Optical technologies, thin films and interfaces for organic photonics

In the last decades, organic compounds brought a technological revolution in the field of miniaturised light sources and photovoltaic devices. Organic electroluminescent devices have become increasingly important because of their potential applications for large area flat-panel displays and solid-state lighting. The tuneable emission and high fluorescence quantum efficiency make organic materials attractive also for the development of optical amplifiers, lasers and photo-detectors, as well as for photovoltaic devices and solar cells. All these novel organic devices are based on thin-film hetero-structures. In the last years, in the framework of the national research project TECVIM, we have focused on growth methods of organic and nano-structured thin films for organic photonics, spectral investigation of their light-emission properties and degradation mechanisms, laser imaging and patterning technologies, methods of electronic characterisation of interfaces, optical modelling of multilayers, which are critical for the overall improvement of multi-layered device performances. These technologies are suitable for the realisation and characterisation of functionalised optical and electronic thin-film devices of highest efficiencies and open interesting perspectives in the fast-growing field of organic photonics and its application in alternative energy and sustainable developments

Tecnologie ottiche, film sottili e interfacce per la fotonica organica

Negli ultimi 25 anni, i materiali organici hanno rivoluzionato la tecnologia delle sorgenti di luce miniaturizzate e dei dispositivi fotovoltaici. La rilevanza tecnologica dei dispositivi organici elettroluminescenti è fortemente cresciuta per le loro potenziali applicazioni negli schermi piatti a larga area e nell’illuminazione a stato solido. L’accordabilità dell’emissione luminosa e l’elevata efficienza quantica dei materiali organici li rendono particolarmente promettenti anche per lo sviluppo di amplificatori ottici e laser, fotorivelatori a stato solido, nonché’ dispositivi fotovoltaici e celle solari. Tutti questi dispositivi organici innovativi sono basati su etero-strutture a multistrato costituite da film sottili. Negli ultimi anni, nell’ambito del Progetto di Ricerca nazionale TECVIM, abbiamo studiato e sviluppato metodi di crescita di film organici e nano-strutturati per la fotonica organica, la caratterizzazione delle proprietà spettrali dell’emissione luminosa e dei suoi meccanismi di degradazione, le tecnologie di imaging e patterning mediante laser, la caratterizzazione elettronica delle interfacce, la modellistica ottica dei multistrati, che sono critici per il miglioramento delle prestazioni dei dispositivi organici. Queste tecnologie sono adatte alla realizzazione e caratterizzazione di dispositivi ottici ed elettronici funzionalizzati basati su film sottili ed aprono prospettive interessanti nell’ambito della fotonica organica applicata alle energie alternative ed allo sviluppo economico sostenibile

 

Rosa Maria Montereali, Francesca Bonfigli, Doriano Brogioli, Antonino Santoni, Maria Aurora Vincenti,
Enrico Nichelatti

(download pdf)

 

In the last decades, organic compounds brought a technological revolution in the field of miniaturised light sources. In particular, since Tang and Van Slyke in 1987 [1] demonstrated efficient electroluminescence (EL) from an organic light-emitting diode (OLED) based on thin film technology, there has been large interest in these materials and devices for flat and flexible full-colour panel displays [2] and for low-consumption solid-state illumination.

Organic semiconductors are a broad class of materials that comprises small molecules and conjugated polymers. The relative easiness of handling, the chemical tuning of their emission properties, the low synthesis cost combined with their high fluorescence quantum efficiency, make organic materials attractive for the development of optical amplifiers [3], lasers [4] and photo-detectors [5]. In recent years, there has been a great deal of interest in organic semiconductors also for photovoltaic (PV) devices, solar cells [6], and field-effect transistors. All these novel organic devices [1-6] are based on thin-film hetero-structures. One of their main advantages is that they can be manufactured at low temperatures on flexible substrates by simple fabrication processes like coating, spraying and printing techniques. Moreover, by defining structures at sub-micron level into organic media, i.e., on a spatial scale comparable with their emission wavelengths, the interaction between light and matter can be exploited to engineer novel effects in the emerging field of nano-photonics [7].

