Let’s backtrack for a moment and discuss something mentioned in our earlier post: semiconductors.
What Are Semiconductors & How Do They Work?
A semiconductor is a material that has a conductivity between that of insulators (i.e. plastics, rubber, Styrofoam, etc.) and conductors (copper, iron, gold, etc.). These semiconductors exhibit the photoelectric effect. This effect causes the semiconductors to absorb photons of light, and the energy from the photons is then transferred to electrons. With an electron’s new gain in energy, it can escape from its normal position in the atom and integrate into the electrical current, which can be used to produce electricity. However, when the electron escapes from its normal position (the valence band), and moves into the conduction band, it leaves a “hole” in the semiconductor (as depicted in picture to the right).
Semiconductors and Solar Panels: What’s the correlation?
Due to the holes that are formed within the semiconductors, they are ideal to use within photovoltaic cells. “Why?” you may ask. In order to induce an electric field inside a photovoltaic cell, two semiconductors are put together, one P-type semiconductor and one N-type semiconductor. The P and N correspond to positive and negative respectively. In a P-type semiconductor, there is an abundance of holes while an N-type semiconductor has an excess of electrons. By putting a P and N-type semiconductor together, a P-N junction is formed where they meet.
The excess electrons from the N-type semiconductor flow to the P-type semiconductor to fill the holes. Now, holes are present in the N-type semiconductor and electrons from the P-type semiconductor move to the N-type material. This process/motion of electrons between the two types of semiconductors occurs back and forth, and as a result of this back and forth motion of electrons, an electric field is created at the junction. This electric field causes the electrons to jump out from the semiconductor to the surface of the material, and from there, they can move to the electrical current. Other electrons fill the vacated holes and then move out to the circuit, and in this way, the current continues. No electrons are technically “used up” throughout this process. The photons merely give their energy to create electron-hole pairs. The electrons transfer this energy to the electrical circuit in the form of electrical power, and then they return to the cell to recombine with the positive holes. Simply put, this process conveys the idea of the conservation of energy.
Silicon & What Makes It a Great Material for Solar Panels
In the earliest successful solar panels, silicon was the semiconductor material that was used, and no surprise, it is still a commonly used material for solar panels today.
Silicon is a perfect example of a semiconductor. On an atomic level, silicon has four valence electrons and has the ability to form four covalent bonds. When it does so with four neighboring silicon atoms, the resulting crystalline silicon solid is implemented into solar panels.
However, pure crystalline silicon is a metalloid that looks shiny, but is brittle and a fairly poor conductor since its electrons are not free to move around. In order to solve this issue, the silicon that is found in photovoltaic cells is not just silicon alone–it has impurities. Usually when one hears of “impurities,” it has a negative connotation, but that is not the case here. In a process calleddoping, impurities (other atoms) are purposely mixed with silicon atoms. Typically, silicon is mixed with phosphorus and boron. For approximately every 1 million silicon atoms, there is 1 phosphorus atom or 1 boron atom. Why phosphorus and boron?
Let’s start with phosphorus. Phosphorus has five valence electrons instead of four. It can still bond with neighboring silicon atoms, but what makes it different is that one of its electrons does not bond with another atom. In comparison to if it was pure silicon, it takes much less energy to cause this extra electron to break free. With several phosphorus atoms mixed with the silicon atoms, several free electrons, or free carriers, become available, and the silicon becomes an N-type material. N-type doped silicon serves as a good conductor.
Mixing silicon with boron forms P-type doped silicon. Boron has three valence electrons, and only forms three bonds with nearby silicon atoms, leaving one “incomplete bond.” This incomplete bond is capable of capturing one of the free electrons from the phosphorus. The capture of a free electron leaves a “hole,” so another electron moves to fill this “hole.” This causes the aforementioned motion of electrons, the creation of the electric field, and the flow of electric current.
In the end, if doping is not done, silicon photovoltaic cells would not work.
A visual representation of how silicon P-N junctions work in solar panels can be found here.
Although silicon is so widely used, it is not the only material that is used for solar panels. Additionally, it does not absorb light as efficiently as other materials do, so in our next post, we will dive deeper into the usage of other materials that enhance the efficiency of the solar panel.