Influence of Alq3 and/or Al2O3 layers at the C60/aluminum interface on the I –V characteristics of CuPc/C60-based solar cells

The electrical properties of organic solar cells based on the couple copper phthalocyanine/fullerene (CuPc/C60) have been studied as a function of the interface fullerene/aluminum (C60/Al). It is shown that, in the case of direct contact, the cells properties depend on the ageing of the cells. Indeed, when the cell is in contact for 5 min with air before encapsulation, i.e. when a thin natural alumina (Al2O3) layer is present between the C60 and Al there is an increase of the open-circuit voltage, which is attributed to an increase of the shunt resistance. When a tris(8-hydroxyquinoline) (Alq3) thin film is introduced between the C60 and the Al, ageing of the devices, that is to say the contact or not for 5 min with air before encapsulation, does not strongly modify the cells properties. The presence of such exciton blocking film improves the cell performance. It is shown that in the presence of Alq3 the forward bias adhered qualitatively to expectations for p–n junctions at the CuPc/C60 interface. Without this blocking exciton film, the analysis of the variation of the current with the forward bias shows that it fails to behave like a p–n junction. In this case it is shown that the current flow can be seen as nearly ohmic at low fields and close to space-charge limited at higher fields. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)