Bistable Amphoteric Native Defect Model of Perovskite Photovoltaics

The past few years have witnessed unprecedented rapid improvement of the performance of a new class of photovoltaics based on halide perovskites. This progress has been achieved even though there is no generally accepted mechanism of the operation of these solar cells. Here we present a model based on bistable amphoteric native defects that accounts for all key characteristics of these photovoltaics and explains many idiosyncratic properties of halide perovskites. We show that a transformation between donor-like and acceptor-like configurations leads to a resonant interaction between amphoteric defects and free charge carriers. This interaction, combined with the charge transfer from the perovskite to the electron and hole transporting layers results in the formation of a dynamic <i>n-i-p</i> junction whose photovoltaic parameters are determined by the perovskite absorber. The model provides a unified explanation for the outstanding properties of the perovskite photovoltaics, including hysteresis of <i>J–V</i> characteristics and ultraviolet light-induced degradation

Computational Study of Halide Perovskite-Derived A₂BX₆ Inorganic Compounds: Chemical Trends in Electronic Structure and Structural Stability

The electronic structure and energetic stability of A₂BX₆ halide compounds with the cubic and tetragonal variants of the perovskite-derived K₂PtCl₆ prototype structure are investigated computationally within the frameworks of density-functional-theory (DFT) and hybrid (HSE06) functionals. The HSE06 calculations are undertaken for seven known A₂BX₆ compounds with A = K, Rb, and Cs; and B = Sn, Pd, Pt, Te, and X = I. Trends in band gaps and energetic stability are identified, which are explored further employing DFT calculations over a larger range of chemistries, characterized by A = K, Rb, Cs, B = Si, Ge, Sn, Pb, Ni, Pd, Pt, Se, and Te; and X = Cl, Br, I. For the systems investigated in this work, the band gap increases from iodide to bromide to chloride. Further, variations in the A site cation influences the band gap as well as the preferred degree of tetragonal distortion. Smaller A site cations such as K and Rb favor tetragonal structural distortions, resulting in a slightly larger band gap. For variations in the B site in the (Ni, Pd, Pt) group and the (Se, Te) group, the band gap increases with increasing cation size. However, no observed chemical trend with respect to cation size for band gap was found for the (Si, Sn, Ge, Pb) group. The findings in this work provide guidelines for the design of halide A₂BX₆ compounds for potential photovoltaic applications

Morphology-Independent Stable White-Light Emission from Self-Assembled Two-Dimensional Perovskites Driven by Strong Exciton–Phonon Coupling to the Organic Framework

Hybrid two-dimensional (2D) lead halide perovskites have been employed in optoelectronic applications, including white-light emission for light-emitting diodes (LEDs). However, until now, there have been limited reports about white-light-emitting lead halide perovskites with experimental insights into the mechanism of the broadband emission. Here, we present white-light emission from a 2D hybrid lead chloride perovskite, using the widely known phenethylammonium cation. The single-crystal X-ray structural data, time-resolved photophysical measurements, and density functional theory calculations are consistent with broadband emission arising from strong exciton–phonon coupling with the organic lattice, which is independent of surface defects. The phenethylammonium lead chloride material exhibits a remarkably high color rendering index of 84, a CIE coordinate of (0.37,0.42), a CCT of 4426, and photostability, making it ideal for natural white LED applications

Investigating the Role of Copper Oxide in Electrochemical CO₂ Reduction in Real Time

Copper oxides have been of considerable interest as electrocatalysts for CO₂ reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu₂O nanoneedles, Cu₂O nanocrystals, and Cu₂O nanoparticles. We show experimentally that the selective formation of gaseous C₂ products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu₂OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu₂O is present at the surface because Cu₂OR is kinetically and energetically more favorable than CO2R