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High Efficiency LIght-emitters with Organic Spintronics

Final Report Summary - HELIOS (High Efficiency LIght-emitters with Organic Spintronics)

The aim of this project is to investigate the intrinsic spin transport properties in organic semiconductors (OS) and to take advantage of them to improve power consumption in light emission devices. In particular we are interested in the effect of spin polarized current on the efficiency of light emitting organic field effect transistors (LE-OFETs). In conventional organic light emitting diodes (OLED) the optical emission is due to the radiative decay of singlet excitons whereas triplet excitons produce phosphorescence reducing the radiative efficiency. Spin polarized injection changes the relative population of singlets and triplets. Thus, the efficiency can be increased by injecting spin polarized currents into the OS.
The original proposal suggests using pentacene among other reasons because of its polymorphic nature. The work plan scheme was: 1.- Studying pentacene single crystal transistors; 2.- Studying pentacene polymorphic thin films; 3.- Development of spin polarized LE-OFET. The first two items would be developed at Massachusetts Institute of Technology (USA), and the third one at CIC nanoGUNE (Spain). The Marie Curie Fellowship involves the Fellow will be trained in some different techniques relevant for the project (numbered T1 to T5).

During the first year at Massachusetts Institute of Technology (MIT), we performed studies on the electrical conduction of organic single crystals. Anthracene, Cu-phthalocyanine and rubrene have been used instead of pentace. We have been forced to take some corrective actions from the very beginning of the project. The main one was changing the OS we were planning to work with. Pentacene was a poor selection for spin injection purposes. This is due to the singlet fission effect that takes place in this material, with no thermal activation, due to the difference of energy levels between singlets and triplets. Further more, pentacene shows some different quenching effects that prevent its use as a transport layer in OLEDs.
The second year of the Marie Curie Fellowship was devoted to the interfacial engineering or organic/ferromagnetic for spin injection, and spin injection studies on thin films. In particular we have clarified the effects of LiF on the spin injection both in organic spin valves and inorganic magnetic tunnel junctions. This research was completed during the ingoing phase by performing some additional characterization on the effect of LiF on ferromagnetic materials by using neutron reflectometry at ISIS (UK).
The study on thin films was completed by producing tris (8‐hydroxyquinoline)‐aluminum (Alq3) OLEDs. They allowed us to study the effect of magnetic films on the efficiency of these devices.
During the first two years the training plan was completed: T1 (optical characterization), T2 (e-beam lithography), T4 (management of organic single crystals), and T5 (training in X-Ray techniques). Finally, item T2 (training in SAM) was not considered for the project.
At MIT the Fellow started to use a non-standard lithographic process based on fluorinated chemicals (orthogonal process), which is compatible with OS. The idea was to developed devices on top of the organic semiconductors to get good interfaces between these materials and the ferromagnetic electrodes used for spin injection. This approach was important for its use in single crystals and thin films with engineered interfaces. This optimization has been completed during the ingoing phase at CIC nanoGUNE. Using this technique, we have developed top gated transistors with organic thin films (CuPc) and organic single crystals (rubrene) as conductive layers, and poly(4-vinyl phenol) (PVP) as insulating layer for the gate. These transistors outperform some of the characteristics (reduced channel lengths, gate voltage thresholds) of organic devices produced by other techniques due to the clean interface between organic and metal. Despite this achievements, we have been unable to reduce the channel length below 750 nm, while a maximum of 100nm is required to observe spin injection effects.
As suggested in the research methodology of the original proposal (section B1.2) we have also produced at CIC nanoGUNE devices based on an organic/inorganic semiconductor heterostructure. The idea was to mix the orthogonal and common e-beam lithography techniques to get the lateral spin valves. Despite the device produced do not show any luminescence, they show interesting photovoltaic effects.

Socio-economic impact of the project
Orthogonal patterning is a group of recently developed lithography techniques whose aim is to overcome the chemical incompatibility problems of common lithography techniques and organic semiconductors. We have developed a new experimental procedure allowing us to use orthogonal patterning by e-beam lithography to produce electronic devices. We have developed an alternative fabrication process for by e-beam lithography based on this kind of patterning. This is a technological need for the industrial integration of organic semiconductors in microprocessors or displays. This is of great interest not only from an industrial point of view but for basic research in systems including organic electronics with reactive electrodes, devices on organic single crystals and organic spintronics.
We have also shown that the sign of the magnetoresistance in organic spin valves and inorganic magnetic tunnel junctions with intercalated LiF layers can be tuned by simply changing the order of deposition of the different layers. Apart from possible applications in devices, we recognize a possible source of MR sign inversion in organic spin valves that at the moment is disregarded by the spintronic community.
We also have developed organic/2D inorganic semiconductor photovoltaic cells. This is the first time that this kind of heterojunction is characterized. It could be of importance in flexible electronic applications.