Blue Emitting Organic Light Emitting Diodes

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Organic light emitting diodes (OLEDs) [1] can be fabricated on a range of materials such asglass, silicon or flexible plastic substrates. This can be exploited for the realization ofintegrated OLED-based fluorescence chemical sensors [2] and microfluidic systems [3] forapplication in areas such as biotechnology, life sciences, pharmaceuticals, public health anddefense. These devices hold promises to be cost effective, ultra-compact (including thepossibility to be fabricated into large bidimensional arrays), and capable to handle smallersample volumes in order to achieve high throughput. Blue light is advantageous because it isstrongly absorbed by most sensing molecules attached to biological samples. In thiscontribution we report the fabrication and characterization of blue emitting OLEDs based onmolecular materials. N,N’-bis-(1-naphtyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (-NPB)is used as emitting material, 2,9-dimethil-4,7-biphenyl-1,10-phenanthroline (BCP) as holeblocking layer and tris(8-hydroxyquinoline)aluminum complex (Alq3) as electron injectionlayer. Devices are fabricated on ITO-coated glass by vacuum thermal evaporation with aLiF/Al cathode.Single-layer devices with ITO/-NPD (60 nm)/ LiF (1 nm)/ Al (100 nm) structure have thetypical luminance L - current density J - voltage V (LJV) characteristics shown in Fig.1. Theluminance vs current density is non linear and light emission is poor. This is due to a strongcharge imbalance caused by different energy barrier heights for electron and hole injectionfrom the electrodes. In fact -NPB is a hole transporting material and, due to a smaller energybarrier height at ITO anode, hole injection starts before electron injection from the metalcathode. As a consequence, the recombination zone is close to metal cathode where defect inthe -NPD layer caused by the vacuum deposition of Al act as non radiative centres(electroluminesce quenching). This shortcoming can be overcome inserting a BCP holeblocking layer to confine holes in the -NPD emitting layer and a Alq3 electron injectionlayer to increase electron injection by lowering barrier energy height at the cathode.Triple-layer devices with ITO/-NPD (50 nm)/ BCP (15 nm)/ Alq3 (30 nm)/ LiF (1 nm)/ Al(100 nm) structure have the typical LJV characteristics shown in Fig.2. Light emission is verystrong reaching values above 1000 cd/m2 for current density greater than 150 mA/cm2. Fig.3shows a picture of a blue triple-layer OLED under operation. The emission spectrum (notshown) is centred around 470 nm and is coincident to the photoluminescence spectrum of-NPD film demonstrating that the radiative recombination zone is indeed confined in the-NPD layer. The external quantum efficiency (EQE) of single-layer and triple-layer OLEDsare shown in Fig. 4 confirming that electroluminescence quenching is dominant for singlelayer devices. Due to its very good performance, the triple-layer OLED is the best candidateto be used for the fabrication of a complete integrated fluorescence sensor.References[1] L.S. Hung et al., “Recent progress of molecular organic electroluminescent materials and devices”, MaterialsScience and Engineering, R: Reports 39 (2002) pp.143-222[2] V. Savvate’ev et al., “Integrated organic light-emitting device/fluorescence-based chemical sensors”, Appl.Phys. Lett. 81 (2002), pp. 4652-4654.[3] Bo Yao et al., “A microfluidic device using a green organic light emitting diode as an integrated excitationsource”, Lab Chip 5, (2005), pp. 1041-1047
Original languageEnglish
Title of host publicationSIE2019 51st Annual Meeting - Book of abstracts
Number of pages2
Publication statusPublished - 2019

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