MTL Annual Research Report 2013

Organic Light Emitting Devices on Plastic and Paper Substrates

The successful fabrication of optoelectronics on economical, flexible, and lightweight substrates such as plastic and paper could pave the way for an exciting, novel breed of disposable electronics. Here, we demonstrate phosphorescent organic light emitting devices (OLEDs) on various substrates, including polyethylene terephthalate (PET), tracing paper, and copy paper. Poly(3,4-ethylenedioxythiophene) [PEDOT] is deposited onto the substrates via a dry oxidative chemical vapor deposition (oCVD) process, avoiding the difficulties of processing PEDOT on chemically absorbing substrates using conventional wet methods. The completed OLEDs comprise a bottom 50 nm PEDOT anode, a 5 nm molybdenum oxide (MoO₃) hole injection layer, a 100 nm tris(4-carbazoyl-9-ylphenyl)amine [TCTA] hole transport layer, a 15 nm iridium, tris(2-phenylpyidine) [Ir(ppy)3]/TCTA (10%) emissive layer, a 60 nm 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene [TPBi] electron transport layer, and a top 100 nm lithium fluoride (LiF)/aluminum (Al) cathode (all layers except PEDOT deposited via thermal evaporation at ~10^-6 Torr). The green light output is measured through the bottom anode and substrate and is readily observable by eye. As a comparison, control OLEDs are made on indium tin oxide (ITO)-on-glass substrates, and these devices demonstrate external quantum efficiencies (EQEs) of ~14%. For OLEDs on PEDOT-on-plastic/paper, EQEs of up to 2% on tracing paper and 6% on PET have been measured to date, and there remains much room for improvement. For instance, at present, many devices suffer from electrical shorts that arise either from the underlying roughness of paper fibers or from large iron chloride (FeCl₃) residues from oCVD on the surface of the PEDOT. Current work focuses on improving the reproducibility of devices by solving this shorting problem. Furthermore, the mechanical flexibility of devices will be measured in the near future. Like the rest of the OLED layers and unlike ITO, PEDOT is not prone to brittle fracture under mechanical stress, so it is expected that OLEDs made on PEDOT-on-plastic/paper can remain operational under bending, folding, and/or twisting – an attractive asset for portable electronics.