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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Due to their special properties, organic semiconductors enable the development of large-area, low-cost devices, paving the way for flexible and nomadic applications that advantageously replace those made with traditional semiconductors. In this second volume, we study the main applications of organic semiconductors, such as organic light-emitting diodes (OLEDs), solar cells (OPVs) and organic field-effect transistors (OFETs). The commercialization of these new devices is then discussed within the Brabec triangle framework, in which yield, stability and production costs are the key factors. We also address the environmental impact of organic devices for their future development. This book presents the application side of organic electronics from a technological, economic and environmental perspective. It is intended for researchers and students in university programs or engineering schools specializing in electronics, energy and materials.
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Contents
Cover
Title Page
Copyright
Introduction
1 Organic Light-Emitting Diodes
1.1. Introduction
1.2. Reminders on optics
1.3. OLED operating principle
1.4. OLED applications
1.5. Conclusion
2 Organic Solar Cells
2.1. Introduction
2.2. Solar spectrum
2.3. Operating principle
2.4. Characteristic parameters of solar cells
2.5. Organic materials
2.6. P3HT:PCBM
2.7. Perovskite
2.8. Solar cells based on organic, hybrid and silicon materials
2.9. Strategies to improve the performance of organic and hybrid solar cells
2.10. Conclusion
3 Organic Transistors
3.1. Introduction
3.2. Operating principle
3.3. Principal OFET parameters
3.4. Materials
3.5. Ambipolar transistors and semiconductors
3.6. Light-emitting transistors
3.7. OFET applications
3.8. Conclusion
4 The Brabec Triangle
4.1. Introduction
4.2. Device efficiency
4.3. Stability of materials and devices
4.4. Organic device production cost and marketing
4.5. Synthesis on Brabec’s criteria
4.6. Environmental dimension
4.7. Prospects and developments
List of Acronyms
References
Index
Other titles from iSTE in Electronics Engineering
End User License Agreement
List of Tables
Chapter 2
Table 2.1. Comparison of the characteristics of organic, hybrid and silicon mate...
Chapter 3
Table 3.1. Relative permittivity of inorganic insulators
Table 3.2. Relative permittivity of organic insulators
List of Illustrations
Chapter 1
Figure 1.1.Parameters of light sources and receivers
Figure 1.2.CIE chromaticity diagram. For a color version of this figure, see www...
Figure 1.3.P–N junction LED operating principle
Figure 1.4. Principle of obtaining white light from chromophores or emitting mat...
Figure 1.5.Structure and operating principle of a basic organic light-emitting d...
Figure 1.6. OLED p-i-n structure with hole and electron blocking layers. For a c...
Figure 1.7.Inverted fluorescence process
Figure 1.8. Main fluorescent materials: a) poly(phenylene-vinylene) (PPV) – gree...
Figure 1.9. Main phosphorescent materials: a) Ir(MDQ)2(acac) – green; b) Ir(ppy)...
Figure 1.10.RGB light emitters
Figure 1.11.Emission of white light: a) by mixing the colors of stacked RGB-emit...
Figure 1.12. Optical losses during extraction of light emitted towards the outsi...
Figure 1.13. The diode structure of: a) a standard OLED; b) a standard TOLED; c)...
Figure 1.14. Structure of a transparent OLED. For a color version of this figure...
Figure 1.15. Structure of an OLET. For a color version of this figure, see www.i...
Figure 1.16.Switching circuit of a pixel of the screen: a) with PMOLED passive m...
Chapter 2
Figure 2.1. Solar spectra for air mass AM0, AM1.5 and the emission spectrum of a...
Figure 2.2.The stages of the photovoltaic effect: a) absorption of a photon and ...
Figure 2.3.Dissociation of charges in a) a P–N junction solar cell; b) an organi...
Figure 2.4.Concepts of solar cell structure: a) bilayer structure; b) bulk heter...
Figure 2.5.Structure of a solar cell using an absorber comprising nanorod arrays...
Figure 2.6. Photovoltaic effect in a complete solar cell. For a color version of...
Figure 2.7. Equivalent circuit of a solar cell: a) ideal cell in the dark; b) id...
Figure 2.8. J(V) characteristics and photovoltaic parameters of a solar cell in ...
Figure 2.9.Examples of electron-donor organic materials: a) pentacene; b) triphe...
