Benzylamine is introduced as a surface passivation molecule that improves the moisture-resistance of the perovskites while simultaneously enhancing their electronic properties. Solar cells based on benzylamine-modified formamidinium lead iodide perovskite films exhibit a champion efficiency of 19.2% and an open-circuit voltage of 1.12 V. The modified FAPbI films exhibit no degradation after >2800 h air exposure.

Methylammonium lead iodide perovskite, CH 3 NH 3 PbI 3 (MAPbI 3 ), has made great progress in its efficiency as used in solid-state solar cells during recent years. Meanwhile, the degradation of its performance in moisture has attracted great attentions, but the specific mechanismis not yet fully established. The water effects on the detailed structure and properties of the perovskite CH 3 NH 3 PbI 3 have been carefully explored based on first-principles calculations. The results reveals that the water adsorption energy on the CH 3 NH 3 PbI 3 (001) surface is about 0.30 eV, while the water can easily penetrate into the surface in the form of molecular state owing to the huge interspace of CH 3 NH 3 PbI 3 , which can further corrode down the whole structure gradually. More importantly, the deformation of the structure greatly affects the electronic structure, which decreases the optical absorption. Such work paves an important way to understand the initial degradation progress of the perovskite structure under the humidity condition, which should help to optimize the structure to prevent the penetration of water in the system. The conversion of solar energy into electricity has attracted great attentions because of the increasing energy demands of future generations without negatively impacting the global environment. 1-2 On the other hand, dye-sensitized solar cells (DSCs) based on nanocrystalline metal oxides like TiO 2 3-4 are a promising photovoltaic device for a renewable energy source. In recent years, new organic-inorganic hybrid perovskite compounds (MAPbX 3 , X=halogen; MA=CH 3 NH 3 ) 5-11 have been used as light harvesters for solid-state DSCs. These MAPbX 3 compounds stand out for their low cost, wide light absorption, ferroelectric properties and high efficiency. 12-18 In fact, since the first reported perovskite solar cell with power conversion efficiency (PCE) of 3.81% by Kojima and co-workers in 2009, 19 the amazing growth rate of PCE about these perovskite materials has been made in the following years. In 2011, Park et al. fabricated MAPbI 3 perovskite solar cells with PCE of 6.54%. 20 Then Kim et al. achieved a PCE of up to 9.7% based on spiro-MeOTAD as hole transport materials in 2012. 21 In 2013, Noh et al. demonstrated highly efficient solar cells of a PCE of 12.3% as a result of tunable composition for MAPb(I 1-x Br x ) 3 . 22 In 2014, Grätzel and co-workers reported an efficiency of 17.01% by controlling the size of MAPbI 3 cuboids during their growth. 23 Up to now, the PCE of perovskite-based solar cells reaches to nearly 20%. 7 Although the methylammonium lead iodide MAPbI 3 perovskite shows an outstanding performance and tantalizing prospect in solar cells, there are deficiencies needed to overcome at the same time. One vital problems is that MAPbI 3 perovskite films are extremely sensitive to moisture in air. 7-8, 24-27 Many experiments have demonstrated that the effect of moisture on MAPbI 3 plays a crucial role in the performance of perovskite solar cells. 22, 28-30 In spite of various...

Conductive confinement of sulfur and polysulfide via carbonaceous blocking layers can simultaneously address the low conductivity, volume expansion of sulfur during charge/discharge process and polysulfides shuttling effect in lithium-sulfur (Li-S) batteries. Herein, conductive and porous nitrogen and phosphorus dual doped graphene (p-NP-G) blocking layer is prepared via a thermal annealing and subsequent hydrothermal reaction route. The doping levels of N and P in p-NP-G measured by the X-ray photoelectron spectroscopy are ca. 4.38% and ca. 1.93 %, respectively. The dual doped blocking layer exhibits higher conductivity than N or P single doped blocking layer. More importantly, the density function theory (DFT) calculation demonstrates that P atoms and -P-O groups in the p-NP-G layer offer stronger adsorption to polysulfides than the N species. The electrochemical evaluation results illustrate that the p-NP-G blocking layer could deliver superior initial capacity (1158.3 mA h/g at the current density of 1 C), excellent rate capability (633.7 mA h/g at 2 C), and satisfactory cycling stability (ca. 0.09% capacity decay per cycle), which are better than the N or P single doped graphene. This work suggests that this synergetic combination of conductive and adsorptive confinement strategies induced by the multi-heteroatoms doping scheme is a promising approach for developing high performance Li-S batteries.

