Solar devices

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Introduction
Solar devices are devices that depends on light for their operation in other words, are devices whose electrical characteristics vary according to the amount of light that strikes them. There are two basic types made from semiconductors: photovoltaic cells and photoconductive cells. A phototube, an older type of photoelectric cell, is a type of electron tube. A photovoltaic cell is a semiconductor device that converts light into electric energy. The photovoltaic cells in a solar battery typically consist of silicon wafers treated to form an n-type semiconductor covered by a very thin layer of p-type material. When light strikes the wafer, it creates pairs of holes and free electrons. Some of the light penetrates the surface of the wafer and creates holes and free electrons near the pn junction. The electric field that naturally exists across the junction tends to push electrons into the n-type region and holes into the p-type region. If the two regions are connected through an external circuit, electrons will flow from the n-type region through the external circuit to the p-type region. Photoconductive cells or photoresistive are light-sensitive semiconductor devices that do not have a pn junction.



CHAPTER TWO
Photoelectric cells ( photocells ), are devices whose electrical characteristics vary according to the amount of light that strikes them. There are two basic types made from semiconductors: photovoltaic cells (solar cells) and photoconductive cells. A phototube, an older type of photoelectric cell, is a type of electron tube.

2.0 Solar cells
Solar cell is a wide area electronic device that converts solar energy into electricity by the photovoltaic effect. Therefore, it is also known as a photovoltaic cell. The solar cells are classified into three types which indicate the order of which each became prominent. The three types are the crystalline silicon, thin-film cell, low cost solar cell. The crystalline silicon solar cell was one of the first types to be developed and it is still the most common type in use today. These are high quality and single junction devices. They involve high energy and labour inputs which prevent any significant progress in reducing production costs. “Single junction silicon devices are approaching the theoretical limiting efficiency of 33% and achieve cost parity with fossil fuel energy generation after a payback of 5 to 7 years”. They do not pollute the atmosphere and they leave behind no harmful waste products. Photovoltaic cells work efficiently even in cloudy weather and unlike solar heaters, are more efficient at low temperatures. They do their job silently and there are no moving parts to wear out.


Advantages of photovoltaics
• Electricity produced by solar cells is clean and silent. Because they do not use fuel other than sunshine, PV systems do not release any harmful air or water pollution into the environment, deplete natural resources, or endanger animal or human health.
• Photovoltaic systems are quiet and visually unobtrusive.
• Small-scale solar plants can take advantage of unused space on rooftops of existing buildings.
• PV cells were originally developed for use in space, where repair is extremely expensive, if not impossible. PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time with virtually no maintenance.
• Solar energy is a locally available renewable resource. It does not need to be imported from other regions of the country or across the world. This reduces environmental impacts associated with transportation and also reduces our dependence on imported oil. And, unlike fuels that are mined and harvested, when we use solar energy to produce electricity we do not deplete or alter the resource.