The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer

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The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses Steponas Raisys, Karolis Kazlauskas, Saulius Juršenas, and Yoan C. Simon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03888 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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ACS Applied Materials & Interfaces

The Role of Triplet Exciton Diffusion in LightUpconverting Polymer Glasses Steponas Raišys,† Karolis Kazlauskas,† Saulius Juršėnas, † Yoan C. Simon*‡,§

†

‡

Institute of Applied Research, Vilnius University, Saulėtekio 3, LT-10222 Vilnius, Lithuania.

Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland.

§

School of Polymers and High Performance Materials, the University of Southern Mississippi, 118 College Dr., Hattiesburg, MS 39406, USA

E-mail: [email protected]

Keywords Sensitized light upconversion, triplet-triplet annihilation, triplet exciton diffusion, exciton diffusion length, diphenylanthracene, platinum octaethylporphyrin.

1

ACS Paragon Plus Environment


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Abstract Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15 – 40 wt%). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time-scale degrades TTA-UC performance at high emitter loadings (>25 wt%) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

1. Introduction Fueled by the many opportunities that it offers in terms of enhancement of solar harvesting,1-2 photocatalysis3 or bioimaging4-5 for instance, sensitized photon upconversion mediated by triplet-triplet annihilation (TTA-UC) has attracted increasing attention in the past decade.6-7 TTA-UC presents two obvious advantages over alternative UC schemes, namely the abilities to 2


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operate at subsolar irradiance power densities (~10 mW/cm2) and with noncoherent light.8 For comparison, widely used UC via generation of high harmonics requires not only coherent excitation, but also needs to fulfill stringent constraint of phase matching. Succinctly, TTA-UC relies on a cascade of photophysical events whereby lower-energy photons are harvested by a sensitizer molecule(s), followed by intersystem crossing and triplet-triplet energy transfer (TTET) to emitter molecules. These triplet excitons then diffuse until they encounter each other such that a singlet excited emitter is formed by means of TTA and blue-shifted light is eventually emitted (Scheme 1). Such TTA-UC scheme, exploiting strong-oscillator-strength of singlet manifold for absorption and emission with long-lived triplet state for intermediate energy storage, also surpasses lanthanoid-based UC scheme in terms of efficiency. Lanthanoid-based upconverting phosphors use the same electronic manifold for all the processes leading to low absorptivity and inherent competition of non-radiative losses with the UC emission.7, 9 There have been numerous reports demonstrating TTA-UC in conjugated oligomeric thinfilms,10-11 elastomeric (rubery) doped-matrices,12-13 gels,14-15 composites16 or glassy materials such as polymer films.17,18,19-22 Undoubtedly the latter offer multiple advantages in terms of transparency, mechanical stability and are much more attractive from a device point of view, however, maximum quantum efficiency of TTA-UC achieved is an order of magnitude lower as compared to that found in solution or liquid medium (~30%) (note that a multiplication factor of 2 is used here to attain a maximum quantum efficiency of 100%).23 The main factors limiting

upconversion efficiency in disordered glassy films are not fully understood,24 still they are believed to be mainly related to the limited triplet exciton diffusion, which hinders exciton encounter and thus TTA. Note that in a liquid state exciton encounter is realized via physical molecule motion (Brownian motion). Increasing emitter concentration to address this limitation 3



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in polymer films is very challenging due to the poor emitter solubility in the matrix (often leading to aggregation) as well as detrimental concentration quenching effects. Improving molecular ordering, indeed, was found to alleviate the problem and facilitate exciton migration resulting in a reasonably high UC quantum yield in the solid state (1.9%) as reported by Mahato et al.25 While triplet exciton mobility can be responsible for some of the UC losses, it is unclear whether it is truly the limiting factor in the amorphous polymer films. Therefore, to clarify the role of triplet exciton diffusion in upconverting polymer glasses, the triplet-exciton diffusion coefficient (D) and diffusion length (LD) were evaluated as a function of emitter concentration. For this evaluation time-resolved photoluminescence bulk-quenching technique26 was employed with phenyl-C61-butyric acid methyl ester (PCBM, Chart 1) serving as appropriate triplet exciton quencher.27 A dye pair (Chart 1.) consisting of 9,10diphenylanthracene (DPA) as an emitter and platinum octaethylporphyrin (PtOEP) as a triplet sensitizer (Figure S1 in SI for their respective absorption, emission and excitation spectra) was chosen due to its high reported solution upconversion efficiency and widespread use, making it an excellent model system for this study. In the present work, the advantage of the meltprocessing methodologies developed by Lee et al. was used to attain homogeneous disordered poly(methyl methacrylate) (PMMA) glasses with high emitter concentrations so as to clarify the role of triplet exciton diffusion in the TTA-UC in such systems.19 Interestingly, the present studies unravel the limited triplet exciton diffusion to be the non-governing loss mechanism of UC efficiency and points to alternate loss channel. 2. Experimental Section 2.1. Materials

4


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All compounds are commercially available and were used as received. PMMA, PtOEP and PCBM were purchased from Sigma-Aldrich, DPA from ABCR.

Scheme 1. TTA-UC energy diagram showing the various energy transfers between the platinum octaethylporphyrin (PtOEP) and the 9,10-diphenylanthracene (DPA): intersystem crossing (ISC), triplet-triplet energy transfer (TTET), triplet exciton diffusion (TED) and triplet-triplet annihilation (TTA).

Chart 1. Molecular structures of PtOEP sensitizer, DPA emitter and PCBM quencher. 2.2. Preparation of the upconverting films Two series of DPA-doped PMMA films were prepared and investigated. PMMA/DPA series contained only emitter molecules with concentration varying from 15 to 40 wt % by increments of 5 wt %. This series was used to evaluate the homogeneity of DPA distribution in PMMA as well as concentration quenching effects. Upconverting films of the PMMA/DPA/PtOEP series 5



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were similar to those of the first series with respect to DPA doping. However, they additionally contained sensitizer PtOEP at a fixed concentration at 0.05 wt %, for which the maximum efficiency of TTA-UC was previously reported.19 Gratifyingly, this value is below the onset concentration (0.2-0.3 wt%) where aggregation-induced phosphorescence quenching occurs (Figure S2 in SI).28 The preparation procedure of the films was the following: PMMA (30 mg), DPA (15%, 20%, 25%, 30%, 35% and 40% w/w) and PtOEP (0.05% w/w), in the case of PMMA/DPA/PtOEP series, were dissolved in chlorobenzene solvent (300 µL) by stirring for 4 h at 65°C. For evaluation of triplet exciton diffusion in PMMA/DPA/PtOEP films, varying amounts of PCBM quencher (0 – 2 wt%) were additionally introduced into the solvent. The mixture of all the compounds were drop casted on pre-cleaned 150 µm thick glass substrate at 200°C and left for half an hour to evaporate solvent. Thereafter the samples were covered with second glass substrate and hot-pressed with Carver press at 240°C under the pressure of ~100 kg/cm2 for 5 min. After the hot-pressing samples were immediately cooled down in the ice-bath. It is important to notice that higher DPA content could be obtained here as compared to the studies of Lee et al.19 This difference is due to the several factors: (i) immersion of the samples into an ice-bath immediately after hot-pressing ensured more rapid thermal quenching, (ii) the samples utilized for this study were 2-3 times thinner as compared to the previous work (70 µm vs 100 – 180 µm)19 and (iii) the Kapton cover sheets were replaced with glass substrates exhibiting 10-fold higher thermal conductivity.29-30 All the factors enabled faster and more homogeneous cooling of the films and therefore better DPA distribution even at higher loadings. 2.3. Optical techniques

