Main-Group Halide Semiconductors Derived from Perovskite: Distinguishing Chemical, Structural, and Electronic Aspects.

Main-group halide perovskites have generated much excitement of late because of their remarkable optoelectronic properties, ease of preparation, and abundant constituent elements, but these curious and promising materials differ in important respects from traditional semiconductors. The distinguishing chemical, structural, and electronic features of these materials present the key to understanding the origins of the optoelectronic performance of the well-studied hybrid organic-inorganic lead halides and provide a starting point for the design and preparation of new functional materials. Here we review and discuss these distinguishing features, among them a defect-tolerant electronic structure, proximal lattice instabilities, labile defect migration, and, in the case of hybrid perovskites, disordered molecular cations. Additionally, we discuss the preparation and characterization of some alternatives to the lead halide perovskites, including lead-free bismuth halides and hybrid materials with optically and electronically active organic constituents.

Temperature-Dependent Polarization in Field-Effect Transport and Photovoltaic Measurements of Methylammonium Lead Iodide.

While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid organic-inorganic perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI3) as the active material and conducted a series of temperature-dependent measurements. Field-effect transistor, capacitor, and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temperature-dependent polarization susceptibility in (MA)PbI3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI3 photovoltaic cells is observed to reduce drastically with decreasing temperature, suggesting that such polarization effects could be a prerequisite for high-performance device operation.

Synthesis, crystal structure and optical properties of Na2RE(PO4)(WO4) (RE = Y, Tb-Lu).

Phase-pure samples of sodium rare-earth phosphate tungstates Na(2)RE(PO(4))(WO(4)) (RE = Y, Dy-Lu) and Na(2)Y(PO(4))(WO(4)):Ln(3+) (Ln = Eu, Tb) were obtained by reaction of the respective rare-earth oxide with ammonium hydrogen phosphate with sodium tungstate Na(2)WO(4)·2H(2)O as a flux at 1220 K. According to X-ray single-crystal investigations Na(2)RE(PO(4))(WO(4)) (RE = Y, Tb-Lu) crystallise orthorhombically in space group Ibca (no. 73) (RE = Y, Z = 8, a = 1799.7(4), b = 1210.2(2), c = 683.82(14) pm, wR(2) = 0.040, R(1) = 0.037, 661 reflections, 62 parameters). The crystal structure contains non-condensed phosphate and tungstate tetrahedra with the (3 + 3)-coordinate sodium and eight-coordinate rare-earth ions in the voids. Relevant absorption and infrared spectra as well as the fluorescence spectra of Na(2)Y(PO(4))(WO(4)):Ln(3+) (Ln = Eu, Tb) are presented. An FP-LAPW band-structure calculation confirms the assignment of the main absorption as well as the optical band-gap (ε(calc) = 5.1 eV; ε(meas) = 5.2(1) eV) of Na(2)Y(PO(4))(WO(4)).

CCl3(+) and CBr3(+) salts with the [Al(OR(F))4]- and [((F)RO)3Al-F-Al(OR(F))3]- anions (R(F) = C(CF3)3).

The CX3(+) salts [CCl(3)](+)[Al(OR(F))(4)](-)1, [CCl(3)](+)[(R(F)O)(3)Al-F-Al(OR(F))(3)](-)2, [CBr(3)](+)[Al(OR(F))(4)](-)3, [CBr(3)](+)[(R(F)O)(3)Al-F-Al(OR(F))(3)](-)4 (R(F) = C(CF(3))(3)) were prepared in 56 to 85% yield from CX(4) (X = Cl, Br) and the corresponding silver salts (weight balance, NMR, IR, X-ray structure of 1). The most convenient solvent for the preparation of 1 and 2 is SO(2)ClF but for 3 and 4 it is SO(2). The reactions are complete after about three days stirring at -30 to -40 °C. The salts are stable for weeks in solution at -40 °C and stable for a few hours at RT in the solid state. In SO(2)ClF (1, 2) or SO(2) (3, 4) solution they decompose slowly at -20 °C and within several hours at RT; in general the CBr3(+) salts are more stable than the CCl3(+) homologues. The decomposition products were assigned as CCl(3)F and primarily CBr(2)F(2) (which likely forms as a Lewis acid induced disproportionation product of the initial CBr(3)F). The C-X vibrations of the salts were found in the expected range and the assignments were made based on experimental and calculated data. The IR spectrum of a CBr3(+) salt is for the first time reported here.

Synthesis, structure, bonding, and properties of Sc(3)Al(3)O(5)C(2) and ScAl(2)ONC-unique compounds with ordered distribution of anions and cations.

Single crystals of the new compounds Sc(3)Al(3)O(5)C(2) and ScAl(2)ONC were obtained by reacting Sc(2)O(3) and C in an Al-melt at 1550 degrees C. Their crystal structures continue the row of transition metal oxide carbides with an ordered distribution of anions and cations with ScAlOC as the first representative. In the structure of Sc(3)Al(3)O(5)C(2) (P6(3)/mmc, Z = 2, a = 3.2399(8) A, c = 31.501(11) A, 193 refl., 23 param., R(1)(F) = 0.024, wR(2)(I) = 0.058) the anions form a closest packing with five layers of oxygen separated by two layers of carbon atoms. Sc is placed in octahedral voids and Al in tetrahedral voids thus forming layers of AlOC(3) tetrahedra and ScC(6)- and ScO(6)-octahedra, respectively. Surprisingly the layers of ScO(6) octahedra are connected by an additional layer of undistorted trigonal bipyramids AlO(5). The structure of ScAl(2)ONC (space group R3m, Z = 3, a = 3.2135(8) A, c = 44.636(1) A, 187 refl., 21 param., R(1)(F) = 0.023, wR(2)(F(2)) = 0.043) can directly be derived from the binary nitrides AlN (wurtzite-type) and ScN (rocksalt-type). The anions form a closest packing with alternating double layers of C and O separated by an additional layer of N. Again, Al and Sc occupy tetrahedral and octahedral voids, respectively. All compositions were confirmed by energy dispersive X-ray spectroscopy (EDXS) measurements on single crystals. According to band structure calculations Sc(3)Al(3)O(5)C(2) is electron precise with a band gap of 0.3 eV. Calculations of charges and charge densities reveal that the mainly ionic bonding contains significant covalent contributions, too. As expected Sc and C show higher covalent shares than Al and O. The different coordinations of O, Al, and Sc are clearly represented in the corresponding p and d states.