What Limits Photon Upconversion on Mesoporous Thin Films

Subscriber access provided by SELCUK UNIV

Article

What Limits Photon Upconversion on Mesoporous Thin Films Sensitized by Solution-Phase Absorbers? Jonas Sandby Lissau, Djawed Nauroozi, Marie-Pierre Santoni, Tomas Edvinsson, Sascha Ott, James M. Gardner, and Ana Morandeira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5118129 • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 10, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

What Limits Photon Upconversion on Mesoporous Thin Films Sensitized by Solution-Phase Absorbers? Jonas Sandby Lissau,†,¶ Djawed Nauroozi,† Marie-Pierre Santoni,† Tomas Edvinsson,† Sascha Ott,† James M. Gardner,‡ and Ana Morandeira∗,† Department of Chemistry - Ångström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden, and Department of Chemistry - Division of Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden E-mail: [email protected]

∗ To

whom correspondence should be addressed University. ‡ KTH Royal Institute of Technology. ¶ Currently at KTH Royal Institute of Technology † Uppsala

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 47

Abstract Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a promising strategy for breaking the Shockley-Queisser limit for efficiency of single-threshold solar cells, and in particular dye-sensitized solar cells (DSSCs). Here, we report on a heterogeneous UC system, where the annihilating dyes (“emitters”) are bound to a ZrO2 nanostructured film and the light absorbing dyes (“sensitizers”) are free in solution. A comparative study of four different emitter dyes was conducted, all of them derivatives of the well-known UC-STTA emitter dye 9,10-diphenylanthracene (DPA), and in every case the sensitizer dye was platinum(II) octaethylporphyrin (PtOEP). The physical separation of emitter and sensitizer molecules in two different phases makes homogeneous triplet–triplet annihilation among sensitizers in solution a significant loss channel at high excitation intensity and low emitter surface coverage. For the studied emitter dyes, the number and type of anchor groups, and the solubility of the emitter dye in the employed solvents, are the determining factors of the UC output. The signal evolves in time and with light exposure due to emitter desorption and light-induced endoperoxide formation. These results can guide the way towards a better understanding of UC-STTA on nanocrystalline metal oxides and its development for solar energy applications.

Introduction Light from the sun is the most abundant source of energy available to humans. 1 Harvesting solar energy can potentially be done in a fashion which does not significantly harm human society. Most commercial solar cells are confined by the Shockley-Queisser limit for solar energy conversion efficiency, which at present limits the potential for widespread use of these technologies. 2 During the past decade, the photophysical mechanism of photon upconversion based on sensitized triplettriplet annihilation (UC-STTA, see Scheme 1), has been recognized as an approach to overcome the Shockley-Queisser limit by converting low energy photons, that otherwise would not be absorbed, into high energy photons and excited-states that can be used. 3–6 The recent acceleration of publications within the UC-STTA research field is documented in a number of reviews, 7–13 and 2 ACS Paragon Plus Environment

Page 3 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the first examples of proof-of-principle improvements of solar conversion efficiencies have been demonstrated for a number of solar cell technologies based on low cost materials. 14–16 There are however substantial problems that need to be overcome before UC-STTA can be effectively utilized in dye-sensitized solar cells (DSSCs). The potential of applying UC-STTA in DSSCs as a molecular approach to the intermediate band solar cell, was first discussed by Ekins-Daukes and Schmidt. 17 It was shown that such an implementation would raise the theoretical efficiency limit of low cost DSSCs to above 40%. 17 The raised efficiency limit can be achieved by increasing the photocurrent through the absorption of photons with energies below the solar cell bandgap, while preserving a high photovoltage.

Scheme 1: Schematic representation of the processes involved in photon upconversion based on sensitized triplet-triplet annihilation (UC-STTA). S is sensitizer, E is emitter, S0 is ground state, S1 , first excited singlet state, T1 , first excited triplet state, ISC is intersystem crossing, TET is triplet energy transfer, and TTA is triplet-triplet annihilation. See text for more details. Our approach is to address this challenge in a systematic way. Instead of using mesoporous TiO2 , the metal oxide usually employed in the DSSCs photoanodes, we have opted to work with ZrO2 films. ZrO2 has structural properties and a refractive index very similar to TiO2 , but its bandgap is higher in energy. 18 The high energy bandgap prevents electron injection from the excited dye molecules into the semiconductor, and thereby allows the study and optimisation of UC-STTA by simple fluorimetry methods. Experimentally, we showed in previous works that UC3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 47

STTA can take place when the emitter and sensitizer molecules are physisorbed on a mesoporous ZrO2 film. 19 The efficiency of the process was however very low due to, among other reasons, aggregation of the sensitizer molecules. More recently, an alternative sample architecture was tested. The annihilating molecules (the “emitters”) were chemically bound to the nano-surface, while the absorbing molecules (the “sensitizers”) were free to diffuse in the solution surrounding the nanostructure (see Chart 1), 20 minimising the formation of sensitizer aggregates. This system was contained in cells sealed in a glove-box, and showed a 60 times improvement in the UC-STTA quantum yield compared to the initial proof-of-principle system, where both emitter and sensitizer dyes were co-physisorbed on the nanostructured surface. Sensitizer

Emitter

ZrO2

Chart 1: Structural composition of the samples.

The more recent system architecture, with sensitizer and emitter molecules in two separate phases, is reminiscent of DSSCs that include so-called “relay” dyes or quantum dots in the solar cell electrolyte. 21–25 The function of these dyes is to absorb photons that would otherwise have been transmitted by the solar cell and transfer the excitation energy to the surface bound dyes, where the energy can be collected by injection of an electron to the TiO2 conduction band. Similarly, in the herein presented system, energy transfer from the sensitizer dyes in solution to the emitter dyes on the surface is required. It is noteworthy though, that while “relay” dye systems can noticeably improve the light harvesting of DSSCs, their maximum theoretical efficiency will


