Toward inkjet printing of small molecule organic light emitting diodes H. Gorter a, ⁎, M.J.J. Coenen a , M.W.L. Slaats a , M. Ren a , W. Lu a , C.J. Kuijpers b , W.A. Groen a, c a Holst Centre, PO BOX 8550, 5605 KN Eindhoven, Netherlands b Eindhoven University of Technology, Applied Physics, Eindhoven, Netherlands c Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629HS Delft, Netherlands abstract article info Available online 31 January 2013 Keywords: OLED Organic light emitting diode Small molecule Inkjet printing Organic electronics Thermal evaporation is the current standard for the manufacture of small molecule organic light emitting di- odes (smOLEDs), but it requires vacuum process, complicated shadow masks and is inefficient in material uti- lization, resulting in high cost of ownership. As an alternative, wet solution deposition can provide significant cost savings by enabling high-volume, large-area electronics on flexible substrates at low fabrication costs. In this report we present inkjet printing as a method to produce three active layers in a smOLED stack: a hole-injection layer, a hole transport layer and an emissive layer. The OLED lighting application sets high de- mands to a uniform light output over an area. This requires homogeneous deposition of the electro-active layers and this poses a significant challenge. OLED device efficiency is greatly influenced by the printed layer morphology and the quality of the deposited layers. Therefore inkjet processed smOLED device efficien- cy will be compared with reference devices made via vacuum deposition. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Unlike all other light sources, organic light emitting diodes (OLEDs) are flat and emit light over an area. OLEDs can form a large area diffuse light source with homogenous light emission. This cre- ates an added value for indoor lighting. OLEDs with efficient light emission offer the benefits of low heat generation, high power effi- ciency and high color rendering index. It is expected that OLEDs will become an attractive alternative for the traditional lighting technolo- gies such as fluorescent lighting [1]. High-efficiency (20 lm/W light emission with life time up to 10,000 h) OLED panels with an area of 55 cm 2 are now commercially available from Philips Lighting [2]. These products are manufactured through thermal evaporation and vacuum deposition of phosphores- cent small molecules that yield much higher light emission efficiency compared to fluorescent polymer materials. However, the commer- cially available OLED lighting tiles and panels are still manufactured in a batch to batch way: in small quantities and sizes resulting in high cost of production. This is an important reason why the lighting market is still hardly penetrated by OLED lighting. Reduction of the OLED production costs is therefore essential for OLEDs to conquer a position in the lighting market. Wet solution deposition can potentially provide an alternative to vacuum deposition techniques. Cost savings can be achieved by en- abling high volume production of large-area electronics on flexible substrates [1,3]. For example, semiconducting organic materials, that comprise the active materials in OLED devices, are currently still very expensive and evaporation technology is not a very efficient way to use these materials. Solution processing in a roll-to-roll ma- chine provides a continuous deposition method at atmospheric con- ditions that are much more efficient in material use [4]. One of the upcoming methods is inkjet printing of the electro-active layers. It is a contactless deposition technology, that can be used to deposit ho- mogeneously patterned layers of a wide range of different materials. In this paper we report inkjet printing of smOLED materials, while avoiding usage of halogenated solvents. The inkjet printing process has the advantage of being a non- contact technique that offers ease of patterning in various industrial processes and low costs of materials and equipment. Furthermore, the past few years have seen the development of reliable and robust inkjet printer heads/processes, making inkjet technology more and more mature for single-pass production printing [5]. This technologi- cal development opens many opportunities for using industrial inkjet technology in manufacturing of printed electronics including OLEDs [6–8] and thin film transistors (TFTs) [9]. Polymer OLEDs generally are less efficient (6–8 cd/A) [10] com- pared to their small-molecule counterparts (up to 84 cd/A reported in [11]). Over the last decades, there has been an intensive search for the polymer materials for light emitting diodes. However, the market growth of the polymer OLEDs has been limited due to issues with too low power efficiency and limited life time. On the other hand, a smOLED with phosphorescent emissive materials requires a multilayer structure to achieve its highest efficiencies [11]. It is diffi- cult to produce multilayer devices using the solution-processing Thin Solid Films 532 (2013) 11–15 ⁎ Corresponding author. Tel.: +31 404020452; fax: +31 404020699. E-mail addresses: Harrie.Gorter@tno.nl (H. Gorter), c.j.kuijpers@student.tue.nl (C.J. Kuijpers). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.01.041 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf