RICERCA ITALIANA     

Photo-active Molecular and Polymeric Materials for Optoelectronics and Photonics

Università della Calabria

Abstract

Different photonic and optoelectronic technologies have already reached the stage of commercial production, in some cases by a long time, such as for example photovoltaic cells. The dominant approach is still based upon crystalline or amorphous inorganic semiconductors, whose properties of electronic conduction and interaction with electromagnetic radiation have been understood since long ago. Developments regarding molecular or polymeric materials are more recent and based on a series of considerations:
- With Carbon-based chemistry it is possible to design and synthesize structures with tailored properties
- Non-covalent intermolecular forces can be used to obtain complex functionalities
- Thin films with controlled nano-morphology can be obtained via different techniques, including ink-jet printing
- Low temperature processing compatible with flexible, lightweight substrates is available
- Materials and processes are potentially low cost, even for large area manufacturing
However, all such properties must be associated with suitable functional (optical and electronic) performance. In this respect, organic materials still lag behind their inorganic counterpart, although they are constantly improving and the first niche commercial products have appeared. The lower performance is certainly tied to the lower level of understanding of some of the most important physical phenomena relevant for their use. In turn, this is due to intrinsic experimental and theoretical problems that slow down the structure/property and the structure/function rationalization processes.
This proposal is centered in this area of photonics and optoelectronics, regarding the study of photo-activated or photo-correlated phenomena in molecular and polymeric materials: electronic conduction (including photoconduction), luminescence, photorefractivity and non-linear optical properties. The main aspects of the proposed research can be summarized as follows:
- Three different sets of new generation materials will be considered, all exhibiting charge photogeneration and transport and/or other photo-induced effects: two of them (photoconducting cyclometalated complexes and chirooptic multifunctional polymers) have been chosen because of the promising performances already demonstrated in the recent past. The work with the third set (luminescent chiral metal complexes, of completely new design) is one of the challenges of the project, as their functional properties are an open question.
- The main objective orienting the synthesis of new photoconducting cyclometalated complexes will be a full understanding and the optimization of the photogeneration process. A complete photophysical characterization of the obtained complexes will be performed, as well as photoconductivity, photogeneration efficiency and charge mobility measurements. The same materials will be considered for thin-film photovoltaic cells.
- New chirooptic polymers will be synthesized and their photoconduction will be characterized. The influence of light ellipticity and handedness will also be investigated. The control of the supermolecular organization of the chiral structure using light will be refined and its influence on transport properties investigated. The photorefractive performance will be studied below Tg.
- New chiral metal complexes will be synthesized and their luminescent and liquid crystalline properties will be characterized. Their use as pure mesogens, or as high concentration dopants of chiral mesophases, in systems exhibiting an optical band-gap for Distributed Feed-Back (DFB) Lasing will be assessed.
- Although basic research will be carried out, the activities will be oriented towards applications and attention will be paid to the relevance of results in the general areas of molecular electronics and of organic photonics/optoelectronics.
- Research will be interdisciplinary, with good perspectives for technologically relevant results deriving from the interaction of researchers with experience in the synthesis of molecular and polymeric materials, in spectroscopic analyses, in different preparation and fabrication techniques and in the photophysical characterization of optoelectronic properties.
- A particular emphasis will be devoted to the development of multifunctional materials.
- Supramolecular structure and organization within amorphous, crystalline or mesomorphic materials (including chiral ones) will be used as a means to induce new functionalities or to increase the ones already exhibited at the molecular level. <<<

Principal Investigator

Attilio Golemme Università della CALABRIA

Research Objectives

The performances of optoelectronic and photonic devices based on organic materials, when compared to their more traditional inorganic counterpart, are still low. This is a consequence of the poorer level of several functional features such as, for example, charge mobility or photogeneration efficiency. The aim of this project is the synthesis, the understanding of the basic properties and the development of improved functionalities of molecular and polymeric materials for photonics and optoelectronics. Given the similar functionalities required by different applications, results will be relevant in a variety of areas including photovoltaic, Light Emitting Devices (LED’s), photorefractive and Organic Thin Film Transistor (OTFT) based technologies.
The need of complex and time consuming (and therefore expensive) processes for assembling different materials with different functionalities in multilayered structures, in connection with the poor understanding of the physical properties of the related interfaces, is another factor affecting negatively the use of organic materials. For this reason, the development of multifunctional materials will be one of the main targets of this project. In addition, particular attention will be given to the structural organization of molecular materials, since the functional levels connected with nano-aggregates, liquid crystalline order or supramolecular organization go well beyond the sum of the contributions of the single molecules.
The success of this project will require the tight interaction of different research groups with complementary instrumentation and competences in the synthesis of molecular and macromolecular compounds, in the preparation of samples and in functional, spectroscopic and photophysical characterizations. Considering classes of materials, the objectives of the project will be the following:

