Nanoscale Photoluminescence Manipulation in Monolithic Porous Silicon Oxide Microcavity Coated with Rhodamine-Labeled Polyelectrolyte via Electrostatic Nanoassembling

Porous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters, still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO2) microcavity (MC) can be conformally coated with a uniform nm-thick layer of a cationic light-emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH-RhoB), leveraging the self-tuned electrostatic interaction of the positively-charged PAH-RhoB polymer and negatively-charged PSiO2 surface. It is found that the emission of PAH-RhoB in the PSiO2 MC is enhanced (≈2.5×) and narrowed (≈30×) at the resonant wavelength, compared with that of PAH-RhoB in a non-resonant PSiO2 reference structure. The time-resolved photoluminescence analysis highlights a shortening (≈20%) of the PAH-RhoB emission lifetime in the PSiO2 MC at the resonance versus off-resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor Fp = 2.82 is achieved. This is further confirmed by the enhancement of the photoluminescence quantum yield of the PAH-RhoB in the PSiO2 MC with respect to the reference structure. Application of the electrostatic nanoassembling approach to other emitting dyes, nanomaterials, and nanophotonic systems is envisaged

Perovskite solar cell resilience to fast neutrons

The high power-per-weight ratio displayed by metal-halide perovskite solar cells is a key advantage of these promising devices for applications that require low payload, such as in space and avionics. However, little is known about the effect of the outer space radiation environment on these devices. Here, we report the first in operando study on fast neutron irradiation of perovskite solar cells. We show the remarkable resilience of these devices against one of the most hazardous forms of radiation that can be found at flight altitude and in space. In particular, our results highlight a comparable in operando degradation pattern between light soaked and light + neutron irradiated devices. However, whereas light-induced degradation is fully reversible, fast neutrons lead to permanent effects likely originating from atomic displacement in the active material. We also propose that such irreversible worsening is alleviated by the formation of neutron-induced shallow traps, which act as dopants and contribute to the increase of open circuit voltage and the decrease of leakage current in light + neutron irradiated devices. The high radiation dose that perovskite-based solar cells can potentially withstand renders these devices highly appealing for space and avionic applications

Nanoscale Characterisation and Neutron Damage Testing of Organic semiconductors

In this thesis work, I employ a wide range of morphological, structural and dynamical tools to investigate how the intermolecular interactions in a variety organic semiconductors (OSCs) can influence heavily the nanoscale arrangement and govern the tolerance against external stimuli (i.e. radiation) of these promising materials and related devices. Among the experimental techniques used, this work emphasises on the use of neutron scattering, due to the strong propensity of highly hydrogenated materials to scatter these particles, and thus reveal precious information about their inner structural and dynamical features. I exploit the power of intermolecular interactions, to grow large and solvent-free single crystals of the electron-acceptor of choice of organic solar cells, the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The structure and quality of these crystals have been characterised by X-rays diffraction and metrological atomic force microscopy experiments. High-quality and solvent-free crystals of organic semiconductors can be a reliable platform for studying the intrinsic optical and electronic properties of these materials. The nanoscale structure and dynamics of poly (thiophene):fullerene blends has been investigate by means of neutron reflectivity and quasi-elastic neutron scattering. In particular, I show that the intercalation of fullerene particles in between polymer side-chain depends on degree of order of the polymer. Furthermore, I find that polymer:fullerene interaction has a great effect on polymer dynamics, leading to a retardation of side-chain motion. The interaction between polymer and fullerene particles and their nanoscale mutual arrangement is of great interest for improving the charge phogeneration process in organic solar cells. Finally, I studied for the first time the neutron radiation tolerance of two benchmark poly(thiophenes), observing that polymer crystallinity has a prominent role in determining the neutron radiation tolerance of conjugated polymers. Radiation hardening studies on organic electronics can pave the way to the integration of this class of devices in space and avionic applications

Neutron Radiation Tolerance of Two Benchmark Thiophene-Based Conjugated Polymers: the Importance of Crystallinity for Organic Avionics

Aviation and space applications can benefit significantly from lightweight organic electronics, now spanning from displays to logics, because of the vital importance of minimising payload (size and mass). It is thus crucial to assess the damage caused to such materials by cosmic rays and neutrons, which pose a variety of hazards through atomic displacements following neutron-nucleus collisions. Here we report the first study of the neutron radiation tolerance of two poly(thiophene)s-based organic semiconductors: poly(3-hexylthiophene-2,5-diyl), P3HT, and the liquid-crystalline poly(2,5-bis (3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT. We combine spectroscopic investigations with characterisation of intrinsic charge mobility to show that PBTTT exhibits significantly higher tolerance than P3HT. We explain this in terms of a superior chemical, structural and conformational stability of PBTTT, which can be ascribed to its higher crystallinity, in turn induced by a combination of molecular design features. Our approach can be used to develop design strategies for better neutron radiation-tolerant materials, thus paving the way for organic semiconductors to enter avionics and space applications

Inorganic caesium lead iodide perovskite solar cells

The vast majority of perovskite solar cell research has focused on organic-inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current-voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current-voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current-voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices

Inorganic caesium lead iodide perovskite solar cells

The vast majority of perovskite solar cell research has focused on organic-inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current-voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current-voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current-voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices