On the efficiency limit of triplet–triplet annihilation for photochemical upconversion

Cheng, Yuen Yap; Khoury, Tony; Clady, R.; Tayebjee, Murad J. Y.; Ekins-Daukes, N. J.; Crossley, Maxwell J.; Schmidt, Timothy W.

doi:10.1039/b913243k

Cited by 363 publications

(478 citation statements)

References 17 publications

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“…[9][10][11][12][13][14] Effective upconverters have been demonstrated using combinations of a triplet sensitizer such as platinum octaethylporphyrin (PtOEP) [3] and a triplet acceptor/annihilator such as 9,10-diphenylanthracene (DPA), [15] perylene, [16] or rubrene. [17] TTA-upconversion (TTA-UC) quantum yield (ratio of upconverted photons to absorbed photons) of 30% was observed in solution phase. [18] This suggests that the intrinsic efficiency of the TTA-UC reaction (i.e., the being built up during the process.…”

Section: Doi: 101002/adma201605987mentioning

confidence: 99%

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

et al. 2017

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probability of a triplet pair forming a singlet) in solution is >60%, much higher than the spin-statistical prediction of 25% (one pair of triplets collides to form one of the four states: one singlet S 1 and three triplets T 1 , assuming higher triplet and quintet states are inaccessible). Practical application of solar energy conversion requires the TTA-UC material to be in solid state rather than in solution phase. However, the TTA-UC quantum yield of solidstate systems remains low, typically below 5%, [10] and is moderately high (about 10%) in only one example. [19] To achieve high efficiencies, high excitation intensities (200 mW cm −2 or above) are commonly required. These imply that in solid state, the experimentally observed reaction efficiency of TTA-UC was up to about 20%, below the 25% spin-statistical limit.To achieve highly efficient triplet fusion (TTA-UC) in OLEDs and other TTA upconverters, we consider four criteria for the selection of emitters: (1) high fluorescence quantum yield, (2) short singlet lifetime, (3) long triplet lifetime, and (4) the energy of two triplet excitons, 2E(T 1 ) lies slightly above that of the singlet exciton, E(S 1 ), but below the second triplet state, E(T 2 ) (and also the energies of any spin-quintet states) (E(S 1 ) ≲ 2E(T 1 ) < E(T 2 )). The first and second criteria are prerequisites for efficient fluorescence. The third criterion is essential for the accumulation of a sufficiently high triplet population density required for rapid triplet-triplet collision processes. The fourth criterion ensures that higher-lying triplet or quintet states do not provide loss channels. The spin states of the triplet excitons give in principle nine spin configurations for the interacting triplet exciton pair, five associated with a quintet, three with a triplet, and one with the singlet state. It is generally considered that the quintet is always higher in energy than the initial triplet pair, so is neglected. If there are no energetically accessible higher-lying triplet states at E(T 2 ), we expect only the S 1 and T 1 excitons to form. Triplets produced from this reaction can be recycled and participate in a further fusion reaction. [7] The basic working principle of a triplet fusion LED (FuLED) is illustrated in Figure 1a. The initial stage (Stage I) of the device operation includes charge injection and exciton formation. Exciton formation on the emissive molecules may occur directly or indirectly through an additional exciton transfer step from a host material. If a host material is present, the S 1 and T 1 of the host are required to be higher than that of the emitter to allow efficient host-emitter energy transfer and to ensure long triplet lifetime of the emitter (Criterion 3 discussed above). The 25% singlet population can be converted to light emission (and nonradiative losses) from the singlet channel immediately, resulting in prompt electroluminescence (EL). The 75% triplet excitons remain nonemissive, but the population of triplets is When an organic light-emitting dio...

