Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.

[1]  Malte C. Gather,et al.  Advanced Device Architecture for Highly Efficient Organic Light‐Emitting Diodes with an Orange‐Emitting Crosslinkable Iridium(III) Complex , 2008 .

[2]  R. Friend,et al.  Blue-Shifted Electroluminescence From a Stable Precursor to Poly(p-Phenylene Vinylene) , 1992 .

[3]  R. N. Marks,et al.  Light-emitting diodes based on conjugated polymers , 1990, Nature.

[4]  David G Lidzey,et al.  Device degradation of polymer light emitting diodes studied by electroabsorption measurements , 1999 .

[5]  Smith,et al.  Direct measurement of conjugated polymer electronic excitation energies using metal/polymer/metal structures. , 1996, Physical review letters.

[6]  J. deMello,et al.  Internal Field Screening in Polymer Light‐Emitting Diodes , 2004 .

[7]  Donal D. C. Bradley,et al.  Ohmic hole injection in poly(9,9-dioctylfluorene) polymer light-emitting diodes , 2003 .

[8]  Klaus Meerholz,et al.  Multi-colour organic light-emitting displays by solution processing , 2003, Nature.

[9]  Malte C. Gather,et al.  Enhanced efficiency of multilayer organic light-emitting diodes with a low-refractive index hole-transport layer: An effect of improved outcoupling? , 2007 .

[10]  Damodar M. Pai,et al.  Trap-controlled hopping transport , 1984 .

[11]  Alison B. Walker,et al.  The internal electric field distribution in bilayer organic light emitting diodes , 2002 .

[12]  L. Díaz,et al.  Method for the determination of optical constants of thin films: dependence on experimental uncertainties. , 1992, Applied optics.

[13]  U. Scherf,et al.  Efficient Polymer Electrophosphorescent Devices with Interfacial Layers , 2006 .

[14]  James L. Auld,et al.  Degradation in blue-emitting conjugated polymer diodes due to loss of ohmic hole injection , 2004 .

[15]  Richard A. L. Jones,et al.  Interfacial structure in semiconducting polymer devices , 2003 .

[16]  D. Bradley,et al.  Influence of carrier injection on the electromodulation response of trap-rich polymer light-emitting diodes , 2006 .

[17]  R. N. Marks,et al.  Light-emitting diodes based on conjugated polymers , 1990, Nature.

[18]  D. Bradley,et al.  Elimination of hole injection barriers by conducting polymer anodes in polyfluorene light-emitting diodes , 2006 .

[19]  Klaus Meerholz,et al.  New crosslinkable hole conductors for blue-phosphorescent organic light-emitting diodes. , 2007, Angewandte Chemie.

[20]  A. Elschner,et al.  Green polyfluorene-conducting polymer interfaces: Energy level alignment and device performance , 2006 .

[21]  Dieter Neher,et al.  Highly Efficient Single‐Layer Polymer Electrophosphorescent Devices , 2004 .

[22]  I. Campbell,et al.  Direct measurement of the internal electric field distribution in a multilayer organic light‐emitting diode , 1995 .

[23]  T. Tsutsui,et al.  Electroabsorption spectroscopy on tris-(8-hydroxyquinoline) aluminum-based light emitting diodes , 1999 .

[24]  D. Bradley,et al.  Influence of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) in polymer LEDs , 2006 .

[25]  Oakley H. Crawford,et al.  Radiation from oscillating dipoles embedded in a layered system , 1988 .

[26]  Richard H. Friend,et al.  Interference effects in anisotropic optoelectronic devices , 2000 .

[27]  Richard H. Friend,et al.  Spin-cast thin semiconducting polymer interlayer for improving device efficiency of polymer light-emitting diodes , 2005 .

[28]  G. R. Webster,et al.  Optical studies of electric fields in poly(2-methoxy-5-ethyl (2′-hexyloxy) para-phenylene vinylene) light-emitting diodes , 1999 .

[29]  Dieter Meissner,et al.  Electroabsorption studies of phthalocyanine/perylene solar cells , 2000 .

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

[31]  Klaus Meerholz,et al.  Highly Efficient Polymeric Electrophosphorescent Diodes , 2006 .

[32]  William R. Salaneck,et al.  Characterization of the PEDOT-PSS system by means of X-ray and ultraviolet photoelectron spectroscopy , 1999 .

[33]  Andrew J. deMello,et al.  Role of electron injection in polyfluorene-based light emitting diodes containing PEDOT:PSS , 2005 .

[34]  Klaus Meerholz,et al.  Highly efficient solution-processed phosphorescent multilayer organic light-emitting diodes based on small-molecule hosts , 2007 .

[35]  Malte C. Gather,et al.  On the Origin of the Color Shift in White‐Emitting OLEDs , 2007 .

[36]  R. H. Friend,et al.  Efficient light-emitting diodes based on polymers with high electron affinities , 1993, Nature.

[37]  L. Alcácer,et al.  Use of cross-linkable polyfluorene in the fabrication of multilayer polyfluorene-based light-emitting diodes with improved efficiency , 2006 .

[38]  Zhaoxin Wu,et al.  Investigation of the spectra of phosphorescent organic light-emitting devices in relation to emission zone , 2005 .

[39]  T. Marks,et al.  Enhanced Polymer Light‐Emitting Diode Performance Using a Crosslinked‐Network Electron‐Blocking Interlayer , 2004 .

[40]  Yun Chi,et al.  Crosslinkable Hole‐Transport Layer on Conducting Polymer for High‐Efficiency White Polymer Light‐Emitting Diodes , 2007 .

[41]  Malte C. Gather,et al.  Solution‐Processed Full‐Color Polymer Organic Light‐Emitting Diode Displays Fabricated by Direct Photolithography , 2007 .