Available online at www.sciencedirect.com Journal of Photochemistry and Photobiology A: Chemistry 195 (2008) 39–46 IV–VI Nanocrystal–polymer solar cells Karolina P. Fritz a , Serap Guenes b,1 , Joseph Luther c , Sandeep Kumar a , N. Serdar Sariciftci b,∗ , Gregory D. Scholes a,∗∗ a Department of Chemistry and Institute for Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ont. M5S 3H6, Canada b Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria c National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401, USA Received 28 May 2007; received in revised form 17 August 2007; accepted 3 September 2007 Available online 7 September 2007 Abstract Processes are investigated for the fabrication of hybrid bilayer photovoltaic (PV) devices consisting of IV–VI nanocrystals (NCs) having near- infrared optical gaps and an organic conductive polymer. Our design utilizes PbS NCs spin-cast onto an indium tin oxide (ITO) coated glass slide, covered with a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer. We demonstrate here that the removal of the surface ligand, by pre-rinsing the NCs and subsequent annealing, generates a smoother film with a greater degree of cross-linking between the NCs. Additionally, a post-production treatment enhances the interfacial layer and improves charge carrier separation and mobility, rendering better performing solar cells. Further, results using PbSe NCs indicate the method can be adapted to similar systems. The method proposed here was designed to optimize more independently the polymer and nanocrystal components of the device and, to an extent, the interface between them. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocrystals; PbS; Solar cells; Photovoltaic devices; Near-infrared 1. Introduction Solar cells based on conjugated polymer active layers have attracted considerable attention in recent years as a way of decreasing the cost of photovoltaic energy conversion [1]. While the low cost and ease of processing is driving further research in this type of organic solar cell, the underlying science has gener- ated much interest across a range of disciplines [2]. Successful devices are typically based on the use of bulk heterojunctions or hybrid blends [1,3–12] which provide a large contact area between the donor and acceptor species. The combination of many factors determine the energy conversion efficiency of solar cells [13,14], however, it is clear that the incident spectrum captured by a device is controlled, and limited by, the absorp- tion of the active layer. Recent work has explored possibilities ∗ Corresponding author. ∗∗ Corresponding author. Tel.: +1 416 946 7532; fax: +1 416 978 8775. E-mail addresses: serdar.sariciftci@jku.at (N.S. Sariciftci), gscholes@chem.utoronto.ca (G.D. Scholes). 1 Current address: Department of Physics, Yildiz Technical University, Davut- pasa Campus, 34220 Davutpasa, Esenler, Istanbul, Turkey. for expanding the cross-section of absorbed light by designing near-infrared (NIR) absorbing polymers [15–17], incorporating NIR-absorbing semiconductor nanocrystals (NCs) into hybrid devices [18–20], or adding an intrinsic “antenna” layer [21,22]. Effective harvesting of sunlight for solar energy conversion ideally captures part of the near-infrared spectral region. One way to achieve that is through the use of nanostructured PbS or PbSe materials, which have absorption edges that can be size-tuned through the near-infrared [23]. Furthermore, the pre- diction [24] and subsequent experimental evidence [25–27] for multiple exciton generation (carrier multiplication), whereby the absorption of one photon results in the formation of more than one electron–hole pair, further motivates the fabrication of lead chalcogenide-based solar cells. For example, it is important to discover whether multiple exciton generation can increase the energy conversion efficiency of a device—that is, can the multi-exciton states be harvested to provide free carriers? Despite the fact that PbS and PbSe appear to pos- sess the required fundamental properties to be used in a high performance hybrid NC/conjugated polymeric solar cell, studies to date of PbS/MEH-PPV [18] (poly[2-methoxy-5- (2-ethylhexyloxy-p-phenylenevinylene)]) or PbSe/P3HT [19] 1010-6030/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2007.09.004