Über die Autorin bzw. den Autor

Professor Klaus Mullen joined the Max-Planck-Society in 1989 as one of the directors of the Max-Planck-Institue for Polymer Research in Mainz, Germany. he obtained his diploma in chemistry in 1969 from the University of Cologne, and completed his PhD at the University of Basel, Switzerland, in 1972. He joined the ETH Zurich and was appointed lecturer (Privatdozent) after finishing his habilitation in 1977, and moved on to a professorship at Cologne University two years later. He followed a call to the chair of organic chemistry at Mainz University in 1983. He received the Max Planck research prize in 1997 and the Phillip-Morris research prize in 1999, and has been visiting scientist at Osaka, Shanghai, Leuven, Jerusalem, Cambridge and other distinguished universities.

Prof. Dr. Ullrich Scherf studied chemistry at Friedrich Schiller University of Jena, Germany, obtaining his Ph.D. in 1988 on the synthesis of PPV-type organic semiconductors and carbonization of polymer films. He subsequently spent one year at the Institute for Animal Physiology of the Saxonian Academy of Sciences in Leipzig isolating and characterizing of cockroach hormones. He joined the Max Planck Institute for Polymer Research, Mainz, in 1990 and completed his habilitation in 1996 on polyarylene-type ladder polymers. He followed a call to the University of Potsdam, Germany, onto a professorship for polymer chemistry. In 2002, he became full professor for Macromolecular Chemistry at Bergische Universitat Wuppertal, Germany. He has published over 350 refereed papers and received the Meyer-Struckmann Research Award in 1998.

Von der hinteren Coverseite

This book reflects a decade of intense research on organic light emitting devices (OLEDs), culminating in excellent successes over the last few years which have resulted in the first commercializations of organic displays. The contributions from both academia as well as the industry leaders combine the fundamentals and latest research results with application know-how and industrial insights.

In five sections, the authors cover introductory topics, fundamental
physics, materials synthesis, device processing and optimization as
well as an outlook on new directions for organic electroluminescence.

Organic and polymer chemists, electrochemists, physical chemists, physicists, materials scientists and electronics engineers will find a broad perspective and in-depth coverage on OLEDs.

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Organic Light Emitting Devices

John Wiley & Sons

Chapter One

E. Fred Schubert, Thomas Gessmann, and Jong Kyu Kim

1.1 Introduction

During the past 40 years, light-emitting diodes (LEDs) have undergone a significant development. The first LEDs emitting in the visible wavelength region were based on GaAsP compound semiconductors with external efficiencies of only 0.2 %. Today, the external efficiencies of red LEDs based on AlGaInP exceed 50 %. AlGaInP semiconductors are also capable of emitting at orange, amber, and yellow wavelengths, albeit with lower efficiency. Semiconductors based on AlGaInN compounds can emit efficiently in the UV, violet, blue, cyan, and green wavelength range. Thus, all colors of the visible spectrum are now covered by materials with reasonably high efficiencies. This opens the possibility to use LEDs in areas beyond conventional signage and indicator applications. In particular, LEDs can now be used in high-power applications thereby enabling the replacement of incandescent and fluorescent sources. LED lifetimes exceeding > [10.sup.5] h compare favorably with incandescent sources (~ 500 h) and fluorescent sources (~ 5000 h), thereby contributing to the attractiveness of LEDs.

Inorganic LEDs are generally based on p-n junctions. However, in order to achieve high internal quantum efficiencies, free carriers need to be spatially confined. This requirement has led to the development of heterojunction LEDs consisting of different semiconductor alloys and multiple quantum wells embedded in the light-emitting active region. The light-extraction efficiency, which measures the fraction of photons leaving the semiconductor chip, is strongly affected by the device shape and surface structure. For high internal-efficiency active regions, the maximization of the light-extraction efficiency has proven to be the key to high-power LEDs.

This chapter reviews important aspects of inorganic LED structures. Section 1.2 introduces the basic concepts of optical emission. Band diagrams of direct and indirect semiconductors and the spectral shape of spontaneous emission will be discussed along with radiative and nonradiative recombination processes. Spontaneous emission can be controlled by placing the active region in an optical cavity resulting in a substantial modification of the LED emission characteristics. Theory and experimental results of such resonant-cavity LEDs (RCLEDs) are discussed in Section 1.3. The electrical characteristics of LEDs, to be discussed in Section 1.4, include parasitic voltage drops and current crowding phenomena that result in nonuniform light emission and shortened device lifetimes. Due to total internal reflection at the surfaces of an LED chip, the light-extraction efficiency in standard devices is well below 100%. Section 1.5 discusses techniques such as chip shaping utilized to increase the extraction efficiency. A particular challenge in achieving efficient LEDs is the minimization of optical absorption processes inside the semiconductor. This can be achieved by covering absorbing regions, such as lower-bandgap substrates, with highly reflective mirrors. Such mirrors should have omnidirectional reflection characteristics and a high angle-integrated, TE-TM averaged reflectivity. A novel electrically conductive omnidirectional reflector is discussed in Section 1.6. Section 1.7 reviews the current state of the art in LED packaging including packages with low thermal resistance.

