I fabricated PBDB-T:ITIC OSCs by using the standard recipe : "1:1 blend ratio, 20mg/ml Conc, 0.5% DIO" in an inverted structure of ITO/ZnO/Active layer (100nm)/ MoO3/Ag. The solution solve well and spin-coated active film looks nice. However, the final device was suffer from low FF below 0.5 with high Rs and low Rsh.
In the same batch, by using another active layer PTB7-Th:ITIC, the final device showed good results with high FF over 0.6. which means that the problem may not coming from the ITIC material, ETL or HTL layer and fabrication process.
I purchased PBDB-T from two different company with different molecular weight and I also tried different the solution string temperature, however, the results are the same with low FF.
Should I try different blend ratio??
Dear Leiping,
Your active layer of devices is too thin for OPVs. Due to high optical absorbance required, we normally usearound 200nm thickness of an active layer within an OPV device. Sometimes tuning the thickness of an active layer has a big impact on improving device efficiency. I agree that the ratio of electron donor and acceptor can be optimised too.
Hope it helps.
Xuhua
I also optimized active layer thickness each time from 70 nm to 400 nm. All device showed poor FF.
It could be materials problem either purity or oxidation. How did you store the materials?
Material is stored in N2 filled glove box. I purchased three different batches of PBDB-T from two different company. The PTB7-Th from the same company has the good quality with high FF.
Low Rsh may be due to the formation of pinholes in the active layer such that HTL can touch the ETL and metal of the anode Ag. It is due to morphology of the active layer.
Small fill factor may be due to high ohmic interface between the active layers and the transport layers.
You may need some investigations to locate the cause of the bad fill factor.
If you display your i-v curve one can get an insight about the causes.
Best wishes.
Leiping Duan I agree with Abdelhalim abdelnaby Zekry and Xuhua Wang
Low FF may be due to pinholes in the active layer. I would like to add some points. LOW FF might be due to interface misalignment between ETL/Active layer, I think the problem is because of ETL or HTL, I suggest you optimize ZnO ETL (conc., additive, temperature, rpm) and HTL layer thickness.
I think the active layer morphology was not so good. May be when you made the films, pinholes were formed. You should be careful about film morphology, thickness, concentration, temperature of the layers.
Thanks for answers! I have found out the problem. It's because that the PBDB-T materials purchased from Solarmer Company has bad quality and low purity. After using PBDB-T purchased from other company, the problem has been solved!
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