Abstract: We presentlow-cost texturing methods to produce differentsurface roughnesses on glass substrates. Using sand blasting, abrasion andwet etching we achieve roughnesses of about 50 nm to 250 nm (root meansquared roughness Rq). These textured substrates are used as extractionelements for guided modes and substrate modes in organic light-emittingdiodes (OLEDs). We evaporate 50 nm of the high index material Ta2O5 onthe textured substrate, which acts as waveguide layer, and flatten it withthe transparent photoresist SU-8. On top of that, we fabricate indium tinoxide (ITO)-free OLEDs, which are characterized by electroluminescenceand photoluminescence measurements. The devices with rough interfacesobtain an up to 37.4% and 15.5% (at 20 mA/cm2) enhanced emission and itis shown that the enhancement is due to an increased outcoupling efficiency.
OCIS codes:(250.3680) Light-emitting polymers; (310.6845) Thin film devices and applica-tions; (310.1860) Deposition and fabrication; (260.3800) Luminescence; (240.5770) Rough-ness; (290.5880) Scattering, rough surfaces.
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1. Introduction
Organiclight-emitting diodes (OLEDs) have raised much interest during the last decade. Therehave been OLED based displays being used in consumer electronics for a few years and thefirst steps towards efficient, long lifetime general lighting devices have been made [1]. Theadvantages of OLED technology lie in the low power consumption, the wide variety of col-ors, the possibility to use flexible substrates and the cost-efficient production. In 2009, Reinekeet al. showed a complex and optimized OLED stack emitting white light with an efficiencycomparable to fluorescent tubes [2]. However, commercially available OLEDs still lack theseefficiencies. While the conversion efficiency of injected electrons into photons (internal quan-tum efficiency) has reached almost 100 % over the last years [3], there is still a lot of roomfor improving the light extraction efficiency. The loss mechanisms in OLEDs are well under-stood [4–6]. The major part (about 80 %) of the photons being generated in the emission layercannot exit the device. About 50 % are trapped as guided modes in the indium tin oxide (ITO)anode and the organic layers or as surface plasmon polaritons at the metal/organic interface,where they are finally absorbed. Another 30 % of the photons are lost due to total internal re-flection at the substrate/air interface. Thus, only about 20 % of the generated photons can leavethe device as useful light [4,7,8]. There have been great efforts to extract the guided modes fromthe device. A well-known method is to corrugate the metal/organic or ITO/organic interface toextract the waveguide modes [9–13]. Other groups have reported on devices containing low-index aerogel layers between the emissive layer and the glass substrate [14] or low index gridswithin the device [15] to increase the extraction efficiency. There have also been approaches to
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A632
embed nanoparticles inside the organic layers to increase the internal quantum efficiency [16]or to scatterout guided modes and substrate modes [17]. By modifying the substrate/air inter-face with microlenses [18], meshed surfaces [19] or sandblasting [20] the substrate modes maybe extracted efficiently [21].
Here, we report on a device stack design and method, which may extract light trapped in thewaveguided modes and the substrate modes by scattering [22]. We textured the glass substratesurface with three different roughening methods. On top of the rough surface a highly transpar-ent Ta2O5-layer, which exhibits a refractive index ofn = 2.1, is evaporated. This layer acts asthe waveguide layer, confining the light due to its high refractive index. In order to smooth therough surface of this layer, a transparent photoresist layer with a smaller index of refraction isbrought on top. The ITO-free OLEDs that are fabricated upon these modified substrates possessan enhanced efficiency compared to OLEDs fabricated on flat substrates (but also featuring theTa2O5- and photoresist-layer) (Fig. 1).
This article is structured as follows. In section 2 we describe the three different fabricationmethods that lead to rough substrate surfaces. In section 3 we show how the rough surfacesare finished and characterized via atomic force microscopy (AFM). Section 4 deals with thefabrication and characterization of the ITO-free OLEDs.
Aluminum
Calcium
Super Yellow
PEDOT:PSS
SU-8
Glass
Ta O2 5
200 nm
50 nm
70 nm
80 nm
50 nm
Fig. 1. Schematic diagram of a structured OLED. Please note, that the lateral and verticallengths arenot true to scale.