Figure 2.10.Examples of non-fullerene acceptor materials: a) perylene diimide (P...
Figure 2.11. Structure of hybrid perovskite. For a color version of this figure,...
Figure 2.12. Perovskite networks: a) 1D; b) 2D; c) 3D
Figure 2.13. Structure of a perovskite solar cell in standard configuration. For...
Figure 2.14. Hysteresis effect of the current–voltage characteristic. For a colo...
Figure 2.15.Marketing criteria for solar cells: a) Brabec triangle; b) Krebs dia...
Figure 2.16. Summary of the physical processes in an organic solar cell in opera...
Figure 2.17. Orbital mixing mechanism for reducing the D–A polymer gap
Figure 2.18.Examples of low bandgap organic materials: a) sexithiophene (6T); b)...
Figure 2.19. Principle of tandem cells. For a color version of this figure, see ...
Figure 2.20. Electrical connection of the sub-cells of a tandem cell: a) in seri...
Figure 2.21. Silicon–perovskite-based tandem cell configurations: a) two-wire te...
Chapter 3
Figure 3.1. Structure of an OFET. For a color version of this figure, see www.is...
Figure 3.2. (a) Bipolar NPN transistor in common emitter configuration; (b) curr...
Figure 3.3. The operating modes for MIS structures: (a) flat band; (b) accumulat...
Figure 3.4. Field effect in a p-channel OFET: (a) in the linear regime; (b) in t...
Figure 3.5. Types of OFET structure: (a) TG/BC structure; (b) TG/TC structure; (...
Figure 3.6.Parameters of the transfer characteristic of a P-type OFET, used in t...
Figure 3.7. Transfer characteristics of an OFET: (a) P-type; (b) N-type showing ...
Figure 3.8.Effect of gate-bias stress V
GS
on the transfer characteristic
Figure 3.9.Examples of insulating and SAM materials used in OFETs
Figure 3.10A. Examples of active materials used in OFETs
Figure 3.10B. Examples of active materials used in OFETs (cont.)
Figure 3.11. Schematics of (a) the output characteristic and (b) the transfer ch...
Figure 3.12.Single-SC ambipolar OLET structures: (a) monolayer; (b) bilayer; (c)...
Figure 3.13. Structure of a vertical organic light-emitting transistor (VOLET). ...
Figure 3.14.Structure of a chemical field-effect transistor: (a) TFT structure; ...
Chapter 4
Figure 4.1. The possible optical coupling modes in an OLED structure. For a colo...
Figure 4.2.Shift of the threshold voltage as a function of time due to the volta...
Figure 4.3. Lifetime of an organic light-emitting device: a) different definitio...
Figure 4.4.Diagram of the life-cycle assessment of a solar cell
Figure 4.5. Revised Brabec triangle. For a color version of this figure, see www...
Guide
Cover
Table of Contents
Title Page
Copyright
Introduction
Begin Reading
List of Acronyms
References
Index
Other titles from iSTE in Electronics Engineering
End User License Agreement
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Series Editor
Robert Baptist
Organic Electronics 2
Applications and Marketing
Thien-Phap Nguyen
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd
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London SW19 4EU
UK
www.iste.co.uk
John Wiley & Sons, Inc.
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Hoboken, NJ 07030
USA
www.wiley.com
© ISTE Ltd 2021
The rights of Thien-Phap Nguyen to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2021939365
British Library Cataloguing-in-Publication Data
A CIP record for this book is available from the British Library
ISBN 978-1-78630-610-4
Introduction
Organic electronics can be defined as a branch of general electronics that focuses on studying the properties and applications of organic semiconductors. These materials, referred to in common usage as plastics, are, in fact, a distinct class separate from that of ordinary plastic materials. They may be small molecules, or conjugated polymers, which have an electronic structure comparable to that of conventional semiconductors. Therefore, their physical properties are very similar to the properties of these latter materials. However, organic materials are generally amorphous and their electrical conductivity is much lower than that of traditional semiconductors. As a result, they have been neglected for a long time despite the discovery of some of their notable physical properties, such as the discovery of electroluminescence in anthracene in the 1960s.
