2] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395 – 398. [3] R. A. Marcus, N. Sutin, Biochim. Biophys. Acta 1985, 811, 265 – 322. [4] B. O’Regan, M. Gratzel, Nature 1991, 353, 737 – 740. [5] D. C. Look, Mater. Sci. Eng. B 2001, 80, 383 – 387. [6] S. Pearton, Prog. Mater. Sci. 2005, 50, 293 – 340. [7] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauport, D. Lincot, T. Oekermann, D. Schlettwein, H. Tada, D. Wçhrle, K. Funabiki, M. Matsui, H. Miura, H. Yanagi, Adv. Funct. Mater. 2009, 19, 17 – 43. [8] S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska, P. Comte, P. Pchy, M. Grtzel, Chem. Commun. 2008, 5194 – 6. [9] J. Falgenhauer, C. Richter, H. Miura, D. Schlettwein, ChemPhysChem 2012, 13, 2893 – 2897. [10] E. Rohwer, C. Richter, N. Heming, K. Strauch, C. Litwinski, T. Nyokong, D. Schlettwein, H. Schwoerer, ChemPhysChem 2013, 14, 132 – 9. [11] A. Furube, R. Katoh, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2003, 107, 4162 – 4166. [12] A. Furube, R. Katoh, T. Yoshihara, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 12583 – 12592. [13] R. Katoh, A. Furube, K. Hara, S. Murata, H. Sugihara, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2002, 106, 12957 – 12964. [14] R. Katoh, A. Furube, T. Yoshihara, K. Hara, G. Fujihashi, S. Takano, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 4818 – 4822. [15] T. Yoshihara, R. Katoh, A. Furube, M. Murai, Y. Tamaki, K. Hara, S. Murata, H. Arakawa, M. Tachiya, J. Phys. Chem. B 2004, 108, 2643 – 2647. [16] G. Burdzinski, J. Karolczak, M. Ziolek, Phys. Chem. Chem. Phys. 2013, 15, 3889 – 3896. [17] A. El-Zohry, A. Orthaber, B. Zietz, J. Phys. Chem. C 2012, 116, 26144 – 26153. [18] A. M. El-Zohry, B. Zietz, J. Phys. Chem. C 2013, 117, 6544 – 6553. [19] M. Fakis, P. Hrobrik, E. Stathatos, V. Giannetas, P. Persephonis, Dye. Pigment. 2013, 96, 304 – 312. [20] P. W. Lohse, J. Kuhnt, S. I. Druzhinin, M. Scholz, M. Ekimova, T. Oekermann, T. Lenzer, K. Oum, Phys. Chem. Chem. Phys. 2011, 13, 19632 – 19640. [21] K. Oum, P. W. Lohse, O. Flender, J. R. Klein, M. Scholz, T. Lenzer, J. Du, T. Oekermann, Phys. Chem. Chem. Phys. 2012, 14, 15429 – 15437. [22] C. Strothkmper, A. Bartelt, P. Sippel, T. Hannappel, R. Schtz, R. Eichberger, J. Phys. Chem. C 2013, 117, 17901 – 17908. [23] T. Le Bahers, T. Pauport, G. Scalmani, C. Adamo, I. Ciofini, Phys. Chem. Chem. Phys. 2009, 11, 11276 – 84. [24] U. B. Cappel, S. M. Feldt, J. Schçneboom, A. Hagfeldt, G. Boschloo, J. Am. Chem. Soc. 2010, 132, 9096 – 9101. [25] A. Fattori, L. M. Peter, H. Wang, H. Miura, F. Marken, J. Phys. Chem. C 2010, 114, 11822 – 11828. [26] M. Rudolph, T. Yoshida, D. Schlettwein, J. Electroanal. Chem. 2013, 709, 10 – 18. [27] J. Zhang, L. Sun, K. Ichinose, K. Funabiki, T. Yoshida, Phys. Chem. Chem. Phys. 2010, 12, 10494 – 10502.
TAS experiments were conducted with the same pump–probe setup described in a previous paper:[10] Complete ZnO solar cells sensitized with either Indoline DN91, DN216, DN285 or D149 and coupled with an iodide tri-iodide redox solution were excited along the S1 S0 transition around 530 nm with 100 nJ, 50 fs pulses generated in a non-collinearly phase-matched optical parametric amplifier (NOPA). The transient absorption spectrum was measured by a supercontinuum (450–700 nm) pulse generated in a 3 mm sapphire crystal and recorded by a 1024 pixel linescan camera with a 1 KHz readout rate (Ingenieurbro Stresing) to match the repetition rate of the Clark-MXR laser system that powers the setup. A beam chopper was incorporated into the pump beam path and blocked every other pulse. A software recording the absorption spectrum of the sample from the readout of the pixel array in the focal plane of the spectrometer that the probe pulse was coupled into binned every alternate pulse spectrum into a pumped (beam block open) and unpumped (beam block closed) spectrum. The software then divided these spectra by each other to elucidate the spectral signatures that had !
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ARTICLES Pump–probe spectroscopy studies in the ps to sub-ps regime are performed to reveal the role of different anchor groups at indoline-type sensitizer molecules in the charge-carrier dynamics of ZnO-based dye-sensitized solar cells.
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E. Rohwer, I. Minda, G. Tauscher, C. Richter, H. Miura, D. Schlettwein, H. Schwoerer* && – && Ultrafast Charge-Transfer Reactions of Indoline Dyes with Anchoring Alkyl Chains of Varying Length in Mesoporous ZnO Solar Cells
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