a) Photoconductive Cyclometallated Complexes.
One of the targets of this part of the project will be the search for a deeper understanding and the optimization of the charge photogeneration behaviour of cyclometallated (palladium and platinum) complexes, recently identified as excellent photogenerators. This entails the carrying through of the following intermediate objectives:
- Synthesis of new complexes with (N^N) metallation through ligands such as 2-(2’-pyridil)pyrroles, featuring different sostituents in 3 and 5 positions.
- Synthesis of Ni(II) analogues of the previously investigated Pd(II) and Pt(II) complexes.
- The introduction of both hole and electron donating moieties on the periphery of the cyclometallated Pd(II) and Pt(II) core complexes.
- The addition of suitable peripheral chemical functionalities (-Si(OEt)3) allowing the grafting of these photogenerators onto specific surfaces, such as nanocrystalline electrodes (e.g. TiO2).
- Measurements of photogeneration efficiency, photoconductivity, charge mobility, photorefractive performance of the resulting complexes.
- A complete photophisical characterization of the compexes, including excited states life-times and dynamics.

b) Chiral Complexes with Emission Properties.
The relevance of this research line is twofold: one objective is to obtain polarized emission from mesophases and the other one to improve the lasing performance in DFB systems with an optical band-gap. The intermediate targets are:
- Synthesis of new luminescent chiral (possibly mesogenic) metal complexes obtained from salen-like ligands (salen = N,N’-bis(salicylidene-1,2-ethylenediamine) and M(II) salts (M = Zn, Pt).
- Characterization of their luminescent, chirooptical and liquid crystalline properties.
- Use of the complexes either as pure chiral mesogens or as high concentration dopants of chiral mesophases in systems exhibiting an optical band-gap and measurement of Distributed Feed-Back (DFB) Lasing performance.

c) Photoactive Multifunctional Polymers.
Due to the presence of side-chain chiral groups, when irradiated with CP light these polymers may assume either of two stable chiral conformations (depending of the helicity of the light) characterized by one prevailing helicity of the chromophores and detectable by circular dichroism (CD). As the polymers contain other functional groups (photocromic, NLO and photoconducting), they can be extremely interesting for a variety of applications, where photo-controlled functionalities can modulate a series of properties, ranging from NLO and optical storage to chirooptical switching and photorefractivity. The targets of this research line will be:
- Synthesis of side-chain methacrylic polymers with i) an azoaromatic photochromic chromophore, ii) a carbazole moiety and iii) a chiral group of one prevailing configuration interposed between the main chain and the chromophore in each repeating unit.
- Synthesis of chiral main-chain regioregular polythiophenes.
- Chirooptical and NLO characterization of oligomers and polymers.
- Control of charge photogeneration and transport properties using light, via its influence on the supramolecular organization of the chiral structure.
- Photorefractive performance assessment for the non-poled polymers below the glass transition temperature.

From a methodological point of view, the objectives of the projet include the development of preparation and fabrication techniques and methods for:
a) Preparation of thick films by capillary absorption between conducting substrates for characterization involving the application of electric fields.
b) Preparation of thin films at controlled temperature by spin coating with or without solvents.
c) Production of thin films of the synthesized organic substances by high vacuum thermal deposition.
d) Deposition of electrodes by thermal evaporation in high vacuum.
e) Controlling chain conformations in chiral polymers.

