This invention explores ways to make Organic Light Emitting Diodes (OLEDs) emit more light and be more efficient. OLEDs create light through particles called singlet excitons. However, only about 25% of these particles produce light directly, while the other 75% do not. This limits the OLED's efficiency. To overcome this, certain mechanisms, like Thermally Activated Delayed Fluorescence (TADF) and Triplet-Triplet Annihilation (TTA), are used to convert non-light-emitting particles into light-emitting ones. The focus here is on improving a specific type of OLED made from polymers, called Polymeric LEDs (PLEDs). By designing these PLEDs to better manage and convert excitons, we can significantly boost their light output and efficiency.
Figure (1) A typical device structure of the inverted F8BT based Polymer Light Emitting Diode (PLED) and the chemical structure of F8BT; (2) The experimental setup incorporating a dual probe (electrical and optical) for detecting the dynamics of spin excitons in bipolar as well as unipolar (hole-only) PLED
OLEDs are highly valued for their use in displays and lighting, but their efficiency has an inherent upper limit (~5%). This is because only 25% of the particles created during the light-emitting process actually produce light. To improve this, we need to find ways to convert more of these particles into light emitters.
Current OLEDs often use mechanisms like Triplet-Triplet Annihilation (TTA) and Thermally Activated Delayed Fluorescence (TADF) to achieve this. However, a detailed analysis of these mechanisms and the factors that reduce efficiency is needed. By studying these interactions and designing better molecular structures, we aim to create more efficient Polymeric LEDs (PLEDs) and other similar devices.
- Utilizing TADF and TTA mechanisms, the OLED improves singlet exciton yield. This leads to increased radiative recombination, thereby boosting Electroluminescence (EL) and Photoluminescence (PL).
- The combination of a zinc oxide (ZnO) electron injection layer and a barium hydroxide (Ba(OH)2) hole blocking layer forms an efficient cathode terminal. This setup, along with MoO3/Au as the Ohmic hole injection contact, ensures effective charge carrier injection and transport.
- The OLED generates singlet and triplet excitons in a controlled ratio of 1:3 at the cathode terminal. These excitons decay exponentially towards the anode, enhancing the overall efficiency of the device.
- The molecular design includes predetermined steric hindrance, causing minimization of orbital overlap, which helps to prevent singlet-triplet interactions and annihilation. This design strategy increases the excited state lifetime of triplet excitons, allowing them to convert into singlet excitons more efficiently at room temperature.
- Adopting a planar molecular structure causes enhanced charge transport and optimal orientation, improving light emission.
- These mechanisms enable the PLED to exceed the theoretical Enhanced Quantum Efficiency (EQE) limit of 5% predicted, achieving EQE values in the range of 17-18%. This makes the OLEDs more efficient for use in displays and lighting devices, providing brighter and more reliable performance.
The prototype is a single-layer PLED based on 9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) with an active layer thickness of about 200 nm, achieving an EQE of 17-18% and a luminance efficiency of approximately 30 cd/A.
The prototype has achieved an EQE of 17-18% and a luminance efficiency of approximately 30 cd/A. The concept has been successfully demonstrated using a single pixel emitting green light. However, it can be scaled up for full displays utilizing backplane thin-film transistors (TFTs) and other colours.
4
PLEDs are more energy-efficient than traditional lighting technologies like incandescent bulbs and even fluorescent lamps. The high EQE (17-18%) of the prototype means it can produce the same amount of light with less electricity consumption, potentially reducing energy bills for consumers and lowering overall energy demand. Also, PLEDs do not contain mercury or other hazardous materials found in traditional fluorescent lamps, making them environmentally friendly over their lifecycle.
The demonstrated concept can be used for molecular engineering as well.
Display and lighting devices, electronics
201621001280
502294