Differences in ITO Surfaces According to the Formation of Aromatic Rings and Aliphatic Self-Assembled Monolayers for Organic Light-Emitting Diode Applications

Abstract

In this study, we investigated the effects on the characteristic changes in OLED devices of using self-assembled monolayers with different functional groups as the hole injection layer, resulting in changes in their performance. Thus, we confirmed that it is possible to control the wetting properties, surface roughness, and work function of the indium tin oxide (ITO) surface by introducing self-assembled monolayers (SAMs). The contact angle measurements confirmed that the substrate surface contact angle tended to increase with SAM deposition. In addition, AFM measurements confirmed that the substrate surface roughness tended to decrease when SAM was deposited on the surface. Finally, it was confirmed through the work function measurement results that the work function increased when the ITO surface was modified by SAM. Furthermore, compared to OLEDs using only the ITO anode, the SAM-modified device showed a higher current density (359.68 A/cm2), improved brightness (76.8 cd/cm2), and a smaller turn-on voltage (7 V). This approach provides a simple route for fabricating organic light-emitting diode applications.

The Optimization of Hole Injection Layer in Organic Light-Emitting Diodes

“…Ultraviolet/ozone (UVO) treatment is proved to be an efficient method for HILs to increase the WF, smooth the defects of films, and improve the hole injection [ 57 , 108 , 109 ]. WS x and MoS x nanodots were synthesized and used as hole injection layers by Quyet Van Le et al After the UVO treatment, the WF of the nanodots was raised from 4.3–4.4 to 5.0–5.1 eV.…”

“…The self-organized GraHIL has excellent hole injection/electron-blocking capability because of the gradient WF constructed by the self-organization of PFI, which hindered the exciton quenching at the HIL/EML interface. The self-assembled monolayers can also increase the surface contact angle, decrease root-mean-square (RMS) surface roughness, and improve WF [ 57 ]. Similarly, Cheng-Chieh Lo et al enhanced the performance of OLEDs with acetone and deionized water-modified PEDOT:PSS as HILs.…”

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

“…For the aromatic ring structures, the energy levels were decreased due to the increased numbers of πelectrons, leading to lower turn-on voltage in the devices. 469 Silane-based SAMs are widely used in ITO surface modification to improve hole injection, luminescence intensity, and EQE. However, the silanol groups are easily selfcondensed in solution, rendering difficulty in forming a uniform monolayer.…”

“…Therefore, F10SAM 206 SAM showed well-aligned film with a high contact angle and a high substrate work function. For the aromatic ring structures, the energy levels were decreased due to the increased numbers of π-electrons, leading to lower turn-on voltage in the devices …”