Bi2O3 thin films deposited by RF magnetron sputtering have been studied in situ by using photoelectron spectroscopy. UV/VIS transmission spectroscopy and XRD measurements were carried out to determine the optical and structural properties of the films. Thin film solar cells were built up with ITO|Bi2O3|Au layer structures. These devices were characterized by current–voltage and capacitance–frequency measurements. Open-circuit voltages up to 680 mV and short-circuit current densities of about 0.3 mA cm−2 were observed. In addition, relative permittivities of approximately 45 have been measured. In order to determine the energy band alignment of Bi2O3 with different contact materials, interface experiments were carried out. With stepwise depositions of the contact material combined with in situ observation of the Fermi level shift via X-ray photoelectron spectroscopy, it is possible to measure the energy barrier height between a semiconductor and a metallic contact. The work functions of the different materials were determined by UV photoelectron spectroscopy.