6


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Absorption spectra of the samples were recorded by UV-Vis-NIR spectrophotometer Lambda 950 (Perkin Elmer). Emission of the samples was excited by 405 nm and 532 nm wavelength semiconductor laser diodes and measured using back-thinned CCD spectrometer PMA-11 (Hamamatsu). Fluorescence excitation spectra were measured for the samples with high DPA loadings, where absorption measurements were impossible due to high optical density. Xenon lamp coupled to the monochromator (FWHM < 7 nm) was used as an excitation source in these measurements. Fluorescence quantum yield was estimated by utilizing integrating sphere (Sphere Optics) coupled to the CCD spectrometer via an optical fiber. Fluorescence transients were recorded using time-gated intensified CCD camera New iStar DH340T (Andor) coupled with spectrograph SR-303i (Shamrock), while frequency-doubled (532 nm) Nd3+:YAG laser (EKSPLA) with a pulse duration of 25 ps and a repetition rate of 10 Hz served as an excitation source. 3. Results and discussion 3.1. Evaluation of concentration quenching in PMMA/DPA films

Figure 1. Fluorescence quantum yield of melt-processed PMMA/DPA films at different DPA concentrations (λex = 405 nm). 7



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Figure 2. a) Emission spectra of Series B films excited at 532 nm as a function of DPA concentration. Note that the vertical axis has a logarithmic scale. For clarity purposes, the spectral region in the vicinity of the excitation wavelength (532 nm) was removed for the films with 35 and 40 wt% of DPA due to some notable scattering of the incident light. b) A picture of green-to-blue upconversion in the polymer film. c) Emission intensity ratio of DPA and PtOEP bands. Concentration effects that could originate from inhomogeneous DPA distribution in upconverting PMMA films due to high DPA loadings were assessed from fluorescence quantum 8


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yield (ΦFL) measurements. Figure 1 depicts ΦFL data for PMMA/DPA films measured as a function of DPA content. The data clearly show monotonous decrease in ΦFL, from 87 to 57% with increasing DPA concentration from 15 to 40 wt%. Taking into account homogeneous appearance of PMMA/DPA films (Figure S3 in SI) and the previously reported thermal characterizations,19 the moderate and constant decrease of ΦFL is an indicative of an even distribution of DPA throughout the polymer matrix. This is attributed to the melt-processing route followed by rapid cooling, which were used for the preparation of polymer films. Massive aggregation would have otherwise resulted in significant concentration quenching.31

3.2. Evaluation of triplet energy transfer in PMMA/PtOEP/DPA films Triplet energy transfer rate from the photoexcited PtOEP to DPA must be high to ensure large efficiency of overall TTA-UC process. The TTET rate was evaluated by analyzing UC and phosphorescence intensity dynamics at different DPA concentrations. Figure 2a shows such dynamics (400 – 700 nm) in a semi-logarithmic plot as a function of emitter concentration of PMMA/DPA/PtOEP films excited at the Q band of the sensitizer (532 nm) with a 30 mW power green laser diode. At each concentration two distinct signals can be observed on either side of the excitation band: a blue-shifted emission at ~440 nm, corresponding to the upconverted light and a red-shifted peak at 647 nm, corresponding to some residual phosphorescence from the porphyrin moiety (Figure 2a). A bright blue UC signal could be observed readily with the naked eye by irradiating the film containing 25 wt% of DPA under the same experimental conditions (Figure 2b). As expected, a monotonous decrease of PtOEP phosphorescence intensity is observed with increasing DPA concentration (Figure 2a), which is consistent with enhanced TTET to DPA. Concomitantly, the UC intensity increases with DPA concentration up to 25 wt% 9



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of DPA, and then decreases as the DPA concentration further increases. The non-monotonous behavior of UC can be clearly observed from Figure 2c, where the peak maximum intensity ratio of DPA and PtOEP emission bands is displayed as a function of DPA concentration. While this is not an absolute measurement of the quantum yield of the process, it provides a good qualitative indication of the overall UC efficiency at each concentration given the identical processing conditions and PtOEP loadings. The initial increase of UC intensity with the DPA load could be attributed to the increased triplet exciton concentration in the DPA due to the enhanced triplet energy transfer from PtOEP. The reduction of UC intensity to a small extent could potentially be ascribed to the concentration quenching of singlet excitons in DPA (Figure 1). However, as will be described further in the text, additional nonradiative decay phenomena of the DPA triplet excitons must be at play to account for the significant UC intensity decrease at high DPA contents. Figure 3a shows the accelerated decay in PtOEP phosphorescence transients with increasing DPA loadings, which further supports the fact that TTET from sensitizer to emitter is in fact quite efficient. These transients were obtained using a variable optical window method, which allowed for different delays and exposure times of measurable optical signal,32 thus enabling to probe time evolutions ranging from 100 ns to 10 ms. A reference film of PtOEP embedded in a PMMA matrix was shown to possess an intrinsic phosphorescent decay time constant of ca. 100 µs (see transient with 0 wt% of DPA in Figure 3a). Several orders of magnitude lower intensity long-lived decay component observed in the transients on a millisecond time-scale (Figure 3a), and which is missing in the transient of the reference sample, is attributed to the back-energy-transfer from DPA to PtOEP triplet. Similar observations were previously reported for blue phosphorescence materials.33 The intrinsic phosphorescence lifetime of PtOEP evidently 10