In all scientific or technological disciplines, an idea, concept or creation can arise with a change or progression and profoundly alter the course of the evolution of the discipline – and sometimes even the course of the history of science. Such was the case in 1947 when the researchers John Bardeen, William Shockley and Walter Brattain invented the transistor, a device that would revolutionize traditional electronics. This invention is what first allowed for the design of circuits that provided logical functions, then for these circuits to be integrated into complex systems capable of handling sophisticated tasks, and finally for these miniaturized systems to be incorporated into the portable electronic devices that we now use every day. A similar progression occurred in organic electronics in 1987, when Tang and Van Slyke used the evaporation of small molecules in a vacuum to create diodes that emitted light in the form of a thin film that operated with a turn-on voltage lower than 10 V. Their work demonstrated that, in practice, organic devices are suitable for optoelectronic applications in the same way as their inorganic counterparts. A few years later, Burroughs et al. (1994) demonstrated that by using conjugated polymers deposited as thin films from a solution, they could also produce light-emitting diodes with a low operating voltage. This work paved the way for film depositing techniques using solutions and led to the production of devices through printing developed later in laboratories. For this domain, we will use the descriptors “flexible” or “printed” to indicate the discipline of organic electronics. Advances in research have been very rapid and have shown remarkable results, not only in the understanding of physical processes in materials and components but also for the creation of new electronic devices now available on the market. Today, organic electronics has become its own field, one that will prove to be important technologically and economically in the near future.
In France, education on organic electronics at the university level is developing, but remains limited in comparison with other European countries. It should be noted that these courses are generally offered in universities that carry out industrial research and development activities on organic electronic materials and devices. With regard to the books written on this discipline, there are very few titles available in French.
This book, divided into two volumes, was written with the goal of introducing organic electronics to students and researchers who are interested in this new discipline. It is organized into the following sections: Chapter 1 of Volume 1 provides an overview of the basic notions of the theory of traditional semiconductors, of which some of the material presented will be used later on. The materials used for the creation of devices are described in Chapter 2. The physical processes, which take place in the volume and interface of the layers of devices, are presented and explained in Chapters 3, 4 and 5. Volume 2 presents the primary applications of organic materials in optoelectronic devices. The first chapter of this volume centers on organic light-emitting diodes (OLEDs), the second on organic solar cells (OSCs or OPVs) and the third on organic field-effect transistors (OFETs). Chapter 4 deals with the practical and economic aspects of the industrialization of organic components. It also includes a discussion on the environmental aspects of the use of organic materials and devices.
As the title indicates, this book is not intended to provide a detailed and complete description of the materials, physical processes and applications involved in organic electronics. This would be far beyond the scope of what can be addressed within a single book. The choice of topics and of the extent to which the subjects are discussed were made by the author, based on his own experience from research and as an instructor. Interested readers will be able to find more detailed discussions on certain topics using the references provided. A list of acronyms used in the text is also included at the end of this book.
I would like to thank the people who devoted their time to proofreading and providing comments and suggestions, which allowed me to improve the writing of this book and the way in which certain areas are presented: Philippe Lerendu, Serge Lefrant, René Leparoux, Agnès Bournigal-Giret, Maxime Bayle and Jean-Luc Duvail. I would also like to thank ISTE for proposing this project, which I hope will help to arouse public interest in the new and promising possibilities offered by organic electronics in the scientific and technological communities in France and around the world.
1Organic Light-Emitting Diodes
1.1. Introduction
According to the International Energy Agency (IEA), worldwide electricity consumption in 2015 represented 18% of total energy consumption, just behind oil at 41%. Almost one-fifth of electricity consumption is devoted to lighting, whether public, private, domestic or industrial. Until recently, incandescent lamps or fluorescent tubes were universal sources of lighting, with low efficiency and short lifetimes. These lamps have gradually been replaced by new inventions in the field of lighting, such as compact fluorescent lamps, semiconductor light-emitting diodes (LEDs), and then organic light-emitting diodes (OLEDs). These sources not only have good optical performance, but are also economical to use, as their electricity consumption is low compared to older generation lamps under the same lighting conditions.
OLEDs result from organic electronic technology and symbolize a new concept of lighting. This concept is based on a modern design, presenting evolving shape and volume, and on energy saving and sustainable development. Indeed, OLED lamps are composed of emitting surfaces, not point sources such as traditional lamps. In addition, they are lightweight and produced on thin substrates, enabling them to be mounted at the desired locations without any particular constraints. The materials used to manufacture the devices are available, non-toxic and not harmful to nature.
