The spectral sensitivity of organic solar cells can be sensitized in the near-IR and IR by incorporating Yb/Er-doped MoO3 up-conversion nanocomposites. Poly-3-hexyl thiophene (P3HT) and [6,6] phenyl C-61 butyric acid methyl ester (PC61 BM) were chosen for the active layer. Yb/Er-doped MoO3 nanoparticles with dual functionality were solution processed as charge selective interface layer. On the one hand, the electrical functionality of MoO3 as hole extraction layer is demonstrated. On the other hand, up-conversion emission from Yb3+ sensitized Er3+ dopants in the MoO3 nanoparticles is observed. Since the photocurrent generated from up-conversion with 975 nm excitation was found to be in competition with photocurrent contributions from a charge transfer complex absorbing also in the near IR, upconversion from a Yb/Er doped MoO3 layer within an organic solar cell is proven spectroscopically and demonstrated by analyzing the photocurrent response of the Yb/Er doped cell and the undoped cell to the excitation intensity. The concept to integrate up-conversion functionality into metal oxide (MOx) based charge extraction layer suggests a promising and practical pathway for improving the near IR and IR sensitivity of thin film solar cells. (c) 2012 Elsevier B.V. All rights reserved.