Since solar cells convert light to electricity it might seem odd to measure the photovoltaic cells in the dark. However, dark IV measurements are invaluable in examining the diode properties. Under illumination, small fluctuations in the light intensity add considerable noise to the system making it difficult to reproduce. Dark IV measurements inject carriers into the circuit with electrical means rather than with light generated carriers. In most cases the two are equivalent and the Dark IV measurements give extra information about the cell for diagnostic purposes. Even in the absence of noise, there is a wealth of information in comparing the illuminated and dark IV curves.
A solar cell in the dark is a large flat diode. A simple dark IV measurement produces the exponential curve so characteristic of a diode.
Dark IV curve with a linear scale. One exponential looks much like another.
The linear graph of current vs. voltage reveals very little information about the diode, much more information is revealed from a semilog plot.
Semilog plot of the same IV curve above revealing much more information about the diode. Different regions of the IV curve are dominated by different loss mechanisms.
Dark IV measurement of a PERL cell. While the cell had a record Voc of 702 mV the fill factor was only 0.81, lower than the 0.83 for earlier cells. Additionally, there was no evidence of parasitic series or shunt resistance. The dark IV measurement showed an unusual curve where the J0 was changing and was explained by a changing recombination velocity at the rear surface 1.
Limitations of Dark IV Measurements
The use of Dark IV curves in solar cell analysis relies on the principle of superposition. That is, in the absence of resistive effects, that the light IV curve is the dark IV curve shifted by the light generated current. While this is true for most cells it is not always the case.
A second problem is that in dark IV measurements the current is flowing in the opposite direction and the current paths are different. The change in the current path causes a lower series resistance in the dark measurements to the light measurements.
Comparison of current paths under illumination and in the dark. In both cases the currents are the same. In the dark case the current flows into the cell and in the illuminated case the current flows out of the cell. Since in the dark case, most of the current crosses the junction under the contact it has a lower series resistance than for the illuminated case. (move the mouse over the image to see dark current flows)
- 1. , “Improvements in Silicon Solar Cell Performance”, 22nd IEEE PV Specialists Conference. pp. 399-402, 1991.