In these last years, in the framework of the national research project TECVIM, we have focused on the development of thin-film growth techniques, spectral investigation of their light-emission properties and degradation mechanisms, laser patterning technologies, optical modelling, and electronic characterisation methods of interfaces, which are critical for the improvement of OLED performances. They can be extended to other kinds of functional photonic and electronic devices that are based on organic-dielectric-metallic thin film hetero-structures. An overview of the main results is here reported.

 

Growth and optical characterisation of organic thin films and OLED

The most studied low-molecular-weight molecule is a metal-chelate, tris (8-hydroxy-quinoline) aluminium (Alq3), commonly vacuum deposited as the active layer in green OLEDs in the form of thin film of typical thickness less than 100 nm.

The basic device structure of an OLED consists of one or more organic layers sandwiched between a high-work-function and Transparent Conducting Oxide (TCO) anode, generally Indium Tin Oxide (ITO), and a low-work-function metallic cathode. A small electron injection barrier height at the organic/metal electrode interface is favoured to increase the OLED efficiency. Al thin films are preferred as cathode due to their better environmental stability and their straightforward use as contact material in Si-based integrated circuits. Insertion of an ultra-thin lithium fluoride (LiF) layer – of typical thickness below 15 nm – between the organic film and the Al cathode was found to significantly reduce the threshold voltage in Alq3-based [8] OLEDs.

In 2001, for the first time in Italy, a double-layer Alq3-based OLED with a LiF/Al cathode was realised and characterised by our group [9] at ENEA Frascati Research Centre. Currently the experimental activities about thermal evaporation of organic (Alq3), dielectric (LiF) and metallic (Al) thin films [10] are finalised to investigate their optical and morphological characteristics and improve growth reproducibility.

 

3 thin films for organic photonics

Many efforts have been devoted to increase the lifetime of organic encapsulated devices by studying the effects of several degradation processes, primarily associated with the degradation mechanisms of the electrode materials and electrode/organic interfaces [11].

The exposure of transporting molecular layers to ambient factors, like moisture, oxygen and light induces degradation phenomena and the quenching of their green photoluminescence (PL) [12]. Thermally evaporated Alq3 thin films were used to study the degradation behaviour under thermal treatments in different atmosphere [13] by means of optical absorption and photoluminescence linear spectroscopy [14].

The photo-bleaching effects under selective ultraviolet (UV) and blue light illumination on the photoluminescence spectra of thermally evaporated Alq3 thin films were also investigated [15]. Alq3 thin films of different thicknesses, from 50 to 80 nm, were thermally evaporated onto amorphous glass and fused silica substrates. They were kept in the evaporation chamber under low vacuum pressure (10-3 mbar) in the dark for a few days and, after their first exposure to air and the measurement of their initial optical characteristics, the time evolution of their PL spectra was monitored under selective illumination with UV and blue LEDs. UV light illumination at 385 nm induced a strong and fast decrease in the Alq3 films PL intensity. In Figure 1a, several PL spectra measured at different irradiation times under UV LED illumination were reported. In Figure 1b, the experimental values of the peak intensity of the PL spectra, measured every 5 min, were reported in a logarithmic scale as a function of the irradiation time.

Figura1Montereali.jpg

FIGURE 1

a) PL spectra of an Alq3 film (50 nm thick) thermally evaporated on a silica substrate, measured at different irradiation times under UV LED illumination: 1-0 min, 2-5 min, 3-15 min, 4-30 min, 5-60 min. b) PL peak intensity at different irradiation times under UV LED illumination. The best fit (solid line) of the experimental points with a double exponential decay curve is reported

 

The observed behaviour can be described as the sum of two exponential decays, as confirmed by the best fit. We also deduced that the significant reduction in the PL intensity is accompanied by a slight modification of the spectral characteristics of the Alq3 film broad emission (blue shift). The spectral position of film fluorescence is found to be correlated with both the molecular density of the packing and the length of interligand contacts between neighbouring Alq3 molecules: the shorter the contacts, i.e., the denser the film, the more the fluorescence is red-shifted.

