Discovery of a Wurtzite-like Cu2FeSnSe4 Semiconductor Nanocrystal Polymorph and Implications for Related CuFeSe2 Materials.

Abstract

I2-II-IV-VI4 and I-III-VI2 semiconductor nanocrystals have found applications in photovoltaics and other optoelectronic technologies because of their low toxicity and efficient light absorption into the near-infrared. Herein, we report the discovery of a metastable wurtzite-like polymorph of Cu2FeSnSe4, a member of the I2-II-IV-VI4 family of semiconductors containing only earth-abundant metals. Density functional theory calculations on this metastable polymorph of Cu2FeSnSe4 indicate that it may be a superior semiconductor for solar energy and optoelectronics applications compared to the thermodynamically preferred stannite polymorph, since the former displays a sharper dispersion of energy levels near the conduction band minimum that can enhance electron mobility and suppress hot electron cooling. The experimental optical band gap was measured by the inverse logarithmic derivative method to be direct, in agreement with theory, and in the range of 1.48-1.59 eV. Mechanistic studies reveal that this metastable phase derives from intermediate Cu3Se2 nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu2FeSnSe4 to the related CuFeSe2 material system. Our experimental and computational comparisons between Cu2FeSnSe4 and CuFeSe2 help explain both the crystal chemistry of CuFeSe2 that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu2FeSnSe4. This work provides insight into the importance of elemental composition when designing syntheses for metastable materials.