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Magnetic Field Effects on Singlet Fission and Fluorescence Decay Dynamics in Amorphous Rubrene
Abstract
Picosecond time-resolved fluorescence experiments are used to study the dynamics of singlet fission in highly disordered films of rubrene. The fluorescence spectral lineshapes are not temperature-dependent, indicating that intermolecular excitonic effects are absent in these films. The temperature-dependent fluorescence decays in the amorphous films are nonexponential, containing both prompt and delayed components. The kinetics are qualitatively consistent with the presence of singlet fission, but to confirm its presence, we examine the effects of magnetic fields on the fluorescence decay. A quantum-kinetic model is developed to describe how magnetic fields perturb the number of triplet pair product states with singlet character and how this in turn affects the singlet state kinetics. Simulations show that the magnetic field effect is very sensitive to mutual chromophore alignment, and the direction of the effect is consistent with a local ordering for rubrene molecules that participate in fission. From our analysis, the dominant fission rate is 0.5 ns–1, about 10 times slower than that observed in polycrystalline tetracene films, but we still estimate that 90% of the initially excited singlets undergo fission. Kinetic modeling of our fluorescence decay data and magnetic field dependence reveals that at the low laser intensities used in this experiment geminate triplet pairs do not interact with each other, and that spin–lattice relaxation between triplet sublevels is not complete on the 100 ns time scale. When both exciton fission and fusion are occurring, dynamic measurements in the presence of a magnetic field can elucidate molecular-level details of both processes.
... This observation is contrasted by reports of coherent SF in both materials [2,20] and has be discussed to result from a high sensitivity of the excited state dynamics on sample preparation or by differences in time-resolution and sensitivity of the used experimental techniques [23]. While for TET there is a consensus that SF occurs in the order of 100 ps [6,19,23,24], the details of the SF process are less clear for RUB [2,6,20,27,28]. In the following we adopt the assignment of reference [6] with the formation of 1 (TT) on the order of 30 ps and the triplet separation on the order of nanoseconds. ...
... For the analysis of the excited state dynamics, we fitted the temporal evolution of the intensity decay by a convolution of a biexponential function and the instrument response function (IRF, see figure 3(c)). For both neat compounds, the multiexponential decay agrees with literature [6,19,23,24,27,28]. In TET, the initial fast decay with a rate of 8.0 ns −1 can be assigned to SF [19,23,24], while neat RUB exhibits a much slower, biexponential PL decay with rates of 0.91 ns −1 and 3.6 ns −1 and a 53% relative amplitude of the faster decay [28], see figure 3(a). ...
... In TET, the initial fast decay with a rate of 8.0 ns −1 can be assigned to SF [19,23,24], while neat RUB exhibits a much slower, biexponential PL decay with rates of 0.91 ns −1 and 3.6 ns −1 and a 53% relative amplitude of the faster decay [28], see figure 3(a). We assign the biexponential PL decay in RUB to a decay in S 1 population caused by the conversion of S 1 to 1 (TT), and subsequent separation of 1 (TT) [6,27]. ...
The application potential of singlet fission (SF), describing the spontaneous conversion of an excited singlet into two triplets, underlines the necessity to independently control SF rates, energetics and the optical band gap. Heterofission, whereby the singlet splits into triplets on chemically distinct chromophores, is a promising approach to control the above-mentioned parameters, but its details are not yet fully understood. Here, we investigate the photophysics of blends of two prototypical SF chromophores, tetracene (TET) and rubrene (RUB) using time-resolved photoluminescence spectroscopy and time-correlated single photon counting (TCSPC) to explore the potential for heterofission in combinations of endothermic SF chromophores.
... Under pulsed illumination, triplet exciton fusion leads to a "delayed fluorescence" that contrasts the "prompt fluorescence" resulting from the radiative decay of the initial singlet exciton population [1][2][3] . This delayed fluorescence has been investigated on several time scales in various materials [4][5][6][7][8][9] , and the time-evolution of the triplet exciton density has also been observed using other techniques like photo-induced absorption 10 . Recent interest in the topic has grown because of the interest of multi-exciton generation processes for photovoltaics 11 . ...
... The rarity of such events can be compensated by appropriately long accumulation times and the sensitivity of single-photon detection. Previous studies done in this regime were confined to amorphous materials or thin films 8,9,13,14 , where the discussion of the dimensionality of diffusion is complicated either by intrinsic disorder or by the possible presence of nanocrystals 15 . ...
The geminate annihilation of two triplet excitons created by singlet exciton fission is affected by the dimensionality of transport as determined by typically anisotropic triplet exciton mobilities in organic molecular crystals. We analyze this process using a random-walk model where the time-dynamics of the geminate annihilation probability is determined by the average exciton hopping times along the crystallographic directions. The model is then applied to the geminate fluorescence dynamics in rubrene, where the main channel for triplet-triplet annihilation is via triplet fusion and subsequent photon emission, and we identify the transitions between transport in one, two, and three dimensions.
... When B increases, the MEL responses increase rapidly in the |B| < 25 mT range, increase slowly (50 µA and 100 µA) and then decrease rapidly (200 µA and 400 µA) in the 25 mT < |B| < 300 mT range. Clearly, all of the low magnetic field range MEL responses originated from the ISC process [31][32][33]; this process is expressed as T +T S +S ) [19][20][21] are the dominant processes at a high magnetic field range in devices A1-A4. In addition, the MEL amplitude of devices A1-A4 decreases with the bias I increase, which belongs to the normal I dependence. ...
... When B increases, the MEL responses increase rapidly in the |B| < 25 mT range, increase slowly (50 µA and 100 µA) and then decrease rapidly (200 µA and 400 µA) in the 25 mT < |B| < 300 mT range. Clearly, all of the low magnetic field range MEL responses originated from the ISC process [31][32][33]; this process is expressed as PP S → PP T . The ISC process, SF process ( S 1,Rub + S 0 → T 1,Rub + T 1,Rub ) [34,35], and TTA process ( T 1,Rub + T 1,Rub → S 1,Rub + S 0 ) [19][20][21] are the dominant processes at a high magnetic field range in devices A1-A4. ...
Although the effect of the conductive polymers PEDOT:PSS on the electroluminescence performance of doped-type organic light-emitting diodes (OLEDs) has been studied, the process of PEDOT:PSS regulation of exciton recombination region and concentration within the deoxyribonucleic acid (DNA)-based doped-type BioLEDs is still obscure. In this study, we fabricated Bio-devices with and without PEDOT:PSS using varying spin-coating speeds of PEDOT:PSS. The Alq3:Rubrene-based BioLEDs achieve higher luminance (44,010 cd/m2) and higher luminance efficiency (8.1 cd/A), which are increased by 186% and 478%, respectively, compared to the reference BioLEDs without PEDOT:PSS. Similarly, the maximum luminance and efficiency of blue TCTA:TPBi exciplex-type BioLEDs are increased by 224% and 464%. In particular, our findings reveal that with an increasing thickness of PEDOT:PSS, the region of exciton recombination shifts towards the interface between the emitting layer (EML) and the hole transport layer (HTL). Meanwhile, the concentration of singlet exciton (S1,Rub) and triplet exciton (T1,Rub) increases, and the triplet-triplet annihilation (TTA) process is enhanced, resulting in the enhanced luminescence and efficiency of the devices. Accordingly, we provide a possible idea for achieving high performance doped-type BioLEDs by adding conductive polymers PEDOT:PSS, and revealing the effect of exciton recombination and conversion in BioLEDs given different PEDOT:PSS thicknesses.
... 40 For simplicity, we take S·D inter ·SB ≈ XS·SB. 43 A convenient basis set for diagonalizing Ĥand obtaining the triplet-pair spin wave functions |ψ l ⟩ comprises the nine product pair states |xx⟩, |xy⟩, ..., |zz⟩, where we have dropped the A, B subscripts for clarity. We note that because the xyz coordinate systems of molecules A and B do not in general coincide, a rotation operation must be applied to Ĥz ero-field,B . ...
... As discussed above, the spin character of weakly exchangecoupled triplet-pair states is dependent on the relative orientation of the two molecules involved. 40,43,46 This has a knock-on effect on the spin statistical factor, as shown in Figure 9c. Rotation of one molecule of the pair with respect to the other causes increased singlet−triplet-quintet mixing. ...
