Patterned OLEDs: effect of substrate corrugation pitch and height

An ongoing OLED challenge is cost-effective enhancement of light extraction, i.e., increasing the external quantum efficiency (EQE similar to 20% in conventional devices). OLEDs on corrugated substrates often show enhanced EQEs providing insight into light emission processes. In particular, patterned plastic substrates directly imprinted easily at room temperature and amenable to low-cost R2R production are ideal for studying/optimizing various structures, further elucidating the extraction process. We show new semi-quantitative data of the effect of the pitch (a) and height/depth (h) of plastic substrate patterns on the OLEDs' stack and EQE, focusing on new designs, interestingly, some showing surprisingly enhanced EQEs that were neither reported nor discussed before. These include: (i) shallow (h < 200 nm) convex polycarbonate with a similar to 750 versus similar to 400 nm, where the h gradually decreases as the OLED stack is built and (ii) concave PET/CAB with large a (similar to 2.8 and similar to 7.8 mu m), where the EQE enhancement of conformal OLEDs may be due largely to scattering. EQEs of green, blue, and white phosphorescent OLEDs were measured. OLEDs on substrates with narrow a similar to 400 nm and low h < 200 nm showed no enhancement, resembling flat devices. In contrast, OLEDs on substrates with comparable or smaller h, but larger a similar to 750 nm show significant EQE enhancement despite h reduction across the stack. Green OLEDs with a similar to 750 nm and h similar to 160 to similar to 180 nm, showed EQEs similar to 30%, reaching similar to 58% with substrate mode extraction. Surprisingly, fully conformal OLEDs on a PET/CAB substrate with a similar to 7.8 mu m showed blue and white EQEs reaching similar to 33%, without substrate mode extraction. The enhancing patterns increase the OLEDs' EQE by reducing surface plasmon excitation and internal waveguiding.