Optical Materials 34 (2012) 716–723

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Optical Materials

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Improving the contrast ratio of OLED displays: An analysis of various techniques

Ranbir Singh a, K.N. Narayanan Unni a,⇑, Ankur Solanki a, Deepak a,b

a Samtel Centre for Display Technologies, Indian Institute of Technology, Kanpur 208 016, Indiab Department of Materials Science & Engineering, Indian Institute of Technology, Kanpur 208 016, India

a r t i c l e i n f o

Article history:Received 27 April 2011Received in revised form 11 August 2011Accepted 17 October 2011Available online 15 November 2011

Keywords:Organic light emitting diodesContrast ratioPolarizerBlack matrix

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.10.005

⇑ Corresponding author.E-mail address: unni@iitk.ac.in (K.N. Narayanan U

a b s t r a c t

Organic light emitting diode (OLED) based displays have matured into commercial products. However,while we consider OLED for a low-cost high-resolution and high-contrast displays with a long life span,still there are performance gaps. This review addresses various techniques used for increasing the ambi-ent contrast ratio of OLED displays. There are techniques which are integral to the OLED device, such asblack cathodes and absorbing transport layers. In contrast, anti-reflection (AR) coatings and circular pola-rizer are applied externally to the device. This review provides a brief overview of each technique alongwith a discussion on its merits and demerits. The choice of a particular contrast enhancement techniquefor a display depends on the ambient where the same is intended to be used. Accordingly, for indoor andoutdoor applications, the best possible methods are suggested.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Organic light emitting diode (OLED) based displays exhibitmany interesting properties such as light weight, fast responsetime, low driving voltage, wide color gamut, wide viewing angle,high brightness and low power consumption [1–5]. As now-a-daysmany of the electronic devices with displays are targeted for out-door applications (i.e. cameras, telephones, music players, etc.), itbecomes important to solve the problem of contrast of the displayunder strong external lighting, more particularly under sunlight.The sunlight readability of display is still quite limited because ofthe strong reflection from the metal electrodes in the case ofOLEDs, i.e. OLED with a reflective (metallic) cathode (with bottomemission architecture) or anode (with a top emission architecture)results in reduction of the display’s ambient contrast ratio.

The new technologies that are emerging or have recently be-come common, must all be compared with the well establisheddisplay technology, the cathode ray tube (CRT). Motivation for amove away from CRT to newer flat panel technologies is necessi-tated by a demand for larger, therefore, thinner and lighter dis-plays, and lower power consumption. Nonetheless, in terms ofthe display quality, CRT still commands the best rendering of pic-tures. The color gamut is the widest and both OLEDs and LiquidCrystal Displays (LCDs) strive to achieve the same. Similarly, thecontrast, the subject of this review, is the best in CRT.

In CRT, the grey scales are controlled by the electron flux emit-ted from a cathode in an electron gun. This control is executed by

ll rights reserved.

nni).

manipulating the voltage at control grid (Grid 1, which is one of theanode grids and is a standard name used in CRT). Application of alarge negative bias to Grid 1 reduces the electron flux to nearlyzero value and transition to this value is rather sharp uponreducing the voltage at Grid 1. Consequently, a pixel has nearlyno optical emission at the zeroth grey level, besides due to opticalcross-talk, if any. Thus, a black screen (pixels at zero grey level) haslittle or no emission, leading to a high contrast ratio in CRTs.

In comparison, a LCD uses a back light, and any leakage of thislight through the liquid crystal in the display set at zero grey levelresults in a poor contrast ratio. The voltage across the liquid crystalcontrols the transmission of the back-light through the pixel. Thetransmission between the low voltage (zero grey level) and a highvoltage (highest grey level) is in form of a S-curve, with nonzeroleakage at the zero grey level. Consequently, the contrast ratio inLCD remains poor, forcing the technology to seek additional mea-sures such as dynamic contrast ratio which is an overall contrastratio that a display can produce over time. When a dark image isto be displayed the display would reduce the back light of the pixelor decrease the aperture of the projector’s lens using an iris but willproportionately amplify the transmission through the LCD panel.

OLED displays, show much better dark room contrast, butperform poorly in day-light readability. This paper discusses thecauses and measures that can improve the contrast.

Before discussing various approaches to improve the contrast inOLED displays, it will be useful to define the ambient contrast of adisplay. The ambient contrast of a display can be expressed as a ra-tio of the brightest and darkest element of the display, taking intoaccount the ambient light reflected from it. For liquid crystal dis-plays, having a white backlight source, this contrast is related to

R. Singh et al. / Optical Materials 34 (2012) 716–723 717

the transmittance values of the on and off pixels, whereas for lightemitting devices, the transmittance is replaced by the luminance ofthe brightest and darkest pixel. The ambient contrast ratio (whichwill be referred to as contrast ratio (CR) or just contrast for theremaining part of this paper) can be expressed as [6]:

CR ¼ LON þ RDLambient

LOFF þ RDLambientð1Þ

where LON and LOFF are the luminance values from lighted and darkregions of a display, respectively, Lambient/a (a is defined further be-low) is the ambient illuminance on the display, and RD is the lumi-nous reflectance of the display, given by:

RD ¼R k2

k1aVðkÞRðkÞSðkÞdkR k2

k1aVðkÞSðkÞdk

¼R k2

k1aVðkÞRðkÞSðkÞdk

Lambientð2Þ

where V(k) being the photopic curve (an eye sensitivity spectrumstandard defined by Commission Internationale de l’Éclairage(CIE)) [7], S(k) spectral irradiance on the display from a standardsource in test conditions (for contrast measurements CIE standardsuch as D65 source is used). Thus, for R(k) as the spectral reflectancefrom the display, aR(k) S (k) represents the reflected spectral radi-ance from it due to ambient light, where a geometric factor a con-verts radiant exitance to radiance. According to the VESA 308-2illumination standard [8], the CR of a display is measured at anambient illuminance of 500 lx.

The reflected spectral radiance due to ambient light consists ofspecular and diffuse components. Specular reflection refers to lightthat is reflected at an angle matched to the incident light. The inci-dent light has its origins in a distant point source such as the sun oran incandescent light bulb. Diffuse light is typically the light re-flected from surfaces, such as walls and ceilings or extendedsources. There are different strategies to deal with specular or dif-fuse ambient light.

