Enhanced cyanine solar cell performance upon oxygen doping Bin Fan a , Roland Hany a , Jacques-Edouard Moser b , Frank Nu ¨ esch a, * a Laboratory for Functional Polymers, Swiss Federal Laboratories for Materials Testing and Research, Empa, U ¨ berlandstrasse 129, CH-8600 Du ¨ bendorf, Switzerland b Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015 Lausanne, Switzerland Received 29 July 2007; received in revised form 25 September 2007; accepted 30 September 2007 Available online 13 October 2007 Abstract The effect of exposing cyanine–fullerene C 60 bilayer solar cells to ambient atmosphere is investigated. For exposure times of a few hours and concomitant light soaking, the device performance experiences a drastic power efficiency increase going from 0.14% to 1.2% measured at 30 mW/cm 2 simulated solar irradiation. The 10-fold enhancement is attributed to the photoinduced doping involving oxygen and water leading to the formation of reactive superoxide anions and mobile holes in the cyanine layer. The influence of water and dry oxygen are investigated separately. While water deteriorates the device performance, dry oxygen leads only to a partial increase of efficiency. Annealing does not ameliorate the perfor- mance of doped devices. Although then the cyanine layer features more crystallinity, the considerable morphological changes cause diffusional loss in charge carrier collection. Doping of not annealed devices brings a sizeable efficiency enhancement that highlights the importance of charge carrier transport in cyanine dye based solar cells. Ó 2007 Elsevier B.V. All rights reserved. PACS: 72.80.Jc; 84.60.Jt; 78.40.Me; 73.61.Ph; 27.40.tw; 78.30.Jw; 73.50.Pz Keywords: Organic optoelectronic devices; Photovoltaic cells; Solar cells; Cyanine dyes; Fullerene; C 60 ; Doping; Oxygen; Annealing; Charge transport; Superoxide anion; Conductivity 1. Introduction Cyanine dyes were developed at the beginning of the 20th century, mainly as sensitizers for silver halide emulsions in the photographic process [1]. Above all, cyanines exhibit extraordinarily high extinction coefficients and tunable absorption spec- tra throughout the visible and near infrared domain. Because of their unique optical properties, they have more recently been applied in non-linear optics [2– 4], in data storage devices [5], as fluorescent probes in biomolecules [6] or as contrast agents in optical imaging of tissue [7]. Redox properties of cyanine dyes have also been extensively studied with respect to their role as sensitizer or desensitizer for silver halides [8]. The wide range of oxidation and reduc- tion potentials allows cyanines to act as electron donors as well as electron acceptors in photoin- duced electron transfer processes. 1566-1199/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2007.09.008 * Corresponding author. Tel.: +41 44 823 4740. E-mail address: frank.nueesch@empa.ch (F. Nu ¨ esch). Available online at www.sciencedirect.com Organic Electronics 9 (2008) 85–94 www.elsevier.com/locate/orgel