# Problem obtaining I-V characterisation of solar cell

1. Aug 26, 2009

### feel luck

Hi guys,

I am currently doing a university project on the reliability studies of a a-Si thin-film solar cell with specs of 3V@25mA. However, I am stuck at the first step, which requires me to obtain the I-V characterisation curve to derive the parameters of Isc,Voc and Pmax. My lab has a Keithley's 2636A Sourcemeter, which I am having trouble to operate. I am using a Philips PL E-C 11W bulb to illuminate my solar cells as the previous Philips 100W tungsten bulb was melting the encapsulation of the solar cell. Both bulbs also did not manage to produce the standard I-V curve of a solar cell.

Is there anyone who is using the Keithley's 2636A sourcemeter and knows how to obtain the I-V characterisation curve of a solar cell with the TSP-link software?

If not, are there any other simpler ways to obtain the I-V curve of the 3V@25mA solar cell?

feel luck

2. Aug 26, 2009

### f95toli

I haven't used a sourcemeter in a while, but they are quite easy to use.

Do you understand how to obtain the IV-curve of a resistor using the sourcemeter?

I guess what I am asking is if you are having problems because you don't know how to use the instrument, or if the problem has something to do with this specific measurement.

3. Aug 26, 2009

### uart

Yes this should not be at all difficult to do with just a voltmeter an ammeter and a variable resistor (or even a hand full of fixed resistors at a pinch). I'd use a 2k variable resistor and just take about a dozen measurements with current varying from minimum resistance (short circuit) to maxium resistance (giving about 1.5 mA at 3 volts) along with one extra open circuit measurement. The characteristics are very cose to straight line near both the short circuit and open circuit ends of the characteristic, so it should be no problems just joining the open circuit point with the measurement taken at the highest resistance value (eg 2k).

If you can get reasonably consistant natural sun-light then I think that would be far preferable. The efficiency could be quite different with the wavelengths of the lamps you're using. Just do it around the middle of the day, make sure there is no cloud about and do the test reasonably quickly so that the illumination doesn't change significantly during the test. If you are well prepared I cant imagine the measuremnts taking more than about 5 minutes so this really shouldn't be a problem.

4. Aug 26, 2009

### feel luck

Hi f95toli,

thanks for the reply. well,i'm not too familiar with using sourcemeter, and tt's why I am having problems with it. the sales engineer had came down to check out the problem but the iv curve for the non-illuminated test and the illuminated test is abt the save, which looks like a diode iv curve. he is unable to explain to me how come i am not able to get solar cell iv curve which is needed by me to derive the parameters.

i also do not understand how to obtain the IV-curve of a resistor using the sourcemeter as I have no experience using a sourcemeter prior to this project.

hope you can help me with this problem. thank you very much.

5. Aug 26, 2009

### uart

It should look like a diode VI curve, that's what it is (but with the current offset by Igen).

6. Aug 26, 2009

### feel luck

Hi uart,

i thought it should look like the picture i attached? wat i got is the other one..

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7. Aug 26, 2009

### Topher925

I replied to your post in the engineering systems forum. Please do not double post.

8. Aug 26, 2009

### uart

Yes both are correct. The first one is still like a diode except with the current reversed (reflected about the x axis) and then shifted upwards. That's how it should look, the current is "reversed" because what we refer to a positive generated current in the PV cell is actually a negative current with respect to normal diode operation.

The other image is kind of still correct but very under-illuminated (and incorrectly illuminated) as pointed out above by both Topher and myself. You need to flip the reference direction of current in that second image for it to make sense ok.

PS. Perhaps one of the mods could merge the other thread in the engineering systems forum.

9. Aug 26, 2009

### vk6kro

It used to be possible to get "tanning lamps". These cause skin cancer so they may not be available, but they might be a good alternative light source if used carefully.

You should be able to put a voltmeter across your solar cell and get zero voltage output at low or zero light levels. Put your hand over it. When you take it out in the sun, it should give some voltage out, which may be 3 volts. With normal silicon cells it is about 0.6 volts per cell and this is achieved quite easily.

As you put smaller resistors across this panel, the current will increase, but the voltage will drop only slowly until a critical current is reached where the voltage starts to drop rapidly.
This critical current depends on how bright the light source is.
If your 3 volt and 25mA figures are correct then any resistance below 120 ohms should cause the voltage to collapse. Even a 500 ohm rheostat might allow you to find this point by slowly reducing the resistance.

That is very much like your first diagram, but it is easier to understand if you reverse the axes and put current as the independent variable along the horizontal axis.

Your second diagram doesn't seem to make much sense at all, especially if you consider the scales on the axes.

Try this by just walking it out into the sun. If it works OK then you have a good solar panel and just a lamp problem on the instrument.