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Charge carrier recombination plays a vital role in the CH 3 NH 3 PbI 3 perovskite solar cell. By investigating a possible synergy between ion migration and charge carrier recombination, we demonstrate that the nonradiative recombination accelerates by an order of magnitude during iodide migration. The migration induces lattice distortion that brings electrons and holes close to each other and increases their electrostatic interactions. The wave function localization in the same spatial region, and the enhanced lattice and iodide movements increase the nonadiabatic coupling.At the same time, quantum coherence lasts longer, because electron and hole energy levels become correlated. All these factors greatly increase the recombination rate. Moreover, the energy level of the iodide vacancy created during the migration moves from inside the conduction band in the equilibrated structure into the band gap, acting as a typical efficient nonradiative charge recombination center. Our work shows that the different dynamic processes are strongly correlated in halide perovskites, and demonstrates that defects, considered to be benign, can become very detrimental under non-equilibrium conditions. The reported results strongly suggest that ion migration should be avoided in halide perovskites, both for own reasons, such as large currentvoltage hysteresis, and because it greatly accelerates charge carrier losses.

Hybrid organic–inorganic perovskites have attracted considerable interest due to their impressive performance in solar energy applications. Many experiments show that a slight excess of PbI2 significantly enhances the properties of the most studied CH3NH3PbI3 compound. We use real-time time-dependent density functional theory and nonadiabatic molecular dynamics to demonstrate that the effect arises due to decreased electron–phonon interactions responsible for nonradiative charge recombination. The fast organic CH3NH3 + (MA) cations, present on surfaces of stochiometric and MAI-rich perovskites, are particularly mobile and introduce high-frequency phonons and strong electric fields that couple to the charge carriers and create large nonadiabatic coupling. Excess PbI2 decreases MA surface coverage, reduces the nonadiabatic coupling by up to an order of magnitude, and extends the charge carrier lifetime. Generally, charges in perovskites are long-lived because the nonadiabatic coupling is very small, less than 1 meV, and quantum coherence formed during charge recombination is short, less than 10 fs. Our results rationalize why decreasing the concentration of the organic cations on perovskite surfaces can suppress nonradiative charge carrier recombination and improve material performance.

Hybrid organic−inorganic perovskites, and particularly CH 3 NH 3 PbI 3 (MAPbI 3 ), have emerged as a new generation of photovoltaic devices due to low cost and superior performance. The performance is strongly influenced by current−voltage hysteresis that arises due to ion migration, and the challenge remains how to suppress the ion migration and hysteresis. Our first-principles calculations demonstrate that the energy barriers to diffusion of the I − , MA + , and Pb 2+ ions are greatly affected by dipole moments of the MA species. The energy barriers of the most mobile I − ion range from 0.06 to 0.65 eV, depending on MA orientation. The positively charged MA + and Pb 2+ ions diffuse along the dipole direction, while the negatively charged I − ion strongly prefers to diffuse against the dipole direction. By influencing ion migration, the arrangement of MA molecules can effectively modulate the current−voltage hysteresis intensity. The current work contributes to the fundamental understanding of the microscopic mechanism of ion migration in MAPbI 3 and suggests means to suppress the hysteresis effect and optimize perovskite performance. By demonstrating in detail how the arrangement of the organic molecules can efficiently influence ion migration and, hence, amplitude of the current−voltage hysteresis, our results suggest that the hysteresis effect can be suppressed and the long-term performance of perovskites can be improved, if the organic molecules are arranged and stabilized in an antiferroelectric order.

Methylammonium lead iodide perovskite, CH 3 NH 3 PbI 3 , is one of the most promising photovoltaic materials for low-cost and clean source of energy. In this work, the first-principles calculations were carried out to investigate the different composition of CH 3 NH 3 PbI 3 (001), including both methylammonium iodide terminated (MAI-T) and PbI 2 terminated (PBI 2 -T) surfaces. The calculated surface energies show that the MAI-T is thermodynamical more stable than the PBI 2 -T one under the equilibrium growth condition. The electronic properties of the two types of surfaces are also different. The band gap of PBI 2 -T is obviously smaller than that of MAI-T due to the surface Pb states. Band gaps of MAI-T decrease with increasing thickness, while band gaps of PBI 2 -T are insensitive to the slab thickness. The calculated optical absorption coefficients suggest that both terminations are effective solar energy absorbers in the visible light spectrum.