As the world seeks cleaner and more efficient energy solutions, solar power has emerged as a key renewable option. Among the various technologies, III-V multi-junction solar cells absorb an extended range of the solar spectrum and thus achieve the highest conversion efficiencies in photovoltaics today. Further enhancing solar cell performance requires adding more subcells to the III-V layer stack while improving material quality and doping confinement at specific heterointerfaces, targeting at precise energy band structure design to enhance absorption efficiency and ensure efficient current extraction. Enabling these developments requires a high-resolution probe of crystalline structure and chemical composition, as well as for dopant potential and minority carrier distributions. In this thesis, in situ electrical biasing transmission electron microscopy (TEM) is employed to investigate the electronic properties of specific III-V p-n hetero-junctions, as a step towards enabling quantitative functional characterisation of working solar cells with nanometer resolution. Off-axis electron holography (EH) is employed to measure electrostatic potential distributions in biased GaAs and InP p–n junctions, enabling to develop a methodology for quantifying depletion widths within complex III–V heterostructures at nanometer resolution. To ensure reproducibility across specimens of varying electron-transparent thickness, electrically inactive surface layers from focused ion beam (FIB) preparation are quantified by correlating EH-derived built-in potentials with thickness measured by convergent beam electron diffraction. Electrical contacting is achieved using W and Pt deposition in the FIB. The EH-measured contact characteristics have been found to match the theoretically predicted Schottky barrier heights. Electrostatic potential mapping under in situ reverse bias confirmed the electrical functionality of the GaAs p–n junctions via depletion region widening. Complementary electron beam induced current (EBIC) measurements in scanning TEM (STEM) mode are used to map charge carrier generation and collection in electrically biased GaAs p–n junctions. These measurements validated the method’s utility for assessing electrical integrity under bias by visualising the increase in junction resistivity and EBIC signal corresponding to enhanced electric field strength during reverse biasing. Low-magnification STEM-EBIC enabled the detection of long-range resistivity variations, while secondary electron-induced surface charging effects in n- and p-doped GaAs are resolved. The results highlight the unique capability of STEM-EBIC to probe electron–specimen interactions at the nanoscale, enabled by the signal’s comprehensive electrical sensitivity. The tunnelling efficiency of p-AlGaAs/n-GaInP hetero-junctions in MJSCs depends on the epitaxial growth direction. To understand this behaviour, comparative in situ biasing EH electrostatic potential profiling is used with high spatial-resolution and sensitivity to quantify the depletion widths as a proxy for active dopant distribution and tunnelling performance in operating solar cells. Despite the change in compositional mean inner potential, the p-AlGaAs/n-GaInP depletion width is reproducibly quantified with high accuracy, consistent with electrostatic potential simulations. Finally, in situ biasing EH is used to study the p-AlGaAs/n-GaInP tunnel junction within a working III–V tandem cell. Complementary STEM-EBIC mapping has been used to assess the electrical integrity and functional biasing of the device junctions. This work advances in situ biasing EH and STEM-EBIC methodologies for III-V hetero-junction analysis, promoting comprehensive, quantitative electrical measurements at high-resolution, which is essential for characterisation and further development of high-efficiency III–V MJSCs.
V. Mergner (Fri,) studied this question.