Aggregation-Enhanced Light-Harvesting and ... - ACS Publications

Article pubs.acs.org/JPCC

Templated Assembly of Betanin Chromophore on TiO2: AggregationEnhanced Light-Harvesting and Efficient Electron Injection in a Natural Dye-Sensitized Solar Cell Nicholas A. Treat, Fritz J. Knorr, and Jeanne L. McHale* Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164-4603, United States S Supporting Information *

ABSTRACT: Enhanced light-harvesting and photoconversion efficiency of nanocrystalline TiO2 sensitized with the plant pigment betanin is observed in the presence of spectral signatures which reveal self-assembly of betanin on the surface. Though aggregation of sensitizing chromophores is generally considered detrimental to dye-sensitized solar energy conversion, solar cells constructed with aggregated betanin show a 2.5-fold increase in power conversion efficiency compared to those using mostly monomeric betanin. Dye aggregation results in a broadened absorbance spectrum with extended light harvesting at blue and red wavelengths. Variation in solution conditions and soaking times for film sensitization enable control of the relative amounts of adsorbed monomer and aggregate. Self-consistent modeling of the absorption spectra of betanin-sensitized TiO2 films as a function of dye loading suggests the templated formation of a betanin dimer on the TiO2 surface and associated splitting of the excited state. We show that dye aggregation increases the light-harvesting efficiency as well as the incident photon-to-current conversion efficiency (IPCE). Measurement of the absorbed photon-to-current conversion efficiency (APCE) reveals that electron injection and collection of the putative betanin dimer is more efficient than that of the monomer. Not only is this the first report of a dye-sensitized solar cell performance increase upon dye aggregation, but it also constitutes a record power conversion efficiency for a natural dye-based solar cell of 3.0%.

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INTRODUCTION Ongoing efforts to optimize solar energy conversion are frequently inspired by Nature, which uses finely tuned assemblies of chromophores to harness sunlight.1,2 In photosynthetic organisms, light-harvesting complexes of chlorophyll and bacteriochlorophyll derivatives are optimized to collect and funnel solar energy to reaction centers where charge separation takes place. TiO2-based dye-sensitized solar cells (DSSCs), on the other hand, depend on adsorbed dyes which perform both the light-harvesting and interfacial charge-separation steps.3−5 Although self-assembly of dyes on the metal oxide surface is prevalent owing to their high surface density,6,7 aggregation of dyes on TiO2 is frequently reported to lower the rate and yield of electron injection, resulting in lower photocurrents.8−12 Consequently, high-performing DSSCs often employ spacer molecules,13−16 competitively binding molecules such as organophosphates,17 functionalized dyes with steric hindrance,18−20 or surface treatments21 to prevent dye aggregation. The reduced photocurrents that result from sensitizer aggregation are variously attributed to decreased excited-state lifetime, self-quenching, attenuation of light by a thicker dye layer, and weak electronic coupling of dyes to TiO2 as a result of greater distance from the surface. On the other hand, dye aggregation can be beneficial to solar energy conversion in that it is often accompanied by spectral broadening which can improve the overlap of the device optical © 2016 American Chemical Society

absorption with the solar spectrum. Structured biomimetic chromophore assemblies are being pursued with the goal of exploiting energy transfer in solar energy conversion,22−26 and there is additional incentive to replace costly synthetic sensitizers with natural pigments.27−29 Unfortunately, none of these biomimetic approaches have resulted in energy conversion efficiencies that rival those of DSSCs using monomeric organic or metallorganic sensitizers. Better understanding of the influence of aggregation on TiO2-based solar energy conversion should permit the enhanced light harvesting of dye assemblies to be exploited without diminished yield of photoelectrical conversion. Recently, we have used betanin (Bt) extracted from beet root as a sensitizer and achieved power conversion efficiencies up to 2.7% and high incident photon-to-current conversion efficiency (IPCE).30 Bt (Figure 1) is a light-harvesting plant pigment belonging to the betalain family, the pigments of which serve photoprotective and antioxidant roles in plants of the order Caryophyllales.31,32 Betalain pigments can be separated into two subclasses of molecules based on Schiff’s base condensation with betalamic acid and a nitrogen heteroatom. The simpler subclass of betalains is the betaxanthins which are condensation Received: March 10, 2016 Revised: April 14, 2016 Published: April 15, 2016 9122

DOI: 10.1021/acs.jpcc.6b02532 J. Phys. Chem. C 2016, 120, 9122−9131

Article

The Journal of Physical Chemistry C

Figure 1. Structure of betanin (left) and 3D representation of optimized geometry (right) obtained using DFT as described in ref 42

conversion compared to analogous DSSCs containing monomeric Bt.

