Projects

This collaborative effort by the joint team between Yale University and the Ilmenau University of Technology will explore the use of efficient semiconductive photoabsorbers to achieve organic synthesis. Semiconductors are known to convert light into charges efficiently, and photocatalysts control the charge transfer processes to drive redox reactions to form stable products. Photocatalytic organic synthesis resembles photoredox catalysis for the coevolution of reductive and oxidative reactions locally, but produces surface-bound organic molecules during coevolution. However, the relationship of the chargetransfer energetics and kinetics with surface reactivity is less known, which limits the design of synthesis pathways by using photocatalysts. To address this gap, the team investigates a model reaction of photocatalytic para-xylene oxidation to produce terephthalic acid, driven by TiO2-coated planar aluminum gallium indium phosphide semiconductors. Theory calculations will corroborate with the measured energetics and product selectivity, and elucidate the surface reaction pathways. The collaborative research thrusts include i.) the correlation between the surface chemistry and the holetransfer energetics, ii.) the coevolution to achieve product selectivity control, and iii.) the vapor-phase reactor implementation. This cross-disciplinary investigation should engage future-generation workforce in both US and Germany. The simple reactor implementation fills the gap of public outreach for chemical production, and also enriches interdisciplinary training plans under this newly initiated international outreach effort.

The joint project "Development of future solar cell technologies and materials for space applications (SMART)" addresses the key technologies for the production of next-generation space solar cells, which can be used to meet the requirements of future satellite platforms in terms of increased performance, cost efficiency and application diversity. The planned technology and material development will focus in particular on metamorphic semiconductor materials for multi-junction solar cells with high radiation hardness as well as the processes for realising ultra-light solar cells and solar cell assemblies. The project consortium consists of AZUR SPACE Solar Power GmbH (project management), the academic partners TU Ilmenau and Philipps-Universität Marburg as well as Fraunhofer ISE.

This project is concerned with the identification and localization of the causes for the limitation of the optoelectronic performance data of semiconductor nanowire structures. The aim is to achieve an intensive correlation of macroscopic device data with spatially very high resolution microscopy data. The devices feature axial and coaxial nanowire homo- and hetero-contacts for charge separation, consisting of GaAs- and InGaP-based pn-contacts, which are fabricated by MOVPE. The relationship between interface formations and recombination paths will be determined by a combination of in-system four-point measurements, scanning probe microscopy and optical methods. The current-voltage characteristics of upright axial versus radial nanowire structures as well as high-resolution scanning tunnelling microscopy will be measured locally. The investigation of the optoelectronic properties is performed with a streak camera system. The metrological detection and localization encounters inherent limitations in nano-devices, which are therefore processed for physical modeling using the simulation software package Silvaco Atlas. The performance data for the determination of conversion efficiencies will be determined taking into account generation and recombination mechanisms as well as minority and tunnel transport across homo- and hetero-junctions.

The project aim is to determine the qualitative and quantitative relationship between nanowire growth, device design, interface formation and surface passivation with respect to the quality of charge-separating pn-junctions in nanowires. From this, concepts are to be presented and tested which allow a significant increase in optoelectronic performance characteristics in light-nanowire interaction.

This project seeks to elucidate the electronic structure and energetic band alignment at the hetero-interfaces of photoelectrochemical multi-junction devices. Comprehension of the band energy diagrams of photoelectrochemical devices in the vicinity of the electrolyte with respect to their relative energetic position as well as the formation of electronic surface states will help to un-derstand efficiency-limiting factors of the overall device. The coupling of absorbers to chemical and electronic passivation layers as well as co-catalysts will be systematically studied, primarily by electrochemical methods coupled in-vacuo to photoelectron spectroscopy. Density functional theory will allow an in-depth interpretation of experimental data, finally providing an atomistic view on the origin of energetic alignments. As the elementary processes of light absorption, charge-separation and -transfer, as well as multi-electron catalysis are highly interrelated, we will focus on two established water splitting multi-junction devices that have already demonstrated high effi-ciencies, but still have not reached the physical limits: silicon-based multi-junction as well as III-V compound semiconductor-based tandem cells. The hereby identified routes to modify the elec-tronic coupling of the hetero-interfaces will, in close cooperation with the other partners, also be studied and evaluated under operating conditions. For the long-term perspective of the Research Consortium, this project will provide generalised research approaches that can be transferred to other high-efficiency multi-junction systems.

