Energy‐transfer dynamics of blue‐phosphorescent iridium and rhodium complexes doped in fluorescent molecules

Abstract— The temperature‐dependent photoluminescence features of polycarbonate thin films doped with blue‐phosphorescent molecules, either bis[(4,6‐difluorophenyl)‐pyridinato‐N,C^2′] (picolinate) iridium (Flrpic) or bis(2‐phenylpyridinato‐N,C^2′) (acetylacetonate) rhodium [(ppy)²Rh(acac)], which have an equivalent triplet energy of 2.64 eV, have been studied. The photoluminescence intensity of the Flrpic‐doped polycarbonate thin film did not show any dependence on temperature. On the other hand, as for the (ppy)²Rh(acac)‐doped polycarbonate thin film, decreasing photoluminescence intensity with increasing temperature (especially above 100K) was clearly visible. These results reflect that the internal heavy‐atom effect of (ppy)²Rh(acac) is weaker than that of Flrpic. Furthermore, the steady‐state and time‐resolved photoluminescence spectra of tris(8‐hydroxyquinoline) aluminum (Alq₃) thin films heavily doped with Flrpic or (ppy)²Rh(acac) (50 wt.%) at 8K was studied. It was found that the enhanced phosphorescence from Alq₃ is mainly due not to the external heavy‐atom effect by doping with the phosphorescent molecule but to the exothermic triplet energy transfer from the phosphorescent molecule to Alq₃.     

Highly efficient blue organic light‐emitting diode with high color purity using 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) doped with 1,4‐bis[2‐(3‐N‐ethylcarbazoryl) vinyl]benzene (BCzVB)

Abstract— An efficient pure blue multilayer organic light‐emitting diode employing 1,4‐bis[2‐(3‐N‐ethylcarbazoryl)vinyl]benzene (BCzVB) doped into 4,4′‐N,N′‐dicarbazole‐biphyenyl (CBP) is reported. The device structure is ITO (indium tin oxide)/TPD (N,N′‐diphenyl‐N,N′‐bis (3‐methylphenyl)‐1,1′biphenyl‐4,4′diamine)/CBP:BCzVB/Alq₃ (tris‐(8‐hydroxy‐quinolinato) aluminum)/Liq (8‐hydroxy‐quinolinato lithium)/Al; here TPD was used as the hole‐transporting layer, CBP as the blue‐emitting host, BCzVB as the blue dopant, Alq₃ as the electron‐transporting layer, Liq as the electron‐injection layer, and Al as the cathode, respectively. A maximum luminance of 8500 cd/m² and a device efficiency of 3.5 cd/A were achieved. The CIE co‐ordinates were x = 0.15, y = 0.16. The electroluminescent spectra reveal a dominant peak at 448 nm and additional peaks at 476 nm with a full width at half maximum of 60 nm. The Föster energy transfer and, especially, carrier trapping models were considered to be the main mechanism for exciton formation on BCzVB molecules under electrical excitation.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

High performance a‐IGZO thin‐film transistors with mf‐PVD SiO2 as an etch‐stop‐layer

In this work, we report on high‐performance bottom‐gate top‐contact (BGTC) amorphous‐Indium‐Gallium‐Zinc‐Oxide (a‐IGZO) thin‐film transistor (TFT) with SiO₂ as an etch‐stop‐layer (ESL) deposited by medium frequency physical vapor deposition (mf‐PVD). The TFTs show field‐effect mobility (μ_FE) of 16.0 cm²/(V.s), sub‐threshold slope (SS^?1) of 0.23 V/decade and off‐currents (I_OFF) < 1.0 pA. The TFTs with mf‐PVD SiO₂ ESL deposited at room temperature were compared with TFTs made with the conventional plasma‐enhanced chemical vapor deposition (PECVD) SiO₂ ESL deposited at 300 °C and at 200 °C. The TFTs with different ESLs showed a comparable performance regarding μ_FE, SS^?1, and I_OFF, however, significant differences were measured in gate bias‐stress stability when stressed under a gate field of +/?1 MV/cm for duration of 10⁴ s. The TFTs with mf‐PVD SiO₂ ESL showed lower threshold‐voltage (V_TH) shifts compared with TFTs with 300 °C PECVD SiO₂ ESL and TFTs with 200 °C PECVD SiO₂ ESL. We associate the improved bias‐stress stability of the mf‐PVD SiO₂ ESL TFTs to the low hydrogen content of the mf‐PVD SiO₂ layer, which has been verified by Rutherford‐Back‐Scattering‐Elastic‐Recoil‐Detection technique.     