products with various amino acids and amines. Betacyanins, including betanin, are more specifically the condensation products of cyclo-dopa derivatives with betalamic acid. Although betalamic acid and the betaxanthins exhibit a strong absorption in the blue, with molar absorptivity ε = 45 000 M−1 cm−1 at 470−500 nm, betacyanins absorb in the green, ε = 65 000 M−1cm−1 at 525−535 nm.33,34 Like their cousins the anthocyanin plant pigments, betalains have been explored as natural sensitizers in DSSCs by our group30,35,36 and others.37−39 Though anthocyanins are known to self-assemble in solution and in vivo,40,41 we are not aware of any studies pointing to aggregation of betanin or other betalains in vivo or in vitro. However, we have noted previously that the absorption spectrum of Bt on TiO2 is considerably broader than that of aqueous Bt, and we have speculated that dye aggregation may be responsible for enhanced light-harvesting.36 Though previously we have considered that preferential absorption of betaxanthins on TiO2 might have contributed to the blue shift, in the present work HPLC analysis confirms the absence of betaxanthins or other yellow pigments. As shown in Figure 1, the optimized ground-state geometry of Bt as obtained by a DFT calculation is decidedly nonplanar, which would inhibit intermolecular interactions via π-stacking. However, the presence of three carboxylic acid functions and two nitrogen heteroatoms on Bt provides ample opportunity for intermolecular interactions though hydrogen-bond formation, perhaps assisted by templated adsorption on the TiO2 surface. We have reported femtosecond and nanosecond transient absorption spectroscopy of Bt-sensitized TiO2 along with spectroelectrochemical measurements and DFT calculations, revealing that excited-state Bt undergoes a two-electron, oneproton oxidation.42,43 In the course of investigating the electrochemical oxidation of Bt on TiO2 as a function of dyeloading, we observed spectral changes suggestive of aggregation. In this work, we use internal and external quantum efficiencies for electron collection, fluorescence measurements, and numerical modeling to understand the evolution of quantum efficiency and light-harvesting as a function of dyeloading. We report here a record high power conversion efficiency, 3%, for a natural-dye based DSSC, sensitized with aggregated Bt. Evidence from numerical modeling and theory will be shown to lead to the conclusion that an excitonically coupled dimer of Bt forms on the surface of TiO2, leading to enhanced light harvesting, electron injection, and energy

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EXPERIMENTAL METHODS Purification. Betanin was extracted from red beet (Beta vulgaris L.) via a modified procedure from ref 44. Red beet root was crushed and juiced by a macerating juicer. The juice was mixed 1:1 with ethanol and filtered to remove proteins. This mixture was adjusted to pH 4 with HCl. Betanin was isolated, on a Sephadex DEAE A25 column at pH 4, and washed with a 0.05 M pH 4 acetate buffer. Betanin was desorbed from the column with a 0.05 M pH 7 phosphate buffer. All excess salts in the eluent were removed. The product was further purified by solid phase extraction on a Phenomenex Strata C18 SPE column and washed with five volumes of water acidified with HCl to a pH of 2. The betanin was desorbed with methanol acidified with a small amount of HCl and dried under reduced pressure at 30 °C. Purity was confirmed by analytical HPLC.44 HPLC. Analytical HPLC was performed using a binary gradient with solution A (0.25% aqueous trifluoroacetic acid) and solution B (acetonitrile). The gradient is as follows: 0−1 min 7.2%B, then ramped to 10%B at 5 min, then to 15%B at 7 min, followed by a rinse of 100%B at 8 min for 1 min, then equilibrated for 2 min at 7.2%B.45−47 Film Preparation and Sensitization (TiO2 and ZrO2). Films of TiO2 were prepared on conductive substrate, fluoride doped tin oxide (FTO), for DSSC construction, while films of TiO2 and ZrO2 for absorption and emission spectroscopy were prepared on glass microscope slides. For DSSC construction, a blocking layer of titanium dioxide was prepared by heating FTO coated slides (Tech15, Hartford Glass) in a solution of dilute 0.1 M TiCl4 to 60 °C for 30 min. The coated films were dried in an oven at 100 °C for 30 min before being heated in a muffle furnace to 500 °C for 30 min. Films of titanium dioxide were prepared by the doctor blade method with Dyesol 18NRT paste on FTO glass slides. The film consists of 20 nm nanoparticles of TiO2 in the anatase phase. Typical SEM images of the resulting films are shown in Supporting Information (Figure S1), along with side views which show the film thicknesses. Films with a thickness of 3.5 μm were prepared with a 50% dilution of the TiO2 paste with ethanol, 7 μm films were prepared by using undiluted paste, and 14 μm films were prepared by two successive coatings of paste. The films were placed in a drying oven at 100 °C for 1 h to smooth 9123