As seen in Eq. (1), because LON > LOFF, always LONþRDLambientLOFFþRDLambient

< LONLOFF

,which implies that in a dark room (Lambient = 0) the contrast ratiois higher than in lit conditions and no contrast is available whenRDLambient is much larger than LON. This dark room CR is limitedby the darkness of the off pixel which can be nearly zero in OLED,unlike in the case of a liquid-crystal display which exhibits leakagefrom the backlight. It is important to note that often display man-ufacturers report the dark room CR as the display CR, which obvi-ously is a large value. However, with typical light levels fordifferent environments given in Table 1, the contrast in actualviewing conditions is much lower; thus reporting only the darkroom CR can be misleading to a viewer. But, technically it can bea good indicator of quality of display with respect to LOFF value,which may be related to cross-talk in display as elaborated furtherin Section 2.2.

Eq. (1) indicates, the strategies for improving CR can be basedon (a) improving LON, (b) reducing LOFF and (c) reducing RD underthe ambient conditions. Ultimately, what value of CR is adequatedepends on the viewer, but a value in few hundreds is generallyacceptable for even televisions in indoors viewing conditions. Out-doors (see Table 1), for mobile phone like displays, a CR of 3–4 isreadable in shade, 10 is readable well in sunlight and 20 is excep-tional [9].

Table 1Typical ambient light levels of different environments.

Environment Ambient light level (lx)

Home 100Office 500Heavy outcast day 2000Sunny day 100,000

2. Approaches to improving contrast ratio

2.1. Enhancing LON

The luminance of a device (LON) can be improved by two basicapproaches, involving increased internal radiative efficiency andout-coupling of light. The external quantum efficiency (gexternal)of an OLED device is related to the internal quantum efficiency(ginternal) and the external coupling efficiency (gcoupling) accordingto the following relation [13];

gexternal ¼ ginternal � gcoupling ð3Þ

In the early days, devices were based on fluorescence, with low effi-ciency. There have been numerous attempts to increase the bright-ness of the devices by employing several techniques to improve thisefficiency. Among these, notables are using highly efficient phos-phorescent materials (see review in [10]) and modifying the devicestructures, for example, P–I–N diodes with doped transport layers(see review in [11]) and tandem structures [12]). With these re-views available of the subject, the approaches to improve internalquantum efficiency are not discussed any further. In spite of devicesthat are nearly 100% efficient, unfortunately most light still remainstrapped in the OLED device. Out-coupling this light, thus, representsa major challenge. Thus, we briefly also indicate the literature onthis subject.

External coupling efficiency of the devices has been improvedby light out-coupling techniques, such as use of scattering medium[14,15], micro-lens arrays [16,17], photonic crystals [18–21],microcavity effect [22–25], surface modification methods [26–28]and surface plasmon for enhancement [29–31]. In certain cases,the luminescence efficiency of the device could be improved bymore than 50%. The readers are also referred to reviews [32,33].

2.2. Reducing LOFF

In off condition, no emission of light from a pixel in a display isdesirable. However, unintended emission in displays arises fromoptical and electrical cross-talk, the latter being more importantin OLED displays. This issue is especially pervasive in passive ma-trix addressed displays. The electrical cross-talk occurs becausewhen current is input to a pixel, unintended forward current flowsthrough other pixels on account of reverse bias leakage currentpathways through the OLED elements because of shared elec-trodes. This results in unintended emission with correspondingloss of contrast.

Work in our group shows [34], in dc conditions, as a best case,when one row is selected and only one pixel in it is addressed bykeeping rest of the columns open, the reverse leakage current al-lows current flow through the non-selected pixels.

According to the calculations, if the reverse saturation currentreduces by an order of magnitude, the electrical cross-talk is con-siderably reduced. Thus, key to reducing loss of CR due of electricalcross-talk is OLED of high on–off ratio. However, alternative strat-egy is also possible through display driving techniques. If a specificbias is applied to all non-selected rows, the electrical cross-talk isfurther reduced. In general, the larger the number of pixels in a dis-play, the greater will be the electrical cross-talk.

Unfortunately, however, even if the electrical cross-talk is re-duced to zero because of high on–off ratio of the OLED or biasingthe non-selected lines, under dynamic conditions, which repre-sents real conditions of operation in a display, the capacitanceassociated with the OLED allows electrical cross-talk [35]. Impactof this can be significant, and is mitigated by addressing a rowfor as long as possible in the driving conditions.

718 R. Singh et al. / Optical Materials 34 (2012) 716–723

Another approach for improving contrast is based on reducingthe reflection (RD) from the display. Despite the importance of thistopic, there are only very few articles covering the progress madeon this front [36,37]. Thus, in the following, we focus our attentionon the attempts to increase the CR by reducing the ambient lightreflection (RD) from a device/display.

2.3. Reducing RD

In the case of a bottom emitting OLED, the devices have severallayers of organics sandwiched between a transparent anode and ametallic reflecting cathode. The top emitting OLED, instead, willhave a transparent cathode. In more common bottom emitting dis-plays, typical metallic cathode (such as Ca, Mg/Ag, and LiF/Al) has ahigh reflection of the ambient light, resulting in low CR values.

Improving CR also improves the perceived brightness of an im-age. This is due to the effect known as inhibitory simultaneous con-trast which helps to improve legibility of the device withoutincreasing the brightness at the expense of power.

Eq. (1) suggests that significant reduction in luminous reflec-tance (RD) of OLEDs is crucial to increase their contrast. Therefore,several approaches are discussed below to reduce the reflectanceand increase the CR.

2.3.1. Antireflection coatingsAntireflection (AR) optical coatings have long been developed

for a variety of applications including glass lenses, eyeglasses, la-sers, mirrors, solar cells, infra red (IR) diodes, multipurpose broadand narrow band-pass filters, architectural and automotive glassand displays such as cathode ray tubes, as well as plasma, liquidcrystal and flat panel displays [38–41]. Anti-reflective film on thesurface of a display is used in order to prevent reflection of externallight. Various types of AR coatings [42–44] have been theoreticallyand experimentally investigated. Most of the work, however, hasbeen done on achieving the AR effect either for a single wavelengthor for a range of wavelengths. AR coatings are an excellent way toimprove transmission, reduce glare and improve readability.

