Improving Light Outcoupling Efficiency for OLEDs with Microlens Array Fabricated on Transparent Substrate

Low light outcoupling efficiency restricts the wide application of organic light-emitting diodes in solid state light market although the internal quantum efficiency of the device could reach near to 100%. In order to improve the output efficiency, different kinds of microlens array on the substrate emission surface were designed and simulated using light tracing method. Simulation results indicate that the microlens array on the substrate could efficiently improve the light output efficiency and an enhancement of 1.8 could be obtained with optimized microlens structure design.Themicrolens array with semicircle shape using polymer material was fabricated on glass substrate by a facile approach. Finally, the organic device with microlens array substrate was manufactured and the light output of the devicewith surfacemicrolens structure could increase to 1.64 times comparingwith the devicewithoutmicrolens.


Introduction
Due to high power efficiency, simple fabrication processes, and wide-color covering, organic light-emitting diodes (OLEDs) are the focus of attentions in applications of display and solid state lighting [1,2].Because of the difference among the refractive indices of the substrate, anode, organic materials, and the air, two wave guiding phenomena become obvious in the anode/organic layers and substrate/air layers of OLEDs [3].Only 20% of the total emitted light from the organic layers can escape into the front side of substrate to form useful emission [4,5].Therefore the greatest potential for enhancing OLEDs external efficiency is to improve the light outcoupling from substrate.
Different kinds of microlens array on the substrate surface have also been employed in some researches and showed obvious enhancement in device efficiency.The microlens array using prepolymer NOA65 material was fabricated on the substrate which was prepatterned by microcontact printing of hydrophobic self-assembled monolayers, and the light outcoupling efficiency was improved by 24.5% without any apparent color change [16].Spherical microlens pattern using photoresist material was designed on the backside of OLEDs substrate by conventional etching method to minimize the total internal reflection loss at substrate/air interface, and an enhancement of 1.65 could be obtained with high refractive index glass as substrate [17].Flexible microlens arrays using a mixture of polydimethylsiloxane prepolymer by breath figure method were poured on organic substrate surface and a 34% of external quantum efficiency enhancement for OLEDs [18].An irregular hemispherical microlens system made of flat polyethylene terephthalate film with microcavity (stacks of SiO 2 and SiNx layers) was used for organic light-emitting devices and the external out-coupling factor of the devise increases by a factor of ∼1.8 with wide viewing angles [19].The numerical simulations of microlens array could also be used effectively in the infrared region with the ray tracing simulation method [20][21][22].
In this paper, a facile approach is proposed to enhance the light outcoupling for organic devices using traditional photolithography method.Transparent substrate with microlens array is simulated with ray trace software.The photoresist is spun coating on the substrate to form microlense array and then the light outcoupling results of organic device are discussed with normal and the modified substrate.With respect to many previously reported studies on microlense fabrication method, here the easy achieved method using polymer photoresist is proposed, and by which ∼85% enhancement of the light extraction for OLED is obtained.

Simulation
The mechanism of outcoupling enhancement with microlens array on substrate for organic device was proposed in Figure 1.Some light emitting from organic material and transmitting to glass-air interface would be reflected to glass because of the mismatch of refractive index and the incident angle higher than critical angle.With microlens array modification of the glass-air interface, the light emitting with large incident angle could be extracted out because the surface of glass was changed by the array.A portion of the light may reflect at the interface and back to the organic layer, but that portion could reflect at the metal organic interface and reach the glass-air interface again.Therefore, more light is extracted out due to morphology changing by the microlens array.The geometric structure and fill factor of the microlens array would affect the outcoupling of emission light.
In order to identify the influence of geometric structure and fill factor of the lens array to the light outcoupling efficiency, three-dimensional models of organic device with the structure shown in Figure 1 were setup and simulated with ray tracing method using TracePro software package.The structure of organic device was glass substrate (50 * 50 * 1 mm)/ITO (30 * 30 * 0.0001 mm)/multiorganic layers (30 * 30 * 0.00012 mm)/aluminum cathode (30 * 30 * 0.0001 mm).For simplicity, the metal cathode layer was regarded as ideal reflecting layer, and the absorption parameter of substrate material was set to zero.Refractive index of glass substrate was 1.5 [23,24], which was very near to that of the AZ series photoresist used to form microlens.Five kinds of microlens with different geometrical parameters were designed and simulated, and those parameters were listed in Table 1.The design of microlens size is dependent on some reported microlens results and the dimension of the microlens we fabricated.Types 1, 2, and 3 microlens arrays were different in the contact angle with substrate; types 1, 4, and 5 microlens array were different in the fill factor at the output plane surface of substrate.
Those organic devices with different substrate and same organic device structure were simulated with ray tracing method.Illuminance maps of the substrate output surface are shown in Figures 2(a), 2(b), and 2(c), corresponding to devices 1, 3, and 4, respectively.The maximum illuminance value is 442 lux for device 1, which is much higher than that of device 3 (321 lux).The light outcoupling efficiencies of different contact angle organic devices (types 1, 2, and 3) microlens array on substrate are 38.7%,34.8%, and 31.4%,respectively.As the contact angle of microlens with substrate is increased, light outcoupling efficiency is improved.But the microlens array with 90 ∘ contact angle is uneasy to fabricate in the realizing process, and feasible angle should be considered combining with the manufacture method.Comparing the microlens with same contact angle and different fill factor (types 1, 4, and 5), the light outcoupling efficiencies of device are 38.7%,38.5%, and 30.3%, respectively.The microlens fill factor on the substrate of type 1 is near to type 5, just the pattern arrangement difference, so that the outcoupling efficiencies of two devices are very comparative.For the device with lower fill factor (57.7%, device 5), the light outcoupling efficiency became obvious down to 30.3%.It's apparent that higher fill factor microlens array design would be the better choice to promote the light outcoupling.The illuminance value and uniformity of three organic devices could be obtained from Figure 2. The heterogeneity of light intensity in the substrate surface center (30 mm × 30 mm) is caused by the quantitative limitation of light ray used in the simulation process.Comparing with the output efficiency of 21.5% for normal glass substrate, adding microlens array in the glass substrate could efficiently improve the light outcoupling and an enhancement of ∼1.8 could be obtained from the simulation results with optimized microlens array structure on the substrate.

