21In water, transparency seems an ideal concealment strategy, as testified by the variety of transparent 22 aquatic organisms. By contrast, transparency is nearly absent on land, with the exception of insect 23 wings, and knowledge is scarce about its functions and evolution, with fragmentary studies and no 24 comparative perspective. Lepidoptera (butterflies and moths) represent an outstanding group to 25 investigate transparency on land, as species typically harbour opaque wings covered with coloured 26 2 scales, a key multifunctional innovation. Yet, many Lepidoptera species have evolved partially or 27 fully transparent wings. At the interface between physics and biology, the present study investigates 28 transparency in 123 Lepidopteran species (from 31 families) for its structural basis, optical 29 properties and biological relevance in relation to thermoregulation and vision. Our results establish 30 that transparency has likely evolved multiple times independently. Efficiency at transmitting light 31 is largely determined by clearwing microstructure (scale shape, insertion, colouration, dimensions 32 and density) and macrostructure (clearwing area, species size or wing area). Microstructural traits -33 density, dimensions -are tightly linked in their evolution, with different constraints according to 34 scale shape, insertion, and colouration. Transparency appears highly relevant for vision, especially 35 for camouflage, with size-dependent and activity-rhythm dependent variations. Links between 36 transparency and latitude are consistent with an ecological relevance of transparency in 37 thermoregulation, and not so for protection against UV radiation. Altogether, our results shed new 38 light on the physical and ecological processes driving the evolution of transparency on land and 39 underline that transparency is a more complex than previously thought colouration strategy. 40 41

Defining low-temperature engineering protocols for efficient planar perovskite solar cell (PSC) preparation is important for fabrication simplification and low-cost production. In the present work, we have defined a low-temperature (123 °C) protocol for the preparation from a solution of SnO layers which are efficient for an application as an electron transporting layer (ETL) in PSCs. Thin, conformal, and transparent layers have been obtained. The related PSCs have shown best devices with a power conversion efficiency of 18.22% and low-hysteresis J- V curves (a hysteresis index of 6.7%). Charge injection has been thoroughly studied by photoluminescence decay measurements. The decay curves followed a biexponential function. The injection of holes into the spiro-OMeTAD layer was found very fast and is a no-limiting step. On the other side, the charge injection into the oxide ETLs depends on its structure and on the oxide. The time constant for the low-temperature SnO layers is close to that of the mesoporous benchmark layers with a fast (surface) and a slow (bulk) component at 11 and 129 ns with relative contributions calculated at 13% and 87%, respectively. The phenomena occurring at a longer time scale have been investigated by impedance spectroscopy. The SnO cell spectra showed no intermediate-frequency inductive loop. The very low frequency part of the spectra was characterized by the beginning of an arc of a circle at the origin of a very large resistance over a large applied potential range. This resistance, along with an intermediate-frequency resistance, has been assigned to a recombination resistance and explains the very large  V achievable with SnO PSCs. The existence of a capacitance at the intermediate frequency with a noticeable low value at about 0.2 mF·cm is linked with the low hysteresis of the devices.

Iridescent colours are colours that change with viewing or illumination geometry. While they are widespread in many living organisms, most evolutionary studies on iridescence do not take into account their full complexity. Few studies try to precisely characterize what makes iridescent colours special: their angular dependency. Yet, it is likely that this angular dependency has biological functions and is therefore submitted to evolutionary pressures. For this reason, evolutionary biologists need a repeatable method to measure iridescent colours as well as variables to precisely quantify the angular dependency. In this study, we use a theoretical approach to propose five variables that allow one to fully describe iridescent colours at every angle combination. Based on the results, we propose a new measurement protocol and statistical method to reliably characterize iridescence while minimizing the required number of time-consuming measurements. We use hummingbird iridescent feathers and butterfly iridescent wings as test cases to demonstrate the strengths of this new method. We show that our method is precise enough to be potentially used at intraspecific level while being also time-efficient enough to encompass large taxonomic scales.