The Root Causes of the Limited Electroluminescence Stability of Solution-Coated Versus Vacuum-Deposited Small-Molecule OLEDs: A Mini-Review

Abstract

Using solution-coating methods for the fabrication of organic light-emitting devices (OLEDs) offers a tremendous opportunity for enabling low-cost products and new applications. The electroluminescence (EL) stability of solution-coated (SOL) OLEDs, however, is significantly lower than that of vacuum-deposited (VAC) OLEDs, causing their operational lifetimes to be much shorter-an issue that continues to hamper their commercialization. The root causes of the lower EL stability of these devices remain unclear. This article briefly reviews and summarizes some of the work that has been done to-date for elucidating the root cause of the lower EL stability of SOL OLEDs, giving special attention to studies where side-by-side comparisons of SOL and VAC devices of the same materials have been conducted. Such comparisons allow for more-reliable conclusions about the specific effects of the solution-coating process on device stability to be made. The mini-review is intended to introduce the work done to-date on the causes of lower stability in SOL OLEDs and to stimulate further work for the purpose of closing the existing knowledge gap in this area and surmounting this long-standing challenge in the SOL OLED technology.

Keywords: OLEDs; extrinsic; intrinsic; solution-coating; stability; vacuum-deposition.

FIGURE 1

Extrinsic and intrinsic factors behind the lower EL stability in SOL versus VAC OLEDs.

FIGURE 2

(A) Fluorescence microscopy images of neat and guest-doped CBP films subjected to UV irradiation for 18 h and of non-irradiated control films. 2,2′,2″-(1,3,5-benzinetriyl)tris(1-phe-nyl-1H-benzimidazole) [Ir (ppy)₃] and tris(1-phenylisoquinoline)iridium [Ir (piq)₃] used as guests. All films were thermally annealed at 100°C for 10 min to enhance crystallization. Reprinted with permission from Yu and Aziz (2020). Copyright 2020 American Chemical Society. (B) EL spectra (normalized to the guest emission peak intensity) collected initially (i.e., at t = 0) and after reaching the LT50 point of SOL or VAC EML devices. Reprinted with permission from Samaeifar et al. (2021). Copyright 2021 American Chemical Society.