its an equivalent circuit of PV cell and which has to exhibit the Current (I)-Voltage(V), Power (W) - Voltage (V) characteristics similar to the PV cell. The current source indicates the power generation and PV cell and diode to exhibit the characteristics. 

If the diode is not connected in reverse then it will a short circuit path across the current source. 

Hope this answer may be of use.. 

I don't think most commercial diodes would do this because the junction won't be exposed to ambient light. However in theory of course it would work because it's just a silicon solar cell designed to be a diode!

Dear Ahmad,

Well, Solar Cell it self actually p-n junction semiconductor (one kind of diode type) with anode being exposed to the sunlight. So it can be modeled as diode. You can check the attached link of youtube video for clearer explanation of the model.

https://www.youtube.com/watch?v=4imVOP_i_LI

actually the PV cell is a current source as a results of the photo-generated current.

notice that in the dark the current source symbol is removed and the remaining is only the diode.

why the diode in reverse? because part of the photo generated current is lost as a diode current (depends on the reverse saturation current, temperature, idealty factor and the terminal voltage)