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Figure 2. Absorption spectra of Bt on (A) ZrO2 and (B) TiO2 as a function of sensitization time in a saturated betanin solution at pH 2. A 900s film on TiO2 was prepared, but it was too dark to determine the absorbance.

and then heated in a muffle furnace to 500 °C for 30 min. Some of the films were selected for an additional treatment with 0.1 M TiCl4 after cooling, as in ref 30. Prior to sensitization films were equilibrated either in purified water or in 0.5 M hydrochloric acid in ethanol. For absorption and fluorescence measurements, films of TiO2 and ZrO2 were prepared by doctor-blading directly onto glass slides. TiO2 films for spectroscopy were 3.5 μm, leading to sufficient transmission for accurate absorption measurement. Owing to lower tendency of Bt to adsorb on ZrO2 compared to TiO2, 7 μm thick ZrO2 films were used in order to obtain sufficient dye loading for spectroscopy measurements. Zirconium dioxide films were prepared from Zr-Nanoxide Z/ SP paste purchased from Solaronix. Prior to sensitization, films for fluorescence measurements were equilibrated in a solution of 0.25% aqueous trifluoroacetic acid. Because the fluorescence measurements (described below) were done using front face emission and corrected for absorptance, the film thickness is not critical to the comparison of fluorescence of Bt on TiO2 and ZrO2. Bt-sensitized ZrO2 films were used only for absorption and emission, not for fabrication of dye-sensitized solar cells. Films were sensitized in solutions with varying amounts of betanin powder dissolved in 25 mL of purified water, and the pH was adjusted by dropwise addition of aqueous 1 M hydrochloric acid. Films were sensitized for varying times to produce a range of aggregation. To produce films with very low amounts of aggregation, 1 mM sodium phosphate was used in place of purified water to prepare the betanin sensitizing solution. Phosphate ions are known to bind strongly to TiO2 in contact with aqueous solutions,48 and thus, we expected competitive binding would limit self-assembly in the presence of sodium phosphate. Absorption and Fluorescence Spectra. Fluorescence spectra of Bt-sensitized TiO2 and ZrO2 films were excited using a Lexel 95L-UV laser operating at 514.5 nm, using a 45° front facing geometry. The incident laser power was attenuated to 20 ± 2 μW using a neutral density filter. Elastically scattered laser radiation was rejected with a holographic notch filter. Emission from 400 to 900 nm was detected with a thermoelectrically cooled CCD camera. All spectra were taken within 10 min of removal of films from the sensitizing solution. Films were otherwise kept in the dark to minimize photodegradation of betanin.

Absorption spectra were taken on a Shimadzu UV-2550 spectrometer before and after fluorescence measurements to check for dye degradation. Fluorescence spectra were scaled by the absorptance at the excitation wavelength, 1−10−A, where A is the absorbance of the film, to correct for the variations in the percent light absorbed for different samples. IPCE and APCE. Incident photon-to-current conversion efficiency was recorded as a function of wavelength, IPCE(λ), as described in a previous publication.30 Absorbed photon-tocurrent conversion efficiency APCE(λ) was obtained from IPCE(λ) by dividing by the absorptance. IPCE(λ) =

Jsc (λ) e Φ(λ)

APCE(λ) =

η=

Jsc Voc FF Pin

(1)

IPCE(λ) 1 − 10−A(λ)

(2)

Pmax Pin

(3)

=

In the above, Jsc is the short-circuit current, Φ(λ) is the incident photon flux at wavelength λ, A(λ) is the absorbance at wavelength λ, η is the total energy conversion efficiency, Voc is the open circuit voltage, FF is the fill factor, and Pin is the incident radiation power. Solar Cell Construction. Platinum counter electrodes were prepared by heating an FTO glass slide to 40 °C and adding six drops of platinum hexachloride dissolved in isopropyl alcohol. Once dry, the films were sintered at 450 °C for 30 min. Solar cells were constructed by sandwiching the sensitized TiO2 film and the platinum electrode, and then sealed on three sides with hot-melt glue. On the fourth side, a drop of 0.50 M lithium iodide and 0.05 M iodine dissolved in acetonitrile was applied and allowed to fill the gap between electrodes. Hot melt glue was then applied to the remaining side. The films were tested via the method published in ref 30. Numerical Methods. Non-negative matrix factorization (NMF) was used to determine the number of components and their spectra for Bt-sensitized films as a function of dye-loading. NMF is a self-modeling data interpretation method that resolves a data set into its individual components. The structure of the data must be a linear combination of the components, which in the present case are spectra of distinct forms of Bt on the TiO2 surface and the background spectrum 9124


