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

Abstract

I 2 −II−IV−VI 4 and I−III−VI 2 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 Cu 2 FeSnSe 4 , a member of the I 2 −II−IV−VI 4 family of semiconductors containing only earth-abundant metals. Density functional theory calculations on this metastable polymorph of Cu 2 FeSnSe 4 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 Cu 3 Se 2 nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu 2 FeSnSe 4 to the related CuFeSe 2 material system. Our experimental and computational comparisons between Cu 2 FeSnSe 4 and CuFeSe 2 help explain both the crystal chemistry of CuFeSe 2 that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu 2 FeSnSe 4 . This work provides insight into the importance of elemental composition when designing syntheses for metastable materials.

Formation Pathway of Wurtzite-like Cu2ZnSnSe4 Nanocrystals

“…Cu 3 Se 2 nanocrystals (space group P4̅ 2 1 m) possess a nearly hexagonally close-packed framework of Se 2− that can template the formation of ternary or quaternary nanocrystalline products via fast cation exchange reactions. 25,29 Herein, when the tin and zinc precursors were hot-injected, the Cu 3 Se 2 nanocrystals converted into quaternary hexagonal intermediates (3 and 5 min aliquots, Figure 2a,b), similar to those observed at low temperatures in Figure 1. The hexagonally close-packed structure of these intermediates is indicated by the persistence of reflections that originate from the hexagonal selenium sublattice of Cu 3 Se 2 .…”

“…At lower temperatures and earlier times in the reaction, the nanocrystalline intermediates produce diffraction patterns that are not easily indexed to any well-defined crystalline compound but are nearly identical to diffraction patterns that have been observed during reactions of Cu 3 Se 2 nanocrystals with tin and iron. 25 Thus, from the results shown in Figure 1, we hypothesized that Cu 3 Se 2 may be the initial transient binary intermediate that generates wurtzite-like Cu 2 ZnSnSe 4 . We postulated that the observation of well-defined Cu 3 Se 2 intermediate nanocrystals by XRD is precluded by fast reactions of Cu 3 Se 2 with tin and zinc since both are present from the onset of nanocrystal nucleation.…”

“…Furthermore, we demonstrated that the reaction of Ph 2 Se 2 with Cu(oleate) 2 in oleylamine yields the requisite metastable Cu 3 Se 2 intermediate at moderate temperatures (i.e., ∼220 °C). 25,26 Knowing this, we sought to establish the formation pathway of wurtzite-like Cu 2 ZnSnSe 4 nanocrystals within a similar synthetic framework with diselenide precursors. Therefore, we adapted our base synthesis of Cu 2 ZnSnSe 4 from the synthesis reported by Wang et al 17 Here, we combined stoichiometric amounts of Cu(oleate) 2 , Zn(oleate) 2 , Sn(ethylhexanoate) 2 , and Ph 2 Se 2 in oleylamine and heated the reaction to 255 °C, removing aliquots as they were heated and after they reached the target temperature.…”

“…17,19 Such compositional tunability has also been used to tune the low-temperature photoluminescence of wurtzitelike CZTSSe 21 and may also be important in tuning the type of concentration of defects, which significantly affect solar cell performance; the champion efficiency CZTSSe solar cells were found to be slightly Zn-rich and Cu-poor; such Cu-poor stoichiometries are known to introduce a higher concentration of copper vacancies (V Cu ), which gives way to improved p-type conductivity in CZTSSe. 11 There is also mounting evidence for various tetrahedral semiconductors, including CZTSSe, 22,23 CuInSe 2 , 24 and Cu 2 FeSnSe 4 , 25 where alternate wurtzite-like phases exist that have superior electronic band structures for harvesting photoexcited electrons in solar cells and photocatalysts as compared to the polymorphs adopted in the bulk of these materials. Given the potential benefits of employing wurtzite-like CZTSSe nanocrystals in solar cells and considering the sensitivity of the properties of this material family to changes in composition, defect concentration, and/or crystalline structure, a detailed formation pathway of wurtzite-like Cu 2 ZnSn(S 1−x Se x ) 4 could provide insight into how best to tune the synthesis of this material for solar applications.…”

“…There is also mounting evidence for various tetrahedral semiconductors, including CZTSSe, , CuInSe 2 , and Cu 2 FeSnSe 4 , where alternate wurtzite-like phases exist that have superior electronic band structures for harvesting photo-excited electrons in solar cells and photocatalysts as compared to the polymorphs adopted in the bulk of these materials. Given the potential benefits of employing wurtzite-like CZTSSe nanocrystals in solar cells and considering the sensitivity of the properties of this material family to changes in composition, defect concentration, and/or crystalline structure, a detailed formation pathway of wurtzite-like Cu 2 ZnSn­(S 1– x Se x ) 4 could provide insight into how best to tune the synthesis of this material for solar applications.…”