Study of the Light Coupling Efficiency of OLEDs Using a Nanostructured Glass Substrate

We study theoretically the enhancement of the light extraction from an OLED (Organic Light-Emitting Diode) with nanoairbubbles embedded inside a glass substrate. Due to such a nanostructure inside the substrate, the critical angle which limits the light extraction outside the substrate from the OLED is increased. The theoretical results show that the nanoair bubbles near by the substrate surface can improve the efficiency of the light extraction by 7%. Such a substrate may also be suitable for photovoltaic cells or display screens.


Introduction
Over the last decade, organic materials have attracted much attention due to their huge potential of applications such as OLED (Organic Light-Emitting Diode) and organic photovoltaic cells.As promising materials, the enhancement of their efficiency has widely been studied [1][2][3][4].Although the internal quantum efficiency of OLEDs can be achieved near 100% [5][6][7][8], the light emitted by the OLED is largely wasted because of the total internal reflection inside the substrate.The most common substrate is glass substrate coated with an ITO (Indium Tin Oxide) layer.The ITO layer is used as a transparent anode electrode, but its refractive index is higher than that of organic materials.Therefore, much of light is trapped in the layer due to the internal reflection.Moreover, the light must again cross the glass-air boundary.This produces a total reflection of the light emitted from the organic materials beyond the critical angle.Therefore only small amount of light is coupled out of the ITO layer and glass substrate, and the rest is trapped inside the ITO layer and substrate as wave-guided modes.It has been reported that the external coupling efficiency of light from conventional OLEDs is only 20% [6][7][8].A number of studies have been done to improve the coupling efficiency from OLEDs or photovoltaic cells such as addition of a diffusive layer on the substrate [9], nanomesh electrodes [10], nanoparticles [11], and microlens array [12,13].Corrugated structure and a quasiperiodic buckling structure were also proposed with an excellent outcoupling efficiency [14,15].A low-index grid embedded in the organic layer and a monolayer of SiO 2microspheres are also suggested for this purpose [7,16].Recently, patterning nanostructures on the substrate surface has been developed in order to increase the light extraction efficiency in [17].This method may also be applied to OLEDs on glass substrate.However, as they are exposed to air directly, such structures are vulnerable to external shocks.So the structure can be destroyed or damaged if mechanical or chemical cleaning process of such substrates is applied, thus reducing the coupling efficiency.
In this work, we study numerically a nanostructure inside the glass substrate in order to enhance the coupling efficiency of OLEDs.The 2D air holes are positioned inside the substrate near the surface.The extraction efficiency of such a nanostructure in a glass substrate for the light emitted from OLED is studied numerically taking into account three main parameters: the period of air holes, the air hole diameter, and the depth of air holes from the substrate surface.The incident angle at which the light is escaped from the substrate is also investigated.The nanostructure embedded in the glass substrate proposed in our work can avoid any damage or deterioration of the structure due to mechanical or chemical cleaning process as the nanoair bubbles are protected in the glass substrate.

Nanostructure in Glass Substrate
The structure of an OLED studied in our work is illustrated schematically in Figure 1.The organic layer is made of Alq 3 (Tris(8-hydroxyquinolinato)aluminium) with a thickness of 120 nm and its refractive index is  3 = 1.7.The ITO layer is found on the organic layer with a thickness of 100 nm and a refractive index of  2 = 2.02.It is used to inject the current as the anode whilst the aluminium layer at the bottom ensures the current flow through the organic layer as the cathode.The glass substrate has an index of  1 = 1.5 and a thickness of 2 mm.The air holes with a unity refractive index are positioned near the substrate surface.The light emitted by the organic layer is collected in the air with a unity air index.
In fact, the typical structure consists of hole transport and electron transport layers.But we have neglected these layers for the sake of simplicity in our simulation as they do not affect much the coupling efficiency.
In order to investigate the external coupling efficiency of the light emitted from the organic layer, three main parameters are taken into account in the simulation: the period of air holes , the air hole diameter , and the depth of air holes from the substrate surface .In the simulation,  >  is maintained to avoid overlaps between the air holes. > /2 is also kept, so that the air holes remain inside the substrate.The angle  is the incident angle of the light emitted from the organic layer with respect to the normal of the substrate surface.We have adapted the RCWA (Rigorous Coupled Wave Analysis) method in 2 dimensions to our light coupling simulation [18,19].The emission wavelength of Alq 3 is known to be 520 nm at a thickness of 120 nm [20].The FWHM (Full Width Half Maximum) of the spectrum is 50 nm and the broad spectrum is integrated to calculate the OLED output intensity.The light transmissions of the TE and TM modes are all summed up on the incident angles between 0 ∘ ≤  ≤ 89 ∘ .The extraction efficiency is investigated from the light transmissions with and without the air hole structure.

