I-V Characteristics of Photovoltaic cells

[vw_g60t]

Hi, could someone care to explain two things:

1) Whats happens when the resistance load of a circuit powered by a PV cell generates a higher voltage then that produced by the PV cell (does band gap get affected).

2) Why does current drop near the Open Circuit Voltage? Surely the generation of current depends on the irradiance on the PV cell? So why does current stop at the Voc point?

Example IV Charaeteristics of a typical photovoltaic cell:
http://www.tfp.ethz.ch/PV/HESC/IV.jpg

Thanks

 

[Redbelly98]

Hi, could someone care to explain two things:

1) Whats happens when the resistance load of a circuit powered by a PV cell generates a higher voltage then that produced by the PV cell (does band gap get affected).

Well, actually that never happens. You can plot the i-v curve of a resistor on the same plot as the photovoltaic cell. It will be a straight line through the origin, with positive slope -- in accordance with Ohm's Law.

Where the two curves intersect will be the operating point for that particular load resistance. This happens in the "+v,+i" quadrant, so it will have voltage less than the open-circuit voltage and current less than the short-circuit current.

2) Why does current drop near the Open Circuit Voltage? Surely the generation of current depends on the irradiance on the PV cell? So why does current stop at the Voc point?

That's a general property of all power supplies. Open circuit means just that, i.e. the load resistance becomes (for all practical purposes) infinite. i=v/R=v/∞=0

Incidently, the i-v curve of a photovoltaic cell is related to that of a diode. Take a diode's i-v curve and flip it about the v-axis (equivalent to defining the current's polarity in the opposite sense). Then shift the curve upward by an amount equal to the short-circuit current. Works for photodiodes as well as photovoltaic cells, as they are the same thing but with different applications.

 

[vw_g60t]

Thanks Redbelly, most useful response I've had on this site.

Excuse me if I am being dumb, but say I have a 60W PV module with Vmax point of 9v and I have it connected to charge a 12v battery?

Is this possible? If it is than surely the 12v voltage from the battery will be going into the 9v PV module - would'nt this have an effect on the PV module>?

 

[Redbelly98]

It is possible, though not useful. To put it simply, the battery will be drained as it runs current through the PV module, in the negative direction, until the battery voltage has dropped to 9V. This is assuming the possibly high current does not damage the PV module.

To explain this, imagine drawing the i-v of the 12V battery on the same plot as the PV's i-v curve. Where do the 2 curves intersect? At a negative current, and at 12V less the voltage drop due to the battery's internal resistance.

I know your graph did not show negative currents, but the full i-v curve does indeed extend to negative currents, as well as to negative voltages.

 

[Enthalpy]

Another way to predict the behaviour of a photovoltaic cell is to model it by a current generator with a diode as a shunt. It's exactly the same as the shifted diagram described above, so pick the one you feel more comfortable with.

The current is proportional to sunlight. It is lost in the diode if the user tries to exploit the current at a voltage exceeding the diode's knee voltage. An optimum can be searched (more complicated electronics) if operating at the beginning of the knee.

As the current density (created by sunlight) is low, so is the knee voltage, especially with poly or amorphous silicon. With single-crystal silicon, one may hope some 0.45V or a bit more. This voltage drops by 2.1mV/K, an important effect.

Also, nearly every single photon of energy >1.12eV is converted in an electron (and a hole) in single-crystal silicon cells, but this electron is available at 0.45V and this limits the efficiency. Expensive materials with a "direct gap" like GaAs have a knee voltage closer to the gap energy, and this improves efficiency.

The next limit is that much of the Sun's power is in infrared, which silicon doesn't harvest. For that, a smaller bandgap would be better, but then more energetic photons are badly used because their electron is available at the small knee voltage. One solution is to stack several cell materials specialised on different wavelengths.