III-V Material Properties

Welcome to this section on III-V PV technology.

In this video we will discuss the material properties of the semiconductors used for this technology.

In this video, you will learn what the III-V PV technology exactly is and how it is different from other solar cell technologies. We will then discuss the structural properties of III-V semiconductors. In the next video we will look into the electrical and optical properties of III- V materials.

The III-V PV technology, as the name implies, utilizes elements of the 3-A group and the 5-A group in the periodic table of elements. As you might recall, group 3 elements have 3 electrons in their outer valence shell, and group 5 elements have 5 valence electrons. One of the most common III-V absorber materials is formed by bonding the group 3 material gallium, with the group 5 material arsenic. Several such combinations are possible however, and many different III-V alloys are used as an absorber material by the PV industry.

Examples include Gallium phosphide, Indium phosphide, indium arsenide, Gallium indium arsenide, gallium indium phosphide, aluminium-gallium-indium-arsenide and aluminium- gallium-indium-phosphide.

An important question for III-V technology is how abundant are the III- and V- elements.

As discussed at the start of this course, of these five elements, only aluminium and phosphorus are relatively abundant in nature. Gallium, arsenide and germanium are not rare or precious metals, but they are much less abundant than silicon. Indium is rare and in high demand. In addition the processing methods to deposit high quality III-V alloys are relatively expensive.

As a result, III-V materials like GaAs are very expensive materials in reference to other PV materials like silicon. Moreover, Arsenic is highly toxic.

So why do researchers still pursue these III-V technologies?

The answer to this question can be found in the NREL chart for best research cell efficiencies. The multijunction technologies based on III-V absorber materials are indicated here in purple. As we can see, the solar cells with III-V absorbers strongly outperform the other PV technologies.

The current record triple junction solar cell without light concentration, developed by Sharp, has an efficiency of 37.9%. The record four-junction solar cell with light concentration, is developed by Fraunhofer ISE, and has an efficiency of 46%.

III- V semiconductor materials are in principle the most ideal materials for PV applications. As I will discuss in the coming videos, they are direct band gap materials, have excellent

electrical properties and can approach the Shockley-Queisser limit as both Auger and SRH recombination can be eliminated under standard test operation conditions.

This makes the III-V PV technology an attractive option for a niche where a very high power output per unit area is crucial. The high-cost-high-performance III-V technology is therefore primarily used for space applications and concentrator photovoltaics. As example, this picture shows a solar panel array mounted on the international space station. For such space applications the metrics of modules with high efficiency in combination with light weight are the most important.

The inset shows a concentrator PV device, where lenses are used to focus the sunlight on a very small solar cell. A tracking system is used to follow the sun throughout the day. We will discuss concentrator PV and PV used for space exploration later in this section.

We will now first focus on the structural properties of III-V semiconductor materials.

Let us begin with a comparison between the crystal structure of gallium arsenide and silicon. As you may notice from the animations, both gallium arsenide and silicon lattice have a similar tetrahedral structure.

The major difference between the two however is that silicon is pure diamond cubic crystalline structure based on solely the element silicon, whereas GaAs is a crystalline structure in which every gallium atom is neighbouring four arsenic atoms and the other way around, every arsenic atom is neighbouring four gallium atoms.

The GaAS lattice structure is a so-called zinc blende crystal lattice structure. Important to note is that both gallium and arsenide are more than twice as heavy as silicon, and therefore the density of a cubic cell of gallium arsenide is more than twice as heavy as crystalline silicon. This means that a gallium arsenide wafer having the same dimensions and thickness as a silicon wafer will weigh almost 4 times as much as the silicon wafer.

In addition, the lattice constant is different. The lattice constant denotes the edge length of a cubic cell and is around 20 pico metres greater in gallium arsenide in reference to silicon. Interestingly, the lattice constant of GaAs is very close to that of germanium.

Most III-V alloys have different lattice constants. As will be discussed later, the lattice constants become very important if we start processing multi-junction solar cells that are based on different combinations of III-V materials for each junction.

In summary, in this video you learnt about the different elements used to create III-V absorber layers. Since most these elements are relatively rare, and processing is relatively expensive, III-V semiconductors are very expensive.

A large advantage of the III-V technology is that it strongly outperforms the other PV technologies, which makes it the best choice for applications that require a high output power density, such as space exploration and concentrator PV.

We also learnt that a unit cell of gallium arsenide is denser and longer than a unit cell of crystalline silicon and these structural properties impact the operation of III-V solar cells. In the next video we will take a look at the electrical and optical properties of the III-V materials.