Page 5 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


still be restricted by the Shockley-Queisser limit (∼ 30 %). In DSSCs the use of additional light harvesting molecules in the electrolyte has the advantage of increasing the number of absorbed photons without compromising light harvesting from dyes attached to the nanostructured surface. For the use of this type of system architecture for UC-STTA in solar energy applications, the spatial dependence represents a bottle-neck to the overall efficiency of the photophysical process. Under sunlight-like illumination, i.e. non-coherent continuous excitation at 8 mW/cm2 , the UCSTTA quantum yield of a system like the one shown in Chart 1 was estimated to be ∼0.04 %. 20 It is noteworthy that, as a consequence of the system architecture and spatial UC dependence, the observed UC output is expected to originate from a subpopulation of less than 1% of the probed sensitizer molecules. A related observation was that the square dependence of the UC signal on excitation intensity showed a negative deviation at rather low intensities ( UC(DMAB) = 0.66. The shown TABA|ZrO2 sample belongs to a different set and therefore is not included in this comparison. 34 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 47

Another factor to consider was that the UC intensity of the samples showed variations with time (sample aging) and irradiation. The changes were dependent on the studied emitter dye, with MTAB and ADBA exhibiting opposite behaviour (Figure 10a and b). Comparison of Figure 10a-c and Figure 7a-c, shows that the variations in the intensities of UC signal and PtOEP phosphorescence with time and irradiation follow the same trend.

Excitation Intensity Dependence The magnitudes of the steady-state UC-STTA emission signals of the four studied emitter systems were monitored as a function of the incident light intensity. The range of applied excitation was limited at low intensities by the onset of an observable signal, and at high intensities by the output of the employed xenon lamp. As previously observed for the MTAB|ZrO2 system, 20 at low intensities, the UC emission of the emitter dyes follows a quadratic dependence on the excitation intensity. At higher intensities, the dependence becomes subquadratic. The most significant difference between the emitter dyes is at which excitation intensity occurs the change from quadratic to subquadratic dependence. Figure 11 shows the intensity dependence of the emitter dyes where the transition occurs at the lowest (DMAB, magenta) and highest (ADBA, red) excitation intensities (see Figure S19 in the ESI for the intensity dependence of TABA and MTAB).

Time-resolved Upconversion Time-resolved UC-STTA studies were carried out for sealed cells containing BuN solutions of PtOEP (∼ 4.4 × 10−5 M) and ADBA|ZrO2 or MTAB|ZrO2 . Figure 12a shows the UC rise and decay of both samples measured at the peak of UC emission (λem = 440 nm). Qualitatively, the samples show a similar behaviour. There is a strong spike at time zero, attributed to emitter prompt fluorescence due to coherent two-photon absorption (see ESI), followed by the appearance of delayed fluorescence (UC emission). The initial rise of the UC signal is very slow and cannot be reproduced with an exponential function. After 1 μs, the observed kinetics can be well reproduced with the sum of a single exponential rise, a single exponential decay and a very weak plateau (see 24 ACS Paragon Plus Environment

Page 25 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10: Short time aging and light irradiation effect on the steady-state UC-STTA emission spectra (λexc = 534 nm) of (a) MTAB|ZrO2 (black) and (b) ADBA|ZrO2 (red), both contained in sealed cells with a dilute (∼ 4.3 × 10−5 M) BuN solution of PtOEP. Solid lines correspond to freshly prepared samples. Dotted lines correspond to samples irradiated for a few hours. (c) Long time aging and light irradiation effect on the steady-state UC-STTA emission spectrum of the sample presented in (b). Dotted line corresponds to the sample irradiated for a few hours. Dashed line corresponds to the sample after being irradiated and then kept two weeks in darkness. Dotted-dashed line corresponds to the sample after being irradiated, then kept two weeks in darkness, followed by a few minutes of additional irradiation. The corresponding steady-state phosphorescence spectra of the samples are shown in Figure 7. See text for more details.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 47

Figure 11: Integrated upconverted emission vs excitation power density for ADBA|ZrO2 (red markers) and DMAB|ZrO2 (magenta markers), respectively, in sealed cells containing BuN solutions of PtOEP (∼ 4.3 × 10−5 M, λexc = 534 nm). The solid lines are quadratic power dependence fits to the data series in corresponding colors. Steady-state absorption and photoluminescence spectra of the samples can be found in Figure 1 and Figure 6. Figure 12b)

UC(t) = a0 + arise exp(−t/τrise ) + adecay exp(−t/τdecay ).

(1)

Table 1 contains the parameters obtained from the fitting procedure. The rise and decay of UC emission in the sample containing ADBA|ZrO2 is markedly slower. Table 1: Time constants, pre-exponential factors and plateau values obtained from fitting Eq. (1) to the observed UC kinetics in Figure 12 Emitter a0 ADBA 0.097 MTAB 0.094

arise -3.42 -23.3

τrise /μs adecay 6.3 3.07 4.3 21.0

τdecay /μs 60 20.8

Time resolved emission spectra were also measured for the sample containing MTAB|ZrO2 (Figure 13). In these, both UC emission and PtOEP phosphorescence were simultaneously monitored. As can be seen in the figure, the decay of PtOEP phosphorescence and the rise of UC emission occur in the same timescale.


Page 27 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12: (a) Kinetic traces of upconverted emission (λexc = 532 nm, Iexc = 22 mJ pulse−1 cm−2 , λem = 440 nm) of MTAB|ZrO2 (black) and ADBA|ZrO2 (red) contained in sealed cells with PtOEP solutions (∼ 4.4 × 10−5 M) in BuN. Inset: The corresponding steady-state UC spectra of the two samples. Full steady-state absorption and photoluminescence characterization of the systems can be found in Figure S4 in the ESI. (b) Same kinetic data as in (a), shown in a logarithmic timescale. After 1 μs, the observed kinetics can be well reproduced with the expression contained in Eq. (1). See also best fit and residuals for the samples containing ADBA|ZrO2 (light red) and MTAB|ZrO2 (grey).



Phosphorescence/a.u.

UC emission /a.u. 2

2

10

10

1

1

10

10

Time/us

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 47

0

0

10

10

−1

10

−1

400

450 Wavelength/nm

500

620

640 660 680 Wavelength/nm

10

Figure 13: Contour plot of the time-resolved emission of MTAB|ZrO2 in a sealed cell containing ∼ 4.4 × 10−5 M PtOEP in BuN (λexc = 532 nm, Iexc = 22 mJ pulse−1 cm−2 ). To the left, rise and decay of UC emission. To the right, decay of PtOEP phosphorescence. To facilitate the comparison, the UC and phosphorescence signals have been normalised to same value. 35 No filters were placed between the detector and the sample.