Coating the AR thin film on the glass substrate creates a doubleinterface, which generates two reflected waves, that is, one at air/AR layer boundary and another at AR layer/glass boundary. Fig. 1shows the schematic diagram of the phenomenon of anti-reflection(AR) coating using single-layer. Light incident on AR-coating istraveling from low to high refractive index medium. If the thick-ness of AR-coating is k/4, where k is the wavelength in question,the relative phase-shift between the two-reflected waves at theupper and lower boundary of thin film is 180�. Hence, in the pres-ent scheme, light wave reflected at the AR coating-glass boundarywill experience a p-phase change with respect to the light wave re-flected at air-AR coating interface due to the path difference

AR Coating nAR coating = (nair × nglass)1/2

nair=1

nAR coating = (nair × nglass)1/2

nglass = 1.55

Transparent Anode

/4

Ambient LightWaves are canceled out due to destructive interference

Hole Transport Layer

Emitting Layer

Electron Transport Layer

Cathode (LiF/Al)

λ

Fig. 1. Schematic diagram of the phenomenon of anti-reflection (AR) coating.

between them. Thus, destructive interference between the two re-flected waves takes place. For complete cancellation, the optimumrefractive index of the coating is calculated by:

nAR coating ¼ ðnair � nglassÞ1=2 ð4Þ

where nair,, nglass are the refractive indices of air and glassrespectively.

Effect of a single layer AR coating is generally limited to a singlewavelength and so it can be useful in the case of a monochromaticreflection. If we want to eliminate reflection of ambient white light(380–780 nm), then we have to use double or multilayer ARcoating.

2.3.1.1. Single Layer AR coatings. Magnesium fluoride (MgF2) iscommonly used for single-layer antireflection coatings. MgF2 coat-ing is almost ideal because of its refractive index (1.38 at 550 nm)and high durability. Note that it is common in design of optical sys-tems to use 550 nm as the design wavelength, that is, in the middleof visible spectrum. For example, design of liquid crystal materialsthickness in a LCD display may be based on this wavelength. Sim-ilarly, for antireflection coating limited by a choice of single layer,the design for best performance may be at 550 nm, with an expec-tation that design will also work for additional spectrum around it.

A single-layer, k/4-thick magnesium fluoride film can act as abroadband coating (400–750 nm) that can be used for substrateswith a refractive index ranging from 1.45 to 2.4. Furthermore, thiscoating is less sensitive to angular and spectral variations com-pared to many multi-layer dielectric coatings [44]. All these fea-tures, along with the ease of deposition, make MgF2 a suitablecandidate for AR coatings.

2.3.1.2. Multilayer AR coatings. In multilayer AR coatings, more thanone layer of coating is applied as a combination of high and lowrefractive index materials. Layers are designed in such a way thatthe reflectance is very low in a certain wavelength range, for in-stance within the visible spectral region (380–760 nm). Baumeister[45] computed reflectance for anti-reflection coatings designed fora wavelength range. If k0 corresponds to wavelength of the centralfrequency in this range and the layers in coating are 1/4th of thisvalue in optical thicknesses, then several comments can be madein situations when refractive indices of individual layers are se-lected such that reflection response is like Chebyshev polynomial.Baumeister shows that for a double layer structure on a substrateof refractive index 4, overall reflection is suppressed with maxi-mum reflection at k0. By increasing the number of layers of specificrefractive indices, the reflection could be further reduced over awide range of wavelengths. However, this method may not be use-ful for displays made on glass of refractive index 1.55 as the anti-reflection coating materials will have to be a series of muchlower refractive indices corresponding to Chebyshev response.Availability of such materials will be limiting. Further, as a conse-quence, the OLED light would also not effectively out-couple.

Gibson et al. [46] measured the performance of a broadbandanti-reflection coating designed to minimize glare by coating sixlayers of Nb2O5/SiO2 on a plastic substrate material. The reflec-tance in the visible range was suppressed considerably.

In specific context of OLEDs, Yang et al. [47] have demonstrateda high contrast top emitting OLED structure with a combination ofthin layers of LiF and TeO2 as AR coating; further they have ensuredthat anode is only partly reflecting. Overall, this device exhibited areflectance of only 3.9% with a standard illumination source, whichis comparable with a bottom emitting device with a circular pola-rizer. As is common, a top emitting device with no provision for ARlayers has a lower current efficiency than a similar bottom emit-ting device. But, their top emitting device with AR coating when

R. Singh et al. / Optical Materials 34 (2012) 716–723 719

compared with bottom emitting device with circular polarizer wasmore current efficient. However, color distortion in a display is alsoaccompanied by adopting this strategy.

These, AR coatings can be thermally evaporated. Or the lami-nates for anti-scratch could also have antiglare properties. How-ever, this method works well to reduce reflection from the glasssurface on which it is applied. Typically 4% light is reflected froma bare glass. But, in OLEDs the major difficulty is not posed by thisreflection, rather it is the reflection from the cathode, which is notsufficiently reduced by AR coatings. These, AR coatings, therefore,generally would only be useful when either illumination is toohigh, as in outdoors conditions, for even 4% reflection from theglass may be significant or in specialized situations as discussedfor a top emitting diode.

2.3.2. Circular polarizerCircular polarizer for reducing reflection is a technology bor-

rowed from the LCD industry [6,48,49] for improving contrast ofOLED displays. This approach does not require introduction ofnew layers in the OLED structure; instead a lamination is pastedonto the display glass.

Circular polarizer is an alternative to AR coating on the surfaceof a flat panel display. Fig. 2 illustrates the basic principle behindusing a polarizer to reduce reflections. Unpolarized light on theupper left goes through a linear polarizer, polarizing the light per-pendicular to the direction of propagation. The light then goesthrough a quarter-wave phase retardation film and becomesright-circularly polarized. Circularly polarized light changes orien-tation when it bounces off a reflecting surface, that is, the reflectedlight becomes left-circularly polarized. When the light goesthrough the quarter-wave film again, it reverts to linear polariza-tion, but this time in an orientation which is not allowed by the lin-ear polarizer. The linear polarizer then blocks the reflected light.

The advantage of using circular polarizers in displays is thatonly the reflected light is nearly blocked. The light emitted by OLEDcan be transmitted, but only with linear polarization. Unfortu-nately, in this process nearly 50% of OLED light is also cut out. Ineffect circular polarizer still works to improve CR because it cutsmore reflected light than the device emitted light. But also note cir-cular polarizers are expensive and generally not bendable for usein flexible displays.

While both circular polarizer and AR coatings are applied out-side the device, on the substrate surface, we now discuss methodsfor increasing contrast which involve structures integral to theOLED.