In addition to their use in lighting, OLEDs are also used in display and visualization devices. Their characteristics make it possible to design screens of higher quality than that of conventional technologies, with advantages due to the nature of the organic materials used: they are lightweight and have large active surfaces and low manufacturing costs. In this chapter, we will study the characteristics of OLEDs and their operations in comparison with conventional semiconductor (SC) LEDs.
1.2. Reminders on optics
1.2.1. Photometry and radiometry
When we use a lamp to light a room, we can distinguish between the values associated with the light source (the lamp) and the receiver (the room). These values are expressed in two different ways, depending on the perspective chosen. From a physical perspective, we consider light as a ray of wavelength λ, and the values associated with this ray are called radiometric quantities. From a vision perspective, light is associated with the perception and sensitivity of the human eye. Each ray of wavelength λ is perceived by the eye as a visible color. The associated values are called photometric quantities. In the definitions of the values, we will specify the units used in radiometry and photometry.
Let us consider a point source emitting a light beam of flux ΔΦ and solid angle ΔΩ. The source intensity in the flux direction is defined by:
The unit of energy flux is the watt (W), and that of luminous flux is the lumen (lm). The unit of radiant intensity is the watt per steradian (W/sr) and that of luminous intensity is the candela (cd).
The luminance of a light source is defined as the source intensity per unit area in a given direction (Δ). It characterizes the source seen by an observer whose eye is placed in direction (Δ). We obtain:
where ΔI is the intensity of the source, ΔS × cos α represents the apparent surface of the source seen by the observer and α is the angle formed by the normal to the surface of the source and direction (Δ). The photometric unit of luminance is the candela per square meter (cd/m2). The radiometric unit of luminance (also called radiance) is the watt per square meter per steradian (W/m2/sr).
For the receiver, we define the illuminance or the irradiance as the incident light flux per unit area. Its unit is the watt per square meter (W/m2). The corresponding photometric unit is the lumen per square meter (lm/m2) or the lux (lx).
Figure 1.1.Parameters of light sources and receivers
Perception of light varies from one individual to another. For the same average observer, perception is maximum for the wavelength λ = 555 nm and the perceived sensation of luminosity is different when the light spectrum varies. To compare the perception of different light sources, we define the luminous efficacy or the visibility factor as the ratio of the visible light flux to the power absorbed by the source. It is expressed in lumen per watt (lm/W).
1.2.2. Colors
1.2.2.1. Chromaticity diagram
The visible light spectrum approximately covers the range of wavelengths from 400 to 800 nm. There are three primary colors, whose wavelengths are, respectively, λ(R) = 700 nm for red, λ(V) = 546 nm for green and λ(B) = 435 nm for blue. Any radiation of the visible spectrum corresponds to a color that can be obtained from a mixture of the three primary colors. For this presentation, we use components that are said to be trichromatic, defined by the following relations:
We then deduce:
However, using these components presents some mathematical disadvantages, so new, fictitious primary colors, X, Y, Z, are proposed to replace them. These colors are mathematical transforms of RGB (red, green and blue) radiations. They take account of the notion of luminance, which introduces the notion of human vision, and the associated new components are defined by:
We obtain:
This is the equation of a plane (P) in space. An arbitrary color, C, is represented in the plane by a point, called the color point, of coordinates (x, y, z). Yet only two of the three coordinates, x and y, for example, need to be known in order to determine the color point (as z = 1 − x − y). The diagram is presented in two dimensions with the x coordinate on the abscissa and y on the ordinate. This diagram is the chromaticity diagram of the CIE (Commission Internationale de l’Eclairage – International Commission on Illumination). The reference color point corresponds to the color white. Its coordinates in the (x, y) plane are . It is located at the center of the diagram. The points of arbitrary color are located between the center and the edges. The dominant color is red when x > y, green when y > x and blue when z > x, y.
Figure 1.2.CIE chromaticity diagram. For a color version of this figure, see www.iste.co.uk/nguyen/electronics2.zip
1.2.2.2. Color temperature
To characterize the color of an incandescent source, we use the color temperature (T) parameter, defined as the temperature of a black body whose radiation color is identical or close to that of the source. In other words, it is the temperature at which a black body needs to be (thermally) heated to obtain the same color as the emitter. The temperature of solar radiations is approximately 5,800 K when the sun is at its zenith. Those of daylight vary between 4,000 and 7,500 K. In lighting, white light is favored for visual comfort. Two shades of white can be distinguished: warm white, with a low color temperature (< 3,500 K), yellowish in tone; and cool white, with a high color temperature (> 4,500 K), bluish in tone.
