The performances of organic photovoltaic cells based on
the layer couple electron donor/electron acceptor (ED/EA) are significantly
improved when an exciton blocking layer (EBL) is inserted between the
organic acceptor and the cathode. A new material, the
(Z)-5-(4-chlorobenzylidene)-3-(2-ethoxyphenyl)-2-thioxothiazolidin-4-one,
that we called (CBBTZ), has been synthesized, characterized and probed as
EBL. The energy levels corresponding to the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the
CBBTZ have been determined from the first oxidation and reduction potential
respectively, using cyclic voltametric (CV) measurements. From CV curves,
CBBTZ in dichloromethane showed a one electron reversible reduction and
oxidation waves. The values of its HOMO and LUMO have been estimated to be
6.42 eV and 3.42 eV respectively. Such values show that CBBTZ could be
probed as EBL in organic solar cells based on the ED/EA couple copper
phthalocyanine(CuPc)/fullerene (C60). The photovoltaic solar cells have
been obtained by sequential deposition under vacuum of the different films.
The different thin film thicknesses were measured in situ by a quartz
monitor. Multilayer solar cells ITO/Au/CuPc/C60/EBL/Al have been
probed, where EBL is the aluminium tris(8-hydroxyquinoline) (Alq3), the
CBBTZ, the 2-(4-byphenylyl)-5-(4-tert-buthylphenyl)-1,3,4-oxadiazole (butyl
PBD) or the bathocuproine (BCP). The optimum film thickness is 8–9 nm
whatever the EBL used. When obtained in the same run, the averaged
efficiency of the cells using the CBBTZ is of the same order of magnitude
than that obtained with BCP and higher than that achieved with Alq3 or
butyl PBD. It is shown by XPS study that some aluminium of the cathode is
present in the buffer layer. This aluminium could justify the ability of the
electrons to cross the insulating exciton blocking layer thick of 9 nm.