Photovoltaic cell - why is I-V curve so strange Today I was doing practicals with PV cells and my team was measuring IV curve of a solar cell. The cell was not homogenously illuminated and the illumination was very strong.
Our measurements were carried out by changing the load in the circuit (look picture below)

The obtained I-V curve was looking like that:

What may be the cause? This doesn't even remotely remind of a photovoltaic cell (IV characteristic for proper PV cell is presented below as a red curve)

 A: First let's make clear that an I-V curve of an illuminated solar cell should not look like the I-V curve of a diode in dark.
Also the way of plotting and the sign of the current are matter of convention and use, and this can differ for different communities as well as on a geographical base.
The curve you plotted is more than reasonable for a solar cell. It just shows a cell whose efficiency (independent of the short circuit current and open circuit voltage) is limited by a low filling factor, the ratio of the maximum power of the solar cell and the product  Isc x Voc. Graphically you see this as a curvature, opposite to the "rectangleness" of an ideal case.
The actual photovoltaic trace of your device can be seen as the sum of IV curve in dark and that of "light only" curve. As such, at least in well established solar cells materials and devices, a nice diode behaviour in dark is a prerequisite for a nice efficient solar cell.
If the cell in dark already show a ohmic component, this will be present under illumination too, reducing the photocurrent every where in the quadrant of interest (except at V = 0). 
Conversely, if the diodes does not rectify in the sense that does not open at all, this can point to high resistance, either due to negligible charge injection or recombination of charges and/ or poor contact at whatever point a contact is required (electrodes, interfaces).
The causes of the above can obviously resides at the material level as well as the constructive one. So it is hard to tell what could be the parameter which mostly affect the filling factor of your cell without knowing the type and its IV characteristics in dark.
This would apply if you would have (properly) illuminated the cell homogeneously, which is not the case. Actually we have now a I-V curve which results from numerous components : the dark curve, and the hypothetical light only ones. Note that the latters will contributes with unknown weights.
If you consider that current, voltage as well FF depends on illumination intensity, it should be clear that a not homogeneously illuminated solar cell is prone to show a less squared (rectangular) trace in the quadrant of concern.
In your case I guess that the result is due to both the non homogeneous illumination as well as an ageing device used for the practical exercise. If it's of interest, you could measure the same under a more homogeneous illumination without forgetting to run a dark measurement. 
