This aim of this dissertation is to fabrication microlens arrays (MLA) by silicon mold using dry etching technique and imprint on the PET substrate by direct imprinting microlens structures on Polyethylene terephthalate (PET) substrates using Si molds.The MLA on PET substrates can be used to increase light emitting efficiency from OLED. The MLA was formed by first etching the silicon wafers using SF6 process gas in an RIE/ECR system using isotropic etching technique.The concave undercuts obtained after the dry etching was removed by wet-etching the wafer in HF and HNO3 solutions.Finally,the fabricated silicon mold was used to imprint the microlens structure on the PET substrates.The microlens array with 10 £gm and 25 £gm radius on PET substrate were successfully fabricated using the technique.The surface coverage of the MLA of beter than 90% was obtained. In addition,the outcoupling efficiency of an OLED can be increased using the MLA.The brightness enhancement factor of 1.67 was achieved using in the MLA comparision to the simulation result of 1.73.

This thesis focuses on the analysis of the external quantum efficiency of quantum dot-enhanced multi-junction solar cells. Divided in four major parts, it uses the experimental methodology developed in the SUNLAB. At first, a model is introduced to calculate the external quantum efficiency of single and multi-junction solar cells. This model takes into account the semiconductor physics governing the electrical property of the solar cell. It furthermore takes into account the optical transmission and reflection in the semiconductor structure using a transfer matrix method. The calculated curve fits a single junction GaAs solar cell's external quantum efficiency to a high degree of precision. Finally, an InGaP/GaAs/Ge solar cell's external quantum efficiency is calculated and it reproduces accurately the behavior of a measured cell. Second, the reflectivity of a solar cell is studied. An analysis technique involving using the fast Fourier transform of the oscillation in the reflectivity is introduced. This technique extracts the thicknesses of the top and middle subcells. The reflectivity is subsequently calculated using the transfer matrix method and it reproduces the behavior of the measured samples. Third, the effect of the addition of quantum dots in the middle subcell is studied. It is demonstrated that they extend the absorption range of the middle subcell. This is completed by first modeling the quantum mechanical behavior of the electrons and holes in the nanostructure. Their emission and absorption properties are derived. Those derived properties are verified by experimentally measured photoluminescence and electroluminescence of the nanostructures. The resulting model is then compared to experimentally measured external quantum efficiencies of single junction and multi-junction quantum dot-enhanced solar cells. Finally, a study of the bottom subcell artifact is completed. Using the fill-factor bias experiment, each of the contribution of the light coupling and the internal voltage biasing is decoupled. For the measured sample, an optimal voltage of 2.1 V is found to minimize the artifact. At this point, the internal voltage biasing creates an artifact of 1 % and the light coupling artifact is 8 %.

abstract: This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (V_OC) and decrease in short circuit current (J_sc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in V_OC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in J_sc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in J_sc, due primarily to poor overall charge transport in the blend. V_OC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C₇₀ solar cells. Due to graphene’s high transparency, graphene devices have enhanced J_sc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. V_OC is raised by using MoO₃ as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C₇₀ layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C₇₀.

General lighting with InGaN light emitting diodes (LEDs) as light sources is of particular interest in terms of energy savings and related environmental benefits due to high lighting efficiency, long lifetime, and Hg-free nature. Incandescent and fluorescent light sources are used for general lighting almost everywhere. But their lighting efficiency is very limited: only 20-30 lm/W for incandescent lighting bulb, approximately 100 lm/W for fluorescent lighting. State-of-the-art InGaN LEDs with a luminous efficacy of over 200 lm/W at room temperature have been reported. However, the goal of replacing the incandescent and fluorescent lights with InGaN LEDs is still elusive since their lighting efficiency decreases substantially when the injection current increases beyond certain values (typically 10-50 Acm-2). In order to improve the electroluminescence (EL) performance at high currents for InGaN LEDs, two approaches have been undertaken in this thesis. First, we explored the preparation and characterization of non-polar and semi-polar GaN substrates (including a-plane, m-plane and semi-polar planes). These substrates serve as promising alternatives to the commonly used c-plane, with the benefit of a reduced polarization-induced electric field and therefore higher quantum efficiency. It is demonstrated that LEDs on m-plane GaN substrates have inherently higher EL quantum efficiency and better efficiency retention ability at high injection currents than their c-plane counterparts. Secondly, from a device structure level, we explored the possible origins of the EL efficiency degradation at high currents in InGaN LEDs and investigated the effect of hot electrons on EL of LEDs by varying the barrier height of electron blocking layer. A first-order theoretical model is proposed to explain the effect of electron overflow caused by hot electron transport across the LED active region on LED EL performance. The calculation results are in agreement with experimental observations. Furthermore, a novel structure called a “staircase electron injector” (SEI) is demonstrated to effectively thermalize hot electrons, thereby reducing the reduction of EL efficiency due to electron overflow. The SEI features several InyGa1-yN layers, with their In fraction (y) increasing in a stepwise manner, starting with a low value at the first step near the junction with n-GaN.

