Zewen Xiao

Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality

Searching for promising nontoxic and air-stable perovskite absorbers for solar cell applications has drawn extensive attention. Here, we show that a promising perovskite absorber should exhibit a high electronic dimensionality. Semiconductors that exhibit a high structural dimensionality, but a low electronic dimensionality have less promise as an absorber, because of barriers to isotropic current flow, enhanced electron/hole effective masses and fundamentally deeper defect states (more effective at causing recombination). Our concept accounts for the device performance of the perovskite-based solar cells reported in literature so far.

Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study

We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.

Thermodynamic Stability and Defect Chemistry of Bismuth-Based Lead-Free Double Perovskites

Bismuth- or antimony-based lead-free double perovskites represented by Cs2 AgBiBr6 have recently been considered promising alternatives to the emerging lead-based perovskites for solar cell applications. These new perovskites belong to the Fm3‾ m space group and consist of two types of octahedra alternating in a rock-salt face-centered cubic structure. We show, by density functional theory calculations, that the stable chemical potential region for pure Cs2 AgBiBr6 is narrow. Ag vacancies are a shallow accepters and can easily form, leading to intrinsic p-type conductivity. Bi vacancies and AgBi antisites are deep acceptors and should be the dominant defects under the Br-rich growth conditions. Our results suggest that the growth of Cs2 AgBiBr6 under Br-poor/Bi-rich conditions is preferred for suppressing the formation of the deep defects, which is beneficial for maximizing the photovoltaic performance.

Intrinsic Instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = Halogen) Double Perovskites: A Combined Density Functional Theory and Experimental Study

Recently, there has been substantial interest in developing double-B-cation halide perovskites, which hold the potential to overcome the toxicity and instability issues inherent within emerging lead halide-based solar absorber materials. Among all double perovskites investigated, In(I)-based Cs2InBiCl6 and Cs2InSbCl6 have been proposed as promising thin-film photovoltaic absorber candidates, with computational examination predicting suitable materials properties, including direct bandgap and small effective masses for both electrons and holes. In this study, we report the intrinsic instability of Cs2In(I)M(III)X6 (M = Bi, Sb; X = halogen) double perovskites by a combination of density functional theory and experimental study. Our results suggest that the In(I)-based double perovskites are unstable against oxidation into In(III)-based compounds. Further, the results show the need to consider reduction-oxidation (redox) chemistry when predicting stability of new prospective electronic materials, especially when less common oxidation states are involved.

Parity-Forbidden Transitions and Their Impact on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites

Using density functional theory calculations, we analyze the optical absorption properties of lead (Pb)-free metal halide perovskites (AB2+X3) and double perovskites (A2B+B3+X6) (A = Cs or monovalent organic ion, B2+ = non-Pb divalent metal, B+ = monovalent metal, B3+ = trivalent metal, X = halogen). We show that if B2+ is not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect band gap nature. Among the nine possible types of Pb-free metal halide double perovskites, six have direct band gaps. Of these six types, four show inversion symmetry-induced parity-forbidden or weak transitions between band edges, making them not ideal for thin-film solar cell applications. Only one type of Pb-free double perovskite shows optical absorption and electronic properties suitable for solar cell applications, namely, those with B+ = In, Tl and B3+ = Sb, Bi. Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications.

n -type conversion of SnS by isovalent ion substitution: Geometrical doping as a new doping route

Tin monosulfide (SnS) is a naturally p-type semiconductor with a layered crystal structure, but no reliable n-type SnS has been obtained by conventional aliovalent ion substitution. In this work, carrier polarity conversion to n-type was achieved by isovalent ion substitution for polycrystalline SnS thin films on glass substrates. Substituting Pb2+ for Sn2+ converted the majority carrier from hole to electron, and the free electron density ranged from 1012 to 1015 cm−3 with the largest electron mobility of 7.0 cm2/(Vs). The n-type conduction was confirmed further by the position of the Fermi level (EF) based on photoemission spectroscopy and electrical characteristics of pn heterojunctions. Density functional theory calculations reveal that the Pb substitution invokes a geometrical size effect that enlarges the interlayer distance and subsequently reduces the formation energies of Sn and Pb interstitials, which results in the electron doping.

Route to n-type doping in SnS

SnS is intrinsically a p-type semiconductor, and much effort has been made to attain n-type conduction. In this letter, we performed density functional theory calculations to seek an effective doping route for n-type SnS. It was found that aliovalent doping of SnS by Sb or Bi is not effective due to their high formation enthalpies; while the isovalent Pb-substitution of the Sn sites largely reduces formation enthalpies of Sn and Pb interstitials, which explain the recently demonstrated n-type conduction in the Sn1−xPbxS films fabricated under low H2S pressures.

Growth of high-quality SnS epitaxial films by H2S flow pulsed laser deposition

SnS epitaxial films were grown on MgO (100) substrates by pulsed laser deposition using a H2S gas as an S source. High growth temperature and high H2S gas flow rate caused re-evaporation and etching of the deposited films; therefore, the optimum condition was limited to a narrow region around 400 °C. The measured bandgap 1.08 eV is consistent with the previously reported theoretical calculation. The films with a S/Sn ratio of ∼1.0 showed the largest mobility of ∼37 cm2/(Vs). The hole transport was dominated by domain boundary potential barriers ∼0.05 eV in height.

Electron Confinement in Channel Spaces for One-Dimensional Electride.

Electrides are characteristic of anionic electrons trapped at the structural voids in the host lattice. Electrides are potentially useful in various technological applications; however, electrides, particularly their inorganic subgroup, have been discovered only in limited material systems, notably zero-dimensional [Ca24Al28O64](4+):4e(-) and two-dimensional [Ca2N](+):e(-) and [Y2C](1.8+):1.8e(-). Here, on the basis of density functional theory calculations, we report the first one-dimensional (1D) electride with a [La8Sr2(SiO4)6](4+):4e(-) configuration, in which the four anionic electrons are confined in the channel spaces of the host material. According to this theoretical prediction, an insulator-semiconductor transition originating from electron confinement in the crystallographic channel sites was demonstrated experimentally, where 10.5% of the channel oxygen was removed by reacting an oxygen stoichiometric La8Sr2(SiO4)6O2 precursor with Ti metal at a high temperature. This study not only adds an unprecedented role to silicate apatite as a parent phase to a new 1D electride, but also, and more importantly, demonstrates an effective approach for developing new electrides with the assistance of computational design.