Enhancing Organic Semiconductor Molecular Packing Using Perovskite Interfaces to Improve Singlet Fission

Singlet fission, a process by which one singlet exciton is converted into two lower energy triplet excitons, is sensitive to the degree of electronic coupling within a molecular packing structure. Variations in molecular packing can be detrimental to triplet formation and triplet–triplet separation, ultimately affecting the harvesting of triplets for electricity in organic photovoltaic devices. Here, six phase-pure molecular packing structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) with varying optoelectronic properties are isolated using 2D lead halide perovskites as tunable, crystalline surfaces for crystallization. Transient absorption spectroscopy reveals that while triplet formation is fast (<100 fs) regardless of template structure, the increased ordering in perovskite-templated samples speeds up triplet–triplet separation and recombination, providing evidence that the benefits of phase-purity offset minor variations in molecular packing. Molecular dynamics modeling of the interface reveals that perovskite-templating allows for closer packing of TIPS-pentacene molecules for all perovskite templates. With an extensive number of organic molecule-perovskite pairings, this work provides a methodology to use ordered, periodic surfaces to elucidate structure–property relationships of small organic molecules in order to adjust structural or optoelectronic responses, such as molecular packing and singlet fission.     

Exciton–Exciton Annihilation in Thermally Activated Delayed Fluorescence Emitter

Recent studies have demonstrated that in thermally activated delayed fluorescence (TADF) materials, efficient reverse intersystem crossing occurs from nonradiative triplet exited states to radiative singlet excited states due to a small singlet–triplet energy gap. This reverse intersystem crossing significantly influences exciton annihilation processes and external quantum efficiency roll‐off in TADF based organic light‐emitting diodes (OLEDs). In this work, a comprehensive exciton quenching model is developed for a TADF system to determine singlet–singlet, singlet–triplet, and triplet–triplet annihilation rate constants. A well‐known TADF molecule, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one (ACRXTN), is studied under intensity‐dependent optical and electrical pulse excitation. The model shows singlet–singlet annihilation dominates under optically excited decays, whereas singlet–triplet annihilation and triplet–triplet annihilation have strong contribution in electroluminescence decays under electrical pulse excitation. Furthermore, the efficiency roll‐off characteristics of ACRXTN OLEDs at steady state is investigated through simulation. Finally, singlet and triplet diffusion length are calculated from annihilation rate constants.     

Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

Triplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.     

Morphological Tuning of the Energetics in Singlet Fission Organic Solar Cells

Effective singlet fission solar cells require both fast and efficient singlet fission as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and efficient singlet fission, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C₆₀ solar cell can be modified dramatically by the morphology of its constituent films. The effect is so pronounced that a crystalline system is able to dissociate and collect triplets generated through singlet fission whereas an as‐deposited amorphous system is not. Furthermore, a novel technique for studying the behavior of this class of devices using external quantum efficiency (EQE) measurements in the presence of a background light is described. When this method is applied to rubrene/C₆₀ solar cells, it is shown that triplet–triplet annihilation makes significant contributions to photocurrent in the amorphous device—enhancing EQE by over 12% at relatively low intensities of background light (4 mW cm^?2)—while detracting from photocurrent in the crystalline device. Finally, the conclusions on how the material system is affected by its morphology are strengthened by time‐resolved photoluminescence experiments.     

Studying singlet fission and triplet fusion by magneto-electroluminescence method in singlet–triplet energy-resonant organic light-emitting diodes

Organic light emitting diodes (OLEDs) utilizing a singlet–triplet energy-resonant (E_S ≈ 2E_T) layer (rubrene) were fabricated to investigate the singlet fission and triplet fusion by the magneto-electroluminescence (MEL) of device from R.T. to 20 K. A large positive MEL (23.5%) was obtained at R.T. due to magnetic-field-suppressed singlet fission. With decreasing temperatures, the MELs changed their signs both at low-field and high-field components because of a gradual decrease in singlet fission simultaneously followed by an increasing triplet fusion, leading to a negative MEL around −7.5% at 20 K. Moreover, transient electroluminescence and MELs from the control devices were used to further confirm the exciton fission and fusion processes in rubrene-based OLEDs. Our findings of MEL may provide a useful pathway to study the microscopic dynamics of excited states in organic optoelectronic devices.     

