Observation of shot noise in phosphorescent organic light-emitting diodes

We employed a cross correlation method to study current noise in phosphorescent organic light-emitting diodes. The noise spectra revealed two frequency-dependent components. The first component displays 1/ f 1.3 dependence and correlates with the light emission of the devices. The second component is dominant in low-bias regime and varies as 1/ f 2.8. It is attributed to inhomogeneities of the barrier height at metal/organic interface. The extended bandwidth of the method allowed us to resolve frequency-independent term in the noise power, which was dominated by the shot noise. At bias voltages from 2.4 to 2.5 V, the Fano factor characterizing shot noise is close to one, confirming that the electron transport in this regime is limited by the carrier injection across metal/organic interface. At higher biases, in the regime where the transport is bulk-limited, the Fano factors drops to 0.5. Possible physical reasons for such behavior are discussed.