large area photo diodes - v2 1586 words

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Large Area Photodiode;

a report

BY

THOMAS DAVID FOXEN(B00133844)


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10th December 2008

Large Area Photodiodes; a report

Dated

1st December 2008

For

Optics and Electronics

(PHYS08002)

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By Thomas David Foxen For Optics and Electronics

(B00133844) (PHYS02008)


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INDEX

Page 1; Index

Page 2; Construction of a Large Area Photodiode

Page 3; Operation of a Large Area Photodiode

Page 6; Applications for a Large Area Photodiode

Page 7; Solar Powered Garden Lights

Page 8; The Circuit

Page 9; The Functions of the Components

Page 10; Conclusion

Page 11; References

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Construction of a Large Area Photodiode

Large area photodiodes (LAPs) used within Solar Panels are usually

constructed in the following way

(2)

;

A) Cover Glass;

This is a layer of protective glass.

B) Ant-reflective Coating;

This can reduce energy loss by refection to less than 5%. It works by having a

refractive index around half-way between the materials either side.

C) Contact Grid;This is the negative contact and is most efficient when providing a balance of

good contact, yet minimal shadowing of the layer below.

D) N-type Silicon, and E) P-Type Silicon;

These layers (which contain Phosphorus and Boron impurities respectively)

are the key parts of the LAP. The N-type Silicon is the area of high electron

concentration and where the photons fall. The P-type Silicon is the area high

electron-hole density.

F) Back Contact;

This is the positive contact and is normally comprised of a conductive sheet

coving the whole of the bottom of the P-type Silicon layer.

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In more detail, we can see the following process is going on at the atomicscale to keep the reaction sustainable as long as a continuous flow of photons issupplied to the N-type Silicon;

Nb. When looking at this diagram and reading the following cycle it is prudent toremember a few key points. Firstly, there is no scale represented in it (for obviousreasons). Secondly, there is an extra electron shown in the cycle, it represents thehole and can be found by looking to the position any electron in the cycle is moving

to.

1) A photon of sufficient energy enters the N-type Silicon Phosphorus and strikesan electron of the Phosphorus atom which is not tied up in a covalent bondwith one of the Silicon atoms. The ensuing photoelectric effect causes it togain enough energy to escape the valence shell orbit (represented as thespace between the solid and dotted lines in the diagram).

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2) At the same time, the hole which has formed from the electron leaving its orbitis taken up by an electron from closer to the PN-Junction. This processcontinues and the amount of holes forming close to the PN-junction causesthe electric neutrality to become more and more unbalanced until the holescross the PN-Junction by way of electrons flowing from the P-type to N-type

Silicon.

3) This causes a potential difference between the sides which is settled by theeasier route for the free electrons (from the photoelectric effect) to take. Thisroute is down the wire through the load, leading to electro-motive force(voltage) cause by the electric field and also current from the flow of electronsaway from the N-type Silicon. This is the origins of the voltage and currentthat is put to use in the load for whatever purpose the load is designed toperform.

4) These electrons, being pushed (through electro-negativity) by the processhappening to the electrons behind them in the circuit then continue round andtake up orbit in the valence shell of the Boron. They then continue to movetowards the Phosphorus, taking up holes that come available from theprocess outlined in step 2 and upon reaching the P-type silicon, one lap of thecycle has been completed.

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Applications for a Large Area Photodiode

Some of the most common places you will find LAPs are as follows;

Solar Powered Garden Lights

Desktop Solar Calculators

Solar Trickle Chargers for use with boat and car batteries

Satellites

This list is not finite as any electrical device can be powered off the chargestored from recharging a battery with an LAP array. The reasons they are not morecommon is due to their low efficiency and the high power consumption required bymost electronics. A good example of this is the following Solar Panels for Laptops(3);

As you can see, it is over twice the size of the Laptop, requires a bulky external 12Vbattery and for that you get an extra 3 to 6 hours of runtime from when the 12Vbattery is fully charged. In other words, even this bulky LAP array (costing over200) running at its peak does not output enough power to run your laptop withoutcharge in the 12V battery pack.

