Monodisperse, indium doped zinc oxide (IZO) nanoparticles were prepared via the polyol-mediated synthesis and incorporated into regular and inverted poly-(3-hexylthiophene-2.5-diyl) and [6,6]-phenyl C(61)-butyric acid methyl ester organic photovoltaic devices as buffer layers between the active layer and the cathode. Efficient hole blocking at the particle buffer layers leads to an enhanced open-circuit voltage of the solar cells. This effect is even more pronounced for inverted device architectures. Device degradation studies revealed a solar cell performance reduction upon sample exposition to ambient atmosphere. However, this degradation is fully reversible under UV illumination. In addition, the n-doped IZO particles form suitable charge carrier transport layers for an efficient recombination in an intermediate recombination zone in tandem solar cells. Accordingly we have fabricated fully solution-processed tandem solar cells and investigated their optoelectronic properties. (C) 2010 Elsevier B.V. All rights reserved.