Periodic Reporting for period 1 - PoTA-LEC (Engineering Phosphorescent and TADF small molecules for white Light-emitting Electrochemical Cells)
Reporting period: 2022-06-01 to 2024-05-31
Light-emitting electrochemical cells (LECs) are getting much attention as single-layered, cost-effective, and moderate performing devices. LEC’s active layer contains an emitter and an electrolyte (ionic liquid or a mixture of ion conductive polymer and inorganic salts). To date, the best emitters are conjugated polymers (CP) or an ionic transition metal complex (iTMC) with average efficiencies of >10 cd/A, and stabilities >1000 h. However, they still encounter serious hitches: i) CPs only harvest singlet excitons (25%) upon recombination (75% triplet excitons are dark), require multistep synthesis/purification, and feature a low device self-stability (spontaneous phase separation); ii) iTMCs harvest both singlet and triplet excitons, but they are based on Ir(III). As alternative Cu(I) complexes have been proposed, reaching moderate efficiencies/stabilities due to their poor redox behavior. Hence, the LEC community exercised to find rainbow emitters like quantum dot (QD), small molecule (SM), and perovskites (P). Since QDs and P contain toxic metal ions (Cd+2 and Pb+4), SMs are the next generation of sustainable emitters due to easy synthesis, high photoluminescence quantum yields (PLQYs) and redox stability, and good carrier motilities.
To date, rainbow phosphorescence and thermally activated delayed fluorescence (TADF) SM emitters have not been explored towards realizing stable and efficient white SM-LECs. In contrast, phosphorescence SMs with PLQYs ca. 10 % covering the visible range are known, while red- or white-emitting TADF-LECs have not been set yet. PoTA-LEC aims at developing a full-fledged phosphorescent and TADF SM design to achieve highly performing rainbow LECs. Based on the SM-LEC and -OLED prior arts, PoTA-LEC focuses on two action lines: i) ionic/neutral phosphorescent SMs with peripheral bulky groups and covalently linked host moieties to enhance PLQYs and device efficiency, and ii) ionic/neutral rainbow TADF emitters used as single emitters or hosts. Optimized monochromatic LECs was targeted.
To date, rainbow phosphorescence and thermally activated delayed fluorescence (TADF) SM emitters have not been explored towards realizing stable and efficient white SM-LECs. In contrast, phosphorescence SMs with PLQYs ca. 10 % covering the visible range are known, while red- or white-emitting TADF-LECs have not been set yet. PoTA-LEC aims at developing a full-fledged phosphorescent and TADF SM design to achieve highly performing rainbow LECs. Based on the SM-LEC and -OLED prior arts, PoTA-LEC focuses on two action lines: i) ionic/neutral phosphorescent SMs with peripheral bulky groups and covalently linked host moieties to enhance PLQYs and device efficiency, and ii) ionic/neutral rainbow TADF emitters used as single emitters or hosts. Optimized monochromatic LECs was targeted.
The project work performed with three main scientific work packages (WP3-5), that is WP3 contains phosphorescent/TADF SM design and synthesis; WP4 contains validation in solution and thin films and WP5 contains fabrication/analysis of SM-LECs.
As per WP3-5, we successfully synthesized few donor-acceptor based small organic molecules, those emitting phosphorescence and fluorescence at room temperature. Though we set reactions with different family of donor molecules, we successfully synthesized and purified few of them due to i) reduced coupling between P and N and, ii) highly basic nature of materials absorbed by silica and there purification was difficult. The synthesis and purification were time consuming and difficult. In the successful molecules, we systematically varied the attachment of donor groups around phosphorescence center, which facilitates the electronic communication between the phosphorescence acceptor and N donors. Therefore, the emission was tuned in powder and thin film associated to photoluminescence quantum yield (PLQY) around 17% in solid powder and 14% in thin film. Though low PLQY, few of these molecules featured long phosphorescence lifetime up to ms to s. Moreover, we have carefully studied the thin film forming behavior of the molecules with and without adding different electrolytes and have investigated their photophysical and optical properties. Though photophysical properties of these molecules were quite interesting, the unusual aggregation formation during thin film even poor the emission and morphology features. Therefore, the first LEC performance was not good (WP5) and in turn, further discussion with companies were not carried out at the end. However, these studies mark a significant advancement toward understanding practical applications of phosphorescent small molecules in optoelectronics and beyond as it is very difficult to obtain room temperature phosphorescence from pure organic small molecules. Therefore, the development of room temperature phosphorescent phsophoramide is very remarkable. The results are analysed and the manuscript for the publication of results is under preparation.
As a second approach, we have developed a proton transfer emitter for LEC. To our best of knowledge, only a few proton transfer emitters are used in LECs. However, their operative mechanism in LECs is not clear. We design a donor-acceptor molecule, which exhibits intriguing emission properties in solid powder and thin films with and without electrolytes. The proton transfer behaviour in thin film was thoroughly investigated, by adding different fraction of ion (NH4PF6) in ethanol/cylcohexane solution. Results revealed that with increase in NH4PF6 ion concentration, the emission is red shifted and PLQY decreases gradually. This indicates, the proton transfer induced charge transfer behavior of the new emitter in thin film. We also observed electroluminescence, however in the spectral sense; it is quite interesting that both photoluminescence and electroluminescence is same region, indicating proton transfer in LECs. The manuscript for the publication of results is under preparation.
