The present invention provides a process for the adjustment of the electron acceptor strength of a transition metal oxide (TMO) to the HOMO of a semiconducting hole transport layer material (HTL material) in a device comprising an anode, a layer of said TMO deposited on said anode and a layer of said HTL material deposited on said TMO layer, comprising: depositing a solution comprising a precursor for said TMO on said anode, wherein the precursor solution has a pH selected so that the acceptor strength of the TMO for which the solution is a precursor is adjusted to the HOMO of said HTL material; drying the deposited solution to form a solid layer precursor layer; depositing a solution of said HTL material onto said solid layer precursor layer; and annealing thermally the resulting product to give the desired device having TMO at the interface between said anode and said HTL.