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Triplet-involved quenching processes lead to significant efficiency roll-off in phosphorescent organic light-emitting diodes (OLEDs). Thermally activated delayed fluorescent (TADF) materials can reduce the triplet exciton density in the emitting layer (EML) via the up-conversion of triplet into singlet exciton. However, the effect of the triplet lifetime of TADF hosts on efficiency roll-off is still under-researched. Inspired by the fast spin–flip process in TADF with a small singlet–triplet energy gap, we herein reported a triazine–carbazole hybrid (Trz–PhCz) with a short triplet lifetime, and extremely low efficiency roll-offs were achieved for green, yellow, orange and red OLEDs with external quantum efficiencies (EQEs) over 20% at 10 000 cd m−2. Particularly, the optimized orange device showed a record-high maximum EQE of 31.4%, and 25.5% at 10 000 cd m−2. Furthermore, unprecedented high PEmaxs of 111.8 lm W−1 and 98.9 lm W−1 were obtained for Ir(ppy)2(acac)- and PO-01-based devices, respectively. Notably, a two-color white OLED based on a single host Trz–PhCz also showed a remarkably high maximum power efficiency (PEmax) of 102.5 lm W−1 with the Commission Internationale de l’Eclairage (CIE) of (0.42, 0.48). So, the Trz–PhCz-hosted phosphorescent OLEDs showed state-of-the-art device performance, and this work provided insight into the design of efficient TADF host materials via controlling their triplet lifetime for practical applications.

Singlet fission (SF) is a process where one singlet exciton splits into two triplet excitons. Utilizing SF may potentially increase the efficiency of solar cells beyond the Shockley–Queisser limit....

Singlet fission (SF) is an exciton dissociation process that generates two triplet excitons from one singlet exciton. Because the exciton dissociation process involves internal conversion between m...

Pentacene thin films are common constituents of organic photovoltaic materials and a prototypical example of a material that undergoes singlet exciton fission, but significant questions remain rega...

Singlet fission (SF) converts a singlet exciton into two triplet excitons and holds great potential in the photovoltaic field. The triplet pair state is proposed to play an important role in SF. Ho...

The modulation of intermolecular interactions upon aggregation induces changes in excited state properties of organic molecules that can be detrimental for some optoelectronic applications but can be exploited for others. The time-dependent density functional theory (TDDFT) is a cost-effective approach to determining the exciton states of molecular aggregates, and it has been shown to provide reliable results when coupled with the appropriate choice of the functional. Here we apply a general procedure to analyze the aggregates’ exciton states derived from TDDFT calculations in terms of diabatic states chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space. We apply the approach to study energy profiles, interstate couplings, and the charge-transfer character of singlet and triplet exciton states of perylene di-imide aggregates (PDI). We focus on the intermolecular displacement along the longitudinal translation coordinate, which mimics different amounts of slip-stacking observed in PDI crystals. The analysis, in terms of symmetry-adapted Frenkel excitations (FE) and charge-resonance (CR) states and their interactions, discloses how the interchange of the H/J character for small longitudinal shifts, previously reported for singlet exciton states, also occurs for triplet excitons. This research was funded by Ministerio de Economía y Competitividad of Spain, grant number PID2019-109555GB-I00 and RED2018-102815-T and by Eusko Jaurlaritza, grant number PIBA19-0004.

Both quantum coherent and incoherent mechanisms have been revealed in singlet exciton fission. Also, in some systems, both mechanisms may concurrently operate. However, the comprehensive picture is...

