engleski

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

Measurements of the diffusion length L for triplet excitons in small molecular-weight organic semiconduc- tors are commonly carried out using a technique in which a phosphorescent-doped probe layer is set in the vicinity of a supposed exciton generation zone. However, analyses commonly used to retrieve L ignore microcavity effects that may induce a strong modulation of the emitted light as the position of the exciton probe is shifted. The present paper investigates in detail how this technique may be improved to obtain more accurate results for L. The example of 4 , 4 -bis carbazol-9-yl 1 , 1 -biphenyl CBP is taken, for which a triplet diffusion length of L = 16 4 nm at 3 mA/ cm2 is inferred from experiments. The influence of triplet-triplet annihilation, responsible for an apparent decrease in L at high current densities, is theoretically investigated, as well as the "invasiveness" of the thin probe layer on the exciton distribution. The interplay of micro- cavity effects and direct recombinations is demonstrated experimentally with the archetypal trilayer structure N , N -bis naphthalen-1-yl -N , N -bis phenyl -4 , 4 -diaminobiphenyl NPB /CBP/ 2, 9-dimethyl- 4, 7-diphenyl-1, 10-phenanthroline named bathocuproine, BCP . It is shown that in this device holes do cross the NPB/CBP junction, without the assistance of electrons and despite the high energetic barrier imposed by the shift between the HOMO levels. The use of the variable-thickness doped layer technique in this case is then discussed. Finally, some guidelines are given for improving the measurement of the diffusion length of triplet excitons in operational OLEDs, applicable to virtually any small molecular-weight material.

OLED; exciton; triplet; diffusion

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