1.2.2.3. Color rendering index
To evaluate the colorimetric quality of a light source, we use a parameter called the color rendering index (CRI), a number with a value that varies between 0 and 100 and which is used to evaluate the source’s ability to reproduce the colors of an object illuminated by this source. The index allocated to a source is compared to that of a reference source with the same color temperature (generally the black body) that has an index of 100. For a high-quality white-light source, the CRI is greater than 85.
1.3. OLED operating principle
1.3.1. P–N junction LED
1.3.1.1. Operating principle
When we apply a voltage, V0, to the terminals of a forward-biased P–N junction, the electrons of the N-type SC diffuse across the space charge region (SCR) in the vicinity of the P-type SC, and the holes of the P-type SC diffuse into the N-type SC. These minority charge carriers recombine with the majority carriers of the SC in which they are to be found after diffusion. The recombination region is limited by the diffusion length of the holes, Lp, in the N-type SC and the diffusion length of the electrons, Ln, in the P-type SC. The active emission region is determined by these lengths from the limits of the junction SCR.
Figure 1.3.P–N junction LED operating principle
1.3.1.1.1. Recombination of excess carriers
In an ideal junction, each electron injected from the N-type SC into the P-type SC generates a photon. If the recombination is radiative, the energy of the photon emitted is equal to the energy of the SC gap:
Let n0 and p0 be the concentrations of electrons and holes of the SC at equilibrium. When the SC is in non-equilibrium through an internal process, the carrier concentrations become n =n0 + dn and p = p0 + dp. The spontaneous recombination rate is defined as the speed at which the concentration of carriers decreases. When an exciton disappears, an electron and a hole disappear. We can therefore write:
The recombination probability of a CB electron with a VB hole is proportional to the concentration of VB holes. We can therefore write R ∝ p. The same reasoning for the recombination of a VB hole with a CB electron leads to R ∝ n. Finally, we can write:
where B is a proportionality coefficient called the bimolecular recombination coefficient. For use in light emission applications, SCs need to have a high recombination coefficient (10-11-10-9 cm3s-1).
1.3.1.1.2. Emission of photons
When a voltage, V0, is applied under forward bias at the junction, the potential barrier between two SCs decreases by a value equal to |e|V0. The diffusion current of majority carriers from one SC to the other proportionally increases to . The diffused carriers originating from an SC are minority carriers and may recombine with the majority carriers of the other SC. If the recombination is radiative, there will be an emission of photons by the junction.
Let n0 and p0 be the concentrations of electrons and holes of the SC at thermal equilibrium, n and p the concentrations of electrons and holes under electrical excitation and Δn and Δp the concentrations of excess electrons and holes. The recombination rates of excess carriers are written as:
By overlooking the second-order term, we obtain the number of recombinations due to the excess carriers created by electrical excitation:
We can write this term in the form:
In this expression, we have:
where τrp is the radiative lifetime of the holes and τrn is the radiative lifetime of the electrons. Other physical processes that do not produce photon emission are referred to as non-radiative recombination and contribute to the overall lifetime, τ, of charge carriers.
We can write:
where τr and τnr are the radiative and non-radiative lifetimes, respectively, of the carriers (holes or electrons).
The carrier lifetimes are used to evaluate the efficiency of the emission of photons by the SC under electrical excitation. We can define a parameter called the internal quantum efficiency using the ratio:
It represents the ratio of radiative recombination to the overall recombination (radiative and non-radiative).
1.3.1.2. Materials and colors
The SCs and their components for the LED emitter are chosen according to the material emission color, that is, according to their gap. The most commonly used materials are compounds of elements belonging to column III (boron, aluminum, gallium and indium) and to column V (nitrogen, phosphorus, arsenic and antimony). By varying the composition of the compounds, we can adjust their gap and thus modify the color emitted. The LEDs marketed can emit wavelengths from 1.43 eV (infrared for the compound GaAs) up to 5.90 eV (ultraviolet for the compound AlN). Diodes emitting the color blue are difficult to produce because their efficiency is low and the compounds are generally unstable. A Japanese team led by three researchers, Akasaki, Amano and Nakamura (Nakamura 1998), succeeded in synthesizing a compound of indium gallium nitride (InGaN) that emitted a stable blue light of high efficacy (300 lm/W). This discovery was awarded the Nobel Prize in Physics in 2014.
