Dear all,
I am trying to prepare inverted solar cells with the following structures: Glass/ITO/ZnO/P3HT:PC60BM/MoO3/Ag. The fabrication of ZnO have been reported by Sun et al. (Adv. Mater. 2011, 23, 1679) who used precursor solution containing zinc acetate dehydrate 0.45 M (ethanolamine as stabilizer, 2-methoxyethanol as solvent). I did read a lot of paper on inverted structures containing P3HT:PCBM blends (about one hundred papers). It appeared that most authors prepared the active solution from orthodichlorobenzen (ODCB) and not chlorobenzene (CB). I also noticed that the D:A ratio is 1.0:1.0 (20 mg : 20 mg) rather than (1.0:0.8; 10 mg: 10 mg), which is mostly used in P3HT:PC60BM solar cells. Why did they use the higher concentration here? They wanted to obtain a thicker layer when spin-casted on ZnO? In this case, can I speculate that the thinner film of P3HT:PCBM may create some shorts? Also, the pre-annealing is often perform instead of post-annealing process. There was a lot of paper which did not use thermal annealing. While the use of solvent can affect the initial morphology of active layer, I do not really know if the use of chlorobenzene (CB) will have a negative effect on the performance of the inverted solar cells. For PTB7:PC70BM, chlorobenzene (with DIO as additive) have been extensively used for creating solar cells. The blends were incorporated in the inverted structure (Glass/ITO/ZnO/PTB7:PC70BM/Hole transport layer/Ag). At this point, can I deduce that chlorobenzene may not affect the film forming properties (because the active layer is spin-coated on ZnO instead of PEDOT:PSS) and the resulting efficiencies of solar cells? I wonder if I confuse something here. The post-annealing, the most used method to improve PCE, can alter negatively the efficiency of inverted devices?
Thank you very much for your help
Best regards,
Minh-Trung
This is a very interesting question which attracted me after reading it to try to add a comment and not an answer. First of all i did not work with organic materials and all my experience is with metallic semiconductor devices.
The most straight forward answer of your question that research is needed to get a more accurate answer. May i add some general advice out of my experience:
- The structure of the such devices is to some extent marginal such that you can consider the donor acceptor ratio and the thicness of the active layer as parameter and need to be optimized. From the rigidity of the material point of view, the uniformity of the film, the freedom from voids and pinholes , as well as the absorption of light, it is preferable that the film be thicker. From the electronic properties point of the view. the thicness is limited by the exciton diffusion length which is about very few hundreds of nanometers. One other important factor is full interdigitation of the the two materials to increase the area of the inerface area between the donor and acceptor.
When you produce a layer, you have to finish it such that it must work properly before depositing the following layer. In addition the processing of the coming layer must not affect the previous layers. So, preannealing concerns the previous layer and preparing the surface for the coming layer. Post annealing is required to solidify the spin on layers and solidify it and remove the solvents and also to change thermally the structure of the deported material. It is normally determined experimentally.
Wish you success
Firstly, the thicker P3HT:PCBM film results a larger current density. Different from lots of low band gap polymers, P3HT tend to be crystalline after thermal annealing and thus achieve a good mobility. So, it will be better to use a thick film around 250 nm.
But I do not know why post annealing do not work, may be the strong crystalline tendency for P3HT lead a significant difference on interfacial contact with electrodes.
Actually, other methods like solvent annealing work good as well.
All the best,
Weidong
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