Single-Step Triplet-Triplet Annihilation: An Intrinsic Limit for the High Brightness Efficiency of Phosphorescent Organic Light Emitting Diodes

We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Förster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of $4,4^{\prime }\mathrm{\text{−}}N,N^{\prime }$-dicarbazole-biphenyl.

Figure 1

(a) Experimental decay of luminescence signal $L(t)$ and monoexponential fits for the sample with highest investigated guest concentration at two initial front-face triplet densities $n_{00}$. Excitation energy densities were $1.2\times {10}^{-4}\mathrm{J}{\mathrm{cm}}^{-2}$ and $1.4\times {10}^{-5}\mathrm{J}{\mathrm{cm}}^{-2}$. (b) Fit residues at $n_{00}=1.1\times {10}^{19}{\mathrm{cm}}^{-3}$ for fits with the single-step Förster transfer model ($R_F=5.2\mathrm{nm}$) and the multistep diffusion model (${\gamma }_0=9.0\times {10}^{-15}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$). (c) Fit residues at $n_{00}=2.4\times {10}^{18}{\mathrm{cm}}^{-3}$ for the single-step model ($R_F=5.2\mathrm{nm}$) and the multistep model (${\gamma }_0=8.5\times {10}^{-15}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$). The fit curves are not shown in panel (a) since they entirely coincide with the experimental curves on this scale.

Figure 2

Fit results for the Förster radius $R_F$ versus initial front-face triplet concentration $n_{00}$ for four different samples with different mol:mol concentration of guest molecules. The density of guest molecules in each sample is shown by vertical lines.