Patterned OLEDs: effect of substrate corrugation pitch and height

Abstract

An ongoing OLED challenge is cost-effective enhancement of light extraction, i.e., increasing the external quantum efficiency (EQE ∼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 ∼ 750 versus∼400 nm, where the h gradually decreases as the OLED stack is built and (ii) concave PET/CAB with large a (∼2.8 and ∼7.8 μm), 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 ∼ 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 ∼ 750 nm show significant EQE enhancement despite h reduction across the stack. Green OLEDs with a ∼ 750 nm and h ∼ 160 to ∼180 nm, showed EQEs ∼30%, reaching ∼58% with substrate mode extraction. Surprisingly, fully conformal OLEDs on a PET/CAB substrate with a ∼ 7.8 μm showed blue and white EQEs reaching ∼33%, without substrate mode extraction. The enhancing patterns increase the OLEDs’ EQE by reducing surface plasmon excitation and internal waveguiding.The experimental results for OLEDs on substrates with a < 2 μm are supported by scattering matrix simulations that assume conformal stacks, incorporating diffraction for internal losses reduction. EQE enhancement not predicted by simulations may be due additionally to scattering mostly for substrates with a significantly larger than the emitting wavelength.