Driving force in organic PV can comes from various sources like built in potential in p-n junction, schottky contact as the interface between metal and semiconductor. Generally built potential in p-n junction is used widely. That means p doping  and n doping  should be introduce in the materials. However, PIN structure is used also used frequently where the main absorbing material is intrinsic.

I hope this will help you. All the best for your research :)  

Thank you for your clarification, but I'm still a bit confused about few things, such as how junction forms in organic sc, is the network is completely continuous to let the movement of charge is possible to the electrode etc... any way thank you 

Dear Vivek,

When contacting two materials with different workfunction, a contact difference of potential will be formed between the two materials. This contact difference of potential phi= phi1- phi2, with phi1 and phi2 are workfunction of the two contacted materials.

Associated with contact of the potential is a built-in electric field E which is formed at the interface of the two materials. If photons with suitable photon energy are incident on the material  and generate electron hole pairs in the field region they will be separated by the built in electric field where the electrons are made to move in the opposite direction of the electric field towards the cathode where they will be collected while the holes will move in the direction of the built in electric field toward the anode where they will be collected. The anode and cathode are made of high conductivity materials buildin ohmic contacts with the active materials 1 and2.

If this structure is left open electrons will accumulate on the cathode while an equal amount of holes will be accumulated on the anode. This will lead to building an electromotive force or the socalled photovoltaic effect.

In case of homo pn junction solar cell such as the silicon solar cells, the contact difference of potential is formed between the p and n type material or the socalled pn junction.

In case of heterojunctions, the contact difference of potential is formed because of the contact difference of potential of the two contacting materials that one of them acts like the p-type and the other acts as an ntype materials. An example is the ZnS on CdTe.

In case organic solar cells the active material is formed from a mixer of DONOR-ACCEPTOR blend sandwiched between a p type hole transport layer htl and n type electron transport layer etl. The  htl and etl are contacted to  metals to form ohmic contacts. The active organic blend acts as an insulator, so the contact difference of potential will be formed due to the difference workfunctionsbetween the etl and the htl. So, the built in field in the organic solar cells is due to the contact difference of potential of the htl and the etl which must be made as high as possible. Theoretically is as large as the difference between the lumolevel of the acceptor material and the homolevel of the donor material minace the binding energy of the excitons.

Best wishes

Let me clarify your question, generally hetero-junction is created in organic solar cell. You will find how hetero-junction works in the following book(attached link) .  

And the network is not continuous in most of cases. For example, the active layer poly(3-hexylthiophene):[6,6]-phenylC61-butyric acid methyl ester (P3HT:PCBM) blend (1:1 by weight ratio)  is sandwiched between 100 nm aluminum layer (cathode) and 45 nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer.

Understanding of band diagram and the hetero-junction characteristics (you can find it in the following book) will help you to clarify these concepts cogently. 

All the best for your research 

Best Wishes :) :D 

https://www.amazon.com/Semiconductor-Physics-Devices-Basic-Principles/dp/0073529583/ref=dp_ob_title_bk