Numerical Results
Figure 2 shows the light transmission as a function of the depth of air holes  with 3 different diameters  and a period of  = 2.5 m.The black straight line is the case without the nanostructure.This case is used as a reference with a unity transmission value in our work.From the figure, the maximum light extraction is obtained as 1.07 at  = 0.6 m and  = 0.8 m.It is clear that the nanostructure with these parameters allows increasing the light coupling by 7%.Other periods are also investigated in the simulation, but the light extraction is a bit lower than that with  = 2.5 m.These parameters should be used in order to fabricate the nanostructure in the glass substrate.
These parameters are also used to calculate the light transmission from the organic layer as a function of the incident angle .In Figure 3, the lights extracted from the OLED are shown with and without the nanostructure.The black curve represents the light extracted from the bulk substrate and the red curve represents the nanostructured substrate.In the case of bulk substrate, the light is transmitted up to 41 ∘ which is the critical angle   for extracting light from the organic layer through the ITO layer.Actually, this angle corresponds to the one calculated by Snell-Descartes Law in such a configuration with three refractive indexes ( 1 = 1.5,  2 = 2.02, and  3 = 1.7).For >   , the light is trapped in both the ITO layer and substrate as wave-guided modes.On the other hand, in the case of nanostructured substrate, 50% of the light is transmitted through the substrate until 61 ∘ .A small amount of light can still be extracted up to 88 ∘ whilst the light is trapped beyond 41 ∘ in the bulk case.Hence, the critical angle is increased to 88 ∘ which is twice greater than that of the bulk substrate case and the overall light intensity is gained due to the wide extraction angle from the nanostructured substrate.It is clearly seen that the nanostructure allows increasing the critical angle to extract more light.This is the promising results for the next generation of OLEDs and photovoltaic cells.

Conclusions
In this work, we have studied theoretically the improvement of the light coupling efficiency using air hole nanostructure inside a glass substrate.In the simulation, we have taken into account three parameters: the air hole period, the air hole diameter, and the depth of the air hole from the substrate surface.Our results show that the efficiency of light extraction increases by 7% due to the air hole nanostructure at specific parameters.Besides, the light emitted from the organic layer is extracted through the ITO glass up to an incident angle of 88 ∘ which is twice higher than that without nanostructure (41 ∘ ).In fact, the wide angle allows higher efficiency of light extraction.Such an embedded nanostructure may improve the efficiency of photovoltaic cells or the quality of display screens.Moreover, it would not suffer much structure deterioration in case of chemical or mechanical cleaning process.In order to realize such a structure, two techniques may be proposed.Femtosecond lasers have been used to create microchannels with a cross-sectional size of around 1 m in glass [21].Higher numerical aperture objectives of femtosecond lasers may be used to fabricate microchannels with a nanometer size.This technique would require only a femtosecond laser and some accessory components such as translation stages and femto-second lasers are now readily available in the market.Therefore, it would not require heavy process.Our first attempts on glass and sapphire show that laser ablation with an 193 nm nanosecond excimer laser is a promising technique for etching microstructures.Photolithography process could also be an alternative method.Microchannels can be etched on the surface of glass substrate.
Then the surface can be covered by a glass membrane by using wafer bonding technique [22].The nanostructure could be embedded in glass by using these techniques to enhance device performance.As this structure offers a wide angle of light extraction, it would be suited to wide angle applications.

Figure 1 :
Figure 1: Schematic illustration of the OLED structure used in our simulation.

Figure 2 :
Figure 2: Light transmission as a function of the depth of air holes  with a period of  = 2.5 m.

Figure 3 :
Figure 3: Light transmission as a function of the incident angle .