Gregor Schwartz

White organic light-emitting diodes with fluorescent tube efficiency

The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60–70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron–photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

Highly efficient white organic light emitting diodes comprising an interlayer to separate fluorescent and phosphorescent regions

White organic light emitting diodes combining the phosphorescent green and orange-red emitting systems fac tris(2-phenylpyridine) iridium doped 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) and iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) doped N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine with the blue fluorescent dye 2,2′,7,7′-tetrakis(2,2-diphenylvinyl)spiro-9,9′-bifluorene (Spiro-DPVBi) are presented. By introducing a thin layer of coevaporated TCTA and 2,2′,2″ (1,3,5-benzenetriyl) tris-[1-phenyl-1H-benzimidazole] between the phosphorescent and the fluorescent region, both singlet and triplet excitons are confined efficiently, whereas charge carriers still pass easily this interlayer. Furthermore, the interlayer suppresses Dexter transfer of the phosphorescent excitons to the nonradiative triplet state of Spiro-DPVBi. Best devices reach a current efficiency of 16.3cd∕A at 100cd∕m2 and a color rendering index of 85 at warm white CIE chromaticity coordinates of (0.47, 0.42). ...

Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an...

Reduced efficiency roll-off in high-efficiency hybrid white organic light-emitting diodes

White organic light emitting diodes harvesting triplet excitons from the fluorescent blue emitter N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) are presented. Direct doping of the phosphorescent orange iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) into 4P-NPD results in a strongly reduced efficiency roll-off as compared to separate emission layers, and yields 49.3lmW−1 total external power efficiency (24.1% quantum efficiency) at a luminance of 1000cdm−2 [CIE 1931 chromaticity coordinates (0.49,0.41)], measured in an integrating sphere. Introduction of an exciton balancing interlayer improves the chromaticity (0.43,0.43) toward the CIE illuminant A warm white point and keeps a high efficiency of 40.7lmW−1, 20.3%.

Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation

The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4,4′,4′-tris(N-carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities jc of 140mA∕cm2 and 270mA∕cm2 defining the current density of half-value IEQE.

Highly phosphorescent organic mixed films: The effect of aggregation on triplet-triplet annihilation

The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.

Balanced ambipolar charge carrier mobility in mixed layers for application in hybrid white organic light-emitting diodes

We investigate the electron and hole mobility in mixed layers of N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine and bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum with different mix ratios, using both space-charge limited currents of single-carrier devices with electrically doped charge transport layers and time-of-flight measurements. Both experimental methods yield consistent results. The 1:1 blend shows balanced ambipolar charge carrier transport, which is advantageous for the application as exciton blocking interlayer in hybrid white organic light-emitting diodes: The electroluminescence spectrum is rather stable against changes in interlayer thickness and driving current. Moreover, the external quantum efficiency is enhanced by a factor of 2.5 as compared to a device without interlayer.

Combination of a polyaniline anode and doped charge transport layers for high-efficiency organic light emitting diodes

Up to now, most organic light emitting diodes (OLEDs) have utilized inorganic materials as transport anodes. In this study, we show that conductive polymers are suitable for this purpose as well. Polyaniline anodes, with a conductivity of 200 S/cm, are used to inject holes into the adjacent organic layers. Due to electrical doping of the electron and hole transport layer with an intrinsic emission layer sandwiched in between (pin-OLED), the devices reach high luminance at low voltage. The phosphorescent emitters Ir(MDQ)2(acac) and Ir(ppy)3, as well as the fluorescent emitter Spiro-DPVBi, are implemented within pin-OLEDs using a polyaniline anode. By the use of different host materials, a green double-emitting OLED is demonstrated and compared to the corresponding single emission layer device. Furthermore, a white OLED combining fluorescent and phosphorescent emitting layers is presented, reaching 8.9 lm/W at 1000 cd∕m2. The results demonstrate an efficient charge carrier injection from the polymer into th...

Increase in internal quantum efficiency in small molecular oligothiophene: C60 mixed heterojunction solar cells by substrate heating

We present small molecule solar cells with α,ω-bis-(dicyanovinylene)-sexithiophene:C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%.

High-efficiency white organic-light-emitting diodes combining fluorescent and phosphorescent emitter systems

We report on white organic light emitting diodes with three stacked emitter layers comprising the fluorescent blue emitter Spiro-DPVBi, the phosphorescent green emitter system TCTA:Ir(ppy)3 and the phosphorescent red emitter system NPB:Ir(MDQ)2(acac). A thin additional layer of mixed TCTA and TPBi separates the fluorescent and phosphorescent emitting regions, simultaneously confining excitons efficiently and letting electrons and holes easily pass. Furthermore, phosphorescence quenching by Dexter transfer to the non-radiative triplet state of Spiro-DPVBi is suppressed. Devices were optimized to get color coordinates very close to the warm white standard illuminant A. Best devices have a current efficiency of 13.8 cd/A, CIE color coordinates of (0.45, 0.42), and a color rendering index of 91 at a brightness of 1000 cd/m2. Due to the use of electrically doped charge transport layers, the voltage needed for 1000 cd/m2 was only 3.0 V, which leads to a power efficiency of 14.4 lm/W assuming Lambertian emission.