Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox ...centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories but clearly observed in ultrafast electro-optical experiments described herein. We show that it is the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing localized charge-transfer states. In contrast to conventional views, our findings suggest that fullerene cluster size, concentration, and dimensionality control charge generation efficiency, independent of exciton delocalization.
Significance We use transient spectroscopy to investigate the mechanism of singlet exciton fission, a quantum mechanical phenomenon in some organic molecules in which a spin-singlet excited state can ...split into two spin-triplet states. This process may be harnessed to boost solar cell efficiencies, but the underlying mechanism remains poorly understood. Central to most models is a triplet pair state, consisting of two triplets entangled into an overall spin-singlet configuration, but it has never before been optically detected. In a solution-based system, we detect a state with simultaneous singlet and triplet exciton character that dissociates to form triplet excitons in 120% yield. We consider that this intermediate constitutes a triplet pair state, and its observation allows important insight into the nature of triplet exciton coupling.
Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction ShockleyâQueisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)TIPSâtetracene we find rapid (<100 ps) formation of excimers and a slower (â¼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.
Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of ...photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ∼200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers. At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole separate freely. This early time behavior is consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.
Liquid phase sintering (LPS) of powder metallurgy (PM) components is a well-recognised strategy to enhance the densification of pressed-and-sintered compacts. This work reports the investigation on ...the liquid phase formation when a Fe-Ni-Mn-C-B master alloy (MA) is used as a boron carrier in combination with two iron base powders pre-alloyed with Mo. Through differential scanning calorimetry tests, quantitation of the microstructure with the help of artificial intelligence, as well as measurement of sintered density and strength as a function of sintering temperature, it was possible to unravel the mechanisms that take place before and during LPS. It was confirmed that a cascade of events takes place in the solid state prior to reaching the temperature necessary for a eutectic reaction to form a liquid. Additionally, the pre-alloyed Mo content was identified as a factor that modifies the initiation of LPS but not the LPS mechanisms per se.
The nanoscale morphology and high charge densities in organic photovoltaics (OPVs) lead to a high rate of bimolecular encounters between spin-uncorrelated electrons and holes. This process can lead ...to the formation of low-energy triplet excitons on the donor polymer that decay nonradiatively and limit the device performance. We use time-resolved optical spectroscopy to characterize the effect of morphology through the use of solvent additives such as 1,8-octanedithiol (ODT) on triplet dynamics and charge recombination in blends of poly2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta2,1-b;3,4-b′-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) and 6,6-phenyl-C71-butyric acid methyl ester. This is an attractive OPV system since the extended absorption of the polymer into the near-infrared gives good coverage of the solar spectrum, but nevertheless, the internal quantum efficiency (IQE) has not been reported to be higher than ∼65% under short circuit conditions. We find that, without ODT, the IQE is 48% and 16% of excitations decay via bimolecular triplet formation. With ODT treatment, which improves crystallinity and carrier mobility, the IQE increases to 65%, but bimolecular triplet formation significantly increases and now accounts for all of the recombination (35% of charges).
We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity, broad-band measurements ...allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ∼90 ps. However, in contrast to pentacene, where fission is effectively unidirectional, we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated “geminate” triplets following fission, leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature, the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together, these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy, and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation.
In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic ...photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.
The efficient transfer of energy between organic and inorganic semiconductors is a widely sought after property, but has so far been limited to the transfer of spin-singlet excitons. Here we report ...efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductors to inorganic semiconductors. We use ultrafast optical absorption spectroscopy to track the dynamics of triplets, generated in pentacene through singlet exciton fission, at the interface with lead selenide (PbSe) nanocrystals. We show that triplets transfer to PbSe rapidly (<1 ps) and efficiently, with 1.9 triplets transferred for every photon absorbed in pentacene, but only when the bandgap of the nanocrystals is close to resonance (±0.2 eV) with the triplet energy. Following triplet transfer, the excitation can undergo either charge separation, allowing photovoltaic operation, or radiative recombination in the nanocrystal, enabling luminescent harvesting of triplet exciton energy in light-emitting structures.
We address the binding energy of charge-transfer excitons at organic semiconductor heterojunctions by investigating a polymer blend where the energy of the intramolecular singlet exciton is just ...sufficient to create separated charge pairs, placing the system at the threshold for photovoltaic operation. At 10 K, we report long-lived photoluminescence arising from charge recombination and triplet-exciton bimolecular annihilation. Both mechanisms regenerate singlet excitons in the electron acceptor, but we demonstrate that charge recombination dominates singlet regeneration dynamics on ≤300 ns time scales. This occurs by tunnelling of separated electron and holes across heterojunctions. The separated charge pairs are therefore degenerate with repopulated singlet states. From the difference of the charge-transfer and intrachain exciton emission energies, we determine that the binding energy of charge-transfer excitons with respect to bulk charge separation is ≥250 meV. Directed charge flow away from the heterojunction would avoid formation of strongly bound charge-transfer excitons, that act as traps and limit current generation in organic solar cells.
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