2.3.3. Black cathode structuresSince the major reason for loss of contrast is reflection from the

cathode, it is natural to conceive of an electrode that does not re-flect light. Therefore, two types of black cathode structures have

Fig. 2. The mechanism behind circular polarizer.

been used. First one relies on a light absorbing material as a cath-ode or a cathode buffer. Whereas, the other employs principle ofdestructive interference between light waves reflected from areflecting cathode and from a semi-reflecting buffer layer.

2.3.3.1. Absorbing cathode. Fig. 3 shows a cross sectional view ofhigh contrast OLED with black cathode. A conducting and lightabsorbing contact is used to reduce the ambient reflection. Theuse of a highly conductive black carbon film in multilayer cathodesystem has been demonstrated by Renault et al. [50]. This blackcathode consists of a thin electron injection layer of Mg, an opti-cally absorbing and electrically conductive carbon layer, and athick Al layer. This multilayer black cathode has a similar chargeinjection property similar to a conventional cathode made of Mg/Al, but it has a much lower reflectivity. The results show that thereflection reduces to 58% compared to devices using conventionalcathode. However the effect of including the carbon layer in the de-vice structure on the device life time is not mentioned and lightoutput measured by current efficiency nearly halved.

The required properties of a light absorbing layer which is to beplaced between the reflecting cathode and electron injection layer(EIL) are high electron mobility, low work function and dark inappearance. Calcium hexaboride (CaB6) is one such material [51]but it could not be incorporated into an OLED structure becauseof the elevated evaporation temperature and subsequent decom-position of the material as reported by Hung and Madathil [52].In absence of a single suitable absorbing layer, black cathode struc-ture with a mixture of metals (Ca, Mg and Ag) and organic materi-als like Tris(8-hydroxyquinolinato)aluminium (Alq3) has been usedto reduce the ambient reflection [53].

A co-evaporated cathode consisting of SiO and Al has been re-ported to be effective as a black cathode [54]. But the luminousefficiency decreased and the operating voltage increased due tothe voltage drop across the SiO + Al layer.

A cathode made of alternating Al–Ag layers was found to behighly absorbing. Each pair of Al–Ag layer was in the form ofAl2O3 nano clusters embedded in an amorphous charge conductingnetwork of silver. These nano clusters were instrumental inabsorbing and scattering the ambient light effectively [55]. Hybridcathodes consisting of semitransparent cathode layers, a passiv-ation layer and a thick light absorbing film have been used to in-crease the contrast ratio [56]. But in this case, at higheroperating voltages, the luminescent efficiency decreases, and nomention is made about the life time of these devices.

Lau et al. [57] have compared the reflectance and contrast ratioof devices with two different cathodes Sm:Ag and Mg:Ag. The CRfor device with Sm:Ag cathode was eight times better than the tra-ditional device with Mg:Ag cathode at an ambient brightness of140 lx. In this case, the luminescence efficiency reduced with theblack cathode, but less than in the cases discussed so far.

A top emitting OLED with high contrast has been developed byusing Au as the anode, copper phthalocyanine (CuPc) as the holeinjection layer (HIL) and Sm as the cathode along with a cathodebuffer layer made of Alq3[58]. The high contrast is attributed tothe moderate reflection of Au at 380–550 nm, low reflection of

Glass

Transparent anode

Organic Stack

Light-absorbing Layer

Metal Cathode

Electron Injection Layer

Fig. 3. Schematic diagram of a bottom emitting OLED with a light-absorbing layer.

720 R. Singh et al. / Optical Materials 34 (2012) 716–723

Sm in the visible range, and high absorption of CuPc at600–700 nm. A low reflectivity cathode has been realized by co-evaporating Pb and LiF [59]. The reflectivity of this cathode is foundto be five times less than that of an Al cathode. A green top emit-ting diode has been reported with low reflection Sm/Ag cathode[60]. Additionally a 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthro-line (BCP) was used to increase the out coupling of the emittedlight.

Since a solar cell, as opposed to OLED, requires light absorbingmaterials, Yang et al. [61] integrated a photovoltaic cell behindthe OLED cathode. In this way, the absorbed light, which improvedOLED contrast, could also be gainfully converted into electrical en-ergy. Further, part of the light from OLED which also gets absorbedby black cathodes, could also be used in the same fashion. This rep-resents an interesting device architecture, but it is not obvious if ithas a clear application. Nonetheless, this device exhibited an ambi-ent reflectance of only 1.4%, which was superior to that achievedwith a polarizer. However, the luminescent efficiency decreasedto 50% of the value for a conventional OLED with the samematerials.

Recently a black cathode consisting of a thin Al layer (10 nm),an organic–metal light-absorbing layer (100 nm) and a thick Allayer (100 nm) has been reported [62]. The best performance ofthe organic–metal light-absorbing layer is obtained using a mix-ture of 25% CuPc, 25% 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and 50%Al. The reflectance could be reduced from 66.2% in a conventionaldevice to 11.3% for a device with the multilayer cathode. In thiscase, the diode performance was not adversely affected due tothe black cathode. However, it may be noted that the black cathodehas maximum absorption in the green and the device emission isin red. Hence this black cathode should be used for a green deviceto ascertain how much loss of luminance is happening due theblack cathode.

Fig. 4 compares the reflectance for different absorbing cathodes,which could be reduced to less than 20% and much lower. Whilethis approach of using a black cathode is relatively simple andstraight forward, it has a serious drawback. In a conventional de-vice, the reflecting electrode serves the purpose of also out-cou-pling the emitted light that is directed toward it (away from theviewing side). That is, ideally, while we would like the ambientlight to not be reflected by this electrode, we still would prefer thatOLED light is indeed reflected by the same electrode. The circular

Fig. 4. Comparison of the ambient light reflections from OLEDs using differentabsorbing cathodes [ Ref. [53], Ref. [56], Ref. [57],Ref. [58], Ref. [61] and Ref. [62]].

polarizer could ensure that by changing the direction ofpolarization. But, in process nearly 50% of useful emission wascut out. But, in case of black cathode, since it prevents reflectionboth of ambient and OLED light, unfortunately, nearly 50% usefullight gets cut out in this case also.

