A photon promotes one chromophore in a 2D cross-section of the organic film to a singlet exciton S1. Singlet fission is a spin-allowed process that converts S1 into a spin-correlated triplet-pair state ¹(TT) on a neighboring molecule — no forbidden intersystem crossing needed, because the pair's overall spin character is still singlet:
S1 + S0 → ¹(TT) → T1 + T1
ΔE = E(S1) − 2·E(T1)
The fission rate depends on whether the reaction is energetically downhill or must borrow thermal energy:
k_fis = k0 if ΔE ≤ 0 (exothermic — fast, ~ps)
k_fis = k0 · exp(−ΔE / kB·T) if ΔE > 0 (endothermic — thermally activated)
The correlated ¹(TT) pair then either separates into two independent, spin-uncorrelated triplets (each free to diffuse and be collected as its own charge carrier) or recombines geminately back to the ground state. Free triplets random-walk across the film lattice via Dexter-type hopping and are extracted as charge at the bottom-row electrode.
- Photon flux — excitation rate per chromophore site.
- ΔE slider — tunes the driving force; negative (exothermic) gives fast, nearly temperature-independent fission, positive (endothermic) makes fission slow and strongly temperature-dependent.
- Temperature — sets the Boltzmann activation factor for endothermic fission and the triplet hopping rate.
- Triplet hop rate — how fast a free triplet random-walks toward the electrode versus decaying before collection.
Because one absorbed photon can yield two extractable charge carriers, a singlet-fission layer (e.g. pentacene or tetracene sensitizers on silicon) can push external quantum efficiency above 100% for high-energy photons — impossible in an ordinary single-exciton absorber, and a distinct mechanism from multiple-exciton generation in quantum-dot solar cells or hot-carrier extraction. Drag to pan the lattice, scroll/pinch to zoom.