Photosensitive ceramics are also called photosensitive resistor ceramics, which belong to semiconductor ceramics. Under the irradiation of light, they absorb light energy and produce photoconductive or photovoltaic effects. The photoconductive effect is used to make photosensitive resistors, which can be used in various automatic control systems; Volt effect can make photovoltaic cells or solar cells, providing new energy for mankind.
1. Semiconductor photoconductivity
The electrons that transition to the conduction band and the holes that appear in the valence band in a semiconductor can move in a directional motion and conduct electricity under the action of an electric field, so they are called carriers. The transition of electrons can have various driving forces. When light is irradiated to the semiconductor, as long as the energy of the photon is greater than the width of the semiconductor forbidden band, Eg, the valence band electrons can transition to the conduction band, and holes are generated in the valence band, that is, the semiconductor Generate photo-generated carriers and increase conductance. This process is called photoconductivity. The light guide of intrinsic semiconductor is called intrinsic photoconductivity. Impurity semiconductors, when the impurity atoms are not completely ionized, light can also make these atoms generate electrons and holes, and can also increase the conductance. This process is called impurity photoconductivity.
When an intrinsic semiconductor is in a state of thermal equilibrium, there is a certain carrier concentration, and this carrier in a state of thermal equilibrium is called equilibrium carrier. When light is injected, the intrinsic semiconductor is in a non-equilibrium state, and the more carriers than the equilibrium state are called non-equilibrium additional carriers. The increase in the carrier concentration will inevitably increase the semiconductor conductance, and the photoconductivity is caused by unbalanced carriers.
2. Photoconductive material technology
Photoresistor material is a kind of photoconductive material. In the past, CdS or CdSe powder was generally doped with a certain amount of Cu, Ag and CdCl2, etc., sintered at a temperature above 600 ℃, so that CdS, CdSe have photosensitive characteristics. The powder is made into flakes under a certain pressure, and then these photosensitive films are oxidized at 300~500℃. The sensitivity of the processed photosensitive film is quite high, but the pressing process in the process is cumbersome and the efficiency is low. In recent years, this process has not been frequently used. Other processes for manufacturing photosensitive materials are described below.
① Non-fired film method
First, the photosensitive CdS is prepared and then mixed with an organic resin adhesive, uniformly mixed, and coated on a certain substrate. After a certain period of heat treatment, a photoconductor can be made. By using this method, a large area of photoconductor thin film can be produced, which has great advantages.
②Sintering film method
Add CdS to the appropriate amount of CuCl2, AgNO3 solution and flux CdCl2, mix with distilled water, grind and dry, synthesize photosensitive CdS in an inert atmosphere above 600℃, then mix with 2%~8% high-purity sulfur, and heat in a nitrogen atmosphere at 200-300℃. Then add water to grind finely, spray on ceramic, quartz, glass or mica substrate, and dry it. Sintered in a nitrogen atmosphere at 600°C. Finally, it was soaked in NH4OH dilute aqueous solution for 8 hours, and then washed thoroughly with warm distilled water to remove chloride and alkali on the surface.
(3) Vacuum coating method
The CdS plus sensitizer CuS and NaCI are sintered in a sulfur-containing atmosphere, crushed and used as an evaporation source, and evaporated to form a thin film on the substrate. Another method is to add an appropriate amount of CuCl2 sensitizer to the CdS to evaporate in the evaporation boat.
Because the evaporation temperature of CdS and the dopant are different, it is not easy to obtain a photosensitive film that meets the requirements. To solve this problem, first evaporate CdS, then evaporate a layer of sensitizer such as Cu, Ag, In, Ga, etc., and then heat it in the air to diffuse the sensitizer into the film and recrystallize the film. Or heat treatment in CdS powder containing Ag, Cu, In, Ga and other sensitizers to make it have good photosensitivity.