Thermally activated singlet exciton fission observed in rubrene doped organic films

The magnetic field effects of photoluminescence (MPL) from rubrene doped organic films were recorded at different temperatures. The measured line shapes were attributed to the field modification on the rate constant of thermally activated singlet exciton fission which occurred between the doped rubrene molecules. And its amplitude exhibited a nonlinear dependence on the averaged intermolecular distance. Such an observation implies that the intermolecular coupling (IMC) which is modulated by changing the intermolecular distance is able to significantly affect the intensity of fission process. Therefore, investigating the variation of singlet fission with different strength of IMC could be an important means to study the dynamics of fission process. Our work reveals the importance of IMC factor which needs to be considered for the design of efficient singlet fission-sensitized organic photovoltaic devices.     

A Multifunctional Blue‐Emitting Material Designed via Tuning Distribution of Hybridized Excited‐State for High‐Performance Blue and Host‐Sensitized OLEDs

Actualizing full singlet exciton yield via a reverse intersystem crossing from the high‐lying triplet state to singlet state, namely, “hot exciton” mechanism, holds great potential for high‐performance fluorescent organic light‐emitting diodes (OLEDs). However, incorporating comprehensive insights into the mechanism and effective molecular design strategies still remains challenging. Herein, three blue emitters (CNNPI, 2TriPE‐CNNPI, and 2CzPh‐CNNPI) with a distinct local excited (LE) state and charge‐transfer (CT) state distributions in excited states are designed and synthesized. They show prominent hybridized local and charge‐transfer (HLCT) states and aggregation‐induced emission enhancement properties. The “hot exciton” mechanism based on these emitters reveals that a balanced LE/CT distribution can simultaneously boost photoluminescence efficiency and exciton utilization. In particular, a nearly 100% exciton utilization is achieved in the electroluminescence (EL) process of 2CzPh‐CNNPI. Moreover, employing 2CzPh‐CNNPI as the emitter, emissive dopant, and sensitizing host, respectively, the EL performances of the corresponding nondoped pure‐blue, doped deep‐blue, and HLCT‐sensitized fluorescent OLEDs are among the most efficient OLEDs with a “hot exciton” mechanism to date. These results could shed light on the design principles for “hot exciton” materials and inspire the development of next‐generation high‐performance OLEDs.     

Temperature-dependent singlet exciton fission observed in amorphous rubrene films

The steady-state/transient fluorescence spectroscopy was used to demonstrate that the dynamics of singlet exciton fission in amorphous rubrene were temperature-dependent (50–300 K). Based on the traditional three-state model of singlet fission, time-resolved fluorescence decay curves measured at different temperatures could be well fitted by using a set of rate equations. The variations of specific rate constants were consistent with the conventional Arrhenius-type, thermally activated process. Additionally, the magnetic field effect of photoluminescence was apparently suppressed at low temperatures. All these findings offer clear evidence that the amorphous rubrene solid undergoes thermally activated singlet exciton fission due to the endothermic nature of fission process in rubrene.     

Trap effect of triplet excitons on magnetoresistance in organic devices

In an organic bipolar device, injected electrons and holes can form spin singlet and triplet excitons, which are manipulated by an applied magnetic field. We suppose that the localized intra-molecule triplet exciton has a blocking effect on charge carrier transport by assuming that the intra-molecule triplet exciton can increase the on-site binding and make the electron states more localized. By considering the magnetic field-dependent transition between singlet and triplet excitons, from the master equation based on the hopping mechanism, we calculate the magnetoresistance (MR) in organic devices and compare the results with some experimental data. Our research reveals the importance of hyperfine interaction in organic magnetoresistance (OMAR). Especially, our investigation indicates that a bipolar organic device should have a larger MR value than a unipolar one due to the trap effect of triplet excitons on hopping electrons or holes, which is confirmed by some experimental observations.     

Achieving a Significantly Increased Efficiency in Nondoped Pure Blue Fluorescent OLED: A Quasi‐Equivalent Hybridized Excited State

Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light‐emitting materials (OLEDS). Charge‐transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi‐equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4‐[2‐(4′‐diphenylamino‐biphenyl‐4‐yl)‐phenanthro[9,10‐d]imidazol‐1‐yl]‐benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record‐setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low‐cost, high‐efficiency fluorescent OLED materials.     