A procedure is described for unbiased identification of all pi-electron chromophore pair geometry choices that locally maximize the rate of conversion of a singlet exciton into a singlet biexciton (triplet pair), using a simplified version of the diabatic frontier orbital model of singlet fission (SF). The resulting approximate optimal geometries provide insight and are expected to represent useful starting points for searches by more advanced methods. The general procedure is illustrated on a pair of ethylenes as the simplest model of a pi-electron system, but it is applicable to pairs of much larger molecules, with dozens of non-hydrogen atoms, and not necessarily planar. We first examine the value of vertical bar T-A vertical bar(2), the square of the electronic matrix element for SF with initial excitation fully localized on partner A, on a grid of several billion geometries within the six-dimensional space of physically realizable possibilities. Several of the optimized pair geometries are somewhat unexpected, but all are found to follow the qualitative guidance proposed earlier. In the neighborhood of each local maximum of vertical bar T-A vertical bar(2), consideration of mixing with charge-transfer configurations and of excitonic interaction between partners A and B determines the SF energy balance and yields squared matrix elements vertical bar T*vertical bar(2) and vertical bar T**vertical bar(2) for the lower and upper excitonic states S* and S**, respectively. Assuming Boltzmann populations of these states, the geometry is further optimized to maximize k, the sum of the SF rates obtained from Marcus theory, and this reorders the suitable geometries substantially. At 87 pair geometries, the vertical bar T*vertical bar(2) and vertical bar T**vertical bar(2) values are compared with those obtained from high-level ab initio nonorthogonal configuration interaction calculations and found to follow the same trend. Finally, the biexciton binding energy at the optimized geometries is calculated. Altogether, 13 significant local maxima of SF rate for a pair of ethylenes are identified in the physically relevant part of space that avoids molecular interpenetration in the hard-sphere approximation. The three best geometries are twist-stacked, slip-stacked, and L-shaped. The maxima occur at the (five dimensional) surfaces of seven six-dimensional "parent" regions of space centered at physically inaccessible geometries at which the calculated SF rate is very large but the two ethylenes interpenetrate. The results are displayed in interactive graphics. The computer code ("Simple") written for these calculations is flexible in that it permits a choice of performing the search for local maxima in six dimensions on vertical bar T-A vertical bar(2), vertical bar T*vertical bar(2), or k.

The literature capturing the unusual electronic structure and photophysics of monomeric azulene and its simple derivatives is briefly reviewed. The principles established on the basis of this model are then used to understand the experimental and predicted spectroscopic and photophysical behaviour of organic compounds containing two or more of the azulene carbon framework. Biazulenes, terazulenes and multiazulenes, both linked and fused, are discussed. The possibilities of singlet exciton fission from the S 2 states of some of these molecules and their aggregates are examined. Gaps in the existing experimental data are identified.

Singlet fission is a photophysical process in which two triplet excitons are produced from one singlet exciton. Understanding the details of this process, especially the properties of intermediates between singlet excitons and free triplet excitons, is particularly important to its optimization and application. Herein, we explore the bound triplet pair, m(T1T1), an important intermediate state in singlet fission. We highlight a growing number of studies indicating there is an energetic stabilization of triplet-pair states relative to free triplets. This stabilization enables endothermic singlet fission processes and often results in long lifetimes of the bound triplet-pair state. However, triplets in close proximity demonstrate heightened recombination and reduced excited-state lifetimes. The ubiquity of bound triplet-pair states in recent reports indicates the importance of this state, the need to discern it from free triplets, and the usefulness of having carefully tuned interactions between triplets produced by singlet fission.

Singlet fission (SF), a spin-conserving process where one singlet exciton is converted into two triplet excitons, may improve the efficiency of organic photovoltaics. Only a few materials have been...

A phenomenological wave-function-based model of vibrationally coherent absorption modulation is proposed and applied to reproduce the triplet–triplet absorption spectra of bis-triisopropylsilylethy...

