Modeling Of Organic Light Emitting Diodes Research Articles

Three-Dimensional Modeling of Organic Light-Emitting Diodes Containing Molecules with Large Electric Dipole Moments

We present the results of a three-dimensional kinetic Monte Carlo (3D KMC) simulation study of the current density and efficiency of a prototypical green phosphorescent organic light-emitting diode within which the TPBi [2,${2}^{\ensuremath{'}}$,${2}^{\ensuremath{''}}$-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)] molecules that comprise the electron transport layer have a large (7 D) static dipole moment. By explicitly calculating the dipole-induced electrostatic field distribution the additional energetic disorder and the additional internal electrostatic field due to the small net dipole-moment orientation in TPBi are included in a mechanistic manner. The simulation results are compared with the results of an experimental study by B. Sim et al. [ACS Appl. Mater. Interf. 8, 33010 (2016)]. Using a set of simulation parameters that provides a fair agreement with the experimental voltage-dependent current density, external quantum efficiency, and emission profiles, the simulations are used to study the sensitivity to the size and net orientation of the dipole moments. We show how the dipole-induced disorder and the dipole orientation affect the current density, the electron injection efficiency, the blocking of holes by the TPBi layer, the shape of the emission profile, and the quantum efficiency. A key element of the validation of the simulations is a comparison with measured emission profiles, which Sim et al. obtained from sense layer experiments. Explicit KMC simulations of these experiments are used to investigate the expected accuracy with which the emission profiles are probed.

Application of Artificial Intelligence for Optimization of Organic Solar Cells Production Process

The study of organic solar cells (OSCs) has been rapidly developed in recent years. Organic solar cell technology is sought after mainly due to the ease of manufacture and their exclusive properties such as mechanical flexibility, light-weight, and transparency. These properties of OSCs are well-suited for unconventional applications with power conversion efficiencies more high than 10%. The flexibility of the used substrates and the thinness of the devices make OSCs ideal for roll-to-roll production. However the organic solar cells still have very low conversion efficiencies due to degradation and stability of the technology. In order to extract their full potential, OSCs have to be optimized. On the other hand the production chain of the organic solar cells (OSC) can take advantage of the use of artificial intelligence (AI). In fact the integration into the production workflow makes solar cells more competitive and efficient. This paper presents some applications of the AI for optimization of OSCs production processes Full Text: PDF ReferencesLo Sciuto, G., Capizzi, G., Coco, S., Shikler, R., "Geometric shape optimization of organic solar cells for efficiency enhancement by neural networks." (2017) Lecture Notes in Mechanical Engineering, pp. 789-796. CrossRef Barnea, S.N., Lo Sciuto, G., Hai, N., Shikler, R., Capizzi, G., Wozniak, M., Polap, D., "Photo-electro characterization and modeling of organic light-emitting diodes by using a radial basis neural network." (2017) Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 10246 LNAI, pp. 378-389. CrossRef Ye, L.; Hu, H.; Ghasemi, M.; Wang, T.; Collins, B.A.; Kim, J.H.; Jiang, K.; Carpenter, J.H.; Li, H.; Li, Z.; et al. "Quantitative relations between interaction parameter, miscibility and function in organic solar cells." Nat. Mater. 2018, 17, 253-260. CrossRef Haralick, R.M., Shanmugam, K., Dinstein, I.: Textural features for image classification. IEEE Trans. Syst. Man Cybern. SMC-3(6), 610-621 (1973) CrossRef Capizzi, G., Sciuto, G.L., Napoli, C., Tramontana, E., Wozniak, M.: Automatic classification of fruit defects based on co-occurrence matrix and neural networks. In: 2015 Federated Conference on Computer Science and Information Systems (FedCSIS), pp. 861-867, September 2015. CrossRef

Scale‐photo‐electro‐thermal model for organic light‐emitting diodes

Organic light-emitting diodes (OLEDs) with large area have a great potential for lighting applications. The knowledge about scale, photometrical, electrical and thermal (SPET) domains of OLEDs is essential to achieve a good performance in lighting systems. This study presents an OLED model based on datasheet information and experimental tests to approach the interaction among SPET domains in OLED devices. The model is based on an equivalent electrical circuit that merges the electrical, thermal and photometrical behaviour of the OLED and that can easily be implemented in any computer-based simulation program. A detailed procedure for obtaining the different parameters of the proposed model is also presented in this study. The model has been verified for two different OLED geometries, with favourable agreement among theoretical, finite element analysis and experimental results. The results show the influence of scale factor on heat transfer mechanism and the temperature effect on electrical and photometrical OLED performance. This model is useful for predicting OLED behaviour and for simulating OLED drivers during the design of the lighting system.

