Co-planar bi-metallic interdigitated electrode substrate for spin-coated organic solar cells Shinobu Nagata a,n , Gary M. Atkinson b , Dmitry Pestov c , Gary C. Tepper a , James T. McLeskey Jr a a Department of Mechanical Engineering, Virginia Commonwealth University, Richmond, VA 23284-3015, USA b Department of Electrical Engineering, Virginia Commonwealth University, Richmond, VA 23284-3015, USA c Nano materials Characterization Center, Virginia Commonwealth University, Richmond, VA 23284-3015, USA article info Article history: Received 11 October 2010 Received in revised form 6 January 2011 Accepted 11 January 2011 Available online 11 February 2011 Keywords: Photovoltaics Solar cells Organic photovoltaics Interdigitated electrode Bulk heterojunction MEH-PPV abstract A bulk heterojunction organic solar cell with co-planar interdigitated electrodes was fabricated and tested. The co-planar electrodes had a separation distance of 1–3 mm and were fabricated from aluminum and nickel on a heavily oxidized silicon wafer using photolithography. The device was prepared by spin-coating MEH-PPV:PCBB in a 1:3 wt ratio with a total donor:acceptor solution concentration of 2.44%. The device demonstrated a strong photovoltaic response under AM1.5 illumination of 80 mW/cm 2 with an open circuit voltage of 0.704 V. The co-planar electrode design offers advantages in terms of electrode material selection and reliability as well as simplified device fabrication. & 2011 Elsevier B.V. All rights reserved. 1. Introduction While the efficiencies of inorganic photovoltaics continue to improve, market penetration is still limited due to their high cost in comparison to non-renewable energy sources. Due to the high energy processing required for silicon, the cost per kilowatt-hour for electric- ity from Si-based solar cells is as high as $0.25–0.65 /kW h [1]. This is roughly 5 times more than the price of electricity produced using fossil fuels. Adding to the high cost of inorganic solar cells, the potential increase in demand for Si crystals can lead to even higher costs for the devices. One estimate on the amount of silicon needed to supply electricity for a family consuming 20 kW h/day using 15% efficient solar cells is approximately 10,000 times more than the amount of silicon in a computer[2]. Clearly, a low cost alternative to inorganic solar cells is needed. Polymer-based solar cells are being widely investigated as a potential low cost alternative to silicon because, in principle, they can be produced on a large scale using inexpensive solution-based processes such as spraying, painting, and roll-to-roll print- ing [3–5]. For example, it has been shown that the manufacturing cost of polymer solar cells can be reduced very quickly (from 35 to 8 Euros/W in one year) [3]. On the other hand, the lifetime of polymer solar cells is still too short to allow comparisons with crystalline silicon [3]. In addition to their lower cost, polymeric materials are lighter, have much greater mechanical flexibility, and are capable of being directly fabricated onto most surfaces including plastics [6]. Bulk heterojunction polymer solar cells convert light into electricity using a straight forward process. Photons are absorbed by an organic semiconductor (the polymer) resulting in the creation of mobile electron–hole bound pairs known as Frenkel excitons [7]. The electron–hole pairs are then separated at a polymer/electron-acceptor interface. Typical acceptors include titanium dioxide, and carbon fullerenes. The holes travel through the polymer to the anode, and the electrons travel through the electron-acceptor toward the cathode resulting in an externally measurable current. The device efficiency depends on a number of important design parameters. For example, electron–hole separa- tion requires a symmetry breaking condition such as using electrode materials with different work functions to provide a preferred direction to the internal electric fields [8]. Polymer solar cells typically take the form of a sandwich structure with the active layer placed in between the anode and cathode electrodes. There are several challenges with this design. For example, at least one of the electrodes must be an optically transparent material such as Indium Tin Oxide (ITO) or Fluori- nated Tin Oxide (FTO) [9]. These materials typically have lower conductivity than metal electrodes and are often deposited using high temperatures, which can be harmful to the polymer. In addition, the device fabrication requires a two-layer coating (at a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.01.005 n Corresponding author. Tel.: +1 8042397483; fax: +1 8048277030. E-mail address: shinobs1@gmail.com (S. Nagata). Solar Energy Materials & Solar Cells 95 (2011) 1594–1597