Influence of electrical operating conditions and active layer thickness on electroluminescence degradation in polyfluorene–phenylene based light emitting diodes

Romero, B.; Arredondo, B.; Álvarez, Antonio García; Mallavia, Ricardo; Salinas, A.; Quintana, Xabier; Otón, J. M.

doi:10.1016/j.sse.2008.11.005

Cited by 11 publications

(6 citation statements)

References 25 publications

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“…The accumulated electrons promote the degradation of the LEDs by electrochemical oxidation of the PFO thin film. 31 The appearance of the 465 nm peak in the EL of the PFO based devices thus arises from new chemical …”

Section: Resultsmentioning

confidence: 99%

Influence of the polymer concentration on the electroluminescence of ZnO nanorod/polymer hybrid light emitting diodes

Zaman

Zainelabdin

Amin

et al. 2012

Journal of Applied Physics

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The effects of the polymer concentration on the performance of hybrid light emitting diodes (LEDs) based on zinc oxide nanorods (ZnO NRs) and poly(9,9-dioctylfluorene) (PFO) were investigated. Various characterization techniques were applied to study the performance of the PFO/ZnO NR hybrid LEDs fabricated with various PFO concentrations. The fabricated hybrid LEDs demonstrated stable rectifying diode behavior, and it was observed that the turn-on voltage of the LEDs is concentration dependent. The measured room temperature electroluminescence (EL) showed that the PFO concentration plays a critical role in the emission spectra of the hybrid LEDs. At lower PFO concentrations of 2-6 mg/ml, the EL spectra are dominated by blue emission. However, by increasing the concentration to more than 8 mg/ml, the blue emission was completely suppressed while the green emission was dominant. This EL behavior was explained by a double trap system of excitons that were trapped in the b-phase and/or in the fluorenone defects in the PFO side. The effects of current injection on the hybrid LEDs and on the EL emission were also investigated. Under a high injection current, a new blue peak was observed in the EL spectrum, which was correlated to the creation of a new chemical species on the PFO chain. The green emission peak was also enhanced with increasing injection current because of the fluorenone defects. These results indicate that the emission spectra of the hybrid LEDs can be tuned by using different polymer concentrations and by varying the current injected into the device.

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Section: Resultsmentioning

confidence: 99%

Influence of the polymer concentration on the electroluminescence of ZnO nanorod/polymer hybrid light emitting diodes

Zaman

Zainelabdin

Amin

et al. 2012

Journal of Applied Physics

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“…Previous reports on the degradation of PLEDs using PFO and its derivatives [37,38] as emitter apply poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) as anode, despite the misalignment in HOMO levels of PEDOT:PSS (−5.0 eV) and PFO (−5.8 eV). From earlier studies it is known that the initially limited hole injection from PEDOT:PSS into PFO is strongly enhanced once injected electrons from the cathode side reach the PEDOT:PSS and get trapped at the PFO/PEDOT:PSS interface.…”

