Singlets lead to photogeneration inC60-based organic heterojunctions

Abstract

Two independent and direct measurements of exciton transport in the fullerene C 60 unambiguously indicate that singlets are responsible for energy transport and ultimately charge generation in organic photovoltaic cells. The singlet exciton diffusion length, L D , was measured using fits to the external quantum efficiency of planar heterojunction photovoltaics, and via C 60 fluorescence, giving a mean value of 34 ± 3 nm. Direct measurement of the C 60 singlet state transient fluorescence decay gives a lifetime of 590 ± 10ps, from which we infer a diffusivity of 0.020 ± 0.004 cm 2 /s. This is at least ten times that typical for organics, which we attribute to the spherical symmetry of the C 60 molecule that promotes highly efficient exciton transfer. Further, L D is used to determine that the C 60 energy gap is 18 ± 5 meV larger than the analogous fullerene, C 70 .2

Submolecular Electroluminescence Mapping of Organic Semiconductors

“…The observed intensity decay lengths agree well with the nonradiative energy transfer reported for C 60 /C 70 nanocrystal domain boundaries on ultrathin NaCl films of 1.5−2 nm. 29 At room temperature, exciton diffusion lengths of 5 nm, 30−32 8 nm 33 and 34 nm 34 have been reported for solid C 60 . The smaller intensity decay lengths of ∼2 nm in our experiment may arise from the much smaller film thickness.…”

On-Surface Chemistry Induced by Long-Lived Excitons: (NO)2 Dissociation on C60

“…The slow channel can be described as follows: initial excitations, which are generated in the C 60 substrate far away from the (NO) 2 /C 60 interface, need to diffuse over a long distance to reach the (NO) 2 adsorbate. Singlet exciton diffusion lengths in C 60 were reported in the range between 30 and 60 nm, depending on the order of the film. , This is already larger than the thickness of the C 60 film grown here, and triplet diffusion lengths are expected to be even larger.…”

Intrinsic Measurements of Exciton Transport in Photovoltaic Cells

“…While capable of yielding a materials-relevant value of L D , this method is only sensitive to diffusive states that can decay radiatively and therefore cannot be applied to materials forming weakly emissive or dark excitons [34][35][36] . Since many high-performing active materials are weakly luminescent or dark 7,12,35,37,38 , it is essential to develop more general means to accurately probe exciton transport in working devices.…”