On the Response Speed of Narrowband Organic Optical Upconversion Devices

Organic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light-emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image-contrast, narrowband OUCs are reported that convert near-infrared (NIR) light at 980 and 976 nm with a full-width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed-versus-light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.

Solution-Processed Organic Optical Upconversion Device

Sina Abdolhosseinzadeh, Matthias Diethelm, Roland Hany, Frank Nüesch, Santhanu Panikar Ramanandan

Imaging in the near-infrared (NIR) is getting increasingly important for applications such as machine vision or medical imaging. NIR-to-visible optical upconverters consist of a monolithic stack of a NIR photodetector and a visible light-emitting unit. Such devices convert NIR light directly to visible light and allow capturing a NIR image with an ordinary camera. Here, five-layer organic solution-processed upconverters (OUCs) are reported which consist of a squaraine dye NIR photodetector and a fluorescent poly(para-phenylene vinylene) copolymer (super yellow)-based organic light-emitting diode (OLED) or light-emitting electrochemical cell (LEC), respectively. Both OLED-OUCs and LEC-OUCs convert NIR light at 980 nm to yellow light at around 575 nm with comparable device metrics of performance, such as a turn-on voltage of 2.7-2.9 V and a NIR-to-visible photon conversion efficiency of around 1.6%. Because of the presence of a salt in the emitting layer, the LEC OUC is a temporally dynamic device. The LEC OUC turn-on and relaxation behavior is characterized in detail. It is demonstrated that a particular ionic distribution and thereby the LEC OUC status can be frozen by storing the device in the presence of a small voltage applied. This provides a test chart for quantitative measurements.

AMER CHEMICAL SOC2019

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

Characterization and Modelling of the Emission Zone and Exciton Dynamics in Doped Organic Light-Emitting Diodes

Markus Regnat

Only recently organic light-emitting diode (OLED) technology has successfully managed the transition from research labs into the consumer market, taking a 60% share of the global mobile display market in 2018. The latest discovery of thermally activated delayed fluorescence attracted a lot of attention in research and industry due to the potential to fabricate fluorescence-based OLEDs with high efficiencies comparable to the currently used phosphorescence-based OLEDs, but with the advantage of possibly cheaper and more sustainable emitter materials (no Ir-, Pt-complexes). For achieving high efficiencies in OLEDs, a substantial number of layers and interfaces of the multilayer stack have to be optimized. A particularly important role is assigned to the emission layer within which light is generated by charge recombination and subsequent energy transfer and radiative decay of excitons. The understanding of charge recombination and exciton dynamics and the determination of the position of light generation are essential for the fabrication of modern OLEDs and are the goal of this thesis. Therefore two different OLED types, phosphorescence-based OLEDs and state-of-the-art TADF exciplex host OLEDs incorporating a fluorescent emitter, are studied by electro-optical characterization and device modelling. In a first step the emission zones are determined and analyzed by angle-dependent steady-state measurements at different biases and optical simulations. In both OLED types split emission zones are obtained with densities of emissive excitons that decay way from both emission layer interfaces toward the center. For the phosphorescence-based OLEDs an additional bias-dependence of the split emission zone is observed, meaning that at low bias the main emission is located at the cathode side and shifts to the anode side for increasing bias. In a second step, with transient EL decay measurements and electro-optical simulations the split emission zones are correlated to an EL peak appearing after OLED turn-off. To study the influence of the emission zone and the exciton dynamics on the OLED efficiency an electro-optical device model is established to reproduce the experimentally obtained measurement data. As the model includes charge carrier dynamics, light outcoupling and time- and position-dependent exciton processes, such as the formation, diffusion, transfer, decay and quenching, the physical mechanisms in the OLEDs are elucidated. For the phosphorescence-based OLED a surprising current efficiency increase of up to 60% for increasing bias as well as a subsequent decrease is explained with the shift of the emission zone and its influence on exciton quenching and light outcoupling. Similarly, for the TADF exciplex host OLEDs a model parameter study illustrates promising EQE enhancement routes, which could lead to EQEs as high as 42%. This thesis emphasizes the need of accurate knowledge of the emission zone and its bias-dependence due to its potentially strong influence on the OLED efficiency and its importance for the optimization of the OLED layer stack. In addition, this thesis shows that full electro-optical device modelling (including electrons, excitons and photons) combined with advanced electro-optical characterization techniques is crucial for elucidating the physical mechanisms in state-of-the-art OLEDs as well as for the prediction of promising routes for future efficiency enhancements.