Future Students - What are Photovoltaic Devices
How do they work?
Solar cells are made using semiconductors such as silicon. Semiconductors have interesting electrical properties, making them useful for electronic devices such as the microprocessors used in computers. One of their properties is that they can be treated in different ways to become either 'positive' (p-type) or 'negative' (n-type).
Photovoltaic Cell under Illumination
A solar cell consists of two layers of semiconductor, one p-type and the other n-type, sandwiched together to form a 'pn junction'. This pn interface induces an electric field across the junction. When particles of light ('photons') are absorbed by the semiconductor, they transfer their energy to some of the semiconductor's electrons, which are then able to move about through the material. For each such negatively charged electron, a corresponding mobile positive charge, called a 'hole', is created. In an ordinary semiconductor, these electrons and holes recombine after a short time and their energy is wasted as heat.
In a solar cell, however, the electrons and holes near the pn junction are swept across in opposite directions by the action of the electric field and others diffuse towards the junction to replace them. This separation of charge induces a voltage across the device. By connecting the device to an external circuit, the electrons are able to flow - and this flow of electrons is what we call electricity.
What are Solar Cells Good For?
In the mid-1950s, the builders of early space craft required an energy source that was reliable, long lasting and required no maintenance. There are no power points in space and, as we all know, batteries can go flat pretty quickly. Solar cells, however, are an ideal choice for this application. They are reliable, maintenance-free and their energy source-sunlight is abundant and virtually everlasting. The first practical solar cells were therefore developed for these space applications.
Unfortunately, these first solar cells were very expensive to produce. In the early 1970s, the oil crisis spurred development of solar cell technology for applications on earth. A terrestrial photovoltaic industry was soon established and has continued to grow ever since, now growing at over 30% per year. Solar cells have been used extensively in remote applications such as to provide electricity for telecommunications equipment or for people who live away from the electricity grid. In such applications, solar cell technology is often cheaper than alternatives such as connecting to the electricity grid or installing a diesel generator and transporting fuel. In recent years though, the main global market has become urban grid-connected modules, led by regions with solar-friendly policies like Japan, Germany and California.
What of the Future?
Recent developments suggest that photovoltaic technology may soon be playing a much larger role in all our lives. Spearheading this push has been the School of Photovoltaic and Renewable Energy Engineering.
While efficiencies have been improved, the cost of the solar cells themselves has steadily fallen with increasing mass production in recent years and should continue to do so over the coming decade. Development of systems technology and integration of solar cells into building materials such as roof tiles is also reducing the overall system costs and making them more attractive to buyers.
As costs continue to fall over the coming decade, we are likely to see solar cells become more commonplace in everyday applications such as on people's roofs. Those who elect to place solar cells on their roof can sell electricity to their local utility during the day when the sun is shining and buy it back at night when it is not. This is the key to the boom in the market in Europe and spectacularly in Germany. These changes should have a significant impact on reducing the emission of greenhouse gases and other pollutants, and will take us one step closer to a cleaner energy future.