Discussion PtOEP Phosphorescence Reveals Bulk Solution Dynamics An important characteristic of the studied samples is that there is a clear separation of the upconverting dye pair in two different physical phases; the emitter dyes are surface bound while the sensitizer, PtOEP, is in solution. The sealed samples or cells are constructed as “sandwiches” with two glass slides separated by a 50 μm sealing layer of Surlyn. Onto one of the slides, a 1-3 μm mesoporous ZrO2 film is deposited by doctor-blading. The emitter dyes are anchored to this ZrO2 surface, while the sensitizer molecules (PtOEP) are dissolved in a BuN solution filling the remaining cell volume. Considering the excited state diffusion length of PtOEP (about 300 nm in deoxygenated solvents) 29 and assuming a homogeneous PtOEP distribution throughout the cell, the described cell architecture means that only ∼3% of the PtOEP molecules contained in the cell will reach the film surface and be involved in the upconverting process. A very important consequence of this fact is that the study of PtOEP phosphorescence, either steady-state or time28 ACS Paragon Plus Environment

Page 29 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


resolved, will not provide information about what happens close to the surface, but instead it will monitor the dynamics of the excited PtOEP molecules in the bulk solution. The triplet lifetime and phosphorescence quantum yield of long-lived triplet sensitizers in solution, such as excited PtOEP, are very sensitive to small concentrations of quenchers. In the upconverting systems presented here, the most probable quenchers are emitter dye molecules desorbed from the ZrO2 film surface and molecular oxygen. Both possibilities will be discussed in the following paragraphs.

Emitter Desorption In spite of rigorous rinsing after the film dye loading to wash off weakly bound emitter dyes (see experimental), the emitter molecules in these systems are not irreversibly bound to the nanoparticles. They rather exist in an equilibrium with the solution phase, with the equilibrium constant depending on the solubility of the emitter molecule in the solvent surrounding the nanostructure. In a previous study of the MTAB|ZrO2 system, the amount of MTAB emitter molecules in the BuN bulk solution phase was estimated to be on the order of ∼1 μM. 20 The exact concentration will depend on the specific ZrO2 surface coverage and film thickness of a given sample. Emitter concentrations in the BuN bulk solution phase on this order can produce phosphorescence quenching (by triplet energy transfer from the excited PtOEP to the desorbed emitter molecule in solution) on the observed order of magnitude (see Figure 6). Due to slow diffusion kinetics in the nanostructured film, it can take hours for such an adsorption–desorption equilibrium to fully establish in the cell. While all the surface bound emitter dyes will endure some degree of desorption into the bulk solution, less desorption was observed in the case of dyes with two anchor groups (DMAB and ADBA).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 47

Photoinduced Removal of Molecular Oxygen Sterically crowded meso-substituted anthracenes, such as the studied emitter dyes, are known to form endoperoxides in the presence of light and molecular oxygen, 36 most probably via the formation of singlet oxygen. While in principle this could be considered an undesired reaction, since it leads to the degradation of the sample, it has the accompanying beneficial effect of decreasing the amount of molecular oxygen present in the sample. We have recently shown that if the right conditions are employed (high concentration of emitter, low initial concentration of molecular oxygen), the decrease of molecular oxygen concentration is significant enough to be observed by spectroscopical means. 37 In the present study, the samples fulfil the conditions to optimize this “molecular oxygen removal” process. There is a large amount of emitter available (the nanostructured nature of the ZrO2 films allows for large areas where the emitter can bind), the initial concentration of molecular oxygen is very low (the samples are prepared in a glove box under Ar atmosphere) and the samples are sealed with Surlyn, which seems to block the entrance of fresh molecular oxygen for at least 1 day. Therefore, it would be expected that in the presence of surface bound emitter and light irradiation, the phosphorescence of PtOEP would grow with time due to the decrease of the molecular oxygen concentration. Such a behaviour has indeed been observed in the case of samples containing ADBA|ZrO2 (Figure 7a and b). Effect of Emitter Desorption and Molecular Oxygen Removal on the Excited PtOEP Bulk Dynamics From the above discussion, it becomes clear that correct analysis of the observed PtOEP phosphorescence in the presence of emitter dye chemisorbed onto ZrO2 , cannot be done without taking into account the following. First, the observed phosphorescence reflects the dynamics of PtOEP in the bulk solution. Second, small amounts of desorbed emitter dye will quench the observed PtOEP phosphorescence due to TET from the excited PtOEP to the desorbed emitter. Third and final, molecular oxygen removal due to endoperoxide formation under light irradiation will lead to an 30 ACS Paragon Plus Environment

Page 31 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increase of the PtOEP phosphorescence. Accordingly, the presence and extent of phosphorescence quenching will depend on which process is dominant: addition (emitter desorption) or removal (endoperoxide formation) of quencher. A key factor will be the solubility of the studied emitter dye. Let us illustrate this point, with the two extreme cases: MTAB (highest solubility) and ADBA (lowest solubility). Due to the very low solubility of ADBA, ADBA|ZrO2 samples consistently have relatively high absorbance compared to the thickness of the ZrO2 film (see for instance Figure 1a and Figure S2 in the ESI). In principle, a large amount of surface bound emitter will enhance emitter desorption, but ADBA solubility is so low that sub-micromolar concentrations of desorbed ADBA are expected. At the same time, the large amount of available ADBA will enhance the formation of endoperoxide. Therefore, under light irradiation conditions, molecular oxygen removal (due to endoperoxide formation) is thought to be the dominant process. This is supported by experimental evidence. As mentioned above, in samples containing ADBA|ZrO2 , PtOEP phosphorescence increases after light irradiation (Figure 7b and c). On the case of MTAB, high dye loadings are more difficult to achieve due to its higher solubility (see as an example Figure 1a and Figure S4 a in the ESI) and a smaller amount of emitter (compared to the case of ADBA) will be available for endoperoxide formation. In spite of the relatively low dye loadings, micro molar or higher concentrations of desorbed MTAB are expected. 20 Even in the case of freshly prepared samples, those containing MTAB|ZrO2 generally show a more pronounced phosphorescence quenching than those containing ADBA|ZrO2 (Figure 6 and Figure S4c in the ESI). Molecular oxygen removal (due to endoperoxide formation under light irradiation) cannot compensate the quenching due to emitter desorption, and in presence of MTAB|ZrO2 , PtOEP phosphorescence decreases after several hours of irradiation (Figure 7a). This phosphorescence quenching is most probably not related to a photoinduced process, but to the slow increase of desorbed MTAB in solution. It can take hours for an adsorption–desorption equilibrium to fully establish in the cell due to the slow diffusion kinetics in the nanostructured film. Time resolved phosphorescence measurements show a faster decay in the microsecond timescale



