GENERALITIES: ORGANIC ELECTRONICS OR PLASTIC ELECTRONICS? WITH WHAT MATERIALS?
Scientifically speaking we tend to use the term “organic electronics” when discussing electronics based on organic materials, notably those with π-conjugation. Nevertheless, in an age when it is often more important to communicate science rather than perform it, it can appear more salient, and thus more saleable, to talk about plastic electronics and flexible electronics (assuming that the supports for the components are themselves flexible). These last two appellations are all the more attractive than the first, as the term “organic” also has the inconvenience of disinteresting a number of students and researchers by associating (organic) chemistry with electronics. This is without any doubt a shame, as the two disciplines are mutually enriching. Despite this set-back, occasionally nature manages to do things well and this argument that has at least two positive assets. First, the ensuing applications are relatively “brilliant”: they result in what some see as visually stimulating objects, for example flexible screens and lighting, and others see as panaceas to our grave environmental problems in the form of solar cells. Second, the subject has been well-profiled in the media through the recognition of the Nobel prize for chemistry awarded in 2000 to the Americans Alan. G. McDiarmid and Alan J. Heeger, and the Japanese H. Shirakawa. In addition, in 2010, the “Millennium Technology Prize” (presented by the Finnish Academy as similar to a Nobel prize but for work in the field of Technology) was given to Michaël Graetzel as the inventor of a third generation of solar cells that employ dyes and to Sir Richard H. Friend whose team discovered electroluminescent diodes based on polymers (PLEDs) and greatly participated in the development of organic thin-film transistors (OTFTs) and organic photovoltaics. One of us, A. Moliton, had great pleasure in working with Sir Friend in the 1990s. The third Laureate, Stephen B. Fürber, worked in a different field and was recompensed for his development of “ARM 32-bit RISC microprocessors”.
With these well-merited prizes the otherwise difficult-to-promote field of organic electronics is now more clearly identifiable and is, one hesitates to say, respected. And while there have been a few “pure” electricians who have found it hard to admit that it is possible to perform electronics with materials other than those found in periodic columns IV, II-V, II-VI, the field of organic physics and electronics is working. Organic electroluminescent diode (OLED) screens have a market place worth $340 million in 2009, while organic photovoltaics are projected at €600 million in 2015 (and €3.4 billion in 2020). Here we shall give a very brief overview, with respect to the aforementioned science, of the general characteristics of the materials used in organic electronics.
Since the beginning, organic materials have been considered (rightly or wrongly) as being readily manufactured and based on easily-sourced resources. Nevertheless, the poor understanding of the origin of their electronic and optical properties has hampered their wider use in electronic and optical applications over many years. In contrast to inorganic semiconductors, organic semiconductors are not atomic solids. In general, they are π-conjugated materials for which the transport mechanisms, as an example, are quite different from “classical” solid-state physics. As for inorganic media, in a different manner, defaults such as traps, along with molecular and macromolecular structural irregularities, play an essential role in the comprehension of transport phenomena.1 It is worth noting that pendent bonds, intrinsic to inorganic materials in for example surface atoms of a silicon crystal that result in a bonding fault, do not systematically appear in organics. In effect, the bonds make up molecular entities and therefore do not give rise to bond rupturing at the extremities unless in an extrinsic manner, for example when subject to degrees of irradiation or heating that break certain molecules.
-conjugated polymers give a double advantage by bestowing good electronic transport properties (with a more relaxed delocalisation of electrons in the medium than their inorganic counterparts) and good optical properties. The energy difference between the π and π* bands, respectively, assimilated as the valence and conduction bands by solid-state physicists, is typically of the order of 1.5 to 3 eV, and in an ideal manner covers the optical domain. So effective is this overlap, it wouldn't be surprising to see life forms elsewhere in this universe that exploit this effect. We can note, in simple terms, that polymers such as polyethylene, which only contain simple σ-bonds, exhibit extremely high resistivities due to the σ-bond localisation of electrons. Their gap, associated with the transition between σ and σ*-bands, is of the order of 5 eV, which no longer corresponds to the optical gap.
Initially, researchers persisted in trying to reproduce the semiconducting properties of inorganic electronics with organic materials, notably those which led to the invention of the transistor in 1948 by Bardeen, Shockley and Brattain. While carbon is in the same column as silicon, organic materials display a notable advantage that has been well-exploited in bringing this about. They are the product of organic chemistry and as such can be varied in a near infinite number of ways. The chemistry of carbon is not exclusively tied to other carbon atoms (this is not an atomic solid as we have briefly indicated) in contrast to what goes on in the crystalline form of silicon, with the exception of doping, but even this only concerns around one atom in a million. Something that many do not know is that the very first silicon crystal doping was not done in a substitutional manner but interstitially with lithium, which resulted in unstable n-type doping. Indeed, doping was not stable until the use of substitutional doping with elements from column V (resulting in n-type materials), and column III (p-type doping). In organic materials, only interstitial doping can be exploited, with charge transfer between alkali dopants and organic materials leading to n-doping, or charge transfer between halogen dopants and organic materials leading to p-doping. Conversely, the usual (as in inorganics) substitutional doping cannot be performed with organics because this process requires the breaking of chemical bond that inevitably lead to the loss of the chemical characteristics of the organic material. Finally, the results of this simple interstitial doping, where the dopants “flee” in the organic lattice is an extremely disparaging drop in conductivity. A step around this is the use of ionic implantation,2, 3 but this technology has not really been adapted to the development of low cost electronics (even if, without doubt, initial studies of p-i-n OLEDs merits being pushed to the limits). In addition, it is by methods different to conventional doping (interstitial by charge transfer), that the adjustment of the electronic transport properties could be carried out properly: an improvement in the intrinsic transport of materials (for example by increasing charge mobilities); doping-dedoping of polyanilines by acid-base equilibration, generation of in-situ charges by photo-excitations or by field effects.
