On the efficiency of perovskite solar cells with a back reflector: effect of a hole transport material.

Organometal halide perovskites are promising, high-performance absorbers in solar cells. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Charge carrier management can be improved, taking into account the transport properties of layers surrounding the absorber. In particular, the choice of an appropriate hole-transport material (HTM) could provide a path towards increasing the device performance of perovskite solar cells (PSCs). The Lambertian reflection on the cell's back-surface reflector could increase the power conversion efficiency (PCE) of PSCs as well. Taking into account these facts, we analyse the absorptance and the PCE of a perovskite thin-film solar cell with the Lambertian reflection on the cell's back-surface reflector for various organic and inorganic HTMs. The analysis is done by means of the Monte-Carlo ray tracing simulations complemented by the transfer-matrix method to account for the interference phenomenon in the local generation rate G of carriers in a thin-film multilayer system. This function is employed further in the transport equations to calculate the current-voltage characteristics of the cell. We show that wide band gap HTMs, that possess negligible absorption, increase the photocurrent in the perovskite, passing reflected photons from the back reflector. In contrast, at the same perovskite thickness the PSC gains less photocurrent with narrow band gap HTMs, where an excessive non-radiative recombination takes place. Our analysis demonstrates that the optimal thickness of the solar cell with the typical absorber CH3NH3PbI3 is ∼300 nm, providing the maximal efficiency ∼18.8% for the wide band gap HTM (CuSCN) at the moderate absorber purity (the diffusion length D ∼ 1 µm).

Klein collimation by rippled graphene superlattice.

The hybridization of [Formula: see text] and [Formula: see text] orbitals of carbon atoms in graphene depends on the surface curvature. Considering a single junction between flat and rippled graphene subsystems, it is found an accumulation of charge in the rippled subsystem due to Klein penetration phenomenon that gives rise to n-p junction. Using this fact, we show that the momentum distribution of electrons in ballisitically propagating beam can be selective without a waveguide, or external electric, and/or magnetic fields in graphene strip under experimentally feasible one-dimensional periodic potential. Such a potential is created with the aid of superlattice that consists of periodically repeated graphene pieces with different hybridizations of carbon orbits, produced by variation of the graphene surface curvature. The charge redistribution and selected transmission of electrons, caused by the superlattice, allows to control the electron focusing in the considered system by simply changing the element properties in the superlattice.

Self-organization of charged particles in circular geometry.

The basic principles of self-organization of one-component charged particles, confined in disk and circular parabolic potentials, are proposed. A system of equations is derived, which allows us to determine equilibrium configurations for an arbitrary, but finite, number of charged particles that are distributed over several rings. Our approach reduces significantly the computational effort in minimizing the energy of equilibrium configurations and demonstrates a remarkable agreement with the values provided by molecular dynamics calculations. With the increase of particle number n>180 we find a steady formation of a centered hexagonal lattice that smoothly transforms to valence circular rings in the ground-state configurations for both potentials.

Reply to "Comment on 'Thomson rings in a disk' ".

We demonstrate that our model [Phys. Rev. E 91, 032312 (2015)PLEEE81539-375510.1103/PhysRevE.91.032312] serves as a useful tool to trace the evolution of equilibrium configurations of one-component charged particles confined in a disk. Our approach reduces significantly the computational effort in minimizing the energy of equilibrium configurations, and it demonstrates a remarkable agreement with the values provided by molecular-dynamics calculations. We show that the Comment misrepresents our paper and fails to provide plausible arguments against the formation hexagonal structure for n≥200 in molecular-dynamics calculations.

Thomson rings in a disk.

We discuss the basic principles of self-organization of a finite number of charged particles interacting via the 1/r Coulomb potential in disk geometry. The analysis is based on the cyclic symmetry and periodicity of the Coulomb interaction between particles located on several rings. As a result, a system of equations is derived, which allows us readily to determine with high accuracy the equilibrium configurations of a few hundred charged particles. For n≳200, we predict the formation of a hexagonal core and valence circular rings for the centered configurations.

Quantum nonequilibrium approach for fast electron transport in open systems: photosynthetic reaction centers.

Creation of electrons or excitons by external fields in a system with initially statistically independent unrelaxed vibrational modes leads to an initial condition term. The contribution of this term in the time convolution generalized master-equation approach is studied in the second order of the perturbation theory in path-integral formalism. The developed approach, applied for the analysis of dynamics in the photosynthetic reaction center, exhibits the key role of the initial condition terms at the primary stage of electron transfer.

Whispering gallery modes in open quantum billiards.

The poles of the S matrix and the wave functions of open two-dimensional quantum billiards with convex boundary of different shape are calculated by using the method of complex scaling. Two leads are attached to the cavities. The conductance of the cavities is calculated at energies with one, two, and three open channels in each lead. Bands of overlapping resonance states appear that are localized along the convex boundary of the cavities and contribute coherently to the conductance. These bands correspond to the whispering gallery modes known from classical calculations.