Triplet–triplet annihilation upconversion (TTA-UC) has great potential to significantly improve the light harvesting capabilities of photovoltaic cells and is also sought after for biomedical applications. Many factors combine to influence the overall efficiency of TTA-UC, the most fundamental of which is the spin statistical factor, η, that gives the probability that a bright singlet state is formed from a pair of annihilating triplet states. The value of η is also critical in determining the contribution of TTA to the overall efficiency of organic light-emitting diodes. Using solid rubrene as a model system, we reiterate why experimentally measured magnetic field effects prove that annihilating triplets first form weakly exchange-coupled triplet-pair states. This is contrary to conventional discussions of TTA-UC that implicitly assume strong exchange coupling, and we show that it has profound implications for the spin statistical factor η. For example, variations in intermolecular orientation tune η from to through spin mixing of the triplet-pair wave functions. Because the fate of spin-1 triplet-pair states is particularly crucial in determining η, we investigate it in rubrene using pump–push–probe spectroscopy and find additional evidence for the recently reported high-level reverse intersystem crossing channel. We incorporate all of these factors into an updated model framework with which to understand the spin statistics of TTA-UC and use it to rationalize the differences in reported values of η among different common annihilator systems. We suggest that harnessing high-level reverse intersystem crossing channels in new annihilator molecules may be a highly promising strategy to exceed any spin statistical limit.
... The case of amorphous systems requires to average the MEL response over all possible spinorientation to each other and to the external magnetic field. This was discussed on the example of fluorescence measurements of amorphous Rubrene in great detail by Piland et al. [17] and Tapping et al. [18] Shown in Supplementary Figure 9 is the simulated MEL response of DiF-TES-ADT using the same rates as were used in the simulations above, as qualitatively larger D and E values will lead to observation of MEL extrema at larger magnetic fields. In both cases the characteristic "M"-like line-shape that was observed for all the isotropic systems is reasonably reproduced and the additional structure that was observable in the aligned films is lost. ...
... et al. and Tapping et al. have chosen tetracene to represent Rubrene in their studies but have argued for a wide range of D and E values.[17][18][19] For this case study the actual values and E/D ratios are of lesser importance as we do not aim to fit the isotropic MEL. ...
Magneto electroluminescence (MEL) is emerging as a powerful tool to study spin dynamics in organic light emitting diodes (OLEDs). The shape of the MEL response is typically used to draw qualitative inference on the dominant process (singlet fission or triplet fusion) in the device. In this study, we develop a quantitative model for MEL and apply it to devices based on Rubrene, and three solution processable anthradithiophene emitters. The four emitters allow us to systematically vary the film structure between highly textured, poly-crystalline to amorphous. We find significant diversity in the MEL, with the textured films giving highly structured responses. We find that the additional structure does not coincide with energy anti-crossings, but intersections in the singlet character between adjacent states. In all cases the MEL can be adequately described by an extended Merrifield model. Via the inclusion of charge injection, we are able to draw additional information on underlying physics in OLED devices.
... Note that these photo-generated T 1 can recombine to S 1 via the bimolecular non-geminate TF process and give rise to delayed PL 47,48 . As shown in Fig. 1a, the spin conversion processes are valid only in solid state, and strongly related to the molecular packing of rubrene 49 . Compared to the thin films where the grain boundaries introduce additional radiative or non-radiative decay routes, the single crystals allow for more efficient spin conversion processes due to the highly ordered molecular packing. ...
Optical detection of magnetic field is appealing for integrated photonics; however, the light-matter interaction is usually weak at low field. Here we observe that the photoluminescence (PL) decreases by > 40% at 10 mT in rubrene microcrystals (RMCs) prepared by a capillary-bridge assembly method. The giant magneto-PL (MPL) relies on the singlet-triplet conversion involving triplet-triplet pairs, through the processes of singlet fission (SF) and triplet fusion (TF) during radiative decay. Importantly, the size of RMCs is critical for maximizing MPL as it influences on the photophysical processes of spin state conversion. The SF/TF process is quantified by measuring the prompt/delayed PL with time-resolved spectroscopies, which shows that the geminate SF/TF associated with triplet-triplet pairs are responsible for the giant MPL. Furthermore, the RMC-based magnetometer is constructed on an optical chip, which takes advantages of remarkable low-field sensitivity over a broad range of frequencies, representing a prototype of emerging opto-spintronic molecular devices.
... In the following years, similar observations were reported for tetracene crystals [92][93][94]. This term was mainly used to explain the occurrence of delayed fluorescence, which was also used to identify and indirectly confirm this phenomenon [95][96][97][98][99][100]. Likewise, SF was later observed in carotenoids and conjugated polymers [101,102]. The conversion efficiency of photoelectric devices can be improved using the SF effect. ...
A photodetector is a type of optoelectronic device with excellent photoelectric conversion abilities, which has especially important applications in many fields such as optical communication, image sensing, aerospace/environmental detection, and military safety. Among these applications, the multiplier effect of optoelectronic devices has been widely explored because photodetectors can convert a very weak optical signal into electrical signal output and offer amazing electron multiplication abilities. To date, hundreds of multiplier effects of photodetectors have been reported. However, there are few reviews on the multiplier effects of such devices. Here, a review of the multiplier effects of photodetectors covering detection spectra from ultraviolet to infrared is presented, including photodetectors based on inorganic materials, organic materials, and organic/inorganic materials. First, we provide brief insights into the detection mechanisms of multiplier effects of photodetectors and introduce the merits that represent key factors for a reasonable comparison of different photodetectors. Then, the multiplier effect on different types of material photodetectors is reviewed. Notably, we summarize the optimization directions of the performance of the multiplier photodetectors, including improving the external quantum efficiency, reducing the dark current, and increasing the response speed and spectral regulation. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high-performance photodetectors with multiplier effects is given to provide a guideline for the future development of this fast-developing field. The bottlenecks of existing multiplier technology are also analyzed, which has strong reference significance for the future development of this field.
... , 因PP 1 和PP 3 的能级几乎简并, [8,20] . 比 S 1,Rub 高 0.1 eV)但 与 T 1,Rub 存 在 大 的 能 级 差 (T 2,Rub 比 T 1,Rub 高1.14 eV ) [15,21] , 因此通过HL- ...
With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existed in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S1,Rub ← T2, Rub) of high-lying triplet excitons (T2, Rub) in Rubrene when Rubrene with a typical content of several percent is doped into a host with high triplet exciton energy and there are also no any energy loss channels of triplet excitons from charge-carrier transporting layers. Furthermore, this HL-RISC process can considerably boost the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e., the pure Rubrene film is used as an emissive layer) with no any energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S1, Rub+S0, Rub → T1, Rub+T1, Rub) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP1 → PP3) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair are generated from the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. Via comparing and analyzing thes tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from B-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from B-mediated HL-RISC process of T2, Rub excitons. Note that such an abnormal and coincidental experiment phenomenon is the physical mechanism why ISC and HL-RISC processes cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has never been observed in the literature. Clearly, this work has further deepened our understanding of some unique microscopic processes and physical phenomena in organic semiconductor "star" material of Rubrene (such as the energy resonance between 2T1 and S1 and the energy approach between T2 and S1).
... The effect arises from the fact that magnetic fields alter the distribution of singlet character among the nine triplet pair states, and was first explained by Merrifield 2,6,64,65 and then revisited by Bardeen and co-workers. [66][67][68][69] See these references for details on the specifics of kinetic and spin models to calculate quantitative effects. In this section, we reframe these previous formalisms to provide an intuitive way in which to understand the qualitative observations and to explain the complexity of the phenomenon. ...
Singlet fission is a form of multiple exciton generation, which occurs in organic chromophores when a high-energy singlet exciton separates into two lower energy triplet excitons, each with approximately half the singlet energy. Since this process is spin-allowed, it can proceed on an ultrafast timescale of less than several picoseconds, outcompeting most other loss mechanisms and reaching quantitative yields approaching 200%. Due to this high quantum efficiency, the singlet fission process shows promise as a means of reducing thermalization losses in photovoltaic cells. This would potentially allow for efficiency improvements beyond the thermodynamic limit in a single junction cell. Efforts to incorporate this process into solar photovoltaic cells have spanned a wide range of device structures over the past decade. In this review, we compare and categorize these attempts in order to assess the state of the field and identify the most promising avenues of future research and development.
... Further explanation on this point is found in the supplementary to the comment of [181], which points out and clears some of the common confusions with molecular orientation. Sup-pose that there are two set of basis one aligned with the lab frame and also with molecule A's intrinsic molecular axis, {|x⟩, |y⟩, |z⟩} ≡ {|x⟩ A , |y⟩ A , |z⟩ A }. ...
