Investigation of interface properties for ClAlPc/C 60 heterojunction-based inverted organic solar cell

Abstract

The interface properties of organic donor-acceptor heterojunction comprising chloroaluminium phthalocyanine (ClAlPc) and C 60 in an inverted organic solar cell configuration have been investigated, with particular emphasis on the molecular orientation and interfacial energy level alignment through the combination of in situ near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet photoelectron spectroscopy (UPS) measurements. In situ NEXAFS measurements reveal that ClAlPc molecules on C 60 film on indium tin oxide (ITO) electrode adopt a lying configuration with their molecular π-plane nearly parallel to the substrate surface. Such lying configuration of ClAlPc films can facilitate the efficient charge transport in heterojunction based solar cell devices. As confirmed by in situ UPS measurements, the large ionization potential of ClAlPc leads to a large energy offset between the highest-occupied-molecular-orbital (HOMO) of ClAlPc and the lowest-unoccupied-molecular-orbital (LUMO) of C 60 at the ClAlPc/C 60 heterojunction interface, as compared with that of the copper phthalocyanine (CuPc)/C 60 heterojunction, thereby resulting in a relatively large open circuit voltage in an inverted solar cell model device. © 2012 American Chemical Society.