Direct Singlet Capture Organic Molecules for Fast OLED Emission
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Solution Overview
Problem
Current organic molecules with intramolecular charge transfer transitions exhibit large energy gaps between the singlet and triplet states, hindering the achievement of short emission decay times and high emission quantum yields required for efficient opto-electronic devices like OLEDs, as they rely on thermally activated delayed fluorescence (TADF) which is temperature-dependent and inefficient.
Innovation Solution
The development of organic molecules with donor and acceptor moieties separated by non-conjugated bridges reduces the overlap of wave functions, minimizing the energy gap between singlet and triplet states, allowing for direct singlet harvesting and significantly shorter emission decay times through the use of molecular structures with reduced hyperconjugation and specific bridge substitutions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If traditional intramolecular charge transfer molecules are used, then emission quantum yield can be achieved, but emission decay time is long due to large energy gap between singlet and triplet states
Solution Approach 1:
The molecule is divided into distinct functional segments: electron-donating moiety, electron-accepting moiety, and non-conjugated bridging units. This segmentation prevents orbital overlap between HOMO (localized on donor) and LUMO (localized on acceptor), minimizing the exchange integral and reducing the singlet-triplet energy gap to enable fast emission decay
Solution Approach 2:
Different parts of the molecule are designed with specific local properties: the donor moiety provides electron-rich character, the acceptor moiety provides electron-deficient character, and the bridging units provide electrical isolation. This local differentiation of electronic properties enables selective orbital localization and controls the energy gap
2Loss of energy
If thermally activated delayed fluorescence (TADF) is used to include triplet state occupation, then emission quantum yield is improved, but emission decay time increases and temperature dependence is introduced
Solution Approach 1:
The design merges the advantages of both prompt fluorescence and TADF by creating a system where the singlet charge transfer state (1CT) is iso-energetic with the triplet charge transfer state (3CT). This merging of energy levels allows rapid thermal equilibration between singlet and triplet states, enabling 100% exciton utilization with fast decay times characteristic of prompt fluorescence
Solution Approach 2:
The key parameter change is reducing the singlet-triplet energy gap (ΔE(1CT-3CT)) to approximately 0.12 meV (10 cm⁻¹) through molecular design. This parameter change transforms the emission mechanism from temperature-dependent TADF to temperature-independent direct singlet harvesting with fast decay
3Use of energy by moving object
If donor and acceptor moieties are closely connected for efficient charge transfer, then energy transfer is improved, but wave function overlap increases leading to large energy gap
Solution Approach 1:
Non-conjugated bridging units are introduced as intermediary elements between the donor and acceptor moieties. These bridges act as electrical insulators that prevent direct orbital overlap while maintaining spatial proximity for efficient charge transfer. The bridges minimize the exchange integral without compromising the charge transfer character of the excited state
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a direct singlet harvesting effect, achieving emission decay times five to ten times faster than traditional TADF emitters, with enhanced emission quantum yields and improved color purity, making them suitable for long-lasting and efficient opto-electronic devices.
Implementation Method 1
The bridges show a reduced hyperconjugation. This allows 100% exciton usage in OLEDs with a short emission decay time. This new mechanism represents direct singlet harvesting
Implementation Method 2
The bridges show a reduced hyperconjugation. This makes it possible to increase the emission quantum yield and to reduce the ΔE(S1-T1) values
Implementation Method 3
singlet harvesting effect with a strongly temperature-dependent thermally activated delayed fluorescence (TADF) takes place in direct singlet-harvesting effect with the intersystem crossing between nearly iso-energetic 3CT and 1CT states with ΔE (1CT-3CT) values in the order of 10 cm−1 (0.12 meV)
Implementation Method 4
singlet harvesting effect with a strongly temperature-dependent thermally activated delayed fluorescence (TADF)
Implementation Method 5
Direct singlet capture organic molecules with short emission decay time and application thereof in opto-electronic devices
Data Source
AI summary
The invention relates to novel pure organic emitter molecules and optoelectronic devices containing these organic emitter molecules. According to the invention, in the optoelectronic device, after the excitation of an organic molecule, relaxation and intersystem crossing processes also result from the almost isoenergetic charge transfer triplet state (3CT) for the direct rapid occupation and emission of the charge transfer singlet state (1CT), so that a 1CT→S0 fluorescence occurs without a thermal activation.