C. N. Carvalho

Ultrasensitive microchip sensor based on boron-containing polyfluorene nanofilms

A fluorene-based π-conjugated copolymer with on-chain dibenzoborole units was used in the development of a nanocoated gold interdigitated microelectrode array device which successfully detects fluoride in a broad range of concentrations (10(-11)-10(-4) M) in aqueous solution, upon impedance spectroscopy measurements. A calibration curve obtained over this range of concentrations and a new analytical method based on impedance spectroscopy measurements in aqueous solution is proposed. The sensor nanofilm was produced by spin-coating and diagnosed via spectroscopic ellipsometry, AFM, and electrically conductivity techniques. Changes in the conductivity due to the boron-fluoride complex formation seem to be the major mechanism behind the dependence of impedimetric results on the fluoride concentration.

Light and temperature effect on pin a-Si: H device performance

Abstract 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(E f )) 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.

Fine Tuning of the Spectral Sensitivity in a-SiC:H Stacked p-i'i-n Graded Cells

It is presented in this work a p(a-SiC:H)/i(a-SiC:H)/i((a-Si:H)/ n(a-Si:H) single junction for application in color sensing domain produced with the PECVD technique. The interest of this device resides in its simplicity of realization and utilization, as it takes advantage from the well known properties of the a-Si:H p-i-n junctions together with the possibility of color filtering by tailoring the optical gap of a-Si:H with the introduction of a small percentage of Carbon. The thicknesses of the i(200 nm) and i (1000 nm) layers are optimized for light absorption in the blue and red ranges, respectively. Measurements of the spectral response under forward and reverse polarization show a dependence of the wavelength of the maximum absorption on the intensity of the applied bias. A comparison of the photocurrent reading with and without a 650 nm background DC optical bias permits a complete separation of blue and red color under reverse and forward applied bias, respectively. The application of the LSP technique (AC regime of the optical bias) permits a complete RGB reading of the incoming light. Simulation results obtained with the program ASCA will support and explain the measurements about spectral response and photocurrent reading under DC and AC regimes.

Fine-Tuning of the Spectral Collection Efficiency in a Multilayer Junction Through the LSP Technique

We report in this paper the recent advances we obtained in optimizing a color image sensor based on the LSP technique. A device structure based on a a-SiC:H/ a-Si:H pin/pin tandem structure has been tested for a proper color separation process that takes advantage on the different filtering properties due to the different light penetration depth at different wavelengths inside the a-Si:H and a-SiC:H absorbers. Under reverse bias the green and the red images give, in comparison with previous tested structures, a weak response, while this structure shows a very good recognition of blue color, leaving a good margin for future device optimization in order to achieve a complete and satisfactory RGB image mapping. The physics behind the device functioning is explained by recurring to a numerical simulation of the internal electrical configuration of the device in dark and under different wavelength irradiations. Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of collected photocurrent.

Effect of different TCO interfaces on the performances presented by hydrogenated amorphous silicon p-i-n solar cells

In this paper we report results concerning the effect of the TCO interface on hydrogenated amorphous silicon (a-Si:H) p-i-n homojunction solar cells. Its correlation with dark current density-voltage (J-V) characteristics and spectral response, before and after white light-soaking degradation, is analysed. From this study, we conclude that the properties and stability of these devices are not only influenced by the a-Si:H film properties, but also by the properties of the transparent conductive electrode and its interface with the a-Si:H layer.

Influence of photodegradation on the υτ and microstructure of pin a-Si:H devices

Abstract 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.

Temperature and light-induced degradation effect on a-Si:H photovoltaic PIN device properties

The purpose of this work is to understand how the recombination of carriers generated by light (at several temperatures) can influence the a-Si:H devices quality. Here we report a comparative analysis between the photovoltaic performances of the PIN diodes and the transport properties of their active i-layers under the same degradation conditions.

Performances presented by a position-sensitive detector based on amorphous silicon technology

A rectangular dual-axis large area position sensitive detector (PSD), with 5 cm X 5 cm detection area, has been developed by using a hydrogenated amorphous silicon (a-Si:H) PIN photodiode produced by plasma enhanced chemical vapor deposition (PECVD). The requirements needed for the fabrication of these devices are the thickness uniformity of the different layers, the geometry, and the contacts location. In this paper we present results on PSD with special emphasis on the linearity as well as on its response time.