Solar cells

The first solar cells were developed in 1883 by the American Charles Fritts. By covering layers of selenium with a thin layer of gold he was able to obtain electric current, at a conversion efficiency of 1%. It was not until 1941 when Russel Ohl produced and patented the first ‘modern’ silicon-based solar cell. The first commercially available solar cells were produced in 1954 and had a conversion efficiency of 4%. These were mainly of interest for the powering of satellites and remote equipment (e.g. a telephone booth), since the efficiency was simply too low to justify residential application. In the following years, scientific research was geared towards improving the conversion efficiency of solar cells. Modern-day commercial solar cells feature a conversion efficiency of roughly 15%, the exact amount depending on the materials used and the structure in which they are placed.

A solar cell is also commonly called a photovoltaic cell, or PV-cell in short. Most commercial solar cells are made up of a thin layer of silicon, which is split into two separate types of semiconductor by doping one side with boron and the other with phosphorus. The result is that one side (the so-called N-type semiconductor) of the layer tends to donate electrons, whereas the other side (the so-called P-type semiconductor) tends to receive them. The surface at which the two layers touch is called the junction (also: depletion layer). To protect the fragile assemblage, a solid transparent glass plate is placed in front of it. This glass is commonly covered with an antireflective coating to minimize loss of incident light.

Converting light into power

A diagram of a solar cell

When a solar cell is irradiated with the right type of light (depending on the material, a solar cell is responsive to a certain frequency band), electrons in the silicon get excited and leave their parent atoms. This process, which is called the photoelectric effect in physics, results in the formation of a so-called electron-hole pair, where the “hole” is the vacancy left behind by the escaping electron. These electrons tend to want to move towards the p-type material, which is intently lacking in electrons. They diffuse through the junction in the p-type material, where they fill a hole. This diffusion does however not continue for long, since the resulting charge imbalance causes an electric field to be established along the junction. This electric field forces the electrons to the far n-type side of the material, whereas the holes are forced to the far p-type side. This results in a small (typically ~0.5V) difference in electric potential (voltage) between the two sides, which can be ‘tapped’ by connecting the two sides with conductive wire.

Note that the voltage and current supplied by a single solar cell are generally very limited. Therefore, large numbers of solar cells are often wired in series in solar panels (also called PV panels). Solar panels in turn can be wired together to form photovoltaic systems, or PV systems.

Video: introduction to solar cells

This long (22 minutes) video gives an excellent and complete introduction into how a solar photovolatic cell works. Both the physical background and the solar cell itself are explained in normal language.

Solar cell materials

The high-grade silicon used for commercial solar cells is a ‘waste product’ from the manufacture of computer circuitry. It essentially comes in three ‘flavors’: monocrystalline, polycrystalline and amorphous.

Other materials offering far higher conversion efficienes are available. Commonly used materials are Gallium Arsenide, Germanium, Copper-Indium Gallium Selenide (CIGS) and Cadmium Telluride. These materials can yield conversion efficiencies of up to 40%, but are hard to acquire and thus very expensive. They are generally used on satellites and interplanetary probes, where sunlight is the only source of power. Recently there have also been interesting developments in the field of organic solar cells, which are very cheap to come by and require no environmentally unfriendly production process.

The limits on efficiency

Each semiconducting material has its own properties which make it more or less suitable for use in a solar cell. One of these properties is the so-called band gap, which is the energy gap an electron must cross in order to be promoted from the valence band to the conduction band. It is this property which determines the part (read: frequency range) of the sunlight that the solar cell absorbs. Parts of the sunlight that are not absorbed are either passed through the material or converted to heat. Because the sun doesn’t shine evenly bright in all wavelengths, certain materials will be able to extract more energy from it than others. The below diagram diagram displays the relation between band gap and maximum efficiency. It also displays the position of several materials, to give you a rough idea of each material’s expected performance.

Band gap versus efficiency for semiconducting materials

Note that the actual efficiency of a material depends on more factors than band gap alone. The reflective properties of the material and crystal matrix defects both play an important role in the conversion between light and power. To overcome all these limitations, solar cells can be constructed using multiple materials. Such a so-called multiple junction solar cell is basically a stack of several materials, which combined absorb a larger portion of the solar radiation spectrum.