Solution-Processed Copper Oxide Thin Film as Efficient Hole Transport Layer for Organic Solar Cells

A simple and cost-effective sol-gel-based spin-coating method for Cu-O film is reported. The solution-processed film has been investigated for its viability as a hole transport layer (HTL) and alternative to poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS) whose hygroscopic nature affects device stability. Cu-O thin films were obtained by spin-coating precursor solution onto indium tin oxide (ITO) substrates followed by thermal annealing in air at 300 degrees C to convert the precursor to oxide form. x-Ray photoelectron spectroscopy (XPS) elemental analysis revealed the presence of both types of phase, namely CuO and Cu2O, at the surface of the film. These films are amorphous in nature, as is evident from their ultraviolet-visible (UV-Vis) absorption spectrum with a considerable amount of subbandgap absorption in the low energy regime. The solar cell performance and Schottky junction properties of devices fabricated using amorphous Cu-O HTL and PEDOT:PSS-HTL were compared and found to be equivalent. Both the PEDOT-PSS reference device and the Cu-O device suffered from poor fill factor originating from surface states present at the Al/polymer interface. The Cu-O/polymer interface does not introduce an additional barrier to charge collection and works as efficiently as the PEDOT:PSS/polymer interface. Solution-processable Cu-O films can thus be used as an alternative to PEDOT:PSS.