Dr Indrachapa Bandara Rajapakshe Mudiyanselage

We report the need for careful selection of anti-solvents for Sn-based perovskite solar cells fabricated through the commonly used anti-solvent method, compared to their Pb-based counterparts. This, in combination with the film processing conditions used, enables the complete removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Sn-based perovskites. Using a Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 perovskite, charge carrier mobilities of 32 ± 3 cm2 V−1 s−1 (the highest reported for such systems through the optical-pump terahertz probe technique) together with ∼28 mA cm−2 short circuit current densities are achieved. A champion efficiency of 11.6% was obtained for solvent extraction using toluene (an 80% enhancement in efficiency compared to the other anti-solvents) which is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. Our work highlights the importance of anti-solvents in managing defects for high efficiency low bandgap perovskite materials and develops the potential for all-perovskite tandem solar cells.

The performance of all solar cells is dictated by charge recombination. A closer to ideal recombination dynamics results in improved performances, with fill factors approaching the limits based on Shockley–Queisser analysis. It is well known that for emerging solar materials such as perovskites, there are several challenges that need to be overcome to achieve high fill factors, particularly for large area lead–tin mixed perovskite solar cells. Here we demonstrate a strategy towards achieving fill factors above 80% through post-treatment of a lead–tin mixed perovskite absorber with guanidinium bromide for devices with an active area of 0.43 cm2. This bromide post-treatment results in a more favorable band alignment at the anode and cathode interfaces, enabling better bipolar extraction. The resulting devices demonstrate an exceptional fill factor of 83%, approaching the Shockley–Queisser limit, resulting in a power conversion efficiency of 14.4% for large area devices.

X-ray detectors currently employed in dosimetry suffer from a number of drawbacks including the inability to conform to curved surfaces and being limited to smaller dimensions due to available crystal sizes. In this study, a hybrid X-ray detector (HXD) has been developed which offers real-time response with added advantages of being highly sensitive over a broad energy range, mechanically flexible, relatively inexpensive, and able to be fabricated over large areas on the desired surface. The detector comprises an organic matrix embedded with high-atomic-number inorganic nanoparticles which increase the radiation attenuation and within the device allows for simultaneous transfer of electrons and holes. The HXD delivers a peak response of 14 nA cm -2 , which corresponds to a sensitivity of 30.8 μC Gy -1 cm -2 , under the exposure of 6-MV hard X-rays generated by a medical linear accelerator. The angular dependence of the HXD has been studied, which offers a maximum variation of 26% in the posterior versus lateral beam directions. The flexible HXD can be conformed to the human body shape and is expected to eliminate variations due to source-to-skin distance with reduced physical evaluation complexities.

Electronic skins (e-skins), which can seamlessly adapt and adhere to the body to mimic the functionality of human skin, are a rapidly emerging research area. Such e-skins have the potential to revolutionize artificial prosthetics, robotics, human-machine interfacing, and health monitoring applications. Powering the e-skin is a critical challenge at present due to strict performance criteria, including flexibility, stretchability, mobility, and autonomous operation. One of the most promising approaches to overcome some of these challenges is to scavenge energy from the human body's movements and its surrounding environment. This paper outlines some of the key potential developments that enable energy harvesting through mechanical, thermal affects, and low light sources, as well as energy management and storage technologies, which could lead toward the construction of autonomous e-skin modules and self-powered sensing systems.

Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 μm due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1–1 mm thickness regime is demonstrated, having a high sensitivity of ∼160 μC mGy–1 cm–3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.

In order to increase the material throughput of aligned discontinuous fibre composites using technologies such as HiPerDiF, stability of the carbon fibres in an aqueous solution needs to be achieved. Subsequently, a range of surfactants, typically employed to disperse carbon-based materials, have been assessed to determine the most appropriate for use in this regard. The optimum stability of the discontinuous fibres was observed when using the anionic surfactant, sodium dodecylbenzene sulphonate, which was superior to a range of other non-ionic and anionic surfactants, and single-fibre fragmentation demonstrated that the employment of sodium dodecylbenzene sulphonate did not affect the interfacial adhesion between fibres. Rheometry was used to complement the study, to understand the potential mechanisms of the improved stability of discontinuous fibres in aqueous suspension, and it led to the understanding that the increased viscosity was a significant factor. For the shear rates employed, fibre deformation was neither expected nor observed.

The dynamic increase in terahertz photoconductivity resulting from energetic intraband relaxation was used to track the formation of highly mobile charges in thin films of the tin iodide perovskite Cs1-xRbxSnI3 and compared to the lead based Cs0:05(FA0:83MA0:17)0:95Pb(I0:83Br0:17)3. Energy relaxation times were found to be around 500 fs, comparable to those in GaAs and longer than the ones of the lead-based perovskite (around 300 fs). At low excess energies the efficient intraband relaxation can be understood within the context of the Frohlich electron-phonon interaction. For higher excess energies the photoconductivity rise time lengthens in accordance with carrier injection higher in the bands, or into multiple bands. The findings contribute to the development of design rules for photovoltaic devices capable of extracting hot carriers from perovskite semiconductors.