Section: Pled Degradation With Reduced Singlet Energymentioning

confidence: 99%

Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

Zee

Paulus

Png

et al. 2020

Adv Elect Materials

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In contrast, small-molecule organic LEDs exploiting triplet excitons using phosphorescent molecules (ph-OLEDs) or thermally activated delayed fluorescence (TADF) clearly outperform fluorescent PLEDs in terms of efficiency. The low efficiency is also detrimental for the operational stability of PLEDs, since higher currents are required to reach a required light-output, which accelerates degradation. However, to reach high efficiencies, both ph-OLEDs and TADF based OLEDs generally have complex device architectures comprising different injection, transport, emission, and blocking layers. Despite dissimilar architectures, the multilayer TADF and phosphorescent OLEDs as well as single-layer PLEDs all show typical degradation features of a voltage increase and a luminance decrease under continuous electrical operation. [4] For multilayer OLEDs the physical processes behind degradation are hard to elucidate due to the presence of many materials and interfaces. [5,6] The standard PLED device structure, being an emitting polymer layer sandwiched between two different work function electrodes, is more basic, making degradation processes more straightforward to analyze. In earlier work on degradation of poly(phenylene vinylene) (PPV) based LEDs the effects of molecular weight, molecular structure, and defects that arise during synthesis on lifetime have been discussed. [7] In particular halogen related defects in PPVs are pointed out to have a negative influence on the lifetime. With regard to physical degradation mechanisms, in 2001, Silvestre et al. [4] were the first to propose that "voltage increase" and "luminance decrease" during degradation shared a common origin, namely the formation of trap states. Furthermore, Pekkola et al. [8] studied the influence of triplet excitons on the electrical stability of conjugated polymers. A triplet sensitizer was introduced into a PPV-based LED and a negative influence of triplet excitons on the lifetime was reported. As a possible mechanism the energy transfer from a PPV triplet to an oxygen triplet state, creating the reactive singlet oxygen molecule, was suggested. [9,10] Singlet oxygen has the ability to attack the vinyl bonds of PPVs, providing a chemical pathway for degrading the material. [11] Degradation due to extrinsic effects, such as oxygen and water, is typically minimized by proper encapsulation of organic LEDs and by working in a controlled environment. [5−7,12] Also in ph-OLEDs the harmful influence of Stability under electrical stress is an important aspect for the function of organic light-emitting diodes (OLEDs). Degradation is currently one of the key topics in this field, concerning all types of OLEDs, including fluorescent-, phosphorescent-, and thermally activated delayed fluorescence-based OLEDs. For single-layer polymer light-emitting diodes (PLEDs) it has recently been found that degradation is the result of hole trap formation due to excitonpolaron interactions. However, whether singlet or triplet excitons are responsible for degradation is an open questi...

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“…Finally, devices were encapsulated under N 2 using a glass cover attached by a bead of epoxy adhesive. A detailed description of the fabrication procedure is found in previous works 10 . Table 1 summarized the main characteristics of devices fabricated in this work.…”

Section: Experimental Techniquesmentioning

confidence: 99%

Extraction of circuital parameters of organic solar cells using the exact solution based on Lambert W-function

Pozo

Romero

Arredondo

2012

Organic Photonics V

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The electrical behavior of organic solar cell (OSC) has been analyzed using a simple circuital model consisting on an ideal diode together with a series and parallel resistances (R S and R P respectively). Applying Kirchhoff's Laws to the circuit leads to a transcendental equation that can be solved numerically without approximations using the Lambert W function. Theoretical expression has been fitted to experimental current-voltage (I-V) curves under forward bias, obtaining fairly accurate values for the electrical parameters. This model has been validated comparing the extracted parameters for dark and illumination conditions of different devices. Results show good agreement for R S , and ideality factor ( ).Electrical parameters obtained in this work are also compared to those ones extracted using an approximated method often employed by other authors 1 . We conclude that approximated method leads to reasonable good values for R S , R P and . However, in the case of R p the voltage range chosen to fit the data with the exact method must be constrained to the fourth quadrant, where the role of parallel resistance is more critical.To validate the model, a bunch of organic solar cells with structure ITO/ poly(3,4-ethylenedioxythiophene)-poly (4styrene sulfonate (PEDOT:PSS)/ poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM)/Al has been fabricated in inert atmosphere. Different active layers were deposited varying the P3HT:PCBM ratio (1:0.64, 1:1, 1:1.55) and the active layer thickness (ranging from 100 to 280 nm). Devices are encapsulated inside the glove-box prior its characterization outside the glove-box. Electro optical characterization has been performed with a halogen lamp.

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Influence of electrical operating conditions and active layer thickness on electroluminescence degradation in polyfluorene–phenylene based light emitting diodes

Cited by 11 publications

References 25 publications

Influence of the polymer concentration on the electroluminescence of ZnO nanorod/polymer hybrid light emitting diodes

Influence of the polymer concentration on the electroluminescence of ZnO nanorod/polymer hybrid light emitting diodes

Role of Singlet and Triplet Excitons on the Electrical Stability of Polymer Light‐Emitting Diodes

Extraction of circuital parameters of organic solar cells using the exact solution based on Lambert W-function

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