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 47

in the presence of MTAB|ZrO2 (Figure 8), further supporting our hypothesis of dynamic quenching due to TET from excited PtOEP to desorbed emitter molecules in the bulk solution. Our analysis not only explains the observed PtOEP phosphorescence behaviour in the presence of MTAB|ZrO2 or ADBA|ZrO2 . It also explains the large variability found between different samples and emitters. For a generic emitter|ZrO2 sample, the amount of emitter that will desorb into the bulk will be strongly linked to the dye loading of the sample. Unfortunately, as mentioned in the results section, this parameter was not easy to control.

Unexpected Subquadratic Dependence of UC-STTA on Excitation Intensity at Low Emitter Surface Coverages Quadratic dependence of UC-STTA on excitation light intensity is expected for TTA processes, 38 it has been used as a proof of UC-STTA existence in solution, 39–41 and it has even been observed in various types of solid state systems. 42–46 In our previous study of UC-STTA using as emitter MTAB|ZrO2 , at low intensities the UC emission of MTAB followed a quadratic dependence on the excitation intensity, whereas at higher intensities, the dependence became subquadratic. The quadratic dependence on incident light intensity was taken as an indication of the TTA nature of the UC mechanism. 20 The deviation from the quadratic dependence of the UC signal at high incident intensities was ascribed to a gradual shift into the so-called “strong-annihilation limit”, where the TTA process is the main decay pathway for triplet emitter excited states and the excitation light intensity dependence of the UC signal goes from quadratic to linear. This behaviour has been observed and modelled in homogenous solution systems, 38,47,48 and there are several reports that this “strong annihilation limit” can be reached at low light intensities provided a sufficiently low oxygen concentration. 49–51 Consequently, it was reasonable to attribute the subquadratic dependence at higher incident light intensities for glove-box sealed cells containing MTAB|ZrO2 , to an approach to the so-called “strong annihilation limit”. 20 The UC signal of the emitter dyes studied in the present work showed a dependence on excitation light intensity similar to the previously reported for MTAB|ZrO2 (quadratic dependence 32 ACS Paragon Plus Environment

Page 33 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at low excitation intensities, subquadratic dependence at higher excitation intensities; see for instance Figure 11). The most significant difference between the emitter dyes was at which excitation intensity the change from quadratic to subquadratic dependence occurred. For the series of studied emitter molecules, the degree of surface coverage varies significantly due to different adsorption/desorption equilibria. As long as the surface coverage does not reach the limit where aggregates start to form, 52 the shorter the distance between chemisorbed emitter molecules, the more efficient triplet energy migration is expected to be, and thereby the more efficient TTA and UC-STTA will be. Hence, samples with large emitter surface coverages and UC signals (as those containing ADBA|ZrO2 ) are expected to present a shift towards linear dependence (“strong annihilation limit”) at lower incident light intensities than samples with low surface coverages and UC signals (such as those containing DMAB|ZrO2 ). Surprisingly, the opposite behaviour was observed (see Figure 11 and Figure 1a), indicating that an approach to the “strong annihilation limit” might not be the only possible reason for the observed dependence of UC emission on excitation intensity in the studied systems.

Homogenous TTA between PtOEP Molecules Limits Chemisorbed Emitter Sensitization As mentioned before, an important characteristic of the studied samples is that there is a clear separation of the upconverting dye pair in two different physical phases; the emitter dyes are surface bound while the sensitizer, PtOEP, is in solution. A consequence of this separation is that TTA between PtOEP molecules in solution can significantly contribute to the decay of excited PtOEP molecules. Because of its quadratic dependence on excitation intensity, at high enough incident light intensities, homogeneous PtOEP TTA can start to noticeably shorten the triplet excited state lifetime of PtOEP and therefore, the diffusion length of the excited sensitizer. This will have a negative impact on the sensitization of chemisorbed emitters, leading to a smaller concentration of triplet excited emitters and therefore smaller UC-STTA, than expected. As a result, the UC-STTA dependence on the incident light intensity will be less than quadratic. Another factor to consider is the lifetime of the excited PtOEP in the absence of significant



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 47

TTA (at very low excitation energies). A long lifetime will lead to efficient homogeneous TTA in the bulk solution at higher excitation intensities, since the excited molecules will have more time to diffuse and meet each other. In the studied systems, the lifetime of excited PtOEP in the bulk is closely related to the concentration of desorbed emitter in solution. This concentration will depend on the solubility of the emitter dye and the amount of dye covering the surface. Low dye solubility and surface coverages will lead to a small concentration of desorbed dye and a long lifetime of PtOEP in the bulk solution. This explains well the behaviour of the sample containing DMAB|ZrO2 , which has the highest steady-state phosphorescence signal (i.e. the longest excited PtOEP lifetime) and shifts towards subquadratic dependence at the lowest excitation intensity (see Figure 6 and Figure 11). It is important to stress that homogenous PtOEP TTA is relevant to the studied upconverting systems because of the physical separation between sensitizer and emitter. In those upconverting systems where sensitizer and emitter are homogeneously distributed, and the concentration of sensitizer is low relative to that of the emitter, triplet energy transfer from the sensitizer to the emitter will be favoured over homogeneous sensitizer TTA. This is the situation most commonly found for UC systems in solution.

