Dear Jusni,

The most common organic solar cell structure is the heterojunction organic solar cell which is composed of a donor organic semiconductor normally organic polymer interfaced to acceptor organic material from short molecules. These donor acceptor structure is the active layers in the organic solar cells. The other layers are mobile charges collecting layers. From the side of the donor layer there exist the hole transport layer and from the acceptor layer there exist the the electron transport layer. Then at both sides there will be proper metallic electrodes. If the collecting layers outside the active layer behaves as ideal collectors of electrons and holes, then the performance of the solar cell will be controlled by the active donor acceptor layers.

This model is very important to understand the operation of the organic heterojunction solar cells.

Let us turn our attention to the donor polymer material, This layer is the main absorbing one of the incident solar radiation. So, it is found that one needs about few hundred nanommeters to absorb the incident solar radiation. These materials are p-type and there hole mobility is very low. 

When the cell is illuminated with the solar radiation, the donor material absorbs photons with the photon energy equal and greater than the energy gap of the material and excitons will be generated. The excitons are bound electron hole pairs belonging to their mother molecule. Because of relatively low dielectric constant of the polymer, the excitons are strongly bound and needs to be separated or dissociated.  Unfortunately, these excitons diffuse in the donor material and have a short lifetime such that their diffusion length is about 10 nanaometer. The material which drives the dissociation of the excitons is the acceptor material.So, one has to limit the thickness of the dodnr material to about 10 nanometers, after which the acceptor material must contact the donor material. The acceptor material it self is an n type organic semiconductor with much higher mobility than the p-type donor one. So it can be made thicker than the donor layer.

If the solar cell is made palaner it would only absorb very small portion of the incident radiation because as said before the donor thickness is only 10 nm and its required thickness is about 200 um. This would make the the photo currrent very small.

In order to overcome this serious limitation one invented the organic heterojunction structure where the both the donor and acceptor materials are mixed with each other such that an interdigitated nanaostructure results with intervening of both layers on the nanometer scale. This results in thin layers for the transport of the excitons while the whole active blend will be few hundred meters thick,

This structure made break through in the photcurrent generation. It increase appreciable. However it still smaller than the exepted current because of:

The loss of the excitons before they they reach to the interface between the donors and acceptors,

The recombination of the separated electron hole pairs at the interface because of the slow motion of mobile charge carriers in the organic materials especially the polymer.

These recombination losses still reduce the expected value of the photocurrent.

Best wishes

Dear Prof. Abdelhalim Zekry

Thank-you so much for your reason prof. then, how was the connected by the active layer thickness actually?. in this case, i using P3HT:PCBM.

afterwards, i got your point about how was made low current density. Thank-you.