This post contains the link between Fermi level and CV energy levels.
setup of cyclic voltametric measurement is as the following:
It has three electrodes in electrolyte configuration:
- Working electrode: Pt, glass carbon disk, ITO coated with polymers
- Counter electrode: Pt, Ag wire
- Reference electrode: Saturated calomel electrode (SCE), Ag/AgCl
- Electrolyte: 0.1M Bu4N+ PF6- /acetonitrile solution (commonly used in OPV materials measurement); trace amount of ferrocene is added in the solution as reference material, as shown in the figure[1].
Three-electrode setup is beneficial. The counter electrode’s role is essentially to ensure that current does not run through the reference electrode, since such a flow would change the reference electrodes potential. That is why its (counter) surface is much larger than the working electrode or the reference electrode. Electrolyte is necessary to assure conductivity of the solution. It will reduce the ohmic drop (resistance of the solution)[2].
Correlation between Fermi level and electrochemical energy levels[3]
For the measurement of OPV materials’ HOMO and LUMO levels, Fc/Fc+ is commonly used as an internal standard because it is stable to store, easy to use and presumed ideal reversible behavior. The oxidation of ferrocene is widely used in cyclic voltammetric studies in highly resistive organic solvents as a means of reference electrode potential calibration.
A semiconductor polymers film is deposited on working electrode and the Eox and Ere will be measured against ferrocene redox. Then these electrochemical energy levels will be converted into Fermi level scale as shown in the following equations:
It is 5.1 or 5.39ev, depending on the determination of the 0V point of NHE and fermi level (-4.5ev or -4.75ev). The Fc/Fc+ 0V is 0.64V more positive than 0V of NHE. 5.1=4.5+0.64; 5.39=4.75+0.64
The other way is to measure the Eox of ferrocene itself first; then measure polymer Eox by same setup. CV of ferrocene shows two peaks at 0.37 and 0.44 V hence the E1/2 (ferrocene) is equal to 0.41 V. HOMO and LUMO of a polymer can be calculated by the following equation:
Where 4.8eV is the 0V of SCE as -4.8V in fermi level scale. In this case, the ferrocene solution is typically 1.0 mM ferrocene in 0.1 M Bu4NPF6 acetonitrile electrolyte.
References:
[1] Angew. Chem. 2010, 122, 1542 –1545
[2] https://www.researchgate.net/post/Is-it-necessary-to-use-a-three-electrode-setup-to-do-cyclic-voltammetry
[3] DOI: 10.1002/adma.201004554