2. Fabrication of rough interfaces
We use standard 1 mm thick soda-lime glass substrates that are cut into pieces of25 mm x 25 mm. Our structuring processes are sandblasting, abrasion and etching. Those threedifferent methods result in rather different surface topographies. Sandblasting is performed withan aluminum oxide abrasive (grain size 50µm, Girrbach Dental, Germany), which is shot ontothe glass surface with a pressure of 3 bar. The emerging surface is shown in the atomic forcemicroscope (AFM) image in Fig. 2(a). The root mean squared roughness is Rq = 250 nm andthe peak-to-valley roughness is about Rt = 1.4µm.
Roughening by abrasion is done with a grinding paste (METADI Diamond Polishing Com-
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A633
pound, grain size 3µm, BUEHLER, Germany), which is spread onto a large glass supportplate together with a droplet of dish washing detergent and a little bit of water. Then the glasssubstrate is put on top of the paste and lapped in the mixture using a ”figure-eight” pattern for15 minutes. The resulting surface topography is shown in Fig. 2(b). It features a Rq of 67 nmand a Rt of about 400 nm.
(a)
(b)
(c)
0 µm 40 µm
60 µm
7 µm
0 µm
0 µm
1.4 µm
0 µm
0 µm 60 µm
40 µm0 µm
0 µm
1 µm
Fig. 2.AFM images of roughened glass substrates textured via sandblasting (a), grindingpaste (b) and glass etching cream (c).
The third roughening method utilizes glass etching cream (Glasotan, CREARTEC trend-design-gmbh, Germany), which is applied onto the glass substrate for 10 seconds. The corre-sponding Rq and Rt are 150 nm and 550 nm, respectively (Fig. 2(c)). All roughness values arealso given in Table 1. After the roughening treatment the substrates are cleaned with acetoneand then with isopropanol in an ultrasonic bath.
3. Finishing and characterization of the rough interfaces
Since the roughnesses of the samples being structured either with glass etching cream or bysandblasting are too large and possess too many pronounced peaks, we polish those texturedsubstrates with a lapping disk on a lapping maschine (PHOENIX 4000 Sample PreparationSystem, diamond grain size 3µm, BUEHLER, Germany ) for 10 minutes. The sample, whichwas roughened with grinding paste, is not polished, as its surface did not exhibit sharp peaks.
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A634
The resulting surface of the sandblasted sample is shown in Fig. 3(a) and reveals a Rq of 200nmanda Rt of 900 nm.
The polished surfaces of samples structured with glass etching cream exhibit a Rq of 24 nmand a Rt of 96 nm (see Fig. 3(b)).
( )a
(b)
60 µm
0 µm
2.5 µm
0 µm
50 µm0 µm
0 nm
600 nm
Fig. 3.AFM images of the polished glass substrates textured via sandblasting (a) and glassetching cream (b).
Now the roughness is in a suitable range; the peaks have vanished and mainly dips are visi-ble (see Table 1). Next, the samples are coated with a 50 nm thick layer of Ta2O5. This is donein high vacuum by electron beam evaporation. Ta2O5 is transparent in the visible region andfeatures a high refractive index ofn = 2.1. It acts therefore as a waveguide layer, confining andseparating light from the cathode where a major part of light is usually lost. The evaporatedTa2O5 layer forms a very good replica of the glass substrate topography. The layer thicknesswas controlled during the evaporation process by an oscillating quartz crystal. SEM measure-ments made afterwards confirmed this thickness. Even after polishing, the roughness is still toohigh to fabricate an OLED on top of it. In order to smooth the surface, a 200 nm thick layer ofthe negative tone photoresist SU-8 (Microchem SU-8 2000.1) is spin coated on top of it. Af-ter a short prebake, the photoresist is illuminated with a uv-exposure unit (proMa TechnologieGmbH) at 365 nm wavelength. The resulting SU-8 layer is transparent and robust. Although theresulting surfaces are not completely flat (see Fig. 4), they are all suitable for OLED fabrication.