Although the proposal can be classified in the area of basic research, the expected results can be relevant for a series of technologies: information and communication (optical data processing and storage, image treatment), illumination and displays (LED’s, Lasing), renewable energy (photovoltaics), control and monitoring (sensors and optical detectors). The evaluation on a lab scale of the properties of the developed materials in connection with such applications is among the aims of this project. <<<

First Results

During the past decades, synthetic organic materials, often referred to as plastics by the public, have increasingly transformed our daily lives. With their light-weight and unique mechanical properties, they are molded and cast into various goods in high volume and low cost. In recent years, new functionalities such as electrical, electro-optical, light harvesting, and light-emitting properties have been built into organic materials and polymers. Advances in the understanding and optimization of these properties through structure-property relationships have fueled the emergence of a new technology that has the potential to lead to a new industrial revolution. Researchers and engineers worldwide see major new opportunities for information technology in this low cost, large area, and flexible alternative to silicon. For instance, displays based on organic light-emitting diodes (OLEDs) have become commercial and are gaining new markets rapidly. Because they are self-emitting, OLEDs consume less power than common liquid-crystal displays that require backlighting. Likewise, organic photovoltaic cells (OPVs) based on organic semiconductors are gaining a lot of attention because of their paper-like form-factors and their ability to be produced on highly flexible substrates. Light-weight flexible organic solar cells are expected to find numerous applications in commercial and consumer markets.
Since it deals with the photo-related properties of a series of molecular and polymeric compounds, the research we propose, and its results, are well within this mainframe. A first summary of the expected results can be presented following the same schematics adopted to illustrate the objectives of the project.

a) Photoconductive Cyclometallated Complexes.
The broad range result we intend to reach with these materials is a deeper understanding of the charge photogeneration behaviour of the cyclometallated (palladium and platinum) complexes that have recently been identified as excellent photogenerators. As these compounds form a new class of charge photogenerators, the understanding of the mechanism of charge separation following photoexcitation is essential for their full exploitation in devices. Materials with optimized properties will be extremely interesting as photosensitizers in organic photovoltaics and as photorefractive materials with lower working tensions. In order to reach these targets, several intermediate results are required, as illustrated in the following.
- The range of compounds exhibiting photogeneration must be widened, with the aim of establishing a structure/property relationship. Considering both complexes with (N^N) metallation and Ni(II) analogues of the previously investigated Pd(II) and Pt(II) complexes, we expect to synthesize at least 15 new compounds belonging to at least three omologous series.
- Complexes with hole and electron donating moieties on the periphery of the cyclometallated Pd(II) and Pt(II) core will be obtained. At least two series of three compounds will be prepared with i) an electron donating group on the HOMO carrying ligand, ii) an electron withdrawing group on the LUMO carrying ligand and iii) both.
- The photophysical characterization of the complexes and the measurements of their functional properties (photogeneration efficiency, photoconductivity, charge mobility) will guide the synthesis in order to obtain complexes with optimized performance. We expect the photogeneration efficiency to increase by at least two orders of magnitude with respect to the state of the art.
- The addition of suitable peripheral chemical functionalities (-Si(OEt)3) allowing the grafting of these photogenerators onto specific surfaces, such as nanocrystalline electrodes (e.g. TiO2), will allow the testing of the cyclometallated complexes as photoreceptors in hybrid organic-inorganic devices. Being a completely new field of research, in this area we just expect a proof-of-concept result.

b) Chiral Complexes with Emission Properties.
The results expected in this area are relevant for two different technologies: OLEDs and Distributed Feed-Back (DFB) Lasing.
- A few emitting zinc (II) complexes are already known but, given the coordination geometries preferred by this metal, the resulting molecular shapes do not favour mesophase formation. A first good result will be the synthesis of Zn (II) and Pt (II) complexes (from salen-like ligands) with molecular shapes suitable for mesophase induction. In addition such complexes must exhibit luminescent properties.
- One important result we expect to achieve is polarized emission from the resulting mesophases. Being chiral phases, the resulting emission might even be circularly polarized. Polarized emission is an importantant added value for the OLEDs technology.
- The complexes will be used as pure chiral mesogens or as high concentration dopants of chiral mesophases in systems exhibiting an optical band-gap and measurement of DFB lasing performance will be carried out. As the active species will be mesogenic, its concentration can be increased if necessary, without the restrictions imposed by mesophase stability. We expect two results from such innovation: i) much higher emission intensities and ii) the possibility of using lower pump energy.

