Solar Cells

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

Fundamentally, the device needs to fulfill only two functions - Photogeneration of Charge Carriers (electrons and holes) in a light-absorbing material, and Separation of the Charge Carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.

Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement. Solar cells are regarded as one of the key technologies towards a sustainable energy supply.

Thin-film solar cells use less than 1% of the raw material (silicon or other light absorbers) compared to wafer based solar cells, leading to a significant price drop per kWh. There are many research groups around the world actively researching different thin-film approaches and/or materials, however it remains to be seen if these solutions can generate the same space-efficiency as traditional silicon processing.

One particularly promising technology is crystalline silicon thin films on glass substrates. This technology makes use of the advantages of crystalline silicon as a solar cell material, with the cost savings of using a thin-film approach. Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET, for example), which opens a new dimension for new applications.

Gas Sensors

Gas sensors interact with a gas to initiate the measurement of its concentration. The gas sensor then provides output to a gas instrument to display the measurements. Common gases measured by gas sensors include Ammonia, Aerosols, Arsine, Bromine, Carbon Dioxide, Carbon Monoxide, Chlorine, Chlorine Dioxide, Diborane, Dust, Fluorine, Germane, Halocarbons or Refrigerants, Hydrocarbons, Hydrogen, Hydrogen Chloride, Hydrogen Cyanide, Hydrogen Fluoride, Hydrogen Selenide, Hydrogen Sulfide, Mercury Vapor, Nitrogen Dioxide, Nitrogen Oxides, Nitric Oxide, Organic Solvents, Oxygen, Ozone, Phosphine, Silane, Sulfur Dioxide, and Water Vapour.

Important measurement specifications to consider when looking for gas sensors include the response time, the distance, and the flow rate. The response time is the amount of time required from the initial contact with the gas to the sensors processing of the signal. Distance is the maximum distance from the leak or gas source that the sensor can detect gases. The flow rate is the necessary flow rate of air or gas across the gas sensor to produce a signal.

Gas sensors can output a measurement of the gases detected in a number of ways. These include percent LEL, percent volume, trace, leakage, consumption, density, and signature or spectra. The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. Also called volume percent or percent by volume, percent volume is typically only used for mixtures of liquids. Percent by volume is simply the volume of the solute divided by the sum of the volumes of the other components multiplied by 100%. Trace gas sensors given in units of concentration: ppm. Leakage is given as a flow rate like ml/min. Consumption may also be called respiration. Given in units of ml/L/hr. Density measurements are given in units of density: mg/m^3. A signature or spectra measurement is a spectral signature of the gases present; the output is often a chromatogram.

Common outputs from gas sensors include analog voltage, pulse signals, analog currents and switch or relays. Operating parameters to consider for gas sensors include operating temperature and operating humidity.

Copper Oxide (CuO) thin films were deposited using a reactive DC sputtering method for gas sensor applications. The structure of the films determined by means of an X-Ray diffraction method indicates that the phase of Copper Oxide can be synthesized in the total pressure and temperature ranges of 6-8.5 mbar and 151-192 °C, respectively. The resistivity of the film synthesized at a substrate temperature of 192 °C increases from 0.104 to 0.51 Ohm-m after absorbing Carbon Dioxide gas at 135 °C. The gas sensitivity of the film synthesized at the substrate temperature of 192 °C increases up to 5.1 in the presence of Carbon Dioxide gas at 160 °C. The gas sensitivity in the presence of Nitrogen gas reaches only 1.43 even at 200 °C.