Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

Abstract

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.

Harvesting singlet fission for solar energy conversion via triplet energy transfer

“…Given the singlet exciton diffusion length of 12 nm (in the direction perpendicular to the a-b plane in Tc) 29 and the light attenuation length in Tc of l a B100 nm at hn 1 ¼ 2.43 eV (ref. 15), we estimate that 24% of the singlet excitons within the 20-nm film can diffuse to the interface with CuPc for charge separation and recombination, whereas 76% can undergo singlet fission. The percentage of the CT loss channel increases as the thickness of the singlet fission layer decreases.…”

“…The quantum superposition in Tc possesses a lifetime of 7 ps (refs 11,17) after which the triplet pair state decouples from the singlet. The triplet excitation energy in Tc is T 1 ¼ 1.25 eV (refs [15][16][17] and that in CuPc is T 0 1 ¼ 1.15 eV (ref. 18); the near-resonant but slightly downhill energetics should facilitate efficient triplet energy transfer from Tc to CuPc.…”

“…5c shows TR-2PPE spectrum for Tc only, without the CuPc overlayer. For Tc, there is an equilibrium between singlet and triplet pair states owing to the quantum superposition on short timescales (7 ps) and to triplet-triplet annihilation on longer times; the triplet population decays on this timescale mainly because of radiative recombination (fluorescence emission) [15][16][17] . Figure 5d summarizes the triplet populations as a function of pump-probe delays for the 2-nm/20-nm CuPc/Tc sample (red line), 2-nm/2-nm CuPc/Tc (blue line) and 20-nm Tc only (grey dots), all exited at hn 1 ¼ 2.43 eV.…”

“…The singlet fission Tc layer is on top, the CuPc layer is in the middle and the electron acceptor at the bottom. On the basis of the published absorption spectra of Tc thin films 15 , we calculate a light attenuation length in Tc of l a ¼ 100 nm at hn 1 ¼ 2.43 eV. The singlet exciton diffusion length in the direction perpendicular to the herringbone a-b plane of crystalline Tc is l d ¼ 12 nm (ref.…”

Visualization of exciton transport in ordered and disordered molecular solids

“…As fusion is a nonlinear process that requires the collision of two triplet excitons ( Supplementary Fig. 1), the triplet exciton density n T (x,t) is calculated from the measured singlet exciton density n S (x,t) using n T ¼ n 1=2 S (see discussion below) 26,27 .…”