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.
The linear graph of current vs. voltage reveals very little information about the diode, much more information is revealed from a semilog plot.
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.
- 1. , “Improvements in Silicon Solar Cell Performance”, 22nd IEEE PV Specialists Conference. pp. 399-402, 1991.