Jsc-Voc

A JSC VOC curve1 is a valuable way of looking at an IV curve in the absence of series resistance. To trace a JSC VOC curve, the illumination on a cell is varied and the cell JSC and VOC measured at each illumination level. The series resistance has no effect on the VOC, since no current is drawn from the cell and so there is no voltage drop across the series resistance. So long as the series resistance is less than 10 Ωcm2 it has little effect on JSC as the IV curve is flat around JSC as illustrated in the series resistance page.

The modeled cell in the graphs below has a J0 of 1e-13 A/cm², a JSC of 35 mA/cm², and VOC of 682 mV at one sun. To highlight the effects on the JSCVOC curve, the series resistance has a high value of 2 ohm cm2 and the shunt resistance has a low value of 1000 ohm cm².

Moving the slider changes the illumination on the solar cell from 0.01 to 1 suns and traces out a JSCVOC curve. JSC changes linearly with light intensity and VOC changes logarithmically.

The top two plots show illustrate how JSCVOC measurements are made, and the bottom two plots show the use of the measurements.

Graph 1 shows the more familiar illuminated one sun IV curve in blue for reference. When the slider is set to one-sun illumination, the JSC and VOC values lie on the one sun curve. Changing the illumination gives a new pair of JSCVOC values that are then plotted on the other graphs.

Graph 2 is a direct plot of the measured JSCVOC measurement. The JSCVOC is very similar to the dark IV except for the effect of series resistance, which affects the dark IV curve but not the JSCVOC curve. The shunt resistance affects both the Dark IV and the JSCVOC measurements.

 

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Moving the slider changes the illumination on a solar cell and traces out a JSCVOC curve. There is more detail on each of the plots in the text above. Reload the page to reset the graphs. Click on a graph to get the data.

Graph 3 shows the same light IV curve in blue as in graph 1. However, instead of plotting Jsc, we now plot the one-sun value of JSC minus JSC against VOC. The result is a pseuduo-IV curve, which is the same as the lightIV curve except but without the effect of series resistance.

$$pseduo J = J_{SC} @1sun - J_{SC}$$

Comparing the light IV curve to JSCVOC gives the series resistance.

$$R_{SERIES} =\frac{\Delta V}{J}$$

Graph 4 is much like graph 2 but plotted on a semilog scale. The JSCVOC curve is linear at high voltages on a semilog plot. At lower voltages, the effect of the shunt resistance causes a large bulge in both the dark IV and JSCVOC curves.

Problems with JSCVOC

JSCVOC is easy to do on existing equipment since the solar cell itself measures the light intensity, but it does have some challenges:

  • Each level of illumination needs a measurement at two very different points on the IV curve, and switching between JSC and VOC is challenging. Usually it is easier to measure the whole IV curve at each illumination and keep only the JSCVOC values.
  • VOC is dependent on temperature and the cell or module temperature can change considerably when the illumination changes. Even on a testing block, it is challenging to keep the temperature of the cell constant or take the JSC and VOC measurements quickly enough so that the temperature does not change. An alternative is to correct for the change in temperature.
  • JSC changes with illumination if the series resistance is high. A high series resistance is particularly common in developmental cells.
  • While the external current is zero at VOC, there may be large internal currents. For instance, a localised shunt may cause considerable currents from surrounding regions causing a variation in VOC across the solar cell.

The first three of these points are largely solved by the SunsVOC technique discussed on the next page.