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

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.

Realization and study of InAs/GaAs quantum dot superluminescent diodes emitting at 1.3µm

Marco Rossetti

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

EPFL2007

Dynamics of quantum dot lasers

Pablo Moreno

The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 µm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.

EPFL2008

Study of the Optical UV-Visible Electron Induced Luminescence of Small Size Selected Metal Clusters in Rare Gas Solids

Alexandre Rydlo

The optical properties of small size selected metal clusters have been studied by fluorescence spectroscopy. Cun (n = 1-5, 8), Agn (n = 1-5, 8, 9, 20), Aun (n = 1-5, 8) and Nin (n = 1-3) clusters embedded in condensed neon are photoionized by VUV photons in the solid, and the fluorescence originating from the reneutralization of the transient cluster ions is recorded. The VUV photons result from the radiative decay of excitons which are generated by the impact of electrons of variable energy on the solid. A new experimental setup has been developed, which permits the excitation of the rare gas solid by electrons with an energy between 0 and 200 eV. A field emission electron source based on carbon nanotubes provides an intense electron beam without any light generation. A custom built cryopumped gas purification system allows the production of rare gas solids with a purity in the order of up to 10-5. The characteristics and a physical descriptive model of electron induced luminescence is presented. The measurement of electron currents through the solid are thoroughly analyzed. Based on localized exciton production and radiative deexcitation of the host rare gas atoms near the surface of the solid, the excitation can be spread over distances as large as 20-30 µm by high energy photons which excite the species contained in the solid. Quenching and amplification effects on the luminescence intensities are observed depending on the material and the size of the dopants contained in the rare gas solids. These effects are believed to originate from the local enhanced electric field induced by the metal clusters which act as antennas. The analysis of the luminescence spectra reveals rich spectroscopic signatures from the UV to the near IR. A certain amount of fluorescent transitions could be identified from conventional excitation fluorescence spectroscopy. This proves that this new method is sensitive and reliable. The so far reported new transitions await interpretation on the basis of sophisticated quantum calculations.

EPFL2010