Increased efficiency of small molecule photovoltaic cells by insertion of a MoO3 hole-extracting layer

We report a ∼60% increase in open circuit voltage (Voc) and power conversion efficiency in a chloroaluminium phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction photovoltaic device after insertion of a MoO3 hole-extracting layer at the interface between the indium tin oxide (ITO) electrode and the ClAlPc donor layer, with an associated improvement in device stability. A similar improvement was observed in heterojunction devices based on mixed ClAlPc/C60 layers. We propose that the improvements in device performance are due to the pinning of the ITO Fermi level to the valance band of the MoO3 interlayer, where the latter is closely aligned with the highest occupied molecular orbital of ClAlPc.