it is just a sign to represent the different from the dark. It can be either positive or negative. The current J-light flow due to illumination is opposite to that of J-dark.

Dear Ram,

Thanks for your response to this question. I know it is a sign for conventional usage, actually I would like to know the working principle and exact physics behind it. 

Dear Shamjid,

At forward bias, Injection of charge at the contacts leads to excess minority carriers. Each time an electron recombines at the P-side (either at contact or at the trap sites) an electron comes from the Val.Band of the P-side flows to the external circuit (J-dark-Recombination current).So whenever a recombination takes place inside the diode an electron is sent to the external circuit. 

Under light, electron generated at the thick base-P to n due to illumination. Now the current flows in the wrong (opposite) direction.This results in shift of the IV curve . in all the case its the minority carrier results current. 

Ramkumar is correct it's essentially just a sign convention. Switch the leads on the JV measurement and you'll get a positive value. NREL tend to do their measurements so that the value is measure positive, see for example http://www.nature.com/ncomms/journal/v4/n2/images_article/ncomms2411-f7.jpg

 However technically as the electron flow is minority carrier diffusion you can consider it a negative, i.e. opposing the majority, flow.

All of the above are correct. I think the simplest explanation is that in a solar cell, photogenerated electrons and holes flow to opposite contacts. The electrons flowing to one contact create an electron current into that contact, AND set up a negative voltage at that contact, i.e. electrons flow to the negative terminal. This is the opposite to normal operation, hence the sign change.

It is just a sign convention adopted to show that the dark current and the generation current are always opposite to one another, this is valid for both forward and reverse biased conditions. Photovoltaic process results from the junction separating the generated HEPs such that holes go into p region, n into n region. Considering a hole current as reference, resulting current will be against the applied voltage (considering forward biased condition), hence a negative current with respect to applied bias. The injection component ( often called diffusion) will follow the direction of applied bias (considering forward bias). Hence a positive dark current with respect to applied bias. Similarly in the reversed biased condition the two components, which are the generation current and this case recombination current will still be opposite to one another. Generation current following same direction as -V (reserve bias), hence still a negative generated current. Recombination current in the space charge region and within the diffusion lengths on opposite sides are replenished by drawing carriers from negative and positive terminals of the battery, resulting in a recombination current opposite to the -V (reverse bias). Hence a positive dark current with respect to applied voltage. For calculation you can either take either current to be negative a or positive as long as consistency is maintained in sign convention.

Regards

Richard

Dear shamjid, few things are well understood, listening to a lecture.

Hope NPTEL works.  otherwise

Chetan singh solanki's "Introduction to solar photovoltaics" , you will definitely get complete picture.

All the Best 

When light falls on the solar cell, short circuit current will increase due to the movement of electrons and holes flowing to the cathode and anode respectively, when load is connected to the solar cell, short circuit current decreases with built in potential at the terminal resulting from absorption of photons and a dark current which flows in the opposite direction and at some point the short circuit current will reach zero crossing the voltage axis.

Essentially, a solar cell is a diode. In the dark, any diode will show a negative current when a negative voltrage is applied and a positive current when a positve voltage is applied. By convention (see Shockley equation) the positive side is "forward bias", where we see the flow of majority carriers and therefore the exponential current increase.

As a consequence, since the photocurrent is caused by photogenerated minority carriers, it has a negative sign. Also note that, in the so-called "power quadrant" where you can draw power from the solar cell, current flow and APPLIED (outer) voltage indeed have opposite signs. It has to be this way, otherwise you could not draw power from the solar cell !

However, in solar cell research, the current is usually plotted vs. the OUTPUT voltage of the solar cell, so that current and voltage have the same sign. In this case, there is no reason to stick to the sign convention of the diode characteristic, so both are usually given positive signs.

This is the simpler answer.

In physics, if the direction of the current through an element is the same as the direction of the voltage on it, the current direction is regulated as positive. If not, it is the negative direction.

So for a resistor, the current direction is always positve (hence the power of I*V is also possitve since it consumes energy).

For a battery (or a solar cell), the current always flows out from the anode, so its direction is negative. The subsequent power of I*V is negative meaning it generates energy.

The net current is always flowing from one contact and towards the other. Thus, it is always positive relative to one contact and negative relative to the other. Thus the direction of the current measured only depends on which electrode you reference to ground. The measured current sign does not depend on or affect the charge physics within the device.

The PV cell is a photodiode.  The presence of light generated charge counteracts the current flow expected due to bias voltage alone.  In the PV field the IV curve is often flipped so that the light generated current is positive (in the first quadrant), where as a typical diode is oriented such that as voltage is increased the current eventually increases.  In a PV cell at Voc (open circuit voltage) the voltage across the electrodes is such that the light generated charge is completely countered and no current flows (hence I = 0 at Voc).  The Voc decreases for lower intensity light because there is less light generated current countering the bias current.  The reason for the "negative" current flow due to light generation is related to the p-n junction physics of a solar cell.  Since the light generated current is what is of interest the sign is generally flipped so that light generated current is considered positive current.  Here is a great resource to learn more  http://pveducation.org/pvcdrom/pn-junction/ideal-diode

The negative or positive value of short circuit current just show that whether  the direction of current flow is opposite or same (respectively) to the applied bias voltage. Photovoltaic cells are photodiode; thus it works exactly like p-n junction diode which is essentially required for charge separation after illumination. The IV curves we generally obtain are for the forward bias case. When we form a p-n junction, conduction and valence band of p and n type material realign themselves( one can find diagram at Google images). When light falls on p type material, minority carriers (electrons) get excited and goes to its conduction band from there it drifts to conduction band of n type material which is at relatively lower level. This leads to generation of photocurrent which is opposite to the applied bias.

Essentially during illumination, the minority carriers on the p- and n- side increase in number all of a sudden, way beyond their equilibrium value and hence Fermi level for electrons in the p-side is higher than electron Fermi level in n-side while reverse is true for holes in the n-side. This makes electrons flow from p-side to n-side while holes travel from n-side to p-side. This is despite electrons and holes being in larger concentration on n- and p-side and this happens because of Fermi energy gradient, so it is sort of an uphill diffusion in terms of concentration gradient. The current is hence negative, opposite to the forward current. In forward bias, electron travel from n-side to p-side and holes from p-side to n-side constituting a positive or opposite current. Then, as you apply bias, this forward J(V) of opposite sign starts opposing the short circuit current and hence J_ph decreases until it becomes zero when you reach Voc where both currents exactly balance each other.

Hope this helps.