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shortens rapidly with the introduction of additional DPA into the polymeric matrix. In order to estimate the efficiency of the TTET step ( ), the following relation was used

% % , %

(1)

where IX%DPA(t) corresponds to the transient obtained at X wt% of DPA.

Figure 3. a) Phosphorescence transients of PMMA/DPA/PtOEP (CPtOEP = 0.05 wt%) films with different DPA loadings (λex = 532 nm). b) Triplet energy transfer rate from PtOEP to DPA. The dashed line corresponds to Dexter-type energy transfer fit of experimental data, where n is the DPA concentration.

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As expected, increases monotonously with increasing DPA concentration and exceeds 75% for a DPA content above 25 wt% implying TTET process to be highly efficient (Figure 3b).

Figure 4. a) Phosphorescence and UC emission intensity vs incident power density of the PMMA/DPA/PtOEP (CDPA = 25 wt%, CPtOEP = 0.05 wt%) film (λex = 532 nm). b) UC intensity threshold (i.e., when UC intensity slope switches from quadratic to linear) at different DPA concentrations. 3.3. Evaluation of upconversion quantum yield Prior to estimating UC quantum yield (ΦUC) in our PMMA/DPA/PtOEP films proper excitation conditions need to be found. To this end, UC emission and sensitizer phosphorescence 12


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intensity was measured as a function of incident power density for all DPA concentrations (Figure S4 in SI). Figure 4a illustrates this dependence for the seemingly most efficient system in a double logarithmic plot. Since the emission spectral shape did not change with increasing excitation power the UC peak intensity could be used instead of the integral value. As expected, the phosphorescence intensity increases linearly with increasing excitation power density, i.e., increasing number of excited PtOEP molecules. Conversely, the UC emission exhibits a dual behavior: (i) a quadratic dependence at low excitation densities and (ii) a linear at higher excitation densities. Such behavior is characteristic of the TTA-UC process and has been previously examined in detail.34-35 Briefly, the linear behavior corresponds to a regime where triplet emitters decay preferentially via TTA, whereas the quadratic regime is consistent with spontaneous decay of the DPA triplet excited states. Consequently, the intensity threshold (Ith),35 whereby the kinetics of UC switches from quadratic to linear is a critical figure of merit as it characterizes the performance of a certain TTA-UC system. The lower this value is, the more performant this system. This threshold value was also determined for our system at different emitter concentrations (Figure S4 in SI) and plotted as a function of DPA content (Figure 4b). It appears clearly that the lowest Ith is obtained for the previously mentioned value of 25 wt% of DPA. Obtaining Ith is also imperative in order to accurately determine UC quantum yields. Indeed, the measurements must be done in a regime where the upconversion depends linearly upon the incident light (i.e., where I > Ith), meaning that the estimated quantum yield value of UC must not depend upon the excitation power density. Consequently, all of the quantum yield measurements were performed at an excitation power density of 155 mW/cm2, which is in the linear regime for

13



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most of concentrations except for the highest one (40 wt%) used in our experiments (Figure 4b and Figure S4 in SI).