Furthermore, organic materials have one of the greatest cards to play, that is through chemical tailoring which can be performed by breaking chemical bonds and replacing chemical groups with others. While this results in the initial loss of the chemical identity of the material, it does mean that there are numerous variations possible. The use of organic chemistry can give an almost infinite number of chemical structures, and it is this that can be used to play with the relative positions of bands (the energy levels), in addition to the all important physical-chemistry properties of the materials (solubilities, processibility and so on). Thus by associating two different materials, gradients favourable to electronic transport (or the dissociation of electron-hole pairs) are accessible. Potential gradients resulting in charge blockages can also be obtained, which can be used to favour radiative recombinations. All these properties are, for example, described in reference 4.
THE BIRTH OF ORGANIC ELECTRONICS AND THE COUPLED DEVELOPMENT OF MACROMOLECULAR CHEMISTRY
Given the interesting properties mentioned above, certain π-conjugated materials were widely studied throughout the 20th century. Notable is the case of anthracene5 for which quantum theory has made possible an interpretation of numerous physical properties. It is an interesting example though because the earlier crystals were too thick and this necessitated the use of too high a tension to observe electroluminescence, and this limited further developments. Similarly, linear polyacetylene has been synthesised since 1968,6 but it was only available as a black powder that did not lend itself to practical use. There was also a premonitory study of the alternating structure of polyene chains in 1962,7 and similarly, laminated organic systems were explored in 1958 for photovoltaic applications.8
In the 1970s, it was shown that two classes of organic materials can exhibit metal-like electrical conductivities. First, molecular crystals based on a mixture of two components, one a donor of electrons, the other an acceptor (for example, the equimolar derivatives of tetracyanoquinodimethane, or TNCQ, and tetrathiafulvalene or TTF). These mono-dimensional conductors9 which can give rise to an interesting superconductivity at low temperatures,10 were not greatly developed due to their poor processibility. Second, π-conjugated polymers. Towards the end of the 1970s, organic π-conjugated materials in the form of thin-films made possible the rise of “conducting polymers”. Thus, decisive progress was made following the synthesis of polyacetylene in films,11 and then by grace of a better comprehension of transport and doping properties of the π-conjugated materials.12 Two articles that can be considered as the foundations of organic electronics are those from J. L. Brédas13 and from A. J. Heeger,14 which played an essential role in proposing interpretations that were both convincing and seductive. In similarity to the theories elaborated for inorganic semiconductors, the electronic and optical properties of organic solids are intimately tied. They both come from interpretations based on quantum mechanics, and the macroscopic phenomena are determined by effects that intervene at molecular and environmental levels, defaults and impurities included. Quasi-particles (polarons and excitons for example) introduced into inorganic media are equally used to interpret the electronic and optical behaviour of organic samples.1 In organics, there are specific characteristics due to the nature of the organic materials such as important electron-lattice interactions.
On from this, the field of “polymer conductors” was enriched with the appearance of the first true organic components that finally gave birth to the field of “organic electronics and optoelectronics”. In effect, the insertion of π-conjugated materials into various components has given them, incontrovertibly, a place in the development of this theme. The first components to really break through were OLEDs, with the pioneering articles appearing in 1987 for “small molecules” such as15 Alq3 and in 1990, for π-conjugated polymers with poly(para-phenylene vinylene).16 The success of OLEDs stimulated the birth of organic photovoltaics. While the use of Schottky-type photodiodes with π-conjugated materials appeared disappointing in its disassociation of electron-hole pairs,17 donor-acceptor based systems was revealed in 1992 to be a more promising route to generating separate photo-induced charges.18 Subsequent and rapid developments demonstrated that a good system could be obtained from a composite of a donor and an acceptor which finally resulted in a system based on a bulk-heterojunction.19, 20 Yields of 3% in 2000 now reach around 8% following recent announcements from Konarka21 (8.3 %) and Solarmer22 (8.1 %), and potential yields of 9% are now being considered.23
For its part, the birth of the organic transistor is in greater flux. For G. Horowitz,24 the first field effects were clearly demonstrated in the 1970s (although at this date, organic photovoltaic components with yields less that 0.1 % had already been published).25–27 Finally, it is above all the groups of R. H. Friend28, 29 and F. Garnier30, 31 who did much to contribute to the emergence of this domain to which one of the authors made a modest contribution.32 Its development has only really come about though since 2000.24, 33, 34
At last, one century after the discovery of the photovoltaic effect in anthracene in 1906 by Pocchetino, organic synthesis has made materials with qualities adapted to the needs of organic electronics and the “old dream” of developing electronics and optoelectronics with thousands and millions5 of organic components has come about.35
Organic chemistry has gained enormously from the stimulation provided by electronics and optoelectronics. As noted above, the first available materials were not easily shaped or even soluble in solvents. Most π-conjugated polymers were prepared via electrochemistry, and prepared from simple repeat units, such as para-phenylene or thiophene. Indeed, most of these structures were highly crystalline, but were also intractable and immobile from their point of formation on an electrode.
