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

Excitation polarization provides structural resolution of individual non-blinking nano-objects.

We propose to combine the method of fluorescence intensity centroid localization with rotation of the plane of excitation polarization. Polarized light interacts selectively with differently oriented fluorophores; thus yielding topological information on the nanometer scale, without any need for fluorophore blinking. The method is applicable to photostable individual systems, when most of the traditional super-resolution methods fail. A theoretical study is supported by experiments on 30 nm long cyclodextrin-encapsulated single polyrotaxane conjugated polymer chains

Inhomogeneous Quenching as a Limit of the Correlation Between Fluorescence Polarization and Conformation of Single Molecules

The photophysical properties of conjugated polymers (CPs) largely depend on the interactions between the CP and its environment. We present a study of two polymers with identical conjugated backbones, bare and insulated, that showed different fluorescence excitation modulation depth histograms. However, the polarization differences are not related to differences in conformation, as commonly believed, but to the existence of "dark" chromophores in the bare polymer that are statically quenched. This results in inhomogeneous quenching of the polymer chain that breaks the correlation between excitation fluorescence polarization and conjugated polymer chain conformation

Colourful luminescence of metal halide perovskites – from fundamentals to applications

This special issue explores the extraordinary luminescence properties of metal halide perovskites. Metal halide perovskites have emerged at the forefront of next-generation, thin-film semiconductors for photovoltaic and light-emitting applications. These materials exhibit remarkable optoelectronic properties such as long charge carrier diffusion lengths and high radiative emission efficiencies, which are unexpected for low-temperature, solution-processed semiconductors. Despite impressive technological advancements in device efficiency, a fundamental understanding of perovskite photophysics and photochemistry is still lagging behind. In particular, perovskites often suffer from unpredictable variations in non-radiative recombination and a range of complex dynamic phenomena that remain poorly understood. This issue contains 26 peer-reviewed papers focused on elucidating and further exploring the intricate relationship between luminescence properties and perovskite synthesis, material composition and structure. The issue traces fundamental structure-function relationships on a microscopic level, which serves as a platform to further understand the photophysical processes in composite heterostructures and devices

Photo-induced fluorescence quenching in conjugated polymers dispersed in solid matrices at low concentration

When isolated conjugated polymer (CP) chains are studied by single molecule spectroscopy, excitation power density in the range of 10-1000 W cm -2 is normally used. We show that at such excitation power densities the fluorescence ability of CPs is significantly reduced. A new methodological approach allowed us to measure the fluorescence quantum yield (QY) of thin matrix polymer films doped with fluorophores at very low concentration using fluorescence microscopy. Fluorescence QYs of different conjugated polymers (P3HT, MEH-PPV, PFBV and cyclodextrin-coated PFBV-Rtx) and a reference perylene diimide dye dispersed in the PMMA matrix were measured as a function of the excitation power density that ranged from ∼10-4 to 100 W cm -2. Already at an excitation power of 0.1 W cm-2 (the power density of the sunlight at the Earth) a detectable reduction of the fluorescence QY was observed for most of the polymers. The origin of the QY reduction is exciton annihilation by photo-generated triplet and/or change-transfer states. Insulation by cyclodextrin was found to decrease significantly the effect of non-emissive quenching states. This journal is © the Partner Organisations 2014

Colourful luminescence of metal halide perovskites – from fundamentals to applications

This special issue explores the extraordinary luminescence properties of metal halide perovskites. Metal halide perovskites have emerged at the forefront of next-generation, thin-film semiconductors for photovoltaic and light-emitting applications. These materials exhibit remarkable optoelectronic properties such as long charge carrier diffusion lengths and high radiative emission efficiencies, which are unexpected for low-temperature, solution-processed semiconductors. Despite impressive technological advancements in device efficiency, a fundamental understanding of perovskite photophysics and photochemistry is still lagging behind. In particular, perovskites often suffer from unpredictable variations in non-radiative recombination and a range of complex dynamic phenomena that remain poorly understood. This issue contains 26 peer-reviewed papers focused on elucidating and further exploring the intricate relationship between luminescence properties and perovskite synthesis, material composition and structure. The issue traces fundamental structure-function relationships on a microscopic level, which serves as a platform to further understand the photophysical processes in composite heterostructures and devices