The Science Behind Photovoltaics

Photovoltaics depend on the electrical properties of certain materials, known as semiconductors, which allow them to transform sunlight into electricity. While a number of materials have this semiconductor property, the one most commonly used in photovoltaics is silicon. On its own, silicon is actually very resistant to electrical current, but its properties can be altered by doping it, or combining it with small amounts of other materials that make it receptive to either a positive or negative electrical charge.

When a positively charged layer of silicon is placed against a negatively charged layer of silicon, it forms an electrical field through which electrical charges can pass. Sunlight, carrying solar energy creates this charge. By connecting the silicon to a conductive metal, this charge can be concentrated into an electrical current, which can then be fed to any device that uses electricity.

Here we will look at the basic properties of semiconductor materials, using silicon as an example, and how these materials work in a photovoltaic system to create electricity.

A Material that Translates Photon Energy into Electricity

The key properties of semiconductor material are determined at the atomic level. Each atom is composed of three types of particles: protons, neutrons, and electrons. Protons, which have a positive electrical charge, and neutrons, which have no electrical charge, form the nucleus, or core of an atom. Electrons, which each have a negative electrical charge, swirl around the nucleus in one or more layers of "shells", shown in the diagram as rings. Different types of atoms are defined by their unique number of protons, neutrons and electrons. It is the electrons that we are particularly concerned with, as these can be disengaged from certain atoms to collectively form an electrical charge.

The atomic characteristic that distinguishes semiconductors from other materials is the number of electrons in its atoms' outermost shells. One thing that all atoms have in common is that they need a certain number of electrons in each of their shells to make them stable. Atoms fill up their inner shells first, and any remaining electrons gather on the outermost shell. Atoms that have less or more electrons than they need in this outermost shell are always looking for other atoms with which they can exchange or share electrons.

A silicon atom by definition has three shells of electrons. Its innermost shell has two electrons and its second shell has eight - the numbers needed to stabilize those shells. But the outermost shell only has four electrons. A silicon atom is always looking to gain four more electrons to fill this outer shell or get rid of its four extra electrons to have only two shells so it can become fully stable. Because all silicon atoms have four electrons and are looking for four electrons, they easily bond with each other in a crystalline structure. In this structure, each silicon atom joins with other silicon atoms, sharing one electron with each and receiving one shared electron from each. In this configuration, each silicon atom has eight electrons in its outer shell. This ability to bond in a crystalline structure is the defining feature of all semiconductor materials.

 

Altering this Material to Create Conductivity

In its crystalline form, silicon is stable because it has no need to add or get rid of electrons in its outer shell. This actually makes it a very poor conductor of electricity on its own, because there are no free electrons to be released into an electrical current. But there is a way to modify a silicon crystal to make it an excellent semiconductor. This is done by introducing other elements whose atoms carry an extra electron or are missing an electron. When these materials are added to a silicon crystal, in a process called doping, they make the crystal receptive to either a positive or a negative charge. A crystal receptive to positive charges is called p-type silicon ("p” stands for "positive"), and silicon receptive to negative charges is called n-type silicon ("n" stands for "negative").

To create n-type, or negatively charged silicon, a material with five electrons in its outer shell is needed to bond with the silicon and have one electron left over. The material most often used in this process is phosphorous. When small numbers of phosphorous atoms are introduced into a silicon crystal, each one displaces a silicon atom and four of its electrons bond with the silicon atoms nearby. This bonding leaves one electron in each phosphorous atom with nowhere to go, and, because all electrons are negatively charged, provides the added negative charge in n-type silicon.

 

 

 

Similarly, p-type, or positively charged silicon, needs a material with three electrons on its outer ring to bond with the silicon but leave a gap in one of its bonds. The material most often used in this process is boron. When small numbers of boron atoms are introduced into a silicon crystal, each one displaces a silicon atom and its three electrons bond with three of the silicon atoms nearby. Because it cannot bond with a fourth atom, a gap is formed where an electron would be needed to make the crystal stable. Since one negatively charged electron is missing, this gap creates the positive charge in p-type silicon.

 

 

 

Creating an Electrical Field

Once both n-type and p-type silicon materials are formed, they can be placed against each other to create a diode, or an electrical field at the juncture of the two materials that only allows electrons to flow in one direction which is essential for creating an electrical current in the materials.