UC Dynamics The upconverting systems studied herein are expected to exhibit complex dynamics. The sensitizers and emitters are physically separated in two phases, implying that the excited PtOEP molecules must diffuse to the ZrO2 surface to sensitise the chemisorbed emitter molecules. Therefore, a time dependent energy transfer rate “constant” (rather termed a coefficient) will be needed to properly describe the initial triplet energy transfer from sensitizer to emitter. 53 Another factor to take into account, is the effect that the dimensional restrictions of the nanostructured ZrO2 film will have on the triplet energy migration through the surface. 54,55 While a rigorous analysis of such dynamics is beyond the scope of this article, important qualitative trends can be deduced from careful inspection of the experimental data. 34 ACS Paragon Plus Environment

Page 35 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Examination of Figure 12 and inset, shows a disparity in the magnitude of the steady-state UC signals compared to the time-resolved UC traces. While the sample containing ADBA|ZrO2 gives a much larger UC signal than the MTAB|ZrO2 sample in steady-state (∼ 20 times larger), the integrated time-resolved UC emission of the same pair of samples is of the same order of magnitude. This disparity is not strange. Under steady-state illumination, at low oxygen concentrations, different triplet state lifetimes (either for the emitter or the sensitizer molecules) will lead to significantly different concentrations of triplet emitter, which will have a strong effect in the UC output. It has been shown that the shorter the excitation pulse, the less pronounced the effect of the triplet state lifetime in the concentration of triplet emitters. 56 The difference in the triplet state lifetimes can be intrinsic (ex: if ADBA has a longer triplet lifetime than MTAB) or due to different amounts of triplet quencher present in the sample (ex: due to endoperoxide formation, there will be variations of molecular oxygen concentration in time and among different samples). This is an important factor to consider when extrapolating time-resolved studies to the development of upconverting materials for solar applications. Inspection of the recorded UC kinetic traces (Figure 12) shows a spike at time zero, attributed to prompt fluorescence from the emitter due to two-photon absorption (see ESI for more details.), followed by a rise and decay in the microsecond timescale. The initial rise cannot be reproduced with an exponential function and is very slow, indicating a slow emitter triplet concentration buildup. After 1 μs, the observed kinetics appear deceptively simple, and can be well reproduced with the sum of a single exponential rise, a single exponential decay and a very weak plateau. In both samples, but especially in the one containing MTAB|ZrO2 , the rise and decay times are close enough (see Table 1, for MTAB|ZrO2 , τdecay ∼ 5 × τrise ) that the processes behind the formation and decay of the UC signal should not be treated independently. Nevertheless, time resolved emission spectra where UC emission and PtOEP phosphorescence were simultaneously monitored (Figure 13), indicate a strong link between the rise of UC emission and the decay of the population of excited PtOEP in the bulk solution. 57 We can therefore establish a qualitative relationship between the lifetime of the excited PtOEP molecules in the bulk (τPtOEP ), the sensitization of the chemisorbed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 47

emitters and the observed rise of UC emission (τrise ). The longer τPtOEP , the longer the time window during which sensitization can occur, and the slower the observed UC rise. As an example, let us compare the data presented in Figure 8 and Figure 12. In the case of the sample containing MTAB|ZrO2 , the average lifetime of PtOEP in solution and the observed UC rise time are 6.4 and 4.3 μs, respectively. On the other hand, in the sample containing ADBA|ZrO2 , τPtOEP is 9.2 μs, appreciably longer (due to less emitter desorption and more efficient molecular oxygen removal, see above), and accordingly, τrise is also longer, 6.3 μs. It is noteworthy that the ratio between the average lifetime of excited PtOEP and the observed UC rise time is the same for both emitter dyes, τPtOEP /τrise = 1.5. The decay of the UC signal occurs in the tens of μs timescale (Figure 12 and Table 1). The slower UC decay in the case of ADBA can be rationalised by the lower concentration of molecular oxygen in the sample (see above).

Reasons Behind the Relative High UC Efficiency of Samples Containing ADBA|ZrO2 Given a set of samples prepared and measured under the same conditions, those containing ADBA (two carboxylic acid groups) as chemisorbed emitter dye consistently gave the largest UC signals (see for instance Figure 9, Figure S3, and Figure S4; additional sets of samples showing the same experimental trend were also measured, but the data are not included). Spectroscopic measurements (electronic steady-state absorption and emission, fluorescence lifetimes), and DFT calculations (see Results section), showed that there were no dramatic differences among the energetics of the four studied emitter dyes. In fact, Stern-Volmer analysis of the quenching of excited PtOEP by the ester emitter molecules (MTAB and DMAB), gave a slightly more favourable rate of TET in the case of the monoester (MTAB). The symmetric emitter dyes do have a larger fluorescence quantum yield (Φf (MTAB) = 0.72 vs. Φf (DMAB) = 0.90 in MeCN), but this isolated fact does not suffice to explain the observed trend of UC efficiencies. As a matter of fact, samples containing chemisorbed MTAB (monoester) gave larger UC signals than those with DMAB (diester). 36 ACS Paragon Plus Environment

Page 37 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The reasons for the relative high UC efficiency of samples containing chemisorbed ADBA seem therefore to lie elsewhere. Careful examination of the steady-state absorption and emission (prompt fluorescence, phosphorescence and upconverted emission) spectra for several sets of data allowed us to extract two important features. Compared to the other emitter dyes of a given set of samples, those containing chemisorbed ADBA typically presented relative high dye loading (high emitter absorbance per film thickness) and large phosphorescence signals (see for instance Figure 1, Figure 6, Figure 9). As discussed in former sections, due to the architecture of the samples, where the sensitizer and emitter molecules are physically separated in two phases, the PtOEP phosphorescence signal (either steady-state or time-resolved) monitors the dynamics of the excited PtOEP in the bulk solution. TET from the excited PtOEP molecules in solution to the chemisorbed emitter, is limited to the small subpopulation of excited PtOEP molecules (∼ 3%) which are close enough to reach the film. Therefore, the counterintuitive conclusion that the larger the observed steady-state phosphorescence signal (and the longer the PtOEP excited state lifetime), the more efficient the sensitization of the chemisorbed emitters by TET. The lifetime of the excited PtOEP molecules will be strongly affected by the presence of quenchers in the bulk solution, mainly desorbed emitter molecules and molecular oxygen. Long lifetimes will be consequently expected in samples with high emitter dye loading (which allows for effective endoperoxide formation and molecular oxygen removal) and low emitter solubility. ADBA is the emitter dye that better fulfills both requirements. The best example for this behaviour can be found in the comparison between samples containing ADBA and MTAB, which usually presents relative low dye loadings and is significantly more soluble (see for instance Figure S4). High emitter dye loading is also expected to increase the UC efficiency by favouring TET (a larger concentration of surface bound emitter should lead to more efficient quenching of the excited PtOEP molecules at the film surface) and triplet energy migration (due to shorter average distances between chemisorbed emitter dyes). The importance of high surface coverage is best illustrated by comparison of samples containing ADBA and DMAB (two ester groups). Both emitter dyes



