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

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

Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH3NH3PbI3 Perovskite Nanocrystals

Photoinduced degradation of individual methylammonium lead triiodide (MAPbI3) perovskite nanocrystals was studied using super-resolution luminescence microspectroscopy under intense light excitation. The photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts in the emission localization position up to a few hundred nanometers were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the three-dimensional MAPbI3 crystal structure smoothly collapses to the two-dimensional layered PbI2 structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is primarily due to migration of methylammonium ions (MA+), which distorts the lattice structure causing alterations to the Pb–I–Pb bond angle and in turn changes the effective band gap

Realistic Instrumentation Platform for Active and Passive Optical Remote Sensing.

We describe the development of a novel versatile optical platform for active and passive remote sensing of environmental parameters. Applications include assessment of vegetation status and water quality. The system is also adapted for ecological studies, such as identification of flying insects including agricultural pests. The system is based on two mid-size amateur astronomy telescopes, continuous-wave diode lasers at different wavelengths ranging from violet to the near infrared, and detector facilities including quadrant photodiodes, two-dimensional and line scan charge-coupled device cameras, and a compact digital spectrometer. Application examples include remote Raman-laser-induced fluorescence monitoring of water quality at 120 m distance, and insect identification at kilometer ranges using the recorded wing-beat frequency and its spectrum of overtones. Because of the low cost this developmental platform is very suitable for advanced research projects in developing countries and has, in fact, been multiplied during hands-on workshops and is now being used by a number of groups at African universities

Microscopic insight into the reversibility of photodegradation in MAPbI3 thin films

Whether optoelectronic devices based on metal-halide perovskite semiconductors will become a commercially viable technology will be determined by their intrinsic and operational stability. Recent results indicate there is some reversibility of perovskite degradation in thin films and devices, although mechanistic insight into the processes driving degradation and recovery are still scarce. We here present a comparative spectroscopic study of methylammonium lead iodide (MAPbI3) films having undergone either photo- or thermal degradation under controlled conditions. We confirm that the degradation mechanism pertaining to each type of stress is inherently different. Our results from photoluminescence microscopy measurements paint a spatially, spectrally and temporarily resolved picture showing that, unlike thermally degraded samples, photodegraded samples are in a state that intermittently recovers to luminescent MAPbI3 upon laser excitation. This indicates that rather than irreversibly decomposing, photoinduced degradation leaves MAPbI3 structurally or compositionally intact but induces defects causing non-radiative recombination losses

Defect-induced local variation of crystal phase transition temperature in metal-halide perovskites

Solution-processed organometal halide perovskites are hybrid crystalline semiconductors highly interesting for low-cost and efficient optoelectronics. Their properties are dependent on the crystal structure. Literature shows a variety of crystal phase transition temperatures and often a spread of the transition over tens of degrees Kelvin. We explain this inconsistency by demonstrating that the temperature of the tetragonal-to-orthorhombic phase transition in methylammonium lead triiodide depends on the concentration and nature of local defects. Phase transition in individual nanowires was studied by photoluminescence microspectroscopy and super-resolution imaging. We propose that upon cooling from 160 to 140 K, domains of the crystal containing fewer defects stay in the tetragonal phase longer than highly defected domains that readily transform to the high bandgap orthorhombic phase at higher temperatures. The existence of relatively pure tetragonal domains during the phase transition leads to drastic photoluminescence enhancement, which is inhomogeneously distributed across perovskite microcrystals