OLEDs on Flexible Substrates

In some applications flexible plastic substrates are advantageous over glass substrates for organic light emitting diode (OLED) fabrication due to properties such as higher refractive index, ease ofnano-patterning for enhanced extraction of light generated in the OLED, and promise in wearable electronics and biochemical sensing. Defects, impurities, and in particular sharp edges that may be formed in patterned plastic substrates such as polycarbonate (PC) can lead to catastrophic shorts when biased due to very high local electric fields. This work evaluated various PC substrates and their treatment with the goal of identifying the best substrates for patterning for enhancing OLED light outcoupling. We investigated the effect of PC planarization and the use of ITO anode, which was sputter-deposited at room temperature. Three green phosphorescent OLEDs based on tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) on three PC types, i.e., as-purchased (device A), planarized (device B), and ITO-coated/planarized (device C), were evaluated. Devices A and B, with a PEDOT:PSS anode, displayed peak brightness of ~47,000 Cd/m2, ~50 Cd/A luminous efficiency, and ~14% external quantum efficiency (EQE). Device C exhibited a maximum brightness of 74,000 Cd/m2, 65 Cd/A luminous efficiency, and 18.8% EQE. The results indicate that planarization does not affect device performance, which was supported by atomic force microscopy imaging that indicated a surface roughness range of 1.5-5.8 nm across substrates A & B; the ITO roughness was similar. A high quality anode, as expected, is essential, which is a challenge when depositing ITO on plastic