Electrically-driven organic lasers

The last example of this section deals with the optical function of hole-transport layers. In OLEDs some light is waveguided in the combined anode/organic layers due to total internal reflection [44], By increasing the thickness of the hole conductor the emitter can be optically decoupled from the contact. In a series of devices with increasing thickness of the hole conductor, deposited subsequently from solution, it was shown that a HTL thickness larger than 300 nm was necessary to accomplish this. As a very sensitive measure the threshold of amplified spontaneous emission (ASE) was used (Fig. 9.18, [45]). This is an important result with respect to a potential electrically driven organic laser diode. 

There are several prerequisites for the realization of an electrically driven organic laser diode. Aside from a low-threshold active organic material a suitable resonator structure incorporating electrical contacts has to be found. 

The challenge on the way to the electrically driven organic laser diode is therefore to find a laser material with a low-threshold and a quantum efficiency that is relatively independent of current density and electric field [14]. 

By combining electron-beam and soft lithography, including room-temperature nanoimprinting, one-dimensional planar photonic crystal (ID PhC) structures were realized with oligothiophene-5,5-dioxide 9 [77, 80], ID PhCs offer much more effective control of the device spectmm than external cavities and for this reason they are considered to be useful for the realization of electrically driven organic semiconductor lasers. 

---