CdSe Quantum Dot-Sensitized TiO 2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment Ne ´stor Guijarro, † Teresa Lana-Villarreal, †, * Iva ´n Mora-Sero ´, ‡ Juan Bisquert, ‡ and Roberto Go ´mez †, * Institut UniVersitari d’Electroquı ´mica i Departament de Quı ´mica Fı ´sica, Apartat 99, E-03080 Alacant, Spain, and Departament de Fı ´sica, UniVersitat Jaume I, Castello ´, Spain ReceiVed: September 11, 2008; ReVised Manuscript ReceiVed: January 9, 2009 We have investigated the sensitization of nanoporous titanium dioxide by previously synthesized CdSe quantum dots (QDs) protected with trioctylphosphine. Covering the nanoporous TiO 2 films with QDs has been achieved using two strategies: (i) direct adsorption from dichoromethane dispersions and (ii) anchoring the QDs through a molecular linker, concretely, mercaptopropionic acid (MPA). In contrast with MPA-mediated adsorption, direct adsorption leads to a high degree of QD aggregation, as revealed by atomic force microscopy (AFM) images obtained with both TiO 2 nanoporous films and monocrystalline surfaces. Importantly, at saturation, only 14% of the real surface area of a 5-μm thick P25 TiO 2 layer is covered for both attachment modes. For MPA attachment, the incident photon-to-current efficiency (IPCE) increases with the loading, whereas a maximum (close to 40% at the QD excitonic peak) is defined for intermediate coverages in the case of QD direct adsorption. In addition, for equivalent QD loading, IPCE values are larger in the case of direct adsorption. Introduction Dye-sensitized solar cells based on mesoscopic wide band gap semiconductors have the potential advantages of lower cost production and versatility in comparison to the conventional solid-state cells. 1 The photoanode in such cells is constituted by a nanoporous TiO 2 layer sensitized to the visible radiation by an adsorbed dye. Instead of using dyes, the sensitization of TiO 2 nanoporous electrodes can be achieved through modifica- tion of the oxide with quantum dots (QDs) of low band gap semiconducting materials. The use of QDs enables band gap tuning through control of the QD size, which allows one to adjust both light absorption and the energetics at the interfaces of the QD with the surrounding media (hole and electron transporting materials). In addition, QD solar cells could benefit from both large QD extinction coefficients and the multiple exciton generation phenomenon, which should lead to an enhancement of the conversion efficiency in solar cells. 2,3 QDs thus have the potential to overcome the energy loss of highly energetic photons caused by carrier thermalization in conven- tional solar cells. The initial works utilized different quantum-sized sulfides to sensitize TiO 2 nanoporous electrodes. 4,5 Many systems have been studied since then, like those based on the following QD sensitizers, in which cadmium chalcogenides play a central role: CdS, 4-10 PbS, 5,11-13 InP, 14 InAs, 15 CdSe, 16-25 and CdTe. 26 In the case of CdSe QDs, several aspects have been analyzed, such as cosensitization with a dye, 17 TiO 2 particle size and shape 21,23,24 and the influence of surface modification of the QDs with either fluoride or ZnS. 23,27 Very recently, the effect of the QD diameter has also been examined. 20 On the other hand, combining both CdSe QD sensitization and nitrogen doping of the TiO 2 matrix has been proposed as an effective and promising way to enhance the response of the photoanode. 28 One of the subjects investigated recently is the type of attachment of the QDs to the oxide matrix. In fact, one of the reasons leading to the poor efficiency of QD-sensitized solar cells is the difficulty of linking the QDs to the mesoporous TiO 2 matrix to obtain a full monolayer on the TiO 2 surface. In most of the reported works, QDs were grown directly onto the nanoporous matrix by chemical bath deposition. Although a direct contact between the oxide and the QDs is achieved in this way, there is no separate control of QD coverage and size. In addition, the deposits could be far from stoichiometric because of, for instance, the possible formation of elemental layers in addition to the sought compound particles. These drawbacks can be avoided if the QDs are synthesized previously, and, later, the oxide layer is modified with them. 14-16,20,25,26,28 In most cases, the attachment of the QDs to the oxide is achieved by using a linker, which is a bifunctional molecule that anchors the QD to the oxide particle, acting as a molecular cable. Different molecular linkers have been inves- tigated, and it has been recognized that the chemical nature of the linker plays a decisive role in determining the efficiency of electron injection into the matrix. Kamat and co-workers reported that mercaptopropionic acid (MPA) is a better linker than thiolacetic or mercaptohexadecanoic acid. 20 In a similar way, we very recently showed that using cysteine as a linker gives rise to more efficient photoanodes than using thioglycolic acid or MPAs. 24 In this paper we compare the behavior of CdSe QD-sensitized TiO 2 electrodes in which the QDs are attached to the oxide either through the use of a linker, such as MPA, or by direct adsorption of the capped QDs. In each case, the dependence of the IPCE on the QD loading is analyzed. The deleterious effect of QD aggregation is evidenced. Experimental Section CdSe QDs, capped with trioctylphosphine (TOP), were prepared by a solvothermal route that permits size control. 29 * Corresponding authors. E-mail: Roberto.Gomez@ua.es; Teresa.Lana@ ua.es; fax: +34 965903537; phone: +34 965903536. † Universitat d’Alacant. ‡ Universitat Jaume I. J. Phys. Chem. C 2009, 113, 4208–4214 4208 10.1021/jp808091d CCC: $40.75  2009 American Chemical Society Published on Web 02/16/2009