It is not at present possible, however, to find compounds emitting white light. Nevertheless, where lighting is concerned, white light provides essential visual comfort as it is close to natural light. To obtain a spectrum covering the entire visible range with a relatively constant intensity, three techniques are available:
– a combination of the three primary colors, red, green and blue, which produces white light. The main difficulty with this technique is the dosage of the intensity of emission colors required to obtain a homogeneous white without any dominant tint or shade (see
Figure 1.4a
);
– use of red, green and blue phosphors excited by UV radiations from an emitter. The resulting emission spectrum practically covers the visible spectrum, enabling white light to be obtained. The color emission efficiency is low owing to the chromophore excitation process, which constitutes an energy transfer (see
Figure 1.4b
);
– use of a blue emitter associated with a yellow-emitting phosphor. This technique is the most widely used in industry, as it uses only two materials to obtain a white emission (see
Figure 1.4c
).
Figure 1.4.Principle of obtaining white light from chromophores or emitting materials. For a color version of this figure, see www.iste.co.uk/nguyen/electronics2.zip
1.3.1.3. Extraction of light emitted
The architecture of an LED must, in principle, be designed such that it enables all of the photons created in the diode to be extracted towards the external medium. In reality, because of the optical processes at the SC/external medium interface, few photons are collected on the outside of the diode.
1.3.1.3.1. Power losses at the semiconductor/air interface
Let us consider an SC of refractive index n, which emits photons to ambient air of index n0 = 1. First, for rays perpendicular to the SC surface, the reflection coefficient is:
For an SC of refractive index n = 3, the value of R is 0.25. The percentage of photons reflected on the surface of the SC is 25%.
For rays of incidence of angle θi with respect to the normal to the surface, the angle of refraction, θr, in the external environment is given by the Snell–Descartes law:
As n > n0, the incident ray will be entirely reflected on the surface of the SC when θr ≥ π/2. The limit incident angle, θl, is such that:
and:
If the incident angle is greater than θl, the ray will be reflected on the surface of the SC and will not be released from the sample. It can then be absorbed by the material or be released, attenuated, to the external environment after several reflections.
The rays emitted in the SC and contained in the volume of a cone, called an extraction cone, with a half-angle at the top, θl, will be released into the medium external to the SC.
Let Ωl be the solid angle corresponding to the extraction cone. We obtain:
For a small θl angle, we can write:
The rays emitted by the SC cover a solid angle equal to Ω = 4π (steradians). The ratio of the solid angle of the rays emitted inside the SC to that collected in ambient air is:
Let Ps be the optical power of the light source. The power available in the extraction cone of this source is:
For an SC of refractive index n = 3, the power available in the extraction cone is approximately 2.70% of the power of the rays emitted in the SC.
However, the power actually collected on the outside of the diode needs to take account of the reflection on the surface of the SC, whose coefficient, R, is given by expression [1.16]. The transmission of the rays emitted is determined by the transmittance, T:
The power actually available on the outside is:
The optical efficiency is defined by the expression:
For n = 3, the optical efficiency of the diode will be ηo ~ 2%.
1.3.1.3.2. Improving extraction efficiency
According to expression [1.25], the optical efficiency depends on the refractive index, n, of the SC and we can improve it by changing the material and choosing an appropriate one. This is not a solution in practice. We can, however, change the external surrounding medium of the SC to modify the limit incident angle, θl, because the latter depends on the refractive index, n0, of the medium external to the SC.
According to expression [1.18], we obtain:
By repeating the same optical efficiency calculations as those used for the case of air (n0 = 1), we obtain the new expression of ηo for an external medium of refractive index n0:
Generally, marketed diodes are wrapped in an epoxy dome to protect the active surface from moisture and mechanical impact. The refractive index of this polymer is n0 ~ 1.50. The optical efficiency of the source with an SC of refractive index n = 3, and covered with an epoxy dome, is ηo = 5.50%. This value assumes that the light collected in the dome is entirely transmitted to the ambient air. To arrive at this, we place the P–N junction at the center of the hemispherical dome such that all of the rays of light arrive perpendicular to the dome surface and are transmitted without being reflected.
