OLEDs are devices which convert electricity into light through the process of electroluminescence. They are made by depositing an organic stack of tens of nanometers between two flat electrodes. When an external voltage is applied to the device electrons are injected from the cathode and holes from the anode. Than they form excitons in the active layer which emit photons, with an energy equal to the energy gap of the active molecules. The structures are realized by doping the transport layers with electron-acceptor or electron-donating molecules (pin structure). Owing to pin structure we are able to finely tune the device without altering the electrical performances, thus allowing to work near the thermodynamic limit (applied voltage near the energy gap of emitting layer per unit charge) and to fabricate LED embedded into an electrically active micro-cavity. The research activities are focused on the following areas:
- Development of hybrid electroluminescent integrated devices
Development of electroluminescent organic devices for industrial applications
Development of electroluminescent hybrid integrated devices
Electroluminescence is the process of generation of light by electrical injection of opposite carriers (electrons and holes) into a semiconductor. We have two main targets:
- Study of electroluminescence process into optical feedback
We are studying electroluminescence form organic compounds working in different light-matter coupling regimes: weak (WC), strong (SC) and ultrastrong (USC). In the WC the Fermi golden rule is changed by managing the photonic density of states (and the related Purcell effect) through the placement of the active material into an optical feedback (i.e. a dielctricmicrovavity). We have exploited this effect to improve the efficiency of white OLEDs owing to the coupling of three metal microcavities. ITO-free devices with anefficiecy of 40lm/W in a full coupled microcavity diodes even on flexible substrateshave been developed.
SC and USC regimes are achieved when the dipoles exchange energy with the cavity at a rate larger than the losses from the cavity and the dephasing of the excitons. In this regime the energy of the cavity and the molecules change generating two hibrid states called polaritons. We work on the study of EL process from polaritons with the aim of developing polariton devices and Bose Einstein condensation under electrical injection.
a) Study of electroluminescence process in perovskites(Pero-LEDs)
We realize LED based perovskite (Pero-LEDs) based on metal-halide perovskite materials, hybrid organic-inorganic compounds, which are recently making inroads in photovoltaics. These materials feature a unique combination of properties: they can be easily be fabricated, both from solution or thermally evaporated, on any substrate and have bright, colour tunable optical emission, like organics, but their electronic conduction properties resemble those of inorganic semiconductors, with large carrier mobility and diffusion lengths. Unfavorably, the important advantages offered by wet processability are accompanied by disconcerting limitations. It is well known, in fact, how the perovskite morphology, strongly depending on the growth conditions, severely impacts on the device performances, causing an intrinsic irreproducibility of the material chemical-physical properties. Therefore huge efforts have been devoted to the optimization of morphology and processing conditions, in particular by controlling the interactions of the material precursors in solution or with the substrate.A further constraint factor in the spin-coated devices is that the number of layers allowed are limited by their solubility in orthogonal solvents. The fabrication of hetero-structured devices may open ways to increase the number of layers allowing to decouple transport from optical characteristics, thus increasing the overall performances of the device, as already shown in small molecules based Organic LEDs (OLEDs). Vacuum-based deposition represents an excellent technique to achieve high-purity layers and potentially allows for a fine control over the stoichiometry and thickness of the perovskite films, thus their reproducibility. Recently we reported for the first time a fully vapor-deposited hetero-structure perovskite light emitting diode exploiting p-i-n technology, as the first step of an innovative approach to electroluminescent perovskite-based devices embedding electrical doping. The approach presented would potentially lead to a better control of the transport and electroluminescent properties of the device, as well as to the possibility of a wide industrial application. Acting on kinetic and thermodynamic conditions during the thermal deposition we study the impact of morphological and structural characteristics on the electro-optical behavior of the perovskite active layer. Several perovskites are explored in infrared and visible windows of electromagnetic spectrum. Analysis of optical characteristics at high current density both in cw and pulsed conditions is done to understand the bimolecular processes responsible for the light emission in perovskites and the aging of the device, which are fundamental to reach the population inversion and lasing. Indeed contrary to organics, in perovskite materials optical recombination is a bimolecular process, not requiring the formation of an exciton. Therefore the radiative efficiency increases with density, as the probability for electrons and holes to find a recombination partner increases, and becomes the dominant recombination channel, with the emission quantum yield approaching unity.
Development of electroluminescent organic devices for industrial applications
Organic light-emitting diodes (OLEDs) are very promising type of technologies having a wide range of applications. In less than two decades, they have become a commercial reality in display technology (AMOLED). The development of large area white OLEDs (WOLEDs) for general lighting with an almost perfect Rendering Colour Index (CRI) near 100 and a theoretical efficiency of 200 lum/W, is still a challenge.
More recently, Organic Light Emitting Field Effect Transistor (OLEFET) combining in a single device, the current modulation functions and electrical switching properties of a field effect transistor with light generation capabilities of an organic LED, have been investigated for display applications.
