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Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors

Organic Opto-electronics represent an appealing emerging technology able to substitute the traditional silicon technology in many applications reducing the production costs, the environmental impact and using full flexible device platforms. Organic Light Emitting Diodes (OLEDs) have already been implemented in smartphones and TV displaysand the next challenge of organic opto-electronics is represented by the transistor platform. In  the future it should be possible a completely new scenario for the integrated electrical circuits by merging in a unique device the opto-electronic functionality, together with the transistor electrical switching functionality at the basis of digital electronics,. In this respect, the tri-layer heterojunction based Organic Light Emitting Transistors (OLET) are emerging devices with far field brightness, efficiency and spatial resolution competitive with the commercial display OLEDs. However, the technology potentials are still limited by the field-effect transport performance. As shown in Fig.1a, the active region of a tri-layer OLET is formed by an emissive layer sandwiched between two field-effect layers, an electron and a hole transporter, respectively. In the tri-layer device configuration, the field-effect transport appears particularly critical in correspondence of the top semiconducting layer. This is strictly correlated to the molecular organization of the film at the interface with the bottom emissive layer. In order to elucidate the mechanism at the basis of the lack in electrical performance at such interface, a multidisciplinary approach based on the correlation of different fundamental investigation tools has been applied to the representative organic heterojunction stack of Fig. 1b. This study was carried out by researchers of the IOM-CNR , the INO-CNR the ISOF-CNR, the ETC S.r.l. and SAES Getters S.P.A. and the University of Modena and Reggio Emilia. The high performance single layer transistor configuration was used as a benchmark, see Fig.1c. The high resolution X-ray Photoemission Spectroscopy (XPS) analysis is performed at the BEAR beamline of Elettra, and as reported in Fig. 2a shows that the molecular integrity of the top semiconducting material (DH4T) is preserved within the tri-layer OLET stack. Measurements of the Near edge X-ray dichroic absorption with linearly polarized light has also been performed at BEAR. The results, reported in Fig. 2b, indicate a mean preferential alignment of the DH4T molecules with the long axis perpendicular to the substrate, with a negligible change by 7° in the DH4T orientation between the heterojunction and the single layer benchmark configuration. This molecular organization is optimal for field-effect transport purposes. X-ray diffraction data reported in Fig. 2d indicate a decreasing of about 15% in the density of the highly ordered crystalline domains for the heterojunction stack.

Figure 1.  Tri-layer heterojunction based Organic Light Emitting Transistor (OLET) architecture. a) Device working principle schematic and image of the OLET emissive stripe. b) Tri-layer organic stack under study and corresponding multiple output curve of the top transport layer. c) Scheme of the benchmark single layer organic thin film transistor (OTFT) and corresponding multiple output curves.    




Figure 2: a) X-ray Photoemission spectra of the  OLET organic stack under study. b) Near edge X-ray dichroic absorption with linearly polarised light as a function of the organic stack configuration. c) X-ray diffraction spectra as a function of the organic stack configuration and schematic of the top layer molecular distribution. d)  X-ray photoemission spectra of the OLET charge injection interface.

Position of pecks are indeed the same for the DH4T grown on PMMA and on Alq3 but, for the latter case, a clear decreasing of the pecks intensity is observed. Although the DH4T molecules are optimally oriented for the field-effect transport, their in plane distribution appear less ordered when the substrate is the Alq3 film. This is the main reason of the loss in transport performances of the tri-layer stack OLET. The reason for the crystallinity decrease at Alq3/DH4T interface is clarified by Atomic Force Microscopy, as reported in Fig. 2b. The fractal-like edges of the DH4T islands on top of the Alq3 layer indicate a strong Alq3/DH4T interaction with respect to the PMMA/DH4T case. This is due to the fact that both Alq3 and DH4T are conjugated system. Thus, at the interface between two conjugated materials, the amorphous character of one layer can compromise the molecular order of the second layer, degrading its transport properties. The DH4T fragmentation at the interface with the Au layer, as evidenced by the XPS spectra in Fig. 2d, is totally unexpected and new in a real device configuration. Based on the consolidated charge injection picture, this appears inconsistent with the optimal electrical working of the single layer DH4T benchmark transistor. In light of that, the definition of an ideal metal/organic interface should be partially revised. In conclusion, through a multidisciplinary approach the crucial interfaces of a real OLET devices have been analysed and a light was shed on their working mechanism.

 

 

This research was conducted by the following research team:

  • Raffaella Capelli, Angelo Giglia, CNR - IOM, Trieste, Italy.
  • Franco Dinelli, CNR- Istituto Nazionale di Ottica, Area della ricerca di Pisa, Pisa, Italy
  • Massimo Gazzano, CNR - Istituto per la Sintesi Organica e la Fotoreattività, Bologna, Italy
  • Riccardo D’Alpaos, Andrea Stefani, Gianluca Generali, ETC S.r.l., Bologna, Italy
  • Mauro Riva, SAES Getters S.P.A. , Lainate (MI), Italy
  • Monica Montecchi, Dipartimento di Ingegneria ‘Enzo Ferrari’, Università di  Modena e Reggio Emilia, Italy and CNR - IOM, Trieste, Italy
  • Luca Pasquali, Dipartimento di Ingegneria ‘Enzo Ferrari’, Università di  Modena e Reggio Emilia,  Italy and CNR - IOM, Trieste, Italy and Department of Physics, University of Johannesburg, South Africa

Contact persons:

Angelo Giglia: angelo.giglia@elettra.eu,
Raffaella Capelli: capelli@iom.cnr.it,
Luca Pasquali: luca.pasquali@unimore.it

Reference

 Raffaella Capelli, Franco Dinelli, Massimo Gazzano, Riccardo D’Alpaos, Andrea Stefani, Gianluca Generali, Mauro Riva, Monica Montecchi, Angelo Giglia, and Luca Pasquali, “Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors (OLETs)”, Advanced Functional Materials, DOI: 10.1002/adfm.201400877 

 

Last Updated on Friday, 10 October 2014 16:14