Orientation of emissive dipoles in OLEDs: Quantitative in situ analysis

Abstract The orientation of the emissive dipole moments in organic light-emitting diodes (OLEDs) has a major impact on the optical outcoupling efficiency and, consequently, on the device performance as well as on possible optimization strategies. In this study we propose and demonstrate a general method to quantify the amounts of parallel and perpendicular emissive sites in OLEDs. The presented in situ -method is based on measurements of the far-field emission of an electrically operating device and corresponding optical reverse simulations. A well adapted OLED stack is utilized, where the contribution of perpendicularly oriented dipoles to the radiation pattern in air is optically enhanced. Additionally, for the reverse simulation we take advantage of the fact that perpendicular dipoles do contribute to transverse-magnetic polarized light emission only. We apply the method to a polymeric OLED and show that the radiation pattern is generated by 93.5% parallel and 6.5 % perpendicular dipoles. Assuming a Gaussian distribution of dipole orientations, the dipoles stagger around the preferred parallel direction with an 1/e-angle of ±22°.

[1]  Andreas Bräuer,et al.  Dipole lifetime in stratified media , 2002 .

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

[3]  Ifor D. W. Samuel,et al.  Light Out‐Coupling Efficiencies of Organic Light‐Emitting Diode Structures and the Effect of Photoluminescence Quantum Yield , 2005 .

[4]  Kristiaan Neyts,et al.  Simulation of light emission from thin-film microcavities , 1998 .

[5]  Daniel Moses,et al.  Anisotropic photoluminescence properties of oriented poly( p -phenylene-vinylene) films: Effects of dispersion of optical constants , 2007 .

[6]  Akio Sakaguchi,et al.  Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films , 2009 .

[7]  Andreas Bräuer,et al.  Fluorescence lifetimes of molecular dye ensembles near interfaces , 2002 .

[8]  William L. Barnes,et al.  Determining the orientation of the emissive dipole moment associated with dye molecules in microcavity structures , 2004 .

[9]  Malte C. Gather,et al.  Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy , 2009 .

[10]  Daisuke Yokoyama,et al.  Spectrally narrow emissions at cutoff wavelength from edges of optically and electrically pumped anisotropic organic films , 2008 .

[11]  Michael Flämmich,et al.  Intrinsic OLED emitter properties and their effect on device performance , 2008, SPIE OPTO.

[12]  Ifor D. W. Samuel,et al.  Effects of dipole orientation and birefringence on the optical emission from thin films , 2000 .

[13]  Richard H. Friend,et al.  Effect of metal films on the photoluminescence and electroluminescence of conjugated polymers , 1997 .

[14]  E. Palik.  Handbook of Optical Constants of Solids , 1997 .

[15]  Andreas Bräuer,et al.  Dispersion-model-free determination of optical constants: application to materials for organic thin film devices. , 2009, Applied optics.

[16]  Malte C. Gather,et al.  An Improved Optical Method for Determining the Order Parameter in Thin Oriented Molecular Films and Demonstration of a Highly Axial Dipole Moment for the Lowest Energy π–π* Optical Transition in Poly(9,9‐ dioctylfluorene‐co‐bithiophene) , 2007 .

[17]  Andrew P. Monkman,et al.  Orientation of triplet and singlet transition dipole moments in polyfluorene, studied by polarised spectroscopies , 2007 .

[18]  Donal D. C. Bradley,et al.  Angular Dependence of the Emission from a Conjugated Polymer Light‐Emitting Diode: Implications for efficiency calculations , 1994 .

[19]  M. Grell,et al.  Film morphology and photophysics of polyfluorene , 2000 .

[20]  Matthias Brinkmann,et al.  Simultaneous color and luminance control of organic light-emitting diodes for mood-lighting applications , 2008 .

[21]  Norbert Danz,et al.  In situ measurement of the internal luminescence quantum efficiency in organic light-emitting diodes , 2009 .

[22]  Ken-Tsung Wong,et al.  Influences of molecular orientations on stimulated emission characteristics of oligofluorene films , 2007 .

[23]  Stefan Nowy,et al.  Efficiency analysis of organic light-emitting diodes based on optical simulation , 2009 .

[24]  Richard H. Friend,et al.  Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations , 2000 .

[25]  R. Silbey,et al.  Molecular Fluorescence and Energy Transfer Near Interfaces , 2007 .

[26]  Stefan Nowy,et al.  Light extraction and optical loss mechanisms in organic light-emitting diodes: Influence of the emitter quantum efficiency , 2008 .

[27]  J. Ferraris,et al.  Optical determination of chain orientation in electroluminescent polymer films , 1995 .

[28]  P. Clemmow,et al.  The plane wave spectrum representation of electromagnetic fields , 1966 .

[29]  Norbert Danz,et al.  Measuring the profile of the emission zone in polymeric organic light-emitting diodes , 2009 .

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

[31]  Charalambos C. Katsidis,et al.  General transfer-matrix method for optical multilayer systems with coherent, partially coherent, and incoherent interference. , 2002, Applied optics.