Diffusion of triplet excitons in an operational organic light-emitting diode

Physical Review B - Condensed Matter and Materials Physics

Volumn 79, Issue 16, 2009, Pages

  Lebental, M ^a   Choukri, H ^a   Chenais S ^a   Forget, S ^a   Siove, A ^a   Geffroy, B ^b   Tutis E ^c  

^a CNRS   (France)

^b LABORATOIRE DE PHYSIQUE DES INTERFACES ET DES COUCHES MINCES   (France)

^c INSTITUTE OF PHYSICS   (Croatia)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 66249101979     PISSN: 10980121     EISSN: 1550235X     Source Type: Journal    
DOI: 10.1103/PhysRevB.79.165318     Document Type: Article

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50. A rough estimate of the exciplex energy, notwithstanding the exciplex binding energy and energetic disorder, is given by the difference between the TCTA HOMO level (-5.9 eV) and the BCP LUMO level (-2.9 eV) and corresponds to a wavelength of about 410 nm.
51. Triplet-to-singlet transfer might be possible if the donor exciton breaks up and reforms on the acceptor via incoherent electron exchange. However, as pointed out by Baldo, (Ref.), this has to be considered as very unlikely since the energy required for dissociation, i.e., the exciton binding energy, approaches 1 eV in most systems.
52. The emitted intensity from DCM and Ir (btp) 2 cannot be directly compared to each other due to their different quantum yields and concentration.
53. In Sec. 2, the triplet excitons diffuse from the CBP/TCTA heterojunction (plane x=0) with a G constant rate and an uniform distribution in the y and z directions. This three-dimensional (3D) problem can thus be solved as an one-dimensional problem thanks to the planar geometry of the source term, but the parameters such as the diffusion length and the diffusion coefficient are still the same (i.e., defined in 3D). In this configuration, the diffusion length is the same in the real three-dimensional system or in its reduction along the x axis. Thus we will only consider the latter case.
54. The quenching among triplet excitons in host is expected when they come close in terms of the Dexter radius, and similarly the quenching of triplet excitons and holes (polarons). This is since otherwise it is difficult to make the spin-flip required for triplet excitons to decay. This implies γTT and γPT being of the same order of magnitude. Therefore frequency of two processes is essentially given by the ratio of the concentrations of triplets and polarons γTP nP nT / γTT nT2 ∼ nP / nT. For typical values of current density, mobility, electric field, and molecular size (μp ∼ 10-4 cm2 /Vs, F∼1 MV/cm, J∼100 mA/ cm2, a∼0.6 nm) the concentration of polarons is of the order of nP ∼ 10-7 per molecule, while the creation rate of excitons or exciplexes at the heterojunction, for the same set of parameters, is estimated to ν∼ 103 s-1 per molecular cross section. The density of triplets nT is obtained upon considering the fraction α of excitons created being triplets, considering their diffusion over length L0, as well as accounting for their decay rate γT. For γT ∼ 106 s-1 and L0 ∼50a, the concentration of triplets per molecule turn to be nT ∼α× 10-4. This gives a plenty of room for the possibility that nT nP, corresponding to the situation where triplet-triplet quenching is much more frequent than the quenching of triplets on polarons.
55. A rough estimation of η∼ 10-2 can be inferred from the red part of the experimental spectra (see Sec. 2), assuming 20% efficiency in extracting photons and an isotropic emission over the upper half-plane. For J=1 mA/ cm2, this implies g∼10.
56. The only small disagreement around d=0 can be explained by excitons diffusing from the CBP/BCP interface as expected.
57. The thickness of the CBP layer was then 10 nm increased to minimize the changes in the optical field.