Ag 16 (SG) 9 Nanoclusters as a Light Harvester for Metal-Cluster-Sensitized Solar Cells Min Soo Kim, †,¶ Muhammad Awais Abbas, ‡,¶ and Jin Ho Bang †,‡,§, * † Department of Bionanotechnology, Hanyang University, Kyeonggi-do 15588, Korea. *E-mail: jbang@hanyang.ac.kr, ‡ Department of Advanced Materials Engineering, Hanyang University, Kyeonggi-do 15588, Korea § Department of Chemistry and Applied Chemistry, Hanyang University, Kyeonggi-do 15588, Korea Received December 29, 2015, Accepted February 19, 2016, Published online May 23, 2016 Keywords: Silver nanoclusters, Metal-cluster-sensitized solar cells, Light harvesting, Electron transfer Metal nanoclusters (NCs) are photoactive nanomaterials that are capable of electron transfer to semiconductor nanoparticles because of their discrete electronic structures, and these mate- rials have opened up new opportunities for third generation solar cells. Noble metal NCs of Pt, Pd, Ag, and Au have been reported to work as sensitizers in solar cells with a device structure similar to dye-sensitized solar cells (DSSCs). 1–4 With Au NCs as sensitizers, a power conversion efficiency (PCE) of 2.4% has been achieved by Kamat’s group. 1 Our group has further enhanced the PCE of NC-sensitized solar cells to 3.8% lately by selecting the optimum size of the Au NCs as sensitizers and by using an efficient redox couple as an electrolyte. 5 Ag NCs also present an interesting alternative to Au NCs because they are lower cost, but little attention has been given to this material to date partly because of their rela- tively poor chemical stability as compared to Au NCs. Recently, a lot of progress has been made in synthesizing sta- ble Ag NCs in aqueous media, 6–9 which initiated the use of Ag NCs in solar cell applications. For example, the Tatsuma group reported the use of Ag NCs as sensitizers, and they achieved a J SC of 120 μA/cm 2 . ,10 Here, we demonstrate the feasibility of Ag 16 (SG) 9 NCs as new sensitizers in solar cells with an I - /I 3 - based redox couple. The resulting solar cell showed an open-circuit voltage (V OC ) of 650 mV with a PCE of 0.26%. A J SC of 617 μA/cm 2 , which is five times higher than the previous report, was also achieved in this work. Ag 16 (SG) 9 NCs had strong absorption in the visible region with an absorption edge at 750 nm and an absorption peak at 489 nm (Figure 1(a)). The absence of a plasmonic peak implies the formation of NCs, which showed molecule-like behavior. When excited with 470 nm light, Ag 16 (SG) 9 showed a strong photoluminescence peak at 647 nm. High resolution transmission electron microscopy (TEM) images (Figure 1(b)) further confirmed the presence of very small NCs with sizes less than 2 nm. This small size is required for the formation of discrete energy levels based on the quantum confinement effect. Photoluminescence (PL) quenching is an empirical way to provide indirect evidence for electron trans- fer in semiconductor heterojunctions. Glutathione has two carboxylic functional groups, which can adsorb onto TiO 2 surfaces under acidic conditions. When a transparent TiO 2 colloidal solution was added to a Ag 16 (SG) 9 NCs solution, the PL was substantially reduced (Figure 1(c)), indicating electron transfer to TiO 2 . These results also suggested that the time for electron transfer to the TiO 2 conduction band was shorter than the radiative recombination within the NCs. PL lifetime measurements were also carried out to further confirm this inference (Figure 1(d)). Ag 16 (SG) 9 NCs showed very short PL lifetimes as compared to the reported Au NCs stabilized by glutathione. 11,12 Different lifetime components were obtained after fitting the exponential decay function to PL lifetime decay curves, and these are summarized in Table S1 in the Supporting Information. The shorter lifetime component (τ 1 ) corresponds to electron transitions inside the metal core, while the longer lifetime component (τ 2 ) origi- nates from the ligand-to-metal charge transfer. Previous investigators suggested that this longer lifetime component is responsible for the photovoltaic properties of noble metal NCs. 1,13,14 When PL lifetime measurements were performed in the presence of TiO 2 , a shorter lifetime was observed, and the contribution of τ 2 to total lifetime decreased substantially. This provides further evidence that the long-lived excited state is responsible for the electron transfer to TiO 2 . Both PL quenching and PL lifetime measurements indi- cate that the lowest unoccupied molecular orbital (LUMO) level of the Ag 16 (SG) 9 NCs lies above the conduction band position of TiO 2 (Figure 2(a)). Hence, the photo-excited electron from the highest occupied molecular orbital (HOMO) to the LUMO level in the Ag 16 (SG) 9 NCs can be injected from the LUMO level of Ag 16 (SG) 9 to the conduc- tion band of TiO 2 . Furthermore, the hole in Ag 16 (SG) 9 can be regenerated by a redox couple (Figure 2(a)). Absorption spectrum of a TiO 2 film sensitized with Ag NCs along with a digital photograph is given in Figure 2(b). The Ag 16 (SG) 9 -sensitized TiO 2 film absorbed very strongly between 400 and 600 nm. Scanning electron microscopy (SEM) image and energy-dispersive X-ray spectroscopy (EDS) spectrum along with a summary of detected ele- ments are given in Figure 2(c) and (d). The EDS spectrum showed the presence of Ag and S, which reflects the thiol groups in glutathione. This observation confirmed the pres- ence of Ag NCs on the mesoporous TiO 2 surface. ¶ These authors contributed equally to this work. Communication DOI: 10.1002/bkcs.10766 M. S. Kim et al. BULLETIN OF THE KOREAN CHEMICAL SOCIETY Bull. Korean Chem. Soc. 2016, Vol. 37, 791–792 © 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley Online Library 791