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Organic Polariton Laser Diode

Periodic Reporting for period 1 - OPLD (Organic Polariton Laser Diode)

Reporting period: 2017-06-01 to 2019-05-31

"-What is the problem/issue being addressed?
Organic semiconductors are low cost and (generally) non-toxic and biodegradable light emitting materials. In addition, their broad spectrum of emission makes them ideal materials to replace inorganic semiconductors in traditional diode laser devices. However, organic semiconductors suffer from poor electrical properties and the injection of high current density required for lasing presents a big challenge. In this project we utilised a conceptually different approach than that of typical lasers namely, polariton lasing. For inorganic semiconductor laser diodes, it has been shown that polariton lasing mechanism can be achieved at threshold power that is hundred times lower than this in typical population inversion lasers. By implementing the same technology to organic semiconductors, the threshold can be decreased, so the various losses are decreased, which can lead to lasing in organic semiconductors by electrical pumping.

-Why is it important for society?
Replacing toxic and difficult to process inorganic semiconductors is one of they key actions needed for reverting global warming. Also, organic semiconductor lasers can potentially offer broader range of emission wavelengths
which will have tremendous impact in research and industry. The demonstration of an organic solid state laser, like the one we propose, would be tremendously important for the European and Global market. Diode-laser technology comprises about 46% of the total revenue (~$10 billion) in the global laser market; Therefore, any steps towards the realization of the first organic laser diode would represent a significant technological and scientific achievement. Furthermore, the proposed device offers a revolutionary bench-top device for fundamental studies of quantum fluids of light, a system that complements experiments in ultracold atomic quantum gases.

-What are the overall objectives?
The main objective of this project was to design and fabricate an electrically pumped organic polariton light-emitting microcavity and to study the polariton formation, thermalization and condensation under electrical injection. Fundamental understanding of the above processes could further lead to the demonstration of a practical organic semiconductor laser device.

-Conclusions of the action:
After evaluating different organic semiconductor emitters and OLED device architectures, we have chosen to proceed with the vertical cavity configuration, as it was proposed in the research plan. We successfully managed to fabricate OLED devices and perform all necessary optoelectronic studies. Moreover, we observed and demonstrate a fascinating new mechanism for converting blue emitting OLEDs to white OLEDs. More details can be found in our recent manuscript with title ""Converting an organic light-emitting diode from blue to white with Bragg modes"" which is under review in high impact peer-reviewed journal. Also, this technology is now a patent currently pending with Aalto University (no. 20195269, filed 3 April 2019).

We have also performed a series of optical studies to understand relaxation, thermalization, condensation and propagation of polariton in organic semiconductors. We performed a series of studies and the results were published in high-impact journals such as Nature Physics, Nano Letters and ACS Photonics. A fifth manuscript was also recently produced entitled ""Sub-picosecond thermalization dynamics in condensation of strongly
coupled lattice plasmons"" and it is under review in high impact peer-reviewed journal and as preprint in arXiv with number 1905.07609."
The project started with WP1 – Utilizing key fabrication techniques. I received training on sophisticated fabrication, characterisation and processing equipment located in the cleanroom at Micronova facilities on the host campus. Training and access to the above equipments resulted in evaluation of the most appropriate methods for the fabrication of the proposed devices. This then enabled me to proceed to WP2 – Fabrication of OPLDs (proposed devices).

In WP2, we focused on fabricating high quality OLED devices. After trying different device configurations and evaluating their performance, I have chosen the device configuration which consists of a 50 nm layer of 2,7-bis [9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl) fluorene (TDAF) sandwiched between a 70 nm Al bottom anode with a 5 nm MoO3 hole-injecting layer and a 10 nm LiF/Al cathode with a 20 nm 4,7-diphenyl-1,10-phenanthroline (BPhen) hole-blocking layer. The top mirror is a DBR consisting of 6 alternating layers of SiO2 and Ta2O5 was directly sputtered on top of the Al anode, thus providing encapsulation of the organic materials.

After WP2 I focused on WP3 in which electrical injection and spectroscopy studies were realised. The result was the optimisation of the k-space and real-space imaging and spectroscopy setup located in the ultrafast lab of the Quantum Dynamics (QD) group at Aalto. I modified this setup in order to be able to use an ultrafast optical parametric amplifier (OPA) and now is capable of spatially filtered transmission, reflection and photoluminescence (PL) measurements. In addition, I integrated a source measurement unit with pulsed capability for performing standard and dynamic electroluminescence measurements.

In parallel with WP3, I started working on the device modeling as proposed in WP4. For this, the help from Prof. Törmä and co-workers was essential. The result is that we have a very good understanding of all the mechanisms namely relaxation, thermalization, condensation and propagation of polariton in organic semiconductors.

Data analysis (WP5) and device optimisation (WP6) were performed also in conjunction with WP4 and WP3.

Support from the patent office at Aalto University was essential for submitting a patent related to the project. This pending patent patent (no. 20195269, filed 3 April 2019) was essentially the WP7 of my project concerning the commercialisation of the OPLED.
As mentioned, beyond the goals of this project we demonstrate a novel technology for converting standard blue OLEDs to White OLEDs. Current methods for the fabrication of white-light OLEDs rely on the combination of multiple organic emitters and/or the incorporation of multiple cavity modes in a thick active medium. These architectures introduce formidable challenges in both device design and performance improvements, namely the decrease of efficiency with increasing brightness (efficiency roll-off) and short operational lifetime. Our work is directly relevant for the state-of-the-art WOLED research, and has the potential to be a disruptive technology. We anticipate that our integrated design has the key elements for solving one of the major problems in WOLEDs namely the efficiency roll-off (decrease of efficiency with increasing brightness), that our technology will enable white OLEDs to be considered as a vial solution of general lighting and to replace current general illumination sources based on non-biodegradable inorganic LEDs. Furthermore, the electric field intensity of Bragg modes can be much higher than that in metallic clad microcavities where electrically generated polaritons have been demonstrated. Thus we expect that this novel utilization of Bragg modes will inspire new research beyond the WOLED framework, such as on strong coupling effects in organic semiconductors.
white emission from organic polariton OLED