Utilising high work function metal oxides as hole extracting layers for organic photovoltaic cells

Abstract

A substantial amount of research has already been undertaken towards creating commercially viable organic photovoltaics (OPVs). This is due to the potential use of OPV cells as an inexpensive source of renewable energy. There are many factors to consider in OPV cell design, including photo-active materials, cell architecture and electrode selection. However, additional interlayers for use between the photo-active materials and the electrodes were identified to be as important and need to be developed to optimise cell performance. The work presented here focuses on the influence of various metal oxide hole extracting layers in different OPV systems. Metal oxides such as molybdenum oxide (MoOx) have shown great promise in polymer cells as a hole extracting layer, and here we investigate their use in small molecule cells. An optimised MoOx layer thickness of 5 nm provides a ~ 60 % increase in overall power conversion efficiency (ηp) for chloroaluminium phthalocyanine (ClAlPc) / fullerene (C60) cells in comparison to those fabricated on bare ITO. A similar improvement of ηp is reported when using the MoOx layer in a boron subphthalocyanine chloride (SubPc) / C60 system. For both high ionisation potential donor materials, the cells containing MoOx achieve a significantly higher open circuit voltage (Voc). Conversely, cells utilising the lower ionisation potential donor materials such as copper phthalocyanine (CuPc) and pentacene produce similar Voc values when deposited on both ITO and MoOx. Hence, the ηp is marginally reduced with the MoOx layer. To attain a deeper understanding, the factors behind these performance differences were explored by UV-vis absorption spectroscopy, ultra-violet photoemission spectroscopy (UPS), X-ray diffraction (XRD) and atomic force microscopy (AFM). Thermally evaporated vanadium oxide (V2Ox) was used as an alternative hole extracting layer to MoOx, achieving analogous performance to MoOx when used in SubPc / C60 and CuPc / C60 cells. The electronic properties of the V2Ox layer are investigated using UPS, and it is demonstrated to have substoichiometric n-type character in contrast to the p-type behaviour previously reported. Additionally, the in-situ fabrication and characterisation of organic layers using UPS indicate Fermi level pinning of the organic to the metal oxide. A solution processed vanadium oxide (V2Ox(sol)) layer was developed and characterised as an alternative method of layer fabrication. The atmospheric processing conditions are found to have a dramatic effect on cell performance, and are studied using x-ray photoelectron spectroscopy (XPS). Layers spin-coated under a nitrogen atmosphere exhibit a larger composition of V4+ states. Kelvin probe and UPS experiments indicate the V2Ox(sol) is also a high work function, n-type layer, with the V2Ox(sol) hole extracting layer producing similar cell performance to the thermally evaporated metal oxide layers. Cells deposited on the V2Ox(sol) layer demonstrate good operational stability characteristics, outperforming a commonly used solution processable hole extracting layer