Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity

Abstract

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.

Nanocomposite of nickel oxide nanoparticles and polyethylene oxide as printable hole transport layer for organic solar cells

“…17 Recently, NiO x layers, obtained through a sol-gel process from nickel organic salts precursors, have been successfully utilized in organic (OPV) and perovskite photovoltaic devices, showing improved device performance and stability. [18][19][20][21][22][23][24] However, the used sol-gel process required a high temperature treatment (250-400 C) to induce the decomposition of the precursor into a crystalline NiO x nanoparticle (NP) layer followed by a plasma treatment which further oxidizes the nanoparticles. The latter treatment enhances the non-stoichiometry of mixed nickel oxidation states, and, in-turn, the layer's conductivity.…”

Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells

“…Surface modification is a simple way to varify the surface properties of thin films, such as work function, wettability and so on . Self‐assembled monolayers (SAMs) are the most popular techniques to modify the surface properties of carrier transport layers.…”

Thermal oxidation of sputtered nickel nano-film as hole transport layer for high performance perovskite solar cells

“…Apparently, NiO shows a significant PL quenching, indicating holes extraction and transfer at NiO/perovskite interface effectively. To understand the behaviour of NiO film that has been oxidized at 450 °C and its apparent effective extraction of holes from perovskite film, we need to consider the study which has been carried by S. Hietzschold et al about the effect of annealing temperature on the work function of NiO films [33]. They have shown that the work function of NiO films at 400 °C was less than the work function at 325 °C, which promote the carrier extraction of NiO film.…”