Solar Cells: An Overview

Almost all of the energy we use today came, initially, from the sun. Fossil fuels may be the remains of ancient plants, but the plants themselves were only able to grow by converting the energy of sunlight into usable sugars and other storage molecules. Today an increasing awareness of the limited future for fossil fuels has driven greater interest in how we might be able to harvest energy directly from sunlight through the use of solar cells. Solar, or photovoltaic, cells take sunlight and convert it directly into usable electricity. Solar cells rely on a phenomenon first noticed in 1839 by Edmond Becquerel. He observed that shining a light on two liquids connected by platinum electrodes caused a weak current to flow between them. This phenomenon became known as the photoelectric effect. Essentially, when light hits some kinds of materials, it causes electrons to be released.

For most kinds of materials, the amount of electricity produced through the photoelectric effect is too small for practical use. Some materials and some arrangements of materials, however, have shown greater flexibility and efficiency. Most people have seen the silicon wafer-based photovoltaic cells that power pocket calculators and satellites. The first modern solar batteries, made by AT&T in 1954, used silicon wafers to capture a modest 6% of the energy from sunlight.

The basic way in which silicon wafers create an electrical current can be illustrated by looking at how a solar cell using crystalline silicon works. Two thin, flat wafers of silicon are placed one on top of the other. The top wafer, called the n-layer, has small impurities in the silicon that contribute extra electrons. The bottom wafer, called the p-layer, has different impurities that lead to a number of “holes,” or places where electrons could be but aren’t. The differences between the two mean that when placed together, electrons migrate from the n-layer to the p-layer, setting up an electrical field with the n-layer being the positive side and the p-layer being the negative side. Next, wires are attached to the top and bottom of these wafers.

When light hits the solar cell, it releases electrons in the p-layer. The electrons flow in the direction of the n-layer, towards the positive pole of the electrical field, and are captured by the wire connected there. This flow of electrons can be used to power anything that uses electricity. The main problem preventing wider use of solar cells is the relatively low efficiency of the process and the difficulty in making cost-effective cells. The fossil fuels used in many of our power plants are much cheaper sources of energy.

New materials are also being explored for their potential use in solar cells. If materials that are cheaper and more flexible than silicon could be used, that would also drive down the cost of solar energy and make solar cells more economically viable. Some examples of new materials being investigated include thin sheets of plastic, nanocrystalline materials made out of oxides like zinc oxide and titanium oxide, and even solid-liquid composite matrices. As research continues, this next generation of solar cells may finally untap the potential energy streaming through our windows every day.