The development and revolution of energy has driven the change and progress of human society, especially after the two industrial revolutions, people have become more and more aware of the importance of energy development.
Today's society is developing rapidly, but the traditional energy sources represented by fossil energy (such as coal, oil, etc.) are becoming increasingly difficult to meet the increasing demand for energy due to long regeneration cycles, decreasing reserves and quality, etc. Therefore, the development and utilization of new energy sources is on the agenda.
Inspiration from the photosynthesis of plants: using solar energy to generate electricity
As we all know, the energy available to all living things on earth basically comes from the photosynthesis of plants.
Plant photosynthesis is a biological process that uses carbon dioxide and water as raw materials to synthesize sugar in plant chloroplasts under light conditions, and because sugar substances can produce energy during metabolism, solar energy is stored in this way.
However, this energy is difficult for us to use directly and generally needs to be transformed into the electrical energy we commonly use. The principles of physics tell us that the process of energy conversion inevitably results in energy loss. Therefore, the topic of direct conversion of solar energy into electricity is on the agenda.
So, can solar energy be converted directly into electricity? What factors are involved in this conversion process? This was a great proposition for scientists in the early 19th century.
Fortunately, this challenge made a huge breakthrough at the end of the 19th century.
With the "strongest brain" he discovered the mystery of light and electricity
In 1887, the famous physicist Hertz (now the unit of frequency is named after him) in a research accidentally discovered: light shining on the surface of certain substances, will cause changes in the electrical properties of the material. Subsequent research proved that this was due to the generation of electron flow, so this phenomenon was called the "photoelectric effect".
It is important to understand that the world works according to the principles of physics. At that time, the classical physics principle established by Newton ruled the minds of people. The principle that light is in the Ether (the ancient Greek philosopher Aristotle envisioned a substance, the 19th century was borrowed by physicists to refer to the medium of light propagation) this medium to pass a wave (imagine the scene of a stone into the lake, the lake surface swirled a circle of water as a medium to pass outward ripples), and the energy of the wave and the amplitude (vibration amplitude) related (the amplitude of the light wave is the intensity of light).
This thing seems to be very consistent with common sense. As you can imagine, the sun is not strong in winter, the sun has a warm feeling on the body; while in summer, the sun is harsh, if not pay attention to protection of the skin are likely to be sunburned. Therefore, under classical physics, whether the photoelectric effect can occur depends on the intensity of light; however, this theory is contrary to a series of experimental results at that time.
Research shows that the same material, some colors of light regardless of the light intensity can not occur photoelectric effect, some colors of light even if the intensity is very low can produce electric current, classical physics then into a crisis: a storm sweeping the entire scientific community is brewing.
The storm breeds destruction, but with it comes new life. Science will not stand still, one scientific giants in the center of the storm to cut the waves, classical physics in the relativistic physics and quantum physics under the double correction again set sail.
The one who solved the problem of photoelectric effect was Albert Einstein, whom we know well.
Einstein is widely known for establishing the theory of relativity, but people may not know that such a great scientist nearly did not get the Nobel Prize, which is known as the highest honor in science (the Nobel Prize is never awarded to controversial discoveries, and the discussion and controversy over the theory of relativity has not ceased to this day).
Einstein won the 1921 Nobel Prize in physics thanks to his creative explanation of the photoelectric effect. He proposed that light is composed of photons, and the nature of photons is a packet of energy, the energy contained in each packet is related to its frequency (the number of changes per unit time (1s)), so whether light shining on an object can produce electrons depends entirely on the energy (frequency) of the packet (photon), and the number of energy packets (light intensity) is not relevant.
Solar cells are like a "sandwich"
We have described the discovery of the photoelectric effect, and we know how to produce the photoelectric effect, but how can the electrons produced be used by us?
This brings us to another concept - energy leap.
The atom is composed of the nucleus and electrons outside the nucleus, the electrons outside the nucleus are not scattered, but follow the physics principle of layered arrangement, near the nucleus of the electron energy is low, the more away from the nucleus of the electron energy is higher, different layers of electron energy is different, these energy values are also known as "energy levels".
Under normal conditions, electrons outside the nucleus always tend to be arranged with the lowest total energy, so that the electron is in the "ground state". When an atom in the ground state receives some form of energy (e.g., a photon), it spontaneously moves to a higher energy level, which is called an energy leap, and the electron is said to be in the "excited state.
But unfortunately, the electron in the excited state is not stable and has a tendency to jump to lower energy levels, and the excess energy of the electron is dissipated in the form of light or heat.
No, the energy is dissipated, we still do not get electricity, right?
Don't worry, in order to conduct the current generated by the photoelectric effect, we need to build a suitable device structure, which is often referred to as solar cells.
The device structure is shaped like a sandwich, the active layer with the photoelectric effect is sandwiched between the electron transport layer and the hole (the local electron deficient part formed after the electron leap is called hole) transport layer, and the two ends are electrode materials, usually metal and indium tin oxide (ITO).
After receiving some form of energy (such as photons), the atoms in the ground state will spontaneously transfer to a higher energy level, which is the energy level jump, and the electron after the jump is said to be in the "excited state". Because the excited state energy level of the electron transport layer is slightly lower than that of the active layer, so the electrons in the excited state of the active layer are easily transferred to the electron transport layer, rather than back to the ground state of the active layer; and the ground state of the hole transport layer is slightly higher than the electron energy of the ground state of the active layer, the electrons have a tendency to transfer to the ground state of the active layer.
This is like setting a small step for the electron, so that the electron just "lift the foot" and step over, rather than a difficult jump (leap), so the whole process is easy to achieve.
With the effective coordination of the electron transport layer and the hole transport layer, the entire device forms a complete circuit and the electrons generated in the active layer can be exported for our use.
