Electronics

The solar cell light IV curve can be traced by altering a variable resistor across the cell and recording the voltage and current at the cell terminals. While this is quite simple, it is time consuming so in practice so more sophisticated electronics is used. The most common method is to use a variable voltage source that is also capable of sinking current. To improve accuracy Isc and Voc are usually measured separately (by respectively setting the voltage to zero and the current to zero) from the rest of the curve. The IV curve has a strongly varing slope that causes additional problems. There are various schemes for improving the accuracy but one of the simplest is to use equally spaced measurement in voltage in two steps. The first section is widely spaced an covers from 0 to 70 % of Voc. The second section has points more closely spaced and covers from 70% to Voc. The second region contains the maximum power point, the open circuit voltage and has a much higher slope.

IV measurement. The second sweep has a higher resolution and covers the more interesting parts of the curve.

For cell testing the difficulty is the high current of large area cells. The trend in silicon substrates is to move to larger substrates of 15 x 15 cm² and an Isc approaching 10 amps. For Dark IV measurements the tester also needs to source current and to be varied over several orders of magnitude, from nanoamps to several amps. There now exist several systems that fulfill these requirements and are integrated into one package with computer control but the users has to specify the size they want.

Equipment rack for cell tester at NREL (Photo: Jim Yost Photography, NREL Picture Exchange)