Electromagnetic radiation, like light, interacts with electrons. This is called the photoelectric effect.
Photoelectric Effect
Light interacts with matter. If a photon with an high enough energy hits an electron, it can knock it out of its place. This leaves a free electron and an electron hole behind. This light-matter interaction is what is known as the photoelectric effect.
In today's article, we are going to look at three different physical phenomenons that are caused by or can be subsumed under the photoelectric effect. This will give us the necessary background knowledge for understanding photoelectric components like light dependent resistors, photodiodes and solar cells.
Photoelectric Emission
Let's start with photoelectric emission which is also called Hallwachs effect. It is the phenomenon, that has lead to the discovery and understanding of the photoelectric effect. Thus, if you search for photoelectric effect, you typically find this effect. However, while this effect is scientifically of utmost importance, it has almost no relevancy in the context of electronics. It's worth having a look anyway, for having better understanding of the photoelectric effect and its properties.
Heinrich Hertz is a German physicist famous for his research on electromagnetic waves. In one of his experiments he noticed that the results differed if his setup was put in a glass box. He discovered, that using quartz instead of glass solved the issue as quartz does not filter out the UV light as glass does. In 1887, he did further experiments on this phenomenon and described it. The physicist Willhelm Hallwachs later continued them. He came up with the following setup:
A zinc plate is placed on top of a gold-leaf electroscope. If charged up, the fine bend gold-leaf attached in center get too. Since its two ends have the same polarity, they repel each other and the gold-leaf opens up.
Hallwachs charged up the zinc plate negatively and discovered, that if the UV-light hits the zinc plate, it slowly discharges. He repeated his experiments with different configurations and found out, that even if the zinc plate is initially uncharged, it will slowly become more and more positively charged. His conclusion was that zinc emits negative charge carriers when exposed to UV-light.
He ought to be right. When the UV-light hits the zinc surface electrons are emitted from it. Nowadays, this is what is known as photoelectric emission or often simply the photoelectric effect. However, this seemingly simple phenomenon posed loads of new questions to the scientific community. Why does it have to be UV-light? Why does the energy of the emitted electrons not depend on the light intensity, but the wavelength?
Notable scientists like Max Planck and Albert Einstein helped to answer these questions during next century. Their answers turned the existing knowledge about electromagnetic waves upside down and took part in the development of quantum physics.
Important for us is, that light has both wave and particle properties. The photoelectric effect is easiest to understand, when describing light as particles called photons. The energy of these photons is dependent on the wavelength. Blue light with a shorter wavelength (~ 450 nm) is more energetic than red light with a higher wavelength (~ 650 nm).
For the photoelectric effect to occur a photon needs to have a higher energy exceeding the electrons binding energy. To set free electrons from zinc, the high energy of UV-light is required.
Setting free electrons from a material is not always the goal. Much less energetic light can already cause the photoelectric effect to happen inside the material. In silicon, wavelengths below 1100 nm, are enough set free electrons from the valence band into the conduction band. This means it occurs for the whole spectrum of visible light, making silicon an ideal material for photo diodes and solar cells.
Photoconductivity
Next up: photoconductivity. When light shines on semiconductor material, this increases its conductivity. The effect is e.g. used in light dependent resistors. But what is causing it?
The answer is of course: the photoelectric effect. The light causes the generation pf new electron-hole pairs inside the material. Consequently, this referred to as the internal photoelectric effect. If an electric field (aka a voltage) is applied, the generated free charge carriers move to the corresponding poles and by doing so transport electrical energy. The more free charge carriers, the more energy is transported per time. The current and conductivity increase in the light.
Photovoltaic Effect
The last effect is the photovoltaic effect. It describes the creation of an electric field inside a material or electric component under the influence of light. The effect was discovered by Alexandre Edmond Becquerel in 1839. He was able to create a very small voltage by using two identical electrodes inside an electrolyte bath. When one of the electrodes was illuminated with light, especially UV-Light, he could measure a voltage of a few microvolts.
Becquerel was only 19 years when he made this discovery in his fathers laboratory. During his lifetime he continued his research on this and many more topics related to the effects of light. However, he was neither able to explain why the photovoltaic effect happens nor was he ever able to profit from it. It took many years, until in the 1950s the first silicon solar cell with a usable efficiency and voltage was build. While first used mainly for space operations, solar cells have now become an important part of power production.
But how does the photovoltaic effect work? As you can probably guess from the long time it took to develop the first solar cells, it is not that easy to explain. What we know as the photovoltaic effect is actually a combination of multiple physical effects and processes. The details can vary for different types of solar cells, however, in general the photovoltaic effect requires two things to happen:
- Charge Carrier Generation:
Due to the photoelectric effect, free charge carriers are generated inside the material. - Charge Separation and Collection:
Positive and negative charges need to be separated and collected at the contacts, between which we can then observe a voltage.
The following image provides a highly simplified depiction of these processes, but without an explanation of what drives the charge separation process:
Be aware that charge separation is not a process that just occurs by itself. Normally, the generated charge carriers would just recombine after some time. Cause for their separation are material specific electrochemical properties that are purposefully combined to build photoelectric devices. These properties also define maximum achievable voltage.
A typical voltage for classic silicon solar cells is 0.6 V. Higher voltages can be achieved by connecting multiple cells in series. The details of how the photovoltaic effect works in classic photodiodes and solar cells is, however, a topic for another blog article.