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Figure 3. Results of NMF analysis of Bt/TiO2 at (A) low and (B) high dye loading. The films in (A) and (B) were sensitized for 300s and overnight (21 h) and contain 25% and 65% aggregate, respectively. The components are dimer (green), monomer (blue), TiO2 extinction (cyan), experimental absorbance (black), and total calculated absorbance (red dashed).

negative conduction band potential of ZrO2 compared to TiO2.42,43 Though Bt appears to absorb less strongly on ZrO2 than on TiO2, it is clear that in both cases longer sensitization times lead to a blue shift that accompanies the increased absorbance of the film. Thus, the spectral changes observed at high dye-loading are not the result of photodegradation of the dye. To confirm that degradation was not the source of the blue-shift, Bt was desorbed from the TiO2 films and the extract submitted to HPLC analysis. As shown in Figure S3 of Supporting Information, HPLC analysis of solutions of material desorbed from the films confirms that betanin is the only component giving rise to any visible light absorption. Similarly, the absorption spectrum of the extract following dye desorption strongly resembles that of the original sensitizing solution. These results strongly indicate that betanin undergoes reversible templated aggregation on the surface of TiO2 and ZrO2 films. It should also be noted that repeated efforts to observe Bt aggregation in aqueous solution were not successful: no deviations from Beer’s law were observed in the absorption spectrum at the highest achievable Bt concentrations. See Supporting Information Figure S4. We conclude that intermolecular interactions of closely packed molecules affect the absorption properties of the dye on the surface of nanocrystalline TiO2 and ZrO2 at high dye loadings. For similar staining times, Bt dye loading is higher and aggregation is more readily observed on TiO2 than on ZrO2. Further confirmation of dye aggregation was obtained by using phosphate ions as coabsorbents.48 As shown in Supporting Information, Figure S5, these inhibit the spectral changes otherwise observed at high dye loadings. We considered the possibility that spectral shifts and inhomogeneous broadening might result from a distribution of binding sites which perturb the transition frequency, versus the formation of discrete aggregates with shifted spectra. We therefore used nonnegative matrix factorization (NMF) to discern the number of components and their resolved spectra which contribute to the total absorption spectrum.49,51 Ninetytwo UV−visible absorption spectra of betanin on TiO2 and ZrO2 films and in solution were compiled and analyzed by NMF. Initial attempts to analyze the data of sensitized films considered a variable number of components and revealed that three components were sufficient to account for the total spectrum. Note that the NMF analysis included a series of spectra: 37 sensitized TiO2 and 19 sensitized ZrO2 films, plus 20 bare films and 16 solution phase spectra. The spectral decomposition and residuals for the entire series is shown in

of TiO2. The component spectra and their weights are required to have positive values. In this case, the absorbance of the betanin-sensitized TiO2 films is found to be the linear combination of three components as discussed below. To apply this method, we construct an 801 × 92 data matrix, D, whose 92 columns are the measured absorbance spectra of the samples and whose 801 rows are the wavelengths (measured every 0.5 nm). Using the algorithm of Lee and Seung,49 D is factored into smaller all-positive matrices: R, an 801 × 3 matrix which contains the normalized spectra of the three individual components in its columns and C, a 3 × 92 matrix which contains the concentration and magnitude information; i.e., D = RC. The spectra in the R matrix are normalized as part of the algorithm, so it is not possible to separate the magnitude of the molar absorptivity from the concentration in the C matrix. The number of components was determined to be three, based on an examination of the residuals of the model of the data and the reproducibility of the components. The residuals are the difference between the measured data and the modeled D matrix, equal to RC calculated using the factored R and C matrices. If not enough components are used, then the data cannot be accurately reproduced; however, if too many components are used, then experimental noise is mistaken for a component.