c) Photoactive Multifunctional Polymers.
Due to the presence of side-chain chiral groups, when irradiated with Circularly Polarized (CP) light these polymers may assume either of two stable chiral conformations (depending of the helicity of the light) characterized by one prevailing helicity of the chromophores. As the polymers contain other functional groups (photocromic, non-linear optical and photoconducting), they can be extremely interesting for a variety of applications, where photo-controlled functionalities can modulate a series of properties, ranging from NLO and optical storage to chirooptical switching and photorefractivity. The results we will obtain could therefore go well beyond what we expect today, that is listed in the following.
- A first result will be the synthesis of side-chain methacrylic polymers with an azoaromatic photochromic chromophore, a carbazole moiety and a chiral group of one prevailing configuration interposed between the main chain and the chromophore in each repeating unit.
- A second results will instead be the synthesis of main-chain polymers: chiral regioregular polythiophenes.
- One important results for applications will be a full control of the chain conformations using light. CP light has already been used for this purpose, and we expect to extend studies to linearly and elliptically polarized light.
- This will offer the opportunity to tune charge photogeneration and transport properties with light. Such a light control of photoconduction is, for what is in our knowledge, unprecedented.
- Photoconduction is one of the necessary requirements for photorefractivity. In photorefractive media, photogenerated charges redistribute in space, setting up an electric field which, in turn, changes the refractive index: the net result is a photogenerated phase hologram. The field-induced refractive index variation is due to reorientation of internal dipoles above the glass transition temperature (Tg), or to NLO effects in pre-poled polymers. As the polymers that will be synthesized exhibit light induced reorientation well below Tg, a photorefractive behaviour can be expected even at such temperatures. This would provide the first photorefractive polymer where stable, erasable, rewritable holograms could be encoded with a simple and time-effective process. In addition, these materials offer the opportunity to study the role of chirality (i.e. of the lack of centrosymmetry) in photorafractive media. <<<

Timescale

24 months

National and international background

Molecular and polymeric semiconductors have recently been the subject of impressive research efforts, with the aim of breaking the dominance of Silicon, and in general of the inorganic materials, in electronic, optoelectronic and photonic devices [1]. Such an activity is certainly driven by a number of properties exhibited by organic materials:
- structural flexibility and lightweight;
- potential low cost;
- low temperature processing;
- compatibility with low-cost fabrication over large areas, such as roll-to-roll manufacturing;
- compatibility with several processing techniques from solution;
- nanomorphology control over large areas, such as with ink-jet printing.
Moreover, by using Carbon-based chemistry, it is possible to design molecules with properties tailored for specific applications and non-covalent intermolecular interactions can lead to important functional added value. The fact that different technologies have manifested a surging interest in molecular materials is therefore not surprising and today organic materials for lasers, light emmitting devices, transistors for flexible electronics, sensors and photovoltaic devices are the target of intense academic and industrial research. In addition, because of the relevance in applications such as image and data treatment and storage and in information and telecommunication technologies, the drive towards an organic approach is particularly relevant when electronic conduction is associated with light sensitivity [2]. Such an association is present in materials with functionalities for light emitting, photovoltaic, non-linear optical (NLO) and photorefractive applications.
Nonetheless, the mentioned appealing features of molecular materials would not be sufficient by themselves, without proper functional performance: good optical and electronic properties are necessary. In this respect, organic materials still lag behind their inorganic counterpart, although progresses are evident and the first commercial devices in niche areas have appeared. This is certainly a consequence of the still poor understanding of the basic physics underlying their functional properties. In the following, the state-of-the-art of research regarding organic materials in fields that are relevant for this proposal will be reviewed.

