Adding to the colleague Martin,

a similar question was asked in the researchgate and this was my answer:

This is an interesting question. The one junction model is assumed to be applied to describe the the I-V characteristic of solar cells. Since the model has adjustable parameters, normally the I-V curves can be satisfactorily fitted with this model.

Returning to the model equations:

Under the dark characteristics

Id = Is( e (V -Id Rs)/ n Vt - 1) + V/Rsh,

with the parameters have their usual meaning.,

Now let us illuminate the cell and measure the its short circuit photo current Iph.

By superposition of the dark current and the The photocurrent , one gets the total current of solar cell, that is:

I= Iph - Id, Here it is assumed that illumination will not change the dar current parameters. This assumption is fairly god for commertial silicon solar cells.

This is because the incident light generated excess electrons and hole concentrations do not affect the the dark current  characteristics in the sense that low injection condition will prevail in the dark and under illumination. 

So, the condition of superposition is valid.

ON the other side when the super position condition fails the diode parameters will be affected by illumination. This results in the intersection of the illuminated I-V curve with the that of the dark. One of the parameters that suffer much from the illumination in the organic solar cells is the conductivity of the material. The material is normally of low conductivity. By illuminating, its conductivity will be modulated becomes much smaller resulting in an appreciable reduction of series resistance. Therefore  one observes the reduction of the solar cell voltage under illumination causing intersection.

The other cause is the excess charges may cause enhanced recombination which is called langevin recombination. Such increased recombination results in the increase of the reverse saturation current under illumination. Such effect decrease the voltage for certain current in the diode and the I-V curve shifts towards the current axis and the illuminated curve will cross the dark curve.

In summary, when the illuminated diode current depends on the illumination, then the condition of super position is no longer valid and the illuminated curve crosses the dark curve. The material properties play the major role in this phenomenon.

wish you success

In organic solar cells, why the dark jV often crosses the light jV? - ResearchGate. Available from: https://www.researchgate.net/post/In_organic_solar_cells_why_the_dark_jV_often_crosses_the_light_jV [accessed Apr 1, 2016].

Dear Navid,

welcome again,

Another important effect which is not mentioned in my previous post is that the the collected photo  current depends on the cell biasing. It decreases as the forward bias voltage increases. The collection of the photogenerated lectrons and holes depends on internal electric field. As the internal electric field decreases with the forward bis, less fraction of the generated electron hole pairs can be swept out side the device.

This phenomenon is normally evident some organic solar cells and in metallic heterojunction solar cells. In organic solar cells the photogenerated excitons are strongly bound and needs higher electric field to dissociate them.

In heterojunction solar cells the recombination in the interface of the junction may sink a portion of the generated electron hole pairs before the electric field sweep them outside the field region.

Organic solar cells may suffer from recombination at the interface as they also heterojunctions.

Best wishes