Guilherme Lavareda

A DC-DC step-up micro power converter for solar energy harvesting applications is presented. The circuit is based on a switched-capacitor voltage tripler architecture with MOSFET capacitors, which results in an, area approximately eight times smaller than using MiM capacitors for the 0.131 mu m CMOS technology. In order to compensate for the loss of efficiency, due to the larger parasitic capacitances, a charge reutilization scheme is employed. The circuit is self-clocked, using a phase controller designed specifically to work with an amorphous silicon solar cell, in order to obtain the maximum available power from the cell. This will be done by tracking its maximum power point (MPPT) using the fractional open circuit voltage method. Electrical simulations of the circuit, together with an equivalent electrical model of an amorphous silicon solar cell, show that the circuit can deliver a power of 1132 mu W to the load, corresponding to a maximum efficiency of 66.81%.

The influence of tin doping on the electrical, optical, structural and morphological properties of indium oxide films produced by radio-frequency plasma enhanced reactive thermal evaporation is studied, as transport properties are expected to improve with doping. Undoped and tin doped indium oxide thin films are deposited at room temperature using both pure In rods and (95-80) % In:(5-20) % Sn alloys as evaporation sources and 19.5 mW/cm(2) and 58.6 mW/cm(2) as rf-power densities. The two most important macroscopic properties - visible transparency and electrical resistivity - are relatively independent of tin content (0-20%). Visible transmittance of about 75% and electrical resistivity around 5 x 10(-4) Omega.cm can be observed in the films. The structural features are similar for all samples. Nevertheless, the surface morphology characterization shows that the homogeneity of the films varies according to the tin content. Moreover this variation is a balance between the rf-power and the tin content in the alloy: i) films with small and compact grains are produced at 58.6 mW/cm(2) from a 5% Sn alloy or at 19.5 mW/cm(2) from a 15% Sn alloy and consequently, smooth surfaces with reduced roughness and similar grain size and shape are obtained; ii) films showing the presence of aggregates randomly distributed above a tissue formed of thinner grains and higher roughness are produced at the other deposition conditions. (C) 2012 Elsevier B. V. All rights reserved.

An alternative technique for production of devices which uses both silicon crystalline wafers (p-type) and heavy doped amorphous silicon thin films (n-type) is reported. The amorphous silicon acts as a finite source of dopant and is deposited (at low temperature, 70 °C) by plasma enhanced chemical vapor deposition on silicon wafers. Afterwards, the process of dopant diffusion into the crystalline silicon occurs in a diffusion furnace at 1000 °C for 2 h, to create p–n junctions. Using SIMS analyses, a dopant (P) transfer into c-Si of about 30% is verified and 87% of the dopant transferred is electrically active. Consequently, n-MOSFET devices are produced using a gate oxide thermally grown at the same diffusion temperature for one hour. The preliminary results of the MOSFET (channel length and width of 0.5 and 5 mm, respectively) show a depletion behavior with a threshold voltage, Vth=−8.2 V and afield-effect mobility, µ_FE=187.8 cm²/(Vs).

TiO2 films with enhanced photosensitivity were deposited on alkali free glass substrates without intentional substrate heating by pulsed DC magnetron reactive sputtering with an average thickness of about 2 mu m. Three dyes, commercial N719 and two new organic dyes were impregnated in order to control the optical spectral selectivity of such films. The type of dye used proved to dramatically influence the device's response to radiation pulses. The practical breakthrough is the use of different dyes according to the region of the electromagnetic spectrum one wants to detect. Devices with photocurrent 6 orders of magnitude higher than the dark current (from similar to 2 x 10(-12) to 2 x 10(-6) A for a 100 V bias) were fabricated with a spectral response within the visible range of the electromagnetic spectrum. In addition, this approach is likely to allow for the fabrication of hybrid photodetectors on cheap heat sensible flexible polymeric substrates. (C) 2011 Elsevier B.V. All rights reserved.