Table 1. Roughnesses Rq and Rt of all samples for each fabrication step.
roughened polishedSample Rq in nm Rt in nm Rq in nm Rt in nm
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A635
(a)
(b)
(c)
500 nm
0 nm
0 µm 50 µm
0 µm
0 µm
1 µm
50 µm
0 nm
500 nm
0 µm 50 µm
Fig. 4. AFM images of the photoresist-smoothed glass substrates textured via sandblasting(a), grindingpaste(b) and glass etching cream (c).
4. Fabrication and measurement of OLEDs
The finished substrates are cleaned with acetone and isopropanol in an ultrasonic bath for10 minutes, respectively. The surface of the photoresist is then exposed to an oxygen plasma tomake it hydrophilic and to remove organic residues. Afterwards, the polymer anode which is amixture of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS; CLEVIOSPH 750 purchased from H.C. Starck) and 5 % of dimethyl sulfoxide (DMSO) is prepared. It isprocessed by spincoating the solution at a speed of 3000 rpm onto the substrate. The thicknessof the resulting layer is 80 nm and it has a conductivity of about 650 S/cm. It is then partlycovered with a stainless steel mask and exposed to an oxygen plasma for 5 minutes to createthe anode pattern in the PEDOT:PSS layer [23]. All following fabrication steps are conductedin a glove box under nitrogen atmosphere. In order to eliminate the residues of water in thepolymer anode, the substrates are baked in a vacuum oven at 130° C for 30 minutes. On topof the PEDOT:PSS anode the emission layer is applied. It consists of phenylene substitutedpoly(para-phenylenevinylene) (Ph-PPV; also known as Super Yellow from Merck OLED Ma-terials GmbH). The Super Yellow is solubilized in toluene with a concentration of 3 mg/ml andthen spincoated at 1000 rpm resulting in a layer thickness of about 70 nm. The OLEDs are thenmetallized with 50 nm of calcium as the cathode and 200 nm of aluminum as a protective layer
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A636
(see Fig. 1).Since theoptical measurement setup is located in normal ambient conditions, the OLEDs
are encapsulated with an epoxy resin adhesive and a glass cover. J-V curves and the electrolu-minescence of all OLEDs are measured with a source-measure unit (Keithley SMU 236) andan integrating sphere (Gigahertz-Optik, UMBB-210) being connected via a multimode fiberto a spectrometer (Acton Research Corporation SpectraPro-300i) with an intensified charge-coupled device (Princeton Instruments PiMax:512).In Fig. 5 the J-V-characteristics of all structured OLEDs and an unstructured reference OLEDare shown. The onset voltage for all devices is the same (typically 2.5 V). Compared to OLEDscontaining an ITO anode this is about 1.5 V smaller, which is due to the smaller difference ofthe HOMOs (highest occupied molecular orbital) of PEDOT:PSS and Super Yellow. Since thelateral resistance of the PEDOT:PSS anode is higher than that of ITO, the slope of the curvesin the J-V-characteristics are smaller. There is a rather large variation in the J-V-characteristicsvisible which is due to the not exactly equally large active OLED areas and particularly dueto the different cross sections of the polymer anode. A possible reason for that might be thatthe surface is partially too rough for an efficient PEDOT:PSS anode resulting in a high voltagedrop along the anode.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
1
10
Voltage / V
Curr
ent D
ensity / m
A/c
m2
Reference
Sandblasting
Grinding paste
Glass etching cream
Fig. 5. J-V-characteristics of the structured OLEDs and an unstructured reference device.The OLEDsreveal almost the same onset voltage. The reason for the difference of thecurves for higher voltages is attributed to the different thicknesses of the PEDOT:PSS an-odes.
To investigate whether this affects the efficiency one has to examine the dependence of theluminous flux on the current density, which is given in Fig. 6.
From this graph, the improvement in the performance of the samples roughened by sand-blasting and grinding paste can be deduced to 37.4 % and 15.5 % at 20 mA/cm2, respectively.Evidently, the voltage drop along the PEDOT:PSS anode cannot account for the poor perfor-mance of the sample treated with glass etching cream in Fig. 5. The same argument is applicablefor the other, better performing modified samples, too. The origin of the increase in the lumi-nous flux must either lie in an improved internal quantum efficiency or in a better outcouplingefficiency due to light scattering within the substrate and the high index layer. Furthermore,there was no change visible in the emission spectra of the different samples.
