Killer impurities in vacuum chamber that affect the lifetime of organic light-emitting diodes

Abstract

We have evaluated a method to investigate killer impurities in vacuum chambers that affect the lifetimes of organic light-emitting diodes (OLEDs) processed in these chambers. In addition to chambers for the deposition of organic and metal films, an exposure chamber was installed to expose the OLEDs to impurities and residual water in a vacuum chamber during device fabrication. We studied a method to investigate the effects of these vacuum chamber impurities after establishing the reproducibility of the device lifetimes. These device lifetimes were affected by the cleanliness of the exposure chamber. Increased exposure times led to shorter device lifetimes, even if the contact angle in the exposure chamber was reduced to less than 5° using plasma cleaning. Furthermore, the device lifetime did not degrade when the partial pressure of water within the exposure chamber was reduced using a cryotrap. We were also able to evaluate the Kapton tape and vacuum greases that were used and determined whether they affected the device lifetime. These results suggest that the influence of residual water and impurities can be separated and it would then be possible to evaluate the influence of the impurities alone on the device lifetime.

Conductive Polymer Work Function Changes due to Residual Water: Impact of Temperature‐Dependent Dielectric Constant

“…[23] In addition, water-driven etching of indium tin oxide (ITO) at the interface with PEDOT:PSS negatively impact the device lifetime, [24] and minute amounts of water, even in a vacuum vessel, have been identified as one of the "killers" during fabrication of organic light-emitting diodes. [25] PEDOT:PSS is commonly used as a hole transport layer in organic optoelectronics because of its comparably high work function. Accordingly, precise knowledge of the work function is crucial to predict the energy level alignment at interfaces, which determines device performance.…”

“…[ 23 ] In addition, water‐driven etching of indium tin oxide (ITO) at the interface with PEDOT:PSS negatively impact the device lifetime, [ 24 ] and minute amounts of water, even in a vacuum vessel, have been identified as one of the “killers” during fabrication of organic light‐emitting diodes. [ 25 ]…”

A Thermally Activated Delayed Fluorescence Green OLED with 4500 h Lifetime and 20% External Quantum Efficiency by Optimizing the Emission Zone using a Single‐Emission Spectrum Technique

“…Device Fabrication: Computer-controlled [44] thermal evaporation of the OLED materials was performed as described in the literature [11] resulting in 1 nm repeatability of microcavity thicknesses (see later section) and high repeatability of lifetimes. [44] Regarding the stack layer thicknesses, all thicknesses were taken from an established stack in the literature.…”

“…Device Fabrication: Computer-controlled [44] thermal evaporation of the OLED materials was performed as described in the literature [11] resulting in 1 nm repeatability of microcavity thicknesses (see later section) and high repeatability of lifetimes. [44] Regarding the stack layer thicknesses, all thicknesses were taken from an established stack in the literature. [5] Devices were deposited under low water pressure of 10 −7 Pa. [45] Masks enabled separate deposition onto the four quarters of the substrate, for each layer deposition stage, allowing four separate-structure devices per deposition run.…”

Single emission spectrum measurement enables control of organic LED emission zone giving ultralong device lifetime

“…Computer-controlled [35] thermal evaporation of the OLED materials was performed as in [30] resulting in 1 nm repeatability of microcavity thicknesses (see below) and high repeatability of lifetimes [35]. Devices were deposited under low water pressure of 10 -7 Pa as detailed in [36].…”