A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor–acceptor organic materials

Busby, Erik; Xia, Jianlong; Wu, Qin; Low, Jonathan Z.; Song, Rui; Miller, John R.; Xy, Zhu; Campos, Luis M.; Sfeir, Matthew Y.

doi:10.1038/nmat4175

Cited by 316 publications

(521 citation statements)

References 47 publications

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“…S18-S21), resulting in a long-lived signature that closely matches the features observed under direct excitation (Fig. 4C, Bottom) (26,27). The remarkably sharp triplet absorption features we observe for the triplet exciton have been observed in a small set of other systems (18,28), where the rigidity of the molecule gives narrow absorption bandwidths, and are a distinct advantage of solution studies of acenes compared with the solid state.…”

Section: Resultssupporting

confidence: 74%

“…In this case, the sharp features allow us to distinguish the absorption signatures of the three species, and in particular track the evolution of the excimer to the triplet excitons. The sensitization measurement also enabled a determination of the triplet absorption cross-section of 5,400 Lmol −1 cm −3 at 1.63 eV, based on the degree of quenching of the triplet excitons on the sensitizer by TIPS-tetracene (27). This value was used to obtain a triplet exciton yield of 120% ± 20% of the singlet exciton concentration.…”

Section: Resultsmentioning

confidence: 99%

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Identification of a triplet pair intermediate in singlet exciton fission in solution

Stern

Musser

Gélinas

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

190

260

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Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spinsinglet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]-tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.T he fission of photogenerated spin-singlet excitons into pairs of spin-triplet excitons is an effective way to generate triplet excitons in organic materials (1, 2). Because the triplets produced are coupled into an overall singlet state, spin is conserved and triplet formation can proceed on sub-100-fs timescales (1, 3-5) with yields of up to 200% (1, 6, 7). Current interest in singlet fission is driven by its potential to improve the efficiency of solar cells by circumventing the Shockley-Queisser limit for single-junction devices (8-10). Incorporating singlet fission material within a lowband-gap solar cell should make it possible to capture the energy normally lost to thermalization following the absorption of highenergy photons (11,12). An external quantum efficiency of 129% (13) and an internal quantum efficiency of >180% (14) have been reported using pentacene as the singlet fission material and fullerene (C 60 ) as electron acceptor. Despite such significant advances, many questions remain about the underlying mechanism of triplet formation, such as the role of intermediate electronic states and the ability of systems to undergo endothermic fission.The basis of most kinetic descriptions of singlet fission is the triplet pair state 1 (TT), which is entangled into an overall singlet and is an essential intermediate for the formation of two free triplet excitons (1, 15). Whether this intermediate state is present only transiently, as expected in exothermic systems such as pentacene, or whether it can be sufficiently long lived to also play a central role in the fission process in slower systems is unclear. Transient absorption measurements of the canonical systems pentacene and tetracene in the solid state allow clear identification of only the singlet and triplet states (3,6,16,17), meaning the character of any intermediate has not been ob...

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Section: Resultssupporting

confidence: 74%

Section: Resultsmentioning

confidence: 99%

Identification of a triplet pair intermediate in singlet exciton fission in solution

Stern

Musser

Gélinas

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

190

260

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“…These authors have obtained nearly identical results for a different DA copolymer PDTP-DFBT [27]. Busby et al have reported triplet exciton generation in picosecond (ps) time scale from a transient absorption measurement of the DA copolymer PBTDO1 [28]. The transient absorption observed is the equivalent of the higher energy PA 2 absorption of Huynh et al [26] (see Fig.…”

