Transmission optimization of multilayer OLED encapsulation based on spectroscopic ellipsometry

• Multilayer, up to 5- (fluorocarbon/co-oxide) bilayers, used for OLED encapsulation.
• (n, k) of components determined from ellipsometry + transmission (SE + T) analysis.
• SE + T analysis of multilayers show surface modification of CFx from co-oxide growth.
• Up to 20% transmission increase by shift of multilayer interference effects to NIR.

Flexible multilayer thin films are promising alternatives for protecting organic light-emitting devices (OLEDs) against moisture and oxygen permeation. However, besides the inherent absorption associated with these encapsulating materials, multiple internal reflections in the multilayer configuration lead to interference effects which further modify the intensity and color balance of the transmitted light with potential detrimental effects for device performance. Accordingly, rational optimization of such system requires detail knowledge of the optical functions [n(λ), k(λ)]. Here we present a spectroscopic ellipsometry (SE) and transmission (T) study of encapsulation systems formed by up to 5 layer Units each consisting of a bilayer of sputtered co-oxide [silicon dioxide (SiO2) and aluminum oxide (Al2O3)] and fluorocarbon (CFx) prepared by plasma-enhanced chemical vapor deposition. The optical properties of each component, co-oxide and CFx, were first determined from films prepared on c-Si and glass substrates. Knowledge of the optical properties of the multilayer components was then used to simulate the transmission of completed encapsulation stacks with 1-, 2-, 3-, 4- and 5- Units. Comparison between experimental and simulated SE + T data strongly suggest that an unintended surface modification of the CFx surface results from exposure to the plasma used for co-oxide growth by magnetron sputtering. The optical functions of new phase formed (~ 18.2 nm thick) resemble CFx but could not, however, be represented by a co-oxide — CFx effective medium approximation. Importantly, this new material was responsible for up to 20% absorption in the visible spectrum of the 5 Unit encapsulation in addition to interference effects. Incorporation of this interface layer enabled analysis (SE + T) of multi-Unit, multiangle-of-incidence analysis. Based on these results, a proposal for an encapsulation layer with average T increase from 59% to 73% over the visible spectral range is proposed by using adequate thickness of the encapsulation system to shift the interference fringes into the NIR region even in the presence of the absorbing interface layer.