Inter-triplet spin–spin interaction effects on inter-conversion between different spin states in intermediate triplet–triplet pairs towards singlet fission

This article reports the experimental studies on the effects of inter-triplet spin interaction on singlet fission by using magnetic field effects of photoluminescence (MFE_PL) based on tetracene. The MFE_PL are compared for three different morphological states based on polycrystalline solid powder, amorphous solid film, and liquid solution. It is observed that the polycrystalline solid powder gives stronger MFE_PL than that of amorphous solid film, while the liquid solution exhibits no detectable MFE_PL. In essence, the MFE_PL are determined by the inter-conversion between different spin states initiated by inter-triplet spin interaction through spin mixing in intermediate triplet–triplet pairs towards the singlet fission. The different MFE_PL amplitudes suggest that the polycrystalline solid powder possesses an enhanced inter-triplet spin interaction in intermediate triplet–triplet pairs as compared to amorphous solid film. As a result, the enhanced inter-triplet spin interaction can cause a larger inter-conversion between different spin states in intermediate triplet–triplet pairs and consequently increases the singlet fission within polycrystalline structures. The absorption spectral characteristics and X-ray diffraction data confirm that the polycrystalline solid powder can indeed exhibits stronger intermolecular electronic interaction relative to amorphous solid film. Here, the stronger intermolecular electronic interaction provides an evidence for the enhanced inter-triplet spin interaction occurring within polycrystalline structures in the solid powder. Our experimental results indicate that increasing the inter-triplet spin interaction can boost the inter-conversion between different spin states in intermediate triplet–triplet pairs and consequently facilitates the singlet fission.     

Anthracene derivatives as efficient emitting hosts for blue organic light-emitting diodes utilizing triplet–triplet annihilation

The molecular design strategies for the host materials suitable for highly efficient, blue fluorescent organic light-emitting diodes (OLEDs) are demonstrated. The device characteristics of blue fluorescent OLEDs are compared with different host materials. Some devices exhibit a highly efficient blue electroluminescence with a high external quantum efficiency of more than 7%. The correlation between OLED efficiency and triplet–triplet annihilation is characterized by measuring the up-conversion of triplet excited states into singlet ones. The host materials require an anthracene unit and a bulky molecular structure to prevent the overlap of anthracene units between adjacent molecules in the film.     

A 9,9′-bianthracene-cored molecule enjoying twisted intramolecular charge transfer to enhance radiative-excitons generation for highly efficient deep-blue OLEDs

The efficiency of fluorescent organic light emitting diodes (FOLEDs) is strongly affected by the fraction of singlet excitons formed. However, the standard statistical value of the single to triplet ratio is 1:3, which implies most of the excitons are invalid in fluorescent emitting devices. Here, we demonstrate the ability of twisted intramolecular charge transfer state (TICT-state) to enhance the occurrence of singlet excitons in a fluorescent emitter that is based on the 9,9′-bianthracene (BA) moiety. The anthracene–anthracene (A–A) linked by a single bond and having perpendicular electronic structure is a charge transfer intersystem crossing π system in excited state. The BA-cored fluorescence emitter (CzBACz) with particular TICT characteristics realizes the electron–hole (e–h) recombination via intramolecular conversion from charge-transfer excitons (immediate precursor) to radiative singlet exciton (final state). For CzBACz-based electroluminescent (EL) device, the singlet generation fraction is more than 25%.     

Photovoltaic Processes of Singlet and Triplet Excited States in Organic Solar Cells

This article reports the respective photovoltaic processes of singlet and triplet photoexcited states in dissociation and charge reactions based on the studies of magnetic‐field effects of photocurrents. The magnetic‐field effects of photocurrents reveal that weak donor‐acceptor interactions lead to a two‐step photovoltaic process: dissociation in polaron‐pair states evolved from singlet excitonic states and exciton‐charge reactions occurred in triplet excitonic states in the generation of the photocurrent. However, strong donor‐acceptor interactions yield a one‐step photovoltaic process: direct dissociation of both singlet and triplet excitons in bulk‐heterojunction organic solar cells. In addition, the magnetic‐field effects of photocurrents indicate that the dissociated electrons and holes form charge‐transfer complexes with singlet and triplet spin configurations at donor‐acceptor intermolecular interfaces. As a result, the magnetic‐field effects of photocurrents can deliver a critical understanding of singlet and triplet photovoltaic processes to design advanced solar‐energy materials and devices.     

