The diffraction limit of light, which is causd by the loss of evanescent waves in the far field that carry high spatial frequency information, limits the resolution of optical lenses to the order of the wavelength of light. We report experimental demonstration of the optical hyperlens for sub-diffraction-limited imaging in the far field. The device magnifies subwavelength objects by transforming the scattered evanescent waves into propagating waves in an anisotropic medium and projects the high-resolution image at far field. The optical hyperlens opens up possibilities in applications such as real-time biomolecular imaging and nanolithography.

https://science.sciencemag.org/content/sci/315/5819/1686.full.pdf

Far-field optical hyperlens magnifying sub-diffraction-limited objects.

Simple rules for the understanding of Heusler compounds

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

Sulfide kesterite Cu2ZnSnS4 provides an attractive low-cost, environmentally benign and stable photovoltaic material, yet the record power conversion efficiency for such solar cells has been stagnant at around 9% for years. Severe non-radiative recombination within the heterojunction region is a major cause limiting voltage output and overall performance. Here we report a certified 11% efficiency Cu2ZnSnS4 solar cell with a high 730 mV open-circuit voltage using heat treatment to reduce heterojunction recombination. This heat treatment facilitates elemental inter-diffusion, directly inducing Cd atoms to occupy Zn or Cu lattice sites, and promotes Na accumulation accompanied by local Cu deficiency within the heterojunction region. Consequently, new phases are formed near the hetero-interface and more favourable conduction band alignment is obtained, contributing to reduced non-radiative recombination. Using this approach, we also demonstrate a certified centimetre-scale (1.11 cm2) 10% efficiency Cu2ZnSnS4 photovoltaic device; the first kesterite cell (including selenium-containing) of standard centimetre-size to exceed 10%. The emerging kesterite Cu2ZnSnS4 solar cell offers a potential low-cost, non-toxic, materially abundant platform for next-generation photovoltaics, yet its efficiency has been mired below 10%. Yan et al. now use post-heat treatment of the heterojunction to show device efficiencies that surpass 10%.

Cu 2 ZnSnS 4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment

Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSn(SxSe1−x)4, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells

We have synthesized off-stoichiometric Ti-Ni-Sn half-Heusler thermoelectrics in order to investigate the relation between randomly distributed defects and thermoelectric properties. A small change in the composition of Ti-Ni-Sn causes a remarkable change in the thermal conductivity. An excess content of Ni realizes a low thermal conductivity of 2.93 W/mK at room temperature while keeping a high power factor. The low thermal conductivity originates in the defects generated by an excess content of Ni. To investigate the detailed defect structure, we have performed first-principles calculations and compared with x ray photoemission spectroscopy measurement. Based on these analyses, we conclude that the excess Ni atoms randomly occupy the vacant sites in the half-Heusler structure, which play as phonon scattering centers, resulting in significant improvement of the figure of merit without any substitutions of expensive heavy elements, such as Zr and Hf.

Improvement of thermoelectric properties for half-Heusler TiNiSn by interstitial Ni defects

In Cu2ZnSnS4 (CZTS) photovoltaic cells, a low open-circuit voltage (VOC) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the VOC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the VOC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%.

https://iopscience.iop.org/article/10.7567/APEX.8.082302/pdf

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

Hirofumi Hazama

Papers

Improvement of thermoelectric properties for half-Heusler TiNiSn by interstitial Ni defects

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

Direct measurement of band offset at the interface between CdS and Cu2ZnSnS4 using hard X-ray photoelectron spectroscopy

Cylindrical thermoelectric generator with water heating system for high solar energy conversion efficiency

First-principles calculations of Ag-Sb nanodot formation in thermoelectric Ag Pb m Sb Te 2 + m ( m = 6 , 14 , 30 )