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Section: Doi: 101002/adma201605987mentioning

confidence: 99%

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

et al. 2017

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“…Intensive efforts are currently being made to synthesize upconversion materials with increasing power upconversion efficiencies 2,7,[14][15][16][17][18][19] . In addition, there have been two recent studies with pulsed laser sources employed for excitation 20,21 , where the excitation power densities in each pulse are much higher than the corresponding timeaveraged excitation power densities. The reported upconversion quantum yields are 16.2 and 16% and the corresponding power upconversion efficiencies are B25 and 19%.…”

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confidence: 99%

Photon energy upconversion through thermal radiation with the power efficiency reaching 16%

et al. 2014

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The efficiency of many solar energy conversion technologies is limited by their poor response to low-energy solar photons. One way for overcoming this limitation is to develop materials and methods that can efficiently convert low-energy photons into high-energy ones. Here we show that thermal radiation is an attractive route for photon energy upconversion, with efficiencies higher than those of state-of-the-art energy transfer upconversion under continuous wave laser excitation. A maximal power upconversion efficiency of 16% is achieved on Yb 3 þ -doped ZrO 2 . By examining various oxide samples doped with lanthanide or transition metal ions, we draw guidelines that materials with high melting points, low thermal conductivities and strong absorption to infrared light deliver high upconversion efficiencies. The feasibility of our upconversion approach is further demonstrated under concentrated sunlight excitation and continuous wave 976-nm laser excitation, where the upconverted white light is absorbed by Si solar cells to generate electricity and drive optical and electrical devices.

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“…Thus, metalcontaining chromophores with long-lived triplet states make desirable triplet sensitizers because the energy and excited state lifetimes of the singlet and triplet states can be rationally tuned by ligand modification [27,28]. Consequently, tremendous advancements have been achieved in photochemical upconversion by using metal-containing triplet sensitizers with a variety of organic-based triplet acceptors/annihilators in both fluid solution [20,22,[29][30][31][32] and various host matrices [33][34][35][36][37][38][39][40][41][42][43]. The ultimate goal of this research is to maximize the efficiency of real-world solar powered devices by imparting sub-bandgap sensitization via integration of UC technology [8][9][10]44,45].…”

Section: Introductionmentioning

confidence: 99%

“…However, there are several critical properties of these systems that must be optimized for this technology to be viable for incorporation into real-world applications. First, because the photon UC via sensitized TTA process occurs through diffusional, bimolecular chemistry, the measurement and improvement of the quantum efficiency in these systems becomes crucial [20,22,35,46]. Second, in order to reap the greatest benefits from these compositions, it is necessary to maximize the anti-Stokes shift that can be achieved.…”

Section: Introductionmentioning

confidence: 99%

“…During the TTA step, one of the acceptor molecules transfers its energy to the other effectively promoting the latter to an excited singlet state that is higher in energy than the incident light resulting in P-type, anti-Stokes delayed fluorescence. Based on this mechanism, it is clear that there are specific energetic and kinetic requirements that enable photon UC to occur [20][21][22][23][24]. Namely, in most cases the triplet state of the acceptor must be lower in energy than the triplet state of the sensitizer and both triplet states must be long lived (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Photon upconversion sensitized by a Ru(II)-pyrenyl chromophore

Deng

Lazorski

Castellano

2015

Phil. Trans. R. Soc. A.

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One contribution of 11 to a discussion meeting issue 'Organic semiconductor spintronics: utilizing triplet excitons in organic electronics' . The near-visible-to-blue singlet fluorescence of anthracene sensitized by a ruthenium chromophore with a long-lived triplet-excited state, [Ru(5-pyrenyl-1,10-phenanthroline) 3 ](PF 6 ) 2 , in acetonitrile was investigated. Low intensity non-coherent green light was used to selectively excite the sensitizer in the presence of micromolar concentrations of anthracene generating anti-Stokes, singlet fluorescence in the latter, even with incident power densities below 500 µW cm −2 . The resultant data are consistent with photon upconversion proceeding from sensitized triplet-triplet annihilation (TTA) of the anthracene acceptor molecules, confirmed through transient absorption spectroscopy as well as static and dynamic photoluminescence experiments. Additionally, quadratic-to-linear incident power regimes for the upconversion process were identified for this composition under monochromatic 488 nm excitation, consistent with a sensitized TTA mechanism ultimately producing the anti-Stokes emission characteristic of anthracene singlet fluorescence.

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On the efficiency limit of triplet–triplet annihilation for photochemical upconversion

Cited by 363 publications

References 17 publications

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light‐Emitting Diodes

Photon energy upconversion through thermal radiation with the power efficiency reaching 16%

Photon upconversion sensitized by a Ru(II)-pyrenyl chromophore

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