One of the fundamental issues for the development of photonics is the acquisition of new skills in the field of non-linear optics, with a particular emphasis on frequency generation, optical phase-conjugation and optical switching. At the same time, the discovery of new classes of materials, which can be more easily designed and synthesized with respect to their inorganic counterpart, has led to a series of new and interesting opportunities. In order to properly design molecular structures, one of the crucial aspects is the assessment of the relationships existing between structure and both linear and nonlinear optical response of the materials [3]. At the moment, the understanding of NLO materials does not allow the implementation of efficient and competitive devices. For this reason, basic research regarding the fundamental properties of candidate materials, as well as applied research for their best use in devices, is needed. Some of the most interesting polymers exhibiting NLO properties are those containing the azoaromatic moiety [4]. Such materials are of interest because of their peculiar optical properties and applications in optical storage [5], holographic memories [6], surface relief gratings [7] and optical switching. An intense interest has also been recently manifested towards the amplification of chirality of polymeric materials, in solution as well as in the solid state [8], since these substances can be used as chiroptical switches and sensors. The noncentrosymmetry inherently present in chiral materials may also allow for the observation of second order NLO properties in thin films [4,9]. In polymeric derivatives containing an azoaromatic moiety, the azo chromophores can assume dissymmetric conformations, thus originating a remarkable enhancement of optical activity [10]. On the basis of interesting examples of chiroptical molecular switches, involving a reading process based on circular dichroism properties [11], the evaluation of the chiroptical features related to the azo-group photoisomerization in polymeric systems with a prevailing chirality appears reasonable. Further research, concerning the photoinduced optical properties of these substrates upon irradiation of thin films with circularly polarized light, has not only shown relevant chiroptical and birefringence properties, but it has also indicated the fascinating possibility of photomodulating the chiroptical properties of polymeric films by using circularly polarized light of the appropriate handedness [12].
A further aspect related to photoresponsive materials concerns the investigation of the chiroptical properties of main chain conjugated polymers, in particular regioregular polythiophenes bearing in the side chain a chiral group of one prevailing configuration. Polythiophenes represent a wide group of materials with readily modulable electronic properties through functionalization with suitable moieties to give photo- and electroactive materials [13]. In particular, chiral polythiophenes have revealed interesting features as NLO materials, circularly polarized light emitters and enantioselective sensors [14]. Recent studies concerning the optical activity of chiral polythiophenes have evidenced that optical activity is strongly affected by regioregularity, as well as by conformational changes induced by temperature and interaction with chemicals [15]. In addition they exhibit the unusual feature of displaying strong dichroic effects in the spectral region related to the - * transition when they are in the microaggregate form or in the solid state, whereas in solution of a good solvent or in the melt these effects are actually negligible. The real nature of this behaviour, as far as its molecular origin is concerned, is still under discussion.

Charge photogeneration is one of the main required functionalities for photorefractive applications and in materials for photovoltaics, photodetection and thin-film transistors. Light absorption in organic materials leads to the creation of excitons that possess a strong binding energy, which is typically on the order of 0.5 eV. In most inorganic materials like Si, excitonic binding energies are smaller than kT at room temperature (0.025 eV) and optically created excitons dissociate efficiently into electron-hole pairs. Due to much larger binding energies, excitons in organic solids do not dissociate in the bulk very efficiently. Fortunately, efficient exciton dissociation can take place at heterojunctions formed between dissimilar materials when energy band (frontier orbital) offsets between hole and electron transport materials are larger (&gt; 0.5 eV) or of the order of the exciton binding energy. Therefore, it is critical for excitons formed in organic solids to diffuse to such heterojunctions where they will undergo efficient dissociation into electron-hole pairs. These separated charges are then transported by diffusion and drift and contribute to the photocurrent: in photovoltaic devices under the influence of the built-in field due to the difference in work function between the electrodes and in other devices under the influence of applied fields.
A relevant contribution to higher photogeneration efficiency can be obtained by using molecular architectures that produce a lower exciton binding energy. In this respect, cyclopalladated complexes, a new class of photoconducting and photorefractive materials [16], can be important. These complexes contain a Palladium (II) or a Platinum (II) atom coordinating two different ligands in a square-planar geometry and they exhibit photoconducting performances of the same level as the best known organic materials. We found by theoretical calculations and confirmed by experimental data, that HOMO and LUMO are located on the two different ligand moieties and therefore physically separated through the metal centre. In addition, it has been observed by modelisation of the excited states that the molecules, which are flat in the ground state, assume a twisted conformation around the metal centre upon excitation. Both effects could contribute to an intramolecular switch, delaying the return to the fundamental state, and consequently leading to relevant charge photogeneration [17]. However, the understanding of the photogeneration mechanism in these materials is still far from complete, and consequently their full potential in photorefractive and photovoltaic devices still unexplored. Photorefractive materials are those in which charges photogenerated by a non-uniform light pattern redistribute in space, setting up an internal electric field that in turn changes the refractive index: the resulting hologram is a replica of the original light distribution.