2.3.3.2. Black cathode with destructive interference structure. Thesecond approach for black cathode enhances the destructive inter-ference of the reflected ambient light. Recently, various black cath-odes based on the principle of destructive optical interference havebeen developed to reduce the reflectivity of cathode and, thereby,to increase the contrast ratio of the OLED. Both metal-inorganic-metal (MIM) and metal–organic-metal (MOM) structures havebeen used for fabricating these black cathodes [63–66]. They com-prise of a thin semitransparent metal layer, a phase-changing layerand a thick reflective metal layer. The component of the light wavereflected from the thin semi-transparent metal layer and the onereflected from the thick reflective cathode interfere destructively,as the latter has a p-phase difference with respect to the formerdue to the phase changing layer. In MIM structures the commonlyused phase changing layers are transparent inorganic materialssuch as oxygen deficient ZnO [64]. Commonly available organicsemiconductors like Alq3 [66], Al doped CuPc [67] or a mixture ofAlq3 and C60 [68] have been used as the phase changing layer inMOM structures. Xie and Hung [64] have demonstrated a high-contrast OLED with a low-reflection cathode. The black cathodewas constructed by depositing a multilayer structure over a semi-transparent layer of Sm. Compared to 81% of luminous reflectanceof a conventional OLED with an Al cathode, the luminous reflec-tance of the OLED with the black cathode can be reduced to 2.7%with Sm/Alq3/Al black cathode structure. Instead, if the cathodeis Sm/Alq3/Sm/Alq3/Al, the reflectance reduces to 0.9%. In these de-vices, cathode structures can be prepared by conventional thermalevaporation and they exhibit reproducible optical and electricalcharacteristics.

A high contrast ratio has been reported for a black film struc-ture, with periodic layers of Al and CuPc, which employs bothdestructive interference and absorption to reduce the ambientlight reflection [69]. A double period structure (Al/CuPc/Al/CuPc/Al) was found to have the highest contrast ratio. Unlike circularpolarizer, this technology can readily be applied to flexible displaysas well [70].

Fig. 5 gives a comparison of the ambient reflection from deviceswith different black cathodes based on destructive interference. Itcan be seen that Sm/Alq3/Sm/Alq3/Al cathode shows the minimumreflectance as compared to other cathodes. In this particular devicesemitransparent Sm, Alq3 and Al have been used for creating thedestructive interference where Alq3 is the phase changing layer.At the same time Sm acts as an absorbing cathode also. Hencethe two methods viz. absorbing cathode and cathode with destruc-tive interference are coupled to realize a high contrast ratio.

When we compare the performance of the devices with andwithout black cathodes it can be seen that while the electricalcharacteristics remained almost the same, the efficiency of blackcathode OLEDs reduced considerably. This because the componentof OLED emitted light directed towards the black cathode is alsonot reflected as in the case of normal reflecting cathodes. Thereis one report [68] which mentions improvement both in contrastand in efficiency. However, it can be seen that their control devicewithout black cathode operates at an abnormally high voltage withpoor efficiency.

Hence, black cathode structures may necessitate that oneshould drive the device harder to compensate for the loss of re-flected light from cathode. This may have an adverse effect onthe life time of the devices, which in most cases has not been re-ported. Also it can be seen that the thickness of the phase changing

Fig. 5. Comparison of ambient reflections from OLEDs having different blackcathodes [ Ref. [64], Ref. [65], Ref. [67] and Ref.[68]].

Table 2Ambient contrast ratios of AMOLEDs bare glass (A), glass with 45%-AR layer and BM(B) and glass laminated with a circular polarizer (C) under different ambient lightillumination when device brightness is 200 cdm�2 (data from Ref. [78]).

Ambient light illumination (lx) ? 150 500 1000 5000 100000

Ambient CR A 24.3 6.0 3.2 1.4 1.2B 199.3 43.3 19.0 4.2 2.5C 182 44 20.4 4.3 2.6

R. Singh et al. / Optical Materials 34 (2012) 716–723 721

layer is designed based on the wavelength in the green region in allthe devices. Similarly, in absorbing black cathode, their efficiency isbest in a limited wavelength range, often tested with green device.Therefore, the same black cathodes may not cause large OLEDbrightness reduction in red or blue devices.

In summary, while the performance of black cathode ap-proaches is no better than a circular polarizer, an alternative maybe possible. For example, in a green OLED, the design of black cath-ode could be to cut out reflections of only red or blue light compo-nents. Similarly, a separate cathode may be used for other primarycolors which only prevents reflections from the remaining two col-ors. In this case, while the OLED output will remain largely unaf-fected, a major fraction of ambient white light will be absorbed,resulting in increased CR. In fact, the papers, for example [62], thatreport only a small loss of luminous power by including black cath-ode is due to this reason.

2.3.4. Absorbing transport layerCR of an OLED device can be reduced by using an absorbing

electron transport layer (ETL) or hole transport layer (HTL), butin this process, the transport characteristics of the device shouldnot be adversely affected. Transport layers with high absorptioncoefficient have been used to increase the contrast ratio, for exam-ple CuPc as HTL and C60 as ETL [71]. This resulted in an increasedcontrast as well as current efficiency. High contrast ratio with goodpower efficiency has been reported for an Alq3 based OLED byincorporating a dual electron transport layer of CuPc and oxotitani-um phthalocyanine (TiOPc) as an absorbing as well as antireflec-tion layer [72].

It may not be possible to achieve high levels of contrast throughthe use of absorbing materials because a decrease in the specularcomponent of the reflectance may be accompanied by an increasein the diffuse component. Also some of the light absorbing materi-als may absorb IR radiation and the device may get heated uponoperation in direct sunlight. Also the transport layer should notbe absorbing in the spectral region where the device is emittingand should have good energy level matching with the emissivelayer.

2.3.5. Black matrixThe pixels in a display are first defined on an insulator layer.

Thus, a portion of the display becomes non-emitting. A partialsolution to reduce reflection of ambient light, therefore, can be

making this non-emitting area black. A black matrix is realizedby depositing a material of high optical density in the non-emittingregions of a display. A few attempts have been made using a blackmatrix (BM) [73–77], which is either solution processed or ther-mally evaporated. In solution process, the materials are spin coatedand then photo-lithographically patterned. Another approach is tomix black carbon particles with photoresist (PR) itself. Some mate-rials such as graphite (commonly used in CRT) Cr:Ni and Cr:SiOx

are thermally evaporated and then patterned by photolithography.Steigerwald [73] optimized the multilayer structure, Cr/Cr2O3/M/Cr2O3, for black matrix with different metals, M. The averagereflectance of the multilayer structure is only 0.1% with Nb asthe metal in the multilayer structure.