Where the positively charged p-type silicon touches the negatively charged n-type silicon, the electrons and gaps on either side start to react to each other. The extra electrons in the n-type silicon are attracted to the positive nature of the p-type silicon and move toward it, some crossing over to create a negative charge on the surface of the p-type silicon. Likewise, the gaps in p-type silicon are attracted to the negative nature of the n-type silicon and move toward it, some crossing over to create a positive charge on the surface of the n-type silicon.

This creates a one way path for electrons to flow through the two materials. They can cross over from the p-type silicon into the n-type silicon because they are attracted by the positive charge on the surface of the n-type silicon. But if they try to move the other direction from the n-type to the p-type silicon, they are repelled back into the n-type silicon by the negative charge on the p-type silicon's surface.

This pairing of n-type and p-type silicon is what makes up a solar cell, the central element in a photovoltaic panel. Now we can see what happens when solar energy, in the form of photons, hits the cell.

Solar Energy Creating an Electrical Charge

Sunlight carries solar energy in the form of photons, or tiny packets of energy. When photons from sunlight hit a photovoltaic panel, they travel uninterrupted through the n-type layer of silicon and hit the atoms in the p-type layer of silicon. The force of the solar photons bumps the electrons in atoms near the diode out of their bond with surrounding atoms. These electrons are now looking for somewhere to go, and because they are attracted to the positive charge on the surface of the n-type layer, begin crossing over into that layer. This movement of electrons from one atom to another is the electrical charge that can be used in an electrical current.

Turning this Charge into a Current

Once they cross over to the n-type silicon, the electrons still have nowhere to go. They are unable to pass back over to the p-type silicon, but are also unable to form any bonds with the atoms in the n-type layer, which have more electrons than they need already.

Here, an additional photovoltaic panel component comes into use. In all photovoltaics, a metal conductor strip is used to collect and concentrate the electrons set free in this process. As the electrons move upward through the n-type layer, they are attracted to one of many conductor strips which aggregate electrons into a current of electricity.

However, if electrons keep moving out of the p-type silicon into the n-type silicon and the metal conductor strip, soon there will not be enough electrons available to continue this process. Instead, electrons need to be fed back into the p-type silicon through another metal conductor strip or plate.

By connecting both conductor strips to an electrical current, a cycle of using and replenishing electrons is formed, and we can store in a battery or connect an electrical load, like a light bulb, building or anything else that uses electricity, to this current to take advantage of the electricity being produced by the photovoltaic panel. In practice, there are several additional steps that the electricity must go through to serve an electrical load, but this is the general concept behind photovoltaic current.

Limitations on Efficiency

Though all semiconductor materials can react to the energy in sunlight, there are limitations to the amount of solar energy each material can use. Each semiconductor material reacts to solar waves within a specific range of wavelengths, and some react to broader ranges than others. This range is represented by a number known as the material's band gap, or conversion efficiency which is calculated as the amount of electricity produced by the material divided by the amount of solar energy hitting the material. A low band gap indicates the photovoltaic material can react to a broader spectrum of wavelengths, while a high band gap indicates the material will react to a more limited set of wavelengths. Conversely, low conversion efficiency correlates to high band gaps and high conversion efficiency correlates to low band gaps.

While this would seem to mean that materials with low band gaps and high conversion efficiency are always better because they can use much more of the sunlight that hits them, materials with very low band gaps have a more difficult time converting their electric charges into usable electricity. At the same time, a material with a high band gap will not react to enough sunlight to be useful. With these limitations, materials with band gaps ranging from 1.1 eV to 1.8 eV are most commonly used in photovoltaics, with 1.4 eV as the ideal band gap.

When waves that fall outside the range of usable wavelengths hit the panel, several things can happen. Some photons from these waves are reflected by other components of the photovoltaic panel before they reach the semiconductor material. Others that reach it can either pass through without dislodging electrons, be absorbed by positively charged atoms instead of disrupting them, or bounce the electron from one atom only to have it be absorbed by another atom.

Because we want to get the most electricity possible from a photovoltaic system, the band gap and conversion efficiency are key factors in selecting panels for installation. The current limitation on photovoltaic panel efficiency is one of the technology's main drawbacks, and its economics will significantly improve as higher efficiency is achieved.

Using this Material

We have taken a look at the basic science behind photovoltaic technology and introduced the concept of the solar cell composed of n-type and p-type silicon. We can now move on to look at the way a cell is integrated into a panel, and explore the different types of panels used today.