We evaluated the influence of impurities in the vacuum chamber used for the fabrication of organic light-emitting diodes on the lifetime of the fabricated devices and found a correlation between lifetime and the device fabrication time. The contact angle of the ITO substrates stored the chamber under vacuum were used to evaluate chamber cleanliness. Liquid chromatography-mass spectrometry was performed on Si wafers stored in the vacuum chamber before device fabrication to examine the impurities in the chamber. Surprisingly, despite the chamber and evaporation sources being at room temperature, a variety of materials were detected, including previously deposited materials and plasticizers from the vacuum chamber components. We show that the impurities, and not differences in water content, in the chamber were the source of lifetime variations even when the duration of exposure to impurities only varied before and after deposition of the emitter layer. These results suggest that the impurities floating in the vacuum chamber significantly impact lifetime values and reproducibility.

We evaluated the influence of impurities in an organic material used for the fabrication of organic light-emitting diodes (OLEDs) on the lifetime of the fabricated devices. Despite no differences in the current-density–voltage characteristics and external quantum efficiencies of the devices, the lifetime was approximately nine times longer for devices with high-purity 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), which was used as a hole-block layer. Chlorine-containing impurities derived from T2T had the greatest influence on the lifetime of the OLEDs even though the amount of halogen in the source material was at most 0.9 ppm. On the other hand, the lifetime was not greatly influenced by other impurities even with concentrations up to 0.2%. Therefore, the purities of materials other than the emitter must also be closely controlled.

Ammonia in the atmosphere can be determined by an improved indophenol method with 1naphthol. Ammonia dissolved in water reacts with sodium hypochlorite in the presence of sodium hydroxide and 1-naphthol to form an indophenol blue dye, which can be measured at 720 nm (maximum absorbance). Since ammonia in the atmosphere can be absorbed in purified water almost completely, a calibration graph using standard aqueous ammonium chloride solutions can be used. The calibration graph showed a good linearity (y 0.37 10 5 x 0.027, R 2 0.9992) and the detection limit was 0.9 10 6 M. For practical samples, the air was collected into a 50 ml plastic syringe with an airtight cap containing 3.0 ml of purified water as the absorbing solution, followed by shaking it by hand. The method was successfully applied to the determination of ammonia in indoor and outdoor air samples.

We evaluated the influence of impurities in the vacuum chamber used for device fabrication on the lifetime of OLEDs and found a correlation between lifetime and the device fabrication time. Our results suggest that impurities including previously deposited materials and plasticizers from the chamber components impact lifetime and reproducibility.