A Generalized Fabry–Pérot Formulation for Optical Modeling of Organic Light-Emitting Diodes Considering the Dipole Orientation and Light Polarization

The Fabry–Perot formulation has been widely used as a simple and intuitive optical modeling method of organic light-emitting diodes (OLEDs). However, because the Fabry–Perot formulation in the optical modeling of OLEDs does not include the effect of the dipole orientation and light polarization on light emission characteristics, rigorous electromagnetic models should be used, in spite of their heavy mathematical and computational complexity. In addition, the validity and the limitation of the Fabry–Perot formulation for OLEDs have yet to be proven. We propose a generalized Fabry–Perot formulation to calculate the dependence of the dipole orientation and light polarization on the angular emission characteristics using a simple analytical equation rather than a complicated electromagnetic model. The generalized Fabry–Perot formulation is derived, together with detailed steps, and becomes equivalent to the current Fabry–Perot formulation in isotropic dipole orientation. In addition, the physical interpretation of the proposed Fabry–Perot formulation is elucidated in comparison with the quantum mechanical model. To demonstrate the applicability and the validity of the generalized Fabry–Perot formulation, the angular emission spectra of a top-emitting OLED are calculated with respect to the horizontal dipole ratio and the light polarization, which can be easily calculated based on a simple analytical equation.

Trap-Assisted Recombination in Disordered Organic Semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.

Diffusion-controlled charge injection model for organic light-emitting diodes

We present an compact model to describe the charge injection for organic light-emitting diodes. By identifying a critical distance where the concentration of carriers in the extended states equals that of the trapped carriers, we obtain a model for the injection current, which links the drift-diffusion and the multiple-trapping theories. This model yields the injection current as a function of electric field, temperature, and barrier height between metal and organic semiconductor. Good agreement with recent experimental data is observed. The effect of the field-dependent mobility on the injection current is also discussed.

Modeling of organic light-emitting diodes with graded concentration in the emissive multilayer

We model the electrical behavior of organic light-emitting diodes whose emissive multilayer is formed by blends of an electron transporting material, tris-(8-hydroxyquinoline) aluminum (Alq3) and a hole transporting material, N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4-diamine. The multilayer is composed of layers of different concentration. The Alq3 concentration gradually decreases from the cathode to the anode. We demonstrate that these graded devices have higher efficiency and operate at lower applied voltages than devices whose emissive layer is made of nominally homogeneous blends. Our results show an important advantage of graded devices, namely, the low values of the recombination rate distribution near the cathode and the anode, so that electrode quenching is expected to be significantly suppressed in these devices.

Organic negative-resistance devices using PPV containing electron-transporting groups on the main chain

A kind of novel structure polymer, a poly(phenylenevinylene) (PPV) derivative containing electron-transporting groups on the main chain (OPPV), was developed according to the polycondensation mechanism. The I–V characteristics at the low-voltage region with negative differential resistance were observed in the same model of organic light-emitting diodes with the OPPV polymer. The different external parameters, including the device structure, layer thickness, morphology, and homogeneity of indium tin oxide and the polymer, were also investigated. The performance improvement in the organic negative differential resistance and a possible mechanism are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 589–593, 2001

Numerical model for organic light-emitting diodes

An extensive numerical model recently developed for the multilayer organic light-emitting diode is described and applied to a set of real devices. The model contains a detailed description of electrical contacts including dipolar layer formation, thermionic and tunneling injection, space charge effects, field dependent mobilities and recombination processes. The model is applied to simulate several single layer devices and the family of bilayer devices made in our group. It provides insight into the energy level shifts, internal electric fields and charge distribution (and consequently recombination) throughout the device. Finally, the analysis is extended to the optimization of bilayer device.

Transport and optical modeling of organic light-emitting diodes

We present detailed theoretical modeling of organic light-emitting diodes. We incorporate both optical and electrical modeling into one and thus achieve a more comprehensive picture of the device operation. The importance of including the exciton generation and emission rate in a transport model is highlighted and we find that in such devices the charge recombination zone is not necessarily identical to the exciton emission zone. Moreover, both the electrical and optical structure affect the exciton density distribution and thus also the externally measured light.