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Solar Powered Garden Lights

For the purposes of this report, Im looking at running a solar powered gardenlight. These are very common, can be found in many shops and often look likethese(4);

The devices comprise of a LAP array which charge cells in the day time andthen discharge them through LEDs at night.

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The Circuit

There are lots of different solar powered LED circuits on the market and Ihave chosen the following because it has a few key point which the designer haspulled together to increase efficiency and keep cost low(5);

The first of these key points is that the circuit runs the LED off a highfrequency plus, rather than a DC supply, meaning that the same brightness is

achieve from the LED while using less than 50% of the energy that a standard DCcircuit would. This allows a single rechargeable power cell to be used, lowering theproduction costs. The second is that the designer has omitted including a dropperresistor for the LED, which also increase its efficiency. Thirdly, another product ofusing one cells, and also having four Solar Panels is that the low minimum chargingvoltage of 1.2v is easily achievable from the LAPs on a dull day. Finally, the circuitturns the LED on by detecting when the voltage from the solar cells drop lower than0.7v, rather than using an extra component to switch between charging and lighting.

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The Functions of the Components

The previous circuit diagram can be represented as follows to help describeits operation(5);

When the output from the four LAP arrays (the Solar Panel) is over 0.7v thecut-off transistor (BC547) keeps the LED part of the circuit off. This allows the energyfrom the LAP arrays to charge the cell through a simple series circuit.

When the output from the LAP arrays is less than 0.7v the oscillator transistor(BC547) along with the resistor (6K8) and the timing component (1n2) createoscillations when connected in this configuration with the high-voltage generator(4R9) and the 10R component. This is the reason the circuit uses less than half thepower of a normal DC circuit. However, how this works is outside the scope of thisreport as it is not related to the function and operation of LAPs.

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Conclusion

We have found how the arrangement of two doped Silicon semiconductorscan take the energy from photons in order to release electrons from their valenceshell orbits and cause them to flow through a circuit in order to get from onesemiconductor to the other and then back into the electron-holes in the semi-conductor they were released from. We have shown this to be done using theprinciples of the photoelectric effect, like charges repelling and electrons flowing fromareas of high to areas of low potential, by way of the easiest path, in order to balancecharge.

All this put together then allows us to create LAPs in order to harness theenergy in photons and use it to free electrons for them to flow through a load, in

order to do useful work for us; either; right then (such as in solar poweredcalculators) or later on by storing chemical potential-energy in a rechargeable battery(for devices such as garden lights).

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References

(1) http ;//en.wikipedia.org/wiki/Photovoltaic_cells

(2) http ;//science.howstuffworks.com/solar-cell3.htm

(3) http ;//www.sunshinesolar.co.uk/khxc/gbu0-prodshow/LAPTOP1.html

(4) http ;//www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Product%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpg

(5) http ;//www.talkingelectronics.com/projects/SolarLight/SolarLight.html

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http://en.wikipedia.org/wiki/Photovoltaic_cellshttp://science.howstuffworks.com/solar-cell3.htmhttp://www.sunshinesolar.co.uk/khxc/gbu0-prodshow/LAPTOP1.htmlhttp://www.sunshinesolar.co.uk/khxc/gbu0-prodshow/LAPTOP1.htmlhttp://www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Product%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpghttp://www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Product%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpghttp://www.talkingelectronics.com/projects/SolarLight/SolarLight.htmlhttp://www.talkingelectronics.com/projects/SolarLight/SolarLight.htmlhttp://science.howstuffworks.com/solar-cell3.htmhttp://www.sunshinesolar.co.uk/khxc/gbu0-prodshow/LAPTOP1.htmlhttp://www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Product%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpghttp://www.greenrewards.co.uk/images/products/FINAL%20PRODUCT%20IMAGES/Product%20images/Eco%20Gadgets/Venetian-Light-Garden-big.jpghttp://www.talkingelectronics.com/projects/SolarLight/SolarLight.htmlhttp://en.wikipedia.org/wiki/Photovoltaic_cells

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