Periodic Reporting for period 3 - PLAS-OLED (Polariton Assisted White Light Generation in Organic Light-Emitting Diodes)
Reporting period: 2022-05-01 to 2023-10-31
Artificial light has been an integral part of our modern life and it would be inconceivable to live without it. However, the increasing need for artificial light has exponentially increased the large-scale production of light-emitting devices and electric power consumption, both exacerbating anthropogenic negative impacts on the environment. Organic light-emitting diodes (OLEDs) are a lighting technology which offers the superior efficiency of semiconductor-based lighting sources, known as LEDs, and uses materials and fabrication processes that promise minimal environmental impacts. Unfortunately, state-of-the-art OLEDs have poor stability and high-power efficiency that have restricted their applications only to low-light, short-operational lifetime display technologies. These OLED problems originate from the fundamental limitations of the materials used in OLEDs; they are inherently poor conductors and rapidly deteriorate in the presence of traces of oxygen and moisture. While OLEDs could counterbalance these limitations by their ability to be fabricated by low-cost methods, consumer OLEDs are based on resource-intensive specialized equipment which drives their cost to 10 times more than standard LEDs, hindering their widespread.
In the PLAS-OLED project, my team is developing low-cost fabrication methods of novel photonic structures that we, in parallel, utilize to investigate approaches for improving the performance and lifetime of white OLEDs (WOLEDs). This challenge that we aim to solve by infusing OLED materials with properties given by photons. One can simply imagine OLEDs as follows: They are devices that consist of multiple layers of nanometer-thick films of materials that enable electrical charges to flow from the device's anode (holes) and cathode (electrons) in opposite directions towards the centre of the device, specifically the emitting layer (EML). In the EML, these charges follow a cascade of relaxation and interaction processes that may result in the emission of photons that then escape out of the device. The EML, therefore, plays a protagonist role in the performance of the device since it is effectively the layer that converts electrical charges into photons and it can consist of a single material or a mixture of materials. The former is usually preferred for keeping the device design simple. A bottleneck in the efficiency of this conversion is governed by spin statistics that define that only 1/4 of the electrical charges can populate electronic states that emit photons. While this might seem demotivating, there have been several methods to modify the kinetics of these processes to harvest back the lost electrical charges namely Föster energy transfer, triplet-triplet annihilation, reverse intersystem crossing (mostly known from thermal activated delayed fluorescence) and hot excitons. Therefore, to increase the efficiency of OLEDs, one needs to utilise one of the above processes and accelerate them further to be able to have highly efficient OLEDs through a broad band of power operations. An additional challenge is also found in WOLEDs because traditional fabrication methods involve the combination or mixing of several types of organic emitters that can physically act as defects in the EML.
In our project we specifically focus on the "emission of photons" and their "escape out of the device". We specifically want to gain control over these relaxation processes that will allow us to engineer them to emit photons rapidly and efficiently. Simultaneously, we design and fabricate photonic structures with modes that allow photons to escape from the OLED without distractions. To achieve this, we design the OLEDs in configurations called optical microcavities which allow us to modify the electromagnetic environment of and confine photons at the EML. If the interaction between the EML and the trapped photon is strong, it leads to the creation of new energy states called polaritons. These are precisely the states we utilize in our project to tackle the WOLED issues. The focus of our first objective is on investigating single-colour EMLs that are more chemically stable and environmentally friendly, and use polaritonics to broaden their emission spectrum to the entire visible spectrum. Or second objective concentrates on using polaritons to accelerate all the charges-to-photon conversion processes within the EML to have efficient emission without degradation at high operating powers.
In the PLAS-OLED project, my team is developing low-cost fabrication methods of novel photonic structures that we, in parallel, utilize to investigate approaches for improving the performance and lifetime of white OLEDs (WOLEDs). This challenge that we aim to solve by infusing OLED materials with properties given by photons. One can simply imagine OLEDs as follows: They are devices that consist of multiple layers of nanometer-thick films of materials that enable electrical charges to flow from the device's anode (holes) and cathode (electrons) in opposite directions towards the centre of the device, specifically the emitting layer (EML). In the EML, these charges follow a cascade of relaxation and interaction processes that may result in the emission of photons that then escape out of the device. The EML, therefore, plays a protagonist role in the performance of the device since it is effectively the layer that converts electrical charges into photons and it can consist of a single material or a mixture of materials. The former is usually preferred for keeping the device design simple. A bottleneck in the efficiency of this conversion is governed by spin statistics that define that only 1/4 of the electrical charges can populate electronic states that emit photons. While this might seem demotivating, there have been several methods to modify the kinetics of these processes to harvest back the lost electrical charges namely Föster energy transfer, triplet-triplet annihilation, reverse intersystem crossing (mostly known from thermal activated delayed fluorescence) and hot excitons. Therefore, to increase the efficiency of OLEDs, one needs to utilise one of the above processes and accelerate them further to be able to have highly efficient OLEDs through a broad band of power operations. An additional challenge is also found in WOLEDs because traditional fabrication methods involve the combination or mixing of several types of organic emitters that can physically act as defects in the EML.
