The figure below shows a cross section of an organic light-emitting diode (OLED device).
The device consists of an organic layer that emits light when a current flows through it, sandwiched between a metal cathode and an ITO transparent anode made of a transparent conducting material such as indium tin oxide (ITO).
These are then formed as laminated layers of nanometer-scale thickness on a glass substrate.

Applying a positive voltage to the anode and a negative voltage to the cathode causes electrons to flow from the metal cathode into the organic layer.
Meanwhile, holes from the ITO anode flow into the organic layer.
The electrons and holes reunite, generating light (photons).

Let’s examine the flow of holes from the ITO anode into the organic layer.
The figure on the right is an energy diagram for the boundary between the ITO and organic layer.
Holes close to the ITO Fermi level flow into the HOMO level of the organic layer.
The energy difference between the ITO Fermi level and the organic layer HOMO level forms a barrier to hole flow.
This corresponds to the difference between the work function (WF) of the ITO and the ionization potential of the organic material.

When designing OLED devices, the capacity to estimate the work function and ionization potential of the material is of critical importance.

Fig.1

This table shows the ionization potential of the typical organic materials used in organic light-emitting diodes, Alq3, α-NPD, and CuPc.
The AC-2 figures were measured with the Riken Keiki AC-2. The UPS figures were measured using ultraviolet photoelectron spectroscopy (UPS).
The UPS measurements are quoted from I.G.Hill and A.Kahn, J. Appl. Phys. 86,4515 (1999).

The AC-2 figures were measured at atmospheric pressure. The UPS figures were measured under vacuum conditions. Both data sets match closely.

Table 1

Table 2

This table shows the ionization potential of organic materials measured using PYSA. The Material column shows the various organic light-emitting diode materials, the IP column shows the ionization potential (work function in the case of Au), and References shows the reference paper(s). PYSA is used by a number of prominent researchers, including Professor Junji Kido at Yamagata University, the Idemitsu Kosan group, Professor Chihaya Adachi, Professor Tetsuo Tsutsui, and Professor Takeshi Yasuda at Kyushu University (as of 2021, Professor Yasuda is affiliated with the National Institute for Materials Science).

The relation between the normalized luminance (L₁₀/L₀) and the ionization potential (Ip) of the hole transport layers (HTMLs).
L₀: the initial luminance,
L₁₀: the luminance after 10h of continuous operation.

Fig.2

This figure shows the relationship between the ionization potential of various different organic materials (hole-transporting layer) and device operating life. These are the results of research by Professor Chihaya Adachi at Kyushu University. With OLED devices, it is important to match the IP for each layer. In this example, materials with a lower ionization potential result in longer device service life.