© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim COMMUNICATION 377 wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de Adv. Energy Mater. 2011, 1, 377–381 Organic solar cells (OSCs) based on polymers and small mole- cules have seen a tremendous increase in interest during the past few years. Significant progress in this field seeded the pros- pect for a cost-effective and easy-to-fabricate photovoltaic tech- nology—typical advantages claimed for organic (opto-)electronic devices. Very recently, certified cell efficiencies in excess of 7% have been reported for polymer based cells. [1] For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifically, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. [2] However, PEDOT:PSS is bur- dened with structural and electrical inhomogeneity [3,4] and has been demonstrated to be an origin of limited device lifetime. [5] Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. [6,7] Very recently, tran- sition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work func- tions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. [8–11] TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. [12–15] The unique energetics of these TMOs has so far been predominantly accessible for films thermally evaporated in high-vacuum. The first results for TMO layers obtained by solution processing from nano-particle (NP) dispersions have been reported only very recently. [16,17] Meyer et al. prepared MoO 3 layers by dispersing MoO 3 NPs using a polymer as dispersing agent. After deposition, the layers had to be treated by an oxygen plasma to remove the polymer. A high WF of the resulting layers of 5.7–6 eV was obtained. A substantial drawback of the approach, however, is the observation of larger NP aggregates with a size of 100 nm and an overall high roughness of 25 nm (rms). Owing to their roughness these NP-layers are critical sources of shorts, especially over a large device area. In contrast, TMO layers (WO 3 , V 2 O 5 and MoO 3 ) have been pre- pared by sol-gel deposition, predominantly for electrochromic, catalytic and sensing applications. [18–21] Post processing of the sol-gel TMO layers at high temperatures (300 °C–600 °C) is routinely applied in order to achieve specific microstructures or crystalline phases in the materials, as required by the particular application. These high processing temperatures are not com- patible with the temperature-sensitive substrates (e.g. polymer foils) envisaged for low-cost, high-throughput fabrication of organic solar cells. In spite of this limitation, Steirer et al. have very recently used NiO prepared via a sol-gel route as a replace- ment for PEDOT:PSS in an organic solar cell. [22] The require- ment of post-deposition annealing at 250 °C and treatment with an oxygen plasma in order to become applicable in OSCs limits the attractivity of the approach. In view of these issues, the application of pristine TMO layers obtained by a sol-gel route would be desirable. As yet, pristine layers have not been success- fully used for organic solar cells. Furthermore, studies of their electrical, optical, and morphological properties (e.g. electrical conductivity, spectral absorption, WF, etc.) relevant for the appli- cation in organic (opto-)electronic devices are missing. In this work, we use liquid-processed V 2 O 5 layers prepared from a vanadium-oxitriisopropoxide/isopropyl alcohol solution as hole extraction inter-layers in polymer:fullerene solar cells. Even without any post-deposition processing the V 2 O 5 layers exhibit a very high WF of 5.6 eV. OSCs with these solution proc- essed V 2 O 5 layers have efficiencies comparable to those of ref- erence devices using PEDOT:PSS and a substantially improved device stability. Atomic force microscopy (AFM) measurements of the sol-gel V 2 O 5 layers on glass/ITO substrates ( Figure 1 ) reveal an extremely smooth surface with a roughness of only 0.4 nm (rms). K. Zilberberg, S. Trost, Prof. T. Riedl Institute of Electronic Devices University of Wuppertal Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany E-mail: t.riedl@uni-wuppertal.de H. Schmidt Institute of High-Frequency Technology Technical University of Braunschweig Schleinitzstr. 22, 38106 Braunschweig, Germany DOI: 10.1002/aenm.201100076 Kirill Zilberberg, Sara Trost, Hans Schmidt, and Thomas Riedl* Solution Processed Vanadium Pentoxide as Charge Extraction Layer for Organic Solar Cells Figure 1. AFM topography image of the solution-processed V 2 O 5 layer (layer thickness 60 nm).