1.2 Optical Emission Spectra

The physical mechanism by which semiconductor light-emitting diodes (LEDs) emit light is spontaneous recombination of electron-hole pairs and simultaneous emission of photons. The spontaneous emission process is fundamentally different from the stimulated emission process occurring in semiconductor lasers and superluminescent LEDs. The characteristics of spontaneous emission that determine the optical properties of LEDs will be discussed in this section.

The probability that electrons and holes recombine radiatively is proportional to the electron and hole concentrations, that is, R [varies] n p. The recombination rate per unit time per unit volume can be written as

R = - dn/dt = - dp/dt = B n p (1.1)

where B is the bimolecular recombination coefficient, with a typical value of [10.sup.-10] [cm.sup.3]/s for direct-gap III-V semiconductors.

Electron-hole recombination is illustrated in Fig. 1.1. Electrons in the conduction band and holes in the valence band are assumed to have the parabolic dispersion relations

E = [E.sub.C] + [??.sup.2] [k.sup.2] /2 [m.sup.*.sub.e] (for electrons) (1.2)

and

E = E.sub.V] - [??.sup.2] [k.sup.2]/2 [m.sup.*.sub.h] (for holes) (1.3)

where [m.sub.e.sup.*] and [m.sub.h.sup.*] are the electron and hole effective masses, ?? is Planck's constant divided by 2[pi], k is the carrier wave number, and [E.sub.V] and [E.sub.C] are the valence and conduction band-edge energies, respectively.

The requirement of energy and momentum conservation leads to further insight into the radiative recombination mechanism. It follows from the Boltzmann distribution that electrons and holes have an average kinetic energy of kT. Energy conservation requires that the photon energy is given by the difference between the electron energy, [E.sub.e], and the hole energy, [E.sub.h], i.e.

h v = E.sub.e] - [E.sub.h] [approximately equal to] [E.sub.g] (1.4)

The photon energy is approximately equal to the bandgap energy, [E.sub.g], if the thermal energy is small compared with the bandgap energy, that is, kT << E.sub.g]. Thus the desired emission wavelength of an LED can be attained by choosing a semiconductor material with appropriate bandgap energy. For example, GaAs has a bandgap energy of 1.42 eV at room temperature resulting in infrared emission of 870 nm.

It is helpful to compare the average carrier momentum with the photon momentum. A carrier with kinetic energy kT and effective mass [m.sup.*] has the momentum

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.5)

The momentum of a photon with energy [E.sub.g] can be obtained from the de Broglie relation

p = [??] k = h v / c = [E.sub.g] / c (1.6)

Calculation of the carrier momentum (using Eq. (1.5)) and the photon momentum (using Eq. (1.6)) yields that the carrier momentum is orders of magnitude larger than the photon momentum. Therefore the electron momentum must not change significantly during the transition. The transitions are therefore "vertical" as shown in Fig. 1.1, i. e. electrons recombine with only those holes that have the same momentum or k value.

Using the requirement that electron and hole momenta are the same, the photon energy can be written as the joint dispersion relation

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.7)

where [m.sub.r.sup.*] is the reduced mass given by

1/[m.sup.*.sub.r] = 1/[m.sup.*.sub.e] + 1/[m.sup.*.sub.h] (1.8)

Using the joint dispersion relation, the joint density of states can be calculated and one obtains

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.9)

The distribution of carriers in the allowed bands is given by the Boltzmann distribution, i. e.

[f.sub.B] (E) = [e.sup.-E/(kT) (1.10)

The emission intensity as a function of energy is proportional to the product of Eqs. (1.9) and (1.10),

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.11)

The emission lineshape of an LED, as given by Eq. (1.11), is shown in Fig. 1.2. The maximum emission intensity occurs at

E = E.sub.g] + 1/2 kT (1.12)

The full width at half maximum of the emission is given by

[DELTA]E = 1.8 kT (1.13)

For example, the theoretical room-temperature linewidth of a GaAs LED emitting at 870 nm is [DELTA]E = 46 meV or [DELTA][lambda] = 28 nm.

The spectral linewidth of LED emission is important in several respects. First, the linewidth of an LED emitting in the visible range is relatively narrow compared with the range of the entire visible spectrum. The LED emission is even narrower than the spectral width of a single color as perceived by the human eye. For example, red colors range from 625 to 730 nm, which is much wider than the typical emission spectrum of an LED. Therefore, LED emission is perceived by the human eye as monochromatic.

Secondly, optical fibers are dispersive, which leads to a range of propagation velocities for a light pulse comprising a range of wavelengths. The material dispersion in optical fibers limits the "bit rate distance product" achievable with LEDs. The spontaneous lifetime of carriers in LEDs in direct-gap semiconductors typically is of the order of 1-100 ns depending on the active region doping concentration (or carrier concentrations) and the material quality. Thus, modulation speeds up to 1 Gbit/s are attainable with LEDs.