Interface and bulk effects for bias—light‐illumination instability in amorphous‐In—Ga—Zn—O thin‐film transistors

Abstract— Positive‐current‐bias (PB) instability and negative‐bias—light‐illumination (NBL) instability in amorphous‐In—Ga—Zn—O (a‐IGZO) thin‐film transistors (TFTs) have been examined. The channel‐ thickness dependence indicated that the Vth instability caused by the PB stress is primarily attributed to defects in the bulk a‐IGZO region for unannealed TFTs and to those in the channel—gate‐insulator interface for wet‐annealed TFTs. The interface and bulk defect densities (Dit and Nss, respectively) are Dit = 4.8 × 10¹¹ cm^?2/eV and Nss = 7.0×10¹⁶ cm^?3/eV for the unannealed TFT, which increased to 5.2×10¹¹ cm^?2/eV and 9.8×10¹⁶ cm^?3/eV, respectively, by the PB stress test. These are reduced significantly to Dit = 0.82×10¹¹ cm^?2/eV and Nss = 3.2×10¹⁶ cm^?3/eV for the wet‐annealed TFTs and are unchanged by the PB stress test. It was also found that the photo‐response of a‐IGZO TFTs begins at 2.3 eV of photon excitation, which corresponds to subgap states observed by photoemission spectroscopy. The origin of the NBL instability for the wet‐annealed TFTs is attributed to interface effects and considered to be a trap of holes at the channel‐gate—insulator interface where migration of the holes is enhanced by the electric field formed by the negative gate bias.     

The reduction of driving voltage in organic light-emitting devices by inserting step barrier layer

The electron injection and transport in OLEDs have been improved by using a tris-[8-hydroxyquinoline] gallium (Gaq) layer as step barrier between tris-[8-hydroxyquinoline]aluminum (Alq₃) (or 4,7-diphyenyl-1,10-phenanthroline (Bphen)) and 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN). Since the LUMO (lowest unoccupied molecular orbital) of Gaq (2.9 eV) lies in between that of Alq₃ (3.1 eV) (or Bphen (3.0 eV)) and TBADN (2.8 eV), step barrier from Alq₃ (or BPhen) though Gaq to TBADN can be formed. The experimental results indicate that the J–V characteristics of both the electron-only and the complete devices show the increase of the current density in devices with step barrier compared with the devices without step barrier. For electron-only devices, the driving voltage at the current density of 20 mA/cm² is reduced from 7.9 V to 4.9 V for devices with Alq₃, and from 4.2 V to 3.1 V for devices with BPhen, respectively, owing to the introduction of step barrier. For the complete devices, when Gaq step barrier is introduced, at 20 mA/cm², the driving voltage is reduced from 7 V to 5.8 V for devices with Alq₃ and from 6.2 V to 5.1 V for devices with BPhen. It has also been observed that for devices with step barrier layer, the luminance at 200 mA/cm² is increased from 1992 cd/m² to 3281 cd/m² for device with Alq₃, and from 1745 cd/m² to 2876 cd/m² for devices with BPhen, respectively. The highest luminance reaches 3420 cd/m² in devices with Alq₃ as ETL and 3176 cd/m² in devices with BPhen as ETL after the introduction of step barrier. The phenomena are explained by using tunnel theory.     

