Please cite this article in press as: M. Vasilopoulou, et al., Engineering of the energetic structure of the anode of organic photovoltaic devices utilizing hot-wire deposited transition metal oxide layers, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.12.130 ARTICLE IN PRESS G Model APSUSC-29365; No. of Pages 6 Applied Surface Science xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Engineering of the energetic structure of the anode of organic photovoltaic devices utilizing hot-wire deposited transition metal oxide layers M. Vasilopoulou a,∗ , N.A. Stathopoulos b , S.A. Savaidis b , I. Kostis a,b , G. Papadimitropoulos a , D. Davazoglou a,∗∗ a Institute of Nanoscience and Nanotechnology, Department of Microelectronics, National Center for Scientific Research Demokritos, POB 60228, 15310 Agia Paraskevi, Attiki, Greece b Department of Electronics, Technological and Educational Institute (TEI) of Piraeus, Petrou Ralli &Thivon, 12244 Aegaleo, Greece a r t i c l e i n f o Article history: Received 30 October 2014 Received in revised form 18 December 2014 Accepted 18 December 2014 Available online xxx Keywords: Interface engineering Organic photovoltaics Hot-wire deposition Metal oxides a b s t r a c t In this work we use hydrogen deposited molybdenum and tungsten oxides (chemically described as H:MO x x ≤ 3 where M = Mo or W) to control the energetics at the anode of bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on poly(3-hexylthiophene):[6,6]-phenyl butyric acid methyl ester (P3HT:PC 71 BM) blends. Significantly improved current densities and open circuit voltages were achieved as a result of improved hole transport from the P3HT highest occupied molecular orbital (HOMO) toward indium tin oxide (ITO) anode. This was attributed to the formation of shallow gap states in these oxides which are located just below the Fermi level and above the polymer HOMO and thus may act as a barrier- free path for the extraction of holes. Consequently, these states can be used for controlling the energetic structure of the anode of OPVs. By using ultraviolet photoelectron spectroscopy it was found that depend- ent on the deposition conditions these gap states and work function of the metal oxides may be tailored to contribute to the precise alignment of the HOMO of the organic semiconductor (OSC) with the Fermi level of the anode electrode resulting in further enhancement of the device performance. © 2014 Elsevier B.V. All rights reserved. 1. Introduction During the last three decades the use of organic semiconductors (OSCs) [1] for the fabrication of various electronic devices such as field-effect transistors (OFETs) [2], light emitting diodes (OLEDs) [3] and photovoltaics (OPVs) [4], is intensively investigated. In all these devices the charge carriers are injected in (or extracted from) the OCS through inorganic (metallic) contacts. The electric contacts between organic and inorganic materials though, are sub- stantially different from those between inorganics only, because the junctions are usually formed with weak Van-der-Waals forces as opposed to the inorganics where they are made by sharing elec- trons of the atoms of the two materials that form the junction [5]. Under such conditions the simple model introduced by Bloch to explain the movement of electrons through a periodic crystal ∗ Corresponding author. Tel.: +30 210 6503269. ∗∗ Corresponding author. E-mail addresses: mariva@imel.demokritos.gr (M. Vasilopoulou), d.davazoglou@imel.demokritos.gr (D. Davazoglou). (based on the delocalization of electrons shared between atoms throughout the entire crystal) cannot be applied. The above model becomes even more inapplicable for holes, which are the dominant carriers in most OCSs, the propagation of which pre-supposes the continuity of the material and, therefore, of the wave functions at the top of the valence band. When a carrier is forced to cross an organic–inorganic material junction, even if this passage is ener- getically favored, in the best case will be delayed and thus create an additional barrier thus blocking the other carriers, while in the worst case it will be completely localized by the random potential created at the interface due to the Van-der-Waals potential. Conse- quently, when designing the contacts of OPVs several requirements must be fulfilled. The first problem which must be faced is the narrowness of the molecular energy levels of OSCs [1,5]. While in inorganic materials valence and conduction bands are several eVs wide, in OSCs the highest occupied and lowest unoccupied molecu- lar orbitals (HOMO and LUMO, respectively) are of the order of a few hundreds of meVs wide only [1,5]. To facilitate charge extraction in OPVs it is therefore necessary to incorporate inorganic mate- rials between the metal contacts and OSC (the so-called charge interfacial layers, CILs) exhibiting energy levels near those of the http://dx.doi.org/10.1016/j.apsusc.2014.12.130 0169-4332/© 2014 Elsevier B.V. All rights reserved.