In our project we specifically focus on the "emission of photons" and their "escape out of the device". We specifically want to gain control over these relaxation processes that will allow us to engineer them to emit photons rapidly and efficiently. Simultaneously, we design and fabricate photonic structures with modes that allow photons to escape from the OLED without distractions. To achieve this, we design the OLEDs in configurations called optical microcavities which allow us to modify the electromagnetic environment of and confine photons at the EML. If the interaction between the EML and the trapped photon is strong, it leads to the creation of new energy states called polaritons. These are precisely the states we utilize in our project to tackle the WOLED issues. The focus of our first objective is on investigating single-colour EMLs that are more chemically stable and environmentally friendly, and use polaritonics to broaden their emission spectrum to the entire visible spectrum. Or second objective concentrates on using polaritons to accelerate all the charges-to-photon conversion processes within the EML to have efficient emission without degradation at high operating powers.
Despite that the PLAS-OLED project started during the peak of the pandemic crisis, an exceptional research team was assembled to tackle the exciting challenges of the PLAS-OLED project. This was an excellent first result that effectively ensured the success of the project. During the first months of the project, we tackled the humongous task of acquiring the necessary infrastructure and building the experiments. This task was running in parallel with other tasks and was finalized with the completion of the ultrafast electro-optical excitation setup that has enabled us to gain access to the relaxation and interaction processed in the OLED's EML. In addition, we have assembled and optimised an ultrahigh vacuum deposition system that allows us to fabricate OLEDs and WOLEDs with excellent control and integrate them into optical microcavities. The OLED devices after fabrication are kept in an inert environment in which we also perform the initial electrical and optical characterization measurement. The main result of this task is that we have the essential infrastructure for fabricating OLED and polariton OLEDs.
Furthermore, we built an ultrafast laser spectroscopy setup that allows us to study steady-state dispersion and emission of polariton microcavities and OLEDs. This setup is coupled to an electro-optical excitation setup and their combination enabled us to gain full access to the dynamics of our polariton OLEDs and for the first time obtain data and control the electrical response of light-emitting of polariton OLEDs. In parallel, we focused on developing a novel method that allows us to fabricate high-quality optical microcavities in distributed Bragg reflector (DBR) configurations using solution-based deposition methods. This method simplified the previously used techniques for fabricating optical microcavities for polariton studies.
Furthermore, we built an ultrafast laser spectroscopy setup that allows us to study steady-state dispersion and emission of polariton microcavities and OLEDs. This setup is coupled to an electro-optical excitation setup and their combination enabled us to gain full access to the dynamics of our polariton OLEDs and for the first time obtain data and control the electrical response of light-emitting of polariton OLEDs. In parallel, we focused on developing a novel method that allows us to fabricate high-quality optical microcavities in distributed Bragg reflector (DBR) configurations using solution-based deposition methods. This method simplified the previously used techniques for fabricating optical microcavities for polariton studies.
We have now an excellent understanding of the limitations of using polaritons for improving the performance and spectrum of OLEDs and WOLEDs. Beyond to what has been previously suggested, we present a viable route to exploit the benefits that the polaritons offer. Importantly we have developed innovative experimental setups that give us unique capabilities in fabricating and studying polariton OLEDs. Using open quantum theory, we also offer a better understanding of the problem of having a cavity mode that interacts with trillions of emitters in the EML. It appears that strong coupling is unlikely to influence the dynamics of a single molecule but instead can positively impact the dynamics in bimolecular species. This gives us a much more coherent approach to how to improve the performance of OLEDs with polaritonics. In simple words, we are certain that polaritons can improve inefficient processes but cannot alter efficient processes due to competition. We plan to look in more detail at how we can use open quantum system approaches to deeply understand these processes and fundamentally engineer devices to harvest the photonic and quantum components of polariton microcavity OLEDs. We anticipate being able to provide a detailed solution to the OLED challenges and demonstrate WOLEDs that have high luminous efficacy even at high operation power.
Furthermore, we demonstrated polaritons in solution-based microcavities and we are further focusing on implementing our novel techniques to easy-to-fabricate polariton OLEDs thus tackling the main challenge found in the current state-of-the-art fabrication methods. In our next steps, we will focus on optimizing the above method to allow us to fabricate a fully solution polariton WOLED which will pave the way in reducing the OLED environmental impact and fabrication costs.
Furthermore, we demonstrated polaritons in solution-based microcavities and we are further focusing on implementing our novel techniques to easy-to-fabricate polariton OLEDs thus tackling the main challenge found in the current state-of-the-art fabrication methods. In our next steps, we will focus on optimizing the above method to allow us to fabricate a fully solution polariton WOLED which will pave the way in reducing the OLED environmental impact and fabrication costs.