Photovoltaics: Introduction to Semiconductor Physics

Welcome to this first post about semiconductor physics. During this course you will learn about the properties of a semiconductor as a photo-active material for solar cells. I will introduce you to the topics that we will cover during the coming weeks. Semiconductors are materials that are widely used for several applications. We can find them in electronic devices such as diodes, transistors or integrated circuits. But for us, they play a crucial role as active materials in solar cells. But, what is a semiconductor? Let’s define this term. We will do this by looking at important material properties that define a semiconductor. When we look at material’s ability to conduct electricity they are generally divided into three classes: conductors, semiconductors, and insulators.

A semiconductor is a material whose electrical conductivity is lower than the one of a conductor, but higher than that of an insulator. One example of a semiconductor material is Silicon. Solar cells made of silicon dominate the photovoltaic market. Therefore, we will focus our attention on silicon as an example of semiconductor material. We will explain its properties and use this material to describe the working principle of a solar cell. A way to control the performance of a Silicon-based device is doping the material. Doping is a process to manipulate the concentration of mobile charges in a semiconductor. This is done by replacing silicon atoms in the lattice by atoms of a suitable element. Doping will increase the electrical conductivity of a semiconductor. Common dopant elements for Silicon are Boron and Phosphorous. The presence of boron, or other Group 3 atoms is known as p-type doping, because it increases concentration of positive charges.

The use of phosphorus, or other Group 5 atoms is called n-type doping, because it increases concentration of negative charges. We will go further into the consequences of doping in the coming week. P means extra positive charge carriers, N means extra negative charge carriers. In order to describe the properties of a semiconductor we make use of energy band diagrams. They are useful representations of the allowed energy states for electrons in a material. We will use them to describe the main properties of Silicon and all the properties involving transport, generation and recombination of the charge carriers.

n a semiconductor, we distinguish transport based on drift and diffusion of carriers. Drift occurs in response to an electric field. Diffusion, on the other hand, is based in thermal motion and is driven by a difference in carrier concentration at different places in device. Both of these processes are important for understanding how charges move around in a solar cell

In most semiconductors, the atoms are kept together by covalent bonds. The covalent bond is formed when two atoms share two electrons, each atom contributing with one electron. We call these electrons valence electrons and they are immobile while forming a bond. The energy states of valence electrons are in a valence band of allowed energies.

Through different forms of excitation such as temperature or illumination, those electron scan be liberated from bonds through a process known as generation. In band diagram this process can be visualized as electron moving from a state in the valence band to a state in the conduction band. Excited electron leaves a vacant place in the valence band and this vacant place is called a hole. You will learn a lot about this process and how it works in solar cells. Generation is in high competition with its opposite process: recombination. This essentially means that excited electrons return to form covalent bonds. Band diagram is a good means to visualize this process. There are three main mechanisms of recombination and you will learn all about each of them in future videos.

This will conclude the second week on semiconductor physics. In the third week, you will learn what happens when we put a p-type semiconductor and n-type semiconductor together. This forms a P-N junction, the extremely important component in semiconductor devices. This is the most important building block of most solar cells. P- and n-type doped layers are commonly applied together in a device. There are several reasons behind this choice.

First of all, as we said, doping enables us to manipulate the concentration of charge carriers. We can use boron atoms to make Silicon p-type .This increases the concentration of holes. Besides, we can use phosphorus atoms to make Silicon n-type. This increases the concentration of mobile electrons that occupy energy states in the conduction band. In the coming week you will learn what happens when these two materials come together. The P-N junction is important for separating photo-generated electrons and holes. The P-N junction’s properties change when voltage or illumination is applied. You will learn how these effects take place in a silicon-based P-N junction and how the junction can be used as a solar cell. P-N junctions are not the only junctions that are present in a solar cell. Photo-active layers are also placed in contact with metals.

This will be one of the main topics of the final week of semiconductor physics in this course. You will also explore other advanced semiconductor topics like there combination current, J_naught and the junctions formed between two different semiconductor materials, so called hetero junctions. Let’s begin to look at the configuration of common solar cells. Generally, a moderately doped semiconductor layer is sandwiched between other highly doped layers. As we mentioned before, this is done to promote separation of photo generated charge carriers within the device and their collection. A solar cell has to be connected to external circuit where the electric current generated by solar cell can do a useful work. This is done by applying two metallic layers, one at the front and one at the back of the device.

They are called front and back contacts. Their presence is necessary to extract photo generated electrons from a solar cell. During last week, we will understand what happens at the junction of a metal and a semiconductor. We will start with an explanation of the energy band structure of metals. By doing so, we will introduce the concepts of the vacuum level, work function and electron affinity. Then, we will explain to you how to make a metal-semiconductor junction and what are its implication on device performance. Lastly, we will focus on the difference between ohmic and Schottky contacts.