Figure 1
(a) Optical absorption spectra and chemical structures of Rubrene and C60. (b) Flat band energy alignment showing highest and lowest occupied molecular orbitals (HOMO-LUMO) and location of triplet states at the rubrene-C60 heterojunction. Note that the position of T1 in rubrene (1.14 eV) is very close to the triplet level of C60 (1.6 eV). (c) The detailed cycle of physical events occurring in rubrene thin films (shown as steps 1–5). A photon absorption event (Step 1) in rubrene is followed by singlet exciton fission (Step 2). Excitation decay to the ground state through two routes - via a prompt emission (Step 3) and delayed emission via triplet-triplet annihilation (Step 4), the non-radiative decay to ground state is also shown (Step 5). (d) In case of rubrene/C60- additional processes are: charge transfer/charge separation state formation (Step 3), a possible excitation back transfer from CT state to T1 (Steps 4 and 5). Finally, two triplets can recombine to generate a singlet via TTA (Step 7), which is identified as an energy up-converted emission processes (Step 9). The competing processes to this step would be CT-emission (Step 6) and non-radiative decay to the ground state (Step 8).