Nitrogen-vacancy (NV) centres are naturally occurring colour centres in bulk diamond crystals that enable quantum sensing via optically active spin ground states. They have demonstrated high sensitivity in thermometry and magnetometry, reaching a temperature precision of 5mK/√Hz and magnetic field sensitivity of a single nuclear spin. Due to their nanometer size and ability to be chemically functionalised, diamond nanocrystals (nanodiamonds) offer sensing in systems inaccessible with bulk diamonds. Of particular interest is magnetometry and thermometry in biological systems. This thesis explores interfacing nanodiamonds host NV centres with complex systems for decoherence spectroscopy, relaxometry and thermometry. In Chapter 1, it is demonstrated that nanodiamonds can perform NMR experiments with nanoscale sensing volume. In this experiment a dynamical decoupling sequence is used to sense the stochastic and stationary magnetic field at the site of the NV created by the target nuclear spins. The spectral resolution of this technique is sufficient to distinguish between two different nuclear species 1H and 19F. Furthermore, a self-referenced calibration scheme is proposed and verified with experimental data to enhance the accuracy of concentration measurements, previously limited by the geometrical variability between nanodiamonds. In Chapter 3, nanodiamonds are used to study triplet exciton diffusion in an organic semiconductor material. In this experiment, NV relaxometry is used to probe the diffusion process of optically generated triplet excitons. It was observed that the longitudinal relaxation time of NV (T1) is related to the optical excitation of the organic material and that the depolarisation rate saturates at large optical excitation power. A stochastic Liouville equation model is used to model the spin diffusion process. In Chapter 4 and 5, nanodiamonds are used in thermometry to measure time-varying signal in biological cells. In this experiment, the theoretical temperature sensitivity is examined using Fisher information formalism and compared with experimental sensitivity. This forms the basis of optimal ODMR frequency sampling in situations where the ODMR lineshape fluctuates. It was also demonstrated that it is possible to simultaneously perform ODMR measurements and track single nanodiamonds, using an orbital tracking method. This enables a single nanodiamond to be measured over hours despite thermal motion. Simultaneously, the temperature can be determined with a sensitivity of 1.5K/√Hz and the position with an accuracy of 7.7 nm. With these technical improvements the intracellular temperature variation in HeLa cells is measured when the cells are subjected to external temperature modulation or chemical stimuli. Finally, despite the success of NV centres in diamond for room-temperature quantum sensing, in Chapter 6, we explore a novel two dimensional material, hexagonal boron nitride, that offers a more scalable platform for future quantum sensing applications. In this experiment, we identified stable spin-addressable single emitters at room temperature. ODMR experiments reveal the fine structure of the defect which was modelled as a spin triplet. Photokinetic properties are investigated with second order intensity correlation measurements and an optical level structure is proposed to provide a brand new understanding of this novel quantum sensing platform. This provides a basis for tackling the issues facing nanodiamond.
... The effect arises from the fact that magnetic fields alter the distribution of singlet character amongst the 9 triplet pair states, and was first explained by Merrifield 2,6,64,65 and then revisited by Bardeen and coworkers [66][67][68][69] . See these references for details on the specifics of kinetic and spin models to calculate quantitative effects. ...
Singlet fission is a form of multiple exciton generation which occurs in organic chromophores when a high energy singlet exciton separates into two lower energy triplet excitons, each with approximately half the singlet energy. Since this process is spin-allowed it can proceed on an ultrafast timescale of less than several picoseconds, outcompeting most other loss mechanisms and reaching quantitative yields approaching 200%. Due to this high quantum efficiency, the singlet fission process shows promise as a means of reducing thermalisation losses in photovoltaic cells. This would potentially allow for efficiency improvements beyond the thermodynamic limit in a single junction cell. Efforts to incorporate this process into solar photovoltaic cells have spanned a wide range of device structures over the past decade. In this review we compare and categorise these attempts in order to assess the state of the field and identify the most promising avenues of future research and development.
... 4,6−8 Related to future applications in photovoltaic devices, rubrene as a derivative of tetracene can be one of the attractive choices due to its pronounced hole mobility, 9 exciton diffusion length, and possibility of constructing flexible electronic devices. 3,10 SF was studied in rubrene single crystals and amorphous/polycrystalline and nanoaggregate thin films using different spectroscopic techniques: femtosecond pump− probe, 4,6,7,11−14 time-resolved photoluminescence, 15−18 magnetic field effect, 19,20 and Raman microscopy. 21 Three different pathways were proposed for triplet formation via SF: (i) ultrafast anti-Kasha/hot SF within 30 fs in films and 200 fs in crystals; 4,6 (ii) thermally activated SF in crystals within 2−20 ps; 7,15 and (iii) charge/electron-transfer state-mediated SF in crystalline films. ...
... Rubrene is also a prototypical organic semiconductor that exhibits efficient fission of a singlet excitation into a pair of triplets and the reverse process of fusion of triplets to a singlet by TTA. It is known that the triplet state T 1 in rubrene is at 1.14 eV above its singlet ground state, whereas the optical energy gap of rubrene, corresponding to the transition to the S 1 state, is 2.23 eV [27,28]. This condition makes TTA favorable. ...
Upconversion (UC) in a molecular system is a process in which excitons produced by a multiple absorption of low-energy photons at long wavelengths undergo fusion to produce high energy excitons that consequently recombine to emit anti-Stokes shifted photons. Molecular systems for UC typically require a sensitizer. However, recent experiments show that UC in rubrene occurs even without the presence of the sensitizer. In this system, intermediate states are assumed to absorb photons at near-infrared wavelengths, which either absorb additional photons to populate the emissive singlet state or undergo fusion to generate triplets. The triplets can again undergo fusion to populate the excited singlet state. The final emission is around 600 nm. These models have been tested against the intensity dependence of the UC emission. Here, we have measured the kinetics of UC in rubrene by using intensity modulated photoexcitation at 800 nm to better understand the underlying mechanism. The models of UC that have been proposed so far do not agree with our measurements. Our results show that the yield of UC lags behind excitation significantly, indicating that triplet states are directly excited from the ground state, and their fusion, which depends on the population, becomes prominent after a certain build up time. While the intermediate states could form dynamically after the UC has been initiated and enhance the process, further sensitive absorption measurements are necessary to understand the role of the intermediate states in the process. Our results are important in finding new routes to enhance UC in pristine organic semiconductors for applications in photovoltaics, lasers, bioimaging, optical devices, and lighting.
... This magneticfield-induced perturbation of the low-spin to high-spin state barrier is explored here for [Fe{H 2 B(pz) 2 } 2 (bipy)]. The influence of magnetic fields on the excitation and de-excitation in molecules has been observed before [13][14][15][16][17][18] and is not unique to spin crossover complexes. These latter studies are not of molecular systems, but organics such as anthracene and rubrene where this phenomenon arises due to the coupling of a molecule in the electronic singlet ground state with an adjacent molecule in the first excited singlet state to form a correlated triplet-pair state. ...
The X-ray-induced spin crossover transition of an Fe (II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior which is measured in the
field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.
... This magnetic field induced perturbation of the low spin to high spin state barrier is explored here for [Fe{H2B(pz)2}2(bipy)]. The influence of magnetic fields on the excitation and de-excitation in molecules has been observed before [13][14][15][16][17][18] and is not unique to spin crossover complexes. These latter studies are not of molecular systems but organics like anthracene and rubrene where this phenomenon arises due to the coupling of a molecule in the electronic singlet ground state with an adjacent molecule in the first excited singlet state to form a correlated triplet-pair state. ...
The X-ray induced spin crossover transition of an Fe(II) molecular thin film in the presence and absence of a magnetic field has been investigated. The thermal activation energy barrier in the soft X-ray activation of the spin crossover transition for [Fe{H2B(pz)2}2(bipy)] molecular thin films is reduced in the presence of an applied magnetic field, as measured through X-ray absorption spectroscopy at various temperatures. The influence of a 1.8 T magnetic field is sufficient to cause deviations from the expected exponential spin state transition behavior that is measured in the field free case. We find that orbital moment diminishes with increasing temperature, relative to the spin moment in the vicinity of room temperature.
Triplet–triplet annihilation photon upconversion (TTA-UC) is a process able to repackage two low-frequency photons into light of higher energy. This transformation is typically orchestrated by the electronic degrees of freedom within organic compounds possessing suitable singlet and triplet energies and electronic couplings. In this work, we propose a computational protocol for the assessment of electronic couplings crucial to TTA-UC in molecular materials and apply it to the study of crystal rubrene. Our methodology integrates sophisticated yet computationally affordable approaches to quantify couplings in singlet and triplet energy transfer, the binding of triplet pairs, and the fusion to the singlet exciton. Of particular significance is the role played by charge-transfer states along the b-axis of rubrene crystal, acting as both partial quenchers of singlet energy transfer and mediators of triplet fusion. Our calculations identify the π-stacking direction as holding notable triplet energy transfer couplings, consistent with the experimentally observed anisotropic exciton diffusion. Finally, we have characterized the impact of thermally induced structural distortions, revealing their key role in the viability of triplet fusion and singlet fission. We posit that our approaches are transferable to a broad spectrum of organic molecular materials, offering a feasible means to quantify electronic couplings.
