Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Device Characteristics of CZTSSe Thin‐Film Solar Cells with 12.6% Efficiency

The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2 . Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn +SnZn ], [VCu +ZnCu ] and [ZnSn +2ZnCu ] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn +SnZn ] clusters, especially in Cu2ZnSnS4. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.

Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers.

Similar to the way that atoms bond to form molecules and crystalline structures, colloidal nanocrystals can be combined together to form larger assemblies. The properties of these structures are determined by the properties of individual nanocrystals and by their interactions. The insulating nature of organic ligands typically used in nanocrystal synthesis results in very poor interparticle coupling. We found that various molecular metal chalcogenide complexes can serve as convenient ligands for colloidal nanocrystals and nanowires. These ligands can be converted into semiconducting phases upon gentle heat treatment, generating inorganic nanocrystal solids. The utility of the inorganic ligands is demonstrated for model systems, including highly conductive arrays of gold nanocrystals capped with Sn2S6(4-) ions and field-effect transistors on cadmium selenide nanocrystals.

http://science.sciencemag.org/content/sci/324/5933/1417.full.pdf

Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands

Beyond 11% Efficiency: Characteristics of State‐of‐the‐Art Cu2ZnSn(S,Se)4 Solar Cells

Inorganic porous materials are being developed for use as molecular sieves, ion exchangers, and catalysts, but most are oxides. We show that various sulfide and selenide clusters, when bound to metal ions, yield gels having porous frameworks. These gels are transformed to aerogels after supercritical drying with carbon dioxide. The aerogels have high internal surface area (up to 327 square meters per gram) and broad pore size distribution, depending on the precursors used. The pores of these sulfide and selenide materials preferentially absorb heavy metals. These materials have narrow energy gaps (between 0.2 and 2.0 electron volts) and low densities, and they may be useful in optoelectronics, as photocatalysts, or in the removal of heavy metals from water.

https://science.sciencemag.org/content/sci/317/5837/490.full.pdf

Porous Semiconducting Gels and Aerogels from Chalcogenide Clusters

A low band gap liquid-processed Cu2ZnSn(Se1−xSx)4 (CZTSSe) kesterite solar cell with x ≈ 0.03 is prepared from earth abundant metals, yielding 10.1% power conversion efficiency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deficit compared to our previously reported higher band gap (Eg = 1.15 eV; x ≈ 0.4) cell with similar record efficiency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examination of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) analogs.

Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency

Admittance spectra and drive-level-capacitance profiles of several high performance Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with bandgap ∼1.0–1.5 eV are reported. In contrast to the case for Cu(In,Ga)(S,Se)2, the CZTSSe capacitance spectra exhibit a dielectric freeze out to the geometric capacitance plateau at moderately low frequencies and intermediate temperatures (120–200 K). These spectra reveal important information regarding the bulk properties of the CZTSSe films, such as the dielectric constant and a dominant acceptor with energy level of 0.13–0.2 eV depending on the bandgap. This deep acceptor leads to a carrier freeze out effect that quenches the CZTSSe fill factor and efficiency at low temperatures.

Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

The p-type Cu2ZnSn(SxSe1–x)4 (with x ≈ 0; CZTSe) thin-film solar cell absorber, made from earth-abundant elements, was substituted with Ge using a hydrazine-based mixed particle-solution approach f...

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Papers

Beyond 11% Efficiency: Characteristics of State‐of‐the‐Art Cu2ZnSn(S,Se)4 Solar Cells

Porous Semiconducting Gels and Aerogels from Chalcogenide Clusters

Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency

Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

Hydrazine-Processed Ge-Substituted CZTSe Solar Cells