Typically, you apply the voltage and measure the current. In this way you get your I-V dataset. Measuring both at the same time makes no sense to me.

I would tried to measure by using Labjack U12 data Acquisition, But the point is the Labjack reads volts only. How can I convert the solar cell current to volt? 

Dear Ahmed

to measure current you would typically use a coil wound round the PV inductor and connect the coil to the second datalogger channel to measure the output voltage on the coil. A calibration is required to associate the coil voltage to amperes. If you check the Sparkfun or similar sites you will find current coils typically under the tag energy meter complements. If you use these then software calibration is not so difficult.

Gerro. 

Dear Ahmed

current coils or clamps can only be used in AC applications. If I understand you right you want to measure the DC current coming directly from the pv module?

In this case, take a resistor, measure the voltage and calculate with Ohms Law.....

To get good results take a resistor with with a small value in Ohms and a high current carrying capacity. And a high accuracy of the ohmic resistance of course. There are special resistors for this measurement application which are called shunts. A small ohmic value is necessary because in this case the impedance of the measuring intstrument has only small effect on the current.

The voltage can be measured in parallel to your load. The ohmic load has to be much larger than the shunt to avoid too much voltage drop on the shunt.

I know you asked for a measurement method for both voltage and current at the same time, but if you need a high accuracy in measurement make independent voltage (without shunt) and current (without voltage instrument) measurements one after another. Like this you can avoid the interactions mentioned above.

I would make a test of independent measurent and measurement of both values at the same time. If you compare the two and you see no difference inside your given tolerance, perform the economic simultaneous measurement.

Hi,

when talking about the "voltage" and the "current" of a solar cell, you typically mean the open-circuit voltage (Voc) and the short-circuit current (Isc). Both quantities cannot be measured at the same time because they correspond to different operating modes of the device.

At open-circuit, which is the case when you connect a voltmeter with a high input impedance (several megohms) to your solar cell, the excess carriers cannot leave the device with the result that the external current is zero. A voltage is created between the contacts because of the charge-separating element of your solar cell (e.g., a p-n junction). Subsequently, all carriers are forced to recombine and the recombination rate equals the generation rate under steady state conditions.

In contrast, when you measure the current, you apply a shunt resistor with a small resistance to your device, as Sven Korte explained in detail. When the resistance is sufficiently small, so that virtually short-circuit conditions are reached, all of the excess carriers are swept outside the solar cell and no voltage appears between the contacts anymore. This would be the case when you connect a ampmeter to your solar cell.

In principle, all other points between Isc and Voc in the forth quadrant of your IV characteristics can be reached when the resistance of the shunt resistor is varied between zero and infinity and correspond to intermediate states between the both scenarios described above.

i am currently trying to implement both sensors

voltage sensor is simply a voltage divider to reduce the solar panel voltage in parallel with this voltage divider we put a small resistance to measure the current after 

The circuits is based on using a small resistance that the current output from the solar panels flow through and measure the voltage across the resistance using differential amplifier (voltage sub tractor) it measures the voltage difference and transform it to a single value.

The second stage is a non-inverting amplifier that amplifies this value. The voltage gain of the non-inverting amplifier i Using these equation the non-inverting amplifier resistances can be designed.

These values are used to feed the ADCs. T

A voltage divider is implemented after the current and voltage sensing circuit to down convert the voltage from 20 volt to 5 volt.

These resources should be helpful; you can perform a voltage sweep while measuring the current

https://www.ossila.com/pages/iv-curves-measurement

https://www.ossila.com/pages/what-is-a-source-measure-unit-source-meter

Beneficial info from Sven Korte.