To explore the role of the energy of the impinging light, similar experiments were performed under illumination of a blue diode, whose emission is peaked at 455 nm, in the spectral range where the optical absorption and emission spectra of Alq3 films overlap. Again, a decrease in the Alq3 PL intensity was observed [15]. The photo-bleaching process was less effective as the blue-light spectrum is outside the main absorption band of Alq3. Indeed, at the wavelengths of the blue LED that are longer than those of the UV one, spectrally centred around the peak of the Alq3 main absorption feature, the excitation probability is reduced. The observed behaviour demonstrates that Alq3 molecules in their excited state are involved in these complex degradation phenomena.

 

Confocal laser scanning microscopy of organic thin films and OLED

At the ENEA Frascati Research Centre, conventional optical and confocal laser microscopies [16] were used to check the quality of light-emitting organic thin films and commercially available OLED displays, as shown in Figure 2.

Figura2Montereali.jpg

FIGURE 2

Optical images of a commercial three-color OLED display, 128x128 dots: wide-field conventional fluorescence under UV excitation, a); confocal fluorescence under argon laser excitation at 457.9 nm with two different magnification levels, b,c); confocal in reflection mode under the same laser illumination, d)

 

3 thin films

Photo-bleaching phenomena can also be exploited for direct laser patterning of Alq3 thin films with a confocal laser scanning microscope. The photo-bleaching of the green PL signal during light and laser exposures is very effective, depending on the selection of the illumination wavelength and the used power [15]. As an example, Figure 3 shows the fluorescence images of thermally evaporated Alq3 thin films. In Figure 3b, the intense green fluorescence, visible at naked eye, is emitted by the organic film with the exception of a dark micrometric pattern that forms to the word TECVIM. It was directly written on the Alq3 film by an argon laser beam operating at 457.9 nm. The irradiation pattern was controlled by using the galvanometric mirrors of the confocal microscope.

 

Figura3Montereali.jpg

FIGURE 3

Fluorescence image, under blue excitation, of an Alq3 film, 50 nm thick, thermally evaporated on a glass substrate at low magnification, a); fluorescence image of a micrometric-sized bleached pattern, the word TECVIM, directly written by an argon laser with a confocal microscope, on an Alq3 thin film, observed under blue illumination at a higher magnification, b)

 

Growth of nano-structured films and electron spectroscopy of interfaces in hybrid organic-semiconductor cathodes

The understanding of the interface properties and the control of the interface formation are crucial for organic device tailoring. Being layered devices, OLED and thin-film solar cells performances are strongly affected by the interface properties as well, and the basic parameters characterising the interface are the band discontinuities at the hetero-junctions between the layers constituting the device stack.

 

X-Ray Photoelectron Spectroscopy of PV Device Cathodes

In their common configuration, polymer solar cells have a TCO electrode acting as the high-work function anode for hole collection and a low-work function metallic cathode acting as the electron collector. As a low-work function metal electron collector is air-unstable and highly reactive, an inverted scheme can be used which allows to use a TCO film as the cathode and a low-reactive, non-oxidizing high-work function metal anode [17]. The performance of these devices (both in common and in reverse configuration) can be improved by using a semiconductor as the electron selective layer [18] which is introduced between the active layer and ITO. In2S3 has gained increasing interest in solar cell technology because it has been identified as a suitable alternative to CdS buffer layer in thin-film solar cells. Copper indium gallium (di)selenide (CIGS) cells with In2S3 buffer layers co-evaporated from In and S powder have shown efficiencies of up to 12.4%. In2S3 can be prepared with low-cost techniques such as Chemical Spray Pyrolysis (CSP) [19]. We have investigated the band discontinuities at the interface between ITO and an In2S3 layer deposited by CSP by means of X-Ray Photoelectron Spectroscopy (XPS). The band discontinuity at the valence band has been evaluated by a direct method involving the measurement of the valence band at a suitable thickness that allows the simultaneous detection of the band edges of both layers [20], and also by an indirect method requiring the acquisition of the binding energy of selected core levels [21].

