No abstract

Dehydrogenation of five species including CH3OH, CH3O, H2COO, NH3, and H2O over clean and oxygen-modified copper surfaces has been investigated by the first-principle density functional calculations within the generalized gradient approximation. The reaction enthalpies and the activation energies have been calculated for 10 elementary steps corresponding to the direct and oxygen-assisted cleavage of X-H bonds (X = O, N, C). The DFT-GGA results showed that the pre-adsorbed oxygen always facilitates the dehydrogenation reaction by decreasing the reaction enthalpies and the activation energies. The obtained results are in general agreement with experimental observations.

Phase‐stable CsSnxPb1−xI3 perovskite quantum dots (QDs) hold great promise for optoelectronic applications owing to their strong response in the near‐infrared region. Unfortunately, optimal utilization of their potential is limited by the severe photoluminescence (PL) quenching, leading to extremely low quantum yields (QYs) of approximately 0.3 %. The ultra‐low sodium (Na) doping presented herein is found to be effective in improving PL QYs of these alloyed QDs without alerting their favourable electronic structure. X‐ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I− and Sn2+ ions upon Na doping, which potentially helps to stabilize Sn2+ and suppresses the formation of I vacancy defects. The optimized PL QY of the Na‐doped QDs reaches up to around 28 %, almost two orders of magnitude enhancement compared with the pristine one.

Low‐dimensional halide perovskites with broad emission are a hot topic for their promising application as white light sources. However, the physical origin of this broadband emission in the sub‐bandgap region is still controversial. This work investigates the broad Stokes‐shifted emission bands in mixed lead‐tin 2D perovskite films prepared by mixing precursor solutions of phenethylammonium lead iodide (PEA2PbI4, PEA = phenethylammonium) and phenethylammonium tin iodide (PEA2SnI4). The bandgap can be tuned by the lead‐tin ratio, whereas the photoluminescence is broad and significantly Stokes‐shifted and appears to be fairly insensitive to the relative amount of Pb and Sn. It is experimentally observed that these low‐dimensional systems show substantially less bandgap bowing than their 3D counterpart. Theoretically, this can be attributed to the smaller spin–orbit coupling effect on the 2D perovskites compared to that of 3D ones. The time‐resolved photoluminescence shows an ultrafast decay in the high‐energy range of the spectra that coincides with the emission range of PEA2SnI4, while the broadband emission decay is slower, up to the microsecond range. Sub‐gap photoexcitation experiments exclude exciton self‐trapping as the origin of the broadband emission, pointing to defects as the origin of the broadband emission in 2D Sn/Pb perovskite alloys.

A-site cations are usually composed of CH 3 NH 3 + (MA + ), CH(NH 2 ) 2+ (FA + ), and monovalent metal cations (such as Cs + ), the B-site cations are Pb 2+ or Sn 2+ , and the X-site anions are halogen ions. Recently, mixed A-site cation perovskite based on FA + and Cs + (FA 1−x Cs x PbI 3 ) becomes popular because of its desired optoelectronic properties and good thermal stability. [8][9][10][11][12][13][14][15] In addition, compared with pure FAPbI 3 with a tolerance factor slightly greater than 1 [16] and prone to phase transition into non-photoactive phase (δ phase) at room temperature, [17][18][19] FA 1−x Cs x PbI 3 shows better phase stability, in which photo active α phase is thermodynamically stable at room temperature. [16,[20][21][22] The improved phase stability of FA 1−x Cs x PbI 3 is stemmed from: 1) the similar crystal structure and volume per stoichiometric unit between α-FAPbI 3 and α-CsPbI 3 , which results in the small internal energy input during the formation of the mixed system α-FA 1−x Cs x PbI 3 ; 2) the slightly increased internal energy and greatly increased mixing entropy lead to a totally reduced free energy for α-FA 1−x Cs x PbI 3 system. [22][23][24] However, as a mixed system with ionic nature, FA 1−x Cs x PbI 3 still suffers from phase instability under operational conditions, e.g., light, heat, electrical field, and humidity. [21,[25][26][27] The phase instability issues of FA 1−x Cs x PbI 3 include phase separation and Phase instability is one of the major obstacles to the wide application of formamidinium (FA)-dominated perovskite solar cells (PSCs). An in-depth investigation on relevant phase transitions is urgently needed to explore more effective phase-stabilization strategies. Herein, the reversible phase-transition process of FA 1−x Cs x PbI 3 perovskite between photoactive phase (α phase) and nonphotoactive phase (δ phase) under humidity, as well as the reversible healing of degraded devices, is monitored. Moreover, through in situ atomic force microscopy, the kinetic transition between α and δ phase is revealed to be the "nucleation-growth transition" process. Density functional theory calculation implies an enthalpy-driven α-to-δ degradation process during humidity aging and an entropy-driven δ-to-α healing process at high temperatures. The α phase of FA 1−x Cs x PbI 3 can be stabilized at elevated temperature under high humidity due to the increased nucleation barrier, and the resulting non-encapsulated PSCs retain >90% of their initial efficiency after >1000 h at 60 °C and 60% relative humidity. This finding provides a deepened understanding on the phase-transition process of FA 1−x Cs x PbI 3 from both thermodynamics and kinetics points of view, which also presents an effective means to stabilize the α phase of FA-dominated perovskites and devices for practical applications.