OLED-on-CMOS - optoelectronic devices with embedded light emitter

Abstract

Highly-efficient, low-voltage organic light emitting diodes (OLED) are well suitable for post-processing integration onto the top metal layer of CMOS devices. The operation of OLEDs on active matrix CMOS substrates requires a top-emitting, low-voltage OLED stack. Low operating voltage devices can be achieved applying the concept of electrical doping of the charge transport layers of the OLED. Devices with very high power efficiency can be prepared when combining the concept of electrical doping with highly efficient emitter systems. The typical device architecture is the so-called p-i-n- stacking (intrinsic emitter layer and blocking layers sandwiched between doped p- and n-type transport layers). Although OLED-on-CMOS micro-displays are on the market already, those devices have not yet been shown with p-i-n technology, i.e. with low operating voltage at high brightness. Red, orange, white, green and blue OLED-stacks with doped charge transport layers were prepared on various CMOS test substrates. The different devices were compared with respect to their performance (current, luminance, voltage, luminance dependence on viewing angle, optical outcoupling etc.). Low operating voltages of 2.4V at 100cd/m² for the red p-i-n type phosphorescent emitting OLED stack, 2.5V at 100cd/m² (3.3V@1000cd/m²) for the orange phosphorescent emitting OLED stack and 3.2V at 100cd/m² (4.7V@1000cd/m²) for the white fluorescent emitting OLED have been achieved. By monolithically combining OLED with CMOS circuitry in a single device, specific OLED advantages (efficiency, low voltage, high brightness, spectral characteristics (VIS/NIR) allow to address several new and alternative applications, going far beyond the current major application in small and medium displays. This is especially based on CMOS capabilities for implementation of various sensing devices (e.g., photodetectors). By OLED-on-CMOS technology, it becomes possible to supplement integrated optical sensors by an efficient and stable light source inside the silicon, both driven, read-out and controlled by embedded CMOS circuitry. Moreover, the light emitter is placed above the CMOS electronics, therefore saving expensive chip area without requiring additional space. That combination allows advanced devices and applications, e.g., OLED microdisplays with embedded image detector (camera), or optoelectronic sensors with embedded light source. Applications address mainly two areas: advanced microdisplays (e.g., head-mounted display with integrated eye-tracking), and optical sensors (e.g., light barriers, lab-on-chip, optocouplers)