This project aims to understand the fundamental processes that govern electron dynamics and energetics of prototypical photoelectrode surfaces, the associated internal interfaces at semiconductor surfaces and related model systems in view of photoelectrochemical hydrogen generation. The detailed mechanisms of interfacial electron transfer processes and their dynamics are still insufficiently understood. We propose to specifically modify the surface electronic and chemical properties of III-V compound semiconductor absorber systems to promote multi-electron processes. Time-resolved two-photon photoemission (tr-2PPE) for explicitly surface-sensitive analysis will be combined with density-functional theory (DFT)-based numerical simulations in order to gain a fundamental understanding of key electron transfer and recombination processes. Tr-2PPE is a unique technique that directly probes the kinetic energy and dynamics of photoemitted electrons accessing at the same time the electronic structure and temporal occupation of surface-near states. III-V compound semiconductors serve as relevant model systems to investigate interfacial dynamics with respect to selected surface modification procedures. Ways to modify III-V surfaces include epitaxial growth of thin films, in-situ surface transformation and catalyst deposition. These methods can produce quasi-two dimensional overlayers to slow down corrosion and enhance photocatalytic activity. The modification with such surface layers will enable tuning of the electron transfer dynamics by selective electronic structure modifications. Studying different types of surface modification will allow us to draw a general picture how interface design benefits electron transport towards the catalytically active surface. In the long-term perspective of the Research Consortium, this project will provide optimum charge separation and transfer for multi-electron catalytic processes in the envisaged new multi-junction absorber systems.

AZURSPACE has many years of industrial experience in the production of solar cells using metal organic vapor phase epitaxy, especially in the field of III-V compounds.

The contamination-free transfer of MOCVD-prepared samples to a UHV-based Multi-Probe-STM available in the group allows the electrical characterization of individual nanowires in vacuo ("as-grown"). The surface of the wires will be either reconstructed, hydrogen terminated or oxidized. These variants should lead to different surface state densities, which will allow their role and characteristics to be better revealed for the first time. In order to enable charge carrier transport over the surface, first completely and partially intrinsic GaAs nanowires are produced. In the first of two work packages, therefore, the charge carrier transport in nanowire structures as a function of surface state density (UHV-transferred vs. oxidized) will be illustrated and investigated in more detail. Experiments are planned in which the wires will be electrically characterized in detail by UHV-transfer without contamination. A modeling should lead to an identification of relevant physical parameters, which serve to reproduce the measurement results first qualitatively and then quantitatively. In the second working point, p-n transitions, which are used as standard for charge carrier separation, will be investigated in more detail. In axial alignment these contacts have already been investigated in GaAs nanowires, but without considering surface contamination or modifications. In this project nanowires with axial p-n junction shall be characterized. In addition, the interaction of nanowires with light will be investigated to verify and apply the previously investigated mechanism.

In the group of photovoltaics, a large-scale facility is currently being put into operation to enable photoluminescence measurements on semiconductor samples for solar energy conversion. Photoluminescence is a process in which a semiconductor emits light characteristic of its optoelectronic properties after illumination. The efficiency of this process is directly linked to the suitability of the material for solar energy conversion. The new measuring station is a so-called streak camera system, whose main components are a femtosecond laser and a streak camera. It will be possible to follow the temporal course of photoluminescence with high temporal resolution as well as spectral resolution. The measuring station represents a substantial extension of the experimental possibilities in the field of photovoltaics and will contribute significantly to the development of next generation semiconductor devices. The large-scale facility was funded by the EU and DFG.