Nonlinear Density Dependence of Singlet Fission Rate in Tetracene Films

Anisotropic Geminate and Non-Geminate Recombination of Triplet Excitons in Singlet Fission of Single Crystalline Hexacene

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley-Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron-phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet-triplet bimolecular process, the rates along the two different optical axes in the a-b crystalline plane differ by a factor of 4. This anisotropy in the triplet-triplet recombination rates was attributed to the interference in the coupling probability of dipole-dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Electrical Detection of Singlet Fission in Single Crystal Tetracene Transistors

We present the electrical detection of singlet fission in tetracene by using a field-effect transistor (FET). Singlet fission is a photoinduced spin-dependent process, yielding two triplet excitons from the absorption of a single photon. In this study, we engineered a more deterministic platform composed of an organic single crystal FET rather than amorphous or polycrystalline FETs to elucidate spin-dependent processes under magnetic fields. Despite the unipolar operation and relatively high mobility of single crystal tetracene FETs, we were able to manipulate spin dependent processes to detect magnetoconductance (MC) at room temperature by illuminating the FETs and tuning the bias voltage to adjust majority charge carrier density and trap occupancy. In considering the crystalline direction and magnetic field interactions in tetracene, we show the MC response observed in tetracene FETs to be the result of the singlet fission process.

Singlet Fission in Weakly Interacting Acene Molecules

The energy conversion in solar cells has conventionally been limited by the Shockley-Queisser limit. Singlet fission (SF), a decay mechanism where a single excited singlet state is converted into two triplet states, can drastically improve this efficiency. For the most part, observation of SF has been limited to crystalline structures in solids and films, where strong ordering was present. Here we report on singlet fission in a disordered system where organic chromophores are distributed on the surface of a rare gas cluster. In this case, the intermolecular distances and degree of excitation can be varied to obtain their effects on the rate of singlet fission. We introduce a kinematic model that takes into account the details of the geometrical arrangement of the system as well as the time-dependent populations of the relevant states of each molecule and evaluate the trends obtained by SF on the experimental observables.

Magnetic dipolar interaction between correlated triplets created by singlet fission in tetracene crystals

Singlet fission can potentially break the Shockley–Queisser efficiency limit in single-junction solar cells by splitting one photoexcited singlet exciton (S₁) into two triplets (2T₁) in organic semiconductors. A dark multiexciton state has been proposed as the intermediate connecting S₁ to 2T₁. However, the exact nature of this multiexciton state, especially how the doubly excited triplets interact, remains elusive. Here we report a quantitative study on the magnetic dipolar interaction between singlet-fission-induced correlated triplets in tetracene crystals by monitoring quantum beats relevant to the multiexciton sublevels at room temperature. The resonances of multiexciton sublevels approached by tuning an external magnetic field are observed to be avoided, which agrees well with the theoretical predictions considering a magnetic dipolar interaction of ∼0.008 GHz. Our work quantifies the magnetic dipolar interaction in certain organic materials and marks an important step towards understanding the underlying physics of the multiexciton state in singlet fission.

The exact mechanism of singlet fission remains unresolved. Here, Wang et al. report a quantitative measurement of the interaction between singlet-fission-induced correlated triplets in tetracene crystals with quantum beat spectroscopy, indicating the role played by exciton delocalization in singlet fission.