Rodrigo Martins

On this paper we report the physical model that supports the theory of the Flying Spot Technique (FST). Through this technique it is possible to determine separately the ambipolar diffusion length (L*) and the effective lifetime (τ*) of the generated carriers, using either Schottky diodes or quasi-ohmic sandwich structures. We also report a new static method based on the Spectral Photovoltage (SPT) that allows to infer the ambipolar diffusion length and to estimate the surface recombination velocity. © 1991 Elsevier Science Publishers B.V. All rights reserved.

We have developed in the past a transient technique called Flying Spot Technique (FST)[1], based on the lateral photoeffect. It allows to determine the ambipolar diffusion length and the effective lifetime of the photogenerated carriers, once the light spot velocity and geometry of the structure are known. We propose to apply this technique backwards in order to detect the path and velocity of an object that is moving toward a light source direction. The light back reflected is analyzed by a p.i.n structure measuring the transient transverse photovoltage which is dependent on the object movement (position and velocity). Details concerning material characterization and device geometry will be presented.

In this work we report experimental results on light induced metastability of a-Si: H p.i.n. devices with different microscopic/macroscopic structures and we discuss them in terms of improved stability through spatial control of charged defects grown during light exposure. By placing a thin (few A) intrinsic layer (i) between both p/i and i/n a-Si: H interfaces we are able to reduce the effective degradation rate through spatial modification of the electric field profile in the device. The electronic transport and the stability changes that accompany the change in microstructure (R) and hydrogen content (CH) of the i- and i′-layer, were monitored throughout the entire light induced degradation process and compared with the corresponding μT product (for both carriers) inferred through steady state photoconductivity and Flying Spot Technique (FST) measurements. Results show that the degradation rate is a function of CH and R of both layers and can be correlated with the density of microvoids and di-hydride bonding. Since the i′-layers have a higher CH bonded mainly as SiF2 radicals (R≈0.4), they act as an hindrance to the growth of the defect, in the active region, generating "gettering centers" whose localisation and density are tailored in such a way that they will control spatially the electric field profile during light exposure. Preliminary results show improvements in film's stability when the interfacial layer is included. So future progress toward more stable and efficient a-Si: H solar cells will depend on a careful engineering design of the devices. © 1994 Materials Research Society.

From the flying spot technique (FST) the ambipolar diffusion length and the effective-lifetime of the carriers photogenerated by a moving light spot that strikes a p-i-n junction can be inferred. In this paper, those properties of a p-i-n junction are used together with an optical triangulation principle to determine the velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analysed through an amorphous or a microcrystalline p-i-n structure. Its transient transverse photovoltage is dependent on the velocity of the object. A comparison between the performances of both kinds of devices is presented.

Using the Flying Spot Technique (FST) we have studied minority carrier transport parallel and perpendicular to the surface of amorphous silicon films (a-Si:H). To reduce slow transients due to charge redistribution in low resistivity regions during the measurement we have applied a strong homogeneously absorbed bias light. The defect density was estimated from CPM measurements. The steady-state photocarrier grating technique (SSPG) is a 1-dimensional approach. However, the modulation depth of the carrier profile is also dependent on film surface properties, like surface recombination velocity. Both methods yield comparable diffusion lengths when applied to a-Si:H.

Tuberculosis (TB) remains one of the most serious infectious diseases in the world and the rate of new cases continues to increase. The development of cheap and simple methodologies capable of identifying TB causing agents belonging to the Mycobacterium tuberculosis Complex (MTBC), at point-of-need, in particular in resource-poor countries where the main TB epidemics are observed, is of paramount relevance for the timely and effective diagnosis and management of patients. TB molecular diagnostics, aimed at reducing the time of laboratory diagnostics from weeks to days, still require specialised technical personnel and labour intensive methods. Recent nanotechnology-based systems have been proposed to circumvent these limitations. Here, we report on a paper-based platform capable of integrating a previously developed Au-nanoprobe based MTBC detection assay - we call it "Gold on Paper". The Au-nanoprobe assay is processed and developed on a wax-printed microplate paper platform, allowing unequivocal identification of MTBC members and can be performed without specialised laboratory equipment. Upon integration of this Au-nanoprobe colorimetric assay onto the 384-microplate, differential colour scrutiny may be captured and analysed with a generic "smartphone" device. This strategy uses the mobile device to digitalise the intensity of the colour associated with each colorimetric assay, perform a Red Green Blue (RGB) analysis and transfer relevant information to an off-site lab, thus allowing for efficient diagnostics. Integration of the GPS location metadata of every test image may add a new dimension of information, allowing for real-time epidemiologic data on MTBC identification. © 2012 The Royal Society of Chemistry.

