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The development of stable materials, processable on alarge area,is a prerequisite for perovskite industrialization. Beyond the perovskiteabsorber itself, this should also guide the development of all otherlayers in the solar cell. In this regard, the use of NiO x as a hole transport material (HTM) offers severaladvantages, as it can be deposited with high throughput on large areasand on flat or textured surfaces via sputtering, a well-establishedindustrial method. However, NiO x may triggerthe degradation of perovskite solar cells (PSCs) when exposed to environmentalstressors. Already after 100 h of damp heat stressing, a strong fillfactor (FF) loss appears in conjunction with a characteristic S-shaped J-V curve. By performing a wide range of analysison cells and materials, completed by device simulation, the causeof the degradation is pinpointed and mitigation strategies are proposed.When NiO x is heated in an air-tight environment,its free charge carrier density drops, resulting in a band misalignmentat the NiO x /perovskite interface and inthe formation of a barrier impeding hole extraction. Adding an organiclayer between the NiO x and the perovskiteenables higher performances but not long-term thermal stability, forwhich reducing the NiO x thickness is necessary.

Perovskite solar cells represent an emerging and highly promising renewable energy technology. However, the most efficient perovskite solar cells critically depend on the use of lead. This represents a possible environmental concern potentially limiting the technologies’ commercialization. Here, we demonstrate a facile recycling process for PbI2, the most common lead-based precursor in perovskite absorber material. The process uses only hot water to effectively extract lead from synthetic precursor mixes, plastic- and glass-based perovskites (92.6 – 100% efficiency after two extractions). When the hot extractant is cooled, crystalline PbI2 in high purity (> 95.9%) precipitated with a high yield: from glass-based perovskites, the first cycle of extraction / precipitation was sufficient to recover 94.4 ± 5.6% of Pb, whereas a second cycle yielded another 10.0 ± 5.2% Pb, making the recovery quantitative. The solid extraction residue remaining is consequently deprived of metals and may thus be disposed as non-hazardous waste. Therefore, exploiting the highly temperature-dependent solubility of PbI2 in water provides a straightforward, easy to implement way to efficiently extract lead from PSC at the end-of-life and deposit the extraction residues in a cost-effective manner, mitigating the potential risk of lead leaching at the perovskites’ end-of-life.

Over the last years, the operational stability of perovskite solar cells has been significantly improved by compositional engineering, interface modification, and improved encapsulation techniques. However, irreversible degradation is still ubiquitously observed during the first 1000 hours of operation, particularly at elevated temperatures. In this work, we elucidate a major mechanism controlling this degradation. For that, formamidinium lead iodide-based perovskite solar cells were stressed under continuous 1 sun illumination at 80 °C in N2, before extensive characterization of their microstructure and composition. The cell active area and hence the photocurrent are found to decrease with time due to the growth of porous PbI2-rich regions. This degradation was observed to originate from a few seed points in the perovskite bulk rather than from the interfaces with the charge-selective layers or from the cell edges. I2 vapor, first released at these defective points and then further released from the decomposition of the perovskite exposed to this vapor, controls the degradation process. Furthermore, this autocatalytic degradation process is shown to locally rupture the top electrode due to vapor pressure build-up. In addition to highlighting the detrimental influence of residual PbI2, we show that such a degradation pathway can be alleviated by reducing the methylammonium and/or iodine content, providing a path to more stable perovskite solar cells.

We describe a novel approach to coherent detection which makes use of a dual polarization coherent receiver. The proposed technique, adapted from telecommunication technology, allows the analysis of the polarization state of an optical signal in addition to its phase. We demonstrate that, with proper digital signal processing, the optical phase between two lasers can be always extracted from a beatnote, regardless the polarization state of the two. In addition, birefringence-induced phase noise can be rejected from the measurement. Such capabilities cannot be achieved with traditional detection based on a single photodiode. This technique is particularly useful in frequency dissemination over fiber, where the polarization of received light changes randomly and on timescales that can be as fast as milliseconds. The proposed approach allows continuous tracking of the fiber-delivered signal as required in many scientific applications, and reduces the phase noise at detection.

Inverted PTB7/PC71BM polymer solar cells are prepared on solution-processed Al:ZnO transparent contacts on PET substrates. Al:ZnO is deposited by a low temperature chemical bath deposition route (T< 100°C at any step) to comply with the temperature sensitive substrate. A maximum conversion efficiency of 6.4% and 6.9% is achieved for the indium-free solar cells on PET and glass substrates, respectively. The devices are relatively stable in air whereby an initial efficiency loss in the order of 15% after storage for 15 days can be fully recovered by light soaking.

