Nickel oxide interlayer films from nickel formate–ethylenediamine precursor: influence of annealing on thin film properties and photovoltaic device performance

Abstract

An organometallic ink based on the nickel formate–ethylenediamine (Ni(O₂CH)₂(en)₂) complex forms high performance NiO_x thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/oxidation chemistries during film formation and corresponding chemical structure-function relationships for energetics, charge selectivity, and transport in photovoltaic platforms. Investigations of as-cast films annealed in air (at 150 °C–350 °C), with and without subsequent O₂-plasma treatment, were performed using thermogravimetric analysis, Fourier transform infrared spectroscopy, ultraviolet and X-ray photoelectron spectroscopy, and spectroscopic ellipsometry to elucidate the decomposition and oxidation of the complex to NiO_x. Regardless of the anneal temperature, after exposure to O₂-plasma, these HTLs exhibit work functions greater than the ionization potential of a prototype donor polymer poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), thereby meeting a primary requirement of energy level alignment. Thus, bulk-heterojunction (BHJ), OPV solar cells made on this series of NiO_x HTLs all exhibit similar open circuit voltages (V_oc). In contrast, the short circuit currents increase significantly from 1.7 to 11.2 mA cm⁻² upon increasing the anneal temperature from 150 °C to 250 °C. Concomitantly, increased conductivity and electrical homogeneity of NiO_x thin films are observed at the nanoscale using conductive tip-AFM. Similar V_oc observed for all the O₂-plasma treated NiO_x interlayers and variations to nanoscale conductivity suggest that the HTLs all form charge selective contacts and that their carrier extraction efficiency is determined by the amount of precursor conversion to NiO_x. The separation of these two properties: selectivity and conductivity, sheds further light on charge selective interlayer functionality.