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RESULTS AND DISCUSSION

Absorption Spectra and Numerical Modeling of Betanin on TiO2 and ZrO2. Figure 2A,B show the absorption spectrum of Bt adsorbed on films of nanocrystalline ZrO2 and TiO2, respectively, as a function of sensitization time. The absorption spectrum of the sensitizing solution is shown in Figure S2 of Supporting Information, compared to that of a sensitized TiO2 film at high dye-loading. For low dye-loading, the absorption spectrum of Bt on TiO2 resembles that of the aqueous solution with an absorption maximum near 530 nm. Films exposed to the sensitizing solution for longer times are seen to exhibit blue-shifted absorption maxima and spectral broadening. Previous evidence showed that the absorption spectrum of Bt on ZrO2 did not exhibit as significant a blueshift and was therefore used to model a film with higher monomer character. Another factor that we considered was that the blue-shift resulted from a degradation of the betanin chromophore to a yellow oxidation product.50 This degradation/oxidation product was observed during time-resolved absorption experiments. We have previously shown that Bt/ ZrO2 films are stable to photooxidation owing to the more 9125


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Figure 4. (A) IPCE and (B) APCE of Bt-based DSSC consisting of 25% aggregate (red), and 65% aggregate (black).

collection, respectively. The light harvesting efficiency, also called the absorptance, is a function of absorbance A: ηLH = 1− 10−A(λ). APCE is obtained from IPCE by dividing by the absorptance at each wavelength and is thus the product of the efficiencies of electron injection and collection. The wavelength-dependent ηLH is of course larger for the film containing more aggregates, owing to higher dye loading. In addition, IPCE(λ) of the aggregate-rich DSSC is broader and attains a higher maximum value than that of the monomer-rich DSSC. Further, IPCE(λ) of the monomer-rich DSSC follows the absorbance much more closely than that of the aggregate-rich DSSC. Comparison of APCE(λ) for the two solar cells, Figure 4B, is more revealing. While APCE(λ) for the monomer-rich solar cell is similar to the corresponding absorption spectrum, that of the aggregate-rich solar cell has pronounced maxima at 450 and 675 nm. While these are close to the maxima in the two bands of the aggregate component absorption spectrum, the red peak in APCE(λ) is almost as intense as the blue peak, whereas in the aggregate absorption spectrum, the blue band is considerably more intense than the red. This correlates to fast relaxation from the higher excited state to the lowest-lying excited state according to Kasha’s rule. The overall higher values of APCE for the aggregate-rich DSSC indicate that improvements to photoconversion efficiency are not merely the result of better light harvesting. Rather, the aggregate exhibits higher efficiency for electron injection and collection than the monomer. Unfortunately, quantification of this phenomena by NMF analysis is not possible because the IPCE(λ) is not a linear combination of the IPCE(λ) of the individual components. However, the increased IPCE correlates very well with the amount of aggregation determined by NMF analysis on the absorbance. Since efforts to develop sensitizers with enhanced red absorption are sometimes thwarted by lower driving force for electron injection,5 it is particularly significant that the lower energy excited state of the dimer has sufficient reducing power for efficient electron injection. Fluorescence of Bt on TiO2 and ZrO2. Ordinarily, a rough estimate of the injection efficiency of photoexcited dye can be determined from the quenching of its fluorescence on TiO2 compared to on ZrO2 where no injection takes place, owing to the higher absolute energy of the ZrO2 conduction band.52 However, there are complications to this approach for the present system. In aqueous solution, Bt displays a weak fluorescence spectrum with a maximum at about 640 nm.30,43,53 Wendel et al. recently reported that the fluorescence quantum yield φfl of Bt in aqueous solution, about 0.0007 in water, increases with solvent viscosity, while the Stokes shift decreases.53 This points to an excited-state conformational change as a significant nonradiative relaxation pathway

Supporting Information, Figure S6−S8. The NMF method cannot separate each component spectrum into the amount and relative intensity of each contribution. Rather, it provides a basis set of three spectra for which each experimental spectrum can be expressed as a linear combination thereof. Figure 3 displays the results of NMF analysis for a film containing a relatively low (Figure 3A) and high (Figure 3B) contribution of aggregate to the total absorption of Bt on TiO2. The three individual components are accounted for as the absorption/scattering of TiO2, and the absorption of monomer and aggregated forms of Bt. The data in Figure 3A are for a sample for which NMF reveals 72% monomer and 23% aggregate contributions, while Figure 3B corresponds to 35% monomer and 65% aggregate. As seen below, the aggregate spectrum is accounted for well if it is attributed to a dimer of Bt. Because spectra were obtained in transmission, the TiO2 contribution consists of the band edge absorption (at ∼390 nm) as well as scattering at visible wavelengths. NMF analysis of solution phase spectra produced only minor amounts of dimer (Figure S7B), consistent with adherence to Beer’s law. We believe the small apparent dimer contribution in the resolved solution phase spectra is a consequence of the inability of the model to separate the relatively similar spectra of the monomer and dimer (see Figure S2). Blank films produced only the TiO2 (or ZrO2) component spectrum (Figure S7C). The consistency with which the NMF method produced component spectra similar to those of the bare films and the monomer in solution lends confidence that the third component is an aggregated form of Bt. Based on the NMF, the absorption spectrum of aggregated betanin consists of two bands which are blue- and red-shifted relative to the monomer spectrum, at about 470 and 607 nm, respectively. This is in complete accord with excitonic splitting of a dimer (vide inf ra), in which transition dipole moment coupling leads to two new absorption bands. Dimer aggregates are quite common in the plant pigments of the anthocyanin family, and it is reasonable to speculate that betalains would exhibit similar behavior.40,41 At long staining times, the amount of aggregate and monomer on TiO2 is approximately equal. Quantum Efficiency of Photon-to-Current Conversion. Figure 4A,B show the wavelength-dependent incident photon-to-current conversion efficiency (IPCE) and absorbed photon-to-current conversion efficiency (APCE) for Bt/TiO2based solar cells containing mostly (75%, red curve) monomer and mostly (65%, black curve) dimer. IPCE is the ratio of the number of electrons collected in the external circuit to the number of incident photons and is the product of three efficiencies: IPCE = ηLHηinjηcoll, where ηLH, ηinj, and ηcoll are the efficiencies of light-harvesting, electron injection, and electron 9126