We present a systematic study on the effects of electrodeposition parameters on the photoelectrochemical properties of Cu2O. The influence of deposition variables (temperature, pH, and deposition current density) on conductivity has been widely explored in the past for this semiconductor, but the optimization of the electrodeposition process for the photoelectrochemical response in aqueous solutions under AM 1.5 illumination has received far less attention. In this work, we analyze the photoactivity of Cu2O films deposited at different conditions and correlate the photoresponse to morphology, film orientation, and electrical properties. The photoelectrochemical response was measured by linear sweep voltammetry under chopped simulated AM 1.5 illumination. The highest photocurrent obtained was −2.4 mA cm−2 at 0.25 V vs RHE for a film thickness of 1.3 μm. This is the highest reported value reached so far for this material in an aqueous electrolyte under AM 1.5 illumination. The optical and electrical properties of the most photoactive electrode were investigated by UV−vis spectroscopy and electrochemical impedance, while the minority carrier lifetime and diffusion length were measured by optical-pump THz-probe spectroscopy.

Two mechanisms have been suggested to explain stretching-induced maximum force depression: a mechanical alteration in the stretched muscle and an impairment of neural activation. Electrical stimulation allows standardization of the level of muscle activation without being limited by neural control. The aim of this study was to evaluate the stretching-induced changes in the electrical and mechanical properties of muscle during electrically elicited contractions. Twelve participants (age 22 +/- 1 years; body mass 75 +/- 2 kg; stature 1.79 +/- 0.02 m; mean +/- standard error) underwent six electrical stimulations of the medial gastrocnemius muscle before and after stretching. During the contractions, surface electromyogram (EMG) and mechanomyogram (MMG) were recorded simultaneously together with force. After stretching we found: (i) no differences in EMG parameters; (ii) MMG amplitude decreased by 4 +/- 1% (P0.05); and (iii) the peak force, the peak rate of force development, and the acceleration peak of force development decreased by 12 +/- 3%, 14 +/- 1%, and 24 +/- 5%, respectively (P0.05). In conclusion, acute passive stretching did not change EMG properties but altered the mechanical characteristics of the contracting muscle. Indeed, muscle force-generating capacity and stiffness of the muscle-tendon unit were significantly impaired.

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C-60 bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H(2) production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0 V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

The utilisation of Cu2O photocathodes for photoelectrochemical water splitting requires their stabilisation due to photocorrosion in aqueous electrolytes. Ultrathin films of wide band gap semiconducting oxides deposited by atomic layer deposition (ALD) on top of cuprous oxide can perform the dual function of both facilitating charge extraction (through the creation of a p–n junction) and protecting the absorber material from the aqueous electrolyte, thereby suppressing corrosion in favor of hydrogen generation. The factors that determine the photocurrent performance as well as the stability of these photoelectrodes are examined. Specifically, the influence of ALD deposition temperature, electrolyte pH, electrolyte composition as well as post-deposition annealing treatments was studied. The successful development of protective overlayers must fulfil the dual requirements of favourable band alignments as well as chemical stability. At long time scales, the deactivation of the photocathodes proceeds through etching of the amorphous overlayer, accompanied by the loss of the platinum catalyst particles. Through the deposition of a semi-crystalline TiO2 overlayer, 62% stability over 10 hours of testing has been demonstrated without re-platinization.

In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.

Simple bilayer organic solar cells rely on very thin coated films that allow for effective light absorption and charge carrier transport away from the heterojunction at the same time. However, thin films are difficult to coat on rough substrates or over large areas, resulting in adverse shorting and low device fabrication yield. Chemical p-type doping of organic semiconductors can reduce Ohmic losses in thicker transport layers through increased conductivity. By using a Co(III) complex as chemical dopant, we studied doped cyanine dye/C60 bilayer solar cell performance for increasing dye film thickness. For films thicker than 50 nm, doping increased the power conversion efficiency by more than 30%. At the same time, the yield of working cells increased to 80%. We addressed the fate of the doped cyanine dye, and found no influence of doping on solar cell long term stability.

Over the last years, the operational stability of perovskite solar cells has been significantly improved by compositional engineering, interface modification, and improved encapsulation techniques. However, irreversible degradation is still ubiquitously observed during the first 1000 hours of operation, particularly at elevated temperatures. In this work, we elucidate a major mechanism controlling this degradation. For that, formamidinium lead iodide-based perovskite solar cells were stressed under continuous 1 sun illumination at 80 degrees C in N-2, before extensive characterization of their microstructure and composition. The cell active area and hence the photocurrent are found to decrease with time due to the growth of porous PbI2-rich regions. This degradation was observed to originate from a few seed points in the perovskite bulk rather than from the interfaces with the charge-selective layers or from the cell edges. I-2 vapor, first released at these defective points and then further released from the decomposition of the perovskite exposed to this vapor, controls the degradation process. Furthermore, this autocatalytic degradation process is shown to locally rupture the top electrode due to vapor pressure build-up. In addition to highlighting the detrimental influence of residual PbI2, we show that such a degradation pathway can be alleviated by reducing the methylammonium and/or iodine content, providing a path to more stable perovskite solar cells.