Finally, the liner Cu (I) activated ionic complex is a significant milestone in the field of optoelectronics. These breakthrough investigations opens up exciting possibilities for designing and development of various new family of molecules. In the realm of optoelectronics, the development of deep blue liner Cu (I) ionic complexes are game-changer. The manuscript for the publication of results is under preparation. Despite of the promising result obtained during this work, the devices made do not fulfil the requirement for industrial market.
The following dissemination activities were performed:
- Participation on post-day, Technical University Munich, Germany campus Straubing (TUMCS) award. Defense of my project and prospect.
- Presentation of the project work an informative video.
- Participation: TUM-Waseda workshop of sustainable technologies for the the future society participation at Technical University Munich, Germany campus Straubing.
- Participation in PhD day and discussion was on state of the art of research fellowships/opportunities. Round Table about my experience as MSCA fellow and the steps toward future position in academy and/or industry.
- Participation in open day doors at TUMCS to show kids and families the evolution and idea of the project.
- Chair of biogenic functional materials (BFM) webpage news at TUMCS.
As per WP3-5, we successfully synthesized few donor-acceptor based small organic molecules, those emitting phosphorescence and fluorescence at room temperature. Though we set reactions with different family of donor molecules, we successfully synthesized and purified few of them due to i) reduced coupling between P and N and, ii) highly basic nature of materials absorbed by silica and there purification was difficult. The synthesis and purification were time consuming and difficult. In the successful molecules, we systematically varied the attachment of donor groups around phosphorescence center, which facilitates the electronic communication between the phosphorescence acceptor and N donors. Therefore, the emission was tuned in powder and thin film associated to photoluminescence quantum yield (PLQY) around 17% in solid powder and 14% in thin film. Though low PLQY, few of these molecules featured long phosphorescence lifetime up to ms to s. Moreover, we have carefully studied the thin film forming behavior of the molecules with and without adding different electrolytes and have investigated their photophysical and optical properties. Though photophysical properties of these molecules were quite interesting, the unusual aggregation formation during thin film even poor the emission and morphology features. Therefore, the first LEC performance was not good (WP5) and in turn, further discussion with companies were not carried out at the end. However, these studies mark a significant advancement toward understanding practical applications of phosphorescent small molecules in optoelectronics and beyond as it is very difficult to obtain room temperature phosphorescence from pure organic small molecules. Therefore, the development of room temperature phosphorescent phsophoramide is very remarkable. The results are analysed and the manuscript for the publication of results is under preparation.
As a second approach, we have developed a proton transfer emitter for LEC. To our best of knowledge, only a few proton transfer emitters are used in LECs. However, their operative mechanism in LECs is not clear. We design a donor-acceptor molecule, which exhibits intriguing emission properties in solid powder and thin films with and without electrolytes. The proton transfer behaviour in thin film was thoroughly investigated, by adding different fraction of ion (NH4PF6) in ethanol/cylcohexane solution. Results revealed that with increase in NH4PF6 ion concentration, the emission is red shifted and PLQY decreases gradually. This indicates, the proton transfer induced charge transfer behavior of the new emitter in thin film. We also observed electroluminescence, however in the spectral sense; it is quite interesting that both photoluminescence and electroluminescence is same region, indicating proton transfer in LECs. The manuscript for the publication of results is under preparation.
Finally, the liner Cu (I) activated ionic complex is a significant milestone in the field of optoelectronics. These breakthrough investigations opens up exciting possibilities for designing and development of various new family of molecules. In the realm of optoelectronics, the development of deep blue liner Cu (I) ionic complexes are game-changer. The manuscript for the publication of results is under preparation. Despite of the promising result obtained during this work, the devices made do not fulfil the requirement for industrial market.
The following dissemination activities were performed:
- Participation on post-day, Technical University Munich, Germany campus Straubing (TUMCS) award. Defense of my project and prospect.
- Presentation of the project work an informative video.
- Participation: TUM-Waseda workshop of sustainable technologies for the the future society participation at Technical University Munich, Germany campus Straubing.
- Participation in PhD day and discussion was on state of the art of research fellowships/opportunities. Round Table about my experience as MSCA fellow and the steps toward future position in academy and/or industry.
- Participation in open day doors at TUMCS to show kids and families the evolution and idea of the project.
- Chair of biogenic functional materials (BFM) webpage news at TUMCS.
The results beyond the prior art are:
-First design of room temperature phosphorescent SMs for lighting with moderate PLQY.
- First direct use of proton transfer molecule in LECs and rationalization via additives
Unfortunately, the device performance was not good enough to impact the market. Thus this fundamental research was not taken into account for IPR and/or industrial transfer. However, it is expected to provide a significant scientific impact with the following publications.
-First design of room temperature phosphorescent SMs for lighting with moderate PLQY.
- First direct use of proton transfer molecule in LECs and rationalization via additives
Unfortunately, the device performance was not good enough to impact the market. Thus this fundamental research was not taken into account for IPR and/or industrial transfer. However, it is expected to provide a significant scientific impact with the following publications.