A millimeter‐wave CMOS power amplifier design using high‐Q slow‐wave transmission lines

A three‐stage 60‐GHz power amplifier (PA) has been implemented in a 65 nm Complementary Metal Oxide Semiconductor (CMOS) technology. High‐quality‐factor slow‐wave coplanar waveguides (S‐CPW) were used for input, output and inter‐stage matching networks to improve the performance. Being biased for Class‐A operation, the PA exhibits a measured power gain G of 18.3 dB at the working frequency, with a 3‐dB bandwidth of 8.5 GHz. The measured 1‐dB output compression point (OCP_1dB) and the maximum saturated output power P_sat are 12 dBm and 14.2 dBm, respectively, with a DC power consumption of 156 mW under 1.2 V voltage supply. The measured peak power added efficiency PAE is 16%. The die area is 0.52 mm² (875 × 600 μm²) including all the pads, whereas the effective area is only 0.24 mm². In addition, the performance improvement of the PA in terms of G, OCP_1dB, P_sat, PAE and the figure of merit using S‐CPW instead of thin film microstrip have been demonstrated. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:99–109, 2016.     

Impact of source/drain contacts formation of self‐aligned amorphous‐IGZO TFTs on their negative‐bias‐illumination‐stress stabilities

In this study, we have compared the performance of self‐aligned a‐IGZO thin‐film transistors (TFTs) whereby the source/drain (S/D) region's conductivity enhanced in three different ways, that is, using SiN_x interlayer plasma (hydrogen diffusion), using calcium (Ca as reducing metal) and using argon plasma (changing the atomic ratio). All these TFTs show comparable characteristics such as field‐effect mobility (μ_FE) of over 10.0 cm²/(V.s), sub‐threshold slope (SS^‐1) of 0.5 V/decade, and current ratio (I_ON/I_OFF) over 10⁸. However, under negative‐bias‐illumination‐stress (NBIS), all these TFTs showed strong degradation. We attributed this NBIS stability issue to the exposed S/D regions and changes in the conductivity of S/D contact regions. The hydrogen plasma‐treated TFTs showed the worst NBIS characteristics. This is linked to increased hydrogen diffusion from the S/D contact regions to the channel.     

Characterization of a SAG‐InGaN/AlGaN LED by mixed‐source HVPE with a multi‐sliding boat system

Abstract— The selective area growth (SAG) of a InGaN/AlGaN light‐emitting diode (LED) is performed by using mixed‐source hydride vapor‐phase epitaxy (HVPE) with a multi‐sliding boat system. The SAG‐InGaN/AlGaN LED consists of a Si‐doped AlGaN cladding layer, an InGaN active layer, a Mg‐doped AlGaN cladding layer, and a Mg‐doped GaN capping layer. The carrier concentration of the n‐type Al_xGa_1?xN (x ～ 16%) cladding layer depends on the amount of poly‐Si placed in the Al‐Ga source. The carrier concentration is varied from 2.0 × 10¹⁶ to 1.1 × 10¹⁷ cm^?3. Electroluminescence (EL) characteristics show an emission peak wavelength at 426 nm with a full width at half‐maximum (FWHM) of approximately 0.47 eV at 20 mA. It was found that the mixed‐source HVPE method with a multi‐sliding boat system is a candidate growth method for III‐nitride LEDs.     

A dual‐band dual‐polarized cubical DRA coupled with new modified cross‐shaped slot for ISM (2.4 GHz) and Wi‐MAX (3.3‐3.6 GHz) band applications