Figure 5. Phosphorescence and UC quantum yield in PMMA/DPA/PtOEP (CPtOEP = 0.05 wt%) films as a function of DPA concentration. Lines are guides to the eye. The absolute measurements of ΦUC and the phosphorescence quantum yield (ΦP) in the UC polymer films were carried out by using an integrating sphere (Figure 5). Again, the decrease of ΦP with increasing DPA concentration confirms enhanced TTET from the sensitizer to the emitter molecules. The evolution of ΦUC is somewhat more intricate. The initial increase in UC quantum yield is in agreement with enhanced TTET to DPA. However, the later dramatic decrease of UC quantum yield from 0.9% to 0.0006% cannot solely be accounted by the concentration quenching of singlet emission in DPA (Figure 1). For the sake of clarity, it is important to mention that UC quantum yield (with the maximum ΦUC of 50%) scales by a factor of 2 as compared to UC quantum efficiency, which maximum value is normalized to 100%).7 The decrease of ΦUC by ca. three orders of magnitude is indeed inconsistent with the previously described 4/5 decrease in DPA emission upon excitation at 405 nm. This diminution can presumably be ascribed to triplet exciton quenching due to enhanced triplet exciton diffusion, 14


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and thus increased probability to reach nonradiative decay sites. Unlike pre-organized systems, amorphous glasses bear indeed a high number of defects which could potentially serve as nonradiative sinks for long-lived triplet exciton.

Figure 6. UC emission transients of the PMMA/DPA/PtOEP (CPtOEP = 0.05 wt%) films at different DPA loadings. The essential role of the triplet exciton quenching is corroborated by the accelerated decay of UC intensity with increasing DPA content in UC transients (Figure 6). The changes in the UC decay rate occurring on a millisecond time-scale indicate that the dominant quenching mechanism is governed by the triplet excitons and not by the singlets. This nonradiative decay channel of the triplet excitons could potentially be related to oxygen (since the polymer films were prepared in air). However, considering the non-emissive character of this channel its identification is challenging. Our latest preliminary results for the films prepared in the inert N2 atmosphere show negligible increase of ΦUC indicating that oxygen-related channel is nondominant.

3.4. Evaluation of the triplet exciton diffusion by addition of a quencher 15



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Triplet exciton mobility in the amorphous upconverting films was elucidated by a timeresolved photoluminescence bulk-quenching technique. Explicitly, varying amounts of randomly distributed quencher molecules were introduced into the PMMA/DPA/PtOEP films and the exciton diffusion coefficient (D) as well as diffusion length (LD) were determined from the emission quenching efficiency. PCBM was chosen as a triplet exciton quencher because of its remarkable aptitude as an electron acceptor instantly leading to exciton dissociation.36 The quenching efficiency was evaluated by modeling experimentally obtained excited state relaxation dynamics (Figure 7) using a Stern-Volmer approach,27 which recently was shown to be the accurate tool for exciton diffusion length evaluation.26

Figure 7.

Triplet

exciton

concentration

dynamics

of

the

PMMA/DPA/PtOEP

(CPtOEP = 0.05 wt%) films at different triplet quencher concentrations from 0 to 2 wt%. Taking into account that the UC is bimolecular process, i.e. that the delayed UC emission originates from TTA, its intensity IUC is proportional to the square of the concentration of emitter triplets :37

∝

,

(2) 16


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where k is the first-order decay constant, which is a combination of the intrinsic radiative and nonradiative decay of the triplet excitons and other possible quenching processes. kTTA is the second-order decay constant originating from the triplet exciton depopulation via TTA process only. Thus, by measuring delayed IUC from the singlet state it is possible to indirectly probe DPA triplet excitons, which are usually non-emissive. In such a way triplet exciton quenching efficiency (Q) can be evaluated by using the following relation: 1

"#

.

(3)

Here IUC(PCBM) stands for UC transients measured with PCBM quenchers present, whereas IUC – unquenched UC transients. Triplet exciton diffusion coefficient was estimated by utilizing modified Stern-Volmer-type quenching analysis, referred to as the “hindered access model”.27 The model allowed to quantitatively describe experimental data where only a fraction (fa) of the emitter molecules is accessible to the quencher due to its aggregation at high concentrations, %&

'( )*+,- ./

.

