Correlation between Photoinduced Electron Transfer and Photovoltaic Characteristics in Solar Cells Based on Hybrid Core-Shell Nanoparticles Asim Guchhait and Amlan J. Pal* Department of Solid State Physics, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed: August 31, 2010; ReVised Manuscript ReceiVed: September 29, 2010 We study the correlation between photoinduced electron transfer and performance of photovoltaic devices. We have considered hybrid core-shell nanoparticles, where such an electron transfer occurs from an inorganic core to an organic shell layer. From photoluminescence (PL) quenching and decrease in PL lifetime, we find that the rate of photoinduced electron transfer depends on the electron-accepting nature of the organic xanthene molecule on the shell layer. Devices based on such hybrid nanoparticles with CdS in the core and different dye molecules on the shell exhibit photovoltaic characteristics. We find that photoinduced electron transfer leads to exciton dissociation followed by photocurrent in the external circuit of the devices. Short-circuit current of the devices depends on the degree of electron transfer. We report a one-to-one correlation between external quantum efficiency of photovoltaic devices and rate of photoinduced electron transfer in hybrid core-shell nanoparticles. 1. Introduction Semiconducting quantum dots were introduced in donor/ acceptor-type organic or polymeric photovoltaic devices for several reasons. In many of the II-VI quantum dots, ultrafast carrier multiplication occurs. That is, several excitons are generated at the expense of one photon, reducing the loss in energy as heat. 1 Quantum dots can also act favorably in exciton dissociation through an ultrafast photoinduced electron-transfer process. The electron transfer may occur from a quantum dot to another dot 2,3 or to an organic molecule. 4 The third advantage that the quantum dots provide in photovoltaic devices is in carrier transport. To facilitate particle-to-particle carrier trans- port, insulating surfactants of quantum dots have been removed through use of weak binding ligands followed by heating. 5 As the interdot coupling is increased in such a course of action, the insulating film turns into a semiconductor, where hopping conduction is a dominant transport mechanism. 6 The semicon- ducting quantum dots hence augment each of the three steps of device operation, (1) exciton generation, (2) exciton dissociation, and (3) carrier transport, which occur in sequence in photovoltaic solar cells. Exciton dissociation in donor/acceptor-type hybrid hetero- structures or bulk-heterojunctions occurs due to type-II band-offset. 7-11 In such a system, both highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of an organic electron donor are above valence- and conduction-band edges of an inorganic electron- acceptor material, respectively. Excitons generated in both components (i.e., donor and acceptor) hence become dissociated. With an assistance from the internal field that arises due to dissimilar work function of the two metal electrodes, carrier separation occurs leading to photocurrent in the external circuit. An additional route of exciton dissociation in hybrid photovoltaic devices based on hybrid core-shell nanoparticles can be through photoinduced electron transfer. 12 In hybrid core-shell nanoparticles, photoinduced electron transfer occurs from an inorganic core to a monolayer of organic shell layer. In suitable systems, electron transfer occurs at an ultrafast time scale (2 ps), which is faster than the exciton annihilation time in quantum dots. 4 In such systems, competition between energy transfer and photoinduced electron transfer is another issue that has been studied by time-resolved fluorescence decay, steady-state emission, and transient absorption measure- ments. 13,14 In hybrid core-shell quantum dots, photoinduced electron transfer has been ascertained through quenching of photoluminescence (PL) emission and a decrease in PL lifetime. 4 It can hence be interesting to know if a correlation exists between characteristics of photovoltaic devices based on hybrid core-shell nanoparticles and the electron-transfer process in the nanostructures. We have chosen a series of hybrid core-shell systems with different organic molecules on the shell that may dictate the degree or quantum of photoinduced electron-transfer process. We have fabricated photovoltaic devices on the basis of such core-shell nanoparticles; characteristics of the devices have been studied in relation to photoinduced electron transfer from the inorganic core to the organic shell layer. 2. Experimental Section 2.1. Growth and Characterization of CdS Nanoparticles. CdS nanoparticles were grown following standard methods. 15 In brief, 0.256 g (2.0 mM) of CdO, 1.6 g (6 mM) of oleic acid, and 9 mL of 1-octadecene were mixed in a 50 mL three-neck flask and heated to 250 °C under continuous flow of nitrogen. When CdO was dissolved completely, the solution was allowed to cool to 220 °C. In a separate vial, 32 mg of sulfur was dissolved in 2 mL of 1-octadecene (60-70 °C) to prepare a stock solution. 2.6 g (10 mM) of octadecylamine (ODA) was added to the stock solution at this temperature. This mixed solution was then swiftly injected to the reaction flask at 220 °C. After the reaction was allowed to continue for 4 min, the temperature of the flask was rapidly cooled to 70 °C. CdS nanoparticles were extracted by repeated precipitation in chloroform/acetone followed by centrifuge at 8000 rpm. Excess * Corresponding author. Phone: +91-33-24734971. Fax: +91-33- 24732805. E-mail: sspajp@iacs.res.in. J. Phys. Chem. C 2010, 114, 19294–19298 19294 10.1021/jp108256t  2010 American Chemical Society Published on Web 10/21/2010