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Figure 5. Fluorescence spectra, excited at 514.5 nm and corrected for absorptance at 514.5 nm, of (A) Bt on TiO2 and (B) ZrO2 as a function of sensitization time. Fluorescence intensity decreases with increasing dye loading, with the exception of the 15s ZrO2 film which shows an initial increase as a result of minimal aggregation.

responsible for the weak fluorescence. Clearly, electron injection from excited-state Bt to TiO2 quenches the fluorescence, such that the emission of Bt is quite weak on TiO2. Nevertheless, the fluorescence spectra of sensitized films are readily determined if stray light is carefully rejected. In addition, independent of fluorescence quenching that results from electron injection to TiO2, there are potentially changes to the fluorescence spectrum and quantum yield on aggregation. To investigate these, the fluorescence spectrum of Bt on both TiO2 and ZrO2 was determined as a function of dye loading as shown in Figure 5. As seen in there, in both cases higher dye loadings lead to reduced intensity and increasing red shift in the fluorescence spectrum. Considering the front-face emission geometry used here and the large Stokes shift (about 3000 cm−1 in solution), the inner filter effect is expected to be negligible. Thus, the results indicate an increasing Stokes shift for the aggregate compared to the monomer. This is expected based on Kasha’s rule that emission takes place from the lowest energy excited state (i.e., the red band of the dimer). In addition, the decrease in intensity that accompanies increased dye-loading on ZrO2 reveals that the dimer has a lower fluorescence quantum yield than the monomer, because fluorescence quenching by electron injection is precluded on ZrO2. Aggregation tends to enhance nonradiative pathways leading to lower fluorescence yields compared to the monomer. Despite this complication, it is clear from the scale in Figures 5A and B that the fluorescence is lower for Bt on TiO2 compared to ZrO2 for similar sensitization times, despite higher dye-loadings on TiO2. Though we cannot make a quantitative estimate of the injection efficiency from this data, the reduced fluorescence on TiO2 compared ZrO2 is evidence of efficient electron injection by both the monomer and the aggregate. Power Conversion Efficiency of Dimer-Based Solar Cells. Several films were prepared at conditions which produced varying amounts of aggregation in the IPCE study, but were too thick to allow direct absorbance spectra to be taken, for efficiency testing with a solar simulator. These films produced efficiencies ranging from 0.5 to 3.0% power conversion efficiency. Shown in Figure 6 are the results for several DSSCs prepared using films in a range of thicknesses from 3.5 to 14 μm. Table 1 displays further information on the percent aggregation, and maximum photocurrent and photovoltage for each solar cell. By using linear combinations of the

Figure 6. Photocurrent versus photovoltage for several Bt-based dyesensitized solar cells. Further information provided in Table 1. The power conversion efficiencies and film thicknesses are as indicated in the legend.

monomer and aggregate components, an approximate aggregate content could be estimated for films that were too dark to effectively analyze with the NMF technique. The darkest films often showed a distortion in the spectrum due to detector saturation, and the median point of the saturated peak was taken as the approximate maximum. All DSSCs were constructed using TiO2 films that had been pretreated with acidic ethanol. The highest efficiencies were produced by films prepared under conditions that produced larger amounts of aggregation as judged by the blue shift in the absorption maximum. The lowest efficiency, 1.3%, was obtained for the DSSC having an absorption maximum (536 nm) consistent with the prevalence of monomers. Unfortunately, despite high current densities, Bt-based DSSCs are limited by low Voc values of about 300−375 mV, which is due to recombination effects. The low voltage does not appear to be a result of acid pretreatment of the films as similar voltages were obtained with films that were not treated with acid prior to sensitization as shown in Supporting Information Figure S9. Despite the low power conversion efficiency, it is quite remarkable that a DSSC created from a naturally occurring pigment can be used to moderate effect in solar energy applications. In our lab, DSSCs constructed similarly to the ones tested in this study but using N3 as the sensitizer produced a maximum conversion efficiency of ∼5%. With better understanding of the role of aggregation in 9127


