Study and characterization of the ZnPc:C60/MoOx interface in organic solar cells by means of photoelectron spectroscopy

Dissertation to obtain the academic degree of Master in materials engineering submitted to the Faculty of science and engineering of Universidade Nova de Lisboa

The search for low cost photovoltaics has led to the use of organic materials as possible candidates to substitute silicon based solar devices. Organic materials have high absorption coe cients, allowing the use of very thin lms and low cost solution phase manufacturing techniques. Control over the electronic structure of organic/inorganic semiconductor interfaces is a key step for the development of these devices. In the present study this issue is addressed for an organic solar cell with an inverted structure: an organic absorber layer of zinc phthalocyanine: Fullerene heterojuntion, sandwiched between a front electrode of zinc oxide doped with aluminium, an intermediate bu er layer of molybdenum oxide and a silver electrode. The objective of this experimental work has been the investigation of the electronic properties of the MoOx/ZnPc:C60 interface. For this purpose photoelectron spectroscopy using di erent types of radiation has been carried out. Results will be presented on the interface analysis carried out by ultraviolet photoelectron spectroscopy (UPS) for the study of the valence electron levels and by X-ray photoelectron spectroscopy (XPS) to probe the core electron levels. Valence band o sets and work function values at MoOx/ZnPc:C60 interfaces have been characterized as a function of MoOx thickness and as a function of annealing temperature. Current-voltage characteristics (I-V) were measured as a function of annealing temperature and the optical properties of the MoOx layer were measured by transmission in the range 250-2500 nm. Near edge X-ray absorption ne structure (NEXAFS)spectroscopy was used to give additional insight into the situation of the electronic structure: spectra were taken from the electron storage ring for synchrotron radiation (BESSY II, Berlin).