BM is also a major component of the color filter used for block-ing light in flat panel displays. In LCD and OLED displays, the inter-spaces between luminous elements are filled by light absorbingblack coating that absorbs external light and scattered radiationfrom adjacent elements. BM provides a distinct tone for each colorpixel and provides more contrast in a color image. In active matrixOLED (AMOLED) displays, BM layer has been used to prevent leak-age of light between pixels and to reduce the ambient reflection.Lee et al. [78] have made BM by mixing carbon black with photo-resist. BM with an optical density of 4.65 was positioned above thenon-emissive areas to block any ambient light being reflected bythe metal. In addition, they used an antireflection layer (having45% transmission) of SiO2 (84 nm)/TiO2 (60 nm)/Cr (11 nm)/SiO2

(21 nm)/Cr (7 nm)/glass. To achieve antireflective properties anda transmittance of 45%, multilayers composed of Cr, TiO2, andSiO2 were deposited by e-beam evaporation on the glass substrate.The ambient contrast ratios of OLEDs containing 45%-AR/BM layersand a circular polarizer were 2.5 and 2.6, respectively, for100,000 lx ambient light (as shown in Table 2). As the 45%-AR/BM layers provide comparable CRs compared to circular polarizers,it can be substituted by such AR/BM layers.

2.3.6. Other techniquesVaenkatesan et al. [79] have used an approach of introducing a

chiral nematic film in the device to improve the CR. Though thischiral nematic film also allows some reflection around 550 nm,outside this reflection band, the CR is high because the chiralnematic film acts like a circular polarizer. This technique doesnot reduce the device luminance as much as a circular polarizerwould. However this technique reduces the viewing angle.

Another technique is to use a neutral density filter over the dis-play. A neutral density filter will absorb incident light as it passesinto the OLED device and will absorb the reflected ambient lightagain as the ambient light is reflected from the back electrode. Incontrast, the emitted light from the OLED will pass through theneutral density filter and will be absorbed only once. Hence, therelative brightness of the emitted light improves in comparisonto the reflected ambient light. If the neutral density filter is a 50%filter (absorbing half of the light passing through it), 75% of theambient light will be absorbed in comparison to only 50% of theemitted light [37]. Color filters which are used along with whiteOLEDs also work more or less the same way but they have aninherent advantage. That is, the color filters absorb incident

Table 3Comparison of the performances of different contrast enhancement techniques.

Serialnumber

Method Strengths Weaknesses

1 Single layer AR coating Easy to apply Not effective for the whole spectral rangeVery effective for a single wavelength

2 Multilayer AR coating Possible to reduce reflection from a broadband Requires multilayer deposition3 Circular polarizer Easy to apply Costly

Effectively reduces ambient reflection Not suitable for flexible displaysMost commonly accepted method Absorbs more than 50% of the emitted light

4 Absorbing black cathode Can be designed for wavelengths other than that of theemitted light

Absorbs a fraction of the emitted light

Effects on device life time are not well understood5 Black cathode with destructive

interferenceCan be designed for green and hence the over-allambient reflection is reduced

A fraction of the emitted light is also dissipatedEffects on device life time are not well understood

6 Absorbing transport layers No separate process step is required Materials selection will be difficultDevice life time may be adversely affected.

7 Black matrix Reduces reflection considerably from non-emittingarea

Should be complemented with another technique forbest results

Easy to integrate with substrate patterningGood for white OLED with color filterSuitable for displays with low aperture ratio

722 R. Singh et al. / Optical Materials 34 (2012) 716–723

radiation just as the neutral density filters do, but not in the fre-quencies at which light is emitted.

A high contrast OLED can also be fabricated using a gradientrefractive index anode to reduce the reflectance of the ambientlight from the device. The concept is based on using an anode witha graded refractive index to minimize the ambient light reflection.For example, a bilayer anode consisting of a thin film of highly oxy-gen deficient indium tin oxide (ITO), and a normal ITO having ahigh work function can be used. The highly oxygen deficient ITOfilm is electrically conducting and optically absorbing. Oxygendeficient ITO layer is inserted between the anode and the substrate[80].

Concept of a multilayer anode comprising an absorbing Au/Agbilayer with a distributed bragg reflector (DBR) that has both highinternal and low outside reflectance has been reported [81]. Thehigh internal reflection is used to create a microcavity effect inthe OLED that is tuned to maximize light outcoupling, and thelow external reflection is used to improve the OLED contrast ratio.It may be noted that microcavity OLED devices employ tuned opti-cal cavities within which light is emitted and constructive interfer-ence is used to purify the desired frequencies of emitted light.These optical structures also have the effect of absorbing incidentlight at the same frequencies thus improving the contrast of thedevice, at least in those frequencies. However, it is difficult to makeoptical cavities for broadband emitters [37].

Yet another approach to increase the CR at the expense of view-ing angle is to employ privacy screens which absorb off-angleemitted light to reduce the viewing angle of a display [37]. Theycan be formed from vertical, high-aspect-ratio light-absorbingstructures within a layer and placed over a display. This arrange-ment effectively absorbs ambient light while maintaining thebrightness of the display at normal angles, sacrificing the viewingangle. Such an arrangement might be useful for devices for whicha limited viewing angle is preferred.

It may also be noted that the ambient contrast ratio does not di-rectly relate to image quality as this metric does not consider hu-man visual characteristics. A more meaningful evaluation methodmust correlate a measurement value to human perception as well.A new technique which relies on measuring perceptual contrastlength (PCL) has been proposed [82]. PCL is defined as the quanti-fied brightness extent of the contrast value corresponding to thehuman visual system. But, contrast on this basis are not yet re-ported widely.

3. Conclusions

Various techniques for enhancement of ambient contrast forOLED displays have been discussed. It can be seen that the ambientcontrast ratio greatly depends on the ambient illuminance. Differ-ent contrast enhancement techniques may be used to get the de-sired contrast ratio for indoor and outdoor applications. Forexample, circular polarizer may be very effective for outdoor appli-cations where the ambient luminance is quite high. A combinationof polarizer and black matrix will give even better results. For in-door applications, white OLED with color filters does not requirethe polarizer, as the color filter transmits only specific frequencies.However, display manufacturers employ a black matrix around thepixels to increase the contrast ratio and this combination of blackmatrix and color filter works fairly well. In this case, the device lifetime may also be improved because one does not need to drive thepixels with twice required luminance, unlike in the case of a circu-lar polarizer, which cuts out half the light.

Polarizer may not be a good choice for flexible displays andhence a combination of black matrix and AR would be better. ForAMOLEDs, the aperture ratio is small, implying large non-emittingareas. Hence, the use of black matrix can be effective by coveringthese areas. Black matrix alone may not satisfy the outdoorrequirements. But again, if the display device is generally nonreflective and only the front reflectivity of the glass dominates,then a combination of black matrix and an AR coating may givethe desired results.

