Singlet Fission, Polaron Formation, and Energy Transfer in Indolo[3,2-b]carbazole Thin Films and Single Crystals

Abstract

The ultrafast excited-state relaxation dynamics of indolo-[3,2-b]carbazole (ICZ) molecules were investigated in thin films and single crystals. In the ICZ film, singlet fission (SF) was directly observed both from the upper excited singlet states S N within 480−850 fs and from the upper vibrational levels of the singlet state S 1 within 6−11 ps, which fully suppressed the intramolecular charge transfer (CT) process in S 1 . In the ICZ crystal, the exciton relaxation through SF and polaron formation compete in the ultrafast timescale (<30 fs), which is accompanied by a thermally induced broadening of the S 1 −S N absorption band in the transient absorption spectra. The triplet-state lifetime is shortened from ICZ crystals (>1 ms) to films (2 μs) due to SF with subsequent triplet−triplet annihilation, suggesting the impact of well-structured crystallinity and molecular packing on the exciton transport dynamics. The efficient SF together with well-characterized energy transfer and CT properties of ICZ solids extends the usage of ICZ-core-based organic materials in photovoltaic and electrochemical applications.

Singlet Fission from Upper Excited States of Bodipy Crystalline Film and Single Crystal

“…The herringbone molecular packing of the PhBDP single crystal (Figure S6) shows various dimer configurations. Therefore, the existence of the rise time can be explained in terms of highly efficient FRET between the neighboring PhBDP dimer moieties with mixed standing stacks . Since the structure of the crystal is much more regular than that of nanoaggregates in films, both incoherent and coherent energy transfer would be responsible for the shortening of the fluorescence lifetimes …”

Effect of Crystallinity on Endoergic Singlet Fission in Perylenediimide Single Crystals and Polycrystalline Films

“…Singlet fission (SF) is a spin-allowed process in which a molecule that has been photoexcited to its excited singlet state (S 1 ) interacts with a neighboring molecule in its ground state (S 0 ) to produce a correlated triplet pair state 1 (T 1 T 1 ) followed by decorrelation to form two free triplet excitons (T 1 ) . SF continues to attract considerable interest because pairing an ideal blue-light absorbing SF chromophore with a complementary red-light absorber raises the Shockley–Queisser limit for photovoltaic efficiency from 33 to 45%. , Many organic chromophores exhibit SF, ranging from small molecules to polymers and from polycrystalline films to single crystals. − One essential criterion for a useful SF chromophore is that it produces triplet excitons rapidly and in high yield. − Equally important, the energy of the triplet excitons should be high enough to match the band gap of silicon, thus capturing the blue photons of the solar spectrum that are degraded to heat in typical silicon solar cells. The SF rate in a chromophoric organic solid depends both on the SF energetics, where rapid rates are achieved when the energies of the singlet and triplet excitons, E S and E T , respectively, are such that E S ≥ 2 E T , and the electronic coupling between adjacent chromophores is favorable.…”