Photoelectric Effect vs Photovoltaic Effect
The ways in which the electrons are emitted in the photoelectric effect and photovoltaic effect create the difference between them. The prefix ‘photo’ in these two terms suggests that both these processes occur due to the interaction of light. In fact, they involve the emission of electrons by the absorption of energy from light. However, they differ in definition as the steps of progression are different in each case. The main difference between the two processes is that in the photoelectric effect, the electrons are emitted to the space whereas, in photovoltaic effect, the emitted electrons directly enter a new material. Let’s discuss that in detail here.
What is Photoelectric Effect?
It was Albert Einstein who proposed this idea in 1905 through experimental data. He also explained his theory on the particle nature of light by confirming the existence of wave-particle duality for all forms of matter and radiation. In his experiment in photoelectric effect, he explains that when light is shunned upon a metal for a period, the free electrons in the metal atoms can absorb energy from the light and come out from the surface emitting itself into space. In order for this to happen, the light has to carry a level of energy higher than a certain threshold value. This threshold value is also called the ‘work function‘ of the respective metal. And this is the minimum energy that is needed to remove the electron from its shell. Additional energy provided will be converted into kinetic energy of the electron allowing it to move freely after being released. However, if only the energy equal to the work function is provided, the emitted electrons will remain on the surface of the metal, unable to move due to the lack of kinetic energy.
For the light to transfer its energy to an electron that is of material origin, it is thought that the energy of light is, in fact, not continuous like a wave, but comes in discrete energy packets that are known as ‘quanta.’ Therefore, it is possible for the light to transfer each energy quanta to individual electrons making them propel out of their shell. Furthermore, when the metal is fixed as a cathode in a vacuum tube with a receiving anode on the opposite side with an external circuit, the electrons that are ejected from the cathode will be attracted by the anode, which is maintained at a positive voltage and, therefore, a current is being transmitted within the vacuum, completing the circuit. This was the base of Albert Einstein’s findings that won him the Nobel Prize in 1921 for Physics.
What is Photovoltaic Effect?
This phenomenon was first observed by French Physicist A. E. Becquerel in 1839 when he tried to produce a current between two plates of platinum and gold, immersed in a solution and that being exposed to light. What happens here is that, the electrons in the valence band of the metal absorb the energy from the light and upon excitation jumps to the conduction band and thus becoming free to move. These excited electrons are then accelerated by a built-in junction potential (Galvani Potential) so that they can directly cross from one material to the other in contrast to crossing a vacuum space like in the case of photoelectric effect, which is more difficult. Solar cells operate on this concept.
What is the difference between Photoelectric Effect and Photovoltaic Effect?
• In the photoelectric effect, the electrons are emitted into a vacuum space whereas, in photovoltaic effect, the electrons directly enter another material upon emission.
• Photovoltaic effect is observed between two metals that are in conjunction with each other in a solution but photoelectric effect takes place in a cathode ray tube with the participation of a cathode and an anode connected via an external circuit.
• The occurrence of the photoelectric effect is more difficult when compared to the photovoltaic effect.
• The kinetic energy of the emitted electrons plays a big role in the current produced by photoelectric effect whereas it is not so important in the case of the photovoltaic effect.
• The emitted electrons via the photovoltaic effect are pushed through a junction potential in contrast to the photoelectric effect where there is no junction potential involved.
Images Courtesy:
- Photoelectric effect by Feitscherg (CC BY-SA 3.0)
- Schematic illustration of the photovoltaic effect by Ncouniot (CC BY-SA 3.0)
