8}Ia3(H2O)a8Ia8}Ia8)La3(H2O)a8IaICYS Annual Report 20232. Research Activities1. Outline of ResearchThi Kim Ngan NGUYENReferences 1) Warnan; J., et al., Chem. Communication, 2012, 48, 675-677. 2) Cordier; S., et al., Inorg. Organomet. Polym. Mater., 2015, 25, 189−204. 3) Renaud; A., Nguyen; T.K.N., et al., Electrochimica Acta, 2019, 317, 737-745. 4) Nguyen ; T.K.N., et al., ACS Appl. Energy Mater. 7, 2, 760–773, 2024.In the dynamics of exploring new potential sensitizers, metal atom clusters were introduced as light-harvesting materials in the metal cluster-sensitized solar cells (MCSSCs). The photosensitizer plays an important role since it absorbs the light energy for supplying photo-excited electrons to generate the photocurrent. Therefore, many efforts have been made to strengthen the function of the sensitizers such as i) absorption in the visible light range which accounts for 46% of total solar light, ii) the quenching of the photoexcited electron, iii) the stabilization and regeneration of sensitizer in the redox mediator, and iv) the beneficial electron transfer and electron lifetime in the redox electrolyte1. A metal cluster family with a unique electron structure has been introduced as a broadly visible light absorber. The face-capped [(Mo6Xi6]-2 cluster (Xi = halogen, La = halogen, functional groups, H2O, etc.) was built on an octahedral Mo6 cluster that bonded with 8 inner ligands in face-capping positions and 6 apical ligands in terminal positions2. The Mo6 cluster was able to create a photoexcited electron under visible light irradiation, however, transfer for optoelectronic applications. For this reason, the incorporation of the Mo6 cluster with a semiconductor has essentially been studied to enhance the charge transfer via covalent bonds or ionic interactions.Recently, a general study revealed the photovoltaic efficiency of the Mo6 cluster as an absorber in n and p-type solar cells prepared by using an electrophoretic deposition process (EPD), resulting in the photovoltaic performance efficiency of 0.44 %3. The lack of knowledge about the photophysical and photoelectrochemical behaviors of the Mo6 cluster in the redox mediator of MCSSCs inspired the investigation of the study. By integrating such Mo6 clusters as sensitizers in n- and p-type MSSCs, a proof of concept was obtained. Such molecular clusters can be used as light harvesters to photoinduce both electron and hole transfers. However, with a short-circuit (Jsc) current of 1.13 mA.cm-2 and an open circuit (Voc) of 529 mV, the recorded performances were low3. They were attributed to a non-effective charge transfer. The and properties, photoluminescence could be modified by the nature of the inner and/or apical ligands and affected by the number of valent electrons in a cluster unit2. This study confirmed a relative relation between the chemical composition of the apical ligands of the cluster and the emission lifetime. A photosensitizer should be luminescent with a favorably long emission lifetime in a range between µs and ns, being faster than the process of charge injection (0.1 ns). With the emission lifetime in the microsecond range, the photoexcited electrons on the Mo6 cluster are able to be injected into the conduction band of the semiconductor electrode in the MCSSCs. Although the Mo6 clusters do not have an optimal absorption in the visible range when compared to porphyrin; for example, a broad absorbing spectrum, limited self-quenching of the excited state at a high concentration and low photobleaching it can compensate for these drawbacks.the material photophysical itself limited charge absorbance, The goal of this study is to introduce functional groups at the terminal positions of the Mo6 cluster for i) improving the interactions of the Mo6-based sensitizer with the semiconductor substrate, ii) improving the efficiency of electron injection by adjusting the emission lifetime of the photoexcited electrons in the suitable range, and iii) limiting the aggregation of adsorbed clusters which can quench the photoexcited electron4. Starting from the Mo6 cluster iodide precursor Cs2[{Mo6Ii6], a few iodide ligands were replaced by naphthalene 2,6-disulfonate (NS) organic molecules, composed of electron-donating naphthalene and electron-withdrawing sulfonate for delocalizing the photoinduced electron. An X-ray photoelectron analysis confirmed the chemical compositions of the as-synthesized compounds including the [Mo6Ii2(-OSO2-C10H6-SO3- Na+)a] clusters and free Na2(-OSO2-C10H6-SO3-) molecules with different constitutions that depended on the molar ratios of the Cs2[{Mo6Ii6] precursor and NS. The replacement of iodide ligands by NS molecules resulted in modification of the band gaps with the shift from 2.08 eV to 2.60 eV and increased the radiative recombination rate that was indicated by a broad peak centered around 640 nm in the photoluminescence spectra. The radiative recombination time of the modified Mo6 cluster was adjusted in the suitable range (ns < τ < µs) for photovoltaic cells, the longer than electron injection time (~ 0.1 ns), and shorter than the cluster regeneration time (~ 0.1 µs) in the electrolyte mediator. The photovoltaic properties were systematically studied to reveal the role of NS at different compositions on isolated and non-isolated Mo6 cluster-based nanocomposites. Both the [Mo6Ii2(-OSO2-C10H6-SO3- Na+)a] clusters and free Na2(-OSO2-C10H6-SO3-) molecules contributed to the enhancement of the homogeneous dispersion of the Mo6 cluster and charge transferring rate such that the power conversion efficiency (PCE = 0.97 %) increased by a factor of 2 compared to a previous study (0.44 %)3. This is a new record of efficiency for Mo6-based cluster-sensitized solar cells. The Mo6 cluster-based cells resulted in an impressive accumulation of the photocurrent while the NS at a good concentration can retain the enhanced photocurrent in the MINS-based cells4. With regard to this, an increase in PCE by about 20 % after the first 5 minutes of the AM 1.5G illumination was recorded until it reached saturation. These promising results open the door for the investigation of the octahedral transition metal clusters as a new nontoxic light harvester for photoelectrochemical devices ranging from sensitizer solar cells to solid solar cells. Research Digest 19Photovoltaic Performance of Molybdenum Cluster-Sensitized Solar Cell
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