The remarkable optoelectronic performance of metal halide perovskites has instigated fundamental research into their photophysics and photochemistry, including the fundamentals of light absorption and emission, and electrical transport. Recent studies have revealed unexpected hot carrier phenomena in metal halide perovskites, whereby at energies high above the band gap, and at high excitation intensities, long-lived hot carriers have been observed1 which can undergo long-range transport.2 This has opened the possibility of harvesting and exploiting hot carriers in semiconductor devices, for instance to overcome the Schockley-Queisser limit in photovoltaic cells.

Perovskite solar cells (PSC) have attracted considerable attention in recent years due to the high efficiencies achieved, their scalability potential and the introduction of record efficiency tandem devices with a perovskite layer on top of silicon. Although the reported efficiencies and scalability approaches demonstrated show great potential for this technology, stability issues are a significant obstacle for PSC before they can compete with established PV technologies. Although efficient encapsulation offers some protection from environmental factors, encapsulation will never be perfect and through the years some permeation of oxygen and humidity into PV modules is inevitable. There are a number of critical factors for PSC degradation, and tests require accurate conditions and measurements in order to distinguish different degradation mechanisms. In situ metrology for ageing tests of perovskite PV devices has been developed at NPL that involves accurately controlled environments that can simulate realistic encapsulation conditions, while applying electrical measurements and spatial characterisation techniques. Environmental conditions including atmospheric composition, spectrum and intensity of illumination, and temperature can be accurately controlled. At the same time, current-voltage measurements, luminescence imaging, and photocurrent mapping of PSC can be performed during degradation. This allows the collection of a significant amount of data, in order to record the failure processes of samples and reveal the degradation mechanisms.

X-ray detectors are critical to healthcare diagnostics, cancer therapy and homeland security, with many potential uses limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitate thick crystals (~1 mm–1 cm), resulting in rigid structures, high operational voltages and high cost. Here we present a disruptive, flexible, low cost, broadband, and high sensitivity direct X-ray transduction technology produced by embedding high atomic number bismuth oxide nanoparticles in an organic bulk heterojunction. These hybrid detectors demonstrate sensitivities of 1712 µC mGy−1 cm−3 for “soft” X-rays and ~30 and 58 µC mGy−1 cm−3 under 6 and 15 MV “hard” X-rays generated from a medical linear accelerator; strongly competing with the current solid state detectors, all achieved at low bias voltages (−10 V) and low power, enabling detector operation powered by coin cell batteries.

Inorganic-organic hybrid metal halide perovskites have shown great promise in realising high performing photovoltaic devices with low fabrication cost. In this regard, Pb-Sn mixed perovskites are considered as highly suitable candidate for all perovskite tandem solar cells and single junction solar cells with narrow bandgaps. However, the maximum achievable performances of Pb-Sn mixed perovskite solar cells (PSCs) are not yet obtained due to the lack of understanding of the material system as well prevailing issues due to unintentional doping as a result of Sn oxidation. In this thesis, the optimisation of a triple cation Pb-Sn mixed perovskite with a formula Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 was investigated. Firstly, the effect of the “anti-solvent” on Pb-Sn mixed perovskites (fabricated through the commonly used anti-solvent dripping method) was investigated and compared to a Pb-only triple cation perovskite analogue, in terms of its effect on the film morphology, crystallisation dynamics and molecular interactions in the films. Secondly, the need for careful selection of anti-solvents for Sn-incorporated PSCs, compared to their Pb-based counterparts is elaborated. This is investigated using Pb-Sn mixed perovskite devices of an inverted architecture, with a device stack of ITO/PEDOT:PSS/Perovskite/PC60BM/ZnO/BCP/Ag. The above studies, in combination with the film processing conditions used is identified to enable the removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Pb-Sn mixed perovskites. Optimisation of the anti-solvent engineering process enables an 80% enhancement in efficiency when toluene is used as the antisolvent, in comparison to the other anti-solvents tested in this work. The champion power conversion efficiency of 11.6% achieved through this is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. This is attributed to the complete removal of Sn4+ dopants following the optimisation process. Following the optimisation of the Pb-Sn mixed perovskite film preparation, a surface post-treatment method is developed based on guanidinium bromide which enables the efficiency to be improved even further to 14.4%. The surface post-treatment especially allows device fill factors of 83%, the highest reported for Pb-Sn mixed PSCs reported so far, approaching the Shockley-Queisser limit for fill factors in Pb-Sn perovskites. This work highlights the importance of careful selection of anti-solvents and post treatments on Pb-Sn mixed perovskite absorber layers, in managing defects and reducing non-radiative recombination for high efficiency low bandgap perovskite materials, and also enables further improvements in device efficiency for all-perovskite tandem solar cells