Abstract

Multi-heterojunction Organic solar cells based on small molecule have been deposited in vacuum by sublimation. The heart of the cell consists in a three superposed multilayers structure. A blend Cu-phthalocyanine:fullerene (Cu-Pc:C 60) or CuPc:perylene derivative (CuPc:PTCBI) deposited by co-evaporation is sandwiched between a CuPc and a C 60 (or PTCBI) layers. The anode is an ITO film deposited onto glass, while the cathode is an Al layer deposited by evaporation. In order to achieve good carrier collection efficiency by the electrode, buffer layers have been introduced between the electrodes and the organic materials. An ultra-thin gold film has been deposited onto the ITO anode to have a good band matching between the work function of the anode and the highest occupied molecular orbital (HOMO) of the electron donor, the CuPc. The bathocuproine (BCP) is used as exciton blocking layer at the interface between the electron acceptor (C60) and the Al cathode. The effect of the substrate temperature during deposition onto the solar cells performance as been studied, using the electron acceptor as parameter. It is shown that when C 60 is used, that is to say when a spherical molecule is used, there is no improvement of the efficiency of the cells when the substrate is heated during deposition. On the contrary, when a planar molecule is used, such as PTCBI, there is a significant improvement of the solar cell efficiency, when the substrate is heated. This behaviour can be attributed to a higher order of the planar molecule in the film due to substrate heating during deposition. © 2011 American Institute of Physics.