(4)

Here KSV is Stern-Volmer constant; [Qc] - quencher concentration. According to the model proposed,27 the diffusion coefficient (D) can be calculated by: +

,, 0 123456

(5)

where r - reactive radius, which in the case of solid films assumed to be a distance between emitters, P – quenching probability upon exciton collision with quencher molecule (assumed to be 1), NA - Avogadro’s number. Reactive radius was estimated from the volume averaged molecular distance, which is a function of emitter concentration.

17



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Figure 8. Relative quenching efficiency as a function of quencher concentration for different DPA concentrations in the PMMA/DPA/PtOEP (CPtOEP = 0.05 wt%) films. Triplet exciton diffusion lengths are indicated. Measured relative quenching efficiency as a function of quencher concentration was modeled using modified Stern-Volmer equation (Figure 8) thereby enabling to estimate exciton diffusion 18


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coefficient D (Table 1). Then exciton diffusion length was calculated from the following relation: 78 √:0;,

(6)

where a is dimensionality constant (a = 3).

Table 1. Main Stern-Volmer fitting parameters. DPA

KSV

D

fa

r

LD

loading cm3·mol-1 cm2·s-1

nm

20 wt% 1.6·106

3.1·10-10 1.0

1.32 5.3 ms 22.2

25 wt% 2.1·106

2.2·10-9

0.97 1.23 1.0 ms 25.8

30 wt% 3.7·106

1.7·10-8

0.97 1.15 240 µs 35.6

35 wt% 9.9·106

5.8·10-8

0.86 1.10 210 µs 60.1

nm

Table 1 summarizes main Stern-Volmer fitting parameters along with the reactive radius, average intrinsic UC emission lifetime and estimated triplet-exciton diffusion length. The application of the time-resolved photoluminescence bulk-quenching technique for the evaluation of diffusion parameters at DPA loadings of 15 and 40 wt % was impossible due to the too weak UC signal (Figure 5). From the Table 1, it is obvious that D increases while the average lifetime of triplets shortens with emitter concentration. However, the increase of D prevails resulting in the final enhancement of LD with increasing DPA content (Figure 9).

19



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Figure 9. Triplet exciton diffusion length in the emitter as a function of its concentration in amorphous PMMA glasses. LD of the triplets was reported to depend very sensitively on the material and its morphology and therefore to range from several nanometers to hundreds of microns.38 Chiefly for purely amorphous films, LD was found to be less than 100 nm, e.g., 54 nm for 4P-NPD,39 22 nm for platinum polyyne polymer (Ph100),27 30 nm for PtOEP film,40 9 nm for SY-PPV,41 28 nm for PdTPPC,42 87 nm for NPD43. In the case of highly-ordered organic crystals LD values well beyond several microns could be obtained, e.g., 20 µm for anthracene crystals,44 8 µm for rubrene,45 and up to 13 µm in anthracene-based metal-organic frameworks25. From a microscopic point of view, exciton diffusion can be described as a series of nearest neighbor hopping events proceeding in a random walk. In highly ordered systems, a simple model for exciton diffusion based on the energy transfer rate formalism can be applied. However, a much more complex treatment of exciton diffusion is required for inherently disordered systems,46 such as polymers, viz. in our case amorphous PMMA/DPA/PtOEP films. In contrast to singlet excitons, which are capable of migrating via long-range (1-10 nm) Förster energy transfers, triplet exciton diffusion is governed by Dexter energy transfer with a typical 20