As an example, Figs. 4a,b show the valence band (VB) XPS spectra measured on an In2S3/ITO interface, which depend on the In2S3 layers composition. Figure 4a shows the XPS VB spectrum taken on the bare In2S3 layers and Figure 4b shows the XPS VB data measured on the In2S3/ITO interface region. The top of the valence band was measured by linear extrapolation of the XPS valence band leading edge. The accuracy of this method has been reported to be usually 0.1 eV or less if the density of states in the valence band region is known. Data show that on the In2S3 layer (Figure 4a) the VB onset occurs at 1.9 eV above the ITO VB onset (Figure 4b). VB data at the interface show the simultaneous presence of both In2S3- and ITO-induced valence bands. By taking into account the uncertainties on the VB onset determination (±0.1 eV) the In2S3/ITO VB offset can be estimated to be (1.9±0.2) eV. The VB offset at the In2S3/ITO interface was also determined by means of the “indirect” method [21]. From our experimental XPS data [22] and accounting for the error propagation, the indirect VB offset, ΔΕV, results to be (1.8±0.3) eV, in good agreement with the direct method result. By knowing the VB offset ΔΕV, also the conduction band offset ΔΕC between the conduction bands can be estimated if the gap energies (differences between the conduction, CB, and valence band, VB, energies) are known. By assuming an energy gap of (2.8±0.1) eV for In2S3 and (3.7±0.1) eV for ITO, ΔΕC results in (1.0V±0.4) eV. In Figure 4c, a sketch of the estimated band diagram is shown.

 

Figura4Montereali.jpg

FIGURE 4

XPS VB spectra of a In2S3 layer (ratio In/S=2.5/3). a), VB top measured on the In2S3 layer; b), Simultaneous detection of the two VB band edges at the In2S3/ITO interface. c), Schematic of the estimated energy level diagram at the In2S3/ITO interface


Growth of nano-structured ultra-thin LiF films for OLED cathodes

An analysis of the role and properties of the LiF ultra-thin layer is complicated by its very limited thickness and its interactions with neighbouring materials [9,10].

Ultra-thin polycrystalline LiF films of controlled nominal thickness below 20 nm were grown by thermal evaporation on several substrates under different conditions; their morphological features were investigated by means of atomic force and scanning electron microscopy and their electronic properties characterised by means of XPS spectra [23]. A comparison among ultra-thin LiF films of different thicknesses indicates that the geometric interaction between the arriving ad-molecules and the roughness of the growing surface is the dominant effect in their nano-structuring and the covering of the substrate increases with thickness [10].

 

Optical Modelling of an Alq3-based Organic Light-Emitting Diode

The internal quantum efficiency of OLED is mainly affected by the non-radiative electron-hole recombination loss [24] and singlet-triplet branching ratio. Most of electro-phosphorescent OLED materials have very small non-radiative loss. On the contrary, the external coupling efficiency of a conventional OLED device remains very low. For the purpose of applications such as illumination and flat-panel displays, light emitted from the substrate surface (external modes) in only 20% of the total emitted light from inside the OLED devices [25].

Among the novel approaches implemented to improve the external efficiency [26], designing suitable boundary conditions have been demonstrated to play a key role in the control of spontaneous emission. The theory of light emission from a single point source, such as an atomic (molecular) system, sandwiched between two plane parallel mirrors – either metallic or Bragg reflectors – is well assessed in both its quantum [27-30] and classical flavours [31-34]. In this context, the classical approach developed by Benisty and co-workers [34] relies on the superposition of linearly polarised fields that, after having been generated by suitable elementary dipole source-terms, interact with the surrounding environment. The radiated electromagnetic field and correspondent power are evaluated in a straightforward way by means of analytical formulas [34].

Recently, Benisty’s theory has been expanded into a more general theory to deal with a collection of point sources so densely packed – with respect to the involved wavelengths – to be viewed as a continuous active volume [35-36]. Still maintaining its analyticity, this generalised theory includes even the cases of partial and total cooperation among the volume elements in the emission process [37]. The case of an Alq3-based multilayer OLED can be dealt with in this theoretical framework. For simplicity, distinct elements of the Alq3 layer are assumed not to cooperate in the emission process, so that the power that is radiated by the device can be simply viewed as the integral of elementary contributions  coming from single dipoles located at each point  of the Alq3 layer [35-36]. The integration is straightforward once the expression of  is taken into account [35-36]. By using the resulting analytical formulas [36], one can theoretically predict the angular and spectral distributions of the radiated power and its total amount; also, an estimation of how the device efficiency depends on various features of the multilayer is feasible.

