Photonics for Energy, lighting & display keywords
The creation of the first incandescent light bulb by Thomas Edison allowed many technologies to be developed such as electrical lamps. However its size cannot be reduced a lot, to create much more effective and smaller devices, a new technology was needed.
Starting in the 30’s, materials known as semiconductors were more and more studied because of their interesting properties.
All atoms are composed of a nucleus, made of protons and neutrons, and a cloud of electrons organized in bands of different levels of energy but in the case of semiconductors their structure (see figure 1) allows either to absorb or emit photons. Indeed non-excited electrons in the Valence Band can go to the Conduction Band when given energy that is to say when absorbing a photon. It creates a positive hole in the Valence Band. On the other hand, excited electrons in the Conduction Band can go to the Valence Band by emitting a photon and fill the hole.
Figure1: structure of a semiconductor
Semiconductors generally have a crystalline architecture and the most used is silicon. That is the material used to create a disc containing peace messages from many countries and left on the moon in 1969.
Knowing these properties, physicists mixed semiconductors with other materials in small quantity to increase the mobility of the electrons (and the ‘holes’, which is the other type of charge carriers in a semiconductor). Introducing impurities in semiconductors is called doping and it is divided in two categories: N and P type.
The energy difference between Valence and Conduction bands is called band gap and defines the level of energy needed to emit or absorb photons: the higher it is the more energy the photon needs to have to be absorbed.
N type materials bring more electrons and artificially decrease the level of energy of the Conduction band and so the band gap.
P type materials steal the electrons of the semiconductor and create holes in the Valence band of the semiconductor. They artificially increase the level of energy of the band and so decrease the band gap.
These 2 types of doping are often combined to create a PN junction. The border between P and N types is a particular area where electrons and holes flow to the opposite band creating a nearly non-conductive area. This property can be changed with the voltage of the junction. For instance a junction diode is a PN junction where the current can flow in one direction (forward voltage) but not in the other (reverse voltage).
Nowadays PN junction are basic components of most of integrated circuits, transistors and Light-Emitting Diodes (LED) that use little electricity, are small and cheap light sources. In addition new PN junctions made of organic materials are now used to create Organic LEDs (OLED), Organic Photodiodes (OPD) or Organic Photovoltaics (OPV). These components have nearly the same efficiency and cost than silicon but their production is cheaper and easier, and they are often biodegradable.
As seen in the “Lighting” chapter, when shining light onto a semiconductor, free (i.e., freely moving) electrons and holes are created. The moving electrons create a current of electricity. It is called photovoltaic effect. The current can be improved when using doped semiconductors and creating a PN junction to increase the number of electrons moving.
The choice of the semiconductor to be used is very important due to its role in the final product characteristics:
– Materials sensible to low light are used to make photodiodes (see module 5);
– Materials with less sensibility but bigger electricity current compose solar panel.
Solar panels are a very interesting technology to replace nuclear plants but also make possible the use of electrical devices in isolated places such as in space. For example Philae, the robot which landed on comet Churyumov in November 2014 has batteries running on solar panels.
All cameras or image creator devices have the same basic component: an array of pixels. A pixel is an area of semiconductor that detects light emitted by the object seen by the camera. An electric circuit receives the information from all the pixels and assembles it to make the image.
The figure 5 shows that enlightened pixels appear white on the screen whereas unlit pixels are black. In a real sensor the color on the screen depends of the intensity created by the pixel. For instance black and white images can have more than 65235 shades of grey.
The size of the pixel is important too. An object cannot be seen if it is smaller than the pixel.
As seen on figure 6, pixels will not detect smaller objects because light can still reach the pixels so they appear white on the screen. At the opposite if the object is bigger than the pixel no light reachs the pixel and it appears black on the screen. The size of a pixel is called resolution.
We have seen that pixels can be used to “capture” an image but they also are used to create an image on screens of computer, smartphones… Nowadays most of the screens are Liquid Crystal Display (LCD) screens. These screens are made of pixels composed of multiple layers:
As said in Module 1 light has an electric field with a particular direction. Polarizers are filters that let pass only some of those directions. In the figure, the crossed polarizers let no light come through. However when changing the voltage of the liquid crystal with the transparent electrodes it changes its physical properties: it becomes a polarizer. This way some light might go through the pixel. Modifying the voltage change the direction of polarization of the crystals and so the amount of light the user can see. It allows to get black and white images.
To obtain colored images red, blue and green filters must be added to groups of 3 pixels.
LCD screens are thin and allow the creation of small devices like smartphones. It is an omnipresent technology but it is surprising to think that the first sales started in the early 2000s in most of the countries.