Suitable structures for organic diode lasers must be integral to a low loss laser resonator and be able to support very high bipolar current densities. At the present time the strongest electroluminescence is achieved in diode structures where the organic semiconductor (OS) is sandwiched between two electrodes29.
Due to the low mobility of disordered organic semiconductors, the current flow through an OS is space charge limited. Child´s law (equation 2-8) predicts that the (unipolar) current density through the organic layer scales with the film thickness, df, according to jSCLC ∝df−3.
In 500 nm thick films up to 300 A/cm2 were obtained using 200 ns pulses of 150V 123. To obtain a current density in the kA/cm2 range, df should not exceed 200 nm 83, similar to half a
wavelength of light in the organic medium, λ 2nf . The optimum thickness of the organic semiconductor layer will be in the order of 100-200 nm. Since the best diode will most probably be a heterostructure, consisting of an emitter layer surrounded by electron and hole transport layers, the thickness of the electron-hole recombination zone, drec, is much lower
than that.
The problem for the optical design of an organic semiconductor laser is to minimize losses arising from the contact layers, while maintaining an excellent diode structure. Thin organic
29 Nevertheless, the recent discovery of ambipolar organic light-emitting field-effect transistors 274 might give
more flexibility in the design of future devices: in contrast to an OLED a bipolar FET structure has all of its electric contacts on one and the same side of the organic layer.
films are compatible with either slab waveguide lasers30 or microcavities. Optical losses arise from the penetration of the electric field of the laser mode into the absorbing electrodes. The potential anode materials are either ITO or a semitransparent film of a high workfunction metal like Pt or Au. Cathodes typically consist of low-workfunction metals or metal alloys, such as Al, Ag, Mg/Ag, or LiF/Al. A recent breakthrough in the development of OLEDs proved that ITO covered with a 5 nm thick transparent layer of bathocuproine can serve as efficient electron injecting electrode 126,275. Hence, both electrodes can be fabricated either from either a metal or ITO.
The most favorable design will have (in order of relevance): (i) a small overlap of the mode with the absorbing electrodes; (ii) as thin as possible a film of the OS;
(iii) a large confinement, Γ, of the mode within the recombination zone.
Fig. 9.2 shows the cross-sections of several potential organic diode laser architectures together with the corresponding optical mode profile, given by transversal distribution of the electromagnetic intensity |Ez|2. The structures (a)-(g) represent potential waveguide designs
while (h) corresponds to a λ 2 microcavity with two DBR mirrors. In a waveguide the evanescent field penetrates up to half a wavelength into the claddings so that -in the best case- the intensity is restricted to a layer with a thickness of ~λ nf 31. In the structures (a)-(d) the guided mode is essentially confined within the diode (including electrodes), whereas it extends well beyond the electrodes in (e)-(h).
Fig. 9.2 (a) shows a waveguide with two metal electrodes. Waveguide losses below 100 cm-1 require that the OS have a thickness of df≥ λ nf . In the case of MeLPPP (λ = 490 nm; nf =
1.7) this implies df≈ 300 nm, too thick for an excellent OLED. Fig. 9.2 (b) is the design of a
typical OLED consisting of a metal and a thick ITO contact (dITO≈ 100 nm). A waveguide is
formed irrespective of the thickness of the OS, thus allowing to optimize the diode setup independently. The mode pattern is slightly different if the ITO layer is at the interface to the substrate (standard diode) or to the air (inverted diode). Irrespective of the small difference, in both cases the largest fraction of light is guided in the ITO layer resulting in large waveguide losses. The losses in the ITO electrode can be strongly reduced if the ITO layer is made thin enough, say ~20 nm thick (structures (c) and (e)). Whereas the inverted diode requires a minimum thickness of the OS to form a low-loss waveguide, the standard diode can be arbitrarily thin. This happens, however, at the cost of a lower confinement of the mode in the OS and a larger overlap with ITO.
30 For DFB and DBR waveguide lasers also the grating must be considered. See Chapter 9.2.3.1.
31 In the following discussion the OS is considered as a homogeneous medium with a constant refractive index. Nevertheless, the confinement factor of the mode in the emitter layer as well as the total waveguide thickness can be optimized if the refractive index of the emitter layer is increased 106, e.g. by doping with TiO
x
104 9 Devices and applications of organic solid-state lasers
The structures in Fig. 9.2 (d) and (f)-(h) use ITO for both anode and cathode. Whereas thick ITO layers (d) induce significant optical losses in a multimode waveguide, thin ITO layers allow to fabricate waveguides with optical losses below 30 cm-1126 without restrictions on df.
Using a dielectric upper cladding (g) it would even be possible to fabricate a symmetric waveguide and to optimize the confinement factor of the mode in the recombination zone. Last but not least, the use of ITO in anode and cathode renders double-DBR microcavities possible (Fig. 9.2 (h)) –today´s lowest threshold organic semiconductor lasers 104. By proper design of the DBR stacks the overlap of the optical mode with the ITO layers can be minimized. Furthermore the thickness of the entire diode λ 2nf would be in the order of 150-200 nm, the optimum thickness for OLEDs.
Which of the above structures will be the best for electrically pumped organic diode lasers depends on the choice of the materials. Apart from (b) and (d) all of the structures in Fig. 9.2 exhibit moderate optical losses in to the electrodes. Criterion (i) can therefore be fulfilled. Assuming that it is more important to optimize the electrical properties of the diode than to maximize the mode confinement 32, the most suitable waveguide design is most probably the one of Fig. 9.2 (g), followed by (f), (e), (c), and (a).
32 According to equation 2-8 threshold current and voltage are related by V
th∝jth1/2df1/2 A reduction of the film
thickness by a factor 2 reduces the mode confinement Γ also by approx. a factor of 2. Consequently, the
(a) (b) (c) (d) (e) (f) (g) Metal Metal ITO Substrate ITO Metal Air / Substrate OS OS ITO Air Metal OS OS Air ITO ITO Substrate Metal OS ITO OS ITO Air Substrate ITO OS ITO Dielectr. Substrate ITO OS ITO (h) n1 n2 n1 n1 n1 n2
Fig. 9.2: Schematic cross-sections of several potential architectures for organic diode laser. The resulting optical mode profile is indicated by the gray-shaded area. (a)-(g) are designs for slab- waveguides, whereas (h) is a microcavity with two dielectric mirrors. In (a)-(d) the mode is essentially confined within the diode (including electrodes), whereas the mode extends well beyond the electrodes in (e)-(h). The contact electrodes can be either made from two metals (a), using an ITO anode and a metallic cathode (b), (c), (e), or with ITO for both electrodes (d),(f)-(h).