The kinetics of the decay (splitting) of the excited singlet -state of rubrene molecules into a pair oftriplet-excitons (T-excitons) in rubrene films, usually represented in terms of the kinetics of the decay of fluorescence(KDF) from the -state, is analyzed in detail. The KDF is known to be significantly controlled bythe process of diffusive migration and annihilation of the generated T-excitons. In the analysis, two migrationmodels are considered: the two-state model (TSM), treating the migration effect as a result of transitionsbetween the [TT] state of coupled T-excitons (at small TT-distances r) and the [T+T]-state of freely migratingЕ-excitons (at large distances r), as well as the free migration model (FMM), neglecting the effect of the [TT]state. Within the TSM and FMM, the expressions for are derived, which are applied to describe the KDF ,measured in amorphous rubrene films. Within the experimentally investigated range of times, , the TSM isshown to reproduce the behavior of the experimental KDF much more accurately than the FMM. At longertimes a substantial difference () between and the FMM-predicted KDF is found, which is far beyond theexperimental error (3%).
В работе детально проанализированы особенности кинетики распада (расщепления) возбужденного синглетного состояния (РСС) на пару триплетных (Т) экситонов (ТТ-пару) в анизотропных молекулярных кристаллах. Эти особенности, как известно, существенно определяются обратной ТТ-аннигиляцией (т.е. аннигиляцией пар Т-экситонов, мигрирующих в объеме кристалла). В предлагаемом анализе кинетика (контролируемых аннигиляцией) процессов РСС описывалась в рамках модели двух состояний (МДС), в которой взаимодействие мигрирующих Т-экситонов ассоциируется с переходами между двумя кинетическими состояниями ТТ-пар: [ТТ]-состояния связанных пар и [Т+Т]-состояния свободно мигрирующих экситонов. Эта модель позволяет представить эффекты миграции и взаимодействия экситонов в РСС-кинетике в терминах решеточных функций Грина, выражения для которых могут быть найдены в аналитическом виде. В данной работе МДС применена для анализа кинетики РСС в кристаллах рубрена, ранее измеренной в широком диапазоне времен. Анализ дал возможность получить важную информацию о кинетических особенностях процессов РСС в анизотропных кристаллах. Показано, например, что формирование [TT]-состояния приводит к заметному искажению формы кинетической зависимости РСС на малых временах порядка времени первичной стадии этого процесса. Показано также, что анизотропия миграции Т-экситонов существенно проявляется в характерных особенностях поведения кинетики РСС на больших временах.
Blends comprising organic semiconductors and inorganic quantum dots (QDs) are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase-separate during film processing, compromising both their structural and electronic properties. Here, we demonstrate a QD surface engineering approach using electronically active, highly soluble semiconductor ligands that are matched to the organic semiconductor host material to achieve well-dispersed inorganic–organic blend films, as characterized by X-ray and neutron scattering, and electron microscopies. This approach preserves the electronic properties of the organic and QD phases and also creates an optimized interface between them. We exemplify this in two emerging applications, singlet-fission-based photon multiplication (SF-PM) and triplet–triplet annihilation-based photon upconversion (TTA-UC). Steady-state and time-resolved optical spectroscopy shows that triplet excitons can be transferred with near unity efficiently across the organic–inorganic interface, while the organic films maintain efficient SF (190% yield) in the organic phase. By changing the relative energy between organic and inorganic components, yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, we provide a highly versatile approach to overcome longstanding challenges in the blending of organic semiconductors with QDs that have relevance for many optical and optoelectronic applications.
Singlet fission (SF) holds great promise for photovoltaic technologies, where tetracenes, with their relatively high triplet energies, play a major role for application in silicon‐based solar cells. However, the SF efficiencies in tetracene dimers are low due to the unfavorable energetics of their singlet and triplet energy levels. In the solid state, tetracene exhibits high yields of triplet formation through SF, raising great interest about the underlying mechanisms. To address this discrepancy, we designed and prepared a novel molecular system based on a hexaphenylbenzene core decorated with 2 to 6 tetracene chromophores. The spatial arrangement of tetracene units, induced by steric hindrance in the central part, dictates through‐space coupling, making it a relevant model for solid‐state chromophore organization. We then revealed a remarkable increase in SF quantum yield with the number of tetracenes, reaching quantitative (196%) triplet pair formation in hexamer. We observed a short‐lived correlated triplet pair and limited magnetic effects, indicating ineffective triplet dissociation in these through‐space coupled systems. These findings emphasize the crucial role of the number of chromophores involved and the interchromophore arrangement for the SF efficiency. The insights gained from this study will aid designing more efficient and technology‐compatible SF systems for applications in photovoltaics.
Singlet fission (SF) holds great promise for current photovoltaic technologies, where tetracenes, with their relatively high triplet energies, play a major role for application in silicon‐based solar cells. However, the SF efficiencies in tetracene dimers are low due to the unfavorable energetics of their singlet and triplet energy levels. In the solid state, tetracene exhibits high yields of triplet formation through SF, raising great interest about the underlying mechanisms. To address this discrepancy, we designed and prepared a novel molecular system based on a hexaphenylbenzene core decorated with 2 to 6 tetracene chromophores. The spatial arrangement of tetracene units, induced by steric hindrance in the central part, dictates through‐space coupling, making it a relevant model for solid‐state chromophore organization. We then revealed a remarkable increase in SF quantum yield with the number of tetracenes, reaching quantitative (196 %) triplet pair formation in hexamer. We observed a short‐lived correlated triplet pair and limited magnetic effects, indicating ineffective triplet dissociation in these through‐space coupled systems. These findings emphasize the crucial role of the number of chromophores involved and the interchromophore arrangement for the SF efficiency. The insights gained from this study will aid designing more efficient and technology‐compatible SF systems for applications in photovoltaics.
Currently, organic light-emitting diodes (OLEDs) have reached the stage of commercialization, and there have been intense efforts to use them in various applications from small- and medium-sized mobile devices to illumination equipment and large TV screens. In particular, room-temperature phosphorescent materials have become core OLED materials as alternatives to conventionally used fluorescent materials because devices made with phosphorescent materials exhibit excellent light-emitting performance with internal electroluminescence efficiency (ηint) of nearly 100%. However, phosphorescent materials have several intrinsic problems, such as their structure being limited to organic metal compounds containing rare metals, for example, Ir, Pt, Au, and Os, and difficulty in realizing stable blue light emission. Therefore, the development of new materials has been anticipated. In this chapter, first, we shortly review the progress of OLED materials and device architectures mainly based on fluorescence (the first generation) and phosphorescence (the second generation) emitters. Then, we mention the third-generation OLED using a new light-emitting mechanism called thermally activated delayed fluorescence (TADF). Recently, highly efficient TADF, which had been difficult to realize with conventional molecular design, has been achieved by very sophisticated molecular structures, indicating unlimited freedom of molecular design in carbon-based materials. This has led to the realization of ultimate OLEDs that are made of common organic compounds without precious metals and can convert electricity to light at nearly ηint = 100%. Further, we mention the recent progress of NIR-OLEDs.
Singlet fission (SF) and its inverse, triplet-triplet annihilation (TTA), are promising strategies for enhancing photovoltaic efficiencies. However, detailed descriptions of the processes of SF/TTA are not fully understood even in...
Dibenzopentacene is a close analogue of pentacene, but it attracted much less attention than its acene cousin, and its solid-state photophysics remains nearly unknown. In this work we present the...
Although the effect of the electron transport layer (ETL) material TmPyPb on the electroluminescence performance of organic light-emitting diodes (OLEDs) has been extensively studied, the process of TmPyPb regulating exciton recombination and annihilation within the device is still unclear. Here, we fabricated devices of various TmPyPb thicknesses with and without ETL. Subsequently, we measured the magneto-electroluminescence (MEL) of these devices. Specifically, at the same luminance, the triplet-charge annihilation (TQA) process is more likely to occur as the thickness of TmPyPb increases, resulting in a decrease in the maximum luminance of devices. Due to electron leakage and exciton recombination region moving towards the cathode, leading to a decrease in luminance efficiency at first and then an enhancement with an increase in the thickness of TmPyPb. Furthermore, at room temperature, the application of a large bias voltage suppresses singlet fission (SF) processes by modulating the dissociation of singlet polaron pairs (PPS) and the concentration of triplet exciton (T1). This leads to the conversion of SF to the TQA process. At low temperatures, the bias voltage and temperature can regulate the concentration and lifetime of PPS and T1. Therefore, as the temperature decreases, the transition of SF → TQA → triplet-triplet annihilation (TTA) and TQA coexistence → TTA process occurs. Moreover, MEL responses of the TmPyPb-ETL device show a W-linear pattern owing to the combined effect of the hyperfine interaction (HFI) and Zeeman splitting at 145 K. Accordingly, we explored the electroluminescence (EL) performance of TmPyPB-ETL OLEDs and investigated the evolution of SF, TQA, and TTA processes using MEL. Our study revealed the effect of exciton recombination and annihilation in OLEDs with varying thicknesses of TmPyPb.
