PHOTOCONDUCTIVITY AND PHOTOVOLTAICS

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In this cell, a thin film of cadmium sulphide is deposited on one side of an iron plate & placed below a transparent foil of metal.  When light radiation of sufficient energy fall on transparent metal foil the electrical resistance of CdS layer gets reduced and hence its electrical conductance is increased & a current starts flowing in the battery circuit connected between the iron plate & the transparent metal foil the external battery is included in the circuit to generate a direction and provide a path for the current to flow.

CHARACTERISITICS & SPECTRAL RESPONE:

Figure shows the illumination characteristics of a CdS cell. It depicts the relationship between illumination & resistance. It may be seen that when not illuminated, the cell has a resistance in the range of 100 kΩ, which is known as dark resistance. When illuminated with strong light the cell resistance falls to only a few hundred ohms. The ratio of ‘dark’ to ‘light’ resistance of the cell is about 1000 : 1.

The spectral response of cadmium sulphide cell is shown in the figure. It closely matches the response of the human eye. Like the human eye, the response is sensitive to visible light. It is best over the visible spectrum & tapers off towards the ultra violet & infrared.

APPLICATIONS:

Since the photocurrent increase linearly with the intensity of illumination & the spectral response of the CdS cell is similar that of the human eye.A photoconductive cell may be used for the following purposes :

1. To measure the intensity of illumination.
2. To work as ON- OFF switch.
3. In street lighting control.
4. In camera exposure settings.
5. In counting applications.

6. In burglar alarm.

ADVANTAGES:

1. High sensitivity.
2. Low cost.
3. Long Life.
4. High dissipation capability.
5. High voltage capability (100 to 300 volts).
6. High ‘dark to ‘light’ resistance ratio  (1000 : 1).

Drawbacks:

1 The current changes with change in light intensity with a time lag.

2. Relatively narrow spectral response.

PHOTO- VOLTAIC IN SEMICONDUCTORS:

The height of the potential barrier is an open circuited dark (non-illuminated) P-N junction adjusts itself such that resultant current is zero. Under this condition, the electric field at the junction is in such a diretion so as to repel the majority carriers.

When light is incident on diode surface, minority carriers get injected & hence the minority current increases. But since the diode is open circuited, the resultant current must remain zero. Therefore majority current should increase by the same amount as the minority carrier current. This increase in majority current is possible if the retarding electric field at the junction is reduced resulting in the lowering of the barrier height. Therefore across the diode terminals there appears voltage which is equal the decrease in the barrier potential. This constitutes the photovoltaic e.m.f. & is of the order of 0.1 volts for the Ge cell & 0.5 volt for Si cell.

DERIVATION OF EXPRESSION:

We have seen that the photovoltaic e.m.f. V_p appears across the diode when the net current I in the diode is zero.

Substituting I= 0. In the volt–ampere characteristics of a photo diode given by

                          I  =  I_s   +  I₀  ( 1- e^{Ve/ ηkT} )

   We get      I_s   +  I₀  ( 1- e^{Ve/ ηkT} )    =  0

1+  ( I_s/I₀)  = e ^{Vpe/ ηkT} 

                  log ( 1+  ( I_s/I₀)  )    =  Vpe/ηkT  .

Therefore photo-voltaic e.m.f.                       

                   Vp =(ηkT/e) log ( 1+  ( I_s/I₀)  )

                      But I_s/I₀     >>  1, except for extremely small light intensities.

                           Vp  =   (ηkT/e) log ( 1+  ( I_s/I₀)  )

This equation shows that the photovoltaic e.m.f V_p increases algorithmically with I_s and hence with illumination it has been shown diagrammatically.

PHOTOVOLTAIC CELLS:

When a pair of electrodes is immersed in an electrolyte & light is allowed to incident on one of them, a potential difference is created between the electrodes this phenomenon is called photovoltaic effect. Devices based on this effect are known as photovoltaic cells. In a photovoltaic cells light energy is used to create a potential difference the potential difference so  developed is directly proportional to the frequency & intensity of incident light.