In this article, a new modified cross‐shaped coupled cubical dielectric resonator antenna (DRA) has been investigated for dual‐band dual‐polarized applications. The linearly polarized (LP) fields in DRA has been generated by using a single slot in the ground plane and kept at either 45° (S_L1) or ?45° (S_L2) from the microstrip feed line. Combining these two slots (S_L1 and S_L2) in the modified ground plane, the proposed structure able to generate circularly polarized (CP) field in DRA. But the generated CP field is not enough to cover ISM 2400 band. To achieve CP in ISM 2400 band, an extra slot (S_L3) to the existing slots and an extra strip (S_T) in the circular ring feed line have been included. This modified final antenna arrangement has been able to produce LP (due to loading effect, ie, slot and DRA) and CP fields (orthogonal modes have been generated, ie, TE ^x₁₁₁ and TE ^y₁₁₁), simultaneously. The measured CP and LP, ?10 dB impedance bandwidths are 11.85% (2.38‐2.68 GHz) and 9.11% (3.25‐3.56 GHz) in combination with the 3‐dB axial ratio bandwidth of 4.11% (2.38‐2.48 GHz). The generated CP and LP fields are used for different wireless communication bands such as ISM 2400 and Wi‐MAX (3.3‐3.7 GHz) bands.     

Flexible thin‐film transistors application of amorphous tin oxide‐based semiconductors

The structural, optical, and electrical properties of Si‐doped SnO₂ (STO) films were investigated in terms of their potential applications for flexible electronic devices. All STO films were amorphous with an optical transmittance of ~90%. The optical band gap was widened as the Si content increased. The Hall mobility and carrier density were improved in the SnO₂ with 1 wt% Si film, which was attributed to the formation of donor states. Si (1 wt%) doped SnO₂ thin‐film transistor exhibited a good device performance and good stability with a saturation mobility of 6.38 cm²/Vs, a large I_on/I_off of 1.44 × 10⁷, and a SS value of 0.77 V/decade. The device mobility of a‐STO TFTs at different bending radius maintained still at a high level. These results suggest that a‐STO thin films are promising for fabricating flexible TFTs.     

Circuits and AMOLED display with self‐aligned a‐IGZO TFTs on polyimide foil

A process to make self‐aligned top‐gate amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) thin‐film transistors (TFTs) on polyimide foil is presented. The source/drain (S/D) region's parasitic resistance reduced during the SiN interlayer deposition step. The sheet resistivity of S/D region after exposure to SiN interlayer deposition decreased to 1.5 kΩ/□. TFTs show field‐effect mobility of 12.0 cm²/(V.s), sub‐threshold slope of 0.5 V/decade, and current ratio (I_ON/OFF) of >10⁷. The threshold voltage shifts of the TFTs were 0.5 V in positive (+1.0 MV/cm) bias direction and 1.5 V in negative (?1.0 MV/cm) bias direction after extended stressing time of 10⁴ s. We achieve a stage‐delay of ~19.6 ns at V_DD = 20 V measured in a 41‐stage ring oscillator. A top‐emitting quarter‐quarter‐video‐graphics‐array active‐matrix organic light‐emitting diode display with 85 ppi (pixels per inch) resolution has been realized using only five lithographic mask steps. For operation at 6 V supply voltage (V_DD), the brightness of the display exceeds 150 cd/m².     

Orange‐red upconversion luminescence of Sm3+‐doped ZnO‐B2O3‐SiO2 glass by infrared femtosecond laser irradiation

Abstract— Near‐infrared‐to‐visible upconversion luminescence was observed in Sm³⁺‐doped ZnO‐B₂O₃‐SiO₂ glass under femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from ⁴G^5/2 to ⁶H^j/2 (j = 5, 7, 9) transition of Sm³⁺. The dependence of the fluorescence intensity of Sm³⁺ on the pump power indicates that a two‐photon absorption process is dominant in the conversion of infrared radiation to the visible luminescence. The analysis of the upconversion mechanism reveals that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic orange‐red emission of Sm³⁺. A three‐dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence.     

