Since the AlGaInN alloy has a continuous direct bandgap from about 0.7 eV (InN) to 6.2 eV (AlN), nitride-based materials can cover most of the electromag netic spectrum from near-infrared to ultraviolet. Based on this feature, nitride based light-emitting diode (LED) devices have been widely used. With the first commercialization of blue LED devices in 1993, the LED industry became more and more important in the field of lighting. As a typical light-emitting material, LED devices are not only determined by their own components, but also their luminous efficiency is one of the focuses of attention. Generally speaking, the standard for measuring the luminous efficiency of LED devices is the ratio of the number of injected carriers to the number of emitted photons, that is, the external quantum efficiency (EQE). In order to obtain a higher EQE, it can be improved from three aspects, namely internal quantum efficiency (IQE), light extraction ef ficiency (LEE) and injection efficiency (IE). However, since LED devices are often grown by vapor phase epitaxy, the epitaxial growth substrate often absorbs the light emitted by the LED device, thereby reducing the EQE of the entire device and affecting the luminous efficiency. Especially as the light-emitting wavelength of LEDs becomes longer and longer, the EQE of LED devices tends to drop from more than 80% to 4% or even lower (the decline of red LEDs will be more signif icant). At the same time, as the size of LED devices decreases, the proportion of damage caused by the mesa etching process and the surface recombination area of devices (such as Micro LED devices) increases accordingly, and EQE will also show a clear downward trend. Therefore, in addition to further improving EQE through internal quantum efficiency, increasing LEE as much as possible through structural changes is also a key point to improve EQE. In our study, based on our group’s own grown red LEDs, we successfully transferred structured vertical InGaN red LEDs from Si(111) substrates to new substrates, achieving further improvements in LEE. At the same time, it also provides options for applying this technology to LED devices and micro-LED devices of various wavelengths in the future. The LED device with the vertical structure has a low turn-on work ing voltage and a small series resistance. The whole process adopts dry etching technology, which makes the process more precise and reliable. Compared to standard LED devices, the operating voltage and series resistance of LEDs are changed from 30Ω to less than 10Ω respectively, and the LEE is improved by 70%, which is mainly attributed to the removal of the light-collecting substrate and the use of metal reflective layers to improve light extraction efficiency. Furthermore, although the process is an improvement over LEE, this structure-based process improvement can be used for LEDs of various wavelengths as well as micro-LEDs in the future. This typical substrate transfer technique can transfer very thin (3 micron) LED structures from one substrate to another without damaging the device itself, thus providing a way to realize flexible substrates in the near future.

The performance of non-fullerene acceptor (NFA)- based organic solar cells (OSC) has shown continuous increase in recent years, reaching power-conversion efficiencies up to 17% through the design and synthesis of efficient acceptor materials. Recent research is directed towards achieving higher efficiency of OSC, which is limited by the open-circuit voltage (Voc) which is lower than the Voc values achieved in inorganic or perovskites solar cells with comparable bandgaps. In this work, voltage losses in NFA based OSC were calculated by investigating charge-transfer state energy (ECT) using electroluminescence spectroscopy and sensitive external quantum efficiency in three polymer:non-fullerene bulk heterojunction solar cells. PCE10:ITIC device acquired the highest ECT with a Voc of 0.82V, and a a power conversion efficiency (PCE) of 7.91%. While PCE10:O-IDTBR obtained the highest Voc of 1.03V, a PCE of 8.02% compared to PCE10:O-IDTBCN solar cell that has a lower Voc of 0.73V with a PCE of 7.98%. Both radiative and non-radiative voltage losses were calculated. In this thesis, the high open circuit voltage of PCE10:O-IDTBR is explained by the low non-radiative voltage losses compared to PCE10:O-IDTBCN and PCE10:ITIC devices.

The solar cells used today have a performance rate of about 30% in theory, but most solar cells on the market only utilize about 20% of the energy provided by sunrays. A prominent reason that the performance rate is far from 100% is the large variety of energies and corresponding wavelengths in white light. Tandem solar cells utilize two different solar cells, where the light not absorbed by the top cell travels through the top cell and onto the bottom cell. This can lead to an efficiency upward of 40%.    The purpose of this thesis was to evaluate how to use the machine Bentham PVE300 optimally for measurements of transmittance, reflectance and EQE (external quantum efficiency) with the aim to calculate the IQE (Internal quantum efficiency).   To optimize the efficiency of the tandem cells, the reflectance, transmittance and EQE needed to be measured. To do this Bentham PVE300 was used. The properties of Bentham PVE300 were explored beforehand to get a better understanding of the equipment. By reading the instrument manual and simultaneously working on the instrument, methods for the measurement of EQE, reflectance and transmittance were compiled into a manual.   The results of measurements performed by Bentham PVE300 were compared to results from other equipment to determine if the measurements were viable. Agilent Cary 7000 was used to validate the measurements of reflectance and transmittance. Bentham PVE300 was ultimately determined to be reliable and in most cases more reliable than the currently used instruments.

