Letter Semi-transparent metal electrode of Cu–Ni as a replacement of an ITO in organic photovoltaic cells D.S. Ghosh a,n , R. Betancur a , T.L. Chen a,n , V. Pruneri a,b , Jordi Martorell a,c a ICFO (Institut de Ci  encies Fot  oniques), Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain b ICREA, Institucio ´ Catalana de Recerca i Estudis Avanc - ats, 08010 Barcelona, Spain c Departament de Fı ´sica i Enginyeria Nuclear, Universitat Polit  ecnica de Catalunya, 08222 Terrassa, Spain article info Article history: Received 12 October 2010 Accepted 16 December 2010 Available online 12 January 2011 Keywords: Ultrathin metal films Transparent conductive oxides Metal electrodes Organic solar cells ITO abstract We demonstrate an indium-free organic photovoltaic cell that incorporates an ultrathin metal film as a semitransparent anode. In the proposed device structure, the indium tin oxide electrode is replaced by an ultrathin Cu–Ni bilayer. When an NiO is used as the hole transporting layer, the characteristic photovoltaic parameters of the cell fabricated with the metal electrode are similar to those of the device fabricated with the indium tin oxide (ITO). Despite the fact that the metal electrode exhibits a transparency that is 65% of the ITO electrode, the short-circuit current for the metallic anode based cell is 77% of the ITO based one, indicating that the photon absorption could be enhanced by the optical microcavity formed between the Cu–Ni and Al electrodes. The overall photo-conversion efficiency for the metallic electrode based cell is 76% of the ITO based one, which was measured to be 3.3%. The obtained performances of ultrathin metals when included in the cell architecture used here, combined with their low cost, high compatibility with other materials, and mechanical flexibility, confirm their potentials for organic photovoltaics. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The potential of organic photovoltaic cells (OPV) to become a viable alternative in the production of energy resides on improving the currently achievable photo-conversion efficiencies, but, also, on establishing new fabrication procedures and the use of materials that would allow for a low cost production of solar modules at large scale. At the moment, a cost reduction in a potential industrial production of OPVs would be severely limited if an indium tin oxide (ITO) were to be used as the transparent electrode [1–3]. Despite its high transparency and low sheet electrical resistance, an ITO suffers from several drawbacks, mainly an increasing cost due to indium shortage, the need of post-deposition treatment and the low flexibility, due to poor mechanical ductility and relatively large thickness. Several alternatives have been applied to OPV cells, including indium-free transparent conductive oxides (TCOs), car- bon nanotubes, Ag nanowires and, more recently, graphene films [4–8]. Ultrathin metal films (UTMFs), i.e. continuous metallic films whose thickness is below 10 nm, are transparent to light, still maintaining large electrical conductivity properties [9]. Contrary to TCOs, they can also be grown with a single step process, possess high compatibility with nearly all organic and semiconductor materials, and are very flexible, due to their low thickness and high mechanical ductility [10–13]. In this paper, we report on the use of ultrathin Cu–Ni bilayers as an anode in a P3HT/PCBM bulk heterojunction solar cell. Recently, it was shown that in such type of cells an NiO could be used as hole transporting layer providing better efficiency and stability than PEDOT:PSS [14–17], in addition allowing for a solution processing in ambient air [14]. Here, we show that an NiO exhibits an excellent work function matching with an Ni layer used as a capping layer for a highly conductive Cu layer. As described below, the OPV cells incorporating such Cu–Ni electrodes perform similarly to those incorporating an ITO electrode. 2. Experimental details Cu and Ni UTMFs were deposited by the DC magnetron sputtering on double-side optically polished UV grade silica substrates. The thickness was calculated based on the deposition rate, which in turn was determined by an MCM-160 quartz crystal. Cross sectional SEM measurements and ellipsometry measure- ments were also carried out in a thicker film to confirm the values inferred from the deposition rate. The deposition rates were 1.5 and 0.57 ˚ A/s for Cu and Ni, respectively. Cu–Ni UTMFs with different Cu Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.12.040 n Corresponding author. Tel.: + 34 93 5534178; fax: + 34 93 5534000. E-mail addresses: tonglai.chen@icfo.es (T.L. Chen), dhriti.ghosh@icfo.es (D.S. Ghosh). Solar Energy Materials & Solar Cells 95 (2011) 1228–1231