Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GreenChalcoCell (Green and sustainable chalcopyrite and kesterite nanocrystals for inorganic solar cells)

Teaser

\"The aim of the project was to develop strategies for mild and \"\"green\"\" synthesis of ternary chalcopyrite (Cu,Ag)InS(Se)2 and quaternary kesterite (Cu,Ag)2ZnSn(S,Se)4 nanocrystals (NCs) as visible-light-sensitive active components of the semiconductor NC-based solar cells...

Summary

\"The aim of the project was to develop strategies for mild and \"\"green\"\" synthesis of ternary chalcopyrite (Cu,Ag)InS(Se)2 and quaternary kesterite (Cu,Ag)2ZnSn(S,Se)4 nanocrystals (NCs) as visible-light-sensitive active components of the semiconductor NC-based solar cells (Fig. 1). The principal idea was in realization of low-temperature (below 100 ºC) syntheses directly in polar solvents, in particular water, using non-volatile and low-toxic sulfur and selenium sources.\"

Work performed

Size-selected series of water-soluble luminescent non-stoichiometric mercaptoacetate-stabilized silver indium sulfide and core/shell AIS/ZnS QDs were produced by the controlled precipitation techniques. Up to 10–11 fractions of size-selected AIS (AIS/ZnS) QDs were isolated differing distinctly in their optical properties and emitting in a broad color range from deep-red (fractions with larger NCs) through yellow-orange (fractions with intermediate sizes) to bluish-green (fractions with smaller sizes). The PL hue can be additionally varied by adjusting the composition of the starting crude AIS (AIS/ZnS) colloids. The PL quantum yield (QY) of size-selected core/shell AIS/ZnS and copper-doped AIS/ZnS quantum dots (Figure 2) varied among the fractions reaching up to 47% for the intermediate (yellow-orange emitting) fractions. The size-selective precipitation/redissolution can be successfully applied to glutathione (GSH) capped aqueous AIS and AIS/ZnS QDs producing broad (8–16 fractions) series of size-selected QDs with different emission colors and a maximum PL QY of 60% for the most populated fraction of the core/shell AIS/ZnS QDs which is among the highest reported for the direct aqueous synthesis of colloidal AIS QDs. We evaluated the efficiency of photoinduced electron transfer in systems comprising mesoporous titania and AIS/ZnS NCs basing on the results of photoelectrochemical and time-resolved PL measurements. We found that the rate of the photoinduced electron transfer as well as the efficiency of the photocurrent generation in the systems based on TiO2/AIS composites increase with a decrease of the AIS NC size (in the studied range of 2-4 nm) simultaneously with an expansion of the bandgap of the AIS NCs. The copper doping was found to result in an enhancement of the photoelectrochemical activity of CAIS/ZnS QDs introduced as spectral sensitizers of mesoporous titania photoanodes of a liquid-junction solar cells. The doping-induced photoactivity increment increases from 24% for the largest QDs to ~60% for the smallest QDs, the photocurrent density correlating closely with the PL QY of original colloidal CAIS/ZnS QDs.
We reported on the aqueous synthesis of ultra-small (~2 nm) copper(I)- and silver(I)-doped CdS and core/shell CdSe/CdS QDs stabilized by Cd(II) complexes with mercaptoacetate anions and ammonia. The doped QDs reveal high stability toward aggregation and oxidation, retain individual character after complete solvent removal and can be easily redispersed in dilute aqueous ammonia solutions producing concentrated luminescent “inks”.
We found that QDs with a similar size reveal the same normalized rate of vibrational relaxation for different broadband-emitting QDs, including CdS, CdSe/CdS, doped CdSe/CdS, Ag-In-S, and Cu-In-S QDs. The reported results show that the broadband PL of different metal-chalcogenide QDs can be described by a general model which does not require the assumption of charge-trapping defects participating in the radiative recombination.
We characterized the structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 NCs produced by a “green” method directly in aqueous solutions in the form of stable and concentrated “inks” with a CZTS content of up to 33 g/L. A size-selective precipitation using 2-propanol as a non-solvent was found to yield a series of around ten fractions of CZTS NCs with an average size d varying from 3 nm to 2 nm and maintaining roughly the same composition. The size-selected CZTS NCs thus obtained showed a phonon confinement effect, with the frequency position of the main phonon shifted by about 4 cm–1 between 3 nm and 2 nm NC diameters, accompanied by a narrowing of the main phonon feature. The conditions of the preparation of samples for optical studies, the nature of the support of the dried CZTS NC film, as well as the conditions of the registration of Raman spectra were found to affect crucially the spectral pro

Final results

The project resulted in an array of new mild methods of the synthesis of nanomaterials with functional properties (intense absorption and emission of light) and tailored characteristics (composition, ligand shell, size, bandgap, etc.). Basing on the project results I showed that the broadband photoluminescence (PL) is most probably an inherent property of NCs produced in mild conditions using metal complexes as surface capping agents and proposed a model explaining the spectral characteristics of light emission by such materials. These notions will be instrumental for the further progress in the understanfing of the PL properties of broadband-emitting NCs.

The results of the project can have several important impacts, including both synthesis and applications of NC materials. In particular, the project expands considerably the assortment of mild aqueous approaches to the well-defined nanomaterials thus paving a way for the green chemistry approaches in the inorganic preparative chemistry. The project provided a line of new highly luminescent NCs with no acutely toxic constituents and tailored properties, in particular the color and emission of PL, ready for various applications in the bio-sensing and light-emitting technologies.
The project provides a unified synthetic approach to the very stable concentrated aqueous inks containing NCs of a defined size and composition that can be used for the ink-jet technologies of the preparation of emissive and photovoltaic devices. This approach can potentially be expanded to a much broader array of compositions including new toxic-metal-free metal chalcogenide compounds and accelerating the progress of sustainable photonics and photovoltaics.
The project shows a feasibility of mild “green” aqueous synthesis of various ternary and quaternary chalcogenide NCs in the form of concentrated inks ready for applications both in the photovoltaic and, in some cases, as luminescent bio-markers for bio-medical sensing. The results showed applicability of the concepts of green chemistry for the needs of photovoltaics and synthetic nano-chemistry and opened new ways to functional materials with a reduced stress on the environment. The synthetic protocols produced during the project are simple and reproducible and can be used in teaching programs for students of different levels. The model solar cells with liquid electrolytes can serve the same purpose and used for laboratory works of students engaged in the studies of physical chemistry, inorganic chemistry, nanotechnologies, etc.