Figure 1

The exciton formation and decay processes are shown for both optical (PL) and electric (EL) excitations. In the PL triplet excitons result from the intersystem crossing of the singlet excitons. In the EL triplet excitons are formed from capture of triplet electron-hole pairs. $\gamma$ is the triplet formation ratio relative to the singlet in EL.Reuse & Permissions

Figure 2

The time-resolved PL of MEH-PPV film. The data can be fit by a single exponential with lifetime 0.64 ns. The inset is the EA spectrum.Reuse & Permissions

Figure 3

The conversion efficiencies CEs for 50 and 100 nm LEDs are shown as a function of the bias $V$. CE rises rapidly as the recombination zone is pushed away from the metal cathode due to enhanced electron mobility at higher field.Reuse & Permissions

Figure 4

The triplet exciton density (EA) is plotted against the singlet exciton density (EL) as the bias of an LED is changed. The curve is highly nonlinear and nearly flat after the LED is turned on. Linear relation between triplet and singlet exciton densities for PL is also shown. PL vs PA is extrapolated to some extent to match EL because of the limited pump intensity after beam expansion. The triplet exciton densities (in mV) for EL and PL at the same singlet exciton density are directly divided to give $\gamma$. The data are shown in raw voltages recorded by the lock-in amplifier.Reuse & Permissions

Figure 5

The frequency dependence of the EA and PA are shown to be the same. Triplet lifetime can be identified to be about $100\mu \mathrm{s}$.Reuse & Permissions

Figure 6

The triplet exciton formation ratios $\gamma$ for two LEDs with 500 and 1000 Å MEH-PPV film thickness are shown. The error bars are determined by the values obtained from different LEDs and different masks (see text). They are the same function of electric field within experimental error.Reuse & Permissions