Uniformity and bias‐temperature instability of bottom‐gate zinc oxide thin‐film transistors (ZnO TFTs)

Abstract— Short‐range uniformity and bias‐temperature (BT) instability of ZnO TFTs with SiO^x/SiN^x stacked gate insulators which have different surface treatments have been investigated. The short‐range uniformity of ZnO TFTs was drastically improved by N²O plasma treatment of the gate insulator. The variation in the gate voltage where a drain current of 1‐nA flows (Vgs at an Ids of 1 nA) was dramatically reduced from ±1.73 V to ±0.07 V by N²O plasma treatment of the gate insulator. It was clarified that the variations in the subthreshold characteristics of the ZnO TFTs could be reduced by N²O plasma treatment of the gate insulator due to a decrease in the variation of trap densities in deep energy levels from 0.9–2.0 × 10¹⁷ to 1.2–1.3×10¹⁷ cm^?3‐eV^?1. From the BT stress tests, a positive shift of Vgs at an Ids of 1 nA could be reduced by N²O plasma treatment of the gate insulator due to a decrease in the charge traps in the gate insulator. When the gate‐bias stress increases, state creation occured in the ZnO TFTs in addition to the charge trapping in the gate insulator. However, N²O plasma treatment of the gate insulator has little effect on the suppression of the state creation in ZnO TFTs under BT stress. The surface treatment of the gate insulator strongly affects the short‐range uniformity and the BT instability of Vth in the ZnO TFTs.     

Ten switched‐beams with 2 × 2 series‐fed 2.4 GHz array antenna and a simple beam‐switching network

This article presents a 2 × 2 series fed 2.4 GHz patch antenna array having multiple beam switching capabilities by using two simple 3 dB/90° couplers to achieve required amplitude and phase excitations for array elements with reduced complexity, cost and size. The beam switching performance with consistent gain and low side lobe levels (SLL) is achieved by exciting the array elements from orthogonally placed thin quarter‐wave (λ_g/4) feeds. The implemented array is capable to generate ten (10) switched‐beams in 2‐D space when series fed elements are excited from respective ports through 3 dB quadrature couplers. The dual polarized characteristics of presented array provide intrinsic interport isolation between perpendicularly placed ports through polarization diversity to achieve independent beam switching capabilities for intended directions. The implemented antenna array on 1.575 mm thick low loss (tan δ = 0.003) NH9450 substrate with ε_r = 4.5 ± 0.10 provides 10 dB return loss impedance bandwidth of more than 50 MHz. The measured beam switching loss is around 0.8 dB for beams switched at θ = ±20°, Ф = 0°, 90°, and 45° with average peak gain of 9.5 dBi and SLL ≤ ?10 dB in all cases. The novelty of this work is the capability of generating ten dual polarized switched‐beams by using only two 3 dB/90° couplers as beam controllers.     

Solution‐processed oxide semiconductors for low‐cost and high‐performance thin‐film transistors and fabrication of organic light‐emitting‐diode displays

Abstract— High‐performance solution‐processed oxide‐semiconductor (OS) thin‐film transistors (TFTs) and their application to a TFT backplane for active‐matrix organic light‐emitting‐diode (AMOLED) displays are reported. For this work, bottom‐gated TFTs having spin‐coated amorphous In‐Zn‐O (IZO) active layers formed at 450°C have been fabricated. A mobility (μ) as high as 5.0 cm²/V‐sec, ?0.5 V of threshold voltage (VT), 0.7 V/dec of subthreshold swing (SS), and 6.9 × 10⁸ of on‐off current ratio were obtained by using an etch‐stopper (ES) structure TFT. TFTs exhibited uniform characteristics within 150 × 150‐mm² substrates. Based on these results, a 2.2‐in. AMOLED display driven by spin‐coated IZO TFTs have also been fabricated. In order to investigate operation instability, a negative‐bias‐temperature‐stress (NBTS) test was carried out at 60°C in ambient air. The IZO‐TFT showed ?2.5 V of threshold‐voltage shift (ΔVT) after 10,800 sec of stress time, comparable with the level (ΔVT = ?1.96 V) of conventional vacuum‐deposited a‐Si TFTs. Also, other issues regarding solution‐processed OS technology, including the instability, lowering process temperature, and printable devices are discussed.     

