Blue Emitting Organic Light Emitting Diodes

Organic light emitting diodes (OLEDs) [1] can be fabricated on a range of materials such as glass, silicon or flexible plastic substrates. This can be exploited for the realization of integrated OLED-based fluorescence chemical sensors [2] and microfluidic systems [3] for application in areas such as biotechnology, life sciences, pharmaceuticals, public health and defense. These devices hold promises to be cost effective, ultra-compact (including the possibility to be fabricated into large bidimensional arrays), and capable to handle smaller sample volumes in order to achieve high throughput. Blue light is advantageous because it is strongly absorbed by most sensing molecules attached to biological samples. In this contribution we report the fabrication and characterization of blue emitting OLEDs based on molecular 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 hole blocking layer and tris(8-hydroxyquinoline)aluminum complex (Alq3) as electron injection layer. Devices are fabricated on ITO-coated glass by vacuum thermal evaporation with a LiF/Al cathode. Single-layer devices with ITO/-NPD (60 nm)/ LiF (1 nm)/ Al (100 nm) structure have the typical luminance L - current density J - voltage V (LJV) characteristics shown in Fig.1. The luminance vs current density is non linear and light emission is poor. This is due to a strong charge imbalance caused by different energy barrier heights for electron and hole injection from the electrodes. In fact -NPB is a hole transporting material and, due to a smaller energy barrier height at ITO anode, hole injection starts before electron injection from the metal cathode. As a consequence, the recombination zone is close to metal cathode where defect in the -NPD layer caused by the vacuum deposition of Al act as non radiative centres (electroluminesce quenching). This shortcoming can be overcome inserting a BCP hole blocking layer to confine holes in the -NPD emitting layer and a Alq3 electron injection layer 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 very strong reaching values above 1000 cd/m2 for current density greater than 150 mA/cm2. Fig.3 shows a picture of a blue triple-layer OLED under operation. The emission spectrum (not shown) 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 OLEDs are 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 candidate to 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”, Materials Science 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 excitation source”, Lab Chip 5, (2005), pp. 1041-1047