As an alternative to methylammonium lead triiodide (MAPbI), formamidinium lead triiodide (FAPbI) perovskites have recently attracted significant attention because of their higher stability and smaller band gaps. Here, based on first-principles calculations, we investigate systematically the intrinsic defects in FAPbI. While methylammonium (MA)-related defects MA and I in MAPbI have high formation energies, we found that formamidinium (FA)-related defects V, FA and I in FAPbI have much lower formation energies. Antisites FA and I create deep levels in the band gap, and they can act as recombination centers and result in reduced carrier lifetimes and low open circuit voltages in FAPbI-based photovoltaic devices. We further demonstrate that through cation mixing of MA and FA in perovskites the formation of these defects can be substantially suppressed.

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...

Based on our earlier works [X. Zheng et al., Phys. Rev. B 75, 195127 (2007); J. S. Jin et al., J. Chem. Phys. 128, 234703 (2008)], we propose a rigorous and numerically convenient approach to simulate time-dependent quantum transport from first-principles. The proposed approach combines time-dependent density functional theory with quantum dissipation theory, and results in a useful tool for studying transient dynamics of electronic systems. Within the proposed exact theoretical framework, we construct a number of practical schemes for simulating realistic systems such as nanoscopic electronic devices. Computational cost of each scheme is analyzed, with the expected level of accuracy discussed. As a demonstration, a simulation based on the adiabatic wide-band limit approximation scheme is carried out to characterize the transient current response of a carbon nanotube based electronic device under time-dependent external voltages.

Detailed balance between photon-absorption and energy loss constrains the efficiency of conventional solar cells to the Shockley-Queisser limit. However, if solar illumination can be absorbed over a wide spectrum by plasmonic structures, and the generated hot-carriers can be collected before relaxation, the efficiency of solar cells may be greatly improved. In this work, we explore the opportunities and limitations for making plasmonic solar cells, here considering a design for hot-carrier solar cells in which a conventional semiconductor heterojunction is attached to a plasmonic medium such as arrays of gold nanoparticles. The underlying mechanisms and fundamental limitations of this cell are studied using a nonequilibrium Green's function method, and the numerical results indicate that this cell can significantly improve the absorption of solar radiation without reducing open-circuit voltage, as photons can be absorbed to produce mobile carriers in the semiconductor as long as they have energy larger than the Schottky barrier rather than above the bandgap. However, a significant fraction of the hot-carriers have energies below the Schottky barrier, which makes the cell suffer low internal quantum efficiency. Moreover, quantum efficiency is also limited by hot-carrier relaxation and metal-semiconductor coupling. The connection of these results to recent experiments is described, showing why plasmonic solar cells can have less than 1% efficiency.

A linear-scaling time-dependent density-functional theory is developed to evaluate the optical response of large molecular systems. The two-electron Coulomb integrals are evaluated with the fast multipole method, and the calculation of exchange-correlation quadratures utilizes the locality of exchange-correlation functional within the adiabatic local density approximation and the integral prescreening technique. Instead of many-body wave function, the equation of motion is solved for the reduced single-electron density matrix in the time domain. Based on its ''nearsightedness'', the reduced density matrix cutoffs are employed to ensure that the computational time scales linearly with the system size. As an illustration, the resulting time-dependent density-functional theory is used to calculate the absorption spectra of linear alkanes, and the linear scaling of computational time versus the system size is clearly demonstrated.