In general, most techniques for enhancing contrast also reducebrightness. Therefore, in order to achieve required brightness, itbecomes necessary to drive display with higher currents. A conse-quence will be loss in life-time. In light of this, the black matrix, ARcoatings or a combination of both are attractive choices. But, thesemethods are specific to the nature of OLED device. Therefore, ifgood life-times can be ensured even at high brightness, methodsof general applicability, such as circular polarizer or neutral densityfilter would be better. Black cathode structures should be usedwith caution because they form an integral part of the deviceand the ramifications of these on the device life time are yet tobe understood.

The visible solar spectrum has the highest intensity in the greenregion. Fluorescent lamps also emulate this spectrum to generatecool white emission, with a color temperature close to 6500 K.Hence, most of the contrast enhancement techniques could rely

R. Singh et al. / Optical Materials 34 (2012) 716–723 723

upon reducing the green component of the ambient light reflectionand thereby increasing the ambient contrast ratio. This thoughtprovides means to choose a contrast enhancement technique.Therefore, best may be to use techniques that cut only the greenreflection. In this approach, green emission from the display willalso decrease, but emission of other colors will remain unaffected.Coupled with the fact that green OLEDs are most efficient, leastcost will have to be paid by increasing current through greenOLEDs to account for loss of green emission due to contrastenhancing technique.

Finally, Table 3 presents the strengths and weaknesses of eachof the major contrast enhancement techniques discussed in thispaper.

Acknowledgement

The authors would like to acknowledge financial support fromSamtel Color Ltd., New Delhi and Department of Science and Tech-nology, Government of India.

References

[1] G. Gu, S.R. Forrest, IEEE J. Sel. Top. Quant. Electron. 4 (1998) 83.[2] J. Shinar (Ed.), Organic Light Emitting Devices: A Survey, Springer Verlag, USA,

2004.[3] H. Kanno, Y. Hamada, H. Takahashi, IEEE. J. Sel. Top. Quant. Electron. 10 (2004)

30.[4] J.N. Bardsley, IEEE. J. Sel. Top. Quant. Electron. 10 (2004) 3.[5] F. So (Ed.), Organic Electronic Materials, Processing, Devices and Applications,

CRC Press, Taylor and Francis Group, USA, 2010.[6] J.A. Dobrowolski, B.T. Sullivan, R.C. Bajcar, Appl. Opt. 31 (1992) 5988.[7] E.J. Schanda, Colorimetry: Understanding the CIE System, Wiley-Interscience,

John Wiley and Sons, USA, 2007.[8] Video Electronics Standards Association (VESA) Flat-Panel Display

Measurement Standard 2.0, Section 308-2.[9] G. Walker, Veritas et Visus 8 (2006) 24.

[10] H. Yersin (Ed.), Highly Efficient OLEDs with Phosphorescent Materials, Wiley-VCH, USA, 2007.

[11] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 107 (2007) 1233.[12] L.S. Liao, K.P. Klubek, C.W. Tang, Appl. Phys. Lett. 84 (2004) 167.[13] S.R. Forrest, D.D.C. Bradley, M.E. Thomson, Adv. Mater. 15 (2003) 1043.[14] T. Yamasaki, K. Sumioka, T. Tsutsui, Appl. Phys. Lett. 76 (2000) 1243.[15] J.J. Shiang, T.J. Faircloth, A.R. Duggal, J. Appl. Phys. 95 (2004) 2889.[16] S. Moller, S.R. Forrest, J. Appl. Phys. 91 (2002) 3324.[17] H. Peng, Y.L. Ho, X.-J. Yu, M. Wong, H.-S. Kwok, J. Disp. Tech. 1 (2005) 278.[18] M. Fujita, T. Ueno, S. Noda, H. Ohata, T. Tsuji, H. Nakada, N. Shimoji, Electron.

Lett. 39 (2003) 121.[19] Y.R. Do, Y.-C. Kim, Y.-W. Song, Y.-H. Lee, J. Appl. Phys. 96 (2004) 7629.[20] C. Liu, V. Kamaev, Z.V. Vardeny, Appl. Phys. Lett. 86 (2005) 143501.[21] K. Ishihara, M. Fujita, I. Matsubara, T. Asano, S. Noda, H. Ohata, A. Hirasawa, H.

Nakada, N. Shimoji, Appl. Phys. Lett. 90 (2007) 111114.[22] V. Bulovic, V.B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R. Forrest,

Phys.Rev. B 58 (1998) 3730.[23] J. Lee, N. Chopra, F. So, Appl. Phys. Lett. 92 (2008) 033303.[24] T.-Y. Cho, C.-L. Lin, C.C. Wu, Appl. Phys. Lett. 88 (2006) 111106.[25] P.-H. Lei, S.-H. Wang, F.S. Juang, Y.-H. Tseng, M.J. Chung, Opt. Commun. 283

(2010) 1933.[26] R. Bathelt, D. Buchhauser, C. Garditz, R. Paetzold, P. Wellmann, Org. Electron. 8

(2007) 293.[27] Y.-H. Cheng, J.-L. Wu, C.–H. Cheng, K.-C. Syao, M.-C.M. Lee, Appl. Phys. Lett. 90

(2007) 091102.[28] B. Riedel, J. Hauss, U. Geyer, J. Guetlein, U. Lemmer, M. Gerken, Appl. Phys. Lett.

96 (2010) 243302.[29] P.A. Hobson, J.A.E. Wasey, I. Sage, W.L. Barnes, IEEE J. Sel. Top. Quant. Electron.

8 (2002) 378.[30] K.Y. Yang, K.C. Choi, C.W. Ahn, Opt. Express 17 (2009) 11495.

[31] S.-Y. Nien, N.-F. Chiu, Y.-H. Ho, J.-H. Lee, C.-W. Lin, K.-C. Wu, C.-K. Lee, J.-R. Lin,M.-K. Wei, T.-L. Chiu, Appl. Phys. Lett. 94 (2009) 103304.

