Achieving Ferroelectricity in a Centrosymmetric High-Performance Semiconductor by Strain Engineering.

Phase engineering by strain in 2D semiconductors is of great importance for a variety of applications. Here, a study of the strain-induced ferroelectric (FE) transition in bismuth oxyselenide (Bi2 O2 Se) films, a high-performance (HP) semiconductor for next-generation electronics, is presented. Bi2 O2 Se is not FE at ambient pressure. At a loading force of ≳400 nN, the piezoelectric force responses exhibit butterfly loops in magnitude and 180° phase switching. By carefully ruling out extrinsic factors, these features are attributed to a transition to the FE phase. The transition is further supported by the appearance of a sharp peak in optical second-harmonic generation under uniaxial strain. In general, solids with paraelectrics at ambient pressure and FE under strain are rare. The FE transition is discussed using first-principles calculations and theoretical simulations. The switching of FE polarization acts as a knob for Schottky barrier engineering at contacts and serves as the basis for a memristor with a huge on/off current ratio of 106 . This work adds a new degree of freedom to HP electronic/optoelectronic semiconductors, and the integration of FE and HP semiconductivity paves the way for many exciting functionalities, including HP neuromorphic computing and bulk piezophotovoltaics.

An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics.

In lead-halide perovskites, antibonding states at the valence band maximum (VBM)-the result of Pb 6s-I 5p coupling-enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4s-Se 4p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~1012 cm-3. We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m-2; and 60 thermal cycles from -40 to 85 °C.

High-throughput screening and classification of layered di-metal chalcogenides.

Through employing the layered-crystal determination program based on the topology-scaling algorithm, 450 MmNnXx (M, N = metal elements, X = S, Se) layered di-metal chalcogenides (LDCs) are identified from the 1 602 011 crystalline materials known in two Material Genome databases (Materials Project and OQMD). Their structures are classified into three types, 104 compounds in standard MmNnXx homo-layered structures, 34 in MmXx1/NnXx2 hetero-layered structures and 312 in large-cation M-intercalated NnXx layered structures. 312 cation-intercalated LDCs are mostly composed of large cations such as K, Rb, Cs, Ba and Tl, and are not easy to be exfoliated into few-layer 2-dimensional (2D) structures because of the strong ionic bonding between the large cations and the negatively charged layers. In contrast, the homo-layered and hetero-layered structures may be exfoliated into stable few-layer 2D structures due to the weak inter-layer van der Waals interaction. The band structure screening identifies 34 direct- and 108 indirect-band-gap layered semiconductors from the 450 LDCs, and 24 of them have small in-plane effective masses and thus high mobility of hole or electron carriers. Two stable, direct-band-gap and high-mobility mono-layer semiconductors MgAl2S4 and ZnIn2S4 are found from group II-III2-VI4 LDCs. Furthermore, 83 LDCs composed of the magnetic metal elements are found, which provides new platforms for the search of 2D magnetic crystals similar to Cr2Ge2Te6 with intrinsic ferromagnetism. This work extends the search of layered materials from metal dichalcogenides to ternary chalcogenides and can serve as a map for the future discovery of novel 2D semiconductors and magnetic materials.

GeSe Thin-Film Solar Cells Fabricated by Self-Regulated Rapid Thermal Sublimation.

GeSe has recently emerged as a promising photovoltaic absorber material due to its attractive optical and electrical properties as well as earth-abundant and low-toxic constituent elements. However, no photovoltaic device has been reported based on this material so far, which could be attributed to the inevitable coexistence of phase impurities Ge and GeSe2, leading to detrimental recombination-center defects and seriously degrading the device performance. Here we overcome this issue by introducing a simple and fast (4.8 µm min-1) rapid thermal sublimation (RTS) process designed according to the sublimation feature of the layered structured GeSe. This new method offers a compelling combination of assisting raw material purification to suppress deleterious phase impurities and preventing the formation of detrimental point defects through congruent sublimation of GeSe, thus providing an in situ self-regulated process to fabricate high quality polycrystalline GeSe films. Solar cells fabricated following this process show a power conversion efficiency of 1.48% with good stability. This preliminary efficiency and high stability, combined with the self-regulated RTS process (also extended to the fabrication of other binary IV-VI chalcogenide films, i.e., GeS), demonstrates the great potential of GeSe for thin-film photovoltaic applications.

Polytypic Nanocrystals of Cu-Based Ternary Chalcogenides: Colloidal Synthesis and Photoelectrochemical Properties.

Heterocrystalline polytype nanostructured semiconductors have been attracting more and more attention in recent years due to their novel structures and special interfaces. Up to now, controlled polytypic nanostructures are mostly realized in II-VI and III-V semiconductors. Herein, we report the synthesis and photoelectrochemical properties of Cu-based ternary I-III-VI2 chalcogenide polytypic nanocrystals, with a focus on polytypic CuInS2 (CIS), CuInSe2 (CISe), and CuIn(S0.5Se0.5)2 alloy nanocrystals. Each obtained polytypic nanocrystal is constructed with a wurtzite hexagonal column and a zinc blende/chalcopyrite cusp, regardless of the S/Se ratio. The growth mechanisms of polytypic CIS and CISe nanocrystals have been studied by time-dependent experiments. The polytypic nanocrystals are solution-deposited on indium-tin oxide glass substrate and used as a photoelectrode, thus showing stable photoelectrochemical activity in aqueous solution. Density functional theory calculation was used to study the electronic structure and the band gap alignment. This versatile synthetic method provides a new route for synthesis of novel polytypic nanostructured semiconductors with unique properties.