Solution-processed photovoltaic (PV) devices based on semiconductor nanocrystals (NCs) such as Cu 2 ZnSnS 4 (CZTS) and CuInS 2 (CIS) are attracting much attention for use in next-generation solar cells. However, the performance of NCbased devices is hindered by insulating surface-capping ligands that limit transfer/transport of charged carriers. Here, to remove surface-capping ligands (long-chain fatty amines) from NCs, we use the strong alkylating agent methyl iodide, which converts primary amines to quaternary amines that have low coordinating affinity to the NC surface. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy analyses confirm the successful removal of capping ligands from the CZTS surface after treatment with methyl iodide without changing the crystal structure of CZTS. CZTS and CIS NC-based devices treated with methyl iodide exhibit a reproducible PV response under simulated sunlight. The developed route can potentially enhance the performance of NC-based devices used in a broad range of applications.

Recently, the process by which energy is transferred from photoexcited semiconductor nanocrystals, called quantum dots (QDs), to other semiconductors has attracted much attention and has potential application in solar energy conversion (i.e., QD-sensitized solar cells). Sensitization of wide band gap polyoxometalates (POMs) to visible light by using CuInS2 QDs dispersed in an organic solution is demonstrated herein. Photoluminescence quenching and lifetime studies revealed efficient electron transfer from the CuInS2 QDs to POMs, such as SiW12 O40 and W10 O32 , that were hybridized with a cationic surfactant. CuInS2 QDs function as an antenna that absorbs visible light and supplies electrons to the POMs to enable certain photocatalytic reactions, including noble-metal-ion reduction. The photoenergy storage capabilities of the QD-POM system, in which electrons photogenerated in QDs by visible-light excitation are trapped and accommodated by POMs to form reduced POM, are also demonstrated. Electrons stored in the POM can be later discharged through reductive reactions, such as oxygen reduction, in the dark.

Polyoxometallates (POMs) have been attracting much attention as homogeneous molecular catalysts because of their excellent photocatalytic activities. However, the poor sensitivities of POMs to visible light limit their utilization of solar energy. Here, we studied photoinduced electron transfer (PET) from a Ru complex to POMs such as SiW10O36 8–, W10O32 4–, SiW12O40 4–, and PMo12O40 3– to use them for photoenergy storage and photocatalysis driven by visible light. A hydrophobic Ru complex ([Ru­(nbpy)3]2+; nbpy = 4,4′-dinoyl-2,2′-bipyridyl) was coupled with POMs that were hybridized with dioctadecyldimethylammonium (DODA) in chloroform. Photoluminescence (PL) quenching and lifetime measurements indicate that PET efficiently occurred from the Ru complex to the POMs/DODA hybrids in chloroform by excitation with visible light. The PET led to the formation of one-electron reduced POMs that store photoexcited electrons. The stored/charged electrons can be discharged in a subsequent reaction that can proceed under dark conditions. The Ru complex-POM hybrid system in chloroform was used to reduce metal ions in a water phase at a liquid/liquid interface under visible light irradiation. The one-electron reduced POM that was formed by PET could reduce metal ions to produce metal particles, suggesting the applicability of this system for photocatalytic reactions under visible light irradiation.