Laser-Induced Forward Transfer of Polymer Light-Emitting Diode Pixels with Increased Charge Injection

Abstract

Laser-induced forward transfer (LIFT) has been used to print 0.6 mm × 0.5 mm polymer light-emitting diode (PLED) pixels with poly[2-methoxy, 5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting polymer. The donor substrate used in the LIFT process is covered by a sacrificial triazene polymer (TP) release layer on top of which the aluminium cathode and functional MEH-PPV layers are deposited. To enhance electron injection into the MEH-PPV layer, a thin poly(ethylene oxide) (PEO) layer on the Al cathode or a blend of MEH-PPV and PEO was used. These donor substrates have been transferred onto both plain indium tin oxide (ITO) and bilayer ITO/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) blend) receiver substrates to create the PLED pixels. For comparison, devices were fabricated in a conventional manner on ITO substrates coated with a PEDOT:PSS hole-transporting layer. Compared to multilayer devices without PEO, devices with ITO/PEDOT:PSS/MEH-PPV:PEO blend/Al architecture show a 100 fold increase of luminous efficiency (LE) reaching a maximum of 0.45 cd/A for the blend at a brightness of 400 cd/m(2). A similar increase is obtained for the polymer light-emitting diode (PLED) pixels deposited by the LIFT process, although the maximum luminous efficiency only reaches 0.05 cd/A for MEH-PPV:PEO blend, which we have attributed to the fact that LIFT transfer was carried out in an ambient atmosphere. For all devices, we confirm a strong increase in device performance and stability when using a PEDOT:PSS film on the ITO anode. For PLEDs produced by LIFT, we show that a 25 nm thick PEDOT:PSS layer on the ITO receiver substrate considerably reduces the laser fluence required for pixel transfer from 250 mJ/cm(2) without the layer to only 80 mJ/cm(2) with the layer.

Improved laser-induced forward transfer of organic semiconductor thin films by reducing the environmental pressure and controlling the substrate–substrate gap width

“…Many of these breakthroughs used an absorbing intermediate layer, termed the dynamic release layer (DRL). The DRL is often an inorganic thermal absorber, such as titanium [16], or a polymer that breaks down in a photon-induced chemical reaction, such as the class of triazene polymers [17].…”

Red-green-blue polymer light-emitting diode pixels printed by optimized laser-induced forward transfer

“…Laser-induced forward transfer (LIFT), also known as laser direct-write, 1 is a class of printing techniques that have already been used to fabricate basic small-molecule organic light-emitting diodes (OLEDs) 2,3 and polymeric OLEDs (PLEDs). 4,5 OLEDs are a form of solid-state lighting, under intense research for commercial applications, 6,7 with OLED electronic displays of particular interest. 8 One challenge which has proved more complex for thin-film electroluminescent (EL) OLEDs than existing liquid crystal display (LCD) technologies is the actual deposition of individual pixels with traditional lithographic techniques requiring specific chemical modification.…”

“…"Dry" OLED printing techniques using lasers have been developed to overcome these problems, 8,[12][13][14] including LIFT. 4,5 In our modification of the LIFT process, a thin photodecomposable triazene polymer layer (TP) acts as sacrificial "dynamic release layer" (DRL). 15 It decomposes under nspulsed UV laser irradiation and propels the overlying layers from the donor to the receiver substrate.…”

“…4,5 In particular, the introduction of a gap between the substrates was viewed as a means to improve the robustness and applicability of the technique. LIFT with the substrates "in contact" limits the application of multiple stages of LIFT to the same receiver substrate.…”

“…Another improvement which this work demonstrates is the addition of polymeric layers onto the receiver substrate to improve adhesion. 5 The combination of the reduced pressure and improved interfacial adhesion means that lower fluences are required to transfer pixels, good both in terms of energy usage and for reducing the photon, thermal and mechanical energy load on the transferred pixel.…”