Ultrafast optical microscopy, generally employed by incorporating ultrafast laser pulses into microscopes, can provide spatially resolved mechanistic insight into scientific problems ranging from hot carrier dynamics to biological imaging. This Review discusses the progress in different ultrafast microscopy techniques, with a focus on transient absorption and two-dimensional microscopy. We review the underlying principles of these techniques and discuss their respective advantages and applicability to different scientific questions. We also examine in detail how instrument parameters such as sensitivity, laser power, and temporal and spatial resolution must be addressed. Finally, we comment on future developments and emerging opportunities in the field of ultrafast microscopy.
Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a "minimal" one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo.
Photochemical upconversion is a process whereby two lower-energy photons are converted into a higher-energy photon by sensitized triplet–triplet annihilation. While recent interest in this process has been motivated by improving the efficiencies of solar cells, many applications are being explored. In this review, we address the underlying physicochemical phenomena that are responsible for photochemical upconversion. We review their kinetics, and the requirements for annihilators and sensitizers to design efficient upconversion systems. We discuss the spin physics of the bi-excitonic interactions and how the spin character of the triplet pairs can fundamentally limit the upconversion efficiency and give rise to the magnetic field effect on delayed photoluminescence. Finally, we address light-matter coupling phenomena that could be employed to enhance photochemical upconversion.
Expected final online publication date for the Annual Review of Physical Chemistry, Volume 74 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Singlet fission (SF) has potential applications in high‐efficiency photo‐energy harvesting applications, but its practical application is hindered by the limited number of materials. In this work, we explored the bay aromatic substitution strategy for the design of new perylenediimide (PDI) based SF materials. A series of PDI derivatives with biphenyl or naphthalene units substituted at the bay positions were designed and synthesized for the investigation of the effects of aromatic substitutes on their photodynamic behaviors. The bay substitutions do not shift the energy level of the PDI core significantly, but give rise to different intermolecular coupling strengths in the thin films and affect the intermolecular SF efficiency. Femtosecond transient absorption (fsTA) spectroscopy reveals that appropriate spacing configuration from the bay aromatic substitution groups enhances the SF yields by promoting the interaction of neighboring PDI cores. Triplet exciton yields of up to 183% have been obtained from these new PDI derivatives, making them potential candidates in future SF‐based optoelectronics. This article is protected by copyright. All rights reserved.
Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes, provided the conjugated backbone is twisted out-of-plane. However, this hypothesis has only been forwarded in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a 'minimal' one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission, despite the canthaxanthin twist in OCPo.
The manifestation of short-time (geminate) and long-time (bimolecular) stages of diffusion-controlled triplet-triplet annihilation (TTA) in the kinetics of singlet fission (SF) (i.e. splitting of the excited singlet state S1∗ into a pair of T-excitons) in molecular organic semiconductors are analyzed in detail. In this analysis, the short- and long-time parts of the SF-kinetics (i.e. kinetics of fluorescence It from S1∗-state), are assumed to be governed by geminate and bimolecular TTA, respectively. Within the analysis, the analytical formula is derived, which is shown to fairly accurately describe the experimental SF-kinetics in the wide range of times.
To develop efficient singlet fission (SF) material with high triplet energy and good stability for the practical application is still a challenge. Anthracene is stable and can carry out SF...
In the third generation photovoltaic device, the main physical mechanism that the photoelectric conversion efficiency is enhanced is the singlet fission (SF). In order to accurately describe the SF and reveal physical process in theoretically, we introduce the anomalous processes (APs) in SF dynamics based on previous model, including anomalous fission, anomalous fusion, anomalous dissociation and combination of triplet pair states, anomalous decay and diffusion of single triplet exciton. The effects of the APs on SF are investigated by the kinetic model with time-dependent coefficient. Further, according to the effects the optimal simulations for the experimental data [G. B. Piland et al., J. Phys. Chem. C, 2013, 117, 1224] are given by adjusting the rate coefficients and exponents in the mended kinetic equations. The results show that the model containing APs is more accurate than previous that to describe SF dynamics, demonstrating that the APs do exist in SF. The model also provides the theoretical foundation for how varies experimentally physical parameters to make SF occur in the required direction.
Singlet fission can double the photon-to-electron conversion efficiency by splitting a singlet exciton into two triplets via an intermediate state of a triplet pair. The spin mixing of tripletpair manifolds with different spin characters is a determining factor for the efficiency of final triplet generation. In this review, we summarize recent studies of magnetic field effects (MFEs) on singlet fission dynamics, from theoretical models to recent experimental results. The analyses of MFEs support a three-step model with dynamic equilibrium between strongly and weakly coupled triplet pairs, suggesting an intermediate regime of intertriplet coupling to be favorable for singlet-quintet mixing toward efficient triplet generation for practical applications.
Intersystem crossing (ISC) and reverse ISC (RISC) are important spin‐mixing processes for obtaining high quantum efficiency in exciplex‐based organic light‐emitting diodes (EB‐OLEDs) and often show normal current (I) dependencies which weaken with increasing I. Surprisingly, herein, using magneto‐conductance (MC) as a fingerprint probing tool, an unreported conversion from normal to abnormal I‐dependent ISC processes is observed at 300 K from relatively‐unbalanced EB‐OLEDs with the low electron‐injection barrier and the high hole mobility. More amazingly, after improving carrier injection balance through replacing the hole‐injection layer, a conversion from abnormal to normal I‐dependent RISC processes with increasing I is observed from the relatively‐balanced EB‐OLEDs for the first time. These two conversions are reasonably analyzed by fitting and decomposing MC traces of the devices. Furthermore, transition I of the conversion from abnormal to normal I‐dependent RISC processes decreases from 50 to 5 μA as the temperature reduces from 300 to 150 K, and only the normal I‐dependent RISC process is observed at 100 and 20 K due to high driving voltages. Obviously, this work deepens the full understandings of I‐dependent ISC and RISC processes in EB‐OLEDs and has the potential applications for the achievement of high‐performance devices. Conversions from normal to abnormal current‐dependent intersystem crossing (ISC) and from abnormal to normal current‐dependent reverse ISC processes are observed from two interfacial exciplex‐based organic light‐emitting diodes with different hole‐injection layers. These two conversions are analyzed by fitting and decomposing magneto‐conductance traces of the devices. This work deepens the understandings of physical microscopic processes for exciplex states formed at the interface.
Kinetics of singlet fission (SF) in molecular semiconductors, i.e., spontaneous splitting of the excited singlet state into a pair of triplet (T) excitons, is known to be strongly affected by geminate annihilation of created TT-pairs. In our work, we analyze in detail the specific properties of SF-kinetics in highly anisotropic molecular crystals (in which T-excitons undergo strongly anisotropic hopping migration) within the earlier proposed two-state model (TSM). This model allows for accurate treatment of the characteristic effects of anisotropic relative migration of T-excitons and TT-interaction on SF-kinetics, describing these effects within the approximation, that assumes kinetic coupling of two states: the [TT]-state of interacting TT-pairs and the [T + T]-state of freely migrating T-excitons. The TSM makes it possible to represent the TT-migration and interaction effects in terms of lattice-migration Green's functions, accurate analytical formulas that are obtained in this work. The TSM is applied to the analysis of SF-kinetics in rubrene single crystals, recently measured in a wide range of times (0.1 ns < t < 104 ns). The analysis enables one to obtain important information on specific properties of SF-kinetics in highly anisotropic crystals. In particular, the observed specific "hump" of SF-kinetics at intermediate times can be treated as a manifestation of the TT-coupling in the [TT]-state. It is also found that the characteristic asymptotic time-dependence of SF-kinetics (∼t-3/2) can markedly be distorted by spin relaxation in TT-pairs.
The conversion of near-infrared photons to visible light through triplet–triplet annihilation upconversion offers an enticing strategy for significantly boosting the efficiency of conventional solar cell technology. Rubrene is widely employed as the acceptor molecule for realising such upconversion, yet in the solid state, the reverse process of singlet fission is believed to hinder efficient upconversion. Consequently, rubrene is sometimes doped at low concentration (0.5 mol%) with the singlet energy collector tetraphenyldibenzoperiflanthene (DBP) which harvests singlet energy via Förster transfer. Although singlet fission is a multi-step process involving various intermediate triplet-pair states, the interplay between it, triplet recombination and singlet energy collection has not been studied in detail to date. Here we use both transient absorption and time-resolved fluorescence spectroscopy to investigate the dynamics of both singlet and triplet species in rubrene-based nanoparticle films. Strikingly, we find that energy transfer from rubrene to DBP does not outcompete the formation of triplet-pairs through singlet fission, despite the fact that DBP doping increases the photoluminescence quantum yield of the nanoparticle films from 3% to 61%. We rationalise this surprising result in the context of the well-known effects of triplet fusion and triplet-quenching defects on the photoluminescence yield of crystalline rubrene.
