The role of metal/metal oxide/organic anode interfaces in efficiency and stability of bulk heterojunction organic photodetectors A. Soultati a,b , D.G. Georgiadou a , A. Douvas a , P. Argitis a , D. Alexandropoulos c , N.A. Vainos c , N.A. Stathopoulos d , G. Papadimitropoulos a , D. Davazoglou a , M. Vasilopoulou a,⇑ a Institute of Microelectronics, NCSR Demokritos, 15310 Aghia Paraskevi, Greece b Department of Chemical Engineering, National Technical University of Athens, 15780 Athens, Greece c Department of Materials Science, University of Patras, 26504 Patras, Greece d Department of Electronics, Technological Educational Institute (TEI) of Piraeus, 12244 Aegaleo, Greece article info Article history: Received 23 October 2013 Received in revised form 29 November 2013 Accepted 5 December 2013 Available online 12 December 2013 Keywords: Organic photodetectors Tungsten oxide Under-stoichiometric Anode interlayer abstract We demonstrate a transition metal oxide based hole extraction layer approach to improve the efficiency, dark current and time stability of organic photodetectors (OPDs). A significant increase in device effi- ciency and over two orders of magnitude lower dark current at a bias voltage of 0.5 V were obtained in OPDs based on the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC 71 BM) bulk heterojunctions (BHJ). This was achieved by introducing an under-stoichiometric tungsten oxide layer, after optimizing its thickness to 10 nm, at the anode/organic layer interface instead of the commonly used poly(styrenesulfonate)-doped poly(ethylenedioxythiophene) (PEDOT:PSS). The increased efficiency and lower dark current were attributed to the formation of a favorable interfacial dipole and the reduction in the device series resistance, when PEDOT:PSS was replaced by these metal oxide layers. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Organic semiconductors (OSCs) offer substantial advantages to be used as photodetectors compared to their inorganic counter- parts, as they may lead to a new generation of consumer devices that can be processed at low cost on large areas, have light weight and conform to flexible substrates. Organic photodetectors (OPDs) have evolved within a decade of intense work from a scientific curiosity to a viable technology [1]. Novel organic materials with improved optoelectronic properties and advanced device struc- tures have been employed to reach this goal [2,3]. State-of-the- art organic photodetectors and solar cells are usually based on a bulk heterojunction (BHJ) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC 71 BM) photoactive layer spin coated on a transparent indium-tin oxide (ITO) layer (bottom electrode), while they use a low work function metal as the cathode contact. Certain challenges for the industrial applica- tion of organic photodetectors are the enhancement of their effi- ciency, the reduction of the dark current under reverse bias – as the dark current limits the detection of low intensity signals – and their environmental stability. A major contribution to the dark current is the injection of charge carriers through the respective counter electrode, for instance electrons being injected through the anode. In order to enhance the efficiency, one should reduce losses occurring within the active layer and also at the respective inter- faces with metal contacts. To this regard, inserting buffer interfa- cial layers with desirable electronic properties at the anode and/ or cathode contacts has proven to be one of the most effective strategies to increase photogenerated charge extraction and oper- ational stability [4,5]. For the anode interface, poly(styrenesulfo- nate)-doped poly(ethylenedioxythiophene) (PEDOT:PSS) has been the workhorse material due to its excellent film formation proper- ties, high conductivity and transparency, high work function of 4.8–5.2 eV and facile processing from solution [6]. However, both its large acidity and hygroscopic nature negatively affect interfacial contact and, consequently, the device stability [7,8]. On the other hand, transition metal oxides (TMOs), such as MoO 3 [9–13] and WO 3 [14,15], both exhibiting a high work func- tion, have been proposed and used as effective PEDOT:PSS substi- tutes to enhance the efficiency of the photodetectors, while at the same time improving the stability of the interfacial organic/ electrode contact. These oxides can be deposited in vacuum by using techniques, such as thermal evaporation or hot wire vapour deposition [15,16], or processed directly from solution to simplify the device fabrication process [17,18]. 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.12.008 ⇑ Corresponding author. Tel.: +30 2106503269; fax: +30 2106511723. E-mail address: mariva@imel.demokritos.gr (M. Vasilopoulou). Microelectronic Engineering 117 (2014) 13–17 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee