Ultrafast Triplet Pair Formation and Subsequent Thermally Activated Dissociation Control Efficient Endothermic Singlet Exciton Fission (1704.01695v1)

Abstract: Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley-Queisser limit in photovoltaics. SF as measured by the decay of S1 has been shown to occur efficiently and independently of temperature even when the energy of S1 is as much as 200 meV less than 2T1. Here, we study films of TIPS-tetracene using transient optical spectroscopy and show that the initial rise of the triplet pair state (TT) occurs in 300 fs, matched by rapid loss of S1 stimulated emission, and that this process is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower 10 ps morphology-dependent phase of S1 decay and TT growth. We observe the TT to be thermally dissociated on 10-100 ns timescales to form free triplets. This provides a model for "temperature independent", efficient TT formation and thermally activated TT separation.

Ultrafast Dynamics of Triplet Pair Formation in Endothermic Singlet Exciton Fission

The paper presented in this paper explores the fundamental photophysical processes of singlet exciton fission (SF) in organic chromophores, particularly focusing on TIPS-tetracene films. Singlet exciton fission, a quantum mechanical process wherein a singlet exciton splits into two triplet excitons, holds potential to surpass the Shockley-Queisser limit imposed on photovoltaic efficiencies. This paper explores the dynamics of this process by investigating the formation and dissociation of triplet pair states (TT) in TIPS-tetracene using transient optical spectroscopy.

Key Findings and Methodology

1. Triplet Pair Formation: The paper demonstrates that in TIPS-tetracene films, the triplet pair state forms within a remarkably short time of 300 femtoseconds (fs), characterized by the rapid loss of singlet (S1) stimulated emission. Notably, this ultrafast TT formation challenges the traditional understanding that endothermic fission systems require activation energy to access higher-lying TT states.
2. Morphology and Dynamics: Two distinct film morphologies, namely disordered and polycrystalline, are compared. A morphology-dependent phase following the initial TT formation is observed, where the decay of S1 and growth of TT state happens over a 10 picosecond (ps) timeframe. This dynamic phase underscores the significance of molecular arrangement and domain size in influencing internal excitonic interactions.
3. Thermally Activated Dissociation: The observation of thermally activated TT dissociation into free triplets occurs on nanosecond to microscale timescales (10-100 ns). This dissociation is markedly efficient in disordered films at room temperature, with TT states in disordered films dissociating into free triplets within 10 nanoseconds (ns). Conversely, TT states remain bound much longer (tens of microseconds) in polycrystalline films due to hindered triplet migration.
4. Spectroscopy: Employing ultrafast broadband transient absorption spectroscopy revealed vibrational coherences that suggest strong coupling between electronic and vibrational states during TT formation. This implies a breakdown of the Born-Oppenheimer approximation, where non-adiabatic processes facilitate the crossing of potential energy surfaces (PES) for S1 and TT states.
5. Vibrational Coupling: The role of vibrational modes, specifically those linked to low-frequency C-C deformations, is highlighted in the ultrafast TT formation process. This finding contrasts with previously suggested strong electronic coupling models for similar systems, suggesting alternative pathways involving vibronic coupling are at play.

Implications and Future Directions

This paper provides critical insights into the ultrafast dynamics and structural influences on singlet exciton fission, particularly emphasizing the efficacy of endothermic systems like TIPS-tetracene in generating multiple excitons. The results pave the way for potential enhancements in photovoltaic technologies through the engineering of materials that leverage similar ultrafast vibrational coupling mechanisms. Practically, the findings advocate for the exploration of molecular geometry and packing strategies to optimize exciton splitting and migration, which are crucial for developing high-efficiency photovoltaic devices.

Theoretically, these findings could influence the ongoing development of computational models to more accurately capture the multi-dimensional potential energy surfaces involved in photon conversion processes. Future work may involve investigating the role of other vibrational modes and potential conical intersections, which could further elucidate the mechanisms behind vibronic state coupling and efficient fission processes in a broader range of organic materials.

In summary, this paper advances our understanding of endothermic singlet exciton fission by highlighting the pivotal role of ultrafast vibrational coherence in facilitating immediate TT formation, providing a potent framework for refining future photovoltaic materials.