Improvement of OLED outcoupling efficiency using graded index layers and diluted semiconductors
Jan 2021
Organic light-emitting diodes (OLEDs) offer several advantages in comparison to inorganic (classical) light-emitting diodes. Due to their small layer thickness and low processing temperature, they can be fabricated cost-effective on arbitrary substrates. Based on that technology, Samsung presented a foldable smartphone in 2019 that attracted a lot of attention. Moreover, about one in two smartphone displays sold in 2020 are based on OLED technology. However, the market share of OLEDs in lighting applications is very small. The major reason for that is lower efficiency compared to inorganic LEDs.
OLEDs are typically composed of several organic semiconductor layers, sandwiched between electrodes. Applying a voltage yields an electrical current that is converted into a photon flux at a certain semiconductor layer. The yield of that process is often close to unity. However, the majority of these photons are trapped in the semiconductor layers, due to their relatively large refractive indices causing total internal reflection. The ratio between escaped photons and generated photons defines the outcoupling efficiency of the OLED.
The aim of this thesis is increasing the outcoupling efficiency. For this purpose two approaches have to be investigated. On the one hand graded index layers (GRI) should be used to match the refractive indices of the substrate and the bottom electrode and therefore reduce Fresnel-reflection [1]. On the other hand the semiconductors can be diluted into polymers, enabling a modification for the refractive indices [2].
Utilizing optical simulations, promising scenarios for an enhanced outcoupling efficiency are identified. Graded index layers are fabricated using atomic layer deposition (ALD), while thermal evaporation is used to fabricate diluted semiconductors. Subsequently, thin-film characterization using ellipsometry and different optical spectroscopy techniques is carried out. Finally, complete optimized OLEDs are fabricated.