To increase the extraction efficiency of rays emitted, we can texture the surface of the SC by employing nanostructures that reflect the incident rays such that they are transmitted to the outside after one or two reflections on the faces of these nanostructures. We can also use distributed Bragg reflectors, which are flat, reflective surfaces with different refractive indices. These reflectors, placed between the SC and the substrate, make it possible to reflect the rays emitted towards the output surface, and then towards the outside, with no notable losses.
1.3.1.4. LED efficiency
For each physical process occurring in the conversion of electrical energy into optical energy of the LED, a corresponding efficiency can be defined in order to be able to evaluate the effectiveness of each operation.
1.3.1.4.1. Injection of charges and emission of photons
The efficiency of the conversion is evaluated by the internal quantum efficiency, which is defined by:
As a function of the measures, this efficiency is written as:
In this expression, Pi is the optical power (in W) of the photons emitted by the junction and I is the current intensity or electric current (in A) that flows in the diode.
1.3.1.4.2. Extraction of photons
The efficiency of the photon extraction is evaluated by the extraction efficiency or optical efficiency, which is defined by:
The optical efficiency is given by expression [1.25].
1.3.1.4.3. Injection of charges and emission of photons to the external medium
The efficiency of the conversion of electrical energy into optical energy is evaluated by the external quantum efficiency, which is defined by:
Its expression is as follows:
Let P0 be the luminous power of the photons emitted by the diode to the external medium. The number of photons emitted to the external medium per second is equal to P0/hν. The expression of the external quantum efficiency is:
1.3.1.4.4. Energy conversion
The efficiency of the energy or power conversion is evaluated by the energy efficiency or the power conversion efficiency (PCE), which is defined by:
This efficiency has no unit as it is equal to the ratio of two powers (in W). However, in terms of visibility, the luminous power, P0, is a function of the luminous flux, Φ, as follows:
The parameter Km is the maximum photopic spectral luminous efficacy and is equal to 683 lm/W for the wavelength λ = 555 nm.
At this wavelength, an optical power of one watt corresponds to a luminous flux of 683 lm.
The function V(λ) is the spectral luminous efficacy function, representing the sensitivity of the human eye to different colors. It is maximum at the wavelength λ = 555 nm, which corresponds to the color yellow-green.
The luminous efficacy ηEL is defined for a lighting source by:
It represents the luminous flux measured by an observer with respect to the electrical power used to produce this light. The unit of ηEL is the lumen per watt (lm/W).
A 60 W lamp producing a power of 900 lm has a luminous efficiency of 15 lm/W. Note that with regard to lighting, luminous efficiency is often referred to as luminous efficacy of the light source.
1.3.2. OLEDs
The basic operating principle of OLEDs is the same as that of inorganic diodes and certain concepts introduced in section 1.3.1 are also applicable to OLEDs.
However, the organic nature of emitters in OLEDs requires the introduction of other parameters, which we examine in this section.
1.3.2.1. Operating principle
1.3.2.1.1. Basic OLED structure
In a basic OLED structure, under the application of a voltage V0 at the diode terminals, the electrons are injected from the cathode and the holes of the anode into the active organic layer. The charge carriers of mobilities μnand μp move and can recombine to emit a photon constituting the light emitted by the diode. Electroluminescence processes comprise the following steps:
– injection of charges into the SC from the electrodes;
– transport of charges injected under the influence of the electric field;
– formation of excitons in the SC;
– radiative recombination of excitons and emission of photons;
– extraction of photons into the external medium.
Figure 1.5.Structure and operating principle of a basic organic light-emitting diode. For a color version of this figure, see www.iste.co.uk/nguyen/electronics2.zip
The transparent conducting anode is very often a glass or plastic substrate covered with a layer of ITO. The metal cathode is often a thin layer of metal of low work function. The active layer is an organic SC of small molecules or conjugated polymer.
The injection, diffusion and recombination processes are described in Volume 1. Note that not all charge carriers recombine as some can cross the active layer without encountering a carrier of the opposite sign. These carriers constitute the leakage current (of holes or electrons). At the origin of these leakage currents are, first, the presence of electrically active traps in the interface regions and in the active layer that capture the carriers. Second, in most organic SCs, mobilities μn and μp are different, leading to two charge carrier currents, one majority and one minority. This dissymmetry reduces the probability of the recombination of injected carriers in the active layer. In both cases, the charge carriers in the leakage currents do not contribute to light emission in the OLEDs.
