Injection of holes and electrons causes oxidation and reduction of the organic semiconductor near the anode and cathode, respectively. Electrons and holes diffuse through the field-free mass and meet at the center of the device, where they recombine.
Objective and overview of the research
Theory of conductivity
Atomic and molecular orbitals of carbon materials
Molecular orbital theory (MO) approximates the molecular orbitals formed when two atoms come together as linear combinations of individual atomic orbitals. The highest occupied molecular orbital (HOMO) is the energy at the top of the band of occupied states, while the lowest unoccupied molecular orbital (LUMO) is the first energy level in the unoccupied band.
Conduction mechanism – Soliton, polarton and bipolaron
Except for the exciton-polaron (Figure 1.5(h)), which is an energy carrier but not a charge carrier, all other charged or neutral entities can be driven by an applied electric field and are therefore called electric carriers, or simply carriers. While two polarons of opposite charge can couple together and spin correlate, creating an exciton polaron (Figure 1.5(h), and then decay to the ground state by emitting a photon, a basic mechanism in OLEDs), two polarons of the same charge can also couple and spin-correlate with each other, creating a relatively stable bipolar entity (a quasiparticle) that can diffuse as a charge carrier carrying two positive (Figure 1.5(c)) or two negative (Figure 1.5(d)) contains. ) unit costs.
Conductivity in conjugated polymers
Optical properties
At room temperature, a photon is absorbed from the lowest vibrational level in the ground state to the vibrational level in the excited state. Absorption spectra are useful for observing the first excited states, while emission spectra enable the observation of fundamental vibrational levels.
Organic based light-emitting diodes (OLEDs)
- Principle of OLED
- Charge transport
- Charge injection
- Device efficiency in OLED
The thickness of the barrier is therefore a function of the applied voltage; the barrier thickness decreases as the voltage increases. Consequently, the electronic structure of the LED can be approximated by the simple band model shown in Figure 1.10.
Hybrid organic-inorganic polymer light emitting-diodes (HyPLEDs)
Inverted structure in PLEDs
The operating mechanism of light-emitting electrochemical cells (LECs) is different from that of LEDs53-56 as depicted in Figure 2.2. Charge carrier injection into the LEC is not sensitive to the work function of the electrode material. Unlike the device with the pure SY active layer, the electroluminescence of the device with the SY:ILM mixture (25 wt % of ILMs) was also observed under reverse bias (0~18 V).
The UV absorption and photoluminescence (PL) characteristics of the host dye “M-blue” and orange emitter are shown in Figure 2.10. The Förster radius, RDA is considered a measure of the efficiency of energy transfer between donors and acceptors. The UV absorption and photoluminescence (PL) characteristics of the host fluorescent polymers and dopant are shown in Figure 3.8.
1996, 'Polymer Light-Emitting Electrochemical Cells: in situ Formation of a Light-emitting p-n Junction', Journal of the American Chemical Society, vol. 1997, 'Cirkularno polarizirana elektroluminiscenca iz polimerne svetleče diode', Journal of the American Chemical Society, vol.
HyPLEDs
Interfacial engineering in HyPLEDs
- Self-assembled dipole molecules (SADMs)
- N-type metal oxide/conjugated polyelectrolyte hybrid charge transport layers
To improve device performance, additional dipole layers at the metal/polymer or metal oxide/polymer interface play a functional role by blocking the injected carriers from crossing the structure and reaching the second contact without recombination. The size and orientation of the dipole moment of the self-assembled dipole molecules (SADM) affect the work function of the adjacent layer, in this case ZnO. As a result, the charge injection barrier between the conduction band of ZnO and , can be effectively controlled, resulting in an extremely enhanced electron injection efficiency.
Schematic energy diagrams for flat band conditions of HyPLEDs with (a) unmodified ZnO, (b) negative and (c) positive SADM-modified ZnO. d) Experimentally measured work functions of ZnO as a function of the _SADM using UPS.41. An interface engineering strategy using n-type metal oxide/conjugated polyelectrolyte (CPE) hybrid charge transport layers was demonstrated for high-efficiency polymer light-emitting diodes (PLEDs). The hybrid metal oxide/CPE layer facilitates electron injection while blocking hole transport, thereby maximizing electron-hole recombination within the emissive layer.
A series of metal oxide/CPE combinations were tested in inverted PLEDs (FTO/metal oxide/CPE/F8BT/MoO3/Au). Specifically, the HfO2/CPE bilayer achieved an electroluminescence (EL) efficiency of up to 25.8 cd/A (@ 6.4 V, one of the highest values reported for fluorescent PLEDs).
Experimental
Device fabrication
Measurement of device performance
Objective and overview of the research
Research background
Polymer light-emitting electrochemical cells (PLECs)
LEC operating mechanism
The p- and n-type regions meet in the bulk of the device to form an undoped 'in situ' p-n junction where the electrons and holes recombine radiatively. Due to the high conductivity of the doped layers, the electric field is small throughout the doped regions and the externally applied potential difference is therefore dropped across the central junction, resulting in a high electric field in the center of the device. Due to the high density of ions, small movements of the ions give rise to very large ones.
