Dear Burak,
If one wants to answer this question, one ha to draw the energy level diagram of the structure before and after he insertion of zinc-phthalocyanine between theP3HT and the PEDOT:PSS .Since the zinc-phthalocyanine has an electron affinity of about 3.7 eV and an energy gap of 2.1 to 2.7 eV then the valence band edge will lay at the level 5.8 to 6.4 eV which is lower than that of the active layer while it must lay higher than it to enhance the extraction of holes from the active layer to the anode. So, energetically, it impedes the the hole flow towards the PEDOT:PSS and therefore decreases the cell current and the cell voltage.
For the energy level diagram of the cell without your material please see the the paper with:
doi: 10.1364/OE.18.00A506
wish you success.
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