Applied Surface Science 302 (2014) 169–176 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Laser prepared organic heterostuctures on glass/AZO substrates Anca Stanculescu a,∗ , Marcela Socol a , Oana Rasoga a , Ion N. Mihailescu b , Gabriel Socol b , Nicoleta Preda a , Carmen Breazu a , Florin Stanculescu c a National Institute of Materials Physics, 105 bis Atomistilor Street, P.O. Box MG-7, Bucharest-Magurele 077125, Romania b National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor Street, P.O. Box MG-36, Bucharest-Magurele 077125, Romania c University of Bucharest, Faculty of Physics, 405 Atomistilor Street, P.O. Box MG-11, Bucharest-Magurele 077125, Romania a r t i c l e i n f o Article history: Received 1 July 2013 Received in revised form 22 January 2014 Accepted 29 January 2014 Available online 6 February 2014 Keywords: Organic semiconductor AZO PLD MAPLE ZnPc NTCDA a b s t r a c t This paper presents some studies about the bi-layer organic heterostructures realized with zinc phthalo- cyanine (ZnPc) as donor layer and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) as acceptor layer, on substrate of glass covered by Al doped ZnO (AZO) layer. These heterostructures have been prepared using laser techniques: pulsed laser deposition (PLD) in an atmosphere of oxygen for AZO films deposition and matrix assisted pulsed laser evaporation (MAPLE) for organic films deposition. The influence of the deposition conditions on the transmission of the organic films has been analysed. The effect of the oxygen plasma treatment, with duration of 5 min and 10 min, on the surface topogra- phy, structural and optical properties of AZO layers deposited by PLD and, as consequence, on the optical and electrical properties of the single layer (ZnPc) and bi-layer (ZnPc/NTCDA) organic heterostructure, deposited by MAPLE, was investigated. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In the search for new materials for a wide area of applica- tions, from optoelectronics to photovoltaics, the heterostructures based on organic compounds containing the conjugated system of  electrons with different substituent groups and aluminium doped zinc oxide (AZO) have shown promising optical and electrical properties. They are considered as alternative for the more expen- sive heterostructures realized with inorganic semiconductors and indium based transparent conductor electrode. The interest in studying doped ZnO as transparent conductor is justified by its non-toxicity and lower cost, which offer the possibil- ity to replace the more commonly used, but expensive, indium tin oxide (ITO). ZnO is an n type conduction II–VI semiconductor com- pound with a band gap around 3.3 eV at room temperature. Pure ZnO films show high transparency in visible range (400–700 nm), but their electrical conductivity is low [1]. A way to improve the electrical properties preserving the optical transparency is to use non-stoichiometric films or films doped with elements from III group [2]. The main disadvantage of the non-stoichiometric ZnO films is represented by the instability of the electrical properties because of the oxygen chemisorption and desorption. ∗ Corresponding author. Tel.: +40 21 2418160; fax: +40 21 3690177. E-mail address: sanca@infim.ro (A. Stanculescu). Like ITO films, AZO films show low electrical resistivities (∼10 -4  cm) [1,3,4]. AZO is a wide band gap semiconductor (E g ∼ 3.4–3.9 eV) characterized by higher chemical (thermal) sta- bility than ITO and good transmission in the visible–NIR region [5]. The physical properties of AZO films depend on the surface topography and microstructure, which are determined by the films’ preparation and post-deposition treatment conditions. We propose a bi-layer organic heterostructure on glass/AZO substrate for potential photovoltaic applications. The donor (D) and acceptor (A) organic semiconductors must satisfy the energetically selection criteria for assuring at the D–A interface a favourable exci- ton dissociation into a hole polaron moving in the donor layer and an electron polaron moving in the acceptor layer, that are bounded by coulomb forces to form the exciton [6,7]: E EB,D < E LUMO,A - E LUMO,D ; E EB,A < E HOMO,A - E HOMO,D (1) where E EB is the exciton binding energy, index A referring to accep- tor and D to donor; E LUMO is the lowest unoccupied molecular orbital energy; E HOMO is the highest occupied molecular orbital energy. We have selected zinc phthalocyanine (ZnPc), characterized by E HOMO = 5.28 eV, E LUMO = 3.34 eV and E EB = 0.6 eV, as donor [8] (Fig. 1a), and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), characterized by E HOMO = 8 eV, E LUMO = 4 eV and E EB = 0.95 eV, as acceptor [9] (Fig. 1b), because E EB,ZnPc < E LUMO,NTCDA - E LUMO,ZnPc and E EB,NTCDA < E HOMO,NTCDA - E HOMO,ZnPc. 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2014.01.181