Photoconductive Cells

A photoconductive cell is a two-way terminal semiconductor device where the terminal resistance varies linearly compared to the intensity of the incident light. Thus photoconductive cells are also known as photoresistive devices.

There are two primary materials, cadmium sulfide as well as cadmium selenide. These are the two primary materials typically used to manufacture photoconductive cells. Both of these cells respond quite slowly to changes in terms of light intensity.

If we take an example to highlight this, cadmium selenide and its response time are around 10ms. In comparison, in the case of cadmium sulfide, it can be about 100ms.

Also, temperature sensitivity is another significant difference between the two materials. For example, there is a marked change related to the resistance of a cadmium selenide cell with respect to changes in the ambient temperature; however, the resistance of cadmium sulphide is relatively stable.

It is important to note that the spectral response of a cadmium sulfide cell is quite similar to that of the human eye; that is, it is responsive to visible light.

The Characteristics and Parameters of Photoconductive Cells

It explains that when a non-illuminated cell has a greater resistance than 100 kΩ. It is also known as the dark resistance of the cell. In an illuminated cell, this resistance falls to a hundred ohms. It is important to note that we present the illuminance on a logarithmic scale.

One of the significant drawbacks of photoconductive cells is that the temperature variation causes considerable variation in resistance in particular light intensity. Hence, these cells are not suitable for analogue applications.

Photoconductive cells are helpful for relay control. So when the cell is dark, its higher resistance reduces the current too down, which is quite low to energize the relay. The resistance is particularly included to limit a certain amount of current to the desired level where the resistance of the particular cell is low.

So, in a dark cell, the base at the transistor is above the emitter level. It indicates the particular device is on. Also, an illuminated cell has lower resistance in the series with R biases; the base voltage of the transistor is below its emitter level. Hence the device is turned off.

Photoconductive Cell Experiment

Let us conduct an experiment to study how photoconductive cells respond to the law of radiations.

For this, we need the following items:

• A photocell mounted in the metal box with connections at terminals
• A lamp holder with a 60W bulb
• Two-coin analogue meters that are moving (500µA & 1000mV) and mounted on a frontal panel
• A wooden bench fitted with that of scale and connecting wires

With the various observations, we come to understand that:

• The inner photoelectric effect forms the basis of a photoconductive cell. Hence, when light falls on a cell, the resistance decreases. Consequently, the current starts flowing in that of an external circuit.
• Also, the change in current is not in proportion to the change in the intensity of light. Further, the response time is quite large; there is a sufficient time lag between the change in the light intensity and the change in the generated photocurrent.

Photoconductive Cell Examples

A photoconductive cell has wide real-life applications; let us look at them. Following are some applications of photoconductive cells.

1. Photoconductive cells are components of light-sensitive alarms, automatic street lights, and lighting control. Generally, they have a beam of infrared light that shines permanently on a light-dependent resistor and produces a steady flow of electric current.
2. The photoconductive cells help detect ships and aircraft locations by the radiations that come out from their exhaust.
3. They are useful for withering switching on as well off the transistors related to electronic circuits.

Why Should We Use Photocells?

The photocells provide an economical and technically superior solution for various applications. Let us consider these characteristics as well as features.

1. Low-cost availability 
2. Near IR photodetector
3. Highly responsive to higher and light levels
4. Resistance to changes of other orders of magnitude
5. Available in a greater range of resistance values

Photoelectricity

Photoconductive cells are an application of the effects of photoelectricity. So let us understand some other concepts related to photoelectricity.

• Photovoltaic

Small solar panels on calculators and digital watches are known as photovoltaic cells. These are like diodes made up of two layers of semiconductor materials placed on top of each other. 

The top layer is rich in electrons, and the bottom layer is electron-poor. So when you shine a light on the top layer, the electrons jump to the top from the bottom.

• Photoemissive 

These are among the oldest and most elaborate ways of turning light into electricity. 

They come in a sealed glass vacuum tube inside which there is a large metal. This metal serves as a negative terminal with a positively-charged cathode with an anode running inside it. This design is known as a phototube. 

In a photoemissive system, the light knocks out electrons from a cathode to that of an anode making the current flow throughout an external circuit.

Conclusion

Hopefully, you have gained a proper understanding of photoconductive cells and their examples and their characteristics. It is important to note that photoconductivity is one of the types of applications of the photoelectric effect.