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The Journal of Physical Chemistry C Table 1. Power Conversion Efficiency, Open Circuit Voltage Voc, Short-Circuit Current Jsc, Wavelength of Maximum Absorption for Sensitized Film λmax, Film Thickness, and Percent Aggregation for Each of the Solar Cells in Figure 6 DSSCa

efficiency, %

Voc, mV

Jsc, mA/cm2

λmax, nm

film thickness, μm

% aggregation

black red green blue cyan

2.5 2.4 2.6 3.0 1.3

375 350 350 375 350

14 15 15 14 7

501 523 --486 536

3.5 7 14 7 3.5

50 25 75 80 0

a All films were sensitized from saturated aqueous Bt except the last one, which used 1 mM Bt for staining. All of the films were pretreated with 0.5M HCl except the last, which was pretreated with purified water.

moment, and û1 and û2 are unit vectors in the direction of the transition dipole moments of molecule 1 and 2 respectively. Since the blue-shifted dimer state corresponds to the more intense band, V12 is positive and the dimer is more H-like than J-like. The transition dipole moment connecting the groundstate φg(1)φe(2) to the two excited states is

enhancing the efficiency of Bt-based DSSC, we are now able to pursue optimization of efficiency through improvements to electrolyte composition and surface treatment. Compared to the typical maximum efficiency obtained with N3 of 11%, it is reasonable to postulate that betanin-based solar cells can be significantly improved by engineering modifications to the solar cell assembly. The total conversion efficiency of 3.0% overtops the maximum obtained for a DSSC based on a natural dye of 2.7% obtained by our lab in 2011.30 Theory. Analysis of the spectra of Bt on TiO2 using NMF reveals a component with two absorption bands at about 470 and 607 nm (21 300 and 16 500 cm−1, respectively), with an intensity ratio of approximately 4:1. The more intense band is shifted 2400 cm−1 to the blue of the monomer absorption band at 530 nm (18 900 cm−1), whereas the less intense component is red-shifted from the monomer by 2400 cm−1. The NMF analysis reveals that these blue- and red-shifted absorption bands belong to a single species, and given the remarkably similar magnitudes of the shifts relative to the monomer, it is reasonable to propose that this species is a dimer that assembles on the TiO2 surface. The coupling of the transition dipole moments μge of two molecules leads to two new excited electronic states, which are shifted from the monomer by an amount dependent on intermolecular distance, magnitude of the transition dipole and relative orientations μge on the two molecules. The relative intensities of the blue- and red-shifted dimer bands depend on the angle between the transition dipoles. As shown next, a reasonable model for this dimer is one in which the transition dipole moments of the two molecules make an angle of about 50°. Excitonic coupling of the excited states of the dimer leads to wave functions which are in-phase and out-of-phase combinations of the basis-states φg(1)φe(2) and φg(2)φe(1), which represent excitation of either molecule 1 or 2 of the dimer with the other molecule remaining in the ground state. These two new dimer wave functions are φ± =

1 [φ (1)φe(2) ± φe(1)φg (2)] 2 g

1 ⇀ ( μ (1) ± ⇀ μge (2)) (6) 2 ge μge is the transition dipole moment of the monomer. Here ⇀ The energies of the two dimer states are μ± =

E± = Emon ± V12

where Emon/hc = 18 900 cm−1 is the energy of the monomer excited state. The intensity of each band of the dimer is proportional to the square of μ+ or μ−, such that the ratio of the band intensities for the blue- and red-shifted components is μ+2 I+ 1 + cos α = 2 = I− 1 − cos α μ−

V12 =

hcr 3

[u1̂ ·u 2̂ − 3(u1̂ ·r )( ̂ u 2̂ ·r )] ̂

(8)

where α is the angle between the transition dipoles on molecules 1 and 2. The results of the NMF analysis result in a blue band for the dimer, which is about 4 times as intense as the red band, based on peak height. This leads to an estimated angle α of approximately 53°. There is no evidence that dimerization takes place in aqueous solution, and thus, we hypothesize that the TiO2 surface acts as a template for the assembly. The distance between bridging oxygens on the ideal (101) surface of anatase TiO2 is on the order of 5 Å, as is the distance between 5-fold-coordinated Ti. Therefore, we chose r = 5 Å to estimate the coupling strength V12. Using the integrated absorption of the solution phase spectrum ∫ ε(ν)dν/ν to estimate the transition dipole results in μge = 8.7 D. (This estimate is based on a maximum molar absorptivity εmax of 65 000 L/mol cm.54) This gives μge2

