Dear Emre,

welcome,

I think it hard to answer this question. In order to reach a decisive answer one has to deposit PEDOT: PSS on glass and measure its conduction type and conductivity. You can use the hot probe and the four point probe or any other suitable method available method in your laboratory.

If the material still retain its original properties you have to investigate the cause of the drop in the conversion efficiency.

If you display the I-V characteristics of the solar cells before and after the pedot:pss one may work out some causes of the reduction.

My question now what is the conduction type of the perovskite and what was the original layer stack?

Best wishes

Maybe there is a connection to the agglomeration of PEDOT:PSS which happens easily.

Did you filter the pedot:pss before using it?

Did you mix it with something?

I don't know the exact conditions, but PEDOT:PSS solutions can get bad after some time. And 5 years is definitely enough time. What usually happens is an agglomeration of the PEDOT:PSS, as mentioned by Nina Feldhofer above.

But filtering might not be enough, if agglomeration has proceeded too much, because in that case you are basically left with small to medium sized flakes of polymer floating in the solution. So technically, it is not even a solution anymore. By filtering you would end up with very low concentration PEDOT:PSS-solutions, or in extreme cases mostly solvent.

As Abdelhalim abdelnaby Zekry mentioned, I would also test the conductivity of the films, to check whether it gives good results or not.

If indeed you are left with agglomerated PEDOT:PSS, you might have to buy a new batch.

Finally, solvent additives for PEDOT:PSS can also be a good idea, as I described in more detail in the following answer:

https://www.researchgate.net/post/How_can_we_fabricate_thin_films_from_PEDOT_PSS_by_spin_coating_technique