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S1 is found to undergo two competing processes?to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, Sx, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield. Funding agencies: Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR-2017-05285]; Linkoping University; Knut and Alice Wallenberg Foundation (KAW)Knut & Alice Wallenberg Foundation; Royal Thai Government Scholarship under the Development and Promotion of Science and Technology Talents Project (DPST); EPSRCUK Research & Innovation (UKRI)Engineering & Physical Sciences Research Council (EPSRC) [EP/M005143/1, EP/P027741/1, EP/L011972/1]; Spanish Government MINECO/FEDERSpanish Government [PID2019109555GB-l00]; Basque governmentBasque Government [PIBA19-0004]; CONACYT-MexicoConsejo Nacional de Ciencia y Tecnologia (CONACyT) [591700]

Intramolecular singlet fission (iSF) has shown potential to improve the power conversion efficiency in photovoltaic devices by promoting the splitting of a photon-absorbing singlet exciton into two triplet excitons within a single molecule. Among different possibilities, the donor-acceptor modular strategy of copolymers has shown great promise in its ability to undergo iSF under certain conditions. However, the number of iSF donor-acceptor copolymers reported in the literature remains remarkably narrow and clear trends for the molecular design of better candidates have not yet been established. In this work, we identify the trade-off between the main iSF requirements of the donor-acceptor strategy and formulate design rules that allow them to be tuned simultaneously in a fragment-based approach. Based on a library of 2944 donor-acceptor copolymers, we establish simple guidelines to build promising novel materials for iSF. These consist in (1st) selecting an acceptor core with high intrinsic singlet-triplet splitting, (2nd) locating a donor with a larger monomer frontier molecular orbital (FMO) gap than that of the acceptor, and (3rd) tuning the relative energy of donor and/or acceptor FMOs through functionalization to promote photoinduced charge transfer in the resulting polymer. Remarkably, systems containing benzothiadiazole and thiophehe-1,1-dioxide acceptors, which have been shown to undergo iSF, fulfill all criteria simultaneously when paired with appropriate donors. This is due to their particular electronic features, which make them highly promising candidates in the quest for iSF materials.

Polariton modes in organic semiconductor microcavities traditionally derive from singlet exciton states that possess no net charge or spin. This talk will explore the properties and prospects of charged polariton states that originate from cationic excitations in a heavily doped organic semiconductor. In addition to new electric and magnetic properties associated with their net charge and spin, charged polaritons are shown to be a useful platform for exploring cavity-modified photoinduced electron transfer and the mechanisms that underlie it.

Energetic conditions that are required for favorable singlet exciton fission, an intermolecular electron correlation effect, are studied with relativistic quantum chemical methods as to investigate the effect of a varying fine-structure constant. Ethylene and derivatives thereof serve as simple model systems, whereas pentacene and perfluoropentacene, which display singlet exciton fission experimentally, are used as specific examples for possible applications. Effects are estimated to be small in this class of compounds, but substitution with heavier halogens might lead to oppositely shifting energy levels and thereby improved sensitivity in narrow resonance situations.

The process of carrier multiplication via singlet fission can potentially exceed the Shockley–Queisser limit on the efficiency of single-junction photovoltaics. In the recent past, theoretical analysis provided the principal guidelines on molecular design strategies for singlet fission. In this Perspective, we focus instead on correlating experimental results for different classes of reported singlet fission materials to identify principles to aid in the design of new molecules for efficient intramolecular singlet fission. Building on an evaluation of several series of multichromophoric and polymeric singlet fission materials, we extract new suggested strategies for molecular design.

Organic semiconductor materials have been widely used in various optoelectronic devices due to their rich optical and/or electrical properties, which are highly related to their excited states. Therefore, how to manage and utilize the excited states in organic semiconductors is essential for the realization of high-performance optoelectronic devices. Triplet-triplet annihilation (TTA) upconversion is a unique process of converting two non-emissive triplet excitons to one singlet exciton with higher energy. Efficient optical-to-electrical devices can be realized by harvesting sub-bandgap photons through TTA-based upconversion. In electrical-to-optical devices, triplets generated after the combination of electrons and holes also can be efficiently utilized via TTA, which resulted in a high internal conversion efficiency of 62.5%. Currently, many interesting explorations and significant advances have been demonstrated in these fields. In this review, a comprehensive summary of these intriguing advances on developing efficient TTA upconversion materials and their application in optoelectronic devices is systematically given along with some discussions. Finally, the key challenges and perspectives of TTA upconversion systems for further improvement for optoelectronic devices and other related research directions are provided. This review hopes to provide valuable guidelines for future related research and advancement in organic optoelectronics.