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

possess two anchoring groups and typically show large phosphorescence signals (see Figure 6), which we attribute to low desorption of the chemisorbed dyes. However, in the case of DMAB, dye loading onto the surface is very inefficient (see Experimental section), and the estimated surface coverage 58 of the sample containing DMAB is very poor (∼ 2%) compared to that of the sample with ADBA (∼ 21 %). This leads to a dramatic drop in UC signal in the case of the DMAB sample (Figure 9). Another possibility that we considered, was the formation of an adduct between chemisorbed ADBA and the sensitizer. For PtOEP, axial coordination of strong ligands to the Pt(II) central ion has been reported. 59 Therefore, if ADBA is bound to the ZrO2 surface by only one carboxylic acid group, the remaining group could potentially coordinate to PtOEP and significantly enhance the emitter sensitization. To test this idea, the absorption spectra of solutions of PtOEP in BuN in the absence and presence of an excess concentration of acetic acid were compared. However, no spectral changes were observed (data not shown), indicating that no significant coordination of carboxylic acid groups to the Pt ion in PtOEP occurred. From the above discussion, it becomes apparent that the relative high UC efficiency observed in samples containing surface bound ADBA, mainly originates in their relatively large surface coverage, and the strength of the emitter attachment to the surface. These are mostly determined by the type and number of anchoring groups (carboxylic acid, two anchoring groups) and the very low solubility of ADBA on the employed solvents.

Conclusions The sample architecture, where sensitizer and emitters are separated in two different phases, has a profound impact on the observed photoinduced dynamics and excitation energy dependence of the UC signal. Equations derived for homogeneous solutions of emitters and sensitizers have to be applied with care in the presented systems. In this case, the observed shift from quadratic dependence to subquadratic (and eventually linear) dependence, usually attributed to a shift toward the strong


Page 39 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


annihilation limit in homogeneous solution, could also be due to an increase of homogeneous sensitizer-sensitizer TTA with increasing excitation intensity. Future experiments to investigate this issue are planned. The physical separation between sensitizer and emitter molecules also affects the sensitization and upconversion efficiencies, which are limited by the diffusion length of the excited sensitizer. A substantial increase in the upconversion efficiency is expected by the use of thinner samples and more soluble sensitizer molecules. A significant advance in isolating and understanding parameters that control the reproducibility of the samples has been achieved: Emitter desorption, that leads to shortening of the lifetime of excited PtOEP molecules in the bulk solution, decreases the UC signal with time. In the presence of light and oxygen, removal of molecular oxygen due to endoperoxide formation, increases the UC signal as long as the sample is air sealed. Comparative study of four derivatives of DPA as surface bound emitters, points out that the most important emitter properties to achieve large UC output are the type and number of anchor groups, and the solubility of the emitter dye in the employed solvents. Given a certain excitation intensity, UC output is mainly determined by how large the surface coverage is and the strength of the binding to the surface. The highest signals were observed with the DPA derivative with two carboxylic acid groups, ADBA. Even though the ester and carboxylic acid groups are expected to lead to the same surface bound species, the carboxylic groups allowed a better control of the loading of the dye onto the surface. Moreover, unattached, free carboxylic acid groups, significantly decreased the solubility of the dye, and therefore, dye desorption. It is our hope that the findings reported herein will contribute to the development and understanding of UC on nanostructured surfaces.

Acknowledgement The authors thank Andreas Orthaber (Uppsala University, Sweden) for his assistance with synthesis, Markus Nordlindh (Uppsala University, Sweden) for his help with the quantum yield measurements, and Erik Göransson (Uppsala University, Sweden) for implementation of a deconvolution 39 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 47

procedure in the Igor Pro software. JMG gratefully acknowledges support from the Swedish government through “STandUP for ENERGY”. This work was supported by the Swedish Research Council (VR).

Supporting Information Available Reaction schemes and procedures for all the syntheses. Spectroscopic data (steady-state and timeresolved) of emitter dyes in solution and additional upconverting samples. Emitter prompt fluorescence following coherent two-photon absorption. DFT and TDDFT results. This material is available free of charge via the Internet at http://pubs.acs.org/.

Notes and References [1] Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A. et al. Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement. Science 2011, 332, 805–809. [2] Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510. [3] de Wild, J.; Meijerink, A.; Rath, J. K.; van Sark, W. G. J. H. M.; Schropp, R. E. I. Upconverter Solar Cells: Materials and Applications. Energy Environ. Sci. 2011, 4, 4835–4848. [4] Huang, X.; Han, S.; Huang, W.; Liu, X. Enhancing Solar Cell Efficiency: The Search for Luminescent Materials As Spectral Converters. Chem. Soc. Rev. 2013, 42, 173–201. [5] Atre, A. C.; Dionne, J. A. Realistic Upconverter-Enhanced Solar Cells with Non-Ideal Absorption and Recombination Efficiencies. J. Appl. Phys. 2011, 110, 034505. [6] Briggs, J. A.; Atre, A. C.; Dionne, J. A. Narrow-Bandwidth Solar Upconversion: Case Studies of Existing Systems and Generalized Fundamental Limits. J. Appl. Phys. 2013, 113, 124509. 40 ACS Paragon Plus Environment

