Langevin and Trap‐Assisted Recombination in Phosphorescent Organic Light Emitting Diodes

Bimolecular and trap-assisted recombination mechanisms are investigated in small molecule-based phosphorescent organic light emitting diodes (PhOLEDs) using the current−voltage−luminance characteristics in the diffusion current region, along with transient electroluminescence and capacitance measurements. Two different PhOLEDs, one with a single host, 4,4′-Bis(carbazol-9-yl)biphenyl, and the other with an exciplex-forming co-host, are studied. Trap-assisted recombination with a large number of trapped charges is dominant in the PhOLED with the single host because of the large energy gap between the host and the dopant state. In contrast, bimolecular Langevin recombination is dominant in the PhOLED with the exciplex forming co-host, where a phosphorescent dye is doped in the co-host. As a result, the accumulated charge density is lower in the co-host system than in the single host emission layer, leading to high efficiency that approaches the theoretical limit, with an extremely low efficiency roll-off.

[1]  Hartmut Yersin,et al.  Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties , 2004 .

[2]  Tae-Woo Lee,et al.  Highly Efficient p‐i‐n and Tandem Organic Light‐Emitting Devices Using an Air‐Stable and Low‐Temperature‐Evaporable Metal Azide as an n‐Dopant , 2010 .

[3]  P. Blom,et al.  Determination of the trap-assisted recombination strength in polymer light emitting diodes , 2011 .

[4]  William Shockley,et al.  The theory of p-n junctions in semiconductors and p-n junction transistors , 1949, Bell Syst. Tech. J..

[5]  R. Hall.  Electron-Hole Recombination in Germanium , 1952 .

[6]  Karl Leo,et al.  Storage of charge carriers on emitter molecules in organic light-emitting diodes , 2012 .

[7]  Donal D. C. Bradley,et al.  Origin of electrophosphorescence from a doped polymer light emitting diode , 2001 .

[8]  Christian Mayr,et al.  Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter , 2013 .

[9]  H. L. Tam,et al.  High Efficiency and Small Roll‐Off Electrophosphorescence from a New Iridium Complex with Well‐Matched Energy Levels , 2008 .

[10]  Stephen R. Forrest,et al.  Transient analysis of organic electrophosphorescence. II. Transient analysis of triplet-triplet annihilation , 2000 .

[11]  S. R. Forrest,et al.  High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer , 2000, Nature.

[12]  I. Hill,et al.  Vacuum level alignment in organic guest-host systems , 2002 .

[13]  W. Read,et al.  Statistics of the Recombinations of Holes and Electrons , 1952 .

[14]  Kwon-Hyeon Kim,et al.  Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency , 2013 .

[15]  Jan Birnstock,et al.  High-efficiency and low-voltage p‐i‐n electrophosphorescent organic light-emitting diodes with double-emission layers , 2004 .

[16]  Hirotake Kajii,et al.  Study of Transient Electroluminescence Process Using Organic Light-Emitting Diode with Partial Doping Layer , 2006 .

[17]  Chihaya Adachi,et al.  Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion , 2012, Nature Photonics.

[18]  Jan Kalinowski,et al.  Quenching effects in organic electrophosphorescence , 2002 .

[19]  Shui-Tong Lee,et al.  Time-resolved transient electroluminescence measurements of emission from DCM-doped Alq3 layers , 2004 .

[20]  Paul W. M. Blom,et al.  Trap-assisted and Langevin-type recombination in organic light-emitting diodes , 2011 .

[21]  Charlotte K. Williams,et al.  Solution-processible conjugated electrophosphorescent polymers. , 2004, Journal of the American Chemical Society.

[22]  Zhuozhi Wang,et al.  Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility , 2011, Science.

[23]  Walter Riess,et al.  Influence of trapped and interfacial charges in organic multilayer light-emitting devices , 2001 .

[24]  Kwon-Hyeon Kim,et al.  Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host , 2013 .

[25]  Zhuozhi Wang,et al.  Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000 cd/m2 , 2011 .

[26]  H. Aziz,et al.  Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching , 2010 .

[27]  S. Forrest,et al.  VERY HIGH-EFFICIENCY GREEN ORGANIC LIGHT-EMITTING DEVICES BASED ON ELECTROPHOSPHORESCENCE , 1999 .

[28]  P. Blom,et al.  Trap-assisted recombination in disordered organic semiconductors. , 2011, Physical review letters.

[29]  Daniel Moses,et al.  Electrophosphorescence from a Polymer Guest–Host System with an Iridium Complex as Guest: Förster Energy Transfer and Charge Trapping , 2003 .

[30]  Wei Ren,et al.  Impedance spectroscopy as a probe for the degradation of organic light-emitting diodes , 2010 .

[31]  D. Morgan,et al.  A theoretical and experimental study of recombination in silicon p−n junctions , 1975 .

[32]  J. Shinar,et al.  Transient electroluminescence spikes in small molecular organic light-emitting diodes , 2011 .

[33]  Jang‐Joo Kim,et al.  Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes , 2013 .

[34]  Jang‐Joo Kim,et al.  Energy transfer from exciplexes to dopants and its effect on efficiency of organic light-emitting diodes , 2011 .

[35]  Dan Davidov,et al.  Transient Electroluminescence from Poly(phenylenevinylene)‐Based Devices , 1999 .

[36]  Jin Jang,et al.  Ideal host and guest system in phosphorescent OLEDs , 2009 .

[37]  Karsten Walzer,et al.  Triplet-exciton quenching in organic phosphorescent light-emitting diodes with Ir-based emitters , 2007 .