mentioning

confidence: 77%

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

et al. 2015

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We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.PACS numbers: 78.66. Qn, 71.20.Rv, 71.35.Cc,The primary photophysical process in polymer solar cells is photoinduced charge transfer, whereby optical excitation at the junction between a donor conjugated polymer and acceptor molecules creates a charge transfer (CT) exciton whose dissociation leads to charge carriers. The donor polymeric materials used to be homopolymers such as polythiophene which absorb in the visible range of the solar spectrum [1]. Homopolymers have recently been replaced by block copolymers whose repeat units consist of alternating donor (D) and acceptor (A) moieties [2][3][4][5][6][7][8][9][10][11]. This architecture reduces the optical gap drastically, and the DA copolymers absorb in the near infrared, where the largest fraction of the photons emitted by the Sun lie. The power conversion efficiencies (PCEs) of organic solar cells with DA copolymers as donor materials have exceeded 10% [11], and there is strong interest in the development of structure-property correlations that will facilitate further enhancement of the PCE. Clearly, this requires precise understanding of the nature of the primary photoexcitations of DA copolymers.Existing electronic structure calculations of DA copolymers are primarily based on the density functional theory (DFT) approach or its time-dependent version (TD-DFT) [12][13][14][15][16][17][18]. The motivations behind these calculations have largely been to understand the localized versus delocalized character of the excited state reached by ground state absorption. Experimentally, DA copolymers exhibit a broad low energy (LE) absorption band at ∼ 700 − 800 nm and a higher energy (HE) absorption band at ∼ 400 − 450 nm [2][3][4]. There is agreement between the computational studies that the LE band is due to CT from D to A, and the HE band is a higher π-π * excitation.Recent optical studies indicate that the above simple characterization of the LE band might be incomplete, and as in the homopolymers [19], electron correlations play a stronger role in the photophysics of the DA copolymers than envisaged within DFT approaches. Grancini et al. determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is c...

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“…In singlet fission, one high energy singlet excited state can decay to form two triplet excitons and is well known to be nearly 100% efficient in many molecule systems. [24,[31][32][33][34][35] This process is thermodynamically endothermic if the triplet energy is less than or equal to the singlet energy, providing the minimum emission wavelength (870 nm), and corresponding PV bandgap, requirement. In this case the UV-only efficiency limits (with emission in the NIR and SF) increases from 3.7% to 5.6% as shown in Figure 4b.…”

Section: Ideal Transparent Luminescent Solar Concentrator Case Imentioning

confidence: 99%

Limits of Visibly Transparent Luminescent Solar Concentrators

Yang

Lunt

2017

Advanced Optical Materials

109

100

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surfaces into energy harvesting resources. This new approach to visibly transparent LSCs was developed by exploiting the nature of excitonic semiconductors to optimize the use of this light-exposed surface area without compromising its existing functionality, thereby avoiding the aesthetic tradeoffs that can hinder adoption. Recent work has demonstrated devices that selectively harvest different parts of the solar spectrum including i) ultraviolet (UV) absorption, with massive downconverted luminescence deeper into the NIR (>400 nm) [4] and ii) near-infrared (NIR) absorption, with even deeper luminescence in the NIR. [3] This work has been subsequently followed by a number of other groups. [8][9][10][11][12] In contrast to transparent photovoltaics, which have also been developed to exploit this untapped area, [13][14][15] this new concentrator approach offers an alternative pathway whereby energy is transported optically, offering a simple route to large-area manufacturing, high defect tolerance, angle independent performance, and low-level energy cost. In this progress report we discuss the theoretical limits of transparent LSCs, idealized configurations, scalability, and properties of enabling materials. Operating Principles of LSCsLuminescent solar concentrators operate based on the following mechanisms (see Figure 1a): a portion of solar spectrum is absorbed in a luminescent dye (or "luminophore") [16] embedded in a transparent waveguide. That absorbed solar energy is then re-emitted at another wavelength isotropically in all directions within the waveguide, provided the oscillators are not preferentially oriented. Due to the index of refraction difference between the waveguide and the ambient environment the re-emitted photons are predominately trapped by total internal reflection, causing them to be directed towards the waveguide edges where they can be converted to electrical power in an edge mounted photovoltaic cell. The overall system power conversion efficiency, η LSC , is then given by:where η Opt is the optical efficiency (number of photons reaching the waveguide edge)/(number of photons incident on the waveguide), R is the front face reflection, η Abs is the solar spectrum Visibly transparent solar harvesting surfaces are an exciting new approach to harvesting solar energy around buildings and mobile electronics to improve their efficiency and autonomy without impacting their appearance. The recent demonstration of ultraviolet (UV) and near-infrared (NIR) selective light harvesters have enabled transparent luminescent solar concentrators (LSCs) that boast unparalleled scalability, flexibility, aesthetic quality, and affordability. Consequently, the question of the efficiency limits has emerged in these new systems. In this perspective, the theoretical efficiency limits of these concentrator systems are reviewed and practical considerations are outlined to approach these limits. For UV and UV/NIR selective harvesting single-junction transparent LSCs are constrained thermodynamically to 21%, which i...

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A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor–acceptor organic materials

Cited by 316 publications

References 47 publications

Identification of a triplet pair intermediate in singlet exciton fission in solution

Identification of a triplet pair intermediate in singlet exciton fission in solution

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

Limits of Visibly Transparent Luminescent Solar Concentrators

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