Page 41 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[7] Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized TripletTriplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560–2573. [8] Zhao, J.; Ji, S.; Guo, H. Triplet-Triplet Annihilation Based Upconversion: From Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields. RSC Adv. 2011, 1, 937–950. [9] Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Low Power, NonCoherent Sensitized Photon Up-Conversion: Modelling and Perspectives. Phys. Chem. Chem. Phys. 2012, 14, 4322–4332. [10] Simon, Y. C.; Weder, C. Low-Power Photon Upconversion Through Triplet-Triplet Annihilation in Polymers. J. Mater. Chem. 2012, 22, 20817–20830. [11] Schmidt, T. W.; Castellano, F. N. Photochemical Upconversion: The Primacy of Kinetics. J. Phys. Chem. Lett. 2014, 5, 4062–4072. [12] Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2014, 115, 395–465. [13] Schulze, T. F.; Schmidt, T. W. Photochemical Upconversion: Present Status and Prospects for Its Application to Solar Energy Conversion. Energy Environ. Sci. 2015, 8, 103–125. [14] Cheng, Y. Y.; Fuckel, B.; MacQueen, R. W.; Khoury, T.; Clady, R. G. C. R.; Schulze, T. F.; Ekins-Daukes, N. J.; Crossley, M. J.; Stannowski, B.; Lips, K. et al. Improving the LightHarvesting of Amorphous Silicon Solar Cells with Photochemical Upconversion. Energy Environ. Sci. 2012, 5, 6953–6959. [15] Schulze, T. F.; Czolk, J.; Cheng, Y.-Y.; Fückel, B.; MacQueen, R. W.; Khoury, T.; Crossley, M. J.; Stannowski, B.; Lips, K.; Lemmer, U. et al. Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells with Photochemical Upconversion. J. Phys. Chem. C 2012, 116, 22794–22801.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 47

[16] Nattestad, A.; Cheng, Y. Y.; MacQueen, R. W.; Schulze, T. F.; Thompson, F. W.; Mozer, A. J.; Fückel, B.; Khoury, T.; Crossley, M. J.; Lips, K. et al. Dye Sensitised Solar Cell with Integrated Triplet-Triplet Annihilation Upconversion System. J. Phys. Chem. Lett. 2013, 4, 2073– 2078. [17] Ekins-Daukes, N. J.; Schmidt, T. W. A Molecular Approach to the Intermediate Band Solar Cell: The Symmetric Case. Appl. Phys. Lett. 2008, 93, 063507. [18] Giaimuccio, J. M.; Rowley, J. G.; Meyer, G. J.; Wang, D.; Galoppini, E. Heavy Atom Effects on Anthracene-Rigid-Rod Excited States Anchored to Metal Oxide Nanoparticles. Chem. Phys. 2007, 339, 146–153. [19] Lissau, J. S.; Gardner, J. M.; Morandeira, A. Photon Upconversion on Dye-Sensitized Nanostructured ZrO2 Films. J. Phys. Chem. C 2011, 115, 23226–23232. [20] Lissau, J. S.; Nauroozi, D.; Santoni, M.-P.; Ott, S.; Gardner, J. M.; Morandeira, A. Anchoring Energy Acceptors to Nanostructured ZrO2 Enhances Photon Upconversion by Sensitized Triplet–Triplet Annihilation Under Simulated Solar Flux. J. Phys. Chem. C 2013, 117, 14493–14501. [21] Hardin, B. E.; Hoke, E. T.; Armstrong, P. B.; Yum, J. H.; Comte, P.; Torres, T.; Frechet, J. M. J.; Nazeeruddin, M. K.; Grätzel, M.; McGehee, M. D. Increased Light Harvesting in Dye-Sensitized Solar Cells with Energy Relay Dyes. Nature Photon. 2009, 3, 406–411. [22] Hardin, B. E.; Yum, J.-H.; Hoke, E. T.; Jun, Y. C.; Péchy, P.; Torres, T.; Brongersma, M. L.; Nazeeruddin, M. K.; Grätzel, M.; McGehee, M. D. High Excitation Transfer Efficiency from Energy Relay Dyes in Dye-Sensitized Solar Cells. Nano Lett. 2010, 10, 3077–3083. [23] Yum, J.-H.; Baranoff, E.; Hardin, B. E.; Hoke, E. T.; McGehee, M. D.; Nuesch, F.; Gratzel, M.; Nazeeruddin, M. K. Phosphorescent Energy Relay Dye for Improved Light Harvesting Response in Liquid Dye-Sensitized Solar Cells. Energy Environ. Sci. 2010, 3, 434–437. 42 ACS Paragon Plus Environment

Page 43 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[24] Odobel, F.; Pellegrin, Y.; Warnan, J. Bio-Inspired Artificial Light-Harvesting Antennas for Enhancement of Solar Energy Capture in Dye-Sensitized Solar Cells. Energy Environ. Sci. 2013, 6, 2041–2052. [25] Adhyaksa, G. W. P.; Lee, G. I.; Baek, S.-W.; Lee, J.-Y.; Kang, J. K. Broadband Energy Transfer to Sensitizing Dyes by Mobile Quantum Dot Mediators in Solar Cells. Sci. Rep. 2013, 3. [26] Gaussian 09, Revision A.02. Gaussian, Inc., Wallingford CT, 2009. [27] Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999–3093. [28] Giri, N. K.; Ponce, C. P.; Steer, R. P.; Paige, M. F. Homomolecular Non-Coherent Photon Upconversion by Triplet–Triplet Annihilation Using a Zinc Porphyrin on Wide Bandgap Semiconductors. Chem. Phys. Lett. 2014, 598, 17–22. [29] Bansal, A. K.; Holzer, W.; Penzkofer, A.; Tsuboi, T. Absorption and Emission Spectroscopic Characterization of Platinum-Octaethyl-Porphyrin (PtOEP). Chem. Phys. 2006, 330, 118– 129. [30] Karpicz, R.; Puzinas, S.; Gulbinas, V.; Vakhnin, A.; Kadashchuk, A.; Rand, B. Exciton Dynamics in an Energy Up-Converting Solid State System Based on Diphenylanthracene Doped with Platinum Octaethylporphyrin. Chem. Phys. 2014, 429, 57–62. [31] Birks, J. B. Photophysics of Aromatic Molecules; John Wiley & Sons Ltd., 1970. [32] Typically, the sample steady-state phosphorescence was measured before, and after timeresolved measurements. The later measurements involved irradiating the sample with high energy pulses for several hours. See experimental section for more details. [33] Xe-lamp irradiation.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