Singlet fission — the conversion of one singlet exciton into two triplet excitons, could improve the efficiency of photovoltaic devices — but its mechanism is still to be fully understood. Now, in films of TIPS-tetracene, it has been shown that the formation of the triplet pair state, which has been proposed to mediate singlet fission, is ultrafast and vibronically coherent in this endothermic fission system.

The fluorescence intensity from rubrene (Rub) in Rub-silver nanoparticle (AgNP) composite film was larger than that in Rub film. The enhancement is most likely attributable to large electric fields...

The emission manners of organic light-emitting diodes (OLEDs) have experienced almost three-decade evolution. In this review, we briefly summarized the emission manners of OLEDs including: (i) emission from singlet exciton; (ii) emission from triplet exciton; (iii) emission from singlet exciton converted from triplet exciton. Then we introduced a new type of OLEDs with the emission from doublet exciton, wherein organic neutral radicals are used as emitters. Due to the spin-allowed transition of doublet excitons, using neutral radicals as emitters is believed to be a new way to break the 25% upper limit of internal quantum efficiency of OLEDs. The progress of emissive stable neutral radicals is also shortly reviewed.

We present a scheme for nonadiabatic direct dynamics simulation of Frenkel exciton diffusion in bulk molecular systems. The fluctuations of exciton couplings caused by the molecular motion can crucially influence exciton transport in such materials. This effect can be conveniently taken into account by computing the exciton couplings along molecular dynamics trajectories, as shown recently. In this work, we combine Molecular Dynamics simulations with a Frenkel Hamiltonian into a combined quantum-mechanical/molecular mechanics approach in order to allow for a simultaneous propagation of nuclear and electronic degrees of freedom using nonadiabatic dynamics propagation schemes. To reach the necessary time and length scales, we use classical force-fields and the semiempirical time-dependent density functional tight-binding method in combination with a fragmentation of the electronic structure. Fewest-switches surface-hopping, with adaptions to handle trivial crossings, and the Boltzmann-corrected Ehrenfest method are used to follow the excitonic quantum dynamics according to the classical evolution of the nuclei. As an application, we present the simulation of singlet exciton diffusion in crystalline anthracene, which allows us to address strengths and shortcomings of the presented methodology in detail.

Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.

The effect of silver nanoplate (AgPL) on the singlet exciton fission (SF) of rubrene (Rub) was verified by examining the magnetic field effects (MFEs) on the SF of Rub through measuring magnetic field dependence of fluorescence intensity of Rub in two thick Rub polymer-composite films in the absence and the presence of AgPL. The fluorescence intensity of Rub in the Rub-AgPL polymer-composite film was larger than that in the Rub polymer-composite film. The enhancement (1.4 times) must be caused by strong electric fields due to localized surface plasmon resonance (LSPR) of the embedded AgPL near the Rub in polymer matrix. The magnitude of the MFEs in the Rub-AgPL polymer-composite film was almost same as those in the Rub polymer-composite film. These results strongly indicate that the efficiency of SF was enhanced by the LSPR due to the embedded AgPL into the polymer matrix in the Rub-AgPL polymer-composite film.

Transient absorption studies conducted on in vitro lycopene aggregates, as well as on lycopene crystalloids inside tomato chromoplasts, reveal the appearance of a long-lived excited state, which we unambiguously identified as lycopene triplet. These triplet states must be generated by singlet exciton fission, which occurs from the lycopene 2Ag state. This is the first time the singlet fission process has ever been shown to occur in a biological material. We propose that the formation of carotenoid assemblies in chromoplasts may constitute a photoprotective process during chromoplast maturation, in addition to their function in signaling processes.