The actualization of a hydrogen economy requires cost-effective and environmentally benign solutions to hydrogen production. Chemical energy in the form of hydrogen is more interesting than electricity to satisfy our ever-increasing energy demand because it can be stored more easily. Photoelectrochemical (PEC) water splitting is a carbon-neutral process which converts solar energy into chemical energy (hydrogen) using a light-absorbing semiconducting material to generate the necessary photovoltage to split water into hydrogen and oxygen. So far, record efficiencies in solar-to-hydrogen conversion have been achieved using p-type semiconductors, but only with very expensive materials. However, the spread of this technology relies on the development of semiconductor electrodes made of cheap and abundant elements that are stable in water for a reasonable long time. The semiconductors investigated in this thesis are p-type copper based oxides. Hydrogen production is measured by the photocurrent of water reduction at the semiconductor surface. In the first chapter a roundup of the basics of photoelectrochemical hydrogen generation is presented. In Chapter 2, ternary oxides presenting the delafossite crystal structure are explored, a class of materials that is expected to be stable in water under reducing conditions and that has never been studied for PEC photocathodes. The main achievements of this thesis are presented in Chapters 3-6. It is shown that CU2O, a semiconductor notoriously unstable in aqueous solutions yet a very efficient light absorber, can be stabilized against photocathodic degradation and thus used for water reduction. A preliminary optimization study is carried out to understand the synthetic parameters that control the photoactivity of cuprous oxide obtained by electrodeposition. The deposition current density, temperature and bath pH are changed to find the conditions where photoactivity is optimized. They most photoactive films are then characterized in terms of their optical and electrical properties, which are relevant to the photon-to-electron conversion efficiency (Chapter 3). The surface protection technique consists in the growth of ultrathin oxide layers (about 10-20 nm) by atomic layer deposition (ALD) on the CU2O electrodes (Chapter 4). This powerful technique enables a uniform and continuous coverage of a substrate and a control of the thickness at the atomic scale. Therefore, the passivation technique developed in this thesis can be applied to CU2O morphologies much more complex than those employed in this study and most probably to other semiconductors as well. Photocurrents up to 7.6 mA cm-2 are achieved using CU2O electrodes protected with amorphous Al-doped ZnO and TiCh. and decorated with a Pt catalyst. This photocurrent equals half of the maximum photocurrent possible for C112O and is the highest photocurrent ever measured for an oxide-based photoelectrode for PEC water splitting under AM 1.5 illumination. Further improvements to extend the performance over longer timescales and improve the overall photocathode stability are presented in Chapter 5. The concept of surface passivation against reductive decomposition of a semiconductor is proved for water splitting and important progresses are made in optimizing and understanding the key parameters for preserving the photoactivity of the material under operation. Particularly, the studies presented in Chapter 5 focus on the protective layers, exploring the relationship between the electrical properties of the ALD layers and their chemical stability in different electrolytes and after different temperature treatments. Although some decay in the hydrogen evolution over time is still observed, the last results on the protected electrodes show conversion efficiencies of 60% of the initial value after 10 hours of testing at the standard potential of hydrogen evolution, while for the unprotected electrodes the efficiency drops to 0% after a few minutes.

A clean and efficient way to overcome the limited supply of fossil fuels and the greenhouse effect is the production of hydrogen fuel from sunlight and water through the semiconductor/water junction of a photoelectrochemical cell, where energy collection and water electrolysis are combined into a single semiconductor electrode. We present a highly active photocathode for solar H-2 production, consisting of electrodeposited cuprous oxide, which was protected against photocathodic decomposition in water by nanolayers of Al-doped zinc oxide and titanium oxide and activated for hydrogen evolution with electrodeposited Pt nanoparticles. The roles of the different surface protection components were investigated, and in the best case electrodes showed photocurrents of up to -7.6 mA cm(-2) at a potential of 0V versus the reversible hydrogen electrode at mild pH. The electrodes remained active after 1 h of testing, cuprous oxide was found to be stable during the water reduction reaction and the Faradaic efficiency was estimated to be close to 100%.

Tandem photovoltaic devices based on perovskite and crystalline silicon (PK/c-Si) absorbers have the potential to push commercial silicon single junction devices beyond their current efficiency limit. However, their scale-up to industrially relevant sizes is largely limited by current fabrication methods which rely on evaporated metallization of the front contact instead of industry standard screen-printed silver grids. To tackle this challenge, we demonstrate how a low-temperature silver paste applied by a screen-printing process can be used for the front metal grid of two-terminal perovskite-silicon tandem structures. Small-area tandem devices with such printed front metallization show minimal thermal degradation when annealed up to 140 degrees C in air, resulting in silver bulk resistivity of