(4)

hcr

Excitonic coupling of the basis states leads to two new states split by 2V12, where V12 is the matrix element of the coupling perturbation with respect to the two basis states: ⟨φg(1)φe(2)| Ĥ ′|φg(2)φe(1)⟩. Using the crude assumption that the coupling results from the interaction of point dipoles, the coupling strength V12 is expressed in wavenumber units as μge2

(7)

3

≈ 3050 cm−1

(9)

Thus, an angular factor û1 · û2 − 3(û1 · r̂)(û2 · r̂) on the order of 0.8 is consistent with the intensities and frequencies of the two dimer absorption bands. Further efforts to estimate the orientation of the dipole moments relative to the intermolecular vector r ⃗ are not warranted in this crude model; however, it is clear that transition dipole coupling provides a reasonable explanation of the observed spectrum of the dimer.

■

(5)

CONCLUSIONS This research represents the first evidence of an aggregated sensitizer improving performance in a DSSC. Based on

Here, r is the intermolecular distance, r̂ is a unit vector in the direction of r,⃗ μge is the magnitude of the monomer transition 9128


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The Journal of Physical Chemistry C APCE(λ) data, the aggregate outperformed the monomer by broadening the absorption spectrum and increasing electron injection and collection over a wider range of wavelengths than the monomer. In addition to this unprecedented result, we have achieved a record efficiency for a DSSC based on a natural dye. As reported previously, betanin-based DSSCs produce large current densities (∼15 mA/cm2). Unfortunately, the Voc is limited by recombination. The large current densities and the broad absorption reinforces the potential of betanin-based DSSCs. Since we have shown that the electron injection of Bt to TiO2 is a two-electron, one-proton process,42,43 we are presently pursuing alternative proton-coupled redox mediators, unlike the iodide/triodide redox couple used here, which may be more efficient at dye regeneration, inhibiting the recombination that appears to limit Voc. In addition, as shown in ref 30, there are efficiency losses over the course of several hours for Bt-based DSSCs, and more work is needed to improve the stability of the dye. It is interesting to consider the mechanism for improved electron injection and collection in DSSCs with more aggregated as compared to monomeric Bt. One factor is the strong splitting that results from coupling of the transition dipole moments, which allows light harvesting over a broader range of wavelengths. The visible absorption band of Bt in solution has an oscillator strength near unity, and the fairly large transition dipole moment facilitates excitonic coupling that broadens the absorption band. It is also reasonable to speculate that, with three carboxylate functions and two nitrogen heteroatoms, there is ample opportunity for hydrogen bonding to play a role in the assembly, in addition to templating on the TiO2 surface. It may be significant that assembly apparently stops at the dimer stage. Larger aggregates present drawbacks mentioned in the introduction, such as reduced electronic coupling to TiO2 and formation of insulating layers of dye. Smaller aggregates would not exhibit these detrimental characteristics. Importantly, we find that it is not mere improvement in light harvesting that makes for better performance of dimer-based DSSCs. However, we are not able to separate the roles of improved electron injection and collection as of yet. We have shown that the photo-oxidation of Bt on TiO2 is a two-electron, one-proton process.42,43 It is conceivable that dimerization enables proton transfer, enhancing electron injection. Alternatively, there may be more spatial separation of the HOMO and LUMO in the dimer than in the monomer, inhibiting electron recombination and enhancing collection.

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■

NMF analysis (Figure S8); characteristic current−voltage plots from films with and without acid pretreatment (Figure S9) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Riley Rex for the film characterization by SEM. The support of the National Science Foundation grant DMR1305592 is gratefully acknowledged.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02532. Scanning electron micrograph of TiO2 films (Figure S1); comparison of absorption spectrum of betanin in solution and on TiO2 (Figure S2); HPLC and absorption spectra of extracted films (Figure S3); Beer’s law plot of aqueous solutions of betanin (Figure S4); absorption spectra of betanin adsorbed on TiO2 from solutions with and without phosphate (Figure S5); unit spectral components of monomer, dimer, and scattering background (Figure S6); NMF component analysis for solutions of Bt and Bt on TiO2 and ZrO2 (Figure S7); linear combinations of monomer and dimer spectra from 9129


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