Volumetric optical imaging of magnetic fields is challenging with existing magneto-optical materials, motivating the search for dyes with strong magnetic field interactions, distinct emission spectra, and an ability to withstand high photon flux and incorporation within samples. Here, the magnetic field effect on singlet exciton fission is exploited to demonstrate spatial imaging of magnetic fields in a thin film of rubrene. Doping rubrene with the high-quantum yield dye dibenzotetraphenylperiflanthene (DBP) is shown to enable optically-pumped, slab waveguide lasing. This laser is magnetic field-switchable: when operated just below lasing threshold, application of a 0.4 T magnetic field switches the device between non-lasing and lasing modes, accompanied by an intensity modulation of +360%. To the best of authors’ knowledge, this is the first demonstration of a magnetically-switchable laser, as well as the largest magnetically-induced change in emission brightness in a singlet fission material to date. These results demonstrate that singlet fission materials are promising materials for magnetic sensing applications and could inspire a new class of magneto-optical modulators.
This article is protected by copyright. All rights reserved
Organic photovoltaics have received active research interest during the past 30 years due to their low cost, flexibility, easy scalability, and robustness. Recently, several efforts have been made to enhance their power conversion efficiency (PCE) and stability by considering advanced photon harvesting technology, utilization of novel donor–acceptor materials, and optimizing device design strategy. Specifically, the photon multiplication process like singlet fission (SF) and design of novel materials, including low‐bandgap conjugated polymers and non‐fullerene acceptors (NFA), have led to the development of advanced organic photovoltaics with PCE close to theoretical Shockley–Queisser (SQ) limit. Here, an up‐to‐date overview of the recent progress during the last five years in advanced organic photovoltaics with a special focus on emerging techniques and materials was reported. Further, various designing and deployment strategies for these processes and materials were explored along with their properties, challenges, and achievements. Finally, a strategy for the next‐stage research directions was analyzed and proposed that could drive this field even further beyond laboratory research to reach the final goal of commercialization.
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.
Molecules with a large local magnetic moment have attracted considerable attention for application in spintronic devices. One candidate of a suitable device goes to the spin crossover molecule, where these 3d transition metal compounds are able to exhibit a robust spin state transition between distinct states. By proper design, the spintronic devices fabricated via spin crossover molecular thin films could achieve novel functionality while retaining flexibility and other traits based on its “organic” nature. Controlling the spin state transition is a key factor of these possibilities. This thesis work investigates the manipulation of the spin state transition in [Fe{H2B(pz)2}2(bipy)] molecular system. The key question is: how can we effectively control the spin state of molecules, and how does the control inspire the future device design? Chapter 1 provides the necessary background knowledge for molecular spintronics and spin crossover molecular systems. Chapter 2 explains relevant experimental techniques involved in the thesis project. Chapter 3 focuses on the bistability and coordination dependence of the spin crossover system. The dielectric substrate will have an influence on the local environment of the spin state occupancy, and a surface to bulk difference in the thin film system indicates the presence of the cooperative effect. Chapter 4 demonstrates the variation of spin state locking effects of the molecular additives to the [Fe{H2B(pz)2}2(bipy)] molecule, and an observation of re-entrant spin state transition. Chapter 5 discusses the isothermal voltage switching of the molecular spin state. Upon forming a heterostructure of [Fe{H2B(pz)2}2(bipy)] with organic ferroelectrics, it is applicable to achieve a room temperature nonvolatile switching of the spin state via external electric fields. This sheds light on the fact that fabricating the molecular thin film spintronics device is indeed possible. Chapter 6 presents an investigation of the energy landscape of the spin state transition. The magnetic field could lower the energy barrier between spin states for the molecular spin crossover thin film fabricated on top of magnetic substrates. Advisor: Peter A. Dowben
Angle dependent magneto-photocurrent in organic single crystal transistors reveals the anisotropy of triplets, verified by a spin-Hamiltonian model with zero-field splitting, providing a basis for metrics of singlet fission–triplet fusion devices.
We report studies on blue and white organic light-emitting devices (OLEDs) based on the deep-blue electrophosphorescent dye iridium(III) bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Using high triplet energy charge transport layers and a dual-emissive-layer structure as well as the p-i-n device structure, we have achieved external quantum efficiencies of 20% and maximum power efficiency of 36 lm/W in these deep-blue OLEDs. White OLEDs with a CRI of 79 and a maximum power efficiency of 40 lm/W were also demonstrated by incorporating red and green phosphorescent dopants together with FIr6.
Polarized superradiant emission and exciton delocalization in tetracene single crystals are reported. Polarization-, time-, and temperature-resolved spectroscopies evidence the complete polarization of the zero-phonon line of the intrinsic tetracene emission from both the lower (F state) and the upper (thermally activated) Davydov excitons. The superradiance of the F emission is substantiated by a nearly linear decrease in the radiative lifetime with temperature, being 15 times shorter at 30 K compared to the isolated molecule, with an exciton delocalization of about 40 molecules.
We report the intrinsic absorption and photoluminescence spectra of rubrene single crystals, deriving them from a series of experiments performed at different excitation wavelengths and in different experimental geometries. We describe the absorption spectra for all three light polarizations in the crystal, and discuss how anisotropic wavelength-dependent absorption and emission affect the characteristics of observed photoluminescence spectra. We identify vibronic progressions both in absorption and emission and discuss their parameters and the main vibrational modes that are responsible for them. We propose that the most commonly measured absorption and emission in rubrene, the one with light polarization perpendicular to the c axis of the crystal, is caused by vibronic-induced depolarization of the c-polarized electronic transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
The excited state dynamics of rubrene in solution and in the single crystal were studied by femtosecond pump-probe spectroscopy under various excitation conditions. Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution. Upon 500 nm excitation, triplet excitons form on the picosecond time scale via fission from the lowest excited singlet state. Upon 250 nm excitation, fission from upper excited singlet states is observed within 200 fs.
Photon up-conversion (UC) and photon-induced multiple-exciton generation (MEG) are proposed directions that are of increasing interest for improving photovoltaic (PV) conversion efficiencies via “photon (or light) management”. Straightforward analysis of these approaches for non-concentrated single-junction cells in the detailed balance limit yields a theoretical PV conversion limit of 49%, instead of 31% without UC and MEG. With what we estimate to be optimistic, maximal realistic efficiencies (25% for UC; 70% for MEG) this limit becomes <40%, i.e., ∼1.25 times the theoretical efficiency of conventional single-band gap cells. While this result does not detract from the fascinating fundamental scientific challenge to make UC and MEG simple and cheap ways to improve PV, such reality checks should be considered when evaluating the short-term promises of these and other options, such as spectral splitting and tandem arrangements.
The excited-state dynamics of the carotenoid zeaxanthin embedded in synthetic bilayers (see picture) are probed with time-resolved resonance Raman spectroscopy. Triplet excitons form with significant yield via a singlet fission mechanism when zeaxanthin self- assembles into multimers. We propose a functional role for singlet fission in biological systems.
We directly measured a spin diffusion length (lambdas) of 13.3 nm in amorphous organic semiconductor (OS) rubrene (C42H28) by spin polarized tunneling. In comparison, no spin-conserved transport has been reported in amorphous Si or Ge. Absence of dangling bond defects can explain the spin transport behavior in amorphous OS. Furthermore, when rubrene barriers were grown on a seed layer, the elastic tunneling characteristics were greatly enhanced. Based on our findings, lambdas in single-crystalline rubrene can be expected to reach even millimeters, showing the potential for organic spintronics development.
The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S(1) state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S(0)→S(1) transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T(1)→T(N) absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene's photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.
Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.
Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here.
Spectroscopic and kinetic properties of tetracene and rubrene in their lowest triplet states have been investigated by laser flash spectroscopy. The formation of triplet rubrene with low quantum yield FISC was detected directly. It was found that triplet rubrene is produced by thermally activated ISC from the S1 state to a higher excited triplet state Tn according to a simple Arrhenius-type relationship. Activation energies and frequency factors were determined in four different solvents. From the activation energies the energy of the Tn state of rubrene can be estimated. By comparing the energy levels of tetracene and rubrene the effect of phenyl substitution is discussed and the analogy to meso-substituted anthracenes is established.
Bimolecular decay of an excited singlet exciton into two triplet excitons is suggested to be an important fluorescence quenching channel in crystalline tetracene and its corresponding rate is estimated to be ∼ 4 × 1010 to 1012 sec−1. The process is tentatively proposed to account for the observed relative fluorescence efficiencies of anthracene, tetracene and pentacene.
Recently the applicability of Suna's theory on triplet-exciton annihilation in molecular crystals to systems other than anthracene has been questioned. In this letter we demonstrate that this discrepancy between theory and experiment is removed, if anisotropic spin relaxation is accounted for.
The room temperature electron paramagnetic resonance spectrum of triplet excitons in single-crystal tetracene is reported. From experiment performed at 24 GHz the exciton spin hamiltonian ≻/sc;* = gβH·S + D* [S2z*→S(S + 1)] + E* [S*→S2v*], is fit to the measured angular dependence in the a'b plane. The spin hamiltonian parameters are g = 2.002,D* = −0.0062 cm−1, and E* = +0.0248 ± 0.0005 cm−1. Extensive spin polarization and emission have been observed and are attributed to the formation of the triplet population from singlet exciton fission.
The fluorescence efficiency of tetracene single crystals may be enhanced by as much as 38% in a magnetic field H>2000 G. The enhancement is anisotropic with respect to the orientation of H in the ab plane. It is shown that a magnetically sensitive coupling of singlet states to a double-triplet-exciton state (T1T1) at ~2.40 eV is an important channel for radiationless decay in crystalline tetracene above 160°K.
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The singlet–triplet (T1 ← S0) excitation spectrum for delayed fluorescence and the prompt fluorescence (S1 ↠ S0) emission spectrum have been measured for a tetracene crystal at room temperature. The origin (0–0 transition) in the two spectra at 10 100 ± 20 and 18 680 ± 30 cm−1, respectively, imply the activation energy for singlet exciton fission, ΔE = 1520 ± 70 cm−1. Raman lines corresponding to the vibrational structure in the above spectra were detected. The fluorescence emission spectra show the need for taking reabsorption effects into account.
Spectrum and decay time of the fluorescence of vacuum sublimed tetracene layers were measured as a function of temperature. The substrate temperature (Tf) during vapor deposition was varied between 80 K and 220 K. The emission spectrum can be fitted by superposition of two gaussians centered near 18.500 cm−1 (band I) and 17.850 cm−1 (band II), respectively, and a broad excimer band with a maximum near 16.400 cm−1. The decay time of band I and II is 6.2 ns, that of the excimer band is 21.3 ns, independent of temperature. Band I appears only with films prepared at Tf ⪢ 140 K and is associated with emission from crystalline areas of submicroscopic size, band II is attributed to (monomeric) defects present in a concentration of about 1.5% as evidenced by absorption studies. Since in low temperature films (Tf < 140 K) the originally excited singlet state does not fluorescence rapid energy transfer to both monomeric defects and incipient dimers capable of excimer formation must occur. The average hopping frequency of the S1-state must be at least 5 × 1010 s−1. Fluorescence excitation spectra support defect and dimer excitation by energy transfer from a common absorbing state.
The magnetic field effect (MFE) on the fluorescence of amorphous and polycrystalline rubrene films has been investigated both experimentally and theoretically. This MFE is known to be due to the annihilation of triplet excitons. The specific features of MFE in these two films turn out to be different. A theoretical analysis has been made within a simple model which takes into account the spin lattice relaxation. The MFE in rubrene films is shown to be strongly affected by the magnetic field dependent spin lattice relaxation.
One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S(1)) into two triplet (T(1)) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S(1). However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2 x T(1), but fission is not activated thermally. Rather, an S(1) <--> ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S(1) <--> ME into 2 x T(1), which leads to a high singlet-fission yield.
Detailed measurements of the magnetic field dependence of the rate of mutual annihilation of triplet excitons in anthracene crystals at room temperature are presented. The field dependence consists of an increase at low fields, with a maximum at ca. 350 Oe, followed by a decrease at higher fields to less than the zero-field annihilation rate. For most field directions, a second maximum occurs at ca. 600 Oe. The amplitude of the field dependence is highly anisotropic. For fields >2000 Oe, resonances in the annihilation rate are found at +76° and -17° with respect to the a axis for fields in the ac plane and at ±23.5° with respect to the b axis in the ab plane. For fields <500 Oe, a second set of resonances occurs at directions bisecting the high-field resonances and at intermediate fields the two sets coexist. These results are discussed in terms of a density-matrix description of the spin states of the interacting triplet pair and of the annihilation process. The field dependence is accounted for on the basis of the field dependence of the pair spin states together with the postulate that annihilation is spin-allowed. The observed resonances result from level crossings among the pair spin states. All of the structure in the field dependence and anisotropy data is satisfactorily reproduced by calculations based on the model, although complete quantitative agreement is not achieved.
A theory is developed for calculating the kinematic part of the exciton-exciton annihilation rate in molecular crystals. The spatial and spin motion of the excitons, as well as the annihilation process itself, is treated phenomenologically. Exciton propagation is assumed to take place as in the hopping model. The importance of the dimensionality of the exciton motion is pointed out; in nearly one- or two-dimensional cases, certain lifetime processes control the exciton collision rate, in contrast to the three-dimensional case. These lifetime processes include motion out of the one- or two-dimensional subspace and, for excitons with spin, spin relaxation. The theory leads to a description of magnetic field effects on the annihilation rate of triplet excitons at room temperature. When applied to triplet excitons in anthracene, this description gives a satsfactory fit to the observed effects and leads to the determination of the nearest-neighbor exciton annihilation rate, the singlet-channel annihilation rate constant, and the exciton diffusion constant for motion perpendicular to the ab plane of anthracene.
We calculate the maximum power conversion efficiency for conversion of solar radiation to electrical power or to a flux of chemical free energy for the case of hydrogen production from water photoelectrolysis. We consider several types of ideal absorbers where absorption of one photon can produce more than one electron-hole pair that are based on semiconductor quantum dots with efficient multiple exciton generation (MEG) or molecules that undergo efficient singlet fission (SF). Using a detailed balance model with 1 sun AM1.5G illumination, we find that for single gap photovoltaic (PV) devices the maximum efficiency increases from 33.7% for cells with no carrier multiplication to 44.4% for cells with carrier multiplication. We also find that the maximum efficiency of an ideal two gap tandem PV device increases from 45.7% to 47.7% when carrier multiplication absorbers are used in the top and bottom cells. For an ideal water electrolysis two gap tandem device, the maximum conversion efficiency is 46.0% using a SF top cell and a MEG bottom cell versus 40.0% for top and bottom cell absorbers with no carrier multiplication. We also consider absorbers with less than ideal MEG quantum yields as are observed experimentally.
Experimental and theoretical studies are reported of the short‐lived and delayed fluorescence of anthracene single crystals, excited by single‐ and double‐photon absorption. A giant‐pulse ruby laser provides the primary source of radiation of 14 400 cm−1 (up to 1027 photons/cm2⋅sec) and is also used to generate second‐harmonic radiation from ADP, as well as stimulated Raman radiation of 12 800 and 17 500 cm−1 from liquid oxygen. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temperature and excitation wavelength. The very intense fast fluorescence with a half‐life of 30 nsec at 300°K, characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are separated using sectored disks. It is concluded that the triplet state at 14 750 cm−1 can be populated (i) by direct absorption of laser photons involving an activation energy of 350 cm−1; (ii) via two‐photon absorption, presumably leading to a vibrationally excited state of the 1B2u exciton, followed by intersystem crossing; (iii) via one‐photon (second‐harmonic) excitation from levels≥700 cm−1 into the singlet absorption band, followed by conversion of the singlet exciton into a triplet pair. The latter process is suggested by the observed activation energy of 700 cm−1. In agreement with these interpretations, the delayed fluorescence intensity is found to vary with the second to fourth power of the laser intensity depending on the experimental conditions. Also, light of 17 500 cm−1 leads exclusively to Process (i), light of 12 800 cm−1 exclusively to (ii). Triplet lifetimes from 2–17 msec are obtained, depending on crystal purity, which indicates that unimolecular triplet decay is an extrinsic, radiationless process. A singlet—triplet intersystem crossing rate constant of about 3×10−5 sec−1 is estimated. The triplet—triplet annihilation rate constant is found to be about 5×10−11 cm3 sec−1. This value considered together with the triplet‐pair creation process suggests a triplet exchange rate ≳ 1013 sec−1 and a triplet diffusion constant ≳o5×10−4cm2/sec.
Triplet exciton fusion and singlet excition fission have been observed
in tetracene crystals, and the magnetic field dependence and the rate
constants for these processes have been measured. At 0°C the singlet
fission rate constant and the triplet-triplet fusion rate constant
(leading to excited singlets) are (6.3+/-0.7) ×108
sec-1 and 9×10-10 cm3 sec-1
(within an order of magnitude), respectively.
Fluorescence quantum yields, γF, lifetimes, τF, and oxygen-quenching rate constants, kQ, are reported for a series of eight aromatic hydrocarbons in benzene at 25 ± 2°. The independence of kQ on oxygen concentration is accepted as a criterion of stationary concentration gradients of the diffusing species over the period of observation, and the viscosity dependence of kQ, reported previously (Ware), and the variation of kQ with fluorescent solute in the same solvent are interpreted in terms of a nondiffusion-limited quenching process.
