1.Characteristics of photoresistor porcelain
After the photoconductive material is added with the electrode, it constitutes a photoresistor. When studying the formula and process of the photoresistor, the characteristics of the photoresistor must be understood. Photoresistor porcelain has the following characteristics:
(1) Spectral characteristics
This refers to the light wavelength range where the photoresistor sensitivity is highest. For example, the sensitivity peak of CdS is at 520nm, and the sensitivity peak wavelength of CdSe is at 720nm. When the two form a solid solution in different proportions, the peak sensitivity ranges continuously from 520 nm to 720nm.
Photoconductive sensitivity refers to the size of the photocurrent generated under certain light conditions, and is related to the number of photogenerated carriers of the material and the distance between the electrodes. The representation method is different in different applications.
The resistance value RD of the photoresistor when there is no light is called the dark resistance. The resistance value Rp after illumination is called bright resistance.
Because the light resistance changes with the light intensity, the resistance sensitivity only makes sense to indicate the light intensity.
The dark resistance and light resistance of the photoresistor can simply and clearly reflect the sensitivity of the photoresistor, so the resistance parameter is also commonly used to indicate the sensitivity of the photoresistor.
(3) Illumination characteristics
The illuminance characteristic of a photoresistor refers to the characteristic that its output signal (voltage, current or resistance value) changes with the change of illuminance.
(4) Response time
The response time of the photoresistor is the rise time required for the bright current to reach a stable value under light and the decay time required for the bright current to disappear after shading. Generally, the time required to reach 63.2% of the stable current is defined as the rise time, and the time required to attenuate 63.2% of the original stable current after shading is defined as the decay time. The response time varies with the intensity of the irradiated light. Response time and sensitivity are two contradictor parameters. Fast response time results in low sensitivity, and high response time becomes longer. Therefore, when manufacturing photoresistors, comprehensive consideration should be given to the use requirements.
(5) Temperature characteristics
The photoconductive and electrical properties of the photoresistor are greatly affected by temperature, and the relationship is more complicated. Generally use the temperature coefficient αT to describe the temperature characteristics of the photoresistor.
2.Other photosensitive materials
(1) Preparation of photosensitive materials for electrophotography
Photosensitive materials for electrophotography require fine particles. Because the fine photosensitive powder can make the photosensitive layer uniform, and at the same time, the contact area between particles is increased, which can prevent the overcurrent from burning the photoelectric device. There are many manufacturing methods for fine photoconductive powder, which can be summarized as follows. One is to further crush the existing photoconductive powder to destroy the crystal and reduce the sensitivity of the photoconductor; the other is to sinter the photoconductive powder with little or no flux during the manufacturing process of the photoconductive powder. Slow grain growth. In the process of making photoconductive powder, the firing temperature should be lower than the melting point of the flux to prevent excessive growth of crystal grains. However, these two methods cannot achieve the purpose of photoconductive powder sensitization, and cannot obtain photosensitive materials with high photoconductive sensitivity.
Another method is to add an appropriate amount of sensitizer and donor dopant to the CdS powder, and then mix the CdS to recrystallize the CdS under high temperature and pressure to obtain a highly sensitive photoconductive fine powder with a particle size of 5 μm or less. However, this method requires too long time under high temperature and high pressure, so it lacks practicality. A better method is to add a dispersant to the ingredients when manufacturing the photoconductive powder. During firing, the dispersant can isolate the powder and obtain fine crystals.
(2) Preparation of target material for color TV camera tube
An important application of photoconductive materials is as a target in color TV camera tubes, and it has been commercialized. The target materials in the color TV camera tube are Sb2S3, PbO, Si, CdSe and so on.
The Sb2S3 target has low photoelectric sensitivity and leaves more images on the phosphor screen.
The PbO target has a complicated manufacturing process, high cost, and low sensitivity to red light.
Since the Si target is a single chip, the image of the phosphor screen is prone to white spots, and because of the use of integrated circuit technology, it forms the matrix-like p-n junction makes the image sharpness bad.
CdSe target High photoconductive sensitivity, low dark current, a little residual image will temporarily stay on the phosphor screen.
The target of general color TV camera tube has lower sensitivity to blue shortwave light than green and red ones. The sensitivity of the entire color TV camera tube to light is determined by the sensitivity to blue. Therefore, as the target of a color TV camera tube, it is required to be sensitive to all visible light, and it is particularly desirable to have high sensitivity to blue light and a fast transient response to light.