Molecular doping of low-bandgap-polymer:fullerene solar cells: Effects on transport and solar cells Ali Veysel Tunc a , Antonietta De Sio a , Daniel Riedel b , Felix Deschler b , Enrico Da Como b , Jürgen Parisi a , Elizabeth von Hauff a,⇑ a Energy and Semiconductor Research Laboratory, Institute of Physics, Carl von Ossietzky Universität Oldenburg, Oldenburg 26111, Germany b Photonics and Optoelectronics Group, Department of Physics, CeNS Ludwig-Maximilians-Universität München, Munich 80799, Germany article info Article history: Received 17 May 2011 Received in revised form 22 November 2011 Accepted 23 November 2011 Available online 7 December 2011 Keywords: Photovoltaics Organic semiconductors Doping Conducting polymer Solar cells Carrier mobility abstract We show how molecular doping can be implemented to improve the performance of solution processed bulk heterojunction solar cells based on a low-bandgap polymer mixed with a fullerene derivative. The molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano- quinodimethane (F4-TCNQ) is introduced into blends of poly[2,6(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1-b:3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) via co-solution in a range of con- centrations from 0% to 1%. We demonstrate that the hole conductivity and mobility increase with doping concentration using field-effect measurements. Photoinduced absorption (PIA) spectroscopy reveals that the polaron density in the blends increases with doping. We show that the open circuit voltage and short circuit current of the correspond- ing solar cells can be improved by doping at 0.5%, resulting in improved power conversion efficiencies. The increase in performance is discussed in terms of trap filling due to the increased carrier density, and reduced recombination correlated to the improvement in mobility. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Solar cells based on polymer:fullerene bulk- heterojunctions (BHJ) can be processed from solution at low temperatures and low production costs to offer a promising technology for flexible, large area photovoltaic applications [1,2]. In the BHJ solar cell [3], the polymer and fullerene form an interpenetrated network of donor and acceptor domains at the nanoscale [4,5]. Excitons are gener- ated upon light absorption by the polymer. The fullerene is used as an electron acceptor, and charge transfer occurs when excitons diffuse to the polymer and fullerene interface. An efficient collection of the photogenerated charge carriers depends on the percolation of holes and electrons towards the anode and cathode with minimized recombination. However, in such thin-film solar cells recombination is a delicate interplay between morphology and carrier lifetime/mobility [6–8]. Several aspects have been explored towards the optimization of morphology [9–11], whereas intrinsic carrier mobility is a material parameter, with limited possibilities for improvement [12]. Both the morphology of the active layer and the charge carrier mobilities in the blend influence the efficiency of BHJ solar cells, which is primarily limited by low photocurrents. The design of conjugated polymers with low bandgaps lying in the near infrared has improved the light harvesting properties and thus the amount of photocarriers potentially capable of generating photocurrent [13]. These novel low bandgap polymers have recently been the focus of intense research [13–15]. However, poor carrier transport 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.11.014 ⇑ Corresponding author. Present address: Institute of Physics, Albert- Ludwigs University of Freiburg, Freiburg 79104, Germany. Tel.: +49 (0)441 7983933; fax: +49 (0)441 7983990. E-mail address: elizabeth.von.hauff@physik.uni-freiburg.de (E. von Hauff). Organic Electronics 13 (2012) 290–296 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel