Solar cells convert light to electricity, so it might seem strange to measure the PV cells in the dark. Yet, dark IV measurements are instrumental in examining diode properties.
Why use dark IV measurements ?
In illumination conditions, small fluctuations in the light intensity introduces significant amounts of noise to the system making it hard to produce accurate readings.Dark IV measurements inject carriers into the circuit with electrical methods rather than the light generated carriers. In general, the two are the same and the Dark IV measurements give additional information about the cell for diagnostic purposes. Even in absence of any noise, there is lots of information in comparing the illuminated and the dark IV curves.
Data from Dark IV measurements
A solar cell in the dark is just a large flat diode. A simple dark IV measurement produces the exponential curve representative of a diode.
The linear graph of current versus voltage exposes little information about the diode, much more information can be revealed from a semilog plot, which can explain how different regions of the IV curve can be dominated by different loss mechanisms.
Importance of dark IV measurements
The IV characteristics of solar cells that are measured under dark and illuminated conditions provide an important tool to assess their performance. The dark characteristics are the simplest way to estimate the quality of the junction, the grid and contact resistances.
Dark IV measurements are typically used to analyze the electrical characteristics of PV cells, providing an effective way to determine the fundamental performance parameters without needing a solar simulator. The dark IV measurement procedure cannot provide information about short-circuit current, but is more sensitive than illuminated measurements in defining the other parameters (series resistance, shunt resistance, diode saturation currents and diode factor) that dictate the performance of any photovoltaic device
Over the years, various methods have been recommended for extracting the parameters from the dark IV characteristics, and illuminated IV curves.
Limitations of Dark IV Measurements
The use of Dark IV curves in analysis of solar cells depends on the principle of superposition. Which states that, in absence of resistive effects, the light IV curve is the same as the dark IV curve, but shifted by the light generated current. Which is not true in all cases.
The second limitation is that in dark IV measurements the current flows in the opposite direction and the paths of the current are different. The change in the current path leads to a lower series resistance in the dark measurements compared to the light measurements. For both cases the currents are the same. In the dark measurement the current flows into the cell while in the illuminated measurement the current flows out of the cell. In the dark, most of the current crosses the junction under the contact, so it has a lower series resistance than in illumination.