We assume that we do a good energy levels alignments for our thin films solar cells, which is based on energy levels of non operating conditions for each material that we use (i.e. the ionization energy level (IE) (or HOMO level) of the hole transport layer (like spiro-OMeTAD) matches with MAPbI3).
How much the IE level of the spiro-MeOTAD will differ under working conditions (stacked with many other layers, with some electric field) from the IE level measured in a non operating conditions? and same question for the perovskite layer energy levels?
Hope my question is clear.
Hi Zafer,
It's a very interesting question, and I always had the question while I was studying BHJ OPVs. I'm not an expert in perovskite solar cells, so I'll comment to the best of my knowledge.
When the materials are far enough from each other, the energy levels of each materials don't interfere with each other; HOMOs and LUMOs can be characterized by CV, UPS, Kelvin probe, etc. When these materials contact each other, these energy levels are affected by each other. Simply put, electrical field from each materials are applied to each other. Thus, there are shift in energies everywhere. Now, how responsive a material is to he externally applied E field is closely related to the dielectric constant of the materials, and the degree of energy shift will roughly correlate to the dielectrics constant. For organic materials, dielectric constant is very low (2~6), thus their density of state is drastically different throughout the layer. This is represnted as space charge depletion or accumulation at the interface/surface of the material, and thus band bending of organic semiconductors are often observed and reported.[1,2]
In case of spiro-OMETAD in a perovskite in a non-working condition, I think it's energy level will still bend at the interface; the HOMO/LUMO level in a non-operaing condition will either accumulate or deplete depending on what material it's in contact. At the bulk of its material, around 30~100 nm from the interface/surface, the change in the band position is minimal and one will observe a flat line.[1] However, note that it's IP probably doesn't change much throughout the entire layer because its vacuum level changes roughly similar to the degree of band bending.[3] (I think what you really want to know is the absolute HOMO level change, not IE. They are related, but should not be interchangeable).
In case of spiro-OMETAD in a perovskite in the working condition, there's a break in the Fermi level through out the device due to the photogenerate volatage and currents. Thus, the charge carrier density changes significantly compared to the device in a dark condition. Thus, one should observe a relatively large change in the absolute HOMO/LUMO energy of spiro-OMETAD roughly correlating to a cell potential.[4] However, note again that IP won't change much because vacuum level will also shift in a same direction to the HOMO/LUMO change. (Think about UPS. The bias one applies to the substrate doesn't interfere with it's work function and IP. It's the surface state it affects the energy structure.) [3] This principle should be applied to all other layers in a perovskite solar cell.
I hope this answers your question to some degree and also help you to understand the OPV device physics. Good luck!
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[1] Lange, I.; Blakesley, J. C.; Frisch, J.; Vollmer, A.; Koch, N.; Neher, D., Phys. Rev. Lett., 2011, 106, 216402/1 - 216402/4.
[2] Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K., Adv. Mater., 1999, 11, 605-625.
[3] Klein, A.; Korber, C.; Wachau, A.; Sauberlich, F.; Gassenbauer, Y.; Harvey, S. P.; Proffit, D. E.; Mason, T. O., Materials, 2010, 3, 4892-4914.
[4] I really hope someone can chime in and enlighten us with a good reference for this point.
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