GaN-Nanodrähte können mit einer hohen strukturellen Perfektion auf verschiedenen kristallinen und amorphen Substraten gewachsen werden. Sie bieten somit faszinierende Möglichkeiten, sowohl zur Untersuchung von fundamentalen Eigenschaften des Materialsystems, als auch in der Anwendung in optoelektronischen Bauteilen. Obwohl bereits verschiedene Prototypen solcher Bauteile vorgestellt wurden, sind viele grundlegende Eigenschaften von GaN-Nanodrähten noch ungeklärt, darunter die interne Quanteneffizienz (IQE), welche ein wichtiges Merkmal für optoelektronische Anwendungen darstellt. Die vorliegende Arbeit präsentiert eine detaillierte Untersuchung der Rekombinationsdynamik von Exzitonen, in selbst-induzierten und selektiv gewachsenen GaN Nanodraht-Proben, welche mit Molekularstrahlepitaxie hergestellt wurden. Die zeitaufgelösten Photolumineszenz (PL)-Experimente werden durch Simulationen ergänzt, welche auf Ratengleichungs-Modellen basieren. Es stellt sich heraus, dass die Populationen von freien und gebundenen Exzitonen gekoppelt sind und zwischen 10 und 300 K von einem nichtstrahlenden Kanal beeinflusst werden. Die Untersuchung von Proben mit unterschiedlichem Nanodraht-Durchmesser und Koaleszenzgrad zeigt, dass weder die Nanodraht-Oberfläche, noch Defekte als Folge von Koaleszenz diesen nichtstrahlenden Kanal induzieren. Daraus lässt sich folgern, dass die kurze Zerfallszeit von Exzitonen in GaN-Nanodrähten durch Punktdefekte verursacht wird, welche die IQE bei 10 K auf 20% limitieren. Der häufig beobachtete biexponentiellen PL-Zerfall des Donator-gebundenen Exzitons wird analysiert und es zeigt sich, dass die langsame Komponente durch eine Kopplung mit Akzeptoren verursacht wird. Motiviert durch Experimente, welche eine starke Abhängigkeit der PL-Intensität vom Nanodraht-Durchmesser zeigen, wird die externen Quanteneffizienz von geordneten Nanodraht-Feldern mit Hilfe numerischer Simulationen der Absorption und Extraktion von Licht in diesen Strukturen untersucht. / GaN nanowires (NWs) can be fabricated with a high structural perfection on various crystalline and amorphous substrates. They offer intriguing possibilities for both fundamental investigations of the GaN material system as well as applications in optoelectronic devices. Although prototype devices based on GaN NWs have been presented already, several fundamental questions remain unresolved to date. In particular, the internal quantum efficiency (IQE), an important basic figure of merit for optoelectronic applications, is essentially unknown for GaN NWs. This thesis presents a detailed investigation of the exciton dynamics in GaN NWs using continuous-wave and time-resolved photoluminescence (PL) spectroscopy. Spontaneously formed ensembles and ordered arrays of GaN NWs grown by molecular-beam epitaxy are examined. The experiments are combined with simulations based on the solution of rate equation systems to obtain new insights into the recombination dynamics in GaN NWs at low temperatures. In particular, the free and bound exciton states in GaN NWs are found to be coupled and affected by a nonradiative channel between 10 and 300 K. The investigation of samples with different NW diameters and coalescence degrees conclusively shows that the dominating nonradiative channel is neither related to the NW surface nor to coalescence-induced defects. Hence, we conclude that nonradiative point defects are the origin of the fast recombination dynamics in GaN NWs, and limit the IQE of the investigated samples to about 20% at cryogenic temperatures. We also demonstrate that the frequently observed biexponential decay for the donor-bound exciton originates from a coupling with the acceptor-bound exciton state in the GaN NWs. Motivated by an experimentally observed, strong dependence of the PL intensity of ordered GaN NW arrays on the NW diameter, we perform numerical simulations of the light absorption and extraction to explore the external quantum efficiency of these samples.