Regulation of Zn/Sn ratio in kesterite absorbers to boost 10% efficiency of Cu_2ZnSn(S,Se)_4 solar cells

The Zn/Sn ratio in Cu2ZnSn（S, Se）4 （CZTSSe） films has been regulated to control the composition-related phase, defect, and photoelectric properties for high performance kesterite solar cells. It is found that the increase in the Zn/Sn ratio can slightly narrow the energy band gap to extend the light absorption range and improve the photocurrent. Optimal Zn/Sn ratio of 1.39 in CZTSSe film is obtained with the least secondary phase, the lowest defect density, and the longest charge recombination lifetime. Up to 10.1% photoelectric conversion efficiency has been achieved by this composition regulation.

Precisely tuning Ge substitution for efficient solution-processed Cu_2ZnSn(S,Se)_4 solar cells

The kesterite Cu2ZnSn（S,Se）4 （CZTSSe） solar cells have yielded a prospective conversion efficiency among all thin- film photovoltaic technology. However, its further development is still hindered by the lower open-circuit voltage （Voc）, and the non-ideal bandgap of the absorber is an important factor affecting this issue. The substitution of Sn with Ge provides a unique ability to engineer the bandgap of the absorber film. Herein, a simple precursor solution approach was successfully developed to fabricate Cu2Zn（SnyGel_y）（SxSe~ x）4 （CZTGSSe） solar cells. By precisely adjusting the Ge content in a small range, the Voc and Jsc are enhanced simultaneously. Benefitting from the optimized bandgap and the maintained spike structure and light absorption, the 10% Ge/（Ge＋Sn） content device with a bandgap of approximately 1.1 eV yields the highest efficiency of 9.36%. This further indicates that a precisely controlled Ge content could further improve the cell performance for efficient CZTGSSe solar cells.

Solvent-free Synthesis of Hexagonal Iron Sulfide Nanoflowers

Employing green and economic solvent-free synthesis route, hexagonal iron sulfide （Fe7Ss） nanoflowers were successfully synthesized for the first time. In the experiment, ferric hexadecylxanthate was used as the precursor, and hexagonal iron sulfide （Fe7Ss） nanoflowers were obtained by thermal decomposition of the precursor at 260 ~C without any additional solvent or inert gas protection. The as-prepared iron sulfide nanoflowers were characterized by means of X-ray diffraction （XRD）, transmission electron microscopy （TEM）, scanning electron microscopy （SEM） and energy dispersive spectrometry （EDS）. The characterization results indicated that the nanoflowers had uniform size distribution with an average size of about 160 rim. The proposed strategy provides a possible general route for the synthesis of other metal chalcogenide nanostructures.