High Current Density in Light-Emitting Transistors of Organic Single Crystals

We measured the external electroluminescence quantum efficiency (${\eta }_{\mathrm{ext}}$) in light-emitting field-effect transistors (LETs) made of organic single crystals and found that, in the ambipolar transport region, ${\eta }_{\mathrm{ext}}$ is not degraded up to several hundreds $\mathrm{A}/{\mathrm{cm}}^2$ current-density range, which is 2 orders of magnitude larger than that achieved in conventional organic light-emitting diodes. The present result indicates the single-crystal organic LET is a promising device structure that is free from various kinds of nonradiative losses such as exciton dissociation near electrodes and exciton annihilations.

Figure 1

(a) Transfer characteristics of the ambipolar tetracene single-crystal field-effect transistor at negative and positive drain voltages (150 V), $V_D$. The channel width and length are 30 and $50\mu \mathrm{m}$, respectively. (b) Transfer characteristics of the ambipolar rubrene single-crystal field-effect transistor at negative and positive drain voltages (160 V), $V_D$. The channel width and length are 400 and $220\mu \mathrm{m}$, respectively.

Figure 2

Electroluminescence (solid red line) and photoluminescence (dashed black line) spectra of (a) tetracene and (b) rubrene single-crystal devices.

Figure 3

(a) Output characteristics and (b) corresponding light output of the tetracene single-crystal LET at different gate voltages, $V_G$.

Figure 4

External electroluminescence quantum efficiency (${\eta }_{\mathrm{ext}}$) vs drain current characteristics of (a) tetracene and (b) rubrene single-crystal LETs at different gate voltages. The upper horizontal axis corresponds to the estimated current density, when we assume the current flow to be confined to a 15 nm, which corresponds to approximately 10 monolayers of the organic molecules [25]. If we take a value of 1 nm as a thickness of accumulation layer, as reported in Refs. 16, 17, the estimated maximum current density reaches more than $2\mathrm{kA}/{\mathrm{cm}}^2$ and $4\mathrm{kA}/{\mathrm{cm}}^2$ for tetracene and rubrene LETs, respectively.