Design of a novel triplet exciton guiding mixed host for lifetime improvement of phosphorescent organic light-emitting diodes

A novel triplet exciton guiding mixed host managing triplet exciton-polaron annihilation by separating the triplet excitons and polarons in the different host was developed. A high triplet energy/narrow gap host and a low triplet energy/wide gap host were mixed to isolate the triplet excitons and polarons, which could improve the extrapolated lifetime of the phosphorescent organic light emitting diodes by more than twice. The triplet exciton-polaron annihilation reducing mechanism was confirmed by triplet energy transfer, single carrier device test and triplet exciton-polaron annihilation rate constant study of the singlet host and mixed host.     

Selective Triplet–Singlet Förster-Resonance Energy Transfer for Bright Red Afterglow Emission

Förster-resonance energy transfer (FRET_T-S) from the lowest excited triplet state (T₁) of a donating sensitizer to a fluorescence acceptor can be used to obtain bright room-temperature afterglow emission at long wavelengths. However, the energy transfer from the lowest excited singlet state of the donor to the acceptor is an undesirable deactivation pathway that prevents FRET_T-S. Herein, heteroatoms in chromophores are shown to allow selective and efficient FRET_T-S for enhanced triplet emission for bright room-temperature afterglow emission at long wavelengths. Different transition characteristics between the lowest singlet excited state and triplet states in heteroatom-containing chromophores accelerate triplet generation, enabling near-zero fluorescence yields. Out-of-plane vibrations of the heteroatoms in aromatic fused rings greatly enhance the radiative rate from T₁ by a factor of 88 relative to non-heteroatom-containing fused chromophore. The compatibility of the near-zero fluorescence and the enhanced triplet emission in a heteroatom-containing fused chromophore enable selective and efficient FRET_T-S pathways, resulting in room-temperature red afterglow emission with a yield of 17%. The bright emission at long-wavelengths allows distinguishable, multiple spectral signals in ambient white light.     

Delayed Fluorescence Emitter Enables Near 17% Efficiency Ternary Organic Solar Cells with Enhanced Storage Stability and Reduced Recombination Energy Loss

Charge transfer state (CT) plays an important role in exciton diffusion, dissociation, and charge recombination mechanisms. Enhancing the utilization and suppressing the recombination process of CT excitons is a promising way to improve the performance of organic solar cells (OSCs). Here, an effective method is presented via introducing a delayed fluorescence (DF) emitter 3,4‐bis(4‐(diphenylamino)phenyl)acenaphtho[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC‐TPDA) in OSCs. The long‐lifetime singlet excitons on APDC‐TPDA can transfer to polymer donors to prolong exciton lifetime, which ensures sufficient time for diffusion and dissociation. Concurrently, the high triplet energy level (T₁) of the DF material can also prevent the reverse energy transfer from CT to T₁. APDC‐TPDA‐containing ternary OSCs shows a high PCE of 16.96% with a reduced recombination energy loss of 0.46 eV. It is noteworthy that the ternary OSC also exhibits superior storage stability. After 55 days of storage, the PCE of the ternary OSC still retains about 96% of its primitive state. Furthermore, this ternary strategy is efficient and universally applicable to OSCs, and positive results can be obtained in different systems with different DF emitters. These results indicate that the ternary strategy provides a new design idea to realize high performance OSCs.     

肿瘤的光动力诊断和光动力治疗的研究进展

HPD等光敏化剂可以有选择地潴留在肿瘤内，激光诱导出的特征光谱可用于肿瘤的光动力诊断。当用一定波长的激光照射光敏物质分子时，它可以从基态跃迁激发单态，通过系际交叉过渡到激发三重态。处于三重态的光敏化剂分子通过能量转移，使三重态的氧变成对肿瘤细胞具有毒化作用的单态氧，从而实现了肿瘤的光动力治疗。     

The Singlet–Triplet Exchange Energy in Conjugated Polymers

Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ΔE_ST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ΔE_ST value close to 0.7 eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum‐chemical calculations are performed to address the origin of the constant exchange energy in phenylene‐based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the π‐conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size‐dependent singlet–triplet energy separation.     

The temperature dependence of the spin relaxation of coupled polaron pairs in organic semiconductors

The coupled polaron pair distributed in adjacent organic molecules has spin relaxation due to molecular vibration. We construct a Hamiltonian with on-site energy fluctuation to investigate its temperature dependence. The relaxation occurs between spin singlet and one of the spin triplet states, but not between spin triplet states. It is found that the polaron pair finally relaxes to a spin state with half probability in spin singlet state and 1/6 probability in each of the spin triplet states, and the relaxation time is inversely proportional to the temperature’s square. The connection to the magneto-electroluminescence experiment is also discussed.