The overall performance of the samples depending on the applied power is shown in Fig. 7.The behavior of the curves is comparable to Fig. 6. The sample which was sandblasted revealsthe highest light output, owing this to some extent to the best current injection (see Fig. 5).In order to find out more about the outcoupling efficiencies of the structured samples photolu-minescence (PL) measurements were performed. The devices were mounted on a goniometer
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A637
Fig. 6. Luminous flux as a function of the current density. Those samples which have beenmodified bysandblastingand grinding paste perform better than the reference sample. Thesample roughened with glass etching cream could not reach the efficiency of the referencedevice.
Fig. 7. Luminous flux as a function of the electrical power.
which wasrotated around the vertical axis. A UV laser (Newport, Explorer Scientific All SolidState UV Laser, EXPL-349-120-CDRH) was coupled into a UV optical fiber being attached tothe goniometer to maintain a constant spot size on the OLED while rotating the goniometer. Afixed multimode fiber was connected to the spectrometer described before. The structured in-terfaces could also increase the amount of exciting laser light coupled into the modified devicesand thus, the number of emitted photons. Therefore, we measured the specular and diffuse re-flectance of all samples in a spectrometer. The difference in the reflectance of the samples wasfound to be small. The results of the PL measurements are shown in Fig. 8.
Apparently, the samples with roughened interfaces exhibit higher outcoupling efficienciescompared to the reference device. Furthermore, there are small differences in the angular emis-sion profile. The sample which was treated by sandblasting reveals an almost Lambertian pro-file closely followed by the sample being treated with grinding paste. The sample which wasstructured with glass etching cream shows the best light output in forward direction, whereasat higher angles its performance forfeits as compared to the other structured samples. Since inthe PL measurement all structured samples revealed a higher outcoupling efficiency, the poorperformance of the sample roughened with glass etching cream is attributed to a drop in theinternal quantum efficiency. The reason for that might lie in the surface of the SU-8 smoothing
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A638
layer, which still might possess local areas where the roughness is too high to guarantee smoothorganiclayers.
Fig. 8. Angle resolved photoluminescence (PL) emission of the structured OLEDs and theunstructured referencedevice. The structured samples exhibit a higher intensity comparedto the reference sample. This proves that roughening inner surfaces in OLEDs increasestheir outcoupling efficiency.
We attribute the improved light outcoupling to both, waveguided modes and substrate modes[22].
5. Conclusion
We demonstrated three different methods for roughening glass substrates for ITO-free OLEDs:abrasion by sandblasting, abrasion by the use of grinding paste and wet etching with glass etch-ing cream. We showed that all three methods can produce substrate roughnesses, which aresuitable for improving the outcoupling efficiency of OLEDs by scattering waveguide modesand substrate modes. When driven electrically, OLEDs with rough interfaces made by sand-blasting and grinding paste revealed an enhancement of 37.4 % and 15.5 % (at 20 mA/cm2),respectively. This enhancement is mainly attributed to a better outcoupling efficiency, whereasthe surface roughness might also have a small impact on the internal quantum efficiency.
Acknowledgment
We acknowledge support by the Bundesministerium fur Bildung und Forschung (BMBF)within the NanoFutur project 03X5514. The authors thank Hans Eisler and the DFG Heisen-berg group ”Nanoscale Science” (Grant No. 442/3-1) of the Light Technology Institute (Karls-ruhe Institute of Technology) for the usage of the AFM. Furthermore, the authors thank HeikeStormer of the Laboratory for Electron Microscopy (Karlsruhe Institute of Technology) for hersupport in roughening glass substrates. B. Riedel and J. Hauss are pursuing their Ph.D. withinthe Karlsruhe School of Optics and Photonics (KSOP).
#135359 - $15.00 USD Received 20 Sep 2010; accepted 19 Oct 2010; published 28 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A639
