For transparent OLED (TOLED), additional technological challenges beyond those of conventional bottom emitting OLEDs (BOLEDs) have to be met. Unlike in BOLEDs, TOLEDs cannot benefit from scattering layers for improved light extraction. If a scattering layer is employed in a TOLED, a frosted look unacceptable for window-integrated lights or head-mounted displays would result. Thus, in TOLEDs, a large amount of light is trapped in wave-guided modes, and their luminous efficacy is significantly lower than that of conventional BOLEDs. Additionally, an ITO top electrode is technologically challenging to fabricate without damaging the underlying organic layers. Other approaches to fabricate a transparent top-electrode using thin metal layers (Ag or Au) promise simplified manufacturing but suffer from strong reflections sacrificing light extraction efficiency and optical transmittance. To increase light extraction through such a metal top electrode, several groups introduced transparent capping layers. Those layers serve as anti-reflective coatings and do not only increase luminous efficacy but also transmittance.
In this work, we use the transfer matrix method (TMM) to optimize TPBi capping layers with respect to light extraction and transmittance in TOLEDs. Our devices are prepared on pre-structured ITO-on-glass substrates using organic vapor phase deposition (OVPD) in an AIXTRON Gen1 OVPD tool. The TOLEDs comprise three organic semiconductors (CBP, Ir(ppy)3 and TPBi) forming an efficient simplified phosphorescent OLED stack. A transparent cathode of 2 nm Cs2CO3, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The refractive indices of all materials in the TOLEDs (glass, ITO, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance measurements. With these spectrally resolved data, we calculate the transmittance of TOLEDs with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm – 1100 nm) by a series of experiments. By chosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated TOLEDs with an optical transmittance which was strongly enhanced from 47 % (reference without capping layer) to 65 %, measured at 555 nm. Simultaneously, the bottom-to-top ratio of luminance is tuned by the capping layer in accordance with our TMM simulations. That ratio was changed from 3.6 (reference device without capping layer) to 2.4 for the transmission-optimized device. A reference BOLED with the same organic stack but with an opaque 2 nm Cs2CO3/120 nm Al cathode reaches a luminous efficacy of 24 lm/W (at 1000 cd/m2). However, our transmission-optimized TOLED has a value of 6 lm/W (1000 cd/m2 total luminance). The reduced efficacy is probably owed to an inferior activation of the Cs2CO3 contact dopant by the 2 nm Al film compared to the flash-evaporated 120 nm Al layer of the BOLED.