[32] K. Saxena, V.K. Jain, D.S. Mehta, Opt. Mater. 32 (2009) 221.[33] N.K. Patel, S. Cina, J.H. Burroughs, IEEE J. Sel. Top. Quant. Electron. 8 (2002) 346.[34] B. Mazhari, M. Tewari, in: Proc. of 8th Asian Symposium on Information

Displays, China 8.1.3-9, 2004, p. 303.[35] B. Mazhari, S. Mehrotra, Proc. Eurodisplay (2002) 635.[36] C.-C. Wu, C.-W. Chen, C.-L. Lin, C.-J. Yang, J. Display Tech. 1 (2005) 248.[37] R.S. Cok, A.D. Arnold, SID Int. Symp. Digest. Tech. Papers 38 (2007) 784.[38] H.A. MacLeod, Thin Film Optical Filters, Hilger & Watts, London, 1969. 37.[39] H.G. Shanbhogue, C.L. Nagendra, M.N. Annapurna, S.A. Kumar, G.K.M.

Thutupalli, App. Opt. 36 (1997) 6339.[40] P. Nubile, Thin Solid Films 342 (1999) 257.[41] V.M. Aroutiounian, K.R. Maroutyan, A.L. Zatikyan, K.J. Touryan, Thin Solid Films

517 (2002) 403.[42] L.A. Catalon, J. Opt. Soc. Am. 52 (1962) 437.[43] B.A. Moys, Thin Solid Films 21 (1974) 145.[44] K. Saxena, D.S. Mehta, V.K. Rai, R. Srivastava, G. Chauhanand, M.N.

Kamalasanan, J. Lumin. 128 (2008) 525.[45] P. Baumeister, Appl. Opt. 25 (1986) 24.[46] D.R. Gibson, I. Brinkley, J.M. Walls, Proc. SPIE 5618 (2004) 156.[47] C.-J. Yang, C.-L. Lin, C.-C. Wu, Appl. Phys. Lett. 87 (2005) 143507.[48] S. Bennet, G. Trapani, Displays 4 (1984) 159.[49] G. Trapani, R. Pawlak, G.R. Carlson, J.N. Gordon, US Patent 6549335, 2003.[50] O. Renault, O.V. Salata, M. Etchells, P.J. Dobson, V. Christou, Thin Solid Films

379 (2000) 195.[51] P. Vonlanthen, E. Felder, L. Degiorgi, H.R. Ott, D.P. Young, A.D. Bianchi, Z. Fisk,

Phys. Rev. B 62 (2000) 10076.[52] L.S. Hung, J. Madathil, Adv. Mater. 13 (2001) 1787.[53] H. Aziz, Y.-F. Liew, H.M. Grandin, Z.D. Popovic, Appl. Phys. Lett. 83 (2003) 186.[54] F.L. Wong, M.K. Fung, X. Jiang, C.S. Lee, S.T. Lee, Thin Solid Films 446 (2004)

143.[55] S.H. Li, H. Liem, C.W. Chen, E.H. Wu, Z. Xu, Y. Yang, Appl. Phys. Lett. 86 (2005)

143514.[56] Z. Wu, L. Wang, Y. Qiu, Opt. Express 13 (2005) 1406.[57] K.C. Lau, W.F. Xie, H.Y. Sun, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 88 (2006)

083507.[58] W.F. Xie, H.Y. Sun, C.W. Law, C.S. Lee, S.T. Lee, S.Y. Liu, Appl. Phys. A 85 (2006)

95.[59] Y. Cong, Z.-J. Chen, Q.-H. Gong, Guangpuxue Yu Guangpu Fenxi 27 (2007) 1051.[60] S. Chen, W. Xie, W. Huang, S. Liu, Opt. Express 16 (2008) 8868.[61] C.-J. Yang, T.-Y. Cho, C.-L. Lin, C.-C. Wu, J. SID 16 (2008) 691.[62] P.Y. Chen, M. Yokoyamal, H.Y. Ueng, Jap. J. Appl. Phys. 49 (2010) 012102.[63] A.N. Krasnov, Appl. Phys. Lett. 80 (2002) 3853.[64] Z.Y. Xie, L.S. Hung, Appl. Phys. Lett. 84 (2004) 1207.[65] X.D. Feng, R.E. Khangura, Z.H. Lu, Appl. Phys. Lett. 85 (2004) 497.[66] J.H. Lee, C.C. Liao, P.J. Hu, Y. Chang, Synth. Met. 144 (2004) 279.[67] Y.C. Zhou, L.L. Ma, J. Zhou, X.D. Gao, H.R. Wu, X.M. Ding, X.Y. Hou, Appl. Phys.

Lett. 88 (2006) 233505.[68] Z.H. Huang, W.M. Su, X.T. Zeng, SIMTech Tech. Reports 8 (2007) 182.[69] J.-J. Huang, Y.-C. Lin, Y.-K. Su, Y.-L. Wu, Juang, F.-S. Juang, J. Nanosci. Nanotech.

8 (2008) 5227.[70] A.N. Krasnov, Inf. Display 18 (2002) 18.[71] W.F. Xie, Y. Zhao, C.N. Li, S.Y. Liu, Opt. Express 14 (2006) 177954.[72] J.-F. Li, S.-H. Su, K.-S. Hwang, M. Yokoyama, J. Phys. D Appl. Phys. 40 (2007)

2435.[73] M.L. Steigerwald, US Patent 5566011, 1996.[74] H. Iwata, US Patent 5976639, 1999.[75] C.W. Park, J.-B. Lee, Y.R. Do, P. Shepeliavyi, K. Michailovs, I. Indutnyy, O.

Kudryavtsev, Semiconduct. Phys. Quant. Electron Optoelectronics 3 (2000)496.

[76] D.L. Matthies, Z. Shem, R.G. Stewart, J.H. Atherton, US Patent 6476783, 2002.[77] Y.-T. Chang, C.-H. Liou, C.-B. Wen, IEEE Circuit. Devices Magazine 21 (2005) 8.[78] B.D. Lee, Y.-H. Cho, M.H. Oh, S.Y. Lee, S.Y. Lee, J.H. Lee, Dong S. Zang, Mater.

Chem. Phys. 112 (2008) 734.[79] V. Vaenkatesan, R.T. Wegh, J.-P. Teunissen, J. Lub, C.W.M. Bastiaansen, D.J.

Broer, Adv. Funct. Mat. 15 (2005) 138.[80] F. Zhu, K.S. Ong, X. Hao, US Patent 0122352, 2008.[81] C. Py, D. Poitras, C.-C. Kuo, H. Fukutani, Opt. Lett. 33 (2008) 126.[82] M.-O. Shin, S.-A. Yang, S.-Y. Kim, J.-H. Chong, S.-B. Lee, S.S. Kim, SID Int. Symp.

Digest Tech. Papers 41 (2010) (1516).