A large electric field can only be maintained at the contacts where the movement of the ions is blocked by the electrodes. An important difference between the two models is the distribution of the internal electric field. The radiative recombination of electrons and holes at the center of the device depletes the charge carriers, leading to the formation of an undoped transition region across which the entire applied bias falls.
The accumulated ions cause a local enhancement of the electric field and - if the ion density is high enough - leave a large part of the device virtually field-free. Electrons and holes diffuse through the field-free mass and meet in the middle of the device, where they recombine [Adapted from deMello, John C.
LECs vs. LEDs: Advantages and disadvantages
Experimental
Results and discussion
White emission was obtained due to the balanced intensity of the blue and orange emission peak in EL spectra based on a weight percentage of 0.5% DCM dye, as shown in Figure 2.11. By attaching an R-CLC reflector to a WPLED device, we observed LCP white emission due to the selective reflection of the CLC film. The strength of this energy transfer depends only on the emission efficiency of the donor and the absorption efficiency of the acceptor at this wavelength.
Thus, white emissions were obtained by partial energy transfer from dilution of SY and M-red polymer concentration in M-blue polymer. The emission from WPLEDs passes through or is reflected from the RH-CLC reflector due to the selective reflection of the CLC film. Hybrid organic-inorganic polymer light emitting diodes (HyPLEDs) with inorganic metal oxide layer have air-stable properties without a low work function metal electrode and compatibility with backplane n-type thin film transistors (TFTs) for active matrix OLED (AMOLED) applications.
The peak luminous efficiency of the white polymer blend polymer LEDs was 4.67 cd/A at 10.8 V. 36] Morii, Katsuyuki, Kawase, Takeo & Inoue, Satoshi 2008, 'High efficiency and stability in air of organic-inorganic hybrid light-emitting diode without encapsulation', Applied Physics Letters, vol.
White emission in HyPLEC
Objective and overview of the research
Among various device architectures, hybrid organic-inorganic polymeric light-emitting diodes (HyPLEDs) using metal oxides as charge transport and injection layers are promising candidates for low-cost, high-performance, and solution-processable flexible displays. One device that takes advantage of this opportunity in an attractive way is the light-emitting electrochemical cells (LECs). Especially white emission devices in hybrid organic-inorganic polymeric light-emitting electrochemical cells (HyPLECs) are cost-effective process as they use white emission fluorescent mixture with ionic compound as single active layer in device and solution processable method to deposit a transition metal oxide (TMO ) low.
Experimental
Results and discussion
Objective and overview of the research
Research background
- Forster and Dexter energy transfer
- Energy transfer in the polymer blend
- Cholesteric liquid crystals (CLCs)
- Unpolarized white light into circularly polarized light
Among the many energy transfer mechanisms, Förster and Dexter energy transfer are two well-known types of transfer. Förster energy transfer, also called Förster resonance energy transfer, describes the mechanism of energy transfer between two fluorescent molecules, as shown in Figure 3.3. When both molecules are fluorescent, the term fluorescence resonance energy transfer (FRET) is also used, although no energy is actually transferred by fluorescence.
The most efficient Förster energy transfer occurs when the acceptor energy gap is well matched to the energy of the photon emitted by the donor, i.e. the transfer coupling may be weak if the acceptor gap is too far compared to the energy of the donor exciton. donors. Also, Förster energy transfer can occur between two sites over 10 nm apart and is sensitive to the orientation of the molecular dipole. Even if the acceptor gap does not match well with the donor exciton emission, Dexter energy transfer can occur as long as the energy offsets between the donor and acceptor are optimal for charge transfer.
The dipole-dipole interaction between two molecules induces a Förster-type non-radiative energy transfer of the excitation energy from a donor to an acceptor. Furthermore, the energy transfer rate between donor and acceptor should be faster than any decay rate of excited states in donor polymer.
Experimental
Results and discussion
2006, 'Highly efficient and stable invert devices for bottom-emitting organic light-emitting devices', Applied Physics Letters, vol. 2007, 'Multilayer polymer light emitting diode using nanocrystalline metal oxide film as charge injection electrode', Advanced Materials, vol. 43] Bolink, Henk J., Coronado, Eugenio, Repetto, Diego & Sessolo, Michele 2007, 'Air-stable organic-inorganic hybrid light-emitting diodes using ZnO as cathode', Applied Physics Letters, vol.
Organic light-emitting diodes using air-stable metal oxides and thick emitting layers', ChemSusChem, vol. 1999, 'Multicolor multilayer light-emitting devices based on conjugated polymers containing pyridine and para-sexyphenyl oligomer', Applied Physics Letters, vol. 1995, 'White light-emitting single-layer organic electroluminescent devices based on dye-dispersed poly(N-vinylcarbazole)', Applied Physics Letters, vol.
1994, 'White light-emitting organic electroluminescent devices using poly(N-vinylcarbazole) emitter layer doped with three fluorescent dyes', Applied Physics Letters, vol. 2001, 'Effects of singlet and triplet energy transfer on molecular dopants in polymer light-emitting diodes and their utility in chromaticity tuning', Applied Physics Letters, vol. S 2006, 'High efficiency pure white light-emitting diodes from a single polymer: polyfluorene with naphthalimide moieties', Advanced Functional Materials, vol.
2000, 'White light emission from a single-layer spin-fabricated organic light-emitting diode', Chemical Physics Letters, vol.