Diamond‐like‐carbon LC‐alignment layers for application in LCOS microdisplays

Abstract— To improve the lifetime and yield of LCOS microdisplays, non‐contact LC alignment techniques using inorganic materials are under investigation. This report focuses on oblique ion‐beam treatment of diamond‐like carbon (DLC) layers, and in particular on the influence of the ion dose on the LC alignment on DLC, keeping the ion‐beam angle (40°) and ion‐beam energy (170 eV) the same. LC alignment on ion‐milled DLC layers is uniform if the ion dose is between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm². Above and below this ion dose range, non‐uniform alignment is observed. NEXAFS experiments show that this is caused by lack of molecular anisotropy on the surface of the ion‐milled DLC layers. By varying the ion dose between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm², LC molecules have an average pre‐tilt between 3° and 5°, which is within the desired range for application in LCOS microdisplays. The lifetime of the LCOS microdisplays with ion‐milled DLC for projection‐TV application is, however, shorter than the lifetime of microdisplays with PI layers. Ion milling probably creates a reactive surface that is unstable under the high light fluxes used in projection TVs. A solution for this problem could be chemical passivation of the ion‐milled alignment layers. Initial experiments with passivation of ion‐milled PI resulted in an increase in lifetime, but the lifetime after passivation was still lower than the lifetime of rubbed PI layers (factor 0.7). Nevertheless, ion‐milling of DLC or PI can be a good alternative LC alignment technique in other LCD applications. LC‐alignment layers based on inorganic layers such as obliquely deposited SiO₂ films would be a better option for application in LCOS microdisplays due to their higher light stability.     

Highly efficient white organic light‐emitting diodes with over 100 lm/W for next‐generation solid‐state lighting

High‐performance two‐unit all‐phosphorescent white devices on a built‐up light extraction substrate that comprised high‐index materials were studied. As a result of suitable optical and electrical design, the device showed an extremely high efficacy of 114 lm/W at 1000 cd/m². The device also showed 102 lm/W with long lifetime (LT70) of over 10,000 h at 3000 cd/m². Outstanding external quantum efficiency of almost 50% was also achieved in a flat panel with an emissive area of 25 cm². Color coordinates of the panel met the Energy Star ® criteria of solid‐state lighting with CIE (Commission Internationale de l'Éclairage) 1931 (x, y) = (0.477, 0.423), and the color rendering index was 81.     

An efficient solution to the subset‐sum problem on GPU

We present an algorithm to solve the subset‐sum problem (SSP) of capacity c and n items with weights w_i,1≤i≤n, spending O(n(m − w_min)/p) time and O(n + m − w_min) space in the Concurrent Read/Concurrent Write (CRCW) PRAM model with 1≤p≤m − w_min processors, where w_min is the lowest weight and , improving both upper‐bounds. Thus, when n≤c, it is possible to solve the SSP in O(n) time in parallel environments with low memory. We also show OpenMP and CUDA implementations of this algorithm and, on Graphics Processing Unit, we obtained better performance than the best sequential and parallel algorithms known so far. Copyright © 2015 John Wiley & Sons, Ltd.     

Low operating-voltage and high power-efficiency OLED employing MoO3-doped CuPc as hole injection layer

Effects of doping molybdenum oxide (MoO₃) in copper phthalocyanine (CuPc) as hole injection layer in OLEDs are studied. A green OLED with structure of ITO/MoO₃-doped CuPc/NPB/10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1)-benzopyropyrano(6,7,8-i,j) quinolizin-11-one (C545T): tris(8-hydroxyquinoline) aluminum (Alq₃)/Alq₃/LiF/Al shows the driving voltage of 4.4 V, and power efficiency of 4.3 lm/W at luminance of 100 cd/m². The charge transfer complex between CuPc and MoO₃ plays a decisive role in improving the performance of OLEDs. The AFM characterization shows that the doped film owns a better smooth surface, which is also in good agreement with the electrical performance of OLEDs.