A spectral width of 1.8kT is expected for the thermally broadened emission. However, due to other broadening mechanisms, such as alloy broadening (i. e. the statistical fluctuation of the active region alloy composition), the spectral width at room temperature in III-V nitride LEDs can be broader, typically (3 to 8)kT. Experimental evidence shown in Fig. 1.3 supports the use of a Gaussian function to describe the spectral power density function of an LED. Therefore,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.14)

where P is the total optical power emitted by the LED. Inspection of Fig. 1.3 indeed reveals that the Gaussian curve is a very good match for the experimental emission spectrum. Giving the line widths in terms of units of kT is very useful as it allows for convenient comparison with the theoretical line width of 1.8kT.

The emission spectra of an AlGaInP red, a GaInN green, and a GaInN blue LED are shown in Fig. 1.4. The LEDs shown in Fig. 1.4 have an active region comprised of a ternary or quaternary alloy, e. g. [Ga.sub.1-x][In.sub.x]N. In this case, alloy broadening leads to spectral broadening that goes beyond 1.8kT. Alloy broadening due to inhomogeneous distribution of In in the active region of green [Ga.sub.1-X][In.sub.x]N LEDs can cause linewidths as wide as 10kT at room temperature. It should be noted, however, that a recent study found inhomogeneous strain distribution in GaInN quantum wells as a result of electron damage during TEM experiments. It was concluded that the damage might lead to a "false" detection of In-rich clusters in a homogeneous quantum-well structure.

Efficient recombination occurs in direct-gap semiconductors. The recombination probability is much lower in indirect-gap semiconductors because a phonon is required to satisfy momentum conservation. The radiative efficiency of indirect-gap semiconductors can be increased by isoelectronic impurities, e. g. N in GaP. Isoelectronic impurities can form an optically active deep level that is localized in real space (small [DELTA]x) but, as a result of the uncertainty relation, delocalized in k space (large [DELTA]k), so that recombination via the impurity satisfies momentum conservation.

During nonradiative recombination, the electron energy is converted to vibrational energy of lattice atoms, i. e. phonons. There are several physical mechanisms by which nonradiative recombination can occur with the most common ones being recombination at point defects (impurities, vacancies, interstitials, antisite defects, and impurity complexes) and at spatially extended defects (screw and edge dislocations, cluster defects). The defects act as efficient recombination centers (Shockley-Read recombination centers) in particular, if the energy level is close to the middle of the gap.

1.3 Resonant-cavity-enhanced Structures

Spontaneous emission implies the notion that the recombination process occurs spontaneously, that is without a means to influence this process. In fact, spontaneous emission has long been believed to be uncontrollable. However, research in microscopic optical resonators, where spatial dimensions are of the order of the wavelength of light, showed the possibility of controlling the spontaneous emission properties of a light-emitting medium. The changes of the emission properties include the spontaneous emission rate, spectral purity, and emission pattern. These changes can be employed to make more efficient, faster, and brighter semiconductor devices. The changes in spontaneous emission characteristics in resonant-cavity (RC) and photonic-crystal (PC) structures were reviewed by Joannopoulos et al.

Resonant-microcavity structures have been demonstrated with different active media and different microcavity structures. The first resonant-cavity structure was proposed by Purcell (1946) for emission frequencies in the radio frequency (rf) regime. Small metallic spheres were proposed as the resonator medium. However, no experimental reports followed Purcell's theoretical publication. In the 1980s and 1990s, several resonant cavity structures have been realized with different types of optically active media. The active media included organic dyes, semiconductors, rare-earth atoms, and organic polymers. In these publications, clear changes in spontaneous emission were demonstrated including changes in spectral, spatial, and temporal emission characteristics.

The simplest form of an optical cavity consists of two coplanar mirrors separated by a distance [L.sub.cav], as shown in Fig. 1.5. About one century ago, Fabry and Perot were the first to build and analyze optical cavities with coplanar reflectors. These cavities had a large separation between the two reflectors, i. e. [L.sub.cav] >> [lambda]. However, if the distance between the two reflectors is of the order of the wavelength, [L.sub.cav] [approximately equal to] [lambda], new physical phenomena occur, including the enhancement of the optical emission from an active material inside the cavity.

At the beginning of the 1990s, the resonant-cavity light-emitting diode (RCLED) was demonstrated, initially in the GaAs material system, shown in Fig. 1.6, and subsequently in organic light-emitting materials. Both publications reported an emission line narrowing due to the resonant cavities. RCLEDs have many advantageous properties when compared with conventional LEDs, including higher brightness, increased spectral purity, and higher efficiency. For example, the RCLED spectral power density at the resonance wavelength was shown to be enhanced by more than one order of magnitude.

The enhancement of spontaneous emission can be calculated based on the changes of the optical mode density in a one-dimensional (1D) resonator, i. e. a coplanar Fabry-Perot cavity. We first discuss the basic physics causing the changes of the spontaneous emission from an optically active medium located inside a microcavity and give analytical formulas for the spectral and integrated emission enhancement. The spontaneous radiative transition rate in an optically active, homogeneous medium is given by (see, for example, ref.)

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