Super-resolution Luminescence Micro-Spectroscopy : A nano-scale view of solar cell material photophysics

Optical microscopy is a fundamental tool in a range of disciplines encompassed by the physical and biological sciences. At the dawn of this millennium, a break-through was made in optical microscopy where super-resolution methods emerged and declared imaging beyond the optical diffraction limit a possibility. Most of these methods are based on fluorescence detection of single molecules. These methods found particular prominence in the life sciences where small structures could be observed inside living organisms, due to the non-invasiveness of light. Currently there is a growing notion that these methods can be applied in physics and chemistry to study photo-induced phenomena in materials with resolution at the nanoscale. The aim of this thesis is to explore and develop these possibilities to study energy and charge transport in functional materials interesting for light harvesting and solar-energy conversion. We present a novel wide-field super-resolution microscopy method adapted from localization microscopy. In combination with fluorescence spectroscopy it allows for an interrogation of a material’s photophysical properties down to the nanometer scale. We call the method super-resolution luminescence micro-spectroscopy (SuperLuMS). One of the examples that we present here is a study of energy migration and trapping in individual molecular J-aggregates. We show that so-called ‘outliers’ (seldomly occurring trapping states) completely determine the exciton transport and dominate the fluorescence response. We also show that hybrid organic-inorganic perovskites are ideal objects for luminescence microscopy. These “hot” solar cell and light-emitting materials possess rich structures at scales just beyond optical diffraction limit making them an ideal “playground” for employing SuperLuMS and demonstrating its abilities.The dynamics of charge carrier recombination in these materials is controlled by trapping and, as we demonstrate here, possess a great spatial inhomogeniety. For the first time we showed that one single trap can control the fate of charge carries in micrometer sized perovskite crystals which has important consequences for optical design of solar cells and other optoelectronic devices. We were also able to observe details of light-induced degradation and crystal phase transition in individual hybrid organic-inorganic perovskite crystals. We believe SuperLuMS is an approach which will continue to evolve and find more diverse applications in material science

Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites

Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450–780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices

The formations of identities in a multicultural world

The aim of this explorative study is to gain knowledge about the question of identities formations in different socio-cultural contexts. This is done by looking at the specific case of the Oromo people living in a larger community (Minneapolis, USA) as compared to living in a smaller group (Stockholm, Sweden). The case of the Oromo people is investigated in light of past historical, political, social and cultural factors that in different ways have targeted the Oromo identities. Postcolonial notions of power imbalances are considered influencing factors. The Dialogical Self is guiding the interpretation of identities as containing multiple voices. By applying a phenomenological approach emphasis is put on understanding the subjective experiences of identities formations in the Oromo people as they are influenced by different socio-cultural factors. Open interviews and self-report forms were conducted and collected from 20 individuals living in Minneapolis and Stockholm. The narratives were analyzed using the MCA-Minerva (phenomenological tool), and Sphinx Lexica (lexical tool) softwares. Outcomes indicate similarities maintaining a kind of Oromo identity and having difficulties with language acquisition, but marked differences in kinds of Oromo identities, in the importance of the Oromo language, in relations with the host culture and in political consciousness. The size of the group seems to be of relevance in how power asymmetry is affecting this process

Staining-free malaria diagnostics by multispectral and multimodality light-emitting-diode microscopy.

We report an accurate optical differentiation technique between healthy and malaria-infected erythrocytes by quasi-simultaneous measurements of transmittance, reflectance, and scattering properties of unstained blood smears using a multispectral and multimode light-emitting diode microscope. We propose a technique for automated imaging, identification, and counting of malaria-infected erythrocytes for real-time and cost-effective parasitaemia diagnosis as an effective alternative to the manual screening of stained blood smears, now considered to be the gold standard in malaria diagnosis. We evaluate the performance of our algorithm against manual estimations of an expert and show a spectrally resolved increased scattering from malaria-infected blood cells