Solar Cell Operation

The solar cell can be represented by an equivalent circuit as shown in the diagram above.  Ignoring R, R_SH and I_SH for now, the core behaviour of a solar cell is represented by the triangular diode and the current source, which is an arrow in a circle.  The triangular diode points in the direction of current flow under forward bias.  I_PH refers to the photo generated current - the current generated under illumination of the solar cell.  I_D refers to the dark current (also known as the leakage current or saturation current) which equals the current under reverse bias.  This can be seen in the following equation, which describes the effective current density J of the solar cell:

J = J_PH - J_dark = J_PH - J₀[ exp(qV/kT) - 1 ]

where current density J = I / Area of the solar cell, J_dark (and J₀) is the I_D equivalent, J_PH is the I_PH equivalent, V is the voltage across the diode, T is the temperature in Kelvin, and k is the Boltzmann's constant.

When a very large reverse bias negative voltage is applied across the diode (the solar cell), exp(qV/kT) becomes zero, and J_dark equals minus J₀, indicating a current that's the reverse of J_dark.

Consider a J-V curve (see diagram above).  The grey shaded area refers to the solar cell under reverse bias, while the yellow area refers to forward bias.  The red graph refers to the solar cell under no illumination, while the blue graph refers to an illuminated solar cell.  The blue curve slopes upwards at about 0.5V because V_bi is overcome by the photo generated voltage in the forward bias direction.  Since the photo generated current density is in the reverse bias direction, it will be negative.



Reference:
3.1 Solar Cell Operation, Delft University of Technology, https://www.youtube.com/watch?v=jBxHY5kMJJA