CONSTRUCTION & WORKING:

A basic photovoltaic cells consist of peace of semi conducting materials bonded to a metal plate. Materials like selenium & silicon are mostly used for preparing photovoltaic cells.

When light is made to fall on semi conducting material, valence electron holes are liberated from its crystal structures the electrons so liberated move towards the metal plate where as holes flow in opposite directions thus a potential difference is created between the semi conducting materials and the metal plate. Consequently a conventional current flows in the external circuit through a load resistor R .

In actual form of photovoltaic cells a thin metallic film of silver,gold or platinum is deposited on a semi conducting layer like cuprous oxide (Cu₂O) or iron selenide. The whole arrangement is than attached to a metal based plate (copper) as shown in the figure.

When external light is allowed to fall on metallic film F, it penetrates easily and at the barrier layer between the metallic film and the semiconductor, photo-electric emission occurs. The photoelectrons so emitted from the layer, move towards the metallic film. Consequently the metallic film  F becomes negatively charged and the copper based plate positively charged. Hence a potential difference is developed between two and a current flows in the external circuit.The strength is proportional to the intensity of light and flows without any bias i.e. without any external source of e.m.f.

USES:

These cells are used in devices like

1. Photographic exposure metre.
2. Direct reading illuminations metre.
3. Operation of relays.

SOLAR CELLS:

A solar cells or solar battery is basically a P–N junction diode which converts solar energy into electrical energy. It is also called a solar energy converter and is simply a photo diode operated zero bias voltage.

CONSTRUCTION:

A solar cell consists of a P–N junction diode generally made of Ge or Si. It may also be constructed with many other semi conducting materials like GaAs, indium arsenide and cadmium arsenide. The P–N diode so formed is packed in a can with glass windows on top so that light may fall upon P & N type materials. The thickness of P region of is kept very small so that electrons generated in this region can deffuse to the junction before the recombination takes place. Thickness of N region is also kept small to allow holes generated near the surface to diffuse to the junctions before they recombine. A heavy doping of P and N regions is recommended to obtain a large photo voltage. A nickel plated ring is provided around the P layer which acts as the positive output terminal. A metal contact at the bottom serves as the negative output terminal.

WORKING:

The working of solar cells may be understood with reference of figure When light is allowed to fall on a P-N junction diode, photons collide with valence electrons and impart them sufficient energy enabling them to leave there parent atoms. Thus electrons hole pairs are generated in both the P and the N sides of the junctions . These electrons and holes reach the depletion region W by diffusion and are then separated by a strong barrier field existing between there. However the minority carriers, electrons in the p-side , slide down the barrier potential to reach the Inside and the holes in the N-side move to P-side. Their flow constitutes the minority current which is directly proportional to the illumination and also depends on the surface area being exposed light.

The accumulation of electrons and holes on the two sides of the jnction gives rise to an open circuit voltage V_oc which is a function of illumination. The open circuit voltage produced for a silicon solar cells is typically 0.6 volt & the short circuit current is about 40 m A / cm² in bright noon day sun light power conversion efficiency of about 15% are obtained with a thin N  diffused layer into a P wafer. Many such cells are interconnected to provide large quantities of electrical power. Solar panels providing 5watt at 12 volt have been built to operate 24 hrs a day by recharging the batteries during day light hrs.

Characteristics:

Typical V- I characteristics of a solar cell corresponding to different levels of illuminations are shown in the figure. It may be seen that for 100 m W/cm² illuminations the open circuit voltage is about 0.57 volt while the short circuit current is 50 m A. maximum power output is however obtained when the cell is operated at the knee of the curve.

USES:

Solar cells are used extensively in satellites and space vehicles to supply power to electronic and other equipments or to charge storage batteries they are receiving attentions even for terrestrial electric power generation. for it, it is planned to orbit big panels of solar cells outside the earth atmosphere for converting solar energy into electrical energy.