Abstract
A facile hydrothermal method to synthesize water-soluble copper indium sulfide (CIS) nanocrystals (NCs) at 150 °C is presented. The obtained samples exhibited three distinct photoluminescence peaks in the red, green and blue spectral regions, corresponding to three size fractions, which could be separated by means of size-selective precipitation. While the red and green emitting fractions consist of 4.5 and 2.5 nm CIS NCs, the blue fraction was identified as in situ formed carbon nanodots showing excitation wavelength dependent emission. When used as light absorbers in quantum dot sensitized solar cells, the individual green and red fractions yielded power conversion efficiencies of 2.9% and 2.6%, respectively. With the unfractionated samples, the efficiency values approaching 5% were obtained. This improvement was mainly due to a significantly enhanced photocurrent arising from complementary panchromatic absorption.
Highlights
In the last decade, colloidal ternary chalcopyrite nanocrystals (NCs), such as CuInS2 (CIS) and CuInSe2, emerged as promising materials for photovoltaic applications
Hydrothermal synthesis is demonstrated to be a method with a high potential for the simple, cost-efficient and eco-friendly fabrication of aqueous CuInS2 NCs suitable for light emission and photovoltaic applications
While the red and green fractions correspond to copper-poor CIS NCs of different size and composition, the third fraction consists of carbon nanodots emitting in the blue region with a high quantum yield, formed as an unexpected side-product of the reaction
Summary
Colloidal ternary chalcopyrite nanocrystals (NCs), such as CuInS2 (CIS) and CuInSe2, emerged as promising materials for photovoltaic applications. Organic synthesis of CIS NCs has several drawbacks, such as the use of solvents, high temperatures, requirement of inert conditions, and most importantly, the inevitable presence of long alkyl chain surface ligands to ensure steric stabilization of the colloids. These ligands considerably impact electron transfer processes from the NCs to the TiO2 photoelectrode in QDSSCs. These ligands considerably impact electron transfer processes from the NCs to the TiO2 photoelectrode in QDSSCs To circumvent this problem, the initial ligands should be replaced by shorter ones prior to their utilization in QDSSCs. To circumvent this problem, the initial ligands should be replaced by shorter ones prior to their utilization in QDSSCs This step generally involves switching from steric stabilization to electrostatic repulsion, achieved for example by transferring the NCs from the organic to the aqueous phase using mercaptocarboxylic acids. Several examples of CIS nanocrystals synthesized in water and used in QDSSCs exist [10,17,19,27,28,29] with a current record efficiency of 8.15% (Table S2) [14]
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