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A formula for the triplet exciton annihilation rate in molecular crystals is obtained with taking into account spin–lattice relaxation of the excitons. Using this formula, some experimentally observed peculiarities in the dependence of the annihilation rate on the magnetic field may be explained at least qualitatively.
[Russian Text Ignored.]
Until now controversies have arisen from the estimates of the triplet energies of rubrene (R) and diphenylisobenzofuran (DPBF), both of which are known reactants with singlet oxygen. To clarify the situation, flash kinetic spectroscopy was used to measure rates of energy transfer between these two molecules and other energy donor and energy acceptor molecules of known triplet energy. The lowest excited triplet state in R is nearly isoenergetic with the corresponding state in zinc phthalocyanine, and the energy value assignment is 9.2 ± 0.2 kilokaysers. Triplet—triplet annihilation is thermoneutral and is consistent with the lowest triplet state having approximately one-half the energy of the lowest excited singlet state. Based on the rates at which azulene and ferrocene quench the lowest excited triplet state of DPBF, the energy of this state can be assigned a value of 11.9 ± 0.1 kilokaysers.
Within a theoretical approach including both electron—phonon and electron—electron interaction terms, we investigate the relaxation processes of photoexcited states in oligomers and long chains of poly(paraphenylene vinylene), PPV. With regard to the vibronic structure, we find that the Huang—Rhys factor decreases monotonically as chain length increases. Our direct calculations indicate that in PPV the difference in binding energy between two polarons and one singlet (triplet) polaron—exciton decreases (increases) with the Hubbard U term; the long-range interaction V terms stabilize both the singlet and triplet polaron—excitons. The theoretically simulated photoinduced absorption spectra agree with experiment both for singlet and triplet states.
The salient features in the absorption and emission spectra of alphaT4 aggregates (see Figure) and films are fully accounted for using a model based on linear exciton-phonon coupling and structural defects. In particular, the Davydov splitting is calculated to be of the order of 1 eV, and the unusually small 0-0 emission intensity is attributed to the high sensitivity of the 0-0 intensity to basal plane structural defects.
A detailed analysis of the oscillations seen in the delayed fluorescence of crystalline tetracene is presented in order to study the mechanism of singlet fission. Three quantum beat frequencies of 1.06 ± 0.05, 1.82 ± 0.05, and 2.92 ± 0.06 GHz are resolved, which are damped on a time scale of 20 ns. The effects of sample morphology, excitation wavelength, and temperature are examined. A density matrix model for singlet fission is developed that quantitatively describes the frequencies, amplitudes, and damping of the oscillations. The model assumes a direct coupling of the initially excited singlet exciton to the triplet pair manifold. There is no electronic coherence between the singlet and triplet pair states, but the rapid singlet decay time of ∼200 ps in solution-grown single crystals provides the impulsive population transfer necessary to create a coherent superposition of three zero-field triplet pair states |xx>, |yy>, and |zz> with overall singlet character. This superposition of the three states gives rise to the three quantum beat frequencies seen in the experiment. Damping of the quantum beats results from both population exchange between triplet and singlet manifolds and pure dephasing between the triplet pair states. By lowering the temperature and slowing the SF rate, the visibility of the oscillations decreases. There is no evidence of magnetic dipole-dipole coupling between the product triplets. Our model provides good overall agreement with the data, supporting the conclusion that singlet fission in tetracene proceeds through the "direct" mechanism without strong electronic coupling between the singlet and triplet pair states.
A photophysical study of the covalently linked tetracene dimer 1,4-bis(tetracen-5-yl)benzene is presented. While the dimer’s steady state spectroscopy is similar to that of monomeric tetracene, it also exhibits a long-lived fluorescence signal in solution and solid polyethylene films, which is absent in the monomer. The behavior of this long-lived component as a function of temperature and oxygenation provides evidence that a small (<1%) fraction of the singlet excited states undergoes fission into two triplet states, which recombine on the order of 100 ns. A kinetic model based on this mechanism fits the fluorescence decay data quantitatively.
Fission or dissociation of singlet excitions into pairs of triplet excitons in tetracene crystals is demonstrated by investigation of the magnetic field dependence of the prompt fluorescence intensity.
Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPT's pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet fission yield, with 1.22 triplets being created per excited singlet. This triplet production occurs over two principal time scales, with ~50% of the triplets appearing within 1 ps after photoexcitation followed by a slower phase of triplet growth over a few hundred picoseconds. To fit these kinetics, we have developed a model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission. Singlet excitons directly excited at these sites can undergo fission rapidly, while singlet excitons created elsewhere in the film must diffuse to these sites to fission.
Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.
The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.
The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ~100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ((1*)TIPS-Pn + TIPS-Pn → 2 (3*)TIPS-Pn) and charge transfer from (1*)TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state (1)(TIPS-Pn(+•)-PDI(-•)) that undergoes radical pair intersystem crossing to form (3)(TIPS-Pn(+•)-PDI(-•)), which then undergoes charge recombination to yield either (3*)PDI or (3*)TIPS-Pn. Energy transfer from (3*)PDI to TIPS-Pn also yields (3*)TIPS-Pn. These results show that multiple pathways produce the (3*)TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.
Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a dynamic interplay between SF and triplet-triplet annihilation.
We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.
We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.
Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Polycrystalline tetracene films have been explored using weak ∼ 30 fs visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay. Adaptive pulse shaping identifies multipulse optimal fields which maximize this absorption feature by ∼ 20%. These are comprised of subpulses separated by time delays well correlated with the period of lattice vibrations suggesting such modes control the yield of SF photochemistry.
Singlet fission is a process in which an organic chromophore in an excited singlet state shares its excitation energy with a neighboring ground-state chromophore and both are converted into triplet excited states. Singlet fission is spin-allowed in the sense that the two resulting triplet excitations produced from an excited singlet are born coupled into a pure singlet state. Like many other internal conversion processes, it can be very fast, particularly in molecular crystals. The wave function of the initially formed pure singlet state 1(TT) is a coherent superposition of the wave functions of these nine sublevels, and their ultimate population will reflect the amplitude of the singlet 1(TT) wave function in each one. As long as the states resulting from singlet fission are of mixed multiplicity, the overall process can also be viewed as a special case of intersystem crossing. Singlet fission does not occur in single small-molecule chromophores, at least not at the usual excitation energies, and is constrained to multichromophoric systems.
Direct observation of triplet absorption and ground-state depletion upon pulsed excitation of a polycrystalline thin solid film of 1,3-diphenylisobenzofuran at 77 K revealed a 200 ± 30% triplet yield, which was attributed to singlet fission.
We report high-yield triplet generation by singlet fission upon photoexcitation of a new aggregate of the carotenoid all-trans 3R,3'R-zeaxanthin. The yield is determined by picosecond time-resolved resonance Raman spectroscopy, which allows direct characterization and quantification of triplet excited-state signatures and ground-state depletion. The technique and analysis reveals that triplets form within picoseconds. A quantum yield of 90-200% is derived with the assumption of weak exciton-coupling in the zeaxanthin aggregate.
Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.
The magnetic field dependence of singlet exciton fission and fluorescence was studied at 300 K in tetracene crystals in several crystal planes for different field intensities and orientations. The positions of the resonances at high field correspond to those predicted by the zero-field splitting tensor as obtained by EPR, indicating that the outcome of the singlet fission is a pair of free migrating triplet excitons. It is shown that the field dependence and the resonance lineshapes can only be fitted, to within the experimental error, with the kinematic theory of Suna assuming a nearly two-dimensional motion of triplet excitons, restricted to the (a prime b) plane. With the assumption of an isotropic triplet exciton diffusion in the (a prime b) plane reasonable sets of values can be deduced from the fit for the in-plane hopping rate, for the anisotropy of the diffusion, and for the nearest neighbor triplet-triplet annihilation rate. In many respects, triplet exciton dynamics in tetracene appears to be similar to that in anthracene, as expected from the calculation of the transfer matrix elements and the similarity of the crystal structures. The dependence of the effects on the exciting light intensity is also well accounted for using the usual kinetic equations for the different processes involved during the lifetime of the triplet excitons.
We report on the amorphous-to-crystalline phase transformation of rubrene thin films. The crystallization of the organic thin films displays disk-like domains whose nucleation and growth follow phase transformation kinetics well-established for inorganic materials under certain time and temperature conditions. We understood that the crystallization of amorphous rubrene thin film shows site-saturated nucleation behavior while the crystalline growth involves both diffusion and interface-controlled kinetics displaying spherulitic disk growth behavior. The activation energy of the transformation kinetics was about 0.78 eV on hexamethyldisilazane-functionalized SiO(2) substrate as mostly consumed at the growth process. The crystallization kinetics changes with the film substrate; more hydrophobic substrate induces a lesser number of crystalline nuclei while causing faster growth of those nuclei.