Diese Arbeit gibt einen tiefgehenden Einblick in verschiedene Aspekte von auf (In,Ga)N/GaN Heterostrukturen basierenden Leuchtdioden (LEDs), mittels Molekularstrahlepitaxie entlang der Achse von Nanodrähten (NWs) auf Si Substraten gewachsen. Insbesondere wurden die Wachstumsparameter angepasst, um eine Koaleszierung der Nanodrähte zu vermindern. Auf diese Weise konnte die durch die NW-LEDs emittierte Intensität der Photolumineszenz (PL) um einen Faktor zehn erhöht werden. Die opto-elektronischen Eigenschaften von NW-LEDs konnten durch die Verwendung von Indiumzinoxid, anstatt von Ni/Au als Frontkontakt, verbessert werden. Zudem wurde demonstriert, dass auch selektives Wachstum (SAG) von GaN NWs auf AlN gepufferten Si Substraten mit einer guten Leistungsfähigkeit von Geräte vereinbar ist und somit als Wegbereiter für eine neue Generation von NW-LEDs auf Si dienen kann. Weiterhin war es möglich, strukturierte Felder von ultradünnen NWs durch SAG und thermische in situ Dekomposition herzustellen. In den durch die NW-LEDs emittierten Elektrolumineszenzspektren (EL) wurde eine Doppellinenstruktur beobachtet, die höchstwahrscheinlich von den kompressiven Verspannungen im benachbarten Quantentopf, durch die Elektronensperrschicht verursachten, herrührt. Die Analyse von temperaturabhängigen PL- und EL-Messungen zeigt, dass Ladungsträgerlokalisierungen nicht ausschlaggebend für die EL-Emission von NW-LEDs sind. Die Strom-Spannungs-Charakteristiken (I-V) von NW-LEDs unter Vorwärtsspannung wurden mittels eines Modells beschrieben, in das die vielkomponentige Natur der LEDs berücksichtigt wird. Die unter Rückwärtsspannung aktiven Transportmechanismen wurden anhand von Kapazitätstransientenmessungen und temperaturabhänigigen I-V-Messungen untersucht. Dann wurde ein physikalisches Modell zur quantitativen Beschreibung der besonderen I-V-T Charakteristik der untersuchten NW-LEDs entwickelt. / This PhD thesis provides an in-depth insight on various crucial aspects of light-emitting diodes (LEDs) based on (In,Ga)N/GaN heterostructures grown along the axis of nanowires (NWs) by molecular beam epitaxy on Si substrates. In particular, the growth parameters are adjusted so as to suppress the coalescence of NWs; in this way the photoluminescence (PL) intensity emitted from the NW-LEDs can be increased by about ten times. The opto-electronic properties of the NW-LEDs can be further improved by exclusively employing indium tin oxide instead of Ni/Au as top contact. Furthermore, the compatibility of selective-area growth (SAG) of GaN NWs on AlN-buffered Si substrates with device operation is demonstrated, thus paving the way for a new generation of LEDs based on homogeneous NW ensembles on Si. Ordered arrays of ultrathin NWs are also successfully obtained by combining SAG and in situ post-growth thermal decomposition. A double-line structure is observed in the electroluminescence (EL) spectra emitted by the NW-LEDs; it is likely caused by compressive strain introduced by the (Al,Ga)N electron blocking layer in the neighbouring (In,Ga)N quantum well. An in-depth analysis of temperature dependent PL and EL measurements indicates that carrier localization phenomena do not dominate the EL emission properties of the NW-LEDs. The forward bias current-voltage (I-V) characteristics of different NW-LEDs are analysed by means of an original model that takes into account the multi-element nature of LEDs based on NW ensembles by assuming a linear dependence of the ideality factor on applied bias. The transport mechanisms in reverse bias regime are carefully studied by means of deep level transient spectroscopy (DLTS) and temperature dependent I-V measurements. The physical origin of the detected deep states is discussed. Then, a physical model able to describe quantitatively the peculiar I-V-T characteristics of NW-LEDs is developed.

Organic semiconductors (OSCs) present an electron mobility lower by several orders of magnitude than the hole mobility, giving rise to an electron-hole charge imbalance in organic devices such as organic light-emitting diodes (OLEDs) and organic light-emitting transistors (OLETs). In this thesis project, I tried to achieve an efficient electron transport and injection properties in opto-electronic devices, using inorganic n-type metal oxides (MOs) instead of organic n-type materials and a polyethyleneimine ethoxylated (PEIE) thin layer as electron transport (ETLs) and injection layers (EILs), respectively. In the first part of this thesis, inverted OLEDs were fabricated in order to study the effect of the PEIE layer in-between ZnO and two different emissive layers (EMLs): poly(9,9-dioctylfluorene-alt-benzothiadiazole) polymer (F8BT) and tris(8-hydroxyquinolinato) aluminum small molecule (Alq3), based on a solution and thermal evaporation processes, respectively. Different concentrations (0.80 %, 0.40 %) of PEIE layers were used to further study electron injection capability in OLEDs. After a series of optimizations in the fabrication process, the opto-electrical characterization showed high-performance of devices. The inverted OLEDs reported a maximum luminance over 104 cd m-2 and a maximum external quantum efficiency (EQE) around 1.11 %. The results were attributed to the additional PEIE layer which provided a good electron injection from MOs into EMLs. In the last part of the thesis, OLETs were fabricated and discussed by directly transferring the energy modification layer from OLEDs to OLETs. As metal oxide layer, ZnO:N was employed for OLETs since ZnO:N-based thin film transistors (TFTs) showed better performance than ZnO-based TFTs. Finally, due to their short life-time, OLETs were characterized electrically but not optically.