There has been an imaginative development of soluble π-conjugated polymers, often by adding alkyl or ether chains to the main body of the polymer that combine various properties, and this has led to massive improvements in properties. They can be dissolved in solvents and cast, the solvent evaporating to leave a film of controllable thickness. Furthermore, processing involving annealing and solvent treatments changed their morphologies and therefore their opto/electronic properties changed considerably too. The recent development of chain-growth condensation polymerisations, has permitted the development of polymers with controlled molar masses and properties.36 The incorporation of π-conjugated segments into block copolymers made possible the exploitation of block copolymer nano-morphologies. These exhibit domains of like-segments self-assembled in various forms, lamellae, gyroid and so on, which elegantly, and again by a fortunate blessing of nature, are of a size commensurate with many of the underlying electronic processes.37–46 Furthermore, such materials can be used to influence the meso-structure.47 For example, domains are of the order of tens of nm, a length equivalent to the mean pathway of an exciton.
It is therefore expected that there will continue to be an enormous growth in this domain. If atoms were musical notes, and macromolecules music, the range of structures attainable would be beyond anything as yet heard by humankind.
THE BASIC ORGANIC ELECTRONIC COMPONENTS
Physically speaking, the function of each of the different components can be seen in terms of the various excitation mechanisms involved in the π-conjugated (macro)molecules. In addition to conventional doping (by charge transfer) other types of excitation can modify the charge density in an organic material, and can be exploited in a range of components. Here we show some examples.
First, excitation by a field effect can be exploited to give thin film transistors as represented in Fig. 1. Electrostatic interactions locally modify the concentration of charge in a thin organic transport film.
Second, by electrical excitation with charge injection either at one electrode i.e., unipolar, or more effectively with hole injection at the anode and electron injection at the cathode i.e., bipolar, for electroluminescent applications in OLEDs and lighting. The steps leading to radiative emission are schematised in Fig. 2. The use of transport layers, for example n and p which are around the emitting layer in a p-i-n structure, and phosphorescent materials permit considerable increases in performances.
Third, optical excitation where for example a solar ray generates electron-hole pairs at the heart of the organic material. These particles are tied to one another by strong Coulombic interactions found in organic materials (where the dielectric permittivity is small, εr ≈ 3), and form an excitonic, electron-hole quasi-particle. Figure 3 shows the steps leading to the formation and disassociation of this quasi-particle and the subsequent collection of charges. The type of organic donor material—organic acceptor material interface plays an essential role. A wide range of methods, such as improving the donor-acceptor morphology, using specific layers, or tandem structures, are leading to an optimisation of the performances of the organic solar cells thus described.
In the articles proposed in this special series, one can measure the considerable effort that has been deployed to obtain new organic components for which industrial development has now commenced.4 These articles are an extension of those that appeared in Polymer International concerning excitons48 and charge transport49, materials,43, 50, 51 OLEDs,52 and organic photovoltaics,53–55 areas that have been well-developed over the last decade. The authors of this series of reviews were all participants in the recent “Dispositifs Electroniques Organiques” (DIELOR) conference, held at the end of 2010 in Presqu'ile de Giens in France, and the articles give an insight into the current fast and almost eclectic way in which semiconducting materials are, or are on their way to, occupying domains classically associated with inorganic materials. They include reviews and discussions on organic56 and hybrid57 photovoltaics, organic transistors and photo-transistors,58 and last but not least, lasers.59 It is hoped that these reviews will give a view of the multiplicities of organic materials in electronic and optoelectronic applications. The first by Ratier et al.56 gives an exceptionally lucid explanation of the basic physical characteristics of organic photovoltaic devices, something that we think that many chemists will appreciate, before going a more specialized but nevertheless highly relevant review of the use of carbon nanotubes in such devices. The next paper by Boucle and Ackermann57 which we warmly recommend gives an extremely useful overview of the history and benefits of combining organic and inorganic materials to produce the so-called hybrid photovoltaic devices. Lucas et al give a careful and generously detailed study of the development of organic transistors which we think will be of great interest to both general and specialist readers.58 And finally, Chénais and Forget give a realistic appraisal of the current state of organic materials in lasing technologies, from fundamental and applied perspectives, and to our mind this prime review transmits a sense of the ingenuity that is being used in this exciting field.59
We hope that these reviews will be of use to those working in both the physics and chemistry of organic materials for electronic devices, a community that has been growing for many years in the domain of organic electronics.