In the past we have developed a transient technique, called the Flying Spot Technique (FST). FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper, we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analyzed through a p.i.n structure being the transient transverse photovoltage dependent on the movement of the object (position and velocity). Assuming that the transport properties of the material and the geometry of the device are known and using a triangulation method we show that it is possible to map the movement of the object. Details concerning material characterization, simulation and device geometry are presented.

PIN devices based on hydrogenated amorphous silicon (a-Si:H) became fundamental elements of many different types sensors, based on either the transverse or the lateral photovoltaic effect. In the past we have developed a transient technique, called the Flying Spot Technique (FST), based on the lateral photoeffect. FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in a light source direction. The light reflected back from the object is analyzed through p.i.n. structure being the transient transverse photovoltage dependent on the object movement (position and velocity). Assuming known the transport properties of the material and the geometry of the device and using a triangulation method we show that it is possible to map the object movement. Details concerning material characterization, simulation and device geometry are presented.

Pin solar cells of a-Si:H were light soaked. The cell characteristics, the optoelectronic properties and the microstructure parameter, as well as the hydrogen content and density of states of the i-layer, were monitored throughout the entire light-induced degradation process and compared with the corresponding μτ product (for both carriers) inferred through steady photoconductivity and FST measurements. Results suggest a correlation between the decrease of μτ product for electrons and the increase of the fraction of hydrogen bonded on internal surfaces, showing structural changes during the degradation process. © 1994.

We report experimental data on light soaking of a-Si: H solar cells as well as the role played by the temperature on the metastable light-induced defect growth. We studied the temperature and intensity dependence on the photoconductivity, μτ product and density of states at the Fermi level (g(Ef)) and we found that the rate of defect growth on the i-layer depends on the quality of the material and on the annealing temperature, resulting from an equilibrium between light-induced and light-annealed defects. The photoresponse of the devices is mainly ruled by its microstructure, and depends on the fraction of hydrogen bounded on internal surfaces. Results suggest a correlation between the decrease of μτ product for electron and the increase of the fraction of hydrogen bonded on internal surfaces, suggesting structural changes during the degradation process. Data show, also, that the thermal annealing effect is worthless up to 70°C because of light-induced defect-generation being the dominant process in recombination mechanisms. © 1994.

PIN solar cells were light soaked up to 60 hours. The cell characteristics, the optoelectronic properties and the microstructure parameter (R = I2100/I2100+I2000) as well as the hydrogen content (CH) and density of states (g(Ef)) of the active i-layer were monitored throughout the entire light induced degradation process and compared with the correspondents μτ product (for both carriers) inferred through steady photoconductivity and FST measurements. Data show a strong correlation between the decrease of μτ product for electron and the increase of the fraction of hydrogen bonded on internal surfaces (R increases from 0.1 to 0.4) suggesting structural changes during the light induced defects' formation. For holes, the μτ product remains approximately constant and only dependent on the initial hydrogen content. As g(Ef) increases, μτ presents an asymmetrical decrease showing that electrons are more sensitive to defects' growth than holes. We also observe that the rate of degradation is faster for samples having the lowest defect densities, R and CH, showing that the amount of degradation is not a simple function of the photon exposure (Gt product) but also depends on the material microstructure.

Nowadays there is a strong demand for intelligent packaging to provide comfort, welfare and security to owners, vendors and consumers by allowing them to know the contents and interact with the goods. This is of particular relevance for low cost, fully disposable and recyclable products, such as identification tags and medical diagnostic tests, and devices for analysis and/or quality control in food and pharmaceutical industries. However, the increase of complexity and processing capacity requires continuous power and can be addressed by the combined use of a small disposable battery, charged by a disposable solar cell, which is able to work under indoor lighting. Herein, we show a proof-of-concept of the pioneering production of thin-film amorphous silicon (a-Si:H) solar cells with an efficiency of 4% by plasma enhanced chemical vapour deposition (PECVD) on liquid packaging cardboard (LPC), which is commonly used in the food and beverage industries. Such accomplishment put us one step closer to this revolution by providing a flexible, renewable and extremely cheap autonomous energy packaging system. Moreover, such Si thin films take advantage of their good performance at low-light levels, which also makes them highly desirable for cheap mobile indoor applications. © The Royal Society of Chemistry.