[34] The TABA|ZrO2 sample should instead be compared to a ADBA|ZrO2 sample made on the same day from the same ZrO2 film (see Figure S3d in the ESI). In this case, the efficiency ratio is UC(ADBA)=2.2 × UC(TABA). [35] The contour plot was built from 10 emission spectra measured at different times after excitation. The monochromator controlled by the iCCD camera was set to a bandpass of 5 nm. The sampling width of each spectrum was 1 μs. 50 measurements were made and averaged per spectrum. Each spectrum was smoothed with a built-in MATLAB procedure. [36] Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade, R. Photodimerization of Anthracenes in Fluid Solution: Structural Aspects. Chem. Soc. Rev. 2000, 29, 43–55. [37] Lissau, J. S.; Söderberg, M.; Gardner, J. M.; Morandeira, A. Submitted to Dalton Trans. [38] Parker, C. A. Photoluminescence of Solutions; Elsevier Publishing Company, Amsterdam, 1968. [39] Kozlov, D. V.; Castellano, F. N. Anti-Stokes Delayed Fluorescence from Metal-Organic Bichromophores. Chem. Commun. 2004, 2860–2861. [40] Islangulov, R. R.; Kozlov, D. V.; Castellano, F. N. Low Power Upconversion Using MLCT Sensitizers. Chem. Commun. 2005, 3776–3778. [41] Zhao, W.; Castellano, F. N. Upconverted Emission from Pyrene and Di-Tert-Butylpyrene Using Ir(ppy)3 As Triplet Sensitizer. J. Phys. Chem. A 2006, 110, 11440–11445. [42] Islangulov, R. R.; Lott, J.; Weder, C.; Castellano, F. N. Noncoherent Low-Power Upconversion in Solid Polymer Films. J. Am. Chem. Soc. 2007, 129, 12652–12653. [43] Singh-Rachford, T. N.; Lott, J.; Weder, C.; Castellano, F. N. Influence of Temperature on Low-Power Upconversion in Rubbery Polymer Blends. J. Am. Chem. Soc. 2009, 131, 12007– 12014.


Page 45 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[44] Merkel, P. B.; Dinnocenzo, J. P. Low-Power Green-to-Blue and Blue-to-UV Upconversion in Rigid Polymer Films. J. Lumin. 2009, 129, 303 – 306. [45] Monguzzi, A.; Tubino, R.; Meinardi, F. Multicomponent Polymeric Film for Red to Green Low Power Sensitized Up-Conversion. J. Phys. Chem. A 2009, 113, 1171–1174. [46] Monguzzi, A.; Frigoli, M.; Larpent, C.; Tubino, R.; Meinardi, F. Low-Power-Photon UpConversion in Dual-Dye-Loaded Polymer Nanoparticles. Adv. Funct. Mater. 2012, 22, 139– 143. [47] Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, F. Upconversion-Induced Fluorescence in Multicomponent Systems: Steady-State Excitation Power Threshold. Phys. Rev. B 2008, 78, 195112. [48] Auckett, J. E.; Chen, Y. Y.; Khoury, T.; Clady, R. G. C. R.; Ekins-Daukes, N. J.; Crossley, M. J.; Schmidt, T. W. Efficient Up-Conversion by Triplet-Triplet Annihilation. J. Phys: Conf. Ser. 2009, 185, 012002 (4pp). [49] Kim, J.-H.; Deng, F.; Castellano, F. N.; Kim, J.-H. High Efficiency Low-Power Upconverting Soft Materials. Chem. Mater. 2012, 24, 2250–2252. [50] Haefele, A.; Blumhoff, J.; Khnayzer, R. S.; Castellano, F. N. Getting to the (Square) Root of the Problem: How to Make Noncoherent Pumped Upconversion Linear. J. Phys. Chem. Lett. 2012, 3, 299–303. [51] Monguzzi, A.; Braga, D.; Gandini, M.; Holmberg, V. C.; Kim, D. K.; Sahu, A.; Norris, D. J.; Meinardi, F. Broadband Up-Conversion at Subsolar Irradiance: Triplet–Triplet Annihilation Boosted by Fluorescent Semiconductor Nanocrystals. Nano Lett. 2014, 14, 6644–6650. [52] Aggregates can behave as energy traps and therefore slow down energy migration. [53] Wöll, D.; Lukzen, N.; Steiner, U. E. Diffusion-Controlled Sensitization of Photocleavage Reactions on Surfaces. Photochem. Photobiol. Sci. 2012, 11, 533–538. 45 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

[54] Kopelman, R. Fractal Reaction Kinetics. Science 1988, 241, 1620–1626. [55] Kopelman, R. Rate Processes on Fractals: Theory, Simulations, and Experiments. J. Stat. Phys. 1986, 42, 185–200. [56] Kondakov, D. Y. Characterization of Triplet-Triplet Annihilation in Organic Light-Emitting Diodes Based on Anthracene Derivatives. J. Appl. Phys. 2007, 102, 114504. [57] In principle, we cannot exclude the possibility of TTA between desorbed excited triplet emitters contributing to the observed UC signal. However, from our previous work, we know that the concentration of desorbed emitters is very low (≤ 1μM) and their contribution to the steady-state UC signal is negligible. 20 It could be argued that the relative contribution of the desorbed excited triplet emitters to the UC signal is larger in the case of time-resolved measurements. However, a detectable time-resolved UC signal was only observed in the case of samples with large steady-state UC signals, which led us to conclude that we are mostly (if not exclusively), monitoring UC on the surface. [58] Gardner, J. M.; Beyler, M.; Karnahl, M.; Tschierlei, S.; Ott, S.; Hammarström, L. LightDriven Electron Transfer Between a Photosensitizer and a Proton-Reducing Catalyst CoAdsorbed to NiO. J. Am. Chem. Soc. 2012, 134, 19322–19325. [59] Nifiatis, F.; Su, W.; Haley, J. E.; Slagle, J. E.; Cooper, T. M. Comparison of the Photophysical Properties of a Planar, PtOEP, and a Nonplanar, PtOETPP, Porphyrin in Solution and Doped Films. J. Phys. Chem. A 2011, 115, 13764–13772.


Page 47 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical TOC Entry


What Limits Photon Upconversion on Mesoporous Thin Films

Recommend Documents