Different scientific approaches are followed to fabricate large area white OLED and efficient OLEFET for practical applications:
– The design and fabrication of a p-i-n microcavity structure (p-i-n MC-OLED) offers the possibility to optimize the outcoupling efficiency and improve the output luminance, through the amplification of the light emission next to the resonance wavelength of the single or coupled microcavity, without changing its electrical behaviour. (link to Ultra High Vacuum (UHV) Kurt. J. Lesker Cluster Tool facility)
– Study of supramolecular aggregates in organic semiconductors, particularly molecular crystal structures, and their integration in OLED devices, in order to minimize typical annihilation phenomena and improve device efficiency at high luminance. (link to Ultra High Vacuum (UHV) Kurt. J. Lesker Cluster Tool facility)
– Implementation of trilayer p-i-n ambipolar OLEFET structure. The doping level results crucial to the capability of emitting light, as well as to the electrical characteristics of the device. The hole and electron current profiles can be tailored in order to create simultaneous flows of opposite charges near and across the active layer, featuring light emission across the whole channel area. Device dimension range from 100um down to a few tens of nanometers. (link to Ultra High Vacuum (UHV) Kurt. J. Lesker Cluster Tool facility; nanofabrication facility)
- Genco , F. Mariano , S. Carallo , V. L. P. Guerra , S. Gambino , D. Simeone , A. Listorti, Silvia Colella , G. Gigli , and M. Mazzeo FullyVapor-DepositedHeterostructured Light-EmittingDiodeBased on Organo-Metal Halide Perovskite, Advanced Electronic Materials, 1500325 (2016) DOI: 10.1002/aelm.201500325.
- Mazzeo, A. Genco, S.Gambino, D. Ballarini, F. Mangione, O. DI Stefano, S. patanè, S. Savasta, D. Sanvitto, G. Gigli, Ultrastrong light-matter coupling in electrically doped microcavity organic light emitting diodes, Applied Physics Letters Vol: 104,Issue:23 (2014)
- Accorsi, S. Carallo, M. Mazzeo, A. Genco, S. Gambino, G. Gigli; A colour tunable microcavity by weak-to-strong coupling regime transition through a light-switchable material Chemical communications Vol: 50,Issue:9(2014)
Other selected Publications
- Gambino, S.; Mazzeo M. Genco, Di Stefano, O; Savasta, S; Patane, S ; Ballarini, D ; Mangione, F ; Lerario, G; Sanvitto, D ; Gigli, G Exploring Light-MatterInteractionPhenomena under UltrastrongCoupling Regime, ACS PHOTONICS (2014) Volume: 1 Issue: 10 Pages: 1042-1048
- Mazzeo, F. Mariano, A. Genco, S. Carallo, G. Gigli, High efficiency ITO-free flexible white organic light-emitting diodes based on multi-cavity technology, Organic electronics Vol: 14,Issue: 11 (2013)
- Maiorano, A.Bramanti, S.Carallo, R.Cingolani and G.Gigli, Organic light emitting field effect transistor based on ambipolar p-i-n layered structure Appl. Phys. Lett., 96, 133305 (2010)
- Mazzeo*, F. Della Sala, F. Mariano, G. Melcarne, S. D’Agostino, Y.Duan, R. Cingolani and G. Gigli Shaping white light through electroluminescent fully organic coupled-microcavities, Advanced Materials, 22, pg. 4696 (2010)
1) Organic light-emitting diode with microcavity including doped organic layers and fabrica-tion process thereof – , B.Dussert-Vidalet, M.Mazzeo, G.Gigli, M.BenKhalifa, F.DellaSala, V.Maiorano, F.MarianoUS 2011079772 (A1) 07/04/2011 ; N° US8969853 (B2) del 03/03/2015
- FR2926677 (A1) 24/07/2009 ; FR2926677 (B1) del 25/04/2014
- EP2235763 (A1) del 06/10/2010
- WO 2009090248 del 23/07/2009
- CA 2712251 23/07/2009
- KR 20110009080 (A) del 27/01/2011; KR101585018 (B1) del 13/01/2016
- CN 101978527 del 16/02/2011
- JP 2011510441 (A) del 31/03/2011 ; JP5594777 (B2) del 24/09/2014
- BRPI0906421 14/07/2015
2)Organic light emitting field effect transistor-V.Maiorano, G.Gigli, EP2545599 A1 16/01/2013
- IT RM20100107 13/09/2011
- WO 2011110664A1 15/09/2011
MAAT: Molecular NAnotechnology for HeAlth and EnvironmenT , PON R&C 2007-2013 – (2012-2015)
FT_WOLED: Flexible Transparent White Organic Light Emitting Device, Executive Programme for scientific and technological cooperation between Italy and China (2013-2015)
PHOEBUS: Plastic tecHnologies for the realization of Organic solar cells and high Efficiency Bright and Uniform Sources, (2009-2011)
OLEDs for lighting, Project MIUR FAR 297 (2006-2009).
OLLA: Organic light emitting diodes for lighting, EUFP6 IP, (2005-2008)