Microlens Fabrication and Device Performance
Before using photoresist as microlens material on the glass substrate surface, the transmittance character of the material in visible spectra area should be identified.The photoresist was spun coating on the glass substrate to form a unique thin film.Then the transmittance of photoresist film with glass substrate in visible region was tested by ultraviolet  spectrometer UV7000 and shown in Figure 3.The average transmittance of the film (including substrate) at 400 nm ∼ 700 nm region is over 95%, so that it's suitable for photoresist to act as microlens array.
The organic device with multiorganic layers structure was fabricated on the glass substrate with microlens array and the concise process was shown in  on the substrate in vacuum chamber Figure 4(e).The scan electron microscope image of the microlens was described in Figure 4(f), which indicated that microlens array with semicircle shape has been formed.The top view photo and 3-dimension image of the microlens array were also shown in the inset of Figure 3.The height and diameter of microlens are ∼2 m and 5 m, respectively, and the gap between the microlens is ∼0.5 m.The size of the microlens array is decreased equal proportion with the former simulated microlens type 1, because the thick film (50 m) material would absorb the emission light obviously and also thick microlens with hemispherical section is uneasy to control.In order to identify the improvement of light outcoupling performance of substrate with microlens array, organic devices with microlens array covered substrate and normal substrate were prepared.Using same white LED diode as light source, the illumination images of the LED through normal glass substrate and the substrate with microlens array are inserted in Figure 5. It's obvious that both the luminance and the emission area of the substrate with microlens array (b) are higher than the normal substrate (a).OLEDs were also fabricated on both substrates with the device structure ITO/NPB (40 nm)/CBP: (tpbi) 2 Ir(acac) (2 wt%, 30 nm)/BCP (10 nm)/Alq 3 (40 nm)/LiF (1 nm)/Al (100 nm).The luminance-voltage characteristics of two organic devices are presented in Figure 5.The luminance value of the organic device with microlens array on substrate is about 1.3 times than that with normal glass substrate at the same driving voltage.A ∼30% of maximum luminance enhancement for OLEDs could be obtained from the microlens array covered substrate.
Not only the luminance but also the emission area would increase from the test result shown in the inset of Figure 5.The organic devices using microlens array substrate and normal substrate (50 mm × 50 mm, the center organic emission area 30 mm × 30 mm) were fabricated with the former structure, and luminance in the center line luminance of both devices was demonstrated in Figure 6.The luminance value of device with microlens array on the substrate was obvious higher than that of device without microlens, and also the emission area extended to the margin of the glass substrate.The luminance integral area of the device covered with microlens array is 1.28 times larger than that of normal substrate device.So the total luminance integral square area of with microlens device is ∼1.64 times larger than that of without microlens.The microlens array on substrate surface could efficiently improve the light outcoupling from OLEDs.

Conclusion
In summary, a facile approach of fabricating microlens array on substrate is introduced to enhance the light outcoupling for organic devices using traditional photolithography method.Different kinds of microlens array are simulated with ray trace software and optimized microlens array model with an enhancement of 80% light output efficiency that is obtained from the simulation results.The microlens array with hemispherical type section and high uniform shape could be easily fabricated using photoresist material on glass substrate.Organic devices were fabricated with the microlens array covered substrate and normal glass substrate.The maximum luminance of organic device with microlens array is ∼1.3 times higher than that without microlens array covered substrate and the light output intensity is about 1.64 times comparing with the device without microlens.

Figure 1 :
Figure 1: Mechanism of light outcoupling enhancement with microlens array for OLEDs.

Figure 4 .Figure 3 :
Figure 3: The transmittance of photoresist film (including substrate) at different wavelength; the inset is top view photo and 3-dimension image of fabricated microlens array.

2 )Figure 5 : 2 )Figure 6 :
Figure 5: Luminance versus voltage character of two organic devices (with and without microlens array covered substrate); the inset is illumination images of the LED through normal glass substrate (a) and the substrate with microlens array (b).

Table 1 :
Microlens arrays with different geometrical parameters.