Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy

Abstract

Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

Effects of Charge Transfer State and Exciton Migration on Singlet Fission Dynamics in Organic Aggregates

“…21573175,91333101,21133007). 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 …”

“…As the SF process potentially allows single-junction photovoltaic devices to surmount the Shockley-Queisser limit in terms of power conversion efficiency, 11,12 it has attracted renewed attentions. [13][14][15][16][17][18][19][20][21] Numerous investigations have revealed that the interaction between the initial singlet exciton (SE) state and TT state, which plays an important role in SF dynamics, may include a direct statestate interaction and an indirect superexchange interaction mediated by a charge-transfer (CT) state. 3,4,[22][23][24][25][26][27][28][29][30][31][32] This interaction in organic crystals can be strong enough to induce the quantum coherence between SE and TT states, 5,7,33,34 resulting in an ultrafast timescale of SF.…”

Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet–Triplet Energy Gap

“…Fluorescence is readily detected for 100's of ns, when triplets are the dominant species (see below). The PL lifetime enhancement for ASFCN is a signature of delayed fluorescence, which typically arises from either triplet-triplet annihilation 22 or thermally activated RISC (akin to that proposed for small molecules 7 ). To differentiate between the two mechanisms, we measured the PL decay dynamics at lower excitation density and performed temperature-dependent steady-state measurements.…”

Triplet transport in thin films: fundamentals and applications

“…[80][81][82][83] As shown in Fig. 4, the spatial spread of the signal at each delay time can be fitted with a Gaussian function parameterized by variance s. The exciton diffusion length L at delay time t is then related to the exciton density characterized by variance s,…”

“…Using this approach, Wan et al studied the triplet diffusion dynamics in teteracene crystals at the picosecond and nanosecond timescales. 81 They selected 633 and 810 nm as the probe wavelengths to selectively monitor the singlet and triplet excitons, respectively. They estimated the diffusion constant for triplets in tetracene crystals with a time resolution of 200 fs, providing significantly more detail than the previous PL-based measurements.…”