Two Alq3-based OLED configurations, shown in Figure 5, are taken into account. In one of them, see Figure 5b, an ultrathin (5 nm) layer of LiF is placed in contact with the Al cathode to lower the threshold voltage of the device [38]. In the following, a 10-nm-thin radiating portion of the Alq3 layer on the TPD side is assumed [36]. Our goal is to check how the LiF layer influences the device not only electrically, but also optically – despite its ultra-thin thickness. The answer is found through the generalised theory: such a thin layer of LiF is able to increase the electroluminescence emerging from the device at, e.g., l= 530 nm by 11% (full solid-angle integration) [36].

One could also wonder whether the design thickness values of the considered OLED are optimal or if they could be suitably changed to improve the overall optical efficiency. Such a parametric study can be performed quite effortlessly thanks to the analytical formulas of the generalised theory. Figure 6 displays a contour map of the theoretical power that emerges from the device of Figure 5b along the normal direction vs. the thicknesses of the Alq3 and TPD layers. One can notice that the maximum of power corresponds to a thickness of ~70-80 nm for both the Alq3 and TPD layers instead of the design’s one (50 nm). For such thickness values, the device improves its optical efficiency by about 60-65%, at least along the normal direction [36].

 

FIGURE 5

OLED based on an active Alq3 layer. Designs containing an Al cathode, a) and a LiF/Al cathode, b)

 

Figura6Montereali.jpg

FIGURE 6

Contour map of the theoretical power (a.u.) at λ = 530 nm radiated by the OLED of Figure 5b along the normal direction for varying thicknesses of the TPD and Alq3 layers

 

Conclusions

Thin films of Alq3, commonly vacuum deposited as the active layer in green OLEDs, were grown by thermal evaporation under controlled conditions and their optical absorption and emission properties were used to study the degradation behaviour under ambient atmosphere and/or light exposure in order to increase their lifetimes.

Conventional optical and confocal laser microscopies were successfully used to check the quality of light-emitting organic thin films and commercially available three-color OLED displays. The photo-bleaching phenomena were also exploited for direct laser patterning of the investigated Alq3 thin films at micrometric scale with a confocal laser scanning microscope.

Being layered electroluminescent devices, OLED and thin-film solar cells performances are strongly affected by the interface properties. X-ray photoelectron spectroscopy was used to investigate some basic parameters that characterise the interface and the band energy discontinuities at the hetero-junctions between the layers constituting the device stack. In electro-luminescent OLED, the insertion of an ultra-thin LiF layer between the organic film and the metallic Al cathode was found to significantly reduce the threshold voltage in Alq3-based OLEDs. Nano-structured polycrystalline LiF films, grown by thermal evaporation on different substrates, were investigated. In polymer solar cells, which feature a TCO electrode acting as the high-work function anode for hole collection, the efficiency can be improved by using a semiconductor, In2S3, as the electron selective layer which is introduced between the active layer and ITO. Their valence band diagram was evaluated.

Finally, an analytical model which describes the effect of optical confinement in a multilayer on light emission from a hybrid organic-dielectric-metal structure was enhanced and successfully applied to a conventional basic OLED device to explain the increase in the external coupling efficiency with the introduction of an ultra-thin LiF layer below the Al cathode.

The above-discussed technologies are suitable for the realisation and characterisation of functionalised optical and electronic thin-film devices of higher efficiencies and open interesting perspectives in the fast-growing field of organic photonics and its application to alternative energy and sustainable development.

 

Acknowledgements

The authors are indebted to S. Almaviva, G. Baldacchini, L. Caneve, A. Mancini, M.R. Rajesh Menon, and A. Rufoloni for fruitful discussions. Useful suggestions given by R. Fantoni are also acknowledged. Many thanks are due to L. Cantarini, R. Giovagnoli, S. Libera for their skilful support. Research carried out within TECVIM (TECnologie per la Visualizzazione di IMmagine) project, founded by MIUR, the Italian Ministry of Education, University and Scientific Research, and with the support of the ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy.

 

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Rosa Maria Montereali, Francesca Bonfigli, Doriano Brogioli, Antonino Santoni, Maria Aurora Vincenti - ENEA, Technical Unit for Radiation Application Development

Enrico Nichelatti - ENEA, Technical Unit for Material Technologies