Ultraviolet photodegradation of tris(8-hydroxy-quinolinate) aluminum (Alq 3 ) thin ﬁlms studied by electron and laser stimulated desorption W.R. Brito a,b,⇑ , W.G. Quirino c,d , C. Legnani c,d , C.R. Ponciano e , M. Cremona d,e , M.L.M. Rocco a a Instituto de Química, Universidade Federal do Rio de Janeiro, UFRJ, Rio de Janeiro, RJ, CEP 21949-900, Brazil b Departamento de Química, Universidade Federal do Amazonas, UFAM, Manaus, AM, CEP 69077-000, Brazil c Departamento de Física, Universidade Federal de Juiz de Fora, UFJF, Juiz de Fora, MG, CEP 36036-900, Brazil d Divisão de Metrologia de Materiais (Dimat), Inmetro, Duque de Caxias, RJ, CEP 25250-020, Brazil e Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ, CEP 22453-970, Brazil article info Article history: Received 2 April 2012 Received in revised form 13 June 2012 Accepted 13 June 2012 Available online 13 July 2012 Keywords: OLED degradation Alq 3 UV irradiation Electron desorption Laser desorption abstract Alq 3 has been the reference material used widely in the fabrication and characterization of efﬁcient OLEDs due to its good properties as electroluminescent and electron transporting layer. Although the inclusion of these devices in commercial displays and lighting devices represents many beneﬁts, the knowledge about the progressive loss of performance and efﬁciency with time for such devices is still limited. Therefore, it is an incentive to understand the mechanisms of Alq 3 degradation when it is sub- jected to the inﬂuence of various extrinsic factors such as UV radiation. In the present work the degrada- tion processes of Alq 3 thin ﬁlms as a result of 254 nm UV light irradiation are presented and discussed. The degradation products produced by the action of UV light were evaluated by time-of-ﬂight mass spec- trometry using electron stimulated ion desorption and laser desorption ionization techniques. Strong evi- dence for carboxylic acid formation after photodegradation was observed by the electron desorption technique. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction In 1987 Tang and VanSlyke described the ﬁrst efﬁcient organic electroluminescent diode employing 8-hydroxyquinoline alumi- num (Alq 3 ) as the luminescent ﬁlm of the device [1]. Since then, there is abundant literature that shows the use of Alq 3 as electro- luminescent or electron transporting layer in Organic Light-Emit- ting Diodes (OLEDs) with high quantum efﬁciency and thermal and chemical stability. However, aspects of interest related to the degradation processes induced by intrinsic and extrinsic factors such as sunlight, humidity, temperature and others are still not completely understood. The comprehension of these processes is one of the keys for the full commercial application of this technology. The incidence of UV light on Alq 3 is an extrinsic effect that may contribute to decrease the lifetime of Alq 3 based-devices. Some studies have estimated the possible degradation mechanisms that lead to the loss of the electroluminescent properties [2–5]. The evolution of a vibrational band at 1697 cm 1 in the infrared spec- trum of Alq 3 ﬁlms exposed to UV light suggested the formation of a carbonyl group [2]. In a recent work the degradation of Alq 3 ﬁlms due to different UV exposures were followed by photoabsorption and photoemis- sion techniques. Comparative studies between exposed and unex- posed Alq 3 thin ﬁlms showed larger degradation processes when Alq 3 is exposed to 254 and 365 nm UV radiation, as observed by to- tal signal intensity of NEXAFS (Near-edge X-ray Absorption Fine Structure) results at the carbon and nitrogen 1s edges [6]. Core le- vel photoemission measurements after 307 nm UV exposure show dramatic changes for nitrogen, oxygen and carbon photoelectric signals [6]. The effects of sunlight radiation on Alq 3 thin ﬁlms was also investigated using valence and core level spectroscopic techniques [7,8]. Another mechanism reports the catalytic role of water mole- cules found in the environment, which favor the loss of the 8- hydroxyquinoline ligand (8-Hq) of the molecular structure of Alq 3 and the formation of intermediary compounds [9–11]. 8-Hq develops into a dark-colored, non-emissive polymer compound due to oxidative polymerization. This is an important factor, since many studies are carried out on open atmosphere. As a part of a systematic study of the inﬂuence of extrinsic ef- fects on molecular ﬁlms used in OLEDs, we employed time-of- ﬂight mass spectrometry coupled to electron stimulated ion desorption (ESID-TOF) and laser desorption ionization (LDI-TOF) techniques in order to raise the knowledge about the Alq 3 degrada- tion mechanisms. To perform this work, 100 nm thin ﬁlms of Alq 3 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.06.010 ⇑ Corresponding author. at: Departamento de Química, Universidade Federal do Amazonas, UFAM, Manaus, AM, CEP 69077-000, Brazil. Tel.: +55 92 3305 4035. E-mail address: wrbrito@ufam.edu.br (W.R. Brito). Optical Materials 35 (2012) 29–32 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat