Dear Zafer

In organic optoelectronics such as OLEDs and OPVs  interfaces are ususlly comprised of a conductive (inorganic) electrode and an organic hole (electron) transporting layer, often denoted as the HTL (ETL) [1]. At such interfaces, the magnitude of the barrier for charge injection depends on the energy level alignment between the Fermi level of the electrode and the highest (lowest) occupied (unoccupied) molecular orbital HOMO (LUMO) of the transport layer. Under the assumption of common vacuum levels (Fermi level alignment / Schottky-Mott limit) the estimates of the hole-injection barrier  can be made when ionization potential (IP) of an HTL and the work function (s) of an anode are known (Similarly, the electron injection barrier can be estimated by the energy difference between the electron affinity EA of the organic and the cathode's work function). However, after contact due to formation of chemical bonds, charge transfer or a “push-back” effect, an interfacial dipole is formed [1,2,3], which manifests itself as a vacuum level offset (denoted as Δ). Therefore, in the process of developing OLEDs and OPVs this effect must be taken into account.

The benefit of a good match between electrode work function s and the IP of an HTL may be hindered by a negative vacuum level shift that occurs at the corresponding interface. Such a shift pushes the occupied electronic levels away from the substrate Fermi level. This effect seems to be particularly significant for the interfaces with  reactive or high work function, atomically clean metal surfaces, i.e., metals with a large surface dipole contribution to the work function. Polymer-based high work function materials, such as the common electrically conducting polymer PEDOT-PSS [or poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid)], have been reported to lead towards more favorable energy level alignment with hole transporting materials. However, significant vacuum level shifts, preventing reduction of hole-injection barrier, have been observed also for polymeric substrates.

[1]H. Ishii, K. Sugiyama, E. Ito and K. Seki, Adv. Mater. 11 (1999), p. 605.

[2] I.G. Hill, D. Milliron, J. Schwartz and A. Kahn, Appl. Surf. Sci. 166 (2000), p. 354.

[3] W.R. Salaneck, K. Seki, A. Kahn and J.-J. Pireaux, Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications, Marcel Dekker, New York (2002).

Hi Zafer,

Dr. Vasilopoulou has answered your question very well. I just wanted to add a little more detail to her response hoping that you feel a bit more comfortable in this field.

In terms of working solar cells, or light emitting diodes, the Fermi level should not, and cannot be aligned through out the whole device. The way these device work relies on the energy gap between relevant donor-acceptor energies. This gap is the source of the generated power or photons for OPVs and LEDs, respectively. Thus, the discontinuity of device Fermi level makes sense.

Also, as Dr. Vasilopoulou pointed out, the interface chemistry almost always occurs and breaks the Fermi level alignment, and many works are trying to resolve this issue by inserting organic, inorganic interfacial materials/states at the electrode surface to enhance device performances. Without these additional layers, Fermi level does not align properly unless the two materials in contact are conductors. In this sense, note that many researchers tune the donor-acceptor interface the opposite way compared to the electrode-active material interface to increase the Voc, for OPVs.

So, the questions is, why do we align the energy levels to the vacuum level? I personally think this happens because there no standard way to line them up. As Ishii et al. (cited above) pointed out, energy levels and vacuum levels of materials are not absolute when they are in close proximity or in contact. Also, it is a very tedious and technically difficult to measure the changes for all materials. Then, since Fermi level cannot be aligned, it makes the most sense to line up the device material orbital energies respect to the vacuum level. However, note that many interfacial studies draw the "broken" vacuum levels to accurately depict the device energy structure more accurately.

In conclusion, one should always be reminded that the energy level diagrams aligned respect to an absolute vacuum level are not entirely accurate for OPVs and LEDs.

I basically agrre with the two previous posts. I would add the following comment.

Because the density of charge carriers in organic semiconductors is very, very low, it is difficult to estimate a Fermi level here. Due to the exponential relationship between Fermi level and charge carrier density, any minute change of the latter would drastically modify the former. So using the vacuum level is the only reliable way to establish the energy diagram of an organic diode. With all the difficulties mentionned above ! (e.g., broken vacuum level at interfaces)