Optoelectronic Processes in Squaraine Dye-Doped OLEDs for Emission in the Near-Infrared
Optoelectronic Processes in Squaraine Dye-Doped OLEDs for Emission in the Near-Infrared
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DOI：
10.1002/adma.201204938
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发表时间：
2013-06-04
影响因子：
29.4
通讯作者：
Pflaum, Jens
中科院分区：
文献类型：
--
作者：
Stender, Benedikt;Voelker, Sebastian F.;Pflaum, Jens
Organic light-emitting diodes (OLEDs) have been the subject of research since the first discovery of electroluminescence in molecular [1] and polymer thin films.[2, 3] Besides vacuum deposited small molecule OLEDs, polymeric OLEDs processed from solution have gained increasing interest due to their potential of large-area fabrication by means of roll-to-roll technique,[4] cheap processing [5] and their tunable emission range.[6] By optimization of the organic compounds, with respect to stability issues, charge carrier transport as well as injection, OLEDs for lighting applications and full-color displays have already entered the market.[7] In contrast, organic-based devices emitting in the near-infrared (NIR) are far behind this development though being of particular interest for utilization in broad-band optical telecommunication [8] and sensors. Recent studies present different approaches for NIR emitting OLEDs with reasonable quantum efficiencies: lanthanide complexes reveal an emission above 1 μm with an external quantum efficiency of 0.3% at low driving current densities of 0.03 mA/cm 2.[9] Organic-inorganic hybrid systems based on nanocrystals as NIR emitters with tunable spectral emission in the telecommunication wavelength range offer quantum efficiencies of 0.83%, but at the expenses of high current densities and high driving voltages.[10] In order to increase the efficiency of NIR emission, electrophosphorescent metal-complexes are applied. By taking advantage of the spin-statistics, efficiencies up to 10.7% have been realized,[11] however allowing only for low repetition rates due to long lifetimes of the excited triplet states. Purely organic NIR emitters are applied as either low bandgap polymers or as molecular dyes imbedded as guest molecules in a polymeric matrix. The former emitters were demonstrated successfully featuring low efficiencies of 0.05%.[12] NIR emission based on molecular dyes has been demonstrated to be reasonably efficient (0.7%)[13] but has barely been investigated due to major limitations imposed by degradation of the organic compounds in their ground-state and even more upon electrical excitation. A key advantage of organic semiconductors in comparison to their inorganic counterparts are the high excitionic binding energies (several 100 meV) preventing non-radiative relaxation of the excited states by coupling to vibrational modes even at room temperature. In this study we present a solution processible NIR-OLED consisting of an organic host-guest system featuring a highly efficient energy transfer from the visible spectral range to the near-infrared (∼ 750 nm) covering an energy range of more than 2 eV. To gain insights in the related processes on microscopic length scales the opto-electronic properties of this blended system are investigated and assigned by comparing photo-and electroluminescence measurements. Our analysis focusses on the energy transfer mechanisms between the polymeric host and guest molecules as well as on the contribution by charge trapping and recombination due to energy level mismatch of the involved transport levels.
