The development of next-generation perovskitebased optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr 3 perovskites that the Pb−Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron−phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of timeresolved and temperature-dependent studies at Br K and Pb L 3 X-ray absorption edges with refined ab initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperaturedependent XAS results rules out thermal effects as the primary source of distortion of the Pb−Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr 3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

With their fast response time and a spatial resolution in the range of a few microns, microchannel plates (MCPs) are a prominent choice for the development of detectors with highest resolution standards. Amorphous silicon-based microchannel plates (AMCPs) aim at overcoming the fabrication drawbacks of conventional MCPs and the long dead time of their individual channels. AMCPs are fabricated via plasma deposition and dry reactive ion etching. Using a state-ofthe-art dry reactive ion etching process, the aspect ratio, so far limited to a value of 14, could be considerably enhanced with a potential for very high gain values. We show first fabricated AMCP devices and provide an outlook for gain values to be expected based on the fabrication results. KeywordsMicrochannel plate, Secondary electron emission, Monolithic device, Deep reactive ion etching, Amorphous silicon Preprint CC-BY-NC-ND Published under https://doi.

For high band gap solar cells, organic molecule based upconverter materials are promising to reduce transmission losses of photons with energies below the absorption threshold. We investigate the approach of embedding the organic upconverter DPA:PtOEP directly into each second layer of a Bragg stack to achieve an enhancement of upconversion performance. The two major effects that influence the upconversion process within the Bragg stack are simulated based on experimentally determined input parameters. The locally increased irradiance is simulated using the scattering matrix method. The variation of the density of photon states is obtained from calculations of the eigenmodes of the photonic crystal using the plane wave expansion method. A relative irradiance enhancement of 3.23 has been found for a Bragg stack of 31 layers including λ/8-layers on both sides. For suppressing the loss mechanism of direct sensitizer triplet decay via variations of the density of photon states, a different design of the Bragg stack is necessary than for maximum irradiance enhancement. In order to find the optimum design to increase upconversion quantum yield, both simulation results need to be coupled in a rate-equation model. The irradiance enhancement found in our simulation is significantly higher than the one found in the simulation of a grating-waveguide structure, which achieved an increase of upconversion quantum yield by a factor of 1.8. Thus, the Bragg structure is very promising for upconversion quantum yield enhancement

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Upconversion of sub-band-gap photons is a promising approach to increase the efficiency of solar cells. In this paper, we review the recent progress in upconverter material development and realization of efficient upconverter silicon solar cell devices. Current published record values for the increase in the short-circuit current density due to upconversion are 13.1 mA/cm2 at a solar concentration of 210 suns determined in a sun simulator measurement. This increase is equivalent to a relative efficiency enhancement of 0.19% for the silicon solar cell. Although this is a considerable enhancement by more than one order of magnitude from values published only a few years ago, further enhancement of the upconversion performance is necessary. To this end, we investigate theoretically the application of resonant cavity and grating photonic structures. Our simulation based analysis considers irradiance enhancement and modified density of photon states due to the photonic structures and their impact on the upconversion dynamics in -NaYF4: 20%Er 3+. It shows that an optimized grating can increase upconversion luminescence by a factor of 3 averaged over the whole structure in comparison to an unstructured reference with the same amount of upconverter material