Electricity from the Sun
Most of us have an idea of what a solar panel is and what they do. In fact most of us use them on a daily basis. Solar panels are commonly found in calculators, satellites, and on road signs around UMD campus. We are becoming more and more environmentally conscientious and finding cleaner and sustainable energy is becoming a popular trend. Today solar panels are being installed on a large scale to provide electricity for communities and are even being installed on electric cars to get a charge while our car sits in the sun all day. Solar power has come a long way since 1953 when scientist at Bell laboratories created the first silicon solar panel that was able to generate a current. For many of us, we use solar panels without really knowing what is happening behind the scene.
A solar panel is an interconnected package of photovoltaic cells or solar cells. The word photovoltaic is derived from "electricity from light" (photo = light and voltaic = electricity). Although solar cells can directly convert sunlight to electricity there is still a high cost associated with solar panels. Each panel needs to be constructed in a certain way to take advantage of the sunlight. The materials used are very important to the energy conversion process, and although most solar panels are constructed in very different ways, they generally follow they same pattern. They are usually formed in layers. Starting from top to bottom generally they all have a glass cover, an antireflection layer, metal electrical contacts, n-type semiconductor, p-type semiconductor, then the back metal contact. Below is a diagram showing the material layers used to construct a solar panel.
The most important parts of a solar panel are the semi-conducting layers. Semiconductors are very important in the electronics world. Semiconductors have properties of both conductors and insulators hence the name semiconductor. There are two types of semi-conducting materials: n-type and p-type. N-type semiconductors have negative electrical charge due to extra electrons in the materials valence band. P-type semiconductors have a positive electrical charge due to absence of electrons or the creation of electron holes. More importantly, when a layer of n-type semiconductor is placed next to a p-type semiconductor, the free electrons from the n-type fill the holes of the p-type semiconductor. With the system eventually coming to equilibrium. Something more interesting happens when energy is introduced to the system, in our case the sun. Tiny packets of light energy from the sun, called photons, strike the solar panel. If the energy from one photon is strong enough, it will knock an electron free. As a result there will be a hole created allowing different electrons to fill it. If we define an electric current as the follow of electrons this is what we have established, and if we provide a path for the electrons to flow they will. The flow of current and the electric field created by the panel will produce useable electric power. The solar panels we are using are a silicon-based semiconductor, which is a common material used in the solar panel fabrication process.
The other layers that are used are for protection and functionality. The top layer is a sheet of glass or plastic. This helps protect the panel from the elements. Even water needs to be kept out of the panel to stop corrosion of the semiconductor layers. The next layer is an antireflection layer. Many semi-conducting materials are reflective in nature, and since we want to absorb energy from the sun we need this layer to decrease the amount of sun reflected away from our panel. Next are metal contacts, basically a wire grid that runs along the semi-conducting layer, allowing electrons to easily leave the material. This grid also connects each separate solar cell together to make a functional solar panel. The last layer is a metal back contact. This acts as the frame for the panel as well a good place to make electrical contacts.
For a more detailed expiation of solar cells check out "How Stuff Works".
Contributed by Brandon Eberle