The second half of the 80’ has seen the start of a surging interest in the synthesis of organic compounds with emissive properties, to be used as thin films in optoelectronic devices [18]. During the following years, improvements in electroluminescence efficiency, operating voltage and color selection allowed the development of Organic Light Emitting Devices (LED) suitable for the market place [19]. Nonetheless, some problems limit a large scale commercial diffusion of this technology: the cost, mainly connected with the complex film deposition techniques, the short life-time due to thermal, morphological and chemical instabilities and the low emissive yields, consequence of the poor charge transport properties.
But LED’s are not the only possible application for emitting organic materials, as they also offer considerable potential for inexpensive visible lasers. There are several reasons why organic substances can be attractive laser materials. The first obvious one is that emission is not uncommon among organics. In addition, often emission spectra are broad, providing the possibility for making tunable lasers. Absorption coefficients can also be quite high, which implies that there is a potential for extremely strong light amplification. Moreover, absorption and emission spectra can be (and often are) well separated, so that absorption of emitted light is negligible. Nonetheless, the full exploitation of organics as lasing materials has yet to come and they are still far from being ready for the market-place, the main problems laying with high threshold energy and the need of optical rather than electrical pumping.



[1] S. R. Forrest, Nature 428 (2004) 911.
[2] see the issues of Materials Today on organic lasers, TFT, photovoltaics and CMOS (September 2004), molecular electronics and interfaces (July-August 2005), flexible electronics and organic devices (April 2006), organic transistors (March 2007) and photovoltaics (November 2007).
[3] M. Albota et al., Science 281 (1998) 1653; Y.-Q. Qin et al., Appl. Phys. Lett. 83 (2003) 1071; P. Innocenzi et al., J. Sol-Gel Sci. Tech. 35 (2005) 225.
[4] H. Katz et al., J. Am. Chem. Soc. 109 (1987) 6561; J. A. Delaire, K. Nakatani, Chem. Rev. 100 (2000) 1817.
[5] W. M. Gibbons et al., Nature 351 (1991) 49; N. C. Holme et al., Opt. Lett. 21 (1996) 902.
[6] T. Todorov et al., Appl. Opt. 23 (1984) 4588; L. Andruzzi et al., Macromolecules 32 (1999) 448.
[7] Y. Wu et al., Macromolecules 37 (2004) 6090; A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 4139.
[8] T. Kajitani et al., J. Am. Chem. Soc. 128 (2006) 708; E. Yashima et al., Chem. Eur. J., 10 (2004) 42; A. J. Wilson et al., Angew. Chem. Int. Ed. 44 (2005) 2275; M. M. Green et al., Nature 268 (1995) 1860.
[9] T. Verbiest et al., J. Mater. Chem., 9 (1999) 2005.
[10] L. Angiolini et al.: Polymer 39 (1998) 6621; Macromol. Chem. Phys. 201 (2000) 533; Polymer 42 (2001) 4005; J. Polymer Sci. Part A Polym. Chem. 37 (1999) 3257; Enantiomer 3 (1998) 391; Synth. Met. 115 (2000) 235.
[11] W. F. Feringa, et al. Angew. Chem. Int. Ed. 34 (1995) 348.
[12] L. Angiolini et al., Chem. Eur. J. 8 (2002) 4241.
[13] T. A. Skotheim et al, Handbook of Conducting Polymers (1998) Dekker, New York; J. Roncali, Chem. Rev. 97 (1997) 173.
[14] L. Pu, Acta Polym. 48 (1997) 116; F. Lebon et al., Mat. Res. Soc. Symp. Proc. 708 (2002) BB10.28.1.
[15] B. M. W. Langeveld-Voss et al., J. Am. Chem. Soc. 118 (1996) 4908; B. M. W. Langeveld-Voss et al., J. Mol. Struct. 521 (2000) 285; M. Catellani et al., Chem. Mater. 14 (2002) 4819.
[16] I. Aiello et al., J. Am. Chem. Soc. 123 (2001) 5598; I. Aiello et al., Adv. Mater. 14 (2002) 1233; R. Termine et al., Adv. Mater. 15 (2003) 723; M. Ghedini et al., Coord. Chem. Rev. (2006), in press.
[17] N. Godbert, D. Dattilo, R. Termine, I. Aiello, A. Golemme, M. Ghedini, M. Amati, F. Lelj, Molmat 2006: International Symposium on Molecular Materials based on coordination and Organometallic Chemistry, June 2006 (Lyon, France).
[18] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913; J.H. Burroughes et al., Nature 347 (1990) 539.
[19] B. D’Andrade, S. R. Forrest, Adv. Mater. 16 (2004) 1585. <<<