DOI: 10.1002/adfm.200500647 The Influence of TiO 2 Particle Size in TiO 2 /CuInS 2 Nanocomposite Solar Cells** By Ryan O’Hayre,* Marian Nanu, Joop Schoonman, Albert Goossens , Qing Wang, and Michael Grätzel 1. Introduction In recent years, the desire for low-cost solar cells has lead to the exploration of new photovoltaic designs based on nano- structured materials. Research by Grätzel and O’Regan, [1] Heeger and co-workers, [2] and others [3–5] has produced a variety of successful solar-cell designs using nanometer-scale blends or interpenetrating systems. Recently, we have reportedon a new, completely inorganic solar-cell design based on nanostructured n-type TiO 2 and p-type CuInS 2 (CIS). [6] In 3D nanostructured TiO 2 /CIS solar cells, as in traditional thin-film CIS solar cells, photons are absorbed in the p-type CIS layer and converted into electron–hole pairs. The holes are conducted through the CIS layer to a back-electrode contact, while the electrons must transport to the p–n junction where they are then transferred to the TiO 2 . Compared to a planar device, the nanostructured interface between the p-type CIS layer and the n-type TiO 2 matrix shortens the average minority-carrier diffusion length, thereby improving the collection efficiency and providing the device with a higher tolerance to the presence of impurities. TiO 2 /CIS nanocomposite solar cells have achieved greater than 5 % energy conversion efficiency under simulated AM 1.5 irra- diation (AM: air mass). In most chalcopyrite solar cells, a buffer layer is typically required between the n-type and p-type regions to control the interfacial properties. In the present 3D nanocomposite solar cells, a thin (ca. 30 nm) In 2 S 3 buffer layer is applied between the TiO 2 and CIS layers. The buffer layer is particularly impor- tant for 3D nanocomposite solar cells because the large interfa- cial junction area increases the probability of recombination. The In 2 S 3 buffer layer has previously been shown to dramati- cally improve the junction rectification and decreases recombi- nation losses in TiO 2 /CIS nanocomposite solar cells, thereby significantly increasing conversion efficiencies. [7] In addition to controlling the interfacial properties, we have recently determined that careful control of the TiO 2 nanostruc- ture—particularly the TiO 2 particle size and layer thickness—is critical in achieving good solar-cell performance. In this paper, the influence of the TiO 2 particle size is examined. Specifically, it is found that ultrasmall (9 nm) TiO 2 particles lead to poor so- lar-cell performance while larger particles (50–300 nm) lead to better performance. By employing current–voltage (I–V), im- pedance spectroscopy, and incident-photon-to-current conver- sion efficiency (IPCE) measurements, and photocurrent/photo- voltage transient measurements, as well as physical sample characterization methods, the relationship between TiO 2 parti- cle size and solar-cell performance is analyzed. As a result of this investigation, it is shown that larger TiO 2 particles lead to better photovoltaic performance owing to greatly improved electron transport. The larger TiO 2 particles may also seconda- rily help improve performance owing to enhanced photon ab- sorption and improved interfacial characteristics. 2. Physical Characterization The fabrication of TiO 2 /CIS nanocomposite solar cells is described in detail elsewhere. [8] For more information, also consult the Experimental section at the end of this paper. To 1566 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 1566–1576 – [*] Dr. R. O’Hayre, M. Nanu, Prof. J. Schoonman, Prof. A. Goossens Delft Institute for Sustainable Energy, Delft University of Technology 2628 BL Delft (The Netherlands) E-mail: rohayre@stanford.edu Dr. Q. Wang, Prof. M. Grätzel Laboratoire de Photonique et Interfaces Ecole Polytechnique Fédérale 1015 Lausanne (Switzerland) [**] This material is based upon research supported by the National Science Foundation under Grant No. 0401817. Optical absorption measurements were acquired with the generous assistance of Ir. An- nemarie Huijser, Optoelectronic Materials Section, Faculty of Applied Sciences, TU Delft. The recently developed CuInS 2 /TiO 2 3D nanocomposite solar cell employs a three-dimensional, or “bulk”, heterojunction to reduce the average minority charge-carrier-transport distance and thus improve device performance compared to a planar con- figuration. 3D nanocomposite solar-cell performance is strongly influenced by the morphology of the TiO 2 nanoparticulate matrix. To explore the effect of TiO 2 morphology, a series of three nanocomposite solar-cell devices are studied using 9, 50, and 300 nm TiO 2 nanoparticles, respectively. The photovoltaic efficiency increases dramatically with increasing particle size, from 0.2 % for the 9 nm sample to 2.8 % for the 300 nm sample. Performance improvements are attributed primarily to greatly im- proved charge transport with increasing particle size. Other contributing factors may include increased photon absorption and improved interfacial characteristics in the larger-particle-size matrix. FULL PAPER