Why Use Solar Cells? Low maintenance, long lasting sources of energy Provides cost-effective power supplies for people remote from the main electricity grid Non-polluting and silent sources of electricity Convenient and flexible source of small amounts of power Renewable and sustainable power, as a means to reduce global warming In 2002, the global market for photovoltaic panels and equipment was valued at 3.5 billion dollars

The Solar Cell The most common type of solar cells are Photovoltaic Cells (PV cells) Converts sunlight directly into electricity Cells are made of a semiconductor material (eg. silicon) Light strikes the PV cell, and a certain portion is absorbed The light energy (in the form of photons) knocks electrons loose, allowing them to flow freely, forming a current Metal contacts on the top and bottom of PV cell draws off the current to use externally as power

The Single Crystalline Silicon Solar Cell Pure silicon is a poor conductor of electricity “Doping” of silicon with phosphorus and boron is necessary to create n-type and p-type regions This allows presence of free electrons and electron-free ‘holes’ The p-n junction generates an electric field that acts as a diode, pushing electrons to flow from the P side to the N side

The Solar Cell

When Light Hits the Cell Light energy (photons) ionizes the atoms in the silicon and the internal field produced by the junction separates some of the positive charges (holes) from the negative charges (electrons) The holes are swept into the p-layer and the electrons are swept into the n-layer The charges can only recombine by passing through an external circuit outside the material Power is produced since the free electrons have to pass through the load to recombine with the positive holes

Efficiency of Solar Cells The amount of power available from a PV device is determined by Type and area of the material The intensity of the sunlight The wavelength of the sunlight Single crystalline solar cells  25% efficency Polycrystalline silicon solar cells  less than 20% Amorphous silicon solar cells  less than 10% Cells are connected in series to form a panel to provide larger voltages and an increased current

Arrays and Systems Panels of solar cells can be linked together to form a larger system – an array (a) a PV panel array, ranging from two to many hundreds of panels; (b) a control panel, to regulate the power from the panels; (c) a power storage system, generally comprising of a number of specially designed batteries; (d) an inverter, for converting the DC to AC power (eg 240 V AC) (e) backup power supplies such as diesel startup generators (optional) (f) framework and housing for the system (g) trackers and sensors (optional);

Solar Cells are used in a wide variety of applications Toys, watches, calculators Electric fences Remote lighting systems Water pumping Water treatment Emergency power Portable power supplies Satellites

Future Applications Looks like denim Can be draped over any shape No rigid, silicon base Made of thousands of flexible, inexpensive solar beads between two layers of aluminum foil Each bead functions as a tiny solar cell The Flexible Solar Cell

Future Applications Based on photosynthesis in plants Use of light-sensitive dyes Cost of manufacture is decreased by 60% Organic Solar Cells New Alloys Indium, gallium, and Nitrogen Converts full spectrum of sunlight from near-infrared to far-ultraviolet

Future Applications Tiny rods are embedded in a semi-conducting plastic layer sandwiched between two electrodes Rods act like wires, absorbing light to create an electric current Nano Solar Cells Tetrapod Nanocrystals May double the efficiency of plastic solar cells Made of cadmium, tellurium