Wenmei Ming

Large dielectric constant, high acceptor density, and deep electron traps in perovskite solar cell material CsGeI3

Many metal halides that contain cations with the ns2 electronic configuration have recently been discovered as high-performance optoelectronic materials. In particular, solar cells based on lead halide perovskites have shown great promise as evidenced by the rapid increase of the power conversion efficiency. In this paper, we show density functional theory calculations of electronic structure and dielectric and defect properties of CsGeI3 (a lead-free halide perovskite material). The potential of CsGeI3 as a solar cell material is assessed based on its intrinsic properties. We find anomalously large Born effective charges and a large static dielectric constant dominated by lattice polarization, which should reduce carrier scattering, trapping, and recombination by screening charged defects and impurities. Defect calculations show that CsGeI3 is a p-type semiconductor and its hole density can be modified by varying the chemical potentials of the constituent elements. Despite the reduction of long-range Coulomb attraction by strong screening, the iodine vacancy in CsGeI3 is found to be a deep electron trap due to the short-range potential, i.e., strong Ge–Ge covalent bonding, which should limit electron transport efficiency in p-type CsGeI3. This is in contrast to the shallow iodine vacancies found in several Pb and Sn halide perovskites (e.g., CH3NH3PbI3, CH3NH3SnI3, and CsSnI3). The low-hole-density CsGeI3 may be a useful solar absorber material but the presence of the low-energy deep iodine vacancy may significantly reduce the open circuit voltage of the solar cell. On the other hand, CsGeI3 may be used as an efficient hole transport material in solar cells due to its small hole effective mass, the absence of low-energy deep hole traps, and the favorable band offset with solar absorber materials such as dye molecules and CH3NH3PbI3.

Chemical instability leads to unusual chemical-potential-independent defect formation and diffusion in perovskite solar cell material CH3NH3PbI3

Methylammonium (MA) lead triiodide (MAPbI3) has recently emerged as a promising solar cell material. However, MAPbI3 is known to have chemical instability, i.e., MAPbI3 is prone to decomposition into MAI and PbI2 even at moderate temperatures (e.g. 330 K). Here, we show that the chemical instability, as reflected by the calculated negligible enthalpy of formation of MAPbI3 (with respect to MAI and PbI2), has an unusual and important consequence for defect properties, i.e., defect formation energies in low-carrier-density MAPbI3 are nearly independent of the chemical potentials of constituent elements and thus can be uniquely determined. This allows straightforward calculations of defect concentrations and the activation energy of ionic conductivity (the sum of the formation energy and the diffusion barrier of the charged mobile defect) in MAPbI3. The calculated activation energy for ionic conductivity due to diffusion is in excellent agreement with the experimental values, which demonstrates unambiguously that is the dominant diffusing defect and is responsible for the observed ion migration and device polarization in MAPbI3 solar cells. The calculated low formation energy of a Frenkel pair and low diffusion barriers of and suggest that the iodine ion migration and the resulting device polarization may occur even in single-crystal devices and grain-boundary-passivated polycrystalline thin film devices (which were previously suggested to be free from ion-migration-induced device polarization), leading to device degradation. However, the device polarization due to the Frenkel pair (which has a relatively low concentration) may take a long time to develop and thus may avoid the appearance of the current–voltage hysteresis at typical scan rates.

Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH3NH3)2CdX4 (X = Cl, Br, I)

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MA2CdX4 (MA = CH3NH3; X = Cl, Br, I). MA2CdI4 is a new compound, whereas, for MA2CdCl4 and MA2CdBr4, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MA2CdX4 solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MA2CdCl4 forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MA2CdBr4 and MA2CdI4 adopt 0D K2SO4-derived crystal structures based on isolated CdX4 tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MA2CdX4 family impact their air stabilities, investigated for the first time in this work; MA2CdCl4 is air-stable, whereas MA2CdBr4 and MA2CdI4 partially decompose when left in air. Optical absorption measurements suggest that MA2CdX4 have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA2CdX4 yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MA2CdX4 following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MA2CdX4. On the basis of our combined experimental and computational results, MA2CdX4 and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.

Doping Y2O3 with Mn4+ for energy-efficient lighting

Developing energy-efficient LEDs that emit warm white light requires new red phosphors with appropriate emission wavelengths and band widths. Mn4+-activated Y2O3 is a potential red LED phosphor with narrow emission and improved emission wavelength compared to previously known Mn4+-activated oxide phosphors. In this work, the dopability and the oxidation state of Mn in Y2O3 are investigated based on the formation energies of native defects, Mn dopants, and divalent co-dopants (i.e., Ca, Sr, Cd, and Zn) calculated using hybrid density functional theory. We found that Mn4+ is difficult to form in Y2O3 without co-doping. Stabilizing Mn4+ on Y3+ sites (forming Mn+Y donors) requires the co-doping of compensating acceptors (Ca or Sr) in oxygen-rich growth environments.

Unraveling luminescence mechanisms in zero-dimensional halide perovskites

Zero-dimensional (0D) halides perovskites, in which anionic metal-halide octahedra (MX6)4− are separated by organic or inorganic countercations, have recently shown promise as excellent luminescent materials. However, the origin of the photoluminescence (PL) and, in particular, the different photophysical properties in hybrid organic–inorganic and all inorganic halides are still poorly understood. In this work, first-principles calculations were performed to study the excitons and intrinsic defects in 0D hybrid organic–inorganic halides (C4N2H14X)4SnX6 (X = Br, I), which exhibit a high photoluminescence quantum efficiency (PLQE) at room temperature (RT), and also in the 0D inorganic halide Cs4PbBr6, which suffers from strong thermal quenching when T > 100 K. We show that the excitons in all three 0D halides are strongly bound and cannot be detrapped or dissociated at RT, which leads to immobile excitons in (C4N2H14X)4SnX6. However, the excitons in Cs4PbBr6 can still migrate by tunneling, enabled by the resonant transfer of excitation energy (Dexter energy transfer). The exciton migration in Cs4PbBr6 leads to a higher probability of trapping and nonradiative recombination at the intrinsic defects. We show that a large Stokes shift and the negligible electronic coupling between luminescent centers are important for suppressing exciton migration; thereby, enhancing the photoluminescence quantum efficiency. Our results also suggest that the frequently observed bright green emission in Cs4PbBr6 is not due to the exciton or defect-induced emission in Cs4PbBr6 but rather the result of exciton emission from CsPbBr3 inclusions trapped in Cs4PbBr6.