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intermolecular exchange radius on the order of 1 nm. Consequently, Dexter mechanism is referred to as short-range energy transfer. This short range stems from the necessary physical exchange of electrons that necessitates spatial overlap of wave functions of the donor and acceptor. Taking this into account, local intermolecular arrangements/interactions play a critical role for triplet excitons and defines their diffusion pathways.47 Designing inhomogeneous diffusion landscapes by proper distribution/alignment/functionalization of molecules or their aggregation is thus foreseen as a way to enhance exciton migration to a desired location, e.g. donor-acceptor interface in a photovoltaic cell. The increase of triplet exciton diffusivity (parameter D) with DPA loading (Table 1) can be intuitively understood to occur as a result of reduced intermolecular separation, i.e. increase of the concentration of nearest-neighbor hopping sites. On the other hand, enhanced exciton mobility ensures access of a larger number of distant sites including those acting as nonradiative decay sites. This explains the apparent shortening of the intrinsic lifetime of triplet excitons with increasing DPA loadings in the PMMA/DPA/PtOEP films. In the studied disordered PMMA/DPA/PtOEP films, LD increased by a factor of 3 (from 22 to 60 nm) as the emitter concentration increased from 20 to 35 wt%. Unfortunately, enhanced triplet exciton diffusion in sensitized polymer glasses with increasing emitter concentration did not result in enhanced UC quantum yield suggesting that LD is not a limiting factor in the amorphous polymer glasses. Evidently, the triplet exciton diffusion in the polymer glasses is found to be sufficiently efficient to not degrade TTA-UC efficiency. As it was discussed above, most likely the main limiting factor is related to nonradiative decay of triplet excitons in emitter molecules. In support of this statement a comparison of the estimated LD values with the average distance between the sensitizer molecules, which is ~14 nm at PtOEP loading of 0.05 wt%, can 21



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be made. A significantly larger LD than the intermolecular distance of sensitizer in PMMA implies a high probability for the triplet excitons generated on the neighboring sensitizer molecules to meet by diffusion process and promote TTA. This observation again confirms that triplet exciton diffusion plays a non-decisive role in limiting TTA-UC efficiency in the disordered PMMA/DPA/PtOEP films. In general, choosing a different quencher with worse quenching ability will result in quantitatively different estimates for diffusion coefficient and diffusion length. Namely, applying a Stern-Volmer model for the quenching analysis of a poorer quencher will underestimate diffusion coefficient, and correspondingly yield shorter diffusion length. However, it shall not affect the overall observed trend pointing to an enhancement of the diffusion length with increasing emitter content, and thus would not alter the main conclusion of this work, i.e. that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor for TTA-UC.

4. Conclusions In conclusion, the present study of a light-upconverting model system consisting of an emittersensitizer pair (DPA-PtOEP) dispersed in glassy PMMA films allowed for the investigation of the factors limiting TTA-UC efficiency with a special focus on triplet exciton diffusion since the latter had been thought to be the major source of loss. The triplet exciton diffusion, i.e. diffusion coefficient and diffusion length in upconverting polymer glasses were evaluated by a timeresolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of obtained experimental data. The analysis has shown that continuous increase of DPA concentration in PMMA/DPA/PtOEP films causes monotonous enhancement of triplet LD but not ΦUC, which rapidly degrades above 25 wt% of DPA. This finding clearly 22


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implies the triplet exciton diffusion to be the non-governing loss mechanism of UC efficiency and points to alternate loss channel. High Dexter-type triplet energy transfer rates from the PtOEP to DPA ( >75% for DPA above 25 wt%), which were estimated from accelerated PtOEP phosphorescence decay measurements with increasing DPA concentration confirmed TTET process to be highly efficient. High ΦFL and minor concentration quenching of emitter even at high loadings (up to 40 wt% in PMMA) achieved via melt-processing route also proved to contribute only mildly to UC efficiency losses. Importantly, the accelerated UC intensity decay with increasing DPA content occurring on a millisecond time-scale unambiguously indicated that the dominant quenching mechanism is nevertheless governed by the triplet excitons in DPA. Moreover, this major loss channel is obviously facilitated by enhanced diffusion of long-lived triplets with increasing DPA content, thus causing increased probability to reach nonradiative decay sites. The identification of these “dark” sites in disordered polymer glasses, which seem to be non-oxygen related, is of prime importance for boosting TTA-UC efficiency, and thus for fostering TTA-UC-based device applications. ASSOCIATED CONTENT Supporting Information. Absorption, excitation and emission spectra of DPA and PtOEP dispersed in PMMA, phosphorescence quantum yield of PtOEP in PMMA, pictures of the glassy polymer samples, UC intensity of the polymer films as a function